GB2386754A

GB2386754A - Tunable optical source

Info

Publication number: GB2386754A
Application number: GB0223052A
Authority: GB
Inventors: Yakov Sidorin; Abdul Hadi Damiriji; Salah A Al-Chalabi
Original assignee: Optitune PLC
Current assignee: Optitune PLC
Priority date: 2001-11-30
Filing date: 2002-10-04
Publication date: 2003-09-24
Anticipated expiration: 2022-10-04
Also published as: GB2386754B; GB0223052D0

Abstract

A diffraction device 115 is used to provide wavelength selective feedback to a laser diode 100. The diffraction device 115 is at least partially fabricated in a material whose refractive index is controllable so as to control the diffraction performance of the device. This allows the diffraction device 115 to be used in tuning the optical source, potentially without using moving parts. For instance, the refractive index can be controlled using a voltage across electrooptic material of the device 115. The diffraction device 115 can be mounted in free space in an external cavity with respect to the laser diode 100. Tunable optical sources of this type can be used for instance in communications, particularly wavelength division multiplexing. The device may also be tuned by temperature control.

Description

TUNABLE FILTER

The present invention relates to tunable filters, and finds particular application in the use of gratings in tuning optical sources.

s Semiconductor laser diodes are known as optical sources. Various techniques are used to obtain single mode, narrow linewidth operation which is desirable in applications such as communications. For instance, unmodified edge emitting laser diodes typically operate with several longitudinal modes lasing simultaneously, leading to low coherence 10 and large linewidths. A technique known for use with edge emitting laser diodes is to use a grating to act as a wavelength filter in providing feedback to the lasing cavity to obtain single mode operation. Examples are the distributed feedback (DEB) and distributed Bragg reflector (DBR) lasers in which a grating extends at least partially along the length of the lasing cavity, or a neighbouring cavity, being formed for instance 15 in the surface of a substrate supporting the lasing cavity and any neighbouring cavity.

An external cavity laser (ECL) is another known arrangement which uses wavelength selective feedback to obtain single mode operation. In this case, the lasing cavity is coupled via an end facet to a further cavity, usually in free space, that contains a 20 wavelength-selective feedback mechanism to provide feedback to the lasing cavity. The wavelengthselective feedback mechanism typically comprises individually mounted wavelength-selective optics in the free space. In the ECL, the "lacing" cavity is sometimes described as a "gain" cavity rather than a lasing cavity as the end facet is anti-reflection coated, giving the laser diode the construction of a gain element rather 25 than a laser.

DFB/DBR and ECL lasers differ in more than one way. Firstly, DFB/DBR lasers are generally monolithic, their lasing and feedback sections being fabricated on the same substrate. This is in contrast to an ECL in which the wavelength-selective optics are 30 usually fabricated separately and mounted in relation to the lasing cavity in a separate mounting operation. Secondly, wavelength selection in an ECL is generally provided by some sort of filtering device having filtering elements arranged to provide a plane or surface which is transverse to the optical path of radiation from the lasing cavity and plays a part in deeming the length of the external cavity, often in free space. This is in

contrast to the distributed feedback arrangement in which the wavelength selection mechanism is distributed longitudinally along the optical path of the radiation in a waveguide. In an ECL, wavelength selection can be provided for instance by a diffraction grating operating in reflection mode and providing a facet in or of the 5 external cavity. The facet may uniquely, or in combination with one or more further feedback elements such as a mirror, define the physical length of the external cavity by providing a discrete change in direction in the optical path. In a DFB/DBR arrangement, wavelength selection is provided at least predominantly by a longitudinally extending, distributed grating such as a distributed Bragg reflector which does not play a part in 10 defining the length of a cavity but instead is part of a waveguiding structure.

In the above, reference is made to a diffraction grating operating "in reflection mode".

This can generally be taken to mean that the radiation incident upon the grating is diffracted by it to the same side of the grating as it was incident upon it. Thus a grating 15 device operating in reflection mode does not have to be transparent to the optical radiation throughout its thickness. In contrast, a diffraction grating device operating "in transmission mode" is generally transparent to the optical radiation to at least some degree throughout its thickness. Thus it can be used to diffract the incident radiation to either side of the grating. In this case, a portion of the incident radiation can be 20 collected on a different side of the grating from the side on which it was incident.

Lasers are also known which are tunable. Tunable optical sources are required in optical communications systems, cable television systems, local area networks and measurement equipment. For instance in wavelength division multiplexing as used in 25 communications it is necessary to provide optical sources which can operate at distinguishable wavelengths. Although an array of separate devices can be used, each tuned to one of the wavelengths, it becomes expensive to maintain a supply of backup lasers since there has to be a backup laser for every device in the array. In this scenario, it has been recognised that it is preferable to have a tunable laser as backup which can be 30 substituted for some, or indeed any, of the devices in the array. Tunable lasers can also provide significant improvement in local wavelength usage so that optical fibre carriers can be used more flexibly in a communications network with less dependence on centralised intelligence.

: - In communications, it would be desirable to have a source tunable over the low loss or low dispersion bandwidth windows of an optical fore for communication. Long distance communication is generally centred on 1310 nm and 1550 nm. In short distance communications such as Local Area Networks (LANs), the equivalent 5 bandwidth window might be centred on 650 rim or 850 nm.

Single mode semi-conductor lasers have been used as tunable sources. For instance, distributed feedback (DFB) lasers have been used but have had a limited tuning range, of the order of 15 nm. This reduces their usefulness in communications. For instance, 10 the International Telecommunications Union (ITU) band of optical channels centred nominally on 1550 nm covers a tuning range of the order of 30 nm.

ECL lasers have also been used as tunable sources. One known configuration uses a diffraction grating in the external cavity to filter a selected wavelength for feedback to 15 the lasing cavity. This is the Littrow configuration. The selected wavelength is diffracted back along the same path as it is incident on the grating so that the grating can provide an end facet of the external cavity in the manner of a retro- reflecting mirror. In the simplest case, wavelength tuning is achieved by controlling the angle of the grating to the incident beam axis. This determines the wavelength diffracted back to the lasing 20 cavity and thus the lasing wavelength.

In tunable sources of this general type, in which feedback is provided by reflection or by a diffraction grating operating in reflection mode, "mode hopping" can arise. This is due to the fact that there will be more than one resonant longitudinal mode for the 25 electromagnetic radiation along the optical path in which oscillation is occurring. To prevent modehopping, tuning without interruption of the phase of oscillation, or so-

called phase-continuous tuning, should be achieved. A way to do that is to keep the number of half-wavelengths in the optical path in the external cavity constant as the wavelength is tuned. There are lcnown techniques to prevent mode hopping.

In the Littrow arrangement, as mentioned above, tuning is achieved by adjusting the position of the grating but this requires great accuracy. Additionally, if there is a risk of mode hopping for instance because the desired tunable range is large enough, this has to be counteracted. The way this has been done is to move the grating to adjust the optical

r.\ _ 4 path length at the same time as carrying out tuning. The various moving parts involved in the optical path are difficult to align in manufacture and to maintain through the working life of the laser and the size of the overall configuration can be simply too large for many applications, being in some cases of the order of tens of centimetres.

There are various aspects of existing tunable sources which could be improved. There is a trade-off between tunable range and power. Some lasers can be configured to replace any of the lasers in today's 40channel wavelength division multiplexed communication systems but they won't sustain very long transmission distances. Other lasers deliver the 10 power but are not tunable over a wide enough range. Manufacturing costs can be high as some tunable sources are at the edge of what can be done in semiconductor technology and reliability and control are often a problem as the characteristics of individual lasers vary and every one then has to be characterized for use.

