We evaluated the energy efficiency per 1-bit transmission of an optical light source on InP substrate to achieve optical interconnection. A semiconductor optical amplifier (SOA) assisted extended reach EADFB laser (AXEL) was utilized as the optical light source to enhance the energy efficiency compared to the conventional electro-absorption modulator integrated with a DFB laser (EML). The AXEL has frequency bandwidth extendibility for operation of over 100Gbit/s, which is difficult when using a vertical cavity surface emitting laser (VCSEL) without an equalizer. By designing the AXEL for low power consumption, we were able to achieve 64-Gbit/s, 1.0pJ/bit and 128-Gbit/s, 1.5pJ/bit operation at 50°C with the transmitter dispersion and eye closure quaternary of 1.1dB.

Effective user accommodation will be more and more important in passive optical networks (PONs) in the next decade since the number of subscribers has been leveling off as well and it is becoming more difficult for network operators to keep sufficient numbers of maintenance workers. Drastically reducing the number of small-scale communication buildings while keeping the number of accommodated users is one of the most attractive solutions to meet this situation. To achieve this, we propose two types of long-reach repeater-free upstream transmission configurations for PON systems; (i) one utilizes a semiconductor optical amplifier (SOA) as a pre-amplifier and (ii) the other utilizes distributed Raman amplification (DRA) in addition to the SOA. Our simulations assuming 10G-EPON specifications and transmission experiments on a 10G-EPON prototype confirm that configuration (i) can add a 17km trunk fiber to a normal PON system with 10km access reach and 1 : 64 split (total 27km reach), while configuration (ii) can further expand the trunk fiber distance to 37km (total 47km reach). Network operators can select these configurations depending on their service areas.

A number of all-optical signal processing schemes based on nonlinear optical effects have been proposed and demonstrated for use in future photonic networks. Since various modulation formats have been developed for optical communication systems, all-optical converters between different modulation formats will be a key technology to connect networks transparently and efficiently. This paper reviews our recent works on all-optical modulation format conversion technologies in order to highlight the fundamental principles and applications in variety of all-optical signal processing schemes.

The influence of pump phase error on phase-sensitive optical amplifier (PSA) repeaters and the waveform degradation due to chromatic dispersion and fiber nonlinearities in the optical multi-relay transmission of quadrature phase-shift keying phase-conjugated twin waves are considered theoretically. First, the influence of noise from the pump phase error, optical local oscillator, receiver, and the amplified spontaneous-emission (ASE) in PSA repeaters is investigated with the assumption that transmission fibers are linear lossy channels. The bit-error rate (BER) is estimated as a function of the signal-to-noise ratio, and the relationship between the number of transmission relays and the fiber launch power is clarified. Waveform degradation due to chromatic dispersion and the optical fiber nonlinearities in transmission fibers are investigated with the noiseless condition, and the maximum repeatable number as a function of the fiber launch power is calculated. Finally, we show the relationship among the maximum repeatable number, standard deviation of pump phase error in PSA repeaters, and the fiber launch power to clarify the optimum transmission condition with consideration of the noise and the waveform degradation.

Recent advances in phase-sensitive amplifiers (PSAs) using periodically poled LiNbO3 are reviewed. Their principles of operation and distinct features are described. Applications in optical communication are studied in terms of the inline operation and amplification of a sophisticated modulation format. Challenges for the future are also discussed.

We propose a novel structure that can reduce the power consumption and extend the transmission distance of an electro-absorption modulator integrated with a DFB (EADFB) laser. To overcome the trade-off relationship of the optical loss and chirp parameter of the EA modulator, we integrate a semiconductor optical amplifier (SOA) with an EADFB laser. With the proposed SOA assisted extended reach EADFB laser (AXEL) structure, the LD and SOA sections are operated by an electrically connected input port. We describe a design for AXEL that optimizes the LD and SOA length ratio when their total operation current is 80mA. By using the designed AXEL, the power consumption of a 10-Gbit/s, 1.55-µm EADFB laser is reduced by 1/2 and at the same time the transmission distance is extended from 80 to 100km.

This paper reviews long optical reach and large capacity transmission which has become possible because of the application of wide-band and low-noise optical fiber amplifiers and digital coherent signal processing. The device structure and mechanism together with their significance are discussed.

