Optical amplifier

In an optical amplifier for amplifying a signal light by propagating the signal light and a pumping light in a rare earth element doped fiber doped with a rare earth element, a diameter of a rare earth element doped portion of the rare earth element doped fiber is gradually reduced in a direction of propagation of the pumping light. With this construction, an adverse rare earth element doped area which does not contribute to optical amplification, but rather attenuates the pumping light, can be eliminated to thereby provide an optical amplifier having increased amplification efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical amplifier employing a rare 
earth element doped fiber or a rare earth element doped optical waveguide 
doped with a rare earth element. 
2. Description of the Related Art 
In optical communication systems used at present, a repeater is inserted at 
fixed distance intervals, so as to compensate the attenuation of an 
optical signal due to a power loss of an optical fiber. The repeater is 
constructed in such a manner that the optical signal is converted into an 
electrical signal by a photodiode followed by amplification of the 
electrical signal by means of an electronic amplifier. Thereafter the 
electrical signal thus amplified is converted into an optical signal by 
means of a semiconductor laser or the like followed by returning the 
optical signal to an optical transmission line, if the optical signal can 
be directly amplified with a low noise without conversion into an 
electrical signal, the optical repeater can be made compact and 
economized. 
In this circumstance, much research has been undertaken to develop an 
optical amplifier capable of directly amplifying an optical signal. The 
optical amplifiers which are the subject of this research are generally 
classified into (a) an optical amplifier employing, in combination, an 
optical fiber doped with a rare earth element (Er, Nb, Yb, etc.) and a 
pumping light; (b) an optical amplifier employing a semiconductor laser 
doped with the rare earth element; and (c) an optical amplifier utilizing 
a nonlinear effect in the optical fiber. 
Above all, the optical amplifier employing the combination of the rare 
earth element doped fiber and the pumping light, as mentioned in the above 
type (a), has excellent features, such as no polarization dependency, low 
noise, and small coupling loss to a transmission line. Accordingly, the 
optical amplifier of this type is expected to remarkably increase a 
repeating distance in an optical fiber transmission system, and it is also 
expected to enable multiple distributions of the optical signal. 
FIG. 1 shows the principle of the optical amplification by the rare earth 
element doped fiber. Referring to FIG. 1, reference numeral 2 designates 
an optical fiber constructed of a core 2a and a clad 2b. Erbium (Er) is 
doped in the core 2a. When a pumping light is input into the Er doped 
fiber 2, Er atoms are excited up to a high energy level. When a signal 
light is input into the optical fiber 2 having Er atoms excited up to the 
high energy level, the Er atoms are shifted to a low energy level. At this 
time, stimulated emission of light is generated, and a power of the signal 
light is gradually increased along the optical fiber, thus effecting 
amplification of the signal light. 
In general, the concentration of atoms doped in the core 2a is uniform with 
respect to a longitudinal direction and a radial direction of the Er doped 
fiber 2. 
In accordance with the above-mentioned principle of the optical 
amplification, when the rare earth atoms in the rare earth element doped 
fiber are excited up to a high energy level by the pumping light, the 
energy of the pumping light is consumed. Therefore, as being propagated in 
the rare earth element doped fiber, the power of the pumping light is 
absorbed. Meanwhile, it is known that if the power of the pumping light is 
less than a certain threshold level, there does not occur the excitation 
of the rare earth atoms enough to effect the optical amplification. 
Accordingly, in the optical amplifier employing the rare earth element 
doped fiber doped with the rare earth element at a uniform concentration 
in the core, the doped rare earth element rather causes a power loss of 
the signal light and the pumping light. Therefore, the conventional 
optical amplifier having the above construction is considered to be 
unsuitable for an increase in amplification efficiency (i.e., a degree of 
amplification of the signal light with respect to the pumping light having 
a fixed power). 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide an optical 
amplifier which can solve the above problem in the conventional technique 
and increase the amplification efficiency. 
