Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-05-20
2002-08-06
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06430340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength tunable filter for use in a wavelength division multiplexing transmission system and apparatus in optical communications, and also relates to a wavelength tunable filter device using the same.
2. Related Art of the Invention
In optical communications, there has been known a technique of wavelength division multiplexing which transmits signals of multiple channels through a single optical fiber. In wavelength division multiplexing, signals of different channels are optically multiplexed at different wavelengths by using a optical multiplexer at the transmitting end, and the thus multiplexed signals are transmitted through a single fiber to the receiving end; at the receiving end, the multiplexed signals are separated according to their wavelengths by means of a wavelength filter, to recover the original signals. In particular, by using a wavelength tunable filter whose wavelength is capable of being varied for signal reception, the light of desired wavelength can be separated from the light on which many wavelengths are multiplexed.
There are various types of wavelength tunable filter; among others, a wavelength tunable filter using a diffraction grating has been studied for applications in wavelength division multiplexing transmission because of its high wavelength selectivity providing good isolation between adjacent wavelengths. In high density wavelength division multiplexing transmission where signals are transmitted at closely spaced wavelengths, enhancing the operational stability and the accuracy in setting the filter for the selected wavelength has been a major challenge in the design of the filter. Since the wavelength selection is performed by rotating the diffraction grating, a minimum angular resolution of one hundred-thousandth revolution (360 degrees/100,000), or 0.05 nm in terms of wavelength, is required of the rotating mechanism if the wavelength selection is to be accomplished for multiplexed light of wavelengths spaced apart on the order of subnanometers. However, such a high resolution characteristic has been unachievable with a conventionally used stepping motor alone, and improvements in the rotating mechanism have been necessary.
FIG. 4
shows the configuration of a prior art wavelength tunable filter device. In FIG.
4
(
a
), reference numeral
105
is a diffraction grating,
501
is a mounting jig,
2
is a lens,
3
is an input fiber,
4
. is an output, fiber,
8
is an optical-to-electrical converter,
9
is an intensity level detector,
100
is a rotational position calculator,
130
is a rotational position storage,
140
is a rotational position detector,
120
is a motor driver,
61
is a stepping motor,
601
is a reduction gear,
71
is an encoder, and
801
and
802
are joints. FIG.
4
(
b
) is a diagram showing the mounting jig
501
with the diffracting grating
105
mounted thereon, as viewed from the top of FIG.
4
(
a
).
Wavelength multiplexed light from the input fiber
3
enters the diffraction grating
105
through the lens
2
and is chromatically dispersed so that light of desired wavelength is coupled into the output fiber
4
though the lens
2
. The light is then converted by the optical-to-electrical converter
8
into an electrical signal. The mounting jig
501
with the diffraction grating
105
mounted thereon is secured to the shaft of the reduction gear
601
which is connected to the stepping motor
61
via the joint
801
. The wavelength of the light to be coupled into the output fiber
4
can be selected by rotating the mounting jig
501
. The speed at which the mount jig
501
rotates about its axis is slower than the speed at which the stepping motor
61
rotates about its axis, which serves to enhance the apparent rotational angular resolution.
When varying the wavelength to be selected, the motor driver
120
supplies a drive current to the stepping motor
61
which thus starts to rotate. The electrical signal from the optical-to-electrical converter
8
is input to the intensity level detector
9
for detection of the received light level. A signal from the encoder
71
connected to the stepping motor
61
by the joint
802
is extracted as rotational position information by the rotational position detector
140
. While monitoring the rotational position information from the rotational position detector
140
and the received light level from the level detector
9
, the rotational position calculator
100
compares them with the rotational position corresponding to the desired wavelength prestored in the rotational position storage
130
and, when the desired rotational position is reached, issues a stop command to the motor driver
120
, whereupon the stepping motor
61
stops and the reduction gear
601
also stops.
In the above prior art wavelength tunable filter device, however, the rotational position of the diffraction grating is controlled via the reduction gear. Accordingly, the hysteresis that the reduction gear has in its rotational direction becomes a factor that limits the rotational angular resolution of the diffraction grating. Furthermore, the rotational resolution of the stepping motor is greatly affected by the connecting condition of the joint between the reduction gear and the stepping motor. Moreover, the use of the reduction gear increases not only the overall size but also the axial length of the rotating mechanism, as a result of which stability cannot be secured for the rotational operation of the mounting jig which has eccentricity as shown in FIG.
4
(
b
).
Furthermore, since the rotation control is such that the rotational motion is stopped abruptly at the desired angular (wavelength) position, the absolute angular precision of the stopping motion may degrade.
SUMMARY OF THE INVENTION
The present invention has been devised to overcome the problems encountered with the above prior art wavelength tunable filter device, and an object of the invention is to provide a wavelength tunable filter device capable of achieving precise wavelength selection for multiplexed light of wavelengths spaced apart on the order of subnanometers.
The present invention of the first invention is a wavelength tunable filter device comprising: at least two optical fibers through which an optical signal is input or output; a wavelength selective element which said optical signal is input to and output from through a lens and which selects a wavelength; a mounting jig to which said wavelength selective element is fixedly secured; a rotating mechanism, comprising an ultrasonic motor and an encoder, for rotating said wavelength selective element; and a motor controller for controlling said ultrasonic motor for driving, wherein said mounting jig is rigidly mounted directly to a rotating shaft of said ultrasonic motor, and the amount of power that does not exceed the driving power necessary to. cause said ultrasonic motor to start rotating from a stopped condition is intermittently applied to said motor controller.
REFERENCES:
patent: 5377286 (1994-12-01), Iida et al.
patent: 5420416 (1995-05-01), Iida et al.
patent: 0 545 735 (1993-06-01), None
patent: 0 682 278 (1995-11-01), None
patent: 5-241083 (1993-09-01), None
Asakura Hiroyuki
Iida Masanori
Font Frank G.
Nguyen Tu T.
Smith , Gambrell & Russell, LLP
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