Method and device for characterizing a resonator element

Abstract

A method for characterizing a resonator element (102) is provided, the method comprising the steps of providing laser light having a tunable carrier frequency and coupling at least a first portion of the laser light into a resonator element (102) having a plurality of carrier resonances (3000) for the carrier frequency of the laser light, wherein adjacent carrier resonances (3000) are spaced apart from one another within a spectral range by a free spectral range (FSR). The method further includes modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element (102) with a first modulation frequency (1001) and a second modulation frequency (1002) to generate, for each carrier resonance (3000), at least two sideband resonances (3001) spaced from the carrier resonance (3000) by the first modulation frequency (1001) and at least two sideband resonances (3002) spaced from the carrier resonance (3000) by the second modulation frequency (1002), wherein the first modulation frequency (1001) and the second modulation frequency (1002) are integer multiples of the free spectral range and are different from each other. The method further includes tuning the carrier frequency of the laser light at a predetermined tuning speed and measuring the intensity of the laser light transmitted and / or reflected by the resonator element (102) while tuning the carrier frequency. Furthermore, the method includes measuring a tuning time elapsed in tuning the carrier frequency across four adjacent sideband resonances (3001, 3002), the tuning time corresponding to the time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to the last.The method further includes determining the spacing between multiple carrier resonances (3000) within the spectral range using the measured intensity of the laser light transmitted and / or reflected by the resonator element (102), the first modulation frequency (1001), the second modulation frequency (1002), and the tuning time measured when tuning the carrier frequency across four adjacent sideband resonances (3001, 3002).

Description

[Technical Field]

[0001] Methods and devices for characterizing resonator elements, methods and devices for obtaining an optical frequency reference, a LIDAR system, and a gas detection system are provided. Accordingly, the present disclosure relates to techniques for obtaining an optical frequency reference. [Background technology]

[0002] Many applications benefit from the ability to measure time and / or spectral differences between multiple signals with high precision, and in many applications, optical frequency combs, such as those described in [DJ Jones, SA Diddams, JK Ranka, A. Stentz, RS Windeler, JL Hall, and ST Cundiff, "Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis," Science 288, 635-639 (2000)] and [R. Holzwarth, Th. Udem, TW Hansch, JC Knight, WJ Wadsworth, and P. St. J. Russell, "Optical Frequency Synthesizer for Precision Spectroscopy," Phys. Rev. Lett. 85, 2264-2267 (2000)], are used for this purpose. These techniques enable single-frequency measurements with up to 18-digit accuracy, as described in Non-Patent Document 3 (M. Takamoto, I. Ushijima, N. Ohmae, T. Yahagi, K. Kokado, H. Shinkai, and H. Katori, "Test of general relativity by a pair of transportable optical lattice clocks," Nat. Photonics 14, 411-415 (2020)).

[0003] In addition to single-frequency measurements, optical frequency combs have also been used for high-precision, high-speed broadband spectroscopy, benefiting from the unique combination of wide bandwidth and high spectral resolution. Over the past two decades, a variety of spectroscopic techniques based on optical frequency combs have been developed, including direct frequency comb spectroscopy, dual comb spectroscopy, and Fourier transform spectroscopy.

[0004] Despite their high precision and widespread use, frequency combs often suffer from low power per comb line and are subject to spectral magnitude fluctuations, which limits their use and poses challenges for further applications, as described in non-patent document 4 (T. Fortier and E. Baumann, "20 years of developments in optical frequency comb technology and applications," Commun. Phys. 2, 1-16 (2019)).

[0005] Furthermore, high-precision measurements based on optical frequency combs often require a frequency comb source with long coherence, which involves sophisticated servo loops, as described in N. Picque and T.W. Hansch, "Frequency comb spectroscopy," Nat. Photonics 13, 146-157 (2019). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] DJ Jones, SA Diddams, JK Ranka, A. Stentz, RS Windeler, JL Hall, and ST Cundiff, "Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis," Science 288, 635-639 (2000). [Non-patent document 2] R. Holzwarth, Th. Udem, T. W. Hansch, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, "Optical Frequency Synthesizer for Precision Spectroscopy," Phys. Rev. Lett. 85, 2264-2267 (2000). [Non-Patent Document 3] M. Takamoto, I. Ushijima, N. Ohmae, T. Yahagi, K. Kokado, H. Shinkai, and H. Katori, "Test of general relativity by a pair of transportable optical lattice clocks," Nat. Photonics 14, 411-415 (2020). [Non-Patent Document 4] T. Fortier and E. Baumann, "20 years of developments in optical frequency comb technology and applications," Commun. Phys. 2, 1-16 (2019). [Non-Patent Document 5] N. Picque and T. W. Hansch, "Frequency comb spectroscopy," Nat. Photonics 13, 146-157 (2019). [Non-Patent Document 6] Baumann, Esther, et al. "Comb-calibrated frequency-modulated continuous-wave ladar for absolute distance measurements." Optics letters 38.12 (2013): 2026-2028 [Non-Patent Document 7] Baumann, Esther, et al. "Comb-calibrated laser ranging for three-dimensional surface profiling with micrometer-level precision at a distance." Optics express 22.21 (2014): 24914-24928; [Non-patent document 8] (Yu, Wenhui, et al. "Comb-calibrated frequency sweeping interferometry for absolute distance and vibration measurement." Optics Letters 44.20 (2019): 5069-5072) [Non-Patent Document 9] Nishiyama, Akiko, Daiki Ishikawa, and Masatoshi Misono. "High resolution molecular spectroscopic system assisted by an optical frequency comb." JOSA B 30.8 (2013): 2107-2112. [Non-Patent Document 10] Rieker, Gregory B., et al. "Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths." Optica 1.5 (2014): 290-298 [Non-Patent Document 11] Herman, Daniel I., et al. "Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy." Science Advances 7.14 (2021): eabe9765 [Non-Patent Document 12] P. Del'Haye et al.: "Frequency comb assisted diode laser spectroscopy for measurement of microcavity dispersion," Nat. Photonics 3, 529-533 (2009) [Non-Patent Document 13] A. Frigg, et al., "Low loss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thin films," Opt. Express 27, 37795-37805 (2019) [Non-Patent Document 14] IE Gordon et al. "The HITRAN2016 molecular spectroscopic database," J. Quant. Spectrosc. Radiat. Transf. 203, 3-69 (2017) [Non-Patent Document 15] J. Liu et al. "Frequency-comb-assisted broadband precision spectroscopy with cascaded diode lasers," Opt. Lett. 41, 3134-3137 (2016) [Non-Patent Document 16] A. Shkarin et al. "Nanoscopic Charge Fluctuations in a Gallium Phosphide Waveguide Measured by Single Molecules," Phys. Rev. Lett. 126, 133602 (2021) [Non-Patent Document 17] E. Baumann et al. "Comb-calibrated frequency-modulated continuous-wave lidar for absolute distance measurements," Opt. Lett. 38, 2026-2028 (2013) [Non-Patent Document 18] E. Baumann et al. "Comb-calibrated laser ranging for three-dimensional surface profiling with micrometer-level precision at a distance," Opt. Express 22, 24914-24928 (2014) [Non-Patent Document 19] Yang et al. "Frequency comb calibrated frequency-sweeping interferometry for absolute group refractive index measurement of air," Appl. Opt. 56, 3109-3115 (2017) [Non-Patent Document 20] V. Brasch et al. "Photonic chip-based optical frequency comb using soliton Cherenkov radiation," Science 351, 357-360 (2016) [Non-Patent Document 21] A. Nishiyama et al. "Precise frequency measurement and characterization of a continuous scanning single-mode laser with an optical frequency comb," Opt. Lett. 39, 4923-4926 (2014) [Non-Patent Document 22] S. Minardi, RJ Harris, and L. Labadie, "Astrophotonics: astronomy and modern optics," Astron. Astrophys. Rev. 29, 6 (2021) Summary of the Invention [Problem to be solved by the invention]

[0007] In prior art, frequency combs are often generated by exciting microresonators with modulated lasers, where the center frequency of the laser radiation coupled to the resonator elements remains unchanged. The carrier frequency of the laser radiation coupled to the resonator elements is generally stabilized. To generate a frequency comb, octave-spanning laser radiations that spectrally overlap each other are often required, resulting in high technical complexity and cost.

[0008] It is therefore desirable to overcome the above limitations. [Means for solving the problem]

[0009] This problem is solved by a method and device for characterizing a resonator element, a method and device for providing an optical frequency reference signal for laser light having a tunable carrier frequency, a LIDAR system, and a gas detection system, having the features of the respective independent claims. Optional embodiments are provided in the dependent claims and the description. In one aspect, a method for characterizing a resonator element is provided, comprising: providing laser light having a tunable carrier frequency; and coupling at least a first portion of the laser light into a resonator element having a plurality of carrier resonances for the carrier frequency of the laser light, wherein adjacent carrier resonances are spaced apart in the spectral range by a free spectral range. The method further comprises modulating the intensity and / or phase of the portion of laser light coupled to the resonator element with a first modulation frequency and a second modulation frequency, the first modulation frequency and the second modulation frequency being integer multiples of a free spectral range and different from each other, to generate at least two sideband resonances spaced from their respective carrier resonances by the first modulation frequency and at least two sideband resonances spaced from their respective carrier resonances by the second modulation frequency. The method further comprises tuning the carrier frequency of the laser light at a predetermined tuning rate, measuring the intensity of the laser light transmitted and / or reflected by the resonator element while tuning the carrier frequency, and measuring a tuning time elapsed in tuning the carrier frequency across four adjacent sideband resonances, the tuning time corresponding to the tuning time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to the last of the four sideband resonances.The method further includes determining spacing between a plurality of carrier resonances in the spectral range based on the measured intensities of laser light transmitted and / or reflected by the resonator element using measurements of tuning time elapsed in tuning the carrier frequency across the first modulation frequency, the second modulation frequency, and four adjacent sideband resonances.

