Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecu- lar spectroscopy, including investigation of large molecules such as C . 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 AgGaSe . 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 ) and the carrier-envelope offset frequency (f ) of the idler wave rep ceo 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. 1 Introduction phased array (VIPA) spectrometry , and dual-comb spectrometer [7, 8] have been developed mainly for appli- High-power frequency comb sources at infrared wavelengths cations of trace gas sensing [9, 10]. Recently, the versatility have become increasingly desirable for a wide range of of direct comb spectroscopy was extended to probe large, applications in spectroscopy. While originally developed complex molecules cooled by buffer gas  and chemical for metrology, comb lasers found immediate applications reaction kinetics involving transient intermediates such as in high-resolution spectroscopy [1, 2]. In direct frequency DOCO molecules produced from the OD + CO reaction [12, comb spectroscopy, frequency combs are used as light 13]. These studies were carried out in the MIR wavelength sources, enabling efficient and multiplexed, high-resolution (3–5 µm), and the next advance for comb-based high-resolu- spectroscopy over a broad spectral bandwidth . Direct fre- tion molecular spectroscopy lies at longer MIR wavelengths quency comb spectroscopy based on a variety of broadband of 5–10 µm, where the fundamental molecular vibration detection schemes including Fourier-transform spectrom- bands display even stronger intensities. In the case of large etry (FTIR) , grating spectrography , virtually imaged molecules, probing at longer wavelengths provides an addi- tional advantage of reducing the impact of intermolecular vibrational redistribution, which causes spectral congestion This article is part of the topical collection “Mid-infrared and THz and dilution of absorption intensity . For detection of Laser Sources and Applications” guest edited by Wei Ren, Paolo transient molecules like DOCO, a strong band intensity is De Natale and Gerard Wysocki. also necessary, since the probing time must be very short, on the order of microseconds, to follow kinetics. * Kana Iwakuni firstname.lastname@example.org Recently, frequency comb lasers in the MIR wavelength region have been reported using a variety of architectures: Department of Physics, JILA, National Institute of Standards difference frequency generation (DFG), optical parametric and Technology and University of Colorado, University oscillators (OPO), quantum cascade lasers (QCL), micro- of Colorado, Boulder, CO 80309, USA resonator, and supercontinuum using waveguide dispersion. IMRA America, Inc., 1044 Woodridge Ave., Ann Arbor, DFG-based MIR frequency comb sources (based on MI 48105, USA periodically polled lithium niobate, PPLN) are known for Present Address: Honeywell International, 303 Technology their relative simplicity (single-pass configuration, passive Court, Broomfield, CO 80021, USA cancelation of the f ) at the expense of relatively low power ceo Present Address: Christian Doppler Laboratory for Mid-IR (100 μW–20 mW at 2.6–5.2 μm) [15–17]. More recently, a Spectroscopy and Semiconductor Optics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria Vol.:(0123456789) 1 3 128 Page 2 of 7 K. Iwakuni et al. higher power (~ 240 mW) DFG comb has been reported at frequency combs [37–39] have been limited to wavelengths 2.7–4.2 μm based on Raman-induced soliton self-frequency no longer than 3 µm. Access to longer infrared wavelengths shift in a nonlinear fiber . Longer MIR wavelengths is mainly hindered by the difficulty of finding suitable which are comparable to those in the present work have materials. been achieved with a GaSe crystal at 7.5–12.5 μm (~ 15 μW) As described above, ideal light sources for molecular  and 8–14 μm (~ 4 mW) , with a A gGaS crystal spectroscopy in the long infrared wavelengths, i.e., fre- at 7.5–11.6 μm (~ 1.55 mW) , and with an orientation- quency comb sources providing a combination of high patterned (OP) GaP at 6–11 μm (~ 60 mW) . power, frequency stability, and broad wavelength tunability, In contrast to DFG-based sources, OPO-based frequency are highly desired for molecular spectroscopy. In this work, combs exhibit higher power while maintaining a wide we have developed a singly resonant A gGaSe -based OPO wavelength tuning range. MIR sources below 6 μm have synchronously pumped with a Tm-fiber comb. We achieved been reported using MgO-PPLN at 2.8–4.8 μm (~ 1.5 W) more than 100 mW average power near the 10 μm region,  and at 2.2–3.7 μm (~ 33 mW) , using OP-GaAs at for the first time to the best of our knowledge. In addition, 2.6–7.5 μm (~ 73 mW)  and at 5.2–6.2 μm (~ 10 mW) we experimentally demonstrate phase stabilization of the , using AgGaSe at 4.8–6.0 μm (~ 17.5 mW) , and frequency comb. This long-wavelength infrared laser will using OP-GaP at 2.3–4.8 μm (~ 30 mW) . There have enable the study of fundamental vibration bands of large been several reported implementations of longer wavelength molecules and molecular clusters, which are important for infrared OPOs at 5–12 μm (~ 15 mW at 8.5 μm) , and investigating structure and dynamics of complex molecules. 