Leanne J. Henry

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.

Enhancement of output power from narrow linewidth amplifiers via two-tone effect--high power experimental results.

Two-tone 1064 nm fiber amplifiers having both cold (16°C) and pump induced temperature zones co-seeded with narrow linewidth 1064 nm and broad linewidth 1040 nm photons have been shown to have a power enhancement factor between 1.6 and 1.8 relative to the optimum single-tone 1064 nm amplifier while maintaining an efficiency of 65% or greater. The output power and efficiency of 1064 nm narrow linewidth two-tone amplifiers is dependent on the length of the gain fiber, the narrow to broad linewidth seed ratio, the wavelength of the broad linewidth seed and the temperature of the gain fiber.

Thermal effects in narrow linewidth single and two tone fiber lasers

Significant effects from heating occur in both single and two tone fiber amplifiers. Single tone 1064 nm amplifiers have highest efficiency when the external environment surrounding the gain fiber is cold while 1064 nm two tone amplifiers co-seeded with broadband 1040 nm have maximum efficiency when the gain fiber is hot. It is shown experimentally that changes in the temperature of the core of the gain fiber have dramatic effects on the 1064 nm/1040 nm power distribution in the output of two tone amplifiers. This has been attributed to temperature dependence of the absorption and emission cross-sections at the wavelengths of interest.

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.

Investigation of the impact of fiber Bragg grating bandwidth on the efficiency of a Raman resonator

Significant spectral power leakage was found to occur around the high reflectivity fiber Bragg gratings (FBGs) defining a 1121 nm Raman resonator cavity comprised of PM 10/125 germanosilicate fiber. This cavity was part of a Raman system pumped with broad linewidth 1069 nm and seeded with narrow linewidth 1178 nm. The 1069 nm upon entering the resonator cavity was Raman converted to 1121 nm which then amplified the 1178 nm as it passed through the cavity. Spectral leakage of 1121 nm light from the resonator cavity resulted in sub-optimal amplification of 1178 nm which forced usage of longer resonator cavities having a decreased threshold for Stimulated Brillouin Scattering. Upon study of 1121 nm linewidth broadening as a function of resonator length for cavities employing 3 nm FBGs, differences in the percentage of 1121 nm power spectrally leaking past the output FBG as a function of the 1121 nm intracavity power propagating in the forward direction are not experimentally discernible for resonator cavities longer than 40 m. But, for cavity’s shorter than 40 m, the percentage of 1121 nm power spectrally leaking past the output FBG decreased significantly for similar 1121 nm intracavity power levels. For all cavity lengths, a nearly linear relationship exists between percent 1121 nm power leakage and intracavity power levels. Also, cavities employing broader bandwidth FBGs experience less 1121 nm power leakage for similar 1121 nm intracavity power levels. Finally, modeling predictions of Raman system performance are greatly improved upon usage of experimentally derived effective FBG reflectivities.

1121 nm resonator properties and impact on the design of a 1178 nm sodium guidestar laser

A dual seeded (pump at 1069 nm and second Stokes at 1178 nm) Raman laser system involving amplification of the second Stokes in a Raman resonator having high reflector Bragg gratings tuned to the first Stokes at 1121 nm is proposed. This system has the potential of generating 50 W of output power at 1178 nm. Because the laser is seeded with the desired output wavelength (second Stokes), outputs having narrow or broad linewidths can be achieved. Highly efficient Raman conversion is achieved in a short Raman fiber (< 20 m) due to high 1121 nm circulating power levels in the Raman resonator. Stimulated Brillouin Scattering, the 1121 nm circulating power level, and the effects of modal instability are constraints on the design of a narrow linewidth system. Length of the resonator cavity, power level of the 1069 nm pump, and power level of the 1178 nm seed are parameters that can be adjusted to optimize the design. Usage of germanosilicate fiber improves the performance of the system. Finally, because of the low power level of available 1178 nm seeds, a two stage system (low and high power stages) is necessary to achieve 50 W of 1178 nm output power.

