With a transparency window up to 6 μm, sapphire can serve as a platform to support silicon photonic integrated circuit in
MWIR. Planar waveguide devices based on silicon-on-sapphire (SOS) are emerging as a bridge between MWIR and
SWIR through frequency band conversion process. While these devices are widely proposed to amplify MWIR signals
and generate MWIR source, it can also be inversely utilized to achieve MWIR light detection. Here MWIR signals are
down-converted to telecommunication wavelength (1.55 μm) through SOS waveguides and indirectly detected by SWIR
detectors. Since detectors at telecommunication wavelengths exhibit superior performances in terms of speed, noise and
sensitivity, the indirect detection scheme can be a promising candidate to improve the detection performance. In this
report, we analyze performance of the indirect detection of MWIR signals by wavelength conversion in SOS
waveguides. Particularly we modeled and compared the noise performance of the indirect detection with direct detection
using state-of-the-art MWIR detectors. We show that, in addition to advantages of room temperature and high speed
operation, the proposed indirect detection can improve the electrical signal-to-noise ratio up to 50dB, 23dB and 4dB
compared to direct detection by PbSe, HgCdTe and InSb detectors respectively. The improvement is more pronounced
in detection of weak MWIR signals.
In this study, we demonstrate method for quasi phase matched silicon-on-sapphire waveguides suitable for MWIR
wavelength conversion to achieve higher conversion efficiency than that can be achieved in uniform waveguide
geometries. In particular we show that periodic change in waveguide width by 0.5μm and hence periodic change in
waveguide dispersion can to reset phase accumulation and provide ever-increasing gain profile. With the fabrication
flexibility of large cross-section of MWIR waveguides, the possibility of using quasi-phase-matching can provide >30dB
conversion efficiency enhancement and increase the conversion bandwidth by 2 times. Such improvement may facilitate
the fabrication of parametric oscillators that can improve the conversion efficiency by 50dB.
Four-wave mixing (FWM) in silicon waveguides is considered to be a promising effect to realize the wavelength
conversion function for wavelength-division-multiplexing optical communication systems. Compared to the degenerate
FWM with a single pump, the nondegenerate FWM with two pumps shows more flexibility in phase-matching condition
and has more opportunities to acquire broader conversion bandwidth. The bandwidth enhancement is theoretically
analyzed for the two-pump FWM and an enhancement of 25% is experimentally demonstrated. Also, an ultra-broadband
wavelength conversion is presented based on two-pump FWM by fixing one pump near the signal and scanning the other
pumps.
Wavelength tunable synchronous pulse sources are highly desirable for spectroscopy and optical diagnostics. The
common method to generate short pulses in the fiber is the use of nonlinear induced spectral broadening which result in
soliton shaping in anomalous dispersion regime. However, to generate ultra-short pulses, broadband gain mechanism is
also required. In recent years, Raman fiber lasers have retrieved strong interest due to their capability of serving as pump
sources in gain-flattened amplifiers for optical communication systems. The fixed-wavelength Raman lasers have been
widely studied in the last years, but recently, much focus has been on the multi wavelength tunable Raman fiber lasers
which generate output Stokes pulses in a broad wavelength range by so called cascaded stimulated Raman scattering. In
this paper we investigate synchronous 1st and 2nd order pulsed Raman lasers that can achieve frequency spacing of up to
1000cm-1 that is highly desired for CARS microscopy. In particular, analytical and numerical analysis of pulsed stability
derived for Raman lasers by using dispersion managed telecom fibers and pumped by 1530nm fiber lasers. We show the
evolution of the 1st and 2nd order Stokes signals at the output for different pump power and SMF length (determines the
net anomalous dispersion) combinations. We investigated the stability of dispersion managed synchronous Raman laser
up to second order both analytically and numerically. The results show that the stable 2nd order Raman Stokes pulses
with 0.04W to 0.1W peak power and 2ps to 3.5ps pulse width can be achieved in dispersion managed system.
Nonlinear silicon photonics has been an immense research subject in the past several years with promising prospects of
delivering chip scale signal modulation, shaping and characterization tools. In particular, broadband parametric process
has been considered for applications ranging from wideband light amplifiers to signal characterization and signal
shaping tools. Although underlying nonlinear effect, Kerr phenomena, in silicon has generated promising result of
wavelength conversion, the success of these devices have been challenged by the presence of nonlinear losses such as
two photon absorption and the two photon generated free carrier absorption. Experimental demonstrations were limited
to conversion efficiencies below -10dB. Here, we present the prospect of ultra wide discrete band conversion schemes
and the prospect of parametric process at mid-infrared wavelengths where nonlinear losses are not present. In particular,
we explore the parametric wavelength conversion scheme at mid-wave infrared wavelength (2μm~6μm) by four-wavefixing
process in silicon waveguides with new cladding materials, such as sapphire, that can provide transparency up to
6μm and facilitate phase matching condition for discrete wavelength bands as far as 60THz away from each other.
Design criteria include the optimization of mode overlap integrals and dispersion engineering for an ultra-wide band
signals. The particular results of wavelength conversion between 2μm bands and 5μm bands, and between 1.8μm bands
and >4μm bands will be presented. Prospects of frequency band conversion in generation of new infrared signals and
low noise, room temperature detection of mid-infrared signals will also be discussed.
