GB2334169A

GB2334169A - Limiting wavelength division multiplex transmission wavelengths

Info

Publication number: GB2334169A
Application number: GB9900717A
Authority: GB
Inventors: Kenichi Nomura
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1998-01-14
Filing date: 1999-01-13
Publication date: 1999-08-11
Also published as: JPH11205289A

Abstract

A plurality of optical transmitters 11, 21, output a plurality of optical signals S1, S2, having particular wavelengths #1, #2. Signals having undesired wavelengths are removed from the plurality of signals, the surviving signals being multiplexed together by a coupler 30. This removal is accomplished either by the use of band pass filters (12, 22, figure 2) or by monitoring each signal 13, 15, 23, 25, and shutting it down if it deviates from an expected wavelength. In the latter embodiment, optical amplifiers 14, 24, may be used to shut down unwanted signals. In this way cross-talk between different channels is minimised.

Description

METHOD AND APPARATUS FOR OPTICAL WAVELENGTH DIVISION NULTIPLEX TRANSMISSION The present invention relates to a method of and apparatus for transmitting a plurality of optical signals, each having a particular wavelength, by an optical wavelength division multiplex system.

An optical wavelength division multiplex transmission apparatus multiplexes a plurality of optical signals of different wavelength and transmits the resulting multiplexed optical signal via a transmission path. With this type of transmission apparatus, it is possible to set a number of discrete channels on a single transmission path and multiplex optical signals without giving consideration to, e.g., synchronization in the time domain. Therefore, this kind of transmission apparatus allows a network to be constructed flexibly.

In a specific wavelength division multiplex transmission apparatus, a first optical transmitter outputs a first optical signal having a first wavelength while a second optical transmitter outputs a second optical signal having a second wavelength. The first and second optical signals are input to a coupler. The coupler multiplexes the two optical signals thereby to output an optical wavelength division multiplex signal including the two signals.

The problem with the above transmission apparatus is that, when the oscillation wavelengths of the optical signals output from the two transmitters are coincident with or extremely close to each other, due e.g., to temperature variation, cross-talk occurs between the multiplexed optical signals, or the signals cannot be separated from each other at a receiving station. The prerequisite for minimising such cross-talk is therefore that the wavelengths of optical signals to be multiplexed be sufficiently remote from each other. However, the optical signals that can be multiplexed should preferably be as many as possible in order to enhance the signal transmission efficiency. It follows that the space between nearby wavelengths should be confined in a minimum necessary range.

Even when the spacing between the wavelengths of optical signals to be transmitted is confined in a minimum necessary range, the above problem is brought about by temperature variation, if the oscillation frequencies of the optical signals are coincident with or extremely close to each other. It is therefore necessary to maintain an adequate spacing between nearby wavelengths and prevent it from varying.

In the light of the above, it has been customary to control directly the wavelength of the optical signal oscillation source of each optical transmitter (Japanese Patent Laid-Open Publication No. 8-265298), to select, e.g., an adequate semiconductor laser serving as an optical signal oscillation source, or to adjust the temperature at which the laser is operated.

However, the direct control of the oscillation source needs a sophisticated circuit. Further, it takes a certain period of time for a semiconductor implementing the oscillation source to reach a stable operation temperature after start-up. Should the oscillation frequency of the optical signal vary during the above period of time due to temperature variation, it might coincide with or become close to the wavelength of the other optical signal, again resulting in the problem stated earlier.

In addition, when the semiconductor laser fails or an error occurs in the operating temperature of the laser while the transmission apparatus is in operation, the cross-talk or the inability to separate the multiplexed signals occurs.

A feature of an optical wavelength division multiplex transmission method and apparatus to be described below, by way of example, in illustration of the invention is that it is capable, when the oscillation wavelength of an optical signal varies during start-up or during operation after start-up, of minimising the likelihood of the oscillation wavelength reaching a coupler, thereby making cross-talk and the inability to separate multiplex signals at a receiving station less likely.

A particular method to be described below, by way of example, in illustration of the invention, of transmitting a plurality of optical signals, each having a particular wavelength, by wavelength division multiplexing includes the steps of limiting the pass band of the wavelength of each of the optical signals being propagated through a particular path thereby to pass only an optical signal having the wavelength of the pass band, and multiplexing the optical signals passed through the respective pass bands.

