Phase-stabilized 100 mW frequency comb near 10 μm

Abstract

Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C₆₀. We present a high power, phase-stabilized frequency comb near 10 μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe₂. The OPO can be continuously tuned from 8.4 to 9.5 μm, with a maximum average idler power of 100 mW at the center wavelength of 8.5 μm. Both the repetition rate (f _rep) and the carrier-envelope offset frequency (f _ceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

Fig. 1

Schematic of the OPO. Output from a 110 MHz Tm-fiber comb is mode-matched into linear cavity consisting of the M1, M2, M3, M4, and M5 mirrors. M2 and M3 are concave mirrors with an ROC of either 50 or 110 mm (see text). The rest are flat mirrors. All mirrors have HR for the signal wave. M1 has high transmittance (HT) for the pump wave and M4 has HT for the idler wave. A part of Tm-fiber comb light enters highly nonlinear fiber after pulse compression by the fused silica rod, and it is spectrally broadened. P + I and spectrally broadened P interfere to obtain f_ceo,I. LPF long pass filter

Fig. 2

OPO oscillation threshold and idler power. a Calculated average pump power at the oscillation threshold of the OPO as a function of pump pulsewidth (FWHM), with a beam size of 30 µm (1/e² intensity radius, solid line). The data points show the measured threshold for each pulsewidth, as shown in Fig. 2b. Inset shows the effective crystal length (L_eff) as a function of the pump pulsewidth (FWHM). L_eff is limited by spatial transverse walk-off at long pulsewidths. b Idler average power as a function of pump average power at different pulsewidths. The dashed lines indicate trends before saturation starts. c Idler average power as a function of pump average power at different beam sizes (1/e² intensity radius), with a pulsewidth of 200 fs (FWHM)

Fig. 3

OPO wavelength tuning. a Idler spectrum and corresponding idler average power. b Reflectivity of input mirror M1 (left) and the line strength of water rovibrational features (right) as a function of signal wavelength

Fig. 4

f_ceo phase locking. a Free-running f_ceo,I beat signal at a resolution band width (RBW) of 100 kHz and sweep time of 5 ms. Inset shows a magnified view of the free-running beat signal at a RBW of 100 kHz and sweep time of 30 ms. b Phase-locked in-loop beat signal at a RBW of 1 kHz. The linewidth is limited by the RBW and the servo bump is at 40 kHz