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 a method for stabilizing the frequency of a laser, and more particularly, to an apparatus and a method for stabilizing the frequency of a laser based on the spectroscopy of a temperature-stabilized volume Bragg grating (VBG).

Background Art

[0002] In applications such as telecommunications and quantum sensors, a frequency reference is required to stabilize a laser having a specific performance profile. In particular, for applications on mobile platforms in outer space, such a frequency reference needs to be as compact as possible, adjustable without mode hopping, stable over a long period of time, and the (reproducible) frequency accuracy should be about 50 MHz. A corresponding frequency reference module for implementing a space-specification frequency reference having such characteristics is not yet known in the prior art.

[0003] Prior art frequency references are generally based on the spectroscopy of fiber Bragg gratings (FBG), and are known to those skilled in the art, for example, as described by Sotor et al. (J. Z. Sotor, A. J. Antonczak and K. M. 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 cited from Sotor et al., the laser beam is injected into the FBG through a beam splitter based on an optical waveguide. The diffracted signal is split through a beam splitter based on an optical waveguide and used to generate an interference signal for stabilizing the frequency of the laser together with the transmitted signal. To ensure the thermal stability of the frequency selection element required to improve the frequency stability of the laser, the FBG is surrounded by a non-thermal housing.

[0004] However, the spectroscopy based on optical waveguides has significant drawbacks. In particular, the characteristics of optical waveguides can change under the influence of irradiation, mechanical stress, and temperature fluctuations during long-term use. This leads to unacceptable measurement errors in the applications of high-sensitivity sensors. When using a beam splitter based on a waveguide, these effects also cause the temperature dependence and polarization dependence of the splitting ratio of the waveguide-based beam splitter. The variation of the splitting ratio results in a misinterpretation of the spectroscopic signal during operation. Such systematic errors cause frequency errors. Using a non-thermal housing is intended to achieve thermal decoupling of the FBG in order to achieve high frequency stability. However, with this method, it is impossible to adjust the frequency of the diffraction grating by temperature control. Therefore, the frequency stabilization based on FBG is not suitable for applications to mobile platforms in space, and other stabilization concepts must be used for this purpose.

SUMMARY OF THE INVENTION

[0005] Accordingly, an object of the present invention is to provide a laser frequency stabilization device that enables sufficient stabilization of a laser for use in space travel having the aforementioned performance profile. Further, it is to provide a corresponding method for laser frequency stabilization.

[0006] These objects are achieved according to the invention by the features of claims 1, 13 and 15. Advantageous configurations of the invention are described in the dependent claims. The features individually recited in the claims can be combined with each other in a technically meaningful way and 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 incident a laser beam emitted by a laser on a frequency selective element, the frequency selective element being temperature controlled, the frequency selective element being a VBG which is a volume holographic diffraction grating having a plurality of diffraction grating structures, before the laser beam is incident on the VBG via 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 forms a measurement beam, which is a laser frequency stabilization device.

[0008] The beam path can be understood as meaning in particular something that guides the laser beam as a "free beam". That is, the laser beam is not incident on the frequency selective element via an optical fiber or a waveguide, but is incident, for example, as a freely propagating Gaussian beam (free beam).

[0009] The temperature of the frequency selection element is controllable, i.e., it can be varied in relation to the environment by appropriate control. Since the filter frequency of the frequency selection element generally depends on its temperature, the temperature control of the frequency selection element includes the control performance of the filter frequency (center frequency) of the frequency selection element. The Peltier element, unlike a heating element with additional cooling, can essentially perform active cooling, so it is advantageously used for rapid temperature control of the VBG. However, when a heating element is used, the thermal connection of the VBG needs to be specially optimized for rapid and effective cooling.

