Method and apparatus for characterizing resonator elements

Abstract

A method of characterizing a resonator element (102) is provided herein. The method includes providing a 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, where adjacent carrier resonances (3000) are spaced from each other in the spectral domain at a free spectral region (FSR). The method further comprises 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), thereby producing, for each carrier resonance (3000), at least two sideband resonances (3001) separated by the first modulation frequency (1001) from the corresponding carrier resonance (3000), and generating at least two sideband resonances (3002) separated from the corresponding carrier resonances (3000) by a second modulation frequency (1002), where the first modulation frequency (1001) and the second modulation frequency (1002) are different from an integer multiple of the free spectral region and are different from each other. Furthermore, the method comprises tuning the carrier frequency of the laser light at a predetermined tuning rate, and measuring the intensity of the laser light transmitted and / or reflected by the resonator element (102) while tuning the carrier frequency. Further, the method includes measuring a tuning time to tune the carrier frequency through the four adjacent sideband resonances, the tuning time corresponding to a tuning time required to change the tunable carrier frequency from a value corresponding to a first one of the four sideband resonances (3001, 3002) to a value corresponding to a last one of the four sideband resonances. Furthermore, the method comprises determining an interval between a plurality of carrier resonances (3000) in the spectral domain from a measured intensity of the laser light transmitted and / or reflected by the resonator element (102) and using the first modulation frequency (1001), the second modulation frequency (1002) and a measured tuning time to tune the carrier frequency through four adjacent sideband resonances (3001, 3002).

Description

Technical Field

[0001] The present disclosure provides a method and apparatus for characterizing a resonator element, a method and apparatus for providing an optical frequency reference, a LIDAR system, and a gas sensing system. Therefore, the present disclosure relates to the technology of providing an optical frequency reference. Background Art

[0002] Many applications benefit from the ability to measure the time and / or spectral differences between multiple signals with high accuracy.

[0003] Optical frequency combs can be used for this purpose in many applications, such as DJ Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall and S.T. Cundiff, “Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis”, Science, vol. 288, pp. 635-639 (2000), and R. Holzwarth, Th. Udem, T.W. J. C. Night, W. J. Wadsworth, and P. St. J. Russell, “Optical Frequency Synthesizer for Precision Spectroscopy,” Phys. Rev. Lett., vol. 85, pp. 2264–2267 (2000). These techniques enable single-frequency metrology with up to 18 digits of accuracy, see 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, vol. 14, pp. 411–415 (2020).

[0004] In addition to single-frequency metrology, optical frequency combs have been used for high-precision and high-speed broadband spectroscopy, thanks to their unique combination of large bandwidth and high spectral resolution. In the past two decades, various spectroscopy methods based on optical frequency combs have been developed, such as direct frequency comb spectroscopy, dual-comb spectroscopy, and Fourier transform spectroscopy.

[0005] Despite their high precision and wide range of uses, frequency combs often suffer from low power per comb line and spectral amplitude fluctuations, which limits their use and poses challenges to their further applications, see T. Fortier and E. Baumann, “20 years of developments in optical frequency comb technology and applications,” Commun. Phys, vol. 2, pp. 1–16 (2019).

[0006] Furthermore, high-precision measurements based on optical frequency combs usually require a frequency comb source with long-term coherence, which involves complex servo loops, see N. Picqué and T.W. “Frequency comb spectroscopy,” Nat. Photonics, vol. 13, pp. 146–157 (2019).

[0007] In the prior art, microresonators are usually pumped with modulated lasers to generate frequency combs, in which the center frequency of the laser radiation coupled into the resonator element remains constant. The carrier frequency of the laser radiation coupled into the resonator element usually needs to be stabilized. To generate a frequency comb, laser radiation with an octave span is usually required, which overlaps each other spectrally, resulting in high technical complexity and high costs.

[0008] Therefore, it is necessary to overcome the above limitations. Summary of the invention

[0009] The above-mentioned problems are solved by a method and a device for characterizing a resonator element, a method and a device for providing an optical frequency reference signal for a laser with a tunable carrier frequency, a LIDAR system and a gas sensing system, all of which have the features of the independent claims. Optional embodiments are provided in the dependent claims and in the description.

[0010] In one aspect, a method of characterizing a resonator element is provided. The method includes providing a laser light source 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 separated from each other by a free spectral region in a spectral domain. The method includes modulating the intensity and / or phase of a portion of the laser light coupled into the resonator element at a first modulation frequency and a second modulation frequency, thereby generating for each carrier resonance at least two sideband resonances separated from the corresponding carrier resonance by the first modulation frequency, and generating at least two sideband resonances separated from the corresponding carrier resonance by the second modulation frequency, wherein the first modulation frequency and the second modulation frequency are different from integer multiples of the free spectral region and are different from each other. In addition, the method also includes 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 when tuning the carrier frequency through 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 a value corresponding to the last of the four sideband resonances. The method also includes determining a spacing between the plurality of carrier resonances in the spectral domain based on a measured intensity of laser light transmitted and / or reflected by the resonator element and based on a measured tuning time of the first modulation frequency, the second modulation frequency, and the tuned carrier frequency through four adjacent sideband resonances.

[0011] In another aspect, a device for characterizing a resonator element is provided. The device comprises a tunable laser light source (for emitting laser light with a tunable carrier frequency) and a coupling element (for coupling at least a portion of the laser light into the resonator element). In addition, the device comprises a modulator for modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element, thereby generating at least two sideband resonances separated from the corresponding carrier resonance by the first modulation frequency for each carrier resonance at a first modulation frequency, and generating at least two sideband resonances separated from the corresponding carrier resonance by the second modulation frequency at a second modulation frequency. In addition, the device 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 also includes a control unit, which is configured to tune the carrier frequency of the laser and determine a tuning time and a spacing between multiple carrier resonances in the spectral domain based on the measured intensity of the laser light transmitted and / or reflected by the resonator element and based on the first modulation frequency, the second modulation frequency and the measured tuning time of the tuned carrier frequency through four adjacent sideband resonances, wherein the tuning time is equivalent to the tuning time required to change the tunable carrier frequency from a value corresponding to the first of the four sideband resonances to a value corresponding to the last of the four sideband resonances.

[0012] In another aspect, a method for providing an optical frequency reference signal for a laser having a tunable carrier frequency is provided. The method includes providing a resonator element having a plurality of carrier resonances for a carrier frequency of the laser, wherein adjacent carrier resonances are spaced apart from each other in a spectral domain by respective predetermined free spectral regions. The method also includes coupling a first portion of the laser into the resonator element and tuning the carrier frequency of the laser at a predetermined tuning rate. In addition, the method includes using a portion of the laser light transmitted and / or reflected by the resonator element as an optical frequency reference signal for spectral scanning, wherein the intensity of the laser light transmitted and / or reflected by the resonator element has local extrema in frequency, and these frequencies are spaced apart from each other in the spectrum by FSR.

[0013] In another aspect, the optical frequency reference signal provided according to the method of the present disclosure may be used as a spectral reference marker.

[0014] In another aspect, a method for spectrally characterizing an analyte is provided, the method comprising providing an optical frequency reference signal according to a disclosed method, and using the optical frequency reference signal as a spectral reference marker for spectrally characterizing the analyte. Using the optical frequency reference signal as a spectral reference marker may include determining relative spectral distances of a plurality of spectral features of the analyte according to one or more FSRs.

[0015] In another embodiment, a device for providing a laser with a tunable carrier frequency as an optical frequency reference signal is provided. The device optionally includes a tunable laser light source, which is used to emit a laser with a tunable carrier frequency. The device also includes a resonator element, which has a plurality of carrier resonances for the tunable carrier frequency of the laser, wherein the carrier resonances are spaced apart from each other by respective predetermined free spectral regions in the spectral domain, and the device is suitable for coupling part of the laser into the resonator element. The device also includes a control unit, which tunes the carrier frequency of the laser at a predetermined tuning rate. The device is used to use part of the laser transmitted and / or reflected by the resonator element as an optical frequency reference signal for spectral scanning, wherein the intensity of the laser transmitted and / or reflected by the resonator element has local extreme values ​​in frequency, and these frequencies are spaced apart from each other in the spectrum by FSR. Optionally, the device also includes a detector unit, which is used to measure the intensity of the part of the laser transmitted and / or reflected by the resonator element. Furthermore, the device optionally comprises a control unit configured to provide a frequency of the tunable carrier frequency having a local extrema in the measured intensity of the portion of the laser light transmitted and / or reflected by the resonator element and serving as a spectral reference marker for an optical frequency reference signal defined by a predetermined free spectral region.

[0016] In another aspect, a LIDAR system is provided. The LIDAR system includes an apparatus for providing an optical frequency reference signal according to the present disclosure.

[0017] In another aspect, a gas sensing system is provided, the gas sensing system comprising an apparatus according to the present disclosure for providing an optical frequency reference signal.

[0018] In another aspect, a method of characterizing a resonator element is provided. The method includes providing a laser light source 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 separated from each other by a free spectral region in the spectral domain. The method also includes modulating the intensity and / or phase of the portion of the laser light coupled into the resonator element with a first modulation frequency so as to generate, for each carrier resonance, at least two sideband resonances separated from the corresponding carrier resonance by the first modulation frequency, wherein the first modulation frequency is different from an integer multiple of the free spectral region. In addition, the method also includes tuning the carrier frequency of the laser light at a predetermined tuning rate and measuring the intensity of the laser light transmitted and / or reflected by the resonator element while tuning the carrier frequency. The method also includes measuring the tuning time between the plurality of carrier resonances separated from each other by at least two times the first modulation frequency. In addition, the method includes determining the spacing between the plurality of free spectral regions in the spectral domain based on the measured intensity of the laser light transmitted and / or reflected by the resonator element using the first modulation frequency and the measured tuning time between the plurality of carrier resonances separated from each other by at least two times the first modulation frequency.

