Laser device and method of operating a semiconductor laser

Abstract

A laser device (1) comprising a semiconductor laser (2) and a driving circuit (3) configured to operate the semiconductor laser (2) in pulsed operation is specified, wherein - the driving circuit (3) comprises a switch (T1) switchable between an open state and a closed state, - the driving circuit (3) comprises a first capacitor (C1) coupled to a power supply (5), - the switch (T1) is coupled to the first capacitor (C1), - the driving circuit (3) comprises a second capacitor (C2), - a p-contact (22) of the semiconductor laser (2) is capacitively coupled to the switch (T1) via the second capacitor (C2), - in the closed state of the switch (T1), the first capacitor (C1) is discharged via the second capacitor (C2) and the semiconductor laser (2) to produce a laser pulse, and - in the open state of the switch (T1), the second capacitor (C2) is discharged. Further, a method of operating a semiconductor laser (2) is specified.

Description

[0001] Description

[0002] LASER DEVICE AND METHOD OF OPERATING A SEMICONDUCTOR LASER

[0003] The present application relates to a laser device and to a method of operating a semiconductor laser .

[0004] For many applications such as LIDAR ( Light Distancing And Ranging) applications , semiconductor lasers are operated in pulsed mode . Field-ef fect transistors can be used in laser driving circuits to produce the pulses . However, the semiconductor laser may be subj ected to a too long electrical pulse or even to a DC current i f the field-ef fect transistor unintendedly remains in the conducting state . This may damage the semiconductor laser or cause eye safety issues .

[0005] It is an obj ect to provide a way to improve the reliability of pulse generation .

[0006] This obj ect is obtained inter alia by a laser device and a method of operating a semiconductor laser according to the independent claims . Further configurations and developments are the subj ect of the dependent claims .

[0007] A laser device comprising a semiconductor laser and a driving circuit configured to operate the semiconductor laser in pulse operation is speci fied . For example , the semiconductor laser comprises a p-contact and an n-contact configured to inj ect charge carriers into an active region where they recombine under emission of electromagnetic radiation . For example , a pulse length of a laser pulse is in a range from 1 ns to 200 ns . An optical power of the semiconductor laser during the pulse is in a range from 20 W to 500 W, for example .

[0008] According to at least one embodiment of the laser device, the driving circuit comprises a switch that is switchable between an open state and a closed state (or on-state) . For example, the switch is a field-effect transistor (FET) , in particular an n-channel FET (nFET) based on gallium nitride (GaN) , for instance .

[0009] For example, the switch is brought into the electrically conductive closed state by applying an electrical voltage to a gate contact of the switch.

[0010] According to at least one embodiment of the laser device, the driving circuit comprises a first capacitor. For example, the first capacitor is coupled to a power supply during operation of the laser device, in particular to a positive pole of a DC power supply.

[0011] In particular, a negative side of the first capacitor is coupled to a negative pole of the power supply.

[0012] A capacity of the first capacitor is in a range from 10 nF to 4.7 pF, for example.

[0013] According to at least one embodiment of the laser device, the switch is coupled to the first capacitor. In particular, the switch is coupled to the positive side of the first capacitor. During operation of the laser device, the first capacitor may be discharged via the switch if the switch is in the closed state. According to at least one embodiment of the laser device , the driving circuit comprises a second capacitor . A capacity of the second capacitor is in a range from 1 nF to 1 pF, for example .

[0014] In particular, a p-contact of the semiconductor laser is capacitively coupled to the switch via the second capacitor . In other words , the switch is arranged on the high side of the semiconductor laser .

[0015] According to at least one embodiment of the laser device , the first capacitor is discharged via the second capacitor and the semiconductor laser to produce a laser pulse in the closed state of the switch .

[0016] According to at least one embodiment of the laser device , the second capacitor is discharged in the open state of the switch . In particular, the second capacitor can be completely discharged before the switch is brought into the closed state to produce the subsequent laser pulse .

