Device and method for stabilizing or measuring frequency of laser

Abstract

The invention relates to a device and a method for stabilizing the frequency of a laser, in particular to a device and a method for stabilizing the frequency of a laser based on a temperature-stabilized volume Bragg grating (VBG) spectrum. The device for stabilizing the frequency of the lasers (D1, D2) comprises a beam path for injecting laser radiation emitted by the lasers (D1, D2) into a frequency selective element, the frequency selective element being temperature-controlled, the frequency selective element being a volume Bragg grating VBG (G1) having a plurality of grating structures (E), before the laser radiation enters the VBG (G1) via the incidence surface, a part of the laser radiation is split into a reference beam, and the part of the laser radiation entering the VBG (G1) forms a measurement beam.

Description

Technical Field

[0001] The present invention relates to an apparatus and method for frequency stabilization of a laser, and in particular to an apparatus and method for frequency stabilization of a laser based on a temperature-stabilized volume Bragg grating (VBG) spectrum.

[0002] Prior art In telecommunication and quantum sensor applications, frequency references are needed to stabilize lasers with a specific performance profile. In particular, for applications on mobile platforms in space, such frequency references must be as compact as possible, tunable without mode hopping, and long-term stable, wherein a (reproducible) frequency accuracy of about 50 MHz must be achieved. Corresponding frequency reference modules for realizing a spatially qualified frequency reference with these properties are not yet known in the prior art.

[0003] Prior art frequency references are usually based on the spectrum of a fiber Bragg grating (FBG) and are known to the person skilled in the art, for example from Sotor et al. (JZ Sotor, AJ Antonczak and KM Abramski, "Fiber Bragg Gratings as References for Frequency Stabilization of Microchip Lasers", 2006 International Conference on Transparent Optical Networks, 2006, pp. 167-169). As shown in the diagram taken from Sotor et al. and reproduced in FIG1 , the laser radiation is here injected into the FBG via a waveguide-based beam splitter. The diffracted signal is separated via the waveguide-based beam splitter and used together with the transmitted signal to generate an error signal for frequency stabilization of the laser. In order to ensure thermal stability of the frequency selective element, which is required to improve the frequency stability of the laser, the FBG is surrounded by an athermal housing.

[0004] However, spectroscopy methods based on optical waveguides have significant disadvantages. In particular, the properties of optical waveguides can change under the influence of radiation, mechanical stress and temperature fluctuations (especially in long-term operation). This leads to measurement errors that are intolerable in high-sensitivity sensor applications. When using waveguide-based beam splitters, these influences also lead to temperature and polarization dependence of the splitting ratio of the waveguide-based beam splitter. Fluctuations in the splitting ratio lead to incorrect interpretation of the spectral signal during ongoing operation. These systematic errors lead to frequency errors. The use of a heatless housing is intended to achieve thermal decoupling of the FBG in order to achieve high frequency stability. However, this approach hinders the possibility of tuning the frequency of the grating by controlling the temperature of the grating. Therefore, frequency stabilization based on FBG is not suitable for applications on mobile platforms in space, and other stabilization concepts must be used for this purpose. Summary of the invention

[0005] Therefore, the object of the present invention is to provide a device for frequency stabilization of a laser, which device is able to provide sufficient laser stability for the above-mentioned mobile applications in space with the above-mentioned performance distribution. In addition, a corresponding method for frequency stabilization of a laser is to be provided.

[0006] According to the invention, these objects are achieved by the features of claim 1, claim 13 and claim 15. Advantageous configurations of the invention are described in the dependent claims. The features listed individually in the claims can be combined with one another in a technically meaningful manner and can be supplemented by illustrative facts from the description and / or details from the drawings, in which further variant embodiments of the invention are indicated.

[0007] A first aspect of the invention relates to a device for frequency stabilization of a laser, the device comprising a beam path for injecting laser radiation emitted by the laser into a frequency selective element, wherein the frequency selective element is temperature controlled, wherein the frequency selective element is a volume Bragg grating VBG having a plurality of grating structures, and before the laser radiation is injected into the VBG via an input facet, part of the laser radiation is branched into a reference beam, and the part of the laser radiation injected into the VBG forms a measuring beam.

[0008] In particular, the beam path is used to refer to guiding the laser radiation as a "free beam", ie the laser radiation is not injected into the frequency selective element via an optical fiber or a waveguide, but for example as a freely propagating Gaussian beam (free beam).

[0009] The temperature of the frequency selective element is controllable, i.e. it can be varied relative to the environment by appropriate control. Since the filtering frequency of the frequency selective element generally depends on its temperature, the temperature control of the frequency selective element also includes the controllability of the filtering frequency (center frequency) of the frequency selective element. Peltier elements are advantageously used for fast temperature control of the VBG, which, unlike heating elements combined with additional cooling, also inherently enable active cooling. However, when using heating elements, the thermal connection of the VBG must be particularly optimized for fast and effective cooling.

