Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-12-03
2003-09-23
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000
Reexamination Certificate
active
06625355
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a waveform dispersive compensation method having a function, such as pulse waveform shaping, and, more particularly, to an adaptive dispersion compensating element used in ultrahigh-speed optical fiber communication.
2. Description of Prior Art
Recently, in optical fiber communication, its introduction into an optical access system is advancing, to say nothing of a trunk line system. In many 1.3 micron band zero dispersion fibers laid at present, when transmission is performed using light having a wavelength in a 1.5 micron band, a wavelength dispersion of about 17 ps/km−nm can be found in an optical fiber. Accordingly, when the transmission distance is made longer or when the transmission rate becomes fast, a means for controlling dispersion becomes necessary to prevent deterioration of an optical signal.
A typical means that controls conventional dispersion is a dispersion compensator that uses a chirp Bragg grating having the structure in which a cycle of refractive index modulation is continuously changed. A fiber Bragg grating that forms a diffraction grating in a core of this optical fiber becomes an element whose position of reflection depends on an optical wavelength by forming a diffraction grating (chirp Bragg grating) that has a characteristic of reflecting light of a specific wavelength and continuously changes a pitch toward the major axis direction of an optical fiber. A dispersion compensator can be constituted using this feature. This chirp Bragg fiber grating becomes compact and has the same function as a dispersion compensating fiber by combining with an optical circulator.
However, in most chirp Bragg gratings, dispersion and reflection characteristics were static. Desirably, they should have a diffraction grating that can change a band or dispersion with satisfactory control against many applications, such as dispersive compensation.
One of the attempts that introduces a dynamically adjustable chirp into a chirp Bragg fiber grating as a conventional example can be found in an “Optical Diffraction Device Having an Adjustable Chirp” disclosed in Japanese Unexamined Patent Publication No. 2000-137197.
FIG. 1
shows a process useful for providing an example in which a chirp diffraction grating is adjusted using a block diagram. In
FIG. 1
, Process A is a “Preparation of a waveguide including a diffraction grating”, Process B is “Coating of a diffraction grating area using a variable-resistance thin film”, or Process C is “Packaging of a device”.
In
FIG. 1
, the operation is described below. As shown in Process A, the first process is to prepare an optical waveguide of a fixed length including an optical diffraction grating. Desirably, a waveguide should be an uncoated fiber, but can include an electrically insulated resistor thin film of uniform resistance. The waveguide ought to be either single mode or multi mode. The diffraction grating ought to be either a Bragg diffraction grating or a long cycle diffraction grating. The next process, as shown in Process B, is to coat a waveguide with a thin film of a resistance material in which local resistance increases substantially in succession along the length of a diffraction grating. The third process (Process C) (this is performed as occasion demands) is to package a device for operation.
FIG. 2
shows a schematic sectional view of a waveguide diffraction grating device having an adjustable chirp as a specific configuration example. In
FIG. 2
, number
10
is an optical fiber,
11
is a diffraction grating,
12
is refractive index perturbation,
13
is a substrate, and
14
,
15
are electrodes.
An optical waveguide diffraction grating having an adjustable chirp includes a waveguide diffraction grating that thermally contacts an electrically controllable thermal conversion substrate whose temperature changes along the length of a diffraction grating. Because a thermal conversion substrate generates a temperature gradient along a diffraction grating, it generates heat on a fiber or can remove the heat from the fiber. As an example, the thermal conversion substrate is a resistive coat in which local resistance changes along the length of the diffraction grating. A current that passes through a thin film generates a temperature gradient along a diffraction grating that is almost proportional to the local resistance of the thin film and the size of a chirp can be adjusted by the current. A device that is obtained is simple and compact, and the power is efficient.
However, in a means that uses the chirp diffraction grating, it is unknown how chirp characteristic control for compensating dispersion is performed in accordance with a change of the transmission state and a change of the transmission distance. Accordingly, the means had a problem that cannot flexibly be solved in accordance with the optical pulse transmission of practical optical communication. Further, a resistor element that generates a temperature gradient has the configuration in which heat output is controlled by changing a value of resistance in accordance with a change in the local thickness of a thin film. However, the means had a problem that it is difficult to control higher order diffusion (exceeding tertiary diffusion) than wavelength diffusion (secondary diffusion) in such configuration.
SUMMARY OF THE INVENTION
The present invention has been made in view of solving the above prior art and provides a device that adaptively performs decentralized control in an optical fiber transmission path, such as performing dispersive compensation and waveform shaping in an optical fiber transmission.
To attain this object, according to an aspect of the present invention, the adaptive dispersion compensating element is provided with a chirp Bragg grating formed in an optical fiber, a temperature gradient impressing means that impresses a temperature gradient along the longitudinal direction of the chirp Bragg grating, a spectral resolving means that spectrally resolves the output light from the chirp Bragg grating, a detecting means that detects the output light from the spectral resolving means, and a controlling means that performs feedback control of the temperature gradient impressing means based on the output from the detecting means.
According to another aspect of the present invention, the adaptive dispersion compensating element can provide a compact and high-stability adaptive dispersion compensating element that adaptively performs dispersive compensation monitoring an optical signal in an optical fiber transmission path, such as performing dispersive compensation or waveform shaping in optical fiber transmission in accordance with the above configuration.
According to another aspect of the present invention, the adaptive dispersion compensating element is provided with a chirp Bragg grating formed in an optical fiber, a temperature gradient impressing means that impresses a temperature gradient along the longitudinal direction of the chirp Bragg grating, a spectral resolving means that spectrally resolves the output light from the chirp Bragg grating, a detecting means that detects the output light from the spectral resolving means, and a controlling means that performs feedback control of the temperature gradient applying means based on the output from the detecting means, and has operation that adaptively performs dispersive compensation monitoring an optical signal in an optical fiber transmission path, such as performing dispersive compensation or waveform shaping in optical fiber transmission.
Further, according to another aspect of the present invention, the adaptive dispersion compensating element is an adaptive dispersion compensating element whose temperature gradient is a nonlinear gradient that is impressed to the longitudinal direction of a chirp Bragg grating and has operation that adaptively performs dispersive compensation monitoring an optical signal in an optical fiber transmission path, such as performing dispersive compensation or
Takeuchi Yoshinori
Wakabayashi Shinichi
Browdy and Neimark , P.L.L.C.
Lee John D.
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