Ultralow-quantum-defect single-frequency fiber laser.

A single-frequency distributed-Bragg-reflector fiber laser at 980 nm with a quantum defect of less than 0.6% was developed with a 1.5-cm 12 wt% ytterbium-doped phosphate fiber pumped by a 974.5-nm laser diode. Linearly polarized single-longitude-mode laser with a polarization extinction ratio (PER) of nearly 30 dB and spectral linewidth of less than 1.8 kHz was obtained. A maximum output power of 275 mW was measured at a launched pump power of 620 mW. The performance of the single-frequency fiber laser pumped at 909 nm and 976 nm was also characterized. This research demonstrated an approach to high-power single-frequency fiber laser oscillators with mitigated thermal effects.

Injection-locked highly Yb3+-doped uncoupled-61-core phosphate fiber laser.

Uncoupled multicore fibers are promising platforms for advanced optical communications, optical computing, and novel laser systems. In this paper, an injection-locked highly ytterbium (Yb3+)-doped uncoupled-61-core phosphate fiber laser at 1030 nm is reported. The 61-core fiber with a core-to-core pitch of 20 µm was fabricated with the stack-and-draw technique. Each core doped with 6-wt.% Yb3+ ions has a diameter of 3 µm and numerical aperture of 0.2. Linearly polarized single-frequency output of 9.1 W was obtained from the injection-locked cavity with a 10-cm-long gain fiber at a pump power of 23.6 W. The injection locking of all 61 cores was confirmed by inspecting the longitudinal modes of the individual lasers with a scanning Fabry-Perot interferometer. The performance of the injection-locked 61-core fiber laser was characterized and compared to that of the free-running operation in terms of optical spectrum, near- and far-field intensity profiles, and relative intensity noise.

Single-frequency fiber laser at 880†nm.

Single-frequency fiber lasers with extremely low noise and narrow spectral linewidth have found many scientific and practical applications. There is great interest in developing single-frequency fiber lasers at new wavelengths. In this paper, we report a single-frequency Nd3+-doped phosphate fiber laser operating at 880 nm, which is the shortest demonstrated wavelength for a single-frequency fiber laser thus far, to the best of our knowledge. An output power of 44.5 mW and a slope efficiency of 20.4% with respect to the absorbed pump power were obtained with a 2.5-cm-long 1 wt.% Nd3+-doped phosphate fiber. Our simulation results show that higher single-frequency laser output can be achieved with 1.5 wt.% or 2 wt.% Nd3+-doped phosphate fiber with mitigated ion clustering.

Magneto-optical properties of highly Dy3+ doped multicomponent glasses.

Due to their large effective magnetic moment, Dy3+-doped materials have attracted much interest for magneto-optical applications. In this paper, we report highly Dy3+ doped multicomponent glasses with concentrations from 40 wt.% to 75 wt.% and their magneto-optical properties. A Verdet constant of -7.4 rad/T/m at 1950 nm was measured with the 75 wt.% Dy3+-doped glass. This is the highest reported Verdet constant around 2 µm for a paramagnetic glass. Our experimental results show that highly Dy3+-doped glasses are promising isotropic magneto-optical materials for applications in the 2 µm wavelength region.

Investigation of ion-ion interaction effects on Yb3+-doped fiber amplifiers.

Ytterbium (Yb3+)-doped materials have been widely used for high efficiency high energy laser sources at the 1 µm wavelength region because of their very low quantum defect and the unique simple energy level structure of Yb3+, resulting in no excited-state absorption and low occurrence probability of deleterious ion-ion interaction processes. It has been generally recognized that these ion-ion interaction processes have very little influence on the operation of Yb3+-doped fiber lasers at low and moderate power levels. However, our recent study shows that the performance of Yb3+-doped fiber amplifiers operating at low power levels is still influenced by the ion-ion interaction processes due to the large amount of population at the upper laser level 2F5/2. In this paper, experimental evidences of the ion-ion interaction effects in Yb3+-doped fiber amplifiers are presented and a new model including these effects is developed for the numerical simulation. Our experimental and numerical investigations on the 976 nm and 1030 nm Yb3+-doped silica and phosphate fiber amplifiers show that ion-ion interaction has non-negligible impact on the performance of Yb3+-doped fiber amplifiers indeed, and compared to Yb3+-doped silica fibers, Yb3+-doped phosphate fibers suffer much less from the ion-ion interaction effects due to the much less clustered ions.

Single-frequency blue laser fiber amplifier.

An all-fiber amplifier for a single-frequency blue laser was demonstrated for the first time, to the best of our knowledge. Over 150 mW continuous-wave single-transverse-mode blue laser output was obtained with a 10 m 1000 ppm thulium-doped fluoride fiber pumped by a 1125 nm fiber laser at a power of 2 W. The output power was limited due to the onset of the competitive lasing at 783 nm. Photodarkening and photo-curing of the thulium-doped fiber amplifier were also studied and analyzed.