To drastically increase the splitting ratio of extended-reach (40km span) time- and wavelength-division multiplexed passive optical networks (WDM/TDM-PONs), we modify the gain control scheme of our automatic gain controlled semiconductor optical amplifiers (AGC-SOAs) that were developed to support upstream transmission in long-reach systems. While the original AGC-SOAs are located outside the central office (CO) as repeaters, the new AGC-SOAs are located inside the CO and connected to each branch of an optical splitter in the CO. This arrangement has the potential to greatly reduce the costs of CO-sited equipment as they are shared by many more users if the new gain control scheme works properly even when the input optical powers are low. We develop a prototype and experimentally confirm its effectiveness in increasing the splitting ratio of extended-reach systems to 512.

Amplification characteristics of the signal and the noise in the semiconductor optical amplifier (SOA), without facet mirrors for the intensity modulated light, are theoretically analyzed and experimentally confirmed. We have found that the amplification factor of the temporarily varying intensity component is smaller than that of the continuous wave (CW) component, but increases up to that of the CW component in the high frequency region in the SOA. These properties are very peculiar in the SOA, which is not shown in conventional electronic devices and semiconductor lasers. Therefore, the relative intensity noise (RIN), which is defined as ratio of the square value of the intensity fluctuation to that of the CW power can be improved by the amplification by the SOA. On the other hand, the signal to the noise ratio (S/N ratio) defined for ratio of the square value of the modulated signal power to that of the intensity fluctuation have both cases of the degradation and the improvement by the amplification depending on combination of the modulation and the noise frequencies. Experimental confirmations of these peculiar characteristics are also demonstrated.

An optical flip-flop circuit with a single semiconductor optical amplifier (SOA) using two orthogonal polarization states is proposed. The optical set / reset input and output signals are at a single wavelength. The flip-flop circuit consists of an SOA, a polarization combiner, a polarization splitter, two directional couplers, and two phase shifters. No continuous light source is required to operate the circuit. In this paper, we theoretically analyze the operation performance. Polarization dependence in SOA is considered in the analysis at a single wavelength operation, and numerically simulated results are presented. We confirm that the flip-flop circuit with a feedback-loop length of 15~mm can be operated at switching time of around 3~ns by 1~ns set / reset pulses. The flip-flop performance is discussed from viewpoints of transient overshoot and contrast at the steady on-off states.

Bit error rate characteristic of negative feedback optical amplifier was investigated by manipulating the negative feedback signal intensity fed into the semiconductor optical amplifier together with the input signal. Consequently, bit error rate was reduced as negative feedback signal intensity increases. Suppression towards the unevenness at the power level `1' and overshoot during rising phase on the output signal eye-diagram was recorded. With negative feedback, through gain decrease of 2.4 dB, power penalty improved remarkably by 15 dB.

We have designed and fabricated a compact 4-array integrated SOA module using a novel parallel optical coupling scheme and polarization-insensitive built-in array isolators. We achieved ultra-high On/Off extinction ratio of more than 60 dB and low cross talk of better than -60 dB as well as high-isolation of over 47 dB in wide wavelength ranges. We also developed a wavelength-insensitive parallel optical coupling scheme and an efficient thermal dissipating structure for a 4-array SOA module. We applied these technologies into 4-array SOA module fabrication and demonstrated a uniform optical coupling with the loss variance of 1 dB over the 140-nm wavelength ranges. We also demonstrated simultaneous operation of 300 mA 4 channels with low thermal degradation of the module gain less than 1 dB.

To characterize and predict the dynamics of the nonlinear polarization rotation in SOAs, an experimental method based on the frequency response technique and a model based on the density matrix and effective index formalisms are presented. Both determine the angular displacement, at the Poincare Sphere, that produces the evolution of the polarization of the output signal.

We propose a new bidirectional gigabit mm-wave wavelength division multiplexed-radio over fiber link which shares the same wavelength. As the downlink, the central station transmits a 30 GHz single sideband wireless signal which is modulated 1.25 Gbps and also transmits a remote 32 GHz local oscillator for down-conversion of a uplink wireless signal by using a mach-zehnder modulator and a fiber bragg grating. As the uplink, the base station transmits a down-converted 1.25 Gbps wireless signal by using a reflective semiconductor optical amplifier. We achieve a BER < 10-9 in the downlink at -14.05 dBm and uplink at -12.5 dBm after 20 km transmission.