[0010] The VBG is a specific type of optical diffraction grating. Mechanical diffraction gratings ("ruler diffraction gratings") and holographically generated planar diffraction gratings ("holographic diffraction gratings") are usually formed as surface diffraction gratings, while the VBG is generally a "thick diffraction grating" inside a material. The VBG has a plurality of diffraction structures arranged adjacent to each other. In this specification, the individual reflective layers of the diffraction grating are denoted as the grating structure. These are, for example, the individual diffraction grating surfaces of a variable band grating consisting of a plurality of diffraction grating surfaces arranged adjacent to each other in a suitable material. However, the individual diffraction grating structures can have a non-planar form, such as a curved surface shape. The spacing between the diffraction grating structures can be varied to provide a diffraction grating with a broadband spectral distribution (referred to as a "chirped diffraction grating"). Also, the VBG may comprise a plurality of gratings that are separate from each other, and at least some may be fused together and have corresponding grating structures. The VBG is usually generated holographically, and the corresponding filter element is also called a volume holographic diffraction grating (VHGs). However, the VBG structure can also be generated by methods other than holography, for example, by engraving fs laser pulses into glass through a phase mask.

[0011] The VBG preferably takes the form of a Bragg grating volumetrically generated in a photothermally refractive glass. In a preferred embodiment, it is also possible to use a volume holographic diffraction grating based on dichroic gelatin, a photopolymer, a photorefractive crystal, or a silver halide emulsion as the VBG medium. One of the important criteria for application to a mobile platform in space is, in particular, the long-term stability of the VBG, and in particular, its resistance to the effects of irradiation, mechanical stress, temperature fluctuations, and aging.

[0012] According to the present invention, a part of the laser beam is branched as a reference beam before entering the VBG. This branching is performed through the input surface of the VBG. The expression "before entering" should be understood in relation to the position along the beam path. Therefore, the branching can also be performed in parallel with the incidence in terms of time, that is, during or within the physical process of incidence. For this purpose, in order to branch the reference beam, a method of directly reflecting it at the input surface of the VBG at the time of incidence is particularly effective. Alternatively, the reference beam can also be branched from the free beam to the beam directed to the VBG by a beam splitter arranged upstream of the VBG. The reference beam can be detected by an appropriate reference detector. On the other hand, a part of the laser beam incident on the VBG forms a measurement beam. The measurement beam can preferably be detected as a measurement beam transmitted downstream of the VBG or as a measurement beam diffracted upstream of the VBG. Both the transmitted measurement beam and the diffracted measurement beam can also be repeatedly reflected (for example, outside the VBG) within the VBG.

[0013] The VBG is preferably configured to be retrofitted. In such a configuration, the laser beam is incident perpendicularly to the diffraction grating structure of the VBG. When the VBG is configured and operated to be retrofitted, the value of the frequency (the center frequency of the VBG) of the device according to the present invention is linearly insensitive to the inclination of the incident laser beam.

[0014] The input surface 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 surface of the VBG has an angle other than 90° with respect to the beam axis of the incident laser beam, a part of the reflected light of the incident laser beam on the input surface of the VBG is directed in a direction other than the incident direction. That is, there is no direct backward reflection to the laser that causes interference. Further, a part of the laser beam can be branched into another reference beam by reflection immediately before entering the VBG. Since the incident beam and the beam reflected by the input surface are spatially separated, there is no need to add a beam splitter. The dielectric beam splitter, that is, has a high-reflection coating with temperature dependence and time-dependent change (aging effect). Particularly preferably, a part of the incident laser beam is branched by Fresnel reflection at the input surface of the VBG. A specific angle can be set between the laser beam and the diffraction grating structure of the VBG according to the refractive index of the VBG medium and the incident angle. In particular, this enables the VBG to be optimally adapted to a specific retro arrangement, in which the beam reflected at the input surface is not reflected in the direction of the incident beam.