[0010] A VBG is a specific kind of grating. In contrast to mechanical gratings ("ruled gratings") or holographically generated planar gratings ("holographic gratings"), which usually take the form of surface gratings, VBGs are usually "thick gratings" inside a material volume. The VBG is in the form of a plurality of grating structures arranged one after another. For the purposes of the present invention, the individual reflective layers of the grating are denoted as grating structures. These can be, for example, individual grating planes of a VBG consisting of a plurality of grating planes arranged one after another in a suitable material. However, the individual grating structures can also have a non-planar form, such as a curved shape. The spacing of the grating structures from one another can be varied to provide a grating with a broad spectral distribution (known as a "chirped grating"). The VBG can also comprise a plurality of gratings separated from one another or at least partially merged into one another and have a corresponding grating structure. The VBG is usually generated holographically, and the corresponding filter element is then also denoted as a volume holographic Bragg grating (VHG). However, the VBG structure can also be generated non-holographically, for example by writing fs laser pulses in glass via a phase mask.

[0011] The VBG preferably takes the form of a volume holographically generated Bragg grating in photothermorefractive glass. In a preferred embodiment, volume holographic Bragg gratings based on dichroic gelatin, photopolymers, photorefractive crystals or silver halide emulsions can also be used as VBG medium. An important criterion for applications on mobile platforms in space is the long-term stability of the VBG, in particular the low sensitivity to radiation, mechanical stress and temperature fluctuations and aging effects.

[0012] According to the invention, before the laser radiation is injected into the VBG via the input face of the VBG, part of the laser radiation is branched into a reference beam. The phrase "before the injection" is to be understood relative to a position along the beam path; the branching can therefore also be performed in time parallel to the injection, i.e. at or during the physical process of the injection. In particular, the branching of the reference beam can be performed by direct reflection at the input face of the VBG at the injection. Alternatively, the reference beam can also be branched from a free beam directed onto the VGB by a beam splitter arranged upstream of the VBG. The reference beam can be detected with a suitable reference detector. On the other hand, part of the laser radiation injected into the VBG forms a measuring beam. The measuring beam can preferably be detected as a transmitted measuring beam downstream of the VBG or as a diffracted measuring beam upstream of the VBG. Both the transmitted measuring beam and the diffracted measuring beam can also be reflected repeatedly within the VBG (e.g. at the outer surface of the VBG).

[0013] The VBG is preferably arranged in a Littrow configuration. In this configuration, the laser radiation impinges perpendicularly on the grating structure of the VBG. If the VBG is operated in a Littrow configuration, the frequency value of the device according to the invention (the center frequency of the VBG) is insensitive to the first-order tilt of the incident laser beam.

[0014] The input facet of the VBG preferably has an angle other than 90° to the beam axis of the incident laser radiation and / or to the grating structure of the VBG. If the input facet of the VBG has an angle other than 90° to the beam axis of the incident laser radiation, the reflected part of the incident laser radiation at the input facet of the VBG points in a direction other than the incident direction. This means that there are no direct back reflections into the laser which could cause interference. Furthermore, parts of the laser beam can be branched off into separate reference beams via reflections immediately before injection into the VBG. The spatial separation of the incident beam and the beam reflected by the input facet can dispense with additional beam splitters. Dielectric beam splitters, i.e., have a highly reflective coating which has temperature-dependent properties and which change over time (aging effects). Some of the incident laser radiation is particularly preferably branched off by Fresnel reflection at the input facet of the VBG. Depending on the refractive index of the VBG medium and the angle of incidence, a specific angle can be set between the laser radiation and the grating structure of the VBG. In particular, this allows the VBG to be optimally adapted to a specific Littrow configuration, in which the beam reflected by the input facet is not reflected in the direction of the incident beam.

[0015] The VBG preferably comprises a mounting surface, a top surface opposite to the mounting surface, and two side surfaces connecting the mounting surface and the top surface, wherein the grating vector of the VBG and the top surface are arranged parallel to the mounting surface and / or the side surfaces are arranged parallel to each other. The VBG may further comprise a front surface connecting the mounting surface and the top surface, through which the laser radiation preferably enters (input surface), and a rear surface connecting the mounting surface and the top surface, the rear surface being opposite to the input surface. The side surfaces of the VBG are preferably oriented parallel to each other with a tolerance of ±100', more preferably ±10', in order to minimize the volume of the VBG. The grating vector orthogonal to the grating structure (e.g. a plane or slightly curved surface with the same refractive index) and the top surface are preferably oriented parallel to the mounting surface of the VBG with a tolerance of ±40', more preferably ±4', in order to simplify the integration process of the device according to the present invention. The angle between the grating vector of the VBG and the beam axis of the refracted beam (the measurement beam after injection into the VBG) preferably has a tolerance of ±40', more preferably ±4'. The tighter the angular tolerance between the grating vector of the VBG and the beam axis of the refracted beam, the greater the degree of freedom during the integration process.

