DE4425358A1 - Method for the optical isolation of a laser-beam source and optical isolator, in particular for carrying out the method - Google Patents

Abstract

The invention relates to a method for the optical isolation of a laser-beam source and an optical isolator, in particular for carrying out this method. The inventive method is distinguished by the fact that, as a result of diffraction, the laser beam is subjected to a beam division and energy division and that, after the diffraction, the energy level of an interference beam retroreflected in the direction of the laser-beam source is lower than the energy level of the laser beam. For the incident laser beam, the diffractive optical element(s) exhibit(s) in the direction of the useful beam a higher diffraction effectiveness (efficiency) than for the beam which returns in the direction of the diffractive optical element after a reflection. Optical isolators preferably have a plurality of diffraction gratings in transmission or reflection.

Description

The invention relates to a method for the optical isolation of a Laser beam source against back reflections from the laser beam source emitted laser beam and an optical isolator, ins special to carry out the aforementioned method.

Optical isolators are used in laser technology Effect of possible object-side back reflections of the emitted La to prevent laser radiation to avoid fluctuations in the laser beam Operation or a destruction of the resonator of the laser ver avoid. In the worst case, the usable light intensity can be too 100% can be fed back in the direction of the laser beam source.

The feedback loss D _R results in

where I _N denotes the usable intensity of the laser beam and I _R denotes an effective as a disturbance on the laser, the feedback intensity of an interference beam returning to the laser in this direction.

There are requirements for the optical isolation of the laser beam source between 30 and 60 dB (decibels).

Insertion loss D _E is defined as the ratio of the intensity I _{O of} the laser to the usable intensity I _N (after passing through an optical isolator)

The total attenuation of the feedback process results

To protect against back reflections of the light in the laser beam source, it is known to use optical isolators which are based on the effect of the rotation of linearly polarized light by a magnetic field, as was discovered by Faraday. In this case, the incident light beam passes through a polarizer and is linearly polarized. Then a Faraday rotator rotates the polarization plane by 45 ° clockwise by a magnetic field, so that the transmission direction of the following analyzer is reached. The back-reflected light hits the analyzer, is polarized in the forward direction and then rotated again by the Faraday rotator by 45 ° clockwise. The back-reflected light thus strikes the polarizer with a polarization plane rotated through 90 ° to the transmission direction. For this purpose, paramagnetic glass or YIG crystals (yttrium iron garnet) are used, which have a large Verdet constant. In this way, optical isolations between 30 and 60 dB are achieved for a wavelength range of 515 nm to 1550 nm and aperture diameters of 1 to 4 mm. The insertion losses D _E are between 0.8 and 2.0 dB. Typical sizes of such optical isolators, which are based on the effect of the rotation of the polarization plane, are 50 mm in length and 40 mm in diameter.

This is the case with laser diodes that couple into fibers (pigtails) refined Faraday principle of optical isolation also used bar. However, the known components are used with laser diodes often too big, too heavy and too expensive.

When using semiconductor lasers in micro-optics and in microsystem technology, optical isolators are required that take account of the specifics of these applications with regard to miniaturization, optical parallel processing, integration options and mass production. Miniaturization requires customized, cost-effective solutions that correspond to the size and size of micro-modules in terms of weight and volume. For various applications, lasers have an excess of intensity, which can be determined by the laser sensitivity catcher (I _O) or the Emp and which can be partially consumed or must. The incident laser beam (useful beam) can and should therefore be subject to a predetermined insertion loss (D _E ), insertion losses of greater than 3 db being possible. For integrated optics, optical isolators are required that are adapted to the conditions of the waveguide. In addition, the micro-optics for optical parallel processing with layered planar optics require isolators that are adapted to the use of laser diode rows. These requirements are not met with conventional insulators based on the Faraday effect or utilizing the polarization properties of crystals.

From US-PS 44 90 021 is a filter element for a coherent Light beam known, with a diffractive grating in the beam path is inclined so that the angle of incidence of the light beam is in the range of 80 to 90 °. This solution is for color Fabric lasers provided high resolution at low To ensure reflection losses.

From US-PS 46 51 315 is an optical isolation device be knows the end of an optical fiber in a telecommunications tion system forms. Here are two ge through a filter mirror separated optical media provided, one Emitter-fiber connection is made in one of the media, while rend the splitting and merging by the recipients in the other optical medium. The above solutions are al but not suitable, the problem of the lowest possible return flexion of emitted laser light into the laser beam source or into to solve the direction of the same.

The invention is therefore based on the object of a method for op to isolate a laser beam source against back reflections give that simple, inexpensive and to the needs of the spe specific geometric conditions of micro-optics, the microsystem  technology and waveguide technology is coordinated.

The invention is also based on the object of an optical iso lator, which allows a high feedback loss, a uncomplicated and inexpensive construction and to the special geometric conditions of micro-optics and microsystem technology as well as the waveguide technology is adaptable.

The above object is fiction, regarding the method solved according to that the laser beam by diffraction of a Beam and energy sharing is subject and following the Diffraction is the energy level towards the laser beam source back-reflected interference beam is less than the energy level of the Laser beam.

The inventive method is based on the surprising, as well as uncomplicated, implementation that it is possible to protect the laser beam source even in applications where there is a risk of 100% reflection of the laser beam back into the source, while avoiding active isolator principles in Adaptation to the integrated optics and the conditions of the waveguide to subject the laser beam to diffractive beam and energy sharing for the incident and returning the beam, such that the intensity of an interference beam returning to the laser beam source is substantially less than the intensity I _o of emitted light beam or the usable intensity I _N.

This method has the advantage of being easily adaptable to the source as well as uncomplicated feasibility through a further variety the use of diffractive optical elements, their optical properties can be precisely adjusted to the properties of the laser beam are.

