Reference wavelength providing device for performance...

Details Reference wavelength providing device for performance... Reference wavelength providing device for performance...

2000-05-12

2003-05-27

Pascal, Leslie (Department: 2733)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06570686

ABSTRACT:

CLAIM OF PRIORITY
This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for “Reference Wavelength Providing Device for Performance Monitor in WDM Optical Transmission System” filed in the Korean Industrial Property Office on May 12, 1999 and there duly assigned Serial No. 99-17015.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a performance monitoring device in an optical transmission system using wavelength division multiplexing (WDM) technologies, and in particular, to a device for providing a reference wavelength that can be used in the performance monitoring device.
2. Description of the Related Art
FIG. 1
illustrates a schematic view of a basic WDM optical transmission system. As shown in
FIG. 1
, the WDM optical transmission system is provided with a transmitter
100
, a transmission line unit
106
, and a receiver
110
. The transmitter
100
includes a plurality of channel transmitters from
102
1
to
102
N
and a wavelength multiplexer (MUX)
104
. The channel transmitters
102
1
to
102
N
are coupled to the SDH (Synchronous Digital Hierarchy) equipment and the ATM (Asynchronous Transfer Mode) equipment that are supported by a particular WDM transmission equipment or exchange equipment for receiving optical data therefrom. Each of the transmitters modulates an optical signal received therein at an assigned wavelength through the optical/electrical/optical conversion and feeds the converted optical signal to the wavelength MUX
104
. The wavelength MUX
104
multiplexes the modulated data at different wavelengths received from the transmitters
102
1
to
102
N
and transmits the multiplexed optical signal to the remote receiver
110
through a transmission line unit
106
.
Generally, in the transmission of optical signals at 2.5 Gbps, an optical amplifier is required for the transmission distance higher than 80 km. Thus, a plurality of optical amplifiers
108
1
to
108
N
is installed in sequence along the transmission line unit
106
. A series of EDFAs (Erbium-doped Fiber Amplifiers) are used as the optical amplifiers
108
1
to
108
N
to serve as power amplifiers, line amplifiers, and preamplifiers. The receiver
110
, which includes a wavelength demultiplexer (DEMUX)
112
and a plurality of channel receivers
114
1
to
114
N
, receives a composite light beam having component light beams of different wavelengths originated from the transmitter
100
through the transmission line unit
106
. The wavelength DEMUX
112
separates the composite input optical signal into component light beams of different wavelengths respectively and feeds the separated light beam to the channel receivers
114
1
to
114
N
. Then, the channel receivers
114
1
to
114
N
subject the received optical signal to the opto-electric conversion for further processing.
When optical signals are multiplexed, fluctuations in the wavelength of the transmitter and the wavelength characteristics of the optical wavelength multiplexer/demultiplexer result in the deterioration of receiver sensitivity. Thus, monitoring the wavelength throughout the system is an essential technique. As the spacing between the wavelengths of the WDM signal light are close to each other, it is important to maintain each signal at its wavelength to avoid crosstalk between channels. To monitor the spacing between the channels, various performance monitors are used in the transmitter, receiver, and optical amplifiers. WDM systems have rapidly developed moving to 8 and 16 (and higher) wavelength systems with the channel transmission rate of 2.5 Gbps and a transmission capacity of 20 or 40 Gbps. Thus, any possible malfunction of the component devices can deteriorate the multitude of communications services in the WDM optical transmission system. This is the reason why it is so important and necessary to monitor the various characteristics of the WDM signal light in future optical wave networks.
For example, it may be necessary to monitor (a) the total input/output optical power, (b) an optical power in each input/output channel, (c) an Optical Signal-to-Noise Ratio (OSNR) of each channel, and (d) a wavelength in each channel. The total input/output power is easily monitored through the implementation of an optical tap coupler and a photo-diode. However, to monitor the optical output, the OSNR, and the wavelength in each channel of WDM signal light, each multiplexed optical signal should be separated into component light beams of different wavelengths respectively. Hence, it is expensive to implement such a monitoring device.
The wavelength of optical signal, the OSNR, and the input light intensity of an optical amplifier are important characteristics to monitor for high accuracy. A slight variation in these values can affect the output of an optical amplifier and accelerate the gain tilt and cause a great output difference in the wavelengths.
Currently, different companies in various countries have deployed their own performance monitoring devices for monitoring the WDM optical transmission system—such as the LG and the KAIST in Korea; the Lucent, the NTT, the Queensgate, and the HP in other countries. These various performance monitoring technologies can be categorized into three types.
(1) The optical signal is demultiplexed using an optical demultiplexer for monitoring each output wavelength. In this area, the Lucent uses fiber gratings as the demultiplexer, the HP uses AWGs (Arrayed Waveguide Gratings) as the demultiplexer, and the LG uses a coupler and a fixed filter as the demultiplexer.
This first type of technologies has shortcomings in that the fiber grating, by the Lucent, still needs to improve its long-term stability and reliability. The AWGs, by the HP, are too expensive, and the use of a coupler and a fixed filter, used the LG, increases the size and cost of the monitoring device. In a further attempt to accurately detect an accurate wavelength, the HP implemented a device for providing a reference wavelength using an He—Ne laser on the market. However, the He—Ne laser, which is a gas laser, is not suitable for the transmission equipment since it has to be replaced whenever the pressure of the filled gas drops below a certain level.
(2) A pilot tone is used by the Lucent and the KAIST to monitor various characteristics of the WDM signal light. An optical transmitter operating at 2.5 Gbps or higher transmits an additional modulation signal in a voice frequency band. Then, the optical output, the OSNR, and the wavelengths at each wavelength are measured using the modulated pilot tone. The problem with the second technology is that the introduction of pilot tone signals along the transmission path can create noises at the receiving end, thus hindering the measurement of each wavelength with high accuracy.
(3) A variable optical filter is used to monitor the WDM signal light, which was disclosed by the NTT at the Optical Fiber Communication Conference (OFC) in 1998. The variable optical filter is implemented in conjunction with the sawtooth waveforms, so as to detect the variations in the optical outputs with respect to wavelengths changed with the passage of time passage are measured. Then, the optical output and the OSNR at each wavelength are measured.
FIG. 2
depicts the performance monitor
204
configured according to the third technology. As illustrated in
FIG. 2
, an input optical signal is applied to a tap coupler
200
. The tap coupler
200
extracts a portion of the optical signal so that the extraction has no influence on the transmission signal. A variable optical filter
206
filters the output of the tap coupler
200
at wavelengths limited by the sawtooth waves received from a sawtooth wave generator
208
. Accordingly, transmission occurs only where the transmission conditions for both filters line up. Thus, for the input of the sawtooth wave, as shown in (a) of
FIG. 3
, the filtering wavelength is varied according to the voltage of the sawtooth wav

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