Laser diode driving device and driving method, and projection-type image display device

Abstract

Detects signs of laser diode failure. [Solution] A drive circuit 21 drives a laser diode 13. A temperature sensor 25 detects a temperature TempA of the laser diode 13. A current sensor 27 detects a current Id flowing through the laser diode 13. A voltage sensor 28 detects a voltage Vd applied to the laser diode 13. A fault detection circuit 22 determines that the laser diode 13 is in a stable operating state if the change in temperature TempA over a predetermined time period T0 is less than a threshold value and the change in current Id over the time period T0 is less than a second threshold value. If the laser diode 13 is in a stable operating state and the change in voltage Vd over the time period T0 is greater than a third threshold value, the fault detection circuit 22 outputs an alert signal indicating that the laser diode 13 is exhibiting signs of failure.

Description

[Technical Field]

[0001] The present disclosure relates to a laser diode driving device and driving method, and a projection-type image display device. [Background technology]

[0002] Laser diodes deteriorate and break down after a certain period of use, for example, several thousand to several tens of thousands of hours. Laser diode failures often occur suddenly, and such sudden failures are also called "sudden death."

[0003] For example, Patent Document 1 discloses measuring the voltage applied to a plurality of light-emitting elements, identifying the number of faulty light-emitting elements based on the measured voltage, and controlling the light emission of the light-emitting elements according to the number of faults. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-035603 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 discloses control of laser diodes after a failure occurs in one of the laser diodes. However, it is desirable to detect symptoms of laser diode failure so that laser diodes that may be at risk of failure can be replaced in advance. Furthermore, Patent Document 1 only considers the case where one of the laser diodes connected in series fails and shorts out. It is desirable to detect symptoms of laser diode failure regardless of whether the laser diode fails and becomes shorted or open.

[0006] An object of the present disclosure is to provide a laser diode driving device and driving method capable of detecting symptoms of a laser diode failure, and a projection-type video display device equipped with such a laser diode driving device. [Means for solving the problem]

[0007] A laser diode driving device according to one aspect of the present disclosure includes: a driving circuit for driving the laser diode; a first temperature sensor that detects a first temperature that indicates a temperature of the laser diode or a temperature in the vicinity of the laser diode; a current sensor for detecting a current flowing through the laser diode; a voltage sensor for detecting a voltage applied to the laser diode; a failure detection circuit that detects a symptom of a failure of the laser diode based on the first temperature, the current, and the voltage; The fault detection circuit determining that the laser diode is in a stable operating state when the change in the first temperature over a predetermined time period is less than a first threshold and the change in the current over the time period is less than a second threshold; If the laser diode is in a stable operating condition and the change in the voltage over the time period is greater than a third threshold, an alert signal is output indicating that the laser diode is exhibiting signs of failure. [Effects of the Invention]

[0008] According to the laser diode driving device according to one aspect of the present disclosure, it is possible to detect symptoms of a failure in the laser diode. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a block diagram showing the configuration of a projection-type image display device 1 according to an embodiment. [Figure 2]2 is a flowchart showing a failure detection process executed by the failure detection circuit 22 of FIG. 1. [Figure 3] 4 is a timing chart illustrating the operation of the projection-type image display device 1 according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. However, in some cases, more detailed explanations than necessary will be omitted. For example, detailed explanations of already well-known matters and duplicate explanations of substantially identical configurations will be omitted. This is to avoid unnecessary redundancy in the following explanation and to facilitate understanding by those skilled in the art. Note that components with the same reference numerals have the same functions in each embodiment.

[0011] It should be noted that the present disclosure provides the accompanying drawings and the following description for the understanding of those skilled in the art, and is not intended to limit the subject matter described in the claims.

[0012] [Embodiment] [Overall configuration] 1 is a block diagram showing the configuration of a projection-type image display device 1 according to an embodiment. The projection-type image display device 1 includes a control circuit 11, a laser diode driver 12, a laser diode 13, an optical system 14, a digital micromirror device (DMD) 15, an optical system 16, and an indicator 17.

