Patent Publication Number: US-2023152463-A1

Title: Light source system

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 16/780,735, filed Feb. 3, 2020, which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a light source system, which may be used, for example, as the light source in a time-of-flight camera system. 
     BACKGROUND 
     Time-of-flight (ToF) camera systems are range imaging systems that resolve the distance between the camera and an object by measuring the round trip time of a light signal emitted from the ToF camera system. The systems typically comprise a light source (such as a laser or LED), a light source driver to control the emission of light from the light source, an image sensor to image light reflected by the subject, an image sensor driver to control the operation of the image sensor, optics to shape the light emitted from the light source and to focus light reflected by the object onto the image sensor, and a computation unit configured to determine the distance to the object by determining the amount of time between an emission of light from the light source and a corresponding reflection from the object. 
     ToF camera systems may measure distances ranging from a few centimetres to 100s or 1000s of metres. Given the high speed of light, a time difference of only 16.66 ns between an emission of light and reception of reflected light corresponds to an object 2.5 m from the camera system. Therefore, ToF camera systems require high levels of temporal precision and control in order to measure distances accurately. 
     SUMMARY 
     The present disclosure relates to a light source system suitable for use in a time of flight camera. The light source system includes a light source, such as a laser, and a driver arranged to supply a drive current to the light source to turn the light source on to emit light. The driver includes a capacitor to store energy and then discharge to generate the drive current, and the driver is integrated into a semiconductor die on which the light source is mounted. Consequently, the driver includes within it the source of energy for the drive current and the light source and driver are very close together, meaning that the light source may be turned on and off very quickly with a relatively large drive current, in order to output a high optical power, short duration light pulse. 
     In a first aspect of the present disclosure, there is provided a light source system comprising: a light source; and a semiconductor die comprising an integrated first driver, wherein the light source is mounted on the semiconductor die and the first driver is coupled to the light source and configured to control supply of a first drive current to the light source for controlling operation of the light source, wherein the first driver comprises a first capacitor for storing electrical energy for use in generating the first drive current. 
     The first driver may be configured to operate in: a charging state, during which the first capacitor stores charge received from a power supply; and an emission state, during which the first capacitor discharges to generate the first drive current, which is supplied to the light source to turn the light source on. 
     The first capacitor may be set to any suitable size (i.e., capacitance), depending on the size of first drive current required and/or the duration of first drive current required and/or the duty cycle ratio between charging state and emission state. 
     The first driver may further comprise a first switch configured to: during the emission state, close a first drive current circuit comprising the first capacitor and the light source to carry the first drive current between the light source and the first driver; and during the charging state, open the first drive current circuit during the charging state to stop the supply of first drive current to the light source. 
     The first switch may be coupled between a cathode terminal of the light source and a reference voltage of the first driver, and wherein the first capacitor is coupled between an anode terminal of the light source and the reference voltage of the first driver. 
     The light source system may further comprise a controller configured to control the first switch, so as to control switching of the first driver between the charging state and the emission state. 
     The first switch may comprise a first transistor, wherein the controller is configured to control operation of the first switch by controlling a first transistor driver signal at a gate/base terminal of the first transistor. 
     The light source system may further comprise a turn-on pre-driver, wherein the controller is configured to turn on the first transistor by applying a turn-on signal to the gate/base terminal of the first transistor using the turn-on pre-driver in order to transition the first driver from the charging state to the emission state. 
     The light source system may further comprise a voltage regulator coupled to the first capacitor and the turn-on pre-driver, wherein the voltage regulator is configured to: receive energy from the first capacitor; and supply a regulated voltage to the turn-on pre-driver at least during transition of the first driver from the charging state to the emission state. 
     The light source system may further comprise a turn-off pre-driver, wherein the controller is configured to turn off the first transistor by apply a turn-off signal to the gate/base terminal of the first transistor using the turn-off pre-driver in order to transition the first driver from the emission state to the charging state. 
     To transition the first driver from the emission state to the charging state, the controller may be configured to apply both the turn-on signal and the turn-off signal to the gate/base terminal of the first transistor for a first period of time, and then apply only the turn-off signal to the gate/base terminal of the first transistor for a second period of time. 
     The light source system may further comprise a photodetector arranged to detect light emitted from the light source, wherein the light source system is further configured to charge the first capacitor only if the photodetector detects light emitted from the light source during the preceding emission state. 
     The light source may comprise a laser, for example a vertical-cavity surface-emitting laser. 
     The light source may comprise at least one terminal of a first polarity on a first surface of the light source and at least one terminal of a second polarity on a second surface of the light source, wherein the first driver is coupled to the at least one terminal of the first polarity and the at least one terminal of the second polarity such that the first drive current can flow through the light source to turn on the light source. 
     The second surface of the light source may be affixed to a first surface of the semiconductor die, and wherein the first driver is coupled to the at least one terminal of the first plurality by a first plurality of bonding wires. 
