Case Study CS-LDTC14 Rev. A

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Figure 4. a) Experimental transmission, below the

lasing threshold, of the ring resonator, showing four

consecutive resonances on the blue side of the pump

wave (P) generated by the ring QCL, into which we

inject signal light (S) to generate idler sidebands (I) by

four-wave mixing. b) Top graphs are the experimental

spectrograms of the ring QCL above the threshold under

external optical injection as the detuning of the signal

is swept across the ring resonance, shown for four

subsequent resonances, 4, 5, 6 and 7 FSRs to the blue of

the ring lasing frequency. Bottom graphs depict optical

spectra for the detuning of the signal wave when the

idler wave is the strongest.1

the lasing regime, it creates a strong single-frequency

unidirectional intracavity field.

Figure 4 shows the

transmission of the ring resonator when sweeping the

wavelength of the injected signal through four adjacent

resonances of the RT resonator. When tuning, an idler

sideband appears symmetrically on the red side of the

pump, showing parametric amplification via four-wave

mixing from coherent interaction of the pump and the signal

waves. As shown in the graphs, a signal and an idler photon

with frequencies of the four interacting waves are generated

from two pump photons.1

The last step was to generate frequency combs with the

ring QCLs as standalone lasers. Once the external probe

laser was removed, the ring QCLs can be characterized.

Previous techniques for mid-IR and THZ frequency comb

generators have revolved around Fabry-Perot (FP) QCLs

due to their compact nature and high power levels in the

10-100 mW range. However, with ring QCLs gaining

momentum, better stability, higher power efficiency, and

larger spectral coverage can be realized.1 Ring QCLs,

compared to FP QCLs, have had very limited output

power in the submilliwatt levels due to the low outcoupling

efficiency of the generated radiation. However, researchers

have solved this problem with RT QCLs with directional

couplers to enable the extraction of optical power above

10 mW at room temperature. This puts RT QCLs on the

same level as FP QCLs as seen in

Figure 5. The ring

resonator was also injected with regimes of bidirectional

and unidirectional lasing. Ultimately, the clockwise

direction leads to lasing at higher pumping levels. Once

the parametric gain is high enough, a frequency comb is

generated (

Figure 5c), proving the versatility of this mid-IR

ring resonator.

Figure 5. a) Output power from the front waveguide

facet of the ring QCL above the threshold as both RT and

WG currents are swept simultaneously. Both WG and RT

are operated under continuous wave electrical current

injection. b) Experimental intensities as function of the

injected current collected simultaneously from both WG

ports on two external detectors, showing the regimes of

bidirectional and unidirectional lasing. c) Experimental

spectrum of the self-starting frequency comb in a ring

QCL, when it operates in a unidirectional regime. PSD,

power spectral density. Inset shows a corresponding

RF spectrum of the intermode beat note. RBW is 750Hz,

sweep time is 1s.1