High-resolution spectroscopy of molecular gases requires sources of mid-infrared laser light combining narrow linewidths and wavelength tunability. Continuous-wave optical parametric oscillators (cw OPOs) fulfill these demands; however, their mid-infrared tuning range has been limited to wavelengths below 5.5 μm so far. Here, we demonstrate the first cw OPO emitting mid-infrared light at wavelengths up to 8 μm. This device is based on a 3.5-mm-diameter whispering gallery resonator made of silver gallium selenide () pumped by a compact distributed feedback laser diode emitting at the 1.57 μm wavelength. The oscillation thresholds are in the mW region, while the output powers range from 10 to 800 μW. By changing the radial mode number of the pump wave, wavelengths of up to 8 μm are achieved. Temperature variation enables 100-nm-wide wavelength tuning. The measured tuning branches are in good accordance with the simulations. Furthermore, the latter show that whispering-gallery OPOs based on with diameters around 2 mm can generate idler waves exceeding the 10 μm wavelength.
© 2017 Optical Society of America
Continuous-wave optical parametric oscillators (cw OPOs) convert cw pump light of the wavelength into signal and idler light at such that energy and momentum conservation are fulfilled [1,2]. Fundamentally, cw OPOs can be tuned to every output wavelength that is larger than the one of the pump wave. This great potential of wavelength tunability is combined with narrow-linewidth emission and even with watt-level output powers of conventional mirror-based systems. Thus, they are ideally suited for high-resolution laser spectroscopy [3,4]. A practical limit for the tunability of cw OPOs is given by the transparency of the nonlinear optical material they are based on. Operated at wavelengths beyond the transparency range, they will possess significantly increased oscillation thresholds as well as decreased conversion efficiencies. The great majority of cw OPOs is based on oxide crystals such as lithium niobate () or potassium titanyl phosphate () and their respective isomorphs. They can be considered as transparent from the visible up to the mid-infrared at wavelengths around 5 μm, while the upper limit is given by multi-phonon absorption starting around the 4 μm wavelength . That is the reason why, even after half a century of existence, cw OPOs are still not generating mid-infrared light at wavelengths larger than 5.3 μm .
Although it has been demonstrated that cascaded optical parametric oscillations  or pump-enhanced devices  based on lithium niobate can generate far-infrared light between 80 and 250 μm wavelengths even in the presence of strong absorption, it is more straightforward to choose nonlinear optical materials that are transparent beyond the 5 μm wavelength in the mid-infrared spectral region. For pulsed OPOs, nonoxide crystals like silver gallium sulfide (), silver gallium selenide (), and zinc germanium phosphide () have been successfully employed for many years , and within the last decade, orientation-patterned gallium arsenide and gallium phosphide (OP-GaAs and OP-GaP) have been tried and tested [10–12]. The tuning range of these devices exceeds the 10 μm wavelength.
However, so far, there are only two experimental realizations of cw OPOs with nonoxide crystals surrounded by a conventional mirror cavity. The first was based on . It generated signal and idler light of the wavelengths and . Both as well as the isomorph crystal material suffer from a very low surface-damage threshold . That is why they are not established as standard crystals for cw OPOs. The second realization employed OP-GaAs  emitting at and . However, the fabrication of low-loss OP-GaAs crystals is far from trivial, such that the oscillation threshold still exceeds 10 W pump power.
Here, we present a cw OPO based on a millimeter-sized whispering gallery resonator (WGR) made of . Such a spheroidally shaped monolithic cavity guides light via total internal reflection. The spatial distribution of the light field is characterized by the three mode numbers, . Here, represents the number of oscillations in the propagation direction. It can be considered as the longitudinal mode number. The other two represent the transverse mode structure, i.e., we have maxima in the radial direction and zeros in the polar direction. In order to achieve optical parametric oscillation in WGRs, the wavelengths and the mode numbers have to fulfill the energy conservation and phase-matching conditions 14]. Thus, the phase-matching condition reads , similar to -quasi-phase matching . Since a WGR OPO is intrinsically triply resonant, the wavelengths have to be close to resonance frequencies , where the latter are a function of the transverse mode numbers . Consequently, the tuning behavior of a WGR OPO depends on and [19,20].