[0015] The VBG preferably comprises an attachment surface, an upper surface facing the attachment surface, and two side surfaces connecting the attachment surface and the upper surface. The diffraction grating vector of the VBG and the upper surface are parallel to the attachment surface, and / or the side surfaces are configured to be parallel to each other. The VBG may further comprise a front surface (input surface) connecting the attachment surface and the upper surface, into which the laser beam preferably enters, and a rear surface connecting the attachment surface and the upper surface and located on the opposite side of the input surface. The side surfaces of the VBG are preferably oriented parallel to each other with a tolerance of ±100', more preferably ±10', in order to minimize the volume of the VBG. The diffraction grating vector orthogonal to the diffraction grating structure (for example, a plane or a slightly curved surface having the same refractive index) and the upper surface are preferably oriented parallel to the attachment surface of the VBG with a tolerance of ±40', more preferably ±4', in order to simplify the integration process of the device according to the present invention. The angle between the diffraction grating vector of the VBG and the beam axis of the refracted beam (the measurement beam after entering the VBG) is preferably a tolerance of ±40', more preferably ±4'. By making the tolerance of the angle between the diffraction grating vector of the VBG and the beam axis of the refracted beam stricter, a larger margin can be provided during the integration process.

[0016] The angle between the beam axis of the incident light beam and the incident 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. By setting the angle between the beam axis of the incident light beam and the input facet of the VBG to 45°, mechanical integration is simplified. On the other hand, by making the tolerance of the angle between the beam axis of the incident light beam and the input facet of the VBG stricter, a detector signal level predictable due to the angular dependence of Fresnel reflection can be obtained, and thus, a smaller and faster detector can be used.

[0017] In a more preferred embodiment, the angle between the beam axis of the incident beam and the input facet of the VBG is not 45°, but instead takes on another value, and the incident beam and the beam reflected by the input facet are spatially separated. In a more preferred embodiment, the beam incident on the input facet of the VBG and the beam reflected by the input facet are spatially overlapping. In this case, the reference beam can be generated by a beam splitter disposed upstream of the VBG.

[0018] The VBG preferably takes the form of a parallelepiped, and the diffraction grating structure of the VBG is arranged perpendicular to any of the mounting surface, upper surface, and side surfaces of the VBG. In this case, not only is the integration process of the device according to the present invention simplified, but also the required space is reduced and the thermal load is reduced. The parallelepiped can itself provide the surface inclination with respect to the grating structure necessary for Fresnel reflection at the input surface.

[0019] The reference beam is preferably branched at the surface of the VBG or at a beam splitter disposed upstream of the VBG. In the case of Fresnel reflection, it is also possible not to apply a dielectric coating to the input surface in order to reduce reflection. By not applying a dielectric coating to the input surface of the VBG, the influence of temperature and aging changes in reflection and transmission can be avoided.

[0020] The device according to the present invention preferably further includes a reference detector set to determine the intensity of the reference beam and a measurement detector set to determine the intensity of the measurement beam, and the intensity of the measurement beam is determined after passing through the VBG. The measurement beam can be repeatedly reflected at the outer surface 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 (transmission signal) transmitted by the VBG. However, alternatively, the light beam diffracted by the VBG can also be detected by the measurement detector (diffraction signal). The detector is preferably a photodiode.

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

[0022] Instead of using the reference signal and the transmission signal to derive the error signal, it is also possible to use the reference signal and the diffraction signal. In a further embodiment, the error signal can be derived using the transmission signal and the diffraction signal, or the reference signal, the transmission signal, and the diffraction signal. Furthermore, it is also possible to derive the error signal using only the diffraction signal or only the transmission signal, but in any case a certain input level is required.

[0023] The VBG is preferably thermally isolated from the surroundings by a diffraction grating housing. A suitable diffraction grating housing can enhance the thermal and optical insulation of the VBG. The diffraction grating housing can be connected to the optical bench on which the VBG is arranged together with other components. The diffraction grating housing may include temperature stabilization. Such an embodiment can reduce the temperature gradient around the VBG related to thermal radiation.

[0024] The VBG is preferably arranged together with other components in a module housing. In particular, the VBG can be arranged in the module housing together with other components (e.g., mirrors, polarizers, and / or the optical bench), regardless of the presence or absence of an additional diffraction grating housing. Such an embodiment improves the thermal and acoustic insulation of the VBG. The module housing is thermally isolated from the surroundings and can include temperature stabilization.

