recording devices such as charge-coupled device (CCD)
and complementary metal oxide semiconductor (CMOS)
image sensors as well as photon-counting detectors such as
photomultiplier tube (PMT) arrays, avalanche photodetec-
tor (APD) arrays, and infrared sensors.
17–19
The second
category includes the development of contrast-enhanced
methods such as quantitative phase imaging, polarization-
sensitive imaging, and chemical imaging as well as the de-
velopment of biomarkers including fluorescent probes for
fluorescence microscopy.
20–22
The last category is focused
on the development of methods for improving spatial reso-
lution such as super-resolved fluorescence microscopy
1–4
and for improving temporal resolution via techniques that
circumvent speed limitations in the conventional image-
recording devices.
23–26
The significance of improving reso-
lution is well recognized by the 2014 Nobel Prize in
chemistry.
One of the emerging methods focused on improving
temporal resolution is optical time-stretch imaging
23,27
—a
method for ultrafast imaging with a single-pixel photodetec-
tor by spectrally encoding and decoding the spatial profile of
the imaging target with dispersive properties of light in both
spatial and temporal domains. Since it was originally demon-
strated by Goda et al. in 2008,
27
it has been shown to be an
effective method for ultrafast imaging that can go beyond
what is possible with traditional detector-array-based image
sensors. It achieves continuous image acquisition at an ultra-
high frame rate of 10–1000 million frames per second
23,27–33
by replacing the traditional image sensor with the single-
pixel photodetector and hence overcoming speed limitations
of the traditional image sensor. By virtue of its inherent af-
finity with optical signal processing, optical time-stretch
imaging can be combined with various optical techniques
such as amplification,
34,35
nonlinear processing,
36–39
com-
pressive sensing,
40,41
and pattern correlation
27,42
to realize
unique capabilities that are not possible with the traditional
image sensors. Applications enabled by such capabilities
include surface inspection,
43–45
surface vibrometry,
46,47
par-
ticle analysis,
48
and cell screening.
49–53
In this paper, we review the principles and applications
of optical time-stretch imaging. Specifically, in Section II,
we discuss the principles and limitations of conventional
imaging systems. In Section III, we show the principles of
optical time-stretch imaging, namely, its schematic, key
components, and design parameters. In Section IV, we dis-
cuss various applications of optical time-stretch imaging. In
Section V, we discuss our future prospective of optical time-
stretch imaging. In Section VI, we summarize this paper. It
is our hope that this review paper will serve as a recipe for
any researcher interested in building an optical time-stretch
imaging system and using it for diverse applications.
II. PRINCIPLES AND LIMITATIONS OF
CONVENTIONAL OPTICAL IMAGING
While various optical imaging techniques have been
developed over the last few decades, virtually all optical
imaging techniques are categorized into two methods of
acquiring multidimensional images with a reasonable signal-
to-noise ratio (SNR): (1) using a detector array such as CCD
and CMOS image sensors (Figure 1(a)) and (2) using a beam
scanner and a single-pixel photodetector (Figure 1(b)).
Speed limitations exist in these imaging methods, but are
manifested in different manners, depending on the architec-
ture of image acquisition, in particular, for microscopy. In
this section, we briefly review the principles of conventional
optical imaging systems and discuss the origin of the speed
limitations.
The first method for image acquisition is the use of a de-
tector array—the most commonly used method not just for
scientific and industrial use but also for consumer electron-
ics, in particular, cameras for photography. All cameras in-
stalled in smart phones are essentially detector arrays, either
CCD or CMOS image sensors. In this method, free electrons
produced by photons striking a silicon surface through
the photoelectric effect are transferred by a shift register
and converted into a voltage by a charge amplifier, and the
resultant voltage is digitized by an analog-to-digital
FIG. 1. Principles of conventional optical imaging systems. CCD: charge-coupled device, CMOS: complementary metal-oxide-semiconductor, PMT: photo-
multiplier tube, and APD: avalanche photodetector. (a) Detector-array-based imaging. Examples of this imaging method are CCD and CMOS image sensors.
The image of the object is formed onto the detector array. (b) Imaging with a single-pixel photodetector and beam scanner. Examples of this imaging method
are a sensitive photodetector such as a PMT or APD with a pair of galvanometric mirrors or acousto-optic deflectors.
011102-2 Lei et al. Appl. Phys. Rev. 3, 011102 (2016)