[0013] Control circuit 11 receives a video signal from an external device (not shown) and controls the overall operation of projection-type video display device 1 based on the video signal.

[0014] The laser diode driver 12 drives the laser diode 13 to generate illumination light under the control of the control circuit 11. The laser diode driver 12 also detects signs of a failure in the laser diode 13 and sends an alert signal to the control circuit 11 indicating that the laser diode 13 is showing signs of a failure.

[0015] The optical system 14 transmits the illumination light generated by the laser diode 13 so that it is incident on the digital micromirror device 15. The optical system 14 includes a combination of multiple optical elements, including lenses, prisms, mirrors, filters, and the like.

[0016] The digital micromirror device 15 receives a video signal from the control circuit 11, and modulates the incident illumination light based on the video signal to generate video light.

[0017] The optical system 16 projects the image light generated by the digital micromirror device 15 onto a screen (not shown). The optical system 16 includes a combination of multiple optical elements, including lenses, prisms, mirrors, and filters.

[0018] Indicator 17 indicates that laser diode 13 is showing signs of failure when control circuit 11 receives an alert signal from laser diode driver 12. Indicator 17 may include, for example, a visual output device such as a light emitting diode, or an audible output device such as a buzzer.

[0019] [Configuration of laser diode driver] The laser diode driving device 12 includes a driving circuit 21, a fault detection circuit 22, a timer 23, a timer 24, a temperature sensor 25, a temperature sensor 26, a current sensor 27, and a voltage sensor .

[0020] The driver circuit 21 drives the laser diode 13. The driver circuit 21 may drive the laser diode 13 using pulse width modulation with a variable duty cycle D.

[0021] The timer 23 measures the elapsed time ta since the drive circuit 21 most recently started driving the laser diode 13 .

[0022] The timer 24 measures the total lighting time tb of the laser diode 13 .

[0023] The temperature sensor 25 detects a first temperature TempA that indicates the temperature of the laser diode 13 or the temperature in the vicinity of the laser diode 13 .

[0024] The temperature sensor 26 detects a second temperature TempB that indicates the temperature around the projection display device 1. The temperature sensor 26 may be provided outside the housing of the projection display device 1, or may be provided at an air intake port of the projection display device 1.

[0025] The current sensor 27 detects the current Id flowing through the laser diode 13 .

[0026] The voltage sensor 28 detects the voltage Vd applied to the laser diode 13 .

[0027] The fault detection circuit 22 detects signs of a fault in the laser diode 13 based on at least the temperature TempA, the current Id, and the voltage Vd.

[0028] The fault detection circuit 22 determines that the laser diode 13 is in a stable operating state if the change in temperature TempA over a predetermined time period T0 is smaller than a first threshold value and the change in current Id over the time period T0 is smaller than a second threshold value. The "stable operating state" means that the laser diode 13 operates at a substantially constant current and is substantially in a thermal equilibrium state. The time period T0 has a duration of, for example, several seconds to several minutes.

[0029] The fault detection circuit 22 outputs an alert signal indicating that the laser diode 13 is experiencing a fault if the change in the voltage Vd over the time period T0 is greater than a third threshold value while the laser diode 13 is in a stable operating state. When the laser diode 13 is in a stable operating state, the voltage Vd is also expected to be substantially constant. However, if the laser diode 13 deteriorates, the voltage Vd may fluctuate, e.g., repeatedly increase and decrease, even when the laser diode 13 is in a stable operating state. Therefore, the fault detection circuit 22 detects that the change in the voltage Vd applied to the laser diode 13 in a stable operating state is greater than a predetermined threshold value as a sign of a fault in the laser diode 13.

[0030] The failure detection circuit 22 may detect signs of failure in the laser diode 13 based on at least some of the duty ratio D, the elapsed time ta, and the temperature TempB in addition to the temperature TempA, the current Id, and the voltage Vd.