According to one aspect of the present invention, there is provided an 
optical amplifier for amplifying a signal light by propagating the signal 
light and a pumping light in a rare earth element doped fiber doped with a 
rare earth element, characterized in that a diameter of a rare earth 
element doped portion of the rare earth element doped fiber is gradually 
reduced in a direction of propagation of the pumping light. The direction 
of propagation of the pumping light may be the same as or counter to a 
direction of propagation of the signal light. 
The gradual reduction of the diameter of the rare earth element doped 
portion in the direction of propagation of the pumping light is attained 
by extending the rare earth element doped fiber with heat to continuously 
change the diameter of the rare earth element doped portion. 
Alternatively, the gradual reduction of the diameter may be attained by 
connecting in series a plurality of rare earth element doped fibers formed 
with rare earth element doped portions having different diameters. 
According to another aspect of the present invention, there is provided an 
optical amplifier for amplifying a signal light by propagating the signal 
light and a pumping light in a rare earth element doped optical waveguide 
doped with a rare earth element, characterized in that a width of a rare 
earth element doped portion of the rare earth element doped optical 
waveguide is gradually reduced in a direction of propagation of the 
pumping light.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
There will first be described the principle of the present invention with 
reference to FIGS. 2A and 2B. FIG. 2A shows a case wherein a signal light 
and a pumping light are propagated in the same direction in a rare earth 
element doped fiber 3, and FIG. 2B shows a case that the signal light and 
the pumping light are propagated in opposite directions in the rare earth 
element doped fiber 3. 
In an optical amplifier for amplifying the signal light by propagating the 
signal light and the pumping light in the rare earth element doped fiber 3 
and 3', a diameter of a rare earth element doped position of the rare 
earth element doped fiber 3 and 3' is gradually reduced in a direction of 
propagation of the pumping light as shown by dashed lines in FIGS. 2A and 
2B. 
Whether the signal light and the pumping light are to be propagated in the 
same direction in the rare earth element doped fiber 3, or whether the 
signal light and the pumping light are to be propagated in opposite 
directions in the rare earth element doped fiber 3', may be selected 
according to construction requirements of an optical communication system 
or the like to which the optical amplifier of the present invention is 
applied. 
Referring to FIG. 2A, a point A denotes an upstream position of the 
direction of propagation of the signal light and the pumping light in the 
rare earth element doped fiber 3; a point C denotes a downstream position 
of the direction of propagation; and a point B denotes an intermediate 
position between the points A and C. FIGS. 3A, 3B and 3C are graphs that 
show the distributions of intensities of the pumping light at the points 
A, B and C shown in FIG. 2A, respectively. In each graph, the ordinate 
represents an electric field amplitude of the pumping light, and the 
abscissa represents a radial position in the rare earth element doped 
fiber 3. 
As apparent from FIGS. 3A, 3B and 3C each graph gives a so-called Gaussian 
distribution such that the electric field amplitude of the pumping light 
at a central position of the fiber 3 in the radial direction is relatively 
high. Further, it is also understood that a maximum electric field 
amplitude of the pumping light is gradually reduced in the direction of 
propagation of the pumping light due to the fact that the rare earth atoms 
doped in the fiber are excited by the pumping light. In FIGS. 3A, 3B, and 
3C reference character Pth denotes a threshold level such that optical 
amplification is effected at levels higher than the threshold level Pth, 
while it is not effected at levels not higher than the threshold level 
Pth. Reference characters Ra, Rb and Rc denote radii of portions giving 
the electric field amplitude higher than the threshold level Pth at the 
points A, B and C, respectively. The relation among these radii is given 
as follows: 
EQU Rc&lt;Rb&lt;Ra 
As to the point B, for example, if the rare earth element is doped at a 
portion radially outside the portion of the radius Rb, such a radially 
outside portion does not contribute to optical amplification at all, but 
rather attenuates the pumping light because of the existence of the rare 
earth element, resulting in difficulty in achieving efficient optical 
amplification. However, according to the present invention, the diameter 
of the rare earth element doped portion of the rare earth element doped 
fiber 3 is gradually reduced in the direction of propagation of the 
pumping light. Accordingly, any adverse rare earth element doped portion 
which does not contribute to optical amplification but rather attenuates 
the pumping light as mentioned above with reference to FIGS. 3A, 3B and 3C 
can be eliminated or reduced, thus providing an optical amplifier suitable 
for an increase in amplification efficiency. 
