OPTICAL DEVICE, PHOTONIC DETECTOR, AND METHOD OF MANUFACTURING AN OPTICAL DEVICE

Abstract

An optical device for an optical sensor comprises a gain element of a semiconductor laser, a wavelength selective feedback element, and a sensing element. At least part of the wavelength selective feedback element and the sensing element are arranged in a common sensor package. The gain element is arranged to generate and amplify an optical signal. The gain element and the wavelength selective feedback element form at least part of an external cavity of the semiconductor laser, thereby providing a feedback mechanism to sustain a laser oscillation depending on the optical signal. The wavelength selective feedback element is arranged to couple out a fraction of the optical signal and direct said fraction of the optical signal towards the sensing element to probe a physical property of the sensing element.

Claims

1. An optical device for an optical sensor, comprising a gain element of a semiconductor laser, a wavelength selective feedback element, and a sensing element, wherein: at least part of the wavelength selective feedback element and the sensing element are arranged in a common sensor package, the gain element is arranged to generate and amplify an optical signal, the gain element and the wavelength selective feedback element form at least part of an external cavity of the semiconductor laser, thereby providing a feedback mechanism to sustain a laser oscillation depending on the optical signal, and the wavelength selective feedback element is arranged to couple out a fraction of the optical signal and direct said fraction of the optical signal towards the sensing element to probe a physical property of the sensing element.

2. The optical device according to claim 1, wherein the gain element and the external cavity are arranged in the common sensor package.

3. The optical device according to claim 1, wherein the wavelength selective feedback element comprises at least one diffractive input grating with a grating period Λ, wherein the external cavity is arranged such that the optical signal has a wavelength adapted to the grating period Λ.

4. The optical device according to claim 1, wherein the gain element comprises an active gain region of an edge-emitting semiconductor laser or an active gain region of a surface-emitting semiconductor laser.

5. The optical device according to claim 3, comprising: at least one further output grating, and a substrate body, wherein the at least one input grating and at least one further output grating are arranged in or on the substrate body and contiguous with a main surface of the substrate body.

6. The optical device according to claim 5, wherein the main surface of the substrate body defines an optical axis running along a longitudinal direction of the substrate body parallel to the main surface, and the at least one output grating is located downstream the at least one input grating with an input side of the input grating facing the gain element and an output side associated with the output grating.

7. The optical device according to claim 6, wherein the semiconductor laser comprises a laser cavity having a laser axis running along a longitudinal direction of the laser cavity, the laser axis is coaxial with respect to the optical axis of the substrate body, or the laser axis is tilted with respect to the optical axis of the substrate body.

8. The optical device according to claim 3, wherein the external cavity comprises a back mirror of the semiconductor laser and the at least one input grating as front mirror.

9. The optical device according to claim 1, wherein the external cavity comprises the back mirror of the semiconductor laser and a semitransparent mirror as front mirror, the input grating is arranged downstream both the back mirror and the semitransparent mirror.

10. The optical device according to claim 6, wherein the input grating comprises at least a first section having a first grating period and a second section having a second grating period, the first section comprises the input side, the second section is located downstream the first section along the optical axis, and the first section is arranged for reflection of the optical signal back into the semiconductor laser, and the second section is arranged to direct the optical signal towards the sensing element.

11. A photonic detector, comprising: at least one optical device according to claim 1, and further comprising: an integrated sensor chip comprising at least one of: an optical sensor, an optical front end, an electrical front end and/or a processing unit; wherein: the at least one optical device and the integrated sensor chip are arranged in the common sensor package.

12. The photonic detector according to claim 11, comprising a reference path and a sensing path arranged for interference of a reference beam and a sensing beam.

13. The photonic detector according to claim 11, wherein the photonic detector comprises one or several of: an integrated optical interferometer, a differential free space optical interferometer, an optical acoustic sensor, an optical audio microphone, an optical audio speaker control device, an optical audio display surface microphone or speaker, a contactless 3D surface mapping and sensing device, a contactless photonic environment sensing device, and/or a pressure sensor.

