WO2006000957A2 - A laser diode drive arrangement - Google Patents

Abstract

The invention relates to a laser diode drive arrangement (100) for an optical storage drive. The laser diode drive arrangement (100) comprises a laser diode drive circuit (103) which provides a drive voltage on an output, and a laser diode (101) for writing to an optical medium. In addition, the laser diode drive arrangement (100) comprises a voltage bias circuit (107) which generates an adjustable bias voltage. The laser diode (101) and the voltage bias circuit (107) are arranged in a serial coupling coupled to the output of the laser diode drive circuit (103). During write operations the drive voltage of the drive circuit (103) is offset by the adjustable bias voltage, thereby providing a laser diode voltage which may be optimised for the current characteristics. The invention provides improved write pulse shaping and thus improved write performance. In addition, the invention provides increased flexibility in driving different types of laser diodes from the same drive circuit.

Description

A laser diode drive arrangement

The invention relates to a laser diode drive arrangement and in particular, but not exclusively, to a laser diode drive arrangement for a DVD (Digital Versatile Disc) or CD (Compact Disc) storage drive.

In latter years, optical data storage has become increasingly widespread and optical storage drives are now ubiquitous in e.g. computers and consumer electronics. For example, optical media such as DVDs (Digital Versatile Discs) or CDs (Compact Discs) have become one of the most common means of distributing and storing audio and video content items such as films and music. Accordingly, much research and development activity has been focussed on improving optical disk drives for reading and writing on optical media. Typically, an optical disk drive comprises a motor for rotating the optical disk as well as one or more motors for moving an Optical Pick-up Unit (OPU) comprising a laser diode reading in a radial direction with respect to the disc, thereby allowing all areas of the disc to be accessed. Writing to an optical disc is accomplished typically by one or more laser diodes to which electrical power is applied, resulting in emitted laser light being directed to a desired area on the disc. The laser diode is typically driven by a driver circuit implemented in an integrated circuit. The supply voltage for the integrated circuit limits the dynamic range for the drive voltage. For example, if the drive circuit is fed by a supply voltage of Vcc=5V, the maximum output voltage swing of the drive circuit is typically from 0 to around 4V. Typically, the drive circuit comprises one or more output Field Effect Transistors (FETs) connected to the supply voltage. These FETs are switched on during a write pulse, thereby feeding current to the laser diode. Hence, the write pulse may be achieved by the laser diode being connected to the supply voltage through the output transistor. The laser diode is typically connected between ground and the output of the drive circuit, resulting in the output voltage from the drive circuit being equal to the laser diode voltage. The current through the laser diode is frequently controlled by a feedback loop, ensuring that a write pulse having the desired energy and intensity is created. A critical issue in the design of laser diode drivers is the choice of the supply voltage. Conventionally, a supply voltage is selected which is guaranteed to be sufficiently high to deliver the maximum required drive power in all circumstances including for all specified operational temperatures and media. Furthermore, the supply voltage is typically selected to be suitable for the drive circuit IC which frequently operates on a standard supply voltage. Specifically, a supply voltage of 5 V is typically used for the laser diode drive circuits in an optical storage device. However, this supply voltage may not be optimal for driving the laser diode. Furthermore, in some applications, such as dual layer DVD applications, increased power levels are required, making it preferable to have higher supply voltages. Similarly, some laser diodes, such as those used for blue lasers, require increased drive voltages. A solution would be to simply increase the supply voltage for the drive circuit. However, this would increase power dissipation in the driver, which is a critical parameter. Furthermore, as driver circuit ICs are typically developed for a standard voltage such as 5V, increased supply voltages may not be possible or suitable for existing drive components, thereby requiring drive components to be manufactured which are specifically aimed at applications requiring increased voltage levels. Another critical issue when driving a laser diode is that of providing a suitable transient performance and write pulse shape. For example, a write pulse is typically generated by switching a transistor connected to the supply voltage of the drive circuit on or off, thereby providing a step function resulting in a substantially square wave write pulse. However, it is well known that reactive components such as capacitors and inductors (including parasitic components) cause the ideal step function to be degraded and in particular may create a step function having a significant overshoot. Such an overshoot may degrade the writing performance significantly. Specifically, the write performance relies on a write laser light pulse being generated having a specific intensity and energy. A significant overshoot (or undershoot) of the laser diode voltage results in a significant deviation from the ideal square wave pulse, resulting in light intensity and energy values deviating from the expected values. Hence, the control of the laser diode is a highly critical parameter for an optical storage device and in particular pulse shape and transient performance of a write pulse are typically suboptimal, resulting in degraded writing performance. Hence, an improved laser diode drive arrangement would be advantageous, in particular an arrangement enabling increased flexibility in selection of supply voltages for the driver circuitry, reduction in power dissipation in one or more elements of the arrangement, an increase of the voltage range for the laser diode, improved pulse shape control, improved transient performance and/or improved write performance.

