Device and method for stabilizing or for measuring the frequency of a laser

Abstract

The invention relates to a device and a method for stabilizing the frequency of a laser, in particular a device and a method for stabilizing the frequency of a laser on the basis of the spectroscopy of a temperature-stabilized volume Bragg grating (VBG). A device for stabilizing the frequency of a laser (D1, D2) comprises a beam path for the incoupling of laser radiation emitted by the laser (D1, D2) to a frequency-selective element, wherein the frequency-selective element is temperature-regulated, wherein the frequency-selective element is a volume Bragg grating, VBG, (G1) having a plurality of grating structures (E) and before incoupling of the laser radiation into the VBG (G1) via an entrance facet a portion of the laser radiation is split off into a reference beam and the portion of the laser radiation that is incoupled into the VBG (G1) forms a measurement beam.

Description

[0001] DEVICE AND METHOD FOR STABILIZING OR MEASURING THE FREQUENCY OF A LASER

[0002] Description

[0003] The present invention relates to a device and a method for frequency stabilization of a laser, in particular to a device and a method for frequency stabilization of a laser based on the spectroscopy of a temperature-stabilized volume Bragg grating (VBG).

[0004] State of the art

[0005] Telecommunications and quantum sensing applications require frequency references for stabilizing lasers with a specific power profile. Especially for applications on mobile platforms in space, such frequency references must be as compact as possible, tunable without mode hops, and long-term stable, while achieving a (reproducible) frequency accuracy of approximately 50 MHz. Corresponding frequency reference modules for implementing space-capable frequency references with these properties are not yet known in the state of the art.

[0006] Prior art frequency references are typically based on the spectroscopy of a fiber Bragg grating (FBG) and are known to those 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 Laser,” 2006 International Conference on Transparent Optical Networks, 2006, pp. 167-169). As shown in the figure taken from Sotor et al. and depicted in Fig. 1, the laser radiation is coupled into an FBG via an optical fiber-based beam splitter. The diffracted signal is split off via an optical fiber-based beam splitter and used with the transmitted signal to generate an error signal for frequency stabilization of a laser.To ensure the thermal stability of the frequency-selective element, which is required for high frequency stability of the laser, the FBG is enclosed in an athermal housing.

[0007] However, a fiber optic-based spectroscopy approach has significant disadvantages. In particular, fiber optics can change their properties, especially during long-term operation, under the influence of irradiation, mechanical stress, and temperature fluctuations. This leads to unacceptable measurement errors in highly sensitive sensor applications. When using a waveguide-based beam splitter, these influences also lead to a 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 spectroscopy signal during operation. These systematic errors lead to frequency errors. The use of an athermal housing is intended to achieve thermal decoupling of the FBG to achieve high frequency stability.However, this approach prevents the possibility of tuning the grating's frequency by controlling its temperature. FBG-based frequency stabilization is therefore unsuitable for applications on mobile platforms in space, and other stabilization concepts must be used.

[0008] Disclosure of the invention

[0009] It is therefore an object of the present invention to provide a device for frequency stabilization of a laser that enables sufficient laser stability for the aforementioned mobile applications in space with the performance profile listed above. Furthermore, a corresponding method for frequency stabilization of a laser is to be provided.

[0010] These objects are achieved according to the invention by the features of patent claims 1, 13, and 15. Expedient embodiments of the invention are contained in the dependent claims. The features listed individually in the patent claims can be combined with one another in a technologically expedient manner and can be supplemented by explanatory facts from the description and / or details from the figures, whereby further embodiments of the invention are shown.

[0011] A first aspect of the invention relates to a device for frequency stabilization of a laser, comprising a beam path for coupling laser radiation emitted by the laser onto a frequency-selective element, wherein the frequency-selective element is temperature-controlled, wherein the frequency-selective element is a volume Bragg grating, VBG, with a plurality of grating structures and, before coupling the laser radiation into the VBG, a portion of the laser radiation is branched off into a reference beam via an input facet and the portion of the laser radiation coupled into the VBG forms a measuring beam.

[0012] A beam path is understood in particular to mean the guidance of the laser radiation as a so-called free beam, i.e. the laser radiation is not coupled to the frequency-selective element by means of an optical fiber or a waveguide but, for example, as a freely propagating Gaussian beam (free beam). The temperature of the frequency-selective element is controllable, i.e. it can be varied relative to the environment by means of appropriate control. Since the filter frequency of a frequency-selective element is generally dependent on its temperature, temperature control of the frequency-selective element therefore also includes controllability of the filter frequency (center frequency) of the frequency-selective element. For rapid control of the temperature of the VBG, the use of a Peltier element is advantageous because, in contrast to a heating element, this also enables inherent active cooling in conjunction with additional cooling.When using a heating element, however, the thermal connection of the VBG must be specifically optimized for fast and effective cooling.

