Simultaneous high idler power and full phase stabilization of a synchronously pumped mid-infrared OPO frequency comb across the 2.8–4.8 µm fingerprint band

A 2009 Optics Letters paper by Adler, Cossel, Thorpe, Hartl, Fermann, and Ye demonstrated a high-power mid-infrared frequency comb based on a fan-out MgO:PPLN optical parametric oscillator (OPO) synchronously pumped by a 10 W Yb:fiber laser: up to 1.5 W average idler power, continuously tunable over 2.8–4.8 µm, with full electronic stabilization of the comb via parasitic sum-frequency mixing signals. The stabilization achieved a comb-tooth linewidth of 40 kHz in the fundamental mid-IR comb, limited by the bandwidth of the piezoelectric cavity-length actuator; offset detection relied on spectral coincidences of parasitic mixing signals and a broadened pump supercontinuum, verified in detail at only one idler wavelength; and no nonlinear loss had been observed up to 9.17 W of pump power, leaving the ultimate power-scaling limits uncharacterized. The open problem is whether a synchronously pumped mid-infrared OPO comb can simultaneously reach watt-to-multi-watt idler power (at least 3 W average idler, at least twice the demonstrated 1.5 W) and full phase stabilization — comb teeth well below 40 kHz (at least an order of magnitude below, i.e. <= 4 kHz, measured out-of-loop with the resolution bandwidth and sweep time stated), offset locking robust across the whole tuning range (verified at no fewer than five idler wavelengths spanning 2.8–4.8 µm) — with the high-power limits (parasitic nonlinear processes, thermal and modal instabilities) characterized.

Background

An optical frequency comb is a spectrum of equally spaced, phase-coherent continuous-wave laser lines whose $n$-th tooth frequency is $\nu_n = f_0 + n f_{\rm rep}$, where $f_{\rm rep}$ is the pulse repetition rate and $f_0$ is the carrier-envelope offset (CEO) frequency, i.e. the slip of the optical carrier phase relative to the pulse envelope between consecutive pulses. The mid-infrared region between roughly 3 and 12 µm contains the fundamental ro-vibrational absorption bands of most molecules, so comb spectroscopy there — for example cavity-enhanced direct frequency comb spectroscopy (CE-DFCS), in which a comb is coupled to a high-finesse optical cavity to multiply the effective path length — is far more sensitive than overtone spectroscopy below 2 µm. Before 2009, two routes existed to mid-IR combs: difference-frequency generation (DFG), which is passively CEO-stable when both input waves derive from the same laser but was limited to about 1 mW of power, and femtosecond optical parametric oscillators (OPOs) based on periodically poled lithium niobate (PPLN, a crystal whose nonlinear susceptibility is periodically inverted to restore the phase matching that ordinary dispersion destroys), which reached several hundred milliwatts but for which a stable idler frequency comb had been demonstrated only below 2.5 µm. Adler et al. (Opt. Lett. 34, 1330, 2009; note that the Crossref-deposited title of this DOI erroneously reads '15 W frequency comb at 28–48 µm' because of dropped decimal points — the published paper is the '1.5-W frequency comb at 2.8 to 4.8 µm') closed this gap with a 5-mirror OPO cavity, singly resonant for the signal wave only, synchronously pumped (cavity round trip matched to the pulse interval) by a 10 W, 100-fs, 136 MHz Yb:fiber laser at 1.07 µm. The fan-out MgO:PPLN crystal (poling period varying across the crystal width, so the wavelength is tuned by translating the crystal) allowed continuous idler tuning from 2.8 to 4.8 µm with up to 0.3 µm simultaneous bandwidth and 1.50 W maximum average idler power. Because the CEO frequencies of pump, signal, and idler satisfy $f_{0p} = f_{0s} + f_{0i}$, the comb could be stabilized entirely electronically: the high crystal nonlinearity emits parasitic, non-phase-matched mixing signals — the sum frequencies $p+s$ and $p+i$ and the doubled signal $2s$ — with milliwatt powers, and heterodyne beats of these against a pump supercontinuum broadened in photonic-crystal fiber reveal $f_{0s}$ and $f_{0i}$; one offset is locked by feeding back to a piezoelectric transducer (PZT) on a cavity mirror, the other via $f_{0s}+f_{0i}=f_{0p}$ by feeding back to the pump-laser diode power. At the idler wavelength 3.09 µm the mixing signals $p+i$ and $2s$ happen to overlap, giving an intrinsic beat at $2f_{0i}-f_{0s}$ without any supercontinuum. The out-of-loop beat of the frequency-doubled idler against a narrow-linewidth diode laser showed an 80 kHz coherent peak, i.e. 40 kHz in the fundamental mid-IR comb, limited by the bandwidth of the PZT feedback; and no nonlinear loss was observed up to 9.17 W of pump power, although the slope efficiency already rolled off above ~4 W from a competing conversion process. Subsequent literature reshaped this landscape: reviews (Schliesser, Picqué, Hänsch, Nat. Photonics 2012) catalogued the field; OPO-comb CEO beats were locked to 15 Hz with ~110 Hz comb-line uncertainty (Ferreiro et al. 2011); a mid-IR comb with 2.0 kHz tooth linewidth at 4330 nm was built by DFG inside a Ti:sapphire cavity (Galli et al. 2013); a fully stabilized mid-IR comb at 2.9–3.4 µm was derived by half-harmonic generation in an OPO, transferring the pump comb's coherence at the sub-20 mHz locking level (Vainio and Karhu 2017); offset-free DFG combs covered 3–5 µm (Maser et al. 2017) and 2.7–4.2 µm at high power (Soboń et al. 2017); a phase-stabilized 8.4–9.5 µm OPO comb with both $f_{\rm rep}$ and $f_{\rm CEO}$ locked was demonstrated (Iwakuni et al. 2018); watt-scale, intrinsically CEP-stable intrapulse DFG spanned 3.7–18 µm (Gaida et al. 2018); a 6.7 W CEP-stable DFG comb at 2.9 µm was reported (Catanese et al. 2020); mid-IR combs with a known, continuously tunable CEO requiring no CEO detection were generated by continuous-wave-seeded parametric generation (Roiz et al. 2021); and most directly in the same platform class, a synchronously pumped OPO tunable from 2.7 to 4.7 µm reached a record 10.3 W of idler power with phase noise, beam quality, and the onset of higher-order transverse modes as the high-power stability limit analyzed (Pecile et al. 2024); that source, however, is not phase-stabilized, and its ~300 kHz free-running idler linewidth exceeds the 2009 stabilized value, so multi-watt idler power and full phase stabilization have not yet been combined on a synchronously pumped OPO.

