We demonstrate that it is possible to obtain a mid-infrared optical frequency comb (OFC) experimentally by using a continuous-wave-pumped optical parametric oscillator (OPO). The comb is generated without any active modulation. It is based on cascading quadratic nonlinearities that arise from intra-cavity phase mismatched second harmonic generation of the signal wave that resonates in the OPO. The generated OFC is transferred from the signal wavelength (near-infrared) to the idler wavelength (mid-infrared) by intracavity difference frequency generation between the OPO pump wave and the signal comb. We have produced a mid-infrared frequency comb which is tunable from 3.0 to 3.4 µm with an average output power of up to 3.1 W.
© 2014 Optical Society of America
An optical frequency comb (OFC) is a light source that emits coherent broadband light, which in frequency space consists of evenly spaced narrow lines. Optical frequency combs have found their way to many different fields. For example, they can be used for accurate measurements in time and frequency metrology, and as light sources in precision molecular spectroscopy [1, 2]. The unique coherence properties of the OFCs allow the use of cavity enhanced methods  to substantially increase the absorption path length in spectroscopy. With two OFCs, it is also possible to construct a fast Fourier transform spectrometer without any moving parts . Spectroscopy with OFCs is well established in the visible and near-infrared region, but has been limited in the mid-infrared region (> 2.5 µm) due to the lack of suitable mid-infrared OFC sources. This is a drawback because the mid-infrared region gives access to fundamental molecular vibrations in the molecular fingerprint region.
Usually, mode-locked lasers are used to generate OFCs. Currently, these lasers work only at wavelengths shorter than 2.5 µm , except quantum cascade lasers (QCLs), which can be tailored to work from the mid-infrared to far-infrared region. Self-mode-locking and active mode-locking of QCLs have been demonstrated [6, 7], although only within narrow spectral regions and with modest output powers. Optical microresonators provide another new method to generate OFCs [8, 9]. In this approach, a comb is generated from a continuous-wave (CW) pump laser by the χ(3) (cubic) nonlinearity and the high finesse of the microresonator. In some cases, microresonator OFCs suffer from material restrictions, which limit their working range. So far, mid-infrared OFCs based on microresonators have not been demonstrated [5, 10, 11].
One option to generate OFCs in the mid-infrared region is to use optical χ(2) (quadratic) nonlinear effects, such as difference frequency generation (DFG) or optical parametric oscillation. In the DFG method, the comb is usually produced in a nonlinear material between a visible or near-infrared OFC and a suitable continuous-wave laser or between the lines of a solitary comb [12–14]. The output power of a DFG-based comb is usually low , although in some cases relatively high mid-infrared output powers have been demonstrated by DFG [14, 15].
The main advantages of optical parametric oscillators (OPOs) are a large tuning range (typically from near-infrared to mid-infrared) and high output power (from milliwatts to several watts). Mid-infrared comb generation by an OPO is usually done by a synchronously pumped OPO (SPOPO) [16–19] or by an actively mode-locked OPO [20, 21]. Actively mode-locked OPOs require intracavity modulators and high-speed electronics, which restricts the available repetition rates and pulse lengths. A synchronously pumped OPO can be used to produce very short pulses and a large spectral bandwidth , but it requires a mode-locked laser for pumping and the OPO cavity length has to be synchronized with the pump repetition rate. Passive modulation of a CW-pumped OPO would remove these limitations. However, it has been shown  that passive mode-locking of a CW-pumped OPO with a traditional saturable absorber is challenging.
We recently demonstrated that it is possible to generate an OFC by a bulk CW-pumped OPO without any active modulation . The physical mechanism behind the comb generation is analogous to the balance between a group velocity dispersion (GVD) and self-phase modulation (SPM) present in microresonator-based combs  but in our case, the SPM is provided by an artificial χ(3) nonlinearity which is caused by a cascading χ(2) effect. The origin of the cascading χ(2) effect  is phase mismatched second harmonic generation (SHG) of the fundamental wave. In a χ(2) medium, the generated second harmonic of the fundamental wave is soon converted back to the fundamental wavelength if the process is phase mismatched. In this process, the back-converted wave obtains an intensity dependent phase difference compared to the unconverted fundamental wave . This effect can substantially increase the effective nonlinear refractive index of the material. Because the sign of the phase mismatch can be adjusted, the sign of the effective nonlinear refractive index can also be changed . Nonlinear phase shift induced by the cascading χ(2) effect has been used in the supercontinuum generation in LiNbO3 waveguides [27, 28] and for Kerr lens mode-locking of solid-state lasers. In the latter, such method is called cascading second order mode-locking (CSM) [29, 30]. Mode-locking can be achieved either with an intracavity hard aperture or with a soft aperture . Partial mode-locking has also been detected in a travelling wave ring cavity solid-state laser , where the observed phenomenon resembles the behavior of the OPO reported here.
