Abstract:
A laser power controller employs: selection circuitry configured to select one of a data input value, a logical high value or a logical low value such that the selection circuitry selects the data input value during a data transmission period during a defined burst period and selects one of the logical high value and the logical low value during an extension time period during the defined burst period and immediately following the data transmission period; drive circuitry configured to apply, to a laser diode, a current corresponding to the value selected by the selection circuitry during the defined burst period or a zero value otherwise, the current being such that the laser diode is configured to provide an optical output; an optical sensor module configured to provide a sensor module output corresponding to the optical output of the laser diode, and configured to provide an electrical output proportional to the laser diode&#39;s optical output corresponding to the logical high value or the logical low value; and a controller configured to receive desired values regarding minimum and maximum optical output power levels of the laser diode and to receive the electrical output from the optical sensor module proportional to the optical output power level corresponding to the logical high and the logical low values; the controller being configured to use the received information to provide control values for the drive circuitry.

Description:
BACKGROUND OF INVENTION 
       [0001]    In a fibre optical communications system, it is important to be able to control the output power of the transmitting laser diode for a number of reasons. Firstly, the average and peak power of the laser must not exceed certain limits in order to avoid damage. Secondly, the different power levels corresponding to binary (or other radix) data values must be set so that the modulation index (alternatively defined as extinction ratio) is within the overall system specifications to ensure reliable reception at the end of the link. One difficulty to be addressed in any control system is that the characteristics of the laser can change significantly with temperature and also over time with ageing, and diverging from an ideal linear response, so that a conventional factory set up of the “high” and “low” drive current levels is not adequate. 
         [0002]    Numerous techniques exist in prior art that describe methods intended to estimate the instantaneous values of the minimum and maximum transmitted optical output and compensate for the changes in device characteristics. Most are limited in their effectiveness due to the restricted bandwidth of the monitor diode and its associated circuitry. 
         [0003]    Monitoring the transmitted output power is even more challenging in an optical communications link that transmits the data in a series of discrete bursts, as the average value of the optical output may vary greatly, and the instantaneous levels are not stable enough for most methods described in prior art to reach adequate estimates of minimum and maximum levels. The temperature related effects are likely to be even more severe, as the transmitting laser diode may be in an off state for a long period before being activated for a data burst, and hence may have cooled to ambient temperature before heating up during a data burst. 
         [0004]    Hence it is desirable to be able to sense the minimum and maximum optical outputs corresponding to logic “1” and logic “0” during data bursts on a near continuous basis. It is further desirable to be able to make such measurements using a transmit power monitoring function with only moderate bandwidth, and by means that do not disturb the transmitted data payload nor compromise the received signal to noise performance. 
       SUMMARY OF INVENTION 
       [0005]    According to an aspect, there is provided a system comprising selection circuitry configured to select one of a data input value, a logical high value or a logical low value such that the selection circuitry selects the data input value during a data transmission period during a defined burst period and selects one of the logical high value and the logical low value during an extension time period during the defined burst period and immediately following the data transmission period; drive circuitry configured to apply a current to a laser diode, the current corresponding to the value selected by the selection circuitry during the defined or a zero value otherwise, the current being such that the laser diode is configured to provide an optical output; an optical sensor module configured to provide a sensor module output corresponding to the optical output of the laser diode; wherein the sensor module output is configured to provide an electrical output proportional to the laser diode&#39;s optical output corresponding to the logical high value or the logical low value; and a controller configured to receive desired values from the optical sensor module regarding minimum and maximum optical output power levels of the laser diode and to receive the electrical output proportional to the optical output power level corresponding to the logical high and the logical low values; wherein the controller is configured to use the received information to provide control values for the drive circuitry. 
         [0006]    The optical sensor module may comprise a photodiode output power detector. 
         [0007]    The optical sensor module may comprise an optical sensor and a trans-impedance amplifier, the trans-impedance amplifier being configured to provide the sensor module output. 
         [0008]    The control values may be configured to control the average power of the optical output of the laser diode. 
         [0009]    The control values may be configured to control the peak power of the optical output of the laser diode. 