Accordingly, the Invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination. According to a first aspect of the invention, there is provided a laser diode drive arrangement for an optical storage drive comprising: a laser diode drive circuit for providing a drive voltage on an output; a laser diode for writing to an optical medium; a voltage bias circuit for generating an adjustable bias voltage; wherein the laser diode and the voltage bias circuit are arranged in a serial coupling, the serial coupling being coupled to the output of the laser diode drive circuit, whereby, in use, the drive voltage is offset by the adjustable bias voltage to provide a laser diode voltage. The inventors of the current invention have realised that the performance of the drive arrangement may be improved by controlling the laser diode voltage with respect to the drive voltage. In particular, the inventors have realised that the pulse shape and transient performance of a drive circuit may depend on controlling an offset of the drive voltage with respect to the laser diode voltage. In particular, the bias circuit may assist in controlling and thus improving the pulse shape transient response of the laser diode drive arrangement and may specifically reduce the overshoot of the laser diode voltage at transitions. For example, the bias circuit may offset the laser diode voltage relative to the output voltage such that the output voltage is closer to the supply voltage, thereby reducing any voltage overhead available for overshoot. Furthermore, the dynamic performance of the drive circuit output may depend on the output voltage level, thereby allowing this to be optimised. Hence, an improved writing performance may be achieved. Furthermore, the bias circuit may effectively separate the design criteria for the drive circuit from the voltage characteristics of the laser diode. Thereby, the invention may provide increased flexibility in the choice of output voltages and thus supply voltages for the drive circuit. In particular, the bias circuit may provide an offset required for a specific drive circuit to drive a specific laser diode. This may in particular allow a given drive circuit to be used with an increased range of laser diodes. For example, a 5V drive circuit IC may be used to drive laser diodes requiring drive voltages higher than the drive circuit IC can provide for a supply voltage of 5 V. The serial coupling comprises the laser diode in series with the voltage bias circuit but may also include other components which are not necessarily in series with the laser diode and the voltage bias circuit. Furthermore, the order of the serial coupling of the laser diode and the voltage bias circuit is not essential and the laser diode and the voltage bias circuit are not necessarily connected directly to each other but may for example include other components in series with the laser diode and the voltage bias circuit and connected in between these. However, in some embodiments the serial coupling comprises only the laser diode and the voltage bias circuit. In some embodiments, the laser diode drive circuit has a single output and the serial coupling is coupled between the output and a fixed voltage such as ground or the supply voltage. However, in some embodiments, the laser diode drive circuit may for example have a differential output and the serial coupling may be coupled between the differential outputs. The optical storage drive may for example be a CD, DVD or BD storage drive. According to a preferred feature of the invention, the laser diode voltage is substantially equal to the sum of the drive voltage and the adjustable bias voltage. The serial coupling may in particular comprise only the voltage bias circuit and the laser diode. This may provide for a simple and efficient implementation. According to a preferred feature of the invention, the adjustable bias voltage has a dynamic range comprising negative and positive voltages. This provides a high degree of flexibility and may for example allow laser diode voltages to be both reduced and increased with respect to the output voltage. According to a preferred feature of the invention, the adjustable bias voltage has a polarity providing a laser diode voltage having a higher absolute voltage than an absolute voltage of the drive voltage. This may in particular allow for a laser diode voltage to be of a larger magnitude than the supply voltage. For example, if the drive voltage is positive, the bias voltage may have a polarity that increases the laser diode voltage. According to a preferred feature of the invention, the laser diode drive is comprised in an integrated circuit not comprising the voltage bias circuit. The invention may provide an easy- to- implement means of separating the requirements and characteristics of a laser diode from the requirements and characteristics of an integrated circuit driving the laser diode. In particular, an integrated circuit having a given supply voltage limiting the output voltage may be used to drive a laser diode suited for a higher drive voltage. Furthermore, the separation of functionality may allow increased design freedom and may specifically allow power dissipation of an integrated drive circuit to be reduced. According to a preferred feature of the invention, the voltage bias circuit comprises a signal input and is operable to adjust the bias voltage in response to a signal on the signal input. The signal input may for example be an analog or digital signal input and the signal may be derived in any suitable way, including in response to a determined characteristic or parameter associated with the optical storage drive, or may be a direct user input. The feature may provide a very flexible laser diode drive arrangement suitable for many different types of laser diodes, media types etc. This may facilitate manufacturing and reduce cost as a standard laser diode drive arrangement may be manufactured and customised to the requirements of the specific application. The signal may furthermore be used to dynamically update the voltage control for example in order to optimise the dynamic (transient) performance. According to a preferred feature of the invention, the voltage bias circuit is operable to adjust the adjustable bias voltage in response to a type of optical medium. This may provide for an improved drive of a laser diode that is specifically suited for the specific characteristics of the type of optical medium being written to. According to a preferred feature of the invention, the voltage bias circuit is operable to adjust the adjustable bias voltage in response to a type of laser diode. This may provide for an improved drive of a laser diode that is specifically suited for the specific characteristics of the laser diode used. According to a preferred feature of the invention, the voltage bias circuit is operable to adjust the adjustable bias voltage in response to a supply voltage of the drive circuit. This may provide for an improved drive of a laser diode that is specifically suited for the specific supply voltage of the drive circuit. As the dynamic and static characteristics of the output voltage may depend strongly on the supply voltage, this allows optimisation of the voltage drive of the laser diode for the characteristics of the drive circuit. According to a preferred feature of the invention, the voltage bias circuit is operable to adjust the adjustable bias voltage in response to a writing power required for writing on the optical medium. This may provide for an improved drive of a laser diode that is specifically suited for the specific characteristics of the writing operation to be performed. According to a preferred feature of the invention, the voltage bias circuit is connected to the output. This provides for a suitable implementation and may facilitate integration of the voltage bias circuit and the laser diode drive circuit. According to another preferred feature of the invention, the laser diode is connected to the output. This provides for a suitable implementation of a separate bias circuit. These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which Fig. 1 is an illustration of a laser diode drive arrangement in accordance with an embodiment of the invention.

