[Paper Review] Widely-tunable mid-IR frequency comb source based on difference frequency generation

Axel Ruehl, Alessio Gambetta+4 more|arXiv (Cornell University)|Mar 12, 2012

Advanced Fiber Laser Technologies4 citations

TL;DR

This paper presents a widely-tunable mid-infrared frequency comb source spanning 3–10 μm via difference frequency generation in a GaSe crystal, pumped by a 151 MHz Yb:fiber frequency comb and seeded with Raman-shifted solitons from a suspended-core fiber. The system achieves up to 1.5 mW average power at 4.7 μm, enabling broad spectral coverage for molecular fingerprinting applications.

ABSTRACT

We report on a mid-infrared frequency comb source of unprecedented tunability covering the entire 3-10 μm molecular fingerprint region. The system is based on difference frequency generation in a GaSe crystal pumped by a 151 MHz Yb:fiber frequency comb. The process was seeded with Raman shifted solitons generated in a highly nonlinear suspended-core fiber with the same source. Average powers up to 1.5 mW were achieved at 4.7 μm wavelength.

Motivation & Objective

• To develop a mid-infrared frequency comb source with broad spectral tunability across the molecular fingerprint region (3–10 μm).
• To enable practical mid-IR spectroscopy applications requiring high spectral brightness and wide coverage.
• To overcome limitations of existing mid-IR sources by combining frequency comb technology with difference frequency generation.
• To achieve high average output power in the mid-IR while maintaining broad tunability and coherence.

Proposed method

• A 151 MHz Yb:fiber frequency comb serves as the pump source for difference frequency generation in a GaSe crystal.
• Raman-shifted solitons generated in a highly nonlinear suspended-core fiber seed the difference frequency process.
• The GaSe crystal is used for efficient mid-IR generation due to its broad transparency and high nonlinearity in the 3–10 μm range.
• The system leverages the comb nature of the pump and seed sources to generate a coherent frequency comb in the mid-IR.
• Phase matching in the GaSe crystal is optimized for difference frequency generation across the target spectral band.
• Power scaling is achieved through careful control of pump and seed intensities and fiber nonlinearities.

Experimental results

Research questions

• RQ1 Can a mid-infrared frequency comb be generated over the entire 3–10 μm molecular fingerprint region using difference frequency generation?
• RQ2 What is the maximum average power achievable in the mid-IR using this approach with a fiber-based pump source?
• RQ3 How does the use of Raman-shifted solitons as a seed improve the efficiency and spectral coverage of the frequency comb?
• RQ4 What are the practical limitations in power and tunability when using GaSe for difference frequency generation?
• RQ5 Can a compact, fiber-integrated system produce a widely-tunable, coherent mid-IR frequency comb suitable for spectroscopy?

Key findings

• The system achieves a continuous tunable mid-IR frequency comb across the entire 3–10 μm spectral range.
• A peak average output power of 1.5 mW is achieved at a wavelength of 4.7 μm.
• The use of Raman-shifted solitons as a seed source enables efficient and broadband difference frequency generation.
• The GaSe crystal supports efficient frequency conversion across the target mid-IR band due to its high nonlinearity and transparency.
• The system demonstrates coherent frequency comb operation in the mid-IR with potential for real-world spectroscopic applications.
• The approach enables a compact, fiber-pumped source with high spectral brightness and wide tunability.