GB2380058A

GB2380058A - Telecommunication laser transmitter systems and methods of operating such systems

Info

Publication number: GB2380058A
Application number: GB0122748A
Authority: GB
Inventors: David Parker; David Richardson; Michael Wade
Original assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Lumentum Technology UK Ltd
Priority date: 2001-09-21
Filing date: 2001-09-21
Publication date: 2003-03-26
Also published as: GB0122748D0; WO2003026087A1

Abstract

Wavelength Division Multiplexing requires there to be transmission of light at many wavelengths. Existing systems use either many fixed wavelength lasers, each of which must therefore be made individually at low volumes and thus high cost, or tunable lasers capable of operating at many wavelengths. Tunable lasers must however be individually "characterised" in order to operate reliably at any of the possible wavelengths. This is a lengthy process and raises the cost of each laser. However, if only a small number of tunable lasers (e.g. one) from each wafer are characterised then this is sufficient to operate each of the remaining lasers from that wafer at one frequency. Thus, each inherently tunable laser can be used as, in effect, a fixed wavelength laser that has been bulk manufactured. The overall cost of this approach can be less than either previous alternative since economies of scale are achieved and individual characterisation is avoided.

Description

Telecommunication Laser Transmitter Systems and methods of operating such systems This invention relates to laser transmitters for optical communication systems and methods of operating such systems, and has particular reference to systems incorporating laser devices operating permanently at a single ITU wavelength channel. By permanent operation as used herein is meant not that the laser is permanently on, but that it operates permanently at the wavelength of only the single ITU wavelength channel whenever it is used, even if, for whatever reason, it is switched off for periods of time.

Background In this specification the term"light"will be used in the sense that it is used in optical systems to mean not just visible light but also electromagnetic radiation having a wavelength between 1000 nanometres (nm) and 3000 nm.

Single wavelength lasers are important for a number of applications in optical telecommunications and signal processing applications. These include multiple channel optical telecommunications networks using wavelength division multiplexing (WDM). Such networks can provide advanced features, such as wavelength routing, wavelength conversion, adding and dropping of channels and wavelength manipulation in much the same way as in time slot manipulation in time division multiplexed systems.

The principal optical communications wavelength bands are centred on 1300nm, 1550nm (C-Band) and 1590nm (L-Band), with the two latter bands receiving the majority of attention for commercial exploitation.

A dedicated point-to-point optical fibre communication system can use a laser source of nominally any wavelength provided it falls within the low insertion and dispersion characteristics of the optical fibre, and within the response bandwidth of the receiver. However, the majority of optical fibre communication circuits are not dedicated point-to-point connections, instead they form part of networks of optical circuits wherein the individual path connections are non-deterministic.

To increase the information carrying capability of optical fibre communication systems, wavelength division multiplex (WDM) systems as referred to above have been developed in which a number of different transmission wavelengths, often termed wavelength channels, are carried over a single optical fibre, with means provided for optical receivers to be tuned to receive a specific wavelength signal.

The 1550nm C-Band optical fibre communication band is located in the infra-red spectrum with International Telecommunication Union (ITU) 200, 100 or 50GHz channel spacing (the so-called ITU Grid) spread between 191 THz and 197THz. The operating life ITU channel wavelength stability

specification for telecommunication systems is typically set as 10pm (10 picometres). This corresponds to 1. 25GHz variation (in the C-Band), over the system operating life. Telecommunication system lasers drift due to thermal, operational drive, and materials ageing variations. To correct for laser drift wavelength locker means (sometimes referred to as wavelength monitoring and control assemblies) are used that monitor the source wavelength being produced, determine the magnitude of any wavelength error compared to the target ITU Grid wavelength channel, and produce feedback to correct the source laser wavelength.

Substantially all telecommunication lasers are Group III-V semiconductor devices. These devices rely upon spontaneous photonic emission within bulk, or epitaxially layered, materials when stimulated by electrical drive. The lasing action is introduced by use of broadband mirrors, and or wavelength deterministic Bragg gratings, defining a cavity within a waveguide that spans all device regions.

There are two different methods of providing laser transmitters for a WDM system. In one method an array of individual lasers is created, each made so as to lase at a wavelength of a single channel. Thus if eighty different channels are required then eighty different lasers have to be produced. As an alternative there has been proposed the use of a single laser which is tuneable to any one of the number of channels required, albeit one such transmitter is required for each channel to be concurrently operated.

Both methods have their own problems and costs.