[0019] A laser with a tunable carrier frequency is a coherent light whose center frequency can be tuned by the light source. Tuning the carrier frequency can be considered to be changing the carrier frequency over time, optionally in a continuous manner. Tuning the carrier frequency can include, for example, sweeping the carrier frequency in a sinusoidal manner.

[0020] A resonator element is an element having specific transmission and / or absorption characteristics for a tunable carrier frequency, wherein for a predetermined value of the tunable carrier frequency, the resonator element shows local minima and / or maxima of the transmission and / or absorption and / or reflection. In particular, the resonator element may comprise a resonant cavity which satisfies resonance conditions for specific values ​​of the tunable carrier frequency. These specific carrier frequency values ​​are considered to be carrier resonances. In the case of a resonant cavity, the transmission may exhibit local minima at these frequencies, wherein the tunable carrier frequency resonates depending on the length of the resonant cavity. Measuring the intensity of laser light transmitted and / or reflected by the resonator element while the carrier frequency is tuned means measuring the intensity and / or power of laser light transmitted through the resonator element and / or reflected by the resonator element while the carrier frequency is continuously tuned.

[0021] Modulating the intensity and / or phase of a portion of the laser light with a first modulation frequency and an optional second modulation frequency means that the intensity and / or phase of the laser light varies periodically with the first modulation frequency and the optional second modulation frequency. Therefore, modulating the laser light intensity with the first modulation frequency and the second modulation frequency will cause the intensity to be periodically weakened and enhanced with the time variation of the first modulation frequency and the second modulation frequency modulation signal. If there is more than one modulation signal and more than one modulation frequency, the modulation may actually be a superposition of the individual modulation signals. This can be achieved by applying the first modulation frequency and the second modulation frequency by a power combiner. The first modulation frequency and the second modulation frequency may be higher than the free spectral region of the laser light and the resonator element. Although the first modulation signal and the second modulation signal having the first modulation frequency and the second modulation frequency, respectively, are explicitly mentioned, it should be noted that, according to some optional embodiments, the laser light may be modulated by more than two modulation signals at more than two modulation frequencies. The first modulation frequency and the second modulation frequency may both be in the radio frequency range. In particular, the first modulation frequency and the second modulation frequency are each in the range of about 100 MHz to about 100 GHz. The free spectral region of the resonator element may be selected to be not less than 1 MHz and not more than 100 GHz.

[0022] The modulation index of the modulation depth of the intensity modulation and / or phase modulation ranges from about 0.6 to 2.

[0023] Sideband resonances resulting from laser intensity and / or phase modulation may result from the superposition of a carrier frequency and one or more modulation frequencies.

[0024] The modulation frequencies are different from integer multiples of the free spectral range and are different from each other, which means that the sideband resonances generated by the modulation frequencies do not completely overlap with the carrier resonances and do not completely overlap with other sideband resonances originating from other respective modulation frequencies.

[0025] Tuning the carrier frequency through the tuning time of four adjacent sideband resonances is equivalent to the tuning time required to change the tunable carrier frequency from the value corresponding to the first of the four sideband resonances to the value corresponding to the last of the four sideband resonances. However, according to other embodiments, the adjustment of the carrier frequency can be performed in a non-uniform but clear manner. In addition, if the time interval between the four adjacent sideband resonances is small enough, detailed knowledge of the tuning characteristics may not be required. Based on the measured tuning time, the spectral difference between the first and the last of the sideband resonances can be determined.

[0026] The tunable laser light source may comprise a tunable continuous wave laser light source.The tunable laser light source may comprise or may consist of a diode laser.

[0027] The modulator may comprise or consist of an acousto-optic modulator and / or an electro-optic modulator, and may also be implemented by modulating the laser current and / or the laser current. The detector unit may comprise one or more photodiodes, which are sensitive within the tuning range of the laser with a tunable carrier frequency. In particular, the modulator may be used to modulate the intensity of the transmitted laser light and / or the phase of the transmitted laser light.

[0028] The control unit may be an electrical and / or electronic device, such as a computer, a smartphone, an integrated circuit and / or a tablet. The control unit may be connected to the detector unit to receive data measured by the detector unit. The control unit may also be connected to the tunable light source and may be configured to send instructions to the tunable light source to tune the carrier frequency in a specific manner.

[0029] A tunable carrier frequency having a local extrema in the measured intensity of the laser light portion transmitted and / or reflected by the resonator element is provided as a spectral reference marker of an optical frequency reference signal defined by a predetermined free spectral region FSR, which means that a specific frequency showing as a local extrema is defined as a spectral reference marker. In other words, the resonator element is used in such a way that its predetermined, well-characterized carrier resonances are used as optical frequency reference values, since their spectral distances can be well determined and characterized according to the characterization process disclosed herein. Well-characterized spectral distances between carrier resonances corresponding to the free spectral region and / or spectral distances between sideband resonances can be provided as optical reference frequencies and thus also as optical frequency reference signals.

[0030] The advantage of the present disclosure is that it can provide a high-precision optical frequency reference signal. In particular, compared with conventional technologies for providing frequency reference signals (such as frequency combs), the present invention can provide a high-precision frequency reference signal at a lower technical cost. According to the disclosed information, the frequency reference signal can be provided based on standard optical and electronic components, which are relatively low in cost compared to conventional frequency combs. It should be emphasized that according to the disclosed content, the optical frequency reference signal can be provided based on a continuous wave laser (such as a diode laser) without the need for octave-spanning spectrum and nonlinear optical processes (which are usually required in frequency combs). Therefore, the present invention can provide an optical frequency reference signal with lower technical complexity, without the need for expensive components, and thus can provide an optical frequency reference signal at a lower cost. In addition, the technical complexity of these devices is low, so that smaller and / or more compact devices can be provided, and these devices can be miniaturized and / or integrated into other compact optical and / or electronic components. In particular, according to the disclosed information, a continuous laser can be used, in which the carrier frequency, i.e. the center frequency of the laser radiation, can be swept within a predetermined spectral region, for example, optionally 10 free spectral regions. During the tuning process, the carrier frequency and the tuning rate may be known, so the carrier frequency can be swept in a predetermined manner within the tuning time. Therefore, the present disclosure can characterize the resonator element and / or provide a frequency reference signal without using a frequency comb or other hardware that is much more expensive and complex. Therefore, according to the technical solution of the present disclosure, there is no need to generate a frequency comb in the resonator element. For spectroscopy applications, the carrier frequency can be tuned within a predetermined tuning range, including tuning within the spectral range required for the spectroscopy application.

[0031] Therefore, compared with the conventional frequency comb, the advantage of the present disclosure is that an optical frequency reference signal can be provided with lower technical complexity, smaller size and lower cost. According to the method and apparatus of the present disclosure, it is not necessary for the octave span and the spectrally stable laser radiation to overlap each other in the spectrum, but only a laser with a tunable carrier frequency is provided, and the carrier frequency is tuned at a predetermined tuning rate within a predetermined spectral region (e.g., 10 free spectral regions). Therefore, compared with the prior art, the technical complexity and requirements can be greatly reduced.

[0032] The difference between the first modulation frequency and the second modulation frequency can be selected to be not less than 10% of the resonance linewidth of the carrier resonance and not more than 50% of the free spectrum area. This can ensure that the sideband resonances are appropriately distinguished from each other and from the carrier resonance because their spacing in the frequency domain is large enough. The first modulation frequency and the second modulation frequency basically correspond to frequencies near n+1 / 2 multiples of the free spectrum area, where n is an integer. "Near" means that the frequency is close to a multiple of the free spectrum area, but not exactly the same. For example, the first modulation frequency and / or the second modulation frequency can maintain a certain spectral spacing with n+1 / 2 multiples of the free spectrum area between 100 MHz and 100 GHz, respectively. This ensures that the sideband resonances are located at appropriate positions between the carrier resonances on the spectrum.

[0033] Two sideband resonances generated by a first modulation frequency from one of the carrier resonances can be located in at least one free spectrum region, and at least two sideband resonances generated by a second modulation frequency from one of the carrier resonances can be located. In this way, the spectral spacing between the first modulation frequency and the second modulation frequency and the related carrier frequency can be used to determine the spectral spacing of two adjacent carrier resonances with high accuracy, thereby determining the spectral spacing of the free spectrum region.

[0034] Measuring the tuning time to tune the carrier frequency through four adjacent sideband resonances may include measuring the tuning time to tune the carrier frequency through four adjacent sideband resonances within a single free spectral region. This has the advantage that the spectral tuning range required for determining information about the spacing of multiple carrier resonances may be limited to a smaller spectral range. This helps maintain a constant tuning rate of the laser light source over the smaller spectral range that needs to be covered, thereby reducing or avoiding unnecessary measurement errors when determining the spacing between carrier resonances.