         [0010]    The control values may be configured to control the modulation index of the optical output of the laser diode. 
         [0011]    The current may comprise a steady element and a variable element. 
         [0012]    The drive circuitry may be configured to set the current applied to the laser diode dependent on a combination of a bias control value and a modulation control value. 
         [0013]    The control values may be configured to control the drive circuitry to set the at least one of a bias current and a modulation current applied to the laser diode. 
         [0014]    The drive circuitry may comprise bias circuitry configured to provide a bias current to the laser diode. 
         [0015]    The drive circuitry may comprise modulation circuitry configured to provide a modulation current to the laser diode. 
         [0016]    The drive circuitry may be configured to set the current applied to the laser diode dependent on a combination of an average value and a modulation value. 
         [0017]    The burst period may be gated by a burst enable signal. 
         [0018]    The duration of the data transmission period may adhere to a standard specification for burst mode operation. 
         [0019]    The control values may control the drive circuitry to deliver the optical output desired values regarding desired minimum and maximum optical output power levels. 
         [0020]    The extension time period may be greater than a settling time of the sensor module output. 
         [0021]    The selection circuitry may alternately select one of the logical high value and logical low value for each consecutive extension time period. 
         [0022]    The selection circuitry may select the logical high value or the logical low value for each consecutive extension time period according to a pre-defined sequence. 
         [0023]    The selection circuitry may select the logical low value immediately after an extension time period where the logical high value has been selected. 
         [0024]    The selection circuitry may comprise a selector switch function. 
         [0025]    The bandwidth of the selection circuitry may be configured to switch between the data input, the logical high value and the logical low value in a time significantly less than that of the extension time period. 
         [0026]    The system may comprise substantially digital circuits. 
         [0027]    The control values may be calculated by a digital calculation function. 
         [0028]    The system may comprise substantially analogue circuits. 
         [0029]    According to another aspect, there is provided a system comprising means for selecting one of a data input value, a logical high value or a logical low value such that the selection circuitry selects the data input value during a data transmission period during a defined burst period and selects one of the logical high value and the logical low value during an extension time period during the defined burst period and immediately following the data transmission period; means for applying a current to a laser diode, the current corresponding to the value selected by the selection circuitry during the defined burst period or a zero value otherwise, the current being such that the laser diode is configured to provide an optical output; means for providing a sensor module output corresponding to the optical output of the laser diode; wherein the sensor module output is configured to provide an electrical output proportional to the laser diode&#39;s optical output corresponding to the logical high value or the logical low value; and means for receiving desired values from the optical sensor module regarding minimum and maximum optical output power levels of the laser diode and to receive the electrical output proportional to the optical output power level corresponding to the logical high and the logical low values; wherein the controller is configured to use the received information to provide control values for the drive circuitry. 
         [0030]    The means for providing a sensor module output may comprise a photodiode output power detector. 
         [0031]    The means for providing a sensor module output may comprise an optical sensor and a trans-impedance amplifier, the trans-impedance amplifier being configured to provide the sensor module output. 
         [0032]    The control values may be configured to control the average power of the optical output of the laser diode. 
         [0033]    The control values may be configured to control the peak power of the optical output of the laser diode. 
         [0034]    The control values may be configured to control the modulation index of the optical output of the laser diode. 
         [0035]    The current may comprise a steady element and a variable element. 
         [0036]    The means for applying a current to a laser diode may be configured to set the current applied to the laser diode dependent on a combination of a bias control value and a modulation control value. 
         [0037]    The control values may be configured to control the drive circuitry to set the at least one of a bias current and a modulation current applied to the laser diode. 
         [0038]    The means for applying a current to a laser diode may comprise bias circuitry configured to provide a bias current to the laser diode. 
         [0039]    The means for applying a current to a laser diode may comprise modulation circuitry configured to provide a modulation current to the laser diode. 
         [0040]    The means for applying a current to a laser diode may be configured to set the current applied to the laser diode dependent on a combination of an average value and a modulation value. 
         [0041]    The burst period may be gated by a burst enable signal. 
         [0042]    The duration of the data transmission period may adhere to a standard specification for burst mode operation. 
