Techniques for stabilizing the carrier-envelope offset (CEO) frequency of an optical frequency comb are prerequisites for precision spectroscopy. To stabilize an optical frequency comb, we need to generate a supercontinuum (SC) light, which broadens its spectrum. A 2f-to-3f self-referencing interferometer (SRI) is used for stabilizing the CEO frequency of an optical frequency comb that has only a 2/3-octave bandwidth of SC light. To stabilize such an optical frequency comb, we constructed a collinear 2f-to-3f SRI with a dual-pitch (DP-) periodically poled lithium niobate (PPLN) ridge waveguide .
The DP-PPLN ridge waveguide has two different pitch sizes (Λ1 and Λ2) to satisfy the quasi-phase matching conditions for generating second- and third-harmonic lights with a wavelength of 600 nm [Fig. 1(a)]. In the first part, second-harmonic light with a wavelength of 900 nm is generated from the SC component at around 1800 nm. In the second part, sum frequency light with a wavelength of 600 nm is generated from the SC component at 1800 nm and 900-nm second-harmonic light. In addition, in the second part, second-harmonic light with a wavelength of 615 nm is involved. If 615-nm second-harmonic light and 600-nm third-harmonic light spectrally overlap, a CEO beat signal is obtained. Figure 1(b) shows a CEO beat signal. The SNR of the CEO beat is 52 dB at the resolution bandwidth of 100 kHz. This is the highest SNR ever reported in a 2f-to-3f SRI.
Then, to evaluate the instability of the CEO frequency, we measured Allan deviations for both in-loop and out-of-loop interferometers, where the former was used for stabilizing an Er-doped fiber laser and the latter was used for monitoring the actual instability. The results showed that both the in-loop and out-of-loop Allan deviations are 7×10-15 (= 1.4 Hz) at gate time of 1 s [Fig. 1(c)]. Furthermore, we measured the Allan deviations after we intentionally placed a small pump on the breadboard near the interferometer. We found that despite the pump vibration, both the in-loop and out-of-loop Allan deviations remained almost the same values. These results confirmed that our collinear 2f-to-3f SRI allows us to stabilize a CEO frequency regardless of environmental perturbation.
This work was supported by KAKENHI.
Fig. 1. (a) DP-PPLN ridge waveguide. (b) CEO beat signal. (c) Allan deviations of CEO frequencies.