Laser Driver

Abstract

A laser driving circuit includes: a high power node for providing a relatively high voltage; a low power node for providing a relatively low voltage; a first laser driving output and a second laser driving output for connecting a laser to provide a driving current to the laser; a high-side switch coupled between the high power node and the first laser driving output; and a low-side switch coupled between the low power node and the second laser driving output; wherein the high-side switch is controlled to adjust the driving current provided to the laser, and the low-side switch is used to be controlled to alternate its state between an on state and an off state, thereby realizing a pulsed operation of the laser.

Description

Technical Field

[0001] The present invention relates to a laser driver, for example, for driving a laser diode (LD) or a laser diode array. The driver can perform time division multiplexed (TDM) driving. The driver can perform pulse driving. The driver can provide temperature compensated peak current control. Background Art

[0002] Pulse LD driver transmitters are typically used to drive LDs for optical ranging using a reflectometer. This can be called light detection and ranging (LiDAR). Such technology can be used to sense a 3D depth map of the environment. To do this, a laser pulse needs to be transmitted (TX) from the device. The laser pulse reflects off objects in the environment. Relative to the time and frequency of transmission, the reception (RX) time of the reflection, and optionally any frequency shift information can be sensed. The range of the reflecting object can be estimated based on the time of flight (TOF) of the pulse.

[0003] It is advantageous to perform this range finding on a two-dimensional array to build up a picture of the environment. To achieve this at a high scan rate, it is advantageous to drive the laser at high power. This places high performance demands on the laser driver.

[0004] Laser diodes can be arranged in arrays. Such arrays provide multiple independent transmission channels. These transmission channels are usually driven through a set of isolated anodes with a common cathode summing node as the return path for the laser current.

[0005] It is necessary to perform spatial laser transmission beam steering to provide directionality of the transmitted signal towards a target. Preferably, this steering is done in a non-mechanical manner. This requires integrated optics to focus the transmitted signal to the target.

[0006] In the case of controlling the peak output optical power from the electro-optical conversion of the LD, it is necessary to be able to set the peak laser current for optimal operation of the TOF system.

[0007] The drive of the LD needs to be controlled to compensate for any temperature dependent characteristics in its performance. Summary of the invention

[0008] According to one aspect, a laser driving circuit is provided, comprising: a high power supply node for providing a relatively high voltage; a low power supply node for providing a relatively low voltage; a first laser driving output and a second laser driving output for connecting a laser to provide a driving current to the laser; a high-side switch coupled between the high power supply node and the first laser driving output; and a low-side switch coupled between the low power supply node and the second laser driving output; wherein the high-side switch is used to be controlled to adjust the driving current provided to the laser, and the low-side switch is used to be controlled to alternate its state between an on state and an off state, thereby realizing a pulsed operation of the laser.

[0009] The high-side switch may be an N-channel metal oxide semiconductor (NMOS) switch, which may provide scale effects.

[0010] The laser can be coupled between a high-side switch and a low-side switch. This can allow the laser to rely on two switches for switching. One of the switches (e.g., the high-side switch) can be used to distinguish the laser from other lasers in a common array that includes lasers. The other switch can be used to modulate the laser and the other lasers.

[0011] The circuit may include a gate driver for controlling the high-side switch and a charge pump for regulating the current supplied to the gate driver under closed-loop control. This may allow the driving of the laser to be controlled.

[0012] The driver circuit is used to selectively drive each of the plurality of driver outputs to the laser array. This can allow the driver to serve multiple lasers.

[0013] The drive circuit may include a current driver for supplying a current to the high supply node, the current driver being configured to temperature compensate the current supplied to the high supply node. This may allow compensation for temperature dependent behavior of the laser.

[0014] The current driver may include a replica circuit for emulating the temperature response of a circuit including the laser. The output of the current driver may depend on the response of the replica circuit. This may allow compensation for the temperature-dependent behavior of the laser.

[0015] The replica circuit can have a current / voltage response that is substantially linear over the temperature-invariant operating region. This can mimic the behavior of a laser diode.

