Laser diode driver circuit techniques

Techniques to achieve higher power/shorter pulses with a laser diode. By initially applying a static reverse bias across the laser diode, the laser diode can turn on at a larger inductor current. When the laser diode is initially reverse biased, depletion charge and diffusion charge can be populated before the laser diode will lase. This causes the laser diode to initially turn on at a larger inductor current, which will reduce the rise time, thereby achieving higher power/shorter pulses.

FIELD OF THE DISCLOSURE

This document pertains generally, but not by way of limitation, to integrated circuits, in particular to circuits for driving laser diodes.

BACKGROUND

Many active optical remote sensing systems such as light detection and ranging (LIDAR), time-of-flight cameras, and range finders utilize pulsed semiconductor light sources to illuminate scenes. Optical detectors (also typically based on semiconductors) collect reflected light to determine the presence, distance, size, and speed of objects in a scene. These optical systems have numerous applications, including autonomous navigation, infrastructure monitoring, medicine, and defense. Semiconductor light sources typically utilize diode structures (P-N junctions) that conduct current when forward biased. For semiconductor materials with direct bandgaps, current carriers (electrons and holes) recombine at the junction to produce light in direct proportion to the current. Electronic driver circuits are used to provide this current, where the compliance voltage, current magnitude, and temporal properties of the continuous or pulsed current source vary depending on the requirements of the application, system design, and power supply constraints.

SUMMARY OF THE DISCLOSURE

This disclosure is directed to, among other things, techniques to achieve higher power/shorter pulses with a laser diode. By initially applying a static reverse bias across the laser diode, the laser diode can turn on at a larger inductor current. When the laser diode is initially reverse biased, depletion charge and diffusion charge can be populated before the laser diode will lase. This can cause the laser diode to initially turn on at a larger inductor current, which can reduce the rise time, thereby achieving higher power/shorter pulses.

In some aspects, this disclosure is directed to a diode driver circuit comprising a laser diode, a power supply configured to apply a static reverse bias across the laser diode, and at least one switch configured to control a current through the laser diode.

In some aspects, this disclosure is directed to a method of operating a laser diode, the method comprising coupling a voltage source to a cathode of the laser diode, applying a current pulse through the laser diode, and applying a static reverse bias voltage across the laser diode using the voltage source to decrease the current after firing the laser diode.

In some aspects, this disclosure is directed to a diode driver circuit comprising a laser diode, a power supply coupled to a diode and configured to apply a static reverse bias across the diode, and at least one switch configured to control a current through the laser diode.

DETAILED DESCRIPTION

Optical systems such as light detection and ranging (LIDAR) systems, time-of-flight cameras, and range finders, can emit one or more pulses of light (e.g., modulated light source) toward one or more objects, and the arrival time of the light reflected from the object(s) is recorded. Based on the arrival time and the speed of light, the distance between the light source and the object(s) can be derived.

A diode can be driven with narrow and high current pulses to emit light pulses onto the object, which can be centimeters to hundreds of meters away. The speed of light is very fast, therefore very short light pulses are needed to achieve meter or centimeter resolution. Accordingly, narrow current pulses are needed to drive the diode to generate the train of short light pulses.

Laser drivers in time of flight (ToF) based LIDAR systems use high powered short pulses from 100 picoseconds (ps) to 100 nanoseconds (ns). The power of these pulses can sometimes be limited by concerns for eye safety. Approaches for architectures for these drivers can include resonant capacitive discharge architectures or hard switching field-effect transistor (FET) architectures. The FETs can be metal-oxide-semiconductor (MOS) FETs or gallium nitride (GAN) FETs, but are not limited to these particular FETs.

For a high-power LIDAR application, the relationship between pulse width and power can be determined by the equation V=L*(di/dt) and some architecture related constants. The voltage V can be set by the voltage of the driver. The inductance L can be determined by the material properties and physical dimensions between the driver and the laser diode. For a given inductance and voltage, the optical power (proportional to current) can be determined by the pulse width. Generally speaking, shorter, higher power pulses can be used to see further, distinguish objects, and stay within the eye safety limit.

In many applications of optical system, e.g., LIDAR system, space is constrained. Space can be limited by the optics, for example, which can require all the laser diodes to be tightly packed. Because the drivers can be physically many times larger than the laser diodes, the drivers will be forced to be spaced far away from the laser diodes, which can increase the loop inductance. The other variable is voltage. If the voltage can be increased with a small impact to area, higher power/shorter pulses can be achieved.

