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.
We demonstrate here for the first time generation of tunable terahertz (THz) radiation based on photomixing continuous wave (CW) radiation of the DFB laser with fixed wavelength and self-sweeping Er-doped fiber one operating near 1560 nm. The latter sequentially generates longitudinal modes with duration of several milliseconds and regular hopping between them. In this case signal accumulation of THz radiation within generation of each mode can be performed to increase signal-to-noise ratio (SNR). We demonstrated the possibility of designing a spectrometer with spectral resolution down to 50 MHz in the tuning range of ~5 GHz near the central frequency of 100 GHz. The proposed approach can be useful in the development of spectroscopic systems, millimeter-wave lidars, and telecom devices.
The effect of broad-range (10 nm) self-sweeping near 2100 nm in Ho-doped fiber laser has been demonstrated experimentally. The narrowband linearly-polarized tunable radiation with average output power of more than 200 mW is obtained. The use of highly-doped holmium fiber allowed to obtain the widest sweeping range. The polarization maintaining elements and temperature stabilization of the active fiber were applied to stabilize the laser operation. The developed source can be used for spectroscopy of Nitrous oxide, having absorption lines in that spectral region.
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.
In this work, a self-sweeping Yb-doped fiber laser generating quasi-CW radiation is demonstrated. The laser is based on a ring configuration allowing to form running and standing waves in separate sections of the cavity. The laser generates a sequence of single-frequency (linewidth of ~0.7 MHz) ultra long (~1 ms) rectangular pulses. The laser frequency changes from pulse to pulse by one inter-mode spacing (~10 MHz). The laser signal intensity is quasi-CW signal accompanied by regular bursts generating during frequency change, and the intensity never drops to zero. The features can be useful for long term averaging when measuring weak signals.
We present results on the first demonstration of self-sweeping effect in a ring Er-doped fiber laser. Reverse self-sweeping (with decreasing wavelength) in a range of ~0.1 nm was obtained. Typical sweeping rate was 4 pm/s, which is sufficiently slower as compared with earlier reported fiber lasers. Dual longitudinal mode self-sweeping (similar to reported earlier in linear cavity Er-doped fiber laser) is obtained. The laser generates sequentially long overlapping pulses. Frequency of each subsequent pulse increases by one inter-mode spacing. However, the typical duration of dual-mode operation is ~10 ms, which is one order longer than in previous works. We demonstrated that increase in pump power causes deceleration of sweeping and elongation of dual-mode pulses up to 500 ms. The developed source can be used for fiber sensor interrogation.
The paper presents the first experimental results on operation of an optical frequency domain reflectometer (OFDR) based on a self-sweeping fiber laser. Frequency tuning in this laser is performed without actively tunable elements. Its intensity dynamics consists of regular microsecond pulses. The self-sweeping laser has high linearity of frequency tuning, which allows us to measure the reflectograms without additional spectral correction. The OFDR demonstrates capability of operation with spatial sampling of ~ 200 μm and sensitivity down to ~ -80 dB with line length of up to 9 meters.
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.
The laser wavelength in self-sweeping laser is linearly changing in time from start to stop wavelength without use of optical elements and electrical drivers for frequency tuning. Absolute difference between the start and stop wavelength values (sweeping span) characterizing the sweeping process is one of the key characteristics of any tunable source. Owing to broad sweeping span (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. Self-induced nature of the sweeping process leads to fluctuations of the sweeping span borders. We demonstrate in this talk implementation of fiber Bragg gratings (FBG) for control and stabilization of start and stop wavelengths in the self-sweeping laser. We showed that the short-wavelength FBG helps to initialize the sweeping process and long-wavelength FBG blocks the laser line sweeping in long-wavelength region. The last effect is associated with mismatch of longitudinal mode structures of the laser and FBG-based selector. As a result, fluctuations of the sweeping span borders decreased by one or two orders of magnitude down to several picometers. In addition, we studied influence of the parameters for FBG-based selectors such as reflections and mode structure on quality of sweeping range stabilization. The results allow to improve the characteristics of self-sweeping fiber laser which can be used for different sensing applications such as atmospheric remote sensing and interrogation of the sensors based on fiber Bragg gratings.
We report on the first demonstration of a linearly polarized cascaded Raman fiber laser based on a simple half-open cavity with a broadband composite reflector and random distributed feedback in a polarization maintaining phosphosilicate fiber operating beyond zero dispersion wavelength (~1400 nm). With increasing pump power from a Yb-doped fiber laser at 1080 nm, the random laser generates subsequently 8 W at 1262 nm and 9 W at 1515 nm with polarization extinction ratio of 27 dB. The generation linewidths amount to about 1 nm and 3 nm, respectively, being almost independent of power, in correspondence with the theory of a cascaded random lasing.
We report on experimental studies of random lasing realized in optical fibers with the use of Brillouin amplification and Rayleigh backscattering employed as a distributed feedback instead of a cavity mirror. In our experiment 25-km-long high quality standard telecom single-mode fiber was employed for Rayleigh reflection uniformly distributed over all fiber length. We have observed a clear competition between a classical Brillouin scattering and Brillouin lasing. Presence of extended fluctuation-free fragments in the recorded oscilloscope traces highlights Stokes power statistics typical for laser radiation rather than for Brillouin process. The results of the experiments are in a perfect agreement with the model of Brillouin - Rayleigh cooperative process in long fibers.
The effect of broad-range self-sweeping in Yb-doped fiber laser has been demonstrated experimentally for the first time.
The self-sweeping effect is observed in an all-fiber laser configuration with a double-clad Yb-doped fiber and a cavity
formed by a broad-band fiber loop mirror and Fresnel reflection from one cleaved end. The sweep range is limited by the
width of the broad-band reflector and reaches up to 16nm. It is found that the self-sweeping effect is related to selfpulsations.
So the sweep rate is increased with an increase in pump power and is decreased with increasing cavity length.
RF and optical spectra (linewidth is measured to be not more than 100 MHz) show that during the evolution of a single
pulse a small number of longitudinal modes take a part in lasing. Based on these results we propose a model describing
dynamics of the laser frequency. The model is based on the spatial hole burning effect and the gain saturation in Yb laser
transition, and takes into account self-pulsations of the laser. Theoretical estimation for pulse to pulse change of lasing
frequency is in good agreement with experimental data.
We present the first experimental demonstration of strong coupling between the core modes in multi-core fibers (MCF)
regardless of large spacing (~28μm) between them. The effect is very sensitive to bending of the fiber and is observed in
the MCF laser as well as in the probe beam schemes. We explain the observed effect by a mechanism of the mode coupling based on their indirect interaction inside the fiber via intermediate cladding mode, analogues to the Bragg mode. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.
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