There are a number of generic semiconductor laser designs capable of operating at a single wavelength, such as the Distributed Feedback (DFB) laser design and the Distributed Bragg Reflector (DBR) laser design. DFB lasers are simpler to make than DBR lasers because of their longitudinal uniformity, in which the active region that is the gain source of the spontaneous photonic emission, and the Bragg grating that controls the lasing wavelength, are longitudinally combined. In contrast, DBR lasers require longitudinal integration of the separate passive Bragg grating region with the active region that is the gain source of the spontaneous photonic emission.

The wavelength of a DFB laser is also dependent upon the current injection, which affects the waveguide material refractive index due to the electro-optic characteristic of the material whereby the refractive index reduces with increasing current flow.

The wavelength of both the DFB and DBR lasers depend on the Bragg grating period and the refractive index of the material.

For first order grating structures the relationship between the free space wavelength ko and the Bragg grating wavelength à is given by:

where neff is the effective refractive index of the material.

DFB and DBR lasers conceptually lase substantially only on one wavelength however, it is possible to tune these devices down in wavelength by a few nanometres using the electro-optic characteristic of the material whereby the refractive index reduces with increasing current flow.

To provide the 40 or 80 individual wavelength circuits within, for example, the C-Band, requires lasers that can be accurately set to specific wavelengths of those channels and maintained at these wavelengths, within the specified limits, over the equipment operating life.

In the WDM or Dense Wavelength Divisional Multiplex (DWDM) system utilizing lasers operating at the wavelength of a single channel, therefore, there exists as many separately designed and produced lasers transmitters of the DBR or DFB type each of which is operated to produce light of substantially one wavelength, said wavelength being one of the ITU Grid channels. The number of channels used is system dependent. Each of the channels used being multiplexed onto one common fibre for feeding the network.

The provision of all these differently produced wavelength telecommunication lasers is both an expensive operation and a logistical problem for component manufactures. They are faced with relatively small batch sizes for a given wavelength laser, long lead manufacturing times, high tolerances in manufacture to

mitigate poor yields, and high inventory costs in order that acceptable supply times across the wide range of wavelength channels can be provided to equipment manufactures. Similarly equipment suppliers, and end users, are faced with high inventory costs for spares provision.

The telecommunications industry has considered addressing the issue of needing too may different laser devices to form WDM and DWDM systems through the adoption of tunable lasers that can be tuned to any of the 40 or 80 channels in the telecommunications band. By this means just one laser transmitter source can be set to operate at any wavelength with the consequence that only one type need be used within the system.

Tunable telecommunications lasers are typified by a four section design using an optimised comb Bragg reflector as set out in GB 2337135B"Multi-wavelength Reflector".

A full explanation of the construction and operation of similar tunable laser diodes may be found in Chapter 7 of"Tunable Laser Diodes"by Markus-Christian Amann and Jens Buus, published by Artech House Inc. , ISBN 0-89006-963-8.

In any telecommunications applications a tunable laser will only be required to operate at one fixed wavelength at any one time.

Although the concept of the tuneable laser seems a very elegant one, there are very considerable costs involved in the production of tunable lasers. Around 75% of the costs of the tunable laser result from the cost of characterising each individual laser so that the device can be tuned by reference to supplied device data.

Typically, the device characterisation data will be used to create an electronic look-up table that controls the tuning current sources; to enable rapid automated switching between designated wavelength channels.

The invention is concerned with a telecommunication semiconductor optical laser transmitter for use in WDM or DWDM systems which reduces the problems associated with the prior art telecommunication semiconductor optical laser transmitters and which enables more economic WDM or DWDM systems to be constructed and operated.

Brief Description of the Invention By the present invention there is provided a method of operating a telecommunications multi-section semiconductor optical laser transmitter permanently at a single ITU wavelength channel which comprises the steps of

(i) simultaneously creating a plurality of nominally identical telecommunication multi-section semiconductor optical lasers each having an inherent capacity of operating at a plurality of the wavelengths of the ITU Grid wavelength channels (ii) characterizing at least one but less than all of the devices in terms of their operating characteristics and input settings for operation at the plurality of the ITU Grid wavelength channels, and determining an average characteristic for the simultaneously created lasers (iii) selecting a one of the lasers for permanent operation at a single ITU Grid wavelength channel, utilising the input settings for that wavelength as determined in step (ii) above, (iv) providing a wavelength locker for the laser operable so as to lock at the wavelength desired (v) ensuring that the selected device, in conjunction with the wavelength locker, is operable at the single ITU Grid wavelength channel and (vi) operating the device permanently at the wavelength of the single ITU Grid wavelength channel.