15 According to a first aspect of the present invention, there is provided a tunable optical source which comprises: i) a gain section for use in generating an optical output, ii) a feedback section to provide optical feedback to the gain section, the feedback section comprising a diffraction device arranged to provide wavelength selection in said 20 optical feedback by diffraction, wherein the tunable optical source further comprises control means for controlling the refractive index of material of the diffraction device to modify said wavelength selection. 25 known type of diffraction device will include a diffractive structure such as a diffraction grating. A diffraction grating can be generally described as an optical component that serves to modulate periodically the phase or the amplitude of incident optical radiation. It is usually constructed as a periodic structure in material of a device, for instance a periodic surface relief or an embedded periodic change in refractive index.

Changing the refractive index of material of the diffraction device can be used to change the diffraction grating performance and thus the spectral content of the feedback.

i 5 Embodiments of the invention have the ability to provide controllable wavelength selectivity without using moving parts. Manufacture can be relatively simple and reliability is less at risk.

S The diffraction device might comprise a reflection or a transmission grating. That is, in use, an incident beam may be diffracted at the diffraction device without passing completely through it (reflection grating) or at least part of the incident beam may be transmitted completely through the diffraction device (transmission grating). In either case, where at least some of the incident beam is reflected, this can be generally 10 described in that the diffraction device has at least one side and is arranged to provide wavelength selection in said optical feedback by diffracting radiation incident on that side back along a path which lies on that same side of the device.

By. I.-., In a preferred embodiment of the invention, the feedback section is an external cavity in 15 relation to the gain section. EIence it may have an end facet for returning radiation emitted by the gain section back to the gain section. The diffraction device can provide a reflecting facet in relation to the external cavity. This reflecting facet might be placed between the gain section and an end facet, to change the path of radiation travelling in the external cavity, for instance to reflect it onto a mirrored end facet. Alternatively, the 20 diffraction device might itself provide an end facet of the external cavity.

The external cavity may be provided in various ways and may comprise, but is not limited to, free space, an optical fibre or other waveguiding device or optical component, or any combination thereof. Where it comprises free space, the diffraction device may 25 be mounted in said free space.

The tunable optical source may further comprise collimating means for collimating optical radiation travelling in the external cavity. This is common practice where the external cavity comprises free space. Where optical radiation incident on the diffraction 30 device, in use, is collimated, the diffraction device extends in a direction which is at least partially transverse to the direction of the incident optical radiation. Although not essential, for most purposes, the diffraction device in an embodiment of the invention will extend wholly across the path of the collimated incident optical radiation. This transverse arrangement is in contradistinction to DEB or DBR arrangements where the

optical radiation is waveguided and a grating is generally disposed along the waveguiding direction of a cavity.

Conveniently, the diffraction device will comprise a grating to provide diffraction of 5 incident optical radiation. Gratings of known type can be used.

In order for the diffraction device to control wavelength selection, it is clearly important that the optical path of the radiation undergoing wavelength selection passes through material of the diffraction device. The wavelength is selected by adjusting the 10 diffraction performance of the diffraction device and this is done by changing the refractive index of at least one material in the diffraction device.

There are several known methods for constructing diffraction gratings and these may result in the device being constructed in one material only or being constructed in two or 15 more different materials. Any of these methods can be used with the proviso that the radiation passes through at least one material of the diffraction device whose refractive index can be controlled so as to adjust the diffraction performance. The grating itself may for instance be a surface relief or a volume grating. Material whose refractive index can be controlled to provide wavelength selectivity in the feedback might be the 20 material in which the grating is formed or might be material which lies in a separate but adjacent layer such as a coating layer on a surface relief grating, or indeed both. In order to control grating performance, however, the material will be material through which the optical radiation is incident on the grating.

25 In a preferred embodiment of the present invention, the diffraction device is arranged in a Littrow configuration with respect to the gain section. There is an entry point where an incident optical beam from the gain section enters the material of the diffraction device whose refractive index is controlled to modify the wavelength. That envy point lies in a plane which is normal to the incident beam in use but the diffractive structure of 30 the diffraction device and the entry point are not in parallel planes. Such an arrangement can be relatively simple in design, fabrication and use.

Although the abovementioned entry point might be provided by a surface of the diffraction device, it may also be provided by an interface between one material and

another, or indeed just by a position in the diffraction device where the refractive index of the material comes under control. For example, if the refractive index is controlled by the use of an electric field, the entry point might be determined by the position of

electrodes for applying the electric field. It has been found important in making the

5 present invention, to support a simple relationship between the refractive index and wavelength selectivity, that the entry point is normal to the incident beam and hence the electric field in such an arrangement must generate a change in refractive index which

occurs only in a direction parallel to the direction of the incident beam.

lO Depending on the characteristics of material selected for the diffraction device, the control means might comprise for instance means, such as electrodes, to apply a voltage or a temperature change across material of the diffraction device. Electrodes used in a diffraction device according to an embodiment of the invention can be fabricated using known techniques such as epitaxy and/or lithography.

The effect of an electric field produced by electrodes will depend on the nature of the

material of the device. In a first embodiment of the invention, the material may be thermo-optic and the effect of the electric field will then be control of the refractive

index of the material via temperature control. In a second embodiment of the invention, 20 the material may be electro-optic and the effect of the electric field is control of the

refractive index of the material in accordance with the electro-optic coefficient of the::; material. Materials which might be used in a diffraction device for use in an embodiment of the 25 invention, whose refractive index can be controlled by the control means, can be selected from (but are not limited to) the group comprising: glasses, photonic crystals, electro-optic polymers, thermo-optic polymers, liquid crystals, organic crystals, semiconductor materials selected from group IV of the periodic table, compound semiconductor materials selected from one or more of the III-V, II-VI and IV-VI groups 30 of the periodic table, and photorefractive materials.

The tunable optical source may be of known type and may comprise for example a laser diode which is constructed as an edge emitting double heterostructure. Materials which might be used in the gain section for use in an embodiment of the invention need to meet

003-1038

the requirement that the source can be tuned over the required range by means of wavelength selective feedback. Suitable materials can be selected from (but are not limited to) semiconductor materials from the III-V groups of the Periodic Table, such as GaInAsP/InP. Other examples of suitable materials can be selected from the group AL 5 xGaxAs, IN xGaxAs ypy7 AlxGayIn-x-yp and IN xGaxAs.

According to a second aspect of the present invention, there is provided a diffraction device for providing wavelength selection in optical radiation, the device comprising: i) a body of material having a controllable refractive index; 10 ii) an entry point to the body of material for an incident beam of optical radiation; iii) a diffractive structure, and iv) control means for controlling said refractive index, wherein the diffractive structure is arranged to extend at least generally transverse to the optical path of the optical radiation in use of the diffraction device, but non-parallel to 15 said entry point, whereby changes in the controllable refractive index provide said wavelength selection.

Examples of diffraction device suitable for use in embodiments of the invention might comprise a body of material which is a prism, having a triangular or trapezoidal cross 20 section, the prism having a first surface providing the entry point and a second surface carrying the diffractive structure.

Electrodes to control the refractive index of the material of the prism might then be disposed on facets of the prism which are not in the optical path of the incident beam, 25 such as end or side facets which might be parallel to the optical path.

The diffractive structure might be provided by a phase or an amplitude grating, for instance as a surface relief grating, or an embedded or volume grating in or adjacent to the second surface.

The grating may in practice be covered in the finished diffraction device by further material and this further material might be used to control the reflectivity of the grating.

For example, the further material might be in the form of a thin-film coating or it might

: have a separately controllable refractive index. This provides a way of controlling the level of output of the tunable optical source independently of wavelength range.

The embedded or volume grating may in practice be adjacent to, even exposed at, a 5 surface of the piece of material it is formed in.

