Case Study CS-LDTC07 Rev. A

Page 2

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SOLUTION DESIGN

To down-convert to the THz range, another mid-IR comb or

a single mode is needed at a different frequency. This must

be generated in the same cavity simultaneously, or it must

be generated with two QCL sources as in Reference 2. It is

difficult for QCLs to emit two separate wavelengths at the

same time owing to the gain competition of the QCL design.

Solutions to this problem can bring dispersion in the cavity

and can suppress comb operation.

Researchers from Northwestern University, Illinois have

developed a THz frequency comb using a mid-IR QCL and

difference-frequency generation. A distributed feedback

(DFB) grating is inserted into the QCL cavity so a single

mode can be created along with the harmonic comb at the

same time. The DFB grating is largely frequency-detuned to

ensure the emission of both the single mode and harmonic

comb. This wavelength is detuned ~80-90 cm-1 with regards

to the comb emission wavelength.1 This generates the least

effect on laser dispersion. This DFB grating design in the

cavity of the QCL does not negatively affect the four-wave

mixing for comb operation.

Figure 2 shows the QCL and

DFB grating design.

Figure 2. Schematic of largely detuned DFB QCL

design for THz frequency comb operation. Both the

single-mode (λ

1) and harmonic (λ2) states are shown in

the QCL cavity to produce the THz frequency comb (λ

THz).

Also shown are the layers of the QCL material and the

DFB addition1

Figure 3 shows the single-mode and multimode (harmonic

comb) state generation from the single mid-IR QCL. The

current is increased from two single-mode states operation

to a stable harmonic comb and a single mode state

operation above 1.5 A.

Figure 3. On-chip generation of single mode and

multimode comb from a single mid-IR QCL. Lasing

mid-IR spectra of a 4-mm long DFB QCL evolving with

currents from 1.2 to 1.60 A at room temperature in

continuous wave operation1

The DFB QCL device generates two distinct states: a

single mode (

λ

1 = 7.25 µm) and a multimode emission

(

λ

2 = 7.81 µm) with mode separation of 14 or 22 times the

FSR depending on the current. Lu states,

The impact of the DFB element is twofold. The single mode

emission resulted from the DFB section forms a spatial

population grating in the cavity and induces an incoherent gain

for multimode operation near the gain peak at higher currents.

On the other hand, the beating between the single DFB mode

and the multimode introduces additional population pulsation

nonlinearity which in turn contributes to the mode skipping of the

multimode emission in the working current range.1

Conventional mid-IR detectors are not useful in this

experiment to analyze the comb operation as the mode