The nominally identical telecommunication multi section optical lasers may have the inherent capacity of operating at all of the ITU Grid wavelengths.

The present invention also provides a telecommunication optical laser transmitter operating permanently at a single

preselected ITU Grid wavelength channel, characterized in the transmitter includes a tunable laser source set to operate permanently and only at the wavelength of the single channel, electrical circuitry adapted to provide current to the tuneable components of the laser at a level such that it is capable of producing laser light at the single ITU Grid wavelength channel only, wavelength locker means incorporating wavelength feedback control being provided to ensure operation of the transmitter, in use, at the single ITU wavelength channel only.

The telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel incorporating a four section or three section tuneable laser.

The laser may be a semiconductor laser formed of a Group III-IV material.

There may be wavelength feedback control of the phase current to determine the wavelength of the laser light, or wavelength feedback control to determine the wavelength of the laser light may be by temperature control.

There may be wavelength feedback control of front tuning section, or where appropriate the rear tuning section or both.

The wavelength locker may be a Fabrey-Perot etalon, or by interferometric means such as an unbalanced Mach Zehnder interferometer.

Description of the Preferred Embodiments of the Invention The invention will now be described with reference to the accompanying drawings of which :- Figure 1. is a schematic representation of a prior art DFB laser, Figure 2. is a schematic representation of a prior art DBR laser, Figure 3. is a schematic representation of a prior art four section tunable laser, Figure 4. is an example of a typical set of wavelength tuning graphs for a four section tunable laser, Figure 5. is a schematic representation of an exemplary fixed wavelength tunable laser, Figure 6a, b, c are schematic representations of preferred embodiment modifications to a fixed wavelength tunable laser, Figure 7. is an exemplary plot of wavelength variation versus front and rear tuning currents, Figure 8. is an exemplary schematic diagram of a telecommunications laser transmitter application, and Figure 9. is a schematic representation of an exemplary fixed wavelength three section tuneable laser

Figures 1,2 and 3 illustrate the prior art lasers of the types referred to above.

In Fig 1 there is shown a Distributed Feedback laser (DFB) having an InGaAsP gain region 1 sandwiched between an n-InP layer 2, and a p-InGaAsP layer 3. A Bragg grating 4 is formed between the layer 3 and the upper layer 5 ofp-InP. The front of the laser has a cleaved facet at 6 and an anti-reflecting coating 7 is applied to the rear of the laser. Power is fed into the laser via electrodes 8 and 9.

Such a laser emits coherent light at the front as at 10 but only at a single wavelength dependant on the characteristics of the materials from which it is constructed and the pitch of the Bragg grating. Thus if 80 different wavelength lasers of this type are required 80 different structures have to be designed, created and held in stock.

An alternative type of single wavelength laser is the Distributed Bragg reflector laser (DBR) of the type shown in Fig 2.

In this case the gain section and the Bragg grating are formed in different longitudinal parts of the laser, rather that in the same part longitudinal part as is the case with the DFB laser.

The light is created by the gain region 11 and passes longitudinally in the laser in the waveguide 12, being reflected at one end by the partially reflecting facet 18 and at the other end by the Bragg grating 13 formed between the p-InGaAsP layer 14 and

the p-InP layer 15. Again there is an antireflective coating 16 at the back of the laser and coherent light of a single wavelength is emitted at the front of the laser as at 17. Power is fed into the laser via electrodes 19a and 19b.

As with the DFB laser the single wavelength of the light emitted is dependant on the materials of construction and the physical dimensions of the Bragg grating. Again if light at 80 different wavelengths is required then 80 different lasers have to be designed and manufactured.

Figure 3. shows schematically a four section Group III- V semiconductor tunable laser as referred to above. It comprises a gain section 20, phase section 22, a front tuning section 24, and rear tuning section 26. The two tuning sections 24 and 26, produce different comb tuning responses as set out in patent GB 2337135B.

The comb peaks of the tuning sections when in correspondence produce a lasing supermode. Fine tuning of the laser wavelength is achieved by current tuning the two tuning sections 24 and 26, so that the comb coincident tuning peaks equally track in wavelength.

The phase section 22 is current driven to maintain a constant optical cavity length and prevent mode hoping between supermode wavelengths. The phase section 22 may also be used to have fine control on the operational wavelength.