Since the interacting light fields are guided by total internal reflection, the round-trip losses are tiny. Therefore, oscillation thresholds down to the μW regime can be reached . All light waves enter and leave the cavity via evanescent-field coupling, i.e., large intensities at the surface and thus possible material damage are avoided. Although WGR OPOs are intrinsically triply resonant, their output wavelengths can be tuned to several MHz-wide resonances in a controlled way . Such devices have been demonstrated so far only with lithium niobate [21,22] and lithium tantalate  with a tunability of up to the 2.8 μm wavelength. Employing in our experiment significantly extends this tuning range into the mid-infrared, even beyond the values that have been reached by mirror-based cw OPOs so far.
The setup, sketched in Fig. 1, comprises a 0.5-mm-thick disk resonator made of with a 1.75 mm major radius and a 0.47 mm minor radius. Its shape is determined by femtosecond laser material processing, and it is polished on a lathe using diamond paste. The optical axis of the material corresponds to the WGR’s symmetry axis. Extraordinarily polarized pump light, i.e., polarized parallely to the optical axis of the WGR material, at wavelength is provided by a distributed feedback (DFB) laser diode and coupled into the resonator via evanescent-field coupling using a silicon prism. The maximum pump power is 12 mW, and it can be reduced by increasing the distance of two adjacent fiber ends between the laser and prism. The gap between the resonator and prism is adjusted by a piezo translator. We can, furthermore, introduce up to variations of the pump frequency by changing the laser driving current. A Fabry–Perot interferometer is employed to measure the actual frequency shift. The temperature of the WGR can be varied from 25°C to 100°C. In the WGR, the pump light is converted to ordinarily polarized signal and idler waves by optical parametric oscillation. After all three waves have been coupled out of the WGR via the silicon prism, the transmitted pump light is separated from the generated waves by a dichroic filter and analyzed in terms of power with a photodetector. A second detector is used to measure the power of the signal wave. Furthermore, we employ a grating spectrometer and a Fourier transform infrared (FTIR) spectrometer in order to determine the output wavelengths.
First, we characterize the frequency spectrum of the WGR with its pump resonances, i.e., its linewidth and radial mode numbers. Figure 2 shows the transmission spectrum at a pump power that is far below the oscillation threshold. Several modes are clearly observed. The narrowest linewidth was determined to be . Assuming that the losses for the pump light are dominated by absorption, we can deduce its absorption coefficient as . With the refractive index  and the vacuum speed of light , we obtain . This value is similar to previous absorption measurements of . In addition, we determine the radial mode numbers of the resonances. This is done by comparing the experimental transmission spectrum with a theoretical simulation  based on the dispersion relation of the resonator  and the Sellmeier equation of the material . Whispering gallery modes with and are successfully identified, and the optical parametric oscillation generated by these modes is studied.
Several processes might occur simultaneously when exciting a single pump mode because the numbers of the generated waves can be chosen freely. Furthermore, in the phase-matching condition doubles the number of possible processes (see Eq. 2). However, the pump power required to start the oscillation strongly depends on the spatial overlap of the interacting waves, i.e., on the actual combination . If the pump power exceeds the oscillation threshold, we measure the power of the signal light with the respective photodetector. Figure 3 shows for . When the pump threshold of 2 mW is overcome, the signal power grows with the increasing pump power. At , we reach 7.5% power conversion efficiency. In order to confirm the frequency conversion, we measure the spectrum of the generated light with the FTIR spectrometer. This requires stable cw operation of the WGR OPO. We achieve this by thermal self-locking [27,28], i.e., by exciting the pump mode on its high-frequency side. By doing so, we achieved less than 10% output-power variation over one minute. We have measured and , confirming parametric oscillation. From the ratio , we determine 12% photon conversion efficiency at a 10 mW pump power. We were able to minimize the oscillation threshold down to 500 μW by adjusting the gap between the resonator and prism. Higher OPO output powers should be achievable using a pump laser with a higher output power, as the data in Fig. 3 do not saturate at 0.8 mW. In order to estimate the idler power, we assume that the outcoupling efficiencies of the two generated waves are equal. Then, . Consequently, the corresponding idler power is expected to grow up to 0.5 mW. By this estimation, we surely underestimate . Due to the longer wavelength, the evanescent field of the idler wave is extended further outside the resonator. Thus, the outcoupling efficiency and hence the idler power will be larger.