According to a preferred embodiment of the method according to the invention The laser beam is then applied to a diffractive optical element leads that for the incident coming from the laser beam source Laser beam (interference beam direction) a higher diffraction activity than  for the reflected in itself, on the diffractive optical element returning beam (direction of reflection).

According to a preferred procedure, depending on the Wavelength and the angle of incidence of the com from the laser beam source emitting, incident laser beam and depending on one characteristic constants of the diffractive optical element the incident laser beam through the diffractive optical element only in the 0th diffraction order and in the +1. Diffraction order under Aus decomposed formation of a diffraction angle, through which after a reflection which in itself goes back to the diffractive optical element de ray of +1. Diffraction order this in the 0th diffraction order that +1. Diffraction order and the -1. Diffraction order is broken down.

The laser beam is turned off to achieve compact optical isolation preferably following a multiple diffraction, preferably on subjected to at least one grid.

It is further preferred that in addition to the optical isolation by Diffraction of the laser beam this also one, preferably two beam enlargement or reduction (beam expansion or beam constriction) in one dimension.

Further preferred configurations of the method according to the invention are set out in the other subclaims.

With regard to an optical isolator of the type mentioned the aforementioned object is achieved in that the ser has at least one diffractive grating arrangement, which in Dependence on a wavelength and an angle of incidence of the Laser beam and a lattice constant of the grating a higher Diffraction effectiveness for the emitted laser beam tung) than for one reflected in itself, back to the grid own beam (interference beam direction).

The optical isolator preferably has at least a diffraction grating a reflection grating and / or a transmission grating on the one linear or also effective in several directions, flat  Grid (such as a cross grid) can be.

At least one is preferably used as the diffractive optical element Grid used, which is designed as a surface relief, preferably a deeply modulated phase grating.

According to a preferred embodiment of the optical according to the invention Isolator, which is preferably realized by a surface relief can be and in its simplest form is a linear phase grating, or has imaging properties and in reflection or trans mission works, the grating only diffracts the incident laser beam into a 0th diffraction order and a +1. Diffraction order, under training of a diffraction angle, through which to an object-side Reflection of the beam of rays returning to itself on the grating +1. Diffraction order in the 0th diffraction order, the +1. Diffraction order and the -1. Diffraction order can be dismantled.

In a preferred embodiment, the opti according to the invention cal isolator on at least two optical elements, of which at least least of a diffractive grating, the elements being prismatic are arranged in planes that are at an angle to each other fen, such that for each diffraction on one in the same plane ordered optical element of the original idea or Beu angle is reproducible.

According to an advantageous embodiment of the optical isolator In the present invention, this forms a prism, the under at least at an acute angle to each other a flat reflection grating and a mirror and / or a wide one Wear a flat reflection grating.

In a further preferred embodiment, which is advantageous for the optical isolation of a plurality of parallel laser beams is suitable net, the optical isolator has a prism with two diffractive Gratings and a mirror surface, this optical isolator also the possibility of swapping the order of the different which have laser beams.  

In the case of maintaining a given direction of the laser beam, possibly with parallel beam displacement, the optical isolator according to a further advantageous embodiment the invention a prism with a transmission grating and a Reflection grating on one beam entry side, one Reflection grating on one side and one Transmission grating on a beam exit side of the prism.

According to a further embodiment of the optical Isolator has a prism with an imaging diffractive optical element (DOE), preferably a holographic optical Element (HOE), for irradiating the laser light beam, e.g. B. from one Laser diode, on, in conjunction with two reflection gratings on the in an acute angle to the sides of the prism, the after beam expansion of the parallel bundle in one dimension has a transmission grating on its beam exit side.

According to a further embodiment of the invention, a diffractive grid in a vertical arrangement in connection with a Layer waveguide provided, the grating structure (surface Chenrelief) perpendicular to the direction of propagation in the layered world len-guided modes is arranged and this lattice structure in Dependence on the wavelength of the laser light, the angle of incidence and the grating parameters also have a higher diffraction efficiency for the emitted laser beam compared to a reflective one th beam returning to the grid.

Further, preferred configurations of the optical device according to the invention Isolators are presented in the remaining subclaims.

The invention is described below using exemplary embodiments and associated drawings explained in more detail. In these show:

Fig. 1 shows an optical isolator as a transmission grating according to a first embodiment of the invention, Fig. 1a shows a beam path up to an object-side reflection and Fig. 1b shows a beam path of the back-reflected beam.

Fig. 2 shows an optical isolator as a reflection grating according to a second embodiment of the invention, Fig. 2a shows a beam path for the emitted laser beam (before an object-side reflection) and Fig. 2b the radiation course of the reflected beam (after an object-side reflection) shows.

Fig. 3 is a diagram of an overall attenuation (D _E) and a rear coupler lung damping (D _R) as a function of the number of bendings for a diffractive grating for different transmission or reflection factors,

Fig. 4 is an illustration of a relative path of the usable intensity of the laser beam in transmission or reflection as a function of the number of inflections in diffractive gratings for cases A1, B1, and B2 of Fig. 3,

Fig. 5 is an optical isolator as a prism having two reflection gratings according to a further embodiment of the invention for explaining the reproduction of the irradiation conditions for each grating,

Fig. 6 shows an optical isolator as a prism, a mirror and a planar relief phase grating according to a further example of execution of the invention,

Fig. 7 an optical isolator with a prism similar to exporting approximate shape shown in FIG. 6, but with two phase relief gratings according to a further embodiment of the invention,

Fig. 8 is an optical isolator with a prism, two grids, and a mirror surface, particularly for a plurality of parallel light beams according to a further embodiment of the invention,

Fig. 9 lung direction an optical isolator with a prism, two reflection gratings and two transmission gratings for Strahlein- -auskopplung and while maintaining the predetermined Einstrah,

Fig. 10 an optical isolator with a prism and simultaneous beamforming in one dimension (beam expansion) according to another embodiment of the invention,

Fig. 11 an optical isolator with a prism and simultaneous beamforming in one dimension (beam reduction) according to another embodiment of the invention,

Fig. 12 is an optical isolator having a plurality of trans missions gratings according to a further embodiment of the invention,

Fig. 13 an optical isolator with a consist of two prisms of the double prism, two mirrors and a transmission grating according to a further embodiment of the invention,

Fig. 14 an optical isolator as a cross grating according to a further embodiment of the invention, and

FIG. 15a an optical isolator in conjunction with a film waveguide according to another embodiment of the OF INVENTION dung,

Fig. 15b shows a detail 1 of FIG. 15a,

Fig. 15c shows the detail of Fig. 15b in perspective.

