Dual-Comb Spectroscopy

Rapid characterization of optical and vibrational spectra with high resolution can identify species in cluttered environments and is important for assays and early alerts. Dual-comb spectroscopy has emerged as a powerful approach to acquire nearly instantaneous Raman and optical spectra with unprecedented resolution. Spectra are generated directly in the electrical domain and avoid bulky mechanical spectrometers. A miniature soliton-based dual comb system can potentially transfer the approach to a chip platform. These devices achieve high-coherence, pulsed mode locking. They also feature broad, reproducible spectral envelopes, which is essential for dual-comb spectroscopy. Our work shows the potential for integrated, high signal-to-noise spectroscopy with fast acquisition rates.

Dual-comb spectroscopy enables rapid acquisition of broad-band Raman and optical spectra in the electrical domain with high accuracy and unprecedented resolution. Miniaturization of dual-comb spectroscopy systems would further extend the application of this technique. Along these lines a miniature comb technology, the frequency microcomb, provides a pathway to both reduce form factor and integrate frequency comb technology with other photonic devices and electronics. Moreover, the recent realization of soliton-mode-locking in microcombs provides phase-coherent femtosecond pulses with well-defined and predictable spectral envelopes, which is essential for dual-comb spectroscopy. In this work we report the first demonstration of dual-microcomb spectroscopy using soliton microcombs.

Fig. 1: (a) Illustration of the soliton dual-comb spectroscopic system. (b) Optical spectra of generated dual solitons. Inset : RF beatnote showing soliton repetition rates. (c) Time domain interferogram (above) and RF spectrum (below) of the dual soliton beatnote. (d) Measured absorption spectra of the H13CN 2ν3 band using a scanning laser and the dual soliton frequency comb. Lower panel is the the residual difference which has a standard deviation of 0.0254.

The principle of microresonator soliton dual-comb spectroscopy is described in figure 1a. Pumped by continuous wave lasers, solitons are generated in the microresonators via the Kerr nonlinearity which both compensates group velocity dispersion and provides parametric amplification. Two soliton pulse trains having slightly different repetition frequencies are combined and coupled into two optical paths. One path produces the signal by passing the pulses through the test sample while the other path produces a reference signal. Two high-Q silica wedge resonators (3 mm in diameter) are used for soliton generation. By precise control of the resonator size during microfabrication, two soliton microcombs with closely-matched repetition rates (21.9842 GHz and 21.9815 GHz) are generated. Typical soliton optical spectra are presented in figure 1b and feature the theoretically predicted sech²spectral envelope. The electrical beatnotes of the directly detected soliton pulse streams (fig. 1b inset) have signal-to-noise ratios exceeding 75 dB, revealing the excellent stability of the corresponding repetition rates.

A beatnote of the co-propagating soliton streams is produced on two photodetectors. The resulting periodic interferogram has a typical form shown in fig. 1c (upper panel) as measured on an oscilloscope. The interoferogram period (386 ns) corresponds to the difference in soliton repetition rates (2.6 MHz). Thermal tuning of one of the resonators was also used to adjust the carrier frequency of the interferogram to be less then 1 GHz (the oscilloscope bandwidth). A fast Fourier transform of the interferogram produces the electrical spectrum (lower panel in fig. 1c). To conduct a dual frequency comb spectral measurement, the electrical spectrum produced in the signal path is normalized by the electrical spectrum in the reference path. Through this process the optical spectrum extending over 4 THz of bandwidth (1535 nm to 1567 nm) is compressed into an electrical spectrum extending over 500 MHz. In the current experiment, the soliton pulse streams are not optically locked and this limits the signal to noise in the electrical spectrum.

The absorption spectrum of the H¹³CN 2ν₃ band acquired by the microresonator soliton dual comb is shown in figure 1d (upper panel). An independent measurement using a wavelength-calibrated scanning laser is also presented for comparison. The 22 GHz repetition rate of the soliton microcombs in this work leads to spectral under-sampling of the absorption. Nonetheless, the characteristic envelope of the H¹³CN 2ν₃ band is clearly resolved, and the residual error between the scanning laser and the dual-comb results shows a small standard deviation of 0.0254. Higher spectral resolution can be achieved by using larger resonators. For instance, a non-soliton microcomb has been demonstrated with comb spacing as narrow as 2.4 GHz.

In conclusion, dual-comb spectroscopy has been demonstrated using soliton microcombs. Good stability and signalto-noise were obtained. It should be possible to extend the soliton spectral coverage anywhere within the transmission window of silica by either operating with a different pumping wavelength or by generation of dispersive waves . Moreover, various material platforms provide microcomb operation with access to mid-infrared spectra. Integrating soliton microcombs with other on-chip devices can one day make possible a dual-comb spectroscopic system-on-a-chip.

Suh, M.G., Yang, Q.F., Yang, K.Y., Yi, X. and Vahala, K.J., 2016. Microresonator soliton dual-comb spectroscopy. Science, 354(6312), pp.600-603

Microresonator Soliton Dual-Comb Spectroscopy