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High-speed and robust optical beam-steering device will be the key component for various applications, such as LiDAR (light detection and ranging) and free-space optical communication. Optical phased arrays (OPAs) integrated on semiconductor chips have recently received increasing attention due to the high-speed operation, compactness, and low cost. In particular, indium phosphide (InP)-based OPAs are advantageous for high-output-power applications at 1.55- $\mu \text{m}$ eye-safe wavelength, owing to their capability of monolithically integrating active components, such as high-power lasers and optical amplifiers. In this letter, we design and fabricate, to the best of our knowledge, the largest-scale InP-based OPA and experimentally demonstrate its beam-steering operations. The fabricated OPA consists of 100 waveguides with carrier-injection-based phase shifters, densely integrated on 7 mm $\times5$ mm footprint. A focused beam width of 0.11° is steered across the free spectral range of 8.88°, corresponding to more than 80 resolvable points. This is the largest number of resolvable points obtained by a monolithic InP OPA. The response time of the OPA is confirmed to be less than 16 ns, which is limited by the driver circuit. This work paves the way for realizing compact beam-steering modules for high-speed and high-output-power imaging applications.

Recent studies have shown that the self-coherent systems, including the bi-directional, the Stokes-vector-modulation direct-detection (SVM-DD), and the Kramers-Kronig (KK) schemes, have the potential to reduce the cost of short-reach networks. One of the most attractive features of the self-coherent systems is that they may eliminate the necessity of a narrow-linewidth laser, which is mandatory in the conventional full coherent systems employing high-order quadrature-amplitude-modulation (QAM) formats. On the other hand, it is also recognized that the laser phase noise may have a significant impact on the system performance if there exists a large length mismatch between the signal and the CW tone in the transmission paths. In this paper, we analyze the impact of laser phase noise on the self-coherent systems numerically and experimentally by considering the bi-directional system as an example case. As a result, we show that the performance of the self-coherent system can be described efficiently by using a limited number of normalized parameters. We then reveal the existence of a distinctive threshold on the path length mismatch that influences the bit-error-rate (BER) performance; the penalty increases in a stepwise manner as the length mismatch exceeds the threshold and converges to a constant value. Based on the results, we suggest the feasibility of employing high-order QAM formats to transmit >800 Gb/s signals per polarization using a 10-MHz-linewidth laser. Finally, proof-of-concept experimental results are presented to verify the theoretical and numerical analyses.

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.

Highly efficient and fabrication-error-resistant InP/ InGaAsP polarization rotator based on the half-ridge waveguide is experimentally demonstrated. A thin InAlAs layer is embedded inside the InGaAsP core layer to provide 20-fold etching selectivity with respect to InGaAsP, enabling precise control of the dry-etched core layer thickness. The designed device is fabricated to show a high polarization conversion efficiency of more than 95% with a large etching time tolerance of ±16%. Demonstrated scheme offers a practical path towards realizing compact and low-cost InP-based monolithic dual-polarization transceivers with on-chip polarization (de)multiplexers.

Direct-detection (DD) Stokes vector receiver (SVR) has rapidly gained interest for the high-speed datacenter and short-reach links, owing to its capability of recovering 3D optical signals without using expensive coherent detection. In order to reduce the size and cost of SVR, we have recently proposed and demonstrated a simple and compact device with single-ended photodetectors (S-PDs), which can be monolithically integrated on InP. In this paper, we provide comprehensive analysis on the receiver sensitivity for different SVR configurations. From rigorous theoretical investigations, we derive optimal design of SVR to maximize the sensitivity as well as explicit analytical expression of the bit-error-rate assuming an unamplified thermal-noise-limited case. From the obtained results, we find that the optimized 4-port S-PD-based SVR exhibits a moderate penalty of 2.1 dB with respect to the conventional more complicated SVR configuration based on 3-port balanced PDs. Since the proposed S-PD-based SVR can reduce the number of PDs, and requires neither a polarization-beam splitter nor precisely tuned optical interferometer, it may be an attractive candidate in realizing a low-cost and compact fully integrated DD-based receiver for the future >Tb/s short-reach systems, where the power budget is not the primary concern.