Preferably, a collimating device is provided to collimate radiation received at the entry point. Where the diffraction device comprises a prism, the collimating device might conveniently be constructed as a lens supported by an input facet of the prism. For 10 example, the lens might be a diffractive lens, of the Fresnel type, or a refractive lens constructed as a convex contour. The lens might be constructed in the material of the prism or might be constructed in material which is additional to but mounted on, the prism. 15 A diffraction device according to this second aspect of the present invention is preferably arranged in a Littrow configuration with respect to the gain section, in use, and the entry point lies in a plane which is normal to the incident beam.

As in embodiments of the invention in its first aspect, the entry point might be provided 20 by a surface of the diffraction device, by an interface between one material and another, or indeed just by a position in the diffraction device where the refractive index of the material comes under control. Again, it is preferable that the entry point is normal to the incident beam and hence, where the entry point is provided by said position in the diffraction device, any change in refractive index should occur only in a direction 25 parallel to the direction of the incident beam.

A novel aspect of embodiments of the present invention is the use of a diffraction grating "back to front". It is known to use a grating device so that optical radiation is incident directly on the grating. The material which carries the grating is disposed either 30 entirely or mostly behind the grating with respect to an incident optical beam. This is done to reduce or avoid for instance optical losses in the material. In a relatively simple form of an embodiment of the present invention, a diffraction grating device is used which is arranged so that material which carries the grating is disposed either entirely or mostly in front of the grating with respect to an incident optical beam. In its most

-: - simple form, the grating might be formed as a surface relief on a surface of a body of material. In this form, the body of material would be arranged so that the surface relief was on a back face of the body of material with respect to incident optical radiation.

5 In a third aspect of the present invention, there is provided a tunable optical source which comprises: i) a gain section for use in generating an optical output, ii) a feedback section to provide optical feedback to the gain section, wherein the feedback section comprises a diffraction device having a diffractive 10 structure arranged to provide wavelength selection in said optical feedback by diffraction, the diffraction device being arranged in relation to the gain section such that, in use, radiation from the gain section is incident on the diffractive structure through material of the diffraction device.

15 The diffractive structure may be formed in or adjacent to a surface of a body of material and the diffraction device can then be arranged in relation to the gain section such that, in use, radiation from the gain section passes through the body of material, or substantially through it, to the diffractive structure. The diffraction device may be mounted in free space and such that the diffractive structure extends primarily in a 20 direction which is transverse to the direction of radiation incident on the diffractive structure in use.

The feedback section of a tunable optical source in an embodiment of the invention might comprise means for modifying optical characteristics of radiation travelling in the 25 external cavity. This might be one or more of devices such as waveplates, filters or apertures for instance. It may be advantageous that the feedback section comprises optical path control means for controlling the length of the optical path of radiation in the feedback section. This can be used for example to avoid mode hopping. For instance, the optical path control means might comprise a material whose refractive 30 index is modifiable and the optical source might then include means to modify said refractive index so as to control the length of the optical path of radiation in the feedback section. Although other configurations could be used, where the optical path control means is moved to control the optical path, such as a rotatable Fabry-Perot etalon, an

arrangement in which the refractive index is modified lends itself to use in a tunable optical source in which tuning is achieved without any moving parts.

An optical source according to the invention may comprise an array of two or more 5 tunable optical sources. In such an array, a diffraction device might provide wavelength selection in the feedback to more than one gain section. However, more conveniently, a set of individual diffraction subdevices might be used to provide wavelength selective feedback to individual respective gain sections. These diffraction sub-devices might be constructed separately or might be monolithically connected to each other.

According to a related aspect of the present invention, there is provided a diffraction device which can be used in a tunable optical source according to the embodiment of the present invention as described above. Such a diffraction device would generally comprise material having a controllable refractive index, diffraction means disposed in 15 or on the material, and control means for controlling the refractive index of the material so as to control wavelength selectivity of the diffraction means.

Although it might be preferable, it is not essential that an embodiment of the present invention is used to provide wavelength selectivity on its own in an optical source. It 20 might be that it is used in conjunction with another technique. For instance, there may be a coarse control over wavelength selectivity provided by a moving part with a diffraction device according to an embodiment of the present invention being used to provide fine tuning.

25 Embodiments of the present invention can also or alternatively be used to retune the output of an optical source which has drifted over time.

A particularly appropriate use of embodiments of the present invention is in wavelength division multiplexing and in a further related aspect of the present invention, there is 30 provided a wavelength division multiplexing system comprising a tunable optical source or diffraction device according to an embodiment of the present invention.

According to a still further related aspect of the present invention, there is provided a method of selecting the output wavelength or wavelength range of an optical source, the

A source comprising a gain section and a wavelength selective feedback section, the feedback section comprising a diffraction device arranged to provide at least part of the wavelength selectivity, which method comprises modifying the refractive index of material of the diffraction device so as to modify one or more diffraction characteristics 5 of the diffraction device.

Said method may comprise modifying the refractive index of first selected material of the diffraction device so as to modify one or more diffraction characteristics of the diffraction device, and may further comprise modifying the refractive index of second 10 selected material of the diffraction device so as to modify reflectivity of the diffraction device. This provides a way of controlling the level of output of the tunable optical source independently of wavelength range.

As well as the aspects of the invention described above, the invention also provides a 15 diffraction device for use in any one or more of said aspects. Further, any feature described in relation to one aspect of the invention may be applied in relation to one or more other aspects of the invention if appropriate.

A tunable source will now be described as an embodiment of the invention, by way of 20 example only, with reference to the accompanying figures in which: Figures 1A and 1B show schematically two external cavity configurations for tuning the output wavelength of an optical source; Figure 2 shows an isometric view of a diffraction device shown in Figure 1; Figure 3 shows an expanded view of the incidence of optical radiation emitted by a laser 25 diode on the diffraction device of Figure 2, operating in transmission mode, Figure 4A shows a trapezoidal version of the diffraction device of Figure 3; Figure 4B shows a three quarter view of a trapezoidal version of the diffraction device of Figure 3; Figure 5 shows the diffraction device of Figure 3 with an integrated lens; 30Figure 6 shows a trapezoidal version of a diffraction device for use in the arrangement of Figure 1 with an embedded grating in place of a surface relief grating; Figure 7 shows schematically an array of tunable optical sources, each generally configured as shown in Figure 1A, having individual respective sub-diffraction gratings; and

Figure 8 shows a contour plot of wavelength tuning ranges achievable in a tunable optical source by means of different temperature changes.

It should be noted that none of the dimensions shown in the figures, such as the 5 dimensions of components or their relationships, is to scale. These figures are schematic only. 1. DIFFRACTION DEVICE: GENERAL CONFIGURATION

10 Referring to Figure 1A, an external cavity optical source according to an embodiment of the present invention comprises a semiconductor laser diode 100 which in use emits electromagnetic radiation into an external cavity with an end facet provided by a diffraction device 115 which comprises a grating operating in reflection mode. The laser 100 has an end facet with high reflectivity 105, provided in known manner, and an 15 end facet with low reflectivity 110, also provided in known manner, for emitting electromagnetic radiation into the external cavity. The laser diode 100 is selected to have a wide gain spectrum in a desired wavelength range and in this example is selected to operate over a range centred on 1550 nm. This is one of the windows within which optical fibres are suited to carry communications signals. Examples of alternatives 20 would be lasers tunable over wavelength ranges centred on 650 nm, 850 rim and 1310 nm, also used in communications. Other spectral regions will also be of interest, for instance for non-communication applications.

Any appropriate laser type could be used but a basic requirement is of course that it 25 should be tunable over the necessary range by means of wavelength selective feedback.