[0016] The drive circuit may include a switch circuit for receiving a first pulse train and a second pulse train having a phase offset therebetween, the switch circuit for controlling the low-side switch to be turned on in response to a first predetermined sensed edge in the first pulse train, and controlling the low-side switch to be turned off in response to a second predetermined sensed edge in the second pulse train. This may provide a convenient way to allow fast switching of the low-side switch.

[0017] The laser drive circuit can be used to drive multiple lasers. For each such laser, the circuit may include: a high power node for providing a relatively high voltage; a low power node for providing a relatively low power voltage; a first laser drive output and a second laser drive output for connecting the corresponding laser to provide a drive current to the laser; a high-side switch coupled between the corresponding high power node and the corresponding first laser drive output; a low-side switch coupled between the corresponding low power node and the corresponding second laser drive output; wherein the corresponding high-side switch is used to be controlled to adjust the drive current provided to the laser, and the low-side switch is used to be controlled to alternate its state between the on and off states, thereby achieving pulsed operation of the laser. Alternatively, there may be a low-side switch shared by multiple lasers and controlled to alternate its state between the on and off states, thereby achieving pulsed operation of the laser. This can provide an effective way to provide laser selection and pulses.

[0018] The driver circuit may include a channel selection circuit for receiving a current source and selectively directing the current source to a corresponding one of the high-side switches. This may allow each of the plurality of lasers to be selectively enabled. The current source may be directed by gating on the high side or the low side of the high-side switch.

[0019] The channel selection circuit may include a plurality of switches for controlling the drive to the respective lasers, the switches being configured such that in the event of a failure of any such switch, the current path to the respective laser will be interrupted. This may provide brownout protection.

[0020] According to another aspect, there is provided a laser unit comprising: a laser driving circuit according to the above; a laser coupled between a first laser driving output and a second laser driving output or between each pair of the first laser driving output and the second laser driving output.

[0021] According to another aspect, there is provided an optical ranging unit comprising a laser unit as above, the laser unit being arranged to intermittently illuminate an area outside the laser unit by switching a corresponding high-side switch, so that the flight time of a signal from the or each laser to an object can be measured. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present system will now be described by way of example with reference to the accompanying drawings. In the drawings:

[0023] Figure 1 The important components of the laser driver are shown;

[0024] Figure 2 The circuit used to mirror and apply gain to the replica laser current is shown;

[0025] Figure 3 shows the timing of signals in the laser driver circuit;

[0026] Figure 4 The overall circuit schematic of the laser driver is shown;

[0027] Figure 5 shows the implementation of the laser driver; and

[0028] Figure 6 A temperature compensated replica circuit is shown. DETAILED DESCRIPTION

[0029] The system to be described below provides a driver suitable for pulse driving of laser diodes, especially semiconductor laser diodes. For example, the driver can be used to drive laser diode arrays for range finding applications; but other applications are also possible, for example, driving lasers for illumination.

[0030] The system described below integrates a pulse modulation switch and multiple beam steering channel selection switches. Such a system can provide drive with relatively high instantaneous current (e.g., greater than 30 amps) and simultaneously relatively high instantaneous voltage (e.g., greater than 15 volts). It may be capable of switching to produce laser pulses with a duration of less than 1 ns.

[0031] In 3D depth sensing using laser ranging, it is often necessary to limit the optical power of the transmitter for safety reasons. For example, acceptable emission limits for suppressing damage to the eyes of bystanders are defined in EN60825-1:2014. Such limits set boundaries on the duration of exposure to laser energy. In order to achieve a good receiver signal-to-noise ratio (SNR), i.e., the ratio of the target reflected photon flux to the ambient photon flux, in the ranging system, while also limiting the laser emission power to an eye-safe level, a trade-off can be made between the acquisition frame rate over the receiver's field of view (FOV) and improving the SNR at the ranging distance. The target irradiance (in watts) can be increased for the same limited output peak optical power output in both 2D scanning mode (raster scanning) and 1D scanning mode (line scanning).

