Abstract:
An external cavity semiconductor laser light source comprises includes a semiconductor gain device operable to provide light amplification; a wavelength selection element including a diffraction grating; and light re-directors. The gain device, light re-directors and grating are arranged so that an optical resonator is established for light portions emitted by the gain device and diffracted by the diffraction grating. The resonator is an external cavity laser resonator. The light source is capable of varying an angle of incidence of radiation circulating in the resonator onto wavelength selection element to select a resonator radiation wavelength dependent on the angle of incidence.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention is in the field of light sources, i.e. of sources of electromagnetic radiation in the infrared, visible, and ultraviolet part of the electromagnetic radiation spectrum. More concretely, the invention concerns a light source, and an optical coherence tomography apparatus as well as a method for generating a light beam of varying wavelength. 
       BACKGROUND OF THE INVENTION 
       [0002]    Tunable light sources are useful for many applications. Among them are Swept-Source Optical Coherence Tomography (SS-OCT) where the wavelength is swept back and forth fast between an upper and a lower wavelength. Other applications require DC tuning (i.e. setting the wavelength to a desired value and maintaining it for a certain time for a measurement) or sweeping at various frequencies and/or with different sweep characteristics etc. 
         [0003]    Various methods have been employed to achieve swept light sources (in this text, generally the term ‘light’ relates to electromagnetic radiation not only in the visible range, but also in the near- and mid-infrared and in the near-UV, in particular to electromagnetic radiation in the range between 300 nm and 2000 nm). Among these are light sources in which a Micro-Electrical-Mechanical System (MEMS) is used for wavelength tuning. As, for example, disclosed in WO 2010/111795 A1, a grating can be used in a Littrow configuration—the incident beam and the diffracted beam are collinear (or coaxial)—but the wavelength sweep is not caused by a movement of the grating but by moving the beam incident on the grating. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    It is an object of the invention to provide a wavelength tunable light source that is tunable over a broad range. It is a further object of the invention to provide a wavelength tunable light source in which the bandwidth of the (instantaneously) outputted light is small. It is yet another object of the invention to provide a light source in which parameters of the output light vary only little over the wavelength range. An even further object of the invention is to provide a light source that is especially compact. 
         [0005]    In accordance with an aspect of the invention, the light source, which in particular can be an external cavity semiconductor laser light source, comprises: a semiconductor gain device operable to provide light amplification; a wavelength selection element comprising a diffraction grating, in particular a transmission diffraction grating; and light re-directors; wherein the gain device, the light re-directors and the diffractive grating are mutually arranged so that an optical resonator is established for light portions emitted by the gain device and diffracted by the diffraction grating; wherein the optical resonator is an external cavity laser resonator; and wherein the light source is capable of varying an angle of incidence of radiation circulating in the resonator onto the wavelength selection element (in particular onto the diffraction grating) to select a resonator radiation wavelength dependent on the angle of incidence. 
         [0006]    The light re-directors comprise resonator end elements between which the light in the resonator travels back and forth. The resonator end elements may be mirrors (of which one mirror may be partially transparent to couple out a portion of the light), or, in an alternative embodiment, one of the end elements may be a grating operated in reflection, e.g., the before-mentioned diffraction grating. In addition to the resonator end elements, the light re-directors may optionally comprise further mirrors, prisms, lenses etc. or other elements influencing the propagation of the light. 
         [0007]    An aspect of the invention is based on the approach that the wavelength λ is selected by tilting the input beam with respect to the wavelength selection element (in particular with respect to the diffraction grating): λ=fct(input angle). And the wavelength selection element directs beams of different wavelengths to different directions, and only for a particular wavelength range the resonator condition is fulfilled, whereas light portions outside this wavelength range are not capable of circulating back and forth in the resonator. 
         [0008]    It can be provided that the diffraction grating is a transmission diffraction grating. This way, an increased resolvance may be achieved while maintaining a very small form factor of the light source, because light can be diffracted twice at the transmission diffraction grating while propagating back and forth once in the optical resonator. In this case, the optical resonator is established for light portions emitted by the gain device and transmitted through the transmission diffraction grating. Alternatively, the diffraction grating can be a reflection diffraction grating. 
         [0009]    It can also be provided that the diffractive grating is static (stationary). This is usually the case. The diffraction grating can be mechanically fixed, e.g., with respect to a substrate. In particular, it can be fixed with respect to the light re-directors and/or to the gain device. With a static diffractive grating, the grating does not move during wavelength scanning. Very rapid wavelength scanning can be achievable with a static diffraction grating and with a direction variation device provided for changing an angle of incidence onto the wavelength selection element (more particularly onto the diffraction grating). Due to typical sizes and masses of suitable diffraction gratings, it is in many cases not possible or at least rather difficult to achieve by moving the diffraction grating high scan rates otherwise (more simply) achievable using a direction variation device. 
         [0010]    In accordance with the present invention, the light source comprises a direction variation device capable of varying a direction of light incident onto the (usually stationary) wavelength selection element. The direction variation device may in particular deflect light coming from the gain device such that its angle of incidence onto the wavelength selection element (and in particular onto the diffraction grating) can be varied. In a group of embodiments, the direction variation device comprises a light deflector with a movable element that deflects the light, such as an actuated mirror, for example a MEMS mirror, or such as a vibrating optical fiber (more details about vibrating optical fibers and about a swept light source comprising a vibrating optical fiber can be found in “A wavelength swept laser with a sweep rate of  150  kHz using vibrations of optical fiber” by R. Isago et al., Proc. of SPIE Vol. 7004 700410-1 which is hereby incorporated by reference in its entirety into the present patent application). Depending on the application, the movable element may move in that it oscillates in a resonant manner or may move in a quasi-DC regime or in a “true” DC regime where the actual position is substantially set, at all times, by a control parameter such as a control voltage. Alternatively, a direction variation device comprises providing an electro-optic beam deflector. Under an electro-optic beam deflector we understand a device comprising a portion of material capable of varying a direction of propagation of light propagating in the material by applying an electrical signal to the material. Therein, in particular, solid materials are of interest, and more particularly, the portion of material can be a semiconductor structure or a non-linear optical crystal such as a KTN crystal. Usually, the portion of material provides an entrance face and an exit face for light. Electro-optic beam deflectors based on a semiconductor structure will be described in more detail further below. More details about electro-optic beam deflectors based on a non-linear optical crystal can be found, e.g., in the publications “A Mechanical-free 150-kHz Repetition Swept Light Source Incorporated a KTN Electro-optic Deflector” by Shogo Yagi et al., Proc. of SPIE Vol. 7889 78891J-1 and “400 kHz Beam Scanning Using KTa 1-x Nb x O 3  Crystals” by J. Miyazu et al., CLEO/QELS, paper CTuG5 (2010), which are both hereby incorporated by reference in their respective entirety into the present patent application. The above definition of electro-optic beam deflectors comprises also devices based on the Pockels effect (Pockels cells). Since, at least according to current knowledge, Pockels-effect based electro-optic beam deflectors appear not suitable for applications in which the dimensions of the light source have to be very small or in which particularly high scan rates are required, such deflectors may be excluded in the invention. 
