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The optical-phased array (OPA) has gained special interest in recent years as a high-speed and compact imaging device. While large-scale OPAs have been demonstrated in single-pixel imaging, the complexity of the driver circuit is becoming a crucial problem as the number of phase shifters increases. Here, we investigate the phase shift requirement of OPA for single-pixel imaging and demonstrate, for the first time, that full 2π phase shifts are not mandatory to generate a set of illumination patterns with a sufficient degree of randomness required to reconstruct the image. Using a silicon photonic OPA chip with 128 carrier-depletion-based phase shifters, we experimentally confirm this property by successfully retrieving an image under a maximum phase shift of only ∼1.5π without affecting the quality of the image. Consequently, the input voltage can be reduced significantly. Since the carrier-depletion phase shifters generally require high driving voltages, this finding paves the way to high-speed OPA-based imaging with a minimum device requirement.

Due to the unique nature of offering minimal invasiveness and high spatial resolution simultaneously, multimode fibers (MMFs) are receiving significant attention in bio-imaging applications. While a spatial light modulator is typically used for controlling the wavefront of the light emitted from the MMF, it makes the system slow, bulky, and expensive. To solve this problem, in this work, we demonstrate the use of a chip-scale integrated optical phased array (OPA) for imaging through an MMF. A silicon OPA with 128 independent phase shifters is fabricated and combined with a 3D waveguide interface to generate speckle patterns at the MMF output. Using the generated speckles and the optical power detected by a bucket detector, we experimentally obtain fine 2D images of the target. From the point spread function analysis of the system, the number of resolvable points is derived to be 1007 points, which is much larger than the number of phase shifters.

Optical phased arrays (OPAs) are promising beam-steering devices for various applications such as light detection and ranging, optical projection, free-space optical communication, and optical switching. However, previously reported OPAs suffer from either an insufficient number of resolvable points, or complicated control requirements due to an extremely large number of phase shifters. To solve this issue, we introduce the non-redundant array (NRA) concept to the OPA devices. Based on this design, we can realize high-resolution OPAs whose number of resolvable points scales quadratically with the number of antennas N . In contrast, that of traditional OPAs scales only linearly with N . Thus, a significant reduction in the number of required phase shifters can be attained without sacrificing the number of resolvable points. We first investigate the impact of employing the NRA theoretically by considering the autocorrelation function of the array layout. We then develop a Costas-array-based silicon OPA and experimentally demonstrate 2D beam steering with ∼ 19 , 000 resolvable points using only 127 phase shifters. To the best of our knowledge, this corresponds to the largest number of resolvable points achieved by an OPA without sweeping the wavelength.

Integrated reconfigurable optical unitary converters (OUCs) are crucial in realizing all-optical spatial mode demultiplexing for mode-division-multiplexed transmission systems and programmable photonic processing for optical neural networks. In this work, we present the first experimental demonstration of 4×4 OUC monolithically integrated on InP. To avoid the difficulty of integrating a large number of Mach-Zehnder interferometer couplers on the InP platform, we apply the concept of multi-plane light conversion and use cascaded stages of 4-port multimode interference couplers, which are more scalable and easier to fabricate on InP. By optimizing the phase shifters, we demonstrate reconfigurable 4-mode sorting as well as error-free switching of 40-Gbit/s signal.

We demonstrate that single-pixel imaging resolution of optical phased array can be enhanced by transmitting through a multimode fiber. Using only 128 phase shifters, >1000 points are resolved, determined by the number of fiber modes.

An integrated Stokes vector receiver (SVR) that can retrieve state of polarization of light in the three-dimensional (3D) Stokes space has widespread applications, such as short-reach communication links, polarization-sensitive imaging, and sensing. While various approaches have been demonstrated to date, monolithic integration of polarization components on InP has been a challenging issue. In this paper, we develop a novel 4-port SVR circuit integrated on a compact InP chip to retrieve complete Stokes parameters of incoming light with various intensity and degree-of-polarization. By judiciously designing the lengths and positions of asymmetric waveguide sections, we demonstrate that the SV of signal can be projected onto four vertices of a regular tetrahedron inscribed in the Poincare sphere. Additionally, we employ this device in decoding 10-Gbaud 4-ary and 8-ary Stokes-vector-modulated signals in the 3D Stokes space.

