GB2402544A

GB2402544A - Electromagnetic Radiation Emission Apparatus

Info

Publication number: GB2402544A
Application number: GB0312638A
Authority: GB
Inventors: Simon Meadowcroft; David Chown
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2003-06-03
Filing date: 2003-06-03
Publication date: 2004-12-08
Also published as: GB0312638D0

Abstract

An electromagnetic radiation emission apparatus comprises a laser 114 thermally coupled to a heating device 116, for example, an electrically resistive material. The apparatus includes a temperature sensor. The heating device heats a laser in response to a temperature being below a threshold of a predetermined parameter for example, a side mode suppression ratio (SMSR), or a wavelength of the laser. A method supporting a WDM channel using the apparatus is also claimed. There is a method where a laser is selected from a plurality of lasers to support a channel of coarse wavelength division mixing CWDM. The method uses a heated laser. The use of a heating device to increase the number of laser devices usable in a CWDM system is also claimed. The CWDM system may operate over a temperature range are -40 - 85{C.

Description

ELECTROMAGNETIC RADIATION EMISSION APPARATUS

AND METHOD THEREFOR

The present invention relates to an electromagnetic radiation emission apparatus, of the type used in a communications system employing, for example, wavelength division multiplexing techniques, such as a Coarse Wavelength Division Multiplexing technique (CWDM). The present invention also relates to a method of supporting a communications channel, for example a channel of a communications system employing a wavelength division multiplexing technique, such as CWDM.

In the field of optical communications, CWDM is a multiplexing technique employed to increase a capacity of a fibre-optic link without incurring the expense associated with Dense Wavelength Division Multiplexing (DWDM).

DWDM uses channels that are spaced 0.8nm or less apart, necessitating fine control of a wavelength of light emitted by a laser device supporting a given DWDM channel. The fine control of the wavelength of the light is achieved by firstly actively controlling a temperature of the laser device in response to a temperature error signal, to achieve a first degree of accuracy, and then actively controlling the temperature of the laser device in response to a wavelength error signal. However, to control the temperature of the laser device, a control loop comprising a wavelength locker, a thermoelectric cooler and supporting electronic circuitry is required and such control loops are complex and hence costly to assemble.

In contrast, CWDM employs channels that are spaced, relatively much further apart, for example 20nm apart, obviating the need for fine wavelength control.

Without the need for fine wavelength control, the temperature of the laser device can be allowed to vary with an ambient temperature of a module within which the laser device can be disposed. Whilst the wavelength of light emitted by the laser device typically varies by 0. 1nm/ C, the 20nm channel spacing would seem sufficiently large to accommodate most ranges of operating - 2 temperatures for the module. However, CWDM requires a wavelength multiplexer, the multiplexer typically using 9nm of the 20nm channel spacing.

Further, as is known, laser devices are manufactured on wafers, each laser device on a given wafer being capable of emitting light at a respectively different wavelength (at a rated temperature) around an expected wavelength for the laser devices on the given wafer. Hence, the specification of each laser on the given wafer differs with respect to other laser devices on the given wafer.

In order to make the given wafer cost-effective, a batch of laser devices from the given wafer is usually selected about a central wavelength, tic, of interest for example tic + 3nm, at a specified temperature (the rated temperature), for example 25 C. The central wavelength, ale, is a central wavelength of a CWDM channel of interest. Consequently, from the 20nm channel spacing, a further 6nm is unavailable to take into account the range of wavelengths of light that can be emitted by laser devices in the batch of laser devices selected from the given wafer.

As a result, only 5nm remains, within the 20nm channel spacing, to accommodate variation in wavelength due to changes in ambient temperature.

As mentioned above, the wavelength of light emitted by a laser device typically varies by 0.1nm/ C, and so the remaining 5nm translates into a range of operating temperatures 50 C wide.

Increasing demands by the market for optoelectronic modules, require the optoelectronic modules to operate over a wide range of temperatures for certain applications. In this respect, 0-50 C is no longer a sufficiently suitable range to meet market requirements, -40-85 C being a preferable range of temperatures.