A first embodiment of the optical isolator 1 is explained in the following with reference to the basic arrangements according to FIGS. 1 and 2, including the underlying physical relationships.

Fig. 1 shows the optical isolator 1 , which here consists of a translucent plate 2 , on the top of which a diffractive grating 3 , which is designed in this case for diffraction of an incident laser light beam Se as a transmission grating 4 , is applied.

While FIG. 1 a shows the incident radiation of the incident laser beam Se emitted by a laser beam source (not shown here), FIG. 1 b shows the reverse beam path after reflection of the diffracted light beam Sa emerging from the optical isolator 1 on an object 5 .

For optical isolation, the property of the grating 3 is used in such an optical isolator 1 , by adapting the grating constants g to the light wavelength λ to suppress diffraction orders, in particular higher diffraction orders depending on the angle of incidence α of the incident laser beam Se. In addition, there is a different diffraction efficiency at different angles of incidence α.

In the simplest case, the grating 3 is a linear phase grating that can be realized, for example, by means of a surface relief or that can have imaging properties.

The grating 3, as shown in FIGS. 1a and 1b for transmission or as shown in Fig. 2a and Fig. 2b, are designed for reflection.

In the context of the present application, the term dif fractive grating or diffractive optical element diffraction elements understood elements or grids of both types, d. H. Diffraction gratings, the are designed for either reflection or transmission.

By tuning the grating constant g of the transmission grating 4 in FIG. 1a to the light wavelength λ of the incident light beam Se and the angle of incidence α, it is achieved that the incident light beam Se only in the 0th and 1st. Diffraction order is broken down and higher diffraction orders do not occur.

This results in the diffraction angle β _e such that when back reflection on the object 5 and the occurrence of a back reflection beam Sr, which is reflected back at the angle of incidence α _r (= diffraction angle β _e ) onto the transmission grating 4 , this back-reflected light beam Sr enters the 0. and -1. Order as well as in the +1. Order is broken down, whereby only the beam part of the +1. Order is reflected in the direction of the laser beam source as an interference beam Ss. For the rest, however, the energy is divided into three beams, so that the portion going in the direction of the laser source is increased by the amount of the intensity of the 0th and -1st Order is weakened, so that there is a different energy balance for the direction of radiation and for the back reflection direction and thus a corresponding optical isolation of the laser beam source.

The relationship of the angles for transmission ( Fig. 1) is given by the lattice equation:

For reflection ( Fig. 2), the grating equation differs only by a sign.

In the grating equation ( 4 ) λ denotes the wavelength, k the diffraction order, α the angle of incidence and β the diffraction angle in transmission.

In its simplest form, the transmission grating 4 is a linear phase grating, which is realized by means of a surface relief. For certain applications, the grid may have imaging properties.

The setting of the diffraction effectiveness of the transmission grating 4 for the angle of incidence α takes place when the transmission grating is designed as a surface relief with a corresponding choice of the ratios of the light wavelength λ to the groove depth of the surface relief h (modulation depth) and to the grating constant g. The angle of incidence α which has only one diffraction order is determined by the following condition:

The relationship between the angle of incidence and the angle of diffraction α, β (here α _E , β _E ) is given by the aforementioned grid equation.

The diffracted light beam Sa (useful beam direction) emerging from the transmission grating 4 in FIG. 1a is weakened by the amount of intensity in the 0th diffraction order. The +1 light diffracted as an interference beam Ss by the transmission grating in the direction of the laser beam source. Diffraction order is by the amount of the intensities in the direction of the 0th diffraction order and the -1. Diffraction order weakened.

The diffraction effectiveness of the grating 3 is also a function of the angle of incidence α of the laser beam S _e and provides the angle of incidence α = α _e = β _r and the angle of incidence of the back-reflected beam Sr ( FIG. 1b) α = α _r = β _e different values of the transmission, ie there are, even if the -1. Diffraction order does not occur, different transmission factors W _a and W _b for different angles of incidence α. The transmission factor or reflection factor (embodiment according to FIG. 2) of the incident light beam Se is designated W _a , W _b denotes the transmission factor or reflection factor ( FIG. 2) of the back-reflected light beam S _r. The usable intensity (I _N ) can in the worst case be completely reflected in the direction of isolation on the laser beam source. For the interference beam Ss (β _r = α _e ) which is deflected back in the direction of the laser beam source, ie in the direction of incidence, a transmission or reflection factor then applies as the intensity factor:

for the proportion of +1 bent back in the direction of incidence. Order.

From these relationships, the above-mentioned relationships (1) to (3) for feedback loss D _R , insertion loss D _E and total loss D _G (D _G = D _R + D _E ) result in the damping quantities. The insulating effect of the optical isolator 1 is determined by W _b , ie by the transmission or reflection factor of the interference beam Ss with the intensity W _b .