The following description focuses on an embodiment of the invention applicable to an optical storage drive for writing to a CD, DVD or BD disc, but it will be appreciated that the invention is not limited to this application and may be applied to other optical storage drives using a laser diode. FIG. 1 is an illustration of a laser diode drive arrangement 100 in accordance with an embodiment of the invention. The laser diode drive arrangement 100 is part of a DVD/CD recorder further comprising functionality required or desired for implementing the storage function, including data interface circuitry, control circuitry, a motor for rotating the disc, motors for moving an OPU etc., as is well known to the person skilled in the art and which for brevity and clarity will not be described in further detail. The laser diode drive arrangement 100 comprises a laser diode 101 which generates a laser light beam that is directed to an optical disc by a suitable arrangement as is well known in the art. The laser diode drive arrangement 100 may comprise laser diodes having different characteristics, such as for example: A red laser diode which typically has an active voltage drop when emitting light of between 1.8 V and 3.0V (depending on the current and thus the intensity of the emitted light), An infrared laser diode which typically has an active voltage drop when emitting light of between 1.6V and 3.0V (depending on the current and thus the intensity of the emitted light), and A blue laser diode which typically has an active voltage drop when emitting light of between 3.2 V and 5.0V (depending on the current and thus the intensity of the emitted light). The laser diode 101 is connected to an output of a laser diode drive circuit 103 which generates a drive voltage for the laser diode. The drive circuit 103 is controlled by control logic (not shown) which controls when write pulses are supplied to the laser diode 101 , as is well known in the art. The drive circuit 103 is operated on a supply voltage, which is generated by a voltage source 105. In the specific exemplary embodiment, the supply voltage is 5V and the drive circuit is implemented in a single integrated circuit (IC). Thus, the drive circuit 103 is contained in a single IC, which is designed for a fixed supply voltage of 5V. In the embodiment, the output of the drive circuit 103 comprises a FET (or equivalently a plurality of FETs coupled in parallel), which is coupled between the positive supply voltage and the output. In the embodiment, the drive circuit 103 generates a positive voltage on the output and the anode of the laser diode 101 is connected to the output. The dynamic voltage of the drive voltage is limited by the supply voltage of the drive circuit 103. In theory, the drive voltage at the output may have a dynamic range between OV and 5V but in practice the upper limit is reduced due to voltage drops internally in the drive circuit 103 and specifically due to the voltage drop of the output FET. Thus, in practice the dynamic range of the drive voltage at the output of the drive circuit 103 is limited to around 4 V. Rather than the cathode of the laser diode 101 being connected between the output of the drive circuit 103 and ground (corresponding to the negative output of the voltage generator 105), it is in the current embodiment connected in series to a voltage bias circuit 107. The voltage bias circuit 107 generates an adjustable bias voltage that biases the laser diode voltage with respect to the drive voltage. Hence, the voltage bias circuit 107 and the laser diode 101 form a serial coupling, wherein the adjustable bias voltage of the voltage bias circuit 107 offsets the drive voltage of the drive circuit 103, thereby providing the laser diode voltage. In the exemplary embodiment, the serial coupling consists of the laser diode 101 and the voltage bias circuit 107; the laser diode is thus directly provided with the difference between the drive voltage and the adjustable bias voltage (using the sign conventions indicated in FIG.l, i.e. that the voltage across an element is considered positive if the voltage potential is higher for the connection furthest removed from ground in the topology). Thus, in the embodiment, the laser diode voltage is given as:

