Towards optical-frequency-comb generation in continuous-wave-pumped titanium-indiffused lithium-niobate waveguide resonators

Abstract

Much progress, both experimentally and theoretically, has recently been made towards optical frequency comb generation from continuously pumped second-order nonlinear systems. Here, we present observations towards finding an integrated solution for such a system, using a titanium indiffused lithium niobate waveguide resonator. These results are compared to recently developed theory for equivalent systems. The system is seen to exhibit strong instabilities, which require further investigation in order to fully determine the suitability of this platform for stable optical frequency comb generation. 1 arXiv:1712.07448v4 [physics.optics] 1 Oct 2018Optical frequency combs (OFC's) are highly broadband coherent light sources that have found uses in a range of applications such as time and frequency metrology and molecular spectroscopy [1,2]. The usefulness of OFC's has led to rapid commercialization of the technology. The most common and reliable method of OFC generation has been through the use of mode-locked lasers, which provide high stability over long time time scales and can span large frequency ranges [3][4][5]. However, these systems tend to be quite large, and therefore there has been a strong drive towards miniaturization of the technology.Recently, most of the work regarding this goal has been directed towards systems involving the third-order nonlinearity. For example, OFC's have been produced in optical fiber cavities [6], whispering gallery mode resonators [7] and a number of microresonator systems [8,9].These are all promising systems, but they are also often plagued by extraneous effects such as mode competition and photothermal instabilities due to the high intensities required for comb generation (typically achieved through the use of strong pulsed light) [8]. A deeper understanding of these processes and possible methods for mitigating them are active areas of research [10][11][12][13].More recently, OFC's have also been observed from continuous-wave (CW) pumped second-order nonlinear systems. These systems typically offer nonlinear interaction strengths that are orders of magnitude larger than those found in third-order nonlinear systems. This may provide one avenue for reducing the amount of intra-cavity power required for comb generation, thereby providing some method of reducing unwanted thermal effects and thermallydriven mode competition. In addition, CW lasers are often simpler to work with than their pulsed counterpart. OFC generation in such systems typically occurs due to a number of cascaded nonlinearities between the various frequency components; second-harmonic generation (SHG) and optical parametric oscillation (OPO) [14]. The general scheme is shown in Fig. 1. CW pumped OFC generation utilising the second-order nonlinearity has been observed in various free-space cavity systems [14][15][16][17][18]. The recent results from CW pumped resonator systems utilising the second-order nonlinearity for comb generation has led to more complete theories that can explain much of the behavior seen f...

Optical Frequency Combs in Quadratically Nonlinear Resonators

“…Scaling the resonator to micrometric size may be beneficial for quadratic combs, allowing for a dramatic reduction of threshold power and a flexible management of the dispersion through a geometric design, allowing for a broader comb emission. As a matter of fact, direct generation of quadratic frequency combs has been very recently observed in chip-scale lithium niobate devices, such as periodically poled linear waveguide resonators [74,75], or exploiting naturally phase-matched SHG in whispering-gallery-mode resonators [76,77].…”

Design of spontaneous parametric down-conversion in integrated hybrid SixNy-PPLN waveguides

“…We can observe from the mode profiles that the optical mode of pump is first confined into the SixNy waveguide, and then lifted to PPLN region in section C. The SPDC light generated from the PPLN region goes down to the SixNy waveguide for convenient output with over 92.3% transmission. Thanks to the high index contrast structure and submicrometer thickness of the LN thin film, the waveguide modes are tightly confined into a mode area that is smaller than that achieved with proton exchange or Ti indiffusion [25,26]. This mode confinement feature directly relates to the refractive index of SixNy.…”

Spectral Broadening in a Continuously Pumped Singly Resonant Second-Harmonic Cavity

“…Soon thereafter, in 2007, OFCs were demonstrated in microresonators pumped by a continuous-wave (cw) laser, exploiting the third-order nonlinear susceptibility χ (3) , in so called Kerr combs [7,8]. Recent theoretical studies and experimental investigations have demonstrated frequency comb generation also in quadratic nonlinear optical cavities, realizing quadratic frequency combs [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. These sources provide an interesting alternative to traditional comb generation techniques.…”

“…These sources provide an interesting alternative to traditional comb generation techniques. They offer numerous advantages in terms of low pump power requirements, spectral versatility, and potential for miniaturization of the device [17][18][19][20][21][22][23]. Interestingly, they can generate simultaneous combs in different spectral ranges.…”