         [0011]    The wavelength selection element may comprise a reflective end surface that constitutes an end element of the resonator (and is thus at the same time one of the mentioned light re-directors). In these embodiments, the wavelength selection element comprises and in particular can be a block that comprises the diffraction grating (in particular a transmission diffraction grating between two prisms) and that constitutes a resonator end element. In most embodiments, the reflective end surface constitutes a mirror. Instead of a mirror, such a reflective end surface may be an additional grating, in particular a grating in a quasi Littrow configuration. 
         [0012]    In alternative embodiments, the resonator end element is separate from the wavelength selection element, a gas gap such as air gap being arranged between the wavelength selection element and the end element. Also in such an alternative embodiment, the end element may comprise an end mirror or may be a retro-reflective grating. 
         [0013]    In any case, it can be advantageous to provide that the wavelength selection element is located close to a resonator end. It can in particular be provided that both, the gain element and an optical retarder (if provided) are located at the other side of the direction variation device than the wavelength selection element. 
         [0014]    In these embodiments, light transmitted through the grating is reflected, by one of the resonator end elements, back to the grating, in particular without passing any light retarding elements, possibly except for a prism and for a possible etalon serving as periodic filter as explained hereinafter. 
         [0015]    Advantages of such a set-up are that a small light deflector can be used (the point of incidence on the light deflector does not depend on the wavelength) as well as the possible compactness of the entire set-up. 
         [0016]    Further, despite the compactness of the double pass concept geometry, which is immanent in the concept of placing a transmission grating within a laser resonator, brings about an increased resolvance. 
         [0017]    The “resolvance” or “chromatic resolving power” of an element to separate wavelengths of light is defined as R=λ/Δλ, where λ is the wavelength and Δλ is the smallest resolvable wavelength difference. For diffractive elements, the limit of resolution may be determined by the Rayleigh criterion as applied to the maxima of separated light portions (diffraction maxima), i.e., two wavelengths are just resolved when the maximum of one lies at the first minimum of the other. 
         [0018]    In further accordance with the present invention, in case of a transmission diffraction grating, a radiation deflection device (which in principle can be embodied like the before-described direction variation device) may be arranged on the “back side” of the grating, i.e. on that side of the transmission diffraction grating opposite to that side on which the gain device is arranged. Accordingly, light transmitted through the transmission diffraction grating propagating towards the radiation deflection device is reflected back to the grating by the radiation deflection device which can be a movable resonator end element. The movable resonator end element then needs a size sufficient for reflecting light portions of the whole light source bandwidth. 
         [0019]    In further accordance with the present invention, the wavelength selection element comprises a grating-on-prism arrangement (GRISM). In this embodiment, the diffraction grating is arranged on a single prism or—more particularly, in case of a transmission diffraction grating—between two prisms. Also set-ups with more than two prisms and/or more than one grating are possible. In a GRISM, the dispersive power of the prism(s) may add to the selectivity of the diffraction grating and/or can be used such that a non-linear relation between input angle and wavelength can be flattened (smoothed). Similarly, the resolvance that in general is not uniform over the whole spectral range due to different angles of incidence onto the diffraction grating, can be flattened (smoothed) so as to show a more homogeneous resolvance over the spectral tuning range. In addition, a prism or prisms can provide mechanical protection for the diffraction grating. It is possible to provide that the grating is a surface relief transmission grating. 
         [0020]    It is also possible to provide a prism (in particular attached to the diffraction grating, e.g., in a GRISM) having a curved entrance surface. Therein, not only concave prism shapes are possible, but also convex prism shapes are possible. In case of concave prism shapes, the entrance surface can in particular describe a circular shape. In case of convex prism shapes, the entrance surface can describe a circular shape, but it can rather describe a non-circular shape. 
         [0021]    It is possible to provide that the wavelength selection element comprises at least one curved diffraction grating, wherein this curved diffraction grating can be identical with or different from the before-addressed diffraction grating anyway comprised in the wavelength selection element. 
         [0022]    In one aspect of the invention, the light source comprises, in addition to the before-addressed first semiconductor gain device, a second semiconductor gain device operable to provide light amplification, wherein the direction variation device is arranged so as to be capable of receiving light amplified in the first semiconductor gain device and light amplified in the second semiconductor gain device and of varying a direction of further propagation of received light. It is, of course, also possible to provide, in addition, a third or even further semiconductor gain devices. Such light sources can make wavelength multiplexing possible. ASE (amplified spontaneous emission) spectra of the different semiconductor gain devices are usually different, they can be substantially overlapping or substantially non-overlapping. In one embodiment, the light source can make possible to simultaneously emit and scan light of two different wavelengths, in particular of clearly spaced-apart wavelengths. The first and the second semiconductor gain devices are comprised in one or in two optical resonators of the light source. Particularly, two different embodiments with (at least) two different semiconductor gain devices are described: 
         [0023]    In a first embodiment, at least one beam splitter is provided for forming at least two separate (partial) beam paths, the first and second semiconductor gain devices being arranged in different ones of the at least two separate beam paths. In this case, both (or all) semiconductor gain devices can be arranged within one and the same optical resonator, but it is also possible to provide that two partially overlapping (or partially identical) optical resonators are formed. It is possible to use only a single direction variation device (for light produced in the first and for light produced in the second semiconductor gain device), and it can be sufficient to provide only a single wavelength selection element, e.g., a single GRISM or one of the other wavelength selection elements described in the present patent application. The separate light paths in which the different semiconductor gain devices are arranged can be considered partial beam paths, in which (at least prevailingly) light of different wavelengths propagates. These separate light paths can be considered logically parallel beam paths. By way of one or more beam splitters, the separate light paths are usually combined so as to convert them into two parallel or, more particularly, into two coaxial light paths, the corresponding parallel or even coaxial light beams impinging on the direction variation device so as to accomplish the before-described variation of direction of light incident on the wavelength selection element. This first embodiment can be particularly suitable in case of overlapping ASE spectra of these semiconductor gain devices. 
         [0024]    In a second embodiment, in addition to the before-addressed first wavelength selection element, a second wavelength selection element is provided. And the second semiconductor gain device, the before-addressed and/or further light re-directors and the second wavelength selection element are mutually arranged so that an additional, second optical resonator is established for light portions emitted by the second semiconductor gain device. And the second optical resonator is an external cavity laser resonator, and the direction variation device is capable of varying a direction of light incident on the second wavelength selection element to select a resonator radiation wavelength dependent on the angle of incidence of the light on the second wavelength selection element. In this second embodiment, two optical resonators are formed, both making use of the same direction variation device. Consequently, it is possible to produce intrinsically synchronized wavelength scanning for light beams of (clearly) different wavelengths. This can be advantageous not only in case of optical coherence tomography applications, but also elsewhere. This first embodiment not only can be particularly suitable in case of (substantially) not-overlapping ASE spectra of the semiconductor gain devices, but also can be applied in case of (substantially) not-overlapping ASE spectra. 
         [0025]    Making use of one and the same direction variation device for simultaneously generating light of different wavelengths does not suffer from problems coming along with using two different direction variation devices. Those problems can, e.g., in case of a movable element deflecting the light (such as an actuated mirror, e.g., a MEMS mirror) be due to manufacturability problems that make it hard to produce or find, e.g., two resonantly operable mirrors having the same (or at least a sufficiently equal) resonance frequency. 
