Nec Corporation

Kazuhito Tajima

Shigeru Nakamura

Yoshiyasu Ueno

Concerning an all-optical switch which makes use of a nonlinear change of a refractive index of a nonlinear waveguide caused by absorption of a control light, a method that an electrostatic field is applied to the nonlinear waveguide is known as an effective one for shortening a lifetime of the carrier which obstructs an improvement of a repetition frequency of a switching operation. The semiconductor nonlinear waveguide is constituted of a pin-structure which comprises a nondoped InGaAsP optical waveguide, an under cladding layer formed of a N-type InP substrate and an upper cladding layer formed of a p-type InP layer and etc. Moreover, the under and upper cladding layers are electrically shortcircuited via upper and lower electrodes. Since a builtin voltage caused by a difference in Fermi level between n and p-type cladding layers is applied to a InGaAsP optical waveguide, and electrostatic field can be applied to the optical waveguide of the pin structure to some extent without using a constant-voltage power supply. By application of the aforementioned electrostatic field, the effective lifetime of the carrier is shortened, and a repetition frequency of an switching operation of the all-optical switch can be improved.

Semiconductor nonlinear waveguide and optical switch

A wavelength converter which has simple configuration and can be easily controlled and operated with stability, and used for a long-distance mass optical communication, comprises a waveguide for causing a change of a nonlinear refractive index. The converter includes a delay interference circuit having two optical paths and of different optical path lengths. A CW light source is input into the waveguide. A coherent length determined by a spectral line width of CW light output from the CW light source is longer than an optical path difference between the two optical paths in the delay interference circuit.

Wavelength converter

An optical switch is provided that utilizes a signal light frequency shift due to the resonance excitation non-linear optical effect in a semiconductor material so that a high extinction ratio optical switching operation can be realized with low energy controlling light pulses. The non-linear optical waveguide,receives a signal light pulse of a frequency of &ohgr;,and a controlling light pulse of a frequency of &ohgr;,and sets the pulse width of the controlling light pulse to a value smaller than that of the signal light pulse. The optical waveguide portion of the non-linear optical waveguide,is formed of a non-linear optical medium of which the non-linear refractive index changes due to absorption or amplification of the controlling light. The frequency &ohgr;,of the signal light is set to the transparent region of the non-linear optical waveguide,When the frequency &ohgr;,of the controlling light is set to the absorption region of the non-linear optical waveguide,some of signal light pulses incident together with the controlling lights partially frequency-shift toward the high-frequency area. The optical frequency filter,outputs only the frequency-shifted components. This allows the optical switching operation to be realized by controlling light pulses with low energies.

Non-linear optical switch

Disclosed is a wavelength converter that is appropriate for long-distance large-capacity optical communication, a wavelength converter according to the present invention includes: waveguides,and,for producing nonlinear changes in refractive indices; light interaction circuits,and,; and light delay circuits,and,. When an input signal pulse interval is denoted by T,and a speed at which signal pulse light proceeds through a delay circuit is denoted by v,, a difference &Dgr;L,for optical paths in the light delay circuits satisfies a relation &Dgr;L,&equals;v,T,/2. As a result, when there is an interaction involving two sequential light beams, the phases of which are demodulated by the input signal pulse, the wavelength chirping affecting the light beams is canceled out.

An optically controlled optical switch operating at a high speed and at a high efficiency can be realized with a simple construction. An optical signal exiting from a nonlinear optical waveguide is passed through an optical frequency filter and then guided into an optical output signal port. The optical frequency filter is a band-pass filter for passing only optical signal pulses having a center frequency of .omega.1. Where the center frequency .omega.0 of controlling light is set within the absorption region of the nonlinear optical waveguide, optical controlling pulses are absorbed, and carriers are excited. This results in a change in the refractive index. Consequently, the center frequency .omega.1 of only the optical signal pulses entered simultaneously with the optical controlling pulses is shifted to .omega.2. The optical frequency filter blocks optical signal pulses whose center frequency has been shifted to .omega.2. Thus, a gate-type optical switching action is accomplished.

Nonlinear optical switch

The invention provides an all-optical switch including (a) polarizing beam splitters for introducing propagation time difference A to vertically and horizontally polarized components of a signal light, (b) a semiconductor waveguide for impressing phase modulation to the signal light; (c) a polarizing beam splitter and mirrors for introducing propagation time difference B, which is opposite to said propagation time difference A, to the vertically and horizontally polarized components of the signal light, (d) a half-wave plate for making polarization of the vertically and horizontally polarized components to coincide with each other, and (e) a half mirror for making the vertically and horizontally polarized components to interfere with each other. The all-optical switch in accordance with the invention has advantages that switch-off time is not limited by the slow longitudinal relaxation time of non-linear effects, and that it has long term stability because it includes only one non-linear portion.

All-optical switch

A semiconductor optical device having an optical waveguide which represents nonlinear refractive index change due to control light absorption. The waveguide has a bandgap wavelength that increases from its entrance to its exit. In the case of the waveguide divided into first to n-th regions, the bandgap wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n-1, .lambda..sub.n of the regions satisfy the relationship .lambda..sub.1 &lt;.lambda..sub.2 &lt; . . . &lt;.lambda..sub.n-1 &lt;.lambda..sub.n. In the case of the waveguide not divided into regions, the bandgap wavelength .lambda. of the waveguide monotonously increases from the entrance to the exit. This device can operate by the control light having a lower energy than the conventional one.

Semiconductor optical device utilizing nonlinear optical effect

The Mach-Zehnder type all-optical device for controlling a signal light with a control light, includes two arms for constituting a Mach-Zehnder configuration, each of the arms being constituted at least partially of material having third order optical nonlinearity, and an introducer of a control light to a portion of each of the arms revealing the optical nonlinearity.

Symmetric Mach-Zehnder all-optical device

A nonlinear optical device for controlling a signal light by a control light comprises a waveguide region consisting of an optical nonlinear semiconductor material, and an electrostatic field generator, which is a DC supply for example. Carriers (electrons and holes) generated by excitation of electrons due to a control light having a predetermined wavelength supplied to the waveguide region are swept away from the waveguide region by an electrostatic field, so that the change of nonlinear refractive index of the waveguide region can follow the change of the control light to increase the operation speed of the nonlinear optical device.

Nonlinear optical device for controlling a signal light by a control

In a wavelength multiplexed optical transmission system, digital signals of separate channels are respectively converted into optical signals of different wavelengths which are assigned to plural optical channels and multiplexed into a beam optical signal for coupling to an optical transmitted medium. Each of the optical channels includes a semiconductor laser for emitting a laser beam and an optical phase or frequency modulator for modulating the laser beam with one of the digital input signals. A light intensity limiter, or a laser diode saturable light amplifier is located in the path of the light from the modulator to maintain the modulated laser beam at a constant amplitude to prevent interchannel crosstalk which would otherwise occur in the transmission medium as a result of variations in the output of the laser and/or modulator.

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Tokyo, Japan

Kazuhito Tajima