AU729252B2

AU729252B2 - Transfer function compensation

Info

Publication number: AU729252B2
Application number: AU28729/97A
Authority: AU
Inventors: Colin Rudolph
Original assignee: Alcatel Australia Ltd
Current assignee: Nokia Services Ltd
Priority date: 1996-07-31
Filing date: 1997-07-18
Publication date: 2001-02-01
Anticipated expiration: 2017-07-18
Also published as: AU2872997A

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "TRANSFER FUNCTION COMPENSATION" The following statement is a full description of this invention, including the best method of performing it known to us:- Technical Field This invention relates to a method and arrangement for compensation of variations in the transfer function of a device.

Background Art Performance characteristics such as the transfer function of electronic devices are subject to variation due to temperature changes and the effects of ageing.

Ageing effects are dependent on a number of factors such as the duration for which the device is in operation and, for some devices such as lasers, the level at which the device is driven. For these slow changes it is known to use manual adjustment eg. of circuit elements such as potentiometers in the bias or feedback circuits to compensate for changes. Manual adjustment has the disadvantages that it is slow, costly, and can only be carried out when a technician is at the location of the equipment. This imposes additional problems for remotely located equipment.

US Patent 5,383,208 discloses an arrangement for compensating for the effects of ageing and temperature in lasers.

In US 5,383,208, a temperature measuring device is used to derive a control signal to connect for the effects of change of temperature (See column 4, line 29 et se of the US patent).

We propose an arrangement to compensate for ageing and temperature changes S 20 without the need to provide means to measure temperature changes.

Disclosure of the Invention This specification discloses a method of compensating for variations in the transfer function of a device, the method comprising: analysing the output of the device to determine variations in the transfer function; and adjusting one or more operational parameters of the device to compensate for at least part of said variations in the transfer function.

In one embodiment the analysis includes comparing the output of the device with predetermined threshold values.

In a second embodiment the analysis includes comparing the output of the device with the input to the device to identify one or more characteristics of the transfer function.

In a preferred embodiment the device is an analog device and the analysis is carried out digitally.

In particular the specification describes: a method of adjusting the performance of a device having a transfer function subject to variation, the device having controllable gain and controllable bias, the method including: defining an upper output limit, max-hi; defining a lower output limit, max-lo; determining an upper output hysteresis range, max-lo to max-hi; determine a lower output hysteresis range, min-lo to min-hi; monitoring the peak output of the device, out-hi, in relation to the upper output hysteresis range; monitoring the minimum output of the device, out-lo, in relation to the lower output hysteresis range; adjusting the gain or bias of the device according to a predetermined algorithm to bring the peak output and the minimum output within the upper output hysteresis range and the lower output hysteresis range respectively; and resuming monitoring of the peak and minimum outputs.

The specification also describes: "a method of adjusting the performance of a device having a transfer function subject to variation, the device having controllable gain and controllable bias, the method including: defining a maximum high output threshold, max-hi; defining a minimum high output threshold, min-hi; defining a maximum low output threshold, max-lo 25 defining a maximum low output threshold, minax-lo; defining a maximum low output threshold, min-bo; whereby satisfactory performance of the device is identified when the high level output of the out-hi device is between the maximum and minimum high output thresholds, and the low level output of the device out-lo, is between the maximum and minimum low output thresholds, measuring the high level output, out-hi; measuring the low level output, out-lo; monitoring out-hi and out-lo for the following conditions: a) out-lo min-lo AND out-hi max-hi; b) out-lo min-lo AND out-hi smax-hi; c) out-lo max-lo AND out-hi min-hi; d) out-lo max-lo AND out-hi min-hi; e) min-lo out-lo max-lo AND out-hi min-hi; f) min-lo out-lo max-lo AND out-hi max-hi; and, in response to condition a), reducing the gain by a first predetermined amount and resuming the monitoring of out-hi and out-lo; and, in response to condition b), increasing the bias by a second predetermined amount and resuming the monitoring of out-hi and out-lo; and, in response to condition c), increasing the gain by a third predetermined amount and resuming monitoring of out-hi and out-lo; and, in response to condition d), decreasing the bias by a fourth predetermined amount and resuming monitoring of out-hi and out-lo; and, in response to condition e), increasing the bias by a fifth predetermined amount and resuming monitoring of out-hi and out-lo; and, in response to condition f), reducing the bias by a sixth predetermined amount and resuming monitoring of out-hi and out-lo.