[0010] In another aspect, a device for characterizing a resonator element is provided. The device comprises: a tunable laser source for emitting laser light having a tunable carrier frequency; and a coupling element for coupling at least a portion of the laser light into the resonator element. The device further comprises a modulator for modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element at a first modulation frequency to generate, for each carrier resonance, at least two sideband resonances spaced from the respective carrier resonance by the first modulation frequency, and at a second modulation frequency to generate, for each carrier resonance, at least two sideband resonances spaced from the respective carrier resonance by the second modulation frequency. The device further comprises a detector unit for measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element. The device further includes a control unit configured to tune the carrier frequency of the laser light, the control unit determining a tuning time and determining spacing between the plurality of carrier resonances in the spectral region based on a measured intensity of the laser light transmitted and / or reflected by the resonator element and based on measurements of the measured tuning time elapsed in tuning the carrier frequency across the first modulation frequency, the second modulation frequency, and four adjacent sideband resonances, the measured tuning time corresponding to the tuning time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to the last of the four sideband resonances.

[0011] In yet another aspect, a method for providing an optical frequency reference signal for laser light having a tunable carrier frequency is provided. The method includes providing a resonator element having a plurality of carrier resonances for the carrier frequency of the laser light, wherein adjacent carrier resonances are spectrally spaced apart by respective predetermined free spectral ranges. The method further includes coupling a first portion of the laser light into the resonator element and tuning the carrier frequency of the laser light at a predetermined tuning rate. The method further includes providing a portion of the laser light transmitted and / or reflected by the resonator element as a spectrally sweeping optical frequency reference signal, wherein the intensity of the laser light transmitted and / or reflected by the resonator element has local extrema at frequencies spectrally spaced apart by an FSR.

[0012] In yet another aspect, there is provided the use of an optical frequency reference signal provided by a method according to the present disclosure as a spectral reference mark.

[0013] In yet another aspect, a method of spectroscopically characterizing an analyte is provided, the method comprising providing an optical frequency reference signal by use of a method according to the disclosure, and using the optical frequency reference signal as a spectral reference mark for spectroscopically characterizing the analyte, wherein using the optical frequency reference signal as a spectral reference mark can include determining relative spectral distances of a plurality of spectral features of the analyte based on one or more FSRs.

[0014] In yet another embodiment, a device is provided for providing laser light having a tunable carrier frequency as an optical frequency reference signal. The device optionally comprises a tunable laser light source for emitting laser light having the tunable carrier frequency. The device further comprises a resonator element having a plurality of carrier resonances for the adjustable carrier frequency of the laser light, the carrier resonances being spectrally spaced apart from one another by respective predetermined free spectral ranges, the device being capable of coupling a portion of the laser light into the resonator element. The device further comprises a control unit configured to tune the carrier frequency of the laser light at a predetermined tuning rate. The device may provide a portion of the laser light transmitted and / or reflected by the resonator element as a spectrally swept optical frequency reference signal, the intensity of the laser light transmitted and / or reflected by the resonator element having local extrema at frequencies spectrally spaced from one another by an FSR. Optionally, the device further comprises a detector unit for measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element. In addition, the device optionally comprises a control unit configured to provide a frequency of a tunable carrier frequency having a local extremum in the measured intensity of the portion of the laser light transmitted and / or reflected by the resonator element as a spectral reference mark of the optical frequency reference signal defined by a predetermined free spectral range.

[0015] In yet another aspect, a LIDAR system is provided, the LIDAR system comprising a device according to the present disclosure for providing an optical frequency reference signal.

[0016] In yet another aspect, a gas detection system is provided, the gas detection system comprising a device according to the present disclosure for providing an optical frequency reference signal.

[0017] In yet another aspect, a method for characterizing a resonator element is provided. The method includes providing laser light having a tunable carrier frequency and coupling at least a first portion of the laser light into a resonator element having a plurality of carrier resonances relative to the carrier frequency of the laser light, wherein adjacent carrier resonances are spaced apart in a spectral range by a free spectral range. The method further includes modulating an intensity and / or phase of the portion of the laser light coupled into the resonator element at a first modulation frequency to generate, for each carrier resonance, at least two sideband resonances spaced apart from the respective carrier resonance by the first modulation frequency, the first modulation frequency being different from an integer multiple of the free spectral range. The method further includes tuning the carrier frequency of the laser light at a predetermined tuning rate and measuring an intensity of the laser light transmitted and / or reflected by the resonator element while tuning the carrier frequency. The method further comprises measuring a tuning time elapsed between a plurality of carrier resonances spaced apart by at least twice the first modulation frequency, and determining a spacing of a plurality of free spectral ranges in the spectral region based on the measured intensity of laser light transmitted and / or reflected by the resonator element using the first modulation frequency and the measured tuning time elapsed between a plurality of carrier resonances spaced apart by at least twice the first modulation frequency.

[0018] A laser light with a tunable carrier frequency is a coherent light having a center frequency that can be tuned by a light source. Tuning the carrier frequency means varying the carrier frequency over time, optionally in a continuous manner. Adjusting the carrier frequency can include, for example, sweeping the carrier frequency sinusoidally.

[0019] A resonator element is an element that has specific transmission and / or absorption characteristics for a tunable carrier frequency, and for a given value of the tunable carrier frequency, the resonator element exhibits local minima and / or maxima of transmittance and / or absorption and / or reflectance. In particular, the resonator element may constitute a resonator cavity that satisfies a resonance condition for a specific value of the tunable carrier frequency. These specific values ​​of the carrier frequency are considered carrier resonances. In the case of a resonator cavity, the transmittance may have local minima at frequencies where the tunable carrier frequency exhibits resonances based on the length of the resonator cavity. Measuring the intensity of laser light transmitted and / or reflected by the resonator element while tuning the carrier frequency means measuring the intensity and / or power of the laser light transmitted and / or reflected by the resonator element while continuously tuning the carrier frequency.

[0020] Modulating the intensity and / or phase of a portion of laser light with a first modulation frequency and, optionally, a second modulation frequency means periodically varying the intensity and / or phase of the laser light with the first modulation frequency and, optionally, the second modulation frequency. Therefore, modulating the intensity of the laser light with the first modulation frequency and, optionally, the second modulation frequency results in a periodic decrease and increase in intensity according to the time evolution of the modulation signal having the first modulation frequency and, optionally, the second modulation frequency. In the case of one or more modulation signals with one or more modulation frequencies, the modulation can be substantially a superposition of the individual modulation signals. This can be achieved by applying the first and second modulation frequencies via a power combiner. The first and second modulation frequencies may be higher than the free spectral range of the laser light and the resonator element. While first and second modulation signals having the first and second modulation frequencies, respectively, are explicitly mentioned, it should be noted that, according to some optional embodiments, the laser light can be modulated by two or more modulation signals at two or more modulation frequencies. The first and second modulation frequencies may each be in the radio frequency range. In particular, the first modulation frequency and the second modulation frequency may each range from about 100 MHz to about 100 GHz.The free spectral range of the resonator element may range from ≧1 MHz to ≦100 GHz.

[0021] The modulation depth of the intensity modulation and / or phase modulation may have a modulation index in the range of about 0.6-2.

[0022] Sideband resonances resulting from modulation of the intensity and / or phase of the laser light can result from the superposition of the carrier frequency with one or more modulation frequencies.

[0023] Modulation frequencies that differ by integer multiples of the free spectral range and from each other mean that the modulation frequencies do not perfectly overlap with the carrier resonance and generate sideband resonances that do not perfectly overlap with each other sideband resonance resulting from each other modulation frequency.

[0024] The tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances corresponds to the tuning time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to the last of the four sideband resonances. However, according to other embodiments, the tuning of the carrier frequency can be performed in a non-uniform, but well-defined manner. Furthermore, if the temporal distance between the four adjacent sideband resonances is sufficiently small, detailed knowledge of the tuning characteristics may not be essential. Based on the measured elapsed tuning time, the spectral difference between the first and last sideband resonances can be determined.

[0025] The tunable laser source may include or consist of a wavelength-tunable continuous wave laser source. The tunable laser source may include or consist of a diode laser.

[0026] The modulator may comprise or consist of an acousto-optical modulator and / or an electro-optical modulator and / or may be realized via modulation of the laser current and / or the laser current. The detector unit may comprise one or more photodiodes sensitive in the tuning range of the laser light having a tunable carrier frequency. In particular, the modulator may modulate the intensity and / or the phase of the transmitted laser light.

[0027] The control unit may be an electrical and / or electronic device such as a computer, a smartphone, an integrated circuit, and / or a tablet computer. The control unit may be connected to the detector unit to receive data measured by the detector unit. The control unit may further be connected to the tunable light source and configured to send instructions to the tunable light source to tune the carrier frequency in a specific manner.

[0028] Providing a frequency of a tunable carrier frequency having a local extremum in the measured intensity of the portion of laser light transmitted and / or reflected by the resonator element as a spectral reference mark of an optical frequency reference signal defined by a predetermined free spectral range means that the specific frequency exhibiting the local extremum is defined as the spectral reference mark. In other words, the resonator element is used in such a manner that its predetermined and well-characterized carrier resonance is used as the optical frequency reference value, since its spectral distance can be well determined and characterized by the characterization process according to the present disclosure. The well-characterized spectral distance between carrier resonances and / or the spectral distance between sideband resonances corresponding to the free spectral range can be provided as the optical reference frequency and, therefore, can be provided as the optical frequency reference signal.

[0029] The present disclosure provides the advantage of being able to provide an optical frequency reference signal with a high level of precision. In particular, the present disclosure enables the provision of a high-precision frequency reference signal with low technical effort compared to conventional techniques for providing frequency reference signals, such as frequency combs. In contrast to conventional frequency combs, the frequency reference signal according to the present disclosure can be provided based on standard optical and electronic components that are readily available and economically feasible. It is emphasized that the optical frequency reference signal according to the present disclosure can be provided based on continuous-wave lasers, such as diode lasers, without the need for octave-spanning spectra and nonlinear optical processes often required for frequency combs. Therefore, the present disclosure significantly reduces technical complexity and enables the provision of an optical frequency reference signal without the need for expensive components, thus enabling the provision of an optical frequency reference signal at low cost. Furthermore, due to the low technical complexity of the devices, these devices can be provided with smaller and / or more compact dimensions, allowing such devices to be miniaturized and / or integrated with other compact optical and / or electronic components. In particular, the present disclosure allows the use of continuous-wave lasers, and the carrier frequency, i.e., the center frequency of the laser radiation, can be swept over a predetermined spectral range, e.g., optionally over a 10-nm free spectral range. The carrier frequency and tuning speed may be known during the tuning process so that a predetermined sweep of the carrier frequency can be performed during the tuning time. Thus, the present disclosure may enable characterizing the resonator element and / or providing a frequency reference signal without the need to use a frequency comb or other hardware that requires significant cost and complexity. Therefore, according to the present disclosure, it is not necessary to generate a frequency comb for the resonator element. For spectroscopic applications, the carrier frequency may be tuned over a predetermined tuning range that includes the desired spectral range for the spectroscopic application.

