Abstract
Since their inception, optical frequency combs have transformed a broad range of technical and scientific disciplines, spanning time keeping to navigation. Recently, dual comb spectroscopy has emerged as an attractive alternative to traditional Fourier transform spectroscopy, since it offers higher measurement sensitivity in a fraction of the time. Midwave infrared (mid-IR) frequency combs are especially promising as an effective means for probing the strong fundamental absorption lines of numerous chemical and biological agents. Mid-IR combs have been realized via frequency down-conversion of a near-IR comb, by optical pumping of a micro-resonator, and beyond 7 μm by four-wave mixing in a quantum cascade laser. In this work, we demonstrate an electrically-driven frequency comb source that spans more than 1 THz of bandwidth centered near 3.6 μm. This is achieved by passively mode-locking an interband cascade laser (ICL) with gain and saturable absorber sections monolithically integrated on the same chip. The new source will significantly enhance the capabilities of mid-IR multi-heterodyne frequency comb spectroscopy systems.
Highlights
Since their inception, optical frequency combs have transformed a broad range of technical and scientific disciplines, spanning time keeping to navigation
Most Midwave infrared (mid-IR) combs have been realized via frequency down-conversion of a near-IR comb through optical parametric oscillation[12,13] or difference frequency generation[14,15,16,17] or by continuous wave optical pumping of a micro-resonator[18,19,20]
Beyond 7 μm, frequency combs based on four-wave mixing in quantum cascade lasers (QCLs) have recently produced high output powers with wide optical bandwidth[21,22]
Summary
The first step is to bombard the saturable absorber sections of the ICL ridges with 350 keV H+ ions at a dose of 5 × 1013 cm−2. A 3-μm-thick gold mask prevents penetration of the high-energy ions into the gain sections. Narrow-ridge waveguides are defined by contact lithography and transferred onto the wafer surface by plasma etching. Current is injected via a narrow metal contact patterned on the top surface of each ridge. A thin dielectric film covers the ridge sidewalls, after which 3 μm of gold is electroplated on top to provide heat extraction and allow wire bonding. The gain section comprises more than 90 % of the total cavity length, with the saturable absorber and non-contacted divider of width ≈100 μm occupying the rest. The wafer is polished to a thickness of ≈100 μm to facilitate cleaving into individual lasers with mirror-like end facets. The lasers are mounted epitaxial-side up on gold-plated BeO submounts to improve the heat extraction and facilitate measurement.
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