We report on a Tm-doped self-sweeping fiber laser operating at 2 μm spectral range with a stabilized pulse repetition rate. Lasers with self-induced wavelength sweeping are bright alternative to narrow-band tunable laser sources. One disadvantage of self-sweeping lasers is fluctuations in the pulse repetition rate. In this paper for the purpose of optimization of intensity dynamics, the technique for pulse train shaping with acousto-optical modulator synchronized with laser pulses is proposed and demonstrated. Synchronization of the AOM frequency with the laser pulses allowed us to improve pulse train stability and reduce relative stability error down to 1%. The developed self-sweeping laser brings up new possibilities for remote gas analysis and LIDAR applications.
Self-sweeping lasers is the simplest type of tunable laser source and have many advantages such as broad tuning range, narrow linewidth, and linear polarization state. However, a main drawback of the laser is the absence of wavelength control. In the present work, automatic control of the tuning parameters (sweeping rate and wavelength) is demonstrated using a Tm-doped fiber self-sweeping laser as an example. The method consists of measuring the laser wavelength, fast data processing to obtain the sweeping direction and the sweeping rate, and subsequent automatic adjustment of the pump power with a feedback loop and high-resolution equipment is not required. The automatic control will provide the possibility of performing more delicate measurements in the field of gas absorption spectroscopy.
Self-sweeping of laser frequency is relatively new effect in fiber lasers. The effect consists in periodic dynamics of the laser frequency without use of tuning elements and electrical drivers for frequency tuning. Owing to broad sweeping range (up to 23 nm) and simplicity, self-sweeping fiber lasers are attractive sources for applications demanding tunable radiation. Currently the self-sweeping effect in fiber lasers was observed in different spectral regions covering range from 1 to 2.1 μm. However, it is difficult to control spectral dynamics due to self-induced nature of the sweeping effect. In the paper, we demonstrated linearly-polarized Tm-doped fiber laser with lasing near 1.9 μm with manually controlled the spectral dynamics with pump power adjustment. The laser operates in three self-sweeping regimes depending on pump power: 1) with normal scanning direction at high rate (~5 nm/sec) and, 2) with reverse one at low sweeping rate (~0.1 nm/sec) and 3) wavelength stopping. In the case of wavelength stopping, the wavelength can be stopped at arbitrary value in the range from 1912 to 1923 nm depending on prehistory of spectral dynamics of the laser. The wavelength stability in case of wavelength stopping is better than 50 pm within 5 minutes. In the case of linear scanning of laser line, sweeping range exceeds 15 nm.
The self-sweeping laser is the simplest sort of tunable laser without use of optical elements and electrical drivers for frequency tuning. Owing to broad sweeping range (more than 10 nm) and simplicity, self-sweeping fiber lasers are attractive sources for applications demanding tunable radiation such as sensors interrogation, spectral analysis, optical frequency domain reflectometry and so on. Currently the self-sweeping effect in fiber lasers was observed in different spectral regions covering range from 1 to 2.1 μm. In the paper, linearly-polarized Tm-doped fiber laser with sweeping range of more than 20 nm in the region of 1.92 μm has been experimentally demonstrated. The laser is based on singlemode polarization-maintaining Tm-doped fiber and pumped by home-made Er-doped fiber laser with wavelength of 1540 nm. The cavity is formed by highly-reflective fiber loop mirror and right-angle cleaved fiber end. The main feature of the laser is generation of periodic μs-scale pulses where each of them contains practically only single longitudinal mode radiation with linewidth of ~1 MHz. The laser frequency is changed from pulse to pulse by one intermode beating frequency of the laser ~8 MHz. The sweeping rate is increased with pump power up to 10 nm/sec. The average output power exceeds 400 mW. The developed laser source can be used for atmospheric remote sensing as well as for interrogation of the sensors based on fiber Bragg gratings and is applied to measure spectrum of water absorption lines in air.
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