[0013] A VBG is a special form of optical grating. In contrast to mechanical gratings (ruled gratings) or holographically produced planar gratings (holographic gratings), which are typically designed as surface gratings, VBGs are generally so-called thick gratings inside a material volume. VBGs have a large number of grating structures arranged one behind the other. In the context of this invention, the individual reflective layers of a grating are referred to as grating structures. These can, for example, be the individual grating planes of a VBG consisting of several grating planes arranged one behind the other in a suitable material. However, the individual grating structures can also have non-planar shapes, for example curved surface shapes.The spacing of the grating structures can vary to provide a grating with a broad spectral distribution (so-called "chirped grating"). A VBG can also comprise several separate or at least partially merging gratings with corresponding grating structures. VBGs are typically generated holographically; the corresponding filter elements are then also referred to as volume holographic Bragg gratings (VHGs). However, VBG structures can also be generated non-holographically, for example, by inscribing them with fs laser pulses via a phase mask in glass.

[0014] The VBG is preferably formed as a volume holographically generated Bragg grating in a photothermorefractive glass. In preferred embodiments, a volume holographic Bragg grating based on dichroic gelatin, photopolymers, photorefractive crystals, or a silver halide emulsion can also be used as the VBG medium. An important criterion for applications on mobile platforms in space is, in particular, the long-term stability of the VBG and, in particular, minimal influence from irradiation, mechanical stresses, temperature fluctuations, and aging effects. According to the invention, a portion of the laser radiation is diverted into a reference beam before coupling the laser radiation into the VBG, which occurs via an input facet of the VBG.The expression “before coupling” is to be understood in relation to positions along the beam path; a branching can therefore also occur at the same time as the coupling, i.e. at or during the physical process of coupling. A branching of the reference beam can occur in particular by a direct reflection at the input facet of the VBG during coupling. Alternatively, the reference beam can also be branched off from the free beam directed at the VBG by a beam splitter arranged in front of the VBG. The reference beam can be detected with a corresponding reference detector. The portion of the laser radiation coupled into the VBG, on the other hand, forms a measuring beam. The measuring beam can preferably be detected either as a transmitted measuring beam behind the VBG or as a diffracted measuring beam in front of the VBG.Both the transmitted and the diffracted measuring beam can be reflected multiple times within the VBG (e.g. at the outer surfaces of the VBG).

[0015] Preferably, the VBG is arranged in a Littrow configuration. In such a configuration, the laser radiation impinges perpendicularly on the grating structures of the VBG. If the VBG is operated in a Littrow configuration, the frequency value of the device according to the invention (center frequency of the VBG) is first-order insensitive to tilts of the incident laser beam.

[0016] Preferably, the input facet of the VBG has an angle other than 90° to the beam axis of the incident laser radiation and / or an angle to the grating structures 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 portion of the incident laser radiation reflected at the input facet of the VBG is directed in a direction deviating from the direction of incidence. This prevents direct reflection back into the laser, which could lead to interference. Furthermore, a portion of the laser radiation can be branched into a separate reference beam via the reflection immediately before coupling into the VBG. The spatial separation of the incident beam and the beam reflected by the input facet eliminates the need for an additional beam splitter.Dielectric beam splitters have highly reflective coatings whose properties are temperature-dependent and change over time (aging effects). Particularly preferred is a portion of the incoming laser radiation diverted by a Fresnel reflection at the input facet of the beam splitter. Depending on the refractive index of the beam splitter medium and the angle of incidence, a specific angle between the laser radiation and the grating structures of the beam splitter can be adjusted. In particular, this allows the beam splitter to be optimally adapted to a specific Littrow configuration, whereby in the Littrow configuration, the beam reflected by the input facet is not reflected in the direction of the incoming beam.

[0017] The VBG preferably comprises a mounting surface, a top surface opposite the mounting surface, and two side surfaces connecting the mounting surface and the top surface; wherein a lattice vector of the VBG and the top surface are formed parallel to the mounting surface and / or the side surfaces are formed parallel to one another. The VBG can additionally comprise a front surface connecting the mounting surface and the top surface, through which the laser radiation preferably enters (input facet), and a rear surface connecting the mounting surface and the top surface, which is opposite the input facet. The side surfaces of the VBG are preferably aligned parallel to one another with a tolerance of + / -100', more preferably + / -10', in order to minimize the volume of the VBG. The lattice vector, which is orthogonal to the lattice structures (e.g.Both the top surface (i.e., flat or slightly curved surfaces with the same refractive indices) and the top surface are preferably aligned 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 invention. The angle between the grating vector of the VBG and the beam axis of the refracted beam (measurement beam after coupling into the VBG) preferably has a tolerance of + / -40', more preferably + / -4'. A narrower tolerance of the angle between the grating vector of the VBG and the beam axis of the refracted beam allows for greater freedom in the integration process.