         [0026]    In one embodiment, the light source comprises a periodic filter (such as a Fabry Perot etalon) arranged within the laser resonator. A periodic filter helps reducing the number of modes admitted in the external laser cavity and thereby to further enhance the resolvance. During a wavelength scan, for example, operation may jump from sharply defined peak to sharply defined peak. 
         [0027]    Such a periodic filter may be constituted by a Fabry-Perot etalon or also by any other suitable devices such as an optical ring resonator etc. It may, in accordance with a first option, be placed on the gain element side of the direction variation device. In accordance with a second option, a Fabry-Perot etalon may also be a part of the wavelength selection element or may be placed between the direction variation device and the wavelength selection element or ‘behind’ the wavelength selection element, i.e. between the wavelength selection element and the resonator end. 
         [0028]    In further accordance with the present invention, the light source further comprises an optical retarder, also shortly referred to as retarder. It is also possible to provide more than one retarder. The retarder may be arranged in the optical resonator on a same side of the wavelength selection element as the side on which the gain device is arranged. It may comprise a block of (usually solid) material, a beam path with a well-defined beam path length being defined for light propagating within the retarder produced by the gain device. The optical beam path length in the retarding device may for example constitute at least 40% of the optical path length of the resonator (an optical beam path length being calculated as the physical length times the index of refraction). Generally, the retarding device comprises a plurality of reflective surfaces for reflecting back and forth light portions propagating in the block of material. In addition or as an alternative, the retarding device may be at least partially defined by a waveguide. 
         [0029]    Such a retarder may be part of a multi-planar resonator design, wherein light guided in the resonator defines a first plane and a second plane different from the first plane, and wherein the deflection arrangement deflects light circulating in the resonator from the first plane to the second plane. The light circulating in the resonator may then, e.g., propagate in the retarder in the first plane and in the wavelength selection element in the second plane. It is furthermore possible to provide that light propagating in the resonator, in addition, propagates in a third plane. The first and second planes can be oriented with respect to each other a generally in any way, wherein a parallel orientation and, in some cases, a perpendicular orientation, can be particularly suitable. The same applies to a third and possibly existing even further planes, relative to the first and/or to the second plane. 
         [0030]    A retarder of a (usually) solid material with a refractive index n&gt;1 contributes less to beam divergence than if a (usually gaseous) material of a smaller refractive index would be used, and, in addition, less space is required for realizing the same optical path length. All this can contribute to enabling to manufacture particularly small light sources. These effects not only apply to retarders possibly present in the light source but also to prisms present there. 
         [0031]    The gain device can be, e.g., a semiconductor optical amplifier (SOA). It can in particular be arranged approximately in the middle of the optical path defined by the resonator (a single trip from one end to the other end), e.g., at a position at 50%±25% of this optical path length, more particularly at a position at 50%±15% of this optical path length. 
         [0032]    The light source may be used for any application in which wavelength tunable light sources are desired. In accordance with an example, the light source may be used as light source for swept-source Optical Coherence Tomography (SS-OCT). An OCT apparatus may comprise, in addition to the light source, an interferometer (or a portion thereof) and one or more detectors as well as further elements such as a k-clock, an absolute wavelength trigger, a scanning mechanism. 
         [0033]    The invention also concerns an OCT module which, in addition to the light source, includes a portion of an interferometer in optical communication with the light source and operable to combine a portion of light produced by the light source and returned from a sample with a portion of light produced by the light source and returned from a reference path, and a detector unit positioned to receive so-combined light from the interferometer. 
         [0034]    In particular, it can in addition include an optics unit, the optics unit suitable of focussing a light portion originating from the light source onto a focus point on a sample, and of performing a scan, in which the focus point and the sample are moved relative to one another. 
         [0035]    The method for generating a light beam of varying wavelength, comprises the steps of providing a wavelength selection element comprising a diffraction grating, in particular a transmission diffraction grating; amplifying light in a semiconductor gain device; establishing an external cavity laser resonator for light portions emitted by the gain device and diffracted by the diffraction grating; varying a direction of light incident on the wavelength selection element to select a resonator radiation wavelength dependent on the angle of incidence of the light on the wavelength selection element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    In the following, embodiments and aspects of the invention are described referring to drawings. The drawings are all schematic and not to scale, and same reference numerals refer to same or analog elements. The drawings show: 
           [0037]      FIG. 1  is an embodiment of an external cavity laser; 
           [0038]      FIGS. 2-4  are embodiments of wavelength selection elements that also constitute cavity ends; 
           [0039]      FIG. 5  is an embodiment of a wavelength selection element with two transmission gratings; 
           [0040]      FIG. 6  is an embodiment of a wavelength selection element with an additional grating operated under quasi Littrow conditions; 
           [0041]      FIG. 7  is an embodiment of a wavelength selection element comprising a separate resonator end constituted by a reflective grating; 
           [0042]      FIGS. 8 and 9  are embodiments of wavelength selection elements with Fabry-Perot periodic filters; 
           [0043]      FIG. 10  is an alternative embodiment of a periodic filter; 
           [0044]      FIGS. 11   a  and  11   b  are a top view and a partial elevation view, respectively, of another embodiment of a laser; 
           [0045]      FIG. 12  is an illustration of a light source system comprising an external cavity laser; 
           [0046]      FIG. 13  is an illustration of a wavelength selection element embodied as a GRISM comprising a prism with a concave face; 
           [0047]      FIG. 14  is an illustration of a wavelength selection element embodied as a GRISM comprising a prism with a convex face; 
           [0048]      FIG. 15  is an illustration of a wavelength selection element embodied as a GRISM plus a separate grating; 
           [0049]      FIG. 16  is an illustration of a wavelength selection element embodied as a GRISM plus a separate grating; 
           [0050]      FIG. 17  is an illustration of a wavelength selection element embodied as a GRISM with two transmission diffraction gratings and three prisms; 
           [0051]      FIG. 18  is an illustration of a wavelength selection element embodied as a GRISM with two reflection diffraction gratings; 
           [0052]      FIG. 19  is an illustration of a light source with a wavelength selection element embodied as a GRISM with one reflection diffraction grating; 
           [0053]      FIG. 20  is an illustration of a light source with a wavelength selection element embodied as a GRISM with a curved diffraction grating; 
           [0054]      FIG. 21  is an illustration of a light source with two gain devices allowing for wavelength multiplexing; 
           [0055]      FIG. 22  is an illustration of a light source with two gain devices allowing for wavelength multiplexing; 
           [0056]      FIG. 23  is an illustration of an electro-optic deflector usable as a direction variation device; 
           [0057]      FIG. 24  is a schematic perspective view of an electro-optic beam deflector; 
           [0058]      FIG. 25  is an elevation view of a semiconductor-based electro-optic beam deflector with one quantum well; 
           [0059]      FIG. 26  is an elevation view of a semiconductor-based electro-optic beam deflector with two quantum wells; 
           [0060]      FIG. 27  is a front or back (facet) view of a semiconductor-based electro-optic beam deflector with a ridge waveguide structure; and 
           [0061]      FIG. 28  is a front or back (facet) view of a semiconductor-based electro-optic beam deflector with a buried waveguide structure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0062]    A laser light source  1  of a first embodiment of the invention is shown in  FIG. 1 . The laser comprises a Semiconductor Optical Amplifier  11  (SOA) as a gain element (or gain device). The SOA is pumped by injecting an electric current onto a p-n junction. Electrically pumped SOAs (and R-SOAs) are known in the art and not described in any more detail here. 