The specification also describes: an arrangement for adjusting the performance of a device having a transfer function subject to variation, the device having an adjustable gain and bias.

The arrangement including: storage means to store values defining upper and lower hysteresis ranges; monitoring means to monitor the peak output and the minimum output of the device; analysing means to determine when the peak and minimum outputs fall outside the upper and lower hysteresis ranges respectively; control means responsive to the analysing means to adjust the gain or bias according to a predetermined algorithm to return the peak and minimum outputs to within their respective hysteresis ranges when the analysing means indicates that either or both of the peak and minimum outputs are outside their respective hysteresis ranges.

The invention will be described in relation to the control of a semiconductor laser. Semiconductor lasers are subject to the effects of temperature drift and ageing, and the ageing effects are increased as the laser is driven harder by increasing the modulating signal thereby increasing the output power of the laser.

Brief Description of the Drawings: Figure 1. shows an exemplary transfer characteristic for a device such as a laser; Figure 2. shows the variation of the characteristic of Fig.1 with temperature for S* 15 age; Figure 3. shows an arrangement embodying the invention for controlling a laster output; Figure 4. shows an algorithm used in an implementation of the invention; Figure 5. shows the operating characteristic for an arrangement embodying 20 the invention; Figure 6. illustrates various output conditions dealt with by the algorithm of **"Figure 4.

Description of the Invention: Figure 1 shows the transfer characteristics of a laser. In telecommunication 25 applications, the operating point is preferably selected so that the laser is operated wholly within the linear range above the "knee" in the transfer characteristic. Figure 1 shows the relationship between the input and the output as determined by the transfer characteristic, and this relationship should preferably remain constant, or at least within nominated tolerances, for the operational lifetime of the laser.

However the effect of ageing is mainly to shift the transfer characteristic to the right so that, as shown, by way of example in curve 2 of Figure 2. Thus, if the driving signal remains at the same level, the operating point moves to and the "knee" may 4~* a. a intrude into the operating range causing non-linearity in the output. The overall amplitude of the output signal may also be reduced.

The effect of a change in temperature is to change the gain of the laser as shown by way of example in curve 5. An increase in gain is indicated by an increase in the steepness of the transfer function characteristic. The increase in temperature shifts the operating point to 03 and increases both the amplitude and peak output power. This may result in the laser operating above its optimum output power level, resulting in premature ageing and early failure of the laser.

The operation of the arrangement shown in Figure 3 will be described with reference to the flow chart shown in Figure 4.

Figure 3 is a schematic representation of an arrangement embodying the invention.

The arrangement includes a laser 101 and a detector 102 which detects a small proportion of the output of the laser. The output of the detector is, in this 15 embodiment, fed to a peak detector 103, which in turn is connected to an analog-todigital converter 104. The output of the A/D converter 104 is applied to a microprocessor 105. The microprocessor 105 analyses the information from A/D converter 103 and generates outputs 106 and 107 to adjust transfer function parameters when the transfer function drifts out of specification.

20 Output 106 is applied to a first digital-to-analog converter 108 which is used to set the bias for laser 101.

Output 107 is used to control the operating point by shifting the DC level of the input signal through amplifier 109.

A third output 110 from gP105 may be used to control the gain of amplifier 109 by controlling the feedback 112 of amplifier 109.

The operation of the arrangement of Figure 3 is described below with reference to the flow chart in Figure 4 and the operating characteristic of Figure Upper and lower limits, max-hi, and min-lo, are defined for the output of the device and a tolerance margin or hysteresis, hyst, is deducted from the upper limit max-hi, and added to min-lo to define first and second zones in which the peak output, out-hi, and lo output, out-lo respectively should fall for satisfactory operation.

Thus if out-hi lies between max-hi and min-hi and out-lo between max-lo and min-lo, the device is operating satisfactorily.