[0016] The angle between the beam axis of the incident beam and the input facet of the VBG is preferably equal to ±10°, more preferably equal to ±1°. The input facet of the VBG preferably has an angle of 45°±1° to the beam axis of the incident beam. Using a 45° angle between the beam axis of the incident beam and the input facet of the VBG simplifies mechanical integration, while tighter tolerances on the angle between the beam axis of the incident beam and the input facet of the VBG allow for more predictable detector signal levels due to the angular dependence of Fresnel reflection, and therefore smaller and faster detectors can be used.

[0017] In another preferred embodiment, the angle between the beam axis of the incident beam and the input face of the VBG is not 45°, but assumes another value, wherein the incident beam and the beam reflected at the input face are spatially separated. In another preferred embodiment, the beam incident on the input face of the VBG and the beam reflected on the input face overlap spatially. In this case, the reference beam can be generated by a beam splitter arranged upstream of the VBG.

[0018] The VBG is preferably in the form of a parallelepiped, and the grating structures of the VBG are arranged perpendicular to the mounting surface, the top surface and the side surface of the VBG. In this case, not only is the integration process of the device according to the present invention simplified, but also the space required for it is reduced, and its thermal load is also reduced. The parallelepiped itself can provide the surface oblique angle of the grating structure required for Fresnel reflection at the input surface.

[0019] The reference beam is preferably branched at the surface of the VBG (preferably via Fresnel reflection at the input face) or by a beam splitter arranged upstream of the VBG. In the case of Fresnel reflection, a dielectric coating for reducing reflection can be omitted on the input face. Omitting a dielectric coating on the input face of the VBG avoids temperature and aging effects on reflection and transmission.

[0020] The device according to the invention preferably further comprises: a reference detector arranged to determine the intensity of the reference beam and a measuring detector arranged to determine the intensity of the measuring beam, wherein the intensity of the measuring beam is determined after passing through the VBG. The measuring beam may be repeatedly reflected at an outer surface within the VBG. In particular, the measuring beam may pass through the VBG once, twice or multiple times. The measuring detector preferably detects a signal transmitted by the VBG (transmission signal). However, alternatively, the beam diffracted by the VBG may also be detected by the measuring detector (diffraction signal). The detector is preferably a photodiode.

[0021] The device according to the invention preferably further comprises a first electronic circuit for deriving an error signal from a reference signal of the reference detector and a measurement signal of the measurement detector. The error signal is taken to represent a signal having a functional dependency associated with a control deviation. Typically, the error signal has a characteristic operating point suitable for the control circuit (e.g. at a zero crossing).

[0022] Instead of using the reference signal and the transmission signal to derive the error signal, the reference signal and the diffraction signal may also be used. In further embodiments, the transmission signal and the diffraction signal, or the reference signal, the transmission signal and the diffraction signal may be used to derive the error signal. Furthermore, the error signal may be derived using only the diffraction signal or only the transmission signal, wherein, however, a constant input level is required in both cases.

[0023] The VBG is preferably thermally separated from its surroundings by a grating housing. An appropriate grating housing can achieve enhanced thermal and optical isolation of the VBG. The grating housing can be connected to an optical bench on which the VBG is arranged along with other components. The grating housing can be temperature stable. Such an embodiment allows for smaller temperature gradients in the surroundings of the VBG associated with thermal radiation.

[0024] The VBG is preferably arranged in a module housing with other components. In particular, the VBG can be arranged in a module housing with other components (e.g., mirrors, polarizers, and / or optical benches) with or without an additional grating housing. Such an embodiment enables better thermal and acoustic insulation of the VBG. The module housing can be thermally separated from its surroundings and have temperature stability.

[0025] At least one device for cooling and / or heating is preferably used for temperature stabilization of the VBG. The device for cooling and / or heating may preferably comprise a Peltier element or a heating element. When a heating element is used, the thermal connection of the VBG (or an element connected thereto) may be particularly optimized for fast and efficient cooling. The cooling may, for example, involve radiative cooling in space. It is further preferred that the detector for signal acquisition according to the invention is arranged with the VBG on a common Peltier element or a suitable common thermal connection platform. Such an embodiment makes it possible to reduce the sensitivity to beam misalignment, thereby improving the accuracy of the device according to the invention.

[0026] The described embodiments may advantageously be combined with one another in whole or in part.