[0032] An example of a vertical cavity surface emitting laser (VCSEL) for performing 1D line scanning is given in Fundamentals, Technology and Applications of Vertical Cavity Surface Emitting Lasers, edited by Rainer Michalzik. Further details of an example of such a laser are disclosed in EP 3 588 702. Such a device may have the advantage of being compact, which is useful in systems intended to be portable, for example, in a vehicle.

[0033] One way to increase the output optical power of a semiconductor laser is to electrically connect multiple laser diodes in series and combine their optical power outputs, treating them as a single transmission source. In this embodiment, the operating bias voltage is increased compared to driving a single laser diode, but the laser current remains unchanged. The traditional quality factor of a laser diode is its slope efficiency (in watts / amperes). The slope efficiency of a device with multiple series-connected LDs increases in multiples of the number of LDs in series. To drive such a device for a direct TOF system, a narrow pulse current is required. This requires a fast step change in the drive current. Such a drive is still needed despite any parasitic effects that hinder this performance. Examples of such parasitic effects can be resistance, inductance, capacitance, and dielectric losses caused by wiring interconnects or transmission lines. Drivers need to be designed for such systems to accommodate the relatively high operating voltages of these diode structures.

[0034] The behavior of VCSELs is discussed in "A comprehensive Verilog-A VCSEL model for>20Gb / s optical interconnect transceiver circuit design" (Wang et al.). Of interest are the variations in optical power with respect to temperature, the variation in threshold current, and the variation in voltage versus current basis characteristics.

[0035] The system to be described below provides an implementation for a time division multiplexed laser diode driver for pulsed current operation with temperature compensated peak current control. Embodiments are suitable for TOF ranging systems and other applications.

[0036] Figure 1The working principle of the present laser driver is shown, which is used for time division multiplexing and guiding pulse current to VCSEL. The circuit includes a charge pump device 1 for providing drive to a channel controller 2. The channel controller 2 can be repeated for each of a plurality of channels. The channel controller uses a replica load device to set the current to be applied to the semiconductor laser 6. The semiconductor laser 6 is arranged between a high-side switch 8 as part of the channel controller and a low-side switch 7 as part of the switch array 3. The switch array may include a plurality of low-side switches for each laser device, as described below. A filter capacitor 9 is arranged between the high side of the laser diode 6 and ground.

[0037] In operation, the charge pump controls the current input to the channel controller. The feedback loop in the charge pump allows the operation of the charge pump to run under closed-loop control. The channel controller forms a signal at 10 to control the high-side switch 8. In addition, the channel selection input 11 allows a higher-level controller to select the channel shown and its corresponding laser to work through switch 12. The current flowing through the high-side switch charges the capacitor 9 according to the required laser current requirements. When the low-side switch 7 is turned on, the capacitor 9 is discharged through the laser, causing it to emit light. The low-side switch 7 can be controlled by a gate pre-driver 13, for example, to achieve fast pulses of laser emission. In practice, there may be multiple such lasers, and one can be selected for operation through the channel selection input 11 (addressed to the appropriate channel controller) and the operation of the corresponding gate pre-driver.

[0038] Figure 2 The mechanism for controlling the laser current is shown as implemented by the charge pump 1. The circuit allows closed-loop regulation of the laser current based on a replication of the laser diode voltage / current (VI) characteristic. Figure 2The replica circuit in the left arm includes a load 14 that simulates the temperature response of the laser diode VI characteristic. The laser diode VI characteristic is approximated as a voltage source plus a resistive element in the load. The characteristics of the laser diode may be different. A batch of laser diodes can be analyzed, and a representative average or median value can be used as the basis for the replica circuit design. The tuning of the replica laser diode VI characteristic is performed by selective programming of three settings: bias voltage at room temperature, bias voltage temperature coefficient, and laser diode mesa resistance. These can be tuned using a bandgap circuit, which can be provided on the chip. It can provide a low temperature coefficient voltage reference (e.g., less than 50ppm), an inverse to absolute temperature current (ICTAT), and a proportional to absolute temperature current (IPTAT). Closed-loop control can be used to apply an absolute laser current set point or an optical output power set point.