14. A method of manufacturing an optical device for an optical sensor, comprising the steps of: arranging at least part of a wavelength selective feedback element and a sensing element in a common sensor package, arranging a gain element with respect to the at least one wavelength selective feedback element and a sensing element such that the gain element is operable to generate and amplify an optical signal, forming at least part of an optical cavity of the semiconductor laser using the gain element and the wavelength selective feedback element, thereby providing a feedback mechanism to sustain a laser oscillation depending on the optical signal, and arranging the wavelength selective feedback element to couple out a fraction of the optical signal and directing said fraction of the optical signal towards the sensing element to probe a physical property of the sensing element.

15. The method according to claim 14, wherein the gain element and the external cavity are arranged in the common sensor package.

16. An optical device for an optical sensor, comprising a gain element of a semiconductor laser, a wavelength selective feedback element, and a sensing element, wherein: at least part of the wavelength selective feedback element and the sensing element are arranged in a common sensor package, the sensing element being operable to transduce a physical measure, such as pressure or temperature, into an optical path change, which path change is sensed by interference via a reference path, the gain element is arranged to generate and amplify an optical signal, the gain element and the wavelength selective feedback element form at least part of an external cavity of the semiconductor laser, thereby providing a feedback mechanism to sustain a laser oscillation depending on the optical signal, and the wavelength selective feedback element is arranged to couple out a fraction of the optical signal and direct said fraction of the optical signal towards the sensing element to probe a physical property of the sensing element.

17. A method of manufacturing an optical device for an optical sensor, comprising the steps of: arranging at least part of a wavelength selective feedback element and a sensing element in a common sensor package, wherein the sensing element is operable to transduce a physical measure, such as pressure or temperature, into an optical path change, wherein the path change is sensible by interference via a reference path, arranging a gain element with respect to the at least one wavelength selective feedback element and the sensing element such that the gain element is operable to generate and amplify an optical signal, forming at least part of an optical cavity of the semiconductor laser using the gain element and the wavelength selective feedback element, thereby providing a feedback mechanism to sustain a laser oscillation depending on the optical signal, and arranging the wavelength selective feedback element to couple out a fraction of the optical signal and directing said fraction of the optical signal towards the sensing element to probe a physical property of the sensing element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In the following, the concept presented above is described in further detail with respect to drawings, in which examples of embodiments are presented. In the embodiments and Figures presented hereinafter, similar or identical elements may each be provided with the same reference numerals. The elements illustrated in the drawings and their size relationships among one another, however, should not be regarded as true to scale, rather individual elements, such as layers, components, and regions, may be exaggerated to enable better illustration or a better understanding.

[0045] FIG. 1 shows an example embodiment of an optical device,

[0046] FIG. 2 shows diffraction modes of an example input grating,

[0047] FIG. 3 shows another example embodiment of an optical device,

[0048] FIG. 4 shows diffraction modes of another example input grating, and

[0049] FIGS. 5A to 5C show cavity configurations from the prior art.

DETAILED DESCRIPTION

[0050] FIG. 1 shows an example optical device. The optical device comprises a gain element 11 of a semiconductor laser 1, a wavelength selective feedback element 2, and a sensing element 3. The wavelength selective feedback element 2 and the sensing element 3 are arranged in a common sensor package 4. The gain element 11 can also be arranged in the common sensor package 4 (not shown) or could be an external component outside the common sensor package 4 as depicted. The common sensor package 4 is arranged to accommodate and align the components of the optical device. For example, the common sensor package 4 comprises a frame body to which at least the wavelength selective feedback element 2 and the sensing element 3 are attached to. The frame body can be molded, for example. Another example is to build the elements 2 and 3 into a common silicon substrate, this substrate thus representing the common sensor package. Furthermore, the common sensor package 4 may comprise further components of a photonic detector, such as an optical sensor, control and data processing units, which complement the optical device to a full photonic detector, for example.