[0025] For the temperature stabilization of the VBG, it is preferable that at least one cooling and / or heating device is used. The cooling and / or heating device preferably comprises a Peltier element or a heating element. When using a heating element, the thermal connection of the VBG (or the element connected thereto) can be specifically optimized for fast and effective cooling. Cooling can include, for example, radiative cooling into space. The detector used for signal acquisition according to the present invention is more preferably arranged on a common Peltier element with the VBG or on a common platform that is properly thermally connected. Such an embodiment can reduce the sensitivity to beam displacement, thereby improving the accuracy of the device according to the present invention.

[0026] The above embodiments can be advantageously combined with each other, whether in whole or in part.

[0027] A second aspect of the present invention relates to a method for stabilizing the frequency of a laser using the device according to the present invention, wherein the error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG (at least once), 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 present invention can be controlled via a cooling and / or heating device. This control may be locally related to the VBG or, alternatively, may be related to other components such as an optical bench on which the VBG is arranged together with other components. Similarly, it is also possible to combine multiple temperature control approaches. The temperature stabilization of the optical bench improves the accuracy of the method according to the present invention.

[0029] The laser beam spectrally filtered by the VBG is preferably incident on the laser again to form an extended resonator of the laser. In particular, a frequency-stabilized diode laser with an extended resonator ("extended-cavity diode laser", ECDL) can be implemented in this way. The associated control circuit can be used, for example, to control the injection current and the temperature of the laser in order to adjust and stabilize the laser frequency.

[0030] A third aspect relates to a method for measuring the frequency of a laser using the device according to the invention, wherein the error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG (at least once), and the temperature of the VBG is adjusted with the error signal via a control loop such that the Bragg frequency of the VBG corresponds to the frequency of the laser, excluding an arbitrarily selected signed frequency difference, and the frequency of the laser is determined by its known relationship with the adjusted temperature of the VBG. Thereby, the Bragg frequency (central frequency) of the VBG can also be directly adjusted to the frequency of the laser.

[0031] The method for measuring the frequency of a laser and the method according to the invention for stabilizing the frequency of a laser are substantially different only in terms of the control variables, but are otherwise based on a common inventive concept. For the purpose of stabilizing the laser frequency, it is possible to control the temperature and / or the injection current of the laser using the error value derived according to the invention in order to adjust and stabilize a specific laser frequency, whereas for the purpose of measuring the frequency of a laser, it is possible to stabilize the temperature of the VBG with the assistance of temperature control. Thereby, the Bragg frequency (central frequency) of the VBG coincides with the frequency of the laser, excluding 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 association and displayed, for example, on a display device.

[0032] Furthermore, the preferred configurations of the method according to the invention are directly revealed by the features described in the description in relation to the device according to the invention.

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

[0034] In the prior art, an optical signal is guided by an optical waveguide, but in the present invention, the optical signal is preferably propagated in free space. The present invention is particularly distinguished by the following structural differences. 1. The VBG is used as a frequency selection element instead of the FBG. 2. In order to separate the optical signal upstream of the FBG, a beam splitter based on an optical waveguide is not used. A part of the incident optical signal is preferably branched from the remaining signal by Fresnel reflection at the input surface of the VBG. For this purpose, the input surface of the VBG can be arranged at an angle with respect 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 branched incident optical signal. 3. The VBG can be arranged on an optical bench or generally a suitable carrier, and its temperature can be stabilized to a predetermined value by a suitable cooling and / or heating device and a temperature sensor.

