2.2 Single-Cavity Dual-Comb Laser
The upper-left part of Fig. 2 shows a free-running SCDCL com-
posed of an all-fiber ring-cavity oscillator, a band-pass filter, and
erbium-doped fiber amplifiers (EDFAs). The dual-comb light
beams with different center wavelengths and different f
rep
were
obtained from the fiber oscillator by multiplexing the mode-
locking operation in the wavelength region.
29,32,34
The fiber os-
cillator has an all-fiber ring cavity in which the dual-comb light
beams propagate along a common-path route. The cavity con-
sists of a hybrid wavelength division multiplexer and isolator
(WDM/ISO), a piece of 0.46-m-long erbium-doped fiber (EDF)
pumped by a 980-nm laser diode, a polarization controller (PC),
a single-wall carbon nanotube saturable absorber (SA), an in-
line polarizer (ILP) with polarization-maintained fiber (PMF)
pigtails, and a 40% fiber output couple r (OC). In addition, a
65-cm-long dispersion compensation fiber was installed to op-
timize the intracavity dispersion at ∼4.0fs∕nm at 1560 nm. The
total length of the cavity was ∼4.21 m. Due to the considerable
birefringence of the PMF and the use of the ILP, spectral filter-
ing and polarization-dependent loss tuning enable dual-comb
lasing oscillation by adjusting the PC.
29
Dual-comb light with
different center wavelengths and different f
rep
(λ
1
-comb with
f
rep1
and λ
2
-comb with f
rep2
) from the oscillator was separated
into two independent combs with different center wavelengths
and different f
rep
by the bandpass filter, and the resulting two
combs were further amplified by EDFAs. The difference in their
center wavelengths is negligible for generation and detection of
THz comb.
2.3 Adaptive Clock Generator
The adaptive clock generator (ACG) provides the adaptive clock
to suppress the residual long-term drift and timing jitter in the
repetition rate difference Δf
rep
of the free-running SCDCL, en-
abling us to recover the temporal linearity of the acquired tem-
poral waveform. To trace the timing fluctuation in real time, we
used photoconductive heterodynin g mixing between a reference
CW-THz radiation and two photocarrier THz combs (PC-THz
comb1 with f
rep1
and PC-THz comb2 with f
rep2
) seeded by the
SCDCL output,
39
as shown in the middle part of Fig. 2.Two
bowtie-shaped, low-temperature-grown GaAs photoconductive
Fig. 1 Principle of operation. (a) Flowchart of time-domain THz-DCS. (b) Acquisition of the tem-
poral waveform using the adaptive sampling method.
Chen et al.: Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy…
Advanced Photonics 036004-3 May∕Jun 2020
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Vol. 2(3)