GB2466212A - A wavelength tuneable laser at a local device is tuned using an optical feedback signal provided from a remote device. - Google Patents

Abstract

A wavelength tunable laser 118A in a slave transceiver module 104A (e.g. an optical network unit (ONU)) transmits an optical signal of a first wavelength to a master transceiver module 102 (e.g. an optical line terminal (OLT)). The master transceiver receives 110A the signal and generates therefrom an optical feedback signal 108A which it transmits to the slave transceiver. The feedback signal has a second wavelength different from the first wavelength. The slave module receives 120A the feedback signal and uses this to control the wavelength of the wavelength tuneable laser 118A. The master module may also provide a retransmission request in the feedback signal. This remote monitoring procedure obviates the provision of monitoring equipment in slave devices.

Description

Improved Optical Telecommunication Module

Field of the Invention

The present invention relates to optical telecommunication modules, particularly for use in passive optical networks.

Background of the Invention

In one-to-one and passive optical network (PON) optical telecommunication systems transmitter-receiver modules (TRM5) communicate along an optical fibre. Long distance links commonly use one-to-one systems. PONs systems are used in fibre to the home applications, in which a common optical line terminal (OLT) (i.e. a central node) communicates through an optical splitter with a number of optical network units (ONU) (e.g. individual subscriber nodes), each having a separate subscriber receiver-transmitter module.

Commonly used PONs systems operate on two optical telecommunications channels, one channel for "downstream" transmission from the OLT to the ONUs and the other for "upstream" transmission from the ONUs to the OLT. Such two channel PONs typically transmit time division multiplexed (TDM) signals in both directions.

The downstream TDM signal is transmitted to all the ON Us, with different parts of the transmission being decoded by the appropriate ONU. Upstream, bursts of transmission from the different ONUs are synchronised, with the synchronised bursts of transmission being interleaved by the optical splitter (functioning as an optical recombiner) to form a composite TDM signal. Disadvantageously, because each channel is used to carry signals to and from many ON Us, the bandwidth available for each ONU is only a fraction of the bandwidth of a channel.

Accordingly, interest has developed in wavelength division multiplexed (WDM) PONs systems, in which each ONU operates on a different channel, enabling each ONU to transmit and receive data at the bandwidth of a channel. Since the provision of different TRMs, each of which is dedicated to transmit on a different single channel, would require a high volume of inventory, "colourless" TRMs have been developed that are usable across many channels, enabling identical TRMs to be deployed across a WDM system, e.g. at ONUs across a WDM PON system.

Colourless TRMs are available that use a widely tunable laser, and they are deployed in dense wavelength division multiplexed (DWDM) networks operating in metro-and long-haul applications. Examples of the widely tunable lasers useable in colourless TRMs are illustrated in US7145923, US4896325 and US6728279.

Advantageously such lasers provide high output power from the ONU across a narrow spectral range, enabling ONUs to operate on closely spaced channels, such that a large number of colourless ONUs with widely tunable lasers could operate on different channels within the same WDN PONs system.

Disadvantageously currently available TRMs comprising widely tunable lasers are very expensive, complex to operate and have a substantial power budget, due to the need for the TRM to contain a "wavelength locker" (e.g. as described in US7161725) that samples the optical output and providing a feedback control signal to the laser.

Further the laser typically needs to be temperature controlled.

A further disadvantage of currently available TRMs comprising widely tunable lasers is that changes in ambient operating temperature, changes in operating wavelength and the effects of ageing can each lead to jumping of a laser between lasing on one longitudinal cavity mode and another. Such "mode hops" cause interruptions in the transmission of data and can lead to the lasing wavelength jumping onto another channel, thereby interrupting the data transmission of two channels. Accordingly, the design and control of widely tunable lasers in existing WDM systems is optimised to reduce the incidence of such mode hops.

Optimisation of the design of a semiconductor laser to avoid mode hops typically involves the provision of a phase control section within the optical cavity of the laser, which is used to provide fine relative tuning of the laser wavelength and cavity modes. However, disadvantageously for existing laser control schemes, lengthening of the laser cavity reduces the spacing between the longitudinal modes of the laser cavity, which increases the likelihood of mode hops.

Optimisation of the laser control scheme requires extensive post-fabrication calibration of the operational characteristics of each widely tunable laser to determine a unique control scheme, for example identifying the operating conditions corresponding with the channels and the boundary regions between cavity modes, where the risk of mode hopping is highest. Such calibration is time consuming and increases the expense operational complexity of the laser drive circuitry.

