• Current Optics and Photonics
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• v.5 no.3
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• pp.306-310
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• 2021

A 200-W continuous-wave thulium-doped all-fiber laser at 2050 nm was developed with a master oscillator power amplifier configuration. For the master oscillator, a single-mode thulium-doped fiber laser was built with fiber Bragg gratings. The operating power of the oscillator was 10.1 W at a pump power of 20.9 W, and the slope efficiency was measured to be 53.0%. All emitted wavelengths of the oscillator were located between 2049.2 nm and 2049.9 nm, and no other peaks in different wavelength ranges were observed. The maximum output power of the final amplified beam was 204.6 W at a pump power of 350.4 W. The slope efficiency of the amplifier was measured to be 58.4%.

• Current Optics and Photonics
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• v.7 no.3
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• pp.297-303
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• 2023

In this paper, we demonstrated a high-energy (285 mJ) mid-infrared flashlamp-pumped electro-optically 𝚀-switched Er:YAG master oscillator power amplifier (MOPA) system and comprehensively investigated its temporal, spectral, and spatial characteristics. To increase the output energy, we optimized the delay between the timings at which the flashlamps of the master oscillator and power amplifier were triggered. In addition, the output energy was improved while minimizing thermal effects by cooling the MOPA system to a temperature slightly above the dew point. Consequently, the MOPA structure boosted the output energy without damaging the lithium niobate Pockels cell, which is a crucial element in 𝚀-switching. This design realized pulses with energies up to 0.285 J and pulse durations of approximately 140 ns at a wavelength of 2,936.7 nm. This high-energy mid-IR Er:YAG MOPA system can be used for various scientific, engineering, and military underwater applications.

• Current Optics and Photonics
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• v.5 no.5
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• pp.544-553
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• 2021

A 40-W 200-ns 300-kHz thulium-doped fiber laser at 2050 nm with a master oscillator power amplifier configuration was developed, for application to lithium-isotope separation. The master oscillator generated a 5.35 W continuous-wave beam, which the pulse generator then broke into 200-ns pulses at 300 kHz. Then, the laser beam was amplified by passing through a two-stage amplifier. The output power finally obtained was 42.0 W at 2050 nm, and was stable for a long time, over 2 hours. In spite of this achievement, mode instability was observed in the output beam. This can be solved in the future by using a method such as tight coiling.

• Korean Journal of Optics and Photonics
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• v.29 no.4
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• pp.159-165
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• 2018

In this paper, we report on the high power amplification of a narrow-linewidth Yb-doped polarization-maintained (PM) fiber laser in a 3-stage, all-fiber master oscillator power amplifier (MOPA) system. The linearly polarized single-mode output power was 400 W with an 85% slope efficiency, with a linewidth of 2.5 GHz (full width at half maximum). Furthermore, mitigation of mode instability (MI) has been demonstrated by tightly coiling the gain fiber to a diameter of 11 cm. In addition, methods for higher power scaling are discussed.

• Korean Journal of Optics and Photonics
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• v.32 no.4
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• pp.187-194
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• 2021

This paper presents a study of laser output characteristics. We fabricated a MOPA (master oscillator power amplifier)-type high-power, narrowbandwidth fiber laser with a bidirectional pumping configuration in its main amplifier. As signal beams, light sources with bandwidths of 3 GHz and 10 GHz-phase-modulated through a PRBS (pseudo-random binary sequence)-were used interchangeably. Furthermore, the characteristics of the SBS (stimulated Brillouin scattering) were analyzed using a signal beam with 3 GHz bandwidth, by adjusting the forward to backward pumppower ratio. Moreover, the characteristics of the transverse mode instability were analyzed by adjusting the forward to backward pump-power ratio, using a signal beam with 10-GHz bandwidth. Finally, the output power from 10 GHz bandwidth was amplified to more than 1 kW using a forward to backward pump-power ratio of 1.6. The beam quality M² was measured to be approximately 1.36, and the optical-to-optical efficiency was 80% at maximum output power.

