By adopting a network architecture in which not only a calling but also a called terminal can select a wavelength, a novel WDM network becomes possible. This we call Wavelength Assignment Photonic Network (WAPN). In this network wavelengths are a kind of network resources and according to requests from terminals, wavelengths are allocated or assigned to calls. In the system a wavelength used for a call is to be used for another call after the call is terminated. By supplying wavelengths to the home, a bitrate-free, protocol free or even transmission method free network can be realized. In this paper, from a viewpoint of S/N or Q factor, WAPN is evaluated with special focus on the node architecture--i. e. , from the viewpoint of node size, number of switching stages, crosstalk level,and losses, because the allowable node size is the crucial issue to decide the whole network capacity. After brief explanations of this proposed system, the model for system evaluations will be established and a node system is to be evaluated for some practical parameter values considering especially traffic characteristics of a node. As a result of this study a node system with capacity more than 100 thousands erl (about 20 Tbps throughput) can be constructed using present available technologies, which will enable us to construct large WAPN network with radius of 2,000 km and subscribers of about 50 millions.

By adopting a network architecture in which not only a calling but also a called terminal can select a wavelength, a novel WDM network becomes possible. This we call Wavelength Assignment Photonic Network (WAPN). In this network wavelengths are a kind of network resources and according to requests from terminals, wavelengths are allocated or assigned to calls. In the system a wavelength used for a call is to be used for another call after the call is terminated. By supplying wavelengths to the home, a bitrate-free, protocol free or even transmission method free network can be realized. In this paper, from a viewpoint of S/N or Q factor, WAPN is evaluated with special focus on the node architecture--i. e. , from the viewpoint of node size, number of switching stages, crosstalk level,and losses, because the allowable node size is the crucial issue to decide the whole network capacity. After brief explanations of this proposed system, the model for system evaluations will be established and a node system is to be evaluated for some practical parameter values considering especially traffic characteristics of a node. As a result of this study a node system with capacity more than 100 thousands erl (about 20 Tbps throughput) can be constructed using present available technologies, which will enable us to construct large WAPN network with radius of 2,000 km and subscribers of about 50 millions.

A novel type of optically clocked all-optical flip-flop based on a coupled-mode distributed Bragg reflector laser diode is proposed. The device operates as a bistable laser, where the two lasing modes at different wavelength are switched all-optically by injecting a clock pulse together with a set/reset signal. We employ an analytical model based on the two-mode coupled rate equations to verify the basic operation of the device numerically. Optically clocked flip-flop operation is obtained with a set/reset power of 0.60 mW and clock power of 1.8 mW. The device features simple structure, small footprint, and synchronized all-optical flip-flop operation, which should be attractive in the future digital photonic integrated circuits.

An 8ch, 400 GHz monolithically integrated WDM channel selector featuring an array of quantum well semiconductor optical amplifiers (SOA) and arrayed waveguidegrating demultiplexer is presented. Reduction of fabrication complexity was achieved by using a single step selective area MOVPE to realize the different bandgap profiles for the SOA array and passive region. The selective growth mask dimensions were optimized by simulation. Dry-etching with short bending radii of 200 µm resulted in compact device size of 7 mm2.5 mm. Static channel selection with high ON-OFF ratio of >40 dB was achieved.

Wavelength Assignment Photonic Switching System (WAPS) provides a wavelength for an end-to-end communication. In this way the features of wavelength can be fully utilized by users. We will give an architectural proposal in which two types of connections over WAPS network are provided and are adaptively used according to service demand by customers. One is a connection established semi-permanently between edge routers and relay-routers and the other is a connection established on flow-by-flow basis between edge routers. We will compare with conventional router networks in terms of data-transfer time. Pre-processing time is a crucial issue in connection-oriented networks, and this is very much reduced thanks to the WAPS network structure.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

Coherent transceivers are now regarded as promising candidates for upgrading the current 400Gigabit Ethernet (400GbE) transceivers to 800G. However, due to the complicated structure of a dual-polarization IQ modulator (DP-IQM) with its bulky polarization-beam splitter/comber (PBS/PBC), the increase in the transmitter size and cost is inevitable. In this paper, we propose a compact PBS/PBC-free transmitter structure with a straight-line configuration. By using the concept of polarization differential modulation, the proposed transmitter is capable of generating a DP phase-shift-keyed (DP-PSK) signal, which makes it directly applicable to the current coherent systems. A detailed analysis of the system performance reveals that the imperfect equalization and the bandwidth limitation at the receiver are the dominant penalty factors. Although such a penalty is usually unacceptable in long-haul applications, the proposed transmitter can be attractive due to its significant simplicity and compactness for short-reach applications, where the cost and the footprint are the primary concerns.

A photon-induced waveguide (PIG) for all-optical switching and wavelength conversion with the functionalities of regeneration and reshaping is proposed. Optical signals are used to switch between lateral optical wave guiding and antiguiding effects. A transfer-matrix method was developed to consider not only the variation of optical signal power along the waveguide, but also the spatial distributions of refractive index and optical confinement factor to explain the switching scheme between guiding and antiguiding. Theoretical analyses show that a threshold-like and sharp input-output response of PIG allows enhancement of the extinction ratio, reshaping, and thus enlargement of noise margin of optical signals in digital all-optical switching and wavelength conversion.