[0018] The angle between the beam axis of the incident beam and the input facet of the VBG is preferably + / -10°, more preferably + / -1°. Preferably, the input facet of the VBG has an angle of 45° + / -1° to the beam axis of the incident light beam. Using a 45° angle between the beam axis of the incident beam and the input facet of the VBG simplifies mechanical integration, whereas a tighter tolerance of the angle between the beam axis of the incident beam and the input facet of the VBG allows for a more predictable detector signal level due to the angular dependence of the Fresnel reflection, thus enabling the use of smaller and faster detectors.

[0019] In a further preferred embodiment, the angle between the beam axis of the incident beam and the input facet of the VBG is not 45°, but takes on a different value, whereby the incident beam and the beam reflected at the input facet are spatially separated. In a further preferred embodiment, the beam incident on the input facet of the VBG and the beam reflected at the input facet spatially overlap. In this case, the reference beam can be generated by a beam splitter arranged in front of the VBG. Preferably, the VBG is designed as a parallelepiped, and the grating structures of the VBG are each arranged perpendicular to the mounting surface, the top surface, and the side surfaces of the VBG. In this case, both the integration process of the device according to the invention is simplified and the required volume and thus its thermal load are reduced.A parallelepiped can already provide the bevel of the surface relative to the grating structures necessary for a Fresnel reflection at the input facet.

[0020] Preferably, the reference beam is branched off at a surface of the VBG (preferably via a Fresnel reflection at the input facet) or a beam splitter arranged in front of the VBG. In the case of a Fresnel reflection, a dielectric coating of the input facet to reduce reflections is not necessary. Eliminating a dielectric coating on the input facet of the VBG avoids temperature and aging effects on reflection and transmission.

[0021] The device according to the invention preferably further comprises a reference detector configured to determine an intensity of the reference beam and a measuring detector configured to determine an intensity of the measuring beam, wherein the intensity of the measuring beam is determined after passing through the VBG. The measuring beam can be reflected multiple times within the VBG on the outer surfaces. In particular, before the measuring beam is detected, it can pass through the VBG once, twice, or more than once. Preferably, the measuring detector detects a signal transmitted by the VBG (transmission signal). Alternatively, however, a light beam diffracted by the VBG can also be detected by the measuring detector (diffraction signal). The detectors are preferably photodiodes.

[0022] Preferably, the device according to the invention 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. An error signal is understood to be a signal with a functional dependency that correlates with the control deviation. Typically, the error signal has a characteristic operating point suitable for a control circuit (e.g., at a zero crossing).

[0023] To derive the error signal, the reference signal and a diffraction signal can also be used instead of the reference signal and a transmission signal. In further embodiments, a transmission signal and a diffraction signal or the reference signal, a transmission signal and a diffraction signal can be used to derive an error signal. Furthermore, only a diffraction signal or only a transmission signal can be used to derive an error signal, although in both cases a constant input level is required. Preferably, the VBG is thermally decoupled from its environment via a grid housing. A corresponding grid housing can enable improved thermal and optical insulation of the VBG. The grid housing can be connected to an optical bench on which the VBG is arranged with other components. The grid housing can comprise temperature stabilization.Such a design enables a lower temperature gradient in the environment of the VBG relevant for thermal radiation.

[0024] Preferably, the VBG is arranged with other components in a module housing. In particular, the VBG can be arranged with or without an additional grid housing with other components (e.g., mirrors, polarizers, and / or an optical bench) in the module housing. Such an embodiment enables better thermal and acoustic insulation of the VBG. The module housing can be thermally decoupled from its environment and include temperature stabilization.

[0025] Preferably, at least one cooling and / or heating device is used to stabilize the temperature of the VBG. A cooling and / or heating device can preferably comprise a Peltier element or a heating element. When using a heating element, the thermal connection of the VBG (or elements connected thereto) can be specifically optimized for rapid and effective cooling. The cooling can, for example, be radiation cooling relative to space. It is further preferred that the detectors used for signal acquisition according to the invention are arranged with the VBG on a common Peltier element or a thermally connected common platform. Such an embodiment enables a reduction in sensitivity to beam misalignment, thereby increasing the accuracy of the device according to the invention.

