Integrated mid-infrared photonics: From quantum cascade lasers to suspended silicon-on-insulator waveguides

Abstract

The mid-infrared, covering the wavelength range 3-30 µm (10 -100 THz) is of particular

importance for chemical species detection, since many atmospheric trace gases and

liquids have fingerprint spectra in this wavelength range. The atmospheric windows

in the wavelength range of 3-5 µm and 8-12 µm, offer a highly desirable low-loss

region for chemical trace gas sensing, high-bandwidth free-space communication, and

laser-based advanced defense countermeasures against IR-guided munitions. Leveraging

the silicon fabrication protocols for CMOS compatible Si-optical integration,

where photonic components are fabricated on a single Si chip, results in improved

yield, compactness and lower manufacturing costs due to the economy of scale.

Quantum Cascade (QC) lasers are intersubband mid-infrared light sources operating

between ~ 3-300 µm, made possible through advances in bandgap engineering

and fabrication techniques, offering promising applications due to their compact size,

range of operation and high output power. Fabry-Perot (FP) QC lasers have a relatively

broadband spectral output due to the wavelength-independent reffectivity of

the end facets and non-linearities such as spatial and spectral hole burning when operating

well above lasing threshold. Narrow-band, high-power operation of QC lasers

is, however, desirable in a variety of applications, such as laser-assisted surgery in

medicine or defense countermeasures.

This thesis focuses first on optimizing the spectral output of FP-QC lasers via

the application of shallow Distributed Bragg Reflectors (DBR), ion-milled after laser

fabrication and characterization. We observed a more than 10-fold reduction in spectral

full-width-half-maximum (FWHM) and up to 20 dB side-mode suppression ratio

(SMSR), maintained to peak optical power. Also observed was a "sweet-spot" in

grating length, ~ 200 µm on a 3mm long laser ridge, and a trade-off between spectral

narrowing and output power, set by the grating depth, varied from 1.8 - 2.5 µm.

Second, this thesis reports on a novel approach to integrating QC lasers into a

silicon platform through evanescent side-coupling of center-cleaved QC dies to custom

suspended Si waveguides (propagation loss 0:5 dB cm􀀀1 at 1:55 µm) in silicon-oninsulator

(SOI). The coupling parameters were optimized numerically, with laserwaveguide

separation upper limit of 2 µm and optimal bending radius for an adiabatic

taper determined to be 110 µm at 5 µm.

The authors envision the work undertaken in this thesis would pave the way for

the fabrication of single-mode mid-infrared lasers at scale and the integration of such

photonic components in a CMOS compatible Si platform, leading to more compact

and economical mid-infrared systems.