The operation of a novel all-optical wavelength converter based on directionally-coupled semiconductor optical amplifiers is described. Merits such as extinction enhancement and digital response are expected through a simple analytical model and a sophisticated transfer matrix method developed to take into account the spatial distributions of the optical power, carrier density, refractive index, propagation constant, and coupling coefficient along device. We fabricated devices operating at 1.55 µm band using an InGaAsP/InP material system and demonstrated successfully the static characteristics of wavelength conversion with the expected advantages. Devices are as small as 1.5 mm and do not need any active/passive integration step during fabrication.

In this paper we modeled and analyzed the ridge type InGaAlAs/InP semiconductor laser with lateral current confinement structure, and optimized the design for the ridge wave guide with the current confinement. We proposed and fabricated the ridge type InGaAlAs/InP laser with a cost effective selective undercut etching method and demonstrated the improvement of the ridge laser performance. This paper provides a solution to solve the cost/yield issue for conventional BH (buried hetero-structure) type laser and performance issue for conventional ridge type laser.

Monolithically integrated four-channel distributed feedback (DFB) laser array has been fabricated by metal organic vapor phase epitaxy (MOVPE) selective area growth for 1.55 µm coarse-wavelength division multiplexing (CWDM) systems. Wide-stripe MOVPE selective area growth and electron-beam lithography are used to obtain wide CWDM channel spacing of 20 nm. Compared to hybrid integration of discrete lasers, monolithic integration of laser array on a single substrate greatly simplifies device alignment and packaging process.

A reliable and automatic parameter extraction technique for DFB lasers is developed. The parameter extraction program which is named "LAPAREX" is able to determine many device parameters from a measured sub-threshold spectrum only, including gain- and index-coupling coefficients, and spatial phases of the grating at front and rear facets. Injection current dependence of coupling coefficients in a gain-coupled DFBlaser is observed, for the first time, by making use of it.

Simulation and fabrication results on back-illuminated 4-channel photodiode (PD) array with a self-aligned micro ball lens are described. The channel pitch and diameter of each photosensitive area are 250 µm and 40 µm, respectively. Measured photocurrent is 1.92 times larger than that without a lens. Alignment tolerance between the single mode fiber (SMF) optical axis and the photodiode is improved from 21.2 µm to 42.7 µm. Moreover, the separation tolerance between the fiber and the lens is 210.5 µm. These large tolerances agree with simulation results, demonstrating that the device configuration is suitable for receivers for multi-channel inter-connection. Frequency response and inter-channel cross talk are also discussed.

Optical buffering has been one of the major technical challenges in realizing optical packet switching routers and interconnects. We demonstrate a compact optical buffer module, comprising an InP 18 phased-array switch and a silica-based delay line circuit. The integrated delay line circuit is fabricated on the silica-based planar-lightwave circuit (PLC) platform, and has the ladder architecture for reducing the size. In addition, variable optical couplers are integrated to achieve effective power equalization. Tunable and uniform buffering of up to 21 ns is obtained with 3-ns temporal resolution.

Integrated InP polarization converters based on half-ridge structure are studied numerically. We demonstrate that the fabrication tolerance of the half-ridge structure can be extended significantly by introducing a slope at the ridge side and optimizing the thickness of the residual InGaAsP layer. High polarization conversion over 90% is achieved with the broad range of the waveguide width from 705 to 915~nm, corresponding to a factor-of-two or larger improvement in the fabrication tolerance compared with that of the conventional polarization converters. Finally we present a simple fabrication procedure of this newly proposed structure, where the thickness of the residual InGaAsP layer is controlled precisely by using a thin etch-stop layer.

We propose a novel 13 planar optical switch using aspheric lenses and carrier-induced tunable prisms on InP. An input light beam is collimated by the aspheric lenses in a slab waveguide. The tunable prism, whose refractive indices are tuned by the carrier plasma effect, deflect the collimated light beam and guide it to the output ports. The switching operations of the 13 optical switch that consists of five lenses and eight prisms with a footprint of 5003500 µm are performed by three-dimensional beam propagation methods. A static switching operation with a 5-dB insertion loss and a 13-dB extinction ratio is obtained with 70-mA current injection for each prism. This device has a simple structure and low power consumption and may be useful for optical packet switching systems.

To meet the demand for continuous increase in data traffic, full usage of polarization freedom of light is becoming inevitable in the next-generation optical communication and datacenter networks. In particular, Stokes-vector modulation direct-detection (SVM-DD) formats are expected as potentially cost-effective method to transmit multi-level signals without using costly coherent transceivers in the short-reach links. For the SVM-DD formats to be practical, both the transmitter and receiver need to be substantially simpler, smaller, and lower-cost as compared to coherent counterparts. To this end, we have recently proposed and demonstrated novel SV modulator and receiver circuits realized on monolithic InP platforms. With compact non-interferometric configurations, relatively simple fabrication procedures, and compatibility with other active photonic components, the proposed devices should be attractive candidate in realizing low-cost monolithic transceivers for SVM formats. In this paper, we review our approaches as well as recent progresses and provide future prospects.

We describe a simple all-optical wavelength converter based on a Fabry-Perot semiconductor optical amplifier (FPSOA). We measure its static characteristics in detail and successfully demonstrate its dynamic wavelength-conversion operation (both inverted and non-inverted) at 2.5 Gbit/s. This is the first demonstration of FPSOA-based wavelength conversion. Quasi-digital response is also observed. Low input power, ease of fabrication and good compatibility with WDM networks are important advantages of FPSOA.