High-power Single-frequency Fiber Research Articles

Ultralow-quantum-defect single-frequency fiber laser.

A single-frequency distributed-Bragg-reflector fiber laser at 980 nm with a quantum defect of less than 0.6% was developed with a 1.5-cm 12 wt% ytterbium-doped phosphate fiber pumped by a 974.5-nm laser diode. Linearly polarized single-longitude-mode laser with a polarization extinction ratio (PER) of nearly 30 dB and spectral linewidth of less than 1.8 kHz was obtained. A maximum output power of 275 mW was measured at a launched pump power of 620 mW. The performance of the single-frequency fiber laser pumped at 909 nm and 976 nm was also characterized. This research demonstrated an approach to high-power single-frequency fiber laser oscillators with mitigated thermal effects.

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.

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.

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.

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.

400-W single-mode single-frequency laser output from homemade tapered fiber

In recent years, the high-power single-frequency fiber lasers have developed rapidly, and they have been used in nonlinear frequency conversion and gravitational wave detection. The main factors limiting the output power of single-frequency fiber lasers are the nonlinear effect and transverse mode instability (TMI) effect. In general, large-core fibers can mitigate nonlinear effects while small-core fibers help to suppress the TMI effect. Owing to the core diameter varying in the longitudinal direction, tapered double clad fiber (T-DCF) is a promising solution to simultaneously suppress the nonlinearity and TMI effects. In the present study, we have fabricated a piece of 2.2-m-long Ytterbium-doped T-DCF. The core diameter and the cladding diameter of this fiber vary gradually from 30.3 μm to 49.3 μm and from 245 μm to 404 μm, respectively. Using this homemade fiber, we constructe an all-fiberized single-frequency master oscillator power amplifier system, which is pumped by laser diodes with a central wavelength of 976 nm. The seed of the system has a central wavelength of 1064 nm, and output power of 30 mW. The T-DCF is coiled on a piece of cooling plate, whose output end is cleaved at a 8° angle. The laser is output to free space and collimated by a free-space collimator. After the collimator, dichroic mirror is utilized to strip out the residual pump power for measuring power, spectrum, time-domain signal and beam quality. The output power increases linearly with the pumping power increasing. When the pumping power is 502 W, the output power reaches 400 W. And there is no stimulated Brillouin scattering (SBS) nor TMI under the power level. The corresponding slope efficiency is 81.7% while the <i>M</i><sup>2</sup> is measured to be 1.29, exhibiting the single-mode output characteristic of the system. When the output power is further increased to 418 W, the TMI effect is observed, which limits further the power scaling of the single-mode output. To the best of our knowledge, this is the highest output power of single-frequency fiber laser based on home-made gain fibers. The results indicate that T-DCFs can simultaneously suppress the nonlinearity and TMI, thus providing a useful reference for further power scaling of single-frequency fiber lasers. Higher output power is expected by optimizing the parameters of T-DCF and the structure of system.

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.

High-efficiency and high-power single-frequency fiber laser at 1.6 μm based on cascaded energy-transfer pumping

In this paper, a technique combining the cascaded energy-transfer pumping (CEP) method and master-oscillator power-amplifier (MOPA) configuration is proposed for power scaling of 1.6-μm-band single-frequency fiber lasers (SFFLs), where the Er 3 + ion has a limited gain. The CEP technique is fulfilled by coupling a primary signal light at 1.6 μm and a C-band auxiliary laser. The numerical model of the fiber amplifier with the CEP technique reveals that the energy transfer process involves the pump competition and the in-band particle transition between the signal and auxiliary lights. Moreover, for the signal emission, the population density in the upper level is enhanced, and the effective population inversion is achieved thanks to the CEP. A single-frequency MOPA laser at 1603 nm with an output power of 52.6 W and an improved slope efficiency of 30.4% is experimentally obtained through the CEP technique. Besides, a laser linewidth of 5.2 kHz and a signal-to-auxiliary laser ratio of 60.7 dB are obtained at the maximum output power. The proposed technique is anticipated to be promising for increasing the slope efficiency and power scaling for fiber lasers operating at L band.

On the Effect of Low Temperatures on the Maximum Output Power of a Coherent Erbium-Doped Fiber Amplifier

The influence of low temperatures on the performance of a high-power single-frequency fiber laser amplifier is evaluated with a numerical simulation. Cooling the fiber can allow to take advantage of both higher damping of the acoustic waves in the silica glass, and higher laser efficiency. We first report on the measurement of the stimulated Brillouin scattering (SBS) threshold in a silica fiber as a function of the temperature from 300~K down to 77~K. We then present the measurements of small-signal absorption and gain spectra of an erbium doped alumino-phosphosilicate fiber, at room temperature and liquid nitrogen temperature. Based on these data, we derive a numerical study of the combined effects of cooling on the SBS threshold and the amplifier efficiency, and conclude on the interest of this technique for SBS limited high power Er doped fiber amplifiers (EDFA). The temperature increase caused by the pump laser in the fiber core is also addressed.