[0035] Compared with the prior art, the device according to the present invention uses no optical waveguide components in areas that are particularly important for measurement accuracy, so it is much more resistant to irradiation, mechanical stress, and temperature changes. Since the branching of the reference beam by Fresnel reflection is prioritized, it is possible to make the splitting ratio substantially independent of the influence of temperature. By using a Peltier element and eliminating the non-thermal housing, it becomes possible to directly adjust the frequency of the VBG via temperature. By miniaturizing the device and appropriately forming the VBG, higher mechanical stability can be achieved, and the weight and form factor can be reduced. With the optional module housing, the device according to the present invention can be very effectively shielded from the effects of sound and heat. Furthermore, for example, by arranging a Peltier element under the VBG and using a diffraction grating housing surrounding the Peltier element, an additional Peltier element for stabilizing the optical bench, and a module housing surrounding the optical bench, it is also possible to achieve high frequency accuracy simultaneously with the option of frequency adjustment by stabilizing the temperature of the VBG step by step.

[0036] A more preferred configuration of the present invention is revealed by the features described in each of the subclaims.

[0037] The various embodiments of the present invention described in this application can be advantageously combined with each other unless otherwise specified.

Brief Description of the Drawings

[0038] Hereinafter, the present invention and the technical background will be described in more detail with reference to the accompanying drawings. It should be noted that the present invention is not intended to be limited by the described exemplary embodiments. In particular, unless otherwise clearly stated, it is also possible to extract secondary aspects of the subject matter illustrated in the figures and combine them with other components and insights in this specification. In the drawings:

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Embodiments for Carrying Out the Invention

[0039] Figure 1 is a schematic diagram showing a laser frequency stabilization device according to the prior art (by Sotor et al.). The laser ("stabilized laser") includes a temperature-controlled laser crystal (Nd:YAG / KTP) whose temperature is controlled by a Peltier element ("thermoelectric cooler": TEC), and emits light by resonating at two wavelengths of 1064 nm and 532 nm. The laser crystal is excited by a diode laser ("excitation diode") with an emission wavelength of 808 nm. A part of the 1064 nm light beam is branched by a dichroic beam splitter ("dichroic mirror") and is incident on an FBG arranged in a non-thermal housing via a fiber coupler. The intensities of the diffracted radiation component ("reflection signal") and the radiation component passing through the FBG ("transmission signal") are determined by a properly arranged PIN photodiode ("PIN diode"), and both signals are supplied to an electronic circuit for temperature control of the laser crystal ("automatic frequency control"). The quality of this laser frequency stabilization mainly depends on the stability of the FBG and the related fiber parts in the device.

[0040] Figure 2 is a schematic diagram showing a first embodiment of the device according to the present invention for laser frequency stabilization. The optical signal is incident on the device via a polarization-maintaining single-mode optical waveguide (PMSF) F1. The PMSF F1 enables the device according to the present invention to be easily integrated into an existing optical system. The incident optical signal becomes parallel light by the optical system L1. Only the s-polarized light is guided to the VBGG1 including the mirrors S1, S1 by the polarizer R1. The light beam is guided to operate with an allowable error of ±1° by configuring the VBGG1 to be retrofitted as shown in Figure 6 (i.e., perpendicular incidence of light on the grating structure E). The polarizer R1 can prevent polarization fluctuations that may lead to systematic measurement errors. The beam guidance through the two mirrors S1, S2 provides greater freedom in positioning the VBG than in the case of arranging the VBG using one mirror or without using any mirror at all.

[0041] VBGG1 can be attached to the temperature sensor N1 and, for example, the first cooling and / or heating device T1 for temperature stabilization. Similarly, 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. PMSFF1, the optical system L1, the polarizer R1, the mirrors S1, S1, and the cooling and / or heating device T1 are preferably adhesively bonded to a common optical bench B1. The diffraction grating housing H1 can be thermally connected to the optical bench B1, thereby improving the temperature stability of the device according to the present invention. The optical bench B1 can also be stabilized to a desired temperature by the second cooling and / or heating device T2.