[0031] For example, the fault detection circuit 22 may determine that the laser diode 13 is in a stable operating state if the change in temperature TempA over the time period T0 is less than a first threshold, the change in current Id over the time period T0 is less than a second threshold, and the change in duty ratio D over the time period T0 is less than a fourth threshold. When the laser diode 13 is in a stable operating state, the duty ratio D is also substantially constant.

[0032] Furthermore, the failure detection circuit 22 may determine that the laser diode 13 is in a stable operating state if the change in temperature TempA over the time period T0 is smaller than a first threshold, the change in current Id over the time period T0 is smaller than a second threshold, and the elapsed time ta is greater than a fifth threshold. Generally, the temperature TempA and the current Id are unstable immediately after the laser diode 13 starts to operate. Therefore, if the elapsed time ta is equal to or less than a predetermined threshold, it is determined that the laser diode 13 is not in a stable operating state. The threshold for the elapsed time ta is set to, for example, several tens of seconds to several minutes.

[0033] Furthermore, the fault detection circuit 22 may determine that the laser diode 13 is in a stable operating state if the change in temperature TempA over time period T0 is smaller than a first threshold, the change in current Id over time period T0 is smaller than a second threshold, and the change in temperature TempB over time period T0 is smaller than a sixth threshold. When the laser diode 13 is in a stable operating state, the temperature TempB will also be substantially constant unless some event occurs that affects the temperature around the projection-type image display device 1 (for example, the operation of an air conditioner). If the temperature TempB around the projection-type image display device 1 changes significantly, the temperature TempA of the laser diode 13 (or the temperature nearby) may also change accordingly.

[0034] The fault detection circuit 22 is, for example, a microcontroller.

[0035] [Operation of the embodiment] FIG. 2 is a flowchart showing the failure detection process executed by the failure detection circuit 22 of FIG.

[0036] In step S1, the failure detection circuit 22 monitors the operation of the drive circuit 21 to detect when the laser diode 13 starts to be driven.

[0037] In step S2, the failure detection circuit 22 reads from the timer 23 the elapsed time ta after the laser diode 13 starts to be driven.

[0038] In step S3, the failure detection circuit 22 determines whether the elapsed time ta is greater than the threshold value Th1, and if YES, the process proceeds to step S4, and if NO, the process returns to step S2.

[0039] In step S4, the fault detection circuit 22 detects temperature TempA, temperature TempB, current Id, duty ratio D, and voltage Vd over time period T0. The fault detection circuit 22 further calculates a change ΔTempA in temperature TempA, a change ΔTempB in temperature TempB, a change ΔId in current Id, a change ΔD in duty ratio D, and a change ΔVd in voltage Vd over time period T0. The change ΔTempA in temperature TempA indicates, for example, the difference between the maximum and minimum values ​​of temperature TempA during time period T0. The change ΔTempB in temperature TempB indicates, for example, the difference between the maximum and minimum values ​​of temperature TempB during time period T0. The change ΔId in current Id indicates, for example, the difference between the maximum and minimum values ​​of current Id during time period T0. The change ΔD in duty ratio D indicates, for example, the difference between the maximum and minimum values ​​of duty ratio D during time period T0. The change ΔVd in voltage Vd indicates, for example, the difference between the maximum and minimum values ​​of voltage Vd during time period T0.

[0040] In step S5, the failure detection circuit 22 determines whether the change ΔTempB in the temperature TempB is smaller than the threshold value Th2, and if YES, the process proceeds to step S6, and if NO, the process returns to step S4.

[0041] In step S6, the failure detection circuit 22 determines whether the change ΔTempA in the temperature TempA is smaller than the threshold value Th3, and if YES, the process proceeds to step S7, and if NO, the process returns to step S4.

[0042] In step S7, the fault detection circuit 22 determines whether the change ΔId in the current Id is smaller than the threshold value Th4 or not, and if YES, the process proceeds to step S8, and if NO, the process returns to step S4.

[0043] In step S8, the failure detection circuit 22 determines whether the change ΔD in the duty ratio D is smaller than the threshold value Th5 or not, and if YES, the process proceeds to step S9, and if NO, the process returns to step S4.