[0030] Thus, the present disclosure provides the advantage that an optical frequency reference signal can be provided with less technical complexity, smaller size, and lower cost than those associated with conventional frequency combs. Instead of requiring octave-spanning and spectrally stabilized laser radiation that overlaps spectrally with one another, with the methods and devices of the present disclosure, it may be sufficient to provide laser light having a tunable carrier frequency and tuning the carrier frequency at a predetermined tuning speed over a predetermined spectral range, such as a 10-GHz free spectral range. Thus, the technical complexity and requirements can be significantly reduced compared to the prior art.

[0031] The difference between the first and second modulation frequencies may be selected to be at least 10% of the resonance linewidth of the carrier resonance and at most 50% of the free spectral range, thereby ensuring that the sideband resonances are appropriately spaced apart in the frequency domain and therefore adequately distinguishable from each other and from the carrier resonance.

[0032] The first modulation frequency and the second modulation frequency essentially correspond to frequencies near an n+1 / 2 multiple of the free spectral range, where n is an integer. "Near" means that the frequencies are close to, but not identical to, an n+1 / 2 multiple of the free spectral range. For example, the first modulation frequency and / or the second modulation frequency may each be spectrally spaced from an n+1 / 2 multiple of the free spectral range between 100 MHz and 100 GHz. This allows the sideband resonances to be spectrally well-placed between the carrier resonances.

[0033] Two sideband resonances generated by a first modulation frequency from one of the carrier resonances and at least two sideband resonances generated by a second modulation frequency from one of the carrier resonances may be located within at least one free spectral range, thereby enabling the spectral spacing of two adjacent carrier resonances, and therefore the spectral spacing of the free spectral range, to be determined with high accuracy using, inter alia, the spectral spacings of the first modulation frequency and the second modulation frequency from each other and from the associated carrier frequency.

[0034] Measuring the tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances may include measuring the tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances located within a single free spectral range. This has the advantage of limiting the spectral tuning range required to determine information used to determine the spacing of multiple carrier resonances to a small spectral range. This makes it easier to keep the tuning speed of the laser source constant over the small spectral range to be covered, thereby reducing or avoiding undesirable measurement errors when determining the spacing of carrier resonances. The method for characterizing a resonator element may further comprise coupling a second portion of the laser light into a calibration element having predetermined absolute transmission and / or reflection characteristics, and measuring the intensity of the portion of the laser light transmitted and / or reflected by the calibration element while tuning the carrier frequency. The method may further include identifying at least one distinct absolute transmission and / or reflection characteristic of a calibration element having a predetermined frequency in the frequency domain that coincides with or has a specified offset from one of the carrier resonance or sideband resonances, and calibrating the absolute frequency of at least one of the carrier resonance or sideband resonances based on the identified distinct absolute transmission and / or reflection characteristic of the calibration element. Thus, these additional steps based on the calibration element allow the spectral frequencies of the carrier resonance and the sideband resonances to be calibrated in an absolute manner. While the above-described steps allow the carrier resonance and the sideband resonances to be calibrated relative to each other, these additional steps allow absolute calibration of at least one of the carrier resonance and the sideband resonance, and calibration of all remaining carrier resonances and sideband resonances via relative calibration based on the modulation frequency via the absolutely calibrated one carrier resonance or sideband resonance. Therefore, this method enables absolute calibration of resonator elements and can be used to provide an absolute optical frequency reference signal.Therefore, the device for characterizing the resonator element may further include a calibration element having predetermined absolute transmission and / or reflection characteristics, and the detector unit may be further configured to measure the intensity of the portion of the laser light transmitted and / or reflected by the calibration element while adjusting the carrier frequency. The control unit may further be configured to identify at least one distinct absolute transmission and / or reflection characteristic of the calibration element having a predetermined frequency in the frequency domain that coincides with or has a specified offset from one of the carrier resonance or the sideband resonance, and to calibrate the absolute frequency of at least one of the carrier resonance or the sideband resonance based on the identified distinct absolute transmission and / or reflection characteristic of the calibration element.

[0035] The calibration element may include a gas cell filled with a predetermined gas having at least one well-defined absolute transmission and / or reflection characteristic. The use of a gas cell offers the advantage that the absorption characteristics of many gases are precisely characterized and well-known from the literature, such as distinct absorption lines. Therefore, the absorption and therefore transmission characteristics can be used as a sufficient basis for absolute calibration when comparing the carrier and / or sideband resonances of the resonator element with each other. Alternatively or additionally, the calibration element may include one or more of a frequency comb, a wave meter, or an element providing atomic and / or molecular transition lines.

[0036] The resonator elements may include fiber cavities and / or integrated waveguide resonators, and / or whispering gallery mode resonators, and / or etalon resonators, and / or Fabry-Perot resonators. This allows the resonator elements to be integrated into conventional optical and / or electronic setups and devices. Furthermore, such resonator elements may enable miniaturization of the devices for implementation in small optical and / or electronic setups.

[0037] The method for providing an optical frequency reference signal to a laser light having a tunable carrier frequency may further include modulating the intensity and / or phase of a first portion of laser light coupled to the resonator element with a first modulation frequency and a second modulation frequency to generate at least two sideband resonances spaced from the respective carrier resonance by a first modulation frequency and at least two sideband resonances spaced from the respective carrier resonance by a second modulation frequency, the first modulation frequency and the second modulation frequency differing from each other by an integer multiple of the free spectral range, and the frequencies of the sideband resonances may serve as spectral reference marks for the optical frequency reference signal. This reflects a calibration process for characterizing the resonator element and can be omitted if the provided resonator element has already been calibrated and does not need to be recalibrated. However, in some embodiments, the method for providing an optical frequency reference signal to a laser light having a tunable carrier frequency may also include a process for calibrating and / or recharacterizing the resonator element. This allows the calibration of the resonator element to be checked periodically, thereby ensuring and possibly improving the accuracy of the supplied optical frequency reference signal. Related methods for characterizing the resonator element may further correspond to those described above. In this case, the first and second modulation frequencies may each be in the radio frequency range, and in particular, the first and second modulation frequencies may each be in the range of approximately 100 MHz to approximately 10 GHz. The difference between the first and second modulation frequencies may be selected to be 10% or more of the resonance linewidth of the carrier resonance and 50% or less of the free spectral range. The first and second modulation frequencies essentially correspond to frequencies near multiples of n+1 / 2 of the free spectral range, where n is an integer.

[0038] Additionally, the method may further include absolute calibration of the provided optical frequency reference signal. This may include coupling a second portion of the laser light into a calibration element having predetermined absolute transmission and / or reflection characteristics and measuring the intensity of the portion of the laser light transmitted and / or reflected by the calibration element while adjusting the carrier frequency. This may further include identifying at least one distinct absolute transmission and / or reflection characteristic of the calibration element having a predetermined frequency in the frequency domain that coincides with or has a specified offset from one of the carrier resonance or sideband resonances, and calibrating the absolute frequency of the carrier resonance and / or sideband resonance based on the identified distinct absolute transmission and / or reflection characteristic. This method thereby enables periodic absolute recalibration of the resonator element and, therefore, the provided optical frequency reference signal. The calibration element may include a gas cell filled with a predetermined gas having at least one distinct absolute transmission and / or reflection characteristic, a frequency comb, a wavemeter, and / or a system providing an atomic transition line.

[0039] The carrier frequency of the laser light can be tuned over a spectral range of at least 10 free spectral ranges or more. The tuning of the carrier frequency is performed according to a predetermined tuning pattern. The predetermined tuning pattern includes sweeping the carrier frequency with a sinusoidal tuning pattern and / or a sawtooth tuning pattern. This allows the optical frequency reference signal to be provided quickly and reliably and / or with reduced technical complexity and low cost.

[0040] The laser light may be continuous wave laser light. The laser light may have a spectral width narrower than the linewidth of the resonator element. The laser light may have a short-term linewidth typically less than 100 kHz within a tuning and / or measurement time of about 5 μs. This may enable highly accurate measurements to be performed.

[0041] Therefore, the device for providing an optical frequency reference signal of laser light having a tunable carrier frequency may perform characterization of the resonator element and / or relative and / or absolute calibration of the resonator element and the carrier resonance and / or sideband resonance. Accordingly, the device for providing an optical frequency reference signal of laser light having a tunable carrier frequency may further comprise a modulator for modulating the intensity and / or phase of a portion of the laser light coupled to the resonator element at a first modulation frequency, the modulator generating, for each carrier resonance, at least two sideband resonances spaced from the carrier resonance by the first modulation frequency. The control unit may be further configured to provide, based on the intensity of the portion of the laser light transmitted and / or reflected by the resonator element, a tunable carrier frequency having local extrema due to the sideband resonance as a spectral reference mark of the optical frequency reference signal defined by the predetermined free spectral range and the first modulation frequency. The resonator element may include a fiber cavity, and / or an integrated waveguide, and / or a whispering gallery mode resonator (WGM), and / or an etalon, and / or a Fabry-Perot resonator. The modulator may include an electro-optic intensity and / or phase modulator.