Problem Statement

For a synchronously pumped, singly resonant femtosecond optical parametric oscillator frequency comb in the mid-infrared fingerprint region (idler continuously tunable from 2.8 to 4.8 µm, as demonstrated by Adler et al., Opt. Lett. 34, 1330, 2009, with 1.5 W average idler power): (1) achieve full electronic phase stabilization of the idler comb with a fundamental comb-tooth linewidth well below the demonstrated 40 kHz (at least an order of magnitude below, i.e. <= 4 kHz, measured as an out-of-loop heterodyne beat of the frequency-doubled idler against a narrow-linewidth reference, with the resolution bandwidth and sweep time stated) — which the source attributes to the bandwidth of the piezoelectric cavity-length actuator — including a characterization of the residual phase noise of the stabilization servo; (2) make carrier-envelope-offset detection and locking robust at all idler wavelengths across the full tuning range (verified at no fewer than five idler wavelengths spanning 2.8–4.8 µm, including both band edges), rather than only where the parasitic mixing signals (p+s, p+i, 2s) or the broadened pump supercontinuum happen to overlap spectrally; and (3) scale the idler average power well beyond 1.5 W (to at least 3 W average idler power, i.e. multi-watt and at least twice the demonstrated value; pump power beyond the 9.17 W at which no nonlinear loss was yet observed) while characterizing the ultimate limits — crystal damage thresholds, onset of parasitic or competing nonlinear conversion, thermal effects, and power-induced modal instabilities (all four required) — and while maintaining the phase stabilization of items (1) and (2) at the higher operating power. The single overarching objective is a synchronously pumped mid-IR OPO comb that is simultaneously multi-watt in idler power and fully phase-stabilized, since these are the jointly required properties for high-signal-to-noise, precision comb spectroscopy (e.g. cavity-enhanced direct frequency comb spectroscopy) of the strong fundamental molecular bands in this region.

References

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