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

Area of Science:

• Fiber optics
• Laser technology
• Nonlinear optics

Background:

• Single-frequency Raman fiber amplifiers (RFAs) are crucial for various applications.
• Power scaling of RFAs is often limited by nonlinear effects like stimulated Brillouin scattering (SBS).

Purpose of the Study:

• To investigate power scaling strategies for core-pumped, polarization-maintaining, single-frequency RFAs.
• To compare single-stage and two-stage RFA designs for improved output power and spectral stability.

Main Methods:

• Utilized commercial-off-the-shelf (COTS) single-mode fiber in a single-stage, counter-pumped RFA with a two-step thermal gradient to suppress SBS.
• Performed pump-probe characterization to determine the Brillouin gain spectrum (BGS) and Brillouin gain coefficient.
• Conducted fiber cutback studies to analyze signal output at the SBS threshold.
• Developed numerical simulations to predict power scaling potential with increased seed power.
• Constructed a two-stage RFA using acoustically tailored fiber with a lower Brillouin gain coefficient and higher seed power.

Main Results:

• Achieved 10 W output power at 1178 nm from a single-stage RFA using COTS fiber.
• Demonstrated a linear dependence of signal output at SBS threshold on pump power, aligning with theoretical predictions.
• Obtained 22 W of single-frequency 1178 nm output from a two-stage, counter-pumped RFA.
• Observed loss of single-frequency spectral bandwidth in a similar co-pumped two-stage RFA.

Conclusions:

• A two-stage RFA design, employing acoustically tailored fiber and higher seed power, significantly enhances output power compared to single-stage systems.
• Suppression of SBS through thermal gradients and careful fiber selection is critical for power scaling in RFAs.
• Counter-pumping configurations appear more suitable for maintaining single-frequency operation in two-stage RFAs.