Formation of ultracold molecules induced by a high-power single-frequency fiber laser

The influence of a high-power single-frequency fiber laser on the formation of ultracold 85Rb2 molecules is investigated as a function of its frequency (in the 1062–1070 nm range) in a magneto-optical trap. We find evidence for the formation of ground-state 85Rb2 molecules in low vibrational levels () with a maximal rate of 104 s−1, induced by short-range photoassociation by the fiber laser followed by spontaneous emission. When this laser is used to set up a dipole trap, we measure an atomic loss rate at a wavelength far from the PA resonances, only four times smaller than that observed at a PA resonance wavelength. This work may have important consequences for atom trapping using lasers around the conventional 1064 nm wavelength.

Limits of Stimulated Brillouin Scattering Suppression in Optical Fibers With Transverse Acoustic Waveguide Designs

A major approach investigated recently for stimulated Brillouin scattering (SBS) suppression in fiber amplifiers for high power single frequency fiber lasers is to explore designs of acoustic waveguide in optical fibers. This acoustic waveguide can be implemented to some extent independent of the optical waveguide by using a combination of dopants which modify the host glass by varying levels in acoustic and optical properties. Although this approach provides some SBS suppressions, the new analysis described in this work, considering the often omitted leaky acoustic modes, demonstrates its limit. A complex acoustic mode solver, reported in details elsewhere, was recently developed to find solutions for simultaneous longitudinal and shear acoustic wave equations which satisfy rigorous boundary conditions in an arbitrary circular acoustic waveguide. By taking advantage of this new tool, it is possible to find the leaky acoustic modes for acoustic waveguides designed for SBS suppressions. In these acoustic waveguides, the guided acoustic modes often have a reduced overlap with the optical modes by design and, sometimes, do not exist at all, while the leaky acoustic mode can have a significant stronger overlap with the optical modes and, consequently, can play a dominant role in the SBS gain spectrum. It is, therefore, critical to consider the leaky acoustic modes in these cases. Two prominent transverse acoustic waveguide designs proposed recently for SBS suppression in optical fibers are studied here. This work reports, for the first time, the limits of these designs for SBS suppression. With consideration of the leakage acoustic modes, some new insights are provided on the underlying physics as well as on how to optimize these designs for optimal SBS suppression.

高功率单频光纤放大器中温度分布对受激布里渊散射的影响

高功率单频光纤放大器中温度分布对受激布里渊散射的影响

Comprehensive Modeling of Single Frequency Fiber Amplifiers for Mitigating Stimulated Brillouin Scattering

A universal model capable of handling various operating conditions has been developed for designing of high power single frequency fiber amplifiers. The model analyzes the impacts of pump configurations, fiber lengths, and fiber temperatures on amplifier performance. It shows that counter-pumped amplifiers are capable of generating several times more output than co-pumped amplifiers. To fully take their advantages and deliver the laser output, a delivery fiber should be properly designed to avoid unwanted SBS which can significantly undermine the amplifier performance. On the other hand, for applications requiring delivery fiber at relative low power, the co-pumped amplifier can be an alternative since it can use the gain fiber as the delivery fiber and thus eliminate requiring an additional delivery fiber. The model also analyzes possible approaches for suppressing SBS, such as applying additional heating, the use of SBS suppressing fiber, and increasing core diameter. It shows that applying additional heating to a co-pumped amplifier can increase the output by a factor of 1.7. Finally, the model concludes that a single frequency amplifier can achieve multiple kilowatts output by carefully choosing amplifier design and utilizing SBS suppression technique.

Narrowband Pulsed THz Source Using Eyesafe Region Fiber Lasers and a Nonlinear Crystal

We report a high-power single-frequency fiber laser system in the eyesafe wavelength region that pumps a GaSe crystal through difference frequency generation, producing narrowband terahertz (THz). Two single-frequency continuous-wave fiber seed lasers are modulated, then amplified with a three-stage single-mode amplifier. The resulting pulses are transform-limited and high power, producing a peak THz power of 26.4 mW with narrow linewidth. This is significantly higher than earlier THz sources based on narrowband fiber lasers in the eyesafe region.

Suppression of stimulated Brillouin scattering in high-power single-frequency fiber amplifiers

The effect of temperature variation along a high-power fiber amplifier on the stimulated Brillouin scattering (SBS) threshold is considered theoretically. We show that for an end-pumped rare-earth-doped double-clad fiber the inhomogeneous distribution of temperature, which is caused by the absorption of pump radiation, may result in total suppression of SBS even for output powers well above 200 W.