Power scalable 10 W 976 nm single-frequency linearly polarized laser source.

A 10 W level 976 nm single-frequency linearly polarized laser source was demonstrated with a two-stage all-fiber amplifier configuration. The continuous-wave output power of 10.1 W was obtained from the second stage amplifier by using a 20/130 µm single-mode, polarization maintaining, 1.5 wt. % ytterbium-doped phosphate double-clad fiber. This all-fiber single-frequency laser source is very promising for watt-level deep ultraviolet laser generation via frequency quadrupling.

Graphene Q-switched Ho(3+)-doped ZBLAN fiber laser at 1190 nm.

We report Q-switched pulse operation of holmium (Ho(3+))-doped ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF (ZBLAN) at ∼1190 nm in an all-fiber ring laser by using a fiber-optic graphene saturable absorber, which was fabricated by depositing graphene onto the flat surface of a side-polished D-shaped fiber. Stable Q-switched operation was established at a pump power of 180 mW with a repetition rate of 24 kHz and pulse width of 5.7 µs. When the pump power was increased to 1125 mW, 0.44 µJ Q-switched pulses with a repetition rate of 111 kHz and a pulse width of 0.8 µs were generated.

Monolithic fiber chirped pulse amplification system for millijoule femtosecond pulse generation at 1.55 µm.

A monolithic fiber chirped pulse amplification system that generates sub-500 fs pulses with 913 µJ pulse energy and 4.4 W average power at 1.55 µm wavelength has recently been demonstrated. The estimated peak power for the system output approached 1.9 GW. The pulses were near diffraction-limited and near transform-limited, benefiting from the straight and short length of the booster amplifier as well as adaptive phase shaping for the overall mitigation of the nonlinear phase accumulation. The booster amplifier employs an Er(3+)-doped large mode area high efficiency media fiber just 28 cm in length with a fundamental mode (LP(01)) diameter of 54 µm and a corresponding effective mode area of 2290 µm(2).

All-fiber fundamentally mode-locked 12 GHz laser oscillator based on an Er/Yb-doped phosphate glass fiber.

We have developed a compact and stable all-fiber fundamentally mode-locked 12 GHz laser system. The passively mode-locked laser centered at 1535 nm has temporal pulse width of ∼2 ps and average power of 5 mW. The timing jitter, which is cumulative from pulse-to-pulse, has been measured using an optical cross-correlation method and found to be 44 fs/pulse. The self-starting, mode-locked laser consists of a semiconductor-based saturable absorber with high modulation depth and a high gain per unit length, polarization-maintaining 0.8 cm long Er/Yb phosphate fiber as a gain medium.

Watt-level short-length holmium-doped ZBLAN fiber lasers at 1.2 µm.

In-band core-pumped Ho3+-doped ZBLAN fiber lasers at the 1.2 µm region were investigated with different gain fiber lengths. A 2.4 W 1190 nm all-fiber laser with a slope efficiency of 42% was achieved by using a 10 cm long gain fiber pumped at a maximum available 1150 nm pump power of 5.9 W. A 1178 nm all-fiber laser was demonstrated with an output power of 350 mW and a slope efficiency of 6.5%. High Ho3+ doping in ZBLAN is shown to be effective in producing single-frequency fiber lasers and short-length fiber amplifiers immune from stimulated Brillouin scattering.

High efficiency, monolithic fiber chirped pulse amplification system for high energy femtosecond pulse generation.

A novel monolithic fiber-optic chirped pulse amplification (CPA) system for high energy, femtosecond pulse generation is proposed and experimentally demonstrated. By employing a high gain amplifier comprising merely 20 cm of high efficiency media (HEM) gain fiber, an optimal balance of output pulse energy, optical efficiency, and B-integral is achieved. The HEM amplifier is fabricated from erbium-doped phosphate glass fiber and yields gain of 1.443 dB/cm with slope efficiency >45%. We experimentally demonstrate near diffraction-limited beam quality and near transform-limited femtosecond pulse quality at 1.55 µm wavelength. With pulse energy >100 µJ and pulse duration of 636 fs (FWHM), the peak power is estimated to be ~160 MW. NAVAIR Public Release Distribution Statement A-"Approved for Public release; distribution is unlimited".

976 nm single-frequency distributed Bragg reflector fiber laser.

