Laser frequency stabilization device, frequency stabilization method, and frequency measurement method

Abstract

The present invention relates to a laser frequency stabilization apparatus and method, and particularly to a laser frequency stabilization apparatus and method based on the spectroscopy of a temperature-stabilized volume holographic diffraction grating (VBG). The apparatus according to the present invention is a laser frequency stabilization apparatus for a laser (D1, D2), comprising a beam path for incident a laser beam emitted by the laser (D1, D2) on a frequency selection element, the frequency selection element being temperature-controlled, the frequency selection element being a VBG (G1) which is a volume holographic diffraction grating having a plurality of diffraction grating structures (E), before the laser beam is incident on the VBG (G1) through an input surface, a part of the laser beam is branched into a reference beam, and a part of the laser beam incident on the VBG (G1) forms a measurement beam. (FIG. 2)

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for frequency stabilizing a laser, and more particularly to an apparatus and method for frequency stabilizing a laser based on temperature stabilized volume Bragg grating (VBG) spectroscopy. [Background technology]

[0002] Telecommunications and quantum sensor applications require frequency standards for stabilizing lasers with specific performance profiles. In particular, for applications on mobile platforms in space, such frequency standards must be as compact as possible, tunable without mode hopping, and stable over time, with a (reproducible) frequency accuracy of around 50 MHz. Corresponding frequency reference modules for implementing space-spec frequency standards with such characteristics are not yet known in the prior art.

[0003] Prior art frequency standards are generally based on fiber Bragg grating (FBG) spectroscopy, and are known to those skilled in the art, for example, by 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 FIG. 1 from Sotor et al., a laser beam is injected into the FBG via a waveguide-based beam splitter. The diffracted signal is split via a waveguide-based beam splitter and used to generate a disturbance signal for stabilizing the laser frequency together with the transmitted signal. To ensure the thermal stability of the frequency-selective element required to improve the laser frequency stability, the FBG is enclosed in an athermal housing.

[0004] However, optical waveguide-based spectroscopy has significant drawbacks. In particular, the properties of optical waveguides can change over time due to irradiation, mechanical stress, and temperature fluctuations. This leads to unacceptable measurement errors in highly sensitive sensor applications. When using waveguide-based beam splitters, these effects also cause the splitting ratio of the waveguide-based beam splitter to be temperature- and polarization-dependent. Variations in the splitting ratio can lead to incorrect interpretation of the spectroscopic signal during operation. Such systematic errors can result in frequency errors. The use of athermal housings aims to achieve thermal decoupling of the FBG to achieve high frequency stability. However, this method does not allow for temperature-controlled tuning of the diffraction grating frequency. Therefore, FBG-based frequency stabilization is unsuitable for applications on mobile space platforms, and other stabilization concepts must be used for this purpose. Summary of the Invention

[0005] It is therefore an object of the present invention to provide a device for stabilizing the frequency of a laser, which allows stabilization of the laser sufficiently for space mobile applications having the aforementioned performance profile, and further to provide a corresponding method for frequency stabilization of a laser.

[0006] These objects are achieved according to the invention by the features of claims 1, 13 and 15. Advantageous configurations of the invention are set out in the dependent claims. The features individually set out in the claims can be combined with one another in a technically meaningful way and can be supplemented by explanatory facts from the description and / or details from the drawings, which show further variant embodiments of the invention.

[0007] A first aspect of the present invention is a laser frequency stabilization device comprising a beam path for causing a laser beam emitted by the laser to enter a frequency selective element, the frequency selective element being temperature controlled and being a VBG which is a volume holographic diffraction grating having a plurality of diffraction grating structures, and before the laser beam enters the VBG through an input surface, a portion of the laser beam is branched into a reference beam, and a portion of the laser beam that has entered the VBG forms a measurement beam.

[0008] The beam path is understood to mean in particular the guidance of the laser beam as a "free beam", i.e. the laser beam is not injected into the frequency selective element via an optical fiber or a waveguide, but rather is injected, for example, as a freely propagating Gaussian beam (free beam).

[0009] The temperature of the frequency-selective element is controllable, i.e., can be varied in relation to the environment with appropriate control. Since the filter frequency of a frequency-selective element generally depends on its temperature, temperature control of the frequency-selective element includes the ability to control the filter frequency (center frequency) of the frequency-selective element. Peltier elements are advantageously used for rapid temperature control of the VBG because they are inherently capable of active cooling, unlike heating elements that require additional cooling. However, when heating elements are used, the thermal connections of the VBG must be specifically optimized for rapid and effective cooling.