[0026] The above-mentioned embodiments can advantageously be combined in whole or in part.

[0027] A second aspect of the present 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 an intensity of the reference beam and an intensity of the measuring beam after passing through the VBG (at least once), and the frequency of the laser is stabilized with the error signal via a control loop.

[0028] In particular, the temperature of the device according to the invention can be controlled via a cooling and / or heating device. This control can affect the VBG locally or other components, such as an optical bench on which the VBG is arranged with other components. A combination of several temperature control approaches is also possible. Temperature stabilization of the optical bench increases the accuracy of the method according to the invention.

[0029] Preferably, the spectrally filtered laser radiation from the VBG is fed back into the laser to form an extended cavity for the laser. In particular, this allows the realization of a frequency-stabilized diode laser with an extended cavity (ECDL). An associated control loop can then be used to adjust, for example, the injection current or the laser temperature to set 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 an intensity of the reference beam and an intensity of the measuring beam after passing through the VBG (at least once). The temperature of the VBG is adjusted via a control loop with the error signal such that a Bragg frequency (center frequency) of the VBG corresponds to the frequency of the laser except for an optionally selected, signed frequency difference. The frequency of the laser is determined from the set temperature of the VBG using a known assignment. The Bragg frequency (center frequency) of the VBG can thus also be adjusted directly to the frequency of the laser.

[0031] The method for measuring the frequency of a laser and the method according to the invention for frequency stabilization of a laser differ essentially only in the controlled variable, but are otherwise based on a common inventive idea. While for frequency stabilization of a laser, the error value derived according to the invention can be used to control the temperature and / or the injection current of the laser in order to set and stabilize a specific laser frequency, for measuring the frequency of a laser the temperature of the VBG can be stabilized with the aid of temperature control such that a Bragg frequency (center frequency) of the VBG corresponds to the frequency of the laser except for an optionally selected, signed frequency difference.The temperature of the VBG determined by a temperature sensor can then be used to determine the frequency of the laser using a known assignment and displayed, for example, in a display device.

[0032] Furthermore, further preferred embodiments of the method according to the invention result directly from the features mentioned in the description of the device according to the invention.

[0033] The present invention can be used in quantum sensors to stabilize lasers more quickly and reliably to a desired wavelength, for example, an atomic transition. The device according to the invention can thus facilitate, for example, the establishment of communication (lock acquisition) between two satellites for optical satellite communications.

[0034] While in the prior art, the light signals are guided in optical fibers, in the present invention they propagate preferably in free space. The present invention is characterized in particular by the following structural differences:

[0035] 1. A VBG is used as the frequency-selective element instead of an FBG.

[0036] 2. The separation of the light signals before the FBG is not performed by an optical fiber-based beam splitter. A portion of the incoming light signal is preferably branched off from the rest of the signal by a Fresnel reflection at the input facet of the VBG. For this purpose, the input facet of the VBG can be positioned at an angle to the incoming beam, so that the Fresnel reflection and the incoming beam are spatially separated. The error signal can preferably be derived from the transmitted and branched incoming light signals.

[0037] 3. The VBG can be arranged on an optical bench or, more generally, on a suitable support, the temperature of which can be stabilized to a predetermined value by means of a suitable cooling and / or heating device and a temperature sensor.

[0038] Compared to the prior art, the device according to the invention is significantly more robust against irradiation, mechanical stress, and temperature changes because no fiber optic components are used at the points particularly critical for measurement accuracy. Preferred branching of the reference beam by Fresnel reflection enables the splitting ratio to be largely independent of temperature influences. With the aid of a Peltier element and the omission of an athermal housing, the VBG's frequency can be tuned directly via temperature. By miniaturizing the device and adapting the shape of the VBG, greater mechanical stability can be achieved, combined with lower weight and a reduced form factor. An optional module housing allows the device according to the invention to be largely shielded from acoustic and thermal influences.Furthermore, high frequency accuracy with simultaneous frequency tuning can be achieved through staggered temperature stabilization of the VBG, for example, using a Peltier element beneath the VBG, a grid housing around the Peltier element, another Peltier element for stabilizing an optical bench, and a module housing around the optical bench. Further preferred embodiments of the invention emerge from the features mentioned in the respective subclaims.