[0042] The optional module housing H2 can be used to shield the optical bench B1 from thermal and acoustic interference from the surroundings. The optical system L1, the polarizer R1, the mirrors S1, S2, and VBGG1 preferably comprise micro-optical components. Due to the related miniaturization, weight reduction, and improvement in the mechanical stability of the device according to the present invention, it is preferable to use micro-optical components. The cooling and / or heating devices T1, T2 preferably comprise Peltier elements or heating elements.

[0043] Figure 3 is a schematic diagram showing a second embodiment of the device according to the present invention for laser frequency stabilization. Since this schematic diagram generally corresponds to Figure 2, the reference numerals are the same. In this embodiment, the intensity of the light beam diffracted by VBGG1 is determined by the measurement detector P3 (diffraction signal). Therefore, an error signal can be generated using the reference signal and the diffraction signal instead of the reference signal and the direct transmission signal. The intensity of the diffracted beam can be measured by the measurement detector P3 after being reflected by the beam splitter A1 upstream of VBGG1.

[0044] FIG. 4 is a schematic diagram showing a third embodiment of the device according to the present invention for laser frequency stabilization. Since this schematic diagram generally corresponds to FIG. 3, the reference numerals are the corresponding ones. When the incident beam and the diffracted beam do not spatially overlap, it is not necessary to provide an additional beam splitter A1 upstream of VBGG1. Instead, the intensity of the diffracted beam can also be directly determined by the measurement detector P4.

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

[0046] FIG. 6 is a schematic diagram showing the geometry of the VBG according to the present invention. The input facet of VBGG1 preferably has an angle of 45° ± 1° with respect to the beam axis of the incident light beam and is not coated. The side surfaces A, B of VBGG1 are preferably oriented parallel to each other with an allowable error of ±10' in order to minimize the volume of VBGG1. The grating vector orthogonal to the grating structure E of equal refractive index, and the upper surface D are preferably with an allowable error of ±4' and are oriented parallel to the mounting surface C of VBGG1. Thereby, the integration process of the VBG according to the present invention into the device is simplified. The incident light beam is preferably Fresnel-reflected at the input facet of VBGG1.

[0047] FIG. 7 is a schematic diagram for implementing a first embodiment of the method according to the present invention for stabilizing the frequency of a laser using the apparatus of FIG. 2. Since this schematic diagram generally corresponds to FIG. 2, the reference numerals shall be the corresponding ones. The light from laser D1 is incident on the optical waveguide F1 of the apparatus according to the present invention here by way of example with the assistance of the optical system L2 so that the frequency of the laser D1 can be stabilized by appropriate control.

[0048] The beam reflected at the input surface of the illustrated VBGG1 impinges, for example, on a reference detector P1 (reference signal) that detects the intensity of the reflected beam. A part of the light beam hitting the input surface refracts when incident on VBGG1. The intensity of the beam transmitted by VBGG1 can be detected by a measurement detector P2 (transmission signal). An error signal that can be used for stabilizing the laser D1 (or measuring the oscillation frequency of the laser D1) can be generated from the reference signal U1 of the reference detector P1 and the transmission 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 by an appropriate electronic circuit E1 to generate an error signal. The error signal is processed by an appropriate electronic circuit E2 and fed back to the laser G1 as a control signal for controlling the frequency of the laser G1, and the frequency of the laser G1 is adjusted to a 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 coefficient and determine the difference between the signal U1 and the signal U2 (see FIG. 8).

[0051] FIG. 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 transmission signal U2 (photodiode signal) of the measurement detector P2 are plotted against the frequency of the laser D1 (laser frequency f). Thereby, the differential signal U2 - U1 can be obtained as an error signal, and as shown as an example in FIG. 8, the first zero-crossing of the error signal can be shown as a control point for the frequency of the laser G1 to become the defined value f0. When the control loop is activated, the frequency of the laser G1 is adjusted to the value f0 at which the error signal zero-crosses with a positive slope or (as shown) a negative slope.