[0010] A VBG is a specific type of optical diffraction grating. While mechanical diffraction gratings ("ruler diffraction gratings") and holographically generated plane diffraction gratings ("holographic diffraction gratings") are typically formed as surface gratings, VBGs are generally "thick diffraction gratings" embedded within a material. A VBG consists of multiple diffraction structures arranged next to each other. Herein, the individual reflective layers of a diffraction grating are referred to as grating structures. These structures are, for example, individual grating planes of a variable band grating, which consists of multiple grating planes arranged adjacent to each other in a suitable material. However, individual grating structures may have non-planar forms, such as curved surface shapes. The spacing between the grating structures can be varied to provide a grating with a broadband spectral distribution (called a "chirped grating"). A VBG may also comprise multiple gratings that are separate from each other, or at least some of which may merge with each other and have corresponding grating structures. VBGs are typically generated holographically, and the corresponding filter elements are also called volume holographic gratings (VHGs). However, VBG structures can also be generated by non-holographic methods, for example by inscribing femtosecond laser pulses into glass through a phase mask.

[0011] The VBG preferably takes the form of a volume holographically generated Bragg grating in photothermorefractive glass. In preferred embodiments, volume holographic diffraction gratings based on dichroic gelatin, photopolymers, photorefractive crystals, or silver halide emulsions can also be used as the VBG medium. One of the important criteria for applications on space-based mobile platforms is the long-term stability of the VBG, especially its insensitivity to irradiation, mechanical stress, temperature fluctuations, and aging.

[0012] According to the present invention, a portion of the laser beam is split off as a reference beam before it enters the VBG. This splitting occurs via the input face of the VBG. The expression "before it enters" should be understood in relation to a position along the beam path. Therefore, splitting can also be performed in parallel with the entrance, i.e., during or during the physical process of the entrance. For this purpose, a particularly effective method for splitting the reference beam is to directly reflect it at the input face of the VBG upon entrance. Alternatively, the reference beam can be split off from the free beam by a beam splitter arranged upstream of the VBG, resulting in a beam directed towards the VBG. The reference beam can be detected by a suitable reference detector. Meanwhile, the portion of the laser beam that enters the VBG forms a measurement beam. The measurement beam can be detected preferably as a transmitted measurement beam downstream of the VBG or as a diffracted measurement beam upstream of the VBG. Both the transmitted measurement beam and the diffracted measurement beam can also be repeatedly reflected within the VBG (e.g., outside the VBG).

[0013] The VBG is preferably configured in a Littrow configuration. In such a configuration, the laser beam is incident perpendicularly on the diffraction grating structure of the VBG. When the VBG is configured and operated in a Littrow configuration, the value of the frequency of the device according to the invention (the center frequency of the VBG) is first-order insensitive to the tilt of the incident laser beam.

[0014] The input face of the VBG preferably forms an angle other than 90° with respect to the beam axis of the incident laser beam and / or the diffraction grating structure of the VBG. When the input face of the VBG has an angle other than 90° with respect to the beam axis of the incident laser beam, a portion of the reflected light of the incident laser beam at the input face of the VBG is directed in a direction other than the incident direction. This means that there is no direct back reflection back to the laser, which would cause interference. Furthermore, a portion of the laser beam can be split into a separate reference beam by reflection just before entering the VBG. Because the incident beam and the beam reflected at the input face are spatially separated, an additional beam splitter is not required. A dielectric beam splitter, i.e., one with a highly reflective coating that is temperature-dependent and changes over time (aging effect), is particularly preferred. Part of the incident laser beam is split by Fresnel reflection at the input face 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 beam and the diffraction grating structure of the VBG. In particular, this allows the VBG to be optimally adapted to a particular Littrow geometry, in which the beam reflected at the input face is not reflected back in the direction of the incident beam.

[0015] 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, with the grating vector and the top surface of the VBG being parallel to the mounting surface and / or the side surfaces being parallel to each other. The VBG may further comprise a front surface (input surface) connecting the mounting surface and the top surface, from which the laser beam is preferably incident, and a rear surface connecting the mounting surface and the top surface opposite the input surface. The side surfaces of the VBG are preferably oriented parallel to each other with a tolerance of ±100°, more preferably ±10°, to minimize the volume of the VBG. The grating vector, which is orthogonal to the grating structure (e.g., a flat or slightly curved surface having 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°, 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 entering the VBG) preferably has a tolerance of ±40', more preferably ±4'. A tighter tolerance on the angle between the grating vector of the VBG and the beam axis of the refracted beam allows for more leeway during the integration process.

