Polarization-maintaining Fiber Amplifier Research Articles

High fidelity amplification of radially-polarized laser in few-mode polarization-maintaining fiber amplifier

A high-power 20/400-μm few-mode polarization-maintaining (PM) fiber amplifier with a radially polarized output beam is firstly demonstrated and investigated. The reconstructed beam profiles and mode contents of the output laser were obtained using a numerical method. Experimentally, a home-made fiberized mode convertor was employed to convert the LP01 mode to be TM01 mode (radially polarized beam) with an average power of 2 W. In addition, the mode purity and polarization purity were 98.76%, and 98.5%, respectively. Based on a few-mode PM fiber amplifier, instead of the commonly used non-PM fiber amplifier, the radially polarized beam was scaled up to 192.1 W, while preserving the donut-shaped intensity distribution. The corresponding mode purity and polarization purity were measured to be approximately 97% and 98.2%, respectively, which indicates the polarization characteristic of the TM01 mode was preserved well. The beam quality factor (M2) was measured to be ∼2.2. Additionally, long-term stability in terms of both output power and polarization state were also verified experimentally. To the best our knowledge, this is the highest output power from a few-mode PM fiber amplifier laser system, while maintaining a high mode purity and a high polarization purity for radially polarized beam.

Nonlinear spectral compression in high-power narrow-linewidth polarization maintaining fiber amplifiers for SBS suppression.

In this paper, we propose a method for narrowing the spectrum in high-power narrow-linewidth polarization-maintaining (PM) fiber amplifiers and investigate its potential for suppressing the stimulated Brillouin scattering (SBS). In this method, in addition to common phase modulation to suppress SBS, precisely designed amplitude modulation is induced to generate self-phase modulation in a high-power PM fiber amplifier. In this co-modulation way, the spectrum can be gradually compressed along the fiber. Compared to phase modulation alone or fiber-Bragg-gratings (FBGs) based narrow-linewidth fiber oscillator schemes, in which the spectrum remains the same or broadens, this scheme can achieve a higher SBS threshold for the same output spectral linewidth. Experiments on a ∼ 3 kW peak power quasi-continuous wave (QCW) fiber amplifier show that the co-modulation scheme can compress the spectrum from 0.25 nm to 0.084 nm as output peak power increases from 13 W to 3.2 kW and enhances the SBS threshold by ∼1.7 times compared to traditional FBGs-based fiber oscillator schemes, and by ∼1.4 times compared to common phase modulation schemes. This co-modulation scheme has the potential for mitigating SBS in high-power fiber amplifiers.

TMI and polarization static energy transfer in Yb-doped low-NA PM fibers.

In this work, we conduct experimental investigations of transverse mode instabilities (TMI) in a large mode area ultra-low numerical aperture polarization maintaining fiber amplifier. This fiber is few mode in the slow-axis (conventional operation mode), but single mode in the fast-axis. We test the stability of the output beam by changing the input polarization angle and systematically investigate the transverse mode instability threshold in the two principal polarization axes. The lowest TMI threshold at 300 W was found when the input polarization angle was aligned parallel to the slow-axis. Detuning the input polarization angle from the slow-axis led to increased TMI thresholds. For input polarization angle of 90° (parallel to the fast-axis), the output signal was stable up to 475 W and further scaling was limited by the available pump power. However, for fast-axis operation a lower polarization ratio compared to slow-axis operation was observed as well as an unexpected static energy transfer from the fast-axis into the slow-axis above 400 W.

Linearly polarized fiber amplifier with narrow linewidth of 5 kW exhibiting a record output power and near-diffraction-limited beam quality.