         [0043]    The control values may control the drive circuitry to deliver the optical output desired values regarding desired minimum and maximum optical output power levels. 
         [0044]    The extension time period may be greater than a settling time of the sensor module output. 
         [0045]    The means for selecting may alternately select one of the logical high value and logical low value for each consecutive extension time period. 
         [0046]    The means for selecting may select the logical high value or the logical low value for each consecutive extension time period according to a pre-defined sequence. 
         [0047]    The means for selecting may select the logical low value immediately after an extension time period where the logical high value has been selected. 
         [0048]    The means for selecting may comprise a selector switch function. 
         [0049]    The bandwidth of the selection circuitry may be configured to switch between the data input, the logical high value and the logical low value in a time significantly less than that of the extension time period. 
         [0050]    The system may comprise substantially digital circuits. 
         [0051]    The control values may be calculated by a digital calculation function. 
         [0052]    The system may comprise substantially analogue circuits. 
         [0053]    According to another aspect, there is provided a method for communications comprising: selecting one of a data input value, a logical high value or a logical low value such that the data input value is selected during a data transmission period during a defined burst period and one of the logical high value and the logical low value is selected during an extension time period during the defined burst period and immediately following the data transmission period; applying a current to a laser diode, the current corresponding to the selected value during the defined burst period or a zero value otherwise, the current being such that the laser diode is configured to provide an optical output; determining an electrical output proportional to a laser diode&#39;s optical output corresponding to the logical high value or the logical low value by using a sensor module output corresponding to the optical output; receiving desired values regarding desired minimum and maximum optical output power levels of the laser diode; and providing control values for the current applied to the laser diode based on the electrical output proportional to the optical output corresponding to the logical high or the logical low value and the received desired values. 
         [0054]    The method may comprise providing the sensor module output using a trans-impedance amplifier. 
         [0055]    The method may comprise providing the sensor module output using a photodiode output power detector. 
         [0056]    The method may comprise controlling the average power of the optical output of the laser diode using the control values. 
         [0057]    The method may comprise controlling the peak power of the optical output of the laser diode using the control values. 
         [0058]    The method may comprise controlling the modulation index of the optical output of the laser diode using the control values. 
         [0059]    The current may comprise a steady element and a variable element. 
         [0060]    The method may comprise setting the current applied to the laser diode dependent on a combination of a bias control value and a modulation control value. 
         [0061]    The method may comprise controlling at least one of the bias control value and the modulation control value applied to the laser diode using the control values. 
         [0062]    The method may comprise setting the current applied to the laser diode dependent on a combination of an average value and a modulation value. 
         [0063]    The burst period may be gated by a burst enable signal. 
         [0064]    The duration of the data transmission period may adhere to a standard specification for burst mode operation. 
         [0065]    The method may comprise controlling the applied current to deliver the optical output desired minimum and maximum optical output power levels using the control values. 
         [0066]    The extension time period may be greater than a settling time of the sensor module output. 
         [0067]    The method may comprise selecting the logical high value and the logical low value alternately for each consecutive extension time period. 
         [0068]    The method may comprise selecting the logical high value or the logical low value for each consecutive extension time period according to a pre-defined sequence. 
         [0069]    The method may comprise selecting the logical low value immediately after an extension time period where the logical high value has been selected. 
         [0070]    The method may comprise selecting using a selector switch function. 
         [0071]    The method may be performed by substantially digital circuits 
         [0072]    The method may comprise calculating the control values using a digital calculation function. 
         [0073]    The method may be performed by substantially analogue circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0074]    The invention will now be described solely by way of example and with reference to the accompanying drawings in which: 
           [0075]      FIG. 1  shows a typical arrangement for a transmitter in a burst-mode optical fibre link. 
           [0076]      FIG. 2  shows a representation of a laser diode output characteristic and temperature effects. 
           [0077]      FIG. 3  shows the limitations of conventional estimation methods where there is curvature in the laser characteristic. 
           [0078]      FIG. 4  shows the structure of a typical data burst with typical allowable laser turn off time. 
