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

3.05 kW, 13.7 GHz linewidth fiber amplifier based on PRBS phase modulation for SBS suppression

In this paper, we establish a multi-stage fiber amplifier with pseudo-random binary sequence (PRBS) phase modulation. The stimulated Brillouin gain spectra of the main amplifier with both the unmodulated and pseudo-random binary sequence phase modulated configuration are measured (with corresponding output power), and the stimulated Brillouin scattering (SBS) threshold is investigated experimentally and theoretically. The pseudo-random binary sequence phase modulation parameters are optimized by theoretical simulation. With a two-stage preamplifier chain and a counter-pumping main amplifier stage, a maximum 3.05 kW output power with a slope efficiency of 85.9% is obtained experimentally. The central wavelength of the fiber amplifier is 1050 nm, associated with a full-width at half-maximum linewidth of 13.7 GHz. The stimulated Brillouin scattering reflectivity is below 0.01% at 3.05 kW at 13.7 GHz, which indicates that stimulated Brillouin scattering can be suppressed efficiently at this power and linewidth level.

10 W super-wideband ultra-low-intensity-noise single-frequency fiber laser at 1 µm

A 10 W super-wideband ultra-low-intensity-noise single-frequency fiber laser (SFFL) at 1 µm is experimentally demonstrated, based on dual gain saturation effects from semiconductors and optical fibers, together with an analog-digital hybrid optoelectronic feedback loop. Three intensity-noise-inhibited units synergistically work, which actualizes a connection of effective bandwidth and enhancement of noise-suppressing amplitude. With the cascade action of the semiconductor optical amplifier and optical fiber amplifier, the laser power is remarkably boosted. Eventually, an SFFL with an output power of 10.8 W and a relative intensity noise (RIN) below -150 dB/Hz at the frequency range over 1 Hz is realized. More meaningfully, within the total frequency range of 10 Hz to 10 GHz exceeding 29 octaves, the RIN is controlled to below -160 dB/Hz, approaching the shot-noise limit (SNL) level. To the best of our knowledge, this is the lowest RIN result of SFFL within such an extensive frequency range, and this is the highest output power of the near-SNL super-wideband SFFL. Furthermore, a linewidth of less than 0.8 kHz, a long-term stable polarization extinction ratio of 20 dB, and an optical signal-to-noise ratio of over 60 dB are obtained simultaneously. This start-of-the-art SFFL has provided a systematic solution for high-power and low-noise light sources, which is competitive for sophisticated applications, such as free-space laser communication, space-based gravitational wave detection, and super-long-distance space coherent velocity measurement and ranging.

Intensity noise transfer properties of a Yb-doped single-frequency fiber amplifier

In this work, the intensity noise transfer properties of a two-stage single-frequency fiber amplifier at 1 µm are systematically investigated in the frequency domain. By applying an artificial modulation signal to the driving current of the first- and second-stage pump sources, the pump and signal transfer functions of the second-stage amplifier are experimentally measured from 10 Hz to 100 kHz. By associating the theoretical model, the effects of pump power, the operating wavelength, and the absorption coefficient of the gain fiber on the pump and signal transfer properties are analyzed based on the experimental measurements. It turns out that the gain dynamics of the last-stage amplifier play an important role in determining the noise performances of the final amplified laser. Because the pump and signal transfer functions essentially behave as a low pass and damped high pass filter, the pump intensity noise of the last-stage amplifier dominates the amplifier system's overall noise performance. In addition, the effects of amplified spontaneous emission (ASE) on the intensity noise transfer properties are nontrivial, although it is not included in the theoretical model. It is believed that the current work provides a useful guideline for optimizing the design of high-power single-frequency fiber amplifiers with low-intensity noise.

Confined-doped active fiber enabled all-fiber high-power single-frequency laser

In this paper, we investigate the performances of an in-house fabricated confined-doped active fiber in the applications of all-fiber high-power single-frequency amplifiers. A 210-W single-frequency single-mode fiber laser is obtained directly, which confirms the excellent performance of the confined-doped active fiber for high-power single-mode operation. To further demonstrate the power scalability of the fiber amplifier, the strategy of applying a temperature gradient along the active fiber is investigated numerically and experimentally, and an up to ∼75% enhancement of the stimulated Brillouin scattering (SBS) threshold is achieved. As a result, a 368-W single-frequency fiber laser is obtained with the beam quality factor of Mx²= 1.19, My²= 1.26. Overall, the technique of the confined-doped active fiber provides a promising approach to scale the output power of single-frequency single-mode fiber lasers.