Approaching the problem from a slightly different direction, 9nm of the 20nm channel spacing, as mentioned above, is reserved for the wavelength multiplexer. The remaining 11 nm available to a given laser device, for example at 25 C, is distributed about a central wavelength, Ac, of a given CWDM channel as follows: - 3 -4nm C'lc c7nm (1), due to position of the range of the 9nm "occupied" by the wavelength multiplexer between adjacent CWDM channels. Taking an operating temperature range of 0-70 C for a transceiver module housing the given laser device, the operating temperature range translates into a wavelength requirement about the central wavelength, ale, at 25 C of: -2.5nm c,1c c4.5nm (2) From the indication of available wavelength from inequality (1) above, it can be seen that, about the central wavelength, ale, the following wavelength "headroom" remains: - 4nm -2.5nm c,1c c 7nm - 4.5nm => -1.5nm c,1c c 2,5nm (3) The range, or wavelength tolerance, of inequality (3) above represents the limits of wavelengths of light about the central wavelength, Tic, of a batch of laser devices that can be selected from a given wafer. Compared with the batch parameters of Ac + 3nm, the batch parameters of inequality (3) constitute a batch size of such small size as to make the manufacture of laser devices on wafers uneconomic for CWDM applications in the operating temperature range of 0- 70 . Further, for wider ranges of operating temperatures, such as -40- 85 C already mentioned above, it can be seen that insufficient wavelength headroom is available from the CWDM channel to support operation of an optoelectronic module over the whole range of operating temperatures that may be required.

Another consideration is the so-called Side Mode Suppression Ration (SMSR) optoelectronic parameter. A Distributed Feedback (DFB) laser device has a gain peak wavelength determined by properties of the material from which the laser device is constructed and a grating wavelength principally determined by - 4 the pitch of a periodic variation of a propagation constant along an active stripe of the laser device. The grating wavelength is relatively insensitive to temperature while the gain peak wavelength is more sensitive to temperature than the grating wavelength by about a factor of six. For optimal performance the detuning, the difference between the grating wavelength and the gain peak wavelength, should be held constant, or allowed to vary over a limited range.

Thus, as the grating wavelength and the gain peak wavelength have different temperature coefficients there is only a limited temperature range within which the grating and gain peak wavelengths will be separated by less than a permitted amount.

A prior solution is to limit the operating temperature range of the laser, thus also restricting the available market for the laser, or to modify the design of the laser device, thereby trading-off other important parameters of the laser device such as output power, and so also restricting the available market for the laser device. Alternatively, the lasers can be screened for acceptable performance over the full temperature range, but which increases the cost of the laser device, and hence of the transceiver module.

According to a first aspect of the present invention, there is provided an electromagnetic radiation emission apparatus comprising: a laser device thermally coupled to a heating device, and a temperature sensor responsive to a temperature associated with the laser device; wherein the heating device is arranged to heat the laser device in response to the temperature measured being below a predetermined temperature threshold, the predetermined temperature threshold corresponding to a threshold of a predetermined parameter.

The apparatus may be a WDM electromagnetic radiation emission apparatus, or the apparatus may, more particularly, be a CWDM electromagnetic radiation emission apparatus. - 5

The temperature of the laser device may be maintained independent of an influence of an ambient temperature with respect to the laser device, thereby maintaining a value of the predetermined parameter below the threshold of the predetermined parameter.

The predetermined parameter may be Side Mode Suppression Ratio (SMSR).

Alternatively, the predetermined parameter may be wavelength.

The heating device may be disposed adjacent the laser device. The heating device may comprise an electrically resistive material. Moreover, the heating device may be integrally formed with the laser device.

The heating device may be arranged to heat an active stripe of the laser device.

The heating device may be a heating element and/or the heating element may be electrically insulated from the laser device.

The temperature sensor may be arranged to measure an ambient temperature with respect to the laser device.

In one embodiment, a transmitter module for a CWDM communications system may comprise the CWDM electromagnetic radiation emission apparatus as set forth above in relation to the first aspect of the present invention.

In another embodiment, a transceiver module for a CWDM communications system may comprise the CWDM electromagnetic radiation emission apparatus.

According to a second aspect of the present invention, there is provided a method of supporting a WDM channel, the method comprising the steps of: providing a laser device; measuring a temperature associated with the laser device; heating the laser device in response to the temperature measured being below a predetermined temperature threshold, the predetermined temperature - 6 threshold corresponding to a predetermined Side Mode Suppression Ratio (SMSR) threshold.

The WDM channel supported may be a CWDM channel.

The method may further comprise the step of: maintaining a temperature of the laser device independent of an influence of an ambient temperature with respect to the laser device, thereby maintaining the SMSR of the electromagnetic radiation above the predetermined SMSR threshold.

The laser device may not be actively cooled.