           [0079]      FIG. 5  shows a burst mode optical signal with high and low reference levels embedded within valid data packets. 
           [0080]      FIG. 6  shows an embodiment of the invention. 
           [0081]      FIG. 7  shows a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0082]    The description is not to be taken in a limiting sense but is made merely for the purposes of describing the general principles of the embodiments of the invention. For example, operations that are illustrated as being performed using digital signals and digital circuits may also be achieved using substantially analogue signals and analogue circuits. 
         [0083]      FIG. 1  shows the typical arrangement in a transmitter suitable for an optical communications system. A laser diode  101  is provided with a current by drive circuitry having a steady element and a variable element. This may be in the form of an average current with a bi-directional modulation current adding and subtracting current to create the optical maxima and minima; or there may be a smaller steady bias current  114  with a modulation current  115  that is disconnected by means of a switching function  110  to indicate a logical low level in the modulation data input  111 . The latter variant is represented in the figure. These currents may be provided by digital-to-analogue converters  106  and  107  having current outputs controlled by digital values  108  and  109  respectively that are set by the controller function  118 . When operating in a burst mode, these currents may be gated in a manner corresponding to the active periods in a data burst by means of a further signal or signals  112  corresponding the length of the prescribed burst. The optical output of the laser diode  101  is sensed by an optical sensor, such as a monitor photodiode  102  to create a current proportional to the sensed optical level and which may be converted to a voltage  105  with a trans-impedance amplifier  103 . The combination of the monitor diode  102  and amplifier  103  typically have a bandwidth that is substantially less than that of the main data channel bandwidth. This monitor value  105  may be converted to digital form  113  by means of an analogue-to-digital converter  104  and these data used by the controller  118  to set the current levels according to some algorithm. The bandwidth limitation of the monitor channel is very significant in the implementation of any transmit optical level control mechanism since it restricts the observability of the peak and trough values of the optical signal. 
         [0084]      FIG. 2  is a diagrammatic representation of the characteristics of a typical laser diode as is used in optical communications systems. When used to generate a modulated optical signal, the current through the laser diode is modulated such that the minimum current is above the threshold value  203 , for the laser, and the maximum current is below the manufacturer&#39;s ratings for the device. When a laser diode is cold, or the current levels are relatively low, a simple linear model  201  may suffice. However, when the laser diode heats up, or as its characteristics change with age, the threshold current may change  204  and the relationship may exhibit a more curved shape  202 . Thus, maintaining the desired optical output and the desired ER during operation over a system&#39;s lifetime is not considered trivial. 
         [0085]    In any given practical system, the maximum current may be set so that the average operating power of the laser is set to a defined level with regard to the required signal level for reliable communications to be established. A critical parameter in such a system is the ratio of the maximum to minimum optical output, usually referred to as the Extinction Ratio (ER), as this affects the signal to noise levels for the receiver. The ER is a function of the minimum and maximum laser diode current values, and is sometimes represented as a simple linear relationship, but in reality this is not an accurate representation. 
         [0086]      FIG. 3  shows how average optical power  301  of a laser diode at an elevated temperature is not suitable as the basis for an accurate estimate of the minimum  302  and maximum  303  optical levels and hence the ER. This also implies that there are problems with controlling the minimum  304  and maximum  305  current levels needed to obtain the desired average power and ER. Where a system operates with a continuous data stream, the average is relatively easy to monitor as the laser can reach a steady state temperature. Further, there is time to gather data from a monitor diode system to measure the peak and trough optical data levels with some kind of averaging of the measurements to provide an estimate of the ER and average optical power. Systems for this purpose are known in prior art (for example, Smith et al, Electronics Letter Vol 14, 1978, and similar derivative arrangements). 
         [0087]      FIG. 4  shows the general form of optical signals intended to transmit data bursts in a system adhering to specifications for burst mode operation, (such as standard ITU-T Recommendation G.984.2). The bias current to the laser is gated by a burst enable signal  112  before data signals  111  are used to modulate the laser output. In such standards the duration T 1  of the data burst  403  is precisely defined, and typically of the order of a few 100 nanoseconds. Note that at the end of a data burst, the logical value may be in a high state (a logical high value) or a low state (a logical low value). Such standards also typically define T 2  a time interval  404  within which the laser output must return to zero. To allow for the bandwidth of practical bias control systems, this interval is of the order of 10 ns. 