Over-20-octaves-bandwidth ultralow-intensity-noise 1064-nm single-frequency fiber laser based on a comprehensive all-optical technique

An over-20-octaves-bandwidth ultralow-intensity-noise 1064-nm single-frequency fiber laser (SFFL) is demonstrated based on a comprehensive all-optical technique. With a joint action of booster optical amplifier (BOA) and reflective Yb-doped fiber amplifier (RYDFA), two-fold optical gain saturation effects, respectively occurring in the media of semiconductor and fiber, have been synthetically leveraged. Benefiting from the gain dynamics in complementary time scales, i.e., nanosecond-order carrier lifetime in BOA and millisecond-order upper-level lifetime in RYDFA, the relative intensity noise (RIN) is reduced to -150 dB/Hz from 0.2 kHz to 350 MHz, which exceeds 20-octaves bandwidth. Remarkably, a maximum suppressing ratio of >54 dB is obtained, and the RIN in the range of 0.09-10 GHz reaches -161 dB/Hz which is only 2.3 dB above the shot-noise limit. This broad-bandwidth ultralow-intensity-noise SFFL can serve as an important building block for squeezed light generation, space laser communication, space gravitational wave detection, etc.

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.

High-power single-frequency single-mode all-solid photonic bandgap fiber laser with kHz linewidth

There have been several demonstrations of single-frequency single-mode ytterbium-doped fiber lasers operating at a few hundred watts of power. A narrow spectral linewidth of these lasers is critical for many applications but has never been properly measured before at high powers. In this work, we report the first spectral linewidth measurement at kHz resolution of high-power single-frequency fiber lasers using a heterodyne technique and can confirm that these lasers can indeed operate at a few kHz spectral linewidth. Furthermore, we have improved the power from single-frequency single-mode all-solid photonic bandgap fiber lasers to 500 W using an improved photonic bandgap fiber.

Compact and low-cost superfluorescent fiber source assisted narrow linewidth Yb-Raman fiber amplifier

Recent work has shown that temporally stable optical sources are required in a narrow linewidth Yb-Raman fiber amplifier to suppress the spectral broadening phenomenon. Superfluorescent fiber sources (SFSs) with different spectral widths are used as the Raman-pumped lasers in a 200-watt level narrow linewidth Yb-Raman fiber amplifier for the first time to the best of our knowledge. The experimental results reveal that the spectral broadening phenomenon could be well controlled by using the broadband SFS. Therefore, the narrow linewidth operation could be well maintained during the power scaling process. Moreover, the suppression of the spectral broadening phenomenon would deteriorate when the spectral width of the SFS decreases. This work could provide a compact, low-cost choice for the Raman-pumped laser in narrow linewidth Yb-Raman fiber amplifiers.

Comprehensive investigation on the power scaling of a tapered Yb-doped fiber-based monolithic linearly polarized high-peak-power near-transform-limited nanosecond fiber laser

An all-fiberized linearly polarized nanosecond master oscillator power amplifier based on polarization-maintaining large-mode-area Yb-doped tapered double cladding fiber (T-DCF) is comprehensively investigated. Firstly, excellent performance of the Yb-doped T-DCF for suppressing nonlinear effects, including stimulated Brillouin scattering (SBS) effect and spectral broadening effects, is experimentally demonstrated and qualitatively analyzed. An SBS-free average output power of 8.8 W is obtained under pulse duration of 3.8 ns and repetition frequency of 80 kHz, with peak power of ∼30 kW, pulse energy of 110 µJ and nearly transform-limited linewidth of < 283.8 MHz respectively. The polarization extinction ratio is > 16 dB and near-diffraction-limited beam quality with M² factor of 1.2 is maintained at the maximal output power. Moreover, the discussion on the optimization of the system for further power scaling is carried out based a nonlinear dynamic model that is capable of simultaneously evaluating the time-domain and frequency-domain evolution properties of the narrow-linewidth linearly-polarized pulsed laser, and meaningful conclusion is obtained.

Tapered Yb-doped fiber enabled monolithic high-power linearly polarized single-frequency laser

The all-fiber high-power linearly polarized single-frequency fiber laser based on the polarization-maintaining tapered Yb-doped fiber (T-YDF) is systematically studied. As a result, a 300 W-level stable output with linear polarization and nearly diffraction-limited beam quality is demonstrated. In particular, the overall properties of the transverse mode instability (MI) effect in such a single-frequency laser system are discussed in detail for the first time, to the best of our knowledge, including temporal, frequency, polarization, and spatial domains. Furthermore, the beam pointing error taking the MI effect into account is investigated. Theoretical analyses covering both stimulated Brillouin scattering and the MI effects reveal the great potential of the T-YDF for further power scaling as well.