Also, a method will be described, by way of example, of transmitting a plurality of optical signals, each having a particular wavelength, by wavelength division multiplexing them, which includes the steps of monitoring each optical signal as to wavelength deviation, and shutting down, if the wavelength of any one of the optical signals deviates from its expected wavelength, the optical signal, thereby to multiplex only optical signals each having a respective expected wavelength.

Further, in order to illustrate the present invention, there will be described an optical wavelength division multiplex transmission apparatus which includes a plurality of optical transmitters, each for outputting an optical signal having a particular wavelength. A coupler combines optical signals output from the optical transmitters, thereby to output a wavelength division multiplex signal. A plurality of optical signal transmission paths each connects one of the optical transmitters and a coupler. A plurality of optical filters is each positioned on one of the optical signal transmission paths for limiting the pass band of the wavelength of the respective optical signal, thereby to pass only an optical signal having the wavelength of the pass band.

Moreover, in one illustrative arrangement to be described an optical wavelength division multiplex transmission system includes a plurality of optical transmitters, each for outputting an optical signal having a particular wavelength. A coupler combines optical signals output from the optical transmitters, thereby to output a wavelength division multiplex signal.

Each of a plurality of monitor circuits is associated with one of the optical transmitters for monitoring the optical signal output from the optical transmitter as to wavelength deviation and outputting a shut-down request signal when the wavelength of the optical signal deviates from its expected wavelength. A plurality of shutdown circuits is each associated with one optical transmitter for outputting, when an associated one of the monitor circuits does not detect a deviation of the wavelength of the optical signal, the optical signal, and for shutting down the optical signal in response to the shutdown request signal.

A previously proposed arrangement will now be described, together with arrangements which are given by way of example in order to illustrate the invention, with reference to the accompanying drawings in which: Fig. 1 is a block schematic diagram showing a previously proposed optical wavelength division multiplex transmission apparatus, Fig. 2 is a block schematic diagram showing an optical wavelength division multiplex transmission apparatus which enables the present invention to be described, and Fig. 3 is a block schematic diagram showing another arrangement which enables the present invention to be described.

A brief reference will first be made to the previously proposed optical wavelength division multiplex transmission apparatus shown in Fig. 1. As shown in Fig.

1, an optical transmitter 51 outputs an optical signal S1 having a wavelength hl. Likewise, an optical transmitter 61 outputs an optical signal 52 having a wavelength X2.

The optical signals S1 and S2 are input to a coupler 70.

The coupler 70 multiplexes the optical signals S1 and S1 thereby to output an optical wavelength division multiplex signal SS including the optical signals S1 and S2. This configuration cannot solve the problems discussed earlier.

Referring to Fig. 2, there is shown an optical wavelength division multiplex transmission apparatus generally designated by the reference numeral 1. As shown, the transmission apparatus 1 has two optical transmitters 11 and 21, two optical band pass filters (BPFs) 12 and 22 respectively associated with the transmitters 11 and 21, and a coupler 30.

The transmitter 11 outputs an optical signal S1 having a wavelength 1 and includes a semiconductor laser. The BPF 12 is positioned on an optical signal transmission path connecting the transmitter 11 and coupler 30 and has a wavelength k1 so as to pass the optical signal S1 therethrough. The transmitter 21 outputs an optical signal S2 having a wavelength A2 and also includes a semiconductor laser. The BPF 22 is positioned on an optical signal transmission path connecting the transmitter 21 and coupler 30 and has a wavelength k2 so as to pass the optical signal S2 therethrough. The coupler 30 combines the optical signals S1 and S2 passed through the BPFs 12 and 22, respectively, and outputs the resulting optical wavelength division multiplex signal SS.

In operation, the optical signal S1 output from the optical transmitter 11 is input to the coupler 30 via the BPF 12. Assume that the oscillation wavelength of the signal S1 varies due to a temperature variation or to a fault in the transmitter 11 at the time of the start, or at the end of the operation of the transmitter 11, producing an unexpected wavelength close to, but different from, the expected wavelength k1. Then, the unexpected wavelength is filtered out by the BPF 12. As a result, only the signal S1 having the expected wavelength k1 is input to the coupler 30.

