Laser light source control

Abstract

The laser circuit has a current source for supplying current to the laser device. A pulse-width modulated laser drive current is used, and the amplitude and duty cycle of the laser drive current are set depending on the estimated junction temperature. In this manner, efficiency can be kept high for a variety of operating temperatures and desired optical output powers.

Description

[Technical field]

[0001] The present invention relates to the control of laser light sources. [Background technology]

[0002] It is generally known that lasers require a drive current higher than the lasing threshold current to emit substantial optical output power. Below this lasing current, the efficiency of photon generation is very low, and therefore the lasing current can be considered as the main contributor to efficiency loss. Therefore, from an efficiency standpoint, it is beneficial to maximize the drive current so that it is significantly higher than the lasing current.

[0003] The maximum drive current for maximum efficiency strongly depends on the junction temperature of a laser, such as a vertical-cavity surface-emitting laser (VCSEL). At high junction temperatures, a faster rollover (after a local maxima) of the optical output power as a function of forward current can be expected, and such maximum efficiency is junction-temperature dependent. The main objective of a laser illumination source is to ensure that a certain amount of light is emitted with the highest possible efficiency. Summary of the Invention [Problem to be solved by the invention]

[0004] It would therefore be desirable to be able to provide a constant output power (corresponding to a desired amount of light) under all operating conditions resulting in different junction temperatures. [Means for solving the problem]

[0005] The invention is defined by the claims.

[0006] According to an example according to an aspect of the present invention, A laser device; a current source adapted to supply a current to the laser device, the current having an amplitude and a duty cycle; a controller for controlling the current source; a sensor device for monitoring a signal making it possible to estimate a junction temperature of the laser device, The controller: - estimating a junction temperature of the laser device; - setting the amplitude of the current to the laser device in dependence on the junction temperature; A laser circuit is provided which is adapted to set the duty cycle of the current to the laser device depending on the power required for the laser device.

[0007] The laser circuit uses both the amplitude and duty cycle of the pulse width modulated laser drive current to enable high efficiency operation at a variety of junction temperatures, which not only saves energy but also reduces power dissipation problems.

[0008] In particular, an increase in temperature will cause the controller to implement a reduction in the amplitude of the current in order to shift to an efficient operating point, and an increase in the duty cycle to maintain a similar average current.

[0009] It is known that the average output power of a laser can be controlled by adapting the duty cycle of the PWM control signal or by adapting the drive current. Reducing the drive current to reduce the power is not beneficial because, as explained above, the efficiency is compromised by the increased contribution of the lasing current. It is also undesirable to reduce the output power of a laser by only controlling the duty cycle of the PWM control signal, since the drive current amplitude for maximum efficiency is temperature dependent.

[0010] The present invention therefore combines these two approaches to not only maintain a desired output, but also allow for highly efficient operation.

[0011] The current source may comprise a switching element coupled in parallel with the laser device or in series with the laser device, the switching element configured to control the duty cycle of the current to the laser device.

[0012] This may function as a shunt switch or a series switch. Alternatively, the current source itself may generate a pulse width modulated output current.

[0013] The controller is adapted to control the current amplitude and the duty cycle of the pulse width modulated laser drive current to achieve, for example, a desired efficiency and a desired optical output power. The desired optical output power may, for example, be constant.

[0014] The controller is adapted to control the current amplitude and the duty cycle of the pulse width modulated laser drive current, for example, to operate at an amplitude corresponding to maximum efficiency and at a duty cycle to provide the desired optical output power. By enabling operation at maximum efficiency, not only is energy saved, but also power loss problems are reduced. The point of maximum efficiency may be estimated based on known characteristics of the laser device, or the efficiency may be monitored to provide feedback control.

[0015] The sensor arrangement may comprise a temperature sensor for measuring a case temperature of the laser device, which may be used to provide an estimate of a junction temperature of the laser device, which may for example use thermal information relating to the device and its casing.

[0016] The sensor arrangement may additionally or alternatively comprise a flux sensor for measuring optical output power. The measured optical output power may be used in combination with data characterizing the optical output power as a function of junction temperature for a particular device. This characterization information may have been obtained, for example, during the manufacturing process of the laser device itself, or during assembly and factory calibration of the entire laser circuit.

[0017] A current amplitude measuring device may also be provided to measure the laser device current. This provides a feedback measurement of the drive current. The laser is driven with a current according to the setting of the current source, but measuring the current allows errors in the current setting to be detected.

[0018] The controller may be further adapted to determine an output power of the laser, and further to set an amplitude and duty cycle of the laser drive current in dependence on the output power.

