Objective   Temperature and wind, as important environmental parameters, characterize the state of the atmosphere. In the region of upper mesosphere and lower thermosphere (UMLT, 75-115 km), due to a lack of effective tools, there is a relative lack of observation data. The resonance fluorescence lidar uses metal atoms in the UMLT region as a neutral tracer, and it is possible to measure temperature and wind by stimulating resonance fluorescence signals of the tracer. Among many in-situ and remote sensing measurement methods, the resonance fluorescence lidar, with its high spatial and temporal resolution, high accuracy and continuous observation, has become a powerful tool. Sodium resonance fluorescence lidar is widely used in the world, while iron resonance fluorescence Doppler lidar (Fe lidar) has the advantage of whole day measurement and is also an effective means to measure temperature and wind profile. The narrow-band, frequency-stabilized laser operating at 372 nm wavelength is one of core technology in the development of Fe lidar, especially for wind measurement. To yield pulsed laser with outstanding characteristic of frequency stabilization, a theoretical and experimental study of the frequency stability of laser sources are presented.

  Methods  The technical solution for the generation of pulsed laser is to use Nd: YAG laser to generate pulsed laser at 1 116 nm wavelength, and then convert to 372 nm wavelength through subsequent second and third harmonic generation. Since the performance of oscillator determines the characteristics of the entire laser system, a theoretical and experimental study of the frequency stability mainly focuses on the oscillator. Frequency stability of pulsed laser at 1 116 nm wavelength from the oscillator is simulated to be less than 1 MHz (RMS) by using the Monte Carlo method (Fig.1). A detailed description for the modified Ramp-Fire method is conducted (Fig.2), and this technology is used in the optical path of the oscillator (Fig.3(a)). In the beat frequency experiments, because of the high-frequency stability of seeder laser, it can be used as a frequency reference, and the frequency difference can indirectly reflect the frequency stability of the pulsed laser by beating with the continuous-wave laser output from seeder laser. To record the beat frequency signal accurately, an indium gallium arsenic (InGaAs) detector with 5 GHz bandwidth is used for photoelectric conversion, and the interference waveform is acquired by a high-speed oscilloscope with a sampling rate of 40 GS/s. A fast Fourier transform algorithm is applied to the digitized beat frequency signal to obtain the spectrum information.

  Results and Discussions  Frequency stability of 543.24 kHz root mean square over 10 min is obtained by using beat frequency experiments (Fig.5). It is verified that injection-seeded technique combined with the modified Ramp-Fire method can meet the requirements of frequency stability. According to the Monte Carlo method (Fig.1), the systematic error of temperature and wind measurement are estimated to be 0.51 K and 0.61 m/s.

  Conclusions   The long-term frequency stability is a prerequisite for the high-precision measurement of temperature and wind. The frequency stability of laser source of Fe lidar is studied in this paper. By Monte Carlo method, the simulation analysis shows that the frequency stability for temperature and wind measurement should be less than 3 MHz at 372 nm wavelength, and thus, it should be less than 1 MHz at 1 116 nm wavelength. In the oscillator, injection-seeded technique combined with modified Ramp-Fire method is applied to maintain resonance with the seeder laser. The frequency stability of pulsed laser output from the oscillator over 10 min was measured to be 543.24 kHz by the beat frequency experiment. This work promotes the practical application of Fe lidar, and it also provides ideas for the development of other lidar systems with frequency stability.