where V_DC is the voltage at the output of the drive circuit 103 and V_BC is the adjustable bias voltage. Hence, the laser diode drive arrangement 100 of the current embodiment provides a voltage bias circuit 107 that allows the active voltage drop range of the laser diode to be offset relative to the dynamic range of the drive voltage. As an example, the voltage bias circuit 107 may provide an adjustable bias voltage that allows the drive circuit to drive a laser diode which has a voltage drop characteristic that does not match the dynamic range of the drive circuit 103. Specifically, a drive circuit 103 having a supply voltage of 5 V may typically be able to generate a loaded voltage in the range of 0 to around 4V. The drive circuit 103 may be used to drive for example a red or infrared laser diode directly. However, it is unsuitable for driving a blue laser diode as this has a voltage drop between typically 3.2 V and 5 V, which exceeds the range of the drive circuit 103. However, by simply adjusting the adjustable bias voltage of the voltage bias circuit 107 to for example -2 V, an effective dynamic range of between 2 V and 6V can be achieved for the laser diode voltage from the dynamic range of 0 to 4V of the drive circuit 103. Thus, in this embodiment, the adjustable bias voltage has a polarity which results in a laser diode voltage having a higher absolute voltage than an absolute voltage of the drive voltage. Accordingly, the voltage bias circuit 107 may be used to modify the drive voltage such that the drive circuit 103 can be used with different types of laser diodes. This is a significant advantage as it may allow a given drive IC being designed for a red or infrared laser diode to be used with an otherwise incompatible (blue) laser diode. This increases flexibility and may facilitate manufacturing and reduce cost as a single drive arrangement can be developed and manufactured for a range of laser diode types. The drive arrangement may then be customised for the specific laser diode type by simply setting the adjustable bias voltage to a suitable level. The adjustable bias voltage may in some embodiments simply be set manually during manufacturing. Hence, the embodiment may provide for increased use of a drive circuit as the voltage bias circuit effectively separates the drive voltage characteristics of the drive circuit 103 from the voltage drop characteristics of the laser diode. The inventors have also realised that controlling the adjustable bias voltage of the voltage bias circuit 107 may provide increased control over the pulse shape of the generated write pulse and may in particular improve the transient response. It is well known that laser diode drive arrangements include parasitic components, as a result of which the write pulse deviates from an ideal square wave pulse. For example, parasitic capacitances of e.g. the output FET(s) and the laser diode may combine with parasitic inductances of the connections to provide a resonating circuit resulting in a distorted pulse shape. For example, the parasitic components may result in an undershoot or overshoot of the drive voltage. A deviation from the ideal waveform may degrade the write performance. For example, an results in increased energy at the start of the pulse, distorting the pulse energy balance over time. This may lead to higher write jitter values. The pulse widths typically decrease at shorter write pulses and therefore the problem increases when shorter pulses are required. This is typically the case at higher speed recording. Hence, in order to increase the writing speed, it is desired to have shorter write pulses and therefore the undesired overshoot may reduce the write speed that can be achieved. In accordance with the preferred embodiment, the adjustable bias voltage is adjusted such that a desired transition is achieved. In particular, the bias voltage may be increased in order to reduce the amount of overshoot. For example, if a 5 V drive circuit, as described above, is used in connection with a red laser diode, a write pulse will be initiated by increasing the driver output current with a steep slope, and hence the output voltage will increase, enabling the parasitic inductance(s) to follow this current step. Depending on the parasitic values, the resulting laser current will show undesired under or overshoot. This effect can be eliminated by controlling the voltage across the laser including its parasitic inductance and capacitance, to an appropriate value. For example, setting the bias voltage to, for example , 0.8V will result in the margin in the drive circuit being substantially reduced, such that at the desired laser diode voltage of 0.8 V, the drive voltage will be 4V, causing clipping to begin. Thus, the bias circuit provides for the overhead being reduced such that any overshoot is clipped by the output circuit at a much lower voltage. In general, the dynamic characteristics of the drive circuit may vary as a