[0019] In this approach, a feedback control loop is provided to allow the output power to be maintained at a desired constant level instead of inferring it based on driving conditions.

[0020] The laser device may comprise a vertical cavity surface emitting laser, or alternatively, the laser device may comprise one or more laser diodes.

[0021] The laser circuit is for example an illumination circuit for providing a constant optical output power.

[0022] The present invention relates to a method of controlling a laser device, the method comprising the steps of: estimating a junction temperature of the laser device; setting an amplitude of a pulse width modulated laser drive current in dependence on said junction temperature; setting a duty cycle of the pulse width modulated laser drive current depending on the power required for the laser device; and providing the laser drive current to the laser device.

[0023] The method may include controlling settings of the current amplitude and the duty cycle of the pulse width modulated laser drive current to achieve a desired efficiency and a desired optical output power.

[0024] The method may then include setting the current amplitude and the duty cycle of the pulse width modulated laser drive current to operate at an amplitude corresponding to maximum efficiency and at a duty cycle to provide the desired optical output power.

[0025] The invention also provides a computer program comprising computer program code means adapted to perform the method defined above when said computer program is run on a computer.

[0026] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments. [Brief description of the drawings]

[0027] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: [Figure 1] 4 shows the output power as a function of forward current as measured for a particular VCSEL. [Diagram 2] 1 shows an example of a PWM drive current with variable current amplitude and duty cycle for different junction temperatures. [Diagram 3] FIG. 2 shows a simplified block diagram of a laser circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will be described with reference to the drawings.

[0029] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the devices, systems and methods of the present invention will become better understood from the following description, appended claims and accompanying drawings. It should be understood that the figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to denote the same or similar parts.

[0030] The present invention provides a laser circuit having a current source for supplying current to a laser device. A pulse-width modulated laser drive current is used, and the amplitude and duty cycle of the laser drive current are set depending on the estimated junction temperature. In this manner, the efficiency can be kept high for a variety of operating temperatures and desired optical output powers.

[0031] The present invention can be applied to any laser that exhibits different output power vs. drive current functions at different junction temperatures. This applies to lasers and laser diodes. As just one example, we will illustrate the invention using measurements made on a Vertical Cavity Surface Emitting Laser (VCSEL).

[0032] 1 shows the optical output power (y-axis) as a function of forward current (x-axis) as measured for a particular VCSEL. Plot 10 shows the optical output power as a function of forward current at a case temperature of 20° C., and plot 20 shows the output power of the VCSEL as a function of forward current at a case temperature of 60° C.

[0033] For a more accurate assessment, Tj =P diss R th,(j-c) +T c The actual junction temperature can be determined by, where T j is the junction temperature of the VCSEL, and P diss is the dissipated power, R th,(j-c) is the thermal resistance between the junction and the case, and T c is the case temperature.

[0034] Figure 1 is instead based solely on case temperature measurements.

[0035] Looking at the main efficiency aspects of VCSELs, broadly speaking,

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[0036] As can be seen in FIG. 1, the output power for drive current levels below the lasing threshold current remains close to zero. Current levels above the lasing current result in a proportional increase in output power. At higher current levels, the proportional relationship between input current and output power is lost and the curve begins to level off. This shows a decrease in efficiency at higher currents, e.g., above 600 mA for plot 10. At higher case temperatures, there is even a rollover effect where the VCSEL output power decreases with increasing forward current, as can be seen, e.g., after 800 mA in plot 20.

[0037] It can therefore be concluded that VCSELs have a maximum operating efficiency that depends on the junction temperature. The decrease in efficiency above a certain drive current is related to the carrier concentration at the junction, which is itself junction temperature dependent. Thus, the peak efficiency is found to be strongly related to the junction temperature.

[0038] Laser lighting applications typically require a certain given average optical output power. With this constraint, the highest efficiency can be achieved by maximizing the forward current so that the fraction of the lasing current is small compared to the forward current, while not exceeding operation where high current densities in the junction reduce the efficiency.

[0039] In practice, this maximum forward current can be expected to be just beyond the linear proportional slope (of output power vs. input current), and thus where the slope begins to decrease slightly. Based on the known characteristic curve of the laser device, a point of (estimated) maximum efficiency can then be determined based on measurements or estimates of the junction temperature.

[0040] The desired average output power can then be obtained by setting the duty cycle of the PWM control signal.

[0041] Depending on the application, the case temperature may vary, which indirectly may also vary the junction temperature, and therefore it is desirable to adapt the control of the laser device depending on the temperature, even for a given application.

[0042] 2 shows an example of a PWM drive current with variable current amplitude and duty cycle to maximize the VCSEL output power efficiency at two different junction temperatures: plot 30 is for a junction temperature of 25° C., and plot 40 is for a junction temperature of 60° C.

