Method of controlling optical output power of a laser diode, control device and related system

Abstract

The invention relates to the field of optical particle sensing, in particular to a method of controlling the optical output power of a laser diode (10). A method of controlling the optical output power of a laser diode (10) is proposed, wherein the laser diode is associated with a photodiode (20) that converts light received from the laser diode into a photodiode current, the method comprising the steps of: obtaining a first optical trimming parameter indicative of a first optical output power of the laser diode (10) at a first laser diode current above a lasing threshold and a second optical output power of the laser diode at a second laser diode current above the lasing threshold different from the first laser diode current under predetermined calibration conditions; obtaining a second electrical trimming parameter indicative of a photodiode characteristic curve of the photodiode current relative to the laser diode current under predetermined calibration conditions; measuring a first photodiode current at a third laser diode current below the lasing threshold; measuring a second photodiode current at a fourth laser diode current below the lasing threshold different from the third laser diode current; determining a slope of the photodiode current relative to the laser diode current below the lasing threshold based on the measurements of the first and second photodiode currents; controlling the output power of the laser diode (10) above the lasing threshold based on the first optical trimming parameter, the second electrical trimming parameter and the slope of the photodiode current relative to the laser diode current below the lasing threshold.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of optical particle sensing, in particular to a method of controlling the optical output power of a laser diode arrangement. The invention further relates to a corresponding control arrangement and to a system, in particular a particle sensor arrangement, comprising a control arrangement adapted to control the optical output power of a laser diode. BACKGROUND

[0002] DE 102 015 207 289 A1 and US 9,857,287 B2 disclose a particle sensor device. The particle sensor device comprises an optical emitter arrangement configured to emit optical radiation such that a volume in which at least one particle can be present is at least partially illuminable, an optical detector arrangement having at least one detection surface which is impinged by at least a portion of the optical radiation scattered on the at least one particle, from which impinging optical radiation an at least one information signal about the intensity and / or the intensity distribution can be output, and an evaluation arrangement with which an information item about the presence of particles, the number of particles, the particle density and / or at least one characteristic of the particles can be identified and output. The particle sensor arrangement further comprises at least one lens element which is arranged such that the emitted optical radiation can be focused onto a focus region within the volume.

[0003] The optical particle sensor device can comprise a VCSEL (Vertical Cavity Surface Emitting Laser) laser diode with an integrated photodiode. A VCSEL is a type of semiconductor laser diode that emits a laser beam from its top surface perpendicular to the surface, in contrast to conventional edge-emitting semiconductor lasers (also in-plane lasers) that emit from the surface of the individual chip formed by cleaving a wafer. Based on the so-called self-mixing interference (SMI) technique, the known optical particle sensor device allows to obtain information about the presence of particles and optionally their velocity.

[0004] The optical particle sensor arrangement should be operated with a well-defined output power. On the one hand, for obtaining a high measurement sensitivity, the optical output power should be as high as possible. On the other hand, the optical output power should be low enough to ensure a safe operation for the human eye.

[0005] However, based on the temperature of the laser diode and aging effects of the laser diode relative to the lifetime of the arrangement, the optical output power can change during operation of the laser diode with a constant laser diode current.

[0006] Therefore, it can be necessary to control the actual output power to a desired or target output power during operation, e.g. by adjusting the laser diode current. The laser diode current can also be referred to as driving current. In principle, the optical output power of the laser diode can be measured via an external optical feedback path. However, this can not be practical, in particular for miniaturized applications, e.g. optical particle sensors integrated in a smartphone.

[0007] German patent application DE 102 018 212 687.9 discloses a method for monitoring the optical output power of a laser diode with an associated photodiode, and a particle sensor device. The application describes the general idea of monitoring the optical output power during operation of the laser diode based on current and / or voltage measurement values at the laser diode and at the photodiode. More precisely, it is suggested to (a) perform a voltage measurement at the laser diode at a predetermined laser diode current, e.g. 10 pA, and (b) perform several current measurements of the photodiode current at different set laser diode currents.

[0008] Although the method disclosed therein can already provide very good performance for monitoring the optical output power of a laser diode, it is desirable to obtain a further improved and preferably simplified method for controlling the optical output power of a laser diode with an integrated photodiode.

[0009] US 6,483,862 B1 discloses a system and method for monolithically integrating a light emitting device and a photodetector using a native oxide semiconductor layer. The provided light emitting device and photodetector combination has a structure in which the photodetector layer in contact with the light emitting device is spaced apart from the light emitting device by a native oxide semiconductor layer that is both insulating and has a lower refractive index than the light emitting device and photodetector. This configuration will result in a light emitting device and photodetector structure that minimizes the capture of spontaneous emission light output from the light emitting device by the photodetector while electrically isolating the light emitting device from the photodetector. The electrical isolation of the light emitting device from the photodetector will result in a four terminal device in which the light emitting device and photodetector can be independently biased and thus can operate at very low bias voltages.

[0010] US 5,757,837 A discloses a vertical cavity surface emitting laser composed of an intracavity quantum well photodetector. The quantum well photodetector is placed at the peak of optical intensity at the Fabry-Perot wavelength. The device can include a current confinement layer in the form of an oxide layer, an air gap, or proton implantation. SUMMARY

[0011] It is therefore an object of the present application to provide a further improved method for controlling the optical output power of a laser diode. In particular, it would be advantageous to provide a method which can provide for a simplified adjustment with respect to temperature and / or aging effects during field operation. In particular, it would be advantageous to provide a control method which can simplify the circuit design. Furthermore, it would be desirable to make the setting or calibration procedure less time consuming during field operation.

[0012] According to a first aspect of the present disclosure, a method of controlling the optical output power of a laser diode is proposed, wherein the laser diode is associated with a photodiode which converts light received from the laser diode into an electronic photodiode current, the method comprising the steps of:

[0013] obtaining a first optical trimming parameter indicative of a first optical output power of the laser diode at a first laser diode current above a lasing threshold and a second optical output power of the laser diode at a second laser diode current above the lasing threshold different from the first laser diode current under predetermined calibration conditions;

[0014] obtaining a second electrical trimming parameter indicative of a photodiode characteristic curve of the photodiode current relative to the laser diode current under predetermined calibration conditions;

[0015] measuring a first photodiode current at a third laser diode current below the lasing threshold;

[0016] measuring a second photodiode current at a fourth laser diode current below the lasing threshold different from the third laser diode current;

[0017] characterized in that

[0018] determining a slope of the photodiode current relative to the laser diode current below the lasing threshold based on the measurements of the first photodiode current and the second photodiode current;

[0019] controlling the output power of the laser diode above the lasing threshold based on the first optical trimming parameter, the second electrical trimming parameter and the slope of the photodiode current relative to the laser diode current below the lasing threshold.

