Coherent light generators – Particular beam control device – Mode locking
Reexamination Certificate
2000-07-27
2003-10-07
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Mode locking
C372S028000
Reexamination Certificate
active
06631145
ABSTRACT:
This application is based on Japanese Patent Application No. 11-215905 (1999) filed Jul. 29, 1999, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a wavelength-tunable mode-locked laser, a wavelength converter and a transmission system that are applicable to, for example, optical communication networks utilizing fast wavelength conversion and wavelength routing.
2. Description of the Related Art
(First Example of Prior Art)
For implementation of optical communication networks that utilize the wavelength division multiplexing method, the wavelength converter plays an important role in changing the wavelength of optical signals. Research conducted by S. J. B. Yoo Wavelength Conversion Technologies for WDM Network Applications Journal of Lightwave Technology, Vol. 14, No. 6, pp. 955-66 (June 1996), for example, is part of the associated wide-range investigation. Among many methods for wavelength conversion, the architecture focused on in this invention particularly relates to technology to convert transmitted optical information into an output light of a different wavelength by the use of a wavelength-tunable light source.
There are two types of methods for wavelength conversion for the above purpose: one is a wavelength selection by a network management system; and the other is that conducted by the transmitted optical signal itself.
FIGS. 34 and 35
illustrate typical configurations of wavelength converters of the two methods.
Referring now to
FIG. 34
, optical signal
15
is guided to a receiver
13
to decode the information signal. By an optical modulator
12
, which is driven by this information signal, the output light from a wavelength-tunable light source
11
can be provided as optical signal
16
of which wavelength has been converted.
The wavelength of light emitted from the wavelength-tunable light source
11
is controlled by control signals sent from a network management system
17
via an information processing circuit
14
. The information processing circuit
14
, interpreting the control information, provides current and voltage which are necessary to change the wavelength of light emitted from the wavelength-tunable light source.
In the case of the wavelength converter shown in
FIG. 35
, the control signal is directly decoded by the information processing circuit
24
, based on the optical signals under transmission. Namely, the optical information signal
25
has both communications information and control information. This control information included in information signals is called “header” and the signal originator can send information about wavelength conversion to intermediate nodes in the network by the use of this header.
The wavelength conversion described in
FIG. 35
is the same as that performed in the wavelength converter of
FIG. 34
, where a receiver
23
corresponds to the receiver
13
, and an optical modulator
22
to the optical modulator
12
.
(Second Example of Prior Art)
The time-to-wavelength mapped laser of prior art is a wavelength-tunable mode-locked laser of which oscillation wavelength is changed by the repetition frequency of the resonator. It has several advantages such as simple structure, fast wavelength conversion and easy wavelength selection.
FIG. 37
illustrates general properties of oscillation
150
of the above laser.
Referring to
FIG. 37
, there are several clock frequencies fi for input clock signals. When a clock signal is applied to laser, a synchronous oscillation (pulse oscillation) occurs at a repetition frequency f
i
and a wavelength of &lgr;
i
.
Since one clock frequency provides one oscillation wavelength, the oscillation frequency of laser is determined by selecting a clock frequency f
i
of the clock signal.
FIG. 27
is a block diagram illustrating the configuration of the time-to-wavelength mapped mode-locked laser (detail explanation will be provided later).
In
FIG. 27
, wavelength mapped delay circuit
1
-
4
in laser resonator
1
-
1
is a circuit that provides a different propagation delay to light of each wavelength (namely, optical path length). The oscillation characteristics are shown in FIG.
38
.
In
FIG. 38
, there are as many as N wavelengths as input wavelength &lgr;
i
. Each wavelength has an intrinsic optical path length &Dgr;L
opt
(&lgr;
i
) and corresponding propagation delay &Dgr;T(&lgr;
i
).
When the light of wavelength &lgr;
i
enters a time-to-wavelength mapped circuit, it travels a length &Dgr;L
opt
(&lgr;
i
) before going out, and the light is given a propagation delay &Dgr;T(&lgr;
i
) corresponding to &Dgr;L
opt
(&lgr;
i
). The optical path length is a product of physical length &Dgr;L(&lgr;
i
) and refractive index n, namely, &Dgr;L
opt
(&lgr;
i
)=n&Dgr;L(&lgr;
i
). With “c” being the speed of light in vacuum, the relationship between propagation delay and optical path length is expressed by &Dgr;T(&lgr;
i
)=&Dgr;L
opt
(&lgr;
i
)/c.
In
FIG. 27
, when a wavelength mapped delay circuit
1
-
4
is inserted in the laser resonator
1
-
1
, the total optical path length L
opt
(&lgr;
i
) in the whole laser resonator and the corresponding primary repetition interval T(&lgr;
i
) (primary repetition frequency f
(T)
i
=1/T(&lgr;
i
)) change depending on each wavelength.
By the modulation conducted in optical modulator
1
-
3
at the frequency equal to the primary repetition interval (namely, repetition interval T(&lgr;
i
)/m, m>0), a mode-locked oscillation occurs at the wavelength &lgr;
i
that corresponds to this interval.
Because the other wavelengths are transmitted in the resonator
1
-
1
at intervals that do not match the modulation interval, oscillation of the other wavelengths is suppressed. In other words, the oscillation wavelength of time-to-wavelength mapped laser is selected by setting the frequency of a clock signal for driving the optical modulator
1
-
3
.
In the laser emission shown in
FIG. 27
, a driver
1
-
15
generates driving signal
1
-
14
. A clock signal generator
1
-
8
in the driver provides clock signal
1
-
9
. One of clock signals of frequencies f
1
, f
2
. . . f
N
, provided by the clock signal generator
1
-
8
, is selected by a clock signal selecting unit
1
-
7
.
The driving signal
1
-
14
comprises clock signal
1
-
9
and DC bias signal
1
-
12
. The DC bias signal
1
-
12
is necessary for setting the operation point of the optical modulator
1
-
3
. DC bias signal
1
-
12
is generated by the DC bias signal generator
1
-
11
and its signal level is adjusted by a DC bias signal adjusting unit
1
-
10
. A synthesizing unit
1
-
13
combines clock signal
1
-
9
and DC bias signal
1
-
12
.
(Third Example of Prior Art)
Since light transmission is performed at a standard transmission rate (for example, 155.52 Mbps in STM1), a pulse light source where the repetition frequency is fixed in accordance with the transmission rate or a light source that emits beams of continuous wave light (cw light source) is employed.
FIG. 21A
shows an example of transmission signals that are sent by the prior art transmission method employing a pulse light source. There is a one-to-one relation between light pulse and data bit.
(First Example of Problems in Prior Art)
First, one of the disadvantages in prior art will be explained below. In the architecture of prior art, the wavelength converters shown in
FIGS. 35 and 36
each require information processing circuit
14
or
24
to select a wavelength.
Signals entered into those circuits are control information that includes information about the wavelength of output light. The control signal is written in binary code, for example. The format of signals required to change the wavelengths of light emitted from the wavelength-tunable light source
11
or
21
depend on each light source.
The wavelength can be controlled by changing the input current and device temperature when a wavelength-tunable semiconductor laser (such as distributed feedback semiconductor laser [G. Soda, Y. Kotaki, H. Ishikawa, S.
Komukai Tetsuro
Nakazawa Masataka
Tamura Kohichi Robert
Alston & Bird LLP
Ip Paul
Nguyen Phillip
Nippon Telegraph and Telephone Corporation
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