Self-seeded reflective semiconductor optical amplifier (RSOA) has been drawing special attention as potentially low-cost colorless wavelength-division-multiplexed transmitter. In this paper, we present numerical and experimental analyses of RSOA to find the optimal condition to induce strong modulation canceling effect over wide temperature range. Three InGaAlAs/InP multiple-quantum-well RSOAs with different length are compared numerically and experimentally to reveal the existence of critical RSOA length to induce efficient modulation-cancelling effect. By using the optimal RSOA length of 1 mm, we experimentally demonstrate 2.5-Gb/s self-seeded transmission at 1550-nm wavelength over a 25-km standard single-mode fiber at both 25 and 50 °C. In addition, we demonstrate feasibility of higher-temperature (70 °C) as well as higher-bitrate (10 Gb/s) operation by employing the amplified self-seeded configuration with appropriately selected RSOAs.

We propose and numerically demonstrate novel imaging scheme using multimode fiber and integrated optical phased array. Thanks to modal dispersion and mixing inside fiber, clear images are obtained even with limited number of phase shifters.

We propose and demonstrate reconfigurable optical unitary converter with non-uniform multimode interference (MMI) couplers. By using compact $182-\mu \mathrm{m}$ -long non-uniform MMIs, desired $8\times 8$ unitary operation is obtained with the crosstalk equivalent to that with $364-\mu \mathrm{m}$ -long uniform-MMIs.

Since the conventional coherent transceiver is costly to be deployed in short-reach networks due to its complicated receiver structure, it is desired to simplify the structure itself. In this paper, we propose a simple polarization-diversity coherent receiver structure by exploiting the concept of the Stokes analyzer. Compared to the conventional architecture, the number of the photodiodes (PDs) is reduced from eight to six without relying on complicated analog circuits. In addition, splitters and combiners for dual-polarization (DP) signals can be replaced with only one polarization beam splitter or combiner (PBS/C). For evaluation of the proof-of-concept (PoC), we developed a prototype of the receiver using free-space optical components. We demonstrate the transmission of 120-Gb/s DP quadrature phase-shift keying (QPSK) and DP 8-ary quadrature-amplitude modulation (8QAM) signals over a 100-km single-mode fiber (SMF). We believe that the demonstrated architecture could potentially be integrated monolithically on silicon-photonic or InP platforms to realize compact and low-cost coherent transceivers for short-reach applications.

We experimentally demonstrate the use of a large-scale silicon-photonic optical phased array (OPA) chip as a compact, low-cost, and potentially high-speed light illuminating device for ghost imaging (GI) applications. By driving 128 phase shifters of a newly developed silicon OPA chip using rapidly changing random electrical signals, we successfully retrieve a slit pattern with over 90 resolvable points in one dimension. We then demonstrate 2D imaging capability by sweeping the wavelength. With the potential of integrating high-speed phase modulators, tunable lasers, grating couplers, and CMOS driver circuit on the same silicon platform, this work paves the way towards realizing ultrahigh-speed and low-cost single-chip GI devices.

We develop large-scale optical phased array (OPA) with 128 phase shifters integrated on a compact silicon chip for single-pixel ghost imaging application. By using speckle illumination pattern generated from OPA, calibration-free robust imaging is demonstrated.

We propose a simple polarization-diversity coherent receiver by taking advantage of the features of the Stokes analyzer. Using a prototype of the receiver, we successfully transmitted 120-Gb/s DP-QPSK signals over a 100-km SMF.

Carrier-injection-based efficient polarization controller with a strained InGaAsP multiple-quantum-well (MQW) layer is demonstrated on a regrowth-free InP platform. We employ a straight-line device configuration by cascading an asymmetric polarization rotator (PR) to provide a fixed polarization conversion and a polarization-dependent phase shifter (PD-PS) to enable tunable polarization rotation. Based on a novel design concept, both the PR and PD-PS sections are integrated monolithically without active-passive integration. Using the fabricated device, we experimentally demonstrate efficient polarization conversion over the entire Poincaré sphere with a total current of less than 40 mA. With the capability of monolithically integrating other InP-based active components, the demonstrated scheme should be attractive for various applications, such-as low-cost coherent communication, microwave photonics, and quantum key distribution.