Generation of 50W of 1178 nm via Amplification of the Second Stokes

A novel 50 W narrow linewidth 1178 nm Raman laser involving amplification of the second Stokes in a 1121 nm resonance cavity is reported. 1121 nm cavity power along with Stimulated Brillouin Scattering are important design considerations.

A novel method of increasing the efficiency of 1064 nm two tone amplifiers through heating of the gain fiber

The efficiency of two tone fiber amplifiers can be changed rather significantly by altering the temperature of the external environment surrounding the gain fiber. It is shown experimentally that changes in the temperature of the core of the gain fiber has dramatic effects on the 1064 nm / 1040 nm power distribution in the output of narrow linewidth 1064 nm two tone amplifiers with a greater percentage of the output being 1064 nm at higher core temperatures. By increasing the environmental temperature of the gain fiber from 20 to 80°C, the efficiency of a 1064 nm two tone amplifier can be increased up to 40% with the greatest increases seen in amplifiers seeded hardest in 1040 nm, i.e., having the smallest 1064 nm / 1040 nm seed ratios. This has been attributed to temperature dependence of the absorption and emission cross-sections at the wavelengths of interest. Finally, the temperature of the gain fiber can be used as a design tool to enable a higher efficiency 1064 nm two tone amplifier.

Investigation of an ultra large mode area power amplifier stage for a pulsed 1550-nm laser system

An Ultra Large Mode Area (ULMA) erbium doped optical fiber having a 52 micron core was investigated as the high power stage of a 1550 nm pulsed fiber laser system. The ULMA fiber was seeded with 1550 nm light from a 4 stage all fiber pulsed laser system with pulse energies of 11.35 and 6.25 μJ at pulse widths of 300 ns and repetition rates of 2 and 10 kHz, respectively. The ULMA fiber stage was counterpumped continuous wave in-band with a 1480 nm Raman laser. Maximum pulse energies (and amplifications) of 360 (15 dB) and 130 μJ (13.2 dB) at 2 and 10 kHz, respectively, were found when pumped with approximately 50 W of 1480 nm pump. More than 90% of the power at both repetition rates was found to reside in the pulse which indicated lower levels of amplified spontaneous emission. A lower conversion efficiency of approximately 53% -- which was significantly less than the theoretical maximum of 95% -- was estimated for the ULMA fiber amplifier. Possible causes are significant contamination on the order of 25% of the 1480 nm pump by lower Stokes orders, low coupling fractions of the 1480 nm pump into the core of the ULMA fiber, as well as pair induced quenching of Er due to clustering. Finally, creation of pump dumps at both the 1480 nm pump and 1550 nm signal ends by roughening the surface of the optical fiber enabled higher pump levels and pulse energies than would otherwise be the case.

Control of pulse format in high energy per pulse all-fiber erbium/ytterbium laser systems

A multi-stage linearly polarized (PM) (15 dB) pulsed fiber laser system at 1550 nm capable of operating at repetition rates between 3 and 20 kHz was investigated. A narrow linewidth seed source was linewidth broadened to approximately 20 GHz and pulses were created and shaped via an electro-optic modulator (EOM) in conjunction with a home built arbitrary waveform generator. As expected, a high repetition rate pulse train with a near diffraction limited beam quality (M2~1.12) was achieved. However, the ability to store energy was limited by the number of active ions within the erbium/ytterbium doped gain fiber within the various stages. As a result, the maximum energy per pulse achievable from the system was approximately 0.3 and 0.38 mJ for 300 ns and 1 μs pulses, respectively, at 3 kHz. Because the system was operated at high inversion, the erbium/ytterbium doped optical fiber preferred to lase at 1535 nm versus 1550 nm resulting in amplified spontaneous emission (ASE) both intra- and inter-pulse. For the lower power stages, the ASE was controllable via a EOM whose function was to block the energy between pulses as well as ASE filters whose purpose was to block spectral components outside of the 1550 nm passband. For the higher power stages, the pump diodes were pulsed to enable strategic placement of an inversion resulting in higher intrapulse energies as well as an improved spectrum of the signal. When optimized, this system will be used to seed higher power solid state amplifier stages.