2.85–8.4 μm , but the idler powers in these studies drop off significantly at wavelengths longer than 8 μm. Finally, the widest wavelength tuning (1.33–20 μm, ~ 40 mW at 2 Apparatus and results 9 μm) has been recently reported by combining OPO and DFG techniques . 2.1 Optical parametric oscillator Additional infrared frequency comb sources include QCL, microresonators, and supercontinuum architectures. Figure 1 depicts the experimental setup. The pump laser is a A QCL-based frequency comb has a high repetition rate mode-locked Tm:fiber laser at 1.95 µm, with a spectral band- (around 10 GHz) and high power (~ 3 mW per comb mode) width of 50 nm, repetition rate of 110 MHz, and a maximum for both 7  and 8 μm  center wavelengths, though output power of 2.5 W. This Tm-fiber laser synchronously it is difficult to tune its wavelength continuously. On the pumps a 5-mirror linear OPO cavity, where all mirrors have other hand, microresonator [34–36] and supercontinuum high reflectivity (HR > 99.5%) at the signal wavelength for 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 con- cave 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 transmit- tance (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 . LPF long pass ceo,I filter 1 3 Phase-stabilized 100 mW frequency comb near 10 μm Page 3 of 7 128 singly resonant operation. Mirror M4 out-couples the gener- 2.2 Idler power optimization ated idler wave with a transmittance of about 92%, while M1 functions as an input mirror with about 99% transmittance The line plot in Fig. 2a shows the calculated average for the pump wave. M5 is attached to a fast piezo (PZT) pump power at the oscillation threshold of the OPO as actuator for f control. The PZT is a commercial 5 × 5 × 2 a function of the pump pulsewidth . This calcula- ceo stack PZT (Phsik Instrumente, PL055.3x), attached to a tion makes the assumption that both the pump and signal 6 mm diameter and 2 mm-thick mirror. We mount the PZT and mirror on a homemade bullet-shaped base, which sup- ports an actuation bandwidth of 100 kHz . The OPO is based on a AgGaSe nonlinear optical crystal with an aperture of 5 × 5 mm and length of 3.6 mm. The crystal is centered at the focal plane of the concave mir- rors, M2 and M3, and mounted on a rotation stage for angle tuning. Both faces of the crystal are anti-reflection (AR) coated for the pump, signal, and idler waves. The AgGaSe crystal has a relatively large second-order nonlinear optical coefficient of 32 pm/V, and it is transparent at 0.76–17 µm −1 (absorption coefficient < 1 cm ) . These characteristics make AgGaSe highly suitable for MIR generation using a 1.95 µm pump wave while avoiding large nonlinear absorp- tion. This is in contrast to GaAs, where the idler power was significantly limited due to nonlinear absorption of the pump and signal . The AgGaSe crystal is cut at an angle of 57° to achieve type II phase matching with pump wave (1.95 µm, extraordinary wave), signal wave (2.53 µm, ordi- nary wave), and idler wave (8.5 µm, extraordinary wave). The generated idler power is determined by a compromise between the nonlinear interaction strength, effective crystal length, and thermal lensing, under the limitation imposed by the crystal’s optical damage threshold. The nonlinear interaction strength increases with peak pump intensity, which in turn is greater for shorter pulsewidth and smaller beam size. However, as the pulse gets shorter and the spot size smaller, other detrimental effects limit the OPO per - formance. First, a shorter pulsewidth reduces the effective interaction length due to temporal walk-off  in the crys- tal. Second, a smaller spot size exacerbates thermal gradi- ents across the crystal, resulting in a thermal lensing effect. The latter effect is more severe in AgGaSe relative to most common nonlinear crystals as it is difficult to cool AgGaSe due to its low thermal conductivity [1 W/(m K)] and large size. The large crystal size is a result of its softness (Mohs hardness of 3), making it challenging for further machining process to yield small pieces. The combination of a