A single-frequency distributed Bragg reflector (DBR) fiber laser at 976 nm was developed with a 2 cm long highly ytterbium-doped phosphate fiber and a pair of silica fiber Bragg gratings. More than 100 mW of linearly polarized output was achieved from the all-fiber DBR laser with a linewidth less than 3 kHz. The outstanding features of this single-frequency laser also include ultralow relative intensity noise and high wavelength stability. This fiber laser is an excellent seeder for high-power 976 nm narrow-linewidth laser amplifiers that can be used for efficient coherent blue-light generation through frequency doubling.

Single-frequency Ho(3+)-doped ZBLAN fiber laser at 1200 nm.

A single-frequency (SF) fiber laser at 1200 nm was developed with a distributed Bragg reflector (DBR) configuration by splicing a 22 mm long highly holmium-doped ZBLAN (ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF) fiber with a pair of silica fiber Bragg gratings. The linewidth was estimated to be less than 100 kHz based on the measured frequency noise. The relative intensity noise was measured to be <110 dB/Hz at the relaxation oscillation peak and the polarization extinction ratio was measured to be >19 dB. Our results highlight the exciting prospect that wavelength coverage of SF DBR fiber lasers can be expanded significantly by using rare-earth-doped ZBLAN fibers.

High peak-power single-frequency pulses using multiple stage, large core phosphate fibers and preshaped pulses.

We have developed a monolithic high power pulsed fiber laser in a master oscillator power amplifier (MOPA) configuration, which is capable of reaching 0.38 mJ pulse energy and 128 kW peak power for 3 ns pulses at ~1550 nm while maintaining transform-limited linewidth. The fiber laser pulse seed was achieved by directly modulating a CW single-frequency fiber laser using an electro-optic modulator. We used an arbitrary waveform generator to preshape the fiber laser pulses before amplification to avoid pulse steepening and dynamic gain saturation. Single-mode, polarization maintaining highly Er/Yb codoped large core phosphate fibers were used in the power amplifier stages to scale the transform-limited fiber laser pulses, avoiding any nonlinearities.

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

We report a unique all fiber-based single-frequency Q-switched laser in a monolithic master oscillator power amplifier configuration at ~1920 nm by using highly Tm-doped germanate fibers for the first time. The actively Q-switched fiber laser seed was achieved by using a piezo to press the fiber in the fiber Bragg grating cavity and modulate the fiber birefringence, enabling Q-switching with pulse width and repetition rate tunability. A single-mode polarization maintaining large core 25 µm highly Tm-doped germanate fiber was used in the power amplifier stage. For 80 ns pulses with 20 kHz repetition rate, we achieved 220 µJ pulse energy, which corresponds to a peak power of 2.75 kW with transform-limited linewidth.

Enhanced terahertz source based on external cavity difference-frequency generation using monolithic single-frequency pulsed fiber lasers.

We demonstrate a resonant external cavity approach to enhance narrowband terahertz radiation through difference-frequency generation for the first time (to our knowledge). Two nanosecond laser pulses resonant in an optical cavity interact with a nonlinear crystal to produce a factor of 7 enhancement of terahertz power compared to a single-pass orientation. This external enhancement approach shows promise to significantly increase both terahertz power and conversion efficiency through optical pump pulse enhancement and effective recycling.

Kilowatt-level stimulated-Brillouin-scattering-threshold monolithic transform-limited 100 ns pulsed fiber laser at 1530 nm.

We demonstrate a high-stimulated-Brillouin-scattering-threshold monolithic pulsed fiber laser in a master oscillator power amplifier configuration that can operate over the C band. In the power amplifier stage, we used a newly developed single-mode, polarization maintaining, and highly Er/Yb codoped phosphate fiber with a core diameter of 25 microm. A single-frequency actively Q-switched fiber laser was used to generate pulses in the hundreds of nanoseconds at 1530 nm. We have achieved peak power of 1.2 kW for 105 ns pulses at a repetition rate of 8 kHz, corresponding to a pulse energy of 0.126 mJ, with transform-limited linewidth and diffraction-limited beam quality.

High SBS-threshold single-mode single-frequency monolithic pulsed fiber laser in the C-band.

We report a high SBS-threshold, single-frequency, single-mode, polarization maintaining (PM) monolithic pulsed fiber laser source in master oscillator and power amplifier (MOPA) configuration that can operate over the C-band. In order to achieve a narrow transform-limited linewidth for pulses longer than 100 ns, we use a single-frequency Q-switched fiber laser seed, which itself can be seamlessly tuned up to 1.24 micros. The Q-switched pulses are amplified in the power amplifier stage of MOPA using a high SBS threshold single-mode PM large core highly Er/Yb co-doped phosphate glass fiber (LC-EYPhF). This seed and amplifier combination represents the first monolithic, all-fiber implementation of a single-frequency pulsed laser with the highest pulse energy of 54 microJ and peak power of 332 W for 153-ns pulses at 1538 nm.