[0016] The angle between the beam axis of the incident light beam and the entrance surface of the VBG is preferably ±10°, more preferably ±1°. The input facet of the VBG preferably has an angle of 45°±1° with respect to the beam axis of the incident light beam. A 45° angle between the beam axis of the incident light beam and the input facet of the VBG simplifies mechanical integration, while a tighter tolerance on the angle between the beam axis of the incident light beam and the input facet of the VBG results in more predictable detector signal levels due to the angular dependence of Fresnel reflections, thus allowing the use of smaller and faster detectors.

[0017] 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 instead has another value, and the incident beam and the beam reflected by 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 by the input facet are spatially overlapped. In this case, the reference beam can be generated by a beam splitter arranged upstream of the VBG.

[0018] The VBG preferably takes the form of a parallelepiped, with the grating structure of the VBG positioned perpendicular to the mounting surface, top surface, or side surface of the VBG. This simplifies the integration process of the device according to the present invention, reduces the required space, and reduces the thermal load. The parallelepiped itself can provide the surface tilt relative to the grating structure required for Fresnel reflection at the input surface.

[0019] The reference beam is preferably split at a beam splitter located on the surface of the VBG or upstream of the VBG. In the case of Fresnel reflection, it is possible to have no dielectric coating on the input surface to reduce reflection. By not having a dielectric coating on the input surface of the VBG, the effects of temperature and aging on reflection and transmission can be avoided.

[0020] The device according to the invention preferably further comprises a reference detector configured to determine the intensity of the reference beam and a measurement detector configured to determine the intensity of the measurement beam, the intensity of the measurement beam being determined after passing through the VBG. The measurement beam can be repeatedly reflected by outer surfaces within the VBG. In particular, the measurement beam can pass through the VBG once, twice, or multiple times. The measurement detector preferably detects a signal transmitted by the VBG (transmission signal). However, alternatively, the light beam diffracted by the VBG can also be detected by the measurement detector (diffracted signal). The detector is preferably a photodiode.

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

[0022] Instead of using a reference signal and a transmitted signal to derive the error signal, it is also possible to use a reference signal and a diffracted signal. In further embodiments, the error signal can be derived using the transmitted signal and the diffracted signal, or the reference signal, the transmitted signal, and the diffracted signal. Furthermore, it is also possible to derive the error signal using only the diffracted signal or only the transmitted signal, although in either case a certain input level is required.

[0023] The VBG is preferably thermally isolated from its surroundings by a grating housing. A suitable grating housing can enhance the thermal and optical isolation of the VBG. The grating housing can be connected to an optical bench on which the VBG is placed along with other components. The grating housing may also include temperature stabilization. Such an embodiment can result in smaller temperature gradients around the VBG associated with thermal radiation.

[0024] The VBG is preferably located within a module housing along with other components. In particular, the VBG can be located within the module housing along with other components (e.g., mirrors, polarizers, and / or optical benches) with or without an additional grating housing. Such an embodiment improves the thermal and acoustic isolation of the VBG. The module housing can be thermally isolated from the surroundings and can include temperature stabilization.

[0025] Preferably, at least one cooling and / or heating device is used for temperature stabilization of the VBG. The cooling and / or heating device preferably comprises a Peltier element or a heating element. If a heating element is used, the thermal connection of the VBG (or elements connected thereto) can be specifically optimized for fast and effective cooling. Cooling can include, for example, radiative cooling to space. It is further preferred that the detector used for signal acquisition according to the invention is located on a common Peltier element or a common, appropriately thermally connected platform with the VBG. Such an embodiment makes it possible to reduce sensitivity to beam deviations and thereby improve the accuracy of the device according to the invention.

[0026] The above embodiments can be advantageously combined with one another, either in whole or in part.

[0027] A second aspect of the invention relates to a method for frequency stabilization of a laser using an apparatus according to the invention, wherein an error signal is derived from the intensity of a reference beam and the intensity of a measurement beam after passing (at least once) through a VBG, and the frequency of the laser is stabilized by the error signal via a control loop.

