Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-07-21
2002-09-03
Mullen, Thomas (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06445471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical communications technology, and more particularly to a technique for making transmission characteristics uniform for transmitting all optical signals with the same transmission characteristics.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) optical communications system, transmitting all optical signals with the same transmission characteristics is referred to as optimization of transmission characteristics. In the wavelength division multiplexing optical communications system, as shown in
FIG. 1
, the deterioration of transmission characteristics occurs in a transmitter, a transmission line, and a receiver. Moreover, the deterioration conditions of transmission characteristics differ between optical signals.
Furthermore, when operating a system, the deterioration conditions of transmission characteristics are different between optical signals, due to various causes, such as the repair of an amplifier or a cable, which results from repairs conducted in a transmission section, and the deterioration of a fiber due to age.
Therefore, pre-emphasis must always be set for each optical signal at the transmitting end, and transmission characteristics must always be optimized at the receiving end. Note that pre-emphasis refers to controlling the power of each optical signal.
The difference in transmission characteristics between optical signals can be observed at the receiving end, as differences in an OSNR (Optical Signal to Noise Ratio), BER (Bit Error Rate), or Q-value.
FIG. 2
shows in the prior art the relation between pre-emphasis and an OSNR after transmission.
Making an OSNR uniform at the receiving end so as to optimize transmission characteristics is a well-known technique in the prior art. In this technique, the OSNR difference between optical signals, which is calculated by monitoring the OSNR for each optical signal at the receiving end, is fed back to the transmitting end as a pre-emphasis value, thereby enabling pre-emphasis to be set at the transmitting end. Adjusting an OSNR by directly changing the output power of each optical signal is a general method for setting pre-emphasis.
As a result, if a transmit light spectrum
1
, for which no pre-emphasis is set, is transmitted on a transmission line, the OSNR of a receive light spectrum
1
, which corresponds to the transmit light spectrum
1
, varies greatly. However, if a transmit light spectrum
2
, for which pre-emphasis is set, is transmitted on a transmission line, the variance of the OSNR of a receive light spectrum
2
, which corresponds to the transmit light spectrum
2
, is reduced.
FIG. 3
shows the configuration of a transmitter of the prior art.
FIG. 4
shows the configuration of a receiver of the prior art.
First, operations of the transmitter, which has a configuration as shown in
FIG. 3
, are explained below.
The transmitter has, for each wavelength, a laser diode driver (LD DRIVER)
1201
, a laser diode (LD)
1202
, an attenuator (ATT)
1203
, a coupler (CPL)
1204
, a post amplifier (POST AMP)
1205
, and a photodiode (PD)
1208
. The laser diode driver
1201
drives the laser diode
1202
, while adjusting the output power and wavelength corresponding to each optical signal. The optical signal outputted from the laser diode
1202
is inputted to the post amplifier
1205
via the attenuator
1203
and the coupler
1204
, and the optical signal is amplified in the post amplifier
1205
. The optical signals, each of which is outputted from the post amplifier
1205
, are multiplexed by an arrayed waveguide grating (AWG)
1206
, and the multiplexed optical signal is outputted to a transmission line via a coupler
1207
.
In the configuration shown in
FIG. 3
, the coupler
1204
branches part of each optical signal to the photodiode
1208
, resulting in part of the optical signal being detected by the photodiode
1208
. The detection results are inputted to a CPU
1210
. Meanwhile, part of the transmitted optical signal, which is outputted from the AWG
1206
to a transmission line, is branched and inputted to an optical spectrum analyzer
1209
by the coupler
1207
. The optical spectrum analyzer
1209
monitors the peak power and wavelength of the transmitted optical signal, and notifies the CPU
1210
of the results. The CPU
1210
controls the laser diode driver
1201
and attenuator
1203
based on the output from the photodiode
1207
and optical spectrum analyzer
1209
, for each optical signal.
Next, operations of the receiver, which has a configuration as shown in
FIG. 4
, are explained below.
At the receiver, an optical signal received through a transmission line is inputted via a coupler
1301
to an AWG
1302
, where the optical signal is demultiplexed into optical signals of various wavelengths.
The receiver has, for each wavelength, a filter
1303
for separating an optical signal of a specific wavelength, an inline amplifier (INLINE AMP)
1304
, a dispersion compensating fiber (DCF)
1305
, an optical-electrical converter (O/E)
1306
, a forward error corrector (FEC)
1307
, and an electric-signal demultiplexer (DEMUX)
1308
.
In the configuration according to the prior art, as shown in
FIG. 4
, the coupler
1301
branches part of a received optical signal into an optical spectrum analyzer
1309
. The optical spectrum analyzer
1309
measures the OSNR for each optical signal received, and notifies a CPU
1310
of the results. The CPU
1310
feeds back the OSNR differences between optical signals received, as a pre-emphasis value, to the transmitting end by using a prescribed communications line.
However, in the transmitter, which has a configuration as shown in
FIG. 3
, the CPU
1210
receives the above-mentioned pre-emphasis value, and controls the laser diode driver
1201
for each optical signal, based on the pre-emphasis value.
As stated above, the prior art is aware of an OSNR so as to optimize transmission characteristics, and makes uniform only an OSNR used for all optical signals. Usually, the most important factor of the transmission characteristics in digital transmission is a transmission error rate. Therefore, it is important to make uniform a transmission error rate for all optical signals in the optimization of transmission characteristics. However, in the prior art, even if an OSNR is made uniform for all optical signals, the transmission error rate is not necessarily made uniform for all optical signals.
Thus, with respect to a transmission error rate, the examples of which are a BER and a Q-value, the prior art has a problem, as shown in
FIG. 5A
, in that even if the OSNR is made uniform for optical signals
1
,
2
and
3
, the transmission error rate does not become uniform because of the difference of Q-values of the optical signals.
Furthermore, in the prior art, to set pre-emphasis at the transmitting end, the CPU
1210
directly changes the output power of the laser diode
1202
by controlling the laser diode driver
1201
, for each optical signal. However, this method has a problem in that the setting of pre-emphasis for each optical signal must be repeated, while maintaining the power balance of optical signals, because the peak power of the other optical signals simultaneously change, resulting in the set value of pre-emphasis for each optical signal deviating from a proper value.
SUMMARY OF THE INVENTION
In view of the above background, the present invention aims at achieving real optimization of transmission characteristics, by making uniform a transmission error rate for all optical signals at the receiving end, based on the adjustment of an OSNR at the transmitting end.
The present invention supposes an apparatus or method for making uniform transmission characteristics in the wavelength division multiplexing optical communications system.
The apparatus according to a first aspect of the present invention has the following configuration.
First, the relation between changes in a signal-to-noise ratio a
Harasawa Shinichiro
Shimokawa Hiroyuki
Fujitsu Limited
Mullen Thomas
Staas & Halsey , LLP
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