[0052] FIG. 9 is a schematic diagram for implementing a second embodiment of a method according to the present invention for stabilizing the frequency of a laser using the apparatus of FIG. 2. Since this schematic diagram substantially corresponds to FIG. 2, the reference numerals shall be the corresponding ones. This is in particular a method for achieving internal frequency stabilization of a diode laser equipped with an extended resonator ("extended cavity diode laser", ECDL) using the apparatus according to the present invention.

[0053] The light beam emitted by the laser D2 is collimated with the assistance of the optical collimation system L3 and is incident on the device according to the present invention with the assistance of the optical system L2. The incident light beam is diffracted frequency-selectively by the VBGG1. The laser configuration thus embodied oscillates at a frequency corresponding to one of the possible longitudinal eigenmodes of the ECDL. The VBGG1 acts as a narrowband reflector and selects one of the oscillatable longitudinal modes. This is usually the longitudinal mode having the frequency closest to the center frequency of the spectrum of the VBGG1 acting as a reflector. An embodiment of the apparatus according to the present invention, which is preferred for using the method described herein, does not include an optical waveguide for injection, but rather a free beam is incident on the apparatus. Thereby, in particular, parasitic feedback from the surface of the optical waveguide to the laser D2 is prevented, otherwise the operation of the ECDL may be interfered with.

[0054] Changes in the air pressure around the laser device, the temperature of the laser D2, the current incident on the laser D2, and / or changes in the temperature around the laser device composed of the laser D2, the optical collimation system L3, and the device according to the present invention cause a spectral detuning between the temperature of the laser setup composed of the system L3 and the device according to the present invention and the frequency of the longitudinal eigenmode of the ECDL and the central frequency of the VBGG1 acting as a narrowband reflector. When this detuning reaches the order of the magnitude of the free spectral range of the ECDL, mode hopping occurs. That is, a change to a different longitudinal mode eigenmode of the ECDL occurs. As a result, the frequency of the laser oscillation changes rapidly.

[0055] The method for stabilizing the frequency of the laser according to the present invention can be used to avoid a rapid change in the frequency of laser oscillation during operation at a constant frequency or when a desired frequency change occurs. 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 an appropriate electronic circuit E1. Then, using an appropriate electronic circuit E2, for example, a control signal for controlling the temperature of a system composed of the current incident on the laser D2, the temperature of the laser D2, or the laser D2, the optical collimation system L3, and the device according to the present invention provided with a cooling and / or heating device T3 can be generated. Combinations of these parameters can also be controlled. In this way, the oscillation frequency of the eigenmode of the ECDL is adjusted to maintain a state in which it is synchronized with 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] FIG. 10 is a schematic diagram for implementing a third embodiment of the method according to the present invention for stabilizing the frequency of a laser using the device of FIG. 2. Since this schematic diagram generally corresponds to FIG. 9, the reference numerals are the same. 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 present invention. In this case, the frequency f0 of the device according to the present invention is stabilized to the oscillation frequency of the ECDL.

[0057] Figure 11 is a schematic diagram for implementing an embodiment of the method according to the present invention for measuring the frequency of a laser using the apparatus according to FIG. 2. Since this schematic diagram generally corresponds to FIG. 2, the reference numerals shall be the corresponding ones. The light from laser D1 is incident on the optical waveguide F1 of the apparatus according to the present invention, for example, with the assistance of the optical system L2. Thereby, the cooling and / or heating device T1 can be driven so that the frequency f0 (corresponding to an arbitrarily selected signed frequency difference of the center frequency of VBG or the Bragg frequency) in the apparatus according to the present invention 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 by an appropriate electronic circuit E1. The error signal, with the assistance of the electronic circuit E2, is processed as a control signal to control the cooling and / or heating device T1 so that the frequency f0 of the apparatus according to the present invention corresponds to the oscillation frequency f1 of laser D1. Depending on the value f1 of the oscillation frequency of laser D1, a temperature T specific to the value f1 of the oscillation frequency of laser D1 is established by the temperature sensor N1. With the assistance of a known association (for example, calibration table K1), the value f1 of the oscillation frequency of laser D1 is thus determined from the value of the temperature T at the temperature sensor N1 and is displayed on the display device Y1.