[0027] A second aspect of the invention relates to a method for frequency stabilization of a laser using a device according to the invention, wherein an error signal is derived from the intensity of a reference beam and the intensity of a measuring beam after (at least one) pass through a VBG and the error signal is used to stabilize the frequency of the laser via a control loop.

[0028] In particular, the temperature of the device according to the invention can be controlled via means for cooling and / or heating. The control can relate locally to the VBG or to other components, such as, for example, an optical bench on which the VBG and other components are arranged. It is also possible to combine a variety of temperature control methods. Temperature stabilization of the optical bench increases the precision of the method according to the invention.

[0029] The VBG spectrally filtered laser radiation is preferably injected again into the laser to form an extended resonator for the laser. In particular, a frequency-stabilized diode laser with an extended resonator ("extended cavity diode laser", ECDL) can be realized in this way. The injection current or the temperature of the laser can then be controlled, for example, using an associated control circuit, in order to adjust and stabilize the laser frequency.

[0030] A third aspect relates to a method for measuring the frequency of a laser using a device according to the invention, wherein an error signal is derived from the intensity of the reference beam and the intensity of the measuring beam after (at least one) pass through the VBG, the temperature of the VBG is adjusted via a control loop using the error signal, so that the Bragg frequency (center frequency) of the VBG corresponds to the frequency of the laser, except for an optionally selected signed frequency difference, and the frequency of the laser is determined by a known correlation with the adjusted temperature of the VBG. Thus, the Bragg frequency (center frequency) of the VBG can also be adjusted directly to the frequency of the laser.

[0031] The method for measuring the frequency of a laser and the method for frequency stabilization of a laser according to the invention differ essentially only in terms of the controlled variables, but are based on a common inventive concept. However, for the purpose of laser frequency stabilization, the temperature of the laser and / or the injection current can be controlled using the error value derived according to the invention in order to adjust and stabilize a specific laser frequency, while for the purpose of laser frequency measurement, the temperature of the VBG can be stabilized by means of temperature control so that the Bragg frequency (center frequency) of the VBG corresponds to the frequency of the laser, except for an optionally selected signed frequency difference. Based on the temperature of the VBG adjusted or established via a temperature sensor, the frequency of the laser can then be determined via a known correlation and, for example, displayed on a display device.

[0032] Furthermore, further preferred configurations of the method according to the invention are revealed directly by the features stated in the description with regard to the device according to the invention.

[0033] The invention can be used in quantum sensors to more quickly and reliably stabilize laser light to a desired wavelength, such as an atomic transition. Thus, the device according to the invention can make it easier for, for example, two satellites to establish communication (acquire lock) for optical satellite communications.

[0034] Although in the prior art, the optical signal is guided in an optical waveguide, in the present invention, it is preferably propagated in free space. The present invention has the following structural differences in particular: 1. Use VBG instead of FBG as the frequency selective element.

[0035] 2. No optical waveguide based beam splitter is used to separate the optical signal upstream of the FBG. Some of the incident optical signal is preferably branched off from the rest of the signal by Fresnel reflection at the input face of the VBG. To this end, the input face of the VBG may be arranged at an angle to the incident light beam such that the Fresnel reflection and the incident light beam are spatially separated from each other. An error signal may preferably be derived from the transmitted and branched incident optical signals.

[0036] 3. The VBG can be arranged on an optical bench or generally on a suitable carrier. Its temperature can be stabilized to a predetermined value using suitable means for cooling and / or heating and a temperature sensor.

[0037] Since no optical waveguide components are used at points that are particularly critical for the measurement accuracy, the device according to the invention is significantly more resistant to radiation, mechanical stress and temperature changes than the prior art. The reference beam is preferably branched by Fresnel reflection, so that the splitting ratio can be made largely independent of temperature influences. By using Peltier elements and dispensing with the athermal housing, the frequency of the VBG can be tuned directly via the temperature. By miniaturizing the device and appropriately shaping the VBG, greater mechanical stability as well as lower weight and reduced form factor can be achieved. The optional module housing can greatly shield the device according to the invention from acoustic and thermal influences. In addition, by staggered temperature stabilization of the VBG (e.g. by means of a Peltier element below the VBG, a grating housing surrounding the Peltier element, a further Peltier element for stabilizing the optical bench and a module housing surrounding the optical bench), high frequency accuracy and frequency tuning can be achieved simultaneously.

[0038] Further preferred configurations of the invention are revealed by the features mentioned in the respective subclaims.

