Control of laser light source

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] This invention relates to the control of a laser light source. [Background technology]

[0002] It is generally known that lasers require a drive current higher than the laser oscillation threshold current to emit their effective optical output power. Below this laser oscillation current, the efficiency of photon generation is very low, and therefore, the laser oscillation current can be considered to contribute significantly to the loss of efficiency. For this reason, from an efficiency standpoint, it is beneficial to maximize the drive current so that it is significantly higher than the laser oscillation current.

[0003] The maximum drive current for maximum efficiency is strongly dependent on the junction temperature of lasers such as vertical-cavity surface-emitting lasers (VCSELs). At high junction temperatures, a faster rollover (after the local maxima) of the optical output power as a function of the 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 specific amount of light is emitted with the highest possible efficiency. [Overview of the project] [Problems that the invention aims to solve]

[0004] Therefore, it would be desirable to be able to supply a constant output power (corresponding to the desired light intensity) under all operating conditions that result in different junction temperatures. [Means for solving the problem]

[0005] The present invention is defined by the claims.

[0006] According to an example of a certain aspect of the present invention, Laser devices and, A current source adapted to supply current to the laser device, wherein the current has amplitude and duty cycle, A controller for controlling the current source, A laser circuit having a sensor device for monitoring a signal that enables estimation of the junction temperature of the laser device, The aforementioned controller - Estimate the junction temperature of the laser device, - The amplitude of the current supplied to the laser device is set depending 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] This laser circuit utilizes both the amplitude and duty cycle of the pulse-width modulated laser drive current to enable high-efficiency operation at various junction temperatures. This high-efficiency operation not only saves energy but also mitigates power loss problems.

[0008] In particular, an increase in temperature causes the controller to reduce the amplitude of the current in order to shift to an efficient operating point, and to increase the duty cycle in order 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 decrease the power is not beneficial because, as explained above, efficiency is impaired due to an increase in the contribution of the laser oscillation current. Since the drive current amplitude for maximum efficiency is temperature-dependent, it is also undesirable to reduce the laser output power simply by controlling the duty cycle of the PWM control signal.

[0010] Therefore, the present invention combines these two approaches to enable not only the maintenance of the desired output but also highly efficient operation.

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

[0012] This can 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, for example, the current amplitude and duty cycle of the pulse-width modulated laser drive current to achieve 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 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 for supplying the desired optical output power. Enabling operation at maximum efficiency not only saves energy but also reduces power loss problems. 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 device may have a temperature sensor for measuring the case temperature of the laser device. The case temperature may be used to provide an estimate of the junction temperature of the laser device. This may be done, for example, by using thermal information related to the device and its casing.

[0016] The sensor device may additionally or alternatively have a light beam sensor for measuring the optical output power. The measured optical output power can be used in combination with data indicating the characteristics of the optical output power as a function of the junction temperature of a specific device. This characteristic information may have been obtained, for example, during the manufacturing process of the laser device itself or during the assembly and factory calibration of the entire laser circuit.

[0017] A current amplitude measuring device may also be provided for measuring 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 makes it possible to detect errors in the current setting.

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

[0019] In this way, a feedback control loop is provided to enable maintaining the output power at a desired constant level instead of inferring the output power based on drive conditions.

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

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

[0022] The present invention is a method for controlling a laser device, comprising estimating the junction temperature of the laser device; setting the amplitude of a pulse width modulated laser drive current depending on the junction temperature; The steps include setting the duty cycle of the pulse-width modulated laser drive current depending on the power required for the laser device, The present invention also provides a method comprising the step of supplying the laser drive current to the laser device.

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

[0024] The method may, in that case, include the step of setting the current amplitude and the duty cycle of the pulse-width modulated laser drive current so that it operates at an amplitude corresponding to the maximum efficiency and at a duty cycle for supplying the desired optical output power.

[0025] The present invention also provides a computer program which includes computer program code means adapted to perform the methods defined above when the computer program is executed on a computer.

