The mid-infrared dual-comb spectral detection system has brought innovative technology to the calibration of extremely low concentration gases due to its high resolution, high sensitivity, and fast measurement characteristics. There are currently three main technologies for generating coherent dual optical comb systems: mode-locked lasers, electro-optic modulation, and nonlinear optical microresonators.
In recent years, single-cavity dual-wavelength lasers, or "single-cavity dual-comb" technology, have attracted much attention. A modulation element is added to a single mode-locked laser resonator, so that the two wavelengths in the resonator can start to oscillate simultaneously, so as to output two pulse sequences with slightly different repetition frequencies to replace two mode-locked lasers. Since the two wavelengths are generated by the common cavity, the common mode noise between the two is well suppressed, the relative frequency stability between the two pulses is high, no external phase-locking system is required, and the frequency jitter standard of the repetition frequency difference between pulses The difference can be reduced to 5.1 mHz, which provides a reliable light source with simple structure, low cost, small size and high stability for the dual optical comb system.
In this paper, asynchronous dual-wavelength pulses output by a single-cavity dual-wavelength laser are used to replace two mode-locked lasers, combined with nonlinear difference frequency technology, an integrated and practical mid-infrared dual-comb system is developed.
experimental device
The mid-infrared dual-comb system based on a single-cavity dual-wavelength laser includes a fully polarization-maintaining fiber structure Ytterbium-doped dual-wavelength laser, a multi-stage cascaded optical amplifier, an optical difference frequency module and a long optical path gas absorption detection module. The overall experimental device is shown in the figure 1(a).
The fully polarization-maintaining ytterbium-doped dual-wavelength laser based on nonlinear amplifying loop mirror (NALM) mode-locked is composed of a nonlinear ring and a Lyot filter bridged by a fiber splitter (CP2), in which the nonlinear ring part is composed of 980/ 1030 nm wavelength division multiplexer (WDM1), ytterbium-doped gain fiber (YSF1), fiber splitter (CP1) and non-reciprocal phase shifter, while fiber coupling mirror (COL1&2), quarter-wave plate (QWP), fully polarization-maintaining fiber and a Lyot filter composed of an optical fiber mirror (OFM) acts as the linear arm part of the NALM cavity. The Out.1 output of the resonator is used for spectrum monitoring, and the Out.2 output is used for mode-locking status and output power monitoring. In the nonlinear loop, the 976 nm pump light with a maximum power of 400 mW is coupled into a 1 m total positive dispersion polarization-maintaining ytterbium-doped gain fiber (YSF1, Nufern, PM-YSF-HI) along the pump end of WDM1. The 1030 nm laser pulse generated by the excitation radiation is transmitted in the resonator. The non-reciprocal phase shifter in the ring can provide a linear phase shift of π/2 for the forward and reverse transmission of the two beams. At the same time, in order to increase the asymmetry, the clockwise transmitted pulse in the ring first passes through the 4.4 m negative dispersion unit mode fiber, and then enters YSF1 for power amplification; while the counterclockwise transmitted pulse first passes through a 0.9 m negative dispersion single-mode fiber, and then enters YSF1 for amplification. The asymmetrically placed gain fiber can introduce nonlinearity for the two beams of light phase shift. For the "9"-shaped cavity with NALM mode locking, when the total phase shift reaches an odd multiple of π, the loss in the cavity is the smallest, and the laser can achieve mode locking. In the linear arm, the pigtail length between COL1 and CP2 is 0.27 m, while the working distance is 30 cm Fiber-coupled mirror pair, quarter-wave plate, 43 cm polarization-maintaining fiber (PM980-XP, Nufern) The polarization-maintaining Lyot filter is formed together with the fiber optic mirrors working in two axes.