In this article, we demonstrate and characterize a tunable mid-infrared OFC. Although the cascading quadratic effect is responsible only for SPM in the near-infrared region (around the signal wavelength of the OPO), the comb structure is also transferred to the mid-infrared region (around the idler wavelength of the OPO) by DFG between the OPO single-mode pump wave and the signal comb. We show that the average optical output power in the mid-infrared region is high, up to several watts. Coarse and fine wavelength tuning of the idler comb are demonstrated. We also describe a method to separately control the offset frequency and mode-spacing of the idler comb.
2. Experimental setup and principle of operation
The OPO cavity is constructed as a bow-tie ring cavity with two nonlinear crystals (Fig. 1). The first nonlinear crystal (PPLN 1) is a 50 mm long periodically poled MgO:LiNbO3 crystal (HC Photonics) with several poling periods ranging from 31.5 to 29.5 µm. The PPLN 1 crystal is responsible for the normal OPO operation, which generates signal and idler photons from the pump photons. The end facets of the crystal are antireflection-coated for the pump, signal, and idler wavelengths. The second crystal (PPLN 2), which provides the phase mismatched second harmonic generation for the signal wave, is also a 50 mm long periodically poled MgO:LiNbO3 crystal (HC Photonics), but the poling periods range from 19.5 to 21.3 µm. The crystal is antireflection-coated for the signal wave and its second harmonic wave. The temperature of the crystals can be independently controlled from room temperature up to 200 °C with a precision of ~10 mK.
The OPO is pumped with a narrow-linewidth laser, which delivers up to 21 W of optical power at λp = 1064 nm. The laser consists of an Yb-fiber amplifier (IPG Photonics YAR-20K-1064-LP-SF), which is seeded with a distributed feedback fiber laser (NKT Photonics Koheras BasiK). Mirrors of the OPO cavity possess high reflectivity for the signal wavelengths (1.55 – 1.7 µm) and high transmission for the pump and idler wavelengths (2.7 – 3.5 µm), as well as for the second harmonic of the signal wave. Without the PPLN 2 crystal, the OPO would be a conventional CW singly resonant OPO [33–35]. The cavity is designed so that the resonating signal beam 1/e2 waist sizes are 55 µm in both crystals, corresponding to the focusing parameter ξ~2. The dispersion caused by the crystals and cavity mirrors were not compensated, and the crystals alone cause a large dispersion, ~11 × 103 fs2 in a single round trip of the signal beam.
In our setup, the signal wave passes through the PPLN 2 crystal and is influenced by the cascading quadratic effect and, therefore, the comb formation is possible because of the balance between the GVD and SPM. The comb structure is transferred from the near-infrared signal wave to the mid-infrared idler wave in the PPLN 1 crystal by DFG between the pump wave and the signal comb (Fig. 2) according to the law of energy conservation (1/λp = 1/λs + 1/λi).
A small fraction of the intracavity power of the signal wave is coupled out after the PPLN 1 crystal. This output beam is used for diagnostics of the signal spectrum. The beam is coupled to a fiber divider and routed to an optical spectrum analyzer (OSA, Ando AQ-6315E) and to a fast InGaAs photodetector (Thorlabs DET01CFC, bandwidth 1.2 GHz). The electronic signal from the photodetector is transmitted to a radio frequency measurement setup, which consists of suitable filters, amplifiers, and a radio frequency (RF) spectrum analyzer (Agilent 4395A, bandwidth 500 MHz). This allows us to characterize the optical and radio frequency spectrum of the signal beam.