In this work, a narrow-linewidth polarization-maintaining (PM) all-fiber amplifier with near-diffraction-limited beam quality and record output power is presented. First, a 4.45-kW PM fiber amplifier with a 3-dB linewidth of 0.08 nm and root mean square (rms) linewidth of 0.22 nm is achieved based on optimized phase modulation. However, the sideband of the spectrum broadens significantly during the amplification process, which is mainly caused by the additional intensity variation of the injected signal. Meanwhile, an up to 5.04-kW linearly polarized fiber laser with a relatively stable spectral bandwidth is achieved by effectively suppressing spectral broadening. At the maximum output power, the rms linewidth is 0.2 nm, the beam quality factor M2 is less than 1.3, the polarization extinction ratio (PER) is 16.5 dB, and the signal-to-noise ratio (SNR) is approximately 53 dB. The further power scaling of the amplifier is mainly limited by the pump power. To the best of our knowledge, this is the maximum output power of a narrow linewidth linearly polarized fiber amplifier to date.

Millihertz band low-intensity-noise single-frequency laser for space gravitational wave detection

A low-noise single-frequency laser is a key component of the space-based gravitational wave detector, and the intensity noise of the laser directly affects the sensitivity of the space-based GW detector. In this work, we report a low-noise single-frequency laser designed for space-based gravitational wave detector. The laser is based on a master oscillator power amplifier (MOPA), which is designed to possess a low-power, narrow-linewidth seed laser acting as master oscillator (MO) and an all polarization-maintaining fiber amplifier acting as power amplifier (PA). The amplifier that uses a robust mechanical design consists of an Yb-doped double-clad fiber forward pumped by wavelength-locked 976 nm pump laser diode (LD) to achieve 2.13 W of output power and 70 dB of signal-noise ratio (SNR). To suppress the relative intensity noise (RIN) in a millhertz regime (1 mHz–1 Hz), we characterize the power stabilization of a pump diode laser based on a proportional-integral-derivative (PID) feedback control loop where an in-loop photodetector is used. The power fluctuation can be converted into the fluctuation of the current signal by the photodiode, the current signal is converted into the voltage signal and amplified by a transimpedance circuit. Then, the voltage signal is compared with the voltage reference signal, and the error signal is achieved to adjust real-timely the drive current of the pump laser diode. This is a good way to significantly suppress the RIN of a laser at low frequencies, but the measured RIN below 4 mHz is still higher than –60 dBc/Hz. In order to further suppress the RIN to lower than 4 mHz, an active precise temperature control technology is used to suppress the thermal noise from pump LD and fiber coupler. To assess the RIN milliertz regime, we design an RIN measurement system consisting of a high-precision signal acquisition card (24 bit) and a computer program based on LabVIEW. The measurement range of the system is 2 μHz–102.4 kHz and the frequency resolution up to 2 µHz, much better than the counterparts of commercial instruments. By stabilizing the fiber amplifier pump LD current and the temperature of pump LD and the temperature of fiber coupler, the out-of-loop RINs are measured to be –63.4 dBc/Hz@1 mHz and –105.8 dBc/Hz@1 Hz , and in a milliertz regime of 1 mHz–1 Hz, the RIN is below –60 dBc/Hz. The results show that the feedback control of the fiber amplifier pump LD current and the temperature control of the key devices can effectively suppress the RIN in the millihertz frequency band, which lays a foundation for further improving the intensity noise performance in the low frequency band.

2.3 W, Linearly-Polarized Superfluorescent Generation From a Polarization-Maintaining Er3+-Doped Fluoride Fiber Amplifier Around 2.8 μm

We present a high-power, polarization-maintaining (PM) Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fiber amplifier, which enables watt- level, linearly-polarized (LP) superfluorescent outputs around 2.8 μm by using a non-PM Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fiber superfluorescent source (FSS) after polarization controlling as the seed. Based on an optimized scheme, the gained superfluorescent signal with a 20 dB bandwidth of 95 nm (2710∼2805 nm) can yield a maximum power of 2.32 W and a polarization extinction ratio (PER) of up to ∼23.6 dB. To our knowledge, this is the first demonstration of a mid-infrared (mid-IR) PM fiber amplifier, and the power also represents, until now, the highest level from a FSS in the mid-IR. Further power scaling is only limited by the seed power, and can be anticipated by boosting seed power or/and adopting multi-stage amplification. This work provides an alternative way for high-power, LP, mid-IR FSS, enabling a range of real-world applications.