     The light source may comprise a first terminal of a first polarity, a second terminal of the first polarity and at least one third terminal of a second polarity, and wherein the first driver is coupled to the first terminal and the third terminal such that the first drive current can flow between the first terminal and the third terminal to turn on the light source, and wherein the semiconductor die further comprises an integrated second driver coupled to the second terminal and the third terminal, wherein the second driver is configured to control supply of a second drive current to the light source such that the second drive current can flow between the second terminal and the third terminal to turn on the light source, wherein the second driver comprises a second capacitor for storing electrical energy for use in generating the second drive current. 
     The first terminal and the second terminal may be are arranged on the light source such that they are substantially symmetrical about a plane of symmetry, wherein the first driver and the second driver are arranged within the die such that they are substantially symmetrical about the plane of symmetry. 
     In a second aspect of the present disclosure, there is provided a time of flight camera system comprising: a light source system comprising: a light source for emitting light towards an object to be imaged; and a semiconductor die comprising an integrated first driver, wherein the light source is mounted on the semiconductor die and the first driver is coupled to the light source and configured to control supply of a first drive current to the light source for controlling operation of the light source, wherein the first driver comprises a first capacitor for storing electrical energy for use in generating the first drive current; and a photodetector for receiving light reflected from the object to the imaged. 
     In a third aspect of the present disclosure, there is provided a driver for coupling to a light source to drive the light source, the driver comprising: at least one capacitor for storing charge; a controllable switch for switching the driver between a charging state and an emission state; a turn-on pre-driver coupled to the controllable switch, wherein the turn-on pre-driver is configured for use in controlling the controllable switch when transitioning from the charging state to the emission state; and a voltage regulator coupled to the at least one capacitor and the turn-on pre-driver and configured to supply a regulated voltage to the turn-on pre-driver, wherein the driver is configured such that during the charging state the at least one capacitor stores charge and during the emission state the at least one capacitor dischargers to supply a drive current to the light source to turn the light source on. 
     The driver may be configured for coupling to a power supply such that during the charging state, the at least one capacitor stores charge received from the power supply. 
    
    
     
       DRAWINGS 
       Aspects of the present disclosure are described, by way of example only, with reference to the following drawings, in which: 
         FIG.  1    shows an example representation of a light source system, in accordance with an aspect of the present disclosure; 
         FIG.  2 A  shows a view of the arrangement of the VCSEL  110  mounted on the semiconductor die  120  of the light source system of  FIG.  1   ; 
         FIG.  2 B  shows a further view of the arrangement of the VCSEL  110  mounted on the semiconductor die  120  of the light source system of  FIG.  1   ; 
         FIG.  3    shows an example graph representing a drive current for the VCSEL and optical power of light output from the VCSEL of the light source system of  FIG.  1   ; 
         FIG.  4    shows a schematic representation of details of an example implementation of the light source system of  FIG.  1   ; 
         FIG.  5    shows an example representation of the first drive current generated by the first driver represented in  FIG.  4   ; 
         FIG.  6    shows a schematic representation of details of a further example implementation of the light source system of  FIG.  1   ; and 
         FIG.  7    shows a schematic representation of details of a further example implementation of the light source system of  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
     Many factors may affect the precision with which a ToF camera system can measure distance to an object. One of those factors is the nature of the light emitted from the light source. The accuracy with which the system knows the moment of light emission may affect how accurately the system can determine distance to the object. For example, if the time difference between the moment the system believes it emitted light and the moment at which the reflected light was received is 20 ns, an object distance of about 3 m will be determined. However, if the emission of light actually took place about 0.4 ns after the system thought it took place, the true time difference between light emission and light reflection is in fact about 19.6 ns, which equates to an object distance of 2.94 m. For many safety critical applications, an error of this size may be significant. 
     The inventors have realised that if the light source is controlled to emit a very short pulse of light (in the order of 100 ps or less), the precision of the ToF camera system may be improved. A relatively long light emission pulse may render it unclear whether reflected light corresponds to photons emitted at the very start, middle or very end of the light emission pulse. To address this, more samples may be gathered to help de-convolve the shape of the laser, but this slows down the system and consumes more power. However, if the light emission pulse is very short, such as less than 100 ps, this represents a maximum uncertainty of 0.1 ns, which may be acceptable for most applications. Consequently, the time between emission of light and corresponding reflection of light may be measured with more certainty. 
     To achieve very fast turn on of the light source, the inventors have determined that a high current light source driving signal may be beneficial. Driving the light source with a relatively high, short duration current pulse should shorten the duration of the output light pulse, and increase the peak optical power output. The inventors have realised that this higher peak optical power may bring a further benefit in improving the precision of the ToF camera system. In particular, a number of safety regulations limit the average optical power emitted from ToF camera systems with no limit on the peak optical power output. By reducing the light emission duration, a system may emit higher optical power whilst staying within safety regulations. Emitting light with higher peak optical power may be beneficial for improving the range and precision of the ToF camera system. 