[0035] The method for characterizing a resonator element may further comprise the steps of coupling a second portion of the laser light into a calibration element having a predetermined absolute transmission and / or reflection characteristic and measuring the intensity of the portion of the laser light transmitted and / or reflected by the calibration element while tuning the carrier frequency. Furthermore, the method further comprises the steps of identifying at least one specific absolute transmission and / or reflection characteristic of the calibration element having a predetermined frequency that coincides with or has a specified offset from one of the carrier resonance or the sideband resonance in the frequency domain, and the control unit being adapted to calibrate the absolute frequency of at least one of the carrier resonance or the sideband resonance according to the identified specific absolute transmission and / or reflection characteristic of the calibration element. Thus, these additional steps based on the calibration element enable the spectral frequencies of the carrier resonance and the sideband resonance to be calibrated in an absolute manner. While the aforementioned steps allow for a relative calibration of the carrier resonance and the sideband resonance, these additional steps allow for an absolute calibration of at least one of the carrier resonance and the sideband resonance and for the calibration of all remaining carrier resonances and sideband resonances by relative calibration according to the modulation frequency through one absolutely calibrated carrier resonance or sideband resonance. Thus, this method allows for an absolute calibration of the resonator element, which can then be used to provide an absolute optical frequency reference signal. Thus, the device for characterizing the resonator element may further comprise a calibration element having a predetermined absolute transmission and / or reflection characteristic, wherein the detector unit is further adapted to 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 is further adapted to identify at least one specific absolute transmission and / or reflection characteristic of the calibration element, which has a predetermined frequency that coincides with or has a specified offset from one of the carrier resonance or the sideband resonance in the frequency domain, and the control unit is adapted to calibrate the absolute frequency of at least one of the carrier resonance or the sideband resonance based on the identified specific absolute transmission and / or reflection characteristic of the calibration element.

[0036] The calibration element may comprise a gas cell filled with a predetermined gas having at least one specific absolute transmission and / or reflection property. The advantage of using a gas cell is that the absorption properties of many gases are precisely characterized and well known from the literature, e.g. distinct absorption lines. Therefore, the absorption and transmission properties can be well used as an absolute calibration reference when comparing the carrier resonance and / or the sideband resonances of the resonator element with each other. Alternatively or additionally, the calibration element may also comprise one or more of the following: a frequency comb, a wavelength meter, an element providing atomic and / or molecular transition lines.

[0037] The resonator element may include an optical fiber cavity and / or an integrated waveguide resonator, and / or a whispering gallery mode resonator, and / or an Etalon resonator and / or a Fabry-Perot resonator. This allows the resonator element to be integrated into conventional optical and / or electronic devices and apparatuses. In addition, such a resonator element may also miniaturize the apparatus so that the apparatus may be mounted in a small-scale optical and / or electronic apparatus.

[0038] The method provides an optical frequency reference signal for a laser with a tunable carrier frequency, comprising modulating the intensity and / or phase of a portion of the laser light coupled to a resonator element with a first modulation frequency and a second modulation frequency, thereby generating at least two sideband resonances separated from the corresponding carrier resonance by the first modulation frequency for each carrier resonance, and generating at least two sideband resonances separated from the corresponding carrier resonance by the second modulation frequency, wherein the first modulation frequency and the second modulation frequency are different from integer multiples of the free spectrum range and are different from each other, and the method provides the sideband resonance frequencies as spectral reference marks of the optical frequency reference signal. If the provided resonator element has been calibrated and no recalibration is required, the calibration process can be omitted. However, in some embodiments, the method for providing an optical frequency reference signal for a laser with a tunable carrier frequency may also include a process of calibrating and / or re-characterizing the resonator element. In this way, the calibration of the resonator element can be checked regularly, thereby ensuring and possibly improving the accuracy of the provided optical frequency reference signal. The related method of characterizing the resonator element may correspond to the method further described above. In addition, in this case, the first modulation frequency and the second modulation frequency can be located in the radio frequency range, in particular, the first modulation frequency and the second modulation frequency can be located in the range of about 100 MHz to about 10 GHz, respectively. The difference between the first modulation frequency and the second modulation frequency can be selected to be not less than 10% of the resonance line width of the carrier resonance and not more than 50% of the free spectrum region. The first modulation frequency and the second modulation frequency substantially correspond to frequencies around n+1 / 2 multiples of the free spectrum region, where n is an integer.

[0039] In addition, the method may further include absolute calibration of the provided optical frequency reference signal. The method may include coupling the second part of the laser into a calibration element having a predetermined absolute transmission and / or reflection characteristic, and measuring the intensity of the laser part transmitted and / or reflected by the calibration element, while tuning the carrier frequency. The method also includes identifying at least one specific absolute transmission and / or reflection characteristic of the calibration element, the absolute transmission and / or reflection characteristic having a predetermined frequency that coincides with one of the carrier resonance or sideband resonance in the frequency domain or has a specified offset, and the control unit is suitable for calibrating the absolute frequency of the carrier resonance and / or sideband resonance according to the identified specific absolute transmission and / or reflection characteristic. In this way, the method can periodically perform absolute recalibration of the resonator element, thereby calibrating the provided optical frequency reference signal. The calibration element includes a gas chamber filled with a predetermined gas, the predetermined gas having at least one specific absolute transmission and / or reflection characteristic, and / or includes a frequency comb and / or a wavelength meter and / or a system providing atomic transition lines.

[0040] The carrier frequency of the laser is tuned within at least ten free spectral regions. The tuning of the carrier frequency is performed according to a predetermined tuning pattern. The predetermined tuning pattern for tuning the carrier frequency includes sweeping the carrier frequency in a sinusoidal tuning pattern and / or a sawtooth tuning pattern. In this way, an optical frequency reference signal can be provided in a fast, reliable and / or low technical complexity and low cost manner.

[0041] The laser may be a continuous wave laser. The spectral width of the laser may be smaller than the line width of the resonator element. With a tuning time and / or measurement time of about 5 μs, the typical short-term line width of the laser light may be 100 kHz or less. This allows high-precision measurements.

[0042] Therefore, the device for providing an optical frequency reference signal for a laser with a tunable carrier frequency can be used to characterize a resonator element and / or to relatively calibrate and / or absolutely calibrate a resonator element and a carrier resonance and / or a sideband resonance. Therefore, the device for providing an optical frequency reference signal for a laser with a tunable carrier frequency can further include a modulator for modulating the intensity and / or phase of a portion of the laser light coupled into the resonator element at a first modulation frequency, thereby generating at least two sideband resonances separated from the corresponding carrier resonance by the first modulation frequency for each carrier resonance. The control unit can also be configured to provide a frequency of the tunable carrier frequency having a local extremum due to the sideband resonance in the measured intensity of the portion of the laser light transmitted and / or reflected by the resonator element, which serves as a spectral reference mark for an optical frequency reference signal defined by a predetermined free spectral region and the first modulation frequency. The resonator element can include an optical fiber cavity and / or an integrated waveguide, and / or a whispering gallery mode resonator and / or an Etalon resonator and / or a Fabry-Perot resonator. The modulator can include an electro-optical intensity and / or phase modulator.

[0043] Furthermore, the device for providing an optical frequency reference signal for a laser with a tunable carrier frequency may further comprise a calibration element having a predetermined absolute transmission and / or reflection characteristic. The detector unit may also 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 is further adapted to identify at least one specific absolute transmission and / or reflection characteristic of the calibration element, the absolute transmission and / or reflection characteristic having a predetermined frequency that coincides with one of the carrier resonance or the sideband resonance in the frequency domain or has a specified offset, and the control unit is adapted to calibrate the absolute frequency of at least one of the carrier resonance or the sideband resonance according to the identified specific absolute transmission and / or reflection characteristic of the calibration element. In this way, an absolute (re)calibration of the resonator element and the optical frequency reference signal is possible. The calibration element may comprise a gas chamber filled with a predetermined gas having at least one specific absolute transmission and / or reflection characteristic, and / or a frequency comb and / or a wavelength meter and / or an element or system providing atomic and / or molecular transition lines.

[0044] The tunable laser light source may include a tunable continuous wave laser light source. The tunable laser light source may include a diode laser. The tunable laser light source is used to emit a tunable laser light source having a spectrum width smaller than the line width of the resonator element.

[0045] The LIDAR system includes a device for providing an optical frequency reference signal according to the present disclosure, which can be used to provide an optical frequency reference signal so that a tunable laser can be referenced to the optical reference signal provided by the device. In some conventional systems known in the prior art, the tunable laser is referenced to a frequency comb in order to achieve high precision, high accuracy and high measurement speed. In this case, the frequency comb can provide an optical frequency reference signal. Such devices refer to the paper “Comb-calibrated frequency-modulated continuous-waveladar for absolute distance measurements” by Baumann Esther et al., Optics Letters, Vol. 38, No. 12, pp. 2026-2028, 2013; the paper “Comb-calibrated laserranging for three-dimensional surface profiling with micrometer-level precision at a distance” by Baumann Esther et al., Optics Express, Vol. 22, No. 21 (2014), pp. 24914-24928; and the paper “Comb-calibrated frequency sweeping interferometry for absolute distance and vibration measurement” by Yu Wenhui et al., Optics Letters, Vol. 44, No. 20 (2019), pp. 5069-5072.

[0046] However, frequency combs generally have disadvantages such as being technically complex, sensitive to environmental influences, and expensive. The disclosed LIDAR system provides a calibrated resonator element, such as an optical fiber cavity characterized and / or calibrated according to the method disclosed herein, to provide an optical frequency reference signal. Compared with a frequency comb, the disclosed LIDAR system can achieve high precision, high accuracy, and high measurement speed, while at a lower cost and with a lower technical complexity. Therefore, using the device disclosed in the present invention to provide an optical frequency reference signal can achieve high precision comparable to that of a frequency comb while greatly reducing technical complexity and cost. Therefore, LIDAR and many other applications can achieve high precision, low complexity, and high stability through the present invention, and at a lower price, while these advantages are economically impossible to achieve using a frequency comb. It should be emphasized that in the LIDAR device, a device for providing a frequency reference signal according to the present disclosure is included, so that only a calibration resonator element needs to be provided, and no modulator needs to be provided. However, according to certain embodiments, an intensity modulator can also be provided, for example for recalibrating a resonator element.

