Case Study CS-LDTC14 Rev. A

Page 2

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Quantum cascade lasers (QCLs) and inter-band cascade

lasers (ICLs) have advanced in the last couple of decades

to provide a promising laser source for the 3-12 µm range

under direct current at room temperature. With these

laser sources, numerous developments have been made

in the mid-IR with integrated waveguides and high quality

passive ring resonators.1 What is needed now is a design

for an active ring resonator to add versatility in function and

application.

METHOD

Researchers from Harvard University, Massachusetts

Institute of Technology, Institute of Solid State Electronics

in Austria, and Institutes in Portugal have developed and

integrated a mid-infrared ring resonator and directional

couplers, using a quantum cascade active region in the

wave core. Because of this unique design, many of the

parameters of the ring resonator can be varied to give

electronic control over the resonance frequency and

the coupling strength between the waveguide and the

resonator. This ring resonator and waveguide design is

seen in

Figure 2.The lasers emit at around 8.2 µm within

the mid-IR range. The active coupler waveguide (WG)

allows for separate control over the injected electrical

currents into the WG and the racetrack (RT).

Asymmetric electrical driving can result in a mismatch of

mode indices inside the ring resonators and waveguide.1

Parameters such as the width of the gap, the index of

material in the gap, the length of the coupling region, the

resonator length and its intrinsic loss can be carefully

altered to define the amount of light coupled into the

resonator at a specific wavelength.1 To affect the resonance

of the ring resonator, the amplitude and the phase response

can be tuned. Thus, the resonance frequency, the coupling

strength, and the intrinsic quality factor of the resonator

can be modified for different effects and applications in the

mid-IR range.

Figure 2b shows the theoretical transmission

intensities of different coupling modes based on the tunable

refractive index (n') and round-trip loss coefficient (α) in

three regimes.

Figure 2c shows an optical microscope

image of the racetrack quantum cascade resonator with an

integrated active directional coupler.

The key to mid-IR operation is the core formed by a QCL

active region. The InGaAs waveguides with a low doped InP

cladding contain this core with intersubband transitions. By

changing the optical gain electrically, different parameters

of the waveguide and coupling regime can be tuned.

Researchers demonstrated the ability to tune parameters

and set its operating regime by measuring the transmission

and output of the designed mid-IR ring resonator.

Figure 2. a) Schematic of a ring resonator with a directional coupler. b) Theoretical intensity transmission of the

ring resonator with a tunable refractive index and the round-trip loss coefficient α in the three coupling regimes. The

depictions to the right of the transmission curves schematize the light intensity distribution in the waveguide and the

ring. c) Optical microscope image of the racetrack quantum cascade (QC) resonator with an integrated active directional

coupler. Integrated components are denoted with RT, for the racetrack, WG, for the waveguide coupler and HT, for the

integrated heater. d) Simulated light intensity distribution (λ=7.9μm, n

RT = 3.323) in the coupling region. The inset shows

an optical microscope image of the cross-section of the coupling region. e) Experimental spectrum of the sub-threshold

emission of a Fabry-Perot QC laser fabricated from the same epitaxial material as the ring resonator shown in c.1