[0039] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0040] Brief description of the drawings

[0041] The invention and the technical environment are explained in more detail below with reference to the accompanying figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments cited. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description. They show:

[0042] Fig. 1 is a schematic diagram of a conventional device for frequency stabilization of a laser according to the prior art (from Sotor et al.);

[0043] Fig. 2 is a schematic representation of a first embodiment of an apparatus according to the invention for frequency stabilization of a laser;

[0044] Fig. 3 is a schematic representation of a second embodiment of a device according to the invention for frequency stabilization of a laser;

[0045] Fig. 4 is a schematic representation of a third embodiment of an apparatus according to the invention for frequency stabilization of a laser;

[0046] Fig. 5 is a schematic representation of a fourth embodiment of an apparatus according to the invention for frequency stabilization of a laser;

[0047] Fig. 6 is a schematic representation of a VBG geometry according to the invention;

[0048] Fig. 7 is a schematic representation for implementing a first embodiment of a method according to the invention for frequency stabilization of a laser using a device according to Fig. 2;

[0049] Fig. 8 is a schematic representation of the signals according to the embodiment of Fig. 7;

[0050] Fig. 9 is a schematic representation for implementing a second embodiment of a method according to the invention for frequency stabilization of a laser using a device according to Fig. 2; Fig. 10 is a schematic representation for implementing a third embodiment of a method according to the invention for frequency stabilization of a laser using a device according to Fig. 2;

[0051] Fig. 11 is a schematic representation for implementing an embodiment of a method according to the invention for measuring the frequency of a laser using a device according to Fig. 2;

[0052] Fig. 12 is a schematic representation of the signals corresponding to Fig. 11; and

[0053] Fig. 13 is a schematic representation of an assignment of the temperature of the VBG to the frequency of the laser according to Fig. 11.

[0054] Detailed description of the drawings

[0055] Figure 1 shows a schematic representation of a conventional device for frequency stabilization of a laser according to the prior art (from Sotor et al.). The laser (“stabilized laser”) comprises a laser crystal (Nd:YAG / KTP) temperature-controlled by a Peltier element (TEC - “thermoelectric cooler”), which emits double-resonant radiation at emission wavelengths of 1064 nm and 532 nm. The laser crystal is pumped by a diode laser (“pumping diode”) with an emission wavelength of 808 nm. The radiation component at 1064 nm is branched off via a dichroic beam splitter (“dichroic mirror”) and coupled via a fiber coupler into an FBG (“ather-housed FBG”) arranged within an athermal housing.Using appropriately arranged PIN photodiodes (PIN diodes), the intensity of the reflected signal and the transmitted signal is determined, and both signals are fed to an electronic circuit for regulating the temperature of the laser crystal (Automatic Frequency Control). The quality of the laser's frequency stabilization depends primarily on the stability of the FBG and the associated fiber sections within the device.

[0056] Figure 2 shows a schematic representation of a first embodiment of an inventive device for frequency stabilization of a laser. A light signal is coupled into the device via a polarization-maintaining single-mode optical waveguide (PMSF) F1. The PMSF F1 allows easy integration of the inventive device into existing optical systems. The coupled light signal is collimated using an optic L1. A polarizer R1 ensures that only s-polarized light is directed onto the VBG G1 by the mirrors S1 and S2. The light beam can be directed such that, as shown in Fig. 6, the VBG G1 can be operated in a Littrow configuration (i.e., perpendicular incidence of the light onto the grating structures E) with a tolerance of + / -1°.The polarizer R1 prevents any polarization fluctuations that could lead to systematic measurement errors. Beam guidance via the two mirrors S1 and S2 offers greater freedom in positioning the VBG than using only one or no mirrors.

[0057] The VBG G1 can be attached to a first cooling and / or heating device T1 with a temperature sensor N1 for temperature stabilization, for example. The reference detector P1 and the measurement detector P2 can also be attached to the cooling and / or heating device T1 to ensure stable beam alignment. The PMSF F1, the optics L1, the polarizer R1, the mirrors S1 and S2, and the cooling and / or heating device T1 are preferably integrated on a common optical bench B1 by means of adhesive bonding. A grid housing H1 can be thermally conductively connected to the optical bench B1 to improve the temperature stability of the device according to the invention. The optical bench B1 can also be stabilized to a desired temperature using a second cooling and / or heating device T2.

[0058] An optional module housing H2 can be used to shield the optical bench B1 from thermal and acoustic interference from the environment. The optics L1, the polarizer R1, the mirrors S1 and S2, and the VBG G1 are preferably micro-optical components. The use of micro-optical components is preferred due to the associated smaller form factor, reduced weight, and increased mechanical stability of the device according to the invention. The cooling and / or heating devices T1, T2 can preferably comprise Peltier elements or heating elements.