[0039] Figure 3 The signaling timing in the system is shown. There may be multiple channels, each channel corresponding to a respective semiconductor laser device. Figure 3 Seven channels are shown in FIG. A channel enable signal 11 is effective when each channel is to be activated. There may be a blanking period between the effectiveness of consecutive enable signals. Then, when the channel is activated, the corresponding laser can be operated in a pulsed manner. It is noteworthy in this circuit that the channel selection is switched explicitly by inserting a blanking period. The order and switching frequency of the switching can also be selectively controlled. The system can provide sequential or non-sequential channel switching: for example, an odd-even numbered staggered order, a random order, or following a recognizable pre-programmed pseudo-random order. Typically, a receiver array (e.g., a CMOS array) is paired with a transmitter array. The receiver array is required to operate in the same order as the transmitter. The laser transmitter device may include a one-dimensional (1D) array. The one-dimensional scanning frequency of such a transmitter can be synchronized with the scanning frequency of the receiver array. The scanning frequency of the receiver array can be provided by a rolling shutter. This synchronization helps to avoid temporal aliasing of the illumination light source (i.e., partial exposure of the sensor).

[0040] Figure 4It is shown that the high-side or channel selection switch 8 is suitably an NMOS device, more preferably an N-channel Laterally-Diffused Metal-Oxide Semiconductor (NLDMOS) device. This can provide the advantage of layout scale efficiency and increase the integration of the multi-channel laser diode array driver. The NLDMOS channel selection switch 8 is used to control the peak laser current. Its gate-source voltage is controlled by the charge pump voltage. When each laser diode channel is paired with a high discharge low inductance capacitor 9, the scale of the NLDMOS channel selection switch can be further reduced (i.e., the requirements for its operating switch resistance are relaxed). Another advantage of using an NLDMOS channel selection switch is that it provides improved transient operation. The source follower configuration provides a low impedance output. In addition, in active operation, a passive bias current can be applied to avoid channel reversal charge when turned on.

[0041] The low-side switch 7 can be an LDMOS switch. It can be used to modulate a pulse state with selectable pulse width and period. This can preferentially reduce switch resistance and drive gate capacitance to achieve high-bandwidth pulse current operation.

[0042] The closed loop regulation of charge pump 1 is hysteretic. The gating of the charge pump is timed to control power delivery by affecting the voltage / current product.

[0043] Figure 2 The replica laser diode circuit shown is operated under ratiometric DC conditions. The term ratiometric refers to the equivalent of tracking scaling relative to the actual applied peak pulsed laser current. This drive mode of operation is called Absolute Current Control (ACC). Because the electro-optical relationship of a laser diode depends on its junction temperature, this approach can provide a means of reflecting the behavior of the laser diode with closed-loop feedback correction.

[0044] Figure 4 The system is shown in more detail and a stacked box is shown in which components can be replicated for each of the multiple channels. Filter capacitors 17 and 18 can filter the voltage sources VHV and VCP. The regulator 15 can perform closed-loop regulation of the charge pump 1. The clamping diode 19 can be set between the voltage source VHV driving the laser and the node between the laser and the low-side switch 7. The gate driver 13 can be controlled by a pulse generator 16. The pulse generator can generate short-duration pulses to turn on the selected laser. Suitably, these short-duration pulses can be generated from two pulse trains 20, 21, each short-duration pulse being initiated by a pulse edge of one of the pulse trains 20, 21 and terminated by a pulse edge of the other of the pulse trains 20, 21.

[0045] The high bandwidth requirement of the drive laser pulse of duration (e.g., less than 1 ns) can be mitigated by using a pair of phase signals at the pulse frequency. These signals are gated so that the edge of one of the signals turns the laser diode on, while the edge of the other signal turns the laser off. The bandwidth of these signals can be much lower than the bandwidth of the desired pulse. The bandwidth can be approximated as 0.35 per signal rise time, or the Fourier transform of the step function (i.e., the pulse waveform) can be used to represent the spectral content in the frequency domain. The phase difference of the edges can be extracted. The generation of high-frequency pulses can be distributed relative to the large aspect ratio of the low-side switch.

