GB2418295A - Temperature compensated optoelectronic device having a laser diode and an electro-absorption modulator - Google Patents

Abstract

A temperature compensated optoelectronic device 100 has a laser diode 3 and an Electro-absorption Modulator (EAM) device 6 to modulate the output of the laser. A method of forming such a device, which may more particularly be a monolithically integrated optoelectronic component is also disclosed. The optoelectronic device 100 comprises a Distributed Bragg Reflector (DBR) device 4 with an electrically-tuneable input 57 for wavelength-tuning of optical radiation 50 generated by the laser 3. The laser diode 3 has an operating wavelength that varies at a different rate than the band edges of the EAM device 6 as temperature changes. The device employs the DBR device 4 to make compensating changes to the laser wavelength in order to extend the operating temperature range of the optoelectronic device 100 without the need for thermoelectric cooling.

Description

Temperature Compensated Optoelectronic Device having a Laser Diode and an

Electro-absorption Modulator The present invention relates to a temperature compensated optoelectronic device having a laser diode and an Electro- absorption Modulator (EAM) device, and to a method of forming such a device, which may more particularly be a monolithically integrated optoelectronic component.

Laser diodes are widely used in optical communications systems. It is known to manufacture such devices as a monolithically integrated optoelectronic device using a III-V semiconductor material based on an InP wafer. Laser diodes used in optical communications systems may be subject in operation to wavelength variability. For example, many types of laser diode used in optical communications systems having a nominal wavelength of around 1310 nm for "metro" networks, or around 1550 nm or 1600 nm for long haul networks, may exhibit wavelength variability of up to about 8 nm owing to laser ageing, carrier density, temperature and power changes in the active semiconductor medium. For example, the Fabry- Perot (FP) gain profile of a typical III-V material semiconductor laser diode will have a maximum at a particular wavelength No, and the value of to will normally vary by an amount AA = 0.5 nm/ C.

In some applications, such as those employing wavelength division multiplexing (WDM), such variability may limit channel separation to about 20 nm, unless the wavelength is stabilized at a desired wavelength for example, by using a distributed feedback (DFB) laser device. - 2

In some applications, the temperature of the laser diode may therefore need to be stabilized, for example with an electrical heater if the laser diode is to be used at low operating temperatures (for example between 0 C and -40 C), or with a thermoelectric cooler if the laser diode is to be used at high operating temperatures (for example between C and 60 C). This, however, adds complexity and cost to an optoelectronic component that includes such a temperature stabilized device. For these reasons it is desirable in low cost applications not to have temperature stabilization of the laser diode. The invention is particularly concerned with such low cost devices, but is also applicable to devices having temperature stabilization where such stabilization is not fully effective in all operating conditions to control the operating temperature of the laser diode, particularly at low temperatures of operation.

However, in many applications, particularly those in which a single optical source is transmitted along an optical fibre, the exact wavelength, the laser wavelength is not stabilized, and is allowed to vary depending on operating conditions.

Direct modulation of a laser diode may be used in relatively low speed optical communication, but in high speed communication, for example between about 1 Gbit/s to Gbit/s, it is common to use an electroabsorption modulator (EAM) device which is then directly modulated to act as a light shutter in order to impart a data signal on the output from the laser diode. The EAM device may be a discrete device, but in order to avoid the difficulty of optically aligning the laser diode with the EAM device, for example by means of a lens arrangement, it is normally preferred if the EAM device is monolithically integrated with the laser diode. An integrated EAM device may therefore be formed on a common InP substrate after epitaxial formation of the laser diode by etching followed by re-growing of doped III-V material layers with properties suitable to form the EAM device. Such EAM layers need to have different properties from those forming the laser device, as the EAM layers must be capable of modulating the optical radiation from the laser diode by switching between substantially transparent and absorbing states upon the application of an electrical current through the EAM device.

Whether or not the EAM device is monolithically integrated with the laser diode device, both devices will be in close proximity, and will therefore have the same or similar ambient temperatures. The EAM device acts as a band edge filter have a rapid transition between transparent and absorbing sections depending on the optical wavelength A. With no current supplied to the EAM device, the wavelengths of the optical radiation and EAM band edge are normally such that the EAM device is transparent to the optical radiation.

Current may then be provided to the EAM device, causing the wavelength of the band edge to shift so that the EAM device absorbs the optical radiation. To achieve rapid and full modulation, it is therefore desirable that optical wavelength of the laser diode is sufficiently closely spaced from the EAM band edge so that the band edge crosses the optical wavelength reliably and fully when the EAM is electrically modulated.

A problem arises in that the EAM device and the laser diode - 4 device will in general experience different wavelength changes depending on ageing, temperature and power changes.

For example, the temperature characteristic of the EAR band edge A will normally vary by an amount HA = 0.5 nm/ C, whereas the DFB lasing mode will vary by an amount LA = 0.15 nm/ C.

