GB2547743A

GB2547743A - Preheating for laser diode drivers

Info

Publication number: GB2547743A
Application number: GB1620951.2A
Authority: GB
Inventors: Borthwick Andrew; Morton Graham; Mcneill Craig
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2015-12-09
Filing date: 2015-12-09
Publication date: 2017-08-30
Anticipated expiration: 2035-12-09
Also published as: GB201620951D0; GB2545216A; GB201521666D0; GB2545216B; GB2547743B

Abstract

Reduction of capacitor ESR in a laser driver by use of low-level discharge current pulses The ESR (equivalent series resistance) of the storage capacitor 3 in a pumping laser diode driver circuit may be too high to allow operation of the pumped laser in polar regions or at high altitude. To address this problem load pulses may be drawn from the capacitor 3 to raise the capacitors temperature by internal dissipation. The laser diode 5 may be used as a pumping energy source for a laser rod, the heating pulses being insufficient to initiate a laser discharge in the rod. The subcritical heating pulses also warm the laser rod and the laser diode. If the measured ESR is too high, or if the ambient or capacitor temperature is too low,

Description

Preheating for Laser Diode Drivers

FIELD

Embodiments described herein relate to devices with pulsed power supplies, and particularly to the field of diode pumped lasers operating at low temperatures.

BACKGROUND

At low temperatures, equivalent series resistance (hereafter ESR) in laser diode driver circuits is classically high. This effect limits the voltage and current that can be obtained from diode driver circuits at such temperatures. In order to offset this voltage drop, the capacitor may be charged to a higher voltage. However, at higher voltages, current control is difficult and capacitor real estate increases. Capacitors also age more quickly the longer that they spend at higher voltages.

As a result, operation of the diode at such temperatures may be not possible without the addition of expensive heaters or expensive driver circuits. This effect is particularly problematic in areas of extremely cold temperatures such as in polar regions or at high altitudes.

Conventional systems designed to work in such environments employ heaters to warm the circuit from the outside. These heat the circuit when the ambient temperature falls below a certain value. Such additional heating systems add a weight, size and cost penalty to the laser diode system. In addition, these systems are prone to a loss of temperature calibration as they age leading to dropouts or inefficiency and long into-action times.

As well as increasing the storage voltage of the capacitor, as explained above, other techniques for overcoming this problem have also included pulsing the laser at a lower current for a longer time.

There is a continuing need to develop laser diode driver systems for use in low temperature environments.

DESCRIPTION OF DRAWINGS

Embodiments will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a driver circuit in accordance with an embodiment;

Figure 2 shows a sequence diagram of the change in voltage with time during the charging and discharge of a capacitor for firing of a laser;

Figure 3 shows a sequence diagram of the change in voltage with time during the emission of a subcritical pulse in accordance with an embodiment;

Figure 4 shows a flowchart of an implementation of ESR measurement according to an embodiment;

Figure 5 shows a flowchart of a method of controlling ESR according to an embodiment; and

Figure 6 shows the change in pulse frequency with calculated ESR error in accordance with an embodiment.

DESCRIPTION OF EMBODIMENTS

According to a first aspect of the present invention, there is provided a method of heating a driver circuit for supplying a pulsed power supply to a load, wherein said driver circuit comprises a capacitor, the method comprising: performing a charging step comprising charging the capacitor to a predetermined voltage; performing an emitting step after said charging step comprising emitting a pulse of charge from the capacitor with predetermined duration and predetermined current, wherein the charge comprised within said pulse is insufficient to activate a load supplied by the driver circuit; and repeating the charging and emitting steps with a predetermined frequency.

This method enables the driver circuit to be heated from the inside, rather than relying on expensive and bulky external heaters.

Heating the driver circuit may mean maintaining the temperature of the driver circuit. Heating the driver circuit may mean increasing the temperature of the driver circuit.

The method may further comprise measuring the voltage of the capacitor before or after the emission of the pulse; measuring the voltage of the capacitor during the emission of the pulse; and calculating the difference between the voltage measured before or after emission of the pulse and the voltage measured during emission of the pulse so as to determine equivalent series resistance in the driver circuit.

The voltage of the capacitor may be measured after the emission of the pulse and during the pulse, substantially at the end of it. The pulse voltage may be measured as close to the end of the pulse as possible with the equipment employed. The pulse voltage may be measured as close to the end of the pulse as permits a satisfactory estimate of ESR from the difference between the two measurements.

The voltage of the capacitor may be measured before the emission of the pulse, and during the pulse, substantially at the beginning of it. The pulse voltage may be measured as near to the start of the pulse as possible with the equipment employed. The pulse voltage may be measured as close to the start of the pulse as permits a satisfactory estimate of ESR from the difference between the two measurements.

The load may be a lasing medium. The load may be an optically pumped laser.

Heating the driver circuit may comprise altering the charging and emitting frequency according to the determined equivalent series resistance. The charging and emitting steps may be repeated with a first predetermined frequency when equivalent series resistance in the capacitor is above a first threshold and with a second predetermined frequency when equivalent series resistance in the capacitor is below a second threshold, wherein said first frequency is higher than said second frequency and the equivalent series resistance at said second threshold is less than or equal to the equivalent series resistance at said first threshold. The second threshold may be less than said first threshold and altering the charging and emitting frequency according to the determined equivalent series resistance may comprise altering the frequency when the equivalent series resistance decreases below the second threshold or increases above the first threshold and maintaining the existing frequency when the equivalent series resistance decreases below the first threshold or increases above the second threshold.