[0046] Figure 5 An example of how the system can be implemented using an integrated circuit is shown.

[0047] The system provides brownout protection. That is, the loss of all power may only be temporary. If Figure 5 If any of the switches indicated by 12 and 22 in the circuit fails open, the high-side switch 8 will not function. Therefore, the active gate driver has built-in passive pull-up and pull-down components to terminate the laser current path in the event of an undervoltage condition. These components may be overloaded in normal operation.

[0048] Compared to some prior art techniques, the system described herein can simplify the functionality of the low-side laser diode switch because the switch is only used for pulse modulation. This can reduce switching losses (IR drop) and increase switching speed. The system uses a high-side switch to implement functional peak current regulation, as well as channel activation selection switching. In the system, the high-side switch is an NLDMOS device. This can provide layout implementation area reduction advantages. These advantages are particularly important when generating high current (e.g., greater than 30 amps) sub-nanosecond pulses.

[0049] Multiple high-side NLDMOS devices can be provided. Such an array can allow multiplexed channel selection. Each channel pre-charges its own discharge capacitor. This can provide good integration. The discharge capacitor can be an MLCC SMD, or integrated on-chip with the rest of the circuit.

[0050] The field of view (FOV) can be sequentially constructed by time-division multiplexed switching of channels in the laser diode array and their signals integrated by receivers operating in series.

[0051] As described above, the pulse modulation switch 7 (which can be shared between multiple laser devices) and the channel selection switch 8 (each laser device can have one) can both be N-type LDMOS devices. These devices can provide a smaller on-resistance (Rds) per device unit area compared to P-type LDMOS devices. This can allow relatively high current switching requirements with relatively low Rds to achieve lower voltage drops. In addition, the switching speed of the modulation switch is inherently limited by the MOSFET gate capacitance (Cg) and can be related to Rds (i.e., lower Rds results in higher Cg).

[0052] In pulsed current operation, there may be relatively high modulation currents (>30 amps) and relatively narrow pulse generation (<1ns). These may be affected by the parasitic inductances present along the transient current path. To accommodate the voltage transients (V=L.di / dt) across these parasitic inductances, the driver may be operated at a higher overvoltage compared to the DC bias condition.

[0053] In operation, to turn on the laser diode, the cathode of the laser diode is switched to ground. This can be done by a high speed pulse modulated switch 7 which may be common to a number of laser diodes. The or each channel select switch 2 is connected to the anode of a single laser diode and isolates operation. Preferably no more than one channel is active at any time. The channel select switch 2 is used to gate the current path from the high side voltage rail to the anode of the or each laser diode. The frequency of channel switching may run at a rate lower than the pulse modulation frequency.

[0054] To facilitate the use of an N-type LDMOS (NLDMOS) channel select switch, a voltage (source voltage) higher than the anode bias of the laser diode at peak laser current may be used as a bootstrap voltage to turn it on. Therefore, the gate-source voltage (Vgs) of the N-type LDMOS may be higher than its threshold voltage. This can be achieved by a regulated charge pump 1. The regulated output voltage of the charge pump effectively sets the peak laser current by regulating the Vgs of the channel select N-type LDMOS switch. The closed-loop feedback regulation of the charge pump can be referenced to a current sensing resistor, the effective voltage drop across which represents the equivalent peak laser current. Figure 6 shows how to achieve this.

[0055] In practice, to compensate for the temperature behavior of the laser diode, a piecewise approximation of the laser diode current-voltage (IV) characteristic can be extracted from the measurement. The mirrored temperature coefficient can be recreated using an on-chip bandgap generator, thereby forming a replica laser diode. The replica laser diode with the approximate IV characteristic is implemented in a closed-loop feedback, working in conjunction with a voltage-regulated charge pump to control the peak laser current.