[0017] In at least one embodiment of the laser device , the laser device comprises a semiconductor laser and a driving circuit configured to operate the semiconductor laser in pulsed operation, wherein the driving circuit comprises a switch switchable between an open state and a closed state . The driving circuit further comprises a first capacitor coupled to a power supply . The switch is coupled to the first capacitor . The driving circuit comprises a second capacitor . A p-contact of the semiconductor laser is capacitively coupled to the switch via the second capacitor . In the closed state of the switch, the first capacitor is discharged via the second capacitor and the semiconductor laser to produce a laser pulse . In the open state of the switch, the second capacitor is discharged .

[0018] The laser device is in particular based on the idea that capacitive coupling of a semiconductor laser to a switch can be used to block too long electrical pulses or even DC currents through the semiconductor laser in a reliable and ef ficient manner . Such pulses or DC currents may occur i f the switch remains in the closed state for a time that is longer than a regular closed state time of the switch . This unintended ef fect is also referred to as " stuck on" . The maximum pulse length can be defined by the capacity of the second capacitor acting as an AC coupling capacitor .

[0019] Consequently, the capacity of the AC coupling capacitor also limits the maximum optical energy emitted during one pulse so that eye safety requirements can be met in a reliable manner .

[0020] According to at least one embodiment of the laser device , a positive side of the first capacitor is coupled to the power supply via a first resistor of the driving circuit . For example , a resistance of the first resistor is in a range from 1 Q to 20 Q .

[0021] According to at least one embodiment of the laser device , the switch is coupled to a negative pole of the power supply via a first parallel branch and a second parallel branch, wherein the first parallel branch comprises the second capacitor and the semiconductor laser and the second parallel branch comprises a second resistor . In the open state of the switch, the second capacitor can be discharged via the second resistor . In the closed state of the switch, the second parallel branch bypasses the semiconductor laser, thereby producing leak currents through the second resistor . However, these leak currents can be reduced by adapting a resistance of the second resistor appropriately . For example , the resistance of the second resistor is in a range from 1 Q to 50 Q .

[0022] According to at least one embodiment of the laser device , the first parallel branch comprises a diode connected in antiparallel to the semiconductor laser . The diode may act as a flyback diode to eliminate voltage spikes caused by fast switching of f .

[0023] According to at least one embodiment of the laser device , a discharge path of the second capacitor extends through the second resistor and the diode . In other words , a positive side of the second capacitor and a negative side of the capacitor are electrically connected via the second resistor and the diode . This helps to ef ficiently discharge the second capacitor between the laser pulses to be produced .

[0024] According to at least one embodiment of the laser device , a capacity of the second capacitor is smaller than a capacity of the first capacitor . For example , the capacity of the second capacitor is , by a factor of at least 1 . 5 or at least 2 or at least 5 , smaller than the capacity of the first capacitor .

[0025] According to at least one embodiment of the laser device , the capacity of the second capacitor defines a maximum pulse length . In particular, the capacity of the second capacitor may be selected such that a damage to the semiconductor laser is prevented and / or the maximum optical energy of the individual pulses remains below a predetermined limit to ensure eye safety .

[0026] According to at least one embodiment of the laser device , a regular closed state time of the switch is shorter than the maximum pulse length . In other words , during regular operation of the laser device , the pulse length of the laser pulses is controlled by the regular closed state time of the switch . I f a " stuck on" of the switch occurs , however, the second capacitor ensures that the pulse length does not exceed the maximum pulse length .

[0027] According to at least one embodiment of the laser device , a regular closed state time of the switch is equal to or larger than the maximum pulse length . In this case , the pulse length of the laser pulses during regular operation is defined by the capacity of the second capacitor acting as the AC coupling capacitor .

[0028] According to at least one embodiment of the laser device , the semiconductor laser is configured for emission of pulsed radiation in the near infrared spectral range . For example , the semiconductor laser is configured for emission of radiation in a range from 800 nm to 1 . 2 pm . For example , the semiconductor laser is based on arsenide semiconductor material .