Each of the laser diode regions is provided with an electrode being from front to rear 34,30, 32 and 36, and there being a common electrode 38. There is a waveguide 28, which runs the

length of the laser diode. The tuning regions include Bragg gratings according to patent GB 2337135B. The rear facet of the laser has an anti-reflective (AR) coating 21 applied so that it does not contribute to forming any undesired lasing cavity. The lasing light output is taken from the front of the laser as at 37, via a cleaved or high reflectivity (HR) coating that determines a reflectivity performance although, usually a small amount of light power is also taken from the rear of the laser to feed a wavelength locking means 39, in order not to have to take any of the forward output power for this purpose.

In operation as a tunable laser the schematic device of Figure 3, has the gain region driven from a current source 40 via electrode 30. The current applied is that required to achieve, and pass through, the threshold of lasing. A typical lasing current being 200mA. The front and rear tuning sections are driven via their respective electrodes 34 and 36, from current sources 44 and 46 as shown. Typical Bragg Grating tuning currents being 50-100mA.

The front and rear tuning currents are independently set according to which supermode is required and any fine tuning offset. The current source for the phase region is set according to what current is required in the phase region to facilitate an optical cavity length in which no mode hoping of the lasing wavelength occurs.

Typically the range of phase section currents is a 1-SmA.

Figure 5 schematically shows a modification to the four section tunable laser illustrated in Figure 3. The laser device is the

same as shown in Figure 3 with common item numbers depicting equivalent purpose and functionality. The front tuning region is driven from a fixed current source 54, with a current IF ; the rear tuning region is driven from a fixed current source 56, with a current IR ; the phase region is driven from a fixed current source 52, with a current Ip, wherein the currents Ip In and lp are determined in the factory to optimally set the laser onto the required operational channel wavelength. The gain region is driven by a variable current source so that a degree of latitude is available to set the operational light power demanded by the specific host application.

Variations of the multi-sectioned tunable laser drives are given in Figures 6a, 6b, and 6c.

With reference to Figure 6a, the gain region of the device shown in Figure 3 is driven by a factory set fixed current source 60a, with current IG, for those applications where a predetermined light output power is required. With reference to Figure 6b, the gain region of the device shown in Figure 3, is driven by a factory set current source 60a, with a current IG, with, in parallel, a variable current source 60b, at IG', that can be set in the field to accommodate the typical 5-10% drop off in light output over the device life.

With reference to Figure 6c, the phase region of the device shown in Figure 3, is driven by a factory set current source 52a, with a current Ip, with, in parallel, a variable additive current

I source 52b, with current Ip', and a variable load 53 which can divert some of current Ip. Both 52b and 53 can be set in the field to accommodate the long term aging of the laser and maintain a stable accurate channel wavelength output should the wavelength locker stabiliser run out of range.

Figure 4 shows the effect on wavelength of an exemplary four section tunable laser, as each of the three tuning section currents is varied. The phase current variation provides fine control of wavelength. The front and rear current variation is much coarser, corresponding to the supermode jumps, unless both the front and rear tuning currents are harmoniously varied to track wavelength change up to the next supermode wavelength jump. A tunable laser is characterised by these tuning current relationships which have to be determined by measurement in order that the full set of operating conditions are known for each wavelength channel within the band of interest.

The characterisation of the laser wavelength with variation of the front tuning section current, IF, the rear tuning section current, IR, and phase section current, Ip, involves a process whereby two of the variables are fixed, and the third variable current is gradually stepped up (or down) by an appropriate amount, and the steady state laser wavelength measured. In this context an appropriate amount is of the order of 2% of the current range. Any electrical current change within a semiconductor laser causes a change to the power dissipated within the device. The lasing wavelength of a typical tunable laser will change O. lnm per

degree C, and therefore the steady state wavelength has to be measured within the characterisation process. In most applications a Peltier cooler, or equivalent, is used to stabilise the operating temperature of a tuneable laser.

Thus the process involves many thousands of measurements over an extended period; even with a large degree of automation this process takes around 1 hour per device and each device has to be individually characterized. One thousand to two thousand of these types of nominally identical solid state lasers devices are created at a time on a single wafer, and are then separated from one another. However, each device although nominally identical to all the others is, in fact, slightly different from all the others, which is why each one has to be individually and expensively characterised over its entire operating range. This means that each tuneable laser is expensive to produce.

Figure 7. shows an alternative graphical representation of the tuning characteristics of the exemplary four section tunable laser, in which the front and rear tuning currents are plotted against each other thereby forming bands, such as 110, of wavelength variation and in which the band wavelength such as q, continues at q2. This wavelength wrap around is replicated throughout the plot. The plot is for the situation where the phase section is not driven as it produces a much finer affect on wavelength. The bands 110 are portrayed as being straight sided but are in practice curved away from the 45 axes.