     However, there are many challenges in driving a light source to emit a very short, high optical power light pulse. A very short, high current driving signal may be required for driving the light source, which may put considerable demands on the power supply/source providing the driving current and may affect other electrical components that are also using that power supply. For example, if the ToF camera system is integrated in a mobile phone, drawing a relatively high current for a very short period of time, without affecting any other components or functionality of the mobile phone, may require a very high quality, costly power system within the mobile phone. Furthermore, if the ToF camera system operates with relatively high current pulses, if a system fault/failure develops, the relatively high currents may present a safety risk to the ToF camera system and/or nearby devices/components and/or operators of the ToF camera system. 
     In addition to this, to achieve such short light emission pulses, the laser source should be turned on and off rapidly and with precision. Inherent resistances and inductances in the light source driver circuits not only contribute to electrical losses (thereby increasing the amount of current needed to achieve a particular optical power output from the light source), but also slows circuit transitions between current levels, thereby slowing turn on and turn off. 
     With all of these challenges in mind, the inventors have devised a light source system that is suitable for use in ToF camera systems. In the light source system, the light source driver is integrated into a semiconductor die and includes a capacitor as the energy source for generating the current to drive the light source. The driver is designed such that the capacitor may be gradually charged using a relatively low current power supply/source, and then rapidly discharged to provide most, if not all, of the energy required for the current driving the light source. Furthermore, the light source is mounted on, or stacked on, the driver such that the physical distance between the driver and the light source is minimised. By minimising the distance between the driver and the light source, the physical length of the current path between the driver and the light source may be reduced, thereby reducing electrical resistance and inductance. Furthermore, by using a capacitor to supply the majority, if not all, of the energy required for the driver current, the physical size of the circuit loop carrying the driver current may be reduced compared with using a separate/external power unit, which even further reduces resistance and inductance. A further benefit of using a capacitor in this way is that if a system fault/failure develops, there is only a limited amount of energy available for generating the high current, which should improve overall device safety. 
       FIG.  1    shows an example representation of a light source system  100 , suitable for use in a ToF system, in accordance with an aspect of the present disclosure. The light source system  100  comprises a light source  110 , in this example a vertical-cavity surface-emitting laser (VCSEL), and a semiconductor die  120 , such as a CMOS silicon die, or a GaN die, or a GaAs die, etc. Throughout this disclose, the light source  110  shall typically be referred to as a VCSEL, although it will be appreciated that any other suitable type of light source may alternatively be used, such as other types of laser or any suitable type of LED. 
     The VCSEL comprises a first surface  112 , from which light is emitted, and a second surface  114 , which opposes the first surface  112 . The first surface  112  comprises a first terminal  116  and a second terminal  118  both of a first polarity, the first polarity in this example being the anode of the VCSEL diode. The second surface  114  comprises one or more third terminals of a second polarity, the second polarity in this example being the cathode of the VCSEL diode. The VCSEL  110  is mounted on, or stacked on, the semiconductor die  120 . The second surface  114  of the VCSEL  110  is mounted on a first surface  122  of the semiconductor die  120  and bonded to the first surface  122  of the semiconductor die  120  using bonding material  150  and electrical interconnects, in such a way as to form an electrical connection between the one or more third terminals of the VCSEL  110  and the first and second drivers  130  and  140 . Any suitable bonding material  150  and bonding techniques may be used for this, for example an epoxy or eutectic bond. 
     The semiconductor die  120  comprises an integrated first driver  130  and an integrated second driver  140 . The first driver  130  is configured to control supply of a first drive current  136  to the VCSEL  110  for controlling operation of the VCSEL  110  (i.e., for controlling light emission from the VCSEL  110 ). The second driver  140  is configured to control supply of a second drive current  146  to the VCSEL  110  for controlling operation of the VCSEL  110  (i.e., for controlling light emission from the VCSEL  110 ). The first driver  130  comprises a first switch  134 , which in this example is a first FET, for controlling the flow of the first drive current  136 , and the second driver  140  comprises a second switch  146 , which in this example is a second FET, for controlling the flow of the second drive current  146 . The first driver  130  is electrically coupled to the first terminal  116  of the VCSEL  110  by one or more first bond wires  152  (although alternatively any other suitable form of electrical coupling may be used, for example depending on the design of the VCSEL  110 ). The second driver  140  is electrically coupled to the second terminal  118  of the VCSEL  110  by one or more second bond wires  154  (although alternatively any other suitable form of electrical coupling may be used, for example depending on the design of the VCSEL  110 ). When a sufficiently large first drive current  136  and/or second drive current  146  flows through the VCSEL  110 , the VCSEL  110  will turn on and be excited to lase and therefore emit light. When no drive current, or insufficient drive current, flows through the VCSEL  110 , no light should be emitted and the VCSEL is effectively turned off. Further details of the operation of the first driver  130  and the second driver  140  shall be given later in this disclosure. 