Accordingly, the colourless TRMs comprising widely tunable lasers that are currently available are unsuitable for use in WDM PONs systems.

An alternative design of colourless TRM that has been proposed for use in WDM PONs systems uses "remote optical seeding", in which a broadband light source upstream is spectrally sliced by being passed through an arrayed waveguide grating (AWG), with each output of the AWG being of a different wavelength range and used to optically pump a Fabry-Perot laser in a TRM at an ONU. The Fabry-Perot laser lases on a mode within the range of the input radiation. However, disadvantageously a remotely optically seeded Fabry-Perot based colourless TRM has a limited operating range, due to the attenuation of the light occurring both from the broadband light source downstream to the ONU, and from the ONU to the OLT.

A further alternative design of colourless TRM that has been proposed for use in WDM PONs systems uses a reflective semiconductor optical amplifier (RSOA) within the TRM at an ONU, in which a filtered downstream continuous wave optical input is reflected, amplified and modulated by the ONU. Disadvantageously the optical input signal requires to have a substantial spectral width, which limits the transmission range of the optical output of the ONU to the OLT, and the number of RSOAs that can be used within a single WDM PONs system. Other impairments due to back scattered light are also very significant.

Thus a need remains in the industry for an alternative design of optical telecommunication module and an improved method of operating telecommunication modules.

Summary of the Invention

It is an object of the present invention to provide an improved optical telecommunication module and an improved method of operating optical telecommunication modules that seek to overcome at least some of the disadvantages described above.

A first aspect of the present invention provides an optical telecommunication slave transmitter-receiver module (slave TRM) comprising a slave transmitter and slave receiver, the slave TRM being adapted to control the transmission of a wavelength tunable slave laser of the slave transmitter at a first wavelength in correspondence with an optical feedback signal at a second wavelength that is received from a master TRM.

A second aspect of the present invention provides an optical telecommunication master transmitter-receiver module (master TRM) comprising a master transmitter and master receiver, the master TRM being adapted to transmit an optical feedback signal at a second wavelength in correspondence with an optical transmission at a first wavelength that is received from a slave TRM.

A third aspect of the present invention provides an optical telecommunication system having an optical telecommunication slave transmitter-receiver module (slave TRM) and an optical telecommunication master transmitter-receiver module (master TRM), the slave TRM being adapted to control the transmission of a wavelength tunable slave laser of the slave transmitter at a first wavelength in correspondence with an optical feedback signal at a second wavelength that is received from the master TRM, and the master TRM being adapted to transmit the optical feedback signal at the second wavelength in correspondence with an optical transmission at the first wavelength that is received from the slave TRM.

A fourth aspect of the present invention provides a method of tuning the lasing wavelength of a slave laser of an optical telecommunication slave transmitter-receiver module (slave TRM) comprising the steps of transmitting an optical signal at a first wavelength from a slave laser of the slave TRM to an optical telecommunication master transmitter-receiver module (master TRM), and transmitting an optical feedback signal from the master TRM to the slave TRM in correspondence with the optical signal.

The optical feedback signal may comprise an instruction to tune the wavelength of the slave transmitter.

The optical feedback signal may comprise an instruction to re-transmit an optical data signal from the slave transmitter.

A wavelength sensitive optical splitter may be deployed between the slave module and the master module. The wavelength sensitive optical splitter may be an arrayed waveguide grating (AWG). The arrayed waveguide may be a cyclic arrayed waveguide grating.

The optical telecommunication system may have a plurality of slave TRMs adapted to operate in a wavelength division multiplexed arrangement.

Brief Descrirtion of the Drawings For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawing, which is a schematic illustration of an optical telecommunication system having master and slave optical telecommunication systems transmitter-receiver modules.

Detailed Description of Preferred Embodiments

Figure 1 of the accompanying drawings, illustrates an optical telecommunication system 100 comprising a master optical telecommunication transmitter-receiver module (TRM) 102 in optical communication with slave TRMs 104A, and 104B through system optical splitter 106. The master TRM 102 has master transmitters 108A and 108B, master receivers 11OA and 11OB, a master optical splitter 112, a transmission wavelength monitor 114, and a master module controller 116. The slave TRMs 104A and 104B have respective slave transmitters 118A and 118B and slave receivers 120A and 120B optically coupled to the system optical splitter 106 through diplexers 122A and 122B, and controlled by slave module controllers 124A and 124B. The master and slave transmitters have master and slave lasers respectively. The lasers are widely wavelength tunable lasers. The master and slave receivers have master and slave photodetectors respectively.