Figure 1 Middle infrared double optical comb system schematic diagram
As shown in Figure 1, the dual-wavelength lasers and cascaded fiber amplifiers of the mid-infrared dual-comb system are connected by direct fusion coupling of optical fibers and optical fiber devices. The optical fiber structure has good flexibility and integration. The fiber-coupled mirror pair in the resonator can be glued with angle deviation, and the spatial distance can be shortened to <1 cm. The optical difference frequency module adopts a spatial structure. For the consideration of system integration, the focal length of the lens used is only 11 mm. Ytterbium-doped dual-wavelength lasers, cascaded fiber amplifiers, and optical difference frequency modules can be integrated in a 3U chassis. Compared with other integrated dual optical comb systems, in 2014, the NIST laboratory used a vehicle-mounted dual optical comb system to realize remote sensing in a km-level open optical path, but its system relies on two sets of independent optical frequency comb sources and supporting phase-locked loops Implementation, does not reflect the advantages of simplicity and integration. The mid-infrared dual-comb system based on the single-cavity dual-wavelength laser in this paper provides a feasible solution for simplifying the system structure, reducing the volume and reducing the cost.
System Performance Characterization
The dual-wavelength laser in the experimental device can be switched between single-wavelength mode-locking and dual-wavelength mode-locking through pump power tuning. When the pump power was increased to ~150 mW, the mode-locked mode showed a single-wavelength multi-pulse state, and the pump power was gradually reduced to ~85 mW to achieve single-wavelength single-pulse mode-locking. Since the transmittances of 1034 nm and 1039 nm are similar, the central wavelength of the mode-locked pulse is random, and the angle of the quarter-wave plate can be rotated to realize switching between the two wavelengths. The multi-peak transmission characteristics of the Lyot filter allow multiple wavelengths to simultaneously oscillate in the resonator, providing the necessary conditions for dual-wavelength or even multi-wavelength mode-locking. When the pump power is increased to ~180 mW, the energy in the cavity is sufficient to support two wavelength pulses to start oscillation, the laser works in a dual-wavelength multi-pulse state, and the pump power is reduced to ~90 mW, and a stable dual-wavelength dual-pulse lock can be obtained mode, whose central wavelengths are 1034 nm and 1039 nm, respectively. The spectral data are shown in Fig. 2(a), and the 3 dB spectral width of each wavelength is ~1.6 nm. Since the spontaneous emission in other passbands of the Lyot filter cannot be completely suppressed, there are still weak spectral components in the adjacent passbands of 1034 nm and 1039 nm. Figure 2(b) is a dual-wavelength pulsed RF domain signal recorded by an RF analyzer (KEYSIGHT, N9320B). Due to the influence of intracavity dispersion, the group velocities of the dual-wavelength pulses are different, so there will be slight differences in their repetition frequencies. The main peak signals at the RF frequencies of 25.5205 MHz and 25.5217 MHz respectively verify the asynchronous characteristics of the dual-wavelength pulses.
Figure 2. Output dual-wavelength pulse feature
In order to verify the relative stability between the pulses of the underlying dual-comb light source, the repetition frequency of the dual-wavelength pulses and their difference were measured. The dual-wavelength pulse output by the resonator is split through the combination of grating and mirror, and the change of repetition frequency is measured by two high-speed photodetectors (FPD 510-FS NIR). Figure 3 records the repetition frequency offset and the corresponding repetition frequency difference of the dual-wavelength pulses within 10 h. As shown in Figure 3, the black and blue lines are the pulse repetition frequency drift curves at 1040 nm and 1034 nm, respectively. During the 10 h measurement time, the pulse repetition frequency offset caused by the environmental disturbance is about 60 Hz, and the repetition frequency of the two wavelength pulses decreases synchronously. The red line is the corresponding repetition frequency difference, and its maximum offset is about 3.5 Hz. At a sampling frequency of 1 Hz, the standard deviation of the two-wavelength repetition frequency difference is 0.45 Hz. Without an external phase-locked loop, the drift of the repetition frequency difference is 1 to 2 orders of magnitude smaller than the respective repetition frequency drift of the dual-wavelength pulses, indicating that the single-cavity dual-wavelength laser used in the experiment can well suppress the common-mode noise Maintain good pulse relative stability over a long period of time.
FIG. 3 Long-term stability of dual-wavelength pulse repetition frequency
The high coherence between pulses is a necessary condition for constituting a dual-comb measurement system. In order to verify the coherence performance between the output pulses of the single-cavity dual-wavelength laser, we used a balanced detector, a high-speed oscilloscope, and a spectrum analyzer to build a coherence test link, and measured and recorded the time-domain beat signal and frequency-domain beat signal of the dual-wavelength pulse. frequency envelope.
