Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-09-03
2001-12-11
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Lens
With variable magnification
C359S281000, C359S337000
Reexamination Certificate
active
06330117
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical isolator module and an optical amplifier using the same. More particularly, the present invention relates to an optical isolator module in which an optical splitter for separating an optical signal input through an input port, an optical detector for detecting the separated light, and a compensator for compensating for polarization mode dispersion are integrated into a single component along with an isolator. Additionally, the present invention relates to an optical amplifier using the optical isolator module.
BACKGROUND ART
An optical fiber which is used for optical communication shows a characteristics of a lower transmission loss in addition to its larger bandwidth compared with other transmission lines such as a copper wire, a coaxial cable, etc. Nevertheless, the transmission loss of the optical fiber can not be completely disregarded, and thus an optical signal which is transmitted should be periodically amplified in order to compensate for the attenuation of the signal. Such an amplification of the optical signal is performed by use of repeaters inserted between the fibers.
In most of optical communication systems currently being used, the repeater is constituted by a detector, an electrical amplifier and a semiconductor laser. In such a repeater, the detector transforms an attenuated optical signal into an electrical signal, the amplifier amplifies the transformed electrical signal, and the semiconductor laser is driven by the amplified signal to transmit a new optical signal to the next stage. However, the repeater has disadvantages in that it increases noise in the signal and the speed of transformations between the optical signal and the electrical signal are restricted by the bandwidth of components such as the detector and the amplifier.
Thus, a pure optical amplifier for amplifying an optical signal as itself has been developed and is being used. Furthermore, such an optical amplifier is used not only for optical communications but also for power amplification for a low-power optical source, signal splitting compensation in a cable TV network, or preamplification with respect to an optical detector.
The most dominating optical amplifier is an Erbium-doped fiber amplifier (hereinafter referred to as “EDFA”), which shows a high gain of 40 dB or above, a high output power, and a low noise figure in a band near 1.55 &mgr;m wavelength.
FIG. 1
is a block diagram of a typical EDFA, wherein
FIG. 1
a
shows a forward amplifier and
FIG. 1
b
shows a reverse amplifier.
The forward amplifier of
FIG. 1
a
includes a first lens
10
for focusing an input light emitted from a first optical fiber (not shown), an optical detector
11
for detecting the intensity of the input light, an optical splitter
12
for coupling the optical detector
11
on a transmission path, a first isolator
14
for enabling an optical signal to flow in only forward direction, a laser diode
16
for generating an optical signal for pumping, a coupler
18
for coupling the laser diode
16
on the transmission path, an Erbium-doped fiber (hereinafter referred to as “EDF”) for amplifying an input optical signal through a stimulated emission by use of photons generated by the pumping operation of the laser diode
16
, a second isolator
22
for enabling the optical signal to flow only in the forward direction, an optical detector
24
for detecting the intensity of an output light, an optical splitter
26
for coupling the optical detector
24
on the transmission path, and a second lens
28
for focusing the output light to output the focused light to a second optical fiber (not shown).
In the forward amplifier having such a configuration, the EDF
20
is formed by doping the core of an optical fiber with Erbium through a modified chemical vapor deposition (CVD) method using an source gas such as Erbium trichloride (ErCl
3
), and has an emission wavelength of 1.536 &mgr;m.
Meanwhile, the laser diode
16
generates a laser light having a wavelength of 1.48 &mgr;m or 980 nm and provides the laser light to the EDF
18
. The laser light pumps electrons of Erbium to cause a distribution inversion, so that the EDF
18
outputs a laser light having a wavelength of 1.536 &mgr;m.
Of two isolators
14
and
22
, the first isolator
14
prevents a degradation amplification efficiency which may results from the propagating of the light amplified in the EDF
20
or a spontaneously emitted light in the reverse direction. The second isolator
22
prevents the optical signal from being reflected by a connector (not shown) at an output port and so on and entering into the EDF
20
.
The reverse amplifier of
FIG. 1
b
has the same configuration as that of the forward amplifier of
FIG. 1
a
except that the pumping laser diode
17
is coupled to the rear side of the EDF
21
by the coupler
19
.
Meanwhile, U.S. Pat. No. 4,548,478 issued Oct. 22 1985 to Masakata Shirasaki and entitled “OPTICAL DEVICE” describes an optical isolator.
FIG. 2
illustrates the optical isolator disclosed by Shirasaki, which is employed in an optical amplifier. The optical amplifier, which is similar to that shown in
FIG. 1
, includes a first lens
31
for focusing an input light emitted from a first optical fiber (not shown), an optical detector
32
for detecting the intensity of the input light, an optical splitter
34
for coupling the optical detector
32
on a transmission path, an isolator
36
for enabling an optical signal to propagate only in one direction.
The optical splitter
34
, which is implemented using a prism or an optical coating, separates the optical signal received therein to output some of the optical signal to the optical detector
32
and the remaining signal to the isolator
36
.
The isolator
36
, which was disclosed by Shirasaki, consists of two tapered plates
37
and
39
which are made of birefringent materials such as rectile and calcite and a 45° Faraday Rotator
38
interposed between the tapered plates
37
and
39
.
However, the isolator
36
brings about polarization mode dispersion arising from the difference in refractive index or propagation velocity of lights. Therefore, a compensator
40
shown in
FIG. 2
is additionally included to compensate for the polarization mode dispersion, which is described in European patent application published with number of 533,398 A1 and assigned to AT&T Bell Laboratories.
Further, the conventional optical amplifier has so many components that the structure is complicated and insertion loss is large. Also, as shown in
FIG. 2
, optical fibers should be spliced in many places such as between the optical splitter
34
and the optical detector
32
, between the optical splitter
34
and the isolator
36
, between the isolator
36
and the compensator
40
, etc. Consequently, the manufacturing process is complicated whereby the unit cost of a product increases Meanwhile, since the light is incident on the optical splitter at an incident angle of 45°, a large polarization dependent loss is resulted in.
DISCLOSURE OF THE INVENTION
To solve the above-described problems, one object of the present invention is to provide an isolator module which is more reliable, reveals improved optical characteristics, and reduces the unit cost of products.
Another object of the present invention is to provide a simplified optical amplifier which employs the above isolator module so that the structure thereof and the manufacturing process are simplified.
To accomplish one of the objects above, there is provided an optical isolator module for splitting and detecting a portion of an incident optical signal while controlling light to propagate only in one direction, the isolator module comprising first focusing means for focusing the incident optical signal; an isolator core including a first birefringent device which has a tapered shape in which a first incident surface forms a first predetermined angle with a first emitting surface from which a polarized light is emitted, wherein the incident surface is coated for partial reflection
Bushnell , Esq. Robert E.
Hughes Deandra
Samsung Electronics Co,. Ltd
Tarcza Thomas H.
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