Likewise, the optical signal S2 output from the optical transmitter 21 is input to the coupler 30 via the BPF 22. Assume that the oscillation wavelength of the signal S2 varies due to the temperature variation or the fault of the transmitter 21 at the time of the start, or at the end of the operation of the transmitter 21, producing an unexpected wavelength close to, but different from, the wavelength k2. Then, the unexpected wavelength is filtered out by the BPF 22. As a result, only the signal S2 having the expected wavelength k2 is input to the coupler 30.

As stated above, when the oscillation wavelength of an optical signal output from any optical transmitter varies due to a temperature variation, or to a fault in the transmitter during operation, the wavelengths around the expected wavelength are filtered out by a BPF before reaching a coupler. This prevents optical signals with unexpected wavelengths from affecting the other wavelengths to be multiplexed.

Consequently, cross-talk between the wavelengths with unexpected frequencies and the other signals and the inability to separate the combined signals are obviated at a receiving station. This advantage is achievable with a simple, low cost circuit arrangement including BPFs.

Reference will now be made to Fig. 3 in which there is shown, an optical wavelength division multiplex transmission apparatus which includes two optical transmitters 11 and 21. The optical transmitters 11 and 21 are connected to photocouplers 13 and 23, respectively. The photocoupler 13 is connected to an optical amplifier 14 and a monitor circuit 15. likewise, the photocoupler 23 is connected to an optical amplifier 14 and a monitor circuit 25. The monitor circuits 15 and 25 are connected to a coupler 30.

The transmitter 11 outputs an optical signal S1 having a wavelength kl and includes a semiconductor laser. The photocoupler 13 delivers the optical signal S1 to the optical amplifier 14 and monitor circuit 15.

The optical amplifier 14 amplifies the optical signal S1 and delivers the resulting amplified signal to the coupler 30. In addition, on receiving a shutdown request signal from the monitor circuit 15, the amplifier 14 shuts down the optical signal S1 output from the photocoupler 13. The monitor circuit 15 monitors the input signal S1 to see if it has deviated from the wavelength Xl or not. If the signal S1 has deviated from the wavelength hl, then the monitor circuit 15 feeds the above shutdown request signal to the amplifier 14.

The transmitter 21 outputs an optical signal S2 having a wavelength 2 and also includes a semiconductor laser. The photocoupler 23 delivers the optical signal S2 to the optical amplifier 24 and monitor circuit 25.

The optical amplifier 24 amplifies the optical signal 52 and delivers the resulting amplified signal to the coupler 30. In addition, on receiving a shutdown request signal from the monitor circuit 25, the amplifier 24 shuts down the optical signal 52 output from the photocoupler 23. The monitor circuit 25 monitors the input signal S2 to see if it has deviated from the wavelength k2 or not. If the signal S2 has deviated from the wavelength k2, then the monitor circuit 25 feeds the above shutdown request signal to the amplifier 24.

The coupler 30 combines the optical signals S1 and 52 output from the optical amplifiers 14 and 24 thereby to output an optical wavelength division multiplex signal 33.

In operation, the monitor circuit 15 does not output the shutdown request signal so long as the optical signal S1 has the expected wavelength k1. Therefore, the signal S1 output from the photocoupler 13 is simply amplified by the optical amplifier 14 and then input to the coupler 30 to be combined with the other optical signal S2. When the signal S1 has a wavelength other than the wavelength k1, the monitor circuit 15 feeds the shutdown request signal to the amplifier 14. In response, the amplifier 14 shuts down the signal S1 and prevents it from reaching the coupler 30.

Likewise, the monitor circuit 25 does not output the shutdown request signal so long as the optical signal S2 has the expected wavelength k2. Therefore, the signal S2 output from the photocoupler 23 is simply amplified by the optical amplifier 24 and then input to the coupler 30 to be combined with the other optical signal S1. When the signal S2 has a wavelength other than the wavelength k2, the monitor circuit 25 feeds the shutdown request signal to the amplifier 24. In response, the amplifier 24 shuts down the signal S2 and prevents it from reaching the coupler 30.