550 W single frequency fiber amplifiers emitting at 1030 nm based on a tapered Yb-doped fiber

In this paper, we report a high power single frequency 1030 nm fiber laser with near-diffraction-limited beam quality based on a polarization-maintaining tapered Yb-doped fiber (T-YDF). The T-YDF has advantages of effectively suppressing stimulated Brillouin scattering (SBS) while maintaining good beam quality. As a result, a record output power of 379 W single frequency, linearly polarized, nearly single-mode fiber amplifier operating at 1030 nm is demonstrated. The polarization extinction ratio is as high as 16.3 dB, and the M² is measured to be 1.12. Further, the dependence of the thermal-induced mode instability (TMI) threshold on the polarization state of an input signal laser is investigated for the first time. By changing the polarization state of the injected seed laser, the output power can increase to 550 W while the beam quality can be maintained well (M²=1.47). The slope efficiency of the whole amplifier is about 80%. No sign of SBS appears even at the highest output power and the further brightness scaling of both situations is limited by the TMI effect. To the best of our knowledge, this result is the highest output power of all-fiberized single frequency fiber amplifiers.

Ultra-low intensity noise, all fiber 365 W linearly polarized single frequency laser at 1064 nm

We demonstrate a robust linearly polarized 365 W, very low amplitude noise, single frequency master oscillator power amplifier at 1064 nm. Power scaling was done through a custom large mode area fiber with a mode field diameter of 30 µm. No evidence of stimulated Brillouin scattering or modal instabilities are observed. The relative intensity noise is reduced down to -160 dBc/Hz between 2 kHz and 10 kHz via a wide band servo loop (1 MHz bandwidth). We achieve 350 W of isolated power, with a power stability < 0.7% RMS over 1100 hours of continuous operation and a near diffraction limited beam (M² < 1.1).

High power, single-frequency, monolithic fiber amplifier for the next generation of gravitational wave detectors

Low noise, high power single-frequency lasers and amplifiers are key components of interferometric gravitational wave detectors. One way to increase the detector sensitivity is to increase the power injected into the interferometers. We developed a fiber amplifier engineering prototype with a pump power limited output power of 200 W at 1064 nm. No signs of stimulated Brillouin scattering are observed at 200 W. At the maximum output power the polarization extinction ratio is above 19 dB and the fractional power in the fundamental transverse mode (TEM ₀₀) was measured to be 94.8 %. In addition, measurements of the frequency noise, relative power noise, and relative pointing noise were performed and demonstrate excellent low noise properties over the entire output power slope. In the context of single-frequency fiber amplifiers, the measured relative pointing noise below 100 Hz and the higher order mode content is, to the best of our knowledge, at 200 W the lowest ever measured. A long-term test of more than 695 h demonstrated stable operation without beam quality degradation. It is also the longest single-frequency fiber amplifier operation at 200 W ever reported.

Real-time investigation of CAP transceivers with hybrid digital equalization for visible light communication

In a practical light emitted diodes (LEDs)-based visible light communication (VLC) system, high-speed transmission is generally limited by the LED bandwidth. To address the bandwidth limitation, a hybrid digital linear and decision-feedback equalization (DFE) is investigated to improve the transmission performance (or spectral efficiency) in the carrier-less amplitude phase modulation (CAP)-based VLC systems. A real-time CAP-VLC transceiver with the hybrid digital equalization is designed, based on which 200 Mb/s transmission is successfully demonstrated over a 15 m VLC link with the commercial red LEDs (bandwidth: 6.5 MHz). In the real-time CAP-VLC system, the baseline wander (BLW) is observed, due to the removal of the low-frequency components with a direct current (DC) block. The BLW effect can be mitigated by increasing the roll-off factor. However, this roll-off factor affects the equalization performance, due to an increased loss in the signal spectrum beyond the system bandwidth. Optimization of the roll-off factor and filter length is required. Experimental results show that, with the optimized real-time transceiver design, the hybrid Wiener/recursive least squares (RLS) and DFE significantly improves the error vector magnitude (EVM) performance compared with the DFE. In addition, the digital signal processing (DSP) complexity is discussed.

Large dynamic range autorefraction with a low-cost diffuser wavefront sensor

Wavefront sensing with a thin diffuser has emerged as a potential low-cost alternative to a lenslet array for aberrometry. Here we show that displacement of caustic patterns can be tracked for estimating wavefront gradient in a diffuser wavefront sensor (DWFS), enabling large dynamic-range wavefront measurements with sufficient accuracy for eyeglass prescription measurements. We compare the dynamic range, repeatability, precision, and number of resolvable prescriptions of a DWFS to a Shack-Hartmann wavefront sensor (SHWFS) for autorefraction measurement. We induce spherical and cylindrical errors in a model eye and use a multi-level Demon's non-rigid registration algorithm to estimate caustic displacements relative to an emmetropic model eye. When compared to spherical error measurements with the SHWFS using a laser diode with a laser speckle reducer, the DWFS demonstrates a ∼5-fold improvement in dynamic range (-4.0 to +4.5 D vs. -22.0 to +19.5 D) with less than half the reduction in resolution (0.072 vs. 0.116 D), enabling a ∼3-fold increase in the number of resolvable prescriptions (118 vs. 358). In addition to being lower-cost, the unique, non-periodic nature of the caustic pattern formed by a diffuser enables a larger dynamic range of aberration measurements compared to a lenslet array.