The idler beam generated in the PPLN 1 crystal is coupled out of the OPO cavity after the PPLN 1 crystal. After the cavity, a small part of the idler beam is aligned to a wavelength meter (EXFO WA-1500-NIR/IR-89) and a spectrum analyzer (EXFO WA-650) to characterize the optical spectrum of the mid-infrared comb. Since fast detectors for the mid-infrared region are not readily available, the mid-infrared idler wave is frequency doubled in a periodically poled MgO:LiNbO3 crystal to the near-infrared region. The optical and RF spectra of the frequency-doubled beam are analyzed using the same setup that is used for the analysis of the signal beam.
3.1 Spectra of the signal and idler
With the PPLN 2 crystal inserted into the OPO cavity, we have previously detected frequency comb generation around the signal wavelength of the OPO . Figure 3 shows the widest signal spectrum we have so far observed. The signal spectrum was also recorded with a high resolution Fourier transform infrared (FTIR) spectrometer (Bruker IFS 120 HR, resolution 55.5 MHz) in order to confirm that the spectrum consists of discrete lines, within the available measurement accuracy. A part of the spectrum measured with the FTIR spectrometer is shown in the inset of Fig. 3. These measurements were done with the following poling periods and temperatures: Λ1 = 30.5 µm and T1 = 75.0 °C for the PPLN 1 crystal, and Λ2 = 19.7 µm and T2 = 120.4 °C for the PPLN 2 crystal. Pump power and depletion were 21 W and ~50%, respectively. The OPO threshold is typically achieved at a pump power of 8 to 10 W. The average output power at signal wavelengths through one cavity mirror is up to 40 mW.
The optical spectrum of the mid-infrared beam is shown in Fig. 4 (a) for the same OPO operating parameters as for the signal spectrum of Fig. 3. As expected, the mid-infrared spectrum resembles a sinc2-function. The width of the spectrum is limited by the phase-matching bandwidth of the DFG process. We believe that the asymmetry of the spectrum is caused by the tight focusing of the Gaussian beams in the crystal  and by the asymmetric signal spectrum (Fig. 3). The idler beam spectrum was also recorded with the FTIR spectrometer; see the inset of Fig. 4 (a). An additional confirmation that the spectrum consists of discrete lines was obtained by measuring the RF spectrum of the frequency-doubled idler beam [Fig. 4 (b)]. The first RF beat note between the comb lines was detected at 207.5 MHz, which is the same as the adjacent peak separation in the high resolution optical spectrum (FTIR spectrum). This is the mode spacing of the comb and in agreement with the calculated free spectral range of the OPO cavity. The full-width at half-maximum (FWHM) of the RF beat notes is ~1.2 MHz in a time scale of 2 s (resolution bandwidth, RBW = 3 kHz). Both the optical and RF spectra, as well as the average output power, remain stable without any active stabilization of the OPO cavity length or power. For example, the output was recorded for 10 minutes with a detector that has a bandwidth of 1 kHz. An average output power of 1.837 W was obtained with a standard deviation of 3 mW (0.16%). However, the broad beat note, the fluctuation of the signal’s envelope amplitude at microsecond timescales, and the large constant background [Fig. 4(c) and (d)] suggest large phase noise when comparing for example to mode locked lasers . Details of the signal’s envelope depend on the OPO operating parameters, but, in general, the observed behavior resembles partial mode locking of a solid state laser under the influence of cascading quadratic nonlinearities  or the OFC generation by CW broadband QCLs  or the Kerr combs obtained by microresonators .
3.2 Tunability and power of the idler beam
The center wavelength of the idler comb is tunable just like the idler output beam of a normal OPO. Coarse tuning is done by choosing a suitable poling period of the PPLN 1 crystal, which were 30.5 and 31.0 µm in this work. Fine tuning is carried out by changing the temperature of the PPLN 1 crystal or by tuning the wavelength of the pump laser. When the desired center wavelength of the idler is obtained, the poling period and temperature of the PPLN 2 crystal are tuned to obtain a proper phase mismatch for SHG of the signal wave. The system is not particularly sensitive to small temperature fluctuations of the PPLN 2 crystal. Temperature changes of several degrees are typically needed in order to observe any clear change in the optical or RF spectra.