[0029] Based on " arsenide compound semiconductor material" means in the present context that a semiconductor layer sequence of the semiconductor laser or at least a part thereof , particularly preferably at least the active region and / or the growth substrate , has a compound semiconductor material with arsenic as group V element , preferably comprising AlxInyGai-x-yAs or consisting thereof , where 0 < x < 1 , 0 < y < 1 , and x + y < 1 . This material does not necessarily have to have a mathematically exact composition according to the above formula . Rather, it may comprise , for example , one or more dopants as well as additional constituents . For the sake of simplicity, however, the above formula includes only the essential constituents of the crystal lattice (Al , Ga, In, As ) , even i f these may be partially replaced and / or supplemented by small amounts of other substances .

[0030] According to at least one embodiment of the laser device , the semiconductor laser comprises a plurality of active regions that are individually addressable via individual p-contacts . In other words , the semiconductor laser provides a plurality of individually addressable channels . For example , the number of individually addressable active regions is in a range from 2 to 200 .

[0031] In particular, the individually addressable active regions may be formed in a single semiconductor chip .

[0032] The n-contact of the semiconductor laser may be a common n- contact for all active regions of the semiconductor laser .

[0033] Each of the individual p-contacts may be provided with a driving circuit as described above . Thus , each of the p- contacts is capacitively coupled to a switch assigned to the respective p-contact . Two or more of the individual p- contacts may be coupled to the same switch, so that one switch may trigger laser pulses in two or more active regions of the semiconductor laser . For example , the laser device is configured for a LIDAR application, in particular for a LIDAR application in a vehicle . However, the laser device can also be used for other application using pulsed laser radiation, in particular i f too long laser pulses or a continuous wave ( cw) laser emission may cause reliability and / or safety issues .

[0034] Further, a method of operating a semiconductor laser is speci fied .

[0035] In at least one embodiment of the method, the method comprises the steps of providing a semiconductor laser wherein a p-contact of the semiconductor laser is capacitively coupled via a capacitor to a switch . The method further comprises the step of bringing the switch in a closed state to supply the semiconductor laser with a current via the switch and the capacitor to produce a laser pulse . The method further comprises the step of discharging the capacitor after the laser pulse .

[0036] According to at least one embodiment of the method, a closed state time of the switch is shorter than a maximum pulse length defined by a capacity of the capacitor . Thus , the switch is controlled such that the closed state time of the switch defines the regular pulse length of the semiconductor laser, whereas , in the event of a failure , the maximum pulse length is limited by the capacity of the capacitor acting as the AC coupling capacitor .

[0037] According to at least one embodiment of the method, a closed state time of the switch is equal or larger than a maximum pulse length defined by a capacity of the capacitor . In this case , the regular pulse length is defined by the capacity of the capacitor acting as the AC coupling capacitor .

[0038] By means of the method, the maximum pulse length of a laser pulse can be controlled in a reliable manner . In particular, the maximum pulse length is not exceeded, even i f a failure in the driving circuit occurs , for example a stuck-on event of a transistor acting as a switch .

[0039] The laser device described above is particularly suited for the method . Thus , features described in connection with the laser device also apply for the method and vice versa .

[0040] Features described above in connection with at least one embodiment of the method or the laser device can be combined with other features described in connection with at least one embodiment of the method or the laser device unless they are contradictory .

[0041] Further configurations and expediencies will become apparent from the subsequent description of the exemplary embodiments in connection with the figures .

[0042] In the exemplary embodiments and figures similar or similarly acting constituent parts are provided with the same reference signs . Generally, only the di f ferences with respect to the individual exemplary embodiments are described . Unless speci fied otherwise , the description of a part or feature in one exemplary embodiment applies to a corresponding part or feature in another exemplary embodiment as well . In the figures :

[0043] Figure 1A shows an exemplary embodiment of a laser device ;

[0044] Figure IB shows a pulse current path for the laser device of Figure 1A;

[0045] Figure 1C shows a discharge path for a laser device of Figure 1A;

[0046] Figure 2 shows an exemplary embodiment of a semiconductor laser ; and

[0047] Figure 3 shows an exemplary embodiment of a method of operating a laser device .

[0048] The elements illustrated in the figures and their si ze relationships among one another are not necessarily true to scale . Rather, individual elements or layer thicknesses may be represented with an exaggerated si ze for the sake of better representability and / or for the sake of better understanding .

