We devise and implement two variants of mid-infrared Optical Parametric Oscillators (OPOs) based on a polarization-maintaining fiber-feedback cavity, which allow to robustly deliver sub-picosecond MIR pulses without the need of active stabilization. The first one integrates an erbium-doped fiber into the OPO cavity as the additional gain medium. The synergistic dual-gain operation significantly reduces the pump threshold to launch a stable MIR pulse formation. The other OPO configuration adopts a chirped-poling nonlinear crystal in a passive-fiber cavity to achieve a broader operational spectral range. The wide phase-matching bandwidth facilitates easy wavelength tuning by simply adjusting the cavity length through the dispersion filtering effect. Therefore, the presented mid-infrared OPO source is featured with high compactness, robust operation, and wide tunability, which would be attractive for subsequent applications, such as infrared photonics, biomedical examination, and molecular spectroscopy.
Sensitive mid-infrared (MIR) detection is in high demand in various applications, ranging from remote sensing, infrared surveillance, and environmental monitoring to industrial inspection. Among others, upconversion infrared detectors have recently attracted increasing attention due to their advantageous features of high sensitivity, fast response, and room-temperature operation. However, it remains challenging to realize high-performance passive MIR sensing due to the stringent requirement of high-power continuous-wave pumping. Here, we propose and implement a high-efficiency and low-noise MIR upconversion detection system based on pumping enhancement via a low-loss optical cavity. Specifically, a single-longitudinal-mode pump at 1064 nm is significantly enhanced by a factor of 36, thus allowing for a peak conversion efficiency of up to 22% at an intracavity average power of 55 W. The corresponding noise equivalent power is achieved as low as 0.3 fW/Hz1/2, which indicates at least a 10-fold improvement over previous results. Notably, the involved single-frequency pumping would facilitate high-fidelity spectral mapping, which is particularly attractive for high-precision MIR upconversion spectroscopy in photon-starved scenarios.
Optical rogue waves are automatically generated in a ultrafast fiber laser utilizing a genetic algorithm. An output of optical rogue waves has been achieved, with a spectral peak intensity exceeding the significant intensity threshold by a factor of 32.8.
A low-loss and wideband silicon polarizing beam splitter is demonstrated with the assistance of a nano-bridge waveguide. Transverse magnetic light can be coupled to the cross port through the nano-bridge waveguide, while the transverse electric light comes out mainly from the through port. The designed device has a coupling length of 19.6 μm, which can realize an extinction ratio of 32.43 dB for TM mode or 34.23 dB for TE mode at a 1565-nm wavelength. Compared with the conventional three-waveguide coupler structure, the proposed device is based on a resonant tunneling principle, which can help to effectively improve the fabrication tolerance. The device is fabricated with a commercial CMOS processing facility, which can achieve an extinction ratio of 23.07 dB for TM or 23.46 dB for TE mode with a low excess loss, and the extinction ratio of more than 10 dB can be realized in the wavelength range from 1525 to 1610 nm for both modes. The device performance can be further improved, which would facilitate its practical applications in commercial integrated optical circuits.
Filament- and plasma-grating-induced breakdown spectroscopy (F-GIBS) was demonstrated as an efficient technique for sensitive detection of metals in water, where plasma gratings were established through synchronized nonlinear interaction of two noncollinear filaments and an additional filament was generated with another fs laser beam propagating along their bisector. A water jet was constructed vertically to the three co-planar filaments, overcoming side effects from violent plasma explosion and bubble generation. Three distinct regimes of different mechanisms were validated for nonlinear couplings of the third filament with plasma gratings. As the third filament was temporally overlapped with the two noncollinear filaments in the interaction zone, all the three filaments participated in synchronous nonlinear interaction and plasma grating structures were altered by the addition of the third filament. As the third filament was positively or negatively delayed, the as-formed plasma gratings were elongated by the delayed third filament, or plasma gratings were formed in the presence of plasma expansion of the ahead third filament, respectively. Using F-GIBS for trace metal detection in water, significant spectral line enhancements were observed.