Figure 5 shows a demonstration of scanning of the idler beam center wavelength from 3.0 to 3.4 µm. Currently, the tuning range is limited by the OPO cavity mirrors and the available poling periods of the PPLN 2 crystal. For each poling period and temperature setting of the PPLN 1 crystal, the poling period and the temperature of the PPLN 2 crystal were changed to find the broadest and the most stable signal and idler spectra. The broadest idler spectrum (0.8 THz, −15 dB width and not accounting the side lobes) was observed around the 3.03 µm. The mid-infrared output power strongly depends on the phase mismatch of SHG in the PPLN 2 crystal, as well as on the cavity alignment. The mid-infrared average output power is up to 3.1 W, which corresponds to an average power of ~3 mW per a comb tooth. As far as we know, these are the highest power levels reported for a mid-infrared frequency comb [14, 15, 17, 19, 37].
3.3 Fine tuning of the idler comb
The possibility of controlling the offset frequency and mode spacing of the frequency comb is needed in many applications. Mode spacing is approximately the free spectral range of the OPO, and can be varied by changing the OPO cavity length. The fine tuning of the mode spacing can be done by adjusting the cavity length with a piezo electric actuator. Such piezo tuning also varies the comb offset frequency, since the cavity resonance frequency is inversely proportional to the cavity length. While these means can be used to control the mode spacing and offset of the signal comb, they also influence the same parameters of the idler comb, since the mode spacing is transferred to the mid-infrared by DFG and since Δνi = -Δνs. The pump laser gives an additional degree of freedom for the control of the offset frequency of the mid-infrared idler comb through the relation Δνi = Δνp. In the following, we demonstrate the fine tuning of the idler comb by both the signal and pump tuning.
To demonstrate the pump tuning method, the frequency-doubled idler beam was combined with a laser beam from a CW external cavity diode laser (ECDL, New Focus Velocity) in a fiber combiner. The frequency-doubled idler and the ECDL beam were detected with the InGaAs photodetector. The RF spectrum of the photodetector signal showed the beat signals between the ECDL beam and the comb teeth. These beat signals could be scanned by scanning the wavelength of the ECDL beam. From the photodetector signal, only the first beat frequency of the ECDL beam with the closest tooth of the frequency-doubled idler comb was transferred to a frequency-to-voltage converter (F/V-converter, bandwidth 4 – 120 MHz). The F/V-converter was used to give a voltage reading which is proportional to the first beat frequency. The output was monitored with an oscilloscope. Figure 6(a) shows an example of a measured scanning of the idler comb relative to the ECDL beam while modulating the pump laser frequency with a sinusoidal signal at a rate of 0.9 Hz. As expected, the idler frequency followed the modulation. We note that the amplitude of the modulation was limited by the measurement setup and not by the pump laser or the OPO. The pump frequency can be scanned more than 4 GHz, which is almost 20 times the comb mode spacing.
In the next demonstration we locked one tooth of the frequency-doubled mid-infrared idler comb to the ECDL frequency. The ECDL frequency was slowly modulated (~200 mHz) and the F/V-converter provided a voltage proportional to the beat frequency between the ECDL beam and the frequency-doubled comb tooth [Fig. 6(b)]. We used the output voltage of the F/V-converter as an error signal for a servo that controlled the OPO cavity length (and hence the signal/idler frequencies) with piezo elements installed on one of the cavity mirrors. A servo bandwidth of ~100 Hz was used. At time 0 s in Fig. 6(b), the lock is turned on and the idler frequency starts to follow the ECDL frequency, thus keeping the beat frequency constant. The same demonstration is also possible to perform more directly by monitoring the beat signal between the signal comb and the ECDL. Another option to stabilize the OPO cavity length, for example, would be to use the beat signal between the signal and frequency doubled idler combs when 2λs ~λi.
We have reported a CW-pumped OPO that can produce a tunable OFC in the mid-infrared region. The OFC is generated around the near-infrared signal wavelength of the OPO by intracavity cascading quadratic effect, and transferred to the mid-infrared idler wavelength by DFG between the resonating signal comb and the OPO pump beam The average output power is up to 3.1 W, which is the highest reported value for a mid-infrared OFC. We have also described that it is possible to control separately the mode-spacing and offset frequency of the comb by changing the cavity length and by tuning the frequency of the pump laser.