650 W All-Fiber Single-Frequency Polarization-Maintaining Fiber Amplifier Based on Hybrid Wavelength Pumping and Tapered Yb-Doped Fibers

Based on hybrid wavelength pumping and tapered Yb-doped fibers (T-YDFs), a 650 W all-fiber single-frequency polarization-maintaining fiber amplifier was demonstrated experimentally at 1030 nm. Different pump power ratios in the T-YDF-based power-amplifier stage were proposed to investigate their influence on the transverse mode instability (TMI) effect. The highest TMI threshold was obtained when the pump power ratio of 940 nm to 976 nm was 1:4.4. A measured M2 factor of 1.7 and a polarization extinction ratio of 14 dB at the maximum output power were obtained. To the best of our knowledge, these results exhibit the highest output power of any all-fiber single-frequency polarization-maintaining fiber amplifiers created up to now.

694 W sub-GHz polarization-maintained tapered fiber amplifier based on spectral and pump wavelength optimization.

The comprehensive suppression of the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) is a critical issue for the power scaling of fiber laser with sub-GHz spectral linewidth. In this manuscript, a narrow linewidth and polarization-maintained (PM) fiber amplifier based on tapered Yb-doped fiber (T-YDF) is established, and the effects of spectral linewidth, spectral shape and pump wavelength on the SBS and/or TMI thresholds are investigated. Up to 694 W polarization-maintained fiber laser with just ∼790 MHz linewidth is obtained by combining the advantages of tapered Yb-doped fiber, near-rectangular spectral injection and 915 nm pump manner. This work could provide a well reference solution for the realization of high-power ultra-narrow linewidth fiber lasers.

3 kW power-level all-fiberized superfluorescent fiber source with linear polarization and near-diffraction-limited beam quality.

In this paper, we demonstrate a high-power linearly polarized all-fiberized superfluorescent fiber source (SFS) with near-diffraction-limited beam quality. The laser system is based on a master oscillator power amplifier configuration, which mainly consists of a polarization-maintained (PM) broadband SFS and multistage PM fiber amplifiers. A linearly polarized fiber laser over 3 kW is obtained at a pump power of about 4.3 kW with the corresponding slope efficiency of 71.9%. The polarization extinction ratio and the beam quality (M2 factor) of the output laser are about 12.7 dB and 1.18, respectively, at the maximum output power. To the best of our knowledge, this is the highest output power of a linearly polarized broadband SFS with near-diffraction-limited beam quality.

Mode Evolution of High Power Monolithic PM Fiber Amplifiers in the Presence of SRS Effect

In this work, mode evolution in the presence of stimulated Raman scattering (SRS) effect has been studied in a 2 kW polarization-maintained (PM) fiber amplifier. It proved that the SRS-induced mode distortion can be observed at 40 dB of signal to Raman noise ratio in a PM fiber amplifier, which is significantly lower than that in a non-PM system. By using mode decomposition (MD) method, the SRS-induced mode distortion has been analyzed quantitatively by decomposing the signal and the Raman light individually, and the signal mode evolution shows a dissimilar phenomenon compared with the SRS-induced transverse mode instability. The SRS effect in large mode area fiber lasers result in continuous excitation of signal high order modes (HOMs), which leads to the signal beam quality deterioration and the output power rollover. The power fraction of HOMs increases by 30&#x0025; with the Raman power ratio only being 1.67&#x0025; at the maximal output power, and the Raman Stokes light mainly exists in fundamental mode distribution, which can be attributed to the beam cleanup effect.

3.25 kW all-fiberized and polarization-maintained Yb-doped amplifier with a 20 GHz linewidth and near-diffraction-limited beam quality.