[0047] Similarly, the gas sensing system of the present invention includes a device for providing an optical frequency reference signal, and the gas sensing system can be adjusted in a manner similar to a conventional greenhouse gas frequency remote sensing system, see Nishiyama, Akiko, Daiki Ishikawa, and Masatoshi Misono., “High resolution molecular spectroscopic system assisted by an optical frequency comb,” Journal of the Optical Society of America B (JOSA B), Vol. 30, No. 8 (2013), pp. 2107-2112; Rieker, Gregory B. et al., “Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths,” Optica, Vol. 1, No. 5 (2014), pp. 290-298; Herman Daniel I. et al., “Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy,” Science Advances, Vol. 7, No. 14 (2021), eabe9765.

[0048] In this case, the frequency comb can also be replaced by a device based on resonator elements, which are calibrated according to the method disclosed in this article and can achieve the same high precision, high accuracy and high measurement speed as the device based on the frequency comb, but with much lower technical complexity, much lower manufacturing cost and higher robustness.

[0049] Those skilled in the art will appreciate that the above features and the features described below and in the drawings are disclosed not only in the form of clearly disclosed embodiments and combinations, but the present disclosure also includes other technically feasible combinations and independent features. In the following, several optional embodiments and specific examples will be described with reference to the drawings to illustrate the present disclosure, but the present disclosure is not limited to the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Optional embodiments and examples will be further described below with reference to the accompanying drawings.

[0051] Figure 1 An apparatus for characterizing a resonator element according to an alternative embodiment is shown.

[0052] Figure 2 Describes the use Figure 1 A method for characterizing a resonator element using a device.

[0053] Figure 3 The carrier resonance and sideband resonance are shown.

[0054] Figure 4 An apparatus for characterizing a resonator element designed according to an alternative embodiment is shown.

[0055] Figure 5 An apparatus for providing an optical frequency reference signal for a laser having a tunable carrier frequency is described.

[0056] Figure 6 A method of providing an optical frequency reference signal for a laser having a tunable carrier frequency is schematically described.

[0057] Figure 7 and Figure 8 Results of a method of characterizing a resonator element according to an alternative embodiment are presented.

[0058] Fig. 9 Results of the method for providing an optical frequency reference signal are presented.

[0059] Fig.10 The results of an absolute calibration using a calibration element are shown.

[0060] Fig.11 A LIDAR designed according to an alternative embodiment is schematically depicted.

[0061] Fig.12 A gas sensing system designed according to an alternative embodiment is schematically described.

[0062] In the different drawings, the same reference numerals are used for corresponding or similar features. DETAILED DESCRIPTION

[0063] Figure 1 An apparatus 100 for characterizing a resonator element 102 according to an alternative embodiment is shown. The resonator element 102 is formed by a fiber cavity resonator and represents a device under test, which is characterized by the apparatus 100.

[0064] The device 100 includes a tunable laser light source 104 for emitting laser light having a tunable carrier frequency. According to the illustrated embodiment, the output of the laser light source 104 is directly coupled into an optical fiber, which is coupled into the resonator element 102. Thus, the optical fiber 106 can act as a coupling element 108 to couple at least a portion of the laser light into the resonator element 102.

[0065] In addition, the device 100 includes a modulator 110 (an electro-optical modulator may be provided), the modulator 110 is used to modulate the intensity and / or phase of the part of the laser light coupled to the resonator element 102, the modulation is performed at a first modulation frequency 1001 and a second modulation frequency 1002, by the modulation of the first modulation frequency 1001, each carrier resonance generates at least two sideband resonances, and the at least two sideband resonances are separated from the corresponding carrier resonance by the first modulation frequency 1001, by the modulation of the second modulation frequency 1002, each carrier resonance generates at least two sideband resonances, and the at least two sideband resonances are separated from the corresponding carrier resonance by the second modulation frequency 1002. The modulator may be an electro-optical modulator, which is used to modulate the intensity of the laser light, and may also be used to modulate the phase of the laser light. One, two or more modulation signals may be applied to the modulator, such as the first modulation frequency 1001 and the second modulation frequency 1002. However, it should be emphasized that according to 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 modulation frequency 1001 and the second modulation frequency 1002 may represent sinusoidal oscillations of the respective modulation frequencies. However, according to some optional embodiments, the modulation signal may be a more complex modulation signal, such as a waveform covering a larger range in the spectral domain, and / or a continuous oscillation deviating from a sinusoidal oscillation, such as a sawtooth signal. In addition, a bias voltage 1004 may be applied to the modulator 110, which bias voltage 1004 is used, for example, to set the basic transmission characteristics of the modulator 110.

[0066] The laser light enters the device under test (ie, the resonator element 102 ) after passing through the modulator 110 .

[0067] Downstream of the resonator element 102, the device 100 comprises a detector unit 112 for measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element 102. According to the illustrated embodiment, the detector unit 112 is used to detect the intensity of the laser light transmitted through the resonator element 102. The detector unit may comprise one or more photodiodes sensitive to the carrier frequency of the laser light. The detector 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 provided by the photodiode 112.

[0068] In addition, the apparatus 100 comprises a control unit 116, which is configured to tune the carrier frequency of the laser light and determine the tuning time and the spacing between the plurality of carrier resonances in the spectral domain 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 to tune the carrier frequency through four adjacent sideband resonances. The control unit 116 is communicatively connected to the detector unit 112 and / or the data logger for retrieving data measured by the detector unit 112. The control unit 116 is also connected to the laser light source 104 for controlling the laser light source 104 to adjust the carrier frequency of the laser light emitted by the laser light source 104. In addition, the control unit 116 is connected to the modulator 110 for providing a modulation signal, such as the first modulation frequency 1001 and the second modulation frequency 1002, to the modulator 110.

[0069] By scanning the carrier frequency (i.e., the central wavelength of the laser) emitted by the laser light source 104, the laser light coupled to the resonator element 102 generates multiple carrier resonances in the resonator element 102 during the tuning process. For the frequency value corresponding to the carrier resonance, a large amount of energy of the laser light is stored in the resonator element 102, and therefore, at the frequency corresponding to the carrier resonance, the transmission intensity through the resonator element 102 is reduced. Therefore, the carrier resonance can be identified by the local minimum of the transmission energy detected by the detector unit 102. In addition, since the intensity and / or phase of the laser light coupled to the resonator element 102 is modulated by the first modulation frequency 1001 and the second modulation frequency 1002, this will generate two frequency sidebands. For each modulation frequency, the two frequency sidebands will generate corresponding two sideband resonances at specific frequencies different from the carrier resonance, and the two frequency sidebands are spaced apart from the carrier frequency in the frequency domain by the first modulation frequency 1001 and the second modulation frequency 1002, respectively, which will be referred to below. Figure 3 Described in further detail.

[0070] Based on the tuning time and the spacing between the multiple carrier resonances in the spectral domain, 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 to tune the carrier frequency through four adjacent sideband resonances, the control unit 116 can determine the spectral spacing between the carrier resonances, thereby determining the spectral range of the resonator element 102. Therefore, the device can accurately describe the spectral characteristics of the resonator element 102.

[0071] The following will refer to Figure 2 Explanation of use Figure 1 The device described is a method for characterizing a resonator element.

[0072] The method comprises a first step 202 of providing a laser having a tunable carrier frequency.

[0073] A further step 204 comprises coupling at least a first part of the laser light into a resonator element 102 which has a plurality of carrier resonances for a carrier frequency of the laser light, wherein adjacent carrier resonances are spaced apart from one another in the spectral domain by a free spectral region.

[0074] 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, thereby generating at least two sideband resonances separated from the corresponding carrier resonance by the first modulation frequency for each carrier resonance, and generating at least two sideband resonances separated from the corresponding carrier resonance by the second modulation frequency. The first modulation frequency 1001 and the second modulation frequency 1002 are different from integer multiples of the free spectral range and are different from each other.

[0075] Step 208 includes tuning the carrier frequency of the laser at a predetermined tuning rate.

[0076] Step 210 includes measuring the intensity of laser light transmitted and / or reflected by the resonator element 102 while the carrier frequency is tuned.

[0077] Step 212 includes measuring the tuning time to tune the carrier frequency through four adjacent sideband resonances.

[0078] Step 214 comprises determining the spacing between the plurality of carrier resonances in the spectral domain based on the measured intensity of 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 to tune the carrier frequency through four adjacent sideband resonances.

[0079] Since the intensity and / or phase of the laser are modulated by the first modulation frequency 1001 and the second modulation frequency, when the carrier frequency is tuned, that is, the frequency of the scanning laser light source is tuned, four sideband resonances will be generated in a free spectrum region FSR between two adjacent carrier resonances in the detection signal, such as Figure 3 As shown. According to the FSR of the resonator element 102, Figure 3 Indicated as f mod1 and f mod2 The two modulation frequencies 1001 and 1002 can be set to approximately (n+1 / 2)×FSR, limited by the spectral bandwidth of the modulator, where n is an integer. According to the structural line width of the resonator element 102, the frequency difference f between the first and second modulation frequencies 1001, 1002 d It can be set at around a few megahertz.