Imaging through a multimode fiber (MMF) is a promising strategy for in vivo endoscopy due to its nature of simultaneously realizing high spatial resolution and minimal invasiveness. In MMF-based speckle imaging systems, a spatial light modulator (SLM) with a large number of pixels is commonly employed to enable independent controls of all the linearly polarized (LP) modes inside the MMF. Here, instead, we show that such an SLM can be replaced by a compact optical phased array (OPA) with a much smaller number of phase shifters without causing any penalty in resolution or sensitivity. Due to the nonlinear process inherent in field-to-intensity conversion, an OPA with N phase shifters is capable of generating ∼ N 2 independent illumination patterns. Since the resolution limit imposed by an MMF with M LP modes per polarization is approximated as 4 M , we show that only 4 M phase shifters are essentially required to utilize the full spatial capacity of MMF. We can therefore enjoy the unique benefits of a low-cost, compact, and high-speed OPA that does not require thousands of phase shifters to achieve high-resolution imaging through an MMF.

Electrically tunable metasurfaces have gained special interest as they can realize ultrathin surface-normal modulators in planar geometries. In this paper, we demonstrate a novel metasurface modulator based on electro-optic (EO) polymer that utilizes bimodal resonance inside a metallic subwavelength grating to increase the modulation efficiency. When two metal-insulator-metal (MIM) resonant modes are excited simultaneously inside the grating, they couple strongly to generate a sharp dip in the reflected spectrum. As a result, efficient intensity modulation with 15-dB extinction ratio can be obtained at the resonant wavelength under a small refractive index change of 8.5 × 10-3, corresponding to modulation voltage of less than 10 V. Due to the low parasitic capacitance of EO polymer and high conductivity of metallic gratings which is also used as the electrodes, the RC bandwidth of the device should easily exceed 100 GHz, potentially applicable to high-speed surface-normal modulators.

We propose a novel reconfigurable integrated MIMO optical mode demultiplexer consisting of cascaded MMI couplers. By configuring phase shifters adaptively, large-scale (>10 modes), low-crosstalk (

We review our recent studies on capsule-shaped InP/InGaAs metallic-cavity lasers. By introducing an optimal curvature at the metallic sidewalls of conventional rectangular metallic lasers, the electric fields of the resonant mode are pushed effectively into the center of the mesa, which allows dramatic reduction of the plasmonic loss. The validity of the scheme is verified both numerically and experimentally. From three-dimensional finite-difference time-domain simulation and rate-equation analysis, we estimate that the threshold current can be reduced to as low as 60 μA with the effective modal volume of 0.45 μm3. Up to 4-fold increase in Q value is confirmed experimentally for the cavity structure with an optimal curvature.

A novel type of multiple-wavelength focusing plasmonic coupler based on a nonperiodic nanoslit array is designed and experimentally demonstrated. An array of nanoslits patterned on a thin metal film is used to couple free-space light into surface plasmon polaritons (SPPs) and simultaneously focus different-wavelength SPPs into arbitrary predefined locations in the two-dimensional plane. We design and fabricate a compact triplexer on a glass substrate with an integrated silicon photodetector. The photocurrent spectra demonstrate that the incident light is effectively coupled to SPPs and routed into three different focal spots depending on the wavelength. The proposed scheme provides a simple method of building wavelength-division multiplexing and spectral filtering elements, integrated with other plasmonic and optoelectronic devices.

We present, to the best of our knowledge, the first experimental demonstration of reconfigurable all-optical on-chip multi-input-multi-output three-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable three-mode demultiplexing is experimentally realized with wavelength-dependent loss of less than 3 dB and modal crosstalk of less than -10 dB over a 23 nm optical bandwidth. Error-free mode demultiplexing of 40 Gbps non-return-to-zero signal is also demonstrated.

