A metalens is the flat and ultrathin surface made of billions of sub-wavelength elements and can bend light like traditional optical lens, which make it attract enormous interests. However, two fundamental issues need to be addressed before metalens can replace traditional lens. First, a large diameter (up-to-cm) metalens is extremely difficult to simulate and optimize, due to the large area, lack of periodicity, and multiple parameters on billions of sub-wavelength elements. Second, to obtain a high-quality image within certain distance, a variable focus length is highly desired in most of modern optical system. In this work, we develop an analytical model based on an optical phased array antenna with the focusing phase profile, and accurately predict the far field radiation pattern for a large-area metalens with significant low computational cost. The beam-width of system and depth-of-focus (DOF) are given with respect to wavelength, element spacing and aperture size. To realize the focus tuning function on silicon metalens, the cascaded PIN junction phase shifters enhanced by Fabry-Perrot cavity are attached to metalens to enable the 2π phase variation. At last, a silicon metalens with a diameter of 400um and a focus of 93um, at 1.55um wavelength is verified in FDTD simulation. The results show that the beam-width, DOF and focus tuning range agree well with the analytical model result. The f/10 axial displacement is achieved with a carrier injection of 1019 cm-3. This focus tuning mechanism could be deployed to many attractive near-infrared applications, such as fluorescence microscopy and LIDAR.
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.
KEYWORDS: Silicon, Waveguides, Antennas, Control systems, Signal attenuation, Refractive index, Wave propagation, Neodymium, Electrons, Near field optics
An optical leaky wave antenna (OLWA) is a device that radiates a light wave into the surrounding space from a leaky
wave (LW) guided mode or receives optical power from the surrounding space into a guided optical mode. In this work,
we propose and provide a 3D analysis of a novel CMOS compatible OLWA made of a silicon nitride (Si3N4) waveguide
comprising periodic silicon perturbations which allow electronic tuning capability. The analysis presented here includes
the effect of the number of semiconductor perturbations, the antenna radiation pattern and directivity. We show that the
number of the silicon perturbations has to be large to provide a long radiating section required to achieve radiation with
high directivity. In other words, the proposed structure allows for a very narrow-beam radiation. Preliminary results are
confirmed by exploiting leaky wave and antenna array factor theory, as well as verified by means of two full-wave
simulators (HFSS and COMSOL). Our purpose is to ultimately use PIN junctions as building blocks for each silicon
implantation for the electronic control of the radiation. In particular, the electronic tunability of the optical parameters of
silicon (such as refractive index and absorption coefficient) via current injection renders itself the ideal platform for
optical antennas that can facilitate electronic beam control, and boost the efficiency of optoelectronic devices such as
light-emitting diodes, lasers and solar cells, and bio-chemical sensors.
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.
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.
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.
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.
We present the fabrication process of straight-ridge Yb3+:Er3+ co-doped Al2O3 waveguides. Thin films are synthesized on silica-on-silicon wafers by middle frequency sputtering (MFS) and microwave ECR (MW-ECR) plasma source deposition. Waveguides are developed by reactive plasma etching employing BCl3 gas. Photoluminescence (PL) spectrum and gain measurements at 1.53 µm are investigated at room temperature: a net gain of 5.225 dB/cm is achieved from a 10.5-mm-long waveguide obtained by MFS, and 0.043 dB/cm is achieved from a MW-ECR with a 980-nm pump power of 62 mW.
Non-uniform designs for Erbium doped waveguide amplifiers (EDWA) and Yb: Er co-doped waveguide amplifiers (YEDWA) were presented based on the standard rate equations including six Er energy levels and two Yb energy levels. Numerical results were shown that the design of the non-uniform EDWA operated by gain value get a gain improvement of 110.3% than uniform one, and designs of non-uniform EDWA and YEDWA operated by pump efficiency get gain improvements of 21.03% and 10.17% than uniform ones, even get gain improvements of 2.25% and 7.26% than the absolute gain maximum of uniform EDWA and YEDWA, respectively. The optimization of population inversion in 4I11/2 could be demonstrated by the photoluminescence (PL) enhancement observed in cascaded Er doped glass measurement at room temperature.
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