Coherent anti-Stokes Raman scattering (CARS) can be used to excite vibrational bonds with chemical selectivity, high spatial and spectral resolution, and high sensitivity, which has many applications in biomedical research. The common way to realize CARS imaging is illuminating sample with two synchronized ultra-short pulses. Recent development of various fiber laser schemes based on nonlinear optical effect provides compact laser source for CARS imaging. However, the nonlinear conversion in optical fiber may inevitably introduce temporal or spectral noise to the newly generated pulses. In this paper, we have proposed a polarization-maintaining (PM) passive-synchronized picosecond fiber laser system that generates dual-color picosecond pulses for CARS. An Er-doped fiber laser and a Yb-doped fiber laser were passively synchronized by cross phase modulation based on master-slave injection scheme. In the experiment, the wavelength of one branch was fixed at 791 nm, which was generated by second harmonic generation of Er-doped fiber laser. The wavelength of the other branch can be continuously tuned from 1017-1047 nm, which was obtained by adding an active spectral broadening module and an optical bandpass filter after Yb-doped fiber laser. As a result, the laser source allows to probe vibrational bonds with frequencies difference between 2809 cm-1 and 3091 cm-1 . Finally, the achieved tunable synchronized pulses enabled us to microscopically image mouse ear samples. The compact optical fiber laser proposed with PM fiber design, stable synchronization and large wavelength tunability would become a promising laser source for CARS imaging in clinical use.
Laser-induced breakdown spectroscopy (LIBS) is a useful tool for determination of elements in solids, liquids, and gases. For nanosecond LIBS (ns-LIBS), the plasma shielding effect limits its reproducibility, repeatability, and signal-to-noise ratios. Although femtosecond laser filament induced breakdown spectroscopy (FIBS) has no plasma shielding effects, the power density clamping inside the filaments limits the measurement sensitivity. We propose and demonstrate plasma-grating-induced breakdown spectroscopy (GIBS). The technique relies on a plasma excitation source—a plasma grating generated by the interference of two noncollinear femtosecond filaments. We demonstrate that GIBS can overcome the limitations of standard techniques such as ns-LIBS and FIBS. Signal intensity enhancement with GIBS is observed to be greater than 3 times that of FIBS. The matrix effect is also significantly reduced with GIBS, by virtue of the high power and electron density of the plasma grating, demonstrating great potential for analyzing samples with complex matrix.
We have proposed and implemented an all-optical passive synchronization fiber laser system, which was used to facilitate high-performance coincidence frequency upconversion detection for infrared photons. Thanks to the intensive peak power and spectro-temporal pulse engineering, the conversion efficiency could be boosted up to 80% for near- and mid-infrared signal photons. Meanwhile, the background noise was substantially suppressed by confining the pump-induced noise within a narrow excitation window. The demonstrated ultra-sensitive upconversion detector would find promising applications in remote sensing, long-distance ranging and trace detection.
InGaAs/InP avalanche photodiodes (APDs) are one of the optimum choices for the practical applications requiring singlephoton detection in the near-infrared. In this paper, we demonstrate a high-speed single-photon detector (SPD) based on an APD working at room temperature with high detection efficiency. Ultrashort pulses are employed as the gating signals applied on the APD to reduce the avalanche time, efficiently reducing the error counts which include the dark count and afterpulses. Low-pass filters are cascaded to remove the spike noise down to the thermal noise level, guaranteeing the extraction of the photon-induced avalanche signal. Finally, the detection efficiency of 50.4% at 1310 nm is achieved with the dark count rate (DCR) of 3.1×10-4/gate and the afterpulse probability (AP) of 5.6% at 1 GHz at the temperature of ~21℃. This room-temperature SPD with such high performance could further expanding the APD’s applications in ranging and imaging systems.
A rotational wave packet (RWP) can be created when a molecule is nonresonantly excited by laser pulses with durations much shorter than the molecular rotational periods. More interestingly, a unidirectional molecular rotation at THz frequency can be initiated by optical kicking. Such phenomenon has attracted many interests in the fields of molecular deflection, generation of molecular vortices, and etc. However, a comprehensive physical picture of the spatiotemporal evolution of the impulsively excited molecular unidirectional rotation has lacked for more than 10 years since its first observation. Here, we directly visualize the spatiotemporal evolution of an impulsively created unidirectional spinning molecular RWP using the coincidence Coulomb explosion imaging technique (CCEIT) in an intense ultrafast laser field for the first time [1]. Both the experimental results and the numerical simulations show rich dynamical information. Depending on the timing or polarization of the pump pulses, the well-confined cigar- or disk-shaped RWP can be impulsively kicked to rotate clockwise or counterclockwise which afterwards disperses and exhibits field-free revivals owing to the time-dependent beating of the coherently populated rotational states. Very recently, the rotational echo of an impulsively excited RWP has been visualized using the CCEIT [2]. The quantum and classical dynamics of the echo phenomena will be discussed. These results will improve the understanding in other fields of physics and trigger the developments in many applications.
Refs:
[1] K. Lin et al., PRA 92, 013410 (2015).
[2] K. Lin et al., PRX 6, 041056 (2016).
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.
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
efficiency.
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.
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