An example of a laser diode 100 type known for use over wavelength ranges centred on 1310 nm and 1550 rim is an edge emitting double heterostructure, using semiconductor materials from the III.V groups of the Periodic Table, such as GaInAsP/InP. Other examples of suitable known materials for use over various wavelength ranges can be 30 selected from the group AL xGaxAs, IN xGaxAs yPy, AlxGayIn x yP and IN xGaxAs.

The optical radiation from the laser 100 is collimated by a lens 120 in the external cavity before reaching the diffraction device 115 as an input beam 145. The lens 120 also collects the optical feedback from the diffraction device 115, which has acted as a

'a wavelength selective filter, and returns it to the laser diode 100. This provides wavelength selective feedback which can be used to select the wavelength of the optical radiation generated in the laser 100.

5 The configuration shown in Figure 1 A is a Littrow configuration in which the first order diffraction beam is returned by the diffraction device 115 along the same path as the incident beam 145. This Littrow configuration is one of several alternatives that might be used and reference might be made to "Tunable External Cavity Semiconductor Lasers" by P Zorabedian, published in the "Tunable Laser Handbook"by Academic 10 Press, 1995, edited by FJ Duarte, San Diego, California, or to "Tunable Laser Diodes" by M-C Amann and J Buus, published by Artech House, 1998, London.

The external cavity in this arrangement extends between the end facet with low reflectivity 110 of the laser diode 100 and a facet of the diffraction device 115 where 15 diffraction occurs. The diffraction device 1 15 is thus providing a second end facet of the external cavity. The optical length of the external cavity, which can be adjusted if required to avoid mode hopping, is the optical distance between the end facet 110 of the laser diode and the facet of the diffraction device 1 15 where diffraction occurs. The lens 120 needs to be at a distance from the laser diode 100 equal to its focal length. The 20 length of the external cavity will therefore have to be greater than the focal length of the lens 120. In practice, the length of the external cavity is likely to be chosen according to other factors combined such as the effect it has on output linewidth and packaging considerations. A typical length for an external cavity which could be used would be on the order of from 1 to 3 ems.

The optical output of the source is obtained as the transmitted beam 146, that is a beam which has passed through the diffraction device 115 from one side to another. This output might be used directly in some applications but in the communications arrangement shown, after passing through an optical isolator 125, it is collected in 30 conventional manner by a lens 135 for coupling into an optical fibre 130. The isolator prevents feedback from the fibre 130 towards the source and again is a known component.

! Referring to Figure 1B, it is not necessary that the optical output be collected from beyond the diffraction device 115. In this arrangement, the output is collected from the end facet 105 of the laser diode 100 which lies away from the external cavity and delivered via lenses 133, 135 and an isolator 125 to a fibre 130. The diffraction device 5 115 is operating here in wholly "reflective" mode, diffracting incident radiation back to the same side of the device as it was incident without transmitting any portion of it.

Although not shown, other intra-cavity elements may be found useful and embodiments of the invention do not exclude them.

Referring to Figures 2A and 2B, the diffraction device 115 itself comprises a prism' optionally referred to herein as a "microprism" in view of its relatively small dimensions, of material having a cross section which is a triangle, in this case rights angled. The incident bean 145 is received at an input facet 200, normal to the beam, 15 and delivered to a diffractive structure 225 on the facet 205 which provides the hypotenuse of the right-angled triangle.

Electrodes 220 are provided on the side facets 215 of the microprism in a symmetrical arrangement for providing an evenly distributed electric field to the material of the

20 microprism. Depending on the characteristics of the material, this field may control the

temperature of the material, and thus its refractive index, or may control the refractively.

index electro-optically.

Referring to Figure 3, the manner in which the diffraction device llS diffracts the 25 optical radiation back towards the laser diode 100 is important. A diffractive structure, in this case comprising a set of grating elements 320 on the "hypotenuse" facet 205, is arranged in a Littrow configuration in relation to the incident beam 145. This means that feedback optical radiation 147 travels back to the laser diode 100 along the same path 145 as it was incident on the diffraction device 115. In order to select the 30 wavelength which is fed back, and thus control the output wavelength of the tunable source as a whole, the refractive index "n" of the material of the diffraction device 115 is controlled. This changes the optical performance of the diffractive structure so that the wavelength of the feedback optical radiation 147 can be modified.

It might be noted that it is not essential that the cross section of the prism of the diffraction device 115 is based on a right-angled triangle. The lowermost facet shown in Figure 3 for example is not directly involved in the optical feedback mechanism and might be at any of various angles with respect to the other facets. Indeed this lowermost 5 face as shown is not necessarily planar, although it is likely to be simpler to fabricate if it is planar.

The diffractive structure 225 is fabricated as a surface relief on the microprism surface 205 and may be subsequently bonded to or overgrown with a coating layer 325 to 10 control reflectivity. For example, a coating layer 325 might be a thin film coating preselected to produce a reflection coefficient at the diffractive structure 225 which meets a desired target. Alternatively, the coating layer 325 might have greater thickness and have a refractive index which is controllable for example by the application of an electric field or heat, independently of the refractive index of the main body of the

15 diffraction device 115. This latter arrangement is further discussed below with reference to Figure 4B.

Suitable materials and fabrication techniques are known for thin film coating layers 325 for this purpose. Where the coating layer 325 has greater thickness and has a refractive 20 index which is independently controllable, then thermo-optic and electro-optic materials such as those disclosed in this specification might be used.

The front facet 200 of the diffraction device 115, or whichever surface optical radiation enters the device 115 through in use, might also be provided with a coating 330, for 25 instance of known type for antireflection purposes.

Referring to Figure 4A, in an alternative arrangement the cross section of the microprism might be that of a truncated triangle, or trapezium. The diffractive structure 225 is disposed on the rear facet with respect to an incident beam 145. Although the 30 electrodes 220 are again best provided on the side facets 215 of the trapezium, in a position equivalent to that shown in Figure 2B, it might be noted that they do not necessarily extend the full length of the diffraction device 115 in the direction of the incident beam. It is important in the embodiment described here though that the change in refractive index they produce in the material of the diffractive device 115 in use is

evenly distributed across the XY plane and only varies, if at all, in the "Z" direction, parallel to the direction of the incident beam 145.

Other configurations of electrode 220 may be found beneficial in different 5 circumstances. For instance another shape of electrode 220 might be used to create an electric field of different distribution in the diffraction device 115.

Referring to Figure 4B, where a coating layer 325 of significant thickness is used over the diffractive structure 225 (not shown in Figure 4B), its refractive index can be 10 controlled by a second pair of electrodes 425, independently of the refractive index of the main body of the diffraction device 115. This can be used to modify the reflectivity of the diffractive structure 225 and thus the balance between a feedback beam 147 and a transmitted beam 146. Firstly, this coating might be used for example to counteract they; change in reflectivity of the diffractive structure 225 which would otherwise occur in air 15 when the refractive index of the material at the diffractive structure 225 is modified to select a different feedback wavelength. Secondly, this coating might be used more generally to change the balance between the feedback beam 147 and the transmitted beam 146 as required.

20 (Although not shown, the two pairs of electrodes 220, 425 must of course be separated by insulation material if they are to be independently operable.) Referring to Figure 5, in a further alternative arrangement the diffraction device 115 comprises a microprism of material 500 with controllable refractive index together with 25 a lens carrying portion 505 of material of constant refractive index. The lens carrying portion 505 is mounted on the input facet 200 of the microprism in order to collimate incident radiation 145 emitted by the laser 100. The distance between the lens 510 and the end facet of the laser 100 is necessarily the focal length of the lens 510.

30 The lens-carrying portion 505 can be made of ordinary optical quality glass with dn/dT 1*10-5, while the microprism 500 can be made of a hybrid glass (further discussed below) with dn/dT -30*10-5. Thus the lens 510, whether it is defined by a curved refractive surface as shown in Figure 5 or by a diffractive surface such as a Fresnel lens, will be only marginally affected when the microprism 500 is heated to achieve tuning.