We note that we have also observed cases where the OPO produces narrow beat notes (FWHM < 10 kHz) in the RF spectrum, but these states have been unstable so far. The source of the instability is currently unknown. Further experimental and theoretical studies are needed to find out if stable, low-noise states with narrow RF beat notes are possible to obtain, similar to what has recently been discovered in the case of Kerr combs produced in optical microresonators [24, 39]. Further studies will also help to include this two crystal case with strong cascading quadratic nonlinearities to the existing theories of CW-pumped singly resonant OPOs .
Currently, the signal and idler tuning range is limited by the cavity mirrors. With different components, one could extend the tuning range of the MgO:LiNbO3 -based OPO beyond 4 µm. Other materials would provide access to even longer wavelengths [19, 41–45]. Use of a shorter crystal would be a straightforward method of increasing the spectral width of the comb, however, at the cost of an increased oscillation threshold of the OPO. This could be circumvented by using a PPLN waveguide  or a whispering gallery mode resonator . These methods, or alternatively the use of a monolithic OPO cavity , would also help to miniaturize the setup. Another option to increase the parametric gain bandwidth is to work closer to the degeneracy of the OPO [48, 49].
We thank IPG Photonics for a generous loan of the fiber amplifier which made these experiments possible. We are grateful to the University of Helsinki, the Academy of Finland, and the Magnus Ehrnrooth Foundation for funding the research reported in this contribution.
References and links
1. R. Holzwarth, T. Udem, T. W. Hansch, J. C. Knight, W. J. Wadsworth, and P. S. J. Russell, “Optical frequency synthesizer for precision spectroscopy,” Phys. Rev. Lett. 85(11), 2264–2267 (2000). [CrossRef] [PubMed]
2. S. T. Cundiff and J. Ye, “Colloquium: Femtosecond optical frequency combs,” Rev. Mod. Phys. 75(1), 325–342 (2003). [CrossRef]
5. A. Schliesser, N. Picque, and T. W. Hansch, “Mid-infrared frequency combs,” Nat. Photonics 6(7), 440–449 (2012). [CrossRef]
6. R. Paiella, F. Capasso, C. Gmachl, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, A. Y. Cho, and H. C. Liu, “Self-mode-locking of quantum cascade lasers with giant ultrafast optical nonlinearities,” Science 290(5497), 1739–1742 (2000). [CrossRef] [PubMed]
7. R. Paiella, F. Capasso, C. Gmachl, H. Y. Hwang, D. L. Sivco, A. L. Hutchinson, A. Y. Cho, and H. C. Liu, “Monolithic active mode locking of quantum cascade lasers,” Appl. Phys. Lett. 77(2), 169–171 (2000). [CrossRef]
8. P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450(7173), 1214–1217 (2007). [CrossRef] [PubMed]
9. A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, I. Solomatine, D. Seidel, and L. Maleki, “Tunable optical frequency comb with a crystalline whispering gallery mode resonator,” Phys. Rev. Lett. 101(9), 093902 (2008). [CrossRef] [PubMed]
10. Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett. 36(17), 3398–3400 (2011). [CrossRef] [PubMed]
11. C. Y. Wang, T. Herr, P. Del’Haye, A. Schliesser, J. Hofer, R. Holzwarth, T. W. Hänsch, N. Picqué, and T. J. Kippenberg, “Mid-infrared optical frequency combs at 2.5 μm based on crystalline microresonators,” Nat Commun 4, 1345 (2013). [CrossRef] [PubMed]
12. P. Maddaloni, P. Malara, G. Gagliardi, and P. De Natale, “Mid-infrared fibre-based optical comb,” New J. Phys. 8(11), 262 (2006). [CrossRef]
14. T. W. Neely, T. A. Johnson, and S. A. Diddams, “High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator,” Opt. Lett. 36(20), 4020–4022 (2011). [CrossRef] [PubMed]
15. I. Galli, F. Cappelli, P. Cancio, G. Giusfredi, D. Mazzotti, S. Bartalini, and P. De Natale, “High-coherence mid-infrared frequency comb,” Opt. Express 21(23), 28877–28885 (2013). [CrossRef] [PubMed]