[0044] In step S9, the failure detection circuit 22 determines whether or not the change ΔVd in the voltage Vd is greater than the threshold value Th6, and if YES, the process proceeds to step S10, and if NO, the process returns to step S4.

[0045] In step S10, the failure detection circuit 22 reads out the total lighting time tb of the laser diode 13 from the timer 24.

[0046] In step S11, the failure detection circuit 22 calculates the probability of failure of the laser diode 13 based on the total lighting time tb. It is considered that the possibility of the laser diode 13 failing increases as the total lighting time tb increases.

[0047] In step S12, the failure detection circuit 22 outputs an alert signal indicating the symptoms and probability of failure of the laser diode 13.

[0048] 3 is a timing chart illustrating the operation of the projection-type image display device 1 according to the embodiment. In the example of FIG. 3, at time t3, the laser diode 13 fails and enters an open state. However, before the laser diode 13 fails, as a symptom of failure, the voltage Vd repeatedly increases and decreases even when the laser diode 13 is in a stable operating state (i.e., the period from time t1 to t2). Therefore, the failure detection circuit 22 can determine that the laser diode 13 has a symptom of failure when the laser diode 13 is in a stable operating state and the change ΔVd in the voltage Vd is greater than the threshold value Th6.

[0049] The laser diode driving device 12 according to the embodiment can detect symptoms of a failure in the laser diode 13, and thereby can replace a laser diode 13 that is likely to fail in advance.

[0050] The laser diode driving device 12 according to the embodiment can detect the symptom of a failure of the laser diode 13, regardless of whether the laser diode 13 has failed and is in a short circuit or open state.

[0051] According to the laser diode driving device 12 of the embodiment, a laser diode 13 that may be malfunctioning can be replaced before the user of the projection-type image display device 1 notices a deterioration in image quality (for example, a decrease in brightness or a change in color), thereby reducing downtime and improving customer satisfaction.

[0052] [Effects of the embodiment] The laser diode driving device 12 according to the embodiment includes a driving circuit 21, a first temperature sensor 25, a current sensor 27, a voltage sensor 28, and a fault detection circuit 22. The driving circuit 21 drives the laser diode 13. The first temperature sensor 25 detects a first temperature TempA indicating the temperature of the laser diode 13 or the temperature near the laser diode 13. The current sensor 27 detects a current Id flowing through the laser diode 13. The voltage sensor 28 detects a voltage Vd applied to the laser diode 13. The fault detection circuit 22 detects signs of a fault in the laser diode 13 based on the first temperature TempA, the current Id, and the voltage Vd. The fault detection circuit 22 determines that the laser diode 13 is in a stable operating state if the change in the first temperature TempA over a predetermined time period T0 is smaller than a first threshold and the change in the current Id over the time period T0 is smaller than a second threshold. The fault detection circuit 22 outputs an alert signal indicating that the laser diode 13 has signs of a fault when the laser diode 13 is in a stable operating state and the change in the voltage Vd over the time period T0 is greater than a third threshold.

[0053] This configuration makes it possible to detect signs of failure in the laser diode 13.

[0054] According to the laser diode driving device 12 of the embodiment, the driving circuit 21 may drive the laser diode 13 using pulse width modulation with a variable duty ratio D. In this case, the fault detection circuit 22 may determine that the laser diode 13 is in a stable operating state if the change in the first temperature TempA over the time period T0 is less than a first threshold, the change in the current Id over the time period T0 is less than a second threshold, and the change in the duty ratio D over the time period T0 is less than a fourth threshold.

[0055] This configuration makes it possible to more reliably detect whether the laser diode 13 is in a stable operating state.

[0056] According to the laser diode driving device 12 of the embodiment, the laser diode driving device 12 may further include a first timer 23 that measures the elapsed time ta since the driving circuit 21 most recently started driving the laser diode 13. In this case, the fault detection circuit 22 may determine that the laser diode 13 is in a stable operating state if the change in the first temperature TempA over the time period T0 is smaller than a first threshold, the change in the current Id over the time period T0 is smaller than a second threshold, and the elapsed time ta is greater than a fifth threshold.