[0049] Figure 1A illustrates a laser device 1 comprising a semiconductor laser 2 and a driving circuit 3 configured to operate the semiconductor laser in pulsed operation .

[0050] The driving circuit 3 comprises a switch T1 switchable between an open state and a closed state . For example , the switch is an n-channel FET based on GaN . The switch T1 can be brought into the closed state by applying a trigger signal to a gate contact 6 of the switch Tl . The driving circuit 3 further comprises a first capacitor Cl , wherein a positive side of the first capacitor Cl is coupled to a positive pole ( + ) of a power supply 5 and a negative side of the first capacitor Cl is coupled to a negative pole ( - ) of the power supply 5 . During operation of the laser device 1 , the first capacitor Cl can be charged via a first resistor R1 arranged between the power supply 5 and the first capacitor Cl . A resistance of the first resistor R1 is in a range from 1 Q to 20 Q, for instance .

[0051] A p-contact 22 ( cf . Fig . 2 ) of the semiconductor laser 2 is capacitively coupled to the switch T1 via a second capacitor C2 . In the closed state of the switch Tl , the first capacitor Cl is discharged via the second capacitor C2 and the semiconductor laser 2 to produce a laser pulse . A pulse current path 35 is illustrated in Figure IB using arrows .

[0052] In the open state of the switch Tl , the second capacitor C2 is discharged . Figure 1C illustrates a discharge path 36 using arrows .

[0053] In the closed state of the switch Tl , the semiconductor laser 2 produces a light pulse until the second capacitor C2 is completely charged . Consequently, a capacity of the second capacitor C2 acting as an AC coupling capacitor defines a maximum pulse length of the laser pulse .

[0054] Even i f switch Tl remains for a longer time , or even permanently, in the conductive closed state , there is no longer any current flow through the semiconductor laser 2 . Rather, the current may flow through a second resistor R2 arranged between the switch 6 and the negative pole of the power supply 5 . In the exemplary embodiment shown, the switch T1 is coupled to the negative pole of the power supply 5 via a first parallel branch 31 and a second parallel branch 32 . The first parallel branch 31 comprises the second capacitor C2 and the semiconductor laser 2 connected in series . The second parallel branch 32 comprises the second resistor R2 .

[0055] Thus , in the closed state of the switch Tl , current flows through the semiconductor laser 2 and through the second resistor R2 . However, this leakage current through the second resistor R2 can be kept at a comparably low value by appropriately selecting the resistance of resistor R2 . A resistance of the second resistor R2 is in a range from 1 Q to 50 Q, for instance .

[0056] Further, the driving circuit 3 comprises a diode DI which may be embodied as a Schottky diode , for instance . The diode DI is connected in anti-parallel to the semiconductor laser 2 . The diode DI can be used as a flyback diode to eliminate voltage spikes due to fast switching of the switch Tl .

[0057] Further, the diode DI is arranged in the discharge path 36 between a positive side of second capacitor C2 and a negative side of second capacitor C2 as illustrated in Figure 1C . During the laser pulse , the charge carriers may be provided by the first capacitor Cl which preferably has a larger capacitance than the second capacitor C2 . For example , the capacity of the first capacitor is , by a factor of at least 1 . 5 or at least 2 or at least 5 , larger than the capacity of the second capacitor C2 . For example , the first capacitor Cl has a capacity in a range from 10 nF to 4 . 7 pF . The second capacitor C2 may have a capacity in a range from 1 nF to 1 pF .

[0058] As described above , the capacity of the second capacitor C2 defines the maximum pulse length . A larger capacity of the second capacitor results in a longer maximum pulse length .

[0059] The maximum pulse length may be selected such that the semiconductor laser 2 is not overheated .

[0060] Alternatively or in addition, the maximum pulse length may be selected such that the energy of the laser pulse does not exceed a predefined level . For example , the predefined level is related to eye safety requirements .

[0061] The actual pulse length during regular operation of the laser device 1 may be equal to the maximum pulse length or shorter than the maximum pulse length .