[0042] Furthermore, the device for providing an optical frequency reference signal of laser light having a tunable carrier frequency may include a calibration element having predetermined absolute transmission and / or reflection characteristics. The detection unit may further measure the intensity of a portion of the laser light transmitted and / or reflected by the calibration element while tuning the carrier frequency. The control unit may further identify at least one clear, absolute transmission and / or reflection characteristic within the frequency range of the calibration element, where a predetermined frequency coincides with one of the carrier resonances or sideband resonances or has a specified offset, and calibrate the absolute frequencies of one or more carrier resonances or sideband resonances based on the identified clear, absolute transmission and / or reflection characteristics of the calibration element. This may enable absolute (re)calibration of the resonator element and the optical frequency reference signal. The calibration element may include a gas cell filled with a predetermined gas and having at least one clear, absolute transmission and / or reflection characteristic, and / or a frequency comb, and / or a wavemeter, and / or an element or system providing an atomic or molecular transition line.

[0043] The tunable laser source may include a tunable continuous wave laser source. The tunable laser source may include a diode laser. The tunable laser source may emit tunable laser light having a spectral width narrower than the linewidth of the resonator element.

[0044] A LIDAR system including a device according to the present disclosure may use the device to reference a tunable laser based on an optical reference signal provided by the device. In some systems known in the prior art, tuning lasers are referenced based on a frequency comb for high-precision, high-accuracy, and high-speed measurements. In this case, the frequency comb provides the optical frequency reference signal. Such devices are described, for example, in non-patent document 6 (Baumann, Esther, et al. "Comb-calibrated frequency-modulated continuous-wave ladar for absolute distance measurements." Optics Letters 38.12 (2013): 2026-2028), non-patent document 7 (Baumann, Esther, et al. "Comb-calibrated laser ranging for three-dimensional surface profiling with micrometer-level precision at a distance." Optics Express 22.21 (2014): 24914-24928), and non-patent document 8 (Yu, Wenhui, et al. "Comb-calibrated frequency sweeping interferometry for absolute distance and vibration measurement." Optics Letters 44.20 (2019): 5069-5072).

[0045] However, frequency combs often suffer from drawbacks such as technical complexity, sensitivity to environmental influences, and cost. The LIDAR system according to the present disclosure provides a calibrated resonator element, such as a fiber cavity characterized and / or calibrated according to the methods disclosed herein, to provide an optical frequency reference signal. This allows for high-precision, high-precision, and high-speed measurements, while being available at a lower cost and technical complexity than frequency combs. Thus, by using a device for providing an optical frequency reference signal according to the present disclosure, high precision comparable to that of a frequency comb can be achieved at significantly lower technical complexity and cost. As a result, the present disclosure can be applied to LIDAR and many other applications, even at low-cost segments where implementing a frequency comb is economically prohibitive, while achieving high precision, low complexity, and high stability. Furthermore, it is emphasized that in LIDAR devices including a device for providing an optical frequency reference signal according to the present disclosure, a calibrated resonator element is sufficient, and a modulator may not be required. However, in some embodiments, an intensity modulator may be provided, for example, to recalibrate the resonator element.

[0046] Similarly, a gas sensing system including a device for providing an optical frequency reference signal according to the present disclosure may be applied in a manner similar to conventional systems for frequency-based remote sensing of greenhouse gases, such as those described in non-patent document 9 (Nishiyama, Akiko, Daiki Ishikawa, and Masatoshi Misono. "High resolution molecular spectroscopic system assisted by an optical frequency comb." JOSA B 30.8 (2013): 2107-2112), non-patent document 10 (Rieker, Gregory B., et al. "Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths." Optica 1.5 (2014): 290-298), and non-patent document 11 (Herman, Daniel I., et al. "Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy." Science Advances 7.14 (2021): eabe9765).

[0047] In this case, the frequency comb can also be replaced by a device based on resonator elements calibrated according to the methods disclosed herein, thereby enabling the same high accuracy, precision, and high speed measurements as frequency comb-based devices, but with significantly lower technical complexity, lower manufacturing costs, and greater robustness.

[0048] It is understood that the present disclosure is not limited to the disclosed embodiments and combinations thereof, and that other technically feasible combinations and individual features are also included in the scope of the present disclosure. In the following, some optional embodiments and specific examples for illustrating the present disclosure will be described with reference to the drawings, but these do not limit the embodiments of the present disclosure.

[0049] Further optional embodiments and examples are described below with reference to the drawings. [Brief explanation of the drawings]

[0050] [Figure 1] 1 illustrates an optional embodiment of a device for characterizing a resonator element. [Figure 2] 2 illustrates a method for characterizing a resonator element using the device described with reference to FIG. 1. [Figure 3] The carrier and sideband resonances are shown schematically. [Figure 4] 1 illustrates a device for characterizing a resonator element according to an optional embodiment. [Figure 5] 1 shows a device for providing an optical frequency reference signal for laser light having a tunable carrier frequency. [Figure 6] 1 illustrates a schematic diagram of a method for providing a laser light optical frequency reference signal having a tunable carrier frequency. [Figure 7] 10 illustrates the results of a method for characterizing a resonator element according to an optional embodiment. [Figure 8] 10 illustrates the results of a method for characterizing a resonator element according to an optional embodiment. [Figure 9] The results of a method for providing an optical frequency reference signal are shown. [Figure 10] The results of absolute calibration using a calibration element are shown. [Figure 11] 1 illustrates a schematic diagram of a LIDAR according to an optional embodiment. [Figure 12] 1 illustrates a schematic diagram of a gas sensing system according to an optional embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0051] In the drawings, the same reference numerals are used between different drawings for corresponding or similar features.

[0052] 1 illustrates a device 100 for characterizing a resonator element 102 according to an optional embodiment. The resonator element 102 is formed as a fiber cavity resonator and represents a device under test to be characterized by the device 100.

[0053] The device 100 comprises a tunable laser source 104 for emitting laser light having a tunable carrier frequency. According to this embodiment, the output of the laser source 104 is directly coupled to an optical fiber, which is coupled to the resonator element 102. Thus, the optical fiber 106 can function as a coupling element 108 for coupling at least a portion of the laser light into the resonator element 102.

[0054] The device 100 further comprises a modulator 110, which may be arranged as an electro-optic modulator, for modulating the intensity and / or phase of a portion of the laser light coupled into the resonator element 102 at a first modulation frequency 1001 to generate at least two sideband resonances separated by the first modulation frequency 1001 from each carrier resonance, and at a second modulation frequency 1002 to generate at least two sideband resonances separated by the second modulation frequency 1002 from each carrier resonance. The modulator may be arranged as an electro-optic modulator and may modulate the intensity of the laser light or, alternatively or additionally, the phase of the laser light. As modulation signals, one, two, or more modulation signals may be applied to the modulator, such as a first modulation frequency 1001 and a second modulation frequency 1002. However, it is emphasized that in some optional embodiments, only one modulation frequency may be used. The modulation signal applied to the modulator 110 may be a superposition of the first modulation frequency 1001 and the second modulation frequency 1002. The first and second modulation frequencies 1001, 1002 may represent sinusoidal oscillations at the respective modulation frequencies. However, in some optional embodiments, the modulation signals may be provided as more complex modulation signals, such as waveforms covering a wide range in the spectral domain or continuous oscillations that deviate from a sinusoidal wave, such as sawtooth waves. Furthermore, a bias voltage 1004, for example, may be applied to the modulator 110 to set the basic transmission characteristics of the modulator 110.

[0055] After the modulator 110, the laser light is incident on the device under test, ie, the resonator element 102.

[0056] Downstream of the resonator element 102, the device 100 comprises a detection unit 112 for measuring the intensity of a portion of the laser light transmitted and / or reflected by the resonator element 102. According to this embodiment, the detection unit 112 is configured to detect the intensity of the laser light transmitted by the resonator element 102. The detection unit may comprise one or more photodiodes sensitive to the carrier frequency of the laser light. The detection unit 112 may further comprise or be connected to a data logger unit 114 for storing and / or visualizing and / or evaluating the output signal obtained by the photodiodes 112.

[0057] The device 100 further comprises a control unit 116 configured to tune the carrier frequency of the laser light, determine a tuning time, and determine the spacing between carrier resonances in a spectral range based on a measured intensity of the laser light transmitted and / or reflected by the resonator element 102 and based on the first modulation frequency 1001, the second modulation frequency 1002, and the measured tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances. The control unit 116 is communicatively connected to the detection unit 112 and / or a data logger and is capable of acquiring data measured by the detection unit 112. The control unit 116 is also connected to the laser light source 104 and may control the laser light source 104 to tune the carrier frequency of the laser light emitted by the laser light source 104. The control unit 116 is further connected to the modulator 110 and may provide modulation signals, such as the first modulation frequency 1001 and the second modulation frequency 1002, to the modulator 110.

[0058] By scanning the carrier frequency, i.e., center wavelength, of the laser light emitted by the laser source 104, the laser light coupled to the resonator element 102 experiences multiple carrier resonances within the resonator element 102 during the tuning process. For frequency values ​​corresponding to these carrier resonances, a significant amount of energy of the laser light is stored within the resonator element 102, and as a result, the intensity transmitted through the resonator element 102 decreases at the frequency corresponding to the carrier resonance. Therefore, the carrier resonances can be identified by local minima in the transmitted energy detected by the detection unit 112. In addition, due to modulating the intensity and / or phase of the laser light coupled to the resonator element 102 with the first modulation frequency 1001 and the second modulation frequency 1002, two frequency sidebands are generated, which experience sideband resonances at frequencies corresponding to the respective modulation frequencies, and these sideband resonances occur at specific frequencies different from the carrier resonances and are spectrally separated from the carrier frequency by the first and second modulation frequencies 1001 and 1002, respectively, as will be described in detail with reference to FIG. 3 .

[0059] Based on the tuning time and spacing between multiple carrier resonances in a spectral region based on the measured intensity of the laser light transmitted and / or reflected by the resonator element 102, and based on the first modulation frequency 1001, the second modulation frequency 1002, and the measured tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances, the control unit 116 can determine the spectral spacing between the carrier resonances and thereby the spectral range of the resonator element 102. In this manner, the device allows the spectral properties of the resonator element 102 to be characterized with high precision.

[0060] A method for characterizing a resonator element using the device described with reference to FIG. 1 is described below with reference to FIG.

[0061] The method includes a first step 202 of providing laser light having a tunable carrier frequency.

[0062] Another step 204 includes coupling at least a portion of the laser light into a resonator element 102 having multiple carrier resonances for a carrier frequency of the laser light, adjacent carrier resonances spaced apart from one another in the spectral range by a free spectral range.