Also in the case shown in FIG. 2B, i.e., in the case of so-called backward 
pumping such that the signal light and the pumping light are propagated in 
opposite directions, the operation is similar to that in the case of 
so-called forward pumping shown in FIG. 2A. 
There will now be described a first preferred embodiment of the present 
invention. 
FIG. 4 shows a construction of an optical fiber amplifier according to the 
first preferred embodiment of the present invention. Referring to FIG. 4, 
a plurality of (two in this preferred embodiment) rare earth element doped 
fibers 21 and 22, different in diameter at the respective rare earth 
element doped portions, are connected together in series, so that the 
diameter of the rare earth element doped portion of the rare earth element 
doped fiber as a whole is gradually reduced in the direction of 
propagation of the pumping light. The connection of the rare earth element 
doped fibers 21 and 22 is carried out by splicing, for example. Further, 
an input optical fiber 4 for propagating a signal light to be amplified 
and an output optical fiber 6 for propagating the signal light amplified 
are connected by splicing or the like to opposite ends of the rare earth 
element doped fibers 21 and 22 connected together in series. 
An optical coupler 8 of a fiber spliced type is formed at a midway portion 
of the input optical fiber 4 by splicing a side surface of another optical 
fiber to a side surface of the input optical fiber 4 and extending a 
spliced portion by heating. The optical coupler 8 includes a first input 
port 8a and a first output port 8c via the input optical fiber 4, and also 
includes a second input port 8b and a second output port 8d via the other 
optical fiber. A semiconductor laser 10 as a pumping light source is 
connected to the second input port 8b. 
In the case wherein the doped rare earth element is erbium (Er), and a 
signal light having a wavelength within the 1.55 .mu.m band is intended to 
be amplified, a wavelength of the pumping light is selected to be within 
the 0.80 .mu.m band, 0.98 .mu.m band, 1.48 .mu.m band, etc. The structural 
parameters of the optical coupler 8 are set so as to efficiently input the 
pumping light and the signal light thus selected into the rare earth 
element doped fiber. That is, so as to introduce substantially 100% of the 
signal light input into the first input port 8a to the first output port 
8c, and similarly introduce substantially 100% of the pumping light input 
into the second input port 8b to the first output port 8c. 
The rare earth element doped fiber 21 located on the upstream side with 
respect to the direction of propagation of the signal light and the 
pumping light will be hereinafter referred to as an upstream fiber 21, and 
the rare earth element doped fiber 22 located on the downstream side will 
be hereinafter referred to as a downstream fiber 22. The upstream fiber 21 
and the downstream fiber 22 are shown in FIGS. 5A and 5B in cross section, 
respectively. The upstream fiber 21 is comprised of a clad 21a and a core 
21b having a refractive index higher than that of the clad 21a. Er is 
doped in the core 21b with a uniform concentration distribution. The 
downstream fiber 22 is comprised of a clad 22a, a first core 22b and a 
second core 22c. A refractive index distribution in the first core 22b and 
the second core 22c of the downstream fiber 22 is the same as that in the 
core 21b of the upstream fiber 21. A refractive index of the clad 22a of 
the downstream fiber 22 is the same as that of the clad 21a of the 
upstream fiber 21. 