[0020] In yet another aspect of the present application, a corresponding control device for controlling the optical output power of a laser diode, as well as a system, in particular an optical particle sensor device, comprising a photodiode, a laser diode and the control device, the control device being adapted to control the optical output power of the laser diode based on the method as described herein, is provided.

[0021] The preferred embodiments of the present application are defined in the dependent claims. It is to be understood that the claimed method, system and control device can have similar and / or identical preferred embodiments as the method, in particular as defined in the dependent claims and as disclosed herein.

[0022] The solution presented herein can provide the possibility to further improve the power control of a laser diode, while it can provide a simplified adjustment with respect to temperature and / or aging effects during field operation.

[0023] The inventors have realized that the light output power of a laser diode above the laser threshold can be controlled with high precision based on the slope of the photodiode current relative to the laser diode current below the laser threshold. Thus, even though it seems counterintuitive to control the laser output power above the laser threshold according to the slope of the photodiode characteristic curve below the laser threshold, it has been found that a high precision power control can be achieved over a range of different light output powers above the laser threshold. For example, the non-linear effects of the photodiode characteristic curve above the laser threshold can be reduced.

[0024] With reference to the aforementioned DE 102018212687.9, wherein it is suggested to evaluate a PD Slope and a PD SlopeT .

[0025] Furthermore, it has been found that by providing a first optical trimming parameter indicative of a first light output power of the laser diode at a first laser diode current and a second light output power of the laser diode at a second laser diode current different from the first laser diode current, the effort of calibration measurements during field operation can be further reduced. Even though the effort to obtain the trimming parameters is increased, these trimming parameters can enable a simplified subsequent calibration during the operation phase. For example, it has been found that additional voltage measurements of the laser diode during field operation can no longer be required. Thus, even though the effort in the trimming phase is increased, the overall effort can be reduced. It should be noted that the trimming measurements are typically performed as one-time measurements during manufacturing under predetermined conditions, whereas the temperature adjustment and / or the adaptation with respect to device aging effects occur frequently during the device lifetime.

[0026] Compared to the method described in the earlier German patent application DE 10 2018 212 687.9, the proposed solution can provide several advantages. For example, where the abcd constants can only be applied when the power level during field operation corresponds to the predetermined power level during calibration, e.g. when setting a light output power of 0.5 mW. Although this method provides very good calibration results to set this particular power level during field operation, the performance and flexibility for setting different power levels can be further improved with the concepts described in this disclosure.

[0027] Further advantages of the proposed solution can relate to a more accurate power control in view of the aging effects of the laser diode. It has been found that the measurement accuracy of conventional methods can provide a poor performance, in particular during the first operating hours of the laser diode. To overcome this problem, prior art methods can perform calibration measurements during manufacturing after a sufficient operating time of the laser diode. Thus, the proposed solution can reduce the calibration time during manufacturing, as there can be no need to wait for the generation of aging effects during manufacturing calibration.

[0028] In general, the proposed method can be divided into two phases: a trimming phase and an operational phase. The trimming phase refers to trimming or calibrating under predetermined, i.e. controlled or defined, conditions, e.g. during manufacturing. The trimming phase can comprise obtaining first and second optical trimming parameters. The first optical trimming parameter is indicative of at least a first optical output power of the laser diode at a first laser diode current above the laser threshold and a second optical output power of the laser diode at a second laser diode current above the laser threshold different from the first laser diode current. Optionally, the optical trimming parameters can comprise or can be derived from a characteristic curve of the laser diode. Actual measurements of the first and second optical diode currents, and subsequent determining and controlling steps can be performed during the operational phase. As used herein, a trimming parameter can refer to a calibration parameter, and the terms trimming parameter, calibration parameter and trimming calibration parameter can be used interchangeably. Similarly, a predetermined condition can refer to a calibration condition, and the terms predetermined condition, calibration condition or predetermined calibration condition can be used interchangeably. As mentioned above, a distinction has to be made between (1) calibrating or trimming under predetermined conditions (trimming phase) and (2) operation of the device in the field (operational phase). I.e. away from a laboratory or manufacturing facility, and not necessarily operating under known conditions. In the operational phase, the steps of measuring the first and second optical diode currents, determining a slope of the optical diode current relative to the laser diode current based thereon, and controlling the optical output power of the laser diode are performed. In the operational phase, extensive optical measurements are not required. While it can be acceptable that extensive device calibration or trimming parameters are measured under predetermined calibration conditions during manufacturing (e.g. one-time calibration measurements), the remaining measurements required during operation in the field should be simple and limited. Thus, during operation, predetermined calibration or trimming parameters can be obtained, e.g. from storage, and used in combination with limited additional measurements.

[0029] In the following, some terms used throughout the application will be briefly explained and defined:

[0030] Obtaining a trimming parameter can refer to receiving, retrieving, determining or measuring the trimming parameter. For example, the trimming parameter can already be stored in a database and retrieved from the database within the device. The trimming parameter can also be received via an optional communication interface. It can also be necessary to measure the trimming parameter as part of the method. Furthermore, the trimming parameter can be calculated from measured values of the first and second optical output powers. Similar considerations apply accordingly for the second electrical trimming parameter.

[0031] The characteristic curve of the photodiode can refer to a curve describing the photodiode current versus the laser diode current. The photodiode slope or slope of the photodiode (curve) can refer to the slope of the characteristic curve of the photodiode. The characteristic curve of the laser diode can refer to a curve of the optical output power of the laser diode versus the laser diode current. The laser diode slope or slope of the laser diode (curve) can refer to the slope of the characteristic curve of the laser diode. The slope can be determined by measuring the output power or the photocurrent at two different laser diode currents.

[0032] In the following, an advantageous embodiment of a method according to an aspect of the application will be described.

[0033] According to one embodiment, obtaining the first optical trimming parameter can comprise measuring a characteristic curve of the laser diode, the characteristic curve being a curve indicating the optical output power of the laser diode versus the laser diode current of the laser diode. In particular, the optical output power can be measured with an (external) power meter (calibrated). Thus, the individual values can be obtained with a very high precision during manufacturing. In view of the fact that the trimming measurement can be performed as a one-time measurement during manufacturing, it is feasible to use a dedicated external device for the high-precision measurement. Additionally or alternatively, the parameter can also be stored in a memory and obtained from said memory. Further, the optical output can be measured using the photodiode associated with the laser diode, e.g. during manufacturing or when the determination device is operated under predetermined calibration conditions. For example, when the determination device is at a desired temperature for performing the calibration, the calibration measurement can be performed with the photodiode associated with the laser diode.