According to a third aspect of the present invention, there is provided a method of manufacturing a laser apparatus, the method comprising the steps of: selecting a laser device from a wafer of a plurality of laser devices to serve as a source of electromagnetic radiation to support a channel of a CWDM I communications system, the channel having a range of acceptable wavelengths associated therewith, and the laser device having a range of temperature- dependent operating wavelengths corresponding to a range of operating temperatures of the laser device; providing a heating device thermally coupled to the laser device; wherein the laser device selected is expressly selected so that over the range of operating temperatures at least part of the range of temperature-dependent operating wavelengths does not exceed a lower limit of the range of acceptable wavelengths.

Over the range of operating temperatures at least another part of the range of temperature-dependent operating wavelengths may not exceed an upper limit of the range of acceptable wavelengths.

According to a fourth aspect of the present invention, there is provided a method of increasing a yield of usable laser devices for a CWDM communications system, the method comprising the steps of: fabricating a wafer comprising a plurality of laser devices, each of the plurality of laser devices having a respective range of temperature-dependent operating wavelengths corresponding to a range of application-specific operating temperatures of the laser device; expressly selecting a laser from the plurality of laser devices so that over the range of operating temperatures at least part of the range of temperature-dependent operating wavelengths of the selected laser device does not exceed a lower limit of a range of acceptable wavelengths corresponding to a channel of a CWDM communications system; and providing a heating device thermally coupled to the laser device selected.

According to a fifth aspect of the present invention, there is provided a use of a heating device to increase a number of laser devices from amongst a plurality of laser devices fabricated that are usable to serve as a source of electromagnetic radiation for a channel of a CWDM communications system.

The heating device may be activated in response to an SMSR of electromagnetic radiation emitted, when in use, by a given laser device from amongst the number of laser devices being below a predetermined SMSR threshold.

It is thus possible to provide a CWDM electromagnetic radiation emission apparatus capable of emitting electromagnetic radiation over a range of desired operating temperatures, for example, -40 - 85 C. The apparatus is also simpler and more economic to manufacture than DWDM laser devices with control loops. Cost of the apparatus is also further reduced by the elimination of a need to test any optoelectronic module comprising the apparatus over a lower sub-range of the range of desired operating temperatures. Additionally, the size of a batch of the apparatuses from a given wafer of laser devices that are useable over the range of desired operating temperatures is increased. The - 8 methods set forth above in accordance with the various aspects of the present invention also afford the above advantages.

At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a subassembly for a transceiver module, constituting an embodiment of the present invention; Figure 2 is a schematic plan view of a laser device constituting a modification of the subassembly of Figure 1; Figure 3 is a circuit diagram of a control circuit for use with the embodiment of Figure 1 or the modification of Figure 2; Figures 4 to 6 are schematic diagrams of possible pin connections for the subassembly of Figure 1; Figure 7 is a schematic diagram of CWDM channel separation and usage; and Figure 8 is a schematic diagram of wavelengths of electromagnetic radiation emitted by laser devices over and around a range of desired operating temperatures with respect to wavelengths corresponding to CWDM channels.

Throughout the following description, identical reference numerals will be used to identify like parts.

Referring to Figure 1, an optoelectronic subassembly comprises a CD header having a base 102 and a first pin 104, a second pin 106 and a third pin 107.

The first second and third pins 104, 106, 107 pass through the base 102, but are electrically insulated from the base 102 by insulating material 108. A further, ground, pin (not shown) obscured by the first and second pins 104, 106 in Figure 1 is electrically connected to the base 102. A photodiode 110 is mounted to the first pin 104 and a plinth 1 12 is attached, or co-formed with, the base 102.

A semiconductor laser 114 is disposed on a heater block 116, and the heater block 116 is optionally mounted on a tile 118 to electrically insulate the heater - 9 - block 116 from the plinth 112. Alternatively, the heater block 116 can be mounted adjacent the laser 114 on the tile 118. The heater block 116 is formed from an electrically resistive material. Alternatively, the heater block 116 can be formed from an electrically insulating material, such as a ceramic, with a heating element formed from an electrically resistive material deposited on a surface of the electrically insulating material. As a further alternative, the heater block 116 can be formed as a thin or thick film resistor and separated from a lower surface of the laser device 114 by a passivation layer (not shown) to electrically isolate the laser device 114 from the heater blocked 116.