large Fig. 2 OPO oscillation threshold and idler power. a Calculated aver- size and low thermal conductivity makes it challenging to age pump power at the oscillation threshold of the OPO as a function of pump pulsewidth (FWHM), with a beam size of 30 µm (1/e inten- remove optically deposited heat from the crystal. In addi- sity radius, solid line). The data points show the measured threshold tion to these trade-offs, the peak intensity is limited by the for each pulsewidth, as shown in Fig. 2b. Inset shows the effective crystal’s damage threshold, which we observed to be about crystal length (L ) as a function of the pump pulsewidth (FWHM). eff 3 GW/cm for ~ 100 fs pulses. In the following, we will dis- L is limited by spatial transverse walk-off at long pulsewidths. b eff Idler average power as a function of pump average power at different cuss our comprehensive experimental investigations of these pulsewidths. The dashed lines indicate trends before saturation starts. competing effects and present a final design that yields an c Idler average power as a function of pump average power at dif- optimal idler power of more than 100 mW. 2 ferent beam sizes (1/e intensity radius), with a pulsewidth of 200 fs (FWHM) 1 3 128 Page 4 of 7 K. Iwakuni et al. waves have 30 µm (1/e intensity radius) beam sizes and 2.3 Wavelength tuning the same pulsewidth. The estimated round-trip loss of the OPO cavity is 14.7%, calculated from mirror reflectivities The OPO’s wavelength can be tuned by changing the crys- and crystal linear absorption. The calculation shows that tal temperature and phase-matching angle. We first studied there is a trade-off between pump pulsewidth and effec- temperature tuning. The crystal refractive index has a small −5 −1 tive crystal length (shown in the inset of Fig. 2a) due to temperature dependence of dn/dT = 8 × 10 K . We temporal walk-off. The experimental OPO threshold under observed a very limited idler wavelength tuning range of three different pulsewidths are measured and plotted in about 1 nm by temperature tuning from 20 to 60 °C due to Fig. 2a with the same plot markers of Fig. 2b. The meas- the narrow safe operating temperature range for the crystal’s urements basically follow the theory prediction and show AR coating. that the lowest threshold is achieved at 700 fs. The small The phase-matching angle provided a significantly larger discrepancy at the pulsewidth of 700 fs can arise from by wavelength tuning capability. Figure 3 shows the output idler considering astigmatism due to finite incident angle to the spectrum recorded with an FTIR  for different phase- concave mirrors. matching angles from 54.2° to 57.2° (solid lines). This cor- Figure 2b shows the measured average idler power as responds to center idler wavelengths of 8.4–9.5 µm with a a function of input pump power at three different pump spectral bandwidth of 200 nm (FWHM). The average idler pulsewidths. At the crystal, the pump and signal beam power corresponding to each spectrum’s center wavelength radii are set to ~ 30 µm using 100 mm radius of curvature is overlaid on the same plot depicted by black square points. (ROC) concave mirrors (M2 and M3). With this beam size, the maximum input pump power is limited to 1.2 W due to the low damage threshold of the crystal AR coating. The 200 fs pulsewidth, obtained by pulse compression with a fused silica rod, is the shortest studied here. This short pulse has a reduced effective interaction length due to temporal walk-off, as shown in Fig. 2a, thus limiting the generated idler power. We also stretch the pulse by inserting ZnSe plates or a fused silica rod to the pump beam’s path (see Fig. 1). When using a 5 mm thickness ZnSe plate, a stretched pulse of 700 fs yields the maximum observed idler power of 105 mW. The photon conversion efficiency is about 60% at the conditions that produces the maximum idler power. We observe saturation of the output idler power at increasing average pump power. This saturation behavior is evident for all three traces in Fig. 2b when the aver- age pump power exceeds 800 mW. A likely explanation is thermal lensing, where the thermal lens focal length is inversely proportional to average intensity. The latter increases for higher average power and smaller beam size. Figure 2c shows the average idler power as a function of pump power for two different beam sizes of 20 and 30 μm, both using a pulsewidth of 200 fs. For a beam size of 20 μm (obtained from using concave mirrors with ROC of 50 mm), there is strong saturation of the idler power above 350 mW pump power, as shown in Fig. 2c, which corre- sponds to the same average intensity for 800 mW at 30 μm beam size. Thermal lensing