[0080] exist Figure 3 In the figure, resonance 3000 represents the carrier resonance, while resonances 3001 and 3002 represent the resonances of the first modulation frequency 1001f. mod1 and the second modulation frequency 1002f mod1 The generated sideband resonance. Since the frequency sweep of the laser light source 104 may have nonlinear behavior, the time intervals t1 (t3) and t2 (t4) between the carrier resonance and the modulation sideband resonance within one FSR are not necessarily equal. However, the frequency interval between the two sideband resonances is constant, and the frequency interval is determined by the frequency f mod1 or mod2 The first or second modulation frequency 1001, 1002 is determined by assuming that the FSR of the resonator element is 2×f mod1 The FSR can be kept constant within a modulation frequency range (which may correspond to tens of GHz), then the FSR can be calculated based on the first modulation frequency 1001f mod1 , based on (2n+1)×FSR=2f mod1 +f ？ To perform calibration, where f ？ is derived from the same modulation frequency f mod1 The frequency interval between two adjacent sideband resonances 3001 is as follows: Figure 3 shown.

[0081] Thus, the frequency interval f ？ becomes the only uncertain variable in nonlinear tunable laser scanning. However, in order to calculate FSR, it is necessary to calculate f ？ For a single modulation frequency f mod1 The method, frequency interval f ？ It can be calculated based on the average scanning speed of the laser light source 104, taking into account the tuning time (t1+t2) and the modulation frequency (2f mod1). This allows the FSR to be determined, although to some extent f ？ There is still frequency uncertainty.

[0082] In contrast, for the first modulation frequency 1001f mod1 and the second modulation frequency 1002f mod2 Dual RF modulation scheme with frequency spacing f ？ It can be used on a shorter time scale T1+T2+T3 (e.g. Figure 3 The fixed frequency spacing f between the two sideband resonances in one FSR is determined only over a small frequency range of a few MHz. d The first modulation frequency 1001f mod1 and the second frequency f mod2 The predetermined frequency difference between them is determined. Figure 3 The corresponding time intervals are marked with T1 3003 and T2 3004. Therefore, the frequency interval f ？ The calculation formula is T3×2f d / (T1+T2). The time interval T3 is denoted by reference numeral 3005. Thanks to the flexibility of the first and second modulation frequencies 1001, 1002, the fixed frequency interval f between the two sideband resonances originating from the different modulation frequencies 1001, 1002 d The time interval T3 can be set to a small value relative to the FSR and the cavity round trip time of the resonator element 102. Therefore, the frequency interval f ？ The uncertainty can be effectively reduced. In addition, by using the low FSR resonator element for dual RF modulation calibration with the first modulation frequency 1001 and the second modulation frequency 1002, the application of the method of the present application can also be extended to characterizing quasi-periodic devices or non-quasi-periodic devices having an FSR greater than the first and second modulation frequencies. Therefore, the method can characterize the resonator element 102 by determining the spectral spacing of the carrier resonance and then determining the free spectral region with high precision. In this way, the resonator element can be used to provide an optical frequency reference signal for a laser with a tunable carrier frequency. It should be emphasized that according to the method shown, it is only necessary to measure the tuning time of tuning the carrier frequency through four adjacent sideband resonances, and the time interval through the four adjacent sideband resonances is T1+T2+T3, which is shown as the reference numerals 3003, 3004 and 3005.

[0083] Figure 4 The apparatus 100 designed according to an optional embodiment is shown, and the apparatus 100 is used to characterize the characteristics of the resonator element 102, and the apparatus is based on a reference Figure 1 The described device 100. According to Figure 4 In the alternative embodiment shown, the device is Figure 1The device differs in that a calibration element 118 is added. The calibration element 118 has a predetermined absolute transmission and / or reflection characteristic, for example a predetermined absorption spectrum consisting of precisely predetermined absorption lines. In addition, the detector unit is capable of measuring the intensity of a portion of the laser light transmitted and / or reflected by the calibration element 118 while tuning the carrier frequency. To this end, the detector unit may comprise a second photodiode 112 for measuring the intensity of the laser light transmitted through the calibration element 118, wherein the measurement signal of the second photodiode 112 is provided to the data recorder 114 and is calculated by the control unit 116. The control unit 116 is further adapted to identify at least one specific absolute transmission and / or reflection characteristic of the calibration element 118, which has a predetermined frequency that coincides with or has a specified offset to one of the carrier resonance or the sideband resonance in the frequency domain, and the control unit 116 is adapted to calibrate the absolute frequency of at least one of the carrier resonance 3000 or the sideband resonances 3001, 3002 according to the identified specific absolute transmission and / or reflection characteristic of the calibration element 118. According to the described embodiment, the calibration element 118 comprises a gas chamber, the gas in the gas chamber having precisely predetermined transmission and / or reflection properties, such as precisely determined absorption lines. This allows an absolute calibration of the spectral properties of the resonator element 116, since the carrier resonance 3000, the sideband resonances 3001, 3002 and the free spectral region can be absolutely determined with reference to the transmission and / or reflection properties of the calibration element. Thus, the resonator element 102 can be precisely characterized to provide an absolute optical frequency reference signal.

[0084] Figure 5 A device 200 for providing an optical frequency reference signal for a laser having a tunable carrier frequency is described. As with the device 100 discussed with reference to the previous figures, the device 200 comprises a tunable laser light source 104 for emitting a laser having a tunable carrier frequency. Furthermore, the device 200 comprises a well-characterized resonator element 102 having a plurality of carrier resonances for a tunable carrier frequency of the laser, wherein the carrier resonances are mutually spaced in the spectral domain by respective predetermined free spectral regions FSR, and the device is adapted to couple a portion of the laser light into the resonator element 102. Furthermore, the device 200 comprises a detector unit 112 for measuring the intensity of the laser light portion transmitted and / or reflected by the resonator element 102, and a control unit 116 for providing a frequency of the tunable carrier frequency having a local extrema in the intensity measurements of the laser light portion transmitted and / or reflected by the resonator element 102 and serving as a spectral reference marker for the optical frequency reference signal defined by the predetermined free spectral region. As shown in FIG. Figure 4 As shown, the device also includes a calibration element 118. Figure 1 and Figure 4Unlike the apparatus and method shown, the use of a well-characterized resonator element 102 to provide an optical frequency reference signal does not necessarily require a modulator 110, because the spectral characteristics of the well-characterized resonator element that can be used can be determined by applying the method of the present disclosure. However, according to different embodiments, the apparatus for providing an optical frequency reference may also include a modulator to recalibrate the resonator element 102 when necessary.

[0085] according to Figure 5 In the illustrated embodiment, the resonator element 102 is used to provide an optical frequency reference signal for other applications. Further applications may include analysis of spectral characteristics of a photonic device 120 (e.g., a micro-ring resonator 120). Due to the provision of an optical frequency reference and an optional calibration element 118, the spectral characteristics (e.g., resonant frequency) of the photonic device 120 may be relatively, optionally absolutely, characterized by comparing the measured frequency with the carrier resonance and / or sideband resonance of the provided resonator element 102.

[0086] Figure 6 A method for providing an optical frequency reference signal for a laser having a tunable carrier frequency is schematically described. The method can use a reference Figure 5 The method comprises providing a resonator element 102 in step 602, the resonator element having a plurality of carrier resonances 3000 for a carrier frequency of the laser light, wherein adjacent carrier resonances 3000 are mutually spaced apart in the spectral domain by respective predetermined free spectral ranges FSR. The method comprises coupling a first portion of the laser light to the resonator element 102 in step 604, and measuring the intensity of the portion of the laser light transmitted and / or reflected by the resonator element 102 in step 606. Furthermore, the method comprises providing in step 608 a frequency of a tunable carrier frequency having a local extrema 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 an optical frequency reference signal defined by the predetermined free spectral range FSR.

[0087] A detailed example is introduced below to demonstrate the proof of concept, but the present embodiments or the present disclosure are not limited to the detailed example.

[0088] As a proof-of-concept demonstration, the described method and apparatus were verified by measuring the optical dispersion of the resonator element 102 to characterize the resonator element 102, which was provided in the form of a fiber cavity. The fiber cavity was made of a 10 dB fiber coupler and a 5 m length of standard telecommunication fiber (SMF-28), with a zero dispersion wavelength of about 1.310 nm. The free spectral region FSR of the fiber cavity was about 39 MHz, and the mode linewidth was about 1 MHz, which limits the measurement speed to about 1 THz / s, see P. Del'Haye et al., "Frequency comb assisted diode laser spectroscopy for measurement of microcavity dispersion", Nat. Photonics, Vol. 3, pp. 529-533 (2009).

[0089] In order to demonstrate the ultra-high frequency resolution of the disclosed method and apparatus, a 1.3 μm tunable laser (tunable range 1.270 to 1.330 nm) was used as a laser light source to resolve small FSR changes near the zero dispersion wavelength. mod1 and the second modulation frequency 1002f mod2 The two modulated signals of 20 GHz with a frequency difference of 4 MHz are provided for modulation. The two modulated signals are combined by a power combiner and applied to an EOM as a modulator 110. The laser light transmitted through the fiber cavity is detected by a detector unit 112, which includes a photodiode (PD) and an oscilloscope as a data recorder 114, which is used to record the measured values, and has a memory depth of 31.25 million.

