220 μJ monolithic single-frequency Q-switched fiber laser at 2 μm by using highly Tm-doped germanate fibers

Single-frequency pulsed fiber laser based on an electro-optic modulator with injection seeding technique

A single-frequency linearly polarization pulsed fiber laser based on an electro-optic modulator with injection seeding technique is demonstrated. The single-frequency performance of the fiber ring-cavity laser is guaranteed by the seed source, which is a distributed-feedback fiber laser based on the π-phase-shifted fiber Bragg grating. The electro-optic modulator triggers active Q-switching of the laser for pulse generation. The devices used in the fiber laser are all polarization-maintaining to ensure linear polarization laser output. Through parameter optimization, the laser generates a single-frequency linearly polarization pulsed laser with a central wavelength of 1064.22 nm, linewidth of 35 MHz, and polarization extinction ratio of better than 40 dB. This type of fiber laser can be applied in lidar, beam combining, nonlinear frequency conversion, and other fields.

414 W near-diffraction-limited all-fiberized single-frequency polarization-maintained fiber amplifier

A high-power 1064 nm single-frequency polarization-maintained fiber amplifier based on an all-fiber master oscillator power amplifier configuration is demonstrated. To mitigate the stimulated Brillouin scattering (SBS) and the mode instability (MI) effect, a polarization-maintained Yb-doped fiber with a high dopant concentration and a 25 μm core diameter is adopted in the main amplifier stage; in addition, step-distributed longitudinal strain is imposed on the active fiber to broaden its effective SBS gain spectrum and further increase the SBS threshold. As a result, a pump-limited 414 W single-frequency fiber laser is obtained without signs of SBS and MI. Experimental results show that the SBS threshold is increased by at least two times. The slope efficiency of the main amplifier is about 80%. The polarization degree is higher than 98% at all the power levels. The beam quality is measured with a M² of 1.34. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on an all-fiber structure.

Dual-wavelength fiber laser operating above 2 μm based on cascaded single-mode-multimode-single-mode fiber structures

A stable dual-wavelength Tm³⁺:Ho³⁺ co-doped fiber laser operating above 2 μm based on cascaded single-mode-multimode-single-mode (SMS) fiber structures is proposed and experimentally demonstrated. Based on the theoretical analysis of the transmission properties of the SMS fiber structure, two cascaded SMS fiber devices with different multimode fiber (MMF) lengths were used in our laser system, where one acted as a long-pass filter to suppress the competitive laser below 2 μm, and the other worked as a band-pass filter to select the specific operating wavelengths of the laser. Dual-wavelength operation of the fiber laser at 2002.8 and 2016.1 nm has been achieved in the experiment with a signal to a noise ratio up to 50 dB.

All-fiber-integrated linearly polarized fiber laser delivering 476 μJ, 50 kHz, nanosecond pulses for ultrasonic generation

We demonstrate a high-energy linearly polarized pulsed fiber laser for ultrasonic generation based on a master oscillator power amplification (MOPA) scheme, which delivers nanosecond pulses with duration of 4.8 ns and pulse energy of 476 μJ at the repetition rate of 50 kHz. The MOPA is seeded by a gain switch semiconductor laser diode at 1064 nm. In the pre-amplification stages, a double-pass amplification structure is designed and successfully applied to amplify the low-power seed laser for the consideration of suppressing amplified spontaneous emission, decreasing the number of amplification stages, and reducing the nonlinear effects. A highly ytterbium-doped fiber is utilized in the main amplifier to shorten the fiber length and reduce the fiber nonlinearity. The average power is finally boosted to 23.8 W with corresponding optical-to-optical efficiency of 66.9% and a polarization extinction rate of ∼10.5  dB. The corresponding peak power is calculated to be 87.1 kW. Finally, the established laser system is successfully used for ultrasonic generation based on a line excitation configuration and grating excitation configuration, and clear surface acoustic wave signals are detected. Many potential applications in laser ultrasonics can be foreseen.