[0063] Step 206 includes modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element 102 with a first modulation frequency 1001 and a second modulation frequency 1002 to generate, for each carrier resonance, at least two sideband resonances spaced from the carrier resonance by the first modulation frequency 1001 and at least two sideband resonances spaced from the carrier resonance by the second modulation frequency 1002. The first modulation frequency 1001 and the second modulation frequency 1002 are different from each other and are integer multiples of the free spectral range.

[0064] Step 208 includes tuning the carrier frequency of the laser light at a predetermined tuning rate.

[0065] Step 210 includes measuring the intensity of the laser light transmitted and / or reflected by the resonator element 102 while tuning the carrier frequency.

[0066] Step 212 includes measuring the tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances.

[0067] Step 214 includes using the measured intensity of the laser light transmitted and / or reflected by the resonator element 102 to determine the spacing between multiple carrier resonances in the spectral region based on the measured tuning time elapsed when tuning the first modulation frequency 1001, the second modulation frequency 1002, and the carrier frequency across four adjacent sideband resonances.

[0068] When tuning the carrier frequency, i.e., scanning the frequency of the laser light source, by modulating the intensity and / or phase of the laser light with the first modulation frequency 1001 and the second modulation frequency 1002, four sideband resonances are generated within one free spectral range (FSR) between two adjacent carrier resonances in the detection signal, as shown in Figure 3. Depending on the FSR of the resonator element 102, mod1 and f mod2 The first modulation frequency 1001 and the second modulation frequency 1002, denoted as , can be set near (n+1 / 2)×FSR, where n is an integer and is constrained by the spectral bandwidth of the modulator. d can be set to about several MHz depending on the structural line width of the resonator element 102.

[0069] In FIG. 3, resonance 3000 represents a carrier resonance, while resonances 3001 and 3002 represent a first modulation frequency 1001 (f mod1 ) and a second modulation frequency 1002 (f mod2 ) are generated due to the sideband resonances caused by the carrier resonance and the modulation sideband resonances within one free spectral range FSRt1 (t3) and t2 (t4). The time intervals between the carrier resonance and the modulation sideband resonances within one free spectral range FSRt1 (t3) and t2 (t4) are not necessarily equal due to the possibility of nonlinear behavior in the frequency scanning of the laser light source 104. However, the frequency f mod1 or f mod2 The frequency spacing between these two sideband resonances, determined by the first or second modulation frequency 1001, 1002, respectively, at 1001, 1002, is constant. The FSR of the resonator element is 2×f, which may correspond to several tens of GHz. mod1 Assuming that the modulation frequency range is constant, the FSR is mod1 ), (2n+1)×FSR=2f mod1 +f ? can be calibrated by f ? is the same modulation frequency f shown in Figure 3 mod1 is the frequency interval between two adjacent sideband resonances 3001.

[0070] This is due to the nonlinear tunable laser scanning, with a frequency interval f ? remains the only uncertain variable. However, to calculate the FSR, f ? It is desirable to calculate a single modulation frequency f mod1 For the method using only the frequency interval f ? is determined based on the average scanning speed of the laser light source 104, the tuning time (t1+t2) and the modulation frequency (2f mod1 ) can be calculated by taking into account f ? Although there remains some frequency uncertainty in , it is possible to determine the FSR.

[0071] In comparison, the first modulation frequency 1001 (f mod1 ) and second modulation frequency 1002 (f mod2 ), the frequency spacing f ? can be determined within a short time scale T1 + T2 + T3 and within a small frequency range of only a few MHz, as shown on the right side of Figure 3. Within one free spectral range, the fixed frequency spacing f between the two sideband resonances d is the first modulation frequency 1001 (f mod1 ) and the second modulation frequency 1002 (f mod2 ) are clearly defined by a predetermined frequency difference between the frequency intervals f1 and f2. The corresponding time intervals are indicated by the symbols T1 3003 and T2 3004 in FIG. 3. As a result, the frequency interval f ? is T3×2f d / (T1+T2). The time interval T3 is shown as 3005. By utilizing the flexibility of the first modulation frequency 1001 and the second modulation frequency 1002, a fixed frequency interval f between two sideband resonances resulting from different modulation frequencies 1001, 1002 can be obtained. d and the time interval T3 can be set to a very small value related to the FSR and the intracavity round-trip time in the resonator element 102. Therefore, the uncertainty of the frequency interval f? can be efficiently reduced. Furthermore, the method is applicable to characterizing quasi-periodic devices with FSRs greater than the first and second modulation frequencies, as well as to characterizing aperiodic devices by using resonator elements with low FSRs and performing dual RF modulation calibration. This method thus enables characterization of the resonator element 102 through determining the spectral spacing of the carrier resonances, i.e., the free spectral range, with high precision. This method allows the resonator element to be used to provide a laser light optical frequency reference signal with a tunable carrier frequency. It is emphasized that, according to the presented method, it is sufficient to measure the time elapsed when tuning the carrier frequency across four adjacent sideband resonances over the time interval T1+T2+T3, as indicated by reference numerals 3003, 3004, and 3005.

[0072] FIG. 4 illustrates an optional embodiment of the device 100 for characterizing the resonator element 102, based on the device 100 described with reference to FIG. 1 . The optional embodiment of the device 100 illustrated in FIG. 4 differs from the device of FIG. 1 in that it further includes a calibration element 118. The calibration element 118 has predetermined absolute transmission and / or reflection characteristics, such as a predetermined absorption spectrum including precisely defined absorption lines. Furthermore, the detection unit can measure the intensity of a portion of the laser light transmitted and / or reflected by the calibration element 118 during tuning of the carrier frequency. For this purpose, the detection unit can include a second photodiode 112 that measures the intensity of the laser light transmitted through the calibration element 118. The signal measured by the second photodiode 112 is supplied to a data logger 114 and evaluated by a control unit 116. The control unit 116 can further identify at least one distinct absolute transmission and / or reflection characteristic of the calibration element 118 that coincides with or has a predetermined offset from either the carrier resonance or the sideband resonance in the frequency domain, and calibrate the absolute frequency of at least one of the carrier resonance 3000 or the sideband resonances 3001, 3002 based on the identified distinct absolute transmission and / or reflection characteristic of the calibration element 118. According to the presented embodiment, the calibration element 118 comprises a gas cell containing a gas having precisely predetermined transmission and / or reflection characteristics, e.g., precisely determined absorption lines. This enables absolute calibration of the spectral characteristics of the resonator element 116, since the carrier resonance 3000, the sideband resonances 3001, 3002, and the free spectral range can be referenced to the absolutely determined transmission and / or reflection characteristics of the calibration element. As a result, the resonator element 102 can be accurately characterized to provide a highly accurate absolute optical frequency reference signal.

[0073] 5 illustrates a device 200 for providing an optical frequency reference signal of laser light having a tunable carrier frequency. Similar to the device 100 described in the previous figure, the device 200 includes a tunable laser source 104 for emitting laser light having a tunable carrier frequency. The device 200 further includes a well-characterized resonator element 102 having multiple carrier resonances for the tunable carrier frequency of the laser light, the carrier resonances being spaced apart from one another in the spectral range by respective predetermined free spectral ranges (FSRs), and the device is capable of coupling a portion of the laser light into the resonator element 102. The device 200 also includes a detection unit 112 for measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element 102, and a control unit 116 configured to provide a tunable carrier frequency having a local extremum in the measured intensity of the portion of the laser light transmitted and / or reflected by the resonator element 102 as a spectral reference mark of the optical frequency reference signal defined by the predetermined free spectral range. The device further comprises a calibration element 118 as described with reference to Figure 4. Unlike the devices and methods described in relation to Figures 1 and 4, when using a well-characterized resonator element 102 to provide the optical frequency reference signal, the modulator 110 is not necessarily required, since a well-characterized resonator element whose spectral characteristics have been determined by applying the method according to the present disclosure can be used. However, according to another embodiment, a modulator can additionally be provided, which makes it possible to recalibrate the resonator element 102 as needed.

[0074] 5, the resonator element 102 is used to provide an optical frequency reference signal for further applications, which may include spectral characterization of a photonic device 120, such as the illustrated micro-ring resonator 120. With the resulting optical frequency reference and optional calibration element 118, spectral characteristics, such as the resonant frequency of the photonic device 120, may be characterized relatively, and optionally absolutely, by comparing the measured frequency to the resulting carrier and / or sideband resonances of the resonator element 102.

[0075] FIG. 6 schematically illustrates a method for providing a laser light optical frequency reference signal having a tunable carrier frequency. This method can be performed using the device described with reference to FIG. 5. The method includes, in step 602, providing a resonator element 102 having a plurality of carrier resonances 3000 for the laser light carrier frequency, wherein adjacent carrier resonances 3000 are spaced apart in a spectral range by respective predetermined free spectral ranges (FSRs). The method further includes, in step 604, coupling a portion of the laser light into the resonator element 102 and, in step 606, measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element 102. In step 608, the method also includes providing a tunable carrier frequency having a local extremum in the measured intensity of the portion of the laser light transmitted and / or reflected by the resonator element 102 as a spectral reference mark of the optical frequency reference signal defined by the predetermined free spectral range (FSR).

[0076] Below, a detailed example is presented to demonstrate proof of concept, without the present disclosure or embodiments being limited to this detailed example.

[0077] As a proof-of-concept demonstration, we characterized the resonator element 102 by measuring its optical dispersion, prepared in the form of a fiber cavity, to verify the method and device described in this disclosure. The fiber cavity consisted of a 10 dB fiber coupler and 5 m of standard telecommunication fiber (SMF-28) with a zero-dispersion wavelength of approximately 1.310 nm. The fiber cavity had a free spectral range (FSR) of approximately 39 MHz and a mode linewidth of approximately 1 MHz, which limited the measurement speed to approximately 1 THz / s, as described in P. Del'Haye et al., "Frequency comb assisted diode laser spectroscopy for measurement of microcavity dispersion," Nat. Photonics 3, 529-533 (2009)."