An InP/InGaAsP integrated 1 times 8 optical phased-array switch is designed, fabricated, and characterized for high-throughput broadband wavelength-division multiplexed (WDM) optical packet switching application. With a single stage of phase shifters utilizing carrier-induced refractive index change, switching to the eight output ports is demonstrated successfully. The polarization-dependent loss is 2.2 dB in the worst case, and the wavelength-dependent loss is less than 2.5 dB across the entire C-band (1520-1580 nm) with the average extinction ratio of 17.7 dB. The measured reconfiguration time is shorter than 6 ns. Low-penalty transmission of a 40-Gb/s non-return-to-zero signal is also demonstrated for the first time. These features make the integrated phased-array switch an attractive candidate to be deployed in the future high-capacity optical packet switching networks.

We derive explicit design rules of optical phased-array planar 1 × N switch to achieve small insertion loss, large extinction ratio, and non-dispersive waveguide structure required for wavelength-independent broadband operation. The minimum number of phase shifters and slab lengths were studied as a function of N and the optimized array shape was derived to minimize the optical path length. We show that the length depends nonlinearly on N, and may become significantly shorter compared with that of a tree-structured switch under certain conditions.

Modulation-cancellation property of InGaAlAs/InP MQW reflective semiconductor optical amplifier (RSOA) and its temperature dependence are investigated for uncooled colorless WDM transmitter applications. Using the optimized RSOA length, we demonstrate self-seeded 2.5-Gb/s transmission over 25-km SSMF at 25oC and 50°C.

The circular-birefringence fiber (CBF) or the so-called twisted fiber provides novel attractive potential for all-optical signal-processing applications. Unlike a standard nonpolarization-maintaining (non-PM) fiber, where the polarization dependence of chi(3) nonlinearity is averaged out by the residual birefringence, the CBF behaves as a perfectly isotropic fiber in terms of nonlinear wave propagation. The CBF, therefore, allows the utilization of the intrinsic absolute polarization dependence of chi (3) nonlinearity in realizing novel functions that can never be obtained with a non-PM fiber. Several types of CBF are successfully fabricated by twisting commercial non-PM fibers at 15 turns/m, and their unique potential is experimentally demonstrated. First, a novel type of all-optical intensity discriminator is realized using the effect of ellipse rotation in CBF, which is absent in a non-PM fiber. The demonstrated device is directly applicable to all-optical data reshaping, pulse compression, pedestral suppression, and enhanced mode-locking of fiber lasers. Next, a 160-Gb/s polarization-insensitive wavelength conversion is demonstrated by using cross-phase modulation in CBF with a circularly polarized probe wave. In addition to its simplicity and robustness, the scheme is readily upgradable to polarization-insensitive all-optical data regeneration

All-optical wavelength conversion is one of the key technologies in constructing future photonic networks. Among many candidates, four-wave mixing in an optical fiber is especially attractive for realizing inherently broadband and modulation-transparent wavelength converters. This paper summarizes the recent research advances of wavelength converter based on fiber four-wave mixing. We first review the basic principle of four-wave mixing in an optical fiber. Then we introduce latest experimental results and discuss design strategies as well as challenges.

A self-seeded reflective semiconductor optical amplifier (RSOA) has been viewed as an attractive candidate for the low-cost colorless transmitter in wavelength-division-multiplexed passive-optical networks (WDM-PON). In this paper, we propose and demonstrate the use of two-section RSOA with segmented electrodes to simultaneously improve the modulation bandwidth and extinction ratio in a self-seeded operation. We first find the optimal driving condition to induce strong modulation cancellation by analyzing the gain dynamics inside the RSOA. In addition, we experimentally confirm the improvement in direct modulation bandwidth by segmenting the electrodes. Finally, we employ the two-section RSOA in a self-seeded system and successfully demonstrate transmission over a 25-km standard single-mode fiber at 5 Gb/s as well as improved back-to-back modulation at 10-Gb/s in C-band by using the optimal driving condition.

We fabricate a capsule-shaped metal-clad wavelength-scale InP/InGaAs cavity and demonstrate room-temperature lasing at a wavelength of ∼1550 nm under pulsed optical pumping. By introducing an optimized curvature to the sidewalls of a conventional rectangular metal-clad laser, we effectively reduce the metallic loss of the transverse-electric mode and experimentally confirm a clear improvement in the side mode suppression ratio and slope efficiency.