[0034] The first optical trimming parameter can comprise an optically determined trimmed laser diode slope (LD Slope_trim_opt), the curve indicating a relationship of the optical output power of the laser diode to the laser diode current of the laser diode under predetermined conditions. One advantage of this approach is that the power control over the lifetime of the device can be further improved. Thus, the laser diode slope can be determined based on an optical measurement of the optical output power of the laser diode, rather than using an electrically determined slope from the photodiode. Although the electrically and optically determined values are quite similar, it has been found that a more precise power control can be achieved over the lifetime of the device. The optically determined laser diode slope can be calculated directly from a first optical output power of the laser diode at a first laser diode current above the lasing threshold and a second optical output power of the laser diode at a second laser diode current above the lasing threshold, different from the first laser diode current, under predetermined conditions. The laser diode slope can be obtained by the difference of the first and second optical output power of the laser diode compared to the difference of the first and second laser diode current. As will be further explained below, the trimmed laser diode slope of the laser diode characteristic curve can be determined non-reciprocally from the optical output power of the laser diode to the laser diode current of the laser diode, or reciprocally from the laser diode current of the laser diode to the optical output power of the laser diode. The reciprocal can be advantageous for implementation in an ASIC, since division terms can be avoided. As used herein, the term "optically determined" can mean that a parameter is determined based on a measurement of the optical output power of the laser diode under predetermined conditions.

[0035] Thus, the first optical trimming parameter can comprise an optically determined trimmed lasing threshold current (I th_trim_opt ), the laser diode characteristic curve indicating a relationship of the optical output power of the laser diode to the laser diode current of the laser diode under predetermined conditions. The optically determined trimmed lasing threshold current (I th_trim_opt ) can be determined by solving a zero optical power equation from one of (a) an optically determined trimmed laser diode slope (LD Slope_trim_opt ) and (b) one of a first optical output power of the laser diode at a first laser diode current and a second optical output power of the laser diode at a second laser diode current. The threshold can be determined as the intersection of a straight line through (a) the first optical output power of the laser diode at the first laser diode current and (b) the second optical output power of the laser diode at the second laser diode current with zero optical output power. As mentioned above, even if quite similar, it has been found that the optically determined threshold can further improve the power control.

[0036] The optical output power of the laser diode is controlled based on the optically determined lasing threshold current (I th_trim_opt ) and the electrically determined lasing threshold current correction value (AI th_el ). In particular, the optical output power of the laser diode is controlled based on the optically determined lasing threshold current (Ith_trim_opt ) and an electrically determined laser threshold current correction value (AI th_el ). The laser threshold current correction value (AI th_el ) can be determined based on electrical measurements. The optically determined threshold can be provided during manufacturing, the electrical determination can be performed during field operation. Thus, during operation, the laser threshold current can be approximated by I th_trim_opt + AI th_el . The specific combination of optically determined laser threshold current and laser threshold correction value considering only electrical determination has the advantage that by this specific combination, the power control can be further improved, while providing a cost-efficient and reliable determination of the correction value during field operation.

[0037] In a further refinement, the laser threshold current correction value (AI th_trim_el ) can be determined based on a difference of (a) a trimmed laser threshold current (I th_el ) derived from a characteristic curve of photodiode current versus laser diode current under predetermined conditions (electrically determined) and (b) an operating laser threshold current (I th_el ) determined during field operation (electrically determined). The operating laser threshold current can be derived, for example, from the characteristic curve of photodiode current versus laser diode current. Thus, instead of using the already available optically determined laser threshold current (I th_trim_opt ), it is proposed to provide a further electrically determined laser threshold current (I th_trim_el ) and to determine the laser threshold current correction value therefrom. Thus, the laser threshold current correction value (AI th_el ) can be determined more reliably and accurately. Optionally, a scaling factor and / or an offset can be applied. For example, the laser threshold current correction value (AI th_el ) can be determined by scaling said difference with a scaling factor B1 and optionally adding an offset factor B2. The scaling factor and the offset can be determined based on empirical measurements.

[0038] The optical output power can be controlled based on an optically determined trimmed laser diode slope (LD Slope_trim_opt ) and an electrically determined laser threshold above laser diode slope correction value (ALD Slope_el ). For example, based on a sum of the optically determined trimmed laser diode slope (LD Slope_trim_opt ) and the electrically determined laser threshold above laser diode slope correction value (ALD Slope_el ). The laser threshold above laser diode slope correction value (ALD Slope_el ) can be determined based on electrical measurements. Thus, during operation, the slope of the laser diode above the laser threshold can be approximated by LD Slope_trim_opt + ALD Slope_el .

[0039] In an advantageous refinement, the laser diode slope correction value (ADLD SlopeSE_el ) above the laser threshold can be determined based on the photodiode slope correction value (ADPD Slope_el ) below the laser threshold. The advantage of this embodiment is that the correction value can be determined from electrical measurements of the photodiode characteristic curve. The photodiode slope correction value (ADPD SlopeSE_el ) below the laser threshold can be determined based on electrical measurements. During operation, the photodiode slope below the laser threshold can thus be approximated as PD SlopeSE_trim_el + ADPD SlopeSE_el . The photodiode slope indicates the relationship of the photodiode current to the laser diode current. It has surprisingly been recognized that using the slope of the photodiode below the laser threshold instead of the slope of the photodiode above the laser threshold provides very accurate results for the laser diode slope above the laser threshold.

[0040] The laser diode slope correction value (ADLD Slope_el ) above the laser threshold is determined based on the following equation:

[0041] ADLD Slope_el = A1*ADPD SlopeSE_el + A2,

[0042] where ADPD SlopeSE_el is the photodiode slope correction value below the laser threshold, A1 is a proportional factor, A2 is an offset factor, ADPD SlopeSE_el denotes the difference between the slope of the photodiode current relative to the laser current below the laser threshold under predetermined conditions and the slope of the photodiode current relative to the laser current below the laser threshold at the time of measuring the first and second photodiode currents; and ADLD Slope_el denotes the difference between the slope of the optical output power of the laser diode relative to the laser current above the laser threshold under predetermined conditions and the slope of the optical output power relative to the laser current above the laser threshold at the time of measuring the first and second photodiode currents. The proportional factor A1 and the offset A2 can be determined based on empirical measurements. It is to be understood that different actual values can differ. Depending on the implementation, the laser diode slope correction value can refer to a non-reciprocal or reciprocal value. As mentioned above, a reciprocal term can be advantageous for implementation in an ASIC since division terms can be avoided. When using a reciprocal term for the laser diode slope correction value, different proportional and offset factors can also be referred to as proportional factor A3 and offset factor A4.