       FIGS.  2 A and  2 B  show different views of the arrangement of the VCSEL  110  mounted on the semiconductor die  120 . As can be seen, in this example there are eight first bond wires  152  and eight second bond wires  154 , although any number of first bond wires  152  and second bond wires  154  may be used (for example, one, two, three, etc). It may be preferable to use the largest number of first bond wires  152  and second bond wires  154  possible for a given size of first terminal  116  and second terminal  118 . By doing so, resistance for the first drive current  136  and the second drive current  146  may be minimised, thereby improving the power efficiency of the light source system  110  and maximising the amount of drive current delivered to the VCSEL  110 . 
       FIG.  3    shows graphs representing drive current (labelled VCSEL Current) and optical power of light output from the VCSEL  110 . It can be seen that in this example, a drive current pulse duration of about 290 ps may be created by the drivers  130 ,  140 , which may excite a light pulse emission from the VCSEL  110  with an effective duration of about 30 ps. It will be appreciated that the drive current pulse duration is longer than the light pulse duration largely as a consequence of lasing delays in the VCSEL  110 . It will also be appreciated that this is merely one non-limiting example of drive current and light pulse duration, and that the light source system  100  may be configured to operate with different durations. 
     It may be counterintuitive to mount the VCSEL  110  on the integrated driver circuits  120 , particularly when the drive currents are intended to be relatively high (for example, &gt;1 A, or &gt;3 A, or &gt;5 A, or &gt;8 A, or &gt;10 A) owing to thermal dissipation difficulties. The VCSEL  110  and the drivers  130  and  140  are likely to generate significant heat when the drive currents are flowing, which should ideally be quickly and effectively dissipated in order to avoid device damage and degradation. It might have been expected that stacking the VCSEL  110  on top of the integrated drivers  130  and  140  would be likely to make heat dissipation considerably more difficult. 
     However, the inventors have recognised that by mounting the VCSEL  110  on the semiconductor die  120  which includes the integrated drivers  130 ,  140 , the physical distance between the VCSEL  110  and the drivers  130 ,  140  may be minimised. Consequently, a first drive current circuit formed by the VCSEL  110 , the first capacitor  132  and the first switch  134  to carry the first drive current  136  may be significantly smaller than other arrangements (for example, arrangements where the VCSEL  110  and the first driver  130  are mounted side-by-side on a PCB substrate with bonding wires carrying current both to and from the VCSEL  110 ). Likewise, the same is also true of the size of the second drive current circuit formed by the VCSEL  110 , the second capacitor  142  and the second switch  144 . This reduction in the size of circuits carrying the relatively high drive currents may reduce the resistance and inductance of the circuit, which may reduce circuit losses and increase the speed with which the drive currents, and therefore the VCSEL  110 , can be turned on and off. This enables the drive currents to be generated as a very short duration pulse, resulting in a very short duration pulse of light from the VCSEL  110 . Thus, by implementing the stacking arrangement represented in  FIGS.  1 ,  2 A,  2 B , a shorter duration (for example &lt;200 ps, or &lt;150 ps, or &lt;100 ps, or &lt;80 ps, or &lt;50 ps), higher optical power pulse of light may be output by the VCSEL  110 . As explained earlier, in order to achieve this relatively short duration optical output from the VCSEL  110 , a slightly longer duration drive current pulse may be required, for example to achieve a 30 ps light pulse, a 300 ps duration first drive current  136  may be required. 
     Not only may this improve the precision of a ToF system that uses the light source system  100 , it has unexpectedly been realised that because of the short duration of current achieved by this arrangement, heat dissipation may not in fact be as significant a problem as might initially have been thought. ToF systems may operate by emitting light for a period of time and then turning off for a period of time. For example, a typical light emission duration for a SPAD direct ToF system may be a pulse duration of about 1-3 ns every 0.2 μs, representing a duty cycle ratio of about 1:100 for light emission:no light emission. A typical light emission duration of a continuous wave (CW) indirect ToF system may be about 100-200 ns every 2 ms, representing a duty cycle ratio between about 1:20 to 1:10. However, in the present disclosure, because such a short, high optical power light pulse has been achieved, a duty cycle of at least 1:1,000, such as 1:5,000, or 1:10,000, may be implemented. For example, the approximately 30 ps light pulse represented in  FIG.  3    corresponds to a duty cycle ratio of about 1:30,000. Consequently, there is a relatively very long period of time during which heat is not being generated by the light source system  100 , during which time the heat generated during the emission part of the cycle may gradually dissipate into the surrounding environment and into the body of the die (and any other material coupled to the die). 
