Clint Zeringue

A theoretical study of transient stimulated Brillouin scattering in optical fibers seeded with phase-modulated light

Beam combining of phase-modulated kilowatt fiber amplifiers has generated considerable interest recently. We describe in the time domain how stimulated Brillouin scattering (SBS) is generated in an optical fiber under phase-modulated laser conditions, and we analyze different phase modulation techniques. The temporal and spatial evolutions of the acoustic phonon, laser, and Stokes fields are determined by solving the coupled three-wave interaction system. Numerical accuracy is verified through agreement with the analytical solution for the un-modulated case and through the standard photon conservation relation for counter-propagating optical fields. As a test for a modulated laser, a sinusoidal phase modulation is examined for a broad range of modulation amplitudes and frequencies. We show that, at high modulation frequencies, our simulations agree with the analytical results obtained from decomposing the optical power into its frequency components. At low modulation frequencies, there is a significant departure due to the appreciable cross talk among the laser and Stokes sidebands. We also examine SBS suppression for a white noise source and show significant departures for short fibers from analytically derived formulas. Finally, SBS suppression through the application of pseudo-random bit sequence modulation is examined for various patterns. It is shown that for a fiber length of 9 m the patterns at or near n=7 provide the best mitigation of SBS with suppression factors approaching 17 dB at a modulation frequency of 5 GHz.

Pump-limited, 203 W, single-frequency monolithic fiber amplifier based on laser gain competition

We present high power results of a Yb-doped fiber amplifier seeded with a combination of broad and single-frequency laser signals. This two-tone concept was used in conjunction with externally applied or intrinsically formed thermal gradients to demonstrate combined stimulated Brillouin scattering suppression in a copumped monolithic, polarization-maintaining (PM) fiber. Depending on the input parameters and the thermal gradient, the output power of the single-frequency signal ranged from 80 to 203 W with slope efficiencies from 70% to 80%. The 203 W amplifier was pump limited and is, to the best of our knowledge, the highest reported in the literature for monolithic, PM single-frequency fiber amplifiers.

Investigation of Nonlinear Effects in Multitone-Driven Narrow-Linewidth High-Power Amplifiers

In this paper, we investigate two approaches to multitone seeding of high-power ytterbium-doped amplifiers using a symbolic and numerical code that solves a two-point boundary problem. Optimization of amplifier action through wavelength separation and/or seed power ratios is considered in relation to the two most dominant nonlinear effects: stimulated Brillouin scattering (SBS) and four-wave mixing. One approach uses a large wavelength separation among the input signals, while the other approach entails that the wavelength separation is set to twice the Brillouin shift. Both techniques are shown to mitigate SBS effects, although for the latter case, four-wave mixing sidebands can carry a substantial amount of power.

Stimulated Brillouin scattering suppression through laser gain competition: scalability to high power

We demonstrate stimulated Brillouin scattering (SBS) suppression in a Yb-doped fiber amplifier by seeding with a combination of broad- and single-frequency laser beams that are separated sufficiently to suppress four-wave mixing and to allow for efficient laser gain competition between the two signals. In the experiment, a monolithic fiber configuration was used. With appropriate selection of seed power ratio, we were able to generate single-frequency 1064 nm light with a slope efficiency of 78% while simultaneously suppressing the backscattered Stokes light. We discuss scalability to high power wherein a large thermal gradient can be induced at the output end of the fiber via quantum defect heating, leading to an SBS suppression factor comparable to counterpumping.

Theoretical analysis of single-frequency Raman fiber amplifier system operating at 1178nm

We analyze the scalability of amplifying the output from a single-frequency diode laser operating at 1178 nm through the utilization of a core pumped Raman fiber amplifier. A detailed model that accounts for stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in relation to the fiber mode field diameter, length, seed power, and available pump power in both co-pumped and counter-pumped configurations is developed. The backward travelling Stokes light is initiated from both spontaneous Brillouin and spontaneous Raman processes. It is found that when fiber length is optimized, the amplifier output scales linearly with available pump power. Although higher amplifier efficiency is obtained with higher seed power, the output power diminishes. In order to mitigate the SBS process for further power scaling, we employ and optimize a multi-step temperature distribution. Finally, we consider the feasibility of generating the D(2a) and D(2b) lines in a sodium guide star beacon from a single Raman amplifier by examining four-wave mixing (FWM).

