Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-12-07
2003-07-08
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular active medium
C372S043010
Reexamination Certificate
active
06590701
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a semiconductor optical amplifier and an optical signal processing apparatus, and more particularly to a semiconductor optical amplifier and an optical signal processing apparatus suitable for long-distance and broadband optical transmission and capable of high-speed operation.
2) Description of the Related Art
Wavelength division multiplexing (WDM) techniques have been developed recently as an optical transmission method to be used in a broadband optical network. Optical time division multiplexing (OTDM) techniques aiming at broadband optical transmission and time wavelength division multiplexing (TWDM) techniques combining WDM techniques and OTDM techniques have been proposed and are now under researches.
FIG. 1
is a conceptual diagram showing a broadband optical network. A plurality of nodes
1
are interconnected by optical channels (optical fibers)
2
. Each node
1
performs optical signal regeneration, drop, add and routing. The optical signal regeneration is generally realized by the functions of amplifying, reshaping and retiming, and is called 3R regeneration.
In a conventional optical transmission method, each node converts once an optical signal into an electric signal, performs signal processing depending upon the state of the electric signal, and reconverts the processed electric signal into an optical signal. However, a response speed of an electric signal is lower than that of an optical signal because the response time is limited by the carrier mobility and CR time constants of electronic components. For example, the speed limit of signal processing using the method of converting an optical signal into an electric signal is 10 to 40 Gb/s. In order to perform signal processing at a speed higher than 10 to 40 Gb/s, it is essential to incorporate overall optical signal processing techniques by which an OTDM signal is processed always in the form of light.
FIG. 2
is a block diagram showing the structure of an optical node at which an OTDM signal is processed in the form of light. An optical node
1
is constituted of a regeneration function block
5
, a drop function block
10
, an add function block
20
and a routing function block
25
.
An optical signal made of a plurality of time-division multiplex channels is input to an amplifier
6
of the regeneration function block
5
. The amplifier
6
amplifies the optical signal. A reshaper
7
reshapes the waveform of the amplified optical signal. A retiming unit
8
corrects the time shifts of pulses of the waveform reshaped optical signal to recover regular positions of the pulses on the time axis. A clock sampler
9
samples a clock signal from the amplified optical signal and supplies the clock signal to each unit of the optical node.
The optical signal subjected to the retiming process is input to a pulse branching unit
11
in the drop function block
10
and to an optical gate
21
in the add function block
20
. The optical signal branched by the pulse branching unit
11
is input to a header reader
12
and a delay memory
13
. The header reader
12
reads header information of the optical signal for each channel. The delay memory
13
delays the optical signal by a predetermined time. A demultiplexer
14
demultiplexes the delayed optical signal of each channel to pick up the optical signal corresponding to the channels to be dropped at the node.
The optical gate
21
in the add function block
20
sets an empty state to a time slot of each channel dropped at the node from the optical signal subjected to the retiming process. A multiplexer
22
multiplexes an optical signal to be added at the node. The multiplex optical signal is added to the empty time slot of the optical signal passing through the optical gate
21
. A header former
23
forms a header of the optical signal added at the node, and adds the header information to a predetermined time slot of the optical signal passing through the optical gate
21
.
The optical signal added with the header information is input to a wavelength converter
24
and to the routing function block
25
. The wavelength converter
24
converts the wavelength of the input optical signal, and inputs the wavelength converted optical signal to the routing function block
25
. The routing function block
25
distributes the optical signal of each channel to the next node in accordance with the routing information of the input optical signal of each channel.
Among those elements realizing the above-described functions of the optical node, it is desired to realize an element having the two functions of the optical amplifier
6
and waveform reshaper
7
(the element having the two functions is called a 2R element) and the elements having the functions of the optical demultiplexer
14
for demultiplexing the optical signal of a plurality of time-division multiplex channels and the wavelength converter
24
.
Such functions can be realized by a semiconductor optical amplifier (SOA).
FIG. 15
is a perspective view showing the outline of a conventional SOA. This SOA has the structure that an active layer
200
having an optical amplification gain is sandwiched between a p-type semiconductor layer
201
and an n-type semiconductor layer
202
. The active layer
200
is a quantum well layer or a semiconductor layer made of semiconductor having a band gap smaller than those of the semiconductor layers on both sides of the active layer
200
.
As a forward bias is applied to the active layer
200
, the carrier distribution in the active layer
200
becomes a reversed distribution state. As an optical signal
203
is incident upon the active layer
200
from one end thereof, the optical signal is amplified in the active layer
200
and outputs from the opposite end. Next, with reference to
FIGS. 16 and 17
, the operation principle of a semiconductor optical amplifier applied to an optical signal processing apparatus will be described.
FIG. 16
illustrates a semiconductor optical amplifier operating as a 2R element. A long-distance-transmitted optical signal is input to the semiconductor optical amplifier
210
. The pulse intensities of an optical signal are irregular because of various factors during transmission, such as generation of noises, external disturbance of a transmission system, and branch. The optical intensity relation between input and output signals of the semiconductor optical amplifier
210
has saturation characteristics. This is called gain saturation.
If the optical signal having irregular pulse intensities is input to the semiconductor optical amplifier
210
having such gain saturation, the optical signal is amplified and the pulse intensities of the output signal become approximately uniform. Namely, the semiconductor optical amplifier
210
has the functions of optical amplifying and waveform reshaping.
FIG. 17
illustrates an operation of a semiconductor optical amplifier as a wavelength converter. An optical signal sig
1
having a wavelength &lgr;
1
and an optical signal sig
2
having a wavelength &lgr;
2
are input to the semiconductor optical amplifier
210
. The optical signal sig
1
is an optical pulse train and the optical signal sig
2
is continuous light. The intensities of the optical signals and the amplification characteristics of the semiconductor optical amplifier
210
are adjusted so that the gain of the semiconductor optical amplifier
210
saturates when both the optical signals sig
1
and sig
2
are input.
Since the on-pulse and off-pulse of the optical signal sig
1
change the gain of the optical signal sig
2
, the intensity of the optical signal sig
2
is modulated. Therefore, the optical signal sig
2
having the wavelength &lgr;
2
output from the semiconductor optical amplifier
210
has the inverted waveform of the input optical signal sig
1
. Namely, it means that the optical signal sig
1
having the wavelength &lgr;
1
is converted into the optical signal sig
2
having the wavelength &lgr;
2
.
As described above, a semiconductor optical amplifier
Armstrong Westerman & Hattori, LLP
Hellner Mark
LandOfFree
Optical signal processing method and apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical signal processing method and apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical signal processing method and apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3017801