Case Study CS-LDTC07 Rev. A

Page 3

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spacing of the harmonic comb is too great for the bandwidth

of the detector. Researchers at Harvard University2 use a

multi-heterodyne beating experiment between two QCLs.

One QCL is in the harmonic comb state, and the other

QCL is in the fundamental comb state as the reference

comb device. At Northwestern University, a reference

Fabry-Pérot (FP) QCL is used as a reference comb in the

fundamental comb state to verify the THz comb operation.

The FP QCL is operated at a current of 1.52 A to establish

the fundamental state. The DFB QCL is operated at a

current of 1.45 A in the multimode harmonic state. Both

are measured using a Fourier Transform Infrared (FTIR)

Spectrometer. These settings can be seen in

Figure 4.

Figure 4. Multi-heterodyne characterizations based

on the DFB and FP QCL combs. (a) Lasing spectrum of

the FP and DFB combs recorded with the FTIR. Multi-

heterodyne beating of the FP comb at 1.52 A and the DFB

comb at 1.45 A (b), and the DFB comb at 1.55 A (c). Total

acquisition time: 100 ms1

The heterodyne spacing is determined by the frequency

difference between mode spacing of the DFB harmonic

comb and the multiple of FP mode spacing.1 This

experiment confirms the harmonic comb generation of the

DFB QCL.

The DFB QCL, with dual wavelength operation, enables

the second-order difference-frequency generation that can

transfer the mid-IR comb into the THz range.

RESULTS

With this innovative technique, the researchers have

achieved a THz harmonic frequency comb at 3.0 THz at

room temperature with a range of 2.2 to 3.3 THz. All testing

was performed on a thermoelectric cooler (TEC) stage

precisely controlled to room temperature (293 K).

Figure 5

shows the THz frequency results at different currents of the

DFB QCL harmonic comb.

Figure 5. On-chip generation of THz frequency comb

from a mid-IR QCL. Lasing THz spectra of the DFB QCL

comb evolving at different currents from 1.20 to 1.55 A at

room temperature continuous wave operation1

The frequency spacing between modes, in

Figure 5, is

157 GHz for currents from 1.3 - 1.5 A and 245 GHz for

currents above 1.5 A. This corresponds to 14 FSR and

22 FSR mode spacings. Because the two mid-IR pumping

sources are simultaneously generated from the same QCL