Christopher Vergien

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.

Investigations of single-frequency Raman fiber amplifiers operating at 1178 nm

We report on core-pumped single-stage and two-stage polarization-maintaining single-frequency Raman fiber amplifiers (RFAs). For a counter-pumped single-stage RFA, commercial-off-the shelf (COTS) single-mode fiber was utilized to generate 10 W of output power at 1178 nm through the application of a two-step thermal gradient in order to suppress SBS. The relatively high output can be explained by the Brillouin gain spectrum (BGS) of the COTS fiber. A pump-probe characterization of the BGS of the fiber provided a Brillouin gain coefficient of 1.2 × 10(-11) m/W with a FWHM of 78 MHz for the gain bandwidth. A fiber cutback study was also conducted to investigate the signal output at SBS threshold as a function of pump power for optimal length. This study revealed a linear dependence, which is in agreement with the theoretical prediction. Furthermore, we present numerical simulations indicating that substantial power scaling can be achieved by seeding at a higher power. Consequently, we constructed a two-stage RFA in order to achieve seed powers at the 1 W level. By utilizing an acoustically tailored fiber possessing a lower Brillouin gain coefficient than the COTS fiber and by seeding at higher powers, 22 W of single-frequency 1178 nm output was obtained from a counter-pumped two-stage RFA. Finally, we show that the single-frequency spectral bandwidth could not be maintained when a similar co-pumped two-stage RFA was utilized.

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).

18 W single-stage single-frequency acoustically tailored Raman fiber amplifier

A single-mode polarization-maintaining fiber doped to increase the Raman gain while suppressing stimulated Brillouin scattering (SBS) was utilized in a single-stage counter-pumped Raman fiber amplifier. The SBS suppression was achieved through the acoustic tailoring of the core. A pump probe experiment was conducted to characterize the Brillouin gain and indicated the existence of multiple Brillouin peaks. When the amplifier was seeded with approximately 15 mW of 1178 nm light, 11.5 W of cw output power was obtained with a linewidth ≤2 MHz. The application of a thermal gradient to further mitigate the SBS process increased the output power to 18 W, thus providing a net amplifier gain >30 dB.

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.

Experimental and theoretical studies of phase modulation in Yb-doped fiber amplifiers

We present detailed studies of the effect of sinusoidal phase modulation on stimulated Brillouin scattering (SBS) in ytterbium-doped fiber amplifiers. Based on a time-dependent SBS model, SBS enhancement factor versus pump linewidth for different modulation depths ranging from 0 to π , and modulation frequencies ranging from 30 MHz to 500 MHz were analyzed. In addition, experimental validation of SBS suppression via sinusoidal phase modulation is presented with experimental results agreeing well with the model and simulations. Furthermore, narrow linewidth coherent beam combining (CBC) of 5 sinusoidal phase modulated lasers is demonstrated via LOCSET.

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.

Single-frequency Yb-doped photonic crystal fiber amplifier with 800W output power

A novel acoustic and gain tailored Yb-doped photonic crystal fiber is used to demonstrate over 800 W single-frequency output power with excellent beam quality at 1064 nm. The large mode area fiber core is composed of 7 individually doped segments arranged to create three distinct acoustic regions and preferential gain overlap with the fundamental optical mode. This design leads to suppression of both stimulated Brillouin scattering and modal instability. To the best of our knowledge, the output power represents the highest power ever reported from a near diffractionlimited single-frequency fiber laser. Furthermore, we show that by using a broadband seed, 1.22 kW of output power is obtained without the onset of the modal instability.