Case Study CS-LDTC14 Rev. A

Page 3

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Figure 3. a) Experimental transmission of the RT coupled to the WG as the wavelength of the probe laser is swept

across the RT resonance. b) Transmitted intensity of the probe signal at 1222cm−1 as function of current density of the

racetrack (RT) and the directional waveguide coupler (WG) and the corresponding least squares model fit. c) Experimental

transmission (dots) and model fit (solid line) as function of the RT current density for the fixed WG current density of

0.52kA cm−2. d) Extracted (dots) and modeled (solid line) RT loaded quality factor (Q) as function of the RT current.

e) Power coupling coefficient, calculated based off the extracted experimental values for the complex indices of the WG

and RT, as function of the WG current for three different values of the RT current. f) Values extracted from the fit shown in

b and c. nRT, background mode index (when the electrical pumping is off) of the RT. Δn, complex index change per unit

of current density. Δn

cross, index change due to thermal crosstalk between WG and RT. |κ|

2

, power coupling coefficient.1

change of the refractive index effectively sweeps the RT

cavity mode resonances. Increasing pumping leads to

narrowing of the resonances, and the mode index mismatch

between the coupled waveguides leads to the change of

the power coupling coefficient |κ|2 between the WG and

the RT.1 By making both simple and complex changes, the

parameters of the active RT resonators and the coupling

regimes can be tuned and controlled.

Each coupling regime can be utilized for appropriate

applications. When the ring resonator is critically coupled,

the extinction ratio on the resonance is useful for sharp

notch filters and intensity modulators. Over-coupled ring

resonators can be used as phase modulators due to the

sharp pi phase change across the resonance and a reduced

resonance contrast.1 These regimes can be chosen on

demand by tuning α.

The next step of testing and proving this mid-IR active ring

resonator is pumping the RT above the lasing threshold to

compensate the loss through the coupler with the stronger

internal gain, creating a nonlinear optical frequency

conversion element. When the ring resonator is operating in

With the RT kept below the lasing threshold, regime control

and tunability of the active quantum cascade resonators are

possible. When the RT is brought to above lasing threshold,

the ring resonator can function as a nonlinear optical

frequency conversion element. When the external probe

laser is removed, the right QCLs can be characterized

as standalone lasers and self-starting frequency comb

generators. In this special experiment, the differences

between RT QCLs with active or directional couplers and

ring QCLs without coupling ports is discovered.

RESULTS

Figure 3 shows the transmission characteristics of the

racetrack-waveguide (RT-WG) system while keeping the

RT below its lasing threshold. By sweeping the wavelength

of the probe laser, a dip in transmission (

Figure 3a) is seen

as a function of probe detuning, and this correlates to an

RT quality factor Q that can be continuously tuned over

two orders of magnitude as population inversion increases

(

Figure 3d). By sweeping the drive currents of both the WG

and the RT (

Figure 3b), the thermal and carrier-induced