In practice, this limits the operating temperature range of such an unstabilized optoelectronic device to a 30 C operating range, for example to between 0 C and 30 C. The conventional solution to this problem is to provide thermoelectric cooling to the devices. Such thermoelectric coolers are also widely used as a means of controlling operating wavelength in closely spaced WDM systems.

Thermoelectric coolers are, however, relatively expensive, and also consume significant additional current. Excessive current consumption can in itself be a problem, as many industry-standard optical communications transmitter modules are specified to consume no more than a set amount of current.

It is an object of the present invention to provide a more convenient optoelectronic component, having an EAR device that is used to modulate the output from a laser diode source of optical radiation.

According to the invention, there is provided a semiconductor optoelectronic device, comprising a laser source of optical radiation including a laser diode and a Distributed Bragg Reflector (DBR) device with an electrically-tuneable input for wavelength-tuning of said optical radiation, and an Electro-absorption Modulator (EAM) device for modulating said optical radiation received from the laser source, the laser diode having an operating wavelength with a first wavelength vs. temperature characteristic, and the EAM device having an absorption band edge with second wavelength vs. temperature characteristic, said first characteristic being different from the second characteristic, wherein the optoelectronic device comprises temperature compensation means arranged to control the DBR input when the temperature of the laser diode and/or the EAM device varies in order to tune the laser diode operating wavelength relative to the EAM band edge and thereby compensate at least partially for said different characteristics.

The semiconductor laser diode will have an active medium that forms a waveguide for optical radiation generated by the laser diode.

The DBR may have a structure that is incorporated in a waveguide that is separate from the waveguide containing the semiconductor lasing medium. The DBR structure may also be monolithically integrated with a laser diode structure on a common semiconductor substrate, or may be a discrete device that is integrated with a discrete laser device, for example being bonded to a common substrate.

The term "wavelength vs. temperature characteristic" as used herein may be represented in the case of the laser diode by the slope of a curve plotting the operating wavelength of the laser diode against operating temperature. The term "wavelength vs temperature characteristic" as used herein may be represented in the case of the EAM device by the slope of a curve plotting the a feature of the EAM band filter against operating temperature, for example the central wavelength at which absorption is equal to transmission.

The temperature compensation means is arranged to vary the DBR input in response to temperature changes of the laser diode and/or the EAM device. Such temperature changes may be sensed directly or inferred from other operational parameters in a number of ways.

In a preferred embodiment of the invention, the optoelectronic device includes a monitoring means for monitoring at least one operational parameter of the optoelectronic device. The, or each, of said operational parameters is correlated with a change in operating temperature of the laser device and/or the EAR device. The monitoring means then provides an output to the temperature compensation means for the control of the DBR input.

For example, the optoelectronic device may include a temperature sensor arranged to monitor the temperature of the laser diode and/or the EAM device. The temperature compensation means then receives an input from the temperature sensor that is used by said means in the control of the DBR input.

Additionally or alternatively, the optoelectronic device may include signal monitoring circuitry for monitoring the electrical drive of the laser diode. The temperature compensation means may then receive an input from this circuitry that is used by the temperature compensation means - 7 in the control of the DBR input. Operational parameters that may be monitored include factors such as the laser duty cycle, bias current or drive current.

Furthermore, the optoelectronic device may include additionally or alternatively signal monitoring circuitry for monitoring the electrical drive of the EAM device. The temperature compensation means may then receive an input from this circuitry that is used by the temperature compensation means in the control of the DBR input. Operational parameters that may be monitored include factors such as the magnitude of the EAM drive current, which will depend on whether or not the optical radiation is being absorbed or transmitted by the EAM device.

In a preferred embodiment of the invention, the laser source and EAM device are monolithically integrated on a common semiconductor substrate.

Most preferably, the temperature compensation means is arranged to maintain a constant separation between the wavelength of the optical radiation and an absorption band edge of the EAM device when the EAM device is passive, said separation being defined, for example, as a separation when the EAM device is un-energised with a drive current.

Also according to the invention, there is provided a method of operating a semiconductor optoelectronic device, said device comprising a laser diode, a Distributed Bragg Reflector (DBR) device with an electrically- tunable input for wavelength-tuning of said optical radiation, an Electroabsorption Modulator (EAM) device with an absorption - 8 band edge for modulating said optical radiation, and a monitoring means, the method comprising the steps of: i) using the laser source to generate optical radiation; ii) using the EAM device to modulate said generated optical radiation; iii) using the monitoring means to monitor one or more operational parameters of the laser diode and/or the EAM device, said operational parameter(s) being correlated with a change in operating temperature of at least one of said devices; iv) using the DBR device to tune the optical radiation in response to said monitored operational parameter(s) in order to compensate at least partially for relative shifts between the wavelength of said optical radiation and the wavelength of said band edge owing to temperature changes of the laser diode and/or the EAM device.

The invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows plots of absorption vs wavelength for an EAM device and also the output wavelength of an unstabilized DFB laser at a nominal operating temperature of 25 C; Figures 2 and 3 show plots similar to that of Figure 1 at respectively low and high operating temperatures of 0 C and 70 C, showing how the relative shifts of the (I - 9 wavelength vs temperature characteristics of the EAM device and DEB laser cause in both cases a degradation in the extinction ratio when the EAM device is modulated; Figure 4 is a schematic plan view of a semiconductor wafer have a plurality of monolithically integrated optoelectronic components for use in an optoelectronic device according to the invention, each of such components having a Distributed Bragg Reflector (DBR) laser source composed of a laser diode section and a DBR section which is butt-coupled with an Electro-absorption Modulator (EAM) device on a common substrate; Figure 5 is a schematic transverse cross-section of the laser section of the DBR laser source of Figure 4, taken along the line III-III, showing a buried heterostructure semiconductor laser junction comprising an active layer within a buried mesa stripe, a current conduction region for channelling current to the active layer, and a pair of current confinement regions either side of the buried mesa stripe; Figure 6 is a schematic transverse cross-section of the DBR section of the DBR laser source of Figure 4, taken along the line IV-IV a DBR waveguide in the buried mesa stripe, a current conduction region for channelling current to the DBR waveguide, and a pair of current confinement regions either side of the DBR waveguide; Figure 7 is a schematic transverse cross- section of the EAM section of Figure 4, taken along the line V-V, a modulation waveguide in the buried mesa stripe, a current conduction region for channelling current to the modulation waveguide, and a pair of current confinement regions either side of the modulation waveguide; Figure 8 is a schematic longitudinal cross-section of the DBR laser source and EAM section taken along the line VI-VI of Figure 24; lo Figures 9, 10, 11 and 12 are schematic plan views similar to that of Figure 2 that illustrate a method according to the invention for forming the monolithically integrated optoelectronic component; and Figure 13 illustrates schematically an optoelectronic device according to the invention having the monolithically integrated optoelectronic component of Figures 4-12, together with a temperature sensor and temperature compensation means for controlling the DBR band edge relative to the laser diode operating wavelength.

Figure 1 shows plots of absorption vs wavelength for an EAM device and also the output wavelength of an unstabilized DFB laser at a nominal operating temperature of 25 C. The EAM device is modulated by means of an applied signal that varies between O V and -3 V and which alters the absorption vs wavelength characteristic of the EAM device between at transmitting curve 13 and an absorbing curve 13' at the operating wavelength 14 of the DFB device. The signal variation therefore changes the absorption curve 13 of the EAM device so light output from a DFB laser operating at about 1500 nm is similarly modulated.

As can be seen from the plots of Figure 1, the optimum or extinction ratio 19 and hence modulation depth occurs at the wavelength where there is a maximum absorption separation between the modulated curves 13,13' . In this example, this is at a wavelength separation 15 of about 60 nm above a point 16 at which the modulation curves 13,13' cross over.

Figures 2 and 3 show plots similar to that of Figure 1 at respectively low and high operating temperatures of 0 C and C, showing how the relative shifts of the wavelength vs temperature characteristics of the EAM device and DFB laser cause in both cases a degradation in the extinction ratio when the EAM device is modulated.

As can be seen from Figure 2, at an ambient operating temperature of 0 C the laser wavelength 114 has moved towards shorter wavelength. The unmodulated and modulated EAM band edges 113, 113' also shift to longer wavelength, but to a lesser degree. As can be seen from Figure 3, at an ambient operating temperature of 70 C the laser wavelength 214 has moved towards longer wavelength. The unmodulated and modulated EAM band edges 213, 213' also shift to longer wavelength, but to a lesser degree. These shifts occur because the laser wavelength 14, 214 changes at LA = 0.15 nm/ C while the curves 13,113,213; 13' ,113' ,213' for the EAM device shift by AA = 0.5 nm/ C. It should be noted that the wavelength of a nonDFB laser will also shift, but typically in discontinuous steps rather than continuously, as operating temperature changes. As a result - 12 of the differences in these characteristics, the laser wavelength at lower operating temperatures is father away from the crossover point 116 and at higher operating temperatures is closer to the crossover point 216 and both these cases the modulation depth 119,219 is reduced. This will result initially in a higher bit error rate of the modulated optical signal, and ultimately, no modulation of the optical signal.

The invention deals with this problem by providing a laser source composed of a laser diode section and a Distributed Bragg Reflector (DBR) section that has an electrical control input. The optical output from this laser source is then provided to an EAR device in an optoelectronic device that comprises also temperature compensation means arranged to control the DBR input when the temperature of the laser diode and/or the EAR device varies in order to tune the DBR band edge relative to the laser diode operating wavelength and thereby compensate at least partially for said different characteristics. By tuning the wavelength of the optical radiation to compensate for the temperature change, it is possible to achieve a wider operating temperature range for the optoelectronic device.