In another embodiment, the heater block 116 is omitted from the subassembly of Figure 1, but the laser device 114 is provided with a heating element 200 (Figure 2). On an upper surface 202 of the laser device 114, the laser device 114 comprises a conventional metallization pattern forming a first bond pad 204 and an electrode 206, the electrode 206 overlying an active stripe 208 of the laser device 114. A second bond pad 210 and a third bond pad 212 are I electrically connected by the heating element 200, a resistive strip in this example. The resistive strip 200 is disposed sufficiently close to the active stripe 208 to heat the active stripe 208 effectively when in use, but not so close as to degrade the performance of the laser device 114, for example by increasing capacitance of the laser device 114.

Although not shown, the laser device 114, the photodiode 110 and the heater I block 116 (of Figure 1), or the resistive strip 200 (of Figure 2) are wire bonded I to the first and second and third pins 104, 106, 107 and the ground pin according to a suitable interconnection configuration.

The subassembly is also provided with a lens cap and receptacle (not shown) to t enable an optical connection, the first, second third and ground pins 104, 106, 107 being connectable to a Printed Circuit Board (PCB) (not shown) so that electrical connection can be made to the laser device 114, the photodiode 110 and the heater block 116 (or the resistive strip 200). ; - 10 Referring to Figure 3, the PCB comprises a circuit 300 for controlling the heater block 116 or resistive strip 200, denoted as a resistor 302. The resistor 302 is shown schematically within a housing 304 comprising the CD header 102, the lens cap and the receptacle (not shown). An emitter terminal of a bipolar non transistor 306 is coupled to ground 308 via the resistor 302, a collector terminal of the transistor 306 is coupled to a positive voltage supply rail 309 maintained at a voltage level of V<x volts. A base terminal of transistor 306 is coupled to an output terminal of a comparator 310, optionally via a first resistor 312 to limit the current flowing into the base terminal of the transistor 306. A first input terminal of the comparator 310 is coupled to a first terminal of a second resistor 314, a second terminal of the second resistor 314 being coupled to the voltage supply rail 309. The first input terminal of the comparator 310 is also coupled to a first terminal of a third resistor 316, a second terminal of the third resistor 316 being coupled to ground 308. A second terminal of the comparator 310 is coupled to first terminals of a fourth resistor 318 and a thermistor 320. A second terminal of the fourth resistor 318 is coupled to the voltage supply rail 309 and a second terminal of the thermistor 320 is coupled to ground 308. Although the thermistor 320 is employed in this example, it should be appreciated that any suitable bolometric component or device can be used.

The values of the second and third resistors 314, 316 are selected so as to correspond to a predetermined SMSR ratio of the laser device 114.

Referring to Figure 4, the CD header 100, the laser device 114, the photodiode 110 are interconnected as follows. A common pin 400 is coupled to a first terminal of the resistor 302, a second terminal of the resistor being coupled to an anode terminal of the laser device 114 and the cathode terminal of the photodiode 110. The first pin 104 is coupled to anode terminal of the photodiode 110, and the second pin 106 is coupled to the cathode terminal of the laser device 114. The ground pin 402 (now shown) is also coupled to the case of the subassembly as well as the second terminal of the resistor 302.

In an alternative configuration (Figure 5), the cathode terminal of the photodiode and the anode terminal of the laser device 114 are coupled to the common pin 400. The anode terminal of the photodiode 110 is coupled to the first pin 104, and the cathode terminal of the laser device 114 is coupled to the second pin 106. An additional pin 500 is provided and coupled to the first terminal of the resistor 302, the second terminal of the resistor 302 being coupled to the ground pin 402.

In a further alternative configuration (Figure 6), the additional pin 500 of Figure 5 is replaced by coupling the common pin 400 to a power rail of an integrated circuit 600 coupled to the resistor 302 to serve as the circuit 300, the integrated circuit 600 and the second terminal of the resistor 302 being coupled to the ground pin 402. Such a configuration retains the four-pin arrangement of known subassemblies, both in terms of electrical and mechanical connections and so possesses additional convenience over the configurations described above.

The manufacture of the above-described apparatus is as follows. One or more wafer of laser devices is manufactured according to any suitable manufacturing process. The laser devices are formed so that at a rated temperature, for example 25 C, the laser devices emit electromagnetic radiation at or around a central wavelength, ale, corresponding to a CWDM channel (Figure 7) to be supported by an optoelectronic subassembly carrying a laser device from the laser devices of the wafer.

Alternatively, the wafer can be fabricated so that the wavelength of electromagnetic radiation of some or all of the laser devices of the wafer are below the central wavelength, ale, of the CWDM channel to be supported.

