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University of Helsinki Faculty of Science

Molecular spectroscopy and theoretical chemistry


Contact Information

Laboratory of Physical Chemistry
Department of Chemistry
A.I. Virtasen aukio 1
(P.O. BOX 55)
FI-00014 University of Helsinki

Group leader
Prof. Lauri Halonen, D.Sc.
Phone: +358(0)2941 50280
email: lauri.halonen


Experimental work in laser spectroscopy

Optical frequency comb generation by cascading quadratic nonlinearities

Fig. 1. CASCHI comb generation in a cw-pumped singly-resonant OPO. The OPO crystal (PPLN 1, periodically poled Mg:O-doped lithium niobate) is placed inside a cavity formed by mirrors that are highly reflective for the signal wavelength of the OPO. The poling period of PPLN 1 is designed for mid-infrared idler operation when pumped at 1064 nm. The resonant (signal) wavelength is typically about 1.5 Ám. A second crystal, PPLN 2, is designed for phase-mismatched second harmonic generation (SHG) of the resonant signal wave, which leads to cascading quadratic nonlinearity. This effect produces the comb around the signal wavelength. The comb is transferred to the mid-infrared idler wavelength by difference frequency generation (DFG) in PPLN 1 - a process that obeys the same phase-matching condition as the initial OPO process. The comb mode spacing is roughly the same as the free spectral range of the OPO cavity.

Direct optical frequency comb (OFC) spectroscopy, such as dual-comb spectroscopy, is one of the most powerful methods for real-time multispecies trace gas analysis. Mid-infrared (? > 3 Ám) frequency combs are needed in order to fully utilize the capabilities of OFC spectroscopy in trace gas detection. While direct mid-infrared frequency comb generation by mode-locked lasers is not yet possible, several methods based on nonlinear optical frequency conversion have been developed during the past years. We are doing research on a novel class of frequency combs, which is particularly suitable for mid-infrared comb generation. Our new method, which is based on cascading quadratic nonlinearities, allows comb generation with a simple cw laser pumping practically at any wavelength from visible to infrared. We have named this new class of OFCs as CASCHI comb (cascading chi-2 comb).

The basic principle of CASCHI comb generation is illustrated in Fig. 1. The cascading quadratic nonlinearity arises from phase-mismatched second harmonic generation in a ?(2) medium like lithium niobate [1-4]. This effect essentially mimics cubic nonlinearity, which leads to self-phase modulation and four wave mixing, and is the basis for, e.g., Kerr comb generation in optical microresonators [5]. While the CASCHI comb shares several aspects of the Kerr combs, it also has some distinct features. Most importantly, the effective cubic nonlinearity, or nonlinear refractive index, arising from the cascading quadratic nonlinearity can be several orders of magnitude larger than the inherent cubic nonlinearity of, say, lithium niobate [6]. This makes it possible to generate CASCHI combs also in bulk systems, not just microresonators. Another feature that makes the CASCHI comb scheme versatile compared to other solutions is that also the sign of the effective nonlinear refractive index can be changed by varying the wavevector mismatch ?k of the SHG process.

Fig. 2. Tuning of the center wavelength of a mid-infrared CASCHI comb generated using the scheme of Fig. 1. The tuning was done by adjusting the temperature and poling period of the PPLN crystals [2, 3].

We typically generate the CASCHI comb inside a cw-pumped optical parametric oscillator (OPO), as shown in Fig. 1. This approach makes it possible to produce a mid-infrared OFC with simple near-infrared laser pumping. Also, the center wavelength of the mid-infrared comb can be tuned by tuning the OPO [2]. This is demonstrated in Fig. 2. We have obtained a mid-infrared output power more than 3 W, which is the highest power ever reported for a mid-infrared OFC.

Fig. 3. The visible/near-infrared output of the CASCHI generated using the scheme of Fig. 1, as measured with a scanning-grating optical spectrum analyzer. Note that the intensities at different wavelengths are not in scale.

While we are mostly interested in the mid-infrared region in our CASCHI comb research, it is worth pointing out the several nonlinear mixing processes inside the OPO cavity inherently lead to comb generation also at many other wavelengths in the visible and near-infrared region, as is illustrated in Fig. 3. These additional features in the output spectrum also provide a means to stabilize the offset frequency of the comb(s), similar to what has previously been reported for OFCs generated by synchronously pumped singly-resonant OPOs [7, 8].

While the figures presented above only show the CASCHI comb envelope spectra, we have confirmed the underlying comb structure with several different measurement techniques. First, we have measured the infrared comb spectra with a Fourier transform infrared (FTIR) spectrum analyzer, which has a sufficiently high resolution (56 MHz) to resolve the adjacent comb modes, which are separated by approximately 208 MHz. The comb structure was also confirmed by beat-frequency measurements against a tunable diode laser, as well as by measuring the intermode RF beat frequency spectrum of the comb at different wavelengths [1]. In our recent research we have confirmed the CASCHI comb equidistance with a measurement accuracy of 1 Hz. We have also shown that the offset frequency and mode spacing of the generated mid-infrared comb can be independently controlled by controlling the pump laser and the OPO cavity length, respectively [2].

[1] V. Ulvila, C. R. Phillips, L. Halonen, and M. Vainio, "Frequency comb generation by a continuous-wave-pumped optical parametric oscillator," Opt. Lett. 38, 4281-4284 (2013).
[2] V. Ulvila, C. R. Phillips, L. Halonen, and M. Vainio, "High-power mid-infrared frequency comb from a continuous-wave-pumped bulk optical parametric oscillator," Opt. Express 22, 10535-10543 (2014).
[3] M. Vainio, V. Ulvila, C. R. Phillips, and L. Halonen, "Mid-infrared frequency comb generation using a continuous-wave pumped optical parametric oscillator," in Photonics West 2014, San Francisco, USA, February 1-6, 2014.
[4] C. R. Phillips, V. Ulvila, L. Halonen, M. Vainio, "Dynamics and Design Trade-Offs in CW-Pumped Singly-Resonant Optical Parametric Oscillator Based Combs," in CLEO 2014, San Jose, USA, June 8-13, 2014. [5] P. Del'Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, T. J. Kippenberg, "Optical frequency comb generation from a monolithic microresonator," Nature 450, 1214-1217 (2007).
[6] C. R. Phillips, C. Langrock, J. S. Pelc, M. M. Fejer, J. Jiang, M. E. Fermann, I. Hartl, "Supercontinuum generation in quasi-phase-matched LiNbO3 waveguide pumped by a Tm-doped fiber laser system," Opt. Lett. 36, 3912-3914 (2011).
[7] J. H. Sun, B. J. S. Gale, 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, 1414-1416 (2007).
[8] F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, J. Ye, "Phase-stabilized, 1.5 W frequency comb at 2.8-4.8 Ám," Opt. Lett. 34, 1330-1332 (2009).