Since the diffraction effectiveness of the grating 3 can only be changed in the same order of magnitude for the two factors W _a and W _b , it is advisable to apply the previously explained isolation principle several times. The following applies to the principle of diffractive optical isolators connected in series:

The overall effectiveness of the optical isolation is determined by the transmission or reflection _factors W _ak and W _bk for k = N, ie by the course of the diffraction effectiveness as a function of the angle of incidence α. W _{ak determines} the usable intensity after k transmissions or reflections and W _{bk determines} the intensity _{fed back} after k transmissions or reflections. The cheapest option is achieved by optimizing the efficiency curve. The profile shapes and the ratios of the wavelength λ to the modulation depth h and the grating constant g interact. This relationship is not trivial in the case of the formation of the grating 3 as a surface relief, since an analytical solution of the wave equation would be required here. Numerical solutions are known under various boundary conditions, but they are very complex and generally not available. Realistic values W _ak and W _bk for a wavelength of λ = 0.633 nm can be obtained from experimental investigations, so that the total attenuation D _G and the insertion loss D _R can be calculated with the aforementioned relationships. Corresponding slide programs are shown in FIGS. 3 and 4.

Fig. 3 shows the course of the total attenuation D _G and the feedback attenuation D _R as a function of the number of diffractions in a diffractive optical isolator for various transmission or reflection factors. For the cases A1, B1 and B2, the total attenuations are D _G or the insertion loss D _E plotted against the number of diffractions (transmissions and / or reflections) N through the diffractive optical element or grating 3 .

Fig. 4 shows the relative course of the useful intensity I _N in Trans mission or reflection as a function of the number of diffractions in diff fractive optical isolators according to the cases A1, B1 and B2.

With the experimentally determined relationships according to FIGS. 3 and 4, the following applies to the efficiency curve, for example for a deeply modulated SIN grating:

h / g = 1.0, g = 0.75 µm, λ = 0.633 µm, α _e = -42 °, β _e = 10.07 °, W _ak = 0.65 and W _bk = 0.20.

The embodiment of FIG. 2 illustrates the design of the diffraction grating 3 as a reflection grating 6 , with FIG. 2a showing the radiation pattern for the irradiation and FIG. 2b the radiation pattern for the back reflection. The designations otherwise follow those of the first exemplary embodiment according to FIG. 1. Also in this case wavelength λ, grating constant g and angle of incidence α _{e of} the incident laser beam Se are chosen such that the diffraction of the incident laser beam Se in reflection with the reflection factor W _a only in the 0th diffraction order and the +1. Diffraction order takes place. The first order diffraction angle is again designated β _e . After reflection on the object side, the retroreflected return beam Sr strikes the reflection grating 6 at the angle of incidence α _r (= diffraction angle β _e ) and is reflected with the reflection factor W _b in three beams of the 0th diffraction order, the -1. Diffraction order and the +1. Diffraction order decomposed, only the light of +1. Diffraction order in the direction of incidence is again conducted as an interference beam to the laser beam source, so that the intensity of the feedback I _{R is} significantly less than the intensity of the laser I _o or the usable intensity I _{N of} the laser beam Sa of the 1st diffraction order in the direction of the useful beam ( Fig ). This also results in a different energy balance of the incident laser light Se and the emerging laser beam 1. Diffraction order Sa in the direction of the useful beam compared to the interference beam Ss in the direction of the laser beam source.

Another embodiment of an optical isolator 1 is shown in Fig. 5, wherein at the same time the optical conditions are explained which lead to the fact that the original radiation conditions are reproduced on each grating 3 .

This exemplary embodiment also relates to an application of the laser light in free space, ie of unguided laser light (for example also in air or transparent media), while an exemplary embodiment for the application in the waveguide region with guided light is explained below with reference to FIG. 14. In general, in the exemplary embodiments explained here, the use of the optical isolators 1 can also be carried out simultaneously by a plurality of parallel light beams.

It has already been pointed out that multiple diffraction in reflection or transmission on a diffractive optical element is desirable for the desired degree of optical isolation. In order to realize multiple diffraction in reflection, the surfaces that carry the diffractive optical elements, here reflection grating 6 , must be oriented at an angle ω to one another such that the original irradiation conditions are reproduced for each diffraction grating, here reflection grating 6 .

In Fig. 5, the optical isolator 1 is configured as a prism 7, the side surfaces 8 and 9 which extend at an acute angle to each other, provided with a reflection grating 6, respectively, wherein an irradiation is carried out a laser beam Se outside the associated grid 6 . For the context for the k. and k + 1. Diffraction in reflection applies:

l = αe _k + βr _{k + 1} and αe _k - βe _k = αe _{k + 1} - βe _{k + 1} k = 1.2

where α _{ek are} the angles of incidence and β _{ek are} the diffraction angles in the useful beam direction, while α _{rk are} the angles of incidence of the back-reflected beam Sr and β _{rk are} the diffraction angles of the interference beam Ss in the direction of the laser beam source (interference beam direction).

The reflection grating 6 can also be arranged at the angle on appropriately arranged plates, in particular glass plates, or can also be arranged in a cantilevered manner in the room or integrated in a solid, as is the case, for example, for a layer waveguide (for the same exemplary embodiments according to FIG. 14 ) or in view of the transmission grating in the exemplary embodiment according to FIG. 11 is shown.

Another embodiment of an optical isolator 1 is shown in FIG. 6. Here, the optical isolator 1 is also designed in the form of a prism 7 , which has two side surfaces 8 , 9 arranged at an acute angle. The first surface 8 is partially provided with a mirror 10 , while the second Be tenfläche 9 is mainly provided with a flat phase relief reflection grating 6 . In the area of the mirror 10 , five reflections of an incident laser light beam Se radiated in outside the reflection grating 6 take place, while five diffractions in reflection take place at the reflection grating 6 (N = 5). The incident light beam Se enters the prism 7 at a tip-side, uncoated surface section of the side surface 8 and is reflected on the mirror 10 on the first side surface 8 , then arrives at the constant angle of incidence α _ek on the reflection grating 6 and becomes a mirror 10 bent. This is repeated until the light emerges from the prism 7 as a failed light beam Sa on a non-mirrored section of the first side surface 8 facing away from the tip of the prism 7 . From the context explained with regard to the embodiment according to FIG. 5

α _ek - β _ek = α _{ek + 1} - β _{ek + 1} = const.

results for a grating 3 and a mirror 10

2α _sk = α _k - β _k ,

where α _sk denotes the angle of reflection at the kth mirror. The constant angle of incidence α _k results from 2α _rk = α _ek - β _rk . The relationship between the angle of incidence α _ek and the prism angle ω is given by ω = 2α _k for this case in which α _ek = β _rk applies.

