AF2K.1.pdf CLEO 2019 © OSA 2019 Microresonator Spectrometer Using Counter-propagating Solitons Qi-Fan Yang 1,† , Boqiang Shen 1,† , Heming Wang 1,† , Minh Tran 2 , Zhewei Zhang 1 , Ki Youl Yang 1 , Lue Wu 1 , Chengying Bao 1 , John Bowers 2 , Amnon Yariv 1 and Kerry Vahala 1,* 1 T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 2 University of California, Santa Barbara, Department of Electrical and Computer Engineering, Santa Barbara, CA * vahala@caltech.edu Abstract: A spectrometer is demonstrated using self-locked counter-propagating soliton frequency combs in a high-Q silica microresonator. Fast tuning laser waveforms and molecular absorption features are measured with kiloHertz to MegaHertz resolution. OCIS codes: (190.4360) Nonlinear optics, devices; (140.3945) Microcavities; (190.5530) Pulse propagation and temporal solitons. Dual-comb spectrometers, in which a frequency vernier is created by the difference in line spacing of the two combs, provide rapid high-resolution spectral measurement of laser sources [1]. Similar to dual-comb spectroscopy [2] the frequency vernier allows the line-spacing ambiguity of the comb to be resolved by setting up a radio frequency grid when the two combs are heterodyned. However, in contrast to dual-comb spectroscopy in which an absorption spectrum is imposed on the combs, the measurement of active signals is performed by interference of the test laser with each comb in conjunction with a cross comb correlation method [1]. Recent developments in the field of high-Q micro- resonators provide a route to miniaturizing frequency combs through soliton mode locking [3]. Here, a high-resolution spectrometer is demonstrated using counter-propagating (CP) soliton microcombs within a single microresonator [4]. The dual comb spectrometer method relies upon high relative frequency stability of the underlying combs, and stability is achieved through a CP soliton phase locking phenomenon [5]. To establish its performance, the microresonator soliton spectrometer (MSS) is applied to measure various laser waveforms including continuously- and step-tuned lasers as well as a fiber mode-locked laser. Finally, absorption spectroscopy of a gas is performed using a tunable laser calibrated by the MSS. Figure 1a depicts the concept of the spectrometer. Counter-propagating soliton microcombs (blue and red) with dif- ferent repetition rates f r1,2 (Δ f r = f r2 - f r1 ) result from applying counter-propagating pumps with distinct frequencies. The pumps are derived from a single laser source by radio-frequency modulation and therefore have high relative sta- bility. For specific tuning of the pumping frequencies, phase-locking occurs at comb order μ = 0 by way of intracavity Power (20 dB/div) Wavelength (nm) 1500 1550 1600 CW soliton CCW soliton Pump Pump (c) (b) resonator Optical frequency Nf r c.w. laser f r =0 =N Chemical absorption (a) Phase locking Residuals (pm) n=54 N=77 (d) Wavelength (nm) 1545 1550 1555 1560 -0.1 0.1 0 Frequency (MHz) 0 4 Correlation (a.u.) RBW 200 Hz 0 1 =n f L f n1 f n2 f n1 -f n2 =nf r pump Fig. 1: Concept and static measurement. (a) Conceptual plot of vernier spectrometer dual comb spectra. (b) Typical correlation showing corre- sponding comb order n = 54. (c) Optical spectra of counter-propagating solitons. (d) Residual deviations between the MSS and a wavemeter in measuring the frequency of a static external-cavity diode laser.