[0051] Typically, the optical device can be used in various optical sensors or photonic detectors. This way the optical device finds applications in integrated optical interferometers, differential free space optical interferometers, optical acoustic sensors, optical audio microphones, optical audio speaker control, optical audio display surface microphones and speakers, contactless 3D surface mapping and sensing, contactless photonic environment sensing, pressure sensors and external cavity lasers, for example. Any sensor composed of a gain element of a semiconductor laser, a wavelength selective feedback element, such as a Bragg grating, and a sensing element may be used together with the optical device described herein. The sensing element 3 can be considered any element which transduces a physical measure, e.g. pressure, temperature, etc. into an optical path change, which may then be sensed by interference with a reference path, for example.

[0052] The gain element 11 of a semiconductor laser 1 comprises an active gain region (or active laser medium), e.g. a semiconductor diode junction. Furthermore, the gain element 11 comprises a laser cavity 12 in which the active laser medium is arranged. The laser cavity 12 in this embodiment comprises a back mirror 13 and a semi-transparent front mirror 14. Such a configuration may be implemented by means of a semiconductor laser, such as an edge-emitting semiconductor laser, a surface-emitting semiconductor laser, e.g. a vertical-cavity surface-emitting laser, or VCSEL.

[0053] The wavelength selective feedback element 2 comprises a substrate body 21. The substrate body 21 may comprise, for example, a semiconductor, such as silicon, or another material, which is transparent for, or in the range of, a target wavelength. For example, silicon is—to a certain degree—transparent in the infrared, IR. Moreover, IR may be the target wavelength or range when using a VCSEL laser which emits in the IR. Similar considerations on substrate, target wavelengths and laser emission apply for other applications as well. Infrared semiconductor laser wavelengths for sensing applications, are in the ranges 780 to 850 nm (AlGaAs) or 900 to 980 nm (InGaAs).

[0054] A first diffraction grating 22, such as a Bragg grating, is arranged in the substrate body 21. In this embodiment the substrate body 21 comprises an optical waveguide and the Bragg grating is a diffraction grating made in the optical waveguide with a periodic variation of the refractive index, e.g. of the substrate 21, or of the waveguide. This periodic variation leads to large reflectance within its bandwidth around a center wavelength which fulfills the Bragg condition for grating reflection. If the Bragg condition is met, the wavenumber of the grating matches the difference of the wavenumbers of the incident and reflected waves. In turn, other wavelengths are only weakly or not affected by the Bragg grating. Similarly, the reflection can nearly or totally disappear when the angle of incidence is modified. These properties allow for using the Bragg grating as an optical filter, for example. Furthermore, the substrate body 21 carrying the first diffraction grating can be used as, or include, a photonic waveguide and allows, thanks to the grating, for selecting wavelengths which are coupled out of or into the waveguide, respectively.

[0055] The same substrate body 21 may have a second diffraction grating 23. The second diffraction grating 23 can be a Bragg grating as well and may be spaced apart from the first diffraction grating 22. For example, the substrate body 21 has a main surface which defines an optical axis 24. The optical axis 24 runs along a longitudinal direction of the substrate body 21. Considering the optical axis 24, the second diffraction grating 23 is located downstream the first diffraction grating 22 with an input side 25 of the first diffraction grating 22 facing the gain element 11 and an output side 26 associated with the second diffraction grating 23. The gain element 11 and the substrate body 21 having the two Bragg gratings 22, 23, are arranged coaxial with respect to the common optical axis 24.

