The Bridge - Issue 1, 2023 - 10

Feature
Ultrafast Lidar Based on Signal Time Stretch and Various Transduction Techniques
cf. Figures 4a and 4b. Temporal interferogram data is
mapped to the frequency domain by using a polynomial
fit function that relates the peak centers of temporal and
spectral interferograms to each other. The ranging data,
which is the distance of the mirror in Figure 1, is encoded
in the temporal interferogram's frequency. An inverse
Fourier transform is applied to the frequency domain
data. By fitting each main sideband that is associated with
a time delay difference to a Gaussian function with the
center of each fitted result, the time delay difference and,
consequently, the displacement of the object (mirror) is
calculated. Using this technique, a range detection of 16
cm with an accuracy of 334 nm is demonstrated [1].
B. Time-stretched Lidar with microwave
processing transduction
Time-stretched Lidar with direct optical transduction,
which uses optical-to-electrical conversion of the detected
signal through a photodetector, usually requires an
analog-to-digital converter with a fast sampling rate. The
measurement data set size is also large for this case. Since
the displacement data is encoded in the frequency of the
time-domain (temporal) interferogram, instantaneous
microwave frequency measurement will bypass the
challenges of the previous technique. In microwave
processing transduction, instead of direct optical detections
of the temporal interferogram, a frequency-to-intensity
mapping known as amplitude comparison function (ACF)
through intensity modulation of a secondary laser could
resolve the issue. This technique allows for a real-time
femtosecond ranging Lidar based on all-optical signal
processing with few-micrometer resolution incorporating
dispersive Fourier transformation and instantaneous
microwave frequency measurement without the need for
a fast ADC [4]. Figure 1b shows the schematic diagram of
this technique.
The main difference between this and the technique
depicted in Figure 1a is the optical transduction
technique used to retrieve the signal information such
as displacements. For this technique, the ultrafast laser
pulses are first stretched by passing through the dispersion
element (DCF) [7]. The stretched amplified signal, then,
passes through an all-fiber MZI similar to the previous
technique. The displacement is encoded in the frequency
variation of the temporal interferogram, and optical
signal processing of the microwave pulse generated on
a photodetector is applied to address the challenges in
storage and real-time processing of the interferogram data.
By an intensity modulator, a carrier wave is modulated by
the time-domain interferogram signal detected by a fast
photodetector, and then the frequency variation of the
microwave pulse is encoded to the first-order sidebands
of the intensity-modulated signal of the secondary laser.
THE BRIDGE
By applying a symmetric-locked frequency discriminator,
the frequency shift of the sidebands is mapped to a
transmission change. Finally, a programmable optical filter
realizes a real-time ranging system with adjustable dynamic
range and detection sensitivity. In summary, using a fast
Fourier transform interferometer, the displacement is
encoded to the frequency shift of microwave pulse that is
uploaded into first-order sidebands of carrier wave under
intensity modulation. The displacement is extracted from
sideband frequency shifts by applying optical frequency
detection. A 15 mm and 45 mm detection range with a
mean error of 19.10 μm and 36.63 μm, respectively, have
been reported using this technique [4].
C. Spectral Lidar with direct
optical transduction
Another ultrafast Lidar technique that incorporates
time-stretch is known as spectral Lidar [8,9]. Current
autonomous Lidar technologies are single-wavelength and
use geometric or spectral beam scanning to generate a
map of the surroundings. The acquisition time of such a
Lidar technology, known as time-of-flight imaging-based
Lidar, is affected by the speed of beam scanning and the
maximum measurable time-of-flight. Using an ultrafast
broadband source and a spatially dispersive element
such as a grating, we can either eliminate beam scanning
or increase the speed of the Lidar image acquisition
for applications such as driverless cars. This technique
uses time encoding of the different spectral bands of
the time-stretched signal, which is adapted from the
time and wavelength multiplexing techniques in optical
communication systems [9]. The spectrally broadened
time-stretched laser pulses are divided into various spectral
bands, and each band is delayed differently (fiber-based
encoding) and detected simultaneously. The combination
of the fiber-based encoder with wavelength-division
multiplexing is used to achieve parallel detection and fast
spectral scanning. To obtain parallel detection of all the
wavelength bands, optical code division multiplexing is
obtained using a fiber-based, all-optical, temporal encoder
in each wavelength multiplexed channel. Correlated
spectro-temporal modulation with a high degree of
freedom enables parallel time-of-flight and, consequently, a
significant increase in the detection speed. A maximum 75
m detection range with a 4.4-fold speedup was obtained
[9]. Figure 1c shows the schematic of an ultrafast Lidar with
a correlated spectro-temporal encoding system.
D. Time-stretched molecular spectroscopy
A time-stretch Lidar technique can also be applied to detect
and characterize molecular properties of the materials,
such as trace gases at ultralow concentrations through
absorption profiles. Fast acquisition time, high sensitivity,
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