Stokes vector modulation and direct detection (SVM/DD) has immense potentiality to reduce the cost burden for the next-generation short-reach optical communication networks. In this paper, we propose and demonstrate an InGaAsP/InP waveguide-based polarization-analyzing circuit for an integrated Stokes vector (SV) receiver. By transforming the input state-of-polarization (SOP) and projecting its SV onto three different vectors on the Poincare sphere, we show that the actual SOP can be retrieved by simple calculation. We also reveal that this projection matrix has a flexibility and its deviation due to device imperfectness can be calibrated to a certain degree, so that the proposed device would be fundamentally robust against fabrication errors. A proof-of-concept photonic integrated circuit (PIC) is fabricated on InP by using half-ridge waveguides to successfully demonstrate detection of different SOPs scattered on the Poincare sphere.

We propose novel 1.55-μm capsule-shaped metallic-cavity lasers with curved facets to reduce plasmonic losses. Significant reduction of threshold current from 291 μA to 60 μA is demonstrated with effective modal volume of 0.45 μm3.

We present an iterative design method for the coupling and the mode conversion of arbitrary modes to focused surface plasmons using a large array of aperiodically randomly located slits in a thin metal film. As the distance between the slits is small and the number of slits is large, significant mutual coupling occurs between the slits which makes an accurate computation of the field scattered by the slits difficult. We use an accurate modal source radiator model to efficiently compute the fields in a significantly shorter time compared with three-dimensional (3D) full-field rigorous simulations, so that iterative optimization is efficiently achieved. Since our model accounts for mutual coupling between the slits, the scattering by the slits of both the source wave and the focused surface plasmon can be incorporated in the optimization scheme. We apply this method to the design of various types of couplers for arbitrary fiber modes and a mode demultiplexer that focuses three orthogonal fiber modes to three different foci. Finally, we validate our design results using fully vectorial 3D finite-difference time-domain (FDTD) simulations. (C)2014 Optical Society of America

We demonstrate a polarization-insensitive all-optical flip-flop based on a multimode-interference bistable laser diode. Polarization-insensitive flip-flop operation is demonstrated with the set power as low as -3 dBm, by applying 0.3% tensile strain to the multiple quantum wells gain medium. The output state of polarization is stabilized to the transverse-magnetic mode irrespective of input polarization, indicating that our device also offers a polarization-stabilizing function. Dynamic switching is achieved without waveform distortion even when the input polarization state is scrambled.

An integrated semiconductor 1times5 InP-InGaAsP optical phased-array switch is fabricated and characterized. Using the carrier-induced refractive index change of InGaAsP waveguide, we achieve successful switching to five output ports with less than 60-mA current injection. Wide operating bandwidth covering the entire C-band (1520-1580 nm) and nanoseconds dynamic response are demonstrated, making it potentially applicable to large-capacity wavelength-multiplexed optical-packet-switching routers.

Polarization-independent all-optical wavelength conversion is demonstrated by using cross-phase modulation in a twisted fiber and subsequent optical filtering. By twisting a 1-km-long fiber at a rate of 15 turns/m and aligning the probe light to circular polarization, we reduce the polarization sensitivity successfully from 3.5 to 0.3 dB. Error-free operation with only 1-dB penalty is realized at 40 Gb/s with the input signal polarization scrambled. The demonstrated scheme also offers a novel function of restoring the degree-of-polarization of the signal light.

We propose and demonstrate a novel scheme of the all-optical, high-speed add-drop multiplexer (ADM) for the optical time-division multiplexed signal. The ADM employs a single nonlinear optical loop mirror (NOLM) composed of an optical fiber. The key technical point of this scheme is the simultaneous use of the reflection mode (add function) and the transmission mode (drop function) of the NOLM, which results in significant simplification of the overall ADM configuration. Experimental results show that simultaneous add-drop operation at 80 Gb/s can be successfully achieved without bit errors.

We propose and demonstrate a novel fiber-based all-optical regenerator, where the wavelength shift induced by cross-phase modulation is employed for level discrimination. Regeneration of a 10-Gb/s signal using the proposed scheme improves the Q-factor by 1.8 dB. The scheme is stable and robust against changes in environmental conditions.

