Silicon nitride is a promising wave-guiding material for integrated photonics applications with a wide transparency bandwidth from visible to mid-infrared, with a superior performance in fiber-coupling and propagation losses, more tolerant fabrication process to the structure parameters variation and compatible with the CMOS technology. Directional coupler (DC) is very popular for realizing beam splitter because of its structural simplicity and no excess loss intrinsically. Here, a conventional silicon nitride directional coupler, three-dimensional vertical coupler, and grating waveguide assisted coupler are designed and fabricated, and compared with each other. A grating waveguide based coupler with a period of 300 nm and coupling length of 26 um, can realize a wideband 3-dB splitter for the wavelength in the range from 1540 to 1620 nm, for a transverse electric (TE) polarized wave. With further optimization of the grating period and duty cycle, the device performance can be further improved with a wider bandwidth.
We demonstrated an efficient way for generating a mid-infrared source at 3.16 μm by coherent coincidence
downconversion. The signal light at 1.04 μm was frequency downconverted by the synchronized pump pulses at 1.55 μm with a conversion efficiency of 65%.
We demonstrated a laser ranging experiment obtained with a Geiger-mode silicon avalanche photodiode (Si GAPD). The Surface-to-surface resolution of 15 cm was achieved with the technique of time-correlated single-photon counting. In the experiment, a mode-locked Yb-doped fiber laser at 1036 nm was applied, as the detection efficiency at 1036 nm of Si GAPDs is much higher than that at 1064nm which was widely applied in remote sensing. Due to the single-photon detector, the laser ranging system was able to measure the reflected photon pulses at single-photon level. We realized 32- m laser ranging experiment with a 135-mm diameter Newtonian telescope in daylight. And the system could measure the non-cooperated object longer than 11.3 km far away, which was tested through inserting the optical loss. It presented a potential for hundreds-of-kilometer laser ranging at low-light level.
Proc. SPIE. 6733, International Conference on Lasers, Applications, and Technologies 2007: Environmental Monitoring and Ecological Applications; Optical Sensors in Biological, Chemical, and Engineering Technologies; and Femtosecond Laser Pulse Filamentation
Two-dimensional multicolored transverse arrays were generated in a quadratic nonlinear medium
under the pump of two crossly overlapped femtosecond beams based on the cascaded non-collinear
quadratic nonlinear couplings between the input pulses and quadratic spatial solitary waves originated
from spatial breakup of one of the input beams with a small asymmetry. A probing supercontinuum
pulse was diffracted and amplified with phase preservation, resulting in the formation of up-converted
multicolor two-dimensional transverse arrays. By seeding with weak second harmonic pulses, the
two-dimensional multicolored transverse patterns could be suppressed through weak beam control of
the induced quadratic spatial solitary waves. At a high-intensity pump, colored conical emissions could
be observed as a result of spatiotemporal collapse of femtosecond pulses in a quadratic medium.
Seeded amplification of colored conical emission was demonstrated to support ultrabroadband
up-conversion with a widely tunable range in wavelength and significantly high energy conversion
A unidirectional intracavity pump scheme was demonstrated efficient for tunable quantum information interface capable of transferring quantum bits between photons of different wavelengths. By means of sum frequency generation in a periodically poled lithium niobate crystal placed inside a diode-pumped Nd:GdVO4 laser cavity, single photons at telecom wavelength were upconverted into replicas around 630 nm with preserved quantum features, which could be easily tuned by adjusting the intracavity pump wavelength.
In our recent experiment, we have designed a novel single-photon detecting module for quantum key distribution using an InGaAs/InP avalanche photodiode with gate-mode quenched photo-detection. At a repetition rate of 100 kHz and the working temperature of -60°C, we obtained the detection efficiency η higher than 10% and 20% at the dark probability Pd about 1.3×10-5 and 1.6×10-5 per nanosecond, respectively. Also at 100 kHz,
we got the best ratio of Pd/η as 1.7×10-3 per pulse (20 ns). And at a lower repetition such as 10 kHz, we obtained
Pd/η as 8.9×10-4 per pulse.
This report proposes a “Plug and Play” quantum key distribution system using differential phase shift, in which any birefringence effects and polarization-dependent losses in the telecom fiber are automatically compensated by using a Faraday mirror. In this system, a light pulse is split into three pulses after traveling through four couplers and a light intensity feedback servosystem is plugged into this system in order to adjust the phase shifter to compensate the variation of these three pulses. The efficiency of key creation in this system is 8/3 higher than the conventional cryptosystem based on the BB84 protocol.
Polarization Mode dispersion (PMD) in single-mode fibers is a common source of problems in quantum key distribution (QKD) as well as all optical communication. Donald S. Bethune and William P. Risk developed a PMD-free phase modulator which is a single port component. We found that it is also necessary to construct a dual-port component. In this report, we demonstrate how PMD affects our single-photon interference experiment, and give a proposal on developing a dual-port PMD-free integrated phase modulator. This new component can be used in QKD as well as in other optical fiber communication schemes. And it will make long-distance information exchange more stable and effective.
A bulk design of optical interleaver with 50 GHz free spectral range (FSR) is experimentally demonstrated. The optical interleaver consists of three cascaded stages of modified Mach-Zehnder interferometers (MZIs), which are fabricated with beam splitters, end-mirror and delay-blocks. Beam splitters are used to split a propagating beam into two beams or coherently recombine two split beams, and delay-blocks located in one arm of each MZI are used to control the phase difference. The interleaver processes square-like spectral response with nearly zero ripple, wide flat-top, and rather low crosstalk, sharp steepness, and ease in fabrication as well. The bandwidth of passband at -0.5 dB and stopband at -15 dB are over 30 GHz and about 25 GHz, respectively. The insertion loss is smaller than 1.5 dB and the polarization dependent loss (PDL) is about 0.15 dB. The crosstalk is lower than -25 dB. The return loss is larger than 45 dB.