The diffraction effectiveness is greater for the incident light beam Se than for the back-reflected light beam Sr, the angle of incidence α _{ek being} equal to the diffraction angle β _rk .

The damping parameters can be found, for example, in FIG. 3. This optical isolator 1 has the advantage that an N-fold diffraction in reflection is realized with only one grating 3 . The number of reflections depends on the angles and the lengths of the first and second surfaces 8 and 9 of the prism 7 , in general a somewhat longer prism base is required for such an optical isolator 1 .

Another embodiment of a diffractive optical isolator 1 is shown in FIG. 7. This embodiment uses, similarly to the embodiment of FIG. 6, a prism 7, the lateral faces 8 and 9 at an angle ω are disposed inclined to each other and each represents a phase relief grating as a reflection grating 6 transmits. In this arrangement, the incident laser beam Se is diffracted five times on each reflection grating 6 , so that the total number of diffractions is N = 10.

The incident laser beam Se enters the prism 7 at a tip-side, uncoated part of the side surface 8 and strikes the lower reflection grating 6 arranged along the prism base at the angle of incidence α _k , the irradiation conditions in connection with that shown in FIG. 7 shown prism 7 running in the cross section as a right-angled triangle, lead to a particularly simple arrangement, such that the angle of incidence α _k, which is reproduced according to each diffraction, is equal to the base angle ω of the prism 7, and the diffraction angle β _k is always 0 ° is. The light beam Sa emerges from the prism 7 at the same angle at which the incident light beam Se strikes the prism 7 .

Compared to the embodiment according to FIG. 6, this arrangement has the advantage that the size of the optical isolator 1 can be reduced.

Another embodiment of a diffractive optical isolator 1 is shown in FIG. 8, consisting of a prism 7 , which has a reflection grating 6 on the first side surface 8 and a reflection grating 6 and a mirror 10 on the further side surface 9 provided as a prism base having.

Such a prism 7 , which, like the other exemplary embodiments, is designed with respect to its diffraction grating (here: reflection grating 6 , grating constant g) and with respect to the depth of modulation in coordination with the light wavelength λ and the angle of incidence α so that the diffraction on a grating 3 (Reflection grating 6 or transmission grating 4 ) only the 0th and +1. Diffraction order occur, so that when the reflected beam Sr strikes the respective grating 3, a diffraction into the 0th diffraction order, the +1. Diffraction order and the -1. Diffraction order are generated, is particularly suitable as an optical isolator for several parallel laser beams len Se, which in the present case strike a grid-free area of the first side surface 8 at right angles and emerge from the prism 7 again at the same angle after reflection on the mirror 9 . In this way, a plurality of light rays Se incident in parallel can use the same optical isolator 1 , and the sequence of the different light beams between the entry and exit sides can be exchanged, as is shown by those in FIG. 8 with 1, 2, 3 and in different line types shown laser beams can be seen. In addition, the distance between the different laser beam bundles 1 , 2 , 3 between the entry and exit sides can be changed, in the present case according to FIG. 8 in the direction of the beam exit can be enlarged.

The mirror 10 can also be replaced by a diffractive optical element, such as a further reflection grating, such an embodiment then leading to a modification of the exemplary embodiment according to FIG. 7. If the mirror 10 is missing in the embodiment according to FIG. 8, the laser beam bundles 1 , 2 , 3 emerge from the underside of the prism 7 , optionally after diffraction on a further transmission grating.

A further embodiment for an optical isolator 1 is shown in FIG. 9, which in turn has a prism 7 forming a right-angled triangle in cross section, the side surfaces 8 and 9 of which, at an acute angle to one another, each have a reflection grating for beam diffraction within the prism 7 6 wear, while in the area of the beam inlet the side surface 8 additionally has a transmission grating 4 , a transmission grating 4 also being arranged on a side surface 11 of the beam outlet. The prism 7 can preferably be arranged in a housing, shown in a similar manner, which only has openings in the area of the beam entrance (transmission grating 4 ) and in the area of the beam exit.

As this exemplary embodiment illustrates, it is possible with such a prism, whose transmission grating 4 and reflection grating 6 follow the diffraction principles explained at the outset, to achieve a beam displacement by the amount a on the beam exit side relative to the beam entrance, the direction between the entering Laser beam Se and emerging laser beam Sa remains unchanged.

By appropriate dimensioning, it is also possible to make the amount of the beam displacement a become 0 and to let the emerging, multiply diffracted laser beam Sa emerge from the prism 7 as an extension of the incoming laser beam Se.

The embodiments according to FIGS. 10 and 11 show a Erwei esterification in relation to the design of the previously discussed optical Iso simulators 1 such that is provided in these cases, the optical isolator 1 with further functional elements, so that in addition to the unverän derten effect of optical isolation at the same time a beam formation of a source, preferably a laser diode 12 , can also take place. In the embodiment according to FIG. 10, the incident laser beam is expanded in one dimension. For this purpose, the optical isolator 1 is in turn designed as a prism 7 in cross section, the side surfaces 8 and 9 of which are provided with reflection gratings 6 for optical isolation.

In the area of entry of the laser beam from the laser diode 12 on the first side surface 8 , the latter has an imaging diffractive optical element DOE, ie a grating with imaging properties, preferably a holographic optical element HOE, by means of which beam shaping takes place in one dimension, whereby the beam exit side of the prism 7, a transmission grating 4 is arranged, via which the expanded, one-dimensionally shaped laser beam emerges in the direction of the arrow in FIG. 10.