[0056] The first diffraction grating 22, or input grating for short, is designed to have at least two diffraction orders to facilitate both sensor and laser operation, respectively. At least one order of diffraction is directed towards the gain element 11, or laser cavity 12. A second diffraction order of the grating 22 is directed firstly towards the sensing element 3 (see arrow A3). Then, after reflection upon sensing element 3, and coupling into the output waveguide through the second grating 23, this beam reaches the output 26. Typically, the output side 26 faces towards an optical sensor or is part of a photonic detector, for example. Thus, the input grating 22 has a period Λ that allows said two diffraction orders in order to transmit light towards the optical sensor or into the photonic detector, and reflect light backwards into the gain element 11 or laser cavity 12. This way the input grating 22, e.g. the Bragg grating, becomes also an external mirror for the semiconductor laser 1. In turn, the semiconductor laser 1 becomes an external cavity laser.

[0057] The second grating 23 may, functionally, not be part of the wavelength selective feedback. The second grating 23 is operable to couple the light beam incoming from the sensing element 3 into the output (waveguide). Physically, the second grating 23 can be either spaced apart, or be contiguous with the first (input) grating 22, for example.

[0058] The external cavity 15 comprises the gain element 11, or laser cavity 12, and the input grating 22. The laser cavity 12 comprises the back mirror 13, which effectively constitutes a back mirror of the external cavity 15, too. The semi-transparent front mirror 14, however, does not affect the lasing process of the external cavity 15, which rather is defined by the input grating 22 as front mirror. In fact, the front mirror 14 can be removed or replaced by an anti-reflecting coating, for example.

[0059] The sensing element 3 is arranged in the common sensor package 4, e.g. along the optical axis 24 opposite and between the input and output gratings 22, 23 (with respect to the optical axis 24). An optical path 27 is established between the input grating 22, the sensing element 3 and the output grating 23. The sensing element 3 can be any element which transduces a physical measure, e.g. pressure, temperature, etc. into a change of the optical path 27, which may then be sensed by interference with a reference path, for example. In this example, the sensing element 3 is implemented by a MEMS membrane arranged in the common sensor package 4.

[0060] The input grating 22 can be considered both the front mirror of the external cavity 15 and a sensor component to probe the sensing element. The semiconductor laser 1 constitutes an external cavity laser which is correlated in wavelength λ with its target, the optical device composed of the sensing element 3 together the two gratings 22 and 23, and, during operation of the optical device and/or optical sensor, tracks an optimal wavelength as will be discussed below.

[0061] An input signal excites the active laser medium of the gain element 11. An optical signal, or laser light, is generated and induces stimulated emission in the external cavity 15. For example, arrows A1 in FIG. 1 represent light which traverses as laser input waves towards the input grating 22. In turn, light is reflected at the input grating 22 which acts as the front mirror of the external cavity 15. In more detail, one order of diffraction of the input grating 22 is used to diffract light backwards into the external laser cavity 15. The reflected order light is indicated by arrows A2 in the drawing. After being reflected at the input grating 22, light traverses back, through the front mirror 14, until the back mirror 13. As a result a laser process and optical amplification of the optical signal is established. This way laser oscillation can be sustained. Thanks to the reflected wave, the input grating 22 becomes an external, wavelength-selective mirror for the semiconductor laser 1.

[0062] A fraction of the optical signal is not reflected back into the external cavity 15 but diffracted by means of the input grating 22 (see arrows A3). This fraction of the optical signal provides a laser output (or laser light) and is diffracted towards the sensing element 3. Another diffraction order of the input grating 22 is used to extract and direct the laser light to the sensing element 3 for sensing purposes. This diffraction order relates to the same beam denoted with arrows A3. The laser light is reflected at the sensing element and eventually deflected towards the output grating 23. Deflection is altered by the sensing element 3. For example, a movement of the sensing element, such as the MEMS membrane discussed above, changes the length of the optical path 27. Depending on the optical path length the deflected laser light strikes the output grating 23 under a characteristic angle of incidence and may be coupled into the substrate body 21 or the output waveguide via the output grating 23 according to its grating equation (see arrows A4).