[0043] As can be seen, at higher junction temperatures, the current amplitude is reduced but the duty cycle ratio is increased. The current amplitude is reduced because the linear portion of the plot in Figure 1 ends at lower drive currents. The duty cycle is increased to maintain the desired optical output power.

[0044] The use of control signals as described above results in the circuit operating at a high current amplitude and low duty cycle at low case temperatures or during initial start-up. As the system heats up, the duty cycle increases while the current amplitude decreases. However, the average optical output power remains constant.

[0045] From FIG. 1 it can be seen that an increase in temperature causes a decrease in efficiency, which can result in increased heating of the semiconductor. Thus, in some circumstances, a thermal runaway condition can occur. Therefore, protection against thermal runaway can be used as part of the laser control technique. The increased heating can be determined, for example, from a determined or estimated junction temperature, as described below.

[0046] FIG. 3 shows a simplified block diagram of a laser circuit 100 having a laser device 102, in this case represented as a series connection of laser diodes D1 through Dn, and a current source 104 for supplying current to the laser device.

[0047] The controller 106 controls the current amplitude I of the pulse width modulated laser drive current supplied to the laser device by the current source 104. dcand a duty cycle. A PWM signal "PWM" is generated that implements this duty cycle. The PWM signal is applied to the switching element 108 such that when the switching element is turned on, the current bypasses the laser device. However, the losses introduced by this current path are minimal. Preferably, the switching element is a transistor, more preferably a metal oxide field effect transistor (MOSFET). It should be noted that the switching element may instead be formed as a series switch between the current source and the laser device. Furthermore, if the current source can directly provide a PWM-based signal, no external PWM switch is required. In such a case, the controller may be considered to be part of the current source circuit of the current source 104.

[0048] The sensor arrangement is used to provide a signal that allows the determination or estimation of the junction temperature of the laser device 102. In the example shown, the sensor arrangement comprises a temperature sensor 110 that measures the case temperature of the laser device. This corresponds to the temperature of the heat sink T hs The present invention provides an indirect measurement of junction temperature based on sensing

[0049] The sensor device instead receives a signal I PD The optical fiber 100 may have a light flux sensor, shown in FIG. 3 as a photodiode 112, that generates

[0050] In this case, the thermal characteristics of the system (power characteristics and heat sink characteristics) can be used as parameters that are stored in registers of the controller such that the junction temperature can be estimated from the measured optical output power and these stored parameters by calculation within the controller.

[0051] In particular, the measured optical output power may be used in combination with data characterizing the optical output power as a function of junction temperature for a particular device, which characterization information may have been obtained, for example, during the manufacturing process of the laser device itself, or during assembly and factory calibration of the entire laser circuit.

[0052] Therefore, the measurement of the junction temperature is performed by an open-loop sensing system.

[0053] However, the detector current I PD The use of a photodiode to generate an .lambda. means that the efficiency can be optimized by a feedback loop: it can be derived from the measured optical output power, and the driving conditions (current and voltage) which determine the input power.

[0054] The forward voltage of the LED or laser is a given parameter so that only the current amplitude needs to be controlled. If the optical power is measured by a photodetector, the drive current does not necessarily need to be measured since the optical output power can be measured. If a closed loop current controller is used, the current level can be set without having to actually measure the current level.

[0055] The drive current is based on the control of the current source 104. However, a current amplitude measuring device may also be provided to measure the laser device current. In the example shown, this is a current sense resistor 114, the voltage across which indicates the magnitude of the current I sense Shows.

[0056] The controller 106 estimates the junction temperature of the laser device and sets the amplitude and duty cycle of the laser drive current depending on the junction temperature, thereby enabling the controller 106 to implement a VCSEL drive scheme with maximum efficiency. As explained above, current sensing is not necessarily required if the optical output power is measured.

[0057] As a minimum, only a temperature estimate is required, i.e., a temperature sensor and / or a light output sensor. The current driving conditions are determined by the current setting I dc and duty cycle. Information about the luminous flux output as a function of temperature (i.e., the information in FIG. 1) is used by the controller, which may be obtained from a factory calibration or from the component datasheet. However, additional current sensing feedback may also be provided.

[0058] Since the effect of lasing threshold current and current density may vary from component to component, a self-learning cycle may be used during factory calibration. In this way, the controller knows the behavior of the laser components over various temperatures. This also compensates for differences in the quality of the cooling interface.