An integrated optical unitary converter (OUC), which can realize arbitrary N × N unitary transformation on chip, is promising for widespread applications in various areas, such as optical communication, quantum information processing, and optical neural networks. Most of the integrated OUCs demonstrated to date comprise cascaded 2 × 2 Mach-Zehnder interferometers (MZIs) based on Reck's scheme or its variation, or cascaded multimode interference (MMI) couplers based on multi-plane light conversion (MPLC). However, these schemes are sensitive to fabrication errors, making it difficult to realize high-performance large-scale OUCs. In this work, we experimentally demonstrate a silicon photonic 4 × 4 OUC that is inherently robust to fabrication errors, based on a novel structure. The proposed structure consists of cascaded multiport directional couplers (DCs) and phase shifter arrays. In this scheme, the multiport DC assures unitary transformation with inherent robustness against fabrication errors. By optimizing all the phase shifters using a simulated annealing algorithm, we experimentally demonstrate reconfigurable 4-mode demultiplexing and error-free demultiplexing of 40-Gb/s non-return-to-zero (NRZ) signals.

Gain-coupled 4 × 25 Gb/s electroabsorption modulated laser (EML) array is implemented by an identical epitaxial layer integration scheme. Gain-coupling is incorporated via absorptive gratings to improve the single mode yield of the device. The EML array exhibits excellent single-mode performance, high extinction ratio, and a modulation bandwidth over 28 GHz. Clear eye opening under 28-Gb/s nonreturn-to-zero modulation has been demonstrated for all four channels. Transmission performance of the EML array over 20- and 40-km single-mode fiber is tested, confirming its potential for 100 GbE applications.

Stokes vector modulation direct-detection (SVM-DD) formats have attained special attention in the short-reach communication links to transmit multilevel signals without using costly coherent receivers. In this paper, we fabricate and experimentally demonstrate a potentially low-cost DD-based SV receiver with on-chip photodetectors (PD) on a compact InP chip. Compressively strained multiple-quantum-well PD array is monolithically integrated with the SV projection circuit, which consists of multimode interference splitter and SV rotators based on self-aligned half-ridge structure. From the photocurrent signals at three on-chip PDs, the SV of input light is retrieved successfully over the entire Poincare sphere. With potentially broad operating bandwidth of InP-based PDs, the demonstrated device may be an attractive candidate to realize low-cost and high-speed receivers for the future short-reach SVM-DD systems.

Of-late research on Stokes vector receivers for the short-reach optical communications has gained significant attention due to its capability to decode information in multidimension using cost-effective direct-detection technique. Here, we consider the use of InP for potentially low-cost direct-detection-based Stokes vector (SV) receiver. It consists of a 1 × 4 multimode interference power splitter with different combination of polarization converters on output arms, so that input SV is projected onto distinct vectors on the Stokes space. By using the fabricated device, we experimentally demonstrate decoding of two-level and four-level SV-modulated signals at 3 Gbaud. With the potential of monolithically integrating high-speed InP photodetector array on the same chip, it paves the way to realize compact and low-cost SV receivers for multilevel SV modulation formats in future short-reach optical communication links.

We demonstrate a waveguide-based polarization controller that monolithically incorporates an active InGaAlAs/ InAlAs multiple-quantum-well phase shifter and a passive half-ridge polarization converter on InP. The butt-joint active/passive integration technique and self-aligned waveguide fabrication steps are employed. Using the fabricated device, efficient rotation of the Stokes vector over $\pi$ radian is obtained on the Poincare sphere by injecting 14-mA current to a 1.4-mm-long phase shifter. Dynamic response time below 5 ns is also confirmed, demonstrating the feasibility of low-power high-speed integrated polarization controller on InP.

Reconfigurable unitary optical mode converters that can convert multiple orthogonal spatial modes into different orthogonal modes without fundamental optical loss are important for mode-division-multiplexed (MDM) optical communication, imaging, and quantum computation. In this letter, we propose a novel integrated reconfigurable unitary optical mode converter, which consists of cascaded multimode interference couplers and phase shifter arrays. We numerically investigate its ability to realize arbitrary unitary mode conversion and demonstrate its applicability to mode unscrambling as well as mode switching in MDM transmission systems.

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.

A broadband optical 2R regenerator based on a single distributed feedback laser is demonstrated for nonreturn to zero signals at a bitrate of 10 Gb/s. A semi-analytical approach for the influence of hysteresis on the transfer function of a 2R regenerator is shown.