[0078] To demonstrate the ultra-high frequency resolution of the disclosed method and device, a 1.3 μm tunable laser with a wavelength range of 1.270-1.330 nm is used as the laser source to resolve small free spectral range (FSR) variations around the zero-dispersion wavelength. In the experiment, the tunable laser is driven at a first modulation frequency 1001 (f mod1 ) and second modulation frequency 1002 (f mod2 ) with a 4 MHz frequency difference. These two modulated signals are combined using a power combiner and applied to an electro-optic modulator (EOM) which also serves as modulator 110. The laser light transmitted through the fiber cavity is detected by a detection unit 112 which includes a photodiode (PD) and an oscilloscope as a data logger 114 for recording measurements with a memory depth of 31.25 million.

[0079] Subsection a) of Figure 7 shows the transmission spectrum of the fiber cavity between 1.270 nm and 1.330 nm, with a detailed zoom around 1.300 nm shown in the inset. The vertical axis represents the transmittance in arbitrary units, and the horizontal axis represents wavelength in nanometers. The deep transmission dip is the fiber cavity experienced by laser light tuned to the carrier frequency and is referred to as carrier resonance 3000. Four additional small dips appearing within one free spectral range (FSR) result from modulation sidebands and are referred to as sideband resonances 3001 and 3002. Subsection b) of Figure 7 shows the measured FSR evolution of the fiber cavity as a function of wavelength around the zero-dispersion region. Trace 7000 (left axis) in subsection b) of Figure 7 shows the calculated results from a dual RF modulation scheme, with the left vertical axis representing FSR minus the 38,906 MHz offset and the horizontal axis representing wavelength in nanometers. With a frequency resolution of less than 15 Hz, trace 7000 clearly resolves small FSR variations (<800 Hz) in the 11 THz range, revealing the complex cavity dispersion of the fiber loop, from normal dispersion at short wavelengths, through zero dispersion, and then anomalous dispersion at long wavelengths. Trace 7002 shows the results of a second-order polynomial fit with a zero-dispersion wavelength of 1.315 nm. For comparison, trace 7004 (right axis) in subsection b) of Figure 7 shows the calculated FSR evolution based on measurements using a single modulation frequency (20 GHz) as the modulation signal, which cannot address small variations in the cavity FSR in as much detail as using two modulation frequencies. The right vertical axis represents the FSR (trace 7004) minus the offset of 38,906 MHz. Subsection c) of Figure 7 shows the frequency difference (trace 7006) by which the measured FSR plotted in trace 7000 deviates from the fitted value plotted in trace 7002, with the vertical axis representing the residual error (in Hz) and the horizontal axis representing the wavelength in nanometers.Subsection d) of Figure 7 shows histograms 7008 and 7010 of the frequency differences based on trace 7006 in subsection c), showing a root-mean-square deviation of 14.2 Hz. These results demonstrate the ultra-high frequency resolution of dual-RF broadband modulation spectroscopy in accordance with the present disclosure.

[0080] Based on the measurements shown in subsection b) of Figure 7, the top panel of Figure 8 shows the calculated group velocity dispersion β2 of the fiber cavity on trace 8000, where the horizontal axis is the wavelength in nanometers and the vertical axis is the group velocity dispersion β2 (ps 2 The results are in good agreement with the dispersion characteristics of standard telecommunication fibers. In the bottom panel of Figure 8, trace 8002 shows the corresponding group delay dispersion (GDD) for a 5 m fiber cavity, including 10 dB coupler dispersion. The vertical axis is the group delay dispersion (fs 2 The graphs represent the GDD (units) of the fiber cavity. By removing 3 m of fiber from the fiber cavity, we can use the dual RF modulation scheme to measure the GDD of the 2 m fiber cavity (trace 8004 in the bottom panel), which has a zero-dispersion wavelength of 1.318 nm. Furthermore, by subtracting the GDD of the 2 m fiber cavity from the GDD of the 5 m fiber cavity, we obtain the GDD of the 3 m fiber, which is plotted as trace 8006 in the bottom panel of Figure 8, showing a zero-dispersion wavelength of 1.312 nm. Trace 8006 shows the difference between traces 8002 and 8004. These measurements confirm that the zero-dispersion wavelength of a fiber cavity with a long fiber length approaches the zero-dispersion wavelength of the fiber itself. This demonstration proves that the method disclosed herein can be used to characterize the optical properties of discrete devices, such as dispersion-engineered broadband mirrors and integrated photonic devices.

[0081] The results presented above demonstrate that the dual RF modulation scheme disclosed herein has ultrahigh frequency resolution for quasi-periodic structures with low FSR (FSR < modulation frequency). Below, we extend the scope of application of the disclosed method and measure the mode spectrum of a device with high FSR (FSR > modulation frequency), such as an optical microresonator, using the device shown in Figure 5. In this measurement, we measure the resonant frequency of a separately fabricated Si3N4 resonator using a 5 m fiber cavity as the resonator element to provide the carrier resonance of the fiber cavity as a frequency marker, i.e., an optical frequency reference signal. The Si3N4 resonator was fabricated by depositing a 750 nm thick Si3N4 thin film on a silicon substrate via a 3 μm SiO2 layer using a low-temperature reactive sputtering method. This fabrication method is described in A. Frigg, et al., "Low-loss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thin films," Opt. Express 27, 37795-37805 (2019)).

[0082] The inset in subsection a) of Figure 9 shows a scanning electron microscope image of the Si3N4 microresonator used in the experiment, with a diameter of 200 μm and a waveguide cross-section of 1.8 μm × 750 nm. The measured FSR and intrinsic optical quality factor are approximately 231 GHz and 2 million, respectively. As shown in Figure 5, a portion of the laser light, optionally modulated at two modulation frequencies, is injected into the fiber cavity, following the same path as in the previous experiment. The transmission signal is detected by a photodiode 112 and recorded on one channel of an oscilloscope 114. Another portion of the laser light is coupled into the Si3N4 resonator through two lensed fibers and emitted from the Si3N4 resonator. The transmission spectrum is recorded by another photodiode 112 and another channel of an oscilloscope 114. While scanning the frequency of the CW laser, the transmission signals from the fiber cavity and the Si3N4 resonator are simultaneously recorded. Subsection a) of Figure 9 shows the normalized transmission spectrum of the Si3N4 resonator, where two distinct mode families are observed. The mode family with the higher optical quality factor is marked with a star marker 9000. The vertical axis shows the transmittance in arbitrary units, and the horizontal axis shows the wavelength in nanometers. Subsection b) of Figure 9 shows one resonance, indicated by trace 9002, near a wavelength of 1.271 nm, along with a frequency marker 9004 from the fiber cavity. The FSR of the fiber cavity is first calculated based on the dual RF modulation scheme described above, and the calculated fiber cavity resonance is used as a frequency marker to measure the mode structure of the Si3N4 resonator (e.g., FSR evolution, resonance linewidth, dispersion, etc.). The vertical axis shows the amplitude in arbitrary units, and the horizontal axis shows the wavelength in nanometers.

[0083] The resonant frequencies of a family of modes in a dispersive resonator can be expressed as a Taylor series as follows:

number

[0084] To further highlight the broad applicability and effectiveness of the disclosed method, we demonstrate its application to nonperiodic structures by elucidating the absorption spectrum of a gas cell. For this demonstration, we use an optical fiber-coupled hydrogen fluoride (HF) gas cell with a pressure of 50 Torr and a path length of 2.7 cm. Again, laser light from a 1.3 μm CW laser source is split into two paths: one path is used to probe the absorption spectroscopy of HF, and the other path is modulated with dual RF signals and coupled into a 5 m fiber loop cavity as a resonator element to simultaneously calibrate the laser frequency sweep.

[0085] Subsection a) of Figure 10 shows the strong HF molecular absorption lines (P and R branches) in the O-band region, with the vertical axis representing transmittance in arbitrary units and the horizontal axis representing wavelength in nanometers. Subsection b) shows the broadened spectrum of the P(2) absorption line (trace 10000), which also includes the carrier resonance of the fiber cavity (10002 as a frequency reference). Because the pressure broadening effect of the HF gas is much larger than its Doppler broadening effect, a Lorentzian function (dashed line 10004 in subsection b) is used to fit the spectral profile.

[0086] Table 1 shows the HF absorption line measurement results compared with the HITRAN database (for the HITRAN database, see Non-Patent Document 14 (IE Gordon et al. "The HITRAN2016 molecular spectroscopic database," J. Quant. Spectrosc. Radiat. Transf. 203, 3-69 (2017))).

[0087] [Table 1] 1. Data from HITRAN show calibrated values ​​for a pressure shift of 50 Torr. 2. Uncertainties in pressure shift and pressure broadening linewidths are calculated based on a 20% uncertainty in pressure. 3. The measured R(2) values ​​are set equal to those from HITRAN.

[0088] The second column of Table 1 shows the absorption line positions calculated from the HITRAN database, corrected for the pressure shift from the vacuum transition wavelength. Columns 3 and 4 show the calculated Gaussian and Lorentzian linewidths (full width at half maximum, FWHM), respectively. The uncertainties are calculated based on a 20% uncertainty in the gas pressure specified by the gas cell manufacturer. Column 5 shows the measured absorption line positions. Because no absolute frequency reference is provided in this demonstration, the measured wavelength of the R(2) absorption line is set equal to the value calculated from the HITRAN database. Column 6 shows the wavelength difference between the measured results and the HITRAN database. The results confirm that the measurements are in excellent agreement with the HITRAN database. This small difference is likely due to the uncertainty in the gas pressure. Using the calculated Gaussian linewidths and Voigt functions shown in column 3, the final column shows the measured Lorentzian linewidths (FWHM) of the different absorption lines. Considering that the measured wavelength of the absorption line position is smaller than the calculated value in the second column and that the measured Lorentzian linewidth is larger than the calculated value, we speculate that the pressure of the HF gas cell used as a calibration element in the experiment may be higher than the 50 Torr specified by the manufacturer.

