Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-08-17
2003-02-18
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S024000, C385S039000, C359S199200
Reexamination Certificate
active
06522809
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling the Bragg wavelength and a method of compensating for temperature dependency of the Bragg wavelength of a waveguide grating, and a waveguide grating device using the same.
2. Description of the Related Art
In a wavelength division multiplexing(WDM) system, an optical multiplexer/demultiplexer that combines or splits wavelength-multiplexed signals, a band pass filter and a dispersion compensator that compensates for dispersion of an optical fiber that transmits signals are key devices, with improvements in the characteristics thereof being viewed as important issues.
Recently, particularly as advancements in the WDM systems have been made, such needs have been mounting to improve the Bragg wavelength control technology and improve the temperature characteristic of the Bragg wavelength of an optical multiplexer/demultiplexer that combines or splits wavelength-multiplexed signals, a band pass filter and a dispersion compensator.
FIG. 25
shows an example of a band pass filter of the prior art that utilizes a waveguide grating. This waveguide grating has a function of reflecting light of a particular wavelength, with the Bragg wavelength &lgr;B being defined by the following formula (1), where n
eff
denotes the effective refractive index of the waveguide grating, and &Lgr; denotes the period of the waveguide grating.
&lgr;B
eff
2
&Lgr; (1)
In the band pass filter shown in
FIG. 25
, light incident on a port P
1
is divided into two portions by a 3 dB coupler
105
. Light of the same wavelength as the Bragg wavelength &lgr;B of a grating
102
, namely the signal light, is reflected from the grating
102
and synthesized in the 3 dB coupler
105
again and is output from the port P
2
. Light having a wavelength different from the Bragg wavelength &lgr;B of the grating, namely noise light, is transmitted through the grating
102
and is output from ports P
3
, P
4
. Thus the band pass filter of
FIG. 25
is capable of cutting off the noise light and extracting only the signal light from the port P
2
.
FIG. 26
shows an example of a multiplexer/demultiplexer that utilizes the waveguide grating. In the multiplexer/demultiplexer of the prior art shown in
FIG. 26
, for example, when signals of different wavelengths (&lgr;
1
, &lgr;
2
, &lgr;
3
, . . . ) enter a port P
1
, only the signal light of a particular wavelength (&lgr;
1
in the drawing) is reflected from the grating
102
and is output from the port P
2
(demultiplexing). Signal light of other wavelengths (&lgr;
2
, &lgr;
3
, . . . ) is output from the port P
4
. Signal light having a particular wavelength (light having wavelength &lgr;
1
) that has entered the port P
3
is reflected from the grating
102
and is output from the port P
4
(multiplexing).
FIG. 27
shows an example of a dispersion compensator that utilizes the waveguide grating. In optical communication, since an optical fiber used as a medium for transmitting optical signals experiences dispersion due to the refractive index changing depending on wavelength, propagation speed along the propagation path varies depending on the wavelength of the light, thus resulting in different amounts of group delay for different wavelengths. Since an optical pulse signal has a certain spread in wavelength, the pulse signal is broadened as shown in
FIG. 28B
after having been propagated over several tens of kilometers, due to the difference in group delay by the wavelength (
FIG. 28A
shows signal waveform before propagation and
FIG. 28B
shows signal waveform after propagation). Therefore, ultra-high speed optical communication requires a technique for compensating for the dispersion in the optical fiber.
Relation between wavelength and group delay during propagation through an optical fiber is shown in
FIG. 29A
, with the dispersion that is given by differentiation thereof being shown in FIG.
29
B. Compensating for this effect requires a device that has such group delay and dispersion characteristics as shown in
FIGS. 30A
,
30
B, which can be achieved by means of a dispersion compensator shown in FIG.
27
. The waveguide grating
102
of the dispersion compensator is formed so that the period thereof becomes longer as the distance from the input port increases. With such a configuration that the period changes as described above, reflecting position changes with the wavelength of the light thus resulting in such a gradient in group delay as shown in FIG.
30
A. That is, dispersion remains a constant and positive value as shown in
FIG. 30B
, thus making it possible to compensate for the dispersion because of the sign of this dispersion reverse to the dispersion of the optical fiber.
FIG. 31
shows an example of a variable dispersion compensator that utilizes a waveguide grating. Since the amount of dispersion that a dispersion compensator is required to compensate varies, depending on the length of an optical fiber used as the propagation path and on the conditions of operation, when a dispersion compensator of a fixed amount of dispersion to be compensated is used, the optimum dispersion compensator has been selected by preparing a plurality of dispersion compensators of different amounts of dispersion and inserting them successively. The variable dispersion compensator shown in
FIG. 31
, by contrast, is capable of making a fine adjustment of the dispersion and therefore makes it possible to solve the problem of the trials and errors being required to determine the optimum dispersion compensator. Specifically, in the variable dispersion compensator shown in
FIG. 31
, heating means
24
a
,
24
b
apply heat to the optical waveguide. At this time, a temperature gradient is generated in the waveguide grating by differentiating the calorific values generated by the heating means
24
a
and the heating means
24
b.
The effective refractive index n
eff
of the waveguide grating is a function of temperature. Consequently, the temperature gradient generated by applying different amounts of heat from the heating means
24
a
and the heating means
24
b
causes a gradient in the effective refractive index n
eff
of the optical waveguide along the longitudinal direction of waveguide. With this configuration, since the Bragg wavelength becomes a function of n
eff
as shown by formula (1), the Bragg wavelength changes with the position along the longitudinal direction of the waveguide grating. As a result, the temperature gradient gives an effect similar to the so-called chirped grating, so that the distance from the reflecting point differs depending on the wavelength, thereby causing dispersion. With this configuration, relations of the temperature gradient with the group delay and the dispersion become as shown in
FIGS. 32A
,
32
B, and
32
C, thus making it possible to adjust the dispersion according to changes in the temperature gradient generated by the heating means.
Now a method of producing the waveguide grating used in the optical devices described above will be described. Typical methods of producing the waveguide grating include one that utilizes light-induced change in refractive index. The light-induced change in refractive index refers to the change that occurs in the refractive index of a silica-based optical waveguide, that is doped with germanium, when irradiated with ultraviolet rays. Specifically, when two rays of ultraviolet light are incident on the waveguide and interfere with each other and form a fringe pattern, the refractive index of the waveguide changes according to the period of the fringe pattern, thus generating a waveguide grating. Though the change in the refractive index is as small as on the order of 0.001, the grating has a very small period of about 500 nm and therefore a waveguide grating of around 20000 periods can be produced with a length of about 1 cm, thus easily achieving about 100% reflectivity at the Bragg wavelength.
The waveguide gratings produced as described above have variations in the grating period du
Hashimoto Minoru
Hoshizaki Junichiro
Maegawa Takeyuki
Matsumoto Sadayuki
Sakai Kiyohide
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Sanghavi Hemang
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