[0059] Figure 3 shows a schematic representation of a second embodiment of an inventive device for frequency stabilization of a laser. The representation shown largely corresponds to Fig. 2; the reference numerals and their assignment therefore apply accordingly. In this embodiment, the intensity of the light beam diffracted by the VBG G1 is determined with the measuring detector P3 (diffraction signal). To generate an error signal, the reference signal and a diffraction signal can thus be used instead of the reference signal and a direct transmission signal. The intensity of the diffracted beam can be determined with the measuring detector P3 after reflection at a beam splitter A1 upstream of the VBG G1.

[0060] Figure 4 shows a schematic representation of a third embodiment of a device according to the invention for frequency stabilization of a laser. The representation shown largely corresponds to Figure 3; the reference numerals and their assignments therefore apply accordingly. If the incident and diffracted beams do not spatially overlap, 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 a measuring detector P4.

[0061] Figure 5 shows a schematic representation of a fourth embodiment of an apparatus according to the invention for frequency stabilization of a laser. The representation shown largely corresponds to Fig. 4; the reference numerals and their assignment therefore apply accordingly. As an alternative to determining the intensity of the diffracted beam with a measuring detector P4, this can also be carried out, for example, via a beam coupled out via a side surface (e.g., side surfaces A, B) of the VBG G1 with a measuring detector P5. The corresponding side surface of the VBG G1, from which the diffracted beam emerges after reflection at the input facet, must in this case have an optical quality.

[0062] Figure 6 shows a schematic representation of a VBG geometry according to the invention. The input facet of the VBG G1 preferably has an angle of 45° + / -1° to the beam axis of the incident light beam and is uncoated. The side surfaces A, B of the VBG G1 are preferably aligned parallel to each other with a tolerance of + / -10' to minimize the volume of the VBG G1. The grating vector, which is orthogonal to the grating structures E of the same refractive indices, as well as the top surface D are preferably aligned parallel to the mounting surface C of the VBG G1 with a tolerance of + / -4' 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 facet of the VBG G1.

[0063] Figure 7 shows a schematic representation for implementing a first embodiment of a method according to the invention for frequency stabilization of a laser using a device according to Figure 2. The representation shown thus largely corresponds to Figure 2; the reference numerals and their assignments therefore apply accordingly. The light from a laser D1 is coupled, for example, into the optical waveguide F1 of a device according to the invention using an optical system L2, so that the frequency of the laser D1 can be stabilized via appropriate control.

[0064] For example, the beam reflected at the input facet of the VBG G1 shown here strikes a reference detector P1 (reference signal), which detects the intensity of the reflected beam. A portion of the light beam incident on the input facet is refracted upon entering the VBG G1. The intensity of the beam transmitted through the VBG G1 can be detected by a measurement detector P2 (transmission signal). An error signal can be generated from the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measurement detector P2, which can be used to stabilize a laser D1 (or to measure the emission frequency of a laser D1).

[0065] For this purpose, the signals U1 and U2 of the reference detector P1 and the measurement detector P2 are processed into an error signal using a suitable electronic circuit E1. The error signal can then be processed using a suitable electronic circuit E2 and fed back to the laser G1 as a control signal to control the frequency of the laser G1 so that the frequency of the laser G1 is set to a defined value f0.

[0066] In the simplest case, the electronic circuit E1 can be realized in such a way that it amplifies the signals U1 and U2 of the reference detector P1 and the measuring detector P2 proportionally with a suitable proportionality factor and determines the difference between the signals U1 and U2 (see Fig. 8).

[0067] Figure 8 shows a schematic representation of the signals according to the embodiment of Fig. 7. Example curves of the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measuring detector P2 (photodiode signals) are plotted against the frequency of the laser D1 (laser frequency f). From this, a difference signal U2-U1 can be derived as an error signal, with Fig. 8 illustrating the first zero crossing of the error signal as a control point for the frequency of the laser G1 at a defined value f0. When the control loop is activated, the frequency of the laser G1 then adjusts to the value f0, at which the error signal has a zero crossing with either a positive or negative (as shown) slope.

[0068] Figure 9 shows a schematic representation for implementing a second embodiment of a method according to the invention for frequency stabilization of a laser using a device according to Fig. 2. The representation shown largely corresponds to Fig. 2; the reference numerals and their assignment therefore apply accordingly. This particularly concerns a method for internal frequency stabilization of an extended cavity diode laser (ECDL) using a device according to the invention.