[0039] Unless stated otherwise, the various embodiments of the invention set forth in this application can advantageously be combined with one another. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention and the technical context are explained in more detail below with reference to the accompanying drawings. It should be noted that the present invention is not intended to be limited to the exemplary embodiments described. In particular, unless otherwise explicitly stated, sub-aspects of the subject matter explained in the accompanying drawings may also be extracted and combined with other components and insights from the present specification. In the accompanying drawings: FIG1 shows a schematic diagram of a conventional arrangement for frequency stabilization of a laser according to the prior art (from Sotor et al.); Figure 2 A schematic diagram showing a first embodiment of a device for frequency stabilization of a laser according to the present invention; Figure 3 A schematic diagram showing a second embodiment of the device for frequency stabilization of a laser according to the present invention; Figure 4 A schematic diagram showing a third embodiment of the device for frequency stabilization of a laser according to the present invention; Figure 5 A schematic diagram showing a fourth embodiment of the device for frequency stabilization of a laser according to the present invention; Figure 6 A schematic diagram showing the geometry of a VBG according to the present invention is shown; Figure 7 Shows the use of Figure 2 A schematic diagram of a first embodiment of a method for frequency stabilization of a laser according to the present invention; Figure 8 Shown according to Figure 7 A schematic diagram of signals corresponding to an embodiment of the present invention; Fig. 9 Shows the use of Figure 2 A schematic diagram of a second embodiment of a method for frequency stabilization of a laser according to the present invention; Fig.10 Shows the use of Figure 2 A schematic diagram of a third embodiment of a method for frequency stabilization of a laser according to the present invention; Fig.11 Shows the use of Figure 2 A schematic diagram of an embodiment of a method for measuring the frequency of a laser according to the present invention; Fig.12 Shown according to Fig.11 A schematic diagram of the signal; and Fig.13 Shown according to Fig.11 Schematic diagram of the association of the VBG temperature and the laser frequency. DETAILED DESCRIPTION

[0041] FIG1 shows a schematic diagram of a conventional arrangement for frequency stabilization of a laser according to the prior art (from Sotor et al.). The laser (“stabilized laser”) comprises a temperature-controlled laser crystal (Nd:YAG / KTP) which is temperature-controlled by means of a Peltier element (TEC—“thermoelectric cooler”) and emits dual resonance at emission wavelengths of 1064 nm and 532 nm. The laser crystal is pumped via a diode laser (“pump diode”) with an emission wavelength of 808 nm. The 1064 nm radiation portion is branched via a dichroic beam splitter (“dichroic mirror”) and injected via a fiber coupler into a FBG arranged in an athermal housing. The intensities of the diffracted radiation component (“reflected signal”) and of the radiation component transmitted by the FBG (“transmitted signal”) are determined via appropriately arranged PIN photodiodes (“PIN diodes”), and both signals are supplied to an electronic circuit to control the temperature of the laser crystal (“automatic frequency control”). The quality of the laser frequency stabilization here depends primarily on the stability of the FBG and the associated fiber sections within the arrangement.

[0042] Figure 2 A schematic diagram of a first embodiment of a device for frequency stabilization of a laser according to the invention is shown. Here the optical signal is injected into the device via a polarization-maintaining single-mode optical waveguide (PMSF) F1. PMSF F1 allows the device according to the invention to be directly integrated into existing optical systems. The injected optical signal is collimated using the optical system L1. Polarizer R1 ensures that only s-polarized light is guided by mirrors S1 and S2 onto VBG G1. The light beam can be guided such that Figure 6 As shown, VBG G1 can be operated in a Littrow configuration (i.e., light is perpendicularly incident on the grating structure E) with a tolerance of ±1°. Polarizer R1 can prevent any potential polarization fluctuations that could cause system measurement errors. Beam steering via two mirrors S1 and S2 provides more freedom in positioning the VBG than when only one mirror or no mirror is used.

[0043] The VBG G1 can be attached to a temperature sensor N1 for temperature stabilization, for example to a first device for cooling and / or heating T1. The reference detector P1 and the measurement detector P2 can likewise be attached to the device for cooling and / or heating T1 in order to ensure a stable beam alignment. The PMSF F1, the optical system L1, the polarizer R1, the mirrors S1 and S2 and the device for cooling and / or heating T1 are preferably glued to a common optical bench B1. The grating housing H1 can be thermally connected to the optical bench B1 in order to improve the temperature stability of the device according to the invention. The optical bench B1 can likewise be stabilized to a desired temperature by means of a second device for cooling and / or heating T2.

[0044] The optional module housing H2 may be used to shield the optical bench B1 from thermal and acoustic disturbances from the surrounding environment. The optical system L1, the polarizer R1, the mirrors S1 and S2 and the VBG G1 preferably comprise micro-optical components. Micro-optical components are preferably used due to the associated smaller form factor, reduced weight and increased mechanical stability of the device according to the invention. The means for cooling and / or heating T1, T2 may preferably comprise Peltier elements or heating elements.