         [0063]    An optical retarder  13  serves for increasing the optical laser cavity length on a limited space. A collimating lens  12 —which may be a Gradient Index lens (GRIN) or another suitable lens (e.g., an aspheric lens or an achromatic lens) or mirror (e.g., an off-axis parabolic mirror)—serves for collimating, on the gain element  11 , the light circulating in the cavity. The retarder  13  comprises a block of transparent material, such as silica, glass or a polymer, with reflective surfaces. The reflective surfaces may be totally reflecting due to total internal reflection and/or due to reflective coatings applied. In addition to the reflective surfaces, the optical retarder  13  also comprises two faces that serve as incoupling and outcoupling surfaces and that are optimized for high transmission. A more detailed teaching concerning such optical retarders and their advantages can be found in WO 2010/111795 A1 incorporated herein by reference in its entirety. 
         [0064]    The laser cavity of the laser  1  is delimited by an outcoupling mirror  15  that transmits a defined portion out of the cavity and by a wavelength tuning arrangement. The wavelength tuning arrangement comprises, as a scanning element, a movable mirror  21  (or scanning mirror) and a wavelength selection element  20  (being a dispersive element in a very broad sense of the word), in particular being in the embodiment of  FIG. 1  a grating and prism arrangement  20 . The grating and prism arrangement  20  (also referred to as GRISM  20 ) comprises two prisms  23 ,  24  with a transmission diffraction grating  25  between them. The transmission diffraction grating  25  may be, e.g., a volume phase holography grating (VPH grating; also called volume phase grating VPG) or any other type of transmission grating, e.g., a surface relief grating with a periodic or possibly with a chirped structure. It is to be noted that it is also possible that the diffractive grating is embodied as a reflective grating, in particular in combination with a prism (reflective GRISM), see also below. 
         [0065]    Light propagating from the gain element side (which in one embodiment also is the retarder side: often it is advantageous if the retarder  13  and the gain element  11  are on a same side of the scanning element  21  (movable mirror), but it is also possible to have them arranged on different sides thereof) towards the wavelength selection element  20  is deflected by the movable mirror  21  and enters the first prism  23 . From there it propagates to the diffraction grating  25 . A first order (or possibly also higher order) diffracted beam portion in the second prism  24  propagates to a reflective end face  26  that at the same time serves as the second (in addition to the outcoupling mirror  15 ) end of the laser cavity. The beam impinges on the reflective end face  26  at a right angle thereto, so that it propagates back on the same path through diffraction grating  25  and first prism  23  back to movable mirror  21 . 
         [0066]    Instead of arranging SOA  11  and collimation optics  12  on one side of retarder  13  and scanning element  21  on the other (all such positions and orders and arrangements, of course relating to the light path), it would also be possible to arrange them between retarder  13  and scanning element  21 . 
         [0067]    Furthermore, beam shaping may not be accomplished solely by collimation optics  12 , but, in addition, further elements or surfaces may contribute thereto, as described in this patent application, e.g., entrance surfaces of prisms or further diffraction gratings. 
         [0068]    When the movable mirror scans (in the illustrated embodiment schematically depicted by a rotational movement around an axis perpendicular to the drawing plane), then the point of incidence and the angle of incidence on the transmission grating  25  change, and consequently, the wavelength for which the condition that the light is incident on the reflective end face  26  at a right angle is fulfilled changes. As a consequence, the resonator wavelength (or resonator wavelength range) can be scanned by movement of the mirror  21 . Refraction at the end faces of the prisms of the grating and prism arrangement  20 , i.e. at those faces of the prisms through which light enters or exits the grating and prism arrangement  20 , may add to the diffractive power of the grating and prism arrangement  20 . In the embodiment of  FIG. 1 , this applies to prism  23  only (namely to that surface of prism  23  which is drawn at the right hand side in  FIG. 1 ), and would apply to prism  24  if reflective end face  26  were not present, but replaced by a mirror not (or at least not fully) attached to prism  24 . Such refractive prism end faces may contribute to “flattening” (smoothening or making more constant) the total dispersion of the wavelength filtering accomplished in the light source  1  in order to provide a more constant resolvance for all wavelengths. Typically, for relatively longer wavelengths the resolvance of the grating is higher than for relatively smaller wavelength (which, generally, could also be vice versa), hence tilted surfaces (such as the prism end faces) can be arranged in such a way that an elliptical beam shape on the grating  25  is achieved from an initially round beam. The thus achieved utilization of a larger number of grating lines can be adjusted for “flattening” resolvance. 
         [0069]    An advantage of a configuration in which a grating (grating  25 ) is passed twice is that the resolving power (resolvance) for a given number of illuminated grating lines is increased. In addition, because of the effect of the prisms and the possible geometry with less flat angles of incidence onto them (cf. the paragraph above), the resolvance of the dispersive element  20  is flatter (more constant) over the spectral range than for a standard reflective grating in a Littrow configuration that is typically oriented so as to be operated with high incidence angles of about 70° and more. The thus achievable flatter (more constant) resolvance characteristics improves the spectral performance relative to the case of a similar reflective grating (in particular with no prism attached). 
         [0070]    A further advantage is that despite being a double pass configuration, the dispersive element is still compact, especially if, as depicted, the end face of the second prism  24  is reflecting, or if a reflecting end of the resonator is placed in close proximity to the GRISM arrangement. A reflective end face  26  may be formed by a mirror (provided by coating or by attaching, e.g., gluing, a pre-fabricated mirror), or may be an integrated or an attached reflection grating. 
         [0071]      FIG. 2  shows in somewhat more detail a GRISM dispersive element of the kind shown in the laser of  FIG. 1 . Also depicted are beam paths of beams of three different wavelengths, a minimal wavelength λ min , a central wavelength λ c  and a maximum wavelength λ max . The indices of refraction n 1 , n 2  of the first and second prisms  23 ,  24  may be equal or different. It is also possible that n 1 =1 and/or that n 2 =1, in which case one or both of the prisms may be dispensed with and replaced by air or a protective gas. In many situations, n 1 &gt;1 and n 2 &gt;1 is preferred, and for practical reasons (but not necessarily) often n 1 =n 2  is chosen. The angle of incidence β on the first prism  23  could be chosen to be different from 90° (for all angles occurring during scanning, and thus for all wavelength from λ min  to λ max ), so as to prevent a further resonator mode by light portions reflected by the prism  23 . In  FIG. 2 , such further resonator modes are not present for a portion of angles under scanning operation; only near λ max , this is not the case. Dashed line in  FIG. 2  shows the normal on the interface between the material constituting the transmission grating  25  and the first prism as well as the normal on the interface between the material constituting the transmission grating  25  and the second prism. The angles γ and δ between the incident beam and the (typically first order) diffracted beam, respectively, and the respective normal may be equal (symmetric configuration with respect to the grating) or different (asymmetric configuration). Note that angles at diffraction gratings reference to the surface normal of the grating, whereas the angles at refractive surfaces are, in the figures, drawn to reference the surface, but when using these angles in calculations, e.g., in the Snellius equation, the angle referencing to the surface normal has to be taken, as is clear to any person skilled in the art. 