[0043] The second electrical trimming parameter can comprise the slope of the photodiode current relative to the laser diode current below the laser threshold (PD SlopeSE_trim_el). The photodiode slope correction value (ΔPD SlopeSE_el ) can be determined based on the difference between (a) the slope of the photodiode current relative to the laser diode current below the laser threshold under predetermined conditions and (b) the slope of the photodiode current relative to the laser diode current below the laser threshold during field operation. The photodiode slope correction value can thus be obtained from the difference between the photodiode's characteristic curve below the threshold of the photodiode's current relative to the laser diode's current of the laser diode during the trimming phase and during operation.

[0044] In an advantageous embodiment, the optical output power of the laser diode is controlled by setting the laser diode current ILD

[0045] I LD (P opt ) = (I th_trim_opt + ΔI th_el ) + P opt / (LD Slope_trim_opt + ΔLD Slope_el ) ;

[0046] wherein I LD (P opt ) is the laser diode current; P opt is the optical output power; I th_trim_opt is the optically determined laser threshold current; ΔI th_el is the electrically determined laser threshold current correction value; LD Slope_trim_opt is the optically determined laser diode slope; and ΔLD Slope_el is the electrically determined laser diode slope correction value. The individual values can be obtained as described above. The advantage of this embodiment is that the method is not limited to adjusting the optical output power of the laser diode during field operation to achieve the same optical output power as used during the trimming phase for calibrating the device. Thus, temperature variations and aging effects for various required output powers P opt can be taken into account. When using the reciprocal values of the parameters LD Slope_trim_opt and ΔLD Slope_el , the above equation can be rewritten as

[0047] I LD (P opt ) = (I th_trim_opt + ΔI th_el ) + P opt / (LD Slope_trim_opt,m1 + ΔLD Slope_el,m1 ) ;

[0048] where the optional suffix ml highlights the use of the reciprocal parameter. The advantage of using the reciprocal parameter is that the implementation in an ASIC can be more efficient as the division term can be cancelled.

[0049] The photodiode can be integrated with the laser diode. In particular, the laser diode can be a vertical cavity surface emitting laser, VCSEL, with an integrated photodiode. Such a device is also referred to as VIP. The photodiode can be thermally coupled with the laser diode.

[0050] In contrast to known solutions in the prior art and the proposed method, controlling the output power of the laser diode can not include measuring the laser diode voltage at a predetermined laser diode current during field operation. It has been found that with the proposed finer optical calibration in the trimming phase, the voltage measurement of the laser diode at a predetermined laser diode current is no longer required. Given that the adjustment during field operation occurs more frequently than the trimming during manufacturing, the overall effort can be reduced.

[0051] It is to be understood that the features described above and those to be explained hereinafter can be used not only in the specified combinations, but also in other combinations or in isolation, without departing from the scope of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0052] These and other aspects of the present application will become apparent from the embodiments described below and with reference to the drawings. In the drawings:

[0053] Figure 1 A schematic diagram of a system according to one aspect of the present disclosure is shown;

[0054] Figure 2 is a graph of the laser diode characteristic, the photodiode characteristic, the laser diode voltage versus the laser diode current;

[0055] Figure 3 A schematic diagram of the laser diode characteristic and the photodiode characteristic with some benefit parameters in the trimming phase under predetermined conditions at a first temperature is shown;

[0056] Figure 4 A schematic diagram of the laser diode characteristic and the photodiode characteristic with some benefit parameters during field operation at a second temperature different from the first temperature is shown;

[0057] Figure 5 A flowchart of a method according to one aspect of the present disclosure is shown;

[0058] Figure 6 A flowchart of exemplary method steps in the trimming phase is shown;

[0059] Figure 7 A flow chart illustrating the first step of the exemplary method during the operational phase is shown;

[0060] Figure 8 A flow chart illustrating the second step of the exemplary method during the operational phase is shown. DETAILED DESCRIPTION

[0061] In the following, exemplary embodiments of a method and a system for controlling the optical output power of a laser diode will be described.

[0062] Figure 1 An embodiment of a system according to the present application is schematically shown. The system is generally denoted by reference numeral 100. The system 100 comprises a laser diode 10, a photodiode 20 and a control device 30. The photodiode 20 is thermally coupled to the laser diode, e.g. by a coupling element 21. In a preferred embodiment, the laser diode 10 is a vertical cavity surface emitting laser (VCSEL) comprising an integrated photodiode 20.

[0063] During operation, the laser diode emits light 11 out of the laser diode, but some light 12 is directed to the photodiode. Some light 11 can be scattered at particles in the field of view of the laser diode 10, possibly backscattered to the device and causing self-mixing interference. The respective signals can be picked up by the photodiode 20 and evaluated by an evaluation unit, e.g. described in more detail in US 9,857,287 B2. The functionality of such an evaluation unit can optionally be performed by the control unit or control device 30. However, a separate evaluation unit can also be provided.

[0064] The control unit 30 is adapted to control the optical output power of the laser diode 10. In particular, the control unit is adapted to control the optical output power of the laser diode based on a method according to the present disclosure. Further details regarding the method steps will be further described below with reference to the flow charts. The control unit 30 comprises an output for providing a drive signal to the laser diode 10. For example, the drive signal can be provided to a laser driver 32, e.g. a laser diode current I LD A current source is provided to the laser diode 10 for operating the laser diode 10. The laser driver 32 can be part of the control unit 30 or can be implemented as a separate device. Advantageously, a rather simple current source can be used, since with the proposed solution, it can not be necessary to measure the laser diode voltage.

[0065] The control unit 30 is connected to the photodiode 20. A port control unit 35 can be provided for supplying a supply voltage to the photodiode and measuring the output current of the photodiode 20. The control unit 30 can be directly connected to the photodiode 20 or via an intermediate unit 36 such as a controlled voltage source or for example a shunt resistor simply for measuring the photodiode current. The photodiode 20 will thus convert the light 21 received from the laser diode 10 into a photodiode current that can be measured directly or indirectly with the control unit 30.

[0066] An external power meter 40 can be used in the trimming phase to calibrate the control device 30 in order to provide a desired, well-controlled light output power of the laser diode 10. It will be understood that the power meter does not necessarily have to be directly connected to the control device but can also be part of the manufacturing machinery and measurement equipment. By using the external power meter 40, a highly reliable first light trimming parameter can be obtained that indicates a first light output power of the laser diode at a first laser diode current above the lasing threshold and a second light output power of the laser diode at a second laser diode current above the lasing threshold and different from the first laser diode current under predetermined conditions. It will be understood that these trimming parameters can also be obtained by the control device 30 from an external database 50. The database 50 can be implemented as an external or internal memory.

