Coherent light generators – Optical isolater
Reexamination Certificate
2002-06-28
2004-11-23
Harvey, Minsun Oh (Department: 2828)
Coherent light generators
Optical isolater
C372S034000
Reexamination Certificate
active
06822997
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser module in which a semiconductor laser chip and an optical fiber are optically coupled. In particular, the present invention relates to a semiconductor laser module having an optical coupling mechanism suppressing light that is reflected back from the optical fiber.
2. Description of Related Art
In the course of the development of optical fiber communication networks, optical communication systems with high quality over long distances have been put into practice. In particular in multi-channel CATV video transmissions, optical communication has become necessary and indispensable.
FIG. 6
is a diagram of a conventional optical communication system. This conventional optical communication system is a send-only system, in which video information is distributed from a broadcasting station (referred to as “HE” (head end) in the following)
21
on the sending end to households
23
on the receiving end. Up to the branching points (referred to as “nodes” in the following)
22
, the video information signals distributed from the HE
21
are optical signals, transmitted over an optical fiber, and from the nodes
22
to the households
23
, they are electrical signals, transmitted over a coaxial cable. A DFB (distributed feedback) laser is used as the semiconductor laser for transmitting the signals in the optical fiber. DFB lasers emit at a single wavelength, so that noise is low, and there is little signal degradation even for long-distance fiber transmissions.
FIG. 7
shows the structure of a conventional semiconductor laser module. The following is an explanation of a conventional semiconductor laser module, with reference to FIG.
7
. This semiconductor laser module uses a DFB laser. Inside a package
1
, a DFB laser chip
2
outputting DFB laser light, a photo diode
3
monitoring the output from the DFB laser chip
2
, an aspheric lens
6
condensing the laser light that is output from the DFB laser chip
2
in order to couple it into the optical fiber
5
, an optical isolator
7
for attenuating the light that is reflected back from the optical fiber
5
, and a thermistor
8
for detecting the temperature of the DFB laser chip
2
and the optical isolator
7
are arranged on a metal base
9
, which is placed on a Peltier cooler
10
.
Here, the optical isolator
7
lets laser light pass from the laser chip
2
to the outside of the package
1
, but attenuates laser light coming in the other direction from the outside of the package
1
. The Peltier cooler
10
, made of a Peltier element, controls the temperature of the components on the base
9
to a certain temperature via the base
9
. The characteristics of the DFB laser chip
2
and the optical isolator
7
change with the temperature, so they are controlled to the optimum temperature. Furthermore, the temperature is detected by the thermistor
8
.
The package
1
is provided with a window
4
through which laser light is emitted to the outside. Moreover, a metal lid
14
is attached to the outer side of the window
4
of the package
1
. The package
1
is filled with inert gas, and a cap
11
is welded to the package
1
, such that the inert gas does not leak out.
The optical fiber
5
is held by a ferrule
12
, which is a stainless steel tube. After the optical fiber
5
has been centered through the ferrule holder
13
such that the laser light emitted through the window
4
is coupled with high efficiency, the optical fiber
5
is held in place by fastening by YAG welding the ferrule
12
to the ferrule holder
13
, and the ferrule holder
13
to the metal lid
14
of the package
1
.
There are two transmission methods for transmitting DFB laser light over an optical fiber. One is direct modulation, in which the DFB laser is directly modulated, and the other is external modulation, in which the DFB laser is driven by a constant current, and modulated by an external modulator optically connected to the after emitting the laser light from the DFB laser module. Differing from direct modulation, there is no laser chirp with external modulation, so that long-distance transmissions of higher quality become possible.
When using direct modulation, a single mode fiber is used for the optical fiber
5
. When using external modulation, a polarization maintaining fiber is used, which can transmit laser light while maintaining its polarization, in order to increase the optical coupling efficiency with the external modulator.
In the semiconductor laser module with the above configuration, the optical isolator
7
is temperature-controlled in the same manner as the DFB laser chip
2
, so that a stable optical isolation ratio can be obtained. Therefore, light reflected back from the optical fiber is reliably attenuated by the optical isolator, and noise due to reflected backlight hardly occurs. It should be noted that the above-described semiconductor laser module is disclosed in JP H3-178181A.
However, in the last couple of years, bi-directional optical communication systems, which can be used for the internet or the like, have been required more than send-only systems.
FIG. 5
is a diagram of a bi-directional optical communication system using direct modulation. It should be noted that
FIG. 5
depicts only the HE
21
serving as the first sending/receiving device and the nodes
22
serving as the second sending/receiving devices, and the households following the nodes
22
have been omitted. As in the system of
FIG. 6
, also in the system of
FIG. 5
, the HE
21
and the nodes
22
are linked by an optical transmission path such as an optical fiber. The aspect where the system in
FIG. 5
differs from the system of
FIG. 6
is that bi-directional communication (i.e. sending and receiving) between the HE
21
and the nodes
22
is possible.
The signals from a sending portion
24
of the HE
21
are converted from electric signals to optical signals by a DFB laser module
26
a
of the HE
21
, and sent to the receiving portions
27
b
of the nodes
22
, where they are converted back from optical signals to electric signals. Furthermore, the optical signals sent from the DFB laser modules
26
b
of the nodes
22
are converted back into electric signals by the light-receiving portions
27
a
of the HE
21
, and sent to the receiving portion
25
.
When the DFB laser modules
26
b
of the nodes
22
do not send a signal, they are simply in a DC driven (unmodulated) state, and the light reflected back from the optical fiber causes noise in the low-frequency region.
FIG. 8
shows the low-frequency noise for a bi-directional optical communication system using an unmodulated DFB laser module. In
FIG. 8
, it can be seen that spike-shaped noise occurs in the low-frequency region. The HE
21
converts the optical signals from the nodes
22
summarily into electric signals and sends them to the receiving portion
25
, so that when signals at frequencies or near the frequencies at which this noise occurs are sent by other nodes
22
, those signals are affected by this noise. That is to say, the transmission characteristics of the video or data are deteriorated.
In order to prevent the deterioration of the transmission characteristics, it is necessary to reduce the reflected backlight more than in conventional semiconductor modules. This is achieved mainly by the following two methods.
A first method is to improve the resistance against reflected backlight by always applying a modulation signal of a different signal frequency than that usually used in the upstream laser. However, with this method, the design of the electric circuit of the entire system needs to be improved.
A second method is to use laser modules that provide sufficient optical isolation against light returning from the optical fiber. This method can be accomplished without modifying the system itself, and exchanging only the DFB laser modules.
Consequently, it is easier to employ the second method as a method to prevent the deterioration of transmission characteristics
Fujihara Kiyoshi
Ohya Jun
Harvey Minsun Oh
Merchant & Gould P.C.
Nguyen Phillip
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