All-optical wavelength conversion and modulation format conversion will be needed in the next generation high-speed optical communication networks. We have proposed and successfully demonstrated the error free operation of all-optical modulation format conversion from NRZ-OOK to RZ-BPSK using SOA based MZI wavelength converter. In this paper, we experimentally investigate the wavelength conversion characteristics of the proposed NRZ-OOK/RZ-BPSK modulation format converter. The results show that error free modulation format conversion is possible over the entire C band.

In a power grid used to distribute electricity, optical fibers can be inserted inside overhead ground wires to form an optical network infrastructure for data communications. Dense wavelength division multiplexing (DWDM)-based optical networks present a promising approach to achieve a scalable backbone network for power grids. This paper proposes a complete optimization procedure for optical network designs based on an existing power grid. We design a network as a subgraph of the power grid and divide the network topology into two layers: backbone and access networks. The design procedure includes physical topology design, routing and wavelength assignment (RWA) and optical amplifier placement. We formulate the problem of topology design into two steps: selecting the concentrator nodes and their node members, and finding the connections among concentrators subject to the two-connectivity constraint on the backbone topology. Selection and connection of concentrators are done using integer linear programming (ILP). For RWA and optical amplifier placement problem, we solve these two problems together since they are closely related. Since the ILP for solving these two problems becomes intractable with increasing network size, we propose a simulated annealing approach. We choose a neighborhood structure based on path-switching operations using k shortest paths for each source and destination pair. The optimal number of optical amplifiers is solved based on local search among these neighbors. We solve and present some numerical results for several randomly generated power grid topologies.

We describe the transmission characteristics of a wavelength independent wavelength division multiplexing passive optical network (WDM-PON) based on a wavelength channel data rewriter (WCDR). The WCDR is composed of a linear amplifier (LA) and a saturated semiconductor optical amplifier (SOA), and by using the WCDR in optical network units (ONUs), we can erase the downstream signal and modulate the same wavelength channel with the upstream signal. In this paper, we analyze the data rewriting characteristic, the frequency chirp characteristic and the bit error rate (BER) degradation occasioned by the use of saturated SOAs. Furthermore, we report high-speed transmission with power penalty of less than 1 dB at bit rates of 1.25 Gbit/s, 2.5 Gbit/s and 10 Gbit/s for downstream signals and 1.25 Gbit/s for upstream signals after transmission through 40 km of single-mode fiber.

We propose a new label recognition system for photonic label switching using self-routing of labels. Binary-coded labels in on-off keying format are considered. The system consists of an all-optical demultiplexer (DeMUX) and an address recognition unit (ARU) consisting of tree-structured switches. The system uses self-routing propagation of an indication bit controlled with address bits. The indication bit is placed in advance of the address bits in the label. In DeMUX, all-optical switches in a configuration of Mach-Zehnder interferometer with semiconductor optical amplifiers (SOA-MZI) are controlled by the indication bit pulse to separate each of the label bits. The indication bit pulse is routed to the destination output port corresponding to the code of the address in ARU. It is shown that all the binary number codes can be recognized with this system. The operation principle is verified by numerical simulation using coupled-mode theory and a rate equation. Moreover, the switching crosstalk is also evaluated.

The design aspects of the bulk InGaAsP semiconductor optical amplifier integrated Mach-Zehnder interferometer (SOA-MZI) optimized for 40 Gbps-NRZ all optical wavelength conversion are described. The dimensions of the SOA active waveguide have been optimized for fast gain recovery by maximizing the gain and adjusting the wavelength-converted NRZ waveforms. Submicron-width buried heterostructure (BH) SOA waveguides were fabricated successfully and showed little leakage current. The experimental wavelength-converted optical waveform agreed well to the numerical simulations, and mask-compliant 40 G-NRZ wavelength-converted waveform was obtained by the optimized SOA-MZI. 40 G-NRZ full C-band operation and polarization-insensitive operation of SOA-MZI were also achieved.

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.