[0090] Figure 7 Part a) of FIG. 4 shows the transmission spectrum of the fiber cavity from 1.270 nm to 1.330 nm, and the inset shows a magnified detail near 1.300 nm. The vertical axis represents the transmission in arbitrary units and the horizontal axis represents the wavelength in nanometers. The deeper dip in the transmission is the fiber cavity resonance generated by tuning the laser to the carrier frequency and is called the carrier resonance 3000. The other four smaller dips within one FSR are formed by the modulation sidebands and are therefore called sideband resonances 3001 and 3002. Figure 7 Part b) shows the measured FSR of the fiber cavity as a function of wavelength near the zero dispersion region. Figure 7Trace 7000 (left axis) in part b) shows the calculated results of the dual RF modulation scheme, where the left vertical axis represents the FSR minus the offset of 38,906 MHz and the horizontal axis represents the wavelength (in nanometers). The frequency resolution of trace 7000 is less than 15 Hz, which can clearly distinguish the small FSR changes in the 11 THz range (<800 Hz) and reveal the complex cavity dispersion of the fiber loop from normal dispersion at short wavelengths to zero dispersion and then to anomalous dispersion at long wavelengths. Trace 7002 is a second-order polynomial fit with a zero dispersion wavelength at 1.315 nm. For comparison, Figure 7 Trace 7004 (right axis) in part b) of FIG. 1 shows the FSR evolution calculated from the measurement results using only a single modulation frequency (20 GHz) as the modulation signal. The right vertical axis represents the FSR (trace 7004) minus the offset of 38,906 MHz. Figure 7 Part c) shows the frequency difference (trace 7006) between the FSR measurement value plotted in trace 7000 and the fitted value plotted in trace 7002, where the vertical axis represents the difference in Hertz and the horizontal axis represents the wavelength in nanometers. Figure 7 Part d) shows histograms 7008, 7010 of frequency differences based on trace 7006 in part c), which shows an RMS deviation of 14.2 Hz. These results demonstrate that the disclosed dual RF broadband modulation spectrometer has ultra-high frequency resolution.

[0091] according to Figure 7 The measurement results in part b) of Figure 8 The upper part of the diagram shows the calculated value of the group velocity dispersion β2 of the fiber cavity as trace 8000, where the horizontal axis represents the wavelength in nanometers and the vertical axis represents the wavelength in ps. 2 The group velocity dispersion β2 is in units of / km. The results are in good agreement with the dispersion of standard telecommunication optical fibers. Figure 8 The trace 8002 in the lower panel of FIG. 8002 shows the corresponding group delay dispersion (GDD) of the 5 m fiber cavity, including the dispersion from the 10 dB coupler. The vertical axis represents the dispersion in fs. 2 By removing the 3-meter fiber from the cavity, the GDD of the 2-meter fiber cavity can be measured using the dual RF modulation scheme (trace 8004 in the figure below), which has a zero dispersion wavelength of 1.318nm. By subtracting the GDD value of the 2-meter fiber cavity from the GDD value of the 5-meter fiber cavity, the GDD value of the 3-meter fiber cavity can be obtained, as shown in Figure 8The zero dispersion wavelength is 1.312 nm as shown by the lower trace 8006. Trace 8006 represents the difference between traces 8002 and 8004. From the measurement results, it can be seen that the longer the fiber length, the closer the zero dispersion wavelength of the fiber cavity is to the zero dispersion wavelength of the fiber itself. This demonstration proves that the disclosed method can be used to characterize the optical properties of a single device, such as a dispersion engineered broadband mirror and an integrated photonic device.

[0092] The above results show that for quasi-periodic structures with low FSR (FSR < modulation frequency), the dual RF modulation scheme according to the disclosed method has ultra-high frequency resolution. Next, the application scope of the disclosed method will be extended to measure the mode spectra of high FSR (FSR > modulation frequency) devices, such as Figure 5 The optical microresonator of the device shown was used for measurement. In this measurement, a fiber cavity was used as a resonator element, and the carrier resonance of a 5-meter fiber cavity was provided as a frequency marker, i.e., as an optical frequency reference signal, to measure the resonant frequency of a separately fabricated Si3N4 resonator. The Si3N4 resonator was made by depositing a 750-nanometer Si3N4 film and a 3-micron SiO2 layer on a silicon substrate by low-temperature reactive sputtering, see A. Frigg et al., “Low loss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thin films,” Optics Express, Vol. 27, pp. 37795-37805 (2019).

[0093] Fig. 9 The inset of part a) shows a scanning electron microscope image of the Si3N4 microresonator used in the experiment, which has a diameter of 200 μm and a waveguide cross section of 1.8 μm × 750 nm. The measured FSR and intrinsic optical quality are about 231 GHz and 2 million, respectively. Figure 5 As shown, part of the laser light maintains the same path as the above experiment, and can be modulated using two modulation frequencies and then injected into the fiber cavity. The transmission signal is detected by a photodiode 112 and recorded by one channel of an oscilloscope 114. Part of the laser light is coupled into the Si3N4 resonator through two lens fibers and transmitted out of the resonator, and the transmission spectrum is recorded by another photodiode 112 and another channel of the oscilloscope 114. When sweeping the frequency of the CW continuous laser, the transmission signals from the fiber cavity and the Si3N4 resonator are recorded simultaneously. Fig. 9Part (a) of FIG. 4 shows the normalized transmission spectrum of the Si3N4 resonator. Two different mode families are observed, with the star 9000 located on the mode family with a higher optical quality factor. The vertical axis represents the transmission in arbitrary units and the horizontal axis represents the wavelength in nanometers. Fig. 9 Part 4(b) of the graph shows a resonance near 1.271 nm in trace 9002 and a frequency marker 9004 of the fiber cavity. The FSR of the fiber cavity is first calculated according to the dual RF modulation scheme described above, and then the calculated fiber cavity resonance is used as a frequency marker to measure the mode structure of the Si3N4 resonator, such as the evolution of FSR, resonance linewidth, and dispersion. The vertical axis represents the amplitude in arbitrary units and the horizontal axis represents the wavelength in nanometers.

[0094] The resonant frequencies of the mode families in a dispersive resonator can be described by Taylor series:

[0095]

[0096] where μ is the mode number relative to the center mode (μ = 0), ω μ is the resonant frequency. D1 / 2π is the FSR of the resonator element (i.e. Si3N4 resonator) in the center mode (μ=0), and D2, D3 and D4 are the second-order, third-order and fourth-order dispersion coefficients, respectively. int is the integrated dispersion, which represents the deviation of the resonant frequency from an equidistant grid spaced at intervals of D1. Fig. 9 Part c) of FIG. 1 shows the measured integrated dispersion curve (shown as circle mark 9006) for the central mode at 1.310 nm, and a second-order polynomial fit curve 9008 derived from the resonance marked 9000 in part a). The vertical axis represents D in gigahertz. int / 2π, the horizontal axis represents the wavelength in nanometers. The dispersion curve shows anomalous dispersion, with mode crossings at 1.280 and 1326 nanometers. To verify the dispersion measurement results, the Si3N4 resonator was tested to generate a soliton frequency comb by pumping the optical mode at 1.310nm. Part d) shows the results with the fitted sech 2 The optical spectrum 9010 of a single bright soliton of the envelope (dashed trace 9012) further verifies the anomalous dispersion mechanism of the pump mode. In addition, the mode crossing at 1.326 nm in part c) generates a dispersive wave (marked by an arrow) at 1.326 nm in part d).

[0097] To further highlight the broad utility and effectiveness of the disclosed method, the application of the disclosed method to non-periodic structures was demonstrated, such as analyzing the absorption spectrum of a gas cell. The demonstration used a fiber-coupled hydrogen fluoride (H19F) gas cell with a pressure of 50 Torr and an optical path of 2.7 cm. Similarly, the laser light from a 1.3μm CW continuous laser source was split into two paths, one for detecting the high-frequency absorption spectrum, and the other modulated by a dual RF signal and coupled into a 5-meter-long fiber ring cavity (as a resonator element) while calibrating the laser sweep.

[0098] Fig.10 Part a) shows the strong high-frequency molecular absorption lines (P and R branches) in the O-band range, where the vertical axis represents the transmission in arbitrary units and the horizontal axis represents the wavelength in nanometers. Part b) shows the enlarged spectrum of the P(2) absorption line (trace 10000), with the carrier resonance 10002 of the fiber cavity as the frequency reference. Since the pressure broadening effect of the high-frequency gas is much larger than the Doppler broadening effect, a Lorentz function (dashed line 10004 in part b) is used to fit the spectral curve.

[0099] Table 1 shows the comparison of the measurement results of HF absorption spectra with the HITRAN database (HITRAN database refers to the paper “The HITRAN 2016 molecular spectroscopic database” by IE Gordon et al., Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 203, pp. 3-69 (2017)).

[0100] Table 1:

[0101]

[0102]

[0103] 1 Data from HITRAN are given after 50 Torr pressure offset calibration

[0104] 2Uncertainties in pressure shift and pressure broadening line width are calculated based on a 20% pressure uncertainty

[0105] 3 The measured R(2) is set equal to the value in HITRAN

[0106] The second column of Table 1 shows the calculated absorption line positions from the HITRAN database, corrected for the pressure offset based on the vacuum transition wavelength. Columns 3 and 4 are the calculated Gaussian full width and Lorentzian full width at half maximum (FWHM), respectively. The uncertainties are calculated based on a 20% uncertainty in the gas pressure specified by the cell manufacturer. Column 5 shows the measured line positions. Since no absolute frequency reference is provided for this demonstration, the wavelength of the measured R(2) line is set equal to the value calculated from the HITRAN database. Column 6 is the wavelength difference between the measured results and the HITRAN database. It can be seen that the measured results are in good agreement with the HITRAN database. This small difference can be attributed to the uncertainty in the gas pressure. Using the Gaussian line widths calculated in column 3 and the Voigt function, the last column shows the measured Lorentzian full widths of the different absorption lines. Considering that the wavelengths of the measured line positions are smaller than the calculated values ​​in column 2, and that the measured Lorentzian line widths are larger than the calculated values, it is inferred that the pressure of the high-frequency cell used as the calibration element in the experiment was higher than the 50 Torr specified by the manufacturer.