Another parametric process generating signal and idler waves at and is observed with . It belongs to different signal and idler modes and can be separated from the one mentioned above by exciting the pump mode at a slightly different frequency that is still within the linewidth. By changing from 4 to 9, we can reach signal wavelengths between 1.97 and 2.54 μm and corresponding idler wavelengths between 4.11 and 7.91 μm at room temperature (see Fig. 4). Such a wide tunability of a cw OPO in the mid-infrared spectral range has not been demonstrated before.
The signal and idler wavelengths of every observed parametric process can be varied by increasing the resonator temperature up to 100°C. Figure 4 shows a collection of idler-tuning branches. They all provide 100-nm-wide tunability. The observations nicely agree with the simulated tuning behavior. For the latter, we took into account different combinations , the thermal expansion of the resonator , and the temperature and wavelength dependence of the refractive index of . The best accordance between the experimentally determined tuning behavior and the simulated is achieved for , which indicates that signal and idler waves propagate in the fundamental polar mode. Here, the outcoupled light is characterized by a single intensity maximum with a nearly Gaussian shape . Thus, we can assume a good beam quality of the generated waves. The tuning range of the WGR OPO based on can be extended further into the mid-infrared. According to the absorption data , output wavelengths of up to at least 10.6 μm should be possible. Simulations of tuning branches of a 2-mm-diameter resonator show that phase matching is fulfilled for idler wavelengths that exceed 10 μm with mode combinations similar to the ones presented here.
Up to pump powers of some mW, all investigated OPO processes generate a single combination of signal and idler waves. Here, we reach signal powers between 30 μW at the 1.97 μm wavelength and 200 μW at 2.54 μm. At a wavelength of 8 μm the idler output power is 10 μW and at a wavelength of 4.1 μm the idler output power is 100 μW. At higher pump powers, additional parametric processes, i.e., multi-mode operation, can occur. However, the simulations show that the resonator could be designed such that tuning branches with output wavelengths around 8 μm have a stronger spatial mode overlap and, consequently, higher output powers would be possible.
We have demonstrated the first cw OPO that emits mid-infrared radiation at wavelengths above 5.5 μm. The millimeter-sized OPO is based on a WGR made of and pumped by a fiber-coupled DFB diode laser emitting in the telecom range. Thus, the whole system can be considered as compact. It delivers output powers of up to 0.8 mW. The presented tuning range with up to 8 μm wavelengths can most likely be extended to values exceeding 10 μm. Keeping in mind that WGR OPOs have been successfully tuned to MHz-wide atomic resonances , and mode-hop-free tuning has been demonstrated as well , we conclude that our work paves the way for the development of miniaturized cw OPOs based on nonoxide crystals ideally suited for high-resolution spectroscopy in the mid-infrared. With orientation-patterned gallium arsenide or gallium phosphide films as a platform, integration on a chip should be possible with even more flexibility due to the freedom in the choice of the pattern structure. Furthermore, chip-integrated and cw-pumped mid-infrared frequency combs could be realized by cascading frequency doubling and optical parametric oscillation [30,31].
Bundesministerium für Bildung und Forschung (BMBF) (13N13648); Deutsche Forschungsgemeinschaft (DFG); Deutsche Telekom Stiftung.
The authors thank F. Kühnemann, P. Schunemann, and K. Zawilski for the fruitful discussions.
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