         [0088]    In such a burst mode system the problem of controlling the average power and ER is difficult. Before the start of a burst the laser will be in a relatively cool state. As soon as the data packets are transmitted, the laser will begin to heat up and will continue to do so during a typical burst. It is a requirement of the standards that the system be operational after only a short number of training bursts, for example 5 or less, in which the system&#39;s operating parameters come under control. 
         [0089]    The requirement to be able to establish operating conditions rapidly after the start of a sequence of bursts is not addressed in this disclosure. Leaving this issue to be solved by other means, there remains a requirement to provide means for accurately controlling the extinction ratio of the laser output after the initial training bursts where the laser has substantially warmed up to an elevated average temperature. Any measurement of the peak and trough values has the same monitor channel bandwidth limitations as in a continuous system, but the demands are further complicated by the intermittent nature of the signal making meaningful averaging more difficult. 
         [0090]    In an embodiment of the invention means are provided to make rapid and accurate estimates of the instantaneous values of the optical output representing data ‘1’ and data ‘0’ values, or other such values as may be defined. Using said estimates, further means are provided that are able to calculate the required values of bias current and modulation current needed to deliver the desired output levels, and to maintain these notwithstanding changes in the laser characteristics due to short term heating and/or long term ageing. 
         [0091]    In  FIG. 4  it will be noticed that the time to turn off of the laser after a burst of data is not a constant but depends on the logical value at the end of the data transmission period  401 . The laser bias turn off time at the end of a data transmission period from a high state  405  is greater than the laser bias turn off time at the end of a data transmission period from a low state  406 . The bandwidth of the modulation circuit  110  in response to the modulation data signal  111  is very fast. Hence rather than use the bias current control to turn off from a high state, the modulation circuit may be used to reduce the laser output very rapidly to the low state first, typically in a time of the order of 10 s of picoseconds. Once the laser output is in said low state, the task of turning off to full extinction becomes much easier. Further, it is not a difficult task to ensure that the bias current  114  responds to the burst enable signal  112  or a substantially equivalent signal in a time interval substantially less than the interval  404  required by the standard. This approach makes available a time interval that while not large is nonetheless greater than the transient settling time typical of such monitor channel circuits. Using this knowledge it is possible to exploit time available in the specified turn off interval  404  to execute valuable measurements of the prevailing optical high and low output levels. 
         [0092]      FIG. 5  shows the optical levels associated with a burst mode system wherein subtle modifications have been made to the transmitted signal that facilitate measurements of the high and low levels. Said modifications are made such that they do not affect the normal transfer of data within the burst packets and do not transgress the specifications set by the relevant standard. 
         [0093]    To provide the framework for said modifications a time interval is first defined to satisfy the conditions that it is substantially less than the laser turn off time  405  allowed by the standard but long enough to be substantially longer than the settling time of the monitor channel output  105  and at the same time allows sufficient remaining time within the period  405  for the bias current control circuits to extinguish the laser completely. A feature of the invention is the replacement of the raw data signal  111  with a modified form of the laser modulation signal  501  wherein at the end of each burst a known logical value is held for an extended time period T 3   502 . At the same time, the bias current to the laser  114  is controlled by a modified version of the burst enable signal (the bias control signal  506 ) such that the bias remains active for a defined period after the data for that burst has ceased. The logical value of this extension of the data burst is made to alternate between a ‘1’ denoted  503  in  FIG. 5  and a ‘0’ denoted  504  in  FIG. 5 . The duration of this logical value holding period  502  is made to be sufficient for the monitor channel output  105  to be able to settle to a substantially accurate measurement result. If the logical value held at the end of the data burst is ‘1’, then the laser modulation current  115  is returned to a ‘0’ at the end of this extension period  502  by means of a command edge  505  to the data modulation circuitry  110 . In this way, the laser current is reduced substantially towards its extinction state by means of a high bandwidth circuit function in some very short time (in this example, 10 s of picoseconds), rather than by a possibly much slower bias current control. Immediately this state has been reached, the bias current  114  is turned off by the bias control signal  506  and decays to zero before the end of the time permitted by the relevant standard. By these or substantially similar means it is therefore possible for the monitor output  105  to deliver substantially accurate representations of the true prevailing optical outputs during both logical ‘1’ and logical ‘0’ data states, without significant restrictions arising from particular data patterns and/or run lengths as is often the case in prior art. From these measurements taken from alternate data bursts the analogue values may be converted into digital form  113  and a simple algorithm may be employed to complete a system to determine the prevailing extinction ratio and the average optical power, and further to determine any required adjustments to the modulation current and the bias current such that the ER and average power correspond with the desired target values for the system. 