[0057] This configuration makes it possible to more reliably detect whether the laser diode 13 is in a stable operating state.

[0058] According to the laser diode driving device 12 of the embodiment, the laser diode driving device 12 may further include a second timer 24 that measures the total lighting time tb of the laser diode 13. In this case, the failure detection circuit 22 may calculate and output the probability of failure of the laser diode 13 based on the total lighting time tb.

[0059] This configuration makes it possible to more reliably detect signs of failure in the laser diode 13.

[0060] The projection-type image display device 1 according to the embodiment includes a laser diode 13 and a laser diode driving device 12.

[0061] This configuration makes it possible to detect signs of failure in the laser diode 13.

[0062] According to the projection display device 1 of this embodiment, the laser diode driving device 12 may further include a second temperature sensor 26 that detects a second temperature TempB that indicates the ambient temperature of the projection display device 1. In this case, the fault detection circuit 22 may determine that the laser diode 13 is in a stable operating state if the change in the first temperature TempA over the time period T0 is smaller than a first threshold, the change in the current Id over the time period T0 is smaller than a second threshold, and the change in the second temperature TempB over the time period T0 is smaller than a sixth threshold.

[0063] This configuration makes it possible to more reliably detect whether the laser diode 13 is in a stable operating state.

[0064] A laser diode driving method according to an embodiment includes detecting a first temperature TempA indicative of the temperature of the laser diode 13 or the temperature near the laser diode 13. The method further includes detecting a current Id flowing through the laser diode 13. The method further includes detecting a voltage Vd applied to the laser diode 13. The method further includes determining that the laser diode 13 is in a stable operating state if a change in the first temperature TempA over a predetermined time period T0 is less than a first threshold and a change in the current Id over the time period T0 is less than a second threshold. The method further includes outputting an alert signal indicating that the laser diode 13 is exhibiting an indication of a fault if a change in the voltage Vd over the time period T0 is greater than a third threshold.

[0065] This configuration makes it possible to detect signs of failure in the laser diode 13.

[0066] [Other embodiments] As described above, the embodiments have been described as examples of the technology disclosed in this application. For this purpose, the accompanying drawings and detailed description have been provided. However, the technology in this disclosure is not limited to these, and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made as appropriate. Furthermore, the components described in the above embodiments can be combined to create new embodiments.

[0067] Therefore, the components shown in the accompanying drawings and detailed description may include not only components essential for solving the problem, but also components that are not essential for solving the problem in order to illustrate the above technology. Therefore, the fact that these non-essential components are shown in the accompanying drawings and / or detailed description should not be interpreted as immediately identifying these non-essential components as essential.

[0068] Furthermore, since the above-described embodiments are intended to illustrate the technology of the present disclosure, various modifications, substitutions, additions, omissions, etc. may be made within the scope of the claims or their equivalents.

[0069] 1 shows only one laser diode 13, the projection-type image display device 1 may include an array of multiple laser diodes 13 connected in series and / or parallel. Even when driving such an array of laser diodes 13, the laser diode driving device 12 according to the embodiment can detect signs of failure in at least one laser diode 13 included in the array, just as in the embodiment described above.

[0070] The failure detection circuit 22 may acquire the set value of the current Id from the drive circuit 21 or the control circuit 11, instead of acquiring the current Id flowing through the laser diode 13 from the current sensor 27. If the set value of the current Id has not changed, the failure detection circuit 22 may determine that the laser diode 13 is in a stable operating state.

[0071] If the set value of the duty ratio D has not changed, the failure detection circuit 22 may determine that the laser diode 13 is in a stable operating state.

