GB2406210A

GB2406210A - control apparatus and method for an electromagnetic radiation device

Info

Publication number: GB2406210A
Application number: GB0321646A
Authority: GB
Inventors: Andrew Thomas Harker; Sonia Ashmore; Jeremey Crouch
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2003-09-16
Filing date: 2003-09-16
Publication date: 2005-03-23
Also published as: GB0321646D0

Abstract

Apparatus for, or a method of controlling an electromagnetic radiation device for example, a laser diode, comprises a sensor 110, and a controller 108 coupled to a memory 112. The controller 108 is arranged to set an operating current. The apparatus is characterised by the memory 112 having two performance models. Temperature data corresponding to a temperature of the device constitutes a first parameter of each one of the performance models. The operating current may be adjusted using an ageing factor which may be calculated or stored in a look up table.

Description

240621 0

CONTROL APPARATUS AND METHOD THEREFOR

The present invention relates to a control apparatus of the type, for example, for controlling an output power of electromagnetic radiation emitted by a device, such as a laser device. The present invention also relates to a method of controlling the output power of the electromagnetic radiation emitted by the device.

In an optical communications system, a plurality of optoelectronic modules is employed, each optoelectronic module being capable of emitting electromagnetic radiation having a respective output power. One example of a known optoelectronic module typically comprises a subassembly having a so-called "CD header" including a disc-like base for supporting a laser diode.

The laser diode is arranged on a heatsink, the heatsink being mounted on a plinth co-formed with the base. A number of pins pass through the base in an electrically insulated manner and provide electrical connectivity to devices provided on the base. A photodiode is mounted to a tab formed on one of the pins.

It is, of course, necessary to maintain the performance of each optoelectronic module. One parameter relating to the performance of an optoelectronic module is the output power of electromagnetic radiation emiffed by the optoelectronic module. The output power of the optoelectronic module varies over the life and the operating temperature range of the optoelectronic module as the output power of the laser diode, incorporated into the optoelectronic module, varies over the life and temperature of the laser diode. Consequently, by controlling the output power of the laser diode so the output power remains constant throughout the life and over an operating temperature range of the optoelectronic module, it is possible to maintain the performance of the optoelectronic module.

One known way of maintaining the output power of a given optoelectronic module throughout the life and over the operating temperature range of the - 2 optoelectronic module is to co-package the laser diode with a back facet monitor, such as a photodiode, as described above, coupled to a so-called "constant power feedback loop".

In use, the photodiode receives light from a back facet of the laser diode and generates a photocurrent dependent on the intensity of the light incident on the photodiode. The photocurrent is used as an input to the feedback loop, the feedback loop adjusting an operating current of the laser diode to maintain the photocurrent at a predetermined value.

However, the operating current of the laser diode has two components: a bias current and a modulation current. To ensure optimum operation of the optoelectronic module, both the bias and modulation currents must be optimised independently to maintain an extinction ratio of the laser diode, and the simple feedback loop described above cannot perform this function as it only has one input parameter so that, for example, only the bias current can be controlled. Also, the above design is relatively expensive to manufacture and does not lend itself well to fabrication by mass-manufacturing processes; the monitor photodiode, being an optoelectronic component, is not inexpensive and is susceptible to failure over a period of time. Furthermore, the formation of the tab of the CD-header on one of the pins and the process of attaching, die bonding and wire bonding, associated with disposing the photodiode on the tab adds further cost and complexity to the sub-assembly.

Also, in order to couple the optoelectronic module to an optical fibre, the subassembly further comprises a receptacle, the receptacle being provided to enable electromagnetic radiation emitted by the laser diode to be received, for example, by a connector ferrule inserted into the receptacle. In order to facilitate launch of the electromagnetic radiation into the connector ferrule, the subassembly is also provided with a lens cap disposed between the receptacle and the laser diode. - 3

However, the output power of the optoelectronic module over temperature is typically also affected by mechanical misalignment, due principally to different coefficients of thermal expansion of the materials forming parts of the subassembly, for example, the lens cap and the receptacle.

Consequently, a variation in an amount of the light emitted by the laser diode and received by the connector ferrule, i.e. a change in coupling efficiency, results. Compensation for such variations in the output power of the optoelectronic module, due to mechanical misalignment, is not made by the above mentioned control loop.

Ageing of optoelectronic modules is also an important consideration and is largely affected by temperature. For example, when optoelectronic modules are mounted in rows and have cooling air blown across them - a typical arrangement in network equipment - a first optoelectronic module has a full benefit of the cooling, but the air is warmed up by successive optoelectronic modules and so the last optoelectronic module in a row may receive very little cooling. Thus, the last optoelectronic module will operate at a significantly hotter temperature than the first optoelectronic module and hence will age faster than the first optoelectronic module.