Each laser device selected for incorporation into an optoelectronic subassembly has a range of temperature-dependent operating wavelengths corresponding to a range of operating temperatures for the laser device 114 selected. - 12

The CD header 102, the first, second and ground pins 104, 106, the insulating material 108, and the plinth 112 are formed and, where appropriate, assembled to create part of the optoelectronic subassembly using standard components and techniques known in the art. Consequently, this initial stage of construction of the optoelectronic subassembly will not be described in any further detail.

The optional insulating tile 118 can then be bonded to the plinth 112, the heater block 116 then being bonded to the insulating tile 118. Alternatively, if no insulating tile is to be used, the heater block 116 is bonded directly onto the plinth 112. The photodiode 110 is then bonded to the plinth 112 a predetermined distance away from a site on the heater block 116 where the laser device 114 is to be located and the laser device 114 is then aligned with the photodiode 110 and bonded to the heater block 116.

If the heater block 116 is to be omitted, the passivation layer (not shown) can be deposited on the plinth 112 and the laser device 114 bonded onto the passivation layer in alignment with the photodiode 110. In such an example, the laser device 114 can be formed with the heating element 200 described above using a technique disclosed in US 5,960,014.

The driving circuit 300 for the heating device 302 is constructed in accordance with known circuit building techniques and can be formed as a PCB circuit or as the integrated circuit 600.

In operation (Figure 7), a given CWDM channel has, at the rated temperature, the central wavelength, ale, associated therewith. The spacing between adjacent channels is 20nm, but due to the need for a wavelength multiplexer, 9nm of the 20nm channel spacing is required to accommodate for drifts in wavelength caused by the wavelength multiplexer. The location of the reserved 9nm between the adjacent CWDM channels is such that the remaining 11 nm of channel space to accommodate drifts in wavelengths of light emitted by the laser device 114 is not distributed symmetrically about the central wavelength, but according to the inequality (1) described previously. - 13

Referring to Figure 8, a transmitter or transceiver module containing the optoelectronic subassembly having the laser device 114 is designed to operate over a range of ambient temperatures, for example, -40 - 85 C. The laser device 114 has an operating characteristic 800 corresponding to the temperature-dependent operating wavelengths over the range of operating temperatures of the laser device 114.

Upon powering-up the module, the second and third resistors of the circuit 300 act as a voltage divider and provide a voltage set point, Vref, at the first input terminal of the comparator 310 of the circuit 300. The voltage set point, Vref, is then compared with a measured voltage, Vm, provided at the second input terminal of the comparator 310. The measured voltage, Vm, is generated by the fourth resistor 310 and the thermistor 320 behaving as another voltage divider, the measured voltage, Vm, being dictated by the resistance generated by the thermistor 320 in response to the ambient temperature of the module. The voltage set point, Vref, corresponds to a predetermined temperature that corresponds to a threshold SMSR of the laser device 114. In order to maintain the SMSR of the laser device 114 at an acceptable level, the temperature of the laser device 114 has to be above the predetermined temperature. However, at some lower ambient temperatures of the module, the temperature of the laser device 114 is below the predetermined temperature and so does not exhibit a sufficiently high performance level. Consequently, the measured voltage, Vm, will be below the voltage set point, Vref, and so a control output signal will be present at the output terminal of the comparator 310 and hence the base terminal of the transistor 306.

In response to the control signal, the transistor 306 permits current to flow from the supply rail 308, through the collector and emitter terminals of the transistor and through the heating device 302, resulting in the heating device 302 heating the laser device 114. - 14

By heating the laser device 114, the SMSR of the electromagnetic radiation emitted by the laser device 114 exceeds the threshold SMSR required, resulting in acceptable performance of the laser device 114. The temperature of the laser device 114 is therefore independent of an influence of the ambient temperature of the module, i.e. the temperature of the laser device 114 no longer follows, or is "decoupled" from, the ambient temperature of the module over a range of temperatures corresponding to the SMSR of the laser device 114 being below the threshold SMSR. Once the ambient temperature exceeds the predetermined temperature, the comparator 310 stops generating the control signal so as to cause the transistor 306 to deprive the heating device 302 of current flow, thereby causing the heating device 302 to cease heating the laser device 114.

The heating device 302 ceases heating the laser device 114, because the ambient temperature is sufficiently high to maintain the laser device 114 above the predetermined temperature necessary to ensure that the SMSR threshold is exceeded.