         [0094]    It is an advantage of the invention that the control system so comprised measures the steady state optical values for both logical ‘1’ and logical ‘0’ free from significant assumptions regarding the performance of other parts of the system and substantially not derived from indirect calculations. 
         [0095]    It is a further advantage of the invention that the intermittent nature of the burst mode signal does not detract from the operation of the control system. 
         [0096]      FIG. 6  shows an arrangement according to an embodiment of the invention. The bias current  114  is set by a current output digital-to-analogue converter (DAC)  106  and the modulation current  115  is similarly set by another DAC  107 . The controlling digital values for said DACs are determined by a digital calculation function  604 , which takes its inputs from the system feedback values and the digital inputs corresponding the desired average power  606  and modulation depth (or ER  605 ). The modulation circuitry  110  is no longer controlled directly by the data input  111  but can now have its input switched between the data input  111 , and logic ‘1’ or logic ‘0’ by means of selection circuitry, for example a selector switch function  610 . When the burst enable signal  112  is asserted to indicate the start of a data burst the logical control function  607  will set the modulation input path using selector  610  to pass the incoming data directly to the modulation circuitry  110 . A modulated optical signal will be generated by the laser  101  and a band-limited representation of same  105  will be created by the monitor diode  102  and its associated amplifier  103 . This monitor signal  105  is converted to a digital value  113  by an analogue-to-digital converter (ADC)  104 . During the payload of the data burst this output  113  may be used but it will be of limited value due to the bandwidth limitations of this channel. At the end of the data payload the burst enable signal will indicate the end of this transmission. In a conventional system, this would disable the modulation  115  and bias  114  currents completely. 
         [0097]    According this embodiment of the present invention, the control logic  607  takes a defined delay time  609  and holds the bias and modulation currents on. An additional burst status signal  601  is provided by the embodiment that changes logical value with each data burst, effectively designating bursts as “HIGH” or “LOW”. As an example embodiment, if the burst is designated as “HIGH” then during the delay at the end of the burst, the modulation input selector  610  is set to a logical ‘1’  503  such that the optical output is held at the high level  303 . This modulation optical value is held for a time period  502  long enough for the monitor channel to make an accurate measurement despite its limited bandwidth; but still short enough that there is time to fully extinguish the laser. The monitor channel output  105  is converted to digital form  113  and then passed at a suitable time instant to a first register  602  via a logical gate  611  enabled by the burst status signal  601 . This register then provides the measured optical high value to the calculation function  604 . 
         [0098]    At the end of this delay period  503  the modulation selector is set to a logical ‘0’ to remove the laser modulation current  115  using the normal modulation circuitry and hence reduce the optical output very rapidly. At the same instant  505 , the control logic  607  commands the bias current DAC  106  and the modulation current DAC  107  to cease outputting current, such that the laser  101  becomes completely extinguished within the period  404  required by the relevant communication standard. 
         [0099]    If the burst is designated as “LOW” by the burst status signal  601  then at the end of the data payload the modulation selector  610  is set to a logical ‘0’  504  such that the laser output is at the low level  302 . Even if the last symbol in the burst data payload required a logical ‘1’ at the end of the burst, then the transition to a logical ‘0’ can be effected with great speed by using the normal modulation circuitry  110 . Again, this modulation optical value is held for a time period  502  long enough for the monitor channel to make an accurate measurement despite its limited bandwidth; but still short enough that there is time to fully extinguish the laser. 
