Laser diode drive circuit, laser transmitting circuit, and lidar system

Abstract

The present disclosure relates to the technical field of electronic circuits. Provided are a laser diode drive circuit, a laser transmitting circuit, and a LiDAR system. The laser diode drive circuit comprises a power supply input end, a control module and at least one group of drive switch units, wherein each group of drive switch units comprises a first switch unit and a second switch unit; the power supply input end is configured to connect to a first preset power source; the power supply input end is grounded by means of at least one laser diode to be driven, and the first switch unit and the second switch unit are connected in series to a power supply circuit of the laser diode; and a first input end of the control module is used for the input of a pulse signal, and a first output end and a second output end of the control module are respectively connected to a control end of the first switch unit and a control end of the second switch unit. By using the laser diode drive circuit provided in the present disclosure, the duration of a pulsed current in a laser diode can be controlled by means of the opening and closing of drive switch units, and thus an ultrashort laser pulse is realized.

Description

Laser diode drive circuit, laser emission circuit and laser radar system

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure claims priority to Chinese patent application number 202311020722.9 filed with the State Intellectual Property Office of China on August 14, 2023, entitled “Laser diode driving circuit, laser emission circuit and laser radar system,” the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present disclosure relates to the field of electronic circuit technology, and in particular to a laser diode driving circuit, a laser emitting circuit, and a laser radar system. Background Art

[0004] A laser diode driver circuit controls a circuit switch to cause a pulsed current to flow through the laser diode. Laser diode driver circuits play a crucial role in lidar. The shorter the duration of the pulsed current flowing through the laser diode, the higher the distance accuracy the lidar can detect.

[0005] As for the two existing laser diode driving circuits, one laser diode driving circuit uses a unilateral pulse width control method based on the resonance principle, and the other laser diode driving circuit uses a bilateral pulse width control method to control the power tube switch. However, neither of these two methods can achieve ultrashort laser pulses.

[0006] Summary of the Invention

[0007] The purpose of the present disclosure is to provide a laser diode driving circuit, a laser emitting circuit and a laser radar system to address the deficiencies in the above-mentioned prior art, so as to solve the problems existing in the prior art.

[0008] An embodiment of the present disclosure provides a laser diode driving circuit, which may include: a power supply input terminal, a control module, and at least one group of driving switch units, wherein each group of driving switch units may include a first switch unit and a second switch unit, and the power supply input terminal may be used to connect to a first preset power supply;

[0009] The power supply input terminal may be grounded through at least one laser diode to be driven, and the first switch unit and the second switch unit may be connected in series to a power supply path of a laser diode;

[0010] The first input terminal of the control module can be used to input a pulse signal, and the first output terminal and the second output terminal of the control module can be connected to the control terminal of the first switch unit and the control terminal of the second switch unit respectively.

[0011] In some optional embodiments, the cathode of the laser diode may be grounded via the first switching unit and the second switching unit connected in series.

[0012] In some optional embodiments, the anode of the laser diode may be connected to the power supply input terminal via the first switch unit and the second switch unit connected in series.

[0013] In some optional embodiments, the laser diode may be disposed between the first switching unit and the second switching unit, such that an anode of the laser diode is connected to the first switching unit and a cathode of the laser diode is connected to the second switching unit.

[0014] In some optional embodiments, each group of driving switch units may further include a sampling resistor, and the laser diode driving circuit may further include a DC-DC converter, and the DC-DC converter may be connected between the power supply input terminal and the first preset power supply;

[0015] The sampling resistor can be connected between the first switch unit, the second switch unit and the ground. The sampling resistor can also be connected to the second input end of the control module. The third output end of the control module can be connected to the control end of the DC-DC converter.

[0016] In some optional embodiments, the laser diode driving circuit may further include a filter capacitor, and the power supply input terminal may be grounded through the filter capacitor.

[0017] In some optional embodiments, the first switch unit and the second switch unit may be switch devices or power switch tubes.