The DBR laser source should have an active layer that is substantially transparent at the optical wavelength of the laser diode. The DBR laser source may therefore be formed by first growing on a substrate the doped III-V material layers forming laser diode, then etching away a section of these grown layers, and then re-growing III-V material layers on the common substrate to form a DBR section having a waveguide layer that is butt-coupled and in-line with the laser cavity A) formed by the laser diode active layers.

The term "butt-coupled'' is used herein to describe a semiconductor material optoelectronic component in which adjacent optoelectronic sections of the component have, on one side of a junction between adjacent sections, one or more semiconductor layers that have been grown (for example using MOVCD techniques) up against other previously formed semiconductor material on the other side of the junction.

In one embodiment of the invention not illustrated in the drawings, the tuneable DBR laser source may then be aligned with an external electroabsorption modulator (EAM) device.

Alternatively, in another embodiment not shown in the drawings it would be possible to form the EAM device on a common substrate by re-growing doped III-V material layers with properties suitable to form the EAM device. Such EAM layers need to have different properties from those forming either the laser section or the DBR section of the DBR laser source, as the EAM layers must be capable of modulating the optical radiation at the operating wavelength of the DBR laser source between substantially transparent and absorbing states upon the application of an electrical current through the EAM device.

Although it is possible to form such a tuneable DBR-EAM device as described above, such approaches introduce significant manufacturing difficulties. In the first approach, it is necessary to align optically discrete DBR and EAM devices and then to bond these to a supporting surface.

In the second approach, it becomes necessary to process the III-V material in two sequential etch and re-growth processes, and to maintain vertical alignment between three separately formed active layers - two in the DBR laser source and a third in the EAM device. These difficulties add significantly to the finished cost of a tuneable optoelectronic transmitter component. The illustrated embodiments therefore show in Figures 4 to 12 one way of forming a monolithically integrated DBR laser source with EAM for use in an optoelectronic component according to the invention.

Figure 4 shows a schematic plan view of a finished III-V semiconductor material wafer 15 on which have been fabricated a number of monolithically integrated components 1 each of which has a DBR laser device 2 butt coupled with an Electro absorption Modulator (EAM) section 6. The DBR laser device 2 has a laser section 3 and a DBR section 4. These sections 3, 4 are elongate and extend along an optical axis 5 which extends in one direction towards a similarly elongate and aligned EAM section 6. The DBR section 4 is butt-coupled with both the laser section 3 and the EAM section 6 at respective junctions 7, 8. In operation, the EAM section 6 receives optical radiation from the DBR section 4.

Reference is now made also to Figures 5 to 8, which show respectively the structures of the laser and DBR sections 3, 4 of the DBR laser source 2, and the EAM section 6. Reference is also made to Figures 9 to 12 which illustrate how these sections 3, 4, 6 are monolithically integrated on a common InP substrate 12. For the purposes of clarity, these Figures are schematic only, and do not show dimensions to scale.

Figure 5 shows a transverse cross section through the laser diode section 3 of the DBR laser source 2. The laser diode section 3 has a buried heterostructure laser diode waveguide layer 9 suitable for use as a transmitter in a fibre-optic link operating at a desired wavelength, which may be between about 1270 nm and about 1610 nm.

Figure 6 shows a transverse cross-section through the DBR section 4 of the DBR laser source 2. The DBR section 4 has a buried grating waveguide 10 suitable for tuning the wavelength of the laser output from the laser diode section Figure 7 shows a transverse cross-section through the EAR section 6. The EAM section has a buried waveguide 11 suitable for modulating the wavelength-tuned output of the DBR laser source 2.

Referring now also to Figure 9, the sections 3, 4, 6 are formed starting from an initial wafer that is around 50 mm in diameter, and that has an nInP substrate 12 doped to around cc, on which is grown a number of e-type and p-type III-V semiconductor layers. These layers are deposited using well-known MOCVD techniques. The p-type dopant is zinc, and the e-type dopant is sulphur.

The first grown layer is a 2 Am thick n--InP buffer layer 18 doped to around 10 cc. Then, using well-known fabrication technology, the processed wafer 25 is coated with an oxide layer. The oxide layer may be SiO2 deposited by a plasma enhanced chemical vapour deposition (PECVD) process. It should, however, be noted that silicon nitride would be a - 16 suitable alternative choice to SiO2. As shown in Figure 9, the oxide layer is photolithographically patterned with a photoresist to leave a patterned mask consisting of pairs of elongate parallel masked areas 16. Each pair of masked areas defines therebetween a confined rectangular stripe area 26 on the exposed surface of the buffer layer 18, as shown in Figure 9.

An active waveguide layer 9, 11 is then grown on the buffer layer 18 of the non-masked portions of the processed wafer 25 according to known techniques for fabricating planar active layers for a laser diode. The active layer could be a bulk region or a strained multiple quantum well (SMQW) structure.

