Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-03-30
2003-07-22
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06597480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength-divided multiplexing (WDM) transmission system for optical communication and to a wavelength tunable filter apparatus used for an apparatus of such a system.
2. Related Art of the Invention
In the optical communication field, a wavelength-multiplexed optical transmission method is already known which is used to transmit multi-channel signals on one optical fiber. A wavelength-multiplexed optical transmission method is a method in which an optical wavelength coupler wavelength-multiplexes signals of respective channels with light having different wavelengths from each other, the signals are transmitted through one optical fiber from the sending side to the receiving side, a wavelength filter separates on the receiving side the multiplexed transmission light with respect to the respective wavelengths and the signals are reproduced. For this reason, a wavelength tunable filter is used which can change a wavelength which is to be received.
In this manner, it is possible to freely extract light having a desired wavelength from light which is formed of a number of multiplexed wavelengths. This is the reason that a wavelength tunable filter apparatus is being explored which is addition of a wavelength-tuning function of correctly selecting a received wavelength to a wavelength tunable filter.
FIG. 10
shows a structure of a conventional wavelength tunable filter apparatus and a flow of a data signal in each part. In
FIG. 10
, denoted at
101
is a wavelength tunable filter, denoted at
102
is a photoelectric conversion part, denoted at
103
is a received light intensity detecting part, denoted at
104
is a wavelength/intensity memory part, denoted at
105
is a peak position detecting part, and denoted at
106
is a wavelength scanning (changing) part, which form the wavelength tunable filter apparatus. Denoted at
107
is an electric signal processing part.
Now, operations of such a conventional apparatus will be described. The photoelectric conversion part
102
converts an optical signal from the wavelength tunable filter
101
into a photoelectrically converted signal. The photoelectrically converted signal is supplied to the wavelength scanning part
107
while partially detected by the received light intensity detecting part
103
to obtain an average value of the photoelectrically converted signal, namely, an average received light intensity. The average received light intensity is stored in the wavelength/intensity memory part
104
together with wavelength position data obtained from the wavelength scanning part
106
.
While the wavelength scanning part
106
changes a wavelength of the wavelength tunable filter
101
, based on the information stored in the wavelength/intensity memory part
104
, the peak position detecting part
105
specifies a wavelength at which the average received light intensity becomes maximum and supplies the position of such a wavelength to the wavelength scanning part
106
, whereby the wavelength is set.
On the other hand, another method already proposed (e.g., Japanese patent no. 2,655,479) requires that wavelength-multiplexed light is supplied to a wavelength tunable filter, received light intensities of light having two wavelengths which are adjacent to a wavelength which is to be selected are monitored and a difference between the intensities is optimized.
However, in the conventional wavelength tunable filter apparatus described above, when a received light intensity at the wavelength tunable filter
101
decreases, in order to determine whether the set wavelength of the wavelength tunable filter
101
has shifted toward the short-wavelength side or the long-wavelength side with respect to a transmitted oscillation wavelength of a light source, it is necessary to detect a received light intensity after shifting the set wavelength of the wavelength tunable filter
101
toward the short-wavelength side and the long-wavelength side and to specify a direction which increases a received light intensity.
Thus, the process described above demands to change a wavelength at least twice, and when a wavelength is changed in a direction which decreases a received light intensity, a signal quality may temporarily degrade owing to the decrease in the received light intensity.
Further, the method which requires to monitor an average received light intensity of light having adjacent wavelengths has a problem that it may become impossible to correctly set a wavelength if original intensity levels of the light having adjacent wavelengths change.
SUMMARY OF THE INVENTION
Noting the problems as above with such conventional wavelength tunable filter apparatuses, the present invention aims at providing a wavelength tunable filter apparatus which allows to accurately set a wavelength once again when a received light intensity decreases, by means of a simple wavelength change using only light which has a wavelength to be selected.
The 1
st
invention of the present invention (corresponding to claim 1) is a wavelength tunable filter apparatus, comprising:
a wavelength tunable filter which can change a wavelength to be selected for an inputted optical signal;
a photoelectric conversion part which converts transmitted light received from said wavelength tunable filter into a received light intensity signal;
a received light intensity detecting part which detects an intensity of said received light intensity signal which is converted;
a signal component detecting part which detects a signal component from said received light intensity signal;
a signal intensity detecting part which detects an intensity of said signal component which is detected;
an intensity variation detecting part which detects a variation in said received light intensity detected by said received light intensity detecting part and an intensity variation in said signal component detected by said signal intensity detecting part; and
wavelength setting means which, on the basis of said detected both variations, causes said wavelength tunable filter to change the wavelength to be selected as to set the wavelength appropriately such that an output from said photoelectric conversion part satisfies a predetermined condition.
The 2
nd
invention of the present invention (corresponding to claim 2) is the wavelength tunable filter apparatus of said the 1
st
invention, wherein a direction of the changing of the wavelength to be selected by said wavelength tunable filter is determined based on said both variations.
The 3
rd
invention of the present invention (corresponding to claim 3) is the wavelength tunable filter apparatus of said the 1
st
invention, wherein the wavelength to be selected by said wavelength tunable filter is changed by a feedback method.
The 4
th
invention of the present invention (corresponding to claim 4) is the wavelength tunable filter apparatus of said the 1
st
invention, wherein said wavelength setting means comprises a wavelength/intensity memory part which stores in advance reference data which describe a state of the intensity variation in said received light intensity signal in a direction in which said wavelength tunable filter changes the wavelength and/or a state of the intensity variation in said signal component, and the wavelength to be selected by said wavelength tunable filter is changed in accordance with comparison of said reference data against at least one of said state of the detected variations.
The 5
th
invention of the present invention (corresponding to claim 5) is the wavelength tunable filter apparatus of said the 1
st
invention, wherein said wavelength setting means sets as a set wavelength a wavelength which maximizes the intensity of said received light intensity signal.
The 6
th
invention of the present invention (corresponding to claim 6) is the wavelength tunable filter apparatus of said the 1
st
invention, wherein said wavelength setting means sets as a set wavelength a wavelength which maximizes the intensity of said signal component.
The 7
th
Asakura Hiroyuki
Iida Masanori
Matsushita Electric - Industrial Co., Ltd.
Pascal Leslie
Smith , Gambrell & Russell, LLP
Tran Dzung
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