The embodiment of Fig. 11 shows the reverse possibility of reduction of beam dimension, wherein the Strahleintritts- and beam exit side of the prism 7 are reversed compared to the embodiment according to Fig. 10 and a laser diode is directed to the in Fig. 11 right-hand side of the prism 7 towards , wherein this side surface 13 carries an imaging diffractive optical element DOE, ie a grating 4 with imaging properties or preferably a holographic optical element. The two side surfaces 8 and 9 are again provided with reflection gratings 6 , while in the region of the beam exit on the left-hand side in FIG. 11, the one-dimensional laser beam emerges via a transmission grating 4 .

In this way, the optical isolator can also be used for beam shaping. The principle of reducing or enlarging a beam dimension can also be used without optical isolation if the diffraction effectiveness of the grating 3 used is suitably designed.

Fig. 12 shows an embodiment of an optical isolator 1 with the implementation of the diffractive isolator principle in Trans mission. In this case, a plurality of prisms 7 with transmission gratings 4 are each arranged in a transparent cuboid 14 in the beam direction at the same inclination angle, here 45 °, so that the angle of incidence α _k 45 ° and the diffraction angle β _{k is} 0 °. The incident laser beam Se passes through a cascade of planar transmission gratings 4 and is bent several times under the conditions set out above with respect to the energy and beam splitting between the useful beam and the returning reflection beam.

Fig. 12 shows in thin and thick line thickness 2 incident laser beams Se to illustrate that this optical isolator 1 can also preferably be used to influence parallel, different laser beams that enter the optical isolator 1 with the distance t _e leave this with an increased distance t _a .

Such an arrangement is advantageous for the production of diffractive optical isolators 1 for the integrated optics and in connection with the diffraction guided light in waveguide technology, in particular for layer waveguides.

This also applies to the optical isolator 1 according to a further exemplary embodiment, which is shown in FIG. 13 and which is constructed as a double prism from two prisms 7 . A transmission grating 4 is arranged along the interface of the two prisms (side surface 9 in the previous exemplary embodiments), while the two outer side surfaces 8 of the double prism arrangement mirrors 9 , which run inclined to one another, for reflecting the laser beam which is diffracted five times on the inner transmission grating Se. If necessary, the mirror 10 can also be replaced by reflection grating 6 . The incident light beam Se strikes the tip-side, non-reflecting part of the side surface 8 of the upper prism 7 in FIG. 13 at an angle and is refracted towards the transmission grating 4 , through this in the manner explained above with regard to the energy balance of the incident and back-reflected light beam diffracted towards the lower outer surface 8 and reflected again by the mirror 10 seen there in the direction of the transmission grating 4 . After multiple reflection by the mirror 10 and diffraction at the internal transmission grating 4 , the multiple-diffracted laser beam Sa emerges from the upper prism 7 on the right-hand side in FIG. 13 at the same angle at which the incident laser beam Se reached the prism 7 .

An optical isolator 1 can also be formed by a diffractive element which has two directions of dispersion. It is preferably formed by periodic phase relief gratings, which enclose an angle that can be advantageously determined by the respective application. In the simplest case, this can be a cross grating 15 , as is shown schematically in FIG. 14.

The cross grating 15 can be understood as a combination of two simple grids 3 , here reflection grids 6 , which are arranged rotated by 90 ° to one another. For the cross grating 15 , the grating equation (4) with corresponding, different signs for reflection or transmission applies in the x and y directions. In this case too, the number of lines of the cross grating 15 in the x- and y-directions are chosen such that the incident laser beam Se (here in reflection) is in the 0th diffraction order and the +1. Diffraction order is broken down.

The laser beam Sr reflected back on the object perpendicularly strikes the cross grating 15 and generates all combinations of the orders k, l = 0.1. This means that the back-reflected beam Sr is broken down into 9 beam parts, so that at most 1/9 of the back-reflected intensity I _{R is} diffracted as an interfering beam Ss in the direction of the laser beam source. With a suitable design of the diffraction effectiveness as a function of the angle of incidence α, this factor can be reduced even further and the optical isolation can thus be improved.

In this case too, the previously explained exemplary embodiments are obsolete visually the lattice arrangements (those for one-dimensional lattices with a direction of dispersion) were applicable, for which Cross gratings have the advantage of reducing the number of bends (Reflections and / or transmissions) at the same time he essential  higher optical isolation (larger beam breakdown of the back reflection tated laser beam) and thus significantly reducing intensity one in the direction of the direction of incidence on the laser source interfering beam Ss.

It is not necessary that on the cross grid in x and y-direction the same number of diffractions of the incident Laser beam occurs. Rather, they can each be different be in ensuring higher diffraction effectiveness Incidence than in the back reflection direction.

The principle of optical isolation by diffraction can be inscribed use special for the integrated optics, as the non-guided Dimension of the light beam corresponds to the open space conditions.

Thus, FIG. 15 schematically shows an embodiment of an optical isolator 1 in conjunction with a planar waveguide 16, and therefore the application of the above-explained principle of the optical isolation by diffraction in the range guided laser beam, the non-run dimension of the use of diffractive optical elements, in particular Grids 3 allowed for optical isolation. While Fig. 15a shows schematically a plan view of a slab waveguide 16, is irradiated into which the laser light beam Se from a laser diode 12, Figs. 15b and 15c respectively schematically in plan view and perspective view of the respective grating structure of the reflection grating 6. Periodic structures 17 are introduced in the layer waveguide 16 such that grating lines as reflection gratings 6 are perpendicular to the direction of propagation of the guided modes of the laser beam and are deeper than the waveguiding region. The light coming from the laser diode 13 is diffracted at these reflection gratings 6 , so that the previously explained different diffraction efficiencies become effective for the direction of irradiation and the direction of reflection back. Correspondingly, the principle of diffractive optical isolation in transmission shown in FIG. 12, ie the use of transmission gratings as a grating structure for waveguides, can also be used.