The present inventors have determined that by initially applying a static reverse bias across the laser diode, the laser diode can turn on at a larger inductor current. When the laser diode is initially reverse biased, depletion charge and diffusion charge can be populated before the laser diode will lase. This causes the laser diode to initially turn on at a larger inductor current, which will reduce the rise time, thereby achieving higher power/shorter pulses.

FIG. 1is a schematic diagram of an example of a laser diode driver circuit that can implement various techniques of this disclosure. The laser diode driver circuit100ofFIG. 1can include a charging inductor L1, a parasitic inductance L2, a switch M1a laser diode LD, an optional capacitor C1, a power source PS, a control logic circuit102, and a gate driver circuit104. The charging inductor L1can be used as a storage element for driving the laser diode LD. The control logic circuit102can control the timing of the switch M1to achieve the desired behavior, e.g., pulse width.

In this disclosure, the switches described can be transistors, such as high-power FETs. The FETs can be metal-oxide-semiconductor (MOS) FETs or gallium nitride (GAN) FETs but are not limited to these particular FETs. Some of the possible implementations of the switch M1are power MOSFETs, GaN FETs, and silicon carbide (SiC) FETs. The laser diodes described in this disclosure can generate visible light and (near) infrared light, for example.

The gate driver104can be coupled to a gate terminal of the switch M1. The gate driver104can provide a high amount of charging current to more quickly charge the gate capacitance, which can reduce the turn on time of the switch.

In some example implementations, the switch M1can be placed tightly with the laser diode LD and the capacitor C1to minimize the parasitic inductance L2, e.g., conductor traces, package parasitic inductances, and the like. InFIG. 1and subsequent figures, the inductance coupled to the laser diode anode can be considered to be a lumped representative of the parasitic loop inductance. Any component that is coupled to the parasitic inductance L2can be considered to be coupled to the laser diode anode. InFIG. 1andFIG. 3, the parasitic inductance refers to L2and inFIGS. 5, 6, 7, 8, 11, and 12, the parasitic inductance refers to L1.

In accordance with various techniques of this disclosure, the circuit100can include a power supply PS, e.g., a voltage source, coupled to the laser diode LD. The power supply PS can be used to set the operating point of the laser diode. The power supply PS can be implemented using various power converter architectures including, but not limited to, a buck-boost DC-DC converter. The power supply PS can also be derived from another power supply, such as by using a ferrite bead, inductor, diode, resistors, capacitors, or some combination thereof.

The rise time can be reduced by initially applying a static reverse bias across the laser diode using the power supply PS. When the laser diode LD is reverse biased, depletion charge and diffusion charge must be populated before the laser will lase, which can allow the laser diode to initially turn ON at a larger inductor current. The fall time can be reduced by the power supply PS applying a larger reverse voltage on inductor L2than the laser diode LD, which has its cathode coupled to ground. In conjunction, the rise and fall times are reduced, thereby achieving higher power/shorter pulses.

In some example configurations, the power supply PS can be adjustable. By adjusting the voltage of the power supply, a pulse width of an output pulse of the laser diode can be modified, as described in more detail below. For a given inductance and voltage, the optical power (proportional to current) can be determined by the pulse width.

In some example configurations, the circuit100can include an optional capacitor C1coupled to the laser diode LD. The optional capacitor C1can provide a lower inductance current return path than the power supply PS to achieve shorter pulses or increase the power for a given pulse width. The capacitor C1can be included and the power supply PS can drive the cathode (or anode, in some configurations) of the laser diode at low frequencies.

In some example configurations, the capacitance of the capacitor can be adjusted to adjust one or both of a charging voltage of the laser diode and a discharging voltage of the laser diode.

InFIG. 1, a charging circuit path can include an inductive element, e.g., inductor L1, and the switch M1. During a charging phase, the control logic circuit102can turn the switch M1ON and allow the inductive element L1to charge. During a firing phase, the control logic circuit102can turn the switch M1OFF and allow the inductive element L1to discharge through the laser diode LD. The operation of the circuit is further described below with respect toFIG. 2.

FIG. 2depicts various waveforms associated with the circuit ofFIG. 1. The x-axis represents time. The top portion110depicts the voltage VRESat the drain of the switch M1. The middle portion112depicts the inductor current L0across the charging inductor L1and depicts the laser diode current D0. The portions are shown in relation to the waveform VCHG, which is the switch M1gate voltage.

As seen inFIG. 2, in the initial charging phase, the control logic circuit102ofFIG. 1can turn ON switch M1and then inductor L1charges to a desired current, as seen in the middle portion112. Then, in the firing phase, the control logic circuit102ofFIG. 1can turn switch M1OFF and a large voltage spike occurs at the drain of switch M1, as seen in the top portion110. This voltage pulse forces a large current into the laser diode LD ofFIG. 1while simultaneously discharging inductor L1, as seen in the middle portion112. After the voltage at the drain of switch M3returns to zero, the current in the laser diode LD discharges based on the voltage of the cathode. There can also be an underdamped LC ring before the circuit settles back to the initial state.