function of the (output) drive voltage and accordingly the dynamic drive of the laser diode may be modified and controlled by offsetting the laser diode voltage, thereby changing the operating range of the drive voltage. The voltage bias circuit may also allow an increased thermal design freedom. For example, a 5V drive circuit directly driving a red laser diode at, for example, 200 mA and with a voltage drop of 3.2V, will dissipate an instantaneous power of P= (5 V - 3.2V) ^• 20OmA = 0.36 W in the output circuit (typically in the output FET(s)). However, by introducing an adjustable bias voltage of e.g. 0.8V, this power may be reduced to (5V - 3.2V - 0.8V) ^• 20OmA = 0.2 W, while the remaining 0.16 W may be dissipated in the voltage bias circuit. Hence, as exemplified, the power dissipation may be shifted from the drive circuit. This may be of major importance in some applications and may facilitate the thermal design of the laser diode drive arrangement. For example, in some applications the drive IC may be mounted on the OPU, whereas the bias circuit is mounted remote from the OPU. As the OPU is very small, the heat dissipation of the drive IC will cause the temperature of the laser diode to increase. The power consumption of a laser diode increases with increasing temperature and it is therefore desirable to reduce the operating temperature of the laser diode. Accordingly, it may be advantageous to reduce the heat dissipation of the drive IC by controlling the adjustable bias voltage accordingly. In the described embodiment, the laser diode is connected to the drive output. However, the order of the laser diode and the voltage bias circuit is not essential and in some embodiments the voltage bias circuit may be connected to the output of the drive circuit. This may facilitate implementation and specifically integrated implementation of the drive circuit and the voltage bias circuit. Furthermore, in some embodiments other components may be placed in series or in parallel with the laser diode and/or the voltage bias circuit. It will be appreciated that any suitable way of generating an adjustable bias voltage may be used, including using a variable voltage regulator. As a specific example, the voltage bias circuit 107 may be implemented as a simple operational amplifier coupled as a follower, i.e. having the output connected directly to the inverting input. The non-inverting input may be coupled to a Digital to Analog Converter (DAC) which sets the bias voltage in accordance with a digital signal being fed to it. Thus, in the preferred embodiment, the voltage bias circuit comprises a digital signal input corresponding to the digital input of the DAC. The DAC may be controlled by suitable control software of the optical storage drive. This may allow the laser diode drive arrangement to be controlled in a simple manner. The adjustable bias voltage may be adjusted in response to a number of parameters and in a semi-permanent way, such as during manufacturing or calibration, or it may be adjusted dynamically in response to current operating conditions. Specifically, as described above, the adjustable bias voltage may be adjusted in response to a type of laser diode. For example, during manufacturing, the adjustable bias voltage is set depending on whether a red, infrared or blue laser diode is used. Alternatively, or additionally, the adjustable bias voltage may be adjusted in response to a type of optical medium used. Typically, different storage media may have different write pulse requirements and, accordingly, the laser diode voltage and the pulse shape may depend on the specific medium. Therefore, the optimal adjustable bias voltage depends on the medium being written to, and the control software of the storage drive may determine the type of medium being inserted and, accordingly, set the DAC to provide the preferred adjustable bias voltage for that medium. Alternatively, or additionally, the adjustable bias voltage may be adjusted in response to a supply voltage of the drive circuit. For example, the supply voltage may be specified as being between 4.75V and 5.25V and the adjustable bias voltage may be dynamically adjusted to follow the variations in the supply voltage. Alternatively, or additionally, the adjustable bias voltage may be adjusted in response to a writing power required for writing on the optical medium. For example, double- layer writing requires a higher writing power than single-layer writing. Therefore, the drive voltage is often increased for double-layer writing and the bias voltage may advantageously be adjusted accordingly. The invention can be implemented in any suitable form. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed among different units and processors. Although the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term comprising does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus, references to "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

