Mid-infrared frequency comb spanning an octave based on an Er fiber laser and difference-frequency generation

TL;DR

This paper presents a coherent mid-infrared frequency comb spanning an octave, generated via difference-frequency generation in GaSe using an Er fiber laser, enabling advanced spectroscopic applications.

Contribution

It introduces a compact, tunable mid-infrared frequency comb based on an Er fiber laser and nonlinear frequency conversion, covering a broad spectral range with high coherence.

Findings

Achieved 700 cm-1 bandwidth covering 4-17 μm

Generated a stable, coherent frequency comb with 500,000 modes

Demonstrated potential for high-resolution spectroscopy and microscopy

Abstract

We describe a coherent mid-infrared continuum source with 700 cm-1 usable bandwidth, readily tuned within 600 - 2500 cm-1 (4 - 17 \mum) and thus covering much of the infrared "fingerprint" molecular vibration region. It is based on nonlinear frequency conversion in GaSe using a compact commercial 100-fs-pulsed Er fiber laser system providing two amplified near-infrared beams, one of them broadened by a nonlinear optical fiber. The resulting collimated mid-infrared continuum beam of 1 mW quasi-cw power represents a coherent infrared frequency comb with zero carrier-envelope phase, containing about 500,000 modes that are exact multiples of the pulse repetition rate of 40 MHz. The beam's diffraction-limited performance enables long-distance spectroscopic probing as well as maximal focusability for classical and ultraresolving near-field microscopies. Applications are foreseen also in studies of transient chemical phenomena even at ultrafast pump-probe scale, and in high-resolution gas spectroscopy for e.g. breath analysis.