The master module controller 116 feeds electrical data signals to the master transmitters 108A and 108B, generating downstream optical data signals that are transmitted to the slave TRMs 104A and 104B through the master optical splitter, the transmission wavelength monitor 114, common optical fibre 126, the system optical splitter 106, and subscriber optical fibres 128A and 128B.

The master transmitters 108A and 108B emit light at different wavelengths corresponding with different telecommunications channels, and the light is optically multiplexed by the master optical splitter 112, which is for example an arrayed waveguide grating (AWG).

The transmission wavelength monitor 114 monitors the downstream optical data signal transmitted by the master TRM 102, generating a feedback signal to the master module controller 116, in response to which the master module controller adjusts the electrical drive control to the master transmitters 108A and 108B to make any necessary adjustments to the lasing wavelength of the master lasers.

The downstream optical data signal received by the system optical splitter 106 is composite, having data signals carried on different channels intended for corresponding different slave TRMs 104A and 104B. The optical splitter 106 is an arrayed waveguide grating (AWG) that separates the different channels, on the basis of wavelength, to different outputs, which are coupled to corresponding subscriber optical fibres 128A and 128B, and onwards to the slave TRMs 104A and 104B. The downstream optical data signal received at each TRM 104A and 104B passes through a respective diplexer 122A and 122B and to a respective slave receiver 120A and 120B, from each of which a corresponding downstream electrical data signal is fed to the respective slave module controllers 124A and 124B.

Each slave module controller 124A and 124B feeds an upstream electrical data signal to each respective slave transmitter 11 8A and 11 8B, which transmit upstream optical data signals that pass through the diplexers 122A and 122B to the system optical splitter 106. From the system optical splitter 106, the composite upstream optical data signal is transmitted along the common optical fibre 126, through the transmission wavelength monitor 114 to the master optical splitter 112. The master optical splitter 112 demultiplexes the composite upstream optical data signal such that different channels are received by different master receivers 11OA and 11OB, with corresponding upstream electrical data signals being fed to the master module controller 116.

In contrast to the master TRM 102, the slave modules 104A and 104B do not have internal wavelength monitors to monitor the emitted wavelength of the slave lasers 118A and 118B. Rather, the upstream optical data signal are monitored by the master TRM 102, which provides a downstream optical feedback signal to the corresponding slave TRM 104A or 104B in the downstream optical signal. The downstream optical feedback signal is detected by the corresponding slave TRM 104A or 104B, which responds in correspondence with that signal.

If the master TRM detects that a slave TRM is transmitting at the wrong wavelength or cannot see any signal from the slave TRM it transmits a downstream optical feedback signal comprising a wavelength tuning instruction, and the slave TRM retunes the slave laser in correspondence with that signal. For example, the slave TRM may scan the lasing wavelength of the slave laser in a continuous (or quasi-continuous) manner, in response to a downstream optical feedback signal from the master TRM. The slave laser then continues to tune the lasing wavelength of the slave laser until the downstream optical data signal from the master TRM indicates that the correct lasing wavelength has been reached, for example by ceasing to transmit a wavelength tuning instruction or by transmitting a stop tuning instruction, such that the slave TRM maintains the wavelength of the slave laser. Similarly when a new slave TRM is inserted into an optical telecommunications system, it can run through a start-up procedure comprising lasing wavelength scanning until optical data transmission from the master TRM indicates that the correct lasing wavelength has been reached. Initiation of the start-up procedure by the slave TRM can be autonomous and commence in the absence of an instruction from the master TRM.

The start-up procedure of the slave TRM can be controlled by a built-in firmware algorithm. In this way the slave laser can be set and maintained at the correct wavelength without the need for a wavelength monitor internal to the slave TRM.

If the master TRM has not received part of an upstream optical data signal, for example due to the occurrence of a mode hop or transmission at an incorrect wavelength, the master TRM transmits a downstream optical feedback signal comprising a re-transmit instruction, and the slave TRM re-transmits the instructed part of the upstream optical data signal. In this way the optical telecommunication system is robust against interruptions in transmission.

To facilitate re-transmission of upstream data, recently transmitted upstream data signals may be buffered in the memory of a slave TRM or an external control system for a slave TRM.

