On-chip dual comb source for spectroscopy (1611.07673v1)

Abstract: Dual-comb spectroscopy is a powerful technique for real-time, broadband optical sampling of molecular spectra which requires no moving components. Recent developments with microresonator-based platforms have enabled frequency combs at the chip scale. However, the need to precisely match the resonance wavelengths of distinct high-quality-factor microcavities has hindered the development of an on-chip dual comb source. Here, we report the first simultaneous generation of two microresonator combs on the same chip from a single laser. The combs span a broad bandwidth of 51 THz around a wavelength of 1.56 $\mu$m. We demonstrate low-noise operation of both frequency combs by deterministically tuning into soliton mode-locked states using integrated microheaters, resulting in narrow ($<$ 10 kHz) microwave beatnotes. We further use one mode-locked comb as a reference to probe the formation dynamics of the other comb, thus introducing a technique to investigate comb evolution without auxiliary lasers or microwave oscillators. We demonstrate broadband high-SNR absorption spectroscopy of dichloromethane spanning 170 nm using the dual comb source over a 20 $\mu$s acquisition time. Our device paves the way for compact and robust dual-comb spectrometers at nanosecond timescales.

• The paper presents the first on-chip dual Kerr comb generation using a single continuous-wave laser, enabling soliton mode-locking with broad spectral coverage.
• The study employs silicon nitride microresonators with integrated microheaters to achieve precise thermal tuning and record-low noise with microwave beat notes under 10 kHz.
• The paper reports high-speed dual-comb spectroscopy of dichloromethane over a 170 nm range in 20 µs, highlighting its potential for compact, field-deployable spectrometers.

An Overview of the On-Chip Dual Comb Source for Spectroscopy

The paper "On-chip dual comb source for spectroscopy" by Avik Dutt et al. presents a significant advancement in the field of integrated optics, describing the first successful generation of dual Kerr frequency combs on a single chip using a solitary laser source. This development addresses a prominent challenge in integrated photonics—producing dual combs with sufficient spectral coherence and bandwidth within a compact format on a chip, a necessity for high-resolution and high-speed spectroscopic applications.

Key Achievements and Results

The authors report the simultaneous generation of two microresonator frequency combs spanning a bandwidth of 51 THz around a 1.56 µm wavelength. The dual combs exhibit mode-locked states with low-noise characteristics, evidenced by microwave beat notes having a narrow linewidth of under 10 kHz. This spectral purity and coherence imply a high relative stability and long coherence time between the two frequency combs, critical parameters for precise spectroscopic measurements.

One of the remarkable achievements herein is the dual-comb spectroscopy of dichloromethane, which was conducted over a 170 nm wavelength range with a rapid acquisition time of 20 µs. This high-speed acquisition was facilitated by the soliton mode-locked states of the combs and their operation from a single fixed-frequency pump laser. This innovation circumvents the usual complexities associated with using separate sources or external stabilizations, thereby simplifying the setup and reducing system bulk.

Technical Approach and Implementation

The paper employs silicon nitride (Si$_3$N$_4$) microresonator rings with slightly different radii to produce dual combs inherently synchronized, driven by a single continuous wave (cw) laser. The actualization of such a system involved intricate thermal tuning via integrated platinum microheaters for resonance alignment of high-Q microcavities, enabling soliton generation without additional frequency tuning of the laser. This approach also allows for the independent tuning of the generated combs to achieve soliton mode-locking, a notable improvement over previous techniques which relied on laser frequency adjustments.

Implications and Future Directions

The discussed technology has pivotal implications for developing compact dual-comb spectrometers, enhancing their potential for on-field applications. The technique can be extended beyond current bandwidths and wavelengths, including access to the mid-infrared "molecular fingerprint" region essential for identifying a broader range of molecular compositions in various media.

Furthermore, future work could benefit from integrating separate outputs for each comb to enable phase-sensitive dispersive measurements, which are crucial for capturing detailed spectroscopic signatures. Potential applications extend to real-time monitoring of dynamic processes including chemical reactions, biological systems, and atmospheric conditions.

Given the innovation this paper presents in integrating dual comb sources onto a chip, it stands to greatly impact the design of future spectroscopic systems, making them more robust, accessible, and efficient by leveraging integrated photonics technology. The groundwork laid by the authors opens pathways for next-generation sensors and analyzers, aptly pushing forward the envelope of spectroscopic capabilities.