[0063] FIG. 2 shows diffraction modes of an example input grating. The input grating 22, being wavelength-selective, forces the semiconductor laser 1 to lase on the wavelength reflected by the input grating. The semiconductor laser 1 is able to displace its lasing frequency thanks to the relatively large bandwidth of the gain medium of the gain element 11, a few nm usually. The sensor input side 25 from FIG. 1 is detailed in FIG. 2 showing the input grating 22, with period Λ, arranged in the substrate body 21 (eventually including a waveguide along its optical axis 24). The substrate (or waveguide) is characterized by an effective refractive index n.sub.eff. Furthermore, FIG. 2 shows the incident wave ki, and the two diffracted waves: one decoupled out of the waveguide (denoted extracted or diffracted wave kd or laser light), and one returned towards the laser cavity 12 (denoted reflected wave kr), and providing the feedback. Arrows A1 to A3 correspond to FIG. 1.

[0064] The embodiment shown in FIG. 1 can be considered a Littrow-type setup, in the extreme case of grazing incidence, i.e. sin θ.sub.i=1 (θ.sub.i is the angle between the incident beam ki and a perpendicular to the grating surface, like in FIG. 4). The laser cavity 12 has a laser axis 16 which in this setup is coaxial with the optical axis 24 of the substrate body 21.

[0065] In order for the input grating 22 to act as a reflector and couple energy from the laser input wave (see arrows A1) into the wave travelling through the same waveguide backwards via substrate body 21 (reflected wave, see arrows A2), towards the external laser cavity 15, the grating period Λ and the wavelength have to be related through the Bragg condition which in this case reads

[00001]$\lambda =\frac{2n_{\mathrm{eff}}\Lambda }{m}$

with m=±1, ±2, ±3, . . . denoting the diffraction order and λ the laser wavelength. The effective index n.sub.eff is defined through the propagation constant of the guided-mode

[00002]${\beta }_{\mathrm{eff}}=\frac{2\pi n_{\mathrm{eff}}}{\lambda },$

wherein n.sub.eff has a value between the guide core and cladding refractive indexes, for example. The equation above illustrates that the semiconductor laser 1 is forced to lase on a wavelength that fulfills the Bragg condition, with m chosen such that the laser wavelength λ falls within the (laser) gain bandwidth.

[0066] In order for the input grating 22 (implemented as Bragg grating, for example) to couple energy from the incident wave ki from the laser into the extracted wave kd, under diffraction angle θ.sub.d, as shown in FIG. 2, the grating period Λ and the wavelength λ additionally are related by the equation:

[00003]$\sin {\theta }_d=n_{\mathrm{eff}}-\frac{q\lambda }{\Lambda }$

with q=±1, ±2, ±3, . . . denoting the diffraction order. If both the Bragg condition and the latter equation hold, and if one chooses m and q such that m/q=2, then

sin θ.sub.d=0.

[0067] This relation is constant or independent of any variations of grating period Λ and effective refractive index n.sub.eff. In other words, if the input grating acts as front mirror of the external cavity 15 for the semiconductor laser 1, it forces the laser wavelength λ to track any variations of grating period Λ and effective refractive index n.sub.eff such that sin θ.sub.d=0 holds. The extracted beam, represented by extracted wave kd, stays perpendicular (θ.sub.d=0) to the input grating 22, independent from the variations of wavelength λ, grating period Λ and effective refractive index n.sub.eff.

[0068] If to the contrary, the Bragg condition would not hold, i.e. the input grating 22 would not be part of the external cavity 15 for the semiconductor laser 1, then, depending on the laser wavelength λ variations on one side, and grating period Λ and effective refractive index n.sub.eff variations on the other, the diffraction angle θ.sub.d would deviate from zero. Consequently, the extracted beam would deviate from its initial direction (perpendicular to the input grating). This is because the laser wavelength λ on one side, and the grating/waveguide parameters on the other, would vary in an unrelated way.