         [0072]    Further, the index of refraction n 1  and the geometry of the set-up have to be chosen so that at the maximum angle γ (at the maximum wavelength λ max  in the depicted configuration) is below the critical angle for total internal reflection (TIR). TIR can occur if the entry prism  23  has a larger refractive index n 1  than the grating structure (or grating layer), or if a bonding material such as optical cement present between prism  23  and grating  25  has a lower refractive index than prism  23 . 
         [0073]    Another variant of the GRISM dispersive element  20  is shown in  FIG. 3 . The entry surface (i.e. the surface of the first prism  23  on which the beam deflected by the movable element is incident) is tilted with respect to the orientation it has in the configuration of  FIG. 2 . Such a tilt like present in the shown orientation, leading to angles of incidence β&gt;90° (with β defined as shown in the figures) for all incident beams occurring during scanning, prevents or at least reduces residual reflections inside the laser cavity and may also help to flatten (smoothen) the resolvance characteristic of the dispersive element  20  (i.e. to make more constant the wavelength-dependence of the resolvance). The angle of incidence on the first prism  23  of smaller wavelength light for which the resonator condition is fulfilled is more obtuse than the angle of incidence on the prism  23  of larger wavelength light for which the resonator condition is fulfilled. As a secondary effect, the dispersion of prism  23  adds to the overall dispersion. The outer surface of the second prism  24  is approximately parallel to the diffracted beam paths (and approximately perpendicular to the reflective coating  26 ), or arranged at a different angle ensuring that an angle a of incidence of the zeroth order diffracted beam is not perpendicular to this outer surface, again to avoid a further undesired laser mode. 
         [0074]      FIG. 4  shows an even further variant. The entry surface is tilted to achieve β&lt;90° (for all angles of incidence occurring during scanning), and incidence on the grating is relatively flat (e.g., γ&gt;50°). Again, this may suppress residual reflections inside the laser cavity. The configuration of  FIG. 4  enhances the maximum resolvance for λ max , because of the occurring deformation of the beam shape caused by prism  23 . E.g., in case of a (cross-sectionally) round beam impinging on the entry surface of the prism  23 , the beam continues inside prism  23  having a slightly elliptical shape. The smaller β (for higher λ), the more pronounced this effect. 
         [0075]    An even further enhanced resolvance (at the cost of a decreased bandwidth and/or an increased assembly size) is achieved by a multi-pass arrangement with more than two passes through one or more gratings as shown in  FIG. 5 . Shown is a bulk assembly (for example cemented) in which three prisms  23 ,  24 ,  27  are attached to each other, with transmission gratings  25 ,  28  between pairs of prisms  23 ,  24  and  24 ,  27 , respectively. The last prism  28  is again provided with a reflective surface  26 . 
         [0076]    Similar effects as the one described for the configuration of  FIG. 5  could also be achieved by placing two or more transmission diffraction gratings in a row, with a reflector at the end, or by placing a transmission GRISM and a reflective arrangement (for example a reflective GRISM as illustrated in one of  FIGS. 2-4 ) in a row. 
         [0077]      FIG. 6  shows a variant in which the second grating  28  serves as a reflective grating (which may, e.g., be a holographic or a surface relief grating) instead of a transmission grating and replaces the end reflector  26  of the previous embodiments, grating  28  being arranged in a quasi Littrow configuration. 
         [0078]    Configurations with more than one grating relax the requirements for the grating line density for a given desired dispersion or increase the resolvance by increasing the number of illuminated grating lines, depending on the needs. 
         [0079]      FIG. 13  shows a wavelength selection element (more particularly a GRISM) comprising a prism  23  with a concave face, more particularly comprising a light entrance face having a concave shape. The entry surface here describes a portion of a cylinder (inner) surface. The shape can be adjusted to the direction variation device, more particularly to the relative position and orientation of the GRISM and the concave prism face. For example, in case of a MEMS mirror as the direction variation device  21  and with a circular shape of radius R of the prism face, the tilting axis of the MEMS mirror can be placed in a distance of R with respect to the circular prism face (the axis of rotation of the MEMS mirror perpendicular to the plane in which the circle of radius R is described). This way, no use of a possible refraction at the concave prism face is made for either increasing and/or flattening the overall resolvance, or for reducing a required maximum deflection angle of the direction variation device  21 . It can be advisable to provide that the beam impinging on the concave prism face is (slightly) convergent (in the plane in which the prism surface is curved), because that way, it can be provided that the beam is collimated (in the plane in which the prism surface is curved) when impinging on the grating  25 . Of course, it is also possible to place the tilting axis of the MEMS mirror (or a corresponding axis or point of a different direction variation device) elsewhere than mentioned above, see, e.g., the case depicted in  FIG. 13 . 
         [0080]      FIG. 14  shows a wavelength selection element (more particularly a GRISM) comprising a prism with a convex face, more particularly a light entrance face describing a convex shape. The entry surface here describes a portion of a cylinder (outer) surface. The shape can be adjusted to the direction variation device, more particularly to the relative position and orientation of the GRISM and the convex prism face. Such a design may require accomplishing a larger (angular) deflection of the incident light and thus a larger scanning amplitude of the direction variation device. But, on the other hand, it is possible to focus the beam onto the direction variation device, in particular onto a corresponding MEMS mirror which again may make possible to use a smaller MEMS mirror which, again, is better suitable for larger scanning amplitudes and high scan rates. The concave surface may describe a circular shape, but the provision of a non-circular (aspheric) shape allows to optimize the local curvature for the one wavelength present at that location allowing the local curvature to function as a wavelength-optimized lens, e.g., for additional collimation. And in addition, the effect of aberrations can be minimized. 
         [0081]    Of course, curved prism faces such as the concave and convex ones described for the embodiments of  FIGS. 13 and 14 , respectively, can also find application in other embodiments described in the present patent application. 
         [0082]      FIG. 15  shows a wavelength selection element, more particularly a GRISM plus a separate (reflective) grating  87 . The resolvance is an important magnitude in the light source, since it considerably influences the coherence length of the generated light. When using a wavelength selection element as shown in  FIG. 15 , an improved (increased) resolvance can be achieved, because an increased number of lines of grating  25  and an increased number of lines of grating  87  can be illuminated. This is due to the grazing incidence on grating  87  (typically above 45° to the grating normal combined with a diffraction angle relatively close to the surface normal). Using an initially round beam, the beam will describe on grating  87  an elliptic shape, thus illuminating more grating lines on grating  87  and especially on grating  25 , an increased number of lines will be illuminated with respect to a configuration as shown, e.g., in  FIG. 2 . 