[0089] In conclusion, this disclosure presents and demonstrates a powerful broadband spectroscopy technique based on a tunable CW laser whose frequency sweep characteristics are calibrated by a fiber cavity with dual RF frequency modulation. Using this method, we can resolve small FSR deviations (800 Hz) of the fiber cavity near the zero-dispersion region over an 11 THz frequency range with a resolution of less than 15 Hz. Furthermore, the demonstrated measurement speed is 1 THz / s, which is limited by the linewidth of the fiber cavity, whereas conventional devices are limited by the measurement system (see non-patent document 12). This measurement speed can be significantly improved to 10 THz / s or more if the resonator elements have a wider linewidth (3.2 MHz or greater). Ultimately, the measurement speed is limited by the tuning speed of the laser source. Furthermore, the demonstrated method can exceed the stringent requirements for spectral flatness, comb line power, and polarization of conventional frequency comb-based spectroscopy. The measurement spectral range of the disclosed method is limited only by the range of the tunable CW laser and can be extended by connecting multiple CW lasers in series, as suggested in [J. Liu et al., "Frequency-comb-assisted broadband precision spectroscopy with cascaded diode lasers," Opt. Lett. 41, 3134-3137 (2016)]. This allows application to previously inaccessible spectral regions where high-resolution wavemeters or frequency combs are unavailable. We further validated the disclosed method for applications in integrated photonic devices and dispersion characterization of molecular absorption spectra of HF gas. By using known atomic / molecular transitions as absolute frequency standards, the method can be applied to high-precision broadband molecular spectroscopy (A. Shkarin et al., "Nanoscopic Charge Fluctuations in a Gallium Phosphide Waveguide Measured by Single Molecules," Phys. Rev. 2016).Lett. 126, 133602 (2021)). Furthermore, the method according to the present disclosure can be applied to LIDAR (see E. Baumann et al. "Comb-calibrated frequency-modulated continuous-wave lidar for absolute distance measurements," Opt. Lett. 38, 2026-2028 (2013)), 3D imaging (see E. Baumann et al. "Comb-calibrated laser ranging for three-dimensional surface profiling with micrometer-level precision at a distance," Opt. Express 22, 24914-24928 (2014))), refractive index measurement (see Yang et al. "Frequency comb calibrated frequency-sweeping interferometry for absolute group refractive index measurement of air," Appl. Opt. 56, 3109-3115 (2017))), precise frequency measurement, and photonic devices (see V. Brasch et al. "Photonic chip-based optical "Frequency comb using soliton Cherenkov radiation," Science 351, 357-360 (2016) and A. Nishiyama et al. "Precise frequency measurement and characterization of a continuous scanning single-mode laser with an optical frequency comb," Opt. Lett. 39, 4923-4926 (2014)), as well as characterization of astrophotonic devices (S. Minardi, RJ Harris, and L.It is poised for widespread application in a variety of scenarios, including the astronomy and modern optics (see Labadie, "Astrophotonics: astronomy and modern optics," Astron. Astrophys. Rev. 29, 6 (2021)).

[0090] Figure 11 illustrates a schematic diagram of a LIDAR system 300 including a device 200 for providing an optical frequency reference signal according to an optional embodiment. Figure 12 illustrates a schematic diagram of a gas sensing system 400 including a device 200 for providing an optical frequency reference signal according to an optional embodiment. [Explanation of symbols]

[0091] 100 Device for characterizing resonator elements 102 Resonator element 104 Laser light source 106 Optical Fiber 108 Coupling element 110 Modulator for modulating intensity and / or phase 112 Detection Unit 114 Data Logger 116 Control Unit 118 Calibration Element 120 Photonic Devices / Microring Resonators 200 Device for providing an optical frequency reference signal 202-214 Method Steps 300 LIDAR System 400 Gas Sensing System 602-608 Method Steps 1001 1st modulation frequency 1002 Second modulation frequency 3000 Carrier Resonance 3001 Sideband resonance caused by the first modulation frequency 3002 Sideband resonance caused by the second modulation frequency 3003Time separation between first sideband resonances scanned from different modulation frequencies 3004 Time separation between scanned second sideband resonances from different modulation frequencies 3005 unknown time interval between adjacent carrier resonances 7000 Trace showing the results of dual RF modulation scheme 7002 Trace showing a quadratic polynomial fit 7004 FSR evolution calculated with single RF modulation scheme 7006 Frequency difference at which the measured FSR deviates from the fit plot 7008, 7010 Histogram of frequency difference 8000 Calculated Group Velocity Dispersion 8002, 8004, 8006 Group delay dispersion at different fiber lengths 9000 Mode family with high optical quality factor 9002 Resonance profile at 1.271nm 9004 Frequency Marker 9006 Bond Dispersion Profile 9008 Fitted Curve 9010 Optical spectrum of a single bright soliton 9012 Fitted envelope curve 10000 absorption line spectrum 10002 Carrier Resonance 10004 Fitted Lorentzian Functions FSR Free Spectral Range

Claims

1. A method for characterizing a resonator element (102), comprising: - providing laser light having a tunable carrier frequency; coupling at least a first portion of the laser light into a resonator element (102) having a plurality of carrier resonances (3000) at a carrier frequency of the laser light, wherein adjacent carrier resonances (3000) are spaced apart from one another within a spectral range by a free spectral range (FSR); modulating the intensity and / or phase of the portion of laser light coupled into the resonator element (102) with a first modulation frequency (1001) and a second modulation frequency (1002) to generate, for each carrier resonance (3000), at least two sideband resonances (3001) spaced from the carrier resonance (3000) by the first modulation frequency (1001) and at least two sideband resonances (3002) spaced from the carrier resonance (3000) by the second modulation frequency (1002), the first modulation frequency (1001) and the second modulation frequency (1002) being integer multiples of the free spectral range and different from each other; tuning the carrier frequency of the laser light at a predetermined tuning speed; - measuring the intensity of the laser light transmitted and / or reflected by the resonator element (102) while tuning the carrier frequency; measuring the tuning time elapsed in tuning the carrier frequency across four adjacent sideband resonances (3001, 3002), the tuning time indicating the time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to the last; and - determining the spacing between a plurality of carrier resonances (3000) in the spectral range using the measured intensity of the laser light transmitted and / or reflected by the resonator element (102), the first modulation frequency (1001), the second modulation frequency (1002), and the tuning time measured when tuning the carrier frequency across four adjacent sideband resonances (3001, 3002); A method comprising:

2. 2. The method of claim 1, wherein the first modulation frequency (1001) and the second modulation frequency (1002) are each within a radio frequency (RF) band.

3. 3. The method according to claim 1 or 2, wherein the first modulation frequency (1001) and the second modulation frequency (1002) are each in the range of about 100 MHz to about 100 GHz.

4. 4. The method according to claim 1, wherein the difference between the first modulation frequency (1001) and the second modulation frequency (1002) is greater than or equal to 10% of the resonance linewidth of the carrier resonance (3000) and less than or equal to 50% of the free spectral range (FSR).

5. 5. The method according to claim 1, wherein the first modulation frequency (1001) and the second modulation frequency (1002) correspond essentially to frequencies close to n+1 / 2 times the free spectral range (FSR), where n is an integer.

6. 6. The method according to claim 1, wherein, within at least one of the free spectral ranges (FSR), two sideband resonances (3001) generated by the first modulation frequency (1001) from one of the carrier resonances (3000) and at least two sideband resonances (3002) generated by the second modulation frequency (1002) from the same carrier resonance (3000) are located.

7. 7. The method of claim 1, wherein measuring the tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances (3001, 3002) comprises measuring the tuning time elapsed when tuning the carrier frequency across four adjacent sideband resonances (3001, 3002) positioned within one of the free spectral ranges.

8. The method according to any one of claims 1 to 7, wherein the free spectral range (FSR) of the resonator element (102) is between 1 MHz and 100 GHz.

9. The method according to any one of claims 1 to 8, further comprising: - coupling a second portion of the laser light into a calibration element (118) having predetermined absolute transmission and / or reflection characteristics; - measuring the intensity of the portion of the laser light transmitted and / or reflected by the calibration element (118) while tuning the carrier frequency; Identifying at least one distinct absolute transmission and / or reflection characteristic of the calibration element (118) in the frequency domain with a predetermined frequency that coincides with or has a specified offset from either the carrier resonance (3000) or the sideband resonances (3001, 3002); and calibrating the absolute frequency of at least one of the carrier resonance (3000) or the sideband resonances (3001, 3002) based on the specific absolute transmission and / or reflection characteristics of the identified calibration element (118); A method comprising:

10. 10. The method of claim 9, wherein the calibration element (118) comprises a gas cell filled with a predetermined gas having at least one well-defined absolute transmission and / or reflection characteristic, and one or more of the following elements: a frequency comb, a wavemeter, or an element providing atomic and / or molecular transition lines.

11. A device (100) for characterizing a resonator element (102), comprising: a tunable laser source (104) for emitting laser light having a tunable carrier frequency; a coupling element (108) for coupling at least a portion of the laser light into said resonator element (102); a modulator (110) for modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element (102) to generate, for each carrier resonance (3000) at a first modulation frequency (1001), at least two sideband resonances (3001) spaced apart from the carrier resonance (3000) by the first modulation frequency (1001), and to generate, for each carrier resonance (3000) at a second modulation frequency (1002), at least two sideband resonances (3002) spaced apart from the carrier resonance (3000) by the second modulation frequency (1002); a detection unit (112) for measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element (102), and a control unit (116) configured to tune a carrier frequency of the laser light, determine a tuning time, and determine a spacing between the carrier resonances (3000) within a spectral range based on a measured intensity of the laser light transmitted and / or reflected by the resonator element (102) and based on the first modulation frequency (1001), the second modulation frequency (1002), and a measured tuning time elapsed in tuning the carrier frequency across four adjacent sideband resonances (3001, 3002), the tuning time corresponding to the time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to the last one; A device comprising:

12. 12. The device (100) of claim 11, wherein the resonator element (102) comprises a fiber cavity and / or an integrated waveguide resonator, and / or a whispering gallery mode resonator, and / or an etalon, and / or a Fabry-Perot resonator.

13. 13. The device (100) according to claim 11 or 12, wherein the modulator (110) comprises an electro-optical intensity and / or phase modulator.