The second core 22c is formed at a central portion of the first core 22b, 
and Er is doped in the second core 22c only with a uniform concentration 
distribution. As a method of doping a rare earth element in a specific 
portion of a core as in the downstream fiber 22, the following method may 
be adopted, for instance. That is, in producing a preform by an MCVD 
process, a first core glass not doped by a rare earth element is formed on 
an inner wall of a silica tube, and a second core glass doped with a rare 
earth element is formed on the first core glass. 
Although the two rare earth element doped fibers, that is, the upstream 
fiber 21 and the downstream fiber 22 are used in this preferred 
embodiment, several (more than two) rare earth element doped fibers formed 
with rare earth element doped portions having different diameters may be 
produced according to the above-mentioned method of producing the 
downstream fiber 22, and these rare earth element doped fibers may be 
connected together in series so that the diameters of the rare earth 
element doped portions become smaller in the direction of propagation of 
the pumping light. Further, although the concentration distribution of Er 
is uniform in the radial direction of the doped portion in this preferred 
embodiment, the concentration distribution of the rare earth element doped 
may be modified such that the concentration is high at the radially 
central area of the doped portion like the intensity distribution of the 
pumping light, so as to make the optical amplification efficient. 
In the optical fiber amplifier shown in FIG. 4, the signal light from the 
input optical fiber 4 and the pumping light from the semiconductor laser 
10 are coupled together by the optical coupler 8, and they are input into 
the upstream fiber 21. In the upstream fiber 21, the signal light is 
amplified by the pumping light which has not yet been absorbed but has a 
sufficient intensity. At the outlet of the upstream fiber 21, the 
intensity of the pumping light becomes relatively small as the result of 
the amplification of the signal light. Then, the pumping light having a 
relatively small intensity and the amplified signal light are input into 
tile downstream fiber 22. In the downstream fiber 22, undesired absorption 
of the pumping light does not occur because the diameter of the Er doped 
second core 22c of the downstream fiber 22 is smaller than the diameter of 
the core 21b of the upstream fiber 21. As a result, the optical 
amplification of the signal light can be efficiently carried out. Also in 
the case of applying this principle to an optical waveguide, the optical 
amplification can be similarly carried out by connecting a series a of 
optical waveguide boards formed with Er doped optical waveguides having 
different widths. 
FIG. 6 shows a construction of an optical fiber amplifier according to a 
second preferred embodiment of the present invention, in which the same 
parts as those in the first preferred embodiment are designated by the 
same reference numerals. Referring to FIG. 6, a rare earth element doped 
fiber 23 is substituted for the upstream fiber 21 and the downstream fiber 
22 used in the first preferred embodiment. The rare earth element doped 
fiber 23 is formed with a rare earth element doped portion having a 
diameter that changes continuously. That is, the diameter of the rare 
earth element doped portion of the rare earth element doped fiber 23 is 
continuously reduced in the direction of propagation of the pumping light. 
As a method of continuously reducing the diameter of the rare earth 
element doped portion, the following method may be employed, for example. 
As shown in FIG. 7A, a rare earth element doped fiber 2 having a given 
length is heated at its substantially axially central portion by a burner 
12, and is then extended in opposite directions as depicted by arrows. As 
a result, a rare earth element doped fiber 2' as shown in FIG. 7B is 
formed. As apparent from FIG. 7B, a diameter of the axially central 
portion of the rare earth element doped fiber 2' is smaller than that of 
the opposite end portions. Then, the rare earth element doped fiber 2' is 
cut at the axially central portion to thereby obtain the rare earth 
element doped fiber 23 (extended fiber) formed with the Er doped portion 
having a diameter continuously reduced. In FIGS. 7A and 7B, elongated 
areas surrounded by dashed lines represent the Er doped portions of the 
rare earth element doped fibers 2 and 2'. 
