Fiber-optic address detector in photonic packet switching device and method for fabricating the same

A fiber-optic address detector comprises fiber-optic delay lines on one surface of which a metal thin film is evaporated, the fiber-optic delay lines being connected in a melting state to fiber-optic couplers, wherein inputting address photonic signals are tapped by the inputting fiber-optic couplers, reflected by the metal thin films at the end portion of the fiber-optic delay line and then re-combined by the inputting fiber-optic coupler, whereby it can reduce the number of the fiber-optic coupler used in the conventional fiber-optic address detector to one half as well as obtain the same address detection effect.

BACKGROUND OF THE INVENTION 
The invention is related to providing a fiber-optic address detector in a 
photo packet switching device and a method for fabricating the same. 
PRIOR ART 
A conventional wired communication network has used copper wires as a 
medium to transmit signals, but the copper wire is on the way to be 
replaced with an optical fiber. The optical fiber enables the great 
expansion of the transmission bandwidth, for the sufficient utilization of 
which the development of the optic switching technique is required. 
A fiber-optic communication technique is a kind of an optical communication 
code division multiple access method which each bit is coded with its 
destination address information to form a code sequence and then 
transmitted to the destination, while other destinations detect addresses 
to receive the transmitted information. Another technique is a kind of a 
photonic packet switching method which payload data to be user information 
is transmitted together with an address header. 
The former method enables the transmission/reception between parties 
together at anytime using the great bandwidth of the single mode optical 
fiber, but it has a disadvantage in that the number of node receivable in 
a communication network is limited. Its adoption is almost not performed 
except for especial cases necessary to keep in secret. The latter method 
for maintaining the payload information of the packet in a photonic form 
during the switching process is widely used because of the processing 
ability of a larger capacity and the high switching speed. 
The typical photonic packet switching device includes an fiber-optic 
coupler for tapping photonic packet signals inputted from an optical fiber 
cable by about 10%; a laser diode optical amplifier for amplifying the 
tapped photonic packet signals; a fiber-optic address detector for 
determining whether its addresses corresponds to the photonic addresses 
which are inputted passing through an fiber-optic delay line, outputting 
auto-correlation pulses when the two addresses are identical to each 
other, otherwise outputting cross-correlation pulses when the two 
addresses are not identical to each other; and an address coherent 
discriminator for controlling optical switches to send the photonic packet 
signals to a receiver if the correlation pulses from the fiber-optic 
address detector each is larger than a predetermined threshold value or to 
by-pass the photonic packet signals to next destination if the correlation 
pulses from the fiber-optic address detector each is smaller than a 
predetermined threshold value. 
In other words, the fiber-optic address detector of the packet switching 
device for processing the very high-speed broad bandwidth signals has the 
configuration of a transmission type which enables the fiber-optic coupler 
to tap part of the photonic packet signal inputted through the optical 
cable line in order to process addresses. The bits of the photonic address 
signals tapped are separated by an inputting coupler, delayed through the 
corresponding delay line and then re-coupled by an outputting coupler. 
Thus, the inputting address signals can be detected according to the 
results of the correlation procedure between bit signals to be coupled. In 
conclusion, the fiber-optic address detector outputs the auto-correlation 
pulses if the inputting address signals correspond to its address signals. 
On the contrary, the fiber-optic address detector outputs the 
cross-correlation pulses if the inputting address signals do not 
correspond to its address signals. 
FIG. 1 shows an example of the fiber-optic address detector including an 
fiber-optic coupler of 3 dB at a array of 2.times.2 and an fiber-optic 
timing delay line (referred to as "an fiber-optic delay line" below), 
which the number of an address bit "1" is 4. The fiber-optic address 
detector comprises the 2.times.2 fiber-optic couplers 1a-1f, the 
fiber-optic delay lines 2a-2d and optical fibers 3a-3l, among which the 
optical fibers 3g-3l is not used. 
Thus, the optical address signals inputted through the optical fiber 3a are 
individually separated by the first, second and third fiber-optic couplers 
1a to 1c which act to divide a photonic power into two by its own two 
outputs. Thereafter, each of the address bit "1" is supplied to the first, 
second, third and fourth outputting fiber-optic delay lines 2a to 2d to be 
re-coupled by the first, second, third and fourth outputting fiber-optic 
couplers 1d to 1f. The correlation procedure enables the fiber-optic 
address detector to detect addresses, optically. In other words, if the 
inputting address signals correspond to the addresses of the fiber-optic 
address detector, the fiber-optic address detector outputs 
auto-correlation pulses. On the contrary, if the two kinds of addresses 
are identical to each other, the cross-correlation pulses appear at the 
outputting optical fiber of the third outputting fiber-optic coupler 3f. 
Herein, it is noted that the identities of corresponding addresses are 
readily detected using a threshold value detector (not shown) because the 
central pulse of the auto-correlation pulses has the amplitude larger than 
that of the cross-correlation pulses. 
In light of these points, it is preferable to remove the unused optical 
fibers as well as to reduce the number of fiber-optic couplers in order to 
simplify the configuration without deteriorating the performance of a 
fiber-optic address detector. 
The object of the invention is to provide a fiber-optic address detector in 
a photonic packet switching device and a method for fabricating the same, 
the configuration of which is simple. 