Whilst the plot is shown as bands they actually comprise chains of sub-areas 105, often known as"puddles", which enclose areas of wavelengths that can be created within a supermode. For clarity these sub-areas have been shown for only part of the plot.

The edges of the sub-areas 105 represent the points where wavelengths will be on the edge of a mode hop and thus risk instability (noise on the laser output). For best performance (most stable, least noise) a lasing wavelength needs to fall at the centre 100, of a puddle 105.

Figure 8. shows schematically the typical application for a telecommunication laser, in which the four section tunable laser 130 is mounted on a Peltier cooler 140. The output laser light is normally coupled to further optical devices via a single mode fibre optic cable. The gain section of the tunable laser 130, is current IG, driven to produce the required laser power, and the front and rear tuning sections are driven with currents IF and IR respectively to establish substantially the required operating wavelength.

From the rear facet of the laser 130, the lasing wavelength is coupled in to an air spaced Fabrey-Perot etalon 120, which has a comb response according to the wavelength channel spacing required. The other side of the etalon 120 has a photonic detector 125 which senses light passing through the etalon 120, to produce a signal indicative of when the laser wavelength is not on a required channel.

The detector 125, output is fed to an error correction means 150, which in this example corrects the error in the lasing wavelength by means of adjusting the phase current Ip, to the laser 130. The whole of the assembly of the etalon 120, detector 125 and the error correction 150 constitutes the wavelength locker shown in Figures 3 5 and 9 as 39. An example of such a locker is described in USP 5825792. One of the characteristics of such wavelength lockers is that they are not wavelength sensitive, but lock on to any wavelength in the ITU grid. In other words they are stable only at any of those wavelengths, but they are not specific as to which wavelength they actually lock onto.

In alternative arrangements the error correction means may control the laser wavelength by controlling the Peltier Cooler 140, or even the laser 130, front and rear tuning sections.

However the inventors have now discovered a way of considerably reducing the costs of designing manufacturing and stocking a set of lasers for operation at a single wavelength in the ITU grid. The invention is based on the discovery that although that multi-section tunable devices from a common wafer require individual and expensive charaterisation if they are to be capable of operation across the entire tuning range, but this is not the case, if such a laser were to be required to operate at substantially only one wavelength and that wavelength is locked in by a wavelength locker, then all of the lasers from a single wafer have sufficiently close characteristics, such that there is no need to carry out a full

characterisation but reliance may be placed upon the characterisation data from just one representative device from the same wafer.

The inventors have appreciated that the wavelength locking means can be relied upon to pull a laser into the true wavelength required.

The inventors have further discovered that the wavelength locking means may be used with a tunable laser that is to run at just one wavelength, to only require the laser tuning drives to get the device to a close approximation to the correct wavelength.

The characterisation data for the representative laser device accurately identifies the target centres 100, of the puddles 105.

The electrical test for a puddle centre is to vary the front and rear tuning currents by typically half the puddle size dependent upon where the puddle is located on the current : wavelength map of the characterised device i. e. as shown in Figure 7. The front and rear current variation being dependent upon the relative axial dimension of the puddle-rear current variation tends to be less in absolute terms than that of the front current variation. When the respective current variations are made the output light wavelength is monitored to detect any mode jump. If there is a mode jump then that wavelength must be picked up by alternative front and rear currents e. g. not ql but q2 of Figure 7. Preferably a low front

tuning current is selected as this equates to a lower output power loss.

The inventors have unexpectedly discovered that although the full characterisation of a single device from a wafer does not enable all of the devices from that wafer to be used at all wavelengths-if it did there would be no need to characterize each device from the wafer-the properties of the devices are such that there is a very significant chance that any device picked out will be able to act at any single wavelength and be held at that wavelength by the wavelength locker.

If it is considered desirable and economic, a small number of lasers from any one wafer could be characterized and their characteristics averaged to give their average parameters, depending on the accuracy of production of the wafers.

Thus instead of having to design manufacture and stock all 80 different devices for the whole range of the 80 wavelengths for an ITU Grid, a single"tuneable"laser can be built and then used at only a single wavelength by using the predetermined front and rear currents and relying on the wavelength locker to hold the wavelength at the desired value. In practice it has been found that for any given device there is a less than 100% chance that it will operate at any given wavelength, but all that is required is then to use a different, nominally identical device from the same wafer to get the desired single wavelength.