The diffraction device 115 is positioned in such a way that its lens is axially aligned with the incident beam 145. Typical dimensions for the lens might be: focal length in the range 0.5 to 2.0 mm 5. clear aperture diameter about 2 mm or so numerical aperture NA O.S Lenses of these dimensional characteristics are commercially available, for instance as products of LightPath Technologies Inc. However, an example of a technique to 10 fabricate such a lens on a diffractive device 115 for use in an embodiment of the present invention is moulding, whether the lens is of refractive or diffractive type.

In this arrangement, electrodes (not shown) may again be mounted on the side facets 215 of the microprism.

1.1 Entry Point In all of the arrangements described above, and particularly with reference to Figures 3 to 6, the incident beam 145 travels through various portions of diffractive devices 115 having different parts and configurations. However, in each case there is an entry point 20 345 at which the incident beam 145 enters the material of the diffraction device llS whose refractive index can be controlled to give wavelength selectivity in the feedback beam 147.

The entry point might lie on a facet of the diffraction device 115, such as the facet 200 25 receiving the incident beam 145 as shown in Figure 3. Alternatively, the entry point 345 might lie at an interface between one part of the diffraction device 115 and another part.

An example of an envy point 345 being at an interface is shown in Figure 5 where the entry point 345 lies at the interface between the front facet 200 of the prism 500 and a lens carrying portion SOS.

Referring to Figures 4A and 4B however, it is important to note that the entry point need not lie at a surface or interface at all. In the configuration shown in Figures 4A and 4B, the entry point 345 is simply defined by the beginning of the region of material in which an electric field is provided in use. Referring in particular to Figure 4B, the beginning of

'A -- j the region of material is defined here by the plane between the ends 420 of the electrodes 220 delivering the electric field. Thus the entry point 345 here lies part way

along the material of the diffractive device 115.

5 In each of these configurations, the entry point 345 lies in a plane which is normal to the axis of a collimated incident beam 145 and this provides a simple and effective arrangement with direct correlation between the controllable refractive index and feedback wavelength, described below.

10 1.2 Diffraction Theory To control the wavelength that is diffracted by the device 1 15, the refractive index of the body of material carrying the diffractive structure 405 (the "incident medium") needs to be modified. The general equation for diffraction, showing the effect of the refractively.

index "n" of the incident medium is: m- = d(sinoinc - sink) where Zinc iS the angle of incidence within the incident medium on the grating elements 320 20 Edify is the angle of diffraction within the incident medium at the grating elements 320 m is the diffraction order 40 is the free space wavelength d is the grating period n is the refractive index of the incident medium through which the incident and 25 diffracted beams are travelling If the first order of diffraction is being considered, in the Littrow configuration, and the incident and diffracted beams follow the same optical path, this resolves to: 30 2 tin singing = 20

fit :' 20 In either case, it can be seen that the refractive index n and the wavelength 20 which travels back to the laser diode 100 have a direct relationship and control over "n" gives control over ''\0'' and thus over the output wavelength of the overall assembly shown in Figures 1 A and 1 B. s The diffraction device 115 is shown in Figures 2 to 5 as being disposed so that the input facet 200 of the microprism is perpendicular to the incident radiation 145. Zinc, the angle of incidence within the incident medium on the grating elements 320, is thus the same as the angle between the facet 205 containing the grating surface and the input lO facet 200. As shown, OinC is approximately 45 .

This angle Zinc can be varied subject to the shape of the diffraction device 115. Higher values of Pine can lead to difficulties with the device construction; on the other hand it can improve the spectral resolution, 670, of the diffraction device 115, and thus the 15 resolution of the tunable optical source as a whole, according to: 6Xo [ho cot(0inc)] / [2nW] where 20 To is the free space wavelength to which the source is currently tuned W is the width of the incident beam Dine and n are as given above.

In a diffraction device 115 fabricated in lithium niobate, for example, a spectral 25 resolution as low as 0.05 rim can be achieved at grazing angles of incidence for a several millimetre wide incident beam at 1550 rim wavelength.

The surface relief gratings described above are conventional binary gratings but other forms can be used, including either phase or amplitude diffraction gratings. Referring to 30 Figure 6, a known alternative form of grating which can be used is a volume grating, where the diffractive elements of the grating might be formed with a different material from that of the material of the diffractive device 115 and embedded. Further alternative grating types include blazed and kinoform (multistep) gratings.

There are many published texts available on diffraction grating theory and fabrication.

Examples are the Diffraction Grating Handbook published by the Richardson Grating Laboratory, fourth edition (2001), "Diffraction Gratings and Applications" by E Popov 5 and EG Loewen, "Digital Diffractive Optics: an Introduction to Planar Diffractive

Optics and Related Technology" by B Kress and P Meyrueis, and "Selected Papers on Diffraction Gratings" from the Milestone Series, volume MS83, published in 1993, edited by D Meyske. Various volume holographic gratings are disclosed in the journals Applied Optics (Balberg et al, volume 37, No. 5, 1998 for example, and Pesach et al, 10 volume 39, No. 5, 2000) and Optics Letters (Pesach et al, volume 23, No. 8, 1998).

It might be noted that Figure 6 shows a further variation of the diffraction device 1 15. It is based on a trapezoidal prism, in the manner of the diffraction device 115 shown in Figure 4, but the entry point 345 lies at the front facet 200 of the prism, facing the gain 15 section (not shown). This version of the diffraction device 115 can be produced by using the arrangement of Figure 4 but extending the electrodes 220 to cover the whole surface of the side facets 215 of the prism.

2. ARRAYS

Referring to Figure 7, there can be significant advantages in providing an array 700 of laser diodes as tunable optical sources. The array 700 may be linear or two-dimensional and it may be monolithic or integrated from separate laser sources. If it is monolithic, the laser diodes will usually have substantially the same optical characteristics, 25 differences being defined only by imperfections of the laser fabrication process.

Referring to Figure 7, in an array 700 in which the laser diodes are all the same, or almost the same, as each other in optical terms, they may be tuned using a set of individual diffraction sub-devices 115a to 115n either separate or monolithically 30 connected to each other. Although not essential, the entry points of some or all of the diffraction sub- devices may be aligned in one plane.

- 22 Where a set of monolithically connected sub-devices are used, these can be considered and manoeuvred as a single diffractive device 115 which can be mechanically advantageous particularly in assembly and alignment.

5 This type of array might be useful, for example, in achieving higher power outputs than would be achievable using a single laser diode, or might be used to produce a spatially spread signal source, or might be used to provide multiple signal sources for coupling into a plurality of separate optical fibres.

10 Such an array may be used as a wavelength division multiplexing source. Individual diffraction sub-devices l l 5 are driven by separate sets of electrodes and potentially have different respective characteristics for tuning purposes. Any one of the elements of the tunable laser source can be tuned or retuned to a different output wavelength as required or can be individually retuned to compensate for changes in performance over time.

As shown in Figure 7, the diffraction device or devices can be operated in a partially transmissive configuration, the tuned output 146 being collected from the far side of the device(s) with respect to the external cavity. This is not however necessarily the case as discussed above with respect to the single laser diode configuration shown in Figure 1 B. The optical arrangement for each laser diode in an array 700 can be seen to be the same or substantially the same as it is for a laser diode operating singly and as described above in relation to any of Figures l to 6. A dotted outline "B" is shown in Figure 7, this picking out the optical path and related elements for an individual laser diode in the 25 relevant array.