• Journal of the Optical Society of Korea
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• v.17 no.2
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• pp.117-129
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• 2013

We briefly review the recent progress in passively mode-locked fiber lasers (PMLFLs) based on semiconductor saturable absorber mirrors (SESAMs) and discuss the detailed characterization of a SESAM-based, passively mode-locked erbium-doped fiber (EDF) laser operating in the 1.5-${\mu}m$ spectral range for various configurations. A simple and compact design of the laser cavity enables the PMLFL to generate either femtosecond or wavelength-tunable picosecond pulses with high stability as the intra-cavity filtering method is altered. All the cavities investigated in our experiments present self-starting, continuous-wave mode-locking with no Q-switching instabilities. The excellent stability of the source eventually enables the wavelength-tunable PMLFL to be used as a master oscillator for a power-amplifier source based on a large-core EDF, generating picosecond pulses of >10-kW peak power and >100-nJ pulse energy.

• Current Optics and Photonics
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• v.8 no.1
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• pp.65-71
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• 2024

We present a Yb-doped narrow-linewidth polarization-maintaining all-fiber amplifier that achieves a high mode-instability (MI) threshold, high output power, and 9.7-GHz spectral linewidth. Six wavelength-multiplexed laser diodes are used to pump this amplifier. First, we construct a high-power fiber amplifier based on a master oscillator-power amplifier configuration for experiments. Subsequently, we examine the MI threshold by individually pumping the amplifier with wavelengths of 976, 974, 981, 974, and 981 nm respectively. The experimental results demonstrate that the amplifier exhibits a high MI threshold (>3.5 kW) when pumped with a combination of wavelengths at 974 and 981 nm. Afterward, we inject an optimized phase-modulated seed with a nearly flat-top spectrum into this amplifier. Ultimately, laser output of 3.2 kW and 9.7 GHz are obtained.

• Korean Journal of Optics and Photonics
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• v.33 no.6
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• pp.245-251
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• 2022

We have developed a multi-channel fiber laser for tiled laser beam combining and a laser output array system for multi-beam alignment. The fiber laser is a master oscillator power amplifier configuration that has a common seed, a preamplifier, and a 7-channel amplifier. The output power of each channel is more than 10 W. The laser output array system is a packed cylindrical configuration for a high fill-factor, and it has capabilities for collimation and tilt control with built-in PZT. Multi-beam alignment to a target is successfully implemented using PZT controlled with a stochastic parallel gradient descent (SPGD) algorithm.

• Korean Journal of Optics and Photonics
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• v.27 no.3
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• pp.95-100
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• 2016

High-power quasi-continuous-wave (qcw) operation in Yb-doped double-clad fiber lasers with near-diffraction-limited quality of the output beam is reported. Based on numerical simulation, we built a simple, all-fiberized Yb fiber laser, and a fiber-based master-oscillator power amplifier (MOPA). Both laser systems have successfully produced qcw output with average power greater than 150 W at 1080 nm and 10 ms pulse duration at 10 Hz repetition rate, corresponding to a peak power greater than 1.5 kW for 205 W of pump power at 976 nm. Laser performance, including beam quality and slope efficiency, was characterized in both configurations. Prospects for power scaling and applications are discussed.

• Korean Journal of Optics and Photonics
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• v.30 no.4
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• pp.167-173
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• 2019

In this paper, we studied characteristics of stimulated Brillouin scattering (SBS) suppression in high-power fiber lasers by using apparatuses applying a temperature gradient (i.e. a step, a sine shape, and random temperature distribution) along the fiber. From the ytterbium-doped polarization-maintaining fiber master oscillator power amplifier built in house, we measured the back-reflection spectrum and power for each temperature gradient, showing that the step shape temperature distribution was the most effective way to suppress SBS. In addition, we investigated the interaction of pseudo-random binary sequence phase modulation conditions and temperature gradients for SBS suppression.