Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-12
2001-02-13
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06188499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength locking method of causing a reception wavelength of an optical receiver in wavelength division multiplexing communication networks or the like to track a wavelength of a light source, a wavelength locking apparatus for performing the wavelength locking method, and a wavelength division multiplexing communication network using this apparatus.
2. Related Background Art
Wavelength division multiplexing communication networks generally have a large number of independent channels in a single transmission line. Therefore, in wavelength division multiplexing communication, transmission speeds of the respective channels need not necessarily be equal to each other since multiplexing on a time axis, such as a frame synchronization, is not required. As a result, information different in their qualities, such as video data and audio data, can be collectively treated in a combined manner. Thus, such communications can be suitably employed for multimedia communications for which network flexibility is required.
As an example of the wavelength division multiplexing communication network, there exists a system in which each terminal includes a set of a tunable optical transmitter and a tunable optical receiver. The physical topology of the system is often a star type. Here, each terminal is connected to a star coupler through an optical fiber, and an optical signal from the optical transmitter in each terminal is distributed by the star coupler to the optical receivers in the respective terminals including the transmitting terminal itself. In this state, the transmitting terminal controls a tunable light source in the transmitter such that its wavelength is coincident with a unused wavelength (or a vacant channel), and the receiving terminal controls a peak wavelength of a transmission spectrum of a tunable filter (also referred to as a wavelength of the tunable filter in the specification and claims) in the receiver such that the wavelength becomes equal to the reception wavelength. The receiving terminal thus receives the optical signal. A wavelength range usable in the system is determined by wavelength changeable or tunable ranges of the transmitter and the receiver, and a wavelength spacing between the channels is determined by a width of the transmission spectrum of the tunable filter in the receiver. The wavelength spacing between the channels can be decreased as the width of the transmission spectrum is narrowed.
As the tunable light source in the transmitter, a tunable semiconductor laser (the semiconductor laser is also referred to as LD) can be used. In order for the system to use a wide wavelength range, research has been made to widen a tunable width of the LD. Semiconductor lasers of distributed Bragg reflector (DBR) and distributed feedback (DFB) types are presently used as a practical LD. Their tunable widths are several nanometers or so. As its example, there exists one that is disclosed in “Long Cavity &lgr;/4 Shifted MQW-DFB Laser with Three Electrodes”, Technical Report OQE (Optical and Quantum Electronics) 89-116, pp.61-66, Electronics Information Communication Association of Japan.
On the other hand, a filter of a Fabry-Perot cavity type can be used as the tunable filter. The filter with a tunable width of several tens nanometers and a transmission spectrum width of about 0.1 nm is considered as a practical one. As its example, there exists one that is disclosed in “A Field-Worthy, High-Performance, Tunable Fiber Fabry-Perot Filter”, Conference Paper ECOC (European Conference on Optical Communication), '90-605, '90-608. The fiber Fabry-Perot filter is also referred to as FFP.
As discussed above, the wavelength range usable in the system is determined by the tunable widths of the transmitter and the receiver, and the wavelength spacing between the channels can be lowered as the spectrum width of the tunable filter is narrowed. In addition, where the wavelength spacing between the channels is small, a larger number of channels can be provided even though the wavelength tunable range, which determines the wavelength range usable in the system, is the same. Here, it should be noted that in order for the wavelength spacing between the channels to be smaller than variation or fluctuation due to wavelength drifts of the tunable LD and the tunable filter, causes of the drifts must be suppressed. For this purpose, wavelength controls of the optical receiver and the optical receiver are performed.
As a wavelength control system of the optical transmitter, there exists a system disclosed in Japanese Patent Application Laid-Open 8-163092. In the system, the transmission wavelength of each optical transmitter is placed at a predetermined wavelength spacing &Dgr; &lgr; on a longer wavelength side (or a shorter wavelength side) along a wavelength axis in the order of start of transmission. A state in which the transmission wavelengths are thus positioned at the predetermined wavelength spacing &Dgr; &lgr; along the wavelength axis, is referred to as a stationary state. Therefore, there is no need of providing an absolute or relative wavelength reference in such a system. Each optical transmitter detects a wavelength spacing between the transmission wavelength itself and its adjacent wavelength on a longer wavelength side, and the transmitter feedback-controls its transmission wavelength based on that detection and maintains the transmission wavelength at a position which attains the predetermined wavelength spacing. The detection of the wavelength spacing is conducted by a wavelength scan of a tunable filter provided in the optical transmitter. The optical transmitter, which emits light at a wavelength placed on the longest wavelength side (or the shortest wavelength side) in the wavelength arrangement, maintains the longest wavelength of the tunable LD in the optical transmitter.
A specific operation of the wavelength control in the optical transmitter will be described with reference to
FIGS. 1A-1E
. Steps illustrated in
FIGS. 1A-1E
show a procedure during which a terminal starts transmission, the stationary state is reached, another terminal stops transmission and the stationary state is again reached. In
FIGS. 1A-1E
, &lgr;
max
and &lgr;
min
are respectively the longest wavelength end and the shortest wavelength end in the usable wavelength range of the wavelength division multiplexing communication system, &Dgr; &lgr; is the wavelength spacing to be maintained by the control, and &lgr;
mar
is a margin for the wavelength setting in the tunable LD and the tunable filter (the margin is needed in the system since no calibration of the absolute value of a wavelength is performed). Due to the margin, the actual usable wavelength range is from &lgr;
min
+&lgr;
mar
to &lgr;
max
−&lgr;
mar
.
In the procedure illustrated in
FIGS. 1A-1E
, it is assumed that four channels (wavelengths &lgr;
c1
, &lgr;
c2
, &lgr;
c3
, and &lgr;
c4
) are used before a terminal starts transmission and that the wavelengths are arranged at the spacing of &Dgr; &lgr; starting from &lgr;
c1
on the longer wavelength side. The starting point of &lgr;
c1
has no adjacent wavelength on its longer wavelength side, so the wavelength &lgr;
c1
is at the longest wavelength end in the tunable range of the tunable LD in the optical transmitter that oscillates at this wavelength. However, this wavelength is not always coincident with &lgr;
max
due to the wavelength setting error, and falls within a range from &lgr;
max
−&lgr;
mar
to &lgr;
max
(see FIG.
1
A).
In this state, the terminal, which is going to start emission, starts the emission at the shortest wavelength end after confirming that no other channels are present in a range of &Dgr; &lgr; from the shortest wavelength end (the wavelength is indicated by &lgr;
c5
in FIG.
1
B). The wavelength &lgr;
c5
is present in a range from &lgr;
min
to &lgr;
min
+&lgr;
mar
due to the wavelength setting error (see FIG.
1
B). The wavel
Canon Kabushiki Kaisha
Chan Jason
Fitzpatrick ,Cella, Harper & Scinto
Leung Christina
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