[0107] In summary, the present disclosure provides and demonstrates a powerful broadband spectroscopy technique based on a tunable CW laser, whose frequency sweep behavior is calibrated by a fiber cavity with dual RF frequency modulation. Using this method, small FSR deviations (800 Hz) of the fiber cavity in a near-zero dispersion state can be resolved with a resolution of less than 15 Hz over a frequency range of 11 THz. The demonstrated measurement speed is 1 THz / s, which is limited by the cavity linewidth of the fiber cavity rather than the measurement system of the conventional device (see P. Del'Haye's paper "Frequency comb assisted diode laser spectroscopy for measurement of microcavity dispersion", Nat. Photonics, Vol. 3, pp. 529-533 (2009)). If the resonator element has a wider linewidth (>3.2 MHz), this speed can be greatly increased to more than 10 THz / s. The measurement speed will ultimately be limited by the tuning speed of the laser light source. In addition, the demonstrated method can also surpass the stringent requirements of conventional frequency comb-based spectrometers for spectral flatness, comb line power, and polarization. The measurement spectral range of the disclosed method is limited only by the tunable range of the CW laser, and can be extended by cascading multiple CW lasers, see J. Liu et al., "Frequency-comb-assisted broadband precision spectroscopy with cascaded diode lasers", Opt. Lett., Vol. 41, pp. 3134-3137 (2016), and can also achieve spectral regions that were previously inaccessible because there were no high-resolution wavelength meters or frequency combs. We further verified the application of the disclosed method in characterizing the dispersion of integrated photonic devices and high-frequency gas molecular absorption spectra. Using well-known atomic / molecular transitions as an absolute frequency reference, this method can be used for high-precision broadband molecular spectroscopy (see A. Shkarin et al., "Nanoscopic Charge Fluctuations in a Gallium Phosphide Waveguide Measured by Single Molecules", Phys. Rev. Lett., Vol. 126, pp. 133-602 (2021)). In addition, the disclosed method can be widely applied to different scenarios, such as lidar (see E.Baumann et al., “Comb-calibrated frequency-modulated continuous-wave lidar for absolute distance measurements”, Opt. Lett., Vol. 38, pp. 2026-2028 (2013)), three-dimensional imaging (see E. Baumann et al., “Comb-calibrated laser ranging for three-dimensional surface profiling with micrometer-level precision at a distance”, Opt. Express, Vol. 22, pp. 24914-24928 (2014)), refractive index measurement (see L. Yang et al., “Frequency combcalibrated frequency-sweeping interferometry for absolute group refractive index measurement of air”, Appl. Opt., Vol. 56, pp. 3109-3115 (2017)), precise frequency measurement and characteristic analysis of photonic devices (see V. Brasch et al., “Photonic chip–based optical frequency comb using soliton Cherenkov radiation”, Science, Vol. 351, pp. 357-360 (2016); A. Nishiyama et al., “Precisefrequency measurement and characterization of a continuous scanning single-mode laser with an optical frequency comb”, Opt. Lett., Vol. 39, pp. 4923-4926 (2014), such as astrophotonic devices (reference S. Minardi, RJ Harris and L. Labadie, “Astrophotonics: astronomy and modern optics”, Astron. Astrophys. Rev., Vol. 29, p. 6 (2021)).

[0108] Fig.11 A LIDAR system 300 is schematically depicted, the system comprising the apparatus 200 for providing an optical frequency reference signal according to an alternative embodiment. Fig.12 A gas sensing system 400 is schematically depicted, the system comprising the apparatus 200 for providing an optical frequency reference signal according to an alternative embodiment.

[0109] Reference numerals list

[0110] 100 Device for characterizing resonator elements

[0111] 102 Resonator Components

[0112] 104 Laser light source

[0113] 106 Fiber Optic

[0114] 108 Coupling element

[0115] 110 Modulator for modulating intensity and / or phase

[0116] 112 Detector unit

[0117] 114 Data Logger

[0118] 116 Control Unit

[0119] 118 Calibration element

[0120] 120 Photonic Devices / Micro Ring Resonators

[0121] 200 Apparatus for providing an optical frequency reference signal

[0122] 202-214 Methods and Steps

[0123] 300 LIDAR system

[0124] 400 Gas Sensing System

[0125] 602-608 Methods and Steps

[0126] 1001 First modulation frequency

[0127] 1002 Second modulation frequency

[0128] 3000 Carrier resonance

[0129] 3001 Sideband resonance generated by the first modulation frequency

[0130] 3002 Sideband resonance generated by the second modulation frequency

[0131] 3003 Time interval between sideband resonances from two different modulation frequencies in the first scan

[0132] 3004 The time interval between the sideband resonances from two different modulation frequencies of the second scan

[0133] 3005 Unknown time interval between adjacent carrier resonances

[0134] 7000 trace showing results of dual RF modulation scheme

[0135] 7002 Displays the trace of a second-order polynomial fit

[0136] 7004 Calculated FSR evolution of a single RF modulation scheme

[0137] 7006 Frequency difference between measured FSR and fitted graph

[0138] 7008, 7010 Frequency difference histogram

[0139] 8000 Calculated group velocity dispersion

[0140] Group delay dispersion of 8002, 8004, 8006 different fiber lengths

[0141] 9000 Mode family with high optical quality factor

[0142] 9002 Resonance curve at 1.271nm

[0143] 9004 Frequency Marker

[0144] 9006 Integrated dispersion curve

[0145] 9008 Fitting curve

[0146] 9010 Optical Spectra of Single Bright Solitons

[0147] 9012 Fitting envelope curve

[0148] 10000 Absorption line spectrum

[0149] 10002 Carrier resonance

[0150] 10004 Fitted Lorentzian function

[0151] FSR Free Spectral Range

Claims

1. A method of characterizing a resonator element (102), the method comprising: -Providing a laser with 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) for a carrier frequency of the laser light, wherein adjacent carrier resonances (3000) are spaced apart from one another by a free spectral range (FSR) in the spectral domain; - 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), thereby generating for each carrier resonance (3000) at least two sideband resonances (3001) spaced apart from the corresponding carrier resonance (3000) by the first modulation frequency (1001), and at least two sideband resonances (3002) spaced apart from the corresponding carrier resonance (3000) by the second modulation frequency (1002), wherein the first modulation frequency (1001) and the second modulation frequency (1002) are different from an integer multiple of the free spectral range and are different from each other; - tuning the carrier frequency of the laser at a predetermined tuning rate; - 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 for tuning the carrier frequency through four adjacent sideband resonances (3001, 3002), said tuning time corresponding to the tuning time required to change the tunable carrier frequency from a value corresponding to a first of the four sideband resonances to a value corresponding to a last of the four sideband resonances; and - Determining the spacing between a plurality of carrier resonances (3000) in the spectral domain based on the measured intensity of laser light transmitted and / or reflected by the resonator element (102) and using the first modulation frequency (1001), the second modulation frequency (1002) and the measured tuning time of tuning the carrier frequency through four adjacent sideband resonances (3001, 3002).

2. The method according to claim 1, wherein the first modulation frequency (1001) and the second modulation frequency (1002) are both within the radio frequency range.

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. The method according to any one of the preceding claims, wherein the difference between the first modulation frequency (1001) and the second modulation frequency (1002) is not less than 10% of the resonance linewidth of the carrier resonance (3000) and not more than 50% of the free spectral range (FSR).

5. A method according to any of the preceding claims, wherein the first modulation frequency (1001) and the second modulation frequency (1002) substantially correspond to frequencies around n+1 / 2 multiples of the free spectral range (FSR), where n is an integer.

6. A method according to any of the preceding claims, wherein two sideband resonances (3001) generated by a first modulation frequency (1001) from one of the carrier resonances (3000) are located within at least one free spectral region (FSR), and at least two sideband resonances (3002) generated by a second modulation frequency (1002) from one of the carrier resonances (3000) are located.

7. A method according to any of the preceding claims, wherein measuring the tuning time to tune the carrier frequency through four adjacent sideband resonances (3001, 3002) comprises measuring the tuning time to tune the carrier frequency through four adjacent sideband resonances (3001, 3002) within a single free spectral region.

8. The method according to any of the preceding claims, wherein the free spectral range (FSR) of the resonator element (102) is not less than 1 MHz and not more than 100 GHz.

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

10. The method according to claim 9, wherein the calibration element (118) comprises a gas chamber filled with a predetermined gas, the predetermined gas having at least one specific absolute transmission and / or reflection property, and / or the calibration element (118) comprises one or more of the following elements: a frequency comb, a wavelength meter, an element providing atomic and / or molecular transition lines.

11. An apparatus (100) for characterizing a resonator element (102), the apparatus (100) comprising: - a tunable laser light source (104), the tunable laser light source (104) being used to emit laser light having a tunable carrier frequency; a coupling element (108) that couples at least a portion of the laser light into the 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) so as to generate, for each carrier resonance (3000), at least two sideband resonances (3001) spaced apart from the corresponding carrier resonance (3000) by the first modulation frequency (1001) and, for each carrier resonance (3000), at least two sideband resonances (3002) spaced apart from the corresponding carrier resonance (3000) by the second modulation frequency (1002); - a detector unit (112) for measuring the intensity of the part of the laser light transmitted and / or reflected by the resonator element (102); - a control unit (116) configured to determine a tuning time and a spacing between a plurality of carrier resonances (3000) in the spectral domain based on a measured intensity of laser light transmitted and / or reflected by the resonator element (102) and based on a first modulation frequency (1001), a second modulation frequency (1002) and a measured tuning time for tuning the carrier frequency through four adjacent sideband resonances (3001, 3002), the tuning time corresponding to a tuning time required to change the tunable carrier frequency from a value corresponding to a first of the four sideband resonances to a value corresponding to a last of the four sideband resonances.

