Unequal-interval frequency comb generation and detection device and method based on single sideband modulator

Abstract

The invention belongs to the technical field of optics, and particularly relates to an unequal-interval frequency comb generation and detection device and method based on a single side band modulator. In order to improve phase coherence between comb teeth, an arbitrary waveform generator generates a modulation signal, the modulation signal is amplified by a power amplifier and then input into a single-side-band phase modulator, a laser is connected with an optical signal input end of the single-side-band phase modulator, and an output end of the single-side-band phase modulator is connected with an optical beam splitter. The light beam splitter divides light into two paths, one path serves as reference light to enter the first detector, the first detector converts the reference light into an electric signal and then carries out FFT observation, the other path serves as detection light, is absorbed by the gas chamber and then is input into the second detector, and the second detector converts the detection light into an electric signal and then carries out FFT observation.

Description

Technical Field

[0001] This invention belongs to the field of optical technology, specifically relating to a device and method for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator. Background Technology

[0002] Traditionally, an optical frequency comb (OFC) appears in the frequency domain as a narrow-linewidth comb with equal-frequency spacing, equivalent to the simultaneous output of thousands of phase-coherent continuous-wave laser sources. OFC has wide applications in numerous fields, such as attosecond science, frequency synthesis, laser generation, exoplanet search, low-noise microwaves, frequency measurement, optical clocks, distance measurement, and lidar. In particular, optical frequency comb spectroscopy offers advantages over previous laser spectroscopy techniques, including wider bandwidth, faster speed, and simultaneous measurement of multiple gases. Currently, there are various methods for generating OFCs, but regardless of whether it's the most mature mode-locked laser source, microcavity-based nonlinear Kerr effect, or external modulation, all these methods produce combs with equal-frequency spacing. After generating an OFC, its detection is even more crucial. Several OFC detection methods have been developed, the most common being Fourier transform spectrometers based on Michelson interferometers. However, this spectrometer system has a complex structure, slow sampling rate, and is susceptible to vibration due to the need for moving components. The second common method is based on dispersive elements such as gratings or virtual phase arrays. These methods offer high sampling rates, reaching the fs level, but suffer from low spectral resolution, unable to distinguish each comb tooth. Dual-comb spectroscopy (DCS) is currently the most popular detection method, offering numerous advantages. However, a key issue with DCS is that high signal-to-noise ratio spectral measurements require extremely high phase coherence between the two combs. Free-running dual combs can also generate spectral frequencies by beat frequency, but suffer from low resolution and frequency accuracy, both only on the order of approximately GHz, limiting signal averaging. In contrast, two phase-locked combs with phase coherence can generate comb-tooth resolution spectra, and averaging can improve the signal-to-noise ratio. Higher phase coherence results in higher resolution and frequency accuracy, reaching MHz, kHz, or even higher levels. Therefore, electro-optic frequency combs are more advantageous for DCS than other generation methods because the two combs originate from the same continuous-wave laser source, exhibiting better coherence. However, the generation of electro-optic dual optical combs still leads to a decrease in coherence due to different propagation paths. Therefore, developing optical frequency comb spectroscopy technology with higher coherence remains an urgent problem to be solved. Summary of the Invention

[0003] To improve the phase coherence between comb teeth, this invention provides a device and method for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator, which features speed, high sensitivity, and high resolution.

[0004] To achieve the above objectives, the present invention employs the following technical solution: This invention provides a device for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator, comprising a laser and an arbitrary waveform generator. The arbitrary waveform generator generates a modulation signal, which is amplified by a power amplifier and then input to a single-sideband phase modulator. The laser is connected to the optical signal input terminal of the single-sideband phase modulator, and the output terminal of the single-sideband phase modulator is connected to an optical beam splitter. The optical beam splitter splits the light into two paths. One path serves as a reference light and enters a first detector. The first detector converts the reference light into an electrical signal and then observes it using FFT. The other path serves as a probe light, which is absorbed by a gas cell and then input to a second detector. The second detector converts the probe light into an electrical signal and then observes it using FFT (9).

[0005] Furthermore, the modulation signal is composed of radio frequency signals with unequal spacing.

[0006] The present invention also provides a method for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator, comprising the following steps: Step 1: Design the RF frequency of the optical frequency comb according to the RF frequency required for subsequent applications, and generate periodic time-domain RF signal data at the above frequencies. Step 2: After turning on the laser switch, import the signal data generated in Step 1 into the arbitrary waveform generator to output the modulation signal. Turn on the RF power amplifier to make it work normally and control the output power. Turn on the power supply of the single-sideband phase modulator and control the bias voltage so that it only outputs one side of the comb teeth and suppresses the carrier frequency and the sideband of the other side of the comb teeth. The frequency comb is split into two paths after passing through the optical beam splitter. One path goes directly into the first detector and FFT is performed on the signal to observe the beat frequency between the comb teeth initially set. The other path goes through the gas chamber and enters the second detector and FFT is performed on the signal to observe the beat frequency between the comb teeth after absorption by the gas to be tested.