Real-time optical imaging and tracking of particles in a complex environment to understand
coordinated events has attracted researchers from various areas such as biomechanics. Here, we report a
way for real time detection and tracking of micron size particles in time-space-wavelength mapping
technology by using a single detector. Experimentally, we demonstrate real time tracking of micron size
glass particles with 50ns temporal resolution and <3μm spatial resolution. Submicron resolution and faster
temporal resolution are achievable with further optimization. The proposed technique utilizes the timewavelength
technology, which has been proven to be very effective in real time digitization of ultra fast RF
signals, and arbitrary waveform generation by random objects. In this work we use a broad band continuum
source generated by a 20MHz fiber laser to emit 50nm short pulses at 1550nm. Following a dispersive time
wavelength mapping in a chirped fiber grating and space-time-wavelength mapping through a diffraction
grating with 600lines/mm, we generate an elliptical beam where each wavelength component corresponds
to different time and position in space. Then the generated beam is focused on an image plane by using
20X-40X microscope objectives. The presence of particles on the image plane induces amplitude
modulation on each pulse which is captured in real time by a high speed digitizing oscilloscope with
20GS/s sampling rate. The trajectory of the particle is extracted from the dynamic amplitude modulation in
a post processing. The same system has also been utilized for imaging of particles by using one
dimensional scanning.
The high-index contrast between the silicon core and silica cladding enable low cost chip-scale demonstration of all-optical
nonlinear functional devices at relatively low pump powers due to strong optical confinement the in silicon
waveguides. So far, broad ranges of applications from Raman lasers to wavelength converters have been presented. This
presentation will highlight the recent developments on ultrafast pulse shaping and pulse characterization techniques
utilizing the strong nonlinear effects in silicon. In particular, pulse compression due to two photon absorption and dual
wavelength lasing and ultrafast pulse characterization based on XPM FROG measurement will be highlighted.
Nonlinear optics in silicon has drawn substantial attention in the recent years. In this research, laser mode-locking and
dual wavelength lasing are achieved in a fiber-ring-cavity using an Erbium-doped fiber amplifier (EDFA) as a gain
medium and a 1.7cm long silicon-on-insular waveguide as pulse compressor, a mode-locker and a Raman gain media.
We show that the transient behavior of two photon absorption (TPA) and TPA induced free carrier absorption can be
used for pulse compression and laser modelocking in the silicon waveguide inside the laser cavity. The proposed
technique takes advantage of spontaneous generation of free carriers and the slow recombination time, >17ns, to
attenuate the trailing edge of the time varying signals passing through the waveguide. When a 5μm2 model area silicon
waveguide is placed inside a fiber ring cavity consisting of an EDFA as a gain media and ~50ps modelocked laser pulses
are generated at 1540nm. We also observe that the generated short pulses also induce stimulated Raman scattering at
1675nm in the same silicon waveguide. We show that engineering the laser cavity facilitates laser modelocking and dual
wavelength laser oscillation at 1540nm and 1675nm. Experimentally we obtain <100ps modelocked pulses at both wavelengths. The average pump threshold power of the Raman laser is measured to be 3.75mW and the Stokes average output power is measured to be 3 μW.
Due to the high-index contrast between the silicon core and silica cladding, the silicon waveguide allows strong optical
confinement and large effective nonlinearity, which facilitates low cost chip scale demonstration of all-optical nonlinear
functional devices at relatively low pump powers. One of the challenges in ultrafast science is the full characterization of
optical pulses in real time. The time-wavelength mapping is proven to be a powerful technique for real time
characterization of fast analog signals. Here we demonstrated a technique based on the cross-phase modulation (XPM)
between the short pulse and the chirped supercontinuum (SC) pulse in the silicon chip to map fast varying optical signals
into spectral domain. In the experiment, when 30 nm linearly chirped supercontinuum pulses generated in a 5 km
dispersion-shifted fiber at the normal regime and 2.4 ps pulse are launched into a 1.7 cm silicon chip with 5 μm2 modal
area, a time-wavelength mapped pattern of the short pulses is observed on the optical spectrum analyzer. From the
measured spectral mapping the actual 2.4ps temporal pulse profile is reconstructed in a computer. This phenomenon can
be extended to full characterization of amplitude and phase information of short pulses. Due to time wavelength
mapping this approach can also be used in real time amplitude and phase measurement of ultrafast optical signals with
arbitrary temporal width. The high nonlinearity and negligible distortions due to walk off make silicon an ideal candidate
for XPM based measurements.
Atherosclerosis is unquestionably the leading cause of morbidity and mortality in developed countries. In the mean time, the worldwide importance of acute vascular syndromes is increasing. Because collagen fiber is a critical component of atherosclerotic lesions; it constitutes up to 60% of the total atherosclerotic plaque protein. The uncontrolled collagen accumulation leads to arterial stenosis, whereas excessive collagen breakdown weakens plaques thereby making them prone to rupture finally. Thus, in this study, we present the first application, to our knowledge, of using polarization-sensitive optical coherence tomography (PS-OCT) in human atherosclerosis. We demonstrate this technique for imaging of intensity, birefringence, and fast-axis orientation simultaneously in atherosclerotic plaques. This in vitro study suggests that the birefringence change in plaque is due to the prominent deposition of collagen according to the correlation of PS-OCT images with histological counterpart. Moreover, we can acquire quantitative criteria based on the change of polarization of incident beam to estimate whether the collagen synthesized is "too much" or "not enough". Thus by combining of high resolution intensity imaging and birefringence detection makes PS-OCT could be a potentially powerful tool for early assessment of atherosclerosis appearance and the prediction of plaque rupture in clinic.
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