In this paper, we demonstrate a high-power, narrow-linewidth, polarization-maintaining fiber amplifier with near-diffraction-limited beam quality. By optimizing the phase modulation signal, a nearly top-hat-shaped spectrum was generated for self-pulsing suppressing. That results in doubling the self-pulsing threshold we got from conventional white noise signal phase modulation with the same optical linewidth. Based on an optimized signal and a high power, polarization-maintaining, counter-pumped fiber amplifier, we obtain a 3.25 kW narrow-linewidth linearly polarized laser output with a linewidth of ∼20GHz, the polarization extinction ratio is about 15 dB, and the M2 is less than 1.22 at the maximum output power. To the best of our knowledge, this is the first demonstration of a narrow-linewidth, linear polarization, all-fiber amplifier with 3.25 kW laser output.

Nonlinear fiber system for shot-noise limited intensity noise suppression and amplification.

We propose a nonlinear fiber system for shot-noise limited, all-optical intensity noise reduction and signal amplification. The mechanism is based on the accumulation of different nonlinear phase shifts between orthogonal polarization modes in a polarization-maintaining fiber amplifier in combination with an implemented sinusoidal transmission function. The resulting correlation between the input intensity fluctuations and the system transmission enables tunable intensity noise reduction of the input pulse train. In the experiment, the noise spectral density of a mode-locked oscillator is suppressed by up to 20 dB to the theoretical shot-noise limit of the measurement at -151.1dBc/Hz with simultaneous pulse amplification of 13.5 dB.

Pedestal analysis of jitter in a 20.97 GHz regeneratively mode-locked laser formed with a short polarization-maintaining fiber amplifier.

We demonstrated a 20.97 GHz regeneratively mode-locked Er fiber laser that has a short ring length of 5.84 m. The short active ring enabled large filter-rejection of adjacent supermodes during extraction of its clock signal for regenerative feedback. Very stable mode locking was observed with the generation of ∼5.5ps Gaussian pulses. A characterization of the noise pedestals in the photodetected first and second harmonics of the mode-locked pulses indicated there was cross-correlated coupling between the timing and amplitude jitter. From an analysis of the noise pedestals in these harmonics, as well as the noise spectrum at baseband, we estimated an amplitude jitter of ∼0.19%, and a timing jitter of ∼60fs. Using a generalized harmonic analysis approach, we estimated a cross correlation of 0.11% between the timing and amplitude jitter.

7.1 kW coherent beam combining system based on a seven-channel fiber amplifier array

Coherently combining of seven-channel narrow-linewidth, linear-polarized, kW-level power all-fiber amplifiers has been successfully demonstrated by using the stochastic parallel gradient descent (SPGD) algorithm. The seven polarization-maintained fiber amplifiers are tiled by a collimator array with a high lenslet filling factor (~95%). Total emitting power is ~8 kW, and ~7.1 kW combined output power is achieved and the contrast of the far-field intensity pattern is measured to be ~86% when the system is in the closed loop. This work could provide practical reference on scaling the output power of the tiled aperture coherent beam combining system.

4 mJ Rectangular- Envelope GHz-Adjustable Burst-Mode Fiber-Bulk Hybrid Laser and Second-Harmonic Generation

We demonstrate a high peak power and high burst energy linear-polarization, narrowband, GHz-adjustable burst-mode fiber-bulk hybrid laser with central wavelength of 1064 nm and its second-harmonic generation. The active modulated burst-mode seed provides the tunability of temporal parameters and the pre-compensation of burst envelope distortion. Three-stage polarization maintained fiber amplifier cascading bulk amplifier promote the burst energy to 4 mJ at a burst duration of 100 ns and intra-burst duty of 50%. To the best of our knowledge, this is the highest reported output energy of burst-mode fiber-bulk hybrid laser with ∼ hundred nanoseconds burst duration. Also, the pre-compensation for rectangular burst envelope is investigated by iteration method. Type I, non-critical phase matching lithium borate (LBO) is employed for second-harmonic generation and 1.8 mJ burst energy is achieved, corresponding to a conversion efficiency of 45%. The 3 dB spectral width of green light at maximal burst energy is 90 pm. The immediate application of this laser system is the generation of high peak power frequency-adaptive microwave.