[0026] These and other aspects of the present invention will be described and clarified with reference to the embodiments described below. [Brief explanation of the drawing]

[0027] For a better understanding of the present invention and to more clearly illustrate how it can be carried out, the accompanying drawings are referenced here, as merely one example. [Figure 1] This shows the output power as a function of forward current, as measured for a specific VCSEL. [Figure 2] Examples of PWM drive currents with variable current amplitude and duty cycle for different junction temperatures are shown. [Figure 3] A simplified block diagram of the laser circuit is shown. [Modes for carrying out the invention]

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

[0029] The detailed descriptions and specific examples illustrate exemplary embodiments of the apparatus, systems, and methods, but are for illustrative purposes only and should not be used to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems, and methods of the invention will be better understood from the following description, the appended claims, and the appended drawings. The figures are for illustrative purposes only and are not drawn to scale. The same reference numerals are used throughout the figures to indicate 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, high efficiency can be maintained for various operating temperatures and desired optical output powers.

[0031] The present invention can be applied to any laser that exhibits different output power and drive current functions at different junction temperatures. This applies to lasers and laser diodes. As just one example, the present invention will be illustrated using measurements performed on a vertical-cavity surface-emitting laser (VCSEL).

[0032] Figure 1 shows the optical output power (y-axis) as a function of forward current (x-axis), as measured for a specific 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 By this, the actual junction temperature can be determined, where T j is the junction temperature of the VCSEL, 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] Instead, FIG. 1 is based only on the measurement of the case temperature.

[0035] Looking at the main efficiency aspect of the VCSEL, roughly,

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

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

[0038] Laser illumination applications typically require a certain predetermined average optical output power. This constraint allows for optimal efficiency to be achieved by maximizing the forward current so that the portion of the laser oscillation current relative to the forward current is small, while ensuring that the operation does not exceed the point where high current density in the junction reduces efficiency.

[0039] In practice, this maximum forward current can be expected to be just above the linear proportional gradient (of output power versus input current), and therefore where the gradient begins to decrease slightly. Thus, based on the known characteristic curve of the laser device, the point of (estimated) maximum efficiency can be determined based on the measurement or estimation of the junction temperature.

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

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

[0042] Figure 2 shows examples of PWM drive currents with variable current amplitude and duty cycle to maximize the efficiency of VCSEL output power 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 decreases, but the duty cycle ratio increases. The current amplitude decreases 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 circuit operation with high current amplitude and low duty cycle at low case temperatures or during initial startup. As the system heats up, the duty cycle increases while the current amplitude decreases. However, the average optical output power remains constant.

[0045] From Figure 1, it can be understood that a rise in temperature leads to a decrease in efficiency, which can result in increased heating of the semiconductor. Therefore, under certain circumstances, a thermal runaway condition may occur. Consequently, protection against thermal runaway can be used as part of laser control technology. The increase in heating can be determined, for example, from the determined or estimated junction temperature, as described below.

[0046] Figure 3 shows a simplified block diagram of a laser circuit 100, which in this case is represented as a series connection of laser diodes D1 to Dn, and has a laser device 102 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 control the duty cycle. A PWM signal "PWM" is generated to perform this duty cycle. The PWM signal is applied to the switching element 108 so that current bypasses the laser device when the switching element is turned on. 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). Note 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 supply a PWM-based signal, an external PWM switch is not necessary. In such cases, the controller can be considered part of the current source circuit of the current source 104.

[0048] The sensor device is used to supply a signal that enables the determination or estimation of the junction temperature of the laser device 102. In the example shown, the sensor device has a temperature sensor 110 that measures the case temperature of the laser device. This means that the temperature of the heat sink T hs It provides indirect measurement of junction temperature based on detection.

[0049] The sensor device, instead, outputs a signal I representing the optical output flux. PD It may also have a light beam sensor, shown as photodiode 112 in Figure 3, which generates the light.

[0050] In this case, the system's thermal characteristics (power characteristics and heatsink characteristics) may be used as parameters stored in the controller's registers, such that the junction temperature can be estimated by calculations within the controller from the measured optical output power and these stored parameters.

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

[0052] Therefore, junction temperature is measured using an open-loop sensing system.

[0053] However, the detector current I PD The use of photodiodes to generate this means that efficiency can be optimized by a feedback loop. Efficiency can be derived from the measured optical output power and the driving conditions (current and voltage) that 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 optical output power can be measured, so the drive current does not necessarily need to be measured. If a closed-loop current controller is used, the current level can be set without actually measuring 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 sensing resistor 114, where the voltage across the current sensing resistor is the current I sense This indicates.

[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. This allows the controller 106 to implement a VCSEL drive scheme with maximum efficiency. As described above, current sensing is not always necessary when the optical output power is measured.