     The VCSEL  110  in the example represented in  FIGS.  1 ,  2 A and  2 B  has two terminals  116  and  118  of the same polarity, arranged to be substantially symmetrical about a plane of symmetry. In this example, the plane of symmetry extends perpendicular to the first and second surfaces  112 ,  114  of the VCSEL  110 , roughly through the middle of the first and second surfaces  112 ,  114 , such that the first terminal  116  is on one side of the plane of symmetry and the second terminal  118  is its mirror image on the other side of the plane of symmetry. The inventors have realised that it is possible to layout within the die  120  the components of the first driver  130  and the second driver  140  in such a way that they are substantially, or approximately, symmetrical about the plane of symmetry (as represented in  FIG.  1   ). Consequently, the direction of the path/loop of the first drive current  136  is substantially opposite to that of the second drive current  146  (for example, the path of the first drive current  136  may be clockwise and the path of the second drive current  146  may be anticlockwise). As a result of this, EM radiation generated by one of the current paths/loops may be substantially, or at least partially, cancelled by EM radiation generated by the other current path/loop, particularly at far-field. Thus, RF emissions from the light source system  100  (caused by the light source system  100  generating pulses of light at RF frequencies) may be reduced and kept acceptably low, even when the drive currents are relatively high. This means that the light source system  100  should not negatively affect other nearby electrical devices/components, or breach RF emissions legislation. It will be appreciated that whilst this symmetrical arrangement may have benefits, such an arrangement is not essential and the first driver  130  and second driver  140  may be relatively arranged in any other suitable way. 
       FIG.  4    shows a schematic representation of example details of the light source system  110 . The representation provides further details of an example implementation of the first driver  130 , to help explain the operation of the first driver  130  in order to control supply of the first drive current  136  to the VCSEL  110 . In this example, the first driver  130  further comprises a voltage regulator  410  (which may be implemented in any suitable way known to the skilled person), a turn-on pre-driver  420 , a turn-off pre-driver  422  and a FET  440  which acts as the first switch  134 . The light source system  430  also comprises a controller  430  configured to control switching of the first driver  130  between a charging state and an emission state. The control may be implemented in any suitable way, for example as logic integrated within the semiconductor die  420  or elsewhere, or using a microcontroller configured to operate as described below, or any other form of processor suitably configured to operate as described below. Likewise, the turn-on pre-driver  420  and turn-off pre-driver  422  may be implemented in any suitable way known to the skilled person, for example as buffers and/or digital buffers and/or amplifiers. 
     The first driver  130  is coupled to a power supply/source PVDD and PVSS, which may be a relatively low current power supply (for example, the first driver circuit  130  may draw less than 50 mA, or less than 30 mA, such as a typical average current of less than 20 mA, from the power supply). When the first driver  130  is in the charging state, the FET  440  is switched off (i.e., the first switch  134  is open), such that the current flowing through the VCSEL  110  is 0 A, or substantially 0 A (i.e., sufficiently low that there is no risk of the VCSEL  110  emitting any light). The power supply may be any suitable power supply to which the first driver  130  may be coupled. For example, if the light source system  100  is included as part of a larger device/system (such as a mobile device), the power supply may be the power supply of that larger device system (such as the battery of the mobile device). During the charging state, the first capacitor  130  is gradually charged by the power supply. 
     When a light pulse emission is desired, the first driver  130  may be transitioned from the charging state to the emission state. To control this transition, the controller  430  may use control lines  434  and/or  436  to control operation of the FET  440 . For example, the controller  430  may receive an instruction via its input line  432  (which may take any suitable form, for example it may be an LVDS differential signal) to start a light pulse emission from the VCSEL  110 . The controller  430  may then drive the control line  434  so that the turn-on pre-driver  420  applies a turn-on signal to the gate of the FET  440  in order to turn on the FET  440  (i.e., close the first switch  134 ). In this example, the turn-on signal is a voltage signal that exceeds the turn-on threshold voltage of the FET  440 . The turn-on pre-driver  420  is used in this example because the first drive current  136  that will flow through the FET  440  when it is turned on should be relatively large (for example, between about 5 A and 12 A, such as &gt;8 A, or &gt;10 A), so the FET  440  should be a relatively high power FET  440 . Most controllers may not be capable of supplying a sufficiently large drive signal to the FET  440  to turn it on, or at least may not be capable of supplying a sufficiently large drive signal to turn the FET  440  on quickly enough to achieve a quick transition from off to on. Therefore, the turn-on pre-driver  420  effectively functions to increase the signal set by the controller  430  on the control line  434  to a level sufficient to drive the FET  440  to turn on quickly. 
     In this arrangement, the power drawn by the turn-on pre-driver  420  and turn-off pre-driver  422  is supplied by the voltage regulator  410 , which in turn draws the majority, if not all, of its power from the capacitor  132  discharging. By arranging it in this way, most, if not all, of the current required to switch the first driver  130  between the charging and emission states is kept within the first driver  130  and is not drawn from elsewhere, such as the power supply PVDD and PVSS. Some of the benefits of drawing the majority, if not all, required energy from the capacitor  132 , rather than external sources, is explained earlier with reference to the first drive current  136 . Furthermore, if the voltage regulator  410  and the pre-drivers are also integrated into the die  120 , they may all be relatively close to each other and to the capacitor  132 , further increasing switching speed and reducing losses. 
     When the FET  440  is turned on (i.e., the first switch  134  is closed), the first driver  130  is in the emission state and the first drive current circuit is closed such that the first drive current  136  flows between the VCSEL  110 . 