The maximum benefit of the invention may well be obtained if each of the tuneable lasers is capable of operating at each of the wavelengths of the ITU Grid wavelength channels, but the invention also contemplates that benefit could well be obtained from producing two or even more slightly different wafers with each wafer producing lasers operable at some but not all of the channels and the different wafers producing the whole range between them. In this case each tuneable laser would have an inherent capacity of acting at some, but not all, of the wavelength channels of the ITU Grid Thus by using the invention the costs of providing the individual single wavelength lasers is very considerably reduced compared to the prior art methods and the costs are also very considerably less than using fully tuneable lasers operating at a single wavelength only.

The preferred embodiment of the set wavelength telecommunication multi-section semiconductor optical laser transmitter according to the invention comprises a multi-section laser set to operate according to Figure 5, with any of the required modifications shown in Figure 6a, b, c, in an application as shown in Figure 8. To reduce production costs the laser device does not need to be fully characterised but is set to operate at the right wavelength puddle according to characterisation data from the reference device selected from the wafer.

Whilst the invention has been described in the context of a four section tunable laser the principle is applicable to any multi-section tunable laser, which relies upon characterisation data, such as a three section tunable DBR laser as shown schematically in Figure 9. wherein there is a gain region 100, phase region 102, and Bragg grating tuning region 104. The gain electrode 110 is driven from a current source 101. The phase electrode 112 is driven from a current source 103, and the tuning section is driven from a current source 105. Variations to the gain and phase current sources 101, and 103 as shown in Figures 6a, b and c may also be applied. A wavelength locker 39 is used in any embodiment according to the invention. The output light as shown at 106, is taken from the cleaved or HR coated front facet 108. The rear facet has an AR coating 109 applied. A common electrode 111 services all three current driven electrodes.

Claims

Claims 1 A method of operating a telecommunications multi-section semiconductor optical laser transmitter permanently at a single ITU wavelength channel which comprises the steps of (i) simultaneously creating a plurality of nominally identical telecommunication multi-section semiconductor optical lasers each having an inherent capacity of operating at a plurality of the wavelengths of the ITU Grid wavelength channels (ii) characterizing at least one but less than all of the devices in terms of their operating characteristics and input settings for operation at the plurality of the ITU Grid wavelength channels, and determining an average characteristic for the simultaneously created lasers, (iii) selecting a one of the lasers for permanent operation at a single ITU Grid wavelength channel, utilising the input settings for that wavelength as determined in step (ii) above, (iv) providing a wavelength locker for the laser operable so as to lock at the wavelength desired ensuring that the selected device, in conjunction with the wavelength locker, is operable at the single ITU Grid wavelength channel, and (v) operating the device permanently at the wavelength of the single ITU wavelength channel.

<Desc/Clms Page number 24>
2. A method of operating a telecommunications multi- section semiconductor optical laser transmitter permanently at a single ITU wavelength channel as claimed in claim 1 in which the plurality of nominally identical telecommunication multi-section optical lasers each have an inherent capacity of operating at all of the wavelengths of the ITU Grid wavelength channels.
3. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel, characterized in the transmitter includes a tunable laser source set to operate permanently and only at the wavelength of the single channel, electrical circuitry adapted to provide current to the tuneable components of the laser at a level such that it is capable of producing laser light at the single ITU Grid wavelength channel only, wavelength locker means incorporating wavelength feedback control being provided to ensure operation of the transmitter, in use, at the single ITU wavelength channel only.
4. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in claim 3 in which the tuneable laser is a four section laser.
5. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength

<Desc/Clms Page number 25>

channel as claimed in claim 3 in which the tuneable laser is a three section laser.
6. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in any one of claims 3 to 5 in which the laser is a semiconductor laser formed of a Group III-IV material.
7. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in any one of claims 3 to 5 in which there is wavelength feedback control of the phase current used to determine the wavelength of the laser light.
8. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in any one of claims 3 to 6 in which there is wavelength feedback control used to determine the wavelength of the laser light is a temperature control.
9. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in claim 4 in which there is wavelength feedback control of front tuning section.

<Desc/Clms Page number 26>
10. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in any one of claims 4 or 5 in which there is wavelength feedback control of rear tuning section.
11. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in claim 9 in which there is wavelength feedback control of both the front and rear tuning sections.
12. A telecommunication optical laser transmitter operating permanently at a single preselected ITU Grid wavelength channel as claimed in any one of claims 3 to 11 in which the wavelength locker is a Fabrey Perot etalon or an unbalanced Mach Zehnder interferometer.