Referring to Figure 7 in more detail, a linear or two-dimensional array 700 of laser diode sources can be used, with the reflectivities of the individual laser diodes' end facets 105 and 110 provided in known manner. A linear or two-dimensional array of lenses 120 30 delivers collimated beams from the laser diode array 700 to the diffraction devices 115, consisting of the set of individual diffractive devices aligned such that their entry points are positioned along a single plane. The output beams 146 are collected after transmission through the diffraction device 115, and may then be delivered as required,

for instance to an array of optical isolators and fibres or to a single fibre for high power output (not shown) or used in a multiplexing system of some sort.

Each diffraction sub-device 115 may be separately controllable to change the refractive 5 index affecting the feedback wavelength 147 to its respective laser diode in the array 700 and the diffraction devices 115 may have different grating periods so that the grating period of the diffraction device 1 15 for the "j"th diode in the array will be dj. In this arrangement, the general equation for diffraction: 10 my = ndj (sin * - sin) can be satisfied differently for different diodes in the array. This means that the array 700 can potentially provide a set of tunable optical sources providing a set of output beams 146 which have wavelengths in any required combination from all the same, 15 through some the same and some different, to all different.

Delivery to a single timbre, or to a smaller number of fibres than there are sources, can be done for instance using fibre-couplers and microlenses. A multiplexing arrangement can be achieved using an array of micro-electro-mechanical system (MEMS) mirrors to 20 control the optical path of radiation from sources to fibres.

It is also possible that all the diffraction sub-devices 115 in an array 700 are under common control to change the refractive index affecting the feedback wavelength 147 to its respective laser diode in the array 700. In this case, simultaneous and synchronous 25 tuning of all the elements of the laser diode array 700 can be achieved, thus providing a multiple output tunable external-cavity laser diode system.

In a further alternative configuration, the arrays of Figure 7 could be arranged so that the diffraction devices 115 operate entirely in a "reflective" manner, as described above 30 with reference to the diffraction device 115 shown in Figure 1B.

3. MATERIALS AND DIMENSIONS

- - Various materials and dimensions can be used for construction of the gratings and electrodes. Examples of suitable materials and dimensions for the grating and electrodes are discussed further below.

5 There are alternative methods for controlling the refractive index "n". It can be done thermo-optically, by applying a temperature differential to the material used in constructing the diffraction device 115, or electro-optically by applying a voltage to the material. In each case, electrodes can be used to apply an input to the diffraction device l l S which will alter the refractive index "n" appropriately.

It is necessary that the material used is selected to meet certain requirements. It has to be transparent over the desired tunable range and, importantly, its refractive index "n" must be controllable through a range appropriate to produce a desired wavelength selectivity in the optical source as a whole, using a practical construction and electrical 1 S control parameters.

3.1 Materials and dimensions: Thermo-OPtical Tuning 3.1.1 Grating materials Examples of materials which can be used in a diffraction device for electro-thermal 20 tuning are: hybrid glass with a thermo-optic coefficient of about -30 x 10-s optical silicone resin which can have a thermo-optic coefficient (An/AT where T is temperature) of approximately 50 x 10-5 optical epoxy which can have a thermo-optic coefficient of approximately -100 to 25 -200 x 1 0-5.

As a reference the thermo-optic coefficient of silica is 1.1 x 1 0-s.

Optical silicone resin and epoxy are known materials but hybrid glass materials are a 30 relatively recent development. A "glass material" in this context is used in the usual way to mean an amorphous or noncrystalline solid. References to a "hybrid glass material" and the like are intended to refer to a glass material having both inorganic and organic components. This makes it possible to select a glass material which has one or more particular properties.

: In the glass material, an inorganic matrix can be provided at least in part by any metal alkoxide or salt that can be hydrolysed, all of these being appropriate inorganic network farmers, including those based on groups 3A, 3B, 4B and SB of the Periodic Table, such S as silicon dioxide, aluminium oxide, titanium dioxide and zirconium oxide. Functional organic components can then be used to modify the inorganic matrix. In general, the glass material of the subskate-based assembly will preferably include an organic component which polymerises by cross- linking. It might for instance be an organic component which polymerises under thermal or photo treatment, such as the functional 10 hydrocarbon compounds comprising acrylates, epoxides, alkyls, alkenes, or aromatic groups which support photopolymerisation.

Reference to suitable glass materials can be found in the following publication: A.TI.

Karkkainen et al, Applied Optics, v.41, pg. 3988, July 2002.

3 1.2 Diffraction device dimensions The grating features in a diffraction device l lS will typically be dimensioned to be of the order of the wavelengths to be selected. Hence in the optical source for use over a wavelength range centred on 1550 nm, the grating elements 320 will have a period in a 20 range such as 250 rim to lSSO nm.

The overall dimensions of the diffraction device 115 will be selected to suit its purpose but linewidth may be an issue. As mentioned above, the area "W" of an optical beam which is incident on a diffraction grating has an inverse relationship with linewidth. In 25 order to get narrow linewidth for communication purposes, a diffraction device 1 1 S may typically be required to provide a grating having dimensions of the order of SxSmrn.

This has an effect on the length of the external cavity and thus of the overall tunable source because the beam spot size of the incident beam 145 needs to be expanded from 30 the typical output of a semiconductor laser 100 to something at least roughly matching the dimensions of the grating 405. The overall length of the tunable optical source might be for instance anything in the range from some hundred micrometres up to several centimetres. Also as mentioned above, in practice, the length of the external cavity is likely to be chosen according to other factors combinedsuch as the effect it has

- - on output linewidth and packaging considerations. A typical length for an external cavity which could be used would then be on the order of from 1 to 3 ems.

3.1.3 Electrode materials and dimensions.

5 Any suitable material could be used but metallic conductors may be preferred for their good thermal conductivity, such as aluminium, molybdenum, nickel and chromium. An alternative is conductive oxides, at least some of which are transparent at wavelengths likely to be of interest.

10 The electrodes will typically be several millimetres wide and long when used on facets of the diffraction device 115.

3.1.4 Polarisation Conventional types of surface relief gratings (binary, blazed, kinoform profiles) which 15 might be used in the diffraction devices 115 of embodiments of the present invention will have a blaze angle which is either low, for instance from 5 to 10 , or very low, for instance less than 5 , while other relief gratings with generally sinusoidal profiles will have zero blazing angle. Classification of angles is given in the Diffraction Grating Handbook (published by Richardson Grating Laboratory).

The diffraction performance of a diffraction device 115 depends on the polarization of the incident light. Transverse electric (TE) -polarized light is polarized parallel to the grating grooves (or other grating elements) while transverse magnetic (TM) -polarized light is polarized perpendicular to the grating grooves. For low and very low blaze 25 angles, TM-polarization is known to result in generally higher diffraction efficiency, with theoretical peak values of 100%, while TE- polarization is diffracted with generally lower efficiency. Hence in embodiments of the present invention it would at least usually be preferable to choose TM-polarization of incident light with respect to the diffraction device, in a direction perpendicular to the grating elements 320.

The polarization of light leaving a laser diode 100 will usually be determined by the orientation of the laser diode 100.

al The preferred mutual orientation of the diffraction device 1 is and the edge-emitting laser diode 100 (or laser diode array 1200, 1300) is that the direction of the primary pn junction(s) in the laser diode(s), that is the direction which crosses the junction from p doped to e-doped material, is parallel to the direction of the grooves of the grating. That 5 is, if each groove extended in a vertical plane, the direction of the pn junction would be vertical and the plane in which the junction was actually formed would be horizontal.

This results in TM polarised radiation being incident on the diffraction device 1 l 5 in a direction perpendicular to the grating elements 320.

10 It is also possible to control polarization of the radiation after it has left the laser diode 100, for instance by using a waveplate or other polarising device in the external cavity.

