The mid-infrared (MIR) band has a wide range of applications in many fields, including security, medical, industrial, food, and environmental monitoring, due to its unique properties. Currently, quantum cascade lasers are an important way to acquire coherent light in the mid-infrared, with excellent performance in terms of wide tuning range and high power output, but they usually operate in continuous mode, and the realization of ultrashort pulse preparation is still challenging. In recent years, fluoride fiber lasers have made great progress in the generation of mode-locked pulses in the mid-infrared, and the mid-infrared output at different wavelengths can be obtained by selecting different gain media, but the problems of limited gain bandwidth and material stability need to be solved. In addition, although the mid-infrared solid-state lasers based on sulfur crystals have broad emission bands, it is still difficult to fully cover the 2-5 µm mid-infrared spectral band with a single gain crystal.
Comparatively speaking, optical parametric oscillator (OPO) utilizes second-order nonlinear crystals as the gain medium, which can effectively break through the limitations of traditional laser gain medium and materials to obtain broad-spectrum and tunable mid-infrared laser output. In particular, synchronously pumped OPO (SPOPO) provides an effective way to obtain a broadly tuned, high-frequency, short-pulse mid-infrared light source with the advantages of low cost, compact structure, and high spectral brightness. In particular, the combination of fiber optic devices can form an all-fiber or fiber-reinforced fiber feedback OPO (FOPO), which avoids the complex configuration of the cavity and cumbersome optical calibration, and is helpful for further reducing the size of the laser as well as improving the output stability. Currently, the FOPO pumping threshold is high, and the wavelength tuning relies on the precise tuning of phase-matching conditions, which involves temperature tuning and mechanical translation in a way that limits the tuning rate.
To this end, Kun Huang and Prof. Heping Zeng's team have constructed two types of FOPOs, which can realize stable output of mid-infrared pulses without active control of the cavity length, and effectively reduce the pumping threshold and broaden the spectral range and tuning rate. First, an erbium-doped fiber is integrated into the FOPO cavity as an additional gain medium, combining the fiber gain and parametric gain to form a synergistic dual-gain structure, which can effectively reduce the pumping threshold. Second, the chirped polarized lithium niobate crystal is used as a nonlinear medium to obtain a wide-band parametric gain bandwidth by taking advantage of its broadband quasi-phase-matching, and the dispersion filtering mechanism can realize the output wavelength tuning by adjusting the cavity length, which avoids the operation of adjusting the phase-matching parameters (such as the crystal temperature and the inversion period, etc.) in the traditional scheme, and simplifies the spectral tuning process, and realizes the rapid switching of the wavelengths. The tuning ranges of signal light and idle frequency light are 1350-1768 nm and 2450-4450 nm, and the experimental results match the predicted curves of the theoretical model, which verifies the accuracy of the system design and the effectiveness of the tuning mechanism.
In the future, this FOPO system can be combined with a high-speed delay tuning device to improve the scanning rate of the laser wavelength, providing strong support for applications such as mid-infrared hyperspectral imaging. Meanwhile, the pulse width can be further compressed by time-frequency domain precision tuning and intra-cavity dispersion management to obtain highly stable, low-threshold, and broadly tuned mid-infrared femtosecond pulses. In addition, highly integrated optical parametric mid-infrared on-chip light sources are expected to be developed by combining advanced photonic integration technologies and emerging nonlinear platforms, such as nonlinear waveguides, thin-film or micro-ring resonators.
The related results were published in Photonics Research 12, 2123 (2024). This work was jointly supported by the Ministry of Science and Technology, the Foundation, Shanghai Municipality, Chongqing Municipality and East China Normal University.
Figure. (a) Device diagram of a broadly tuned mid-infrared fiber feedback optical parametric oscillator; (b, c) Spectrograms of signaling and idler frequency light at different delay positions;
Link to paper: Widely tunable mid-infrared fiber-feedback optical parametric oscillator
