2-μm fiber lasers have become a research topic with an increased emphasis due to a variety of applications including eye-safe LIDAR, spectroscopy, remote sensing, and mid-infrared (mid-IR) frequency generation. We review our latest development on various 2-μm fiber laser sources, including single-frequency fiber lasers, Q-switched fiber lasers, mode-locked fiber lasers, and mid-IR supercontinuum fiber sources. All these fiber laser sources are based on thulium and holmium ions using our proprietary glass fiber technology. Potential applications of these fiber laser sources for sensing are also briefly discussed.
We demonstrate a single-frequency gain-switched Ho-doped fiber laser based on heavily doped silicate glass fiber
fabricated in house. A Q-switched Tm-doped fiber laser at 1.95μm was used to gain-switch the Ho-doped fiber laser via in-band pumping. Output power of the single-frequency gain-switched pulses has been amplified in a cladding-pumped Tm-Ho-codoped fiber amplifier with 1.2m active fiber pumped at 803nm. Two different nonlinear effects, i.e.,
modulation instability and stimulated Brillouin scattering, could be seen in the 10μm-core fiber amplifier when the
peak power exceeds 3kW. The single-frequency gain-switched fiber laser was operated at 2.05μm, a popular laser
wavelength for Doppler lidar application. This is the first demonstration of this kind of fiber laser.
Mode-locked fiber lasers based on Tm-doped and Tm-Ho-codoped silicate fibers have been studied. The first modelocked
fiber laser oscillator beyond 2 μm was demonstrated. The first mode-locked 2μm fiber laser with ~GHz repetition
rate was demonstrated. The broadest spectrum (50nm FWHM) was obtained from dispersion managed 2μm mode-locked
fiber laser. Various operation regimes of mode-locked thulium fiber lasers are presented, as well as all-fiber 2μm pulse
amplifiers.
30ns, 3μJ Q-switched pulse laser with the repetition rate of 10 kHz at 1950 nm was amplified with a 50cm long piece of
thulium(Tm)-doped silicate fiber, and 300μJ pulses with the pulse width of 20 ns were obtained. The heavily Tm-doped
silicate fiber has the cladding-pump absorption of 22dB/m at 793nm and the fiber maintains solid single-mode operation
at 1950nm. To further increase output pulse energy, the Q-switched seed was pre-amplified to 20 μJ with core-pumping
at 1567 nm. The power amplifier features of a 55cm long piece of Tm-doped silicate fiber with 21μm core. ~600μJ
pulses with peak power up to 28 kW have been achieved. The ASE was suppressed 40dB below the laser pulses due to
the short highly Tm-doped silicate fiber. Mode-locked pulses with the pulse energy of 0.3 nJ and the spectral width of 14
nm were amplified with a 30cm long Tm-fiber with 10 μm core, pulses with 16kW peak power as well as 20nJ pulses
were achieved. With another Tm-doped silicate fiber (22μm core), 36nJ pulses with 1.1ps pulse width were obtained in
all-fiber configuration. The high peak power pulses at 2 μm reported here should be useful for mid-IR generation.
A high-spectral-flatness mid-infrared supercontinuum fiber source is presented. It generates highly stable single-mode
broadband beam with a FWHM bandwidth of ~600nm and a 20dB-down spectra spanning from 1.8μm up to ~ 2.7μm.
The supercontinuum pulses are emitted from a single-mode fiber with ~15kW peak power and >20GW/cm2 laser peak
intensity. This compact fiber source, in combination with a commercial mid-infrared optical spectrum analyzer
(Yokogawa), offers a powerful tool for mid-infrared component characterization with high spectral resolution.
Mid-infrared fiber laser sources have attracted a lot of interest in space and defense applications. We review our latest
developments of various fiber laser sources operating near 2μm based on Tm3+ and Ho3+ ions, which include singlefrequency
CW laser sources, Q-switched laser sources, mode-locked laser sources. Potential applications of these fiber
laser sources are also briefly discussed.
A novel fiber optic sensor has been developed to be used in superconducting magnets for fusion reactors and other large
cable-in-conduit superconductor (CICC) magnet applications. These large superconducting magnets need a diagnostic
that can measure the temperature and strain throughout the magnet in real-time, which was not possible until now.
Simultaneous temperature and strain measurements at cryogenic temperatures have been demonstrated, using
spontaneous Brillouin scattering in an optical fiber. Using an extremely narrow (100 Hz) linewidth Brillouin laser with
very low noise as a frequency shifted local oscillator, the frequency shift of spontaneous Brillouin scattered light was
measured using heterodyne detection. A pulsed laser was used to probe the fiber using Optical Time Domain
Reflectometry (OTDR) to determine spatial resolution. The spontaneous Brillouin frequency shift and linewidth as a
function of temperature agree with previous literature on stimulated Brillouin scattering data from room temperature
down to 4 K. For the first time, the spontaneous Brillouin frequency shift, linewidth, and intensity as a function of strain
have been measured down to 4 K. Analyzing the frequency spectrum of the scattered light after an FFT gives the
Brillouin frequency shift, linewidth, and intensity of the scattered light. 65,000 pulses, with 53 ns pulse widths, were
averaged in under one second, providing a 5 meter spatial resolution along a fiber that was about 100 m long. Measuring
these three parameters allow the simultaneous determination of temperature and strain in real-time throughout a fiber
with a spatial resolution on the order of several meters.
Frequency-modulated continuous wave (FMCW) measurements of Rayleigh backscattering and Fresnel reflection from 95-km fiber have been demonstrated by using a compact piezo-electrically-tuned sub-kilohertz-linewidth fiber laser. This technique enables ultra-long fiber fault location in optical transmission systems. The high sensitivity (over-80dBm) of the long-range backscatter measurements without using optical amplifier are benefit from the extremely long coherence length of the sub-kilohertz linewidth fiber laser, which has been estimated to be 210 km in air.
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