SUMMARY OF THE INVENTION 
In order to accomplish this object, the invention comprises a fiber-optic 
address detector including fiber-optic delay lines on one surface of which 
a metal thin film is evaporated, the fiber-optic delay lines being 
connected to fiber-optic couplers, wherein inputting address photonic 
signals are tapped by the inputting fiber-optic couplers, reflected by the 
metal thin films at the end portion of the fiber-optic delay line and then 
re-combined by the inputting fiber-optic coupler, whereby it can reduce 
the number of the fiber-optic coupler used in the conventional fiber-optic 
address detector to one half as well as obtain the same address detection 
effect. Therefore, the invention is exampified as a reflected packet 
address detector.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2, the reference numbers identical to those of a 
conventional fiber-optic address detector show the same elements as those 
of FIG. 1. The reference numbers 4a-4d are a metal thin film. 
According to the invention, an optical fiber 3g located at the inputting 
terminal, but is not being used in first inputting fiber-optic coupler 1a, 
is used as an outputting terminal. 
A method for fabricating a fiber-optic address detector according to the 
invention is as follows: 
An optical fiber having a predetermined length is firstly cleaved at the 
one surface. The optical fiber is arranged in an aluminum case with a 
groove having a predetermined depth and width being formed at the center, 
wherein the cross-section of the optical fiber cleaved is extended by a 
predetermined length of about 5 mm from the end of the aluminum case. The 
aluminum case is sealed with a cover by means of a screw, so that the 
optical fibers are not escaped from the groove. 
Then, the aluminum case is placed in the vacuum chamber of an electronic 
beam evaporation apparatus. The cleaved cross-section of the optical fiber 
is faced to a metal target thereby to evaporate the metal thin film by the 
thickness to be able to obtain a desired reflection factor. 
Thereafter, the aluminum case is picked out from the vacuum chamber and its 
cover is removed to separate the optical fibers therefrom. 
On the other hand, in order to fabricate the fiber-optic address detector 
that the number of the address bit "1" is 4, an outputting optical fiber 
is connected in a melting state to one inputting optical fiber 3a of the 
first fiber-optic coupler 1a, in which the outputting optical fiber is 
pig-tailed to a laser diode (not shown) for amplifying the inputting 
photonic signals and generating single microwave pulses. Each of two 
outputting optical fibers 3b and 3c in the first fiber-optic coupler 1a is 
connected in a melting state to one inputting optical fiber of the second 
fiber-optic coupler 1b and one inputting optical fiber of the third 
fiber-optic coupler 1c. The first, second and third fiber-optic couplers 
1a to 1c are a fiber-optic coupler of 3 dB at an array of 2.times.2. 
The position of fresnel pulses is measured on an oscilloscope after passing 
through a photo-detector, in which the fresnel pulses are reflected from 
the cleaved outputting optical fibers of the second and third fiber-optic 
couplers 1b and 1c and outputted to other inputting optical fiber 3g of 
the first fiber-optic coupler 1a not used in FIG. 1 of a prior art. Thus, 
the lengths of the first to fourth fiber-optic delay lines 2a to 2d from 
the positions at the time axes of these pulses to the position 
corresponding to that of the detector's address code bit "1" are 
respectively calculated according to the following formula: the 
fundamental unit length L of the first to fourth fiber-optic delay lines 
2a to 2d is equal to ct/2n.sub.c, in which the fiber-optic delay lines 
includes the metal thin films 4a to 4d respectively evaporated on the 
surface thereof, wherein c is the light velocity (=3.times.10.sup.8 m/sec) 
passing through the vacuum, t is the time difference between two address 
bits "1" and n.sub.c is the reflection factor of an optical fiber core. If 
the velocity of the address bit is 2.5 Gbps (t=1/(2.5.times.10.sup.8) and 
the reflection factor n.sub.c is a 1.46, the unit length difference of the 
fiber-optic delay line between two address bits becomes 4.109 cm. 
Thus, if the address bit code including four bits "1" is 101101, four 
fiber-optic delay lines are required, in which the metal thin films are 
respectively evaporated on the fiber-optic delay line. For example, on the 
basis of the length of the optical fiber corresponding to MSB (Most 
Significant Bit; first bit) the length of the third fiber-optic delay line 
corresponding to third bit should be longer than that of the first 
fiber-optic delay line by 2.times.4, 109 cm. The fourth delay line be 
longer than 3.times.4, 109 cm over the first delay line. The sixth delay 
line be longer than 5.times.4, 109 cm over the first delay line. 
The fiber-optic delay lines determined as above are cleaved so that they 
are in turn connected in a melting state to each of the outputting optical 
fiber of the second and third fiber-optic couplers 1b and 1c having a 
given delay time. 
The reflected fiber-optic address detector fabricated as described above 
outputs the photonic pulse signals r(t) through the second inputting 
optical fiber 3 g of the first fiber-optic coupler 1a which has the 
correlation function as follow: 
EQU r(t)=i(x)f(t-x)dx 
Wherein, i(x) is the waveform of the inputting address and f(x) is the 
impulse response of the address detector. 
Therefore, if the inputting photonic address code is corresponding to the 
of the address code of the address detector, the auto-correlation pulses 
appear at the second inputting optical fiber 3 g of the first fiber-optic 
coupler 1a. If the two address codes do not correspond to each other, the 
cross-correlation pulses appear at the second inputting optical fiber 3 g 
of the first fiber-optic coupler 1a. The central amplitude of the 
auto-correlation function is in proportion to the number of the bit "1" in 
the address code. Thus, when the two address codes do not correspond to 
each other, the inputting photonic address pulse is higher than the 
central amplitude of the cross-correlation pulses, so that it facilitates 
the detection of addresses. 
As described above, according to the invention the fiber-optic address 
detector has advantages in that it can obtain the same address detection 
effect with less number of the fiber-optic coupler over the prior art.