Case Study CS-LDTC07 Rev. A

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chip, the stability of the THz comb is greatly improved from

the frequency tuning rates that are shared.1 The THz comb

tuning rates of the emitting frequency and carrier frequency

are estimated to be -.56cm-1/A and 6.9 MHz/mA. These

values are over ten times smaller than the mid-IR comb

values, increasing stability.1

The DFB QCL design has the increased current dynamic

range for harmonic comb operation and increased

reproducibility while operating at room temperature. It has

also decreased complexity in the setup - no cryogenic

cooling or external optical elements are needed. Further

research and experimentation could enable monolithic

control and tuning of emission of harmonic combs as well

as realization of fundamental THz frequency comb through

the DFB QCL design.

ALTERNATIVE APPLICATIONS

selF-starting harmonic Frequency comb

Researchers at Harvard University, Massachusetts

have developed frequency combs using QCLs for other

applications based on a self-starting comb generation

design.2 Researchers from Harvard University have

developed a terahertz harmonic frequency comb using a

QCL device.

This experiment studies the harmonic comb state of

the QCL and its possibilities. Two FP QCLs are used to

produce and confirm the presence of a harmonic comb.

Injected current is increased from the lasing threshold

of single-mode operation to harmonic comb state. The

second FP QCL has a higher injection current to operate

at the fundamental comb state for reference to verify the

equidistant spacing between modes in the harmonic comb

QCL.

This multiheterodyne beating technique allows the

measurement of terahertz-scale beatnote frequency of

the harmonic state. This allows for the sample spectrum

to be down-converted from the optical domain to the

radiofrequency (RF) domain. The RF comb that is created

can be measured and verified using electronic frequency

counters.

The harmonic comb QCL operates with a repetition

rate of 400 GHz, and the reference fundamental comb

QCL operated with a repetition rate of 7.7GHz. The

light emitted from the harmonic comb QCL is passed

through the reference QCL. This enables extraction of the

multiheterodyne signal for verification.

The optical carrier frequency of the laser was found to be

f

c = 66.7 THz with improvements of the stability of the signal

due to the self-detection design using a reference QCL.

This concept utilizes intermodal comb spacing of hundreds

of gigahertz up to the terahertz range. Applications

for this compact comb device range from terahertz

wireless communication systems, telecommunications,

radioastronomy, quantum optics, to spectroscopy seen

in the previous design. Due to the QCL's versatile

composition, microwaves can be generated and modulated

to wirelessly transmit information.2

radio Frequency transmitter

Other research from Harvard University realizes a compact

radio frequency transmitter based on a QCL frequency

comb.4 With demand for wireless communication and

devices increasing, the need for higher frequency

operation also increases. Extremely narrow linewidth

can be generated at room temperature, and modulation

and emission of subterahertz waves are attainable. This

can compensate the growing need for high-frequency

communication technology with high-speed data transfer.

In this experiment, a FP QCL operating in the fundamental

frequency comb state is used with a narrow linewidth beat

note at f

B = 5.5 GHz. An Antenna is attached to the top of

the QCL and connected to two top laser contacts, and a

gap is created in the top electrode. This allows researchers

to use the radio frequency alternating currents from within

the QCL to generate into the antenna enabling wireless

microwave emission as well as the mid-IR radiation output

from the QCL. This creates the Laser Radio Transmitter

(LRT).

An audio analog signal can modulate the laser current which

modulates the laser beat note frequency. This encodes the

baseband information onto the 5.5 GHz carrier wave which

is received by a horn antenna away from the laser. This

signal is filtered and down-converted to fit the bandwidth of

a software-defined radio. The audio track can be retrieved

after demodulation. This device shows the success of a

radio frequency transmitter using a QCL device.

Further advancements show wireless frequency sensitivity

capabilities with the LRT. This would allow the laser

beat note to be wirelessly injection locked to an external

microwave reference. This single device can be used in

applications similar to the ones previously listed.4