[0067] Figure 2 A diagram of the laser diode characteristic 60, the photodiode characteristic 70 and the laser diode voltage 80 versus the laser diode current is shown. The horizontal axis represents the laser diode current I LD . The left vertical axis represents the light output power P LD or P out of the laser diode. The right vertical axis represents the photodiode current I PD . The laser diode characteristic 60 thus describes the light output power P LD of the laser diode versus the laser diode current I LD . The photodiode characteristic 70 describes the photodiode current I PD versus the laser diode current I LD .

[0068] Figure 3 A simplified schematic diagram of the laser diode characteristic 61 and the photodiode characteristic 71 with some of the parameters of interest during the trimming phase under predetermined conditions at a first temperature of for example 50°C is shown.

[0069] Figure 4A schematic diagram showing laser diode characteristic curve 61'and photodiode characteristic curve 71'with some of the parameters of interest during a field operation phase at a second temperature, different from the first temperature, e.g. a higher temperature, 60°C.

[0070] As can be seen from the comparison of the graphs, the threshold voltage I th increases with increasing temperature and the slope LD Slope_opt of the laser diode curve decreases with increasing temperature. The same applies to the photodiode slope PD Slope_el . Therefore, a higher laser diode current I LD has to be applied to achieve a desired optical output power P LD of e.g. 0.5 mW. Thus, temperature and aging change the threshold current, the slope and the voltage of the laser diode. Therefore, for field operation it is important to know or at least estimate the difference or delta of the trimming values to fully reconstruct the laser diode curve. From such a curve, the required driving current I th for each required output power (e.g. 0.5 mW) can be calculated. LD An upper limit of the driving current that can be reasonably estimated can be due to the beginning of thermal roll-over.

[0071] Further details will be described with reference to the flowchart shown in Figure 5 to Figure 8 . Figure 5 A flowchart of a method 500 according to one aspect of the present disclosure is shown. Steps S501 and S502 can be performed during a trimming phase during manufacturing under predetermined conditions, e.g. at a defined temperature, e.g. 50°. Steps S503 to S505 can be performed during field operation when the temperature and other operating conditions are not known and / or after aging of the device.

[0072] In step S501, a first optical trimming parameter is obtained. The optical trimming parameter can be indicative of a first optical output power of the laser diode at a first laser diode current above the laser threshold (see item 64 in Figure 3 ) under predetermined conditions, and a second optical output power of the laser diode at a second laser diode current above the laser threshold (see item 65 in Figure 3 ) different from the first laser diode current. Based thereon, an optically determined slope LD Slope_trim_opt of the laser diode characteristic curve 61 during trimming can be obtained. Based on this optical measurement, an optically determined laser threshold current I th_opt can be obtained, as shown in Figure 3 .

[0073] In step S502, a second electrical trimming parameter can be obtained. The second electrical trimming parameter can indicate the photodiode characteristic curve 71 of the photodiode current relative to the laser diode current under predetermined conditions. Specifically, the second electrical trimming parameter can include measurements below the laser threshold, indicated by "SE" for spontaneous emission below the laser threshold, and measurements above the laser threshold, which do not have an SE indication. For example, the first photodiode current indicating a third laser diode current below the laser threshold (see...). Figure 3 Item 76 in the text) and the second photodiode current at a fourth laser diode current below the laser threshold (see item 76 in the text) Figure 3 The parameters of item 77 in [the original text] are used. Based on this, the slope of the photodiode characteristic curve 71 below the determined laser threshold can be obtained during the trimming process. Similarly, further measurements can be performed above the laser threshold (see [the original text]). Figure 3 Items 74 and 75 in the text), and the slope of the photodiode characteristic curve 71 can be obtained during the trimming process, which is above the determined laser threshold slope PD. Slope_trim_el The electrically determined threshold current I of the laser diode. th_el The estimate can be determined from the inflection point of the photodiode characteristic curve, for example, by calculating the intersection of the straight lines passing through terms 76 and 77 and the straight lines passing through terms 74 and 75. It should be noted that the optically determined laser threshold current I... th_opt It may deviate slightly from the electrically determined laser threshold current I. th_el .

[0074] Following this finishing stage during manufacturing, subsequent on-site operations can be performed, for example, in situations leading to... Figure 4 Steps S503 to S505 are performed under the conditions shown in the photodiode characteristic curve and the laser diode characteristic curve. In step S503, the first photodiode current is measured at a third laser diode current below the laser threshold (see...). Figure 4 (Item 76' in the text). Furthermore, the second photodiode current was measured at a fourth laser diode current below the laser threshold, which differed from the third laser diode current (see item 76' in the text). Figure 4 (Item 77' in the text). It can be used in conjunction with... Figure 3 The corresponding photodiode current is measured under the same or different laser diode currents. Accordingly, measurements above the laser threshold at 74' and 75' can be obtained. The corresponding laser diode slopes are shown for illustrative purposes only and are not required to be measured during field operation.

[0075] In step S504, the slope PD of the photodiode current relative to the laser diode current below the laser threshold is determined based on the measurements of the first photodiode current and the second photodiode current. SlopeSE_elFor spontaneous emission, the slope of the photodiode below the lasing threshold is again denoted by "SE".

[0076] In step S505, the optical output power of the laser diode above the lasing threshold can then be controlled based on the first optical trimming parameter, the second electrical trimming parameter and the slope of the photodiode current relative to the laser diode current below the lasing threshold. This will now be described with reference to Figure 6 to Figure 8 Further details of exemplary embodiments are described.

[0077] Figure 6 A flow chart of exemplary method steps 600 during the trimming phase is shown. The trimming process starts in step S601. The left branch denoted by S602 refers to optical measurements, while the right branch of the flow chart, denoted by S603, refers to electrical trimming steps. In step S604, a first laser diode current above the lasing threshold is applied to the laser diode under predetermined conditions. The optical output power of the laser diode at said first laser diode current is measured (see item 64 in Figure 3 ). In addition, a second laser diode current above the lasing threshold, different from the first laser diode current, is applied to the laser diode under predetermined conditions in step S604. The optical output power of the laser diode at said second laser diode current is measured (see item 65 in Figure 3 ). In step S605, a trimming laser diode slope (LD Slope_trim_opt ) of the laser diode characteristic curve indicating the relation of the optical output power of the laser diode relative to the laser diode current of the laser diode under the predetermined conditions is determined based on said measurements. In step S606, a linear equation can for example be solved for zero optical output to determine in 607 an optically determined trimming laser threshold current (I th_trim_opt ) of the laser diode characteristic curve indicating the relation of the optical output power of the laser diode relative to the laser diode current of the laser diode under the predetermined conditions.