Single-frequency fiber laser at 1950 nm based on thulium-doped silica fiber

A single-frequency fiber laser operating at 1950 nm has been demonstrated in an all-fiber distributed Bragg reflection laser cavity by using a 1.9 cm commercially available thulium-doped silica fiber, for the first time, to the best of our knowledge. The laser was pumped by a 793 nm single-mode diode laser and had a threshold pump power of 75 mW. The maximum output power of the single longitudinal mode laser was 18 mW and the slope efficiency with respect to the launched pump power was 11%. Moreover, the linewidth and relative intensity noise at different pump power have been measured and analyzed.

105 W ultra-narrowband nanosecond pulsed laser at 2 μm based on monolithic Tm-doped fiber MOPA

We present a high power ultra-narrowband pulsed fiber amplifier at 2 μm. A single frequency fiber laser was modulated by a phase modulator and an intensity modulator to serve as the ultra-narrowband pulsed seed laser with a bandwidth of 307 MHz. The pulsed seed laser was amplified by a monolithic Tm-doped fiber master oscillator power amplifier (MOPA). The average output power reaches 105 W with a slope efficiency of 0.41. The output pulse train has a repetition rate of 1 MHz and a pulse width of 66 ns. The output power is limited by the onset of stimulated Brillouin scattering. Higher output power can be achieved by further broadening the linewidth or narrowing the pulse width to several nanoseconds. To the best of our knowledge, this is the first demonstration on a monolithic ultra-narrowband nanosecond pulsed MOPA at 2 μm with an average power exceeding 100 W.

High-power operation of silica-based Raman fiber amplifier at 2147 nm

We demonstrated a 2147 nm silica-based Raman fiber amplifier with output power of 14.3 W directly pumped with a 1963 nm CW thulium-doped all-fiber MOPA. The 1963 nm thulium-doped all-fiber MOPA is seeded with a 2147 nm thulium-doped all-fiber laser at the same time. The Raman Stokes power shift from 1963 nm to 2147 nm is accomplished in a piece of 50 m silica-based highly nonlinear fiber (HNLF). The conversion efficiency was 38.5% from 1963 nm to 2147 nm in the HNLF. The output power achieved was only currently limited by available 1963 nm input power and the architecture has significant scaling potential. To the best of our knowledge, this is the highest power operation of a Raman fiber amplifier at >2 µm wavelength region.

Fiber lasers and their applications [Invited]

Fiber lasers have seen progressive developments in terms of spectral coverage and linewidth, output power, pulse energy, and ultrashort pulse width since the first demonstration of a glass fiber laser in 1964. Their applications have extended into a variety of fields accordingly. In this paper, the milestones of glass fiber laser development are briefly reviewed and recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.

1 kW peak power, 110 ns single-frequency thulium doped fiber amplifier at 2050 nm

We report a high power, single frequency, linearly polarized master oscillator power amplifier emitting 110 ns, 1 kW peak power pulses at 2050 nm. A 20% slope efficiency and a beam quality of M2=1.21 are achieved with three-stage double-clad Tm(3+)-doped fiber architecture. Various pump schemes are compared leading to the conclusion that 793 nm pump wavelength is the most efficient for amplification at 2050 nm. Based on numerical simulations, the Brillouin gain coefficient around 2 μm in Tm(3+) highly doped silica fiber is estimated to 1.2×10(-11) m/W. Output peak power is limited by stimulated Brillouin scattering to 535 W without mitigation and to 1 kW with application of a strain distribution along the doped fiber.

The effect of La₂O₃ in Tm³⁺-doped germanate-tellurite glasses for ~2 μm emission

A germanate-tellurite glass (GeO₂-TeO₂-K₂O-Nb₂O₅-La₂O₃) with thulium doping has been investigated for application as a laser material around 2.0 μm regions. Under the 808 nm laser diode pumped, intense 1.8 μm emission is obtained. Based on the absorption spectra, radiative properties are predicted using Judd-Ofelt theory. The maximum value of emission cross-section of Tm³⁺ around 1.8 μm can reach 1.46 × 10⁻²⁰ cm², which indicated that the germanate-tellurite glass may provide high gain as a good medium for efficient 1.8 μm laser system.

