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

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.

Over 250 W low noise core-pumped single-frequency all-fiber amplifier

A high-power linearly-polarized all-fiber single-frequency amplifier at 1 µm based on tandem core-pumping is demonstrated by using a large-mode-area Ytterbium-doped fiber with a core diameter of 20 µm, which nicely balances the stimulated Brillouin scattering effect, thermal load, and output beam quality. A maximum output power of more than 250 W with a corresponding slope efficiency of >85% is achieved at the operating wavelength of 1064 nm without being constrained by the saturation and nonlinear effects. Meanwhile, a comparable amplification performance is realized with a lower injection signal power of the wavelength near the peak gain of the Yb-doped fiber. The polarization extinction ratio and the M² factor of the amplifier are respectively measured to be >17 dB and 1.15 under the maximal output power. In addition, by virtue of the single-mode 1018 nm pump laser, the intensity noise of the amplifier under maximal output power is measured to be comparable to that of the single-frequency seed laser at frequencies higher than 2 kHz, except for the emergence of parasitic peaks that can be eliminated by optimizing the driving electronics of the pump lasers, while the deterioration of the amplification process to the frequency noise and linewidth of the laser is negligible. To the best of our knowledge, this is the highest output power of a single-frequency all-fiber amplifier based on the core-pumping scheme.

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.

Single-polarization single-frequency Brillouin fiber laser that emits almost 5 W of power at 1 µm

We demonstrate a high-power single-polarization single-frequency 1064 nm Brillouin fiber laser (BFL) that is constructed with polarization-maintaining germanium-doped fiber with a core/cladding diameter of 20/400 µm. A maximum output power of 4.9 W is achieved with a slope efficiency of 68% and an optical signal-to-noise ratio of 65 dB. To the best of our knowledge, this is the highest power output from a single-frequency fiber laser. The polarization extinction ratio is over 18.7 dB and the BFL output presents a good transverse mode. The BFL shows a significant reduction (10-15 dB) in both the relative intensity noise and frequency noise of the pump source, while the estimated linewidth is 170 kHz with a measurement time of 2 ms at the maximum output power. It is believed that the high power output in combination with the decreased relative intensity and frequency noise renders the proposed BFL an important candidate for applications in optical sensing and high-purity microwave signal synthesis.

Ultralow-intensity noise, 10 W all-fiber single-frequency tunable laser system around 1550 nm

We report herein on the development of a linearly polarized, single-frequency tunable laser system producing more than 10 W in the 1550 nm range, using a two-stage erbium/ytterbium co-doped fiber-based master oscillator power amplifier (MOPA) architecture. The all-fiber MOPA provides an ultralow intensity noise of -160dBc/Hz beyond 200 kHz between 1533 and 1571 nm (Δλ=38nm) at full output power and a minimum optical signal to noise ratio of 38 dB. A good stability is obtained over 4 h at maximum power for several wavelengths with peak-to-peak fluctuation less than 3% and rms below 0.5%.

Low-threshold 1150 nm single-polarization single-frequency Yb-doped DFB fiber laser

We demonstrate a stable single-polarization single-frequency distributed feedback Bragg (DFB) fiber laser at 1150 nm based on a 5 cm long Yb-doped fiber which, to the best of our knowledge, is the first demonstration of a Yb-doped fiber-based single-frequency laser with a wavelength longer than 1120 nm. The threshold is as low as 10 mW. The measured maximum output power is 10.6 mW, and the spectrum at the highest power shows an excellent optical signal-to-noise ratio of about 70 dB, considering the amplified spontaneous emission in a short wavelength. The polarization extinction ratio is 25 dB, and the spectral linewidth is 20 kHz. This fiber laser is suitable for seeding high-power 1150 nm narrow-linewidth laser amplifiers, which can be used as high brightness pump sources for rare-earth-doped fiber lasers and Raman fiber lasers, or to generate visible radiation in the yellow spectral range, facilitating medical and astronomic applications.

Low noise 400 W coherently combined single frequency laser beam for next generation gravitational wave detectors

Design studies for the next generation of interferometric gravitational wave detectors propose the use of low-noise single-frequency high power laser sources at 1064 nm. Fiber amplifiers are a promising design option because of their high output power and excellent optical beam properties. We performed filled-aperture coherent beam combining with independently amplified beams from two low-noise high-power single-frequency fiber amplifiers to further scale the available optical power. An optical power of approximately 400 W with a combining efficiency of more than 93% was achieved. The combined beam contained 370 W of linearly polarized TEM₀₀-mode and was characterized with respect to the application requirements of low relative power noise, relative beam pointing noise, and frequency noise. The noise performance of the combined beam is comparable to the single amplifier noise. This represents, to our knowledge, the highest measured power in the TEM₀₀-mode of single frequency signals that fulfills the low noise requirements of gravitational wave detectors.

Robust 17 W single-pass second-harmonic-generation at 532 nm and relative-intensity-noise investigation

We demonstrate a 17 W single-frequency, low-intensity-noise green source at 532 nm, by single-pass second-harmonic generation of a 50 W continuous-wave fiber laser in a 30 mm MgO-doped periodically-poled stoichiometric lithium tantalate crystal. The maximum conversion efficiency is about 37%. A nearly Gaussian beam (M²<1.15 at 15 W) and low wavefront distortion are obtained. The system shows stable behavior over 100 h of uninterrupted operation. The evolution of the relative-intensity-noise transfer from the fundamental to the second harmonic is theoretically and experimentally investigated with high resolution.

Performance study of a high-power single-frequency fiber amplifier architecture for gravitational wave detectors

The next generation of interferometric gravitational wave detectors will use low-noise single-frequency laser sources at 1064 nm. Fiber amplifiers are a promising design option because of high efficiency, compact design, and superior optical beam properties compared to the current generation of laser sources for gravitational wave detectors. We developed a reliable 200 W single-frequency fiber amplifier architecture to meet the application requirements regarding relative power noise, relative pointing noise, frequency noise, linear polarization, and beam quality. We characterized several of these amplifiers and discuss performance variations resulting from manufacturing tolerances and variations in amplifier architecture. This study serves as a baseline for further power scaling via e.g., coherent beam combining experiments.

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.