[0007] Compared with the prior art, the present invention has the following advantages: 1. In all DCS measurement schemes, two optical combs are generated before measurement. In this invention, only one beam of light is used before measurement. Therefore, the phase coherence between the comb teeth is high and the phase noise is extremely low, which can detect absorption information at a high rate.

[0008] 2. Benefiting from high phase coherence, it has higher detection sensitivity compared to DCS with direct absorption measurement. Even compared to other single optical comb technologies, such as optical combs generated by electro-optic modulators (EOM) or Mach-Zehnder modulators (MZM), these methods have the same high phase coherence between the comb teeth. However, there is a problem that the sidebands of the comb teeth on both sides of the carrier frequency interfere with each other when the detector beats. This means that these methods often need to add devices such as optical filters to filter one sideband before performing spectral measurements, which greatly reduces the long-term stability of the system. Therefore, this invention has extremely high long-term stability and detection sensitivity.

[0009] 3. Traditional DCS systems face a trade-off between spectral resolution, acquisition rate, and spectral coverage, namely... Where m is the number of comb teeth. f rep It is the repetition frequency of the optical frequency comb, Δ f rep The difference in repetition frequency between the two optical combs is a limitation designed to avoid cross-interference between the comb teeth. In contrast, the present invention does not have the above-mentioned problem. By designing the modulation frequency, the present invention can make the optical frequency and radio frequency correspond one-to-one in the experiment, thereby eliminating the conflict between spectral resolution, acquisition rate and spectral coverage. This is especially beneficial for detecting narrow bandwidth spectral lines, such as saturation absorption and multiphoton absorption, at high rates.

[0010] 4. Compared with the DCS measurement scheme, the present invention can arbitrarily set the modulation frequency, and can set high resolution for the spectrum that requires high attention and low resolution for the spectrum that is relatively uninterested, making it particularly suitable for high-resolution spectral measurements.

[0011] 5. Compared to the DCS measurement solution, the experimental system has a compact structure, can be deployed on-site, and is significantly more cost-effective. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of a frequency comb generation and detection device based on a single-sideband modulator with unequal spacing. Figure 2 A schematic diagram in the time-frequency domain of an unequally spaced optical frequency comb generated by a computer program; Figure 3 A time-frequency domain diagram showing the beat frequency between comb teeth detected by the detector; Figure 4 This is a schematic diagram of gas absorption lines; Among them, there are laser-1, single-sideband phase modulator-2, arbitrary waveform generator-3, radio frequency power amplifier-4, optical beam splitter-5, gas cell-6, first detector-7, second detector-8, and FFT-9. Detailed Implementation

[0013] To further illustrate the technical solution of the present invention, the present invention will be further described below through embodiments. Example 1

[0014] This embodiment provides a non-uniformly spaced frequency comb generation and detection device based on a single-sideband modulator (SSB). It includes a laser 1 and an arbitrary waveform generator 3. The arbitrary waveform generator 3 generates a modulation signal composed of non-uniformly spaced radio frequency signals. The modulation signal is amplified by a power amplifier 4 and then input to a single-sideband phase modulator 2. The laser 1 is connected to the optical signal input terminal of the SSB phase modulator 2, and the output terminal of the SSB phase modulator 2 is connected to an optical beam splitter 5. The optical beam splitter 5 splits the light into two paths. One path serves as a reference light and enters a first detector 7. The first detector 7 converts the reference light into an electrical signal and observes it using an FFT 9. The other path serves as a probe light, is absorbed by a gas chamber 6, and then input to a second detector 8. The second detector 8 converts the probe light into an electrical signal and observes it using an FFT 9.

[0015] Based on the above-described apparatus, this embodiment provides a method for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator, characterized by comprising the following steps: Step 1: Design the RF frequency of the optical frequency comb according to the RF frequency required for subsequent applications, and use a computer program to generate periodic time-domain RF signal data at the above frequencies. Step 2: After turning on the laser 1, import the signal data generated in Step 1 into the arbitrary waveform generator 3 to output the modulation signal. Turn on the RF power amplifier 4 to put it into normal working condition and control the output power. The output power should meet the requirements of subsequent applications. Here, the RF power is not necessarily better the higher it is. When the RF power reaches a certain level, the higher-order sidebands will not be ignored and will affect the final result, so strict control is required. Turn on the power supply of the single-sideband phase modulator 2, control the bias voltage to output only one side of the comb teeth and suppress the carrier frequency and the sidebands of the other side of the comb teeth. This frequency comb is split into two paths after passing through the optical beam splitter 5. One path goes directly into the first detector 7, and FFT9 can be performed on the signal to observe the beat frequency between the comb teeth initially set. The other path goes through the gas chamber 6 and enters the second detector 8, and FFT9 can be performed on the signal to observe the beat frequency between the comb teeth after absorption by the gas under test.