12. The device (100) according to 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 resonator and / or a Fabry-Perot resonator.

13. The device (100) according to claim 11 or 12, wherein the modulator (110) comprises an electro-optical intensity modulator and / or a 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; wherein the detector unit (112) is further adapted to measure the intensity of a portion of the laser light transmitted and / or reflected by the calibration element (118) while tuning the carrier frequency; and The control unit (116) is further adapted to identify at least one specific absolute transmission and / or reflection characteristic of the calibration element (118), the absolute transmission and / or reflection characteristic having a predetermined frequency that coincides with or has a specified offset from a carrier resonance (3000) or one of the sideband resonances (3001, 3002) in the frequency domain, and the control unit (116) is adapted to calibrate the absolute frequency of at least one of the carrier resonance (3000) or the sideband resonance (3001, 3002) based on the identified specific absolute transmission and / or reflection characteristic of the calibration element (118).

15. A device according to claim 14, wherein the calibration element (118) comprises a gas chamber filled with a predetermined gas, the predetermined gas having at least one specific absolute transmission and / or reflection property, and / or comprises a frequency comb and / or a wavelength meter and / or an element providing atomic and / or molecular transition lines.

16. A method of providing a laser with a tunable carrier frequency as an optical frequency reference signal, the method comprising: - providing a resonator element (102) having a plurality of carrier resonances (3000) for a carrier frequency of the laser light, wherein adjacent carrier resonances (3000) are spaced apart from one another in the spectral domain by respective predetermined free spectral ranges (FSR); - coupling a first portion of the laser light into the resonator element (102); Characterized in that the method further comprises: - tuning the carrier frequency of the laser at a predetermined tuning rate; - using the portion of the laser light transmitted and / or reflected by the resonator element (102) as an optical frequency reference signal for spectral scanning, wherein the intensity of the laser light transmitted and / or reflected by the resonator element (102) has local extrema at frequencies which are spectrally spaced apart from each other by the FSR.

17. The method of claim 16, wherein the carrier frequency of the laser is tuned within at least ten free spectral regions.

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

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

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

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

22. The method according to any one of claims 16 to 21, further comprising: modulating the intensity and / or phase of a first portion of the laser light coupled into the resonator element (102) at a first modulation frequency (1001) and a second modulation frequency (1002) to generate, for each carrier resonance (3000), at least two sideband resonances (3001) separated from the corresponding carrier resonance (3000) by the first modulation frequency (1001), and at least two sideband resonances (3002) separated from the corresponding carrier resonance (3000) by the second modulation frequency (1002), wherein the first modulation frequency (1001) and the second modulation frequency (1002) are different from each other and are different from an integer multiple of a free spectral range (FSR); and The frequencies of the sideband resonances (3001, 3002) are provided as spectral reference marks for the optical frequency reference signal.

23. The method of claim 22, wherein the first modulation frequency (1001) and the second modulation frequency (1002) are both within the radio frequency range.

24. The method of 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. 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 not less than 10% of the resonance linewidth of the carrier resonance (3000) and not more than 50% of the free spectral range (FSR).

26. The method according to any one of claims 22 to 25, wherein the first modulation frequency (1001) and the second modulation frequency (1002) substantially correspond to frequencies around n+1 / 2 multiples of 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; - while tuning the carrier frequency, measuring the intensity of the portion of the laser light transmitted and / or reflected by the calibration element (118); - identifying at least one specific absolute transmission and / or reflection characteristic of the calibration element (118), said absolute transmission and / or reflection characteristic having a predetermined frequency, said predetermined frequency coinciding with or having a specified offset from a carrier resonance (3000) or one of the sideband resonances (3001, 3002) in the frequency domain; and - Calibrate the absolute frequencies of the carrier resonance (3000) and / or the sideband resonances (3001, 3002) based on the determined specific absolute transmission and / or reflection characteristics.

28. The device according to claim 27, wherein the calibration element (118) comprises a gas chamber filled with a predetermined gas, the predetermined gas having at least one specific absolute transmission and / or reflection property, and / or comprising a frequency comb and / or a wavelength meter and / or an atomic transition line.

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

30. A method for spectroscopic characterization of an analyte, the method comprising: - providing an optical frequency reference signal using a method according to any one of claims 16 to 29; -Spectral characterization of analytes using an optical frequency reference signal as a spectral reference marker.

31. The method of claim 30, wherein using the optical frequency reference signal as a spectral reference marker comprises determining relative spectral distances of a plurality of spectral features of the analyte based on the one or more FSRs.

32. A device (200) for providing a laser with a tunable carrier frequency as an optical frequency reference signal, the device comprising: - a tunable laser light source (104), the tunable laser light source (104) being used to emit laser light having a tunable carrier frequency; a resonator element (102) having a plurality of carrier resonances (3000) for a tunable carrier frequency of the laser light, wherein the carrier resonances (3000) are mutually spaced apart in the spectral domain by respective predetermined free spectral ranges (FSR), and wherein the device (200) is adapted to couple a portion of the laser light into the resonator element (102); Characterized in that the device also includes: - a control unit, which tunes the carrier frequency of the laser at a predetermined tuning rate; The device is used to use part of the laser light transmitted and / or reflected by the resonator element (102) as a spectrally scanned optical frequency reference signal, wherein the intensity of the laser light transmitted and / or reflected by the resonator element (102) has a local extremum in frequency, and the frequencies are spaced apart from each other in the spectrum by FSR.

33. The apparatus according to claim 32, further comprising: - a detector unit (112) for measuring the intensity of the part of the laser light transmitted and / or reflected by the resonator element (102); - a control unit (116) configured to provide a frequency of a tunable carrier frequency having a local extrema in the measured intensity of a portion of the laser light transmitted and / or reflected by the resonator element (102), said frequency serving as a spectral reference marker for an optical frequency reference signal defined by a predetermined free spectral range (FSR).

34. The device (200) according to claim 32 or 33, further comprising a modulator (110) for modulating the intensity and / or phase of the portion of the laser light coupled to the resonator element (102) at a first modulation frequency (1001) so as to generate for each carrier resonance (3000) at least two sideband resonances (3001) spaced apart from the corresponding carrier resonance (3000) by the first modulation frequency (1001), The control unit (116) is further configured to provide a frequency of a tunable carrier frequency having a local extrema due to a sideband resonance (3001) in a measured intensity of a portion of the laser light transmitted and / or reflected by the resonator element (102), said frequency serving as a spectral reference marker for an optical frequency reference signal defined by a predetermined free spectral range (FSR) and a first modulation frequency (1001).

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

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

37. The 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; wherein the detector unit (112) is further adapted to measure the intensity of a portion of the laser light transmitted and / or reflected by the calibration element (118) while tuning the carrier frequency; and The control unit (116) is further adapted to identify at least one specific absolute transmission and / or reflection characteristic of the calibration element (118), the absolute transmission and / or reflection characteristic having a predetermined frequency that coincides with or has a specified offset from a carrier resonance (3000) or one of the sideband resonances (3001, 3002) in the frequency domain, and the control unit (116) is adapted to calibrate the absolute frequency of at least one of the carrier resonance (3000) or the sideband resonance (3001, 3002) based on the identified specific absolute transmission and / or reflection characteristic of the calibration element (118).

38. A device (200) according to claim 37, wherein the calibration element (118) comprises a gas chamber filled with a predetermined gas, the predetermined gas having at least one specific absolute transmission and / or reflection property, and / or comprises a frequency comb and / or a wavelength meter and / or an element providing atomic and / or molecular transition lines.

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

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

41. The apparatus according to any one of claims 32 to 40, wherein the tunable laser light source is used to emit tunable laser light having a spectral width smaller than a line width of the resonator element (102).

42. A LIDAR system (300) comprising an apparatus (200) according to any one of claims 32 to 41 for providing an 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 of characterizing a resonator element (102), the method comprising: -Providing a laser with a tunable carrier frequency; - coupling at least a first part of the laser light into a resonator element (102) having a plurality of carrier resonances (3000) for a carrier frequency of the laser light, wherein adjacent carrier resonances (3000) are spaced apart from one another in the spectral domain by a free spectral region; - modulating the intensity of the portion of the laser light coupled to the resonator element with a first modulation frequency (1001) so as to generate for each carrier resonance at least two sideband resonances (3001) spaced apart from the corresponding carrier resonance (3000) by the first modulation frequency (1001), wherein the first modulation frequency (1001) is higher than the free spectral range and is different from an integer multiple of the free spectral range; - tuning the carrier frequency of the laser at a predetermined tuning rate; - measuring the intensity of the laser light transmitted and / or reflected by the resonator element (102) while tuning the carrier frequency; - measuring a tuning time (1001) between a plurality of carrier resonances, the plurality of carrier resonances being spaced from each other by at least twice a first modulation frequency; as well as - Determining the spacing between a plurality of free spectral regions in the spectral domain based on the measured intensity of the laser light transmitted and / or reflected by the resonator element (102) using a first modulation frequency and measured tuning times between a plurality of carrier resonances (3000) spaced apart from each other by at least twice the first modulation frequency (1001).

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