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Case Study CS-LDTC14 Rev. A

Active Ring Resonators Using

Mid-Infrared QCLs

March 2024

Page 1

ABSTRACT

Researchers in Massachusetts, Austria, and Portugal have designed an active mid-infrared ring resonator

incorporating a quantum cascade active region in the waveguide core with directional couplers. The

resonance frequency, quality factor, and coupling regime and coefficients can all be tuned electrically

to better fit multiple applications. By changing these parameters, the active ring resonator can act as a

tunable filter, a nonlinear frequency converter, or a frequency comb generator. This design allows active

ring resonator integration into the mid-infrared spectral region (3-12 µm) for a variety of applications in

photonic integrated circuits with significant power output of 10 mW for spectroscopy, communication, and

microwave generation.

RING RESONATORS

Ring resonators are one of the most versatile building

blocks of photonic integrated circuits (PICs).1 They have

contributed to scaling down optical laboratory experiments

and making commercial technologies more portable. The

PIC world is growing in applicability and physically shrinking

in size, and it can enable reduction of global electricity

consumption, improved classical and quantum optical signal

processors, and lightweight and cost-effective devices for

spectroscopy.1 But how do ring resonators work?

High quality optical ring resonators use a set of waveguides

to confine light in a small volume and store it for millions of

round-trips. One of the waveguides is a closed loop coupled

to the input and output light (

Figure 1). Light injected

through the input waveguide is partially coupled into the

closed loop and ring resonator, and the other portion

is transmitted into the output facet. Depending on the

material and characteristics of the ring resonator, the field

experiences gain or attenuation for particular wavelengths

and is controlled by the round-trip loss coefficient (α).

Figure 1. Schematic of a ring resonator with a

directional coupler. The field experiences gain or

attenuation depending on the value of α.1

There are two types of resonators using waveguides:

passive and active. Passive resonators can adjust the

resonance frequency of a transparent dielectric waveguide

at kilohertz rates using thermal tuning of the refractive index

or at gigahertz rates using the electro-optic effect. Active

resonators use an amplifying medium in the waveguide

core instead of a transparent medium. This allows not only

the resonance frequency and coupling strength but also

the intrinsic quality factor of the resonator, to be tuned via

electrical or optical pumping to turn absorption into gain.1

Due to the selective wavelength nature of the ring

resonance within the closed loop, ring resonators can

be utilized as optical wavelength filters, modulators,

and frequency converters. The distance between the

waveguide and the ring resonator, the coupling length, and

the refractive indexes of both materials can be tuned for

different applications.

PROBLEMS AND GOALS

Ring resonators are nothing new to the PIC world in near-

infrared (near-IR) and visible ranges, but the mid-IR range

lacks development of PIC tools and solutions. The mid-

IR range is largely dominated by spectroscopy, chemical

and biological sensing, and free space communications

applications, yet compact and powerful laser sources in this

range have been missing. Ultimately, this resulted in less

manufacturing and design of waveguides, resonators, and

integrated photonic chips for these applications. Simply

extending the wavelength range for well established

techniques in the near-IR and visible ranges is not an option

with large losses and the requirement of unconventional

materials.1 A new, state-of-the-art, mid-IR technique for

photonic integration is crucial for ring resonators.