An example of an SMQW device is discussed in W. S. Ring et al, Optical Fibre Conference, Vol. 2, 1996 Technical Digest Series, Optical Society of America. The type of active layer employed is not critical to the invention.

The active layer does not grow on the paired masks 16. The masks 16 therefore control where the active layer is deposited. This technique is known as selective area growth (SAG). The paired arrangement of masks 16 enhances the growth rate of the semiconductor material and concentration of group III component in region 26 relative to 18.

As can be seen from a comparison of Figures 5 and 7, the laser active waveguide layer 9 therefore is therefore thicker than the EAR active layer 11. In the present example, the laser active waveguide layer 9 grows to be about 200 nm thick, and the EAM active waveguide layer 11 grows to be about 180 nm thick. - 17

In the present example, the laser diode section 3 has a quaternary multiple quantum well (MQW) InxGal-xAsl-yPy active layer 9 that may be between about 100 nm to 300 nm thick. As the EAM section 6 is grown during the same step, the EAM also has a quaternary MQW active layer 11. The EAM active waveguide layer 11 is thinner by around 10% compared to the laser active waveguide layer 9 due to difference in growth rate. The variation in thickness and composition leads to a variation in photoluminescence (PL) wavelength (and bandgap).

The PL wavelength of the laser diode is around 1550 nm (lower bandgap) and the PL wavelength of the EAM is around 1480 nm (higher bandgap). Thus the unmodulated EAM waveguide is transparent at the nominal laser diode operating wavelength of 1550 nm.

The EAM active waveguide layer 11 is thinner than the laser section active waveguide layer 9, and may be about 80 to 250 nm thick.

The rectangular area 26 therefore defines an area of enhanced growth of the active waveguide layer 9 for the laser section 3.

A typical spacing for between the paired masks 16 is about 10 to 30 m, and the typical length is about 300 to 500 m. Each of the masks 16 has a width comparable to the spacing between masks, for example being about 20 Am wide. The width and spacing is engineered to control the SAG enhancement.

There is a transition region between the active waveguide layers 9, 11 for the laser section 3 and the EAM section 6, with the different thickness and compositions grading into (A each other over a distance of about 100 m.

The active waveguide layers 9, 11 are then topped by a cladding layer 22, formed from p±InP material, grown to be between about 100 nm to 1 Am thick. Again the growth is selective because the cladding layer 22 is not formed over the paired masks 16.

The paired masks 16 are then removed with 10:1 buffered HF acid to expose the buffer layer 18 beneath the masks 16 thereby leaving in place of the masks 16 rectangular depressions 16' that are elongate in a direction parallel with the device axis 5, and a second patterned mask consisting of pairs of discontinuous stripes areas 21, 21' centrally aligned above the device axis 5 are deposited on the wafer 35, using similar process steps to that described above for the first masks 16. One of the mask areas 21 lies parallel with and fully between the rectangular depressions 16' in an area where the initial selective growth has been relatively enhanced. The other of the mask areas 21' lies at a distance from the depressions 16'.

The second patterned mask 21, 21' defines and protects the regions of the laser diode and EAM sections 3, 6 illustrated in Figure 10 which are around 10 Am wide and equidistant from the end of the rectangular depressions 16' left by the first masks 16. One of the mask areas 21 extends over enhanced growth active waveguide layer 9 and the other mask area 21' over a portion of the non-enhanced active waveguide layer 11 for the EAM section 6. Figure 10 illustrates the processed wafer 35 at this stage of production.

The exposed active and cladding layers 9, 11, 22 inside an unmasked area 23 defined by the masks 21, 21', are then removed in a wet-etch process that cuts down through the deposited structure into the buffer layer 18. It would, alternatively, be possible to use a reactive ion dry etching process. The DEB section 4 may then be formed from material deposited between the paired stripe areas 21, 21'.

The active waveguide layer 10 for the DBR section 4 is then grown on the buffer layer 18 of the non-masked portions 23 of the processed wafer 35according to known butt-coupling techniques for fabricating planar active layers for a DBR laser source. As shown in Figure 6, this active waveguide layer 10 is thicker than the adjacent active waveguide layers 9, 11 for the laser section 3 and the EAM section 6.

A p±InP material cladding layer 37 is then grown over the DBR active waveguide layer 10. The formation of the DBR cladding layer 37 also involves using known techniques (for example by e-beam or holographic lithography) to form as shown in Figure 8 a DBR grating 39 in the cladding layer, for example by forming a periodically etched layer of a material such as GaInAsP. Alternatively, the grating may be formed in the buffer layer 18 beneath the subsequently deposited DBR active waveguide layer 10.