A balanced photodetector (Thorlabs, PDB410C) was used to receive the pulsed light output from Out. The oscilloscope has both time domain and frequency domain analysis functions. When the sampling time of the oscilloscope is set to 5 ms and the sampling frequency is set to 1 GHz, the oscilloscope can detect the pulse interference signal sequence in the time domain, as shown in Fig. corresponding to the repetition frequency difference. The illustration in Figure 4(a) is a single time-domain interference envelope unfolded around time 0. The blue is a single sampling value, and the red is the average value of 200 times. After the single sampling value is subjected to Fourier transform (FFT), The frequency domain envelope signal of pulse interference can be obtained. Figure 4(b) compares the single-acquisition frequency-domain interference signal envelope (black) with the RF signal after FFT transformation (red), and the traces of the two tend to be consistent. Figure 4(c) is the beat-frequency comb signal obtained after unfolding in the blue frame near ~11.6 MHz in Figure 4(b). The comb tooth interval is equal to the repetition frequency difference of 1.18 kHz, and the signal-to-noise ratio of the comb signal exceeds 25 dB. Limited by the resolution of the oscilloscope, the FWHM of each comb tooth is read at 12 Hz. The beat frequency comb signal with high signal-to-noise ratio proves the high coherence between the dual-wavelength pulse trains, while the narrow linewidth characteristics of the comb teeth confirm the high stability and low-noise performance of the single-cavity dual-wavelength laser.
Figure 4. Pulse beat frequency signal of dual wavelength
Using a high-coherence, high-stability, and low-noise single-cavity dual-wavelength laser as a light source, its power is amplified and then frequency-differenced with 1550 nm continuous light nonlinearly to develop an integrated mid-infrared dual-comb system. The average power of the seed pulse output by the single-cavity dual-wavelength laser is only 1 mW. In order to avoid the introduction of a large amount of spontaneous radiation noise in the amplification due to the too weak seed pulse, two stages of forward pre-amplification and one stage of main amplification were used in the experiment. In a connected structure, the power of the seed light pulses is increased step by step to maintain high coherence between pulses. Both the pre-amplifier and the main amplifier adopt full polarization-maintaining fiber structure, which enhances the environmental immunity of the system. Different from the single-mode ytterbium-doped gain fiber of 1 m each in the forward pre-amplification, in the main amplifier, the double-clad structure of the ytterbium-doped fiber is used as the gain medium to increase the pulse power. The double cladding structure is conducive to carrying higher power pulses. When the pump light power of the main amplifier increases to 3.5 W, the cascaded amplifier can increase the average power of the dual wavelength pulses to 1.1 W. During the amplification process, due to the comprehensive influence of various nonlinear effects, such as self-phase modulation, cross-phase modulation, etc., the output spectrum of the seed pulse is continuously broadened. At the same time, because the optical fiber exhibits positive dispersion characteristics for the 1 μm laser, the pulse width continues to increase. . Figure 5(a) is the recorded output spectrum of the main amplifier after power attenuation. Comparing with Figure 2(a), it can be seen that both the spectra of 1034 nm and 1039 nm are broadened, the spectral overlap of the two wavelengths increases, and the spectrum is relatively continuous. Under the action of nonlinear effects, the 3 dB bandwidth of the amplified spectrum over 20nm. There are many technical means to generate mid-infrared laser, such as direct pumping by gain medium or indirect generation by nonlinear frequency conversion technology. In this experiment, a 1030 nm laser pulse and a 1550 nm continuous laser are used to generate a mid-infrared laser in a non-linear frequency difference method in a PPLN. The advantage of this method is that the generated mid-infrared laser spectrum is wide and the conversion efficiency is high. And the spectral coverage of mid-infrared can be extended by wavelength tuning technology. The amplified laser is coupled to the nonlinear difference frequency module through the collimator lens, and the laser pulse and the continuous laser near 1550 nm are combined at the dichroic mirror (DM) and enter the PPLN crystal together. In order to improve the conversion efficiency of the mid-infrared laser, the temperature of the PPLN crystal is precisely controlled at 125 °C. The CW laser has a linewidth <10 kHz and its center wavelength is tunable. Figure 5(b) is the recorded mid-infrared laser spectrum when the central wavelength of the continuous laser is adjusted to 1549.315 nm. The average power of the generated mid-infrared laser is greater than 3.5 mW, and the spectrum is in the characteristic absorption band of various gases and the range exceeds 50 nm, which can provide a good light source for simultaneous spectral detection of multiple absorption peaks of various gases.