[0072] The behavior of the voltage Vd varies depending on the temperature TempA and the current Id. Therefore, the fault detection circuit 22 may determine a threshold value Th6 for the change ΔVd in the voltage Vd based on the average value of the temperature TempA and the average value of the current Id over the time period T0. In this case, the fault detection circuit 22 may store a table showing the relationship between a plurality of different combinations of the temperature TempA and the current Id and the corresponding threshold value Th6 of the voltage Vd. Alternatively, the fault detection circuit 22 may calculate the threshold value Th6 of the voltage Vd using a predetermined formula based on the temperature TempA and the current Id.

[0073] Instead of calculating the probability of failure of the laser diode 13 based on the total lighting time tb, the failure detection circuit 22 may determine that the laser diode 13 is unlikely to fail and terminate the failure detection process if the total lighting time tb is equal to or less than a threshold value Th1. In this case, the threshold value may be set to, for example, several hundred hours to several thousand hours.

[0074] When the control circuit 11 receives an alert signal from the laser diode driver 12, instead of displaying on the indicator 17 that the laser diode 13 has signs of failure, the control circuit 11 may record the same in a log, execute some application program, or notify an external device (not shown) via a communication interface.

[0075] When the fault detection circuit 22 detects a symptom of a fault in the laser diode 13, it may send a control signal to the drive circuit 21 to stop driving the laser diode 13. Alternatively, when the control circuit 11 receives an alert signal from the laser diode driver 12, it may send a control signal to the drive circuit 21 to stop driving the laser diode 13.

[0076] The fault detection circuit 22 may be integrated into the drive circuit 21 or into the control circuit 11 .

[0077] The laser diode driving device 12 according to the embodiment is not limited to the projection-type image display device 1, but can be applied to any other device equipped with a laser diode 13.

[0078] [Summary of the embodiment] A laser diode driving device according to a first aspect of the present disclosure includes: a driving circuit for driving the laser diode; a first temperature sensor that detects a first temperature that indicates a temperature of the laser diode or a temperature in the vicinity of the laser diode; a current sensor for detecting a current flowing through the laser diode; a voltage sensor for detecting a voltage applied to the laser diode; a failure detection circuit that detects a symptom of a failure of the laser diode based on the first temperature, the current, and the voltage; The fault detection circuit determining that the laser diode is in a stable operating state when the change in the first temperature over a predetermined time period is less than a first threshold and the change in the current over the time period is less than a second threshold; If the laser diode is in a stable operating condition and the change in the voltage over the time period is greater than a third threshold, an alert signal is output indicating that the laser diode is exhibiting signs of failure.

[0079] According to the laser diode driving device according to the second aspect of the present disclosure, in the laser diode driving device according to the first aspect, the drive circuit drives the laser diode using pulse width modulation with a variable duty cycle; The fault detection circuit determines that the laser diode is in a stable operating state if the change in the first temperature over the time period is less than the first threshold, the change in the current over the time period is less than the second threshold, and the change in the duty ratio over the time period is less than a fourth threshold.

[0080] According to the laser diode driving device according to the third aspect of the present disclosure, in the laser diode driving device according to the first or second aspect, the laser diode driving device further includes a first timer that measures the elapsed time since the driving circuit most recently started driving the laser diode; The fault detection circuit determines that the laser diode is in a stable operating state if the change in the first temperature over the time period is less than the first threshold, the change in the current over the time period is less than the second threshold, and the elapsed time is greater than a fifth threshold.

[0081] According to the laser diode driving device according to the fourth aspect of the present disclosure, in the laser diode driving device according to one of the first to third aspects, the laser diode driving device further includes a second timer that measures a total lighting time of the laser diode; The failure detection circuit calculates and outputs the probability of failure of the laser diode based on the total lighting time.

[0082] A projection-type image display device according to a fifth aspect of the present disclosure includes: A laser diode; The laser diode driving device according to one of the first to fourth aspects is provided.

[0083] According to the projection-type image display device according to the sixth aspect of the present disclosure, in the projection-type image display device according to the fifth aspect, the laser diode driving device further includes a second temperature sensor that detects a second temperature that indicates the temperature around the projection-type image display device; The fault detection circuit determines that the laser diode is in a stable operating state if the change in the first temperature over the time period is less than the first threshold, the change in the current over the time period is less than the second threshold, and the change in the second temperature over the time period is less than a sixth threshold.