Because the DBR active waveguide layer 10 is selectively grown in a gap etched between the laser section 3 and EAM section 6, the DBR section 4 is butt-coupled with the adjacent laser section 3 and EAM section 6 to form a monolithically integrated optoelectronic component 1. The active waveguide layers 9, 11 of the laser section 3 and EAM - 20 section 6 are automatically self-aligned in the longitudinal direction along the component axis 5, and therefore the DBR active waveguide layer 10 is also automatically aligned with the adjacent active waveguide layers 9, 11 as long as the DBR active waveguide layer 10 is growth to the correct level above the component substrate 12. This aspect of the invention makes beneficial use of the fact that the thickness of the DBR active waveguide layer 10 can be greater than the thicknesses of the adjacent butt-coupled active waveguide layers 9, 11, as shown in most clearly in Figure 8, in order to achieve this alignment.

After this, the second mask areas 21, 21' are removed with 10:1 buffered HF acid.

A set of third patterned masks 41 is deposited on the wafer as shown in Figure 11, using similar process steps to that described above for the first and second masks 16, 21, 21'.

Each of the third masks 41 covers stripes that run parallel to and between the paired elongate depressions 16' left by the paired first masks 16. The third patterned masks 41 extend fully between neighbouring paired depressions 16' so that each mask stripe 41 extends the full length of each of the DBR laser source 2 and EAT sections 6 that are ultimately formed. Figure 11 illustrates the processed wafer 45 at this stage of production.

The exposed cladding layers 22, 37 outside the masked stripes 41 are then removed in a wet-etch process that cuts down through the deposited layers into the buffer layer 18. It would, however, be possible to use a reactive ion dry etching process. The unmasked grown layers 9, 10, 11, 18, 22 and 37 - 21 are removed in all areas except along a set of parallel mesa stripe structures 24 defined by the mask stripes 41. In Figures 5, 6 and 7, the mesa stripe 24 extends perpendicular to the plane of the drawing, and rises above the level of the surrounding etched buffer layer 18. The mesa stripe 24 has left and right opposite side walls 31, 32 that together with the buffer layers 18 and the cladding layers 22, 37 form current conduction regions 46, 47 and 48 for respective applied currents 56 (IL), 57 (ID) , 58 (IM) to the laser section 3, DBR section 4 and EAM section 6.

As can be seen from the cross-section of Figure 8, the three active waveguide layers 9, 10, 11 are longitudinally aligned at the butt-coupled junctions 7, 8 between the laser section 3, DBR section 4 and EAM section 6. This, together with the mesa structure 24, has the effect of guiding an optical mode along the active waveguide layers 9, 10, 11 within the mesa stripe 24.

The laser current 56 may be applied to pump and drive the laser to generate the optical mode 50. The DBR current 57 may be varied in order to vary the effective refractive index of the DBR active waveguide layer, and hence tune the wavelength 14, 114 of the optical mode 50. The EAM current 58 may be modulated to shift the band absorption edge 13, 13', 113, 113 in the EAM active waveguide layer 11 and impart a similar modulation on the optical mode 50.

The wet etch process produces mesa side walls 31, 32 that slope laterally away from the active layers 9, 10, 11. A dry etch process would produce side walls that are more closely vertical. - 22

The width of the mesa stripe 24 varies depending on the particular device, but for opto-electronic devices such as laser diodes, the mesa stripe 24 is usually between 1 Am and 3 Am wide. The mesa strip 24 rises 1 Am to 3 Am above the surrounding buffer layer 18.

A current confinement or blocking structure 30 is then grown on the etched device up to approximately the level of the patterned stripe mask 41. The structure 30 includes a number of layers adjacent the buffer layer 18 including a first p-doped InP layer 17 having a dopant concentration of about lxlO cc and above this, an e-doped InP layer 28, having a dopant concentration of at least about lxlO cc. The e-doped InP layer 28 preferably has a substantially constant dopant concentration at least as high as the highest dopant concentration in the p-type layer 17. Finally, a second p-doped InP layer 29 having a dopant concentration of about lxlO cc is deposited on the e-doped InP layer 28.

The thicknesses of the e-doped layer 28 is about 0.5 Am and the thickness of the first p-doped layer 17 is about 0.4 m.

These InP layers 17, 28 form a pen junction that in use is reverse-biased and hence insulating when the laser conduction region 46 is forward biased.

The first p-doped layer 17 should be between about one-tenth and one-half the thickness of the e-doped layer 28, that is between about 50 nm and about 250 nm thick.

After deposition of the semiconductor layers 17, 28, 29 used - 23 to form the current blocking structure 30, the oxide layer mask 41 is removed with 10:1 buffered HF from the mesa strip 24 to expose again the cladding layers 22, 37. As shown in Figure 12, this results in an etched and overgrown wafer 55 comprising the substrate 12, the mesa stripe 24, the layers 17, 28, 29 abutting the opposite sides 31, 32 of the mesa stripe 24.

As also shown in Figure 12, the arrangement of the masks 16, 21, 21' is such that the adjacent components 1 along the axis have laser, DBR and EAM sections 3, 4, 6 arranged in opposite order so that one pair adjacent components 1 will have laser sections 3 formed from adjacent portions of the completed wafer 15, and the next adjacent pair of components will have an EAM section formed from adjacent portions of the finished wafer 15.