[0078] Referring to the right branch in Figure 6 , in step S608, a first laser diode current above the lasing threshold is applied to the laser diode under predetermined conditions. The photodiode current at said first laser diode current is measured (see item 74 in Figure 3 ). In addition, a second laser diode current above the lasing threshold, different from the first laser diode current, is applied to the laser diode under predetermined conditions in step S608. The photodiode current at said second laser diode current is measured (see item 75 in Figure 3(Item 75 in the text). Advantageously, steps S604 and S608 can be performed in parallel, i.e., the same first and second laser diode currents above the laser threshold are applied, and their respective optical output power and photodiode currents can be measured in parallel. However, different laser diode currents can also be applied in steps S604 and S608. Based on the measurements, the electrically determined photodiode slope PD during the trimming period can be determined in step S609. Slope_trim_el .

[0079] Accordingly, in step S610, a third laser diode current below the laser threshold is applied to the laser diode under predetermined conditions. The photodiode current at the location of the third laser diode current is measured (see...). Figure 3 (Item 76 in the text). Additionally, in step S610, a fourth laser diode current, different from the third laser diode current and below the laser threshold, is applied to the laser diode under predetermined conditions. The photodiode current of the laser diode under the fourth laser diode current is measured (see...). Figure 3 Item 77 in the text). Based on the measurement, the electrically determined photodiode slope PD can be determined in step S611 below the laser threshold during the trimming period, i.e., during spontaneous emission (SE). SlopeSE_trim_el In step S612, based on, for example Figure 3 The aforementioned measurements can determine, or more precisely estimate, the threshold current I of the laser diode. th_trim_el .

[0080] Figure 7 and 8 The method steps during operating conditions are illustrated, for example, after placing a particle sensor device, including a corresponding VCSEL with an integrated photodiode, in the field. In the method described herein, the laser diode voltage at a predetermined laser diode current is not measured during field operation. Therefore, the calibration process can be simplified. Furthermore, circuit design can be simplified because the device need not be adapted to perform the corresponding voltage measurement of the laser diode.

[0081] exist Figure 7 In, it can be repeated in Figure 6 The electrical measurements described in the right branch, but this time under unknown conditions during field operation of the device. Figure 7 This is done, rather than during manufacturing under controlled predetermined conditions. Figure 6 The adjustment measurement is performed. Therefore, in step S702, a first laser diode current higher than the laser threshold is applied to the laser diode during field operation. The photodiode current at the first laser diode current is measured (see...). Figure 4(Item 74' in the text). Additionally, in step S702, during field operation, a second laser diode current, different from the first laser diode current and higher than the laser threshold, is applied to the laser diode. The photodiode current of the laser diode under the second laser diode current is measured (see item 74' in the text). Figure 4 (Item 75' in the text). Based on the measurement, the slope PD of the photodiode in the field can be determined in step S703. Slope_el .

[0082] Therefore, in step S704, a third laser diode current below the laser threshold is applied to the laser diode during field operation. The photodiode current at the location of the third laser diode current is measured (see...). Figure 4 (Item 76' in the text). Additionally, in step S704, during field operation, a fourth laser diode current, different from the third laser diode current and below the laser threshold, is applied to the laser diode. The photodiode current of the laser diode under the fourth laser diode current is measured (see item 76' in the text). Figure 4 (Item 77' in the text). Based on the measurement, the photodiode slope PD can be determined in step S705 below the laser threshold in the field, i.e., during spontaneous emission (SE). SlopeSE_el In step S706, based on as follows Figure 4 The aforementioned measurements can determine, or more precisely estimate, the threshold current I of the laser diode. th_el .

[0083] The electrically determined laser threshold, also known as the alternative threshold current, can be based on, for example, Figure 3 and Figure 4 The inflection point in the characteristics of the photodiode shown is used to determine this.

[0084] Figure 8 It shows in Figure 7 The flowchart shows the other method steps 800 following the method steps shown. In step S801, based on (a) the slope of the photodiode current relative to the laser diode current below the laser threshold under predetermined conditions (PD... SlopeSE_trim_el ),Right now Figure 6 The results of step S611 are compared with (b) the slope of the photodiode current relative to the laser diode current below the laser threshold during field operation (PD). Slope-SE_el ),Right now Figure 7 The difference between the results of step S705 determines the photodiode slope correction value (ΔPD) below the laser threshold. SlopeSE_el For example:

[0085] △PD SlopeSE_el =PD SlopeSE_el -PDSlopeSE_trim_el .

[0086] In step S802, the laser diode slope correction value (ΔLD Slope_el ) above the laser threshold can be determined based on the following equation:

[0087] ΔLD Slope_el = A1*ΔPD SlopeSE_el + A2,

[0088] where ΔPD SlopeSE_el is the photodiode slope correction value below the laser threshold, A1 is a proportional coefficient, and A2 is an offset coefficient. The coefficients can be determined empirically or based on simulations. Thus, the laser diode slope correction value above the laser threshold is determined based on the photodiode slope correction value below the laser threshold.

[0089] In steps S803 and S804, the laser threshold current correction value ΔI th_trim_el is determined based on the difference between (a) the trimmed laser threshold current I th_el derived from the characteristic curve of the photodiode current versus the laser diode current under predetermined conditions and (b) the operating laser threshold current I th_el derived from the characteristic curve of the photodiode current versus the laser diode current during field operation. In step S803, the auxiliary laser threshold correction value can be calculated, for example, as follows:

[0090] ΔI th_aux_el = I th_el - I th_trim_el .

[0091] In step S804, the laser threshold current correction value can then be determined by applying an optional proportional factor B1 and / or an offset B2 based on:

[0092] ΔI th_el = B1*ΔI th_aux_el + B2.

[0093] In step S805, the desired or target optical output power to be provided by the laser diode is provided. The optical output power is preferably selected such that the device, e.g. the optical particle sensor, can be operated within the eye safety range.

[0094] In step S806, the optical output power of the laser diode is controlled by setting the laser diode current I LD to the value for the desired optical output power. The laser diode current can be calculated from

[0095] I LD (P opt ) = I th + Popt / LD Slope ;

[0096] in,

[0097] I LD (P opt ) is the laser diode current; P opt This refers to the light output power.