[0016] Compared to conventional DCS, this invention uses only the same path during the optical comb generation stage, resulting in extremely high phase coherence and very low noise. This allows for the generation of interferograms with a high signal-to-noise ratio using only a single period of the time-domain signal, thus providing rapid measurement capabilities. The sampling rate depends only on the final time-domain signal period. Furthermore, due to the low noise, it exhibits higher detection sensitivity. Traditional DSC schemes suffer from a trade-off between spectral resolution, acquisition rate, and spectral coverage. ,in m It refers to the number of comb teeth. f rep It is the repetition frequency of the optical frequency comb, Δ f rep The difference in repetition frequency between two optical combs is a limitation of the present invention. The modulation frequency can be set arbitrarily, making it particularly suitable for high-resolution spectral measurements. In addition, compared with other optical comb generation methods, the present invention has a compact structure, can be deployed on-site, and is cost-effective. Example 2

[0017] First, a computer program generates the time-domain radio frequency signal data required for modulation. This signal consists of radio frequency components with unequal spacing. Specifically, the frequency difference between any two modulation frequencies must be unequal; for example, the frequency difference between adjacent comb teeth can be set as follows: .in, f 1 represents the frequency difference between the first two comb teeth, Δ f r This is the modulation frequency parameter, specifically understood as the frequency difference between two adjacent modulation frequencies. If we let... f 1=200MHz, Δ f r =1MHz, and generates 20 comb teeth, then the corresponding comb tooth frequencies are 1000, 1200, 1401, 1603, 1806, 2010, 2215, 2421, 2628, 2836, 3045, 3255, 3466, 3678, 3891, 4105, 4320, 4536, 4753, 4971MHz. Figure 2 The diagram above shows the time-frequency domain representation of the unequal-pitch optical frequency comb generated by the computer program. The left image displays two periods of time-domain signal with a period of 1 μs. The right image is the spectrum after FFT of the left image, clearly showing the unequal-pitch comb teeth. The frequency intervals between adjacent comb teeth are 200, 201, 202, ..., 218 MHz. The optical frequency comb, after passing through the device described in this invention, detects the same frequency components at both detector ends. Figure 3 As shown, the left figure is the time-domain signal of the reference optical path for two cycles, and the right figure is the spectrum of the time-domain signals of the reference optical path and the probe optical path after FFT. The attenuation in the middle of the measured optical path is the absorption of gas in the gas chamber. Finally, according to Figure 3 The absorption lines of the gas are as follows Figure 4 As shown, the gas concentration, temperature, pressure, and other information can be further calculated using this measurement result.

[0018] The foregoing has shown and described the main features and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0019] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A device for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator, characterized in that, Includes a laser (1) and an arbitrary waveform generator (3). The arbitrary waveform generator (3) generates a modulation signal. The modulation signal is amplified by a power amplifier (4) and then input to a single-sideband phase modulator (2). The laser (1) is connected to the optical signal input terminal of the single-sideband phase modulator (2). The output terminal of the single-sideband phase modulator (2) is connected to an optical beam splitter (5). The optical beam splitter (5) splits the light into two paths. One path is used as a reference light and enters the first detector (7). The first detector (7) converts the reference light into an electrical signal and then observes it using FFT (9). The other path is used as a probe light and is absorbed by a gas cell (6) and then input to a second detector (8). The second detector (8) converts the probe light into an electrical signal and then observes it using FFT (9).

2. The device for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator according to claim 1, characterized in that, The modulation signal consists of radio frequency signals with unequal spacing.

3. A method for generating and detecting unequal-pitch frequency combs based on a single-sideband modulator, characterized in that, Includes the following steps: Step 1: Design the optical frequency comb RF frequency according to the RF frequency required for subsequent applications, and generate periodic time-domain RF signal data at the above frequencies. Step 2: After turning on the laser (1) switch, import the signal data generated in step 1 into the arbitrary waveform generator (3) and output the modulation signal. Turn on the RF power amplifier (4) to make it work normally and control the output power. Turn on the power supply of the single sideband phase modulator (2) and control the bias voltage to make it output only one side of the comb and suppress the carrier frequency and the sideband of the other side of the comb. The frequency comb is split into two paths after passing through the optical beam splitter (5). One path directly enters the first detector (7) and the signal is subjected to FFT (9) to observe the beat frequency between the comb teeth initially set. The other path enters the second detector (8) after passing through the gas chamber (6) and the signal is subjected to FFT (9) to observe the beat frequency between the comb teeth after being absorbed by the gas to be tested.

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