It should be appreciated that references to "light" herein refer to electromagnetic radiation of wavelengths between about 300nm and about 10, um, preferably between about 400nm and about Sum, and very preferably between about 300nm and about 1700nm. -

Claims

Claims: 1. An electromagnetic radiation emission apparatus comprising: a

laser device thermally coupled to a heating device, and a temperature sensor responsive to a temperature associated with the laser device; wherein the heating device is arranged to heat the laser device in response to the temperature measured being below a predetermined temperature threshold, the predetermined temperature threshold corresponding to a threshold of a predetermined parameter.
2. An apparatus as claimed in Claim 1, wherein a temperature of the laser device is maintained independent of an influence of an ambient temperature with respect to the laser device, thereby maintaining a value of the predetermined parameter below the threshold of the predetermined parameter.
3. An apparatus as claimed in Claim 1, wherein the predetermined parameter is Side Mode Suppression Ratio (SMSR).
4. An apparatus as claimed in Claim 1, wherein the predetermined parameter is wavelength.
5. An apparatus as claimed in any one of the preceding claims, wherein the heating device is disposed adjacent the laser device.
6. An apparatus as claimed in Claim 5, wherein the heating device comprises an electrically resistive material.
7. An apparatus as claimed in Claim 6, wherein the heating device is integrally formed with the laser device.
8. An apparatus as claimed in Claim 7, wherein the heating device is arranged to heat an active stripe of the laser device. - 16
9. An apparatus as claimed in any one of the preceding claims, wherein the temperature sensor is arranged to measure an ambient temperature with respect to the laser device.
10. A transmitter module for a CWDM communications system, the module comprising the CWDM electromagnetic radiation emission apparatus as claimed in any one of the preceding claims.
11. A transceiver module for a CWDM communications system, the module comprising the CWDM electromagnetic radiation emission apparatus as claimed in any one of Claims 1 to 9.
12. A method of supporting a WDM channel, the method comprising the steps of: providing a laser device; measuring a temperature associated with the laser device; heating the laser device in response to the temperature measured being below a predetermined temperature threshold, the predetermined temperature threshold corresponding to a predetermined Side Mode Suppression Ratio (SMSR) threshold.
13. A method as claimed in Claim 12, wherein the WDM channel supported is a CWDM channel.
14. A method as claimed in Claim 12 or Claim 13, further comprising the step of: maintaining a temperature of the laser device independent of an influence of an ambient temperature with respect to the laser device, thereby maintaining the SMSR of the electromagnetic radiation above the predetermined SMSR threshold.
15. A method as claimed in any one of Claims 12 to 14, wherein the laser device is not actively cooled. - 17
16. A method as claimed in any one of Claims 12 to 15, wherein the temperature sensor is arranged to measure an ambient temperature with respect to the laser device.
17. A method of manufacturing a laser apparatus, the method comprising the steps of: selecting a laser device from a wafer of a plurality of laser devices to serve as a source of electromagnetic radiation to support a channel of a CWDM communications system, the channel having a range of acceptable wavelengths associated therewith, and the laser device having a range of temperature- dependent operating wavelengths corresponding to a range of operating temperatures of the laser device; providing a heating device thermally coupled to the laser device; wherein the laser device selected is expressly selected so that over the range of operating temperatures at least part of the range of temperature-dependent operating wavelengths does not exceed a lower limit of the range of acceptable wavelengths.
18. A method as claimed in Claim 17, wherein over the range of operating temperatures at least another part of the range of temperature-dependent operating wavelengths does not exceed an upper limit of the range of acceptable wavelengths.
19. A method of increasing a yield of usable laser devices for a CWDM communications system, the method comprising the steps of: fabricating a wafer comprising a plurality of laser devices, each of the plurality of laser devices having a respective range of temperature-dependent operating wavelengths corresponding to a range of application-specific operating temperatures of the laser device; expressly selecting a laser from the plurality of laser devices so that over the range of operating temperatures at least part of the range of temperature- dependent operating wavelengths of the selected laser device does not exceed - 18 a lower limit of a range of acceptable wavelengths corresponding to a channel of a CWDM communications system; and providing a heating device thermally coupled to the laser device selected.
20. A use of a heating device to increase a number of laser devices from amongst a plurality of laser devices fabricated that are usable to serve as a source of electromagnetic radiation for a channel of a CWDM communications system.
21. A use as claimed in Claim 20, wherein the heating device is activated in response to an SMSR of electromagnetic radiation emitted, when in use, by a given laser device from amongst the number of laser devices being below a predetermined SMSR threshold.