[0056] Applicants hereby separately disclose, each individual feature herein and any combination of two or more such features, to the extent that such features or combinations can be implemented in accordance with the common general knowledge of those skilled in the art in light of this specification as a whole, regardless of whether such features or combinations of features solve any problem disclosed herein, and without limitation to the scope of the claims. Applicants indicate that aspects of the present invention may consist of any such individual features or combinations of features. In view of the foregoing description, it will be apparent to those skilled in the art that various modifications may be made within the scope of the present invention.

Claims

1. A laser driving circuit, characterized in that: include: A high power node for providing a relatively high voltage; A low power node for providing a relatively low voltage; a first laser drive output and a second laser drive output for connecting to a laser to provide a drive current to the laser; a high-side switch coupled between the high power supply node and the first laser driver output; as well as a low-side switch coupled between the low power supply node and the second laser driver output; wherein the high-side switch is used to be controlled to adjust the drive current provided to the laser, and the low-side switch is used to be controlled to alternate its state between an on state and an off state, thereby achieving a pulsed operation of the laser; a current driver for supplying current to the high power supply node, wherein the current driver is used to temperature compensate the current supplied to the high power supply node; The current driver includes a replica circuit for simulating a temperature response of a circuit including a laser, and an output of the current driver depends on the response of the replica circuit.

2. The laser driving circuit according to claim 1, characterized in that: The high-side switch is an N-type metal oxide semiconductor (NMOS) switch.

3. The laser driving circuit according to claim 1, characterized in that: The laser is coupled between the high-side switch and the low-side switch.

4. The laser driving circuit according to any one of claims 1 to 3, characterized in that: A gate driver is included to control the high-side switch; and a charge pump is included to regulate the current provided to the gate driver under closed-loop control.

5. The laser driving circuit according to any one of claims 1 to 3, characterized in that: For selectively driving each of a plurality of driver outputs to a laser array.

6. The laser driving circuit according to any one of claims 1 to 3, characterized in that: The replica circuit has a current / voltage response that is substantially linear over a temperature-invariant operating region.

7. The laser driving circuit according to claim 6, characterized in that: It includes a switching circuit for receiving a first pulse string and a second pulse string with offset phases between each other, and the switching circuit is used to control the low-side switch to be turned on in response to a first predetermined sensed edge in the first pulse string, and to control the low-side switch to be turned off in response to a second predetermined sensed edge in the second pulse string.

8. The laser driving circuit according to any one of claims 1 to 3, characterized in that: For driving a plurality of lasers, for each of the lasers, the circuit comprises: A high power node for providing a relatively high voltage; A low power node for providing a relatively low power supply voltage; A first laser drive output and a second laser drive output, for connecting to corresponding lasers to provide drive current to the lasers; a high-side switch coupled between a corresponding said high power supply node and a corresponding said first laser driver output; and a low-side switch coupled between a corresponding said low power supply node and a corresponding said second laser driver output; The corresponding high-side switch is used to be controlled to adjust the drive current provided to the laser, and the low-side switch is used to be controlled to alternate its state between on and off states, thereby achieving pulsed operation of the laser.

9. The laser driving circuit according to claim 8, characterized in that: A channel selection circuit is included for receiving a current source and selectively directing the current source to a corresponding one of the high-side switches.

10. The laser driving circuit according to claim 9, characterized in that: The channel selection circuit includes a plurality of switches for controlling the driving of the corresponding lasers, the switches being configured so that in the event of a failure of any of the switches, a current path to the corresponding laser will be interrupted.

11. A laser unit, characterized in that: include: A laser driver circuit according to any one of the preceding claims; as well as A laser is coupled between the first laser drive output and the second laser drive output or each pair of the first laser drive output and the second laser drive output.

12. An optical distance measuring unit, characterized in that:

11. A laser unit according to claim 11 arranged to intermittently illuminate an area outside the laser unit by switching a respective high side switch so as to enable the time of flight of a signal from the or each laser to an object to be measured.

Images