[0069] The radiation emitted by a laser D2 is collimated using collimation optics L3 and coupled into a device according to the invention using optics L2. The coupled radiation is frequency-selectively diffracted by the VBG G1. The laser structure thus constructed oscillates at a frequency that corresponds to the frequency of one of the possible longitudinal eigenmodes of the ECDL. The VBG G1, acting as a spectrally narrowband reflector, selects one of the possible longitudinal eigenmodes for oscillation. Typically, this is the longitudinal eigenmode whose frequency is closest to the center frequency of the spectrum of the VBG G1 acting as a reflector. For use with the method described here, an embodiment of a device according to the invention is preferred in which the device does not comprise an optical fiber for coupling, but in which a free beam is coupled into the device.This avoids, in particular, parasitic feedback from the facets of the optical fiber into the laser D2, which could otherwise disrupt the operation of the ECDL.

[0070] A change in the atmospheric pressure surrounding the laser assembly, the temperature of laser D2, the injection current into laser D2, and / or a change in the temperature of the laser assembly comprising laser D2, collimation optics L3, and a device according to the invention results in a spectral detuning between the frequency of the longitudinal eigenmode of the ECDL and the center frequency of VBG G1, which acts as a spectrally narrowband reflector. If this detuning reaches the magnitude of the free spectral range of the ECDL, a mode hop occurs, i.e., a change to another longitudinal eigenmode of the ECDL occurs. The frequency of the laser oscillation changes abruptly.

[0071] If a sudden change in the frequency of the laser oscillation during operation at a constant frequency or during a desired frequency change is to be avoided, the method according to the invention can be used to stabilize the frequency of a laser. For this purpose, an error signal is generated from the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measurement detector P2 using a suitable electronic circuit E1. A suitable electronic circuit E2 can then be used to generate a control signal from this signal, which, for example, controls the injection current or the temperature of the laser D2 or the temperature of the assembly consisting of the laser D2, collimation optics L3, and the device according to the invention using a corresponding cooling and / or heating device T3. Control can also be based on a combination of these variables.This adjusts the oscillation frequency of the eigenmode of the ECDL so that it remains tuned to the frequency f0 of the device according to the invention. The oscillation frequency of the ECDL thus follows the frequency f0 of the device according to the invention.

[0072] Figure 10 shows a schematic representation for implementing a third embodiment of a method according to the invention for frequency stabilization of a laser using a device according to Fig. 2. The representation shown largely corresponds to Fig. 9; the reference numerals and their assignment therefore apply accordingly. In contrast to Fig. 9, the control signal generated by the electronic circuit E2 is used to control a cooling and / or heating device T1 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.

[0073] Figure 11 shows a schematic representation for implementing an embodiment of a method according to the invention for measuring the frequency of a laser using a device according to Fig. 2. The representation shown largely corresponds to Fig. 2; the reference numerals and their assignment therefore apply accordingly. The light from a laser D1 is coupled, for example, with the aid of an optics L2 into the optical waveguide F1 of a device according to the invention, so that the device for cooling and / or heating T1 can be controlled such that the frequency f0 of the device according to the invention (corresponds to the center frequency of the VBG or the Bragg frequency, except for an optionally selected, signed frequency difference) assumes the value h of the frequency of the laser D1. A specific value of the frequency of the laser D1 can be assigned to the temperature value thus set and measured at the temperature sensor N1.

[0074] For this purpose, the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measuring detector P2 are processed by a suitable electronic circuit E1. The error signal is conditioned by an electronic circuit E2 such that, as a control signal, it controls a cooling and / or heating device T1 such that the frequency of the device according to the invention f0 corresponds to the oscillation frequency T of the laser D1. Depending on the value fi of the oscillation frequency of the laser D1, a temperature T characteristic of the value T of the oscillation frequency of the laser D1 is established at the temperature sensor N1. With the aid of a known assignment (e.g., a calibration table K1), the value T of the oscillation frequency of the laser D1 can therefore be determined from the value of the temperature T at the temperature sensor N1 and displayed on a display device Y1.

[0075] Figure 12 shows a schematic representation of the signals corresponding to Fig. 11. The frequency f0 of the device according to the invention is stabilized to the value of the frequency of the laser D1 (laser frequency f). Exemplary curves of the difference signal U2-U1 (see Fig. 8 for determination) are shown for two different temperatures Ti and T2 of the device according to the invention. Here, too, the first zero crossing of the error signal can be used as a reference point. With the aid of a known assignment, the value fi or f2 of the oscillation frequency of the laser D1 can be determined from the value of the temperature Ti or T2 at the temperature sensor N1 (see Fig. 14).