         [0083]    Similar effects as achievable with the embodiment shown in  FIG. 15 , can also be achieved with the embodiment of  FIG. 16 . Therein, a transmission diffraction grating  88  is used instead of the reflection diffraction grating  87  in  FIG. 15 . 
         [0084]    Again similar effects can be achieved with an embodiment shown in  FIG. 17 . Therein, transmission diffraction grating  88  is attached to prism  23  which is attached to transmission diffraction grating  25 . And, an additional prism  29  is (optionally) attached on the other side of grating  88 . Accordingly, here, the wavelength selection element comprises or rather is a GRISM comprising two gratings and three prisms. 
         [0085]    Also with two reflective gratings, the above-described effects can be achieved, as illustrated in  FIG. 18 . Therein, a direction variation device  21  is sketched, too (in dotted lines). In this embodiment, light traverses the prism of the GRISM on its way between gain device and direction variation device  21 , but traversing the prism on that path, the light beam is not diffracted at a grating. 
         [0086]    In  FIG. 19 , a rather simple embodiment is illustrated in which a GRISM is used as a wavelength selection device which comprises one prism  23  and one reflection diffraction grating  25 . The latter is in Littrow configuration. Also here, the above-described effect of an increased resolvance due to grazing incidence on the grating combined with an optically denser medium, e.g., glass, attached to the grating is achieved. Usually, in such an embodiment, a collimated beam (e.g., produced by way of generally optional collimation optics  12 ) enters the prism  23 . 
         [0087]      FIG. 20  illustrates an embodiment, in which a curved grating  89  is used. In this case, the light beam between direction variation device  21  and curved grating  89  usually is a divergent beam, and between curved grating  89  and the grating  23  under quasi Littrow condition, the beam is a collimated beam (as achieved by way of the curvature of grating  89 ). Of course, it is possible to provide that the two gratings are both attached to a prism, more particularly to one and the same prism (not illustrated in  FIG. 20 ). 
         [0088]    In  FIG. 7 , the second grating  28  of  FIG. 6  that is part of the GRISM and integrated in or attached to (typically cemented to) the second prism  24  is replaced by a separate, classical reflective grating  41 . Instead of a reflection grating  41 , an additional wavelength selecting element, e.g., a prism, plus a reflector could be added for increasing transmission efficiency. Even though this would reduce the advantage of increased resolvance for better spectral bandwidth (and the GRISM would be a transmission and not a reflection GRISM anymore), such an alternative configuration may be advantageous in special circumstances. As a further alternative, one could omit prism  24  and/or prism  23 . 
         [0089]    As a further variation, both indices of refraction n 1 , n 2  could be chosen to be (approximately) equal to 1, so that the dispersive element is not a GRISM anymore but just a stand-alone transmissive grating. 
         [0090]    In accordance with a group of embodiments, the dispersive wavelength tuning arrangement is combined with or comprises a periodic filter. This results in a periodically (in k-space, not in λ-space) increased coherence length by reducing the number of modes admitted in the external cavity laser due to a combination of well chosen spectral characteristics of the dispersive element (GRISM or other) and the periodic filter. 
         [0091]    In the embodiments of  FIGS. 8 and 9 , this periodic filter is a Fabry-Perot etalon that is combined with the wavelength tuning arrangement. 
         [0092]    In  FIG. 8 , the periodic filter is a Fabry-Perot etalon  51  integral with the GRISM. The end reflector  26  of the previously described embodiments is replaced by separate reflecting surface  55  forming a resonator end. The Fabry-Perot etalon  51  has a partially transmissive mirror  52  at the interface to the second prism  24  and a further partially transmissive mirror  53 . The transparent optical medium  54  between the two mirrors  52 ,  53  of the Fabry-Perot filter may have an index of refraction n 3  that can be equal to one or both of the indices of refraction n 1 , n 2  of the prisms  23 ,  24  or different thereto. In a special case, the optical medium may be a gas, e.g., air. The etalon  51  is tilted with respect to the incident light, more particularly to light incident from grating  25  and/or to light incident from reflective surface  55 . 
         [0093]      FIG. 9  shows a variation of the principle illustrated in  FIG. 8 . Again, the etalon  51  is tilted with respect to the incident light, more particularly to light incident from grating  25  and/or to light incident from reflective surface  55 . Between the etalon  51  and the separate reflecting surface  55 , there is a further block of (solid) transparent material  56  with an index of refraction n 4  which may be smaller than, larger than or equal to the index of refraction of the transparent optical medium  54  of the tilted Fabry-Perot etalon  51 . In accordance with an alternative option, the space between the etalon  51  and the separate reflecting surface  55  may be filled by a gas, e.g., by air. 
         [0094]    In embodiments comprising a periodic filter, the periodic filter does not need to be a Fabry-Perot etalon and/or does not need to be a part of the dispersive arrangement, i.e. it may also be on the gain element side of the movable element  21  in the laser resonator. 
         [0095]    A schematic representation of an alternative periodic filter  60  is shown in  FIG. 10 . Light circulating in the cavity is guided in a first optical waveguide  61  that is coupled to an optical ring resonator  62 . The amount of light that is coupled out of the first waveguide  61  and into the ring resonator  62  depends on the wavelength of the light, more particular depends on the wavelength of the light in a (wavelength-) periodic way. Depending on the requirements, the light that is transmitted through such a filter may be constituted by the light transmitted through the first waveguide  61  or by the light coupled, via the ring resonator  62 , into an optional second optical waveguide  63 . In the former case, the first waveguide  61  constitutes a part of the cavity. In the latter case, one branch of the first waveguide  61 , the ring resonator and one branch of the second waveguide  63  constitute part of the cavity. 
         [0096]    Further embodiments of periodic filters (or frequency combs) are possible and can be part of a laser resonator of a laser according to embodiments of the invention. 
         [0097]    Two embodiments suitable for simultaneously generating two light beams of different wavelengths are illustrated in  FIGS. 21 and 22 . In both cases, two separate gain devices  11   a,    11   b  are provided which have different ASE (amplified spontaneous emission) spectra. One and the same direction variation device  21 , e.g., a movable mirror, is used in the light source. This can provide intrinsically synchronized simultaneous wavelength scanning over different wavelength ranges. 
         [0098]    In  FIG. 21 , two logically parallel (partial) beam paths are created, each comprising one semiconductor gain device  11   a  and  11   b,  respectively. This is accomplished using beam splitters  16   a,    16   b,    17   a,    17   b,  wherein the term “beam splitter” is meant in a very broad and general sense. They can all be conventional beam splitters, e.g., based on semi-transparent mirrors, and beam splitters  16   b  and  17   b  could also be (conventional) mirrors, whereas beam splitters  16   a  and  17   a  could also be WDM combiners (wavelength division multiplexing combiners). Outcoupling mirror  15  is common for light produced in any of the gain devices  11   a,    11   b.  Like in other embodiments, a retarder  13  (schematically illustrated using dashed lines) can be a valuable option. At beam splitter  16   a,  light beams of different wavelengths are made parallel or even coaxial, such that two parallel or two coaxial light beams of different wavelengths impinge on direction variation device  21 , and in a before-described way, an angle of incidence of light incident on wavelength selection element  20  is varied (as indicated in  FIG. 21  by dashed lines). Two separate collimating optics (one in each partial beam path) or one common one can be provided. 