Pump wavelength dependence of ASE and SBS in single-frequency EYDFAs

In this Letter, the pump wavelength dependence of the amplified spontaneous emission (ASE) and the threshold of stimulated Brillouin scattering (SBS) in a typical single-frequency continuous wave Er³⁺:Yb³⁺-codoped fiber amplifier is investigated numerically. The Er³⁺:Yb³⁺ system is modeled as coupled two- and three-level systems, linked by a Förster resonance energy transfer process and described by the corresponding rate equations. The evolution of the pump and signal power along the fiber is modeled by differential equations, taking into account the steady-state population densities. Since the absorption at 976 nm can exceed the Yb³⁺-to-Er³⁺ energy transfer rate in high-power operation, unsaturated gain at around 1.0 μm can generate excessive ASE. Off-peak pumping with commercially available pump diodes at 915 or 940 nm spatially distributes the energy over a longer distance. For the studied amplifier configuration, energy transfer bottlenecking is prevented without the onset of SBS.

Ultralong π-phase shift fiber Bragg grating empowered single-longitudinal mode DFB phosphate fiber laser with low-threshold and high-efficiency

Phosphate glass fiber is one of the candidates for building compact fiber lasers because of its capability of high-concentration of rare-earth ions doping in fiber core. Nevertheless, it is challenging for the integration of UV-written intra-core fiber Bragg gratings into the fiber laser cavity due to the low photosensitivity of phosphate glass fiber. The research presented in this paper will focus on demonstration of UV-written Bragg gratings in phosphate glass fiber and its application in direct-written short monolithic single-frequency fiber lasers. A 5-cm-long strong π-phase shift Bragg grating structure is direct-inscribed into the Er/Yb co-doped gain fiber using an excimer laser. The fiber laser device emits output power of 10.44 mW with a slope efficiency of 21.5% and the threshold power is about 42.8 mW. Single-longitudinal mode operation is validated by radio frequency spectrum measurement. Moreover, the output spectrum at the highest power shows an excellent optical signal to noise ratio of about 70 dB. These results, to the best of our knowledge, show the lowest power threshold and highest efficiency among the reports that using the same structure to achieve single-longitudinal mode laser output.

25 W single-frequency, low noise fiber MOPA at 1120 nm

This Letter reports on the development of a 25 W single-frequency, all-fiber master oscillator power amplifier (MOPA) operating at 1120 nm. By heating the gain fiber at 75°C, an output power of 25.3 W is achieved with an optical-to-optical efficiency of 53.5%. The output shows no sign of stimulated Brillouin scattering and the signal to amplified spontaneous emission ratio is close to 40 dB. A M² value of 1.15 and a polarization extinction ratio of 17 dB are measured. The relative intensity noise of the output is also characterized, reaching -155  dBc/Hz at 10 MHz at the maximum output power. The study of the noise dynamics highlights, for the first time to the best of our knowledge, an unpredicted behavior due to the strong backward amplified spontaneous emission.

Ultra-compact waveguide crossing for a mode-division multiplexing optical network

We propose and experimentally demonstrate an ultra-compact multimode waveguide crossing that can process two modes simultaneously. The symmetric Y-junction is introduced to split the high-order modes into fundamental ones, easing the subsequent processing. The footprint of the proposed crossing is as compact as 21  μm×21  μm. The measured results show an insertion loss of ∼1.82  dB for the TE₀ mode and ∼0.46  dB for the TE₁ mode at 1550 nm, as well as a crosstalk of <-18  dB from 1510 to 1600 nm.

High-power and near-shot-noise-limited intensity noise all-fiber single-frequency 1.5 μm MOPA laser

An all-fiber high-power and broad-frequency-band near-shot-noise-limited kHz-linewidth (Δν ~1.7 kHz) single-frequency master-oscillator power amplifier (MOPA) laser at 1.5 μm is demonstrated. To significantly suppress the intensity noise of seed laser and mitigate the detrimental effects of amplified spontaneous emission and stimulated Brillouin scattering in fiber amplifiers, more than 23 W of a stable low noise single-frequency laser output is achieved with a relative intensity noise of < -150 dB/Hz @0.5 mW (near to the shot-noise limit: -152.9 dB/Hz) in the frequency band from 0.1 to 50 MHz. It is believed that the achieved laser performance of ultra-low intensity noise and high-power output make the laser source become a promising candidate in further applications, such as cold atom optical lattice, quantum key distribution, and gravitational wave detection.