[0057] At a minimum, only temperature estimation, i.e., a temperature sensor and / or a light output sensor, is required. The current driving conditions are current setting I provided to the current sensor. dc This can be considered known based on the duty cycle. Information about the luminous flux output as a function of temperature (i.e., the information in Figure 1) is used by the controller, and this information can be obtained from factory calibration or from the component datasheet. However, additional current sensing feedback may be provided.

[0058] Since the effects of laser oscillation threshold current and current density may differ for each component, a self-learning cycle may be used during factory calibration. In this approach, the controller recognizes the behavior of the laser components over various temperatures. This also compensates for differences in the quality of the cooling interface.

[0059] Depending on available detections, a self-learning process may be used over the lifespan of the laser device to adapt to the effects of semiconductor aging. This could, for example, be used to compare detected values ​​with expected values ​​for the laser oscillation threshold and laser oscillation efficiency rollover. The self-learning process involves the use of a computer program that has the capability to track aging trends and apply feedback or feedforward control signals to adapt the duty cycle or current amplitude without continuously scanning the optimal efficiency operating point or each time the device is started / powered on.

[0060] This invention can be applied not only to VCSELs but also to any type of laser, including laser diodes.

[0061] The present invention is of particular interest with respect to low-frequency operation. The operating frequency is, for example, in the range of 10 Hz to 100 kHz, and especially in the range of 1 kHz to 20 kHz. The duty cycle can vary from 0.1 to 0.9, generally within the range of 0.5 to 0.9.

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

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

[0064] Examples of controller components that may be employed in various embodiments of this 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 memory, including RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that perform the required functions when executed in one or more processors and / or controllers. The various storage media may be fixed within the processor or controller, or they may be portable such that one or more programs stored in the storage media can be loaded into the processor or controller.

[0066] A person skilled in the art will be able to understand and achieve, in carrying out the claimed invention, variations to the disclosed embodiments by studying the drawings, specification and appended claims. In the claims, the word “has” does not exclude other elements or steps, and singular nouns do not exclude plural nouns.

[0067] The mere fact that certain means are mentioned in different dependent claims does not mean that combinations of these means cannot be used to one's advantage.

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

[0069] No reference numeral in the claims should be construed as limiting the scope.

Claims

1. Laser devices and, A current source adapted to supply a pulse-width modulated laser drive current to the laser device, wherein the pulse-width modulated laser drive current comprises a current source having amplitude and duty cycle, A controller for controlling the current source, A laser circuit having a sensor device for monitoring a signal that enables estimation of the junction temperature of the laser device, The aforementioned controller The junction temperature of the laser device is estimated, The amplitude of the pulse-width modulated laser drive current to the laser device is set depending 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, depending on the power required for the laser device.

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

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

4. The laser circuit according to claim 3, wherein the controller is adapted to control the current amplitude and duty cycle of the pulse-width modulated laser drive current so that it operates at an amplitude corresponding to maximum efficiency and at a duty cycle for supplying the desired optical output power.

5. The laser circuit according to claim 1, wherein the sensor device has a temperature sensor for measuring the case temperature of the laser device.

6. The laser circuit according to claim 1, wherein the sensor device has a light beam sensor for measuring light output power.

7. The laser circuit current according to claim 1, further comprising a current amplitude measuring device for measuring the laser device current.

8. The laser circuit according to claim 1, wherein the controller is further adapted to determine the output power of the laser and to set the amplitude and duty cycle of the laser drive current depending on the output power.

9. The laser circuit according to claim 1, wherein the laser device has a vertical cavity surface-emitting laser.

10. The laser circuit according to claim 1, wherein the laser device has one or more laser diodes.

11. A laser circuit according to any one of claims 1, having an illumination circuit for supplying a constant light output power.

12. A method for controlling a laser device, A step of estimating the junction temperature of the laser device, The steps include setting the amplitude of the pulse width modulated laser drive current depending on the junction temperature, The steps include setting the duty cycle of the pulse-width modulated laser drive current depending on the power required for the laser device, A method comprising the step of supplying the laser drive current to the laser device.

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

14. The method according to claim 13, further comprising the step of setting the current amplitude and duty cycle of the pulse width modulated laser drive current so that it operates at an amplitude corresponding to maximum efficiency and at a duty cycle for supplying the desired optical output power.

15. A computer program comprising computer program code means adapted to perform the method described in any one of claims 12 to 14 when the computer program is executed on a computer.

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