[0028] In particular, the temperature of the device according to the invention can be controlled via cooling and / or heating devices. This control can be local to the VBG or can be related to other components, such as the optical bench where the VBG is located together with other components. Likewise, it is also possible to combine several temperature control approaches. Temperature stabilization of the optical bench improves the accuracy of the method according to the invention.

[0029] The laser beam spectrally filtered by the VBG is preferably injected back into the laser, forming an extended cavity of the laser. In particular, a frequency-stabilized diode laser with an extended cavity ("extended-cavity diode laser" (ECDL)) can be implemented in this way. Associated control circuits can be used to control, for example, the injection current and the temperature of the laser to adjust and stabilize the laser frequency.

[0030] A third aspect relates to a method for measuring the frequency of a laser using the apparatus according to the invention, in which an error signal is derived from the intensity of a reference beam and the intensity of a measurement beam after passing (at least once) through a VBG, the temperature of the VBG is adjusted with the error signal via a control loop so that the Bragg frequency of the VBG corresponds to the frequency of the laser except for an arbitrarily chosen signed frequency difference, and the frequency of the laser is determined in a known relationship to the adjusted temperature of the VBG, so that 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 stabilizing the frequency of a laser according to the present invention differ substantially only in terms of the controlled variables, but are otherwise based on a common inventive concept. For the purpose of stabilizing the frequency of a laser, the temperature and / or the incident current of the laser can be controlled using the error value derived according to the present invention to adjust and stabilize a specific laser frequency. For the purpose of measuring the frequency of a laser, the temperature control can be used to stabilize the temperature of the VBG. This causes the Bragg frequency (center frequency) of the VBG to coincide with the frequency of the laser except for an arbitrarily selected signed frequency difference. Based on the temperature of the VBG, adjusted or determined via a temperature sensor, the frequency of the laser can be determined by a known correlation and displayed, for example, on a display device.

[0032] Furthermore, preferred configurations of the method according to the invention are directly apparent from the features set out in the description in connection with the device according to the invention.

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

[0034] While in the prior art the optical signals are guided in optical waveguides, in the present invention the optical signals are preferably propagated in free space. The present invention is distinguished in particular by the following structural differences: 1. A VBG is used as a frequency-selective element instead of an FBG. 2. No optical waveguide-based beam splitter is used to separate the optical signal upstream of the FBG. A portion of the incident optical signal is preferably split off from the remaining signal by Fresnel reflection at the input face of the VBG. For this purpose, the input face of the VBG can be positioned at an angle to the incident beam, so that the Fresnel reflection and the incident beam are spatially separated from each other. The error signal can preferably be derived from the transmitted incident optical signal and the split incident optical signal. 3. The VBG can be placed on an optical bench or generally on a suitable carrier and its temperature can be stabilized to a predetermined value by suitable cooling and / or heating devices and temperature sensors.

[0035] Compared to the prior art, the device according to the present invention is much more resistant to radiation, mechanical stress, and temperature changes, since no optical waveguide components are used in areas that are particularly important for measurement accuracy. The splitting ratio can be made nearly independent of temperature effects, since Fresnel reflection prevails in the reference beam splitting. The use of Peltier elements and the elimination of athermal housings allows for direct VBG frequency tuning via temperature. By miniaturizing the device and appropriately shaping the VBG, higher mechanical stability can be achieved, and weight and form factor can be reduced. The optional modular housing allows for very effective shielding of the device according to the present invention from acoustic and thermal influences. Furthermore, high frequency accuracy can be achieved simultaneously with the frequency tuning option by using, for example, a Peltier element placed below the VBG, a diffraction grating housing surrounding the Peltier element, an additional Peltier element for stabilizing the optical bench, and a modular housing surrounding the optical bench for stepwise temperature stabilization of the VBG.

[0036] Further preferred configurations of the invention are revealed by the features set forth in the respective subclaims.