[0018] In some optional embodiments, the first switch unit may be a first power switch tube, the second switch unit may be a second power switch tube, and wherein the drain of the first power switch tube may be connected to the laser diode, the source of the first power switch tube may be connected to the drain of the second power switch tube, the source of the second power switch tube may be grounded, the first output end of the control module may be connected to the gate of the first power switch tube, and the second output end of the control module may be connected to the gate of the second power switch tube.

[0019] In some optional embodiments, each group of driving switch units may further include a current source, and the current source may be connected to a power supply path of the laser diode.

[0020] In some optional embodiments, the current source may be a power switch tube; the power switch tube may be connected to a power supply path of the laser diode, and the control end of the power switch tube may be connected to a second preset power supply.

[0021] The embodiment of the present disclosure further provides a laser emitting circuit, which may include: the laser diode driving circuit described in the above embodiment, at least one laser diode, and a first preset power supply;

[0022] The first preset power supply can be connected to a power supply input terminal of the laser diode driving circuit, and at least one group of driving switch units in the laser diode driving circuit can be respectively connected in series to a power supply path of the at least one laser diode.

[0023] In some optional embodiments, a group of the driving switch units may be provided on the power supply path of each of the laser diodes.

[0024] An embodiment of the present disclosure also provides a laser radar system, which may include: the laser emitting circuit, the laser receiving circuit, and the processing unit described in the above embodiment, and the laser emitting circuit and the laser receiving circuit may be connected to the processing unit respectively.

[0025] The embodiments of the present disclosure provide a laser diode driving circuit, a laser emitting circuit, and a laser radar system. The laser diode driving circuit includes a power supply input terminal, a control module, and at least one group of driving switch units, wherein each group of driving switch units includes a first switch unit and a second switch unit; the power supply input terminal is used to connect to a first preset power supply; the power supply input terminal is grounded through at least one laser diode to be driven, and the first switch unit and the second switch unit are connected in series on the power supply path of a laser diode; the first input terminal of the control module is used to input a pulse signal, and the first output terminal and the second output terminal of the control module are respectively connected to the control terminal of the first switch unit and the control terminal of the second switch unit. According to the embodiment of the present disclosure, the laser diode driving circuit can control the on and off of the first switch unit and the second switch unit through the control module to control the duration of the pulse current flowing through the laser diode, thereby realizing ultrashort laser pulses. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0027] Figure 1(a) and Figure 1(b) are schematic diagrams of two existing laser emission circuits;

[0028] FIG2 is a circuit diagram of a laser diode driving circuit according to some embodiments of the present disclosure;

[0029] FIG3 is a second circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0030] FIG4(a) to FIG4(e) are schematic diagrams of states of laser diode driving circuits provided by some embodiments of the present disclosure;

[0031] FIG5 is a third circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0032] FIG6 is a fourth circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0033] FIG7 is a fifth circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0034] FIG8 is a sixth circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0035] FIG9 is a seventh circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0036] FIG10 is an eighth circuit schematic diagram of a laser diode driving circuit provided by some embodiments of the present disclosure;

[0037] FIG11 is a schematic structural diagram of a laser emission circuit provided by some embodiments of the present disclosure;

[0038] FIG12 is a schematic structural diagram of a lidar system provided in some embodiments of the present disclosure. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the embodiments of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below in combination with the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments.

[0040] Therefore, the following detailed description of the embodiments of the present disclosure provided in the accompanying drawings is not intended to limit the scope of the present disclosure as claimed, but merely represents selected embodiments of the present disclosure. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present disclosure without creative effort shall fall within the scope of protection of the present disclosure.

[0041] In addition, the terms "first," "second," and the like in the specification and claims of the present disclosure and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or precedence. It should be understood that the numbers used in this way are interchangeable where appropriate so that the embodiments of the present disclosure described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having," as well as any variations thereof, are intended to cover non-exclusive inclusions, for example, a process, method, system, product, or apparatus comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products, or apparatus.