In an attempt to obviate, or at least mitigate, some of the above disadvantages, other control schemes have been conceived to control the extinction ratio of the laser diode by measuring or estimating a temperature of the laser diode, the constant power loop using the acquired temperature indication to affect any necessary changes to the bias and modulation currents.

In US 5,844,928, an operating current for the laser diode of a laser transmitter is generated by monitoring the temperature of an integrated circuit that generates the operating current. The temperature sensed at the integrated circuit is transformed to a corresponding temperature at the laser diode. Using knowledge of the corresponding temperature at the laser diode and a look-up table, data which may include a digital representation of a back - 4 facet monitor diode current set point, desired bias and modulation currents at the measured temperature, and a maximum allowed ageing coefficient are retrieved. The accessed information is used by the integrated circuit to generate the operating current for the laser diode.

Nevertheless, whilst the above technique provides information relating to the desired bias and modulation currents at the measured temperature in order to be able to compensate for all variations with temperature of output power and extinction ratio, no compensation for ageing of the laser diode is made.

For this reason, the laser transmitter of US 5,844,928 additionally requires a back facet monitor photodiode and a feedback loop that modifies the retrieved values of desired bias and modulation currents to compensate for laser diode ageing.

Thus, the above apparatus disadvantageously does not provide a reduction in complexity due to the need for the back facet monitor photodiode.

US 5,019,769 discloses a laser diode controller that uses a programmed microcontroller to control accurately the process of turning on and selecting an operating point for the laser diode operating current. The laser diode has a back facet monitor for monitoring the optical output power of the laser diode. Once the back facet of the laser diode is calibrated, the controller can accurately monitor operating characteristics of the laser diode, and can select a best operating current based upon current operating characteristics of the laser diode. Device measurements are taken by the controller periodically during operation of the laser diode, for example, once every ten hours of operation, and a most recent device measurement is stored in a nonvolatile memory. The controller records, in the non-volatile memory, the operating characteristics of the laser diode and the back facet monitor calibration factor when the laser diode is turned on for the first time, and device characteristics after 10, 100, 1000 and 10,000 hours of operation, and, by referring to an elapsed time clock, the number of hours of operation of the laser diode. The microcontroller analyses changes in the recorded characteristics, and generates a failure warning message when the changes match predefined failure prediction criteria.

Whilst the controller of US 5,019,769 provides a way of monitoring the ageing of the laser diode and predicting failure thereof, the periodic device measurements create an unacceptable interruption to data transmission.

Additionally, the controller does not provide a reduction in apparatus complexity.

According to a first aspect of the present invention, there is provided a control apparatus for a device capable of emitting electromagnetic radiation, the apparatus comprising: a sensor and a memory; and a controller coupled to the sensor and capable of accessing the memory, the controller being arranged to set an operating current for the device; characterized in that: a first performance model and a second performance model are stored in the memory, temperature data corresponding to a temperature of the device constituting a first parameter of each of the performance models.

Output power data for the device may be a second parameter of the first model. The second model may comprise a second parameter and, optionally, the second parameter may be operating current data for the device.

The apparatus may further comprise an ageing factor parameter related to the second performance model. In such a case, the operating current may be adjusted using the ageing factor.

The sensor may be a temperature sensor for measuring a temperature corresponding to the temperature of the device. Consequently, the controller may be arranged to retrieve data from the memory in response to a measure of the temperature corresponding to the temperature of the device.

The first and second performance models may be stored as look-up tables. 6

According to a second aspect of the present invention, there is provided a method of controlling a device capable of emitting electromagnetic radiation, the method comprising the steps of: measuring a temperature corresponding to a temperature of the device; generating an operating current for the device; the method being characterized by: a first performance model and a second performance model stored in a memory, temperature data corresponding to the temperature of the device being a first parameter of each of the performance models; retrieving from the memory data associated with the measured temperature; and setting the operating current for the device in response to the data retrieved from the memory.

Output power data for the device may be a second parameter of the first model and/or the second model may comprise a second parameter. The second parameter may be operating current data for the device.

The method may further comprise the step of: using data from the second performance model to determine an ageing factor for the device. In such a case, the method may further comprise the step of: adjusting the operating current using the ageing factor.

The first and second performance models may be respectively stored as a first and a second look-up table, and the step of retrieving the data may comprise the step of: looking-up the measured temperature in the first and second look-up tables.

