Mid-infrared frequency comb generation with silicon nitride nano-photonic waveguides (1704.02478v2)

Abstract: Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Substantial advances in mid-infrared frequency comb generation have been made in recent years based on nonlinear frequency conversion, microresonator Kerr frequency combs, quantum cascade lasers and mode locking regimes. Here we demonstrate a simple, yet effective method for the direct generation of mid-infrared optical frequency combs in the region from ${2.5-4~\mu{\rm m}}$, i.e. ${2500-4000~{\rm cm}^{-1}}$ covering a large fraction of the functional group region, directly from a conventional and compact erbium-fiber-based femtosecond laser in the telecommunication band (i.e. ${1.55~\mu{\rm m}}$). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in large-cross-section and dispersion-engineered silicon nitride (${\rm Si_3N_4}$) waveguides. The long-wavelength dispersive wave, with its position lithographically determined, performs as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such a simple and versatile approach to mid-infrared frequency comb generation is suitable for spectroscopic applications in the first mid-infrared atmospheric window. Moreover, the compactness and simplicity of the approach have the potential to realize compact dual-comb spectrometers. The generated combs have a fine teeth-spacing, making them also suitable for gas phase analysis.

• The paper introduces a novel method for direct mid-IR frequency comb generation from an erbium-fiber femtosecond laser using dispersive wave engineering in silicon nitride waveguides.
• The paper achieves broad wavelength coverage from 2.5 µm to 4 µm, facilitating effective probing of key molecular vibrational modes for enhanced spectroscopy.
• The paper validates comb coherence through optical heterodyne measurements, underscoring its potential for developing compact dual-comb spectroscopic systems.

Mid-Infrared Frequency Comb Generation with Silicon Nitride Nano-Photonic Waveguides

The paper "Mid-infrared frequency comb generation with silicon nitride nano-photonic waveguides" investigates the use of silicon nitride ($\text{Si}_3\text{N}_4$) waveguides for generating mid-infrared (Mid-IR) optical frequency combs. Mid-IR combs are of significant utility in molecular spectroscopy due to the strong absorption of molecular vibrational modes in this region, which enhances spectroscopic techniques such as dual-comb and cavity-enhanced spectroscopy.

Key Contributions

1. Novel Mid-IR Comb Generation Methodology: The authors introduce a technique for directly generating mid-IR frequency combs from an erbium-fiber-based femtosecond laser operating at the telecom band (1.55 µm). This is achieved through dispersive wave generation in large-cross-section, dispersion-engineered $\text{Si}_3\text{N}_4$ waveguides.
2. Wavelength Coverage: The reported comb spans from 2.5 µm to 4 µm, covering a broad spectrum necessary for accessing the functional group region, which includes common vibrational modes such as C-H, O-H, and N-H.
3. Coherence Measurement: The paper provides experimental validation of comb coherence through optical heterodyne measurements, ensuring the generated comb maintains phase coherence inherited from the seed laser.

Technical Highlights

• Dispersive Wave Generation: The dispersive wave contribution to supercontinuum generation (SCG) allows for efficient light conversion across a large frequency range. This approach leverages soliton dynamics, with the position of the dispersive wave lithographically controlled, to extend the frequency comb into the mid-IR.
• Use of Large-Cross-Section $\text{Si}_3\text{N}_4$ Waveguides: This design overcomes previous limitations related to material absorption and waveguide dispersion, extending functionality into the mid-IR range. Waveguides used have been optimized to balance mid-IR confinement with dispersion engineering, enhancing the coherence and broadening potential of the SCG process.

Implications

The simplicity and compactness of the proposed method make it well-suited for practical applications in spectroscopic measurements. The approach holds potential for the development of compact dual-comb spectrometers and could facilitate more sensitive and precise analytical techniques in fields like environmental monitoring and medical diagnostics.

Theoretical and Practical Implications

This work underlines the role of advanced waveguide engineering in optimizing nonlinear optical processes for the generation of coherent broadband spectra. It demonstrates the potential of integrating advanced photonic structures into practical systems that require high coherence and broad frequency coverage, aiding in the advancement of precision spectroscopy and expanding the applicability of frequency comb technologies.

Future Directions

Future research could explore scaling this technology to enhance the efficiency and spectral coverage further. The flexibility of $\text{Si}_3\text{N}_4$ waveguide design presents opportunities for extending these methodologies to other regions of the electromagnetic spectrum and for different types of spectroscopic applications.

In summary, this paper contributes a methodologically sound and efficient approach to mid-IR comb generation, grounded in thorough experimental investigation and coherent spectral synthesis, marking a step towards advanced nanophotonic devices with applications in spectroscopy and beyond. The findings emphasize the sophistication in leveraging nonlinear dynamics in waveguides and corroborate their importance in generating valuable optical tools for diverse scientific and industrial fields.