An upper cladding layer 60 formed from highly doped p±InP is then grown above the cladding layers 22, 37 of the mesa stripe 24 and the second pdoped InP layer 29 of the current blocking structure 30, up to a thickness of about 2 Em to 3 m. The final semiconductor layer is a 100 nm to 200 nm thick ternary cap layer 61 is deposited on the upper cladding layer 60. The cap layer 61 is formed from p+±GaInAs, very highly doped to greater than 10 cc, in order to provide a good low resistance ohmic contact for electrical connection to the three current conduction regions 46, 47, 48 of the mesa stripe 24. As an alternative to a ternary cap layer, it is possible to use a quaternary InGaAsP cap layer, or both InGaAsP and InGaAs layers.

After a further masking step, an isolation etch through the - 24 ternary cap layer is then performed in order to help electrically isolate the three sections 3, 4, 6 from each other and to help prevent cross-talk between the three sections of the device 1. In this isolation etch, the InGaAs cap layer 61 is etched in photolithographically defined areas down to the second p-doped InP layer 29.

Standard metal layers for three electrical 66, 67, 68 contacts to the three portions 3, 4, 6 of each of the optoelectronic components 1 are then vacuum deposited on the cap layer 61 using well known techniques, followed by metal wet etch in photolithographically defined areas. The remaining metal forms three contact pads 66, 67, 68 with good ohmic contact through the cap layer 61.

The resulting completed wafer 15 is then thinned to a thickness of about 70 Am to 100 Am in a standard way, in order to assist with cleaving. Standard metal layers 70 are then deposited by sputtering on the rear surface of the wafer 15, so enabling electrical contact to be made to the e-side of the devices 1.

The wafer is then inscribed and cleaved along cleave lines 80 between adjacent laser sections 3 and cleaved along cleave lines 81 between adjacent EAT sections 6 in a conventional cleaving process that produces transverse bars about 500 Em wide. Then each bar is cleaved into individual devices 200 Am wide. The final individual cleaved device 1 is about 500 Am long (i.e. in the direction of the mesa 24) and about 200 Am wide. ( ' - 25

As shown schematically in Figure 13, each cleaved device 1 is then combined with temperature compensation means 101 to form a semiconductor optoelectronic device 100 according to the invention in which the wavelength of generated optical radiation 50 is tuned to match that of the EAM band edges as temperature varies, for example the band edges 113, 113 ' at an ambient temperature of 0 C and the band edges 213, 213' at an ambient temperature of 70 C. Also shown schematically in Figure 13 is drive circuitry 103 to provide the DC bias current 56 to the laser diode section 2, and drive circuitry to provide the modulation signal 58 to the EAM section 6.

In the illustrated embodiment, the temperature compensation means is circuitry 101 that receives as input a signal 102 from a temperature sensor 104 affixed to a portion of the monolithically integrated device 1, for example affixed to a portion of the earth contact pad 70. The circuitry 101 directly monitors the operating temperature of the device 1, and calculates from this temperature, for example by means of a look-up table in a microprocessor 106, a value for a drive current or voltage 57 which, when applied to the DBR section 4, will result in a compensating shift to the wavelength 14, 114, 214 of the optical radiation 50, so that this wavelength is still centrally bounded by the unmodulated and modulated curves 13, 13', 113, 113', 213, 213' of the EAM device for that operating temperature. The circuitry therefore generates the signal 57 provided to the DBR section 4 in order to tune the optical wavelength 14, 114, 214.

Alternatively or additionally to the use of the temperature sensor 104, the circuitry may receive an input 110 from the modulation drive circuitry 105 indicative of the EAM drive - 26 voltage or current 58, which can be a measure of the temperature or the level of absorption of optical radiation by the EAM device 6.

Optionally, it may also be possible to monitor the laser drive current 56 via an output line 112 to the circuitry 101 which may be used to estimate the temperature of the laser diode 3.

As indicated by arrows 127 and 227 in Figures 2 and 3, the temperature compensation means 101 can then tune the wavelength of the DBR device 2 in order to achieve a more optimum extinction ratio of the EAM device 6. By such means, the invention extends the useful operating temperature range of the device 1, for example from 30 C to 70 C, without having to provide thermoelectric cooling.

Although not shown, after testing the device 1 may be packaged in an industry standard package, with a single mode optical fibre coupled with a spherical lens to an output facet of the laser diode, and with gold bond wires 56, 57, 58 thermal-compression bonded onto the metalised contacts 66, 67, 68.

Although the present invention has been described specifically for the example of an InGaAsP/InP mesa waveguide laser diode in combination with a DFB section for tuning the optical wavelength and EAM device for modulating the optical radiation, the invention is applicable to any optoelectronic waveguide device requiring both wavelength tuning or selection from the DBR laser source in combination with optical modulation from the EAM section and temperature compensation provided by the temperature compensating means 101 which compensates for the different temperature- wavelength characteristics of the laser source and EAR device.