As stated above, in the arrangement of Fig. 3, when the oscillation wavelength of an optical signal output from any optical transmitter varies due to a temperature variation or to a fault in the transmitter during operation, an optical amplifier shuts down wavelengths around the expected wavelength in response to a shutdown request signal output from a monitor circuit. This prevents optical signals with unexpected wavelengths from affecting the other wavelengths to be multiplexed, as in the arrangement of Fig. 2.

Therefore, the arrangement of Fig. 3, as in the arrangement of Fig. 2, obviates cross-talk between wavelengths with unexpected frequencies and the other signals and the inability to separate combined signals at a receiving station.

In summary, in the arrangements described, by way of example in illustration of the present invention, even when the oscillation wavelength of an optical signal output from an optical transmitter varies at the time of the start, or at the end of the operation, of the transmitter, wavelengths other than the expected wavelength are prevented from reaching a coupler. This easily and effectively obviates cross-talk between the optical signals with unexpected wavelengths and the other signals at a receiving station, and prevents the undesired optical signals from obstructing the separation of the combined signals.

It will be understood that, although particular arrangements have been described, by way of example, for use in illustrating the present invention, variations and modifications thereof, as well as other arrangements may be conceived within the scope of the appended claims.

Claims

ClAIMS

1. A method of transmitting a plurality of optical signals, each having a particular wavelength, by wavelength division multiplexing the plurality of optical signals, which method includes the steps of limiting the pass band of each of the plurality of optical signals being propagated through a particular path, thereby to pass only an optical signal having a wavelength within a respective pass band, and multiplexing optical signals passed through respective pass bands.

2. A method of transmitting a plurality of optical signals, each having a particular wavelength, by wavelength division multiplexing the plurality of optical signals, which method includes the steps of monitoring each of the plurality of optical signals as to its wavelength deviation, and shutting down, if a wavelength of any one of the plurality of optical signals deviates from an expected wavelength, the one optical signal, thereby to multiplex only the optical signals having a respective expected wavelength.

3. An optical wavelength division multiplex transmission apparatus including a plurality of optical transmitters, each for outputting an optical signal having a particular wavelength, a coupler for combining optical signals output from the plurality of optical transmitters thereby to output a wavelength division multiplex signal, a plurality of optical signal transmission paths each connecting one of the plurality of optical transmitters and the coupler, and a plurality of optical filters each being positioned in one of the plurality of optical signal transmission paths for limiting the pass band a respective optical signal thereby to pass only an optical signal having a wavelength in a respective pass band.

4. An apparatus as claimed in claim 3, wherein the plurality of optical filters each includes an optical band pass filter.

5. An optical wavelength division multiplex transmission apparatus including a plurality of optical transmitters, each for outputting an optical signal having a particular wavelength, a coupler for combining optical signals output from the plurality of optical transmitters thereby to output a wavelength division multiplex signal, a plurality of monitoring means, each monitoring means being associated with one of the plurality of optical transmitters for monitoring the optical signal output from the one optical transmitter as to wavelength deviation, and outputting a shut-down request signal when a wavelength of the optical signal deviates from an expected wavelength, and a plurality of shutdown circuit means, each shutdown circuit means being associated with a respective optical transmitter for outputting, when an associated one of the plurality of monitoring means does not detect a deviation of the wavelength of the optical signal, the optical signal, and for shutting down the optical signal in response to the shutdown request signal.

6. An apparatus as claimed in claim 5, including a plurality of photocouplers, each photocoupler being associated with a respective one of the optical transmitters for feeding the optical signal output from the one optical transmitter to an associated one of the plurality of wavelength monitoring means and an associated one of the plurality of shutdown circuit means.

7. An apparatus as claimed in claim 5, wherein the plurality of shutdown circuit means each includes an optical amplifier.

8. A method as claimed in claim 1 substantially as described herein with reference to either Fig. 2 or Fig.

3 of the accompanying drawings.

9. An apparatus as claimed in either claim 3 or claim 4 substantially as described herein with reference to either Fig. 2 or Fig. 3 of the accompanying drawings.