If out-hi and/or out-lo falls outside their respective predetermined operating zones, the monitoring arrangement detects the nature of the error and the information is fed to the computer which makes an appropriate incremental adjustment of the gain or bias and monitoring is resumed. Preferably the incremental adjustments are chosen to make a change of 1/2 hyst to the output levels.

The bias is chosen to set the operating point mid-way between max-hi and minlo.

Various conditions which are monitored are shown in Figure 6 as lines a) to f) which identify the following conditions: a) b) c) d) 20 a) Reduce Gain b) Increase Bias c) Increase Gain d) Reduce Bias e) Increase Bias 25 f) Reduce Bias After each response, monitoring is resumed.

Claims

1. A method of adjusting the performance of a device having a transfer function subject to variation, the device having controllable gain and controllable bias, the method including: defining an upper output limit, max-hi; defining a lower output limit, max-lo; determining an upper output hysteresis range, max-lo to max-hi; determine a lower output hysteresis range, min-lo to min-hi; monitoring the peak output of the device, out-hi, in relation to the upper output hysteresis range; monitoring the minimum output of the device, out-lo, in relation to the lower output hysteresis range; adjusting the gain or bias of the device according to a predetermined algorithm to bring the peak output and the minimum output within the upper output hysteresis range and the lower output hysteresis range respectively; and resuming monitoring of 315 the peak and minimum outputs.

2. A method as claimed in claim 1, the method including the steps of: defining a maximum high output threshold, max-hi; "defining a minimum high output threshold, min-hi; defining a maximum low output threshold, max-lhi; defining a maximum low output threshold, min-lo; Whereby satisfactory performance of the device is identified when the high level output of the out-hi device is between the maximum and minimum high output thresholds, and the low level output of the device out-lo, is between the maximum and minimum low output thresholds, measuring the high level output, out-hi; measuring the low level output, out-lo; monitoring out-hi and out-lo for the following conditions: a) out-lo min-lo AND out-hi max-hi; b) out-lo min-lo AND out-hi :max-hi; c) out-lo max-lo AND out-hi min-hi; d) out-lo max-lo AND out-hi min-hi; e) min-lo out-lo max-lo AND out-hi min-hi; f) min-lo out-lo max-lo AND out-hi max-hi; and, in response to condition a), reducing the gain by a first predetermined amount and resuming the monitoring of out-hi and out-lo; and, in response to condition b), increasing the bias by a second predetermined amount and resuming the monitoring of out-hi and out-lo; and, in response to condition c), increasing the gain by a third predetermined amount and resuming monitoring of out-hi and out-lo; and, in response to condition d); decreasing the bias by a fourth predetermined amount and resuming monitoring of out-hi and out-lo; and in response to condition e); increasing the bias by a fifth predetermined amount and resuming monitoring of out-hi and out-lo; and, in response to condition f), reducing the bias by a sixth predetermined amount and resuming monitoring 20 of out-hi and out-lo.

3. A method as claimed in claiml or claim2, wherein the output amplitude is controlled by controlling the amplitude of the input signal to the device.

4. A method as claimed in claim 1 or claim 2, wherein the gain is controlled by controlling device current to the device. 25

5. A method as claimed in any one of claims 1 to 4, wherein controlling the bias controls the operating point of the device.

6. A method of adjusting the performance of a device substantially as herein described with reference to the accompanying drawings.

7. An arrangement for adjusting the performance of a device having a transfer At function subject to variation, the device having an adjustable gain and bias, said arrangement including: storage means to store values defining upper and lower hysteresis ranges; monitoring means to monitor the peak output and the minimum output of the device. analysing means to determine when the peak and minimum outputs fall outside the upper and lower hysteresis ranges respectively; control means responsive to the analysing means to adjust the gain or bias according to a predetermined algorithm to return the peak and minimum outputs to within their respective hysteresis ranges when the analysing means indicates that either or both of the peak and minimum outputs are outside their respective hysteresis ranges.

8. An arrangement as claimed in claim 7, wherein the device is a laser.

9 An arrangement for adjusting the output of a device substantially as herein described with reference to the accompanying drawings. DATED THIS FIFTEENTH DAY OF NOVEMBER 2000 ALCATEL AUSTRALIA LIMITED 000 005 363) *o* *g o*o