It should be noted that the invention is not limited to the use of a mesa current blocking structures of the type described above, and may employ other current confinement techniques and structures.

The invention described above has been described in detail for a device based on an n-InP substrate. However, it is to be appreciated that the invention can also be applied to devices based on a p-InP substrate.

In summary, because the laser diode has an operating wavelength that varies at a different rate than the band edges of the EAR device as temperature changes, the invention employs the DBR device to make compensating changes to the laser wavelength in order to extend the operating temperature range of the optoelectronic device without the need for thermoelectric cooling. The invention therefore provides a convenient optoelectronic component for providing modulated optical radiation with an improved temperature operating range, plus an economical method for manufacturing such a device. (a - 28

Claims (9)

1. Claims 1. A semiconductor optoelectronic device (100), comprising a laser

   source of optical radiation (50) including a laser diode (3) and a Distributed Bragg Reflector (DBR) device (4) with an electrically-tuneable input (57) for wavelength-tuning of said optical radiation (50), and an Electro- absorption Modulator (EAM) device (6) for modulating said optical radiation (50) received from the laser source (3,4), the laser diode (3) having an operating wavelength (14,114) with a first wavelength vs. temperature characteristic, and the EAM device having an absorption band edge (13,13',113,113') with second wavelength vs. temperature characteristic, said first characteristic being different from the second characteristic, wherein the optoelectronic device (100) comprises temperature compensation means (101) arranged to control the DBR input (57) when the temperature of the laser diode (3) and/or the EAM device (4) varies in order to tune the laser diode operating wavelength (14,214) relative to the EAM band edge (13,13', 213,213') and thereby compensate at least partially for said different characteristics.
2. 2. A semiconductor optoelectronic device (100) as claimed in Claim 1, in which the EAM device (6) receives said optical radiation (5) directly from the DBR device (4).
3. 3. A semiconductor optoelectronic device (100) as claimed in Claim 1 or Claim 2, comprising a monitoring means (104) for monitoring at least one operational parameter of the optoelectronic device (100), the or each operational parameter being correlated with a change in operating (A - 29 temperature of the laser diode (3) and/or the EAM device (6) and said monitoring means (104) providing an output (102) to the temperature compensation means (101) for said control of the DBR input ( 57) .
4. 4. A semiconductor optoelectronic device (100) as claimed in any preceding claim, comprising a temperature sensor (104) arranged to monitor the temperature of the laser diode (3) and/or the EAM device (6), wherein the temperature compensation means (101) receives an input (102) from the temperature sensor (104) that is used by said means (101) in the control of the DBR input (57) .
5. 5. A semiconductor optoelectronic device (100) as claimed in any preceding claim, comprising signal monitoring circuitry (103) for monitoring the electrical drive (56) of the laser diode (3), wherein the temperature compensation means (101) receives an input (112) from said circuitry (103) that is used by said means (101) in the control of the DBR input (57).
6. 6. A semiconductor optoelectronic device (100) as claimed in any preceding claim, comprising signal monitoring circuitry (105) for monitoring the electrical drive (58) of the EAM device (6), wherein the temperature compensation means (101) receives an input (110) from said circuitry (105) that is used by said means (101) in the control of the DBR input (57).
7. 7. A semiconductor optoelectronic device (100) as claimed in any preceding claim, wherein the laser source (3) and EAM device (6) are monolithically integrated on a common 1\, - 30 semiconductor substrate ( 12) .
8. 8. A semiconductor optoelectronic device (100) as claimed in any preceding claim, in which temperature compensation means (101) is arranged to maintain a constant separation between the wavelength (14,214) of the optical radiation (50) and the absorption band edge (13,213) of the EAM device (6) when the EAM device is passive.
9. 9. A method of operating a semiconductor optoelectronic device (100), said device comprising a laser diode (3), a Distributed Bragg Reflector (DBR) device (4) with an electrically-tunable input (57) for wavelength- tuning of said optical radiation (50), an Electro-absorption Modulator (EAM) (6) device with an absorption band edge (13,13',113,113', 213,213' ) for modulating said optical radiation (50), and a monitoring means (103,104,105), the method comprising the steps of: i) using the laser source (3) to generate optical radiation (50); ii) using the EAM device (6) to modulate said generated optical radiation (50); iii) using the monitoring means (103,104,105) to monitor one or more operational parameters of the laser diode (3) and/or the EAM device (6), said operational parameter(s) being correlated with a change in operating temperature of at least one of said devices (3,6); iv) using the DBR device (4) to tune the optical radiation ) (50) in response to said monitored operational parameter(s) in order to compensate at least partially for relative shifts between the wavelength (14,114) of said optical radiation (50) and the wavelength of said band edge (13,13',113,113') owing to temperature changes of the laser diode (3) and/or the EAM device (6).