Wavelength converter

Details Wavelength converter Wavelength converter

2002-03-29

2004-09-21

Lee, John D. (Department: 2874)

Optical: systems and elements

Optical frequency converter

C359S330000, C359S332000, C385S122000, C385S002000, C385S008000

Reexamination Certificate

active

06795232

ABSTRACT:

This application is based on Japanese Patent Application Nos. 2001-103552 filed Apr. 2, 2001 and 2001-104943 filed Apr. 3, 2001, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength converter used for optical communication, optical measurement or display devices, and more particularly to a wavelength converter applicable to optical signal processing that requires high speed, high efficiency and low noise wavelength conversion. In addition, the present invention relates to a wavelength converter as a multi-wavelength light source used for wavelength division multiplexing communication requiring low noise signal light with multiple wavelengths and accurate channel spacing.
2. Description of the Related Art
Conventionally, a wavelength tunable laser, which is implemented by irradiating a crystal or a liquid or gas medium, which possesses second order or third order nonlinearity, with a high power laser beam to convert the laser beam to a wavelength region the laser cannot oscillate, is applicable as a wide range wavelength tunable light source. This technique is generally called an optical wavelength conversion using nonlinear optical media. As for materials of the wavelength conversion media utilizing the secondary nonlinear optical effect, inorganic crystals are applied to many wavelength conversion media at present.
To implement such wavelength conversion, an optical waveguide is often employed to make effective use of the nonlinear optical coefficient of the material. The wavelength converters proposed so far include those utilizing the cross gain modulation, cross phase modulation, and four wave mixing (optical mixing using third-order nonlinear polarization) of optical semiconductors.
In addition, the phase matching is considered as an effective method to be applied to inorganic materials such as KTP and LiNbO
3
, and techniques are proposed which utilize temperature tuning, angle tuning, and quasi-phase matching in which less cancellation takes place between a nonlinear polarization wave based on a fundamental wave and a propagation high frequency generated.
As for the wavelength conversion utilizing optical semiconductors that are under development at present, they are inapplicable to optical communication or optical measurement that requires high speed and low noise because they have large noise due to their spontaneous emission light, and their speed limit due to carrier lifetime. In addition, although LiNbO
3
quasi-phase matching devices are proposed as a high-speed, low-noise wavelength converter, they have drawbacks such as insufficient conversion efficiency, requiring an interaction length of at least 5 cm to achieve preferable conversion efficiency. Furthermore, it has a problem of having polarization sensitivity that the conversion efficiency varies sharply depending on the orientation of the crystal.
Moreover, the domain inversion for the quasi-phase matching must undergo poling using a high voltage, offering a problem of low yields. Besides, since the domain inversion by the poling must be formed such that it makes phase matching with a specified wavelength, the wavelength of the pumping light must be fixed.
As a result, the wavelength converter fabricated has a problem in that it can convert only to a fixed wavelength, and hence cannot convert to a wavelength required. The converting function to a desired wavelength is needed for equipment such as optical switching systems and optical routers, which carry out routing using wavelengths as routing information. In addition, the function is important to circumvent blocking of wavelengths, which can occur when multiple wavelength signals are supplied to a single system.
At present, installation of wavelength division multiplexing (WDM) systems is accelerated to implement large capacity communications. The WDM systems can reduce the cost of a system by transmitting multiple signals with different wavelengths through a single optical fiber. Therefore, it can increase the transmission capacity without installing a new fiber.
Although the method has an advantage in the fiber installation cost, it has a problem of requiring many light sources with high wavelength accuracy to achieve high density. Up to now, a method is used which selects semiconductor lasers that precisely fit to the wavelengths of the signal light, and disposes them by the number required. This method, however, has a problem of increasing cost because of the selection of lasers suitable for the wavelengths.
Alternatively, a method using a semiconductor mode-locking laser or fiber ring laser is also proposed. In addition, a spectral slice light source is proposed which slices supercontinuum (SC) light that is generated by the short-pulse light source and nonlinear optical fiber by an arrayed waveguide grating demultiplexer. However, since it requires a long nonlinear fiber to generate the SC light, it has a problem of making it difficult to reduce its size.
SUMMARY OF THE INVENTION
The present invention is implemented considering the foregoing problems. Therefore, an object of the present invention is to provide a high efficiency, low noise wavelength converter that can be implemented without the high voltage poling of a crystal, and that can carry out switching and modulation of converted light by using electric field.
Another object of the present invention is to provide a wavelength converter functioning as a multi-wavelength light source capable of controlling a wavelength band or the number of wavelengths by selecting electrodes to which electric fields are applied.
To accomplish the objects, according to the present invention, there is provided a wavelength converter for producing converted light with a wavelength corresponding to an energy difference between signal light and pumping light with a wavelength different from that of the signal light, by launching the signal light and the pumping light into a crystal material simultaneously, wherein the crystal material consists of a crystal composed of at least one of KTa
1-x
Nb
x
O
3
and K
1-y
Li
y
Ta
1-x
Nb
x
O
3
.
In addition, to accomplish the objects, according to a present invention, there is provided a wavelength converter operating as a multi-wavelength light source including a planar optical waveguide comprising: a core with a high refractive index composed of a crystal material with a composition of at least one of KTa
1-x
Nb
x
O
3
and K
1-y
Li
y
Ta
1-x
Nb
x
O
3
; a cladding surrounding the core; an electrode that is formed on a surface of the optical waveguide and has a fixed electrode period; signal light generating means for generating signal light with at least one wavelength; and pumping light generating means for generating pumping light with a wavelength different from that of the signal light output from the signal light generating means, wherein the signal light and the pumping light are launched simultaneously into the optical waveguide to generate signal light with at least one wavelength.
Thus, the present invention is characterized in that it utilizes the crystal with the composition of KTa
1-x
Nb
x
O
3
and/or K
1-y
Li
y
Ta
1-x
Nb
x
O
3
as a medium for achieving the wavelength conversion. These KTN and KLTN crystals are a cubic system with centrosymmetry in an applied temperature range. Although they have no second order nonlinear effect, they are characterized by exhibiting secondary nonlinear effect in response to an electric field applied. Therefore, it is possible to implement the multiple wavelength generation based on the differential frequency generation by applying the electric field to the electrode with the period that makes phase matching with the signal light and pumping light.
The efficiency of the nonlinear optical effect increases in proportion to the electric field applied, and offers a twice or more efficiency as compared with the conventional LiNbO
3
nonlinear optical crystal within a range of a practical electric field to be applied. Accordingly,

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