High-beam-quality 2-kW-class Spectrally Combined Laser Using Narrow-linewidth Ytterbium-doped Polarization-maintaining Fiber Amplifiers

High-beam-quality 2-kW-class Spectrally Combined Laser Using Narrow-linewidth Ytterbium-doped Polarization-maintaining Fiber Amplifiers

5 W ultra-low-noise 2 µm single-frequency fiber laser for next-generation gravitational wave detectors.

We demonstrated an ultra-low-noise polarization-maintaining (PM) single-frequency fiber laser at 2 µm. Both relative intensity noise (RIN) and frequency noise were improved by suppressing the pump source RIN using feedback control. After a two-stage Tm3+-doped PM fiber amplifier, the output power reached about 5 W, and the amplifier did not introduce any observable extra frequency noise. The frequency noise was less than 100Hz/Hz above 13 Hz, which is comparable to the frequency noise of a typical seed laser of the Advanced LIGO high-power laser. The central wavelength was measured to be 1990.25 nm, with a polarization extinction ratio above 24 dB.

Vector Effects of Dissipative Soliton in All-Fiber MOPA System

The performance of seed laser source will have a great impact on the output pulse quality of the master oscillator power amplifier (MOPA) system. Here, we report the influence of the soliton vector features on the pulse uniformity in an all-fiber MOPA configuration. We demonstrated that, although the intensity of the dissipative soliton seed source is uniform, the intensity of the output pulse train from the MOPA will show fluctuations if the soliton polarization state is not locked, i.e., polarization-rotating vector soliton. It is found that the appearance of non-uniform pulse intensity is caused by the combination of the polarization instability and the polarization dependent gain in the polarization maintaining fiber amplifier. The obtained results indicate that the polarization evolution of seed pulsed source is critical to generation of stable pulse train from the MOPA, and also might be useful to the communities dealing with solitons and high-power pulsed fiber lasers.

Ultra low-noise coherent supercontinuum amplification and compression below 100 fs in an all-fiber polarization-maintaining thulium fiber amplifier.

We report an ultra-low noise, polarization-maintaining, ultrafast Thulium-doped all-fiber chirped pulse amplifier, seeded by a polarized all-normal dispersion (ANDi) supercontinuum (SC) driven by an ultrafast Erbium-fiber laser. The system comprises only polarization-maintaining fibers and delivers 96 fs pulses with 350 mW output power at 100 MHz, centered at 1900 nm. The integrated relative intensity noise (RIN) in the range of 10 Hz - 10 MHz is only 0.047% at the amplifier output, which is virtually identical to the RIN of the Erbium-fiber laser driving the SC. Therefore, neither the SC generation nor the amplification process introduce significant excess noise. The RIN of our system is an order of magnitude lower than similar systems previously seeded with Raman solitons. This highlights the superior noise properties of ANDi SC and their potential as ultra-low noise seed sources for broadband, high power ultrafast fiber amplifiers and frequency combs.

1 μJ nanosecond low-coherent laser source with precise temporal shaping and spectral control

A μJ-level low-coherent pulse with dynamic and precise temporal shaping and spectral control is demonstrated. Low-coherent light delivered by a superluminescent diode is precisely shaped through a wave guide amplitude modulator driven by an arbitrary waveform generator. Utilizing a three-stage axial-alternating polarization-maintaining single-mode fiber amplifier, μJ-level low-coherent pulses with 3 ns durations and 20 nm instantaneous bandwidths at a 1 kHz repetition rate are obtained. In addition, a spectral control unit is used to adjust the spectrum shape. As a result, near flat-top, saddle-type, or other required special spectra can be produced. The low temporal coherence characteristics and output stability are characterized experimentally.