[0059] Figure 12 is a schematic diagram showing the signals according to FIG. 11. The frequency f0 of the apparatus according to the present invention is stabilized to the value of the frequency of laser D1 (laser frequency f). An exemplary curve of the differential signal U2 - U1 (see FIG. 8 for the determination) is shown for two different temperatures T1, T2 of the apparatus according to the present invention. Also in this case, the first zero crossing of the error signal can be used as a reference point. With a known assignment, the values f1 or f2 of the oscillation frequency of 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 plotted against the frequency of laser G1 (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 frequency stabilization device for a laser (D1, D2), comprising a beam path for causing a laser beam emitted by the laser (D1, D2) to enter a frequency selection element (G1). The frequency selection element (G1) is temperature-controlled. The frequency selection element is a VBG (G1) which is a volume holographic diffraction grating having a plurality of diffraction grating structures (E). Before the laser beam enters 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. A frequency stabilization device for a laser, characterized in that.

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

3. The device according to claim 1 or claim 2, wherein The VBG (G1) is configured to be retrofitted.

4. The device according to any one of claims 1 to 3, wherein The input surface of the VBG (G1) forms an angle other than 90° with respect to the beam axis of the incident laser beam and / or the diffraction grating structure (E) of the VBG (G1).

5. The device according to any one of claims 1 to 4, wherein The VBG (G1) includes a mounting surface (C), an upper surface (D) facing the mounting surface (C), and two side surfaces (A, B) connecting the mounting surface (C) and the upper surface (D). The diffraction grating vector of the VBG (G1) and the upper surface (D) are parallel to the mounting surface (C), and / or the side surfaces (A, B) are configured to be parallel to each other.

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

7. The device according to any one of claims 1 to 6, wherein The reference beam is branched at the surface of the VBG (G1) or a beam splitter (A1) arranged upstream of the VBG (G1).

8. The device according to any one of claims 1 to 7, wherein A reference detector (P1, P3) set to determine the intensity of the reference beam. further comprising measurement detectors (P2, P4, P5) configured to determine the intensity of the measurement beam The apparatus, wherein the intensity of the measurement beam is determined after passing through the VBG (G1). **Claim 9** The apparatus according to claim 8, further comprising a first electronic circuit (E1) for deriving an error signal from the reference signal (U1) of the reference detector (P1, P3) and the measurement signal (U2) of the measurement detector (P2, P4, P5). **Claim 10** The apparatus according to any one of claims 1 to 9, wherein the VBG (G1) is thermally isolated from the surroundings by a diffraction grating housing (H1). **Claim 11** The apparatus according to any one of claims 1 to 10, wherein the VBG (G1) is arranged together with other components within a module housing (H2). **Claim 12** The apparatus according to any one of claims 1 to 11, wherein at least one cooling and / or heating device (T1, T2, T3) is used for temperature stabilization of the VBG (G1). **Claim 13** A method for frequency stabilization of a laser (D1, D2) using the apparatus according to any one of claims 1 to 12, wherein the error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG (G1), and the frequency of the laser (D1, D2) is stabilized by the error signal via a control loop. **Claim 14** The method for frequency stabilization of the laser (D2) according to claim 13, wherein the laser beam spectrally filtered by the VBG (G1) is incident again on the laser (D2) to form an extended resonator of the laser (D2). **Claim 15** A method for frequency measurement of a laser (D1) using the apparatus according to any one of claims 1 to 12, wherein the error signal is derived from the intensity of the reference beam and the intensity of the measurement beam after passing through the VBG (G1), the temperature of the VBG (G1) is adjusted by the error signal via a control loop such that the Bragg frequency of the VBG (G1) corresponds to the frequency of the laser (D1) excluding an arbitrarily selected signed frequency difference, and the frequency of the laser (D1) is determined by a known association with the adjusted temperature of the VBG (G1).

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