FIG. 3is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The driver circuit120ofFIG. 3can include a charging inductor L1, a loop inductance L2, switches M1-M3, a laser diode LD, an optional capacitor C1, a power source PS, and a control logic circuit102. The charging inductor L1can be used as the storage element for driving the laser diode LD. The control logic circuit102can control the timing of the switches M1-M3to achieve the desired behavior, e.g., optical peak power. In this structure, a high-voltage switch M3, e.g., a GaN FET, can be placed tightly with the laser diode L2and the optional capacitor C1to minimize the loop inductance L2.

In some configurations, M2can be a diode instead of a switch, e.g., transistor. In some configurations, the switch M3can be a high-voltage FET rated to support high voltage and a large peak current.

The power supply PS can be connected to the laser diode LD, e.g., the cathode of the laser diode LD, in order to set its operating point. The power supply PS can be implemented using various power converter architectures including, but not limited to, a buck-boost DC-DC converter. The power supply PS can also be derived from another power supply, such as by using a ferrite bead, inductor, diode, resistors, capacitors, or some combination thereof.

An inductor L1can be used as the storage element for driving the laser diode LD. The switches M1and M2can be added to drive a current into inductor L1. The control logic circuit102can control the timing of the switches to achieve the desired behavior.

A charging circuit path can include an inductive element, e.g., inductor L1, and the switches M1and M3. During a charging phase, the control logic circuit102can control the switches M1and M3to turn ON and allow the inductive element to charge. During a firing phase, the control logic circuit102can control the switch M3to turn OFF and allow the inductive element to discharge through the laser diode. During a rest phase, the control logic circuit102can control one or both of switches M2and M3to turn ON. An operation of the circuit is further described below with respect toFIG. 4.

FIG. 4depicts various waveforms associated with the circuit ofFIG. 2. The x-axis represents time. The top portion130depicts the voltage VRESat the drain of the switch M3and the voltage at the node VSWbetween the switches M1and M2. The bottom portion132depicts the inductor current L0across the charging inductors L1and L2and depicts various laser diode currents D0at134. The various laser diode currents show how the pulse width can be modulated by changing the nominal cathode voltage, for example, of the laser diode LD. The top and bottom portions are shown in relation to the waveform VM1, which is the switch M1gate voltage, the waveform VM2, which is an example of a switch M2gate voltage, and the waveform VM3, which is the switch M3gate voltage.

As seen inFIG. 4, in the initial charging phase, the control logic circuit102ofFIG. 3can turn ON the switches M1and M3and the inductor L1charges to a desired current. In the firing phase, the control logic circuit102momentarily turns OFF the switch M3, causing a large voltage spike at the drain of the switch M3, as seen in the top portion130. This voltage pulse quickly charges the input current to the laser diode LD while simultaneously discharging the inductor L1, as seen in the bottom portion132. After the voltage at drain of the switch M3returns to zero (VRESin the top portion130), the control logic circuit102ofFIG. 3can turn the switch M3back ON and the turn OFF of the laser diode LD can be dictated by the power supply, e.g., voltage source, and optional capacitor combination tied to the cathode of the laser diode, for example. During the rest phase, the switch M2and/or the switch M3can be ON.

Using the techniques described above with respect toFIGS. 1 and 3, no high voltage inputs are needed to drive the laser diode, which can minimize switching losses that are typically present in voltage-based drivers. The LC settling between the anode and the power supply is a low loss way to recover the energy that was not consumed in the laser diode. Another benefit of this driver circuit is that the fast-rising edge is fixed regardless of pulse width. The larger the current, the higher the voltage that will be forced across the inductor, which can be an advantage in many LIDAR systems that use simplified receiver designs. The architecture is flexible, allowing multiple current levels and pulse widths dictated by the charged inductor current and the bias voltage at the cathode.

FIG. 5is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The laser driver circuit140ofFIG. 5can include a switch M1, a laser diode LD, an optional capacitor C1, a power source PS, and a catch diode D1. A control logic circuit (not depicted) can control the switch M1.