210 W single-frequency, single-polarization, thulium-doped all-fiber MOPA

A high-power single-frequency, single-polarization, thulium-doped all-fiber master-oscillator power-amplifier (MOPA) is demonstrated by using all-polarization-maintaining (all-PM) thulium-doped fiber and all-PM-fiber components. The MOPA yielded 210 W of single-frequency, linear-polarized laser output at central wavelength of 2000.9 nm with a polarization extinction ratio (PER) of >17 dB. No indication of stimulated Brillouin scattering (SBS) could be observed at the highest output power level, and the output power was only currently limited by available pump power. To the best of our knowledge, this is the first demonstration of average output power exceeding 200 W from a single-frequency, single-polarization, thulium-doped all-fiber laser at 2 µm wavelength region.

102 W monolithic single frequency Tm-doped fiber MOPA

We demonstrate a high power all-fiber single frequency Tm-doped fiber amplifier. The maximum output power reached 102 W and the central wavelength was 1.97 μm. The single frequency laser signal from a seed laser was amplified based on a monolithic master oscillator power amplifier (MOPA) configuration. The slope efficiency was about 50% against the absorbed pump power. Neither parasitic lasing nor nonlinear effect was observed in the monolithic fiber amplifier. The SBS threshold of the single frequency Tm-doped fiber amplifier was analyzed and estimated. The output power is not limited by the SBS threshold and could be further improved by increasing the pump power. To the best of our knowledge, this is the first demonstration of average power exceeding 100 W from monolithic all-fiber laser near 2 μm wavelength.

High-power passively Q-switched thulium fiber laser with distributed stimulated Brillouin scattering

We report a novel passively Q-switched 2 μm Tm(3+) fiber laser simply by distributed stimulated Brillouin scattering with an all-fiber configuration. The maximum output power is 2.49 W with an optical-to-optical slope efficiency of 12%, and the laser wavelength is centered at ~1991 nm with a spectral width of >10 nm. The single pulse energy can be over 50 μJ with a pulse width of about 20 ns, and the pulse repetition rate can be tuned from several kilohertz to tens of kilohertz by changing the pump power. To the best of our knowledge, this is the highest-average-power passively Q-switched 2 μm Tm(3+) fiber laser in the nanosecond regime.

High-peak-power, single-frequency, single-mode, linearly polarized, nanosecond all-fiber laser based on self-phase modulation compensation

We demonstrated a high-peak-power, single-frequency, single-mode, linearly polarized, nanosecond all-fiber laser in master oscillator power amplifier configuration. The pulsed seed was generated by modulating a single-frequency continuous wave laser at 1064 nm. The short pulse width (∼3.2  ns) of the pulsed seed prevented the onset of stimulated Brillouin scattering in the 10 μm-core single-transverse-mode polarization-maintaining ytterbium-doped fiber of the main amplifier. In order to compensate self-phase modulation-induced spectral broadening, a phase modulator was employed to control the phase of the pulsed seed, and the linewidth of the output pulses were reduced from ∼1.22  GHz to ∼185  MHz at the peak power of 1.47 kW.

High average power picosecond pulse generation from a thulium-doped all-fiber MOPA system

We report a stable highly-integrated high power picosecond thulium-doped all-fiber MOPA system without using conventional chirped pulse amplification technique. The master oscillator was passively mode-locked by a SESAM to generate average power of 15 mW at a fundamental repetition rate of 103 MHz in a short linear cavity, and a uniform narrow bandwidth FBG is employed to stabilize the passively mode-locked laser operation. Two-stage double-clad thulium-doped all-fiber amplifiers were used directly to boost average power to 20.7 W. The laser center wavelength was 1962.8 nm and the pulse width was 18 ps. The single pulse energy and peak-power after the amplication were 200 nJ and 11.2 kW respectively. To the best of our knowledge, this is the highest average power ever reported for a picosecond thulium-doped all-fiber MOPA system.

150 W high-average-power, single-frequency nanosecond fiber laser in strictly all-fiber format

We demonstrate an all-fiber, single-frequency nanosecond laser with both high peak power and average power based on a master oscillator power amplifier (MOPA) configuration. The MOPA produced a single-frequency pulsed laser with pulse duration of ∼8  ns. The average and peak power were as much as 139.3 W and 1.07 kW, respectively, when the repetition rate was 10 MHz, and 153.1 and 668 W, respectively, when the repetition rate was 20 MHz. Higher output power can be obtained by increasing pump power of the main amplifier.