[0045] Figure 3 A schematic diagram of a second embodiment of the device for frequency stabilization of a laser according to the present invention is shown. The diagram shown corresponds largely to Figure 2 , and the reference numerals are applied accordingly. In this embodiment, the intensity of the light beam diffracted by VBG G1 (diffraction signal) is determined using the measurement detector P3. Therefore, the error signal can be generated using the reference signal and the diffraction signal instead of the reference signal and the direct transmission signal. The intensity of the diffracted light beam can be determined using the measurement detector P3 after reflection at the beam splitter A1 upstream of VBG G1.

[0046] Figure 4 A schematic diagram of a third embodiment of the device for frequency stabilization of a laser according to the present invention is shown. The diagram shown corresponds largely to Figure 3 , so the reference numerals apply accordingly. If the incident beam and the diffracted beam do not overlap spatially, the additional beam splitter A1 upstream of the VBG G1 can be omitted and the intensity of the diffracted beam can instead be determined directly with the measuring detector P4.

[0047] Figure 5 A schematic diagram of a fourth embodiment of the device for frequency stabilization of a laser according to the present invention is shown. The diagram shown corresponds largely to Figure 4 , so the reference numerals are applied accordingly. As an alternative to determining the intensity of the diffracted beam with the measuring detector P4, this can also be performed, for example, with the measuring detector P5 via the beam coupled out of the side surfaces (e.g. side surfaces A, B) of the VBG G1. In this case, the corresponding side surface of the VBG G1, from which the diffracted beam emerges after reflection at the input face, must have optical quality.

[0048] Figure 6A schematic diagram of a VBG geometry according to the invention is shown. The input face of the VBG G1 preferably has an angle of 45°±1° to the beam axis of the incident light beam and is not coated. The side surfaces A, B of the VBG G1 are preferably oriented parallel to each other with a tolerance of ±10' in order to minimize the volume of the VBG G1. The grating vectors orthogonal to the grating structure E of the same refractive index and the top surface D are preferably oriented parallel to the mounting surface C of the VBG G1 with a tolerance of ±4' in order to simplify the integration process of the VBG into the device according to the invention. The incident light beam is preferably Fresnel reflected at the input face of the VBG G1.

[0049] Figure 7 Shows the use of Figure 2 Schematic diagram of a first embodiment of a method for frequency stabilization of a laser according to the present invention. The diagram shown corresponds largely to Figure 2 , so the reference numerals are applied accordingly. The light from the laser D1 is here injected into the optical waveguide F1 of the device according to the invention, for example by means of an optical system L2, so that the frequency of the laser D1 can be stabilized by appropriate control.

[0050] The light beam reflected at the input face of the VBG G1 shown strikes, for example, a reference detector P1 that detects the intensity of the reflected light beam (reference signal). Part of the light beam incident on the input face is refracted upon entering the VBG G1. The intensity of the light beam transmitted by the VBG G1 can be detected with a measurement detector P2 (transmission signal). From the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measurement detector P2, an error signal that can be used to stabilize the laser D1 (or for measuring the emission frequency of the laser D1) can be generated.

[0051] To this end, the signal U1 of the reference detector P1 and the signal U2 of the measuring detector P2 are processed by a suitable electronic circuit E1 to generate an error signal. The error signal can then be processed by a suitable electronic circuit E2 and fed back as a control signal to the laser G1 to control the frequency of the laser G1 so that the frequency of the laser G1 is adjusted to a defined value f0.

[0052] In the simplest case, the electronic circuit E1 can be implemented such that it scales the signal U1 of the reference detector P1 and the signal U2 of the measuring detector P2 using a suitable scaling factor and determines the difference between the signals U1 and U2 (see Appendix Figure 8 ).

[0053] Figure 8 It shows that according to the Figure 7Schematic diagram of signals corresponding to an embodiment of the invention. An exemplary curve of the reference signal U1 of the reference detector P1 and the transmission signal U2 (photodiode signal) of the measuring detector P2 is plotted against the frequency of the laser D1 (laser frequency f). From this, the difference signal U2-U1 can be determined as an error signal, wherein Figure 8 In FIG. 1 , it is shown by way of example that the first zero crossing of the error signal serves as a control point to set the frequency of the laser G1 to a defined value f0. When the control loop is activated, the frequency of the laser G1 is then adjusted to the value f0 at which the error signal has (as shown) a zero crossing with a positive or negative gradient.

[0054] Fig. 9 Shows the use of Figure 2 Schematic diagram of a second embodiment of the method for frequency stabilization of a laser according to the present invention. The diagram shown corresponds largely to Figure 2 , so the reference numerals are used accordingly. Specifically, this is a method for internal frequency stabilization of a diode laser with an extended resonator (“extended cavity diode laser”, ECDL) by means of the device according to the invention.