14. The device (100) according to any one of claims 11 to 13, further comprising a calibration element (118) having predetermined absolute transmission and / or reflection characteristics, The detection unit (112) may further measure the intensity of a portion of the laser light transmitted and / or reflected by the calibration element (118) during tuning of the carrier frequency; and The control unit (116) is capable of identifying at least one clear absolute transmission and / or reflection characteristic of the calibration element (118) having the predetermined frequency in the frequency domain that coincides with or has a specified offset from either the carrier resonance (3000) or the sideband resonances (3001, 3002), and calibrating the absolute frequency of at least one of the carrier resonance (3000) or the sideband resonances (3001, 3002) based on the identified absolute transmission and / or reflection characteristic.

15. 15. The device of claim 14, wherein the calibration element (118) comprises a gas cell filled with a predetermined gas having at least one well-defined absolute transmission and / or reflection characteristic, and / or a frequency comb, and / or a wavemeter, and / or an element providing atomic and / or molecular transition lines.

16. 1. A method for providing laser light having a tunable carrier frequency as an optical frequency reference signal, comprising: providing a resonator element (102) having a plurality of carrier resonances (3000) at a carrier frequency of the laser light, the adjacent carrier resonances (3000) being spaced apart in a spectral range by a respective predetermined free spectral range (FSR); - coupling a first portion of the laser light into the resonator element (102); and the method further comprises: tuning the carrier frequency of the laser light at a predetermined tuning speed; providing a portion of the laser light transmitted and / or reflected by the resonator element (102) as a spectrally swept optical frequency reference signal, the intensity of the laser light transmitted and / or reflected by the resonator element (102) having local extrema at frequencies spectrally spaced from one another by the FSR; A method comprising:

17. 17. The method of claim 16, wherein the carrier frequency of the laser light is tuned in a spectral range spanning at least 10 of the free spectral range (FSR).

18. 18. The method of claim 16 or 17, wherein the tuning of the carrier frequency is performed according to a predetermined tuning pattern.

19. 20. The method of claim 18, wherein the predetermined tuning pattern for tuning the carrier frequency comprises sweeping the carrier frequency through a sinusoidal tuning pattern and / or a sawtooth tuning pattern.

20. The method according to any one of claims 16 to 19, wherein the laser light is a continuous wave laser light.

21. The method according to any one of claims 16 to 19, wherein the laser light has a spectral width that is smaller than the linewidth of the resonator element (102).

22. 22. The method according to claim 16, further comprising the steps of: modulating the intensity and / or phase of the first portion of the laser light coupled into the resonator element (102) with a first modulation frequency (1001) and a second modulation frequency (1002) to generate, for each carrier resonance (3000), at least two sideband resonances (3001) spaced from the carrier resonance (3000) by the first modulation frequency (1001) and at least two sideband resonances (3002) spaced from the carrier resonance (3000) by a second modulation frequency (1002), wherein the first modulation frequency (1001) and the second modulation frequency (1002) are integer multiples of the free spectral range (FSR) and are different from each other; and providing frequencies of the sideband resonances (3001, 3002) as spectral reference marks of the optical frequency reference signal.

23. 23. The method of claim 22, wherein the first modulation frequency (1001) and the second modulation frequency (1002) are each within a radio frequency (RF) band.

24. 24. The method according to claim 22 or 23, wherein the first modulation frequency (1001) and the second modulation frequency (1002) are each in the range of about 100 MHz to about 10 GHz.

25. 25. The method according to any one of claims 22 to 24, wherein the difference between the first modulation frequency (1001) and the second modulation frequency (1002) is greater than or equal to 10% of the resonance linewidth of the carrier resonance (3000) and less than or equal to 50% of the free spectral range (FSR).

26. 26. The method according to any one of claims 22 to 25, wherein the first modulation frequency (1001) and the second modulation frequency (1002) correspond to frequencies substantially close to n+1 / 2 times the free spectral range (FSR), where n is an integer.

27. The method according to any one of claims 22 to 26, further comprising: - coupling a second portion of the laser light into a calibration element (118) having predetermined absolute transmission and / or reflection characteristics; - measuring the intensity of the portion of the laser light transmitted and / or reflected by the calibration element (118) while tuning the carrier frequency; Identifying at least one distinct absolute transmission and / or reflection characteristic of the calibration element (118) in the frequency domain with a predetermined frequency that coincides with or has a specified offset from either the carrier resonance (3000) or the sideband resonances (3001, 3002); and calibrating the absolute frequencies of the carrier resonance (3000) and / or sideband resonances (3001, 3002) based on the determined absolute transmission and / or reflection characteristics; A method comprising:

28. 28. The method of claim 27, wherein the calibration element (118) comprises a gas cell filled with a predetermined gas having at least one well-defined absolute transmission and / or reflection characteristic, and / or a frequency comb, and / or a wavemeter, and / or an atomic transition line.

29. 29. Use of the optical frequency reference signal provided by the method of any one of claims 16 to 28 as a spectral reference mark.

30. 1. A method for spectroscopically characterizing an analyte, comprising: - Providing said optical frequency reference signal using a method according to any one of claims 16 to 29; and - using said optical frequency reference signal as a spectral reference mark for spectroscopically characterizing an analyte; A method comprising:

31. 31. The method of claim 30, wherein using the optical frequency reference signal as a spectral reference mark comprises determining relative spectral distances of a plurality of spectral characteristics of an analyte based on one or more free spectral ranges (FSRs).

32. A device (200) for providing laser light having a tunable carrier frequency as an optical frequency reference signal, comprising: a tunable laser source (104) for emitting laser light having a tunable carrier frequency; a resonator element (102) having a plurality of carrier resonances (3000) for laser light having a tunable carrier frequency, the carrier resonances (3000) being spaced apart from one another in a spectral range by a predetermined free spectral range (FSR), and the device (200) being capable of coupling a portion of the laser light; the device further comprising: - a control unit configured to tune the carrier frequency at a predetermined tuning speed; The device (200) can use a portion of the laser light transmitted and / or reflected by the resonator element (102) as a spectrally swept optical frequency reference signal, wherein the intensity of the laser light transmitted and / or reflected by the resonator element (102) has local extrema at frequencies spectrally spaced from one another by the free spectral range (FSR).

33. 33. The device of claim 32, further comprising: a detection unit (112) for measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element (102), and a control unit (116) configured to provide the frequency of the tunable carrier frequency having the local extremum in the measured intensity of the laser light transmitted and / or reflected by the resonator element (102) as a spectral reference mark of the optical frequency reference signal defined by the predetermined free spectral range (FSR); A device comprising:

34. 34. The device (200) of claim 32 or 33, further comprising a modulator (110) for modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element (102) at a first modulation frequency (1001) to generate, for each carrier resonance (3000), at least two sideband resonances (3001) spaced from the carrier resonance (3000) by the first modulation frequency (1001); The control unit is further configured to provide a frequency of the tunable carrier frequency having the local extremum due to the sideband resonance (3001) in the measured intensity of the portion of the laser light transmitted and / or reflected by the resonator element (102) as the spectral reference mark of the optical frequency reference signal defined by the predetermined free spectral range (FSR) and the first modulation frequency (1001).

35. The device (200) according to any one of claims 32 to 34, wherein the resonator element (102) comprises a fiber cavity and / or an integrated waveguide, and / or a whispering gallery mode resonator, and / or an etalon, and / or a Fabry-Perot resonator.

36. 36. The device (200) of claim 34 or 35, wherein the modulator (110) comprises an electro-optic intensity and / or phase modulator.

37. A device (200) according to any one of claims 32 to 36, further comprising a calibration element (118) having predetermined absolute transmission and / or reflection characteristics, The detection unit (112) may further measure the intensity of the portion of the laser light transmitted and / or reflected by the calibration element (118) during tuning of the carrier frequency; and The control unit (116) is further configured to identify at least one distinct absolute transmission and / or reflection characteristic of the calibration element (118) having a predetermined frequency that coincides with or has a specified offset from either the carrier resonance (3000) or the sideband resonances (3001, 3002) in the frequency domain, and to calibrate the absolute frequency of at least one of the carrier resonance (3000) or the sideband resonances (3001, 3002) based on the identified distinct absolute transmission and / or reflection characteristic of the calibration element (118). Device (200).

38. 38. The device (200) of claim 37, wherein the calibration element (118) comprises a gas cell filled with a predetermined gas having at least one well-defined absolute transmission and / or reflection characteristic, and / or a frequency comb, and / or a wavemeter, and / or an atomic and / or molecular transition line.

39. The device of any one of claims 32 to 38, wherein the tunable laser source (104) comprises a tunable continuous wave laser source.

40. The device of any one of claims 32 to 39, wherein the tunable laser source (104) comprises a diode laser.

41. The device according to any one of claims 32 to 40, wherein the tunable laser light source is capable of emitting tunable laser light having a spectral width smaller than the linewidth of the resonator element (102).

42. A LIDAR system (300) according to any one of claims 32 to 41 for providing said optical frequency reference signal.

43. A gas sensing system (400) comprising a device (200) according to any one of claims 32 to 41 for providing an optical frequency reference signal.

44. A method for characterizing a resonator element (102), comprising: - providing laser light having a tunable carrier frequency; coupling at least a first portion of the laser light into the resonator element (102) having a plurality of the carrier resonances (3000) at the carrier frequency of the laser light; modulating the intensity of the portion of the laser light coupled into the resonator element at a first modulation frequency (1001) to generate, for each carrier resonance (3000), at least two sideband resonances (3001) spaced apart from the carrier resonance (3000) by the first modulation frequency (1001), the first modulation frequency (1001) being higher than a free spectral range (FSR) and different from an integer multiple of the free spectral range; tuning the carrier frequency of the laser light at a predetermined tuning speed; - measuring the intensity of the laser light transmitted and / or reflected by the resonator element (102) while tuning the carrier frequency; measuring the tuning time elapsed between a plurality of said carrier resonances spaced at least twice the first modulation frequency (1001); and determining a spacing of a plurality of free spectral ranges (FSRs) within the spectral region using the measured intensity of the laser light transmitted and / or reflected by the resonator element (102), the first modulation frequency (1001), and the measured tuning time between a plurality of carrier resonances (3000) spaced at least twice the first modulation frequency (1001); A method comprising:

Images