According to the second preferred embodiment, the diameter of the rare 
earth element doped portion of the rare earth element doped fiber is 
continuously changed. Therefore, as compared with the first preferred 
embodiment wherein the diameter of the rare earth element doped portion is 
stepwise changed, the optical fiber amplifier according to the second 
preferred embodiment can further improve the amplification efficiency. 
In the first and second preferred embodiments as described above, the 
signal light and the pumping light are propagated in the same direction in 
the rare earth element doped fiber. However, the signal light and the 
pumping light may be propagated in the opposite directions in the rare 
earth element doped fiber. 
There will now be described a third preferred embodiment of the present 
invention with reference to FIG. 8 employing a rare earth element doped 
optical waveguide which is doped with a rare earth element such as Er. 
Referring to FIG. 8, a signal light input from the input optical fiber 4 
and a pumping light emitted from the semiconductor laser (LD) 10 are 
coupled together by the optical coupler 8. Then, the signal light and the 
pumping light thus coupled are condensed by a pair of lenses 25 and 26 to 
reach an optical waveguide 28 formed on an optical waveguide board 27. As 
shown in FIG. 9, an Er doped portion 29 is formed in the optical waveguide 
28 so as to be gradually reduced in width in the direction of propagation 
of the pumping light. The formation of the Er doped portion 29 in the 
optical waveguide 28, as controlling the width of the Er doped portion 29, 
can be carried out by a thermomigration process, for example. In this 
preferred embodiment, the pumping light input into the optical waveguide 
28 loses an energy upon excitation of Er in the optical waveguide 28 up to 
a high energy level, and a power of the pumping light is attenuated as 
propagating in the optical waveguide 28. According to this preferred 
embodiment, since the width of the Er doped portion 29 in the optical 
waveguide 28 is continuously reduced in concert with the attenuation of 
the power of the pumping light, absorption of the pumping light having an 
intensity lower than a threshold by Er in the optical waveguide 28 can be 
prevented. After being amplified in the optical waveguide 28, the 
amplified signal light is condensed by a pair of lenses 30 and 31 to reach 
the output optical fiber 6. 
FIG. 10 shows a construction of an optical amplifier employing a rare earth 
element doped optical waveguide according to a fourth preferred embodiment 
of the present invention. Referring to FIG. 10, an optical coupler 35 of a 
waveguide type is employed, so as to make a structure of the optical 
amplifier integratable. More specifically, an optical waveguide 33 for 
receiving a signal light from the input optical fiber 4 through the lenses 
25 and 26 and an optical waveguide 34 for receiving a pumping light from 
the semiconductor laser (LD) 10 are formed on an optical waveguide board 
32. The signal light and the pumping light propagated in the optical 
waveguides 33 and 34 are coupled together by the coupler 35, and they are 
then input into the Er doped optical waveguide 28 formed on the optical 
waveguide board 27. Both the optical waveguide boards 27 and 32 are bonded 
together by an optical adhesive or the like so as to align the optical 
waveguide of the coupler 35 to the waveguide 28. 
Although the third and fourth preferred embodiments shown in FIGS. 8 and 10 
have been directed to the forward pumping such that the pumping light and 
the signal light are propagated in the same direction in the rare earth 
element doped optical waveguide, the present invention may be applied to 
the backward pumping such that the pumping light and the signal light are 
propagated in opposite directions in the rare earth element doped optical 
waveguide. 
According to the present invention as described above, since the diameter 
of the rare earth element doped portion of a rare earth element doped 
fiber is gradually reduced in the direction of propagation of the pumping 
light, an optical amplifier suitable for an increase in amplification 
efficiency can be provided. Such a feature can be similarly obtained by 
using a rare earth element doped optical waveguide. 
Such an increase in amplification enables the use of a semiconductor laser 
of a relatively low power as the pumping light source. Furthermore, in the 
case wherein the power of the semiconductor laser is fixed, a length of 
the rare earth element doped fiber to be employed can be reduced owing to 
the increase in amplification efficiency, thereby providing a compact 
optical amplifier.