[0055] The radiation emitted by the laser D2 is collimated by means of an optical collimation system L3 and injected into the device according to the invention by means of the optical system L2. The injected radiation is frequency-selectively diffracted by the VBG G1. The laser equipment implemented in this way oscillates at a frequency corresponding to the frequency of one of the possible longitudinal eigenmodes of the ECDL. The VBG G1, which acts as a spectrally narrowband reflector, selects one of the possible longitudinal eigenmodes for oscillation. This is typically the longitudinal eigenmode whose frequency is closest to the center frequency of the spectrum of the VBG G1 acting as a reflector. An embodiment of the device according to the invention that is preferably used for the method described here is that the device does not include an optical waveguide for injection, but a free beam is injected into the device. In particular, this prevents parasitic feedback from the various faces of the optical waveguide into the laser D2, which otherwise could interfere with the operation of the ECDL.

[0056] Changes in the air pressure around the laser equipment, the temperature of the laser D2, the injection current of the laser D2 and / or the temperature of the laser equipment consisting of the laser D2, the optical collimation system L3 and the device according to the invention lead to a spectral detuning between the frequency of the longitudinal eigenmode of the ECDL and the center frequency of the VBG G1 used as a spectral narrowband reflector. If this detuning reaches the order of magnitude of the free spectral range of the ECDL, a mode jump occurs, i.e. a change of different longitudinal eigenmodes of the ECDL occurs. This leads to a sudden change in the laser oscillation frequency.

[0057] The method for frequency stabilization of a laser according to the invention can be used to avoid sudden changes in the laser oscillation frequency during operation at a constant frequency or in the case of a desired frequency change. To this end, an error signal is generated from a reference signal U1 of a reference detector P1 and a transmission signal U2 of a measuring detector P2 using a suitable electronic circuit E1. Then, with the aid of an appropriate device T3 for cooling and / or heating, a suitable electronic circuit E2 can be used to generate a control signal therefrom, which controls, for example, the injection current or temperature of the laser D2 or the temperature of an equipment consisting of the laser D2, the optical collimation system L3 and the device according to the invention. Combinations of these parameters can also be controlled. In this way, the oscillation frequency of the eigenmode of the ECDL is adjusted so that it remains tuned to the frequency f0 of the device according to the invention. Therefore, the oscillation frequency of the ECDL follows the frequency f0 of the device according to the invention.

[0058] Fig.10 Shows the use of Figure 2 Schematic diagram of a third embodiment of a method for frequency stabilization of a laser according to the present invention. The diagram shown corresponds largely to Fig. 9 , and the reference numerals are used accordingly. Fig. 9 In contrast, the control signal generated by the electronic circuit E2 is used to control the device T1 for cooling and / or heating directly coupled to the device according to the invention. In this case, the frequency f0 of the device according to the invention is stabilized to the oscillation frequency of the ECDL.

[0059] Fig.11 Shows the use of Figure 2 Schematic diagram of an embodiment of a method for measuring the frequency of a laser according to the invention. The diagram shown corresponds largely to Figure 2 , the reference numerals are applied accordingly. The light from the laser D1 is here injected, for example by means of an optical system L2, into the optical waveguide F1 of the device according to the invention, so that the device for cooling and / or heating T1 can be driven in such a way that the frequency f0 of the device according to the invention (corresponding to the center frequency or Bragg frequency of the VBG, except for an optionally selected signed frequency difference) assumes the value f1 of the frequency of the laser D1. The temperature value measured at the temperature sensor N1 adjusted in this way can be assigned a specific value of the frequency of the laser D1.

[0060] To this end, the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measuring detector P2 are processed by means of a suitable electronic circuit E1. The error signal is processed by means of the electronic circuit E2 as a control signal, which controls the device T1 for cooling and / or heating so that the frequency f0 of the device according to the invention corresponds to the oscillation frequency f1 of the laser D1. From the value f1 of the oscillation frequency of the laser D1, a temperature T characteristic of the value f1 of the oscillation frequency of the laser D1 is established at the temperature sensor N1. With the help of a known correlation (for example, a calibration table K1), the value f1 of the oscillation frequency of the laser D1 can thus be determined from the value of the temperature T at the temperature sensor N1 and displayed on the display device Y1.

[0061] Fig.12 Shows the corresponding Fig.11 The frequency f0 of the device according to the invention is stabilized to the frequency value of the laser D1 (laser frequency f). Exemplary curves of the difference signal U2-U1 are shown for two different temperatures T1 and T2 of the device according to the invention (see Figure 8 Determined). In this case, the first zero crossing of the error signal can also be used as a reference point. With the aid of known assignments, the value f1 or f2 of the oscillation frequency of the laser D1 can be determined from the value of the temperature T1 or T2 at the temperature sensor N1 (see FIG. 14 ).