In the example configuration shown inFIG. 5, the power supply PS, e.g., a positive terminal of the power supply, can be coupled to the cathode of the laser diode LD in order to set its operating point. The power supply PS can be implemented using various power converter architectures including, but not limited to, a buck-boost DC-DC converter. The power supply PS can also be derived from another power supply, such as by using a ferrite bead, inductor, diode, resistors, capacitors, or some combination thereof. The power supply PS can be implemented as described above and can be referenced to a power supply rail or to ground. The voltage VPOSof the power supply can be set as high as the reverse breakdown of the laser diode LD and the voltage VINcan be set as high as the maximum voltage rating of the switch. As a non-limiting example, the voltage VPOSof the power supply can be between about 0-40 volts and the supply voltage VINcan be between about 0-100 volts.

A pulse width can be determined by how quickly a current is ramped up to its peak and ramped back down. A high positive voltage across a parasitic inductance L1of the laser diode LD increases the current in a positive direction and a large negative voltage across the inductance L1can quickly decrease current, resulting in a short pulse width.

InFIG. 5, the power supply PS can apply a static reverse bias across the laser diode LD thereby raising the cathode voltage of the laser diode LD. By raising the cathode voltage of the laser diode LD, the effective discharge voltage across a parasitic inductance L1of the laser diode LD becomes the voltage of the power supply PS (VPOS).

When the switch M1turns ON, a large positive voltage VIN−VPOSis applied across the inductor L1, which quickly ramps up current. When the switch M1turns OFF, the voltage VPOSfrom the power supply PS (which can always be ON), effectively applies a large negative voltage across the parasitic inductance L1, and the current flows through the catch diode D1. The additional voltage added by the power supply PS can discharge the inductance L1more quickly. The catch diode D1can be a diode, which does not conduct in the reverse direction. As a diode, the catch diode D1can naturally turn OFF when the current becomes zero, which also turns OFF the laser diode LD, resulting in a self-timing feature.

The circuit140ofFIG. 5can be reconfigured, as shown below inFIG. 6, such that the switch M1is on the low side, rather than on the high side as inFIG. 5.

FIG. 6is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The laser driver circuit160ofFIG. 6can include a switch M1, a laser diode LD, an optional capacitor C1, a power source PS, and a catch diode D1. A control logic circuit (not depicted) can control the switch M1.

In the example configuration shown inFIG. 6, the power supply PS can be coupled to the anode of the laser diode LD in order to set its operating point. The power supply PS can be implemented as described above. The optional capacitor C1can be coupled to the anode of the laser diode LD.

FIG. 7is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The laser driver circuit170ofFIG. 7can include a switch M1, a laser diode LD, an optional capacitor C1, a power source PS, and a catch diode D1. A control logic circuit (not depicted) can control the switch M1.

In the example configuration shown inFIG. 7, the power supply PS, e.g., a negative terminal of the power supply, can be coupled to the cathode of the laser diode LD and the positive terminal can be coupled to the anode of the catch diode in order to set the DC operating point for the catch diode to turn ON. The power supply PS can be implemented as described above and can be referenced to the lowest supply potential of the circuit, e.g., the lower power supply rail or to ground. The negative voltage VNEGcan be set up to the limit set by the laser diode LD and the supply voltage VINcan be set up to the limit of the switch. In a non-limiting example, the voltage VNEGof the power supply can be between about 0 volts to 40 volts and the supply voltage VINcan be between about 0-100 volts.

In the configuration inFIG. 7, the power supply PS can decrease the anode voltage of the catch diode D1. As a result, the drain voltage of the switch M1can drop lower before the catch diode D1turns ON.

The circuit170ofFIG. 7can be reconfigured, as shown below inFIG. 8, such that the switch M1is on the low side, rather than on the high side as inFIG. 7.

FIG. 8is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The laser driver circuit180ofFIG. 8can include a switch M1, a laser diode LD, an optional capacitor C1, a power source PS, and a catch diode D1. A control logic circuit (not depicted) can control the switch M1.

In the example configuration shown inFIG. 8, the power supply PS, e.g., a positive terminal of the power supply, can be coupled to the cathode of the catch diode D1in order to set its DC operating point. The power supply PS can be implemented as described above and can be referenced to the highest supply potential of the circuit, e.g., an upper power supply rail. The voltage VPOSshould be higher than the voltage VIN. The optional capacitor C1can be coupled to the anode of the laser diode LD.

FIG. 9depicts various waveforms associated with the circuit ofFIG. 7. The x-axis represents time and the y-axis represents the current through the laser diode LD. The first curve150represents the current through the parasitic inductance L1without applying an additional voltage using the power supply PS ofFIG. 7. The second curve152represents the current through the parasitic inductance L1after applying the additional voltage using the power supply PS ofFIG. 7. As seen inFIG. 9, the discharge slope of the second curve152is greater than the first curve150, indicating that the additional voltage has discharged the inductance L1more quickly.