We demonstrate that the gentle confinement method is effective for optimizing area-limited photonic crystal cavities. We also show the importance of the boundary and that optimal boundary parameters improves the Q-factor significantly.

We experimentally demonstrate a compact and low-loss InP/InGaAsP polarization converter, fabricated by a simple self-aligned process. The mode-conversion of 96% and the excess loss of less than 1.0 dB are obtained over the entire C-band.

Highly efficient tunable wavelength conversion based on nondegenerate fiber four-wave mixing is demonstrated. We apply simultaneous phase modulation to the two pump waves so that stimulated Brillouin scattering is suppressed without affecting the generated idler wave. The signal light is converted from an arbitrary wavelength to another arbitrary one within the entire C-band with a conversion efficiency higher than -10 dB.

We propose a novel ultra compact electro-absorption optical modulator based on a silicon rib waveguide and numerically demonstrate its performance. The proposed design employs two types of transparent conductive oxide (TCO) layers with different carrier densities to achieve both high modulation efficiency and low optical insertion loss. The thin TCO layer with high carrier density enables efficient modulation through the metal–oxide–semiconductor structure. On the other hand, the upper TCO layer with low carrier density allows low-resistance electrical contact for the top electrode without large optical loss. Using an indium tin oxide bilayer with optimized carrier densities, we numerically demonstrate a 4.3 dB extinction ratio and a 2.6 dB optical insertion loss with 1 µm device length. We estimate that the modulator operates under a low driving voltage of 1.3 V, exhibiting an ultra low energy consumption of 22.5 fJ/bit and a broad RC modulation bandwidth of over 40 GHz.

We propose and demonstrate a simple scheme of a highly efficient polarization-independent wavelength converter utilizing the two-pump fiber optical parametric amplification. First, binary phase-shift-keying modulation is applied to two pump waves, which allows us to suppress stimulated Brillouin scattering without broadening the idler spectrum. Then, polarization states of the two pumps are aligned orthogonally to realize the wavelength conversion which is insensitive to the signal polarization. Using this method, we achieve polarization-independent wavelength conversion with an efficiency larger than 0 dB over a 30-nm signal bandwidth.

We demonstrate a compact optical buffer module, comprising an InP 1×8 phased-array switch and a silica-based delay line circuit. Tunable and uniform buffering of up to 21 ns is obtained with a 3-ns temporal resolution.

We present a novel efficient model to analyze the optical properties of multiple subwavelength apertures arbitrarily located on metal films. The model is verified by computing extraordinary optical transmission through an aperiodic nanoslit array.

Proposes and demonstrates a novel method of suppressing spectral broadening of the idler wave, which is generated in fiber four-wave mixing (FWM) pumped by a continuous wave. In this scheme, binary-phase-shift-keying (BPSK) modulation of the pump wave broadens the pump spectrum so that the Brillouin threshold for the pump power is increased, whereas it never affects the spectrum of the idler wave. By the 2-Gb/s BPSK modulation of the pump wave, we achieve a FWM efficiency as high as 10 dB, maintaining a side-band suppression ratio of the idler wave larger than 25 dB.

We demonstrate large-capacity high-resolution optical buffer, comprising 1?16 InP switch and ultra-compact delay lines based on thin-cladding highly nonlinear fibers. Silica-PLC-based pitch converter is employed to realize uniform coupling from all 16 switch ports simultaneously.

We perform and demonstrate synchronous all-optical flip-flop operations with optical clock injection using Mach-Zehnder interferometer bistable laser diode. Saturable absorbers play the key role which allow broadband wavelength tuning of the clock wavelength.

We demonstrate a 1 ÿ 16 optical packet switch sub-system for 160 Gbps RZ-OOK and 12 ÿ 10 Gbps multi-wavelength DPSK packets. We show error-free operation with maximum penalties of 0.7 dB for 160 Gbps RZ-OOK and 0.6 dB for multi-wavelength DPSK packets.

We demonstrate the first all-optical 2R regeneration of 25 Gbit/s NRZ data based on hysteresis in a DFB laser. The scheme results in BER improvement, exhibits low power consumption and is effective after fiber transmission.