[0098] I th It is the laser threshold current (during operation);

[0099] LD Slope It is the slope of the laser diode (during operation);

[0100] More precisely, this can be achieved by setting the laser diode current I. LD To control the light output power of the laser diode,

[0101] I LD (P opt )=(I th_trim_opt +△I th_el )+P opt / (LD Slope_trim_opt +△LD Slope_el );

[0102] in,

[0103] I LD (P opt ) is the laser diode current; P opt It is the light output power;

[0104] I th_trim_opt It is the optically determined laser threshold current;

[0105] △I th_el It is an electrically determined laser threshold current correction value;

[0106] LD Slope_trim_opt It is the optically determined slope of the laser diode;

[0107] △LD Slope_el It is the electrically determined slope correction value for the laser diode.

[0108] In one embodiment, the reciprocal of the laser diode slope can be used to avoid the division term. The term LD is introduced. Slope,m1 =1 / LD Slope Therefore, I LD (P opt ) = I th +P opt *LD Slope,m1This can allow for an efficient implementation in an application specific integrated circuit (ASIC). Similar to the non-reciprocal term, the reciprocal term can be written as:

[0109] LD Slope,m1 = 1 / LD Slope_trim_opt +△LD Slope_el,m1 = LD Slope_trim_opt,m1 +△LD Slope_el,m1 , where LD Slope_trim_opt,m1 is the reciprocal of the optically determined laser diode slope. In step S605, the reciprocal of the modified laser diode slope (LD Slope_trim_opt,m1 ) can thus be determined which indicates the laser diode characteristic curve of the optical output power of the laser diode relative to the laser diode current of the laser diode under predetermined conditions. Similarly, in step S802, the reciprocal of the laser diode slope correction value above the laser threshold (△LD Slope_el,m1 ) can be determined according to:

[0110] △LD Slope_el,m1 = A3*△PD SlopeSE_el + A4,

[0111] where △PD SlopeSE_el is the photo diode slope correction value below the laser threshold, A3 is a proportional coefficient and A4 is an offset coefficient. Surprisingly, it has been found that the reciprocal of the laser diode slope correction value above the laser threshold (△LD Slope_el,m1 ) can be determined based on the non-reciprocal photo diode slope correction value below the laser threshold (△PD SlopeSE_e ). The coefficients can be determined empirically or based on simulations. Thus, the reciprocal of the laser diode slope correction value above the laser threshold is determined based on the photo diode slope correction value below the laser threshold. It is noted that in general, △LD Slope_el,m1 ≠ 1 / △LD Slope_el . Nonetheless, it has surprisingly been found that the above approach allows for a high precision control of the laser output power while allowing for an efficient implementation in an ASIC. It is further noted that the term describing the threshold current can remain unchanged. Thus, the optical output power of the laser diode can be controlled by setting the laser diode current I LD :

[0112] I LD (P opt ) = (I th_trim_opt +△I th_el ) + P opt *(LD Slope_trim_opt,m1 +△LD Slope_el,m1 )

[0113] = (I th_trim_opt +B1*△I th_aux_el +B2) + P opt *(LDSlope_trim_opt,m1 + A3*△PD SlopeSE_el + A4

[0114] wherein

[0115] I LD (P opt ) is the laser diode current; P opt is the optical output power;

[0116] I th_trim_opt is the optically determined laser threshold current;

[0117] △I th_el is the electrically determined laser threshold current correction value;

[0118] LD Slope_trim_opt,m1 is the inverse of the optically determined laser diode slope;

[0119] △LD Slope_el,m1 is the inverse of the electrically determined laser diode slope correction value.

[0120] As a further advantage of this approach, for a given optical output power P opt , the set of parameters can be further reduced, since B2and P opt *A4can be combined into one constant. Thus, the number of constants can be reduced from four to three.

[0121] In summary, the proposed solution can thus provide a further improved approach for controlling the optical output power of a laser diode. In particular, the circuit design can be simplified, since a voltage measurement of the laser diode voltage can no longer be required during field operation. Moreover, the setup or calibration procedure can be less time consuming in view of the reduced number of measurements required in the field.

[0122] While the application has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the application is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed application, from a study of the drawings, the disclosure, and the appended claims.

[0123] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit can fulfil the functions of several items recited in the claims. 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.

[0124] A computer program can be stored / distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state storage medium supplied in or with other hardware, but can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0125] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method of controlling the optical output power of a laser diode (10), wherein, The laser diode is associated with a photodiode (20) that converts light received from the laser diode (10) into an electronic photodiode current, the method comprising the steps of: - obtaining first optical trimming parameters indicative of a first optical output power of the laser diode (10) at a first laser diode current above the lasing threshold and a second optical output power of the laser diode at a second laser diode current above the lasing threshold different from the first laser diode current under predetermined calibration conditions; - obtaining second electrical trimming parameters indicative of a photodiode characteristic curve of the photodiode current relative to the laser diode current under predetermined calibration conditions; - measuring a first photodiode current at a third laser diode current below the lasing threshold; - measuring a second photodiode current at a fourth laser diode current below the lasing threshold different from the third laser diode current; characterized in that - determining a slope of the photodiode current relative to the laser diode current below the lasing threshold based on the measurements of the first and second photodiode currents; - controlling the optical output power of the laser diode (10) above the lasing threshold based on the first optical trimming parameters, the second electrical trimming parameters and the slope of the photodiode current relative to the laser diode current below the lasing threshold; wherein the first light trimming parameter comprises an optically determined trimming laser diode slope (LD Slope_trim_opt ) of a laser diode characteristic curve, Slope_trim_opt a correction value (△LD Slope_el ) of the laser diode slope above the electrical determined laser threshold in order to control the optical output power by setting a laser diode current I LD as follows: I LD (P opt )=(I th_trim_opt +△I th_el )+P opt / (LD Slope_trim_opt +△LD Slope_el ); wherein I LD (P opt ) is the laser diode current; P opt is the optical output power; I th_trim_opt is the optically determined trimmed laser threshold current; ΔI thel is an electrically determined laser threshold current correction value; LD Slope_trim_opt is the optically determined trimmed laser diode slope; ΔLD Slope_el is the laser diode slope correction value above the electrically determined laser threshold.

2. The method of claim 1, wherein, obtaining the first optical trimming parameters comprises measuring a laser diode (10) characteristic curve indicative of a relationship of the optical output power of the laser diode relative to the laser diode current of the laser diode, the optical output power being measured with a calibrated external power meter.

3. The method according to any of the preceding claims, wherein, The laser diode characteristic curve is indicative of a relationship of the optical output power of the laser diode (10) relative to the laser diode current of the laser diode under predetermined calibration conditions.

4. The method of claim 1 or 2, wherein, The first light trimming parameter comprises an optically determined trimming laser threshold current (I th_trim_opt ) of a laser diode characteristic curve, which indicates a relation of a light output power of the laser diode (10) relative to a laser diode current of the laser diode under predetermined calibration conditions.