Although the example of Figure 1 illustrates the master TRM communicating with two slave TRMs, it will be appreciated that a master TRM can operate on a one-to-one basis with just a single slave TRM or with a larger number of slave TRMs. A master transmitter can be provided for each slave TRM, or a master transmitter may be used sequentially to transmit signals on different downstream channels.

The downstream and upstream optical data signals to and from each slave TRM are carried on different telecommunications channels. The system optical splitter may be a cyclic AWG and be operated with suitable choices of downstream and upstream channels such that the downstream and upstream optical data signals to a slave TRM both couple between the common input port and the same output port of the splitter.

The downstream channels may be in a different optical telecommunications band from the upstream channels, e.g. one direction being in the C-band and the other in the L-band. In a WDM PONs system as described all of the upstream channels can be in one optical telecommunications band, and all of the downstream channels can be in another band.

To increase the number of slave TRMs connectable to each single WDM optical telecommunications system, the wavelength division multiplexed optical data signals carried on each channel may additionally be time division multiplexed (TDM).

Accordingly between the system optical splitter and the slave transmitters and receivers further optical splitters are provided. The further optical splitters split a TDM channel between different receivers, which each decode a corresponding part of the data signal of the channel. Similarly the slave lasers transmit synchronised upstream data signals that are interleaved by the further optical splitter to form a composite TDM signal carried by a single optical channel, which can then be wavelength division multiplexed with other channels. In this way by combining both TDM and WDM, the number of OLTs connectable to a PONs system can be increased compared with existing PONs systems, whilst enabling a useful bandwidth of data transmission both upstream and downstream to each ONU.

Master and slave lasers within the respective transmitters can be directly modulated by an electrical data signal. However, it will also be appreciated that the lasers can be integrated with optical modulators. The optical modulators may be monolithically integrated with the semiconductor laser chip, or may be external modulators optically coupled with the laser chip.

The master and slave lasers can be widely tunable lasers, for example which can wavelength tune across many optical telecommunications channels. Alternatively, the lasers can be narrowly tunable lasers, which can wavelength tune across only one or only a few optical telecommunications channels.

The transmission wavelength monitor may be a wavelength locker. Alternatively it may comprise another wavelength discriminating element, such as an AWG.

The monitoring of the upstream optical transmission wavelengths from the slave transmitters may be monitored by the transmission wavelength monitor and/or the master photodetectors, which provides feedback to the master module controller.

Reducing the susceptibility of a telecommunication system to mode hops by provision of a re-transmission protocol in the master and slave module controllers can permit a considerable relaxation of the manufacturing and operational tolerances of the lasers in the respective transmitters, compared with corresponding existing systems. This can permit the use of a semiconductor laser without a phase control section. Similarly this can permit the use of a slave TRM with lower requirements for temperature stability of the slave laser, which can considerably reduce the cost, manufacturing complexity and operational power consumption and size of the transmitter within the slave TRM. For example, where existing modules typically have a thermoelectric cooler (TEC, also known as a Peltier cooler) to which the laser chip is attached, within the laser package, which seeks to maintain the temperature of the laser chip at a constant level, a more relaxed operating regime can permit the use of a TEC that is external to the laser package, improving thermal dissipation from the TEC and reducing power consumption, or can even enable the operation of a laser without a TEC. Further, a re-transmission protocol permits the use of laser packaging that is less resistant to mechanical deformation and thermal expansion, which also can cause mode hopping of semiconductor lasers, enabling the use of less expensive packaging materials than are currently used.

Hitherto, in existing transmitters the tuning of the laser within regions of hysteresis has been avoided due to the proximity of such regions to operating conditions that cause mode hops and the ambiguity of the operating wavelength within the hysteresis regions, being dependent upon the preceding operating history. However, provision of a re-transmit protocol enables the use of these regions, considerably simplifying the laser operation, removing the need for detailed post-fabrication characterisation, and considerably reducing the associated costs. Further, a significant hysteresis with respect to wavelength tuning can improve stability of a laser against repeated mode hops when operated close to a mode boundary, by reducing the likelihood of the laser hopping repeatedly between two adjacent modes.

In the case of a mode hop, a slave laser may jump to a different lasing wavelength, which risks interrupting other communication channels being transmitted along the same optical fibre. Accordingly, advantageously, the slave module or the optical splitter may be provided with a wavelength sensitive filtering element to filter out such transmissions. The filtering element may be active or passive. Advantageously an arrayed waveguide grating may be used as a passive filtering element.