CLAIMS:

1. A laser diode drive arrangement for an optical storage drive, comprising: a laser diode drive circuit (103) for providing a drive voltage on an output; a laser diode (101) for writing to an optical medium; a voltage bias circuit (107) for generating an adjustable bias voltage; wherein the laser diode (101) and the voltage bias circuit (107) are arranged in a serial coupling, the serial coupling being coupled to the output of the laser diode drive circuit (103), whereby, in use, the drive voltage is offset by the adjustable bias voltage to provide a laser diode voltage.

2. The laser diode drive arrangement as claimed in claim 1, wherein the laser diode voltage is substantially equal to the sum of the drive voltage and the adjustable bias voltage.

3. The laser diode drive arrangement as claimed in claim 1, wherein the adjustable bias voltage has a dynamic range comprising negative and positive voltages.

4. The laser diode drive arrangement as claimed in claim 1, wherein the adjustable bias voltage has a polarity providing a laser diode voltage having a higher absolute voltage than an absolute voltage of the drive voltage.

5. The laser diode drive arrangement as claimed in claim 1, wherein the laser diode drive circuit (103) is comprised in an integrated circuit not comprising the voltage bias circuit (107).

6. The laser diode drive arrangement as claimed in claim 1, wherein the drive circuit (103) has a supply voltage and the drive voltage is limited by the supply voltage.

7. The laser diode drive arrangement as claimed in claim 1, wherein the voltage bias circuit (107) comprises a signal input and is operable to adjust the bias voltage in response to a signal on the signal input.

8. The laser diode drive arrangement as claimed in claim 1, wherein the voltage bias circuit (107) is operable to adjust the adjustable bias voltage in response to a type of optical medium.

9. The laser diode drive arrangement as claimed in claim 1, wherein the voltage bias circuit (107) is operable to adjust the adjustable bias voltage in response to a type of laser diode.

10. The laser diode drive arrangement as claimed in claim 1, wherein the voltage bias circuit (107) is operable to adjust the adjustable bias voltage in response to a supply voltage of the drive circuit (103).

11. The laser diode drive arrangement as claimed in claim 1, wherein the voltage bias circuit (107) is operable to adjust the adjustable bias voltage in response to a writing power required for writing on the optical medium.

12. The laser diode drive arrangement as claimed in claim 1, wherein the voltage bias circuit (107) is connected to the output.

13. The laser diode drive arrangement as claimed in claim 1, wherein the laser diode (101) is connected to the output.