It is thus possible to provide a method of, and apparatus for, controlling the output power of an optoelectronic module throughout the life of the module and over the operating temperature range of the module, whilst also being able to compensate for ageing of a laser diode of the optoelectronic module without interrupting data transmission. A further advantage is the provision of maintenance of the output power of the optoelectronic module by measuring only the temperature of the module, thereby reducing the cost and complexity of the module. In this respect, the avoidance of a back facet monitor photodiode results in a minimum number of low cost and reliable parts, which can be assembled cheaply using standard mass production processes.

At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, in which: Figure 1 is a schematic diagram of a control apparatus constituting an embodiment of the invention; Figure 2 is a flow diagram of a method to determine and set a set point of a device of the apparatus of Figure 1; and Figure 3 is a flow diagram of a method of determining an ageing factor for the device of the apparatus of Figure 1.

Referring to Figure 1, an optoelectronic module 100 comprises a laser diode 102 mounted in a housing 104, the housing 104 comprising a subassembly, a lens cap available from Nippon Electric Glass Co. Ltd., Japan and a receptacle available from Adamant Kogyo Co. Ltd., Japan (not shown). A I Printed Circuit Board (PCB) 106 comprises a microprocessor 108, coupled to a temperature sensor 110, for example a thermistor, a memory 112, a laser diode driver circuit 114 and a clock or timer 116.

Whilst the memory 112 is, in this example, described as being coupled to the microprocessor 108, the memory 112 could be provided as on-board memory of the microprocessor 108. It should also be appreciated that the microprocessor 108 can be any suitable processing unit capable of providing a control function, for example, a controller integrated circuit, such as a microcontroller.

As part of a manufacturing process, a first performance model of the variation of output power of the optoelectronic module 100 with temperature, either obtained empirically for each optoelectronic module individually, or by characterization of a typical optoelectronic module or modules, is loaded into the memory 112. In this example, the first performance model is stored as a - 8 look-up table, but it should be appreciated that the first performance model can be stored as parameters of a linear, polynomial or other equation.

A second performance model is also loaded into the memory 112. The second performance model relates to a rate of aging of the laser diode 102 and typically can be characterized using an ageing model of the form: R(T I) = R(T I) ( ) k (T-T) Where: R(T,I) is a rate of ageing of the laser diode 102 at a temperature T. Kelvin, and operating current, I R(To,lo)is a rate of ageing at a reference temperature, To, and a reference current lo; n is a current ageing exponent; Ea is an activation energy; and I k is Boltzmann's constant.

Values for R(To,lo), n and Ea are obtained experimentally, for example by a multi-cell Retest in which laser diodes are aged at different temperatures and operating currents, because values for n R(To,lo), and Ea differ for different types of laser diode.

With knowledge of the temperature T and operating current 1, the second performance model allows a rate of ageing to be calculated. Knowing the rate of ageing and the elapsed time (recorded by the clock or timer 116), a bias and a modulation current can be increased to compensate for laser diode ageing, and the eventual failure of the laser diode 102 can be anticipated, thereby permitting appropriate action to be taken, for example providing a warning, such as an alarm.

Again, the second performance model can be stored as a look-up table or as parameters of a linear, polynomial or other equation. - 9 -

In operation (Figures 2 and 3), the microprocessor 108 receives (step 200) a temperature signal from the temperature sensor 110 and, using the first performance model stored in the memory 112 computes (step 202) a value of an operating point, the value of the operating point being output to the driver 114. The driver 114 then, using prior knowledge of an ageing factor described in more detail hereinunder (step 204), sets (step 206) the laser diode 102 to the calculated operating point. The microprocessor 108 continually monitors the temperature signal from the temperature sensor 110 to enable the operating point to be adjusted to maintain the output power of the optoelectronic module 100 over an operating temperature range of the optoelectronic module 100.

To compensate for ageing, the clock 116 monitors an elapsed operating time of the laser diode 102 and increments a register (not shown) in the memory 112 to record the elapsed time, for example, by incrementing the register by one every hour to record a number of elapsed hours. The elapsed hours can be used to adjust the operating point of the laser diode 102 to maintain the output power of the laser diode as the laser ages. For example, after a given elapsed time, the bias and modulation currents could be adjusted by a predetermined amount. However, such practice does not take into account the temperature or operating current set point of the laser.

Since the microprocessor 108 possesses the temperature and operating current set point (step 300) of the laser diode 102, the temperature and operating current can be taken into account. The second performance model can therefore be used to calculate (step 302) the rate of ageing R(T,I) of the laser diode 102 for time t,, which is noted (step 304), prior to resetting the timer (step 306). Thereafter, an ageing adjustment, K, (for a real time at which the ageing adjustment is applied) can be retrieved (step 308) using the ageing data calculated at time, t,. The ageing adjustment K, can, of course, be calculated instead of being looked-up, if required. The microprocessor - 10 108 then waits (step 310) a predetermined period of time (step 302), for example, between 10 and 1000 hours.