Instead, or as well as altering the frequency of the pulses, the duration of each pulse of charge may be varied according to the determined equivalent series resistance. Instead, or as well as altering the frequency and/or the duration of the pulses, the current of each pulse of charge may be varied according to said determined equivalent series resistance.

According to another aspect of the present invention, there is provided a system for driving a laser diode comprising a capacitor and configured to perform the method of the first aspect of the invention.

Also disclosed herein is a method of measuring equivalent series resistance in a driver circuit for supplying a pulsed power supply to a load, wherein said driver circuit comprises a capacitor, the method comprising: charging the capacitor to a predetermined voltage; emitting a pulse of charge from the capacitor through the circuit with predetermined duration and predetermined current; measuring the voltage of the capacitor before or after the emission of the pulse; measuring the voltage of the capacitor during the emission of the pulse; and calculating the difference between the voltage measured before or after emission of the pulse and the voltage measured during emission of the pulse so as to determine equivalent series resistance in the driver circuit.

This method enables automatic diagnosis of ESR in driver circuits to be performed without the need for bulky or expensive components in the system.

The predetermined voltage may be the same or different to the voltage to which the capacitor is charged when it is operated to drive the load. The equivalent series resistance in the driver circuit may be taken to be equal to the calculated difference between the voltage measured before or after emission of the pulse.

The voltage of the capacitor may be measured before the emission of the pulse and during the pulse, substantially at the beginning of it. The pulse voltage may be measured as near to the start of the pulse as possible with the equipment employed. The pulse voltage may be measured as close to the start of the pulse as permits a satisfactory estimate of ESR from the difference between the two measurements.

Each measurement may comprise a series of measurements which are then averaged.

The charge comprised within said pulse may or may not be insufficient to activate a load supplied by the driver circuit. The load may be a lasing medium and the charge comprised within said pulse may or may not be insufficient to cause lasing. The lasing medium may be an optically pumped laser rod. The optically pumped laser rod may be pumped by a laser diode over which the capacitor discharges.

Also disclosed herein is a driver circuit for or supplying a pulsed power supply to a load, the driver circuit comprising: a capacitor; a current supply configured to charge the capacitor to a predetermined voltage; a voltage meter for measuring a voltage across the capacitor; and a controller operable to cause the capacitor to discharge a pulse of charge of predetermined duration and current through the circuit, the pulse being insufficient to activate the load; wherein the controller is further operable to gather a measurement from the voltage meter of the voltage across the capacitor either before or after said emission and during said emission and to calculate a difference between capacitor voltage before or after said discharge and during said discharge.

The controller may be operable to determine a measure of equivalent series resistance, ESR, of the capacitor from the calculated difference in capacitor voltage. The controller may be operable to determine from the measure of ESR, if the measure of ESR exceeds a predetermined threshold and, if so, to execute an ESR threshold fault response. The controller may comprise a fault output unit. The controller may be operable to execute an ESR threshold fault response comprising outputting, by way of the fault output unit, a fault output alert. Alternatively, or in addition, the controller may be operable to execute an ESR threshold fault response comprising causing the capacitor to discharge at least one pre-heating pulse of charge through the driver circuit.

Figure 1 shows a circuit for driving a laser according to an embodiment. In this embodiment, a laser diode 5 is arranged to pump the gain medium of a laser (not shown), for example a laser rod. A grounded storage capacitor 3 is connected over a power supply 1 which is configured to charge the storage capacitor. A voltage detection means (not shown) is connected over the storage capacitor 3 in order to measure its voltage. In an embodiment, the voltage detection means comprises a voltage divider circuit having two resistors connected in series with the bottom end of the circuit grounded.

The storage capacitor 3 is configured to discharge over the laser diode 5 which, in this embodiment, is connected to a ground potential via a resistor 9. The discharge of the capacitor 3 over the diode 5 to ground is controlled by MOSFET switches 7.

When charging the storage capacitor 3, the gate voltage applied to the MOSFET switches is such that they do not allow current to pass through. Once the capacitor is charged to the required voltage for firing the laser, the power supply 1 is switched off and the gate voltage applied to the MOSFET switches 7 is altered to allow current to flow through the switches and the capacitor 3 to discharge to ground over the laser diode 5. Thus, the laser diode is pulsed with current which causes it to emit photons.

The gates of the MOSFET switches are connected to a hardware controller 13 with which they are controlled. The skilled person will appreciate that there are a number of suitable arrangements for controlling the MOSFET switches. In one example of a suitable arrangement, the hardware controller 13 comprises a field programmable gate array (FPGA) with transistor-transistor logic (TTL). This arrangement allows for direct driving of the MOSFETs without the need for additional components such as log buffers, etc. or circuitry between the hardware controller 13 and the MOSFETs 7.

In this embodiment, the circuit further comprises a local controller 11 which is connected to the hardware controller 13. The local controller 11 provides an interface with which commands are given to the hardware controller 13. For example, the local controller 11 may provide the hardware controller 13 with a command to fire the laser. When this command is received the hardware controller 13 switches the MOSFET switches 7 such that the storage capacitor 3 discharges over the laser diode 5.

Again, the skilled person will appreciate that there are a number of components suitable for use in the local controller. In