Observation of spectral mode splitting in a pump-enhanced ring cavity for mid-infrared generation

Technical Summary: Observation of Spectral Mode Splitting in a Pump-Enhanced Ring Cavity for Mid-Infrared Generation

Problem Statement
Mid-infrared (MIR) laser sources are critical for applications ranging from free-space optical communication to trace gas detection. While nonlinear frequency conversion, specifically difference frequency generation (DFG), offers a versatile route to generate high-power MIR light at arbitrary wavelengths, achieving high efficiency often requires optical cavities to enhance the pump or signal fields. Periodically poled lithium niobate (PPLN) crystals are commonly used for quasi-phase-matching in these systems. However, the internal structure of PPLN crystals, specifically the domain walls, induces fractional Bragg reflections. The authors investigate how these minute internal reflections affect the performance of pump-enhanced DFG (PE DFG) systems, comparing ring cavities (typically designed for unidirectional running-wave operation) against linear cavities (inherently bidirectional standing-wave operation).

Methodology
The study employed a comparative experimental and theoretical approach using two distinct cavity configurations:

1. Experimental Setup: A pump-enhanced DFG system was constructed using a 1064 nm pump (from a Yb-doped fiber laser) and a 1550 nm signal (from an Er-doped fiber laser). Both sources were amplified to watt-level powers and combined. The system utilized a 50-mm long, 5% MgO-doped PPLN crystal as the nonlinear medium.
2. Cavity Configurations: Two cavity geometries were engineered to maintain identical spatial mode profiles (beam waist radii) to ensure a fair comparison:
   • A ring cavity (bow-tie configuration) designed for unidirectional propagation.
   • A linear cavity designed for bidirectional standing-wave propagation.
   • Both cavities incorporated four mirrors, with one mirror mounted on a piezoelectric transducer (PZT) for active stabilization via dither-locking.
3. Characterization: The researchers recorded output signals from various ports while varying the cavity length to observe spectral responses. They measured the enhancement factors and the resulting MIR power output for both configurations.
4. Theoretical Modeling: Numerical simulations were performed using FINESSE, a frequency-domain cavity simulation software. The models accounted for the intrinsic loss of the PPLN crystal (assumed at 2%) and the fractional reflection induced by the crystal's Bragg grating effect.

Key Contributions and Findings

• Observation of Mode Splitting in Ring Cavities: Contrary to the expectation that ring cavities support only unidirectional running waves, the study observed pronounced spectral mode splitting even with extremely tiny internal reflections (as low as 0.0003 or 0.03%). The Bragg reflection from the PPLN crystal's domain walls broke the unidirectional property, establishing comparable circulating fields in both forward and backward directions.
• Impact on Enhancement Factor: The transition to bidirectional operation in the ring cavity caused a dramatic reduction in the power enhancement factor. Theoretical and experimental results showed that a fractional reflection of just 0.01% could reduce the enhancement factor by 25%. As reflection increased, the enhancement factor dropped rapidly, converging to a low asymptotic value.
• Robustness of Linear Cavities: In contrast, the linear cavity configuration demonstrated immunity to these small internal reflections. Due to its intrinsic bidirectional nature, the linear cavity maintained stable intra-cavity power and did not exhibit mode splitting. It retained a high enhancement factor even with internal reflections up to 1%.
• Performance in MIR Generation: In the final PE DFG experiments:
  • The ring cavity achieved a slope efficiency of 2.32% (approx. 10x improvement over single-pass), limited by the mode splitting and reduced enhancement.
  • The linear cavity achieved a slope efficiency of 5.8% (approx. 20x improvement over single-pass).
  • With 1 W of pump and 1 W of signal, the linear cavity generated approximately 60 mW of mid-infrared light, a value three times higher than previously reported results using ring cavities.

Significance
The paper claims that these findings provide a critical insight into the engineering of enhancement cavities for frequency conversion using periodically poled nonlinear crystals. The study reveals that the "exotic feature" of concatenated optical cavities—specifically the sensitivity of running-wave ring cavities to minute internal reflections—can severely degrade performance. The authors conclude that for systems where non-negligible Bragg reflections exist (e.g., >0.1%), the linear cavity layout offers a more expedient and stable solution, enabling higher conversion efficiencies. This work serves as a practical guide for optimizing cavity-based frequency conversion systems, suggesting that linear cavities may be preferable to ring cavities in specific PPLN-based applications to avoid the detrimental effects of mode splitting.