16. J. H. Sun, B. J. S. Gale, and D. T. Reid, “Composite frequency comb spanning 0.4-2.4 microm from a phase-controlled femtosecond Ti:sapphire laser and synchronously pumped optical parametric oscillator,” Opt. Lett. 32(11), 1414–1416 (2007). [CrossRef] [PubMed]
18. S. T. Wong, K. L. Vodopyanov, and R. L. Byer, “Self-phase-locked divide-by-2 optical parametric oscillator as a broadband frequency comb source,” J. Opt. Soc. Am. B 27(5), 876–882 (2010). [CrossRef]
19. N. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, “Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser,” Opt. Express 20(7), 7046–7053 (2012). [CrossRef] [PubMed]
20. S. A. Diddams, L. S. Ma, J. Ye, and J. L. Hall, “Broadband optical frequency comb generation with a phase-modulated parametric oscillator,” Opt. Lett. 24(23), 1747–1749 (1999). [CrossRef] [PubMed]
22. J. Khurgin, J. M. Melkonian, A. Godard, M. Lefebvre, and E. Rosencher, “Passive mode locking of optical parametric oscillators: an efficient technique for generating sub-picosecond pulses,” Opt. Express 16(7), 4804–4818 (2008). [CrossRef] [PubMed]
23. V. Ulvila, C. R. Phillips, L. Halonen, and M. Vainio, “Frequency comb generation by a continuous-wave-pumped optical parametric oscillator based on cascading quadratic nonlinearities,” Opt. Lett. 38(21), 4281–4284 (2013). [CrossRef] [PubMed]
24. T. Herr, K. Hartinger, J. Riemensberger, C. Y. Wang, E. Gavartin, R. Holzwarth, M. L. Gorodetsky, and T. J. Kippenberg, “Universal formation dynamics and noise of Kerr-frequency combs in microresonators,” Nat. Photonics 6(7), 480–487 (2012). [CrossRef]
25. R. DeSalvo, D. J. Hagan, M. Sheik-Bahae, G. Stegeman, E. W. Van Stryland, and H. Vanherzeele, “Self-focusing and self-defocusing by cascaded second-order effects in KTP,” Opt. Lett. 17(1), 28–30 (1992). [CrossRef] [PubMed]
26. G. I. Stegeman, “χ(2) cascading: Nonlinear phase shifts,” Quantum Semicl. Opt. 9(2), 139–153 (1997). [CrossRef]
27. C. R. Phillips, C. Langrock, J. S. Pelc, M. M. Fejer, J. Jiang, M. E. Fermann, and I. Hartl, “Supercontinuum generation in quasi-phase-matched LiNbO3 waveguide pumped by a Tm-doped fiber laser system,” Opt. Lett. 36(19), 3912–3914 (2011). [CrossRef] [PubMed]
28. C. R. Phillips, C. Langrock, J. S. Pelc, M. M. Fejer, I. Hartl, and M. E. Fermann, “Supercontinuum generation in quasi-phasematched waveguides,” Opt. Express 19(20), 18754–18773 (2011). [CrossRef] [PubMed]
29. G. Cerullo, S. De Silvestri, A. Monguzzi, D. Segala, and V. Magni, “Self-starting mode locking of a Cw Nd:YAG laser using cascaded second-order nonlinearities,” Opt. Lett. 20(7), 746–748 (1995). [CrossRef] [PubMed]
30. M. Zavelani-Rossi, G. Cerullo, and V. Magni, “Mode locking by cascading of second-order nonlinearities,” IEEE J. Quantum Electron. 34(1), 61–70 (1998). [CrossRef]
31. S. J. Holmgren, V. Pasiskevicius, and F. Laurell, “Generation of 2.8 ps pulses by mode-locking a Nd:GdVO4 laser with defocusing cascaded Kerr lensing in periodically poled KTP,” Opt. Express 13(14), 5270–5278 (2005). [CrossRef] [PubMed]
32. J. J. Zondy, F. A. Camargo, T. Zanon, V. Petrov, and N. U. Wetter, “Observation of strong cascaded Kerr-lens dynamics in an optimally-coupled cw intracavity frequency-doubled Nd:YLF ring laser,” Opt. Express 18(5), 4796–4815 (2010). [CrossRef] [PubMed]
34. M. Vainio, J. Peltola, S. Persijn, F. J. M. Harren, and L. Halonen, “Thermal effects in singly resonant continuous-wave optical parametric oscillators,” Appl. Phys. B-Lasers O. 94(3), 411–427 (2009). [CrossRef]
35. M. Siltanen, M. Vainio, and L. Halonen, “Pump-tunable continuous-wave singly resonant optical parametric oscillator from 2.5 to 4.4 microm,” Opt. Express 18(13), 14087–14092 (2010). [CrossRef] [PubMed]
36. G. D. Boyd and D. A. Kleinman, “Parametric interaction of focused Gaussian light beams,” J. Appl. Phys. 39, 3597 (1968).
