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.

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.

We have proposed and theoretically verified a 2R (reshaping and regeneration) limiter circuit using continuous wave (CW) holding beam for cross-gain modulation (XGM) wavelength converter, through simulation. The gain clamping effect of semiconductor optical amplifier (SOA), which is caused by CW holding beam injected into SOA, was used to obtain the accurate optical gain and phase conditions for SOA's in 2R limiter circuit. XGM wavelength converter with the proposed 2R limiter circuit provides higher extinction ratio (ER) as well as more enhanced operation speed than any other wavelength converter. Our numerical results show that after the wavelength-converted signal from XGM wavelength converter passed the 2R limiter circuit, it was re-inverted with the improved ER of 30 dB at 5 Gb/s. In case of high-speed operation, great enhancement to decrease power penalty of about 12 dB was shown at 10 Gb/s.

Various types of wavelength-selectable light sources (WSLs) and wavelength-tunable laser diodes (LDs) have been developed, and the one based on an array of distributed feedback (DFB) laser diodes (LDs) has the advantage of tuning that is both simple and stable tuning. It requires only the selection of a DFB-LD and a temperature control. We report on monolithically integrated WSLs with a DFB-LD array, multimode interference (MMI) coupler, semiconductor optical amplifier (SOA), and electro-absorption (EA) modulator. To make them compact, we introduced microarray structures and to ensure that they were easy to fabricate, we used selective area growth. For the WSL with an integrated EA modulator, we developed a center-temperature-shift method that optimizes the detuning wavelength between the lasing wavelength and the absorption edge wavelength of the EA-modulator. Using this method, we obtained a uniform extinction ratio and were able to demonstrate error-free 2.5-Gb/s transmission over a 600-km fiber span. A CW-WSL without an EA-modulator should provide enough output power to compensate the loss caused by the external modulators, but the high-power operation of a CW-WSL is sensitive to optical feedback from the front facet. We therefore used an angled facet in our WSLs and eliminated a mode hop problem. More than 20 mW of fiber-coupled power was obtained over 23 ITU-T channels on a 50-GHz grid.

Integration of spot-size converters (SSCs) with semiconductor optical amplifiers (SOAs) that improves chip-fiber optical coupling is inevitable for realizing high performance SOA modules. In this paper SSCs that can be easily integrated with SOAs and have little influence on the polarization sensitivity have been studied. We found that polarization insensitive active width-tapered SSCs can be realized by an optimum waveguide design of tensile-strained bulk structures. The SOA module exhibited large fiber-to-fiber gain (> 19 dB), small polarization sensitivity (< 0.4 dB), high fiber-coupled saturation output power (> +11.7 dBm) and record low module noise figure (< 6.3 dB) for the signal wavelength range of 1530-1560 nm.

This paper describes the recent progress made in developing wavelength tunable semiconductor light sources for WDM applications. Wide and quasi-continuous wavelength tunings were investigated for a wavelength-selectable laser and a wavelength tunable distributed Bragg reflector (DBR) laser having a super structure grating (SSG). A wavelength-selectable laser consisting of a DFB laser array, a multi-mode interferometer (MMI), and a semiconductor optical amplifier (SOA) demonstrated a quasi-continuous tuning range of 46.9 nm by using temperature control. A wavelength-tunable DBR laser with SSG exhibited a quasi-continuous tuning range of 62.4 nm by using three tuning current controls. Wavelength stabilization was also demonstrated under the temperature variations of 5.

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.

The authors propose and experimentally demonstrate an all-optical exclusive OR (XOR) logic gate based on self-phase modulation (SPM) of a semiconductor optical amplifier (SOA). The scheme is insensitive to the polarization of the input signal and requires no additional synchronized clock. The output of the XOR gate showed the contrast ratio of more than 17 dB for the input signal at 2.5 GHz.

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.