[0069] In other embodiments it may not be possible to implement an input grating with two or more diffractions orders, i.e. at least one for the sensing element and another for the external cavity as described above. However, the input grating 22 can be split into sections. For example, the input grating may have sections with different grating periods, respectively. One section is arranged with a first period to be used for reflection back into the semiconductor laser, and a second section with a second grating period arranged to direct laser light towards the sensing element. Typically, both sections share the same substrate 21 in order to have possible deformations under varying parameters (such as temperature or mechanical stress) correlated with each other. At least one section of the input grating is arranged in the common sensor package 4.

[0070] FIG. 3 shows another example embodiment of an optical device. The optical device comprises a gain element 11 of a semiconductor laser 1, a wavelength selective feedback element 2, and a sensing element 3. A part of the wavelength selective feedback element 2 and the sensing element 3 are arranged in a common sensor package 4. The whole input grating 22 can also be arranged in the common sensor package 4 (not shown). The gain element 11 can also be arranged in the common sensor package 4 (not shown) or can be an external component, outside the common sensor package 4, as depicted.

[0071] The various components of the optical device are similar to their respective counterparts discussed with respect to the embodiment of FIGS. 1 and 2. Thus, the following discussion will focus on the differences. If not stated otherwise properties of the components discussed above with respect to FIGS. 1 and 2 apply equally to the embodiment of FIGS. 3 and 4.

[0072] In this embodiment the gain element 11, or laser cavity 12, can be the same as in FIG. 1, i.e. comprises a back mirror 13 and a semi-transparent front mirror 14. However, the gain element 11, or laser cavity 12, is tilted with respect to the optical axis 24 of the substrate body 21. That is, the laser axis 16, common to the gain element 11 and laser cavity 12 of the semiconductor laser 1, is not coaxial but tilted with respect to the substrate axis 24 of the substrate body 21. The substrate body 21 has the two Bragg gratings 22, 23. The wavelength selective feedback element 2 comprises the input grating 22. The input grating 22 and the output grating 23 are arranged in the common substrate body 21, along its optical axis 24. The output grating 23 is located downstream the input grating 22 with the input side 25 facing the gain element 11, and the output side 26 associated with the output grating 23. The input grating 22 comprises two sections 28 and 29. A first section 28 of the input grating comprises the input side 25 and a second section 29 of the input grating is located downstream the first section along the optical axis 24.

[0073] The first section 28 is arranged to couple energy from the incident wave ki from the laser (arrows A1) partly into the guided wave propagated through the waveguide/substrate to the second section 29, and partly into the wave kr (arrows A2) reflected backwards, to the gain element 11. Hence the input grating 22 defines also the external cavity 15 of the semiconductor laser 1. The first section 28 of the input grating may (but does not have to) be spatially separate from the second section 29 and/or the output grating 23, and also may have a grating period different from the second section and/or the output grating. Nevertheless, all the gratings (the sections of the input grating and the output grating) are arranged in the common substrate body 21, such that their grating periods are in a constant and well-defined ratios. If any one of the gratings expands by a factor (due to temperature variation, for instance), the other gratings should expand by the same factor.

[0074] FIG. 4 shows the waves and their corresponding diffraction orders of the input grating 22 in the embodiment example illustrated in FIG. 3. The drawing from FIG. 4 represents the two sections (28 and 29) of the input grating 22, with their respective periods and beam angles. The first section 28 of the input grating (on the left side of the figure) can be considered the external laser cavity for the semiconductor laser 1 by returning or reflecting, through a diffraction order, the wave represented by kr (arrow A2) backwards into the laser cavity 12, i.e. towards the back mirror. The incident wave ki from the laser strikes the first section 28 under a first incidence angle θ.sub.i with respect to the normal to the grating surface. The wave kr is reflected backwards (as a diffraction order), into the laser cavity, hence under the same angle θ.sub.i with respect to the normal to the grating surface. The laser wavelength λ is hence defined by the Bragg condition on the section 28 of the grating, which reads

[00004]$\lambda =\frac{2{\Lambda }_i\sin {\theta }_i}{m}$

with m=±1, ±2, ±3, denoting the diffraction order, Λ.sub.i the grating period of the first section 28, and A the laser wavelength. Note that this equation does not depend on n.sub.eff. Light strikes the grating and is diffracted back into the laser from the “air” side of the grating, which thus acts like a classical, open-space, diffraction grating. If the light came through the optical waveguide the equation would be different and had n.sub.eff contribution.

