Optical: systems and elements – Light interference
Reexamination Certificate
2003-01-14
2004-10-19
Dunn, Drew A. (Department: 2872)
Optical: systems and elements
Light interference
C359S584000, C359S589000, C359S868000, C359S884000, C398S081000, C398S147000
Reexamination Certificate
active
06807008
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Filed of the Invention
The present invention relates to an apparatus which is intended to apply to a wavelength division multiplex transmission apparatus, generates a dispersion different from one another according to a channel signal wavelength, and compensates a wavelength dispersion slope accumulated on an optical fiber transmission network.
(2) Description of Related Art
A light transmitter transmits a light pulse to a light receiver over an optical fiber in a conventional optical fiber communication system which transmits information using light. However, the wavelength dispersion of the optical fiber, which is also referred to as a “chromatic dispersion,” degrades the quality of a signal in the system.
More specifically, as a result of the wavelength dispersion, the propagation velocity of signal light in the optical fiber depends on the wavelength of the signal light in the optical fiber. For example, when a light pulse with a longer wavelength (such as a light pulse presenting red) propagates faster than a light pulse with a shorter wavelength (such as a light pulse presenting blue), its dispersion is referred to as a normal dispersion. On the other hand, when a light pulse with a shorter wavelength (such as a blue pulse) propagates faster than a light pulse with a longer wavelength (such as a red pulse), its dispersion is referred to as an abnormal dispersion.
Thus, if a signal light pulse includes a red pulse and a blue pulse when the signal light pulse is transmitted from a transmitter, the red pulse and the blue pulse are split from each other as the signal light pulse propagates over an optical fiber, and the individual light pulses are received by a receiver at different moments.
As another example of the light pulse transmission, when a signal light pulse having wavelength components continuously changing from blue to red is transmitted, since the individual components propagate at different speeds in an optical fiber, the width in time of the signal light pulse increases in the optical fiber, and a distortion is generated. Since any pulse includes components within a finite wavelength range, the generation of this wavelength dispersion is extremely general in the optical fiber communication system.
Thus, it is necessary to compensate the wavelength dispersion so as to obtain high transmission capability especially in a high speed optical fiber communication system. Consequently, a “reverse dispersion component” which adds a wavelength dispersion opposite to the wavelength dispersion generated in the optical fiber is necessary in the optical fiber communication system.
Some conventional apparatuses may be used as this “reverse dispersion component”. For example, a dispersion compensation fiber (DCF) has a specific cross section refractive index profile, provides a wavelength dispersion opposite to one generated in the conventional transmission line, and is used as the “reverse dispersion component”.
However, it costs high to manufacture the dispersion compensation fiber, and simultaneously, a relatively longer fiber is necessary for sufficiently compensating the wavelength dispersion generated in the transmission line. For example, a dispersion compensation fiber with a length of about 20 km to 30 km is necessary for completely compensating a wavelength dispersion generated in a transmission line of 100 km. Thus, the optical loss increases, and simultaneously the size increases.
A chirped fiber grating is used as the “reverse dispersion component” for compensating the wavelength dispersion as well. The fiber grating uses a phenomenon where the refractive index of germanium oxide used for doping the core changes as a result of ultraviolet irradiation, and forms a grating which changes the refractive index at a cycle of a half of a wavelength, and a longer wavelength component is reflected through a longer distance so as to propagate a distance longer than that of a shorter wavelength component as a result of gradually changing the interval of the grating in the lengthwise direction of the fiber. Thus, the chirped fiber grating also provides a light pulse with a reverse dispersion.
However, since the chirped fiber grating has a very narrow range in terms of the light to be reflected, it is impossible to provide a sufficient range for light including many wavelengths such as a wavelength division multiplex transmission signal. Though it is possible to cascade multiple chirped fiber gratings for a wavelength division multiplex transmission signal, the system becomes expensive.
In view of these conventional apparatuses, Published Japanese Translation of a PCT Application 2000-511655 (Japanese Unexamined Patent Application Publication HEI10-534450) and Published Japanese Translation of a PCT Application 2002-514323 (Japanese Unexamined Patent Application Publication HEI11-513133) propose an optical apparatus including a device called as a virtually imaged phased array (VIPA), for example.
This VIPA is a device which receives light having a respective wavelength within a continuous range of wavelengths, and generates output light continuously corresponding to the input light, and comprises parallel flat plates placed such that two reflection surfaces oppose to each other at a predetermined interval, the one reflection surface has light reflectance of 100%, and the other reflection surface has light reflectance smaller than 100% (about 98%) as disclosed in Japanese Patent Application Publication HEI 9-43057, for example.
A light incident window (a transparent area) for introducing light from the outside is provided on a part of the reflection surface with reflectance of 100%, and when light having a respective wavelength within a continuous range of wavelengths is obliquely introduced into the VIPA (between the parallel flat plates) from this light incident window, reflection is repeated between the parallel flat plates, and a partial light is released continuously from the multiple positions on the reflection surface with reflectance smaller than 100% to the outside of the VIPA.
Since the transmission light released at the multiple positions from the VIPA in this way travels while spreading radially at a certain angle, interference of light having a large number of different travel directions for the same wavelength occurs. Thus, only a component with a specific traveling direction depending on the wavelength is enhanced, light flux is formed, and consequently, it is possible to provide light having a respective wavelength within a certain continuous range of wavelengths toward directions different from one wavelength to another (namely, it is possible to provide the output light with an angle dispersion). In other words, the output light of the VIPA is spatially discriminated from one another having a different wavelength within the continuous range of wavelengths of the input light.
The technology described in Published Japanese Translation of a PCT Application 2000-511655 and Published Japanese Translation of a PCT Application 2002-514323 relates to technology using the characteristic of the VIPA so as to generate a wavelength dispersion. Specifically, the technology relates to an optical apparatus (a wavelength dispersion generating apparatus) constituted to return the light provided from the VIPA to the VIPA, and to generate the multiple reflection again in the VIPA.
Namely, this optical apparatus comprises a collimating lens
100
, a cylindrical lens
200
, a VIPA
300
, a focusing lens
400
, and a mirror
500
as shown in
FIG. 18
, input light from an optical fiber
600
is collimated by the collimating lens
100
, the cylindrical lens focuses only one way of the light wave, the light enters into the VIPA
300
through a light incident window
301
, the focusing lens
400
focuses (condenses) output light from the VIPA, the mirror
500
reflects the light, and the light is introduced into the VIPA
300
again.
The VIPA
300
and the focusing lens
400
are positioned such that the light proceeding from the VIPA
300
to the foc
Boutsikaris Leo
Dunn Drew A.
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