         [0099]    It is also possible to replace beam splitters  17   a  and  17   b  and outcoupling mirror  15  by one common outcoupling mirror or by two separate outcoupling mirrors (one for each partial beam path) or by mirrored (reflective) ends of the gain devices  11   a,   11   b.  In that case, two partially identical (partially overlapping) optical resonators are formed. 
         [0100]    The beam splitters can be polarization-dependent ones or polarization insensitive. 
         [0101]    In particular in case the maxima of the ASE spectra are rather far apart, it can be provided to use a higher diffraction order beam for light produced in a first one of the gain devices  11   a,   11   b  (in order to cope with the limited wavelength range within which a diffraction grating is operable. 
         [0102]    In  FIG. 22 , two optical resonators are formed, each comprising a gain device ( 26   a/   26   b ), possibly a collimator ( 12   a/   12   b ) and a wavelength selection element ( 20   a/   20   b ). Both resonators share direction variation device  21 , e.g., a MEMS mirror. The gain devices  11   a,    11   b  can be mirrored like illustrated in  FIG. 22  (reflective end faces  26   a,    26   b ), but separate mirrors or even a common mirror or others could also be provided so as to form resonator ends. 
         [0103]    From the above, it is clear, how the embodiments of  FIGS. 21 and 22 , respectively, can be generalized to three or even more gain devices, thus making possible the emission of simultaneously wavelength-scanned light or two or more than two different wavelengths. Wavelength multiplexing can be useful, e.g., in optical coherence tomography apparatuses, but also elsewhere. 
         [0104]    Furthermore, the optical amplifiers can be modulated in time, e.g., in order to reduce ASE background while not lasing, and/or for spectrally flattening the outputted light, and/or for other reasons. 
         [0105]    Special features of, for example, especially compact embodiments of lasers are depicted in  FIGS. 11   a  and  11   b.    FIG. 11   b  shows the wavelength scanning arrangement of the laser of  FIG. 11   a  in a schematic side view. 
         [0106]    A first feature of the laser of  FIG. 11   a  is that the gain element  11  is a Reflective Semiconductor Optical Amplifier (R-SOA). The cavity end mirror  15  is constituted by a partially reflective coating of the gain element  11 . This first feature brings about the advantage that less coupling losses are encountered in the cavity. Embodiments with separate mirrors (as in the case of non-reflective SOAs as gain elements), on the other hand, have less tendency to encounter spatial hole burning and, in general, are less sensitive to multi-mode operation (coupled laser cavities). 
         [0107]    A second feature of the laser of  FIGS. 11   a  and  11   b,  that can be implemented independent of the first feature (i.e. one can implement only the first, only the second, both or none of these features) is a multi-layer planar design. While the light is guided through the gain element  11 , collimation optics  12  and retarder  13  essentially in a first plane, a deflection arrangement  71  guides the light to a second plane that is parallel to the first plane, wherein, in general, an arbitrary mutual alignment or orientation of the first and second planes is possible. In the wavelength tuning arrangement comprising a dispersive wavelength selection element  20 , such as a GRISM  20 , and a movable mirror  21 , the light is guided in the second plane. 
         [0108]    In particular, the GRISM  20  may be placed on top of the retarder  13 . For example, it may be attached to it, for example by gluing. 
         [0109]    A schematic illustration of a light source that comprises a laser  1  of the herein discussed kind is shown in  FIG. 12 . Outside the laser cavity of the external cavity laser  1  with the outcoupling mirror  15  being a reflective coating of the gain element or a separate element, a couple of components are arranged, including a collimation arrangement  81 , an optional optical isolator  82 , a beam splitter  83 , beam steering re-directors  85 , a coupling lens  87 , and a fiber  88 . The collimation arrangement  81  (for example a collimation lens or a plurality of lenses) together with the gain element serve to collimate the beam for beam propagation to a fiber or optical feedthrough, see below. The collimation arrangement may be or comprise an aspherical lens. Also, a GRIN lens or mirror could be used. In principle, also direct coupling into the fiber would be possible. The optional optical isolator  82  is preferably close to the laser  1  to prevent any reflections back into the laser cavity. The beam splitter  83  reflects a portion of the beam into a wavemeter  84  (or k-clock), such as a wavemeter described in WO 2010/111795.The beam steering re-directors  85 , which are are optional, can be realized with different approaches, such as by prisms, wedges, mirrors, combination of lenses to form a telescope, etc. . . . In general, such re-directors are passive but could also be active components like 1D or 2D MOEMS (micro-opto-electro-mechanical systems) for adjusting coupling efficiency. The closer the optical feed through is to laser  1 , the less important is optical beam steering provided by such re-directors  85 . The coupling lens  87  couples the beam into the optical feedthrough  88 . Depending on how the feedthrough (which can be, e.g., a waveguide or a window) is realized, the lens is required or not. Again, mirrors or other types of focusing optics can be used. The fiber  88  serves as optical feedthrough. The fiber can be angle cleaved or polished in order to prevent back reflection back towards the laser  1 . It is also possible to combine the fiber with a lens, for example a GRIN lens (straight or angled) glued onto a fiber, the coupling lens  87  and the fiber feedthrough  88  would together be conceived as a single component. 
         [0110]    The above components may be used in combination, each alone, or in any sub-combination, depending on the needs. 
         [0111]    All described elements in the depicted configuration are carried by a common substrate  90 . The substrate can also be realized with several levels, e.g., to make possible different heights of optical axes above substrate  90 , e.g., if, inside the laser a different beam diameter is preferred than for the fiber coupling, or to accommodate sub-assemblies on separate substrates (of different thicknesses). The substrate is preferably made of ceramics and relatively thick, for mechanical stability, e.g., at least 0.5 mm and/or at most 10 mm, or at least 1 mm and/or at most 6 mm. 
         [0112]    The components held by the substrate are encased by a casing that provides mechanical protection. All constituents of the optical resonator can be arranged in the casing. The casing can in particular be hermetically closed, which can make possible to realize various environmental circumstances therein, e.g., the presence of a certain gas/gas mixture or a vacuum (of a certain degree). In addition to the optical feedthrough  88 , the casing comprises a plurality of electrical feedthroughs  91 , in particular for controlling the laser  1  and/or one or more others of the components. 
         [0113]    As direction variation devices  21 , various elements, devices or arrangements may be used, not only MEMS mirrors, but, e.g., vibrating optical fibers or electro-optic beam deflectors. In case of a vibrating optical fiber, a light beam from the gain element is fed into a first end of an optical fiber, and light exiting the optical fiber at a second end of the optical fiber propagates (under variation of its direction) to the wavelength selection element, wherein the direction variation is accomplished, e.g., by moving (vibrating) the second end of the fiber. In case of electro-optic beam deflectors, the light path in a material is varied using an electrical signal to which the material is exerted. The application of an electrical field, e.g., by applying a voltage to the material, or the injection of charge carriers can provoke a variation of the way light propagates in the material, more particularly, properties of the material are changed in such a way that an angle between a light beam incident on the material and the light beam exiting the material after having propagated through the material can be varied. This is schematically sketched in  FIG. 23  in a top view, in which α 1 ≠α 2  indicates the deflection. The direction of the arrows indicates a light path from the light-generating element, i.e. from a gain device, through a semiconductor structure  21  towards a wavelength selection element. The return path is the same but not indicated in this figure. 