[0062] For example , the regular pulse length can be shorter than the maximum pulse length i f a trigger pulse applied to the switch T1 is selected such that the regular closed state time of the switch T1 is shorter than the maximum pulse length . In this case , the maximum pulse length is only reached i f the switch T1 remains in the conductive closed state for a longer time than intended .

[0063] Alternatively, the regular closed state time of the switch T1 may be equal to or even longer than the maximum pulse length so that the maximum pulse length corresponds to the pulse length during regular operation of the laser device . For example , the laser device is configured to produce laser pulses in a range from 1 ns to 200 ns In particular, the pulse length may be equal to or shorter than 5 ns .

[0064] For example , the laser device 1 may be used as a light source in a LIDAR application . For example , the laser device 1 is configured for being mounted in a vehicle such as a car or a truck .

[0065] It is noted that in the context of the present application, the term " light" does not limit the radiation of the semiconductor laser to radiation in the visible spectral range . Rather, the radiation may be in the infrared spectral range . For example , a peak wavelength of the semiconductor laser 2 is in a range from 800 to 1200 nm .

[0066] Figure 2 illustrates an exemplary embodiment of a semiconductor laser 2 . The semiconductor laser 2 comprises a semiconductor layer sequence with an n-layer 25 , a p-layer 26 and an active region 20 arranged between the n-layer 25 and the p-layer 26 .

[0067] In the exemplary embodiment shown, the active region 20 is subdivided into individually addressable active regions , wherein each active region 20 is assigned to an individual p- contact 22 . These active regions 20 represent individually addressable channels of the semiconductor laser 2 . However, a single active region may also be suf ficient .

[0068] The active regions 20 may be connected to a common n-contact 21 . In the exemplary embodiment shown, the n-contact 21 is arranged on a side of a substrate 29 facing away from the active region 20 . For example , the substrate is a growth substrate of the semiconductor layer sequence comprising the active region .

[0069] For operating a multichannel semiconductor laser 2 , each of the p-contacts 22 may be provided with a driving circuit 3 as described in connection with Figure 1A. Consequently, each of the p-contacts 22 may be capacitively coupled to an individually assigned switch Tl . Thus , the number of switches T1 may be equal to the number of individually addressable active regions 20 of the semiconductor laser 2 . Alternatively, a single switch Tl may also trigger two or more channels of a semiconductor laser 2 . For example , a 16 channel laser bar may be operated using four switches Tl so that each switch Tl is assigned to four laser channels .

[0070] By means of the driving circuit 3 , the semiconductor laser 2 may be operated such that the emission of too long laser pulses or even the emission of continuous laser light can be prevented in a highly reliable and cost ef ficient manner . In particular, a single switch Tl is suf ficient to trigger the laser pulses of the channel or channels assigned to the respective switch Tl . This helps to reduce the triggering ef fort and the costs .

[0071] Figure 3 illustrates an exemplary embodiment of a method of operating a semiconductor laser 2 . For better understanding, the structural features included in the method steps are provided with the reference signs shown in Figures 1A to 1C and 2 .

[0072] In a step S I , a semiconductor laser 2 is provided wherein a p-contact 22 of the semiconductor laser 2 is capacitively coupled via a capacitor C2 to a switch Tl . In a step S2 , the switch T1 is brought into a closed state to supply the semiconductor laser with a current via the switch T1 and the capacitor C2 to produce a laser pulse . For example , the current may flow as described in connection with Figure IB .

[0073] In a step S3 , the capacitor C2 is discharged after the laser pulse . The discharge path of the current may be configured as described in connection with Figure 1C .

[0074] By means of the capacitor acting as the AC coupling capacitor, a highly reliable and safe operation of the semiconductor laser can be obtained, in particular compared to the case where the semiconductor laser is DC-coupled to the switch so that a switch unintendedly remaining in the closed state may cause long pulses or even continuous wave laser emission .

[0075] This patent application claims the priority of German patent application 10 2024 103 280 . 4 , the disclosure content of which is hereby incorporated by reference .

