Widely tunable mid-infrared fiber-feedback optical parametric oscillator

Technical Summary: Widely Tunable Mid-Infrared Fiber-Feedback Optical Parametric Oscillator

Problem Statement
Synchronously pumped optical parametric oscillators (OPOs) are recognized as versatile platforms for generating ultrafast mid-infrared (MIR) pulses beyond the reach of conventional mode-locked lasers. However, traditional OPOs based on bulk crystals suffer from complex optical alignment requirements and large physical footprints. While fiber-feedback OPOs (FOPOs) have been developed to improve compactness and robustness, previous iterations often required high pump intensities to overcome intra-cavity losses and reach the parametric gain threshold, particularly at high repetition rates. Furthermore, spectral tuning in existing FOPOs typically relies on altering the phase-matching conditions of the nonlinear crystal (e.g., via temperature control or mechanical translation of the poling period), which severely limits tuning speed due to thermal equilibrium times or mechanical inertia.

Methodology
The authors present two novel variants of FOPO systems designed to address these limitations. Both systems utilize a hybrid free-space/fiber architecture where a section of optical fiber replaces the free-space beam path for synchronous pumping, enhancing stability and reducing alignment complexity.

1. Dual-Gain FOPO: The first variant integrates an erbium-doped fiber (EDF) into the OPO cavity as an additional gain medium. The system is pumped by a 1032 nm ytterbium-doped fiber laser (YDFL). The generated near-infrared (NIR) signal is coupled into a polarization-maintaining (PM) fiber, passes through the EDF (pumped by a 976 nm laser diode), and is recirculated into a periodically-poled lithium niobate (PPLN) crystal. The cavity length is tuned via a delay line to achieve passive synchronization based on group-velocity dispersion.
2. Broadband Tunable FOPO: The second variant replaces the PPLN crystal with a chirped-poling lithium niobate (CPLN) crystal and removes the EDF. The CPLN crystal features a ramping poling period (25–32 µm) over 20 mm, providing a broad parametric gain window. Tuning is achieved solely by adjusting the cavity length (time delay), leveraging the group-velocity dispersion of the fiber to select specific wavelengths that satisfy the phase-matching condition at different points along the chirped crystal.

Key Contributions

• Threshold Reduction: By introducing an additional fiber gain medium (EDF), the authors demonstrate a significant reduction in the pump threshold. The system achieves stable MIR generation with a pump threshold as low as 200 mW, substantially lower than previous bulk-crystal-based FOPOs.
• Passive Synchronization and Stability: The use of polarization-maintaining fibers and the dual-gain mechanism enables robust operation without active stabilization. The system exhibits high tolerance to cavity-length variations, maintaining tight passive synchronization between pump, signal, and idler pulses.
• Rapid, Wide Spectral Tuning: The implementation of the CPLN crystal allows for wide spectral coverage without the need to change the crystal's temperature or mechanical position. Wavelength tuning is realized simply by adjusting the optical delay line, decoupling the tuning speed from thermal or mechanical constraints.

Results

• Performance of Dual-Gain FOPO:

  • Threshold: The pump threshold was reduced to approximately 0.2 W (with 83 mW LD power) compared to 0.4 W without the gain fiber and 0.6 W for a single-pass OPG configuration.
  • Efficiency: At a pump power of 0.77 W, the system generated 83.5 mW of MIR power, corresponding to a 32% energy conversion efficiency.
  • Stability: The FOPO demonstrated superior pulse stability compared to single-pass OPG. The pulse-to-pulse fluctuation was measured at 2.2% for the signal and 2.3% for the idler, compared to 37–39% for OPG at similar power levels.
  • Synchronization: The relative repetition rate fluctuation between pump and signal/idler pulses was measured at 98 mHz (standard deviation), indicating tight temporal synchronization over a one-hour period.
  • Tuning Range: The signal band was tuned from 1553 to 1586 nm, with corresponding idler wavelengths inferred from energy conservation. Pulse durations were measured at approximately 380 fs (signal) and 390 fs (idler).

• Performance of CPLN-Based FOPO:

  • Tuning Range: By removing the gain fiber and utilizing the CPLN crystal, the system achieved a signal tuning range of 1350–1768 nm and an idler (MIR) range of 2450–4450 nm.
  • Tuning Mechanism: The central wavelengths were tuned by varying the time delay over a range exceeding 60 ps. The output wavelengths agreed well with theoretical models based on the group-velocity dispersion and the local poling period.
  • Speed Potential: The authors note that the use of high-speed optical delay lines could facilitate spectral sweeping rates up to 31,000 nm/s, enabling real-time applications.

Significance and Claims
The paper claims that the presented FOPO sources offer a combination of high compactness, robust operation, and wide tunability, making them attractive for subsequent applications in infrared photonics, biomedical examination, and molecular spectroscopy. Specifically, the authors highlight that the dual-gain configuration lowers the barrier for accessing MIR pulsed light sources by reducing pump power requirements. Furthermore, the CPLN-based system provides a method for agile wavelength sweeping without the slow thermal or mechanical tuning mechanisms typical of previous FOPOs. The authors suggest that while current pulse durations are in the sub-picosecond regime, they can be further optimized to the femtosecond level with intra-cavity dispersion management. They conclude that these paradigms could stimulate further developments in MIR sensing, material inspection, and spectroscopy, potentially leading to high-integration systems using emerging nonlinear platforms like waveguides and microring resonators.