Single-frequency Fiber Amplifiers Research Articles

Theory and experiment of transient two-wave mixing in Yb-doped single-frequency fiber amplifiers induced by frequency modulation

This paper presents a theoretical and experimental characterization of an instability phenomenon observed in single-frequency fiber amplifiers when the frequency of the seed laser is modulated. The instability manifests itself as fluctuating elastic back-reflections that occur only when the frequency is decreasing with time. The theory is a generalization of a coupled-mode model developed for a single-frequency fiber amplifier back-seeded with a constant frequency shift relative to the main signal. It can explain most observed features of the experiments in a qualitative and semi-quantitative way. Open questions and directions for further developments are also discussed.

Single frequency pulsed Er,Yb co-doped fiber amplifier with 2 mJ of output energy

High-energy single-frequency pulsed light with low noise is generated from a master oscillator power amplifier (MOPA) system at 1.55 μm eye-safe wavelength using an ultra large mode area Er-Yb gain fiber. At a pulse repetition rate of 2 kHz, the maximum pulse energy without stimulated Brillouin scattering (SBS) reaches 2.07 mJ with 375 ns pulse duration for a pump power of 56.4 W. The pulse energy fluctuation is approximately 0.72 % within 21.5 min. The signal noise ratio (SNR) from the single-frequency fiber amplifier, at the highest pulse energy, centered at 1551.1 nm, is approximately 30 dB. The spectral half width of the signal light is calculated to be 1.28 MHz, which is close to the transform limit.

Polarization-maintaining large-mode-area solid-core anti-resonant fiber for high-power fiber lasers

A polarization-maintaining large-mode-area solid-core anti-resonant fiber is investigated in this work with double-layer anti-resonant elements for improved capability on fundamental mode guidance and higher order mode suppression, and structural and material asymmetry on inner-layer cladding tubes for high birefringence. Mode coupling between fundamental core mode in one polarization direction and mode on two high-index inner cladding tubes is used to enhance the fiber birefringence under large core diameter. Eventually, an optimized design of polarization-maintaining large-mode-area solid-core anti-resonant fiber with the currently largest mode field area of 3026 μm2 and birefringence of 1.20 × 10−5 can be anticipated with the confinement losses for all the higher order modes exceeding 10 dB/m. Moreover, confinement loss below 0.1 dB/m can still be obtained for the fundamental core mode in both polarization direction even under the mode-coupling effect. Active polarization-maintaining anti-resonant fiber is analyzed and the refractive index of the central rare-earth-doped region is optimized for the first time for both well guided signal and pump laser. Large pump absorption coefficient can thus be anticipated from the multimode laser core-pumped active large-mode-area fiber, which facilitates short active fiber length needed for sufficient laser gain. The fiber proposed here offers great potential for the development of high-performance fiber laser sources including high-power single-frequency fiber amplifiers and high-peak-power ultrashort pulsed fiber amplifiers.

Phase noise of pumping in single-frequency fiber amplifier

Phase noise of pumping in single-frequency fiber amplifier

High-power single-frequency fiber amplifiers: progress and challenge [Invited

High-power single-frequency fiber amplifiers: progress and challenge [Invited

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 1µm 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 10Hz to 100kHz. 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.

Modeling of the thermal-lens induced mode coupling in fiber amplifiers

In this paper, a novel model for describing the thermal-lens induced mode coupling (TLMC) effect in the high power fiber amplifier is proposed firstly, to the best of our knowledge. Based on the rate equation and couple-mode equation, the model is built with both consideration of the thermal-lens induced mode field variation and the gain effect. With this model, the mode coupling behaviors with various pump schemes are numerically investigated and the impact of the TLMC is quantitatively discussed in a finite-length single-frequency fiber amplifier for paradigm. It is found that the higher-order mode (HOM) can be induced by the TLMC even with pure single-mode input signal and this effect is most severe with the co-pumped scheme while weakest with the counter-pumped scheme. It is also revealed that the TLMC will enhance the average HOM content and will keep predominant until the incident HOM is larger than a certain content. Such an enhancement of HOM content can play an important role as the seed triggering the transverse mode instability (TMI) and the pertinent study is helpful on investigating the relationship between the thermal-lens effect and the TMI.

Observation of two-wave mixing in a single-frequency fiber amplifier induced by frequency modulation.

We report on the observation of unstable two-wave mixing in a Yb-doped optical fiber amplifier induced by frequency modulation of a single-frequency laser. What is believed to be a reflection of the main signal experiences a gain much higher than that provided by the optical pumping and potentially limits power scaling under frequency modulation. We propose an explanation for the effect based on the dynamic population and refractive index gratings formed by the interference between the main signal and its slightly frequency-detuned reflection.

700 W single-frequency all-fiber amplifier at 1064 nm with kHz-level spectral linewidth

In this work, a single-frequency fiber amplifier with output power of 703 W was demonstrated at 1,064.4 nm in an all-fiber configuration. Cascaded Yb3+-doped fiber structure with different dopant concentration and hybrid 915/976 nm pump scheme were employed in power scaling stage to improve the gain saturation for higher transverse mode instability threshold. An overall optical efficiency of 67.5% was achieved at the maximum output power and the M2 was measured to be ∼1.4. A spectral linewidth of 2 kHz was obtained from the 703-W single-frequency laser. To the best of our knowledge, this is the first time that a single-frequency all-fiber amplifier with kHz-level spectral linewidth is achieved at such high output power.