As indicated above, the circuits of this disclosure can include an optional capacitor C1. This capacitor can be added in parallel or shared among many laser diodes to create a return path of the proper impedance. The power supply PS can be implemented using various power converter architectures including, but not limited to, a buck-boost DC-DC converter. The power supply PS can also be derived from another power supply, such as by using a ferrite bead, inductor, diode, resistors, capacitors, or some combination thereof. The power supply required can be low current and can be shared among all the lasers diode drivers.

For smaller values of capacitors, the laser current can charge up the capacitor and increase the reverse bias on the laser diode LD. In these cases, rising edge performance can be traded for falling edge performance. Using various switch timings, capacitor sizing can be used to shape the laser pulses. The charging voltage and the discharging voltage can be adjusted by changing the capacitance of the capacitor CL. For example, by adjusting a capacitance of the capacitor C1, the shape of a laser pulse can be adjusted, e.g., such as approximating a Gaussian shape, as shown inFIG. 10.

FIG. 10depicts examples of various laser pulse shapes using various techniques of this disclosure. The x-axis represents time and the y-axis represents the laser diode current. Various laser diode pulse shapes185are shown inFIG. 10that can be achieved by adjusting a capacitance of a capacitor in a laser driver circuit. As the capacitance decreases, the fall time decreases, resulting in shorter pulse widths.

FIG. 11is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The laser driver circuit190ofFIG. 11can include switches M1and M2coupled in series, a laser diode LD coupled between the switches M1and M2, an optional capacitor C1, and a power source PS. A control logic circuit (not depicted) can control the switches M1and M2. In some configurations, rather than be a switch, M2can be a diode.

In the example configuration shown inFIG. 11, the power supply PS, e.g., a positive terminal of the power supply, can be coupled to the cathode of the laser diode LD in order to set its DC operating point. The power supply PS can be implemented as described above. The optional capacitor C1can be coupled between the cathode of the laser diode LD and the source of the switch M2.

FIG. 12is a schematic diagram of another example of a laser diode driver circuit that can implement various techniques of this disclosure. The driver circuit200ofFIG. 12can include switches M1and M2coupled in series, a laser diode LD coupled between the switches M1and M2, an optional capacitor C1, and a power source PS. A control logic circuit (not depicted) can control the switches M1and M2. In some configurations, rather than be a switch, M1can be a diode.

In the example configuration shown inFIG. 12, the power supply PS, e.g., a positive terminal of the power supply, can be coupled to the anode of the laser diode LD in order to set its DC operating point. The power supply PS can be implemented as described above. The optional capacitor C1can be coupled between the anode of the laser diode LD and the drain of the switch M1.

In any of the configurations described above, the capacitor C1in conjunction with the power supply PS can be used to measure the average current through the laser diode LD, as described with respect toFIG. 13.

FIG. 13is a schematic diagram illustrating an example of a laser diode current measurement circuit. Firing the laser diode D1can store charge on the capacitor C1. However, because firing happens so quickly, a current waveform can be difficult to measure.

Using various techniques of this disclosure, charge during the firing phase can be removed slowly by the power supply PS. The charge removed can provide an average current through the laser diode LD. For example, a measurement circuit210can measure a voltage across a sense resistor RShaving a known resistance. Using Ohm's law (V=I×R) which the measurement circuit can use to calculate an average current through the laser diode LD. Then, the measurement circuit can use the determined average current through the laser diode LD to calculate an energy per pulse value, which can be used to ensure that the energy stays within an eye safety limit.

In some implementations, multiple laser driver circuits, e.g., forming an array of laser driver circuits, can be driven from a single power source, such as shown inFIGS. 14 and 15.

FIG. 14is an example of an array of laser driver circuits driven by a single power source PS. InFIG. 14, an array300of laser driver circuits302coupled in parallel is shown. Any of the laser driver circuits described in this disclosure can be used in the array300. The array300can be coupled to a shared component R where R can be one or more ferrite beads, diodes, inductors, capacitors, resistors or combinations thereof. The shared component R can be coupled to a power source PS shared by the array300that drives all the C1-LD nodes.

FIG. 15is another example of an array of laser driver circuits driven by a single power source PS. InFIG. 15, an array400of laser driver circuits402coupled in parallel is shown. Any of the laser driver circuits described in this disclosure can be used in the array400. Each laser driver circuit402can include a component R where R can be one or more ferrite beads, diodes, inductors, capacitors, resistors or combinations thereof. The component R can be coupled to a power source PS shared by the array400that drives all the C1-LD nodes.

NOTES

Each of the non-limiting aspects or examples described herein may stand on its own or may be combined in various permutations or combinations with one or more of the other examples.