[0059] Depending on the available sensing, a self-learning process may be used over the life of the laser device to adapt to the effects of semiconductor aging. This may be used, for example, to compare sensed values ​​with expected values ​​of lasing threshold and lasing efficiency rollover. The self-learning process involves the use of a computer program capable of tracking aging trends and applying feedback or feedforward control signals to adapt the duty cycle or current amplitude without scanning for optimal efficiency operating points continuously or at every start-up / power-up.

[0060] The present invention may be applied to all types of lasers, including laser diodes, not just VCSELs.

[0061] The invention is particularly interesting for low frequency operation: the operating frequency is, for example, in the range of 10 Hz to 100 kHz, in particular in the range of 1 kHz to 20 kHz, and the duty cycle can vary from 0.1 to 0.9, typically in the range of 0.5 to 0.9.

[0062] The present invention may be used in laser-based lighting systems, but also in other laser systems, such as industrial laser-based heating systems.

[0063] As described above, the embodiments use a controller. The controller can be implemented in software and / or hardware in numerous ways to perform the various functions required. The processor is one example of a controller employing one or more microprocessors that can be programmed using software (e.g., microcode) to perform the functions required. However, the controller may be implemented with or without a processor, or as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.

[0064] Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field programmable gate arrays (FPGAs).

[0065] In various implementations, a processor or controller may be associated with one or more storage media, such as volatile and non-volatile computer memories, such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed in the one or more processors and / or controllers, perform the required functions. The various storage media may be fixed within a processor or controller, or may be portable such that one or more programs stored on the storage media can be loaded into a processor or controller.

[0066] Those skilled in the art can understand and effect variations to the disclosed embodiments in the practice of the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular form "a" does not exclude a plurality.

[0067] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0068] Please note that when the term "adapted to" is used in the claims or specification, the term "adapted to" is intended to be equivalent to the term "configured to."

[0069] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A laser device; a current source adapted to provide a pulse width modulated laser drive current to the laser device, the pulse width modulated laser drive current having an amplitude and a duty cycle; a controller for controlling the current source; a sensor device for monitoring a signal making it possible to estimate a junction temperature of the laser device, The controller: Estimating a junction temperature of the laser device; setting the amplitude of the pulse width modulated laser drive current to the laser device in dependence on the junction temperature; A laser circuit adapted to set the duty cycle of the pulse width modulated laser drive current to the laser device dependent on a power required for the laser device.

2. 2. The laser circuit of claim 1, wherein the current source comprises a switching element coupled in parallel with the laser device or in series with the laser device, the switching element configured to control the duty cycle of the pulse width modulated laser drive current to the laser device.

3. 10. The laser circuit of claim 1, wherein the controller is adapted to control a current amplitude and a duty cycle of a pulse width modulated laser drive current to achieve a desired efficiency and a desired optical output power.

4. 4. The laser circuit of claim 3, wherein the controller is adapted to control the current amplitude and the duty cycle of the pulse width modulated laser drive current to operate at an amplitude corresponding to maximum efficiency and at a duty cycle to provide the desired optical output power.

5. 2. The laser circuit of claim 1, wherein the sensor arrangement comprises a temperature sensor for measuring a case temperature of the laser device.

6. 2. The laser circuit of claim 1, wherein the sensor device comprises a flux sensor for measuring optical output power.

7. 10. The laser circuit of claim 1 further comprising a current amplitude measuring device for measuring the laser device current.

8. 2. The laser circuit of claim 1, wherein the controller is further adapted to determine an output power of the laser and further adapted to set an amplitude and a duty cycle of the laser drive current in dependence on the output power.

9. 2. The laser circuit of claim 1, wherein the laser device comprises a vertical cavity surface emitting laser.

10. 2. The laser circuit of claim 1, wherein the laser device comprises one or more laser diodes.

11. 2. A laser circuit as claimed in any one of claims 1 to 10, comprising an illumination circuit for providing a constant optical output power.

12. 1. A method of controlling a laser device, comprising: estimating a junction temperature of the laser device; setting an amplitude of a pulse width modulated laser drive current in dependence on said junction temperature; setting a duty cycle of the pulse width modulated laser drive current depending on the power required for the laser device; and providing the laser drive current to the laser device.

13. 13. The method of claim 12, comprising controlling settings of the current amplitude and the duty cycle of the pulse width modulated laser drive current to achieve a desired efficiency and a desired optical output power.

14. 14. The method of claim 13, comprising setting the current amplitude and the duty cycle of the pulse width modulated laser drive current to operate at an amplitude corresponding to maximum efficiency and at a duty cycle to provide the desired optical output power.

15. A computer program comprising computer program code means adapted to perform the method according to any one of claims 12 to 14 when said computer program is run on a computer.

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