[0076] Figure 13 shows a schematic representation of the assignment of the temperature of the VBG (G1) to the frequency of the laser according to Fig. 11. In the illustration, the temperature T of the temperature sensor N1 (TNI) is plotted against the frequency of the laser G1 (laser frequency f).

[0077] D1, D2 lasers

[0078] G1 Volume Bragg Grating (VBG, frequency-selective element)

[0079] E Lattice structures (e.g. lattice surfaces or planes)

[0080] A, B side surfaces

[0081] C Mounting surface

[0082] D Top surface

[0083] A1 beam splitter

[0084] P1, P3 reference detectors

[0085] P2, P4, P5 measuring detectors

[0086] U1 Reference signal (reference detector)

[0087] U2 measuring signal (measuring detector)

[0088] E1, E2 electronic circuits

[0089] H1 grille housing

[0090] H2 module housing

[0091] T1, T2, T3 Cooling and / or heating devices

[0092] K1 calibration table

[0093] Y1 Display device

[0094] L1, L2, L3 optics

[0095] S1, S2 mirror

[0096] F1 fiber optic cable

[0097] R1 Polarizer

[0098] N1 temperature sensor

[0099] B1 optical bench

Claims

Patent claims 1. Device for frequency stabilization of a laser (D1, D2), comprising a beam path for coupling laser radiation emitted by the laser (D1, D2) onto a frequency-selective element (G1), characterized in that the frequency-selective element (G1) is temperature-controlled, and the frequency-selective element is a volume Bragg grating, VBG, (G1) with a plurality of grating structures (E) and, before coupling the laser radiation into the VBG (G1), a portion of the laser radiation is branched off into a reference beam via an input facet and the portion of the laser radiation coupled into the VBG (G1) forms a measuring beam.

2. Device according to claim 1, wherein the VBG (G1) is formed as a volume holographically generated Bragg grating in a photothermorefractive glass.

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

4. Device according to one of the preceding claims, wherein the input facet of the VBG (G1) has an angle other than 90° to the beam axis of the incident laser radiation and / or has an angle to the grating structures (E) of the VBG (G1).

5. Device according to one of the preceding claims, wherein the VBG (G1) comprises a mounting surface (C), a top surface (D) opposite the mounting surface (C), and two side surfaces (A, B) connecting the mounting surface (C) and the top surface (D); wherein a grid vector of the VBG (G1) and the top surface (D) are formed parallel to the mounting surface (C) and / or the side surfaces (A, B) are formed parallel to one another.

6. Device according to claim 5, wherein the VBG (G1) is designed as a parallelepiped and the grid structures (E) of the VBG (G1) are each arranged perpendicular to the mounting surface (C), to the top surface (D) and the side surfaces (A, B) of the VBG (G1).Device according to one of the preceding claims, wherein the reference beam is branched off at a surface of the VBG (G1) or a beam splitter (A1) arranged in front of the VBG (G1). Device according to one of the preceding claims, further comprising a reference detector (P1, P3) configured to determine an intensity of the reference beam and a measuring detector (P2, P4, P5) configured to determine an intensity of the measuring beam, wherein the intensity of the measuring beam is determined after passing through the VBG (G1). 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 measuring signal (U2) of the measuring detector (P2, P4, P5). Device according to one of the preceding claims, wherein the VBG (G1) is thermally decoupled from its surroundings via a grid housing (H1).Device according to one of the preceding claims, wherein the VBG (G1) is arranged with other components in a module housing (H2). Device according to one of the preceding claims, wherein at least one device for cooling and / or heating (T1, T2, T3) is used to stabilize the temperature of the VBG (G1). Method for frequency stabilizing a laser (D1, D2) using a device according to one of claims 1 to 12, wherein an error signal is derived from an intensity of the reference beam and an intensity of the measuring beam after passing through the VBG (G1), and the frequency of the laser (D1, D2) is stabilized with the error signal via a control loop. Method for frequency stabilizing a laser (D2) according to claim 13, wherein the laser radiation spectrally filtered by the VBG (G1) is fed back into the laser (G1) to form an extended resonator for the laser (D2).Method for measuring the frequency of a laser (D1) using a device according to one of claims 1 to 12, wherein an intensity of the reference beam and an intensity of the measuring beam after passing through the VBG (G1). an error signal is derived, the temperature of the VBG (G1) is adjusted via a control loop with the error signal in such a way that a Bragg frequency of the VBG (G1) corresponds to the frequency of the laser (D1) up to an optionally selected, signed frequency difference, and the frequency of the laser (D1) is determined from the set temperature of the VBG (G1) by means of a known assignment.

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