         [0100]    The monitor channel output  105  is then converted to digital form  113  and then passed at a suitable time instant to a second register  603  via a logical gate  612  enabled by the logical complement of the burst status signal  601 . This register then provides the measured optical low value to the calculation function  604 . 
         [0101]    A convenient and efficient arrangement will be to designate the bursts as “HIGH” and “LOW” in an alternating manner. However, the invention may also employ some other sequence of “HIGH” and “LOW” states where there may be a need to obtain an estimate of one level faster than the other, or to take account of some other requirements of the system. 
         [0102]    The calculation function  604  then takes the required target value inputs for the average  606  and ER  605  and using a simple calculation derives the new bias current control value  108  and the new modulation current value  109  such that the errors between the calculated ER and average values and the corresponding required ER and average values are minimised and brought to negligible or acceptable levels. This process may take several iterations of “HIGH” and “LOW” bursts and the precise rate of convergence of the system will depend on coefficients and scale factors chosen for a particular application. 
         [0103]      FIG. 7  shows an arrangement according to a second embodiment of the invention. In this arrangement, the derivation of the corrections to the bias and modulation currents are performed with more analogue processing. The laser modulation and monitor circuits are substantially as in the previous arrangement according to  FIG. 6 . Instead of converting the output of the monitor channel  105  into digital form, the analogue value is compared directly with another analogue value derived from reference analogue values generated by DACs from user defined input values. The operation is as follows: 
         [0104]    The desired optical high value  701  and desired optical low value  702  are supplied from the user in explicit form and used to control two DACs  703  and  704  respectively. The outputs  705  and  706  of these DACs are equivalent to the desired monitor photodiode amplifier  105  outputs for optical ‘1’ and optical ‘0’ under ideal optical bias conditions and desired modulation value. A person skilled in the art will also immediately recognise that the desired operating current may also be supplied as an average value and an ER value, and then converted to equivalent high and low values by means of simple arithmetic circuits. 
         [0105]    The voltages  105  and  705  should be substantially identical when the laser is operating in the logical high state under ideal conditions. The voltages  105  and  706  should be substantially identical when the laser is operating in the logical low state under ideal conditions. The comparators  703  and  704  are used to determine the sign of any difference between the indicated levels and the desired levels. 
         [0106]    When the data burst is designated “HIGH”, then at the end of the holding period  502  the comparator  707  output is passed via logic gate  611  controlled by the burst status signal  601  to a counter  712  wherein it is used to control a counting process either up or down, depending on the sign of the output of the comparator  707 . If the monitor signal  105  is less than the reference signal  705  from the DAC  703  at this instant, then the counter will decrement indicating a negative error for the high optical state. If the monitor signal  105  is greater than the reference signal  705  then the counter  712  will increment. 
         [0107]    Similarly, when a data burst is designated as “LOW” then at the end of the holding period  502  then the comparator  708  output is passed via logic gate  612  controlled by the complement of the burst status signal  601  to a counter  713  wherein it is used to control a similar counting process either up or down, depending on the sign of the output of the comparator  708 . If the monitor signal  105  is less than the reference signal  706  from the DAC  704  at this instant, then the counter will decrement indicating a negative error for the low optical state. A corresponding increment will take place if the monitor output is higher than the replica at this instant. 
         [0108]    From the values from the counters  712  and  713  at any given time the logical arithmetic block  604  can easily calculate the bias control value  108  and the modulation value  109  needed to correct the error observed between the monitor output  105  and the replica path  710 . Over a number of data bursts, the system will adjust the currents so that the errors are minimised, and hence the laser will be operating at substantially the desired average optical output and with substantially the desired ER. 
         [0109]    Whilst this invention has been described with reference to particular examples and possible embodiments thereof these should not be interpreted as restricting the scope of the invention in any way. It is to be made clear that many other possible embodiments, modifications and improvements may be incorporated into or with the invention without departing from the scope and spirit of the invention as set out in the claims.