[0084] A laser diode driving method according to a seventh aspect of the present disclosure includes: Detecting a first temperature indicative of a temperature of the laser diode or a temperature proximate the laser diode; detecting a current flowing through the laser diode; detecting a voltage applied to the laser diode; determining that the laser diode is in a stable operating state when the change in the first temperature over a predetermined time period is less than a first threshold and the change in the current over the time period is less than a second threshold; and outputting an alert signal indicating that the laser diode is exhibiting signs of failure if the laser diode is in a stable operating state and the change in the voltage over the time period is greater than a third threshold. [Industrial Applicability]

[0085] A laser diode driving device according to an aspect of the present disclosure is applicable to, for example, a projection-type image display device. [Explanation of symbols]

[0086] 1 Projection-type image display device 11 Control circuit 12 Laser diode driver 13 Laser diode 14 Optical system 15 Digital Micromirror Device (DMD) 16 Optical system 17 Indicators 21 Drive circuit 22 Fault detection circuit 23 Timer 24 Timer 25 Temperature Sensor 26 Temperature Sensor 27 Current Sensor 28 Voltage Sensor

Claims

1. a driving circuit for driving the laser diode; a first temperature sensor that detects a first temperature that indicates a temperature of the laser diode or a temperature in the vicinity of the laser diode; a current sensor for detecting a current flowing through the laser diode; a voltage sensor for detecting a voltage applied to the laser diode; a failure detection circuit that detects a symptom of a failure of the laser diode based on the first temperature, the current, and the voltage; The fault detection circuit determining that the laser diode is in a stable operating condition if the change in the first temperature over a predetermined time period is less than a first threshold and the change in the current over the time period is less than a second threshold; outputting an alert signal indicating that the laser diode is exhibiting an indication of a failure if the laser diode is in a stable operating state and the change in the voltage over the time period is greater than a third threshold. Laser diode driver.

2. the drive circuit drives the laser diode using pulse width modulation with a variable duty cycle; the fault detection circuit determines that the laser diode is in a stable operating state when the change in the first temperature over the time period is less than the first threshold, the change in the current over the time period is less than the second threshold, and the change in the duty ratio over the time period is less than a fourth threshold.

2. The laser diode driving device according to claim 1.

3. the laser diode driving device further includes a first timer that measures the elapsed time since the driving circuit most recently started driving the laser diode; the fault detection circuit determines that the laser diode is in a stable operating state if the change in the first temperature over the time period is less than the first threshold, the change in the current over the time period is less than the second threshold, and the elapsed time is greater than a fifth threshold.

2. The laser diode driving device according to claim 1.

4. the laser diode driving device further includes a second timer that measures a total lighting time of the laser diode; the failure detection circuit calculates and outputs a probability of failure of the laser diode based on the total lighting time.

2. The laser diode driving device according to claim 1.

5. A laser diode; and a laser diode driving device according to any one of claims 1 to 4. Projection-type image display device.

6. the laser diode driving device further includes a second temperature sensor that detects a second temperature that indicates the temperature around the projection-type image display device; the fault detection circuit determines that the laser diode is in a stable operating state when the change in the first temperature over the time period is less than the first threshold, the change in the current over the time period is less than the second threshold, and the change in the second temperature over the time period is less than a sixth threshold.

6. The projection type image display device according to claim 5.

7. Detecting a first temperature indicative of a temperature of the laser diode or a temperature proximate the laser diode; detecting a current flowing through the laser diode; detecting a voltage applied to the laser diode; determining that the laser diode is in a stable operating condition if the change in the first temperature over a predetermined time period is less than a first threshold and the change in the current over the time period is less than a second threshold; and if the laser diode is in a stable operating condition and the change in the voltage over the time period is greater than a third threshold, outputting an alert signal indicating that the laser diode is exhibiting an indication of failure. Laser diode driving method.

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