[0037] The various embodiments of the invention described in this application can be advantageously combined with one another, unless otherwise specified. [Brief explanation of the drawings]

[0038] The present invention and technical background will be described in more detail below with reference to the accompanying drawings. It should be noted that the present invention is not intended to be limited by the exemplary embodiments described. In particular, unless otherwise clearly stated, it is also possible to extract sub-aspects of the subject matter described in the drawings and combine them with other elements or insights of this specification. In the drawings: [Figure 1] FIG. 1 is a schematic diagram showing a conventional apparatus for frequency stabilization of a prior art laser (Sotor et al.). [Figure 2]FIG. 2 shows a schematic diagram of a first embodiment of a device according to the invention for frequency stabilization of a laser. [Figure 3] FIG. 3 shows a schematic diagram of a second embodiment of the device according to the invention for frequency stabilization of a laser. [Figure 4] FIG. 4 shows a schematic diagram of a third embodiment of the device according to the invention for frequency stabilization of a laser. [Figure 5] FIG. 5 shows a schematic diagram of a fourth embodiment of the device according to the invention for frequency stabilization of a laser. [Figure 6] FIG. 6 is a schematic diagram illustrating the geometry of a VBG according to the present invention. [Figure 7] FIG. 7 shows a schematic diagram for implementing a first embodiment of the method according to the invention for frequency stabilization of a laser using the device of FIG. [Figure 8] FIG. 8 is a schematic diagram showing signals corresponding to the embodiment of FIG. [Figure 9] FIG. 9 is a schematic diagram for implementing a second embodiment of the method according to the invention for frequency stabilization of a laser using the device of FIG. [Figure 10] FIG. 10 is a schematic diagram for implementing a third embodiment of the method according to the invention for frequency stabilization of a laser using the device of FIG. [Figure 11] FIG. 11 shows a schematic diagram for implementing an embodiment of the method according to the invention for measuring the frequency of a laser using an apparatus according to FIG. [Figure 12] FIG. 12 is a schematic diagram showing the signals according to FIG. [Figure 13] FIG. 13 is a schematic diagram showing the relationship between the temperature of the VBG and the frequency of the laser according to FIG. DETAILED DESCRIPTION OF THE INVENTION

[0039] Figure 1 shows a schematic diagram of a prior art laser frequency stabilization device (Sotor et al.). The laser ("stabilized laser") contains a temperature-controlled laser crystal (Nd:YAG / KTP) whose temperature is controlled by a Peltier element ("thermoelectric cooler") and resonates and emits at two wavelengths: 1064 nm and 532 nm. The laser crystal is pumped by a diode laser ("pump diode") emitting at 808 nm. A portion of the 1064 nm beam is split by a dichroic beam splitter ("dichroic mirror") and directed via a fiber coupler to a fiber Bragg grating (FBG) located in an athermal housing. The intensities of the diffracted radiation component ("reflected signal") and the radiation component transmitted through the fiber Bragg grating ("transmitted signal") are determined by appropriately positioned PIN photodiodes ("PIN diodes"). Both signals are fed to an electronic circuit that controls the temperature of the laser crystal ("automatic frequency control"). The quality of this laser frequency stabilization depends primarily on the stability of the fiber Bragg grating and the associated fiber section within the device.

[0040] FIG. 2 is a schematic diagram illustrating a first embodiment of an apparatus for frequency stabilization of a laser according to the present invention. An optical signal enters the apparatus through a polarization-maintaining single-mode optical waveguide (PMSF) F1. The PMSF F1 allows for easy integration of the apparatus according to the present invention into existing optical systems. The incident optical signal is collimated by an optical system L1. A polarizer R1 guides only s-polarized light to a VBGG 1, which includes mirrors S1 and S2. The optical beam is guided by configuring the VBGG 1 in a Littrow configuration (i.e., with light perpendicular to the grating structure E) to operate with a tolerance of ±1°, as shown in FIG. 6. The polarizer R1 prevents polarization fluctuations that could lead to systematic measurement errors. Beam guidance through two mirrors S1 and S2 provides greater flexibility in VBG positioning than positioning a VBG using a single mirror or no mirrors at all.

[0041] For temperature stabilization, the VBGG1 may be attached to a temperature sensor N1, e.g., a first cooling and / or heating device T1. The reference detector P1 and the measurement detector P2 may also be attached to the cooling and / or heating device T1, thereby ensuring stable beam alignment. The PMSFF1, optical system L1, polarizer R1, mirrors S1 and S1, and cooling and / or heating device T1 are preferably adhesively bonded to a common optical bench B1. The diffraction grating housing H1 may be thermally connected to the optical bench B1, thereby improving the temperature stability of the apparatus according to the present invention. The optical bench B1 may likewise be stabilized to the desired temperature by a second cooling and / or heating device T2.

[0042] An optional module housing H2 can be used to shield the optical bench B1 from ambient heat and acoustic interference. The optical system L1, polarizer R1, mirrors S1, S2, and VBGG1 preferably comprise miniature optical components. The use of miniature optical components is preferred due to the associated miniaturization, weight reduction, and improved mechanical stability of the device according to the present invention. The cooling and / or heating devices T1, T2 preferably comprise Peltier elements or heating elements.