FIG. 5 Dual-wavelength pulse spectra at different positions
According to the Lambert-Beer law, the light absorption intensity of gas molecules is proportional to the gas concentration and the interaction length. Therefore, after the PPLN crystal, a multi-pass long path path gas cell is placed. The gas pool is composed of two high-reflection mirrors. After precise optical path adjustment, the mid-infrared laser is reflected back and forth in the gas pool for 50 times before exiting, which can increase the contact path between the gas and the laser to 10 m, which is more conducive to extremely low concentrations. gas detection. It is worth noting that for dual-wavelength pulses, the coherence is strong in the overlapping part of the spectrum. For the mid-infrared laser generated by nonlinear difference frequency, the coherence near the center wavelength is strong, while the coherence in the sideband is weak. The sideband part of the mid-infrared laser will introduce noise, increase the base of beat frequency envelope detection, and reduce the signal-to-noise ratio of the comb signal. Therefore, at the exit end of the gas cell, a diffraction grating is used to spatially distribute the mid-infrared laser according to the wavelength, and combined with an aperture to take out the central part and filter out the sideband part, the spectral filtering can be realized.
The filtered frequency-domain beat envelope signal is measured with a mercury cadmium telluride detector, as shown in Figure 6. Expanding the envelope around 2.6 MHz, the mid-infrared dual optical comb beat frequency comb signal in the inset of Figure 6 can be obtained. Compared with Figure 4(c), it can be seen that the frequency interval, signal-to-noise ratio and line width of the comb signal do not deteriorate significantly, indicating that the mid-infrared dual optical comb maintains the coherence of the underlying dual optical comb. Therefore, we use a 1030 nm single-cavity dual-wavelength laser as a light source, and convert its output band frequency to mid-infrared through nonlinear difference frequency technology, and develop an integrated mid-infrared dual-comb system, and the mid-infrared dual-comb system It can maintain the high coherence of the underlying dual optical combs, providing a good light source for highly sensitive trace gas detection in complex environments.
FIG. 6 Spectrum of middle infrared beat comb teeth
in conclusion
The mid-infrared dual optical comb technology combines the high resolution and high precision of spectral technology, the rapid measurement, high sensitivity and mid-infrared band characteristic fingerprint of dual optical comb technology, providing an innovative means for the calibration of extremely low concentration gas molecules. At present, the technical means of constructing a mid-infrared dual optical comb system have problems such as complex structure, high cost, small number of comb teeth, and poor practicability. Aiming at the above problems, this paper builds a fully polarization-maintaining single-cavity dual-wavelength laser based on NALM mode-locking, and replaces two mode-locked lasers with its output asynchronous dual-wavelength pulses as a simple dual-comb light source. By building a multi-passband Lyot filter in the resonator, the laser can output dual-wavelength pulses of 1034 and 1039 nm, and the repetition frequency difference is 1.18 kHz. Using this as a seed pulse, the average power of the laser output pulse is increased step by step from 1 mW to 1.1 W by using cascaded amplification technology while introducing low noise. After the amplified laser beam is combined with the 1549.315 nm continuous light beam, the nonlinear frequency difference is made in the PPLN crystal to expand the output band of the laser to the mid-infrared. Through precise crystal temperature control and quasi-phase matching modulation, the power of mid-infrared laser can reach 3.5 mW. In order to verify the coherence of the dual optical comb system, the beat frequency comb signals of the dual optical combs in the 1 μm and 3 μm bands were measured respectively. In comparison, the frequency interval, line width and signal-to-noise ratio of the mid-infrared dual optical comb beat frequency comb signal generated by the nonlinear difference frequency have no obvious degradation, and the characteristics of the underlying dual optical comb can be maintained. In this paper, based on single-cavity dual-wavelength lasers, combined with nonlinear difference-frequency technology, an integrated mid-infrared dual-comb system is developed, which is small in size, low in cost, and simple in structure.
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