38. F. Ferdous, H. Miao, D. E. Leaird, K. Srinivasan, J. Wang, L. Chen, L. T. Varghese, and A. M. Weiner, “Spectral line-by-line pulse shaping of on-chip microresonator frequency combs,” Nat. Photonics 5(12), 770–776 (2011). [CrossRef]
39. K. Saha, Y. Okawachi, B. Shim, J. S. Levy, R. Salem, A. R. Johnson, M. A. Foster, M. R. E. Lamont, M. Lipson, and A. L. Gaeta, “Modelocking and femtosecond pulse generation in chip-based frequency combs,” Opt. Express 21(1), 1335–1343 (2013). [CrossRef] [PubMed]
40. C. R. Phillips and M. M. Fejer, “Stability of the singly resonant optical parametric oscillator,” J. Opt. Soc. Am. B 27(12), 2687–2699 (2010). [CrossRef]
41. Z. Zhang, D. T. Reid, S. Chaitanya Kumar, M. Ebrahim-Zadeh, P. G. Schunemann, K. T. Zawilski, and C. R. Howle, “Femtosecond-laser pumped CdSiP₂ optical parametric oscillator producing 100 MHz pulses centered at 6.2 μm,” Opt. Lett. 38(23), 5110–5113 (2013). [CrossRef] [PubMed]
42. J.-B. Dherbecourt, A. Godard, M. Raybaut, J.-M. Melkonian, and M. Lefebvre, “Picosecond synchronously pumped ZnGeP2 optical parametric oscillator,” Opt. Lett. 35(13), 2197–2199 (2010). [CrossRef] [PubMed]
43. M. A. Watson, M. V. O’Connor, D. P. Shepherd, and D. C. Hanna, “Synchronously pumped CdSe optical parametric oscillator in the 9-10 microm region,” Opt. Lett. 28(20), 1957–1959 (2003). [CrossRef] [PubMed]
44. R. K. Feaver, R. D. Peterson, and P. E. Powers, “Longwave-IR optical parametric oscillator in orientation-patterned GaAs pumped by a 2 µm Tm,Ho:YLF laser,” Opt. Express 21(13), 16104–16110 (2013). [CrossRef] [PubMed]
45. C. R. Phillips, J. Jiang, C. Mohr, A. C. Lin, C. Langrock, M. Snure, D. Bliss, M. Zhu, I. Hartl, J. S. Harris, M. E. Fermann, and M. M. Fejer, “Widely tunable midinfrared difference frequency generation in orientation-patterned GaAs pumped with a femtosecond Tm-fiber system,” Opt. Lett. 37(14), 2928–2930 (2012). [CrossRef] [PubMed]
46. T. Beckmann, H. Linnenbank, H. Steigerwald, B. Sturman, D. Haertle, K. Buse, and I. Breunig, “Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators,” Phys. Rev. Lett. 106(14), 143903 (2011). [CrossRef] [PubMed]
47. C. R. Phillips, J. S. Pelc, and M. M. Fejer, “Continuous wave monolithic quasi-phase-matched optical parametric oscillator in periodically poled lithium niobate,” Opt. Lett. 36(15), 2973–2975 (2011). [CrossRef] [PubMed]
48. M. Vainio, C. Ozanam, V. Ulvila, and L. Halonen, “Tuning and stability of a singly resonant continuous-wave optical parametric oscillator close to degeneracy,” Opt. Express 19(23), 22515–22527 (2011). [CrossRef] [PubMed]