     It can be seen that in this example arrangement of the first driver  130 , the first capacitor  132  is coupled between an anode terminal of the VCSEL  110  and a reference voltage (ground in this example, but the reference voltage could alternatively be any other suitable voltage level) of the first driver  130  (i.e., one terminal of the capacitor  132  is coupled to the VCSEL  110  and the other terminal of the capacitor  132  is coupled to the reference voltage). Whilst this particular configuration is not essential, it has a benefit that the VCSEL  110  can be driven by positive voltages within the first driver  130 , such that driving the VCSEL  110  does not require the generation of negative voltages within the first driver  130 . This may be beneficial to simplify operation of the first diver  130  and enables the components of the first diver  130  to be integrated in the semiconductor die  120  using, for example junction isolation, and not require dielectric isolation. 
       FIG.  5    shows an example representation of first drive current  136  flowing through the first drive current circuit. During the emission state, the first capacitor  132  discharges to generate the first drive current  136 . Consequently, the majority, if not all, of the energy required for the first drive current  136  is supplied by the first capacitor  132  during the drive time, such that the draw on the power supply PVDD and PVSS is very low (for example &lt;5%, or &lt;1%, of the first driver current  136  may be drawn from the power supply), if not effectively zero. As such, the majority, if not all, of the first drive current  136  is kept within the first driver  130  (i.e., no current, or no significant current, is drawn from sources elsewhere), within a relatively small first drive current circuit. In one alternative implementation, a further switch may be used to isolate the first driver  130  from the power supply when the first diver  130  is in the emission state, although in most implementations this should not be necessary since the first capacitor  130  provides so much energy during the emission state that the draw on the power supply is insignificant. 
     By utilising the first capacitor  132  in this way, the first capacitor  132  may be gradually, slowly charged by the power supply during the charging state, which should not affect the power supply in any significant way. The first capacitor  132  may then discharge to generate the first drive current  136  such that the power supply may be effectively, or entirely, unaffected by the relatively high first drive current  136 . Consequently, it may be possible for the power supply to be of a standard specification, thereby minimising costs, and may be used by other components/systems without being affected by the light source system  100 . Furthermore, because the first drive current  136  is generated effectively entirely by the integrated first capacitor  132  acting as a source of power, the first current driver circuit is kept relatively small (compared with the case where the first drive current  136  is drawn from an external power supply), which reduces delays in the first drive current  136  commencing and increases the speed of operation. 
     During the emission state, the first driver  130  may be configured such that the first capacitor  132  may be completely, or only partially, discharged in the process of generating the first drive current  136 . In some implementations, generating the first drive current  136  may result in only a partial discharge of the charge stored on the first capacitor  132 , such that the voltage across the first capacitor  132  is reduced (for example, by a few volts), but there is still a non-zero voltage across the first capacitor  132  at the end of the emission state. In this case, the voltage across the capacitor  132  may be monitored before, during and/or after the emission state, such that the amount of first drive current  136  supplied to the VCSEL  110  during the emission state may be determined. The size of the first capacitor  132  may be set to any suitable value depending on the amount of voltage headroom desired (i.e., the desired voltage across the capacitor at the end of the emission state) and/or the voltage of the power supply PVSS and PVDD and/or the desired first drive current level and/or the duration of the first drive current and/or the duty ratio of emission state to charging state. By way of non-limiting example, if a first drive current  136  of about 10 A is desired, for a first drive current duration of about 300 ps, with a desired reduction of voltage across the first capacitor  132  during the emission state of about 3V, the first capacitor  132  may have a capacitance of about 3 nF. 
     Furthermore, because the capacitor  132  can store only a finite amount of energy, if there is a fault or failure in the system, the relatively high first driver current  136  can only be sustained for a finite, relatively short, period of time. Consequently, the light source system  100  may have improved safety, compared with other devices that draw drive current from a less limited supply. 
     In order to transition the first driver  130  from the emission state to the charging state, the FET  440  is turned off (i.e., the switch  134  is opened), thereby opening the first drive current circuit and stopping the first drive current  136  from flowing through the VCSEL  110 . To control this transition, the controller  430  may use control lines  434  and/or  436  to control operation of the FET  440 . For example, the controller  430  may receive an instruction via its input line  432  to stop a light pulse emission from the VCSEL  110 . The controller  430  may then set control line  434  to a level that will turn off the FET  440 , for example to a voltage that is below the turn-on threshold voltage of the FET  440 . The controller  430  may also apply a turn-off signal to the gate of the FET  440  using the turn-off pre-driver  422 , which may be designed to have a very high drive strength and speed in one direction (i.e., pulling down the gate of the FET  440 ). For example, because in this implementation the turn-off pre-driver  422  is an inverting type pre-driver, the controller  430  may set the voltage at control line  436  to a ‘high’ voltage (e.g., 3.3V, or 5V), resulting in the output of the turn-off pre-driver  422  going ‘low’ (e.g., to 0V). Using the turn-off pre-driver  422  in this way, the speed with which the relatively high current FET  440  may switch off may be improved. The inventors have recognised that whilst this control of the turn-on pre-driver  420  and the turn-off pre-driver  422  may take place at the same time and a relatively fast turn off speed be achieved for the FET  440 , an even faster turn-off speed may be achieved by applying the turn-off signal to the gate of the FET  440  first (such that both the turn-on signal and turn-off signal are applied to the gate of the FET  440  for a first period of time) and then subsequently removing the turn-on signal such that then only the turn-off signal is applied to the gate terminal of the FET  440  for a second period of time. This effect may be particularly realised when the turn-off pre-driver  422  is designed to have a relatively high drive strength compared with the drive strength of the turn-on pre driver  420 , which results in the turn-off pre-driver  422  overdriving the turn-on pre-driver  420 . The superior drive strength of the turn-off pre-driver  422  may thus result in pulling down the gate of the FET  440  faster than could otherwise be achieved. It may be counter intuitive to drive the gate of the FET  440  with both the turn-on and turn-off signal for a first period of time in order to turn off the FET  440 , but the inventors have nevertheless realised that such driving may increase the turn off speed of the FET  440 . 