Gain-tailored SBS suppressing photonic crystal fibers for high power applications

We present experimental studies of PM Yb-doped photonic crystal fibers possessing acoustic and Yb-ion concentration tailoring. In the initial design, the concentration of dopants in two regions of the core were selected such that the corresponding Brillouin shifts were sufficiently separated to allow for further stimulated Brillouin scattering suppression through thermal effects. The Yb-ion concentration was maintained uniformly throughout the entire core. When this fiber was utilized in a counter-pumped amplifier configuration, ~500 W of single-frequency (kHz linewidth) output was obtained in a 10 m long fiber. Further power scaling with good beam quality beyond 500 W was limited by modal instabilities. A second fiber design was developed in which the Yb-ion concentration was modified to have preferential overlap with the fundamental mode as well as reduced pump absorption. The onset of the modal instabilities was sufficiently suppressed to allow for an output of 990 W with a nominal linewidth of 300 MHz and good beam quality.

Experimental and theoretical investigations of photonic crystal fiber amplifier with 260 W output

We report on a polarization-maintaining narrow-linewidth high power ytterbium-doped photonic crystal fiber amplifier with an output as high as 260 W and a slope efficiency of approximately 74%. Measurements of the beam quality yielded M2 values in the range of 1.2-1.3. The linewidth was determined at two different powers using an optical heterodyne detection technique and yielded values that were less than 10 KHz. Our maximum output power was pump limited and measurements of the reflected light indicated that we operated below the stimulated Brillouin scattering (SBS) threshold. Using a pump-probe technique, we estimated the Brillouin gain bandwidth to be approximately 68 MHz. In addition, the Brillouin gain spectrum revealed secondary peaks lying at the high-frequency side. In order to study the power limitations of our amplifier, we developed a detailed model that included a distributed noise source for the SBS process and a temperature gradient obtained via quantum defect heating. Our simulations indicated that for this particular fiber amplifier configuration an output power approaching 1 KW can be achieved. We also found that for forced air cooling the SBS threshold saturates regardless of the operating temperature of the polymer coating. Finally, we show that relatively small enhancement is obtained if a continuous transverse acoustic velocity gradient was implemented in conjunction with the thermal gradient. The latter conclusions drawn from our simulations also hold true for conventional fibers.

Experimental and theoretical studies of single frequency PCF amplifier with output of 400 W

We report on experimental and theoretical investigations of single frequency high power PCF amplifiers. A model describing the interplay among laser gain, thermals effects, and SBS was developed to study the power limitations of single frequency amplifiers in general, and PCF amplifiers in particular. A distributed noise term was used to initiate the SBS process with the Stokes light spanning multi-frequency channels. The use of thermal and acoustic gradients in conjunction was considered and indicated marginal improvement. In the set of experiments, slope efficiencies as high as 77% were obtained with a maximum output of 427 W. The linewidth was measured and yielded values that were less than 10 KHz. A pump-probe measurement of the Brillouin gain spectrum revealed secondary peaks lying at the highfrequency side. Measurements conducted on a novel PCF, specifically designed to utilize thermal and acoustic gradients in conjunction, showed the existence of two primary gain peaks.

SBS suppression through seeding with narrow-linewidth and broadband signals: experimental results

We present experimental verification of a novel technique to suppress stimulated Brillouin scattering (SBS) in single frequency fiber amplifiers. This technique relies on seeding with a combination of broadband and single frequency laser beams to allow for efficient laser gain competition between the two signals. In the experiment, a monolithic fiber configuration was used. Broadband 1045 nm light and single frequency 1064 nm light were coupled into an Yb-doped gain fiber. With appropriate selection of seed power ratio, we were able to generate an output signal predominantly comprised of 1064 nm light while simultaneously suppressing the back-scattered Stokes light. The slope efficiency for the two-tone amplifier was approximately 78%; slightly below that of a single-tone amplifier. The SBS threshold for the former, on the other hand, was appreciably higher than that of the latter which is in excellent agreement with the theory. In preliminary implementation of this technique at high power, we generated close to 100 W without encountering the SBS threshold. Finally, we show numerically that due to a favorable thermal gradient much higher powers can be obtained.

Suppression of stimulated Brillouin scattering in optical fibers through phase-modulation: a time dependent model

We describe a time-dependent model that describes the evolution of stimulated Brillouin scattering (SBS) in fibers under phase-modulated pump conditions. In order to accurately model fast modulations, the triply-coupled system of differential equations describing the interaction of SBS through optical and phonon fields is solved numerically. SBS is initiated from noise by using a Langevin term. We initially consider single-frequency sinusoidal modulations as a function of modulation amplitude and frequency. We then investigate the effects of SBS mitigation when a single-frequency seed is phase modulated with a broad-band white-noise source (WNS).