The Bridge - Issue 1, 2023 - 11

Ultrafast Lidar Based on Signal Time Stretch and Various Transduction Techniques
Feature
high resolution, and broad spectral bandwidth are topics
of interest. Existing techniques, such as Fourier transform
spectroscopy (FTS), provide a broadband spectrum and
well-resolved data in nonintrusive diagnostics of molecular
structures in various media but suffer from slow spectral
acquisition rates. Wavelength-swept spectroscopy has
the advantage of better SNR but suffers from repetitive
measurements. Time-stretch has been demonstrated to
improve these techniques and address their shortcomings
through high-resolution parallel detection [10]. One of
the techniques that incorporate time-stretch and time-tofrequency
mapping is known as femtosecond imbalanced
time-stretch spectroscopy (FITSS) [2] where the optical
frequency domain signal is converted to a microwave or
radio frequency domain comb. The resulting temporal
interferogram and its Fourier transform will demonstrate
a frequency comb from which material properties can be
extracted. Time-stretched femtosecond laser pulses enter
an MZI as shown in Figure 2. The reference arm of the MZI
includes an optical delayer followed by a second dispersive
element. The probing arm includes the material under test.
The Fourier transform of the beat signal between the two
arms of the MZI demonstrates a frequency comb in the
radio frequency domain with a comb spacing the same
as the laser repetition rate [2,11]. The RF comb structure
Figure 3: Temporal (top) and spectral (bottom left) interferograms, and
fast Fourier transform of the temporal interferograms (bottom right).
Figure 2: Time-stretched spectroscopy for ultrafast material (gas, solid, and liquid)
characterization.
includes the molecular spectrum from the optical
frequency comb. The advantage of such a technique
over the existing techniques is fast acquisition time and
high-resolution detection.
III. Experimental Results
We used the time-stretch ultrafast Lidar technique
explained in the previous section to detect the small
displacements of a mirror. Figure 1a demonstrates the
schematic of the optical ranging based on dispersive
frequency-modulated interferometry using 120 km of
dispersive optical fiber. A picosecond laser source is used in
the experiment with a repetition rate of 10 MHz, a center
wavelength of 1550 nm, and a pulse width of 7 ps. The
reflected signal from the mirror passes through the same
circulator to combine with the reference signal and is set
to have parallel polarization with the reference arm using
polarization controllers and in-line polarizers before they
are combined with each other. The beat signal is then
amplified and passed through 120 km of DCF with a total
dispersion of 2048 ps/nm to generate high-frequency
interferograms. We use a 45 GHz photodetector and
a 12 GHz oscilloscope, as well as an optical spectrum
analyzer to detect the temporal and spectral interferograms.
Figure 4 demonstrates the interferograms and fast Fourier
transformation for various locations of the mirror. Both
reference and probe signals go through the dispersive
element, are stretched in the time domain, and divided
into two parts: one enters the photodetector and then
connects to an oscilloscope, and the other connects
to an optical spectrum analyzer. The former forms
the temporal interferogram, and the latter forms
the spectral interferogram. The optical delayer is
adjusted to generate a temporal interferogram
with a frequency of 6 GHz (half of the maximum
oscilloscope frequency) as the initial frequency
at x=0. By moving the positioning stage (mirror), the
interferogram signal frequency changes. The displacement
value can be extracted from frequency variations. The
results for three different mirror positions of -5 mm, 0, and
Figure 4: Measured temporal (left) and spectral (right) interferograms
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The Bridge - Issue 1, 2023

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