[0042] It should also be noted that, in the description of this disclosure, unless otherwise expressly specified or limited, the terms "disposed" and "connected" should be understood in a broad sense. For example, they may refer to a fixed connection, a detachable connection, or an integral connection. They may refer to a direct connection, an indirect connection via an intermediate medium, or internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in this disclosure based on specific circumstances.

[0043] It should be noted that, in the absence of conflict, the features in the embodiments of the present disclosure may be combined with each other.

[0044] As a long-range ranging method, LiDAR (Light Detection and Ranging) has played an increasingly important role in autonomous driving, unmanned aerial vehicles, robots, augmented reality and other fields in recent years.

[0045] LiDAR consists of a laser transmitter circuit, a laser receiver circuit, and a processing unit. Simply put, the operating principle of LiDAR is that the processing unit records the time when the laser transmitter circuit emits a laser pulse and the time when the laser receiver circuit senses the laser pulse reflected by the target object; the distance between the LiDAR and the target object is calculated based on the time difference between the two.

[0046] The laser emission circuit includes a laser diode and a laser diode driver circuit. When a current above the laser diode's threshold flows through it, laser light is generated, providing the laser pulses necessary for LiDAR operation. The laser diode driver circuit controls the circuit switches, allowing the pulsed current to flow through the laser diode. The shorter the duration of the pulsed current flowing through the laser diode, the more accurate the LiDAR's range detection capability.

[0047] Figures 1(a) and 1(b) are schematic diagrams of two existing laser emission circuits. As shown in Figures 1(a) and 1(b), there are two main types of existing laser diode drive circuits. The first is the unilateral pulse width control method based on the LC resonance principle, as shown in Figure 1(a). This method has a fixed pulse width and cannot achieve ultrashort laser pulses. The second is the bilateral pulse width control method, as shown in Figure 1(b), which controls the power tube switch. This method requires simultaneous control of the rising and falling edges of the pulse width. Although the pulse width is adjustable, it is still unable to achieve ultrashort laser pulses due to the limitations of the rising and falling edges.

[0048] Therefore, in order to solve the above problems in the prior art, the present disclosure provides a laser diode driving circuit, a laser emitting circuit, and a laser radar system. The laser diode driving circuit provided by the embodiments of the present disclosure can solve the above problems in the prior art.

[0049] The following describes the laser diode driver circuit, laser emission circuit, and lidar system provided by the present disclosure in detail through multiple examples, with reference to the accompanying drawings. It should be noted that the laser diode D1 in the accompanying drawings is intended to facilitate understanding of the laser diode driver circuit provided by the present disclosure; the laser diode D1 does not actually exist in the laser diode driver circuit.

[0050] FIG2 is one of the circuit schematic diagrams of the laser diode driving circuit provided by some embodiments of the present disclosure. It should be noted that, in actual applications, there may be multiple laser diodes connected in parallel in the laser emission circuit, and correspondingly, there may also be multiple groups of driving switch units in the laser diode driving circuit, where FIG2 is only a schematic diagram of a group of driving switch units. In actual applications, the number of driving switch units in the laser diode driving circuit can be set according to the number of laser diodes. For example, FIG3 is the second circuit schematic diagram of the laser diode driving circuit provided by some embodiments of the present disclosure. As shown in FIG3, a group of driving switch units can be set on the power supply path of each laser diode.

[0051] As shown in FIG2 , the laser diode driving circuit may include a power supply input terminal VCC, a control module, and at least one group of driving switch units, wherein each group of driving switch units includes a first switch unit S1 and a second switch unit S2, and the control unit is any type of controller that can control the on and off of the driving switch units.