[0075] The same section 28 of the input grating is ensuring the coupling between the input wave ki and the guided-wave mode kg of effective index n.sub.eff (see arrows A4). Therefore

[00005]$\sin {\theta }_i=n_{\mathrm{eff}}-\frac{q\lambda }{{\Lambda }_i}$

with q=±1, ±2, ±3, . . . denoting the diffraction order.

[0076] The second section 29 of the input grating (on the right side of FIG. 4) couples the guided-wave mode kg out, into the wave kd (arrow A3), towards the sensing element, at a second diffraction angle θ.sub.d with respect to the normal. Therefore

[00006]$\sin {\theta }_d=n_{\mathrm{eff}}-\frac{p\lambda }{{\Lambda }_d}$

with p=±1, ±2, ±3, . . . denoting the diffraction order. These equations express the diffraction or coupling angles θ.sub.d and θ.sub.i as functions of the other parameters, e.g. the laser wavelength λ, or grating periods Λ.sub.i, Λ.sub.d, which vary the most with the temperature. The diffraction or coupling angles θ.sub.i and θ.sub.d denote [0077] θ.sub.i: the “incident” angle formed between the normal to the surface of the first grating section 28 of the input grating 22 and the incident light wave A1 incoming from the laser. It is identical to the angle between the same perpendicular and the light wave A2 reflected (actually diffracted) by the first section 28 of the input grating backwards, towards the gain element 11. [0078] θ.sub.d: is the “diffraction” angle formed between the perpendicular to the surface of the second section 29 of the input grating 22 and the light wave A3 extracted (or diffracted) towards the sensing element 3 by this section 29.

[0079] Without the constraint of the input grating being equally the external cavity of the laser, the laser wavelength λ variation (with the temperature, for example) would be independent from the gratings expansions. Consequently, extraction angle θ.sub.d would vary with the temperature, and the light path directions inside the sensor would fluctuate, with impact on the sensor performance. However, in this embodiment the various parameters are interlinked by the Bragg condition, and the input grating is used as external cavity to the semiconductor laser. This constrains the laser wavelength λ to adapt to the Bragg condition at the input grating 28. By combining the Bragg condition and equations for (de-)coupling angles θ.sub.d and θ.sub.i, one obtains for the injection (coupling) and the extraction (decoupling) angles the following equations

[00007]$\sin {\theta }_i=\frac{n_{\mathrm{eff}}}{1+2\frac{q}{m}}$$\mathrm{and}$$\sin {\theta }_d=n_{\mathrm{eff}}\left(1-\frac{2p}{m+2q}\right)\frac{{\Lambda }_i}{{\Lambda }_d}$

[0080] The last two equations show that the injection and extraction angles are constant and independent from the laser wavelength λ, and the grating periods, as the grating period variations at injection (Λ.sub.i, grating 28) and extraction (Λ.sub.d, grating 29) are correlated (Λ.sub.i/Λ.sub.d=constant), and as the laser wavelength λ is constrained to match the Bragg condition at input grating 28, corresponding to the injection angle θ.sub.i. Hence the light path directions inside the sensor stay unchanged, independent from wavelength and grating period variations (following temperature or stress fluctuations, for instance). It remains, however, a temperature dependence by way of the effective index n.sub.eff. This temperature dependence usually has less impact on the coupling angles than temperature variations of laser wavelength in regular Fabry-Perot or VCSEL laser setups. It has been found that the proposed optical device shows improved stability under varying temperature and/or mechanical stress. Having two grating sections 28 and 29 on the input grating 22 supports a more general application, e.g. with p≠q and the incident and diffracted angles not being equal.