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Broadband Degenerate Femtosecond OPO 时间:2018-09-09 22:58 来源:未知作者:admin 点击: 次 Abstract:We demonstrate a broadband degenerate femtosecond optical parametric oscillator (OPO) around 2060 nm. Synchronically pumped by a commercially available 75.6 MHz Yb:KGW femtosecond laser, it is able to generate self-phase-locked 2 μm pulses with 82 mW output power and 3 dB bandwidth of 105 nm. The degenerate operation is naturally stable in free-running condition, with only 0.8% rms fluctuation in 3.5 hours. We also obtain broadband spectrum extending from 1600 nm to 2500 nm with the 3 dB bandwidth of 452 nm by increasing the intracavity power density.
1. Introduction
Femtosecond sources in the range of 2 μm, which is close to the strong water absorber lines, are of great interests for numerous applications, such as “eye-safer” remote sensing [1], high harmonic generation [2], and driven particle acceleration [3] as well as optical frequency combs [4]. For the optical frequency comb in particular, extending it to wavelength around or beyond 2 μm is attracted for a variety of applications including coherent molecular spectroscopy [5] and trace gas detection in the fingerprint region [6], however, which still faces technical challenging. Currently, there are several methods to generate ultrafast 2 μm pulses including mode-locked solid-state lasers based on Tm or Ho doped crystal [7, 8], Tm-doped fiber lasers [9], and nonlinear parametric process [10, 11]. Among them, degenerate femtosecond optical parametric oscillator (OPO) can clearly transpose near-IR frequency combs to the mid-IR, for example, transfer the properties of a frequency comb based on commercial Yb laser to the 2 μm wavelength range. In addition, a synchronically pumped OPO operated at degeneracy not only provides phase and frequency locking of the mid-IR comb lines to the pump but also results in enhanced bandwidth. Hence, degenerate femtosecond OPO is a simple approach to generate coherent and broadband mid-IR frequency combs and has been demonstrated at several different pump wavelengths ranging from 775 nm to 4 μm [12–16]. In 2012, Rudy et al. report generation of 48 fs pulses around 2070 nm from a degenerate OPO; however the average output power is only ~10 mW [17].
In this letter, we report the demonstration of a broadband degenerate femtosecond OPO synchronically pumped by a commercially available Yb:KGW femtosecond laser. With a 3 mm long magnesium oxide-doped, periodically poled, lithium niobate crystal (MgO:PPLN), self-phase-locked pulses with average output power of 82 mW are obtained, whose spectrum has a 3 dB bandwidth of 105 nm. By further improving the intracavity power density, as broad as extending from 1600 nm to 2500 nm spectrum with 3 dB bandwidth of 452 nm is also achieved.