In another embodiment, the ageing acceleration factor, R(T,I), is evaluated every time the register is to be incremented, the register being incremented by the value of the ageing acceleration factor, rather than by one. For example, if Ea=0.4eV and To=25 C, R(65 C,lo)=6.3R(To,lo), i.e. the laser ages 6.3 times faster at 65 C relative to 25 C. In a further embodiment, the register can be incremented by one but at intervals dependent upon the value of the ageing acceleration factor, R(T, I). Using the values of the previous example, the register would be incremented every l/6.3 hours, i.e. every 9.5 minutes. In this example, the register need only store integers.

Thus, the register stores the effective age of the laser diode 102 had the laser diode 102 been ageing at To and lo.

As the ageing characteristic of the laser diode 102 at To and lo is known, for example from experiment, the bias and modulation currents can be continually adjusted to compensate for ageing of the laser diode 102, irrespective of the variation of T and I as the laser diode 102 operates.

In another embodiment, the temperature of the laser diode 102 can be kept substantially constant, for example by use of a thermoelectric cooler, and so the second performance model can be simplified by the omission of a temperature term. Alternatively, where the laser is not cooled, the temperature term may dominate the second performance model and so the output current term can be omitted to simplify the second performance model.

Whilst the above example has been described in the context of a laser diode, it should be appreciated that the invention is applicable to other devices capable of emitting electromagnetic radiation.

Alternative embodiments of the invention can be implemented as a computer program product for use with a computer system, the computer program product being, for example, a series of computer instructions stored on a tangible data recording medium, such as a diskette, CD-ROM, ROM, or fixed disk, or embodied in a computer data signal, the signal being transmitted over a tangible medium or a wireless medium, for example microwave or infrared. The series of computer instructions can constitute all or part of the functionality described above, and can also be stored in any memory device, volatile or non-volatile, such as semiconductor, magnetic, optical or other memory device. - 12

Claims

Claims: 1. A control apparatus for a device capable of emitting

electromagnetic radiation, the apparatus comprising: a sensor and a memory; and a controller coupled to the sensor and capable of accessing the memory, the controller being arranged to set an operating current for the device; characterized in that: a first performance model and a second performance model are stored in the memory, temperature data corresponding to a temperature of the device constituting a first parameter of each of the performance models.
2. An apparatus as claimed in Claim 1, wherein output power data for the device is a second parameter of the first model.
3. An apparatus as claimed in Claim 1 or Claim 2, wherein the second model comprises a second parameter.
4. An apparatus as claimed in Claim 3, wherein the second parameter is operating current data for the device.
5. An apparatus as claimed in any one of the preceding claims, further comprising: an ageing factor parameter related to the second performance model.
6. An apparatus as claimed in Claim 5, wherein the operating current is adjusted using the ageing factor.
7. An apparatus as claimed in any one of the preceding claims, wherein the sensor is a temperature sensor for measuring a temperature corresponding to the temperature of the device. - 13
8. An apparatus as claimed in Claim 7, wherein the controller is arranged to retrieve data from the memory in response to a measure of the temperature corresponding to the temperature of the device.
9. An apparatus as claimed in any one of the preceding claims, wherein the first and second performance models are stored as look-up tables.
10. A method of controlling a device capable of emitting electromagnetic radiation, the method comprising the steps of: l O measuring a temperature corresponding to a temperature of the device; generating an operating current for the device; the method being characterized by: a first performance model and a second performance model stored in a memory, temperature data corresponding to the temperature of the device being a first parameter of each of the performance models; retrieving from the memory data associated with the measured temperature; and setting the operating current for the device in response to the data retrieved from the memory.
11. A method as claimed in Claim 10, wherein output power data for the device is a second parameter of the first model.
12. A method as claimed in Claim 10 or Claim 11, wherein the second model comprises a second parameter.
13. A method as claimed in Claim 12, wherein the second parameter is operating current data for the device. - 14
14. A method as claimed in any one of Claims 10 to 13, further comprising the step of: using data from the second performance model to determine an ageing factor for the device.
15. A method as claimed in Claim 14, further comprising the step of: adjusting the operating current using the ageing factor.
16. A method as claimed in any one of Claims 10 to 15, wherein the first and second performance models are respectively stored as a first and a second look-up table, and the step of retrieving the data comprises the step of: looking-up the measured temperature in the first and second lookup

tables. g
17. A computer program element comprising computer program code means to make a computer execute the method as claimed in any one of Claims 10 to 16.
18. A computer program element as claimed in Claim 17, embodied on a computer readable medium.