3.1.5 Diffraction Order To maintain diffraction efficiency while keeping the design of the diffraction device l 15 15 relatively simple, it is best to use the first order of diffraction; Imp = 1. Operation in higher diffraction orders is possible but may require additional fabrication steps such as coating of the grating surface to enhance the diffraction efficiency.

3.1.6 Diffraction Angle 20 Diffraction at small angles O4jff, under otherwise equal circumstances, may result in decreased spectral resolution, which is not usually desirable. There will generally be an upper limit on the diffraction angle O0jff for a diffraction device 115 in a preferred embodiment of the invention: it cannot be extreme, to avoid the loss of optical radiation by total internal reflection within the grating. A typical range might be for example O0jff 25 = 20 to 60 .

3.1.7 Tuning Range Example In an example, a thermally tunable diffraction device made of hybrid glass with refractive index n = 1.5000 (at a room temperature of 20 Celsius and at a wavelength 30 of 1550 nm) and linear thermo-optic co-effcient n/5T = -30 x 10-5 is used to operate in a 20 = 1. 55 Am spectral window.

c - -? ::j Examples of a configuration in which the above material could be used are of the general type shown in Figures 1A or 1B, with the diffraction device 115 operating to give first order diffraction in a Littrow configuration. The angular orientation of the diffraction device 115 with respect to the incident light 145 gives an angle Dine (as shown 5 in Figure 3) in the general range from about 20 to about 60 with respect to the normal to the grating. The plane of the pn junction in the laser diode 100 is normal to the plane in which the grating elements 320 extend so as to give TM polarization in the direction perpendicular to the grating elements 320.

10 The refractive index controlling voltage is applied to the diffraction device 115 in the direction parallel to the grating plane (the plane in which the grating elements lie).

Then, according to the following relationship (which is easily obtainable from the diffraction equation), it is possible to calculate AL which is the change in diffracted 15 wavelength achieved when a temperature change AT is applied: MA, = 70 On AT n AT where 70 is initial free space wavelength, before a temperature change AT is applied n is the initial refractive index, before a temperature change AT is applied Thus a local temperature change of about 50 would produce a 1% change in refractive index and thus an approximately 1% shift in diffracted wavelength, which would amount to about 15.5 rim in this example. The values of voltage required to achieve this temperature variation would depend on the material of the electrodes and their 25 arrangement.

Since there is direct dependence of the shift in diffracted wavelength on local temperature change, using a temperature change of 100 gives a wavelength shift of 3 lam and using a lower temperature change, say 33 , gives a smaller wavelength shift of 30 the order of 1 Onm.

: 1) - Figure 8 shows a contour plot of the free space wavelength tuning ranges achievable by varying the temperature of the material adjacent a diffractive structure 225 in an embodiment of the invention.

5 It will be understood that, in using the above equation, the changes in and n are so small in percentage terms that the values used for them in the equation can be left at the central values for the ranges involved.

It might be noted that there is a secondary effect of changing the temperature of the 10 material carrying the diffractive structure 225 and this is a dimensional change.

However, in most cases this is not significant: for example the coefficient of thermal expansion for the hybrid glass material is of the order of 1 x 1 o-6.

Rather than starting at room temperature, it may be preferred to bring the diffraction 15 device to a selected temperature (for example 50 C) and then to heat or cool it from there to achieve appropriate wavelength tuning.

3.2 Materials and dimensions: Electro-Optical Tunine It will be understood by a person skilled in the art that the following known electro-optic 20 materials are examples of materials which could be used to give refractive index control, and thus feedback wavelength control, in embodiments of the present invention.

Most common photonic crystals may be used, including but not limited to LiTaO3 (lithium tantalate), LiNbO3 (lithium niobate), KH2PO4 (KDP) and CdTe, depending on 25 the spectral region of operation of the diffraction device. Electro-optic polymers such as azo-dye doped polymethyl methacrylate, modified nitroanilines, polythiophenes, liquid crystals, organic crystals such as 2-methyl-4-niroaniline, or photorefractive and/or semiconductor materials could also be used. Examples of semiconductor materials might be selected from group IV of the periodic table, or compound semiconductor 30 materials might be selected from one or more of the III-V, II-VI and IV-VI groups of the periodic table, such as InP (indium phosphide) or GaAs (gallium arsenide). Doped semiconductor materials in particular are capable of carrying charges and are thus electro-optically tunable.

\ -; 30 General dimensions of the grating, diffraction arrangements, electrode configuration etc where electro-optic materials are used will be similar to those where thermo-optic materials are used.

5 4. PHASE-CONTINUOUS TUNING

Tuning without interrupting the phase of oscillation, or so-called phasecontinuous tuning, is realized provided that the number of halfwavelengths, in the overall laser cavity, of the radiation being tuned remains constant during tuning. To achieve this, the 10 longitudinal modes of the radiation in the cavity must be shifted at the same rate as the peak wavelength, irk, of the pass bandwidth of the tuning element, which as described above is the diffraction device 115. This is described by the following condition: Glen 64pk Leff Ok where Let is the effective optical length of the external cavity laser.

Fortunately, the nature of TO-tuning can naturally satisfy the equation above so that no mechanical movement has to be utilized. However, there may be effects which change the external cavity length in an unpredictable way, such as transmitted vibration or movement, and it may still be advisable to introduce another controllable element in the 20 cavity to try to miminise these other effects.

S. FABRICATION TECHNIQUES

Diffraction gratings of most of the types described above are known and can be made 25 using suitable known fabrication techniques such as epitaxy and etching, or by exposure of a photosensitive material of the grating device to UV-light through an appropriate lithographic mask.

Another aspect of fabrication in embodiments of the present invention is the mounting 30 of the components of the source in relation to one another, movably or otherwise.

Again, suitable mounting arrangements for components of the optical source are known.

Soldering or welding might typically be used. Other available techniques include precision-fabrication of holders with tolerances of the order of micrometres, clueing and

l - 31 thermo-compression with manipulation at nanometre precision. The laser diode and diffraction device may conveniently be mounted on a common substrate such as silicon, quartz, ceramic material, metal, metal alloy or glass. Suitable arrangements are described for example in pending European patent application 02256515.4 filed on 19th 5 September 2002 in the name Optitune plc.

As mentioned above, the feedback section of a tunable optical source in an embodiment of the invention might comprise means for modifying optical characteristics of radiation travailing in the external cavity. This might be one or more of devices such as 10 waveplates, filters or apertures for instance. It may be advantageous that the feedback section comprises optical path control means for controlling the length of the optical path of radiation in the feedback section. This might be used for instance to avoid mode hopping. Such optical path control means might comprise a material whose refractive index is modifiable and the optical source might then include means to modify said 15 refractive index so as to control the length of the optical path of radiation in the feedback section. Although other configurations could be used, where the optical path control means is moved to control the optical path, such as a rotatable Fabry-Perot etalon, an arrangement in which the refractive index is modified lends itself to use in a tunable optical source in which tuning is achieved without any moving parts.

To summarise, embodiments of the present invention are capable of providing a tunable-

source of optical radiation in which there are no moving parts, the tuning being provided via controlled changes in refractive index of a material of a diffraction device in a phase-continuous fashion.

Claims

. À j CLAIMS

1. A tunable optical source which comprises: 5 i) a gain section for use in generating an optical output; ii) a feedback section to provide optical feedback to the gain section, the feedback section comprising a diffraction device arranged to provide wavelength selection in said optical feedback by diffraction, wherein the tunable optical source further comprises control means for controlling the 10 refractive index of material of the diffraction device to modify said wavelength selection.

2. A tunable optical source according to Claim I wherein the feedback section

comprises an external cavity.

3. A tunable optical source according to Claim 2 wherein the external cavity comprises free space and the diffraction device is mounted in said free space.

4. A tunable optical source according to either one of Claims 2 or 3 wherein the 20 diffraction device provides an end facet of the external cavity.