     It will be appreciated that each of the turn-on pre-driver  420  and the turn-off pre-driver  422  may be inverting or non-inverting type and the signals applied by the controller  430  to the control lines  434  and  436  set accordingly. Furthermore, it will be appreciated that the turn-off pre-driver  422  is optional and in an alternative it may be omitted entirety, with the FET  440  being turned off merely by changing the voltage applied to its gate so that it is below the turn-on threshold of the FET  440 . 
       FIG.  6    shows an example alternative implementation of the first driver  130 . This implementation is similar to that represented in  FIGS.  4  and  5   , but includes an additional turn-on pre-driver driver  610 , an additional control line  612  from the controller  430  and an additional FET  620 . The FET  620  is a relatively low current FET (for example, rated at around 400 mA, compared with about 10 A for the FET  440 ) and acts as a VCSEL pre-bias, to enable a relatively low, sub-lasing threshold, current to flow through the VCSEL  110  before the VCSEL  110  is fully turned on by the FET  440 . As such, prior to desiring the turn-on of the VCSEL  110 , the controller  612  may apply a turn-on voltage to the gate of the additional FET  620  (i.e., a voltage exceeding the turn-on threshold voltage of the additional FET  620 ) using the additional turn-on pre-driver  620 . Once the additional FET  620  is turned on, a relatively low current may flow through the additional FET  620  (for example, about 400 mA). Because the current is relatively low, and below the lasing threshold of the VCSEL  110 , the VCSEL  110  will not yet start emitting light. However, ideally, the current level will be only just below the lasing-threshold. When the VCSEL  110  is to be turned on, the FET  440  may be turned on as described above so that the relatively high first drive current  136  may flow through the first drive current circuit and turn on the VCSEL  110 . The additional FET  620  may then be turned off by the controller  430  by reducing the gate voltage at the additional FET  620 , for example once the FET  440  is fully turned on. It will be appreciated that using the additional FET  620  in this way may speed up the time between the controller  430  starting to turn on the FET  440  and light being emitted from the VCSEL  110 , since the VCSEL  110  will already be close to lasing at the time the FET  440  starts to switch on. 
       FIG.  7    shows an example of a further alternative implementation of aspects of the light source system  110 . This implementation shows a simplified implementation of the first driver  130  that includes just the first capacitor  132 , the turn-on pre-driver  420  and the FET  440 . However, it will be appreciated that the first driver  130  may alternatively be implemented in the ways represented in  FIG.  4 ,  5  or  6   . 
     In this implementation, a photodetector  710  is configured to receive light emitted from the VCSEL  110 . For example, the photodetector  710  may be the photodetector used for ToF imaging and may receive some stray light emitted from the VCSEL  110  and reflected directly back to the photodiode by device optics packaging, such that it detects light emitted from the VCSEL  110  (and not just light reflected from the object being imaged). Alternatively, it may be arranged in such a way as to directly receive light emitted from the VCSEL  110 . The photodetector  710  is coupled to a detector  720 , which is configured to output a signal  722  indicative of whether or not the photodetector  710  has received light output from the VCSEL  110 . The controller  430  may then be configured to control the operation of a charge pump  740  based on the received signal  722 . In particular, if the controller  430  transitions the first driver  130  to the emission state and subsequently receives a signal from the detector  720  indicating that the photodetector  710  received light output from the VCSEL  110 , the controller  430  may then enable the charge pump  740  using control line  732 . In this instance, the light source system can be assumed to be working correctly and the charge pump  740  will then be operable to charge the first capacitor  132  when the first driver  130  is next in the charging state. If, however, the detector  720  does not output a signal  722  indicative of the photodetector  710  having received light output from the VCSEL  110 , the controller  430  may disable the charge pump  740  using control line  732 . The charge pump  740  may then not charge the first capacitor  132  when the first driver  130  is next in the charging state, such that no further energy will be stored by the first capacitor  132  for use in trying to drive the VCSEL  110 . In this instance, the photodetector  710  may not have received any light emitted from the VCSEL  110  because there may be some error or failure in the light source system that is preventing the VCSEL  110  from emitting light properly. Since the light source system may be used in safety critical ToF applications, or the fault may be of a type that would be electrically dangerous to the light source system and/or a nearby system and/or an operator, preventing further capacitor charging and attempted emissions from the VCSEL  110  may improve the overall safety of the light source system. Optionally, the controller  430  may be configured to output a failure warning to any other suitable systems/entities. 