         [0114]    Light enters the device  21  (comprising a suitably structured material) at first interface  91  (front facet) under an angle α 1 . The angle of incidence (AOI) α 1  can be zero, which would refer to normal (perpendicular) incidence, or can be a few degrees (e.g., 1° to 7°) in order to minimize unwanted reflections at this first interface. In order to suppress unwanted reflections, an anti-reflection coating (ARC) can applied to this first interface. Inside device  21 , the light travels along an optical waveguide. At the first interface  91 , the optical waveguide has a tilt angle of β 1 . Optimum coupling (from free space/ambient air) into device  21  (more particular into the waveguide thereof) occurs when α 1  and β 1  obey the diffraction law of Snellius: 
         [0000]        n   ext  sin(α1)= n   eff  sin(β1)
 
         [0000]    Where n eff  is the effective mode index inside the semiconductor device  21 ), and n ext  is the index of refraction of the material outside device  21  (at the first interface), which usually is ambient air (or another gas or vacuum), the index of refraction n ext  thus being around unity. For semiconductor devices, n eff  is typically in the range of 3.0 to 3.5, depending on the design of the optical waveguide (such as the waveguide dimensions and the surrounding semiconductor material). Thus, the tilt angle β 1  of the optical waveguide at the first interface  91  is typically in the range of 0° to 2°. 
         [0115]    The light travelling inside the optical waveguide is entering the second interface  92  under an AOI β 2  larger than the angle β 1 . The exiting light from the electro-optical deflector  21  is diffracted under an angle α 2  with β 2  and α 2  obeying the law of Snellius: 
         [0000]        n   eff  sin(β2)= n   ext  sin(α2)
 
         [0000]    In a possible realization of an electro-optical deflector, the angle β 2  is in the range of 10°-20°, depending on the value and range of n eff . This means that the exiting angle α 2  then is in the range of 30° to 90°, where larger angles may be particularly helpful in the present invention. 
         [0116]    Importantly, the effective mode index of device  21  can be changed by applying an electrical signal to device  21 , for example a voltage signal or a current signal. A voltage signal is usually applied by applying an electrical signal in reverse direction, such as by connecting a positive (plus) terminal of an electrical power source to an n-layer and the negative (minus) terminal to a p-layer; and a current signal is usually applied by applying an electrical signal in forward direction, such as by connecting a positive (plus) terminal of an electrical power source to a p-layer and the negative (minus) terminal to an n-layer. Index changes through electrical voltages in reverse direction are known and can be described by the Franz-Keldysh effect. However, this effect is generally more effective in changing the optical absorption by applying an electrical signal than in changing refraction. For practical realizations of semiconductor-based electro-optic beam deflectors, the effective mode index will therefore rather be varied by current injection (applying an electrical signal in forward direction. Note that the p and n contacts are illustrated, e.g., in  FIGS. 24 ,  27  and  28 , wherein it is to be noted that p and n contacts could, in principle, be interchanged with respect to what is described. 
         [0117]    In an example, deflection variation angles Δα of 3° to 10° are desired, that means by changing the electrical drive signal, the exiting angle α 2  shall change by 3° to 10°. The amount of index change usually is limited by the design of the active region (in which the light propagates in device  21 ) and by the applicability of electrical signals. But an increased deflection variation angle Δα may be achievable by increasing the waveguide tilt angle β 2 . Consequently, the exiting angle α 2  may increase up to its maximum possible value of 90°. When design parameters are properly adjusted, it is possible to achieve, e.g., that small index changes of 0.1 can already be sufficient to generate deflection variation angles Δα of 5° to 10°. 
         [0118]    The light exiting from the deflector  21  is launched, e.g., towards a diffraction grating or an assembly of diffraction gratings and prisms described in the present patent application. Besides the angle of propagation α 2 , the exiting light has a divergence angle that is described by the numerical aperture (NA) of the optical waveguide of electro-optical deflector  21 . This means that the exiting light has a beam diameter increasing while propagating. For that reason, the exiting light should to be collimated, e.g., by way of appropriate optical elements, when being used in combination with diffraction gratings. This may be accomplished, e.g., by using ball lenses. Another option is the use of a concave diffraction grating, e.g., like described for  FIG. 20 , accomplishing beam collimation for divergent beams. Electro-optic beam deflectors based on non-linear optical crystals not comprising a waveguide usually do not show an increased divergence of deflected beams. 
         [0119]    The active region of the semiconductor optical beam deflector  21  can be realized with a single quantum well (QW), having a thickness in the range of, e.g., 2 nm to 40 nm, or with multiple QWs having the same or different thickness values. The optical waveguide can be realized, e.g., with ridge waveguide devices or with buried waveguide devices, the latter typically producing a rounder mode profile which may be advantageous (in view of beam collimation and interfacing to other optical components). 
         [0120]      FIGS. 24 to 28  further illustrate possible semiconductor-based electro-optical beam deflectors. 
         [0121]      FIG. 24  is a schematic perspective view of an electro-optic deflector  21  with an active region  2 . 2  in which the light to be deflected propagates, a top layer  2 . 4  typically acting as the p-contact, and a bottom layer  2 . 3  typically acting as the n-contact. 
         [0122]      FIG. 25  is side view of possible realization of a semiconductor deflector  21 , showing top layer  3 . 4 , active region  3 . 2  and bottom layer  3 . 3 . The active region consists of one quantum well  3 . 5  surrounded by two barrier layers  3 . 7 . 
         [0123]      FIG. 26  is side view similar to  FIG. 25 , but the active region comprises more than one quantum well, specifically one quantum well  4 . 5  and at least one other one  4 . 6 . The two or more quantum wells may have different stochiometric compositions and/or different thicknesses. 
         [0124]      FIG. 27  is a front or back (facet) view of semiconductor deflector device  21  with a ridge waveguide structure, showing a ridge  5 . 1  which usually is formed by etching of semiconductor material, an upper layer  5 . 4 , an active region  5 . 2  and a lower layer  5 . 3 . Typically, the electrical p-contact is realized on top of the ridge  5 . 1 , and the electrical re-contact is realized at the bottom of the lower layer  5 . 3 . The bottom of the lower layer might be a semiconductor substrate on which the semiconductor epitactically grown structure is grown on, for example a GaAs, InP or GaN substrate. 
         [0125]      FIG. 28  is a front or back (facet) view of a semiconductor deflector device  21  with a buried waveguide structure, showing an upper layer  6 . 4 , an active region  6 . 2  and a lower layer  6 . 3 , lower layer  6 . 3  usually serving as an n-contact. The waveguide can be formed, e.g., by etching through the active region down to the lower layer  6 . 3  and then regrowing another semiconductor material  6 . 5  having a lower refractive index, followed by another regrown top (cap) layer  6 . 1  acting as an electrical contact layer, typically the p-contact.