5. The method of claim 4, wherein, Controlling optical output power of a laser diode (10) based on an optically determined trimming laser threshold current (I th_trim_opt ) and an electrically determined laser threshold current correction value (△I th_el ), the laser threshold current correction value (△I th_el ) being determined based on an electrical measurement.

6. The method of claim 5, wherein, The laser threshold current correction value (ΔI th_el ) is determined based on a difference between (a) a trimmed laser threshold current (I th_trim_el ) derived from a characteristic curve of photodiode current versus laser diode current under predetermined calibration conditions and (b) an operating laser threshold current (I th_el ) determined during field operation.

7. The method of claim 3, wherein, The laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on electrical measurements.

8. The method of claim 4, wherein, The laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on electrical measurements.

9. The method of claim 5 or 6, wherein, The laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on electrical measurements.

10. The method of claim 7 or 8, wherein, a laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on a photodiode slope correction value (ΔPD SlopeSE_el ) below the laser threshold, the photodiode slope correction value (ΔPD SlopeSE_el ) below the laser threshold is determined based on electrical measurements.

11. The method of claim 9, wherein, a laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on a photodiode slope correction value (ΔPD SlopeSE_el ) below the laser threshold, the photodiode slope correction value (ΔPD SlopeSE_el ) below the laser threshold is determined based on electrical measurements.

12. The method of claim 10, wherein, The laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on the following equation: ΔLD Slope_el = A1*ΔPD SlopeSE_el + A2, wherein, ΔPD SlopeSE_el is a photodiode slope correction value below the laser threshold, A1 is a proportional coefficient, A2 is an offset coefficient, ΔPD SlopeSE_el represents the difference between the slope of the photodiode current relative to the laser current below the laser threshold under predetermined conditions and the slope of the photodiode current relative to the laser current below the laser threshold at the time of measuring the first and second photodiode currents; ΔLD Slope_el represents the difference between the slope of the optical output power of the laser diode relative to the laser current above the laser threshold under predetermined conditions and the slope of the optical output power relative to the laser current above the laser threshold at the time of measuring the first and second photodiode currents.

13. The method of claim 11, wherein, The laser diode slope correction value (ΔLD Slope_el ) above the laser threshold is determined based on the following equation: ΔLD Slope_el = A1*ΔPD SlopeSE_el + A2, wherein, ΔPD SlopeSE_el is a photodiode slope correction value below the laser threshold, A1 is a proportional coefficient, A2 is an offset coefficient, ΔPD SlopeSE_el represents a difference between a slope of the photodiode current relative to the laser current below the laser threshold under predetermined conditions and a slope of the photodiode current relative to the laser current below the laser threshold at the time of measuring the first and second photodiode currents; ΔLD Slope_el represents a difference between a slope of the optical output power of the laser diode relative to the laser current above the laser threshold under predetermined conditions and a slope of the optical output power relative to the laser current above the laser threshold at the time of measuring the first and second photodiode currents.

14. The method of claim 10, wherein, The second electrical trimming parameter comprises a slope (PD SlopeSE_trim_el ) of the photodiode current relative to the laser diode current below the laser threshold under calibration predetermined conditions. and The photodiode slope correction value (ΔPD SlopeSE_el ) below the laser threshold is determined based on the difference between (a) the slope of the photodiode current relative to the laser diode current below the laser threshold (PD SlopeSE_trim_el ) under predetermined calibration conditions and (b) the slope of the photodiode current relative to the laser diode current below the laser threshold (PD SlopeSE_el ) during field operation.

15. The method of any one of claims 11-13, wherein, The second electrical trimming parameter comprises a slope (PD SlopeSE_trim_el ) of the photodiode current relative to the laser diode current below the laser threshold under calibration predetermined conditions. and The photodiode slope correction value (ΔPD SlopeSE_el ) below the laser threshold is determined based on the difference between (a) the slope of the photodiode current relative to the laser diode current below the laser threshold (PD SlopeSE_trim_el ) under predetermined calibration conditions and (b) the slope of the photodiode current relative to the laser diode current below the laser threshold (PD SlopeSE_el ) during field operation.

16. The method of any one of claims 1-2, 5-8, 11-14, wherein, The photodiode (20) is integrated with the laser diode (10) that is a vertical cavity surface emitting laser, VCSEL, with the integrated photodiode (20).

17. The method of any one of claims 1-2, 5-8, 11-14, wherein, Controlling the output power of the laser diode (10) does not comprise measuring a laser diode voltage at a predetermined laser diode current during field operation.

18. A control device (30) for controlling the optical output power of a laser diode (10), wherein The laser diode is associated with a photodiode that converts light received from the laser diode into an electronic photodiode current, the control device (30) being adapted to perform the steps of: - obtaining first optical trimming parameters indicative of a first optical output power of the laser diode (10) at a first laser diode current above the lasing threshold and a second optical output power of the laser diode at a second laser diode current above the lasing threshold different from the first laser diode current under predetermined calibration conditions; - obtaining second electrical trimming parameters indicative of a photodiode characteristic curve of the photodiode current relative to the laser diode current under predetermined calibration conditions; - measuring a first photodiode current at a third laser diode current below the lasing threshold; - measuring a second photodiode current at a fourth laser diode current below the lasing threshold different from the third laser diode current; characterized in that - determining a slope of the photodiode current relative to the laser diode current below the lasing threshold based on the measurements of the first and second photodiode currents; - determining a slope of the photodiode current relative to the laser diode current below the laser threshold based on the measurements of the first and second photodiode currents; - controlling the output power of the laser diode (10) above the laser threshold based on the first optical trimming parameter, the second electrical trimming parameter and the slope of the photodiode current relative to the laser diode current below the laser threshold; wherein the first light trimming parameter comprises an optically determined trimming laser diode slope (LD Slope_trim_opt ) of a laser diode characteristic curve, Slope_trim_opt ) and an electrically determined laser diode slope correction value (△LD Slope_el ) above the laser threshold, in order to control the optical output power by setting a laser diode current I LD ​ I LD (P opt )=(I th_trim_opt +△I th_el )+P opt / (LD Slope_trim_opt +△LD Slope_el ); wherein, I LD (P opt ) is the laser diode current; P opt is the optical output power; I th_trim_opt is an optically determined trimmed laser threshold current; ΔI thel is an electrically determined laser threshold current correction value; LD Slope_trim_opt is the optically determined trimmed laser diode slope; ΔLD Slope_el is the laser diode slope correction value above the electrically determined laser threshold.

19. An optical particle sensor device comprising a photodiode, a laser diode (10) and a control device (30) adapted to control the optical output power of the laser diode based on the method of any one of claims 1 to 18.

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