Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2001-09-11
2004-06-01
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C359S483010
Reexamination Certificate
active
06744554
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength conversion apparatus capable of converting a wavelength of input light to another wavelength.
2. Description of the Related Art
In the field of communications using optical fibers, large-capacity and high-speed data transmission is required. Particularly, wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) are considered promising in that the transmission capacity of the optical fibers can be significantly increased, and a wavelength control technology for precisely controlling a plurality of carrier wavelengths and a wavelength conversion technology for converting a carrier wavelength to another carrier wavelength are important.
For example, in existing optical communication networks, single-wavelength optical transmission is mainstream that uses as the carrier wavelength the 1.3-&mgr;m band in which the loss of optical fibers is small. Networks of this type are generally constructed for the purpose of substituting for telephone communication networks within cities. In trunk optical communication networks connecting cities, wavelength division multiplexing optical transmission is mainstream that uses as the carrier wavelength the 1.5-&mgr;m band suitable for wavelength division multiplexing transmission.
When these types of optical communication networks are connected, since the carrier wavelengths are different from each other, it is necessary to temporarily convert the optical signals flowing on one network to electric signals and then convert the electric signals to optical signals using the carrier wavelength conforming to the other network. Then, the optical communication performance is limited according to the electric signal processing capability.
Therefore, by making it possible to directly convert the carrier wavelength of one network to the carrier wavelength of the other network, no electric signal processing is involved, so that the high performance of the optical communication can be effectively maintained. To do this, a light mixing technology for converting carrier wavelengths is essential.
In such wavelength conversion, since second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), parametric conversion or the like by a nonlinear optical effect is used, a material with a high nonlinear optical effect is desired.
Examples of the related conventional art include Japanese Unexamined Patent Publication JP-A 10-213826 (1998), Japanese Unexamined Patent Publication JP-A 2000-10130 (2000), and a literature (IEICE Trans. Electron. Vol. E83-C, No. 6, pp. 869-874 (2000)).
The nonlinear optical effect largely depends on the polarization condition of the input light and the bearing of the nonlinear optical material. For example, when a light linearly polarized in a predetermined direction passes through an optical fiber, since the light is affected by the dispersion and the like of the optical fiber, the polarization condition at the exit of the optical fiber generally cannot be identified.
Moreover, since nonlinear optical materials are generally polarized light dependent in connection with wavelength conversion, when the polarization condition of the input light varies among carrier wavelengths, the wavelength conversion efficiency is inconstant, so that the intensity of the wavelength-converted output light is unstable.
SUMMARY OF THE INVENTION
An object of the invention is to provide a wavelength conversion apparatus capable of realizing stable wavelength conversion not depending on polarization conditions of signal light.
The invention relates to a wavelength conversion apparatus comprising:
a wavelength conversion element for carrying out wavelength conversion of a linearly polarized light component in a first direction; and
polarization rotating means including a reflecting element for reflecting light having passed through the wavelength conversion element to return the light to the wavelength conversion element, for making a polarization direction difference of 90 degrees between a light traveling from the wavelength conversion element to the reflecting element and a light reflected at the reflecting element so as to return to the wavelength conversion element.
According to the invention, the wavelength conversion element is polarization dependent, namely, the wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in a second direction perpendicular to the first direction, and by disposing the polarization rotating means behind the wavelength conversion element, when a pumped light and a signal light are input, the first-direction components of the pumped light and signal light are wavelength-converted by the wavelength conversion element, and the second-direction components perpendicular to the first direction are not wavelength-converted.
Then, when the lights are reflected at the polarization rotating means, the first-direction components which are perpendicular to the first direction of the wavelength conversion element are not wavelength-converted, and the second-direction components which are parallel to the first direction of the wavelength conversion element are wavelength-converted. Consequently, the first-direction components are wavelength-converted on the way to the reflecting element and the second-direction components are wavelength-converted on the way back, so that the composite intensity of the wavelength-converted outputs is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, in the invention, it is preferable that the wavelength conversion apparatus comprises a wavelength selection reflecting element, disposed between the wavelength conversion element and the polarization rotating means, for selectively reflecting a pumped light.
According to the invention, by disposing the wavelength selection reflecting element for selectively reflecting the pumped light between the wavelength conversion element and the polarization rotating means, when the pumped light and the signal light are input, the first-direction components of the pumped light and signal light are wavelength-converted by the wavelength conversion element, and the second-direction components perpendicular to the first direction are not wavelength-converted. Then, only the pumped light is returned to the wavelength conversion element by the wavelength selection reflecting element and the signal light passes through the wavelength selection reflecting element as it is.
Then, after being reflected at the polarization rotating means, the first-direction component and the second-direction component are again rotated by 45 degrees about the optical axis by a 45-degree polarization rotating element, and pass through the wavelength selection reflecting element as they are. Then, the first-direction component of the signal light perpendicular to the first direction of the wavelength conversion element is not wavelength-converted, the second-direction component of the signal light parallel to the first direction of the wavelength conversion element is wavelength-converted, and at this time, the first-direction component of the pumped light contributes to wavelength conversion. Consequently, since the first-direction component of the pumped light can contribute to wavelength conversion on both ways, the wavelength conversion efficiency is improved. Preferably, by using a linearly polarized light in the first direction as the pumped light, the wavelength conversion efficiency can be further improved.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element made of a birefringent material, for separating, by beam walk-off, a light to be wavelength-converted into two linearly polarized light components perpendicular to each other so as to travel a
Shichijyo Shiro
Yamada Kazuhiro
Burns Doane , Swecker, Mathis LLP
Lee John D.
Mitsui Chemicals Inc.
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