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.

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Wavelength converters using quasi-phase matched LiNbO₃ (QPM-LN) waveguides are recognized as key devices for future wavelength division multiplexing (WDM) systems. These converters provide unique characteristics such as the simultaneous conversion of WDM channels, a large signal bandwidth, and transparency as regards modulation format. In this review, we describe waveguide fabrication using the annealed-protonexchange and direct-bonding techniques. A direct-bonded ridge waveguide shows a high resistance to photorefractive damage and a -4.6 dB wavelength conversion using a χ⁽²⁾-cascaded pumping scheme is successfully demonstrated. Polarization independent wavelength conversion using a LPE-LN substrate is also described.

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All-optical wavelength conversion based on cross-gain modulation (XGM) using semiconductor optical amplifiers (SOA’s) is attractive because of its high-speed, simple configuration, wide operation wavelength range, and large optical gain capability. In this paper, first, operation principle of XGM and the wavelength conversion performance are described analytically and experimentally. It is pointed out that the dominant factor for operation speed is carrier recovery time when the input signal turns from 1 level to 0 level. Next, the investigation of optimized condition for XGM in terms of power penalty are explained. The optimized condition for minimizing the power penalty results from the degradation of the extinction ratio at small signal power and the slow carrier recovery time at high signal power. Then, application to an all-optical regeneration system using XGM is presented. The improvement of received power is realized by a cross-gain modulation-type wavelength converter-cascaded optical regenerator (CGCOR).

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This paper describes the operating principle, structure, fabrication technology, and basic characteristics of newly developed wavelength converter named SIPAS. In SIPAS, a Mach-Zehnder interferometer with semiconductor optical amplifier in each arm is monolithically integrated with a Sagnac interferometer. Filter-free wavelength conversion with output pulse width of 13 ps due to differential-phase-modulation was achieved using the fabricated SIPAS. The power penalty was as low as 0.9 dB at a bit rate of 10 Gbit/s. All-optical 3R that retains input wavelength is also demonstrated using SIPAS at a bit rate of 40 Gbit/s.

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Optical wavelength conversion utilizing cross-absorption modulation and cross-phase modulation in an electroabsorption modulator (EAM) were investigated. Error-free all-optical wavelength conversion at 40 Gbit/s and 100 Gbit/s were successfully demonstrated employing bulk-type EAM and MQW-type EAM, respectively, in conjunction with delay interferometer. Electro-optical wavelength conversion with re-timing capability was also demonstrated by rf-driven EAM.

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The usefulness was examined of the replica method for observing the shapes of long drill holes several microns in diameter. The results closely agreed with the results of conventional sectional observation. Replicas of multiple drill holes in the same sample can be taken at the same time without the need for specialized techniques, with the process requiring only 10-20 minutes. A laser scanning microscope gives a clear 3D image of the replica. The replica method is thus suitable for the observation and evaluation of long drill holes several microns in diameter.

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A collimated light beam from a He-Ne Laser, is transformed into two beams by using a concave lens and a beam-splitter cube, in a Twyman-Green-type interferometer. When a diffusing surface is placed at the focal point of one of the two beams, a diverging spherical wave front containing large-sized speckle patterns is scattered. If a smooth surface reference mirror is placed in the vicinity of the focal point of the other beam, the reflected beams have essentially the same shaped wave front. The two beams can be combined to form concentric interference fringes, which are fairly regular. By observing the concentric fringes flowing out or sinking in, it is shown that the normal component of the displacement of the diffused surface can be measured with an accuracy of a fraction of a half wavelength.

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