Single-Frequency 336 W Spliceless All-Fiber Amplifier Based on a Chirally-Coupled-Core Fiber for the Next Generation of Gravitational Wave Detectors

Specialty fibers such as chirally-coupled-core fibers show a high potential for further power scaling of single-frequency fiber amplifiers. For the first time, we demonstrate a spliceless all-fiber amplifier, where all optical components are directly integrated in a single Yb<inline-formula><tex-math notation="LaTeX">$^{3+}$</tex-math></inline-formula>-doped 3C-fiber. Such a spliceless laser design enables a compact and robust architecture using specialty fibers, while maintaining excellent beam properties. At an output power of 336 W operating at 1064 nm, a fundamental mode content of 90.4&#x0025; at a polarization extinction ratio above 13 dB was measured without any impact of transverse mode instabilities or other parasitic effects. This work emphasizes the field of applications of 3C-fibers in high-power laser systems.

Influence of Pump Properties on the Tunability of 1.5 μm Low-Noise Single Frequency Fiber Amplifier

In this work, we investigate the impact of the pump configuration (wavelength and pump scheme i.e., backward forward) on the tuning range in a watt level low intensity noise single-frequency Erbium/Ytterbium doped fiber amplifier. Two different pump wavelengths, 976 nm and 940 nm, and three different pumping schemes were numerically and experimentally investigated. Tunability over 47 nm (1533 nm-1580 nm) with relative intensity noise level as low as −155 dB/Hz is achieved using an unconventional pumping scheme where the active fiber is pumped in both directions with two different pump wavelengths.

Quasi-CW Pumping of a Single-Frequency Fiber Amplifier for Efficient SHG in PPLN Crystals with Reduced Thermal Load

Single-frequency lasers are essential for high-resolution spectroscopy and sensing applications as they combine high-frequency stability with low noise and high output power stability. For many of these applications, there is increasing interest in power-scaling single-frequency sources, both in the near-infrared and visible spectral range. We report the second-harmonic generation of 670 µJ at 532 nm of a single-frequency fiber amplifier signal operating in the quasi-continuous-wave mode in a 10-mm periodically poled Mg-doped lithium niobate (MgO:PPLN) crystal, while increasing compactness. To the best of our knowledge, this is the highest pulse energy generated in this crystal, which may find applications in the visible and UV such as remote Raman spectroscopy.

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.

High-power narrow-linewidth fiber laser technology

In this paper, the research progress of single-frequency fiber oscillators are introduced in the terms of wavelength expansion, the development of single frequency fiber amplifiers are introduced in the terms of power scaling. Besides, the research achievements of 1 μm-band high-power narrow-linewidth fiber laser are summarized based on the techniques of generating narrow-linewidth seed sources. Then the development trend and main challenges of high-power single-frequency and narrow-linewidth fiber laser are analyzed. The key technologies of high-power narrow-linewidth fiber laser are summarized and discussed. Finally, applications in various fields based on the current development status of high-power narrow-linewidth fiber laser are introduced.

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.

10 W low-noise green laser generation by the single-pass frequency doubling of a single-frequency fiber amplifier

A high-power low-noise continuous-wave green laser at 532 nm is developed by the single-pass frequency doubling of a single-frequency 1064 nm Yb-doped fiber amplifier in a MgO-doped periodically-poled stoichiometric lithium tantalate crystal. More than 10 W is obtained with a conversion efficiency of 38.3%. The linewidth of the green laser is estimated at about 3.4 kHz according to the linewidth of 1.7 kHz of the fundamental laser. The relative intensity noise integrated from 100 Hz to 10 MHz is less than 0.03%. The laser is a promising candidate for applications in quantum simulation with ultracold atoms and the pumping of Ti:sapphire lasers etc.

18 W single-frequency 1550 nm Er:Yb co-doped fiber amplifier cladding-pumping at 1018 nm

A continuous wave (CW) 1550 nm single-frequency fiber amplifier based on a double-clad Er:Yb co-doped fiber, featuring off-peak cladding-pumping at 1018 nm, is demonstrated for the first time to our knowledge. Compared with the traditional single-mode Er:Yb co-doped fiber amplifiers (EYDFA) pumping at 9xx nm, for which the threshold of Yb-band amplified spontaneous emission (ASE) at 1.0μm region was several watts level, the Yb-band ASE threshold was enhanced to 17.0 watts. Experimentally, a homemade 1018 nm fiber laser was used as the pump source, which can provide an average power of ∼76 W. The 1550 nm signal power of 18.0 W was achieved, when the pump power was 71 W. This result indicates that off-peak pumping at 1018 nm is a promising scheme for achieving high-power single-frequency EYDFA at 1.5μm region.

Relative intensity noise comparison of fiber laser and amplified spontaneous emission sources

The performance of Raman fiber laser and amplifier largely depends on the temporal stability of the pump sources. Here the relative intensity noise of three different sources are experimentally investigated and compared, which are fiber laser oscillator, master oscillator power amplifier, and amplified spontaneous emission. The amplified spontaneous emission source exhibits the lowest intensity noise. The fiber laser oscillator and amplifier have inevitable relaxation oscillation and longitudinal mode beating, which results in higher intensity noise and frequency dependent features. The results firmly explain the recent demonstrations in the performance improvement with amplified spontaneous emission as pump sources and the beneficiation of backward pumping for single frequency fiber amplifier.

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.