Coexisting frequency combs spaced by an octave in a monolithic quantum cascade laser

TL;DR

This paper demonstrates a monolithic terahertz quantum cascade laser that coexists with two octave-spaced frequency combs, enabling broad spectral coverage and internal self-referencing for compact mid-infrared and terahertz applications.

Contribution

It introduces a heterogenous quantum cascade laser with two active regions spaced by an octave, capable of simultaneous comb operation and serving as a test bed for self-referencing techniques.

Findings

Achieved dual comb operation with narrow RF beatnotes (~15 kHz).

Demonstrated broad spectral coverage up to 4.6 THz.

Showed potential for compact, broadband frequency comb sources.

Abstract

Quantum cascade lasers are proving to be instrumental in the development of compact frequency comb sources at mid-infrared and terahertz frequencies. Here we demonstrate a heterogeneous terahertz quantum cascade laser with two active regions spaced exactly by one octave. Both active regions are based on a four-quantum well laser design and they emit a combined 3~mW peak power at 15~K in pulsed mode. The two central frequencies are 2.3~THz (bandwidth 300~GHz) and 4.6~THz (bandwidth 270~GHz). The structure is engineered in a way that allows simultaneous operation of the two active regions in the comb regime, serving as a double comb source as well as a test bench structure for all waveguide internal self-referencing techniques. Narrow RF beatnotes ($\sim$ 15~kHz) are recorded showing the simultaneous operation of the two combs, whose free-running coherence properties are investigated by means of beatnote spectroscopy performed both with an external detector and via self-mixing. Comb operation in a highly dispersive region (4.6~THz) relying only on gain bandwidth engineering shows the potential for broad spectral coverage with compact comb sources.