[0062] Fig.13 Shown according to Fig.11 Schematic diagram of the relationship between the temperature of VBG (G1) and the frequency of the laser. In this diagram, the temperature T (T N1 ) and the frequency of laser G1 (laser frequency f).

[0063] Reference numerals list D1, D2 laser G1 Volume Bragg Grating (VBG, frequency selective element) E grating structure (e.g., grating surface or plane) A, B side surface CMounting surface D Top surface A1 Beam Splitter P1, P3 reference detector P2, P4, P5 measurement detector U1 reference signal (reference detector) U2 measurement signal (measurement detector) E1, E2 electronic circuit H1 grating housing H2 module housing T1, T2, T3 devices for cooling and / or heating K1 Calibration Table Y1 Display Device L1, L2, L3 optical system S1, S2 reflectors F1 optical waveguide R1 Polarizer N1 Temperature Sensor B1 Optical Station

Claims

1. A device for frequency stabilization of a laser (D1, D2), said device comprising a beam path for injecting laser radiation emitted by said laser (D1, D2) into a frequency selective element (G1), in, The frequency selective element (G1) is temperature controlled, and The frequency selective element is a volume Bragg grating VBG (G1) having a plurality of grating structures (E), and before the laser radiation is injected into the VBG (G1) via an input facet, part of the laser radiation is branched into a reference beam and the part of the laser radiation injected into the VBG (G1) forms a measuring beam.

2. The device according to claim 1, wherein: The VBG (G1) takes the form of a volume holographically generated Bragg grating in photothermorefractive glass.

3. The device according to claim 1 or 2, wherein: The VBG (G1) is arranged in a Littrow configuration.

4. A device according to any one of the preceding claims, wherein: The input facet of the VBG (G1) is at an angle other than 90° to the beam axis of the incident laser radiation and / or is at an angle to the grating structure (E) of the VBG (G1).

5. A device according to any one of the preceding claims, wherein: The VBG (G1) includes a mounting surface (C), a top surface (D) opposite to the mounting surface (C), and two side surfaces (A, B) connecting the mounting surface (C) and the top surface (D), wherein a grating vector of the VBG (G1) and the top surface (D) are configured to be parallel to the mounting surface (C), and / or the side surfaces (A, B) are configured to be parallel to each other.

6. The device according to claim 5, wherein: The VBG (G1) is in the form of a parallelepiped, and the grating structure (E) of the VBG (G1) is arranged perpendicular to the mounting surface (C), the top surface (D) and the side surfaces (A, B) of the VBG (G1).

7. A device according to any one of the preceding claims, wherein: The reference beam is branched at the surface of the VBG (G1) or at a beam splitter (A1) arranged upstream of the VBG (G1).

8. The device according to any one of the preceding claims, further comprising: A reference detector (P1, P3) arranged to determine the intensity of the reference beam and a measurement detector (P2, P4, P5) arranged to determine the intensity of the measurement beam, wherein the intensity of the measurement beam is determined after passing through the VBG (G1).

9. The device according to claim 8, further comprising a first electronic circuit (E1) for deriving an error signal from a reference signal (U1) of the reference detector (P1, P3) and a measurement signal (U2) of the measurement detector (P2, P4, P5).

10. The device according to any one of the preceding claims, wherein: The VBG (G1) is thermally separated from its surroundings by a grating housing (H1).

11. The device according to any one of the preceding claims, wherein: The VBG (G1) and other components are arranged in a module housing (H2).

12. The device according to any one of the preceding claims, wherein: At least one device (T1, T2, T3) for cooling and / or heating is used for temperature stabilization of the VBG (G1).

13. A method for frequency stabilization of a laser (D1, D2) using a device according to any one of claims 1 to 12, wherein: An error signal is derived from the intensity of the reference beam and the intensity of the measuring beam after passing through the VBG (G1), and the frequency of the laser (D1, D2) is stabilized via a control loop using the error signal.

14. The method for frequency stabilization of a laser (D2) according to claim 13, wherein: The laser radiation spectrally filtered by the VBG (G1) is re-injected into the laser (D2) to form an extended resonator for the laser (D2).

15. A method of measuring the frequency of a laser (D1) using a device according to any one of claims 1 to 12, wherein: An error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG (G1), and the temperature of the VBG (G1) is adjusted via a control loop using the error signal so that the Bragg frequency of the VBG (G1) corresponds to the frequency of the laser (D1), except for an optionally selected signed frequency difference, and the frequency of the laser (D1) is determined by a known correlation with the adjusted temperature of the VBG (G1).

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