Random distributed feedback fiber lasers (RDFFL) are now attracting more and more attentions for their unique cavity-free, mode-free and structural simplicity features and broadband application potentials in many fields, such as long distance sensing, speck free imaging, nonlinear frequency conversion as well as new pump source. In this talk, we will review the recent research progresses on high power RDFFLs. We have achieved (1) More than 400 W RDFFL with nearly Gaussian beam profile based on crucial employment of fiber mismatching architecture. (2) High power RDFFL with specialized optical property that include: high power narrow-band RDFFL, hundred-watt level linearly-polarized RDFFL, hundred-watt level high-order RDFFL. (3) Power enhancements of RDFFL to record kilowatt level are demonstrated with the aid of fiber master oscillator power amplifier (MOPA) with different pump schemes.
Thermal effect of the gain fiber is one of the main factors which limit the power improvement of high power fiber lasers. Using traditional temperature measurement methods, we can only obtain the surface temperature of fiber while the core temperature cannot be detected. In this paper, the temperature of the gain fiber core in all-fiber laser is measured by optical frequency domain reflectometry(OFDR). The fiber laser is operating at 1080nm and the wavelength of detecting light emitted by OFDR is approximately 1550nm. The different wavelength of the two lasers has ensured the proper operating of fiber laser without effecting the operating lasers at 1080nm. The maximum output power of the fiber laser is 58.95W and results show that OFDR is a practical way to monitor the temperature of fiber laser system.
Fiber lasers are now attracting more and more research interest due to their advantages in efficiency, beam quality and flexible operation. Up to now, most of the high power fiber lasers have random distributed polarization state. Linearlypolarized (LP) fiber lasers, which could find wide application potential in coherent detection, coherent/spectral beam combining, nonlinear frequency conversion, have been a research focus in recent years. In this paper, we will present a general review on the achievements of various kinds of high power linear-polarized fiber laser and its application. The recent progress in our group, including power scaling by using power amplifier with different mechanism, high power linearly polarized fiber laser with diversified properties, and various applications of high power linear-polarized fiber laser, are summarized. We have achieved 100 Watt level random distributed feedback fiber laser, kilowatt level continuous-wave (CW) all-fiber polarization-maintained fiber amplifier, 600 watt level average power picosecond polarization-maintained fiber amplifier and 300 watt level average power femtosecond polarization-maintained fiber amplifier. In addition, high power linearly polarized fiber lasers have been successfully applied in 5 kilowatt level coherent beam combining, structured light field and ultrasonic generation.
Ytterbium-doped fiber laser (YDFL) and Thulium doped fiber laser (TDFL) have been two kinds of the most widely
studied fiber laser in recent years. Although both silica-based Ytterbium-doped fiber and Thulium doped fiber have wide
emission spectrum band (more than 200 nm and 400 nm, respectively), the operation spectrum region of previously
demonstrated high power YDFL and TDFL fall into 1060-1100 nm and 1900-2050nm. Power scaling of YDFL and
TDFL operates at short-wavelength or long-wavelength band, especially for extreme wavelength operation, although is
highly required in a large variety of application fields, is quite challenging due to small net gain and strong amplified
spontaneous emission (ASE). In this paper, we will present study on extreme wavelength operation of high power YDFL
and TDFL in our group. Comprehensive mathematical models are built to investigate the feasibility of high power
operation and propose effective technical methods to achieve high power operation. We have achieved (1) Diodepumped
1150nm long wavelength YDFL with 120-watt level output power (2) Diode-pumped 1178nm long wavelength
YDFL operates at high temperature with 30-watt level output power (3) Random laser pumped 2153nm long wavelength
TDFL with 20-watt level output power (4) Diode-pumped 1018nm short wavelength YDFL with a record 2 kilowatt
output power is achieved by using home-made fiber combiner.
A high power single-frequency fiber amplifier with linear polarization is demonstrated based on the master oscillator power amplifier configuration, consisting of a single-frequency seed laser at 1064.1 nm and three-stage amplifiers. To suppress stimulated Brillouin scattering, a short piece of polarization-maintaining active fiber with large core diameter of 25 μm and high pump absorption coefficient is adopted in the main amplifier. Besides, step-distributed longitudinal strain is intentionally imposed on the active fiber to broaden the effective SBS gain spectrum and correspondingly improve the SBS threshold. As a result, a pump-limited single-frequency output of 414 W is obtained with no sign of SBS and mode instability, experimentally showing that the SBS threshold is improved by at least two times through introducing extra strain.. The slope efficiency of the main amplifier is about 80 %. The polarization degree is better than 98 % at all the power levels. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on all-fiber structure.