     Whilst a charge-pump  740  is used in this particular example, any other suitable circuit/component may be used to control connection/disconnection of the first driver  130  to the power supply based on detection of light emitted from the VCSEL  110 . For example, the charge pump  740  could be replaced with a switch (such as a transistor) controlled by the controller  430  to connect or disconnect the first driver  130  from the power supply PVDD. In a further alternative, rather than using a charge pump  740 , the power supply could be a controllable power supply, such as a switch mode power supply, which may be controlled by the controller  430  to provide power, or not, to the first driver  130 . 
     Whilst  FIGS.  4  to  7   , and the above explanations, relate only to the first driver  130 , it will be appreciated that the second driver  140  may be implemented in exactly the same ways. The light source system  100  may have a single controller  430  that is configured to control the operation of the first driver  130  and the second driver  140  such that they each generate the first drive current  136  and the second drive current  146  at substantially the same time, or each of the first driver  130  and second driver  140  may have their own controller that operates as described above. Likewise, each of the first driver  130  and the second driver  140  may have a respective voltage regulator, or a single voltage regulator may be used to power the pre-drivers in both the first driver  130  and the second driver  140 . 
     The skilled person will readily appreciate that various alterations or modifications may be made to the above described aspects of the disclosure without departing from the scope of the disclosure. 
     For example, whilst the above example first driver  130  includes one or more pre-drivers  420 ,  422 , etc, in an alternative, the pre-drivers may be omitted entirely and the FET  440  (and optionally also additional FET  620 ) may be driven directly by the controller  430 . In this alternative, the voltage regulator  410  may be omitted entirely. Furthermore, even when the first driver  130  includes one or more pre-drivers, whilst it may be beneficial to power the pre-drivers using the voltage regulator  410 , at least one of the one or more pre-drivers may alternatively be powered in any other suitable way and the voltage regulator omitted from the first driver  130 . 
     Whilst in the above examples, FETs are used (for example, FET  440  and additional FET  620 ), any other suitable type of controllable switch may alternatively be used, for example any other type of transistor, such as BJTs, etc. Therefore, whenever the gate of a FET is referred to, this should be understood to be the gate/base of a transistor. 
     In the above disclosure, two drivers (the first driver  130  and the second driver  140 ) are described. Using two drivers may be particularly beneficial for the VCSEL  110  design represented in  FIGS.  1 ,  2 A and  2 B , where there are two anode terminals on the surface of the VCSEL  110 , so that the two drivers may be arranged symmetrically within the die  120 . However, in an alternative, only a single driver (for example, only the first driver  130 ) may be implemented to provide all of the drive current required to drive the VCSEL  110 . For example, the VCSEL  110  may be of a design where there is only one anode terminal. Alternatively, the VCSEL  110  may be of a design where there are two or more anode terminals, in which case the single driver may be coupled to any one or more of the anode terminals. 
     In a further alternative, the die  120  may comprise more than two integrated drivers arranged to drive the VCSEL  110 . For example, it may comprise four drivers, each of the same design as the first driver  130  described above. These plurality of drivers may be arranged in any suitable way within the die  110 , for example first and second drivers may be symmetrical to each other with reference to a first plane of symmetry, and the third and fourth drivers may be symmetrical to each other with reference to a second plane of symmetry that is perpendicular to the first plane of symmetry. 
     Whilst the above light source system  110  is described particularly with reference to use with ToF camera systems, the light source system  110  is not limited to this use and may be used for any other purpose. 
     The terms ‘coupling’ and ‘coupled’ are used throughout the present disclosure to encompass both direct electrical connections between two components/devices, and also indirect electrical coupling between two components/devices where there are one or more intermediate components/devices in the electrical coupling path between the two components/devices. 
     Whilst the first driver  130  is described as having a switch  134  for use in controlling the transition or switching between the charging state and emission state, it will be appreciated that the first driver  130  may be configured in any other suitable way, using any other suitable components to switch or transition the first driver  130  between a charging state, where the first capacitor  132  gradually stores charge received from a power supply and the VCSEL  110  is turned off, and an emission state where the first capacitor  132  discharges to generate the first drive current  136  to turn on the VCSEL  110 . Likewise, it is not essential that a controller  430  is used to control the switching or transition of the first driver  130  between the charging state and the emission state. Any other suitable arrangement or circuit could alternatively be used for that purpose, for example a timer circuit configured to transition the first driver  130  at regular intervals, or a circuit configured to transition the first driver  130  based on the amount of charge stored on the first capacitor  132  (for example, switching to the emission state when the charge stored on the first capacitor  132  reaches a predetermined level), etc.