[0076] The invention described herein is not restricted by the description given with reference to the exemplary embodiments . Rather, the invention encompasses any novel feature and any combination of features , including in particular any combination of features in the claims , even i f this feature or this combination is not itsel f explicitly indicated in the claims or exemplary embodiments . References

[0077] 1 laser device

[0078] 2 semiconductor layer

[0079] 20 active region

[0080] 21 n-contact

[0081] 22 p-contact

[0082] 25 n-layer

[0083] 26 p-layer

[0084] 29 substrate

[0085] 3 driving circuit

[0086] 31 first parallel branch

[0087] 32 second parallel branch

[0088] 35 pulse current path

[0089] 36 discharge path

[0090] 5 power supply

[0091] 6 gate contact

[0092] Cl first capacitor

[0093] C2 second capacitor

[0094] DI diode

[0095] R1 first resistor

[0096] R2 second resistor

[0097] S 1 , S2 , S3 step

[0098] T1 switch

Claims

Claims1. A laser device (1) comprising a semiconductor laser (2) and a driving circuit (3) configured to operate the semiconductor laser (2) in pulsed operation, wherein- the driving circuit (3) comprises a switch (Tl) switchable between an open state and a closed state,- the driving circuit (3) comprises a first capacitor (Cl) coupled to a power supply (5) ,- the switch (Tl) is coupled to the first capacitor (Cl) ,- the driving circuit (3) comprises a second capacitor (C2) , a p-contact (22) of the semiconductor laser (2) is capacitively coupled to the switch (Tl) via the second capacitor (C2 ) ,- in the closed state of the switch (Tl) , the first capacitor (Cl) is discharged via the second capacitor (C2) and the semiconductor laser (2) to produce a laser pulse, and- in the open state of the switch (Tl) , the second capacitor (C2) is discharged.

2. The laser device according to claim 1, wherein a positive side of the first capacitor (Cl) is coupled to the power supply (5) via a first resistor (Rl) of the driving circuit (3) .

3. The laser device according to claim 1 or 2, wherein the switch (Tl) is coupled to a negative pole of the power supply (5) via a first parallel branch (31) and a second parallel branch (32) , wherein- the first parallel branch (31) comprises the second capacitor (C2) and the semiconductor laser (2) ; and- the second parallel branch (32) comprises a second resistor (R2) .

4. The laser device according to claim 3, wherein the first parallel branch (31) comprises a diode (DI) connected in antiparallel to the semiconductor laser (2) .

5. The laser device according to claim 4, wherein a discharge path (36) of the second capacitor (C2) extends through the second resistor (R2) and the diode (DI) .

6. The laser device according to any one of the preceding claims , wherein a capacity of the second capacitor (C2) is smaller than a capacity of the first capacitor (Cl) .

7. The laser device according to any one of the preceding claims , wherein the capacity of the second capacitor (C2) defines a maximum pulse length.

8. The laser device according to any one of the preceding claims , wherein a regular closed-state time of the switch (Tl) is shorter than the maximum pulse length.

9. The laser device according to any one of claims 1 to 7, wherein a regular closed-state time of the switch (Tl) is equal to or longer than the maximum pulse length.

10. The laser device according to any one of the preceding claims , wherein the semiconductor laser (2) is configured foremission of pulsed radiation in the near infrared spectral range .

11. The laser device according to any one of the preceding claims , wherein the semiconductor laser (2) comprises a plurality of active regions (20) that are individually addressable via individual p-contacts (22) .

12. A method of operating a semiconductor laser (2) comprising the steps of: a) providing a semiconductor laser (2) , wherein a p-contact (22) of the semiconductor laser (2) is capacitively coupled via a capacitor (C2) to a switch (Tl) , b) bringing the switch (Tl) into a closed state to supply the semiconductor laser (2) with a current via the switch (Tl) and the capacitor (C2) to produce a laser pulse; and c) discharging the capacitor (C2) after the laser pulse.

13. The method according to claim 12, wherein a closed-state time of the switch (Tl) is shorter than a maximum pulse length defined by a capacity of the capacitor (C2 ) .

14. The method according to claim 12, wherein a closed-state time of the switch (Tl) is equal to or longer than a maximum pulse length defined by a capacity of the capacitor (C2) .

15. The method according to any one of claims 12 to 14, wherein the method is performed using a laser device (1) according to any one of claims 1 to 11.

Images