Objective Modern spectroscopic analysis technology has developed into a discipline that studies the absorption, emission or scattering spectra of substances, and has played a key role in many fields. In the field of spectroscopy, spectral scanning is performed by adjusting slits in spectroscopic instruments, but there are problems such as wavelength drift and frequency instability caused by mechanical movement. The stability of mechanical movement and the performance of spectroscopic elements gradually limit the development of spectrometers. In the field of gas detection, with the development of laser technology, gas detection lidar based on optical frequency comb locking can achieve high-resolution remote sensing of various gas spectra. The optical frequency comb reference locking scheme has the advantages of high stability and wide tuning range, but it also faces the problems of complex system and long tuning process. In summary, whether it is adjusting the slit or the laser cavity, the current spectrum analysis technology operates and detects the spectrum in the optical frequency.

Methods This paper proposes a spectral analysis technology based on time dispersion gating to achieve fast and high-resolution spectral analysis without locking. First, a large amount of dispersion is introduced through time stretching technology to stretch femtosecond laser pulses to nanosecond pulses, and achieve one-to-one mapping between spectrum and time. Secondly, picosecond pulses are used in the time to modulate the time stretched ns pulses to achieve selective passage of light frequency, similar to the role of slits in spatial dispersion spectrometers. Finally, spectrum scanning is achieved by adjusting the delay of the ps pulse. Since the ps pulse is generated by electronic devices and loaded into the modulator, the ps pulse width and delay are controlled by the electrical signal generator. Its one-to-one corresponding spectral resolution and scanning speed can break through the limits of traditional spectral analysis technology, without mechanical scanning making this spectral analysis technology inherently frequency stable.

Results and Discussions Through the time dispersion gating experiment, it can be obtained that the spectral signal and time signal of the gas-free absorption line composed of ps pulses under different delays (Fig.4). The time and frequency information of the time pulse vertex and the corresponding frequency spectrum vertex are obtained, the coordinates of (time, frequency) form and fitting is performed to obtain the time-frequency mapping equation (Fig.5). The experimentally obtained ps pulse time signals containing gas absorption lines at different delays can be mapped through the above time-frequency mapping equation to obtain the corresponding spectral data. The error of the spectral data obtained by time-frequency mapping inversion is compared with the actual spectrum (Fig.6). The standard deviation of the available error is only 0.006 5. Only one of the 51 points collected is not within the ideal range, and the maximum deviation ratio is only 1.54%. The probability of up to 98% proves that the spectral inversion results are consistent with the actual results. This shows that the HCN gas absorption spectrum retrieved from this experiment is in good agreement with the measured data, and further proves that high-speed, lock-free spectral scanning can be achieved in the time.

Conclusions This paper proposes a spectral analysis technology based on time- dispersion gating to achieve a lock-free, fast, high-resolution spectral analysis method in the time, and experimentally verifies its feasibility in the field of gas spectrum remote sensing. This paper designed a time- dispersion gating experiment and calculated the time-frequency mapping equation. Through the time-frequency mapping equation, it can be concluded that the spectral resolution under this method is 6.2 GHz and the spectral scanning interval is 1.5 GHz. Based on this mapping relationship, the time data passing through the gas cavity can be collected to invert the optical frequency data to achieve spectral analysis. The spectrum analysis experiment based on time- dispersion gating adopts an all-fiber system design scheme, which has a streamlined and stable structure. In order to expand the practical application of this method, the team will build a system with shorter modulation pulse width and faster scanning speed in the future, and amplify the gated pulses and emit them into the atmosphere for remote sensing of atmospheric gas spectra.