[0052] Specifically, the power supply input terminal VCC can be used to connect to a first preset power supply (not shown in the figure, the first preset power supply is used to power the laser diode driving circuit and the laser diode D1), and the power supply input terminal VCC can also be grounded through at least one laser diode D1 to be driven. The first switch unit S1 and the second switch unit S2 can be connected in series on the power supply path of a laser diode. For example, as shown in Figure 2, the cathode of the laser diode is grounded through the first switch unit and the second switch unit connected in series. The first input terminal of the control module can be used to input a pulse signal VG, and the first output terminal and the second output terminal of the control module can be connected to the control terminal of the first switch unit S1 and the control terminal of the second switch unit S2, respectively. The control module can output drive signals VG1 and VG2 to the first switch unit and the second switch unit, respectively, based on the first pulse signal VG, so that the first switch unit S1 and the second switch unit S2 are turned on.

[0053] 4(a) to 4(e) are schematic diagrams of the state of the laser diode driving circuit provided in some embodiments of the present disclosure. The laser diode driving circuit in the working state is described in detail below in conjunction with FIG. 4(a) to FIG. 4(e).

[0054] Figure 4(a) corresponds to state ① in Figure 4(e). In this state, the first input end of the control module receives the input pulse signal VG, while the first output end and the second output end of the control module have not yet output the driving signal to the first switch unit S1 and the second switch unit S2. The first switch unit S1 and the second switch unit S2 are both in the disconnected state, and the power supply path of the laser diode is not energized.

[0055] Figure 4(b) corresponds to state ② in Figure 4(e). In this state, according to the pulse signal VG, the first output end of the control module outputs the drive signal VG1 to the first switch unit S1, and the second output end does not output the drive signal to the second switch unit S2. The first switch unit S1 is turned on, and the second switch unit S2 is still in the off state. The power supply path of the laser diode is not energized.

[0056] Figure 4(c) corresponds to state ③ in Figure 4(e). In this state, according to the pulse signal VG, the first output end of the control module outputs a drive signal VG1 to the first switch unit S1, and the second output end of the control module outputs a drive signal VG2 to the second switch unit S2. The first switch unit S1 and the second switch unit S2 are both turned on, the power supply path of the laser diode is energized, and current flows through the laser diode.

[0057] Figure 4(d) corresponds to state ④ in Figure 4(e). In this state, according to the pulse signal VG, the first output end of the control module does not output the drive signal VG1 to the first switch unit S1, and the second output end outputs the drive signal to the second switch unit S2. The first switch unit S1 is disconnected, the second switch unit S2 is turned on, and the power supply path of the laser diode is not energized.

[0058] After the laser diode driving circuit completes the power-on from ① to ④, as shown in FIG4(e), the first input terminal of the control module receives the input pulse signal VG again, and the laser diode driving circuit repeats the process from ① to ④. The above process is repeated multiple times to make the laser diode generate a pulse current.

[0059] In summary, the control module can control the time d that the laser diode drive circuit is in state ③ t2 The shorter the time the laser diode drive circuit is in state ③, the shorter the duration of the pulse current flowing through the laser diode, and the higher the distance accuracy that the laser radar can identify. The laser diode drive circuit provided in the disclosed embodiment controls the on-off switching of the first switch unit and the second switch unit through a control module, thereby controlling the duration of the pulse current flowing through the laser diode and achieving ultrashort laser pulses.

[0060] Figure 5 is a third circuit diagram of a laser diode driver circuit provided by some embodiments of the present disclosure. As shown in Figure 5 , the laser diode driver circuit may further include a filter capacitor C1, and the power supply input terminal VCC may be grounded via the filter capacitor. The filter capacitor can filter out AC components in the circuit, thereby smoothing the current in the laser diode's power supply path.

[0061] Figure 6 is a fourth circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure. As shown in Figure 6, in the laser diode driving circuit, the anode of the laser diode can be connected to the power supply input terminal through a first switching unit and a second switching unit connected in series, or alternatively, the laser diode can be located between the first switching unit and the second switching unit.

[0062] In some embodiments, the first and second switch units may be power switch tubes in addition to the switch devices described in the above embodiments. FIG7 is a fifth circuit diagram of a laser diode drive circuit provided by some embodiments of the present disclosure. As shown in FIG7 , the drain of power switch tube M1 is connected to laser diode D1, the source is connected to the drain of power switch tube M2, the source of M2 is grounded, and the first and second output terminals of the control module are connected to the gates of M1 and M2, respectively.