[0043] 3 is a schematic diagram showing a second embodiment of an apparatus according to the invention for frequency stabilization of a laser. This schematic diagram generally corresponds to FIG. 2, so the reference numerals are used accordingly. In this embodiment, the intensity of the light beam diffracted by VBGG1 is determined by measurement detector P3 (diffracted signal). Therefore, instead of the reference signal and the directly transmitted signal, the reference signal and the diffracted signal can be used to generate the error signal. The intensity of the diffracted beam can be measured by measurement detector P3 after reflection by beam splitter A1 upstream of VBGG1.

[0044] 4 is a schematic diagram of a third embodiment of the device according to the invention for frequency stabilization of a laser. This diagram generally corresponds to FIG. 3, so the reference numerals correspond. If the incident and diffracted beams do not spatially overlap, there is no need to provide an additional beam splitter A1 upstream of VBGG1; instead, the intensity of the diffracted beam can also be determined directly by the measurement detector P4.

[0045] 5 is a schematic diagram of a fourth embodiment of the device for frequency stabilization of a laser according to the invention. This schematic diagram generally corresponds to FIG. 4, so the reference numerals are corresponding. Instead of determining the intensity of the diffracted beam by measurement detector P4, for example, the beam separated via a side surface (e.g., side A, B) of VBGG1 can also be measured by measurement detector P5. In this case, the corresponding side surface of VBGG1, from which the diffracted beam emerges after reflection at the entrance surface, must be of high optical quality.

[0046] FIG. 6 is a schematic diagram illustrating the geometry of a VBG according to the present invention. The input facet of the VBGG1 is preferably uncoated and has an angle of 45°±1° relative to the beam axis of the incident light beam. The side faces A and B of the VBGG1 are preferably oriented parallel to each other with a tolerance of ±10° to minimize the volume of the VBGG1. The grating vector orthogonal to the equal-index grating structure E and the top face D are preferably oriented parallel to the mounting surface C of the VBGG1 with a tolerance of ±4°. This simplifies the integration process of a VBG according to the present invention into a device. The incident light beam is preferably Fresnel-reflected at the input facet of the VBGG1.

[0047] Figure 7 is a schematic diagram for implementing a first embodiment of the method according to the invention for frequency stabilization of a laser using the device of Figure 2. This diagram generally corresponds to Figure 2, so the reference numerals correspond. Light from a laser D1 is injected into an optical waveguide F1 of the device according to the invention, here by way of example with the aid of an optical system L2, so that the frequency of the laser D1 can be stabilized by appropriate control.

[0048] The beam reflected from the input face of the illustrated VBGG1 impinges, for example, on a reference detector P1 (reference signal), which detects the intensity of the reflected beam. A portion of the light beam striking the input face is refracted as it enters VBGG1. The intensity of the beam transmitted by VBGG1 can be detected by a measurement detector P2 (transmitted signal). An error signal that can be used to stabilize the laser D1 (or measure the oscillation frequency of the laser D1) can be generated from the reference signal U1 of the reference detector P1 and the transmitted signal U2 of the measurement detector P2.

[0049] For this purpose, the signals U1, U2 of the reference detector P1 and the measurement detector P2 are processed in a suitable electronic circuit E1 to generate an error signal, which is then processed in a suitable electronic circuit E2 to generate an error signal for the laser. D as a control signal to control the frequency of the laser D 1, and the laser D The frequency of 1 is adjusted to the defined value f0.

[0050] In the simplest case, the electronic circuit E1 can be implemented to proportionally amplify the signals U1, U2 of the reference detector P1 and the measurement detector P2 using an appropriate proportionality factor and determine the difference between the signals U1 and U2 (see Figure 8).

[0051] 8 is a schematic diagram showing signals corresponding to the embodiment of FIG. 7. Exemplary curves of the reference signal U1 of the reference detector P1 and the transmitted signal U2 (photodiode signal) of the measurement detector P2 are plotted against the frequency of the laser D1 (laser frequency f). This allows the difference signal U2-U1 to be determined as the error signal, and the first zero crossing of the error signal is calculated by subtracting the first zero crossing of the laser D1 from the first zero crossing of the laser D1, as shown by way of example in FIG. 8. D The control loop can be shown as a control point for the laser frequency to reach a defined value f0. D The frequency of 1 is adjusted to a value f0 at which the error signal crosses zero on a positive slope or (as shown) a negative slope.