5. A tunable optical source according to any one of Claims 2, 3 or 4 further comprising collimating means for collimating optical radiation travelling in the external cavity.

6. A tunable optical source according to any one of the preceding claims wherein optical radiation incident on the diffraction device, in use, is collimated and the diffraction device extends primarily in a direction which is transverse to the direction of the incident optical radiation.

7. A tunable optical source according to Claim 6 wherein the diffraction device extends wholly across the path of the collimated incident optical radiation.

8. A tunable optical source according to any one of the preceding claims wherein the diffraction device comprises a grating to provide diffraction of incident optical radiation. 5

9. A tunable optical source according to any one of the preceding claims wherein the control means comprises means to apply a voltage across material of the diffraction device.

10. A tunable optical source according to any one of the preceding claims wherein 10 the control means comprises means to apply a temperature change across material of the diffraction device.

11. A tunable optical source according to any one of the preceding claims wherein tt the diffraction device comprises a grating to provide diffraction of incident optical 15 radiation and the diffraction device comprises more than one layer of material, the material whose refractive index is controlled by the control means being material of a layer adjacent to or containing the grating and through which optical radiation is incident on the grating in use.

20

12. A tunable optical source according to Claim 11 wherein the material of a first layer of the diffraction device is different from the material of a second layer thereof.

13. A tunable optical source according to any one of the preceding claims wherein the diffraction device comprises a grating which operates in transmission mode.

14. A tunable optical source according to any one of Claims 1 to 12 wherein the diffraction device comprises a grating which operates in reflection mode.

15. tunable optical source according to any one of the preceding claims wherein 30 material of the diffraction device whose refractive index is controlled by the control means is selected from the group comprising glasses, photonic crystals, electro-optic polymers, thermo-optic polymers, liquid crystals, organic crystals, semiconductor materials selected from group IV of the periodic table, compound semiconductor

/ _ 34

materials selected from one or more of the III-V, II-VI and IV-VI groups of the periodic table, and photorefractive materials.

16. A tunable optical source according to any one of the preceding claims wherein 5 the gain section comprises a compound semiconductor material selected from the III-V groups of the periodic table.

17. A tunable optical source according to Claim 16 wherein the gain section comprises a material selected from the group All xGaxAs' Ins xGaxAs yPy, AlxGayIn x yP 10 and IN xGaxAs.

18. A tunable optical source according to any one of the preceding claims tunable to provide an optical radiation output tunable over a wavelength range of at least l O nm.

15

19. A tunable optical source according to any one of the preceding claims tunable to provide an optical radiation output tunable over a wavelength range of at least 30 nm.

20. A tunable optical source according to any one of the preceding claims, comprising an array of two or more tunable gain sections.

21. A tunable optical source according to Claim 20 wherein, in use, a common diffraction device provides wavelength selection in the feedback to more than one gain section. 25

22. A tunable optical source according to Claim 20 wherein, in use, at least two diffraction devices provide wavelength selection in the feedback to two respective gain sections.

23. A tunable optical source according to any one of the preceding claims wherein 30 the feedback section comprises optical path control means for controlling the length of the optical path of radiation in the feedback section.

24. A tunable optical source according to Claim 23 wherein the optical path control means comprises a material whose refractive index is modifiable and the optical source

- comprises means to modify said refractive index so as to control the length of the optical path of radiation in the feedback section.

25. A diffraction device for use in an optical source according to any one of the 5 preceding claims.

26. A wavelength division multiplexing system comprising an optical source according to any one of the preceding claims.

10

27. A method of selecting the output wavelength or wavelength range of an optical source, the source comprising a gain section and a wavelength selective feedback section, the feedback section comprising a diffraction device arranged to provide at least part of the wavelength selectivity, which method comprises modifying the refractive index of material of the diffraction device so as to modify one or more diffraction 15 characteristics of the diffraction device.

28. A method according to Claim 27 which further comprises modifying the refractive index of material of the diffraction device so as to modify reflectivity of the diffraction device.

29. A tunable optical source which comprises: i) a gain section for use in generating an optical output; ii) a feedback section to provide optical feedback to the gain section, wherein the feedback section comprises a diffraction device having a diffractive 25 structure arranged to provide wavelength selection in said optical feedback by diffraction, the diffraction device being arranged in relation to the gain section such that, in use, radiation from the gain section is incident on the diffractive structure through material of the diffraction device.

30 30. A tunable optical source according to Claim 29 wherein the diffractive structure is formed in or adjacent to a surface of a layer of material and the diffraction device is arranged in relation to the gain section such that, in use, radiation from the gain section passes through the layer of material, or substantially through it, to the diffractive structure.

i -L i

31. A tunable optical source according to either one of Claims 29 or 30 wherein the diffraction device is mounted in free space and such that the diffractive structure extends primarily in a direction which is transverse to the direction of radiation incident on the 5 diffractive structure in use.

32. A diffraction device for providing wavelength selection in optical radiation, the device comprising: i) a body of material having a controllable refractive index, 10 ii) an entry point to the body of material for an incident beam of optical radiation; iii) a diffractive structure; and iv) control means for controlling said refractive index, wherein the diffractive structure is arranged to extend at least generally transverse to the optical path of the optical radiation in use of the diffraction device, but non-parallel to 15 said entry point, whereby changes in the controllable refractive index provide said wavelength selection.

33 A diffraction device according to Claim 32 wherein the incident beam is incident in a direction normal to the plane of said entry point.

34. A diffraction device according to either one of Claims 32 or 33 wherein the entry point is provided at a surface of the body of material.

35. A diffraction device according to any one of Claims 32 to 34 wherein the entry 25 point is provided at an interface between the body of material and a further material of the diffraction device.

36. A diffraction device according to any one of Claims 32 to 35 wherein the body of material is at least partly defined by the extent of a controllable electric field in the

30 diffraction device in use.

37. A diffraction device according to any one of Claims 32 to 36 wherein the body of material is provided as a prism, having a triangular or trapezoidal cross section, the

^: 1' prism having a first surface providing the entry point and a second surface carrying the diffractive structure.

38. A diffraction device according to Claim 37 further comprising electrodes to 5 control the refractive index of the material of the prism, said electrodes being disposed on facets of the prism which are not in said optical path.

39. A diffraction device according to Claim 38, said electrodes being disposed on side facets of the prism which are parallel to the direction of the optical path.

40. A diffraction device according to any one of Claims 32 to 39 wherein the diffractive structure is mounted in a Littrow configuration with respect to the incident beam. 15

41. A diffraction device according to any one of Claims 32 to 40, comprising a phase grating as the diffractive structure.

42. A diffraction device according to any one of Claims 32 to 40, comprising an amplitude grating as the diffractive structure.

43. A diffraction device according to any one of Claims 32 to 42, comprising a surface relief grating as the diffractive structure.

44. A diffraction device according to any one of Claims 32 to 42, comprising a 25 volume grating as the diffractive structure.

45. A diffraction device according to any one of Claims 32 to 44, wherein the grating is covered by further material, the refractive index of the further material being selected or controlled to determine the reflectivity or transmissivity of the grating.

46. A diffraction device according to Claim 45 wherein the further material is provided as a thin film coating.

A 3 8

47. A diffraction device according to Claim 45 wherein the refractive index of the further material is controllable separately from that of the body of material.

48. A diffraction device according to any one of Claims 32 to 47, further comprising 5 a collimating device to collimate radiation received at the entry point.

49. A diffraction device according to any one of Claims 36 to 48 wherein the collimating device is provided as a lens constructed from material supported by the first surface of the prism.

50. A diffraction device according to Claim 49 wherein the lens is a refractive lens.

51. A diffraction device according to Claim 49 wherein the lens is a diffractive lens.