[0063] FIG8 is a sixth circuit diagram of a laser diode driving circuit provided by some embodiments of the present disclosure. As shown in FIG8 , each group of driving switch units may further include a sampling resistor R SEN The laser diode driving circuit may further include a DC-DC converter (DC-DC converter in the figure), the DC-DC converter is connected between the power supply input terminal VCC and the first preset power supply (VIN in Figure 8 is used to connect the first preset power supply), the sampling resistor is connected between the first switch unit and the second switch unit and the ground, the sampling resistor is also connected to the second input terminal of the control module, and the third output terminal of the control module is connected to the control terminal of the DC-DC converter.

[0064] In these embodiments, the control module can SEN The voltage V SEN , obtain the pulse width and amplitude information in the laser diode drive circuit, and output the control signal V to the DC-DC converter based on the pulse width and amplitude information REF , so that the DC-DC converter can adjust the size of VCC. The control module can also adjust the value of VCC according to the V SEN , change the width of the input pulse signal VG.

[0065] FIG9 is a seventh circuit diagram of a laser diode driving circuit provided in some embodiments of the present disclosure. As shown in FIG9 , each group of driving switch units may further include a current source A, which is connected to a power supply path of a laser diode.

[0066] Figure 10 is the eighth circuit schematic diagram of the laser diode driving circuit provided by some embodiments of the present disclosure. As shown in Figure 10, the current source can be a power switching tube, and the power switching tube is connected to the power supply path of a laser diode. The control end of the power switching tube is connected to the second preset power supply, that is, the gate of the power switching tube is connected to the second preset power supply with a fixed potential.

[0067] In these embodiments, the current flowing through the laser diode can be controlled by setting a current source. Typically, the current value provided by the current source is set to be smaller than the value when the current source is not set.

[0068] FIG11 is a schematic diagram of the structure of a laser emission circuit provided by some embodiments of the present disclosure. As shown in FIG11 , in a possible implementation example, some embodiments of the present disclosure further provide a laser emission circuit, which may include the laser diode drive circuit provided by the above embodiments, at least one laser diode, and a first preset power supply, wherein the first preset power supply may be connected to a power supply input terminal of the laser diode drive circuit, and at least one set of drive switch units in the laser diode drive circuit may be respectively connected in series to the power supply path of at least one laser diode.

[0069] Figure 12 is a schematic diagram of the structure of a lidar system provided by some embodiments of the present disclosure. As shown in Figure 12, in a possible implementation example, some embodiments of the present disclosure further provide a lidar system, which may include the laser emitting circuit, laser receiving circuit, and processing unit provided by the above embodiments. The laser emitting circuit and the laser receiving circuit may be connected to the processing unit, respectively. The processing unit may record the time when the laser emitting circuit emits a laser pulse and the time when the laser receiving circuit senses the laser pulse reflected by the target object, and calculate the distance between the lidar and the target object based on the time difference between the two.

[0070] The above are only specific embodiments of the present disclosure, but the scope of protection of the present disclosure is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this disclosure should be included in the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure should be based on the scope of protection of the claims. Industrial Applicability

[0071] The embodiments of the present disclosure provide a laser diode driving circuit, a laser emitting circuit, and a laser radar system. The laser diode driving circuit includes a power supply input terminal, a control module, and at least one group of driving switch units, wherein each group of driving switch units includes a first switch unit and a second switch unit; the power supply input terminal is used to connect to a first preset power supply; the power supply input terminal is grounded through at least one laser diode to be driven, and the first switch unit and the second switch unit are connected in series on the power supply path of a laser diode; the first input terminal of the control module is used to input a pulse signal, and the first output terminal and the second output terminal of the control module are respectively connected to the control terminal of the first switch unit and the control terminal of the second switch unit. According to the laser diode driving circuit of the embodiment of the present disclosure, the on and off of the first switch unit and the second switch unit can be controlled by the control module to realize the control of the duration of the pulse current flowing through the laser diode, thereby realizing ultrashort laser pulses.