[0052] Figure 9 is a schematic diagram for implementing a second embodiment of a method according to the invention for frequency stabilizing a laser using the apparatus of Figure 2. This diagram corresponds substantially to Figure 2, so the reference numerals correspond. This is in particular a method for internal frequency stabilization of a diode laser with an extended resonator ("extended cavity diode laser", ECDL) using the apparatus according to the invention.

[0053] The beam emitted by laser D2 is collimated with the aid of optical collimation system L3 and then incident on the device of the present invention with the aid of optical system L2. The incident beam is frequency-selectively diffracted by VBGG1. The resulting laser configuration oscillates at a frequency corresponding to the frequency of one of the possible longitudinal eigenmodes of the ECDL. VBGG1 acts as a narrow-band reflector, selecting one of the possible longitudinal modes. This is typically the longitudinal mode whose frequency is closest to the center frequency of the spectrum of VBGG1 acting as a reflector. A preferred embodiment of the device of the present invention for use with the method described herein does not include an injection optical waveguide, but rather uses a free beam as the input to the device. This, in particular, prevents parasitic feedback from the facet of the optical waveguide back to laser D2, which could otherwise interfere with the operation of the ECDL.

[0054] Changes in the atmospheric pressure around the laser setup, the temperature of laser D2, the current incident on laser D2, and / or the temperature around the laser setup consisting of laser D2, optical collimation system L3, and the inventive device can cause a spectral detuning between the frequencies of the longitudinal eigenmodes of the ECDL and the center frequency of VBGG1, which acts as a narrowband reflector. When this detuning reaches an order of magnitude of the ECDL's free spectral bandwidth, mode hopping occurs, i.e., a shift to a different longitudinal eigenmode of the ECDL occurs, resulting in an abrupt change in the laser oscillation frequency.

[0055] The laser frequency stabilization method according to the present invention can be used to prevent abrupt changes in the laser oscillation frequency during constant frequency operation or when a desired frequency change occurs. To this end, an error signal is generated using a suitable electronic circuit E1 from the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measurement detector P2. A suitable electronic circuit E2 can then be used to generate control signals, for example, for controlling the input current to the laser D2, the temperature of the laser D2, or the temperature of the system comprising the laser D2, the optical collimation system L3, and the cooling and / or heating device T3. A combination of these parameters can also be controlled. In this way, the oscillation frequency of the ECDL's eigenmodes is adjusted to maintain its tuning to the frequency f0 of the device according to the present invention. In this way, the oscillation frequency of the ECDL follows the frequency f0 of the device according to the present invention.

[0056] Figure 10 is a schematic diagram for implementing a third embodiment of the method according to the invention for frequency stabilizing a laser using the apparatus of Figure 2. This schematic diagram generally corresponds to Figure 9, so the reference numerals correspond. In contrast to Figure 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 apparatus according to the invention. In this case, the frequency f0 of the apparatus according to the invention is stabilized to the oscillation frequency of the ECDL.

[0057] FIG. 11 is a schematic diagram for implementing an embodiment of the method according to the invention for measuring the frequency of a laser using an apparatus according to FIG. 2. This diagram generally corresponds to FIG. 2, so the reference numerals correspond. Light from laser D1 is incident on the optical waveguide F1 of the apparatus according to the invention, for example with the aid of an optical system L2. This allows the cooling and / or heating device T1 to be driven so that the frequency f0 in the apparatus according to the invention (corresponding to an arbitrarily chosen signed frequency difference of the center frequency of the VBG or Bragg frequency) assumes the value f1 of the frequency of laser D1. The temperature value measured by the temperature sensor N1 adjusted in this way can be assigned to a specific value of the frequency of laser D1.

[0058] For this purpose, the reference signal U1 of the reference detector P1 and the transmission signal U2 of the measurement detector P2 are processed in a suitable electronic circuit E1. With the help of the electronic circuit E2, the error signal is processed as a control signal for controlling the cooling and / or heating device T1 so that the frequency f0 of the device according to the invention corresponds to the oscillation frequency f1 of the laser D1. Depending on the value f1 of the oscillation frequency of the laser D1, a temperature T specific to 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 (e.g., a calibration table K1), the value f1 of the oscillation frequency of the laser D1 is thus determined from the value of the temperature T at the temperature sensor N1 and displayed on the display Y1.