could be mitigated by cooling of the crystal. However, this strategy is not effective due to the crystal’s large thermal mass and low thermal con- ductivity. These results suggest that larger ROC mirrors Fig. 3 OPO wavelength tuning. a Idler spectrum and corresponding could further relax this thermal effect and increase idler idler average power. b Reflectivity of input mirror M1 (left) and the power, especially in combination with a more powerful line strength of water rovibrational features (right) as a function of pump laser. signal wavelength 1 3 Phase-stabilized 100 mW frequency comb near 10 μm Page 5 of 7 128 The strong absorption lines present in the spectra are from the ν fundamental vibrational band of atmospheric water. Fringes on the spectra are due to etalon effects from optics outside of the OPO cavity. Access to longer idler wavelength is limited by the HR coating of the M1 mirror for the signal, while the shorter idler wavelength is limited by absorption of atmospheric water at the signal wavelength, which increases the loss of the OPO cavity and hence degrades the idler power (Fig. 3b). 2.4 f and f phase stabilization rep ceo The idler frequency comb can be completely stabilized by phase locking f and f to a radio frequency (RF) reference rep ceo signal. To lock f , a small fraction of the output light from rep the Tm comb oscillator is detected by a fast photodiode. The 9th harmonic of f (990 MHz) is mixed with a 1 GHz refer- rep ence frequency from a Cs-stabilized quartz oscillator. The resulting 10 MHz beat signal is measured by a phase detec- tor, where it is mixed with a reference RF signal generated by a DDS slaved to a 10 MHz quartz oscillator stabilized to a Cs clock. The phase error signal from the mixer is low-pass filtered and fed back to a PZT in the Tm comb oscillator to lock f (Fig. 1). The fractional frequency instability is rep −13 measured by a frequency counter to be 1.6 × 10 at 1 s. Stabilization of the carrier-envelope offset frequency of the idler wave, f , is achieved by the heterodyne optical ceo,I beat of (1) the sum frequency of the pump and idler wave (P + I) generated in the OPO cavity and (2) spectrally broad- ened pump wave (P) by a highly nonlinear fiber . The P + I wave at 1.6 μm, along with many other parasitic waves, are generated in the crystal by nonphase-matched nonlinear processes. An optical band pass filter (1550–1600 nm) is used to make sure only the P + I wave reaches the detector. The bottom insets of Fig. 1 show the broadened pump spec- trum from a 24 cm long, 4 μm core diameter nonlinear fiber and the P + I spectrum from the OPO. To broaden the pump Fig. 4 f phase locking. a Free-running f beat signal at a resolu- ceo ceo,I tion band width (RBW) of 100 kHz and sweep time of 5 ms. Inset spectrum efficiently, the pulsewidth is narrowed to 200 fs shows a magnified view of the free-running beat signal at a RBW of with a 30 cm fused silica rod. The pump spectrum broadens 100 kHz and sweep time of 30 ms. b Phase-locked in-loop beat signal to 1.6 µm at 300 mW input power into the nonlinear fiber. at a RBW of 1 kHz. The linewidth is limited by the RBW and the The f beat signal is detected with a single photo- servo bump is at 40 kHz ceo,I detector with an input power of approximately 50 μW. The signal-to-noise ratio of the beat signal is limited by can be controlled by adjusting the cavity length . the amplitude noise from the spectrally broadened pump wave , which was suppressed by 5 dB using balanced Figure 4b shows the in-loop beat signal of f with f ceo, I rep phase-locked. The servo bandwidth is about 40 kHz based detection. We achieved a final signal-to-noise ratio of 35 dB at 100 kHz resolution bandwidth. Figure 4a shows on Fourier analysis of the error signal. The coherent peak is about 30 dB with a spectral width limited by the instru- the observed free-running f beat signal. f is phase- ceo,I ceo,I stabilized to the same Wenzel quartz oscillator used for mental resolution bandwidth of 1 kHz and averaging time of 85 ms. The estimated modified Allan deviation of the f locking. The error signal from the phase detector is rep fed back to both a fast PZT (M5) and slow PZT (M4) in idler wave is about the same as that of the quartz oscillator −13 (5 × 10 at 1 s) according to the stability measurement of the OPO cavity (Fig. 1). In the synchronously pumped OPO cavity, the signal carrier-envelope offset frequency the repetition rate . 1 3 128 Page 6 of 7 K. Iwakuni et al. 5. R. Grilli, G. Méjean, C. Abd Alrahman, I. Ventrillard, S. Kassi, 3 Conclusions D. 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Published: Jun 6, 2018
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