Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-06-27
2003-11-04
Glick, Edward J. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000, C385S008000
Reexamination Certificate
active
06643430
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature control system for a grating used in dispersion equalizing in an ultra-high-speed optical communication system. Particularly, the temperature control system includes a temperature control device for a grating, a method of storing a temperature control pattern in a storage device, a method of automatically controlling the temperature control device for a grating, and a variable dispersion equalizer.
2. Description of the Prior Art
In an optical communication system using an optical fiber cable as a transmission path, since an optical pulse is distorted by wavelength dispersion (also called dispersion, to be referred to as “dispersion” hereinafter) of the optical fiber, a signal is degraded. A reason why dispersion occurs when the optical fiber cable is used as described above will be described below. In a material constituting a general optical fiber cable, a group velocity of a wave packet of optical pulses depends on a wavelength, and a time required to propagate the wave packet, i.e., a group delay time (unit: ps) changes. An inclination of the group delay time to the wavelength is dispersion (unit: ps
m). In a single mode fiber used in a general optical fiber transmission path, dispersion generated for a transmission path of 1 km has a value of about 16 ps/(nm·km) at a wavelength of about 1550 nm. This means that the difference between delay times required to propagate optical pulses having wavelengths which are different from each other by 1 nm through a single mode fiber (hereinafter referred to as SMF) of 1 km is 16 ps. For example, a group delay time when optical pulses having wavelengths which are different from each other by 1 nm are propagated through an optical fiber cable of 100 km is 1600 ps which is 100 times the group delay time obtained in the above case.
On the other hand, modulated optical pulses have the spreads of several spectra determined by a modulation method or a bit rate, and an envelope for the optical pulses is of a Gaussian distribution type. For example, in an RZ (return-to-zero) modulation method, when a bit rate (transmission speed) is 10 Gbit/s, intervals between the respective line spectra are 0.08 nm each. However, when the bit rate is 40 Gbit/s, intervals are 0.32 nm each. More specifically, the spread of the line spectrum increases in proportion to a bit rate. In an NRZ (non-return-to-zero) modulation method, the spread of a line spectrum is half the spread of the line spectrum in the RZ modulation method. In this manner, as a bit rate increases, the interval between line spectra which are the components of optical pulses increases. For this reason, the difference between group delay times when the optical pulses are propagated through an optical fiber transmission path increases, distortion of the optical pulses increases. In addition, an influence of a dispersion of an optical fiber transmission path received by optical pulses increases in proportion to the square of a bit rate. For this reason, a device having dispersion which cancels the dispersion of the optical fiber transmission path is inserted into the transmission path, and the dispersions are made close to zero. This technique is a dispersion compensation technique. In particular, a dispersion of a transmission path at a bit rate of 40 Gbit/s or more must be made precisely close to zero. At a bit rate of 80 Gbit/s or more, a dispersion slope which is a rate of a change in dispersion caused by a wavelength must be compensated for.
As a device for equalizing such a dispersion, a variable dispersion equalizer using a chirp grating is known. For example, as shown in a perspective view in
FIG. 19
, Japanese Laid-Open Patent Publication No. 10-221658 discloses a variable dispersion equalizer using a chirp grating. In a fiber grating
1
serving a chirp grating, circularly cylindrical compact thick-film heaters
3
1
,
3
2
, . . . ,
3
N
(N is an integer) consisting of tungsten, NiCr, or the like are arranged in a capillary
2
such as a hollow ceramics consisting of an insulator and having a through hole for fixing a fiber having a relatively large diameter such that the circularly cylindrical compact thick-film heaters
3
1
,
3
2
, . . . ,
3
N
are coaxial with the through hole of the capillary
2
. Here, the heaters
3
1
,
3
2
, . . . ,
3
N
are arranged at equal intervals in the longitudinal direction of the capillary 2. When currents are flowed into the heaters
3
1
,
3
2
, . . . ,
3
N
such that the currents increase by a predetermined value, the fiber grating
1
is gradually heated in a micro-section, but is heated with a predetermined temperature gradient as a whole. The equivalent refractive index of the fiber grating
1
changes depending on an applied voltage to realize a linear chirp characteristic. The equivalent refractive index is also called an effective refractive index, is an equivalent refractive index which is received by light propagated through an optical fiber cable, and is a refractive index generated by an interactive function between the refractive indexes of a core and a cladding and a propagation path of light. Although, exactly, the grating pitch of the fiber grating
1
also changes depending on a change in temperature, the change of the grating pitch is neglected because the influence of the change of the grating pitch is smaller than that of the change of the equivalent refractive index.
There was no temperature control device for grating which appropriately controlled a plurality of heaters disposed near the grating to give an appropriate temperature distribution to the grating. More specifically, when powers applied to the heaters disposed near the grating are not appropriately controlled, a temperature distribution given to the grating is incorrect to adversely affect chirp characteristics such as a dispersion and a dispersion slope given to a reflected light component. In this case, a group delay ripple which is a shift from an almost linear relationship between a group delay time and a wavelength is generated. On the other hand, when a temperature distribution given from each heater to the grating is a linear distribution, a group delay ripple caused by a manufacturing error inherent in the grating may occur. In addition, in a conventional variable dispersion equalizer, a group delay ripple which adversely affects transmission quality occurs due to a gradual temperature distribution generated by the plurality of heaters disposed near the grating. In addition, the cycle of the group delay ripple is dependent on the numeral distribution of the heaters. The existence of the group delay ripple having a predetermined cycle or more considerably influences the numeral distribution of the heaters at a high bit rate.
In a variable dispersion equalizer disclosed in Japanese Laid-Open Patent Publication No. 10-221658, a case in which a linear chirp characteristic is given exemplified. However, control of each heater is not concretely described. The heaters are arranged at equal intervals in the longitudinal direction, and the numeral distribution of the heaters is not considered. The cycle of a group delay ripple is not described.
SUMMARY OF THE INVENTION
Therefore, the first object of the present invention to provide a system of variable dispersion equalizer with grating, which gives a predetermined dispersion and a dispersion slope with suppressed a group delay ripple inherent. It is the second object of the present invention to provide a variable dispersion equalizer having the small cycle of a group delay ripple.
In accordance with one aspect of the present invention, there is a temperature control device for controlling a plurality of temperature variable device for grating which are disposed near the grating of a variable dispersion equalizer constituted by an optical waveguide forming the grating and the temperatures of which can be independently controlled. The temperature control device includes a controller for controlling the plurality of t
Hoshizaki Junichiro
Matsumoto Sadayuki
Matsuno Shigeru
Ohira Takuya
Takabayashi Masakazu
Barber Therese
Glick Edward J.
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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