[0072] Furthermore, it is understood that the laser diode driver circuit, laser emission circuit, and lidar system provided by the present disclosure are reproducible and can be used in a variety of industrial applications. For example, the laser diode driver circuit, laser emission circuit, and lidar system provided by the present disclosure can be used in radar ranging applications.

Claims

1. A laser diode driving circuit, wherein: The laser diode driving circuit comprises: a power supply input terminal, a control module and at least one group of driving switch units, wherein each group of driving switch units comprises a first switch unit and a second switch unit, and the power supply input terminal is used to connect a first preset power supply; The power supply input terminal is grounded through at least one laser diode to be driven, and the first switch unit and the second switch unit are connected in series on a power supply path of a laser diode; The first input end of the control module is used to input a pulse signal, and the first output end and the second output end of the control module are respectively connected to the control end of the first switch unit and the control end of the second switch unit.

2. The laser diode driving circuit according to claim 1, wherein: The cathode of the laser diode is grounded through the first switch unit and the second switch unit connected in series.

3. The laser diode driving circuit according to claim 1, wherein: The anode of the laser diode is connected to the power supply input terminal through the first switch unit and the second switch unit connected in series.

4. The laser diode driving circuit according to claim 1, wherein: The laser diode is disposed between the first switching unit and the second switching unit such that an anode of the laser diode is connected to the first switching unit and a cathode of the laser diode is connected to the second switching unit.

5. The laser diode drive circuit according to any one of claims 1 to 4, wherein: Each group of the driving switch units further includes a sampling resistor, and the laser diode driving circuit further includes a DC-DC converter, and the DC-DC converter is connected between the power supply input terminal and the first preset power supply; The sampling resistor is connected between the first switch unit, the second switch unit and the ground. The sampling resistor is also connected to the second input end of the control module. The third output end of the control module is connected to the control end of the DC-DC converter.

6. The laser diode drive circuit according to any one of claims 1 to 5, wherein: The laser diode driving circuit also includes a filter capacitor; the power supply input end is also grounded through the filter capacitor.

7. The laser diode driving circuit according to any one of claims 1 to 6, wherein: The first switch unit and the second switch unit are switch devices or power switch tubes.

8. The laser diode drive circuit according to any one of claims 1 to 6, wherein: The first switch unit is a first power switch tube, and the second switch unit is a second power switch tube, wherein the drain of the first power switch tube is connected to the laser diode, the source of the first power switch tube is connected to the drain of the second power switch tube, the source of the second power switch tube is grounded, the first output end of the control module is connected to the gate of the first power switch tube, and the second output end of the control module is connected to the gate of the second power switch tube.

9. The laser diode driving circuit according to any one of claims 1 to 8, wherein: Each group of driving switch units further includes: a current source; The current source is connected to the power supply path of the laser diode.

10. The laser diode driving circuit according to claim 9, wherein: The current source is a power switch tube; The power switch tube is connected to the power supply path of the laser diode, and the control end of the power switch tube is connected to a second preset power supply.

11. A laser emitting circuit, wherein: The laser emission circuit comprises: a laser diode driving circuit according to any one of claims 1 to 10; at least one laser diode; and a first preset power supply; The first preset power supply is connected to a power supply input terminal in the laser diode driving circuit, and at least one group of driving switch units in the laser diode driving circuit is respectively connected in series to a power supply path of the at least one laser diode.

12. The laser emitting circuit according to claim 11, wherein: A group of driving switch units is arranged on the power supply path of each laser diode.

13. A laser radar system, wherein: The laser radar system includes: a laser emitting circuit according to claim 11; a laser receiving circuit; and a processing unit, wherein the laser emitting circuit and the laser receiving circuit are respectively connected to the processing unit.

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