[0059] FIG. 12 is a schematic diagram showing the signals according 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 T1, T2 of the device according to the invention. Again, the first zero crossover of the error signal can be used as a reference point. By known assignment, the value of the oscillation frequency f1 or f2 of the laser D1 can be determined from the values ​​of the temperatures T1, T2 at the temperature sensor N1 (see FIG. 14).

[0060] FIG. 13 is a schematic diagram showing the relationship between the temperature of the VBG (G1) and the laser frequency according to FIG. 11. In this diagram, the temperature T(T N1 ) is a laser D The frequency is plotted against the frequency of 1 (laser frequency f). [Explanation of symbols]

[0061] D1, D2: Laser G1: Volume holographic diffraction grating (VBG, frequency selective element) E: diffraction grating structure (e.g., grating plane or plane) A,B: Side C: Mounting surface D:Top surface A1: Beam splitter P1, P3: Reference detector P2, P4, P5: Measurement detectors U1: Reference signal (reference detector) U2: Measurement signal (measurement detector) E1,E2:Electronic circuit H1: Diffraction grating housing H2: Module housing T1, T2, T3: Cooling and / or heating device K1: Calibration table Y1:Display device L1, L2, L3: Optical system S1, S2: Mirror F1: Optical waveguide R1: Polarizing plate N1: Temperature sensor B1:Optical bench

Claims

1. A laser frequency stabilization device, comprising: a beam path for directing a laser beam emitted by the laser to a frequency selection element; the frequency selective element is temperature controlled; A laser frequency stabilization device in which the frequency selective element is a volume holographic diffraction grating (VBG) having a plurality of diffraction grating structures, and before the laser beam is incident on the VBG through an input surface, a portion of the laser beam is branched into a reference beam, and a portion of the laser beam incident on the VBG forms a measurement beam.

2. 10. The apparatus of claim 1, The device wherein the VBG takes the form of a volume holographically generated Bragg grating in a photothermographic refractive index glass.

3. 10. The apparatus of claim 1, The apparatus, wherein the VBG is configured in a Littrow configuration.

4. 10. The apparatus of claim 1, An apparatus wherein the input face of the VBG is at an angle other than 90° with respect to the beam axis of an incident laser beam and / or the grating structure of the VBG.

5. 10. The apparatus of claim 1, the VBG is composed of a mounting surface, an upper surface facing the mounting surface, and two side surfaces connecting the mounting surface and the upper surface; The device wherein the grating vector of the VBG and the top surface are parallel to the mounting surface, and / or the side surfaces are configured parallel to each other.

6. 6. The apparatus of claim 5, the VBG has the form of a parallelepiped; The device, wherein the diffraction grating structure of the VBG is disposed perpendicular to any one of the mounting surface, the top surface, and the side surface of the VBG.

7. 10. The apparatus of claim 1, The apparatus, wherein the reference beam is split at a beam splitter located at the surface of the VBG or upstream of the VBG.

8. 10. The apparatus of claim 1, a reference detector configured to determine the intensity of the reference beam; a measurement detector configured to determine an intensity of the measurement beam; The apparatus wherein the intensity of the measurement beam is determined after passing through the VBG.

9. 9. The apparatus of claim 8, The apparatus 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.

10. 10. The apparatus of claim 1, The apparatus wherein the VBG is thermally isolated from the surroundings by a diffraction grating housing.

11. 10. The apparatus of claim 1, The apparatus, wherein the VBG is disposed with other components within a modular housing.

12. 10. The apparatus of claim 1, The apparatus, wherein at least one cooling and / or heating device is used for temperature stabilization of the VBG.

13. 10. A method for stabilizing a laser frequency using the device of claim 1, comprising: an error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG; The method wherein the frequency of the laser is stabilized by the error signal via a control loop.

14. 14. A method for stabilizing a laser frequency according to claim 13, comprising: The laser beam spectrally filtered by the VBG is then re-injected into the laser, forming an extended cavity for the laser.

15. 10. A method for measuring the frequency of a laser using the device of claim 1, comprising: an error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG; the temperature of the VBG is adjusted with the error signal via a control loop so that the Bragg frequency of the VBG corresponds to the frequency of the laser, except for an arbitrarily selected signed frequency difference; The method wherein the frequency of the laser is determined by a known relationship with the tuned temperature of the VBG.

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