As with commonly used protocols for PONs the receivers will typically operate in a burst mode' wherein the receiver quickly resynchronises to extract the clock signal in the data. This will operate advantageously should a mode hop occur, as this could cause a phase jump in the signals The optical re-imaging of light in an AWG is a function both of the wavelength of the light and the port to which it is input. Consequently, in the case that the system optical splitter is an AWG, the wavelength of the upstream and downstream channels are determined by the ports to which the slave TRMs are optically coupled. So, in the case that the system optical splitter is an AWG, if a slave laser drifts off from the channel wavelength the optical coupling to the master TRM through the AWG will be attenuated, enabling the master TRM to sense the wavelength drift and to send an optical feedback instruction to the corresponding slave TRM, as well as preventing interference of the drifted signal with other upstream channels.

The lasing wavelength of a slave laser can be controlled such that a wavelength is chosen at which the intensity of the upstream signal received at the master TRM is above a threshold level, and if it falls beneath that threshold the master TRM sends an optical feedback instruction to the slave TRM to retune the slave laser.

Advantageously the facility of remote optical tuning of a slave laser by a master TRM and the re-transmit protocol enable lasers to be deployed in ONUs with only a minimal post-fabrication performance inspection, with the expectation that they can be quickly tuned to the required operating wavelength once coupled into an optical telecommunication system. Avoiding lengthy post-fabrication burn-in and calibration can substantially reduce the manufacturing cost of TRMs.

Claims (16)

1. Claims 1. An optical telecommunication slave transmitter-receiver module (slave TRM) comprising a slave transmitter and slave receiver, the slave TRM being adapted to control the transmission of a wavelength tunable slave laser of the slave transmitter at a first wavelength in correspondence with an optical feedback signal at a second wavelength that is received from a master TRM.
2. 2. The slave TRM according to claim 1, wherein the optical feedback signal comprises an instruction to tune the wavelength of the slave transmitter.
3. 3. The slave TRM according to one of claims 1 and 2, wherein the optical feedback signal comprises an instruction to re-transmit an optical data signal from the slave transmitter.
4. 4. An optical telecommunication master transmitter-receiver module (master TRM) comprising a master transmitter and master receiver, the master TRM being adapted to transmit an optical feedback signal at a second wavelength in correspondence with an optical transmission at a first wavelength that is received from a slave TRM.
5. 5. The master TRM according to claim 4, wherein the optical feedback signal comprises an instruction to tune the wavelength of the slave transmitter.
6. 6. The master TRM according to one of claims 4 and 5, wherein the optical feedback signal comprises an instruction to re-transmit an optical data signal from the slave transmitter.
7. 7. An optical telecommunication system having an optical telecommunication slave transmitter-receiver module (slave TRM) and an optical telecommunication master transmitter-receiver module (master TRM), the slave TRM being adapted to control the transmission of a wavelength tunable slave laser of the slave transmitter at a first wavelength in correspondence with an optical feedback signal at a second wavelength that is received from the master TRM, and the master TRM being adapted to transmit the optical feedback signal at the second wavelength in correspondence with an optical transmission at the first wavelength that is received from the slave TRM.
8. 8. An optical telecommunication system according to claim 7, wherein a wavelength sensitive optical splitter is deployed between the slave module and the master module.
9. 9. An optical telecommunication system according to claim 8, wherein the wavelength sensitive optical splitter is an arrayed waveguide grating.
10. 10. An optical telecommunication system according to any of claims 7, 8 and 9, wherein optical telecommunication system has a plurality of slave TRMs adapted to operate in a wavelength division multiplexed arrangement.
11. 11. A method of tuning the lasing wavelength of a slave laser of an optical telecommunication slave transmitter-receiver module (slave TRM) comprising the steps of transmitting an optical signal at a first wavelength from a slave laser of the slave TRM to an optical telecommunication master transmitter-receiver module (master TRM), and transmitting an optical feedback signal from the master TRM to the slave TRM in correspondence with the optical signal.
12. 12. The method of claim 11 wherein the optical feedback signal comprises an instruction to tune the wavelength of the slave transmitter.
13. 13. The method of one of claims 11 and 12 wherein the optical feedback signal comprises an instruction to re-transmit an optical data signal from the slave transmitter.
14. 14. An optical telecommunication slave transmitter-receiver module substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.
15. 15. An optical telecommunication master transmitter-receiver module substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.
16. 16. An optical telecommunication system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.