With the rapid growth in Internet traffic, demands for a high-capacity switching network with low end-to-end latency are being felt. Optical-label switching, in general, and optical burst switching (OBS), in particular, can provide such a solution with realizable device technology. This paper is a summary of the Photonic Network Project undertaken in Japan, wherein we have implemented a prototype OBS-node testbed based on novel device concepts. A three-node OBS core testbed and edge nodes operating over payload bit rates of 10 Gb/s to 40 Gb/s were developed, demonstrating feasibility of the OBS concept. We report newly developed key photonic building blocks of the OBS node, namely: a fast matrix switch based on lead-lanthanum-zirconate-titanate, a bit-rate transparent tunable wavelength converter based on a monolithically integrated wavelength converter; a fast tunable wavelength laser diode; compact arrayed waveguide amplifiers; and athermal, flat-top multiplexers. Together, with an optical-label recognition scheme implemented in a field-programmable gate array (FPGA), we have integrated the building blocks into an optical burst switching-node prototype that can potentially be scaled up to 64 times 64 ports. We have confirmed error-free optical burst forwarding for more than 10 hops in an OBS network with variable length, asynchronous burst arrival. We have also achieved a mixed bit-rate optical burst forwarding and contention resolution with wavelength conversion. We have also demonstrated coordination of OBS with upper-layer protocols by implementing an Ethernet-frame-to-optical-burst converter edge node, which successfully transmitted real application data over the OBS network at 40 Gb/s. Similar results were achieved in a multinode testbed under the field fiber environments, with a low frame error rate of less than 1 times 10-6.

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 flip-flops (AOFFs) have recently received increased attention as elements for all-optical packet-switched networks. In this letter, we use a single off-the-shelf distributed- feedback laser as AOFF to switch 40-Gb/s packets with a guard time as low as 150 ps.

We report 92-km 2-hop field trial of an optical burst-switching (OBS) network testbed, consisting of a (Pb, La)(Zr, Ti)O3 (PLZT) space switching matrix and a contention-resolving wavelength converter. Owing to the low polarization-dependent loss of the PLZT switch, we achieve stable forwarding of 40/10-Gb/s mixed-bit-rate signals with wavelength conversion. We also perform Layer-2 evaluation at a line rate of 40 Gb/s by using specially developed Ether-frame media converters. Loss-free transport of encapsulated Ether-frames is demonstrated successfully over the 2-hop OBS field testbed.

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.

We developed an optical burst switching (OBS) network testbed with three nodes, where each node consisted of a fast 5 5 (Pb,La)(Zr,Ti)O optical matrix switch. A newly developed Ether frame media converter was successfully used to evaluate Layer-2 performance of the testbed, with a line rate of 40Gb/s. A dynamic deflection routing scheme for contention resolution is implemented. Error-free switching with 40-Gb/s payload was achieved over the three nodes, promising low frame loss due to dynamic operation. By comparing effects of random collision with and without deflection routing for varying loads, we demonstrate that near-theoretical low frame loss is achievable in fast Layer-2 (Ether frame) transport over OBS networks.

We report a bit-rate transparent optical burst switching (OBS) router prototype using a fast 5 times 5 PLZT [(Pb,La)(Zr,Ti)O3 ] optical matrix switch. Dynamic switching in a two-wavelength, 2 times 2 OBS switch is demonstrated. Contention resolution using a tunable Mach-Zehnder interferometer wavelength converter for both 40- and 10-Gb/s bursts is demonstrated for the first time. Error-free operation was achieved for both bit rates under the same settings, as required in autonomous networks

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.

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 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.

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 present a theoretical and numerical framework to analyze optical properties of subwavelength apertures that are distributed arbitrarily on the 2-D plane of a metal film. Using the radiation patterns linked to individual eigenmodes inside the aperture, the coupling between multiple apertures is described efficiently by a small-rank linear system of equations. The complicated contributions from both the surface plasmon polariton (SPP) and creeping wave are inherently included in the analysis. Three-dimensional fully vectorial finite-difference time-domain calculations are used to verify the model in several test cases. The model accurately predicts all the salient features in extraordinary optical transmission (EOT) spectra of 2-D nanoslit arrays, including the surface plasmon resonances and Rayleigh-Wood anomalies. We also explore the effects of finite array size on EOT and discover a novel regime where the EOT efficiency decreases with an increasing number of apertures. Finally, we apply the model in calculating SPP excitation by an arbitrarily patterned nanoslit array. The presented method not only provides deeper physical insights into EOT and related phenomena, but should also be useful in designing a variety of novel aperiodic plasmonic devices with drastically less computational cost.