With regard to the previously described exemplary embodiments, it is also pointed out that it is possible to arrange the grating 3 for multiple diffraction in an inclined arrangement relative to one another both on a prism and, for. B. on glass plates arranged in appropriate angles of inclination or freely in space or in a transparent solid body ( Fig. 11).

It is also noted that for optical isolation by setting up a beam splitting and different Energy balance for the irradiation and the feedback direction is possible when using polarized light for the Polarization state DE (E vector) in the direction parallel to the Grating grooves or lines of the diffraction grating and for the Polarization state DM (E vector perpendicular to the lattice furrows or grating lines) to the same diffraction effectiveness of the grating reach so that the optical isolator regardless of the Direction of polarization in both parallel and orthogonal Arrangement can be used.

The arrangement of the reflection gratings 6 and / or the transmission gratings 4 is possible individually or in any combination with one another and in connection with other optical components, in particular other diffractive or refractive optical elements and / or with elements of the integrated optics. Metallic vapor deposition and / or a coating for optimizing the intensity ratios (transmission factors) W _a and W _b can be carried out for reflection gratings 6 , in particular when they are integrated into a layer waveguide (see FIG. 15).

Those explained for the free space (non-guided light) Embodiments can be found in the known Manufacturing technologies for optical or computer-generated Holograms (HOE or CGH) are executed. With that is also the Application of appropriate techniques used there Reproduction, such as copying, embossing, injection molding, etc. possible.  

The corresponding lattice relief structures can be introduced into the layer waveguide (see exemplary embodiment in FIG. 15) perpendicular to the waveguide using the known etching techniques by means of masks or by direct exposure in photoresist. A special possibility of the execution results from the modulation of the periodically bordered waveguide in photoresist and the thus possible production of a tool for embossing into suitable materials such as polymers, PMMA, photoresist. Metallic vapor deposition can serve to increase the diffraction efficiency for reflection.

Further embodiments of the design of optical isolators through in particular multiple diffraction of the incident light beam diffractive optical elements and their different combinations tion are possible depending on the specific application. A there is a particularly favorable effect if the Lattice constants depending on the light wavelength used and under the choice of the angle of incidence of the light beam on the first grating higher diffraction orders are suppressed and for the back-reflected beam has a larger beam distribution than for the incident beam occurs. When guaranteeing different Diffraction effectiveness for the laser beam in the direction of the useful beam and The direction of reflection is differentiated in both directions Liche angle of incidence can possibly also on the suppression higher diffraction orders can be dispensed with. Can also be beneficial both effects (suppression of higher diffraction orders for the one falling laser beam, different angles of incidence for the one falling and back-reflected laser beam of the diffraction used order) to increase the insulation effect with each other be combined.

The subject of registration is therefore not on the suppression of higher ones Diffraction orders (as the 0th and +1 st diffraction orders) limited. Rather, higher diffraction orders for the incident Laser beam (and even the same diffraction orders between and back-reflected laser beam) as long as in Direction of incidence a higher diffraction efficiency (and radiation energy) than in the back reflection direction due to higher decomposition of the  back-reflected beam and / or different angles of incidence for the incident and back-reflected beam by tuning of lattice constants, light wavelength, angle of incidence and given if the depth of modulation remains guaranteed.

Claims (36)

1. A method for the optical isolation of a laser beam source against back reflections of a laser beam emitted by the laser beam source, characterized in that the laser beam (Se) is subjected to a beam and energy division by diffraction and, following the diffraction, the energy level is reflected back in the direction of the laser beam source Interference beam (Ss) is lower than the energy level of the laser beam (Se, Sa). 2. The method according to claim 1, characterized in that the laser beam (Se) on at least one diffractive optical element ( 3 , 4 , 6 ) is guided. 3. The method according to claim 1 or 2, characterized in that the diffractive optical element ( 3 , 4 , 6 ) for the coming from the laser beam source, incident laser beam (Se) a higher diffraction activity than for the reflected in itself on the diffractive op table element ( 3 , 4 , 6 ) returning beam (Sr). 4. The method according to at least one of the preceding claims 1 to 3, characterized in that, depending on the wavelength (λ) and the angle of incidence (α) of the coming from the laser beam source, the incident laser beam (Se) and a characteristic constant (g) of the diffractive optical element ( 3 , 4 , 6 ) the incident laser beam (Se) only in the 0th diffraction order (β _e ) and +1. Diffraction order is broken down to form a diffraction angle, by means of which, after reflection, the beam (Sr) of +1, which in itself returns to the diffractive optical element ( 3 , 4 , 6 ). Diffraction order in the 0th diffraction order, the +1. Diffraction order and the -1. Diffraction order is broken down. 5. The method according to at least one of the preceding claims 1 to 4, characterized in that different diffraction efficiencies are provided by different incidence and back reflection incidence angles (α _e , α _r ) for the incident and back reflected laser beam (Se, Sa) the diffraction effectiveness in the direction of the retroreflected beam (Sr) is as low as possible by choosing the smallest possible reflection angle (α _o ). 6. The method according to at least one of the preceding claims 1 to 5, characterized in that the diffractive optical element for polarized light regardless of irradiation in parallel or the same diffraction effect perpendicular to its diffractive structure activity. 7. The method according to at least one of the preceding claims 1 to 6, characterized in that the laser beam (Se) sequentially one undergoes multiple diffraction. 8. The method according to at least one of the preceding claims 1 to 7, characterized in that the laser beam (Se) is subjected to a diffraction on a grating ( 3 , 4 , 6 ). 9. The method according to claim 8, characterized in that the laser beam (Se) is subjected to multiple diffraction in a prism ( 7 ) between a flat reflection grating ( 6 ) and a mirror ( 10 ). 10. The method according to claim 8, characterized in that the laser beam (Se) following at least two diffractive optical elements te is guided, which are arranged at an angle (ω) to each other are such that for each diffraction at the same diffractive op table element the original angle of incidence or diffraction right is produced. 11. The method according to claim 10, characterized in that the laser beam is diffracted between at least two grids ( 3 , 4 , 6 ) arranged at an angle to one another. 12. The method according to at least one of the preceding claims 1 to 11, characterized in that the laser beam in connection with the Diffraction undergoes beam shaping. 13. Optical isolator to protect a laser beam source against back reflections of a laser beam emitted by the laser beam source  les, in particular for performing the method according to claim 1, characterized by at least one diffractive grating arrangement which depending on a wavelength (λ) and an angle of incidence the laser beam (Se) and a lattice constant (g) a higher one Diffraction effectiveness for the emitted laser beam (Se, Sa) than for a reflected beam that returns to the grating (Sr). 14. Optical isolator according to claim 13, characterized in that the grating ( 3 ) is a reflection grating ( 6 ). 15. Optical isolator according to claim 13, characterized in that the grating is a transmission grating ( 4 ). 16. Optical isolator according to at least one of the preceding claims 13 to 15, characterized in that the grating ( 3 ) is a linear or an effective in several directions, flat grating ( 4 , 6 , 15 ). 17. Optical isolator according to at least one of the preceding claims 13 to 16, characterized in that the grating ( 3 ) is a surface relief. 18. Optical isolator according to claim 17, characterized in that the grating ( 3 ) is a deeply modulated phase grating. 19. Optical isolator according to at least one of the preceding An Proverbs 13 to 18, characterized by a diffraction of the simplicity laser beams (Se) only in a 0 diffraction order and a +1. Diffraction order, with the formation of a diffraction angle, by that after an object-side reflection of the in itself on the Grid returning beam of +1. Diffraction order in the 0th Beu regulations, the +1. Diffraction order and the -1. Diffraction order zer can be laid. 20. Optical isolator according to at least one of the preceding claims 13 to 19, characterized by at least two optical elements, at least one of which is a diffractive grating ( 4 , 6 ), the elements being arranged in planes which are at an angle ( ω) to each other, such that the original angle of incidence or diffraction (α _e , β _e ) can be reproduced for each diffraction on an optical element arranged in the same plane. 21. An optical isolator according to claim 20, characterized by a prism ( 7 ), the side surfaces ( 8 , 9 ) of which at an acute angle (ω) are at least one flat reflection grating ( 6 ) and a mirror ( 10 ). 22. An optical isolator according to claim 20, characterized by a prism ( 7 ), the side surfaces ( 8 , 9 ) of which at an acute angle (ω) are at least one flat reflection grating ( 6 ). 23. Optical isolator according to claim 22, characterized in that at least one side of the prism ( 7 ) additionally has a mirror ( 10 ). 24. Optical isolator according to at least one of the preceding claims 13 to 20, characterized by a prism ( 7 ) with a transmission grating ( 4 ) and a reflection grating ( 6 ) on a beam entry side, a reflection grating ( 6 ) a further side ( 9 ) and a transmission grating ( 4 ) on a beam exit side of the prism ( 7 ). 25. Optical isolator according to claim 24, characterized in that at least one of the transmission gratings ( 4 ) is an imaging grating. 26. Optical isolator according to claim 25, characterized in that the transmission grating ( 4 ) is a holographic-optical element. 27. Optical isolator according to at least one of the preceding An sayings 13 to 20, characterized in that depending on a ratio of wavelength (λ) to grating constant (g) and a Mo dulation depth (h) to the lattice constant (g) for a given profile shape the surface relief of the grating this for polarized light  the same diffraction effectiveness perpendicular and parallel to the grating having. 28. Optical isolator according to at least one of the preceding claims 13 to 20, characterized in that the grating ( 3 ) on a surface of a prism ( 7 ) or a parallelepiped ( 14 ) is arranged or integrated in a light-guiding solid. 29. Optical isolator according to at least one of the preceding An Proverbs 13 to 20, characterized in that the grid is flat in Space is arranged. 30. Optical isolator according to at least one of the preceding claims 13 to 20, characterized by a plurality of planar transmission gratings ( 14 ) are arranged in the beam direction in succession at the same inclination angles to one another. 31. Optical isolator according to claim 30, characterized in that the angle of inclination is 45 °. 32. Optical isolator according to claim 31, characterized in that four alternating prisms ( 7 ) and four transmission gratings ( 4 ) are arranged in succession with pairwise arrangement of the prisms ( 7 ) along their longest sides and the angle of incidence of the laser beam (Se) , which is equal to a base angle of the prisms, is reproduced to the same size on each transmission grating ( 4 ). 33. Optical isolator according to at least one of the preceding claims 13 to 20, characterized by an intermediate layer of a transmission grating ( 4 ) between two prisms ( 7 ) formed double prism, the opposite, mutually inclined outer surfaces ( 8 ) each have mirrors ( 10 ) or reflection grating ( 6 ). 34. Optical isolator according to at least one of the preceding claims 13 to 20, characterized in that the grating ( 3 , 15 ) has a plurality of diffraction directions. 35. Optical isolator according to claim 34, characterized by a cross grating ( 15 ) by the depending on the wavelength and the angle of incidence of the laser beam source coming from the incident laser beam and the grating constants of the cross grating in the X and Y directions of the laser beam only in the 0th and +1. Diffraction order and formation of a diffraction angle is disassembled by the beam of +1 returning in itself perpendicular to the cross grating after an object-side reflection. Diffraction order in the X and Y directions in the 0th diffraction order, the +1. Diffraction order and the -1. Diffraction order can be dismantled. 36. Optical isolator according to at least one of the preceding claims 13 to 35, characterized in that the grating is arranged in a vertical arrangement in connection with a layer waveguide and a grating structure is provided perpendicular to the direction of propagation of modes guided in the layer waveguide ( 16 ).