High-Efficiency Detection of Carrier-Envelope Offset Frequency with a Dual-Pitch PPLN Ridge Waveguide


Kenichi Hitachi, Atsushi Ishizawa, and Masaki Asobe
Optical Science Laboratory

   A carrier-envelope offset (CEO) stabilized optical frequency comb provides dramatic progress in the field of precision spectroscopy and femtosecond pulse shaping. We have studied a 25-GHz-mode-spacing optical frequency comb provided by an externally phase-modulated laser diode, in which each frequency mode can be separated. This is an advantage compared to a common mode-locked laser whose repetition rate is smaller than a few hundred megahertz. For stabilizing a frequency comb, it is common to utilize af-to-2f self-referencing interferometer (SRI), in which one-octave bandwidth of the supercontinuum (SC) spectrum is required [1]. However, it is difficult to use this method for a phase-modulated frequency comb, because phase noise in the SC spectrum increases as the frequency departs from the original frequency. For this reason, we focus on a 2f-to-3f SRI, in which 2/3-octave bandwidth of the SC spectrum is used. By utilizing a fiber laser for generating a SC spectrum and a dual-pitch (DP) periodically poled lithium niobate (PPLN) ridge waveguide for third-harmonic (TH) generation, we succeeded in detecting the CEO frequency with a high signal-to-noise ratio (SNR) (> 30 dB).
   A DP-PPLN ridge waveguide device consists of two monolithically integrated segments with different quasi-phase matching (QPM) pitch sizes (1, 2) [Fig. 1(a)]. First, the second-harmonic (SH) light of the SC component at the frequency of f1 is generated in the first segment with the pitch size of 1. Then, the sum frequency of the SH light (2f1) and SC component of f1f is generated in the second segment with 2 [2]. Confinement of the SC light with a ridge waveguide structure and monolithically integrated design free from optical coupling loss enables TH generation with high efficiency.
   In the experiment, a SC light (~300 mW) was generated by injecting the output from the fiber laser into a high nonlinear fiber, and spectrally separated with a dichroic mirror at the wavelength of 1500 nm. The SH (TH) light is generated from the short- (long-) wavelength component [~1200 (1800) nm] of the SC light. By detecting the heterodyne beat of the two outputs with a photodetector, we detected CEO signals with a SNR > 30 dB, which is high enough for stabilizing the CEO frequency [Fig. 1(b)]. In the future, we will detect and stabilize the CEO frequency of our developed externally phase-modulated laser with this technique.

[1] A. Ishizawa et al., Opt. Express 16 (2008) 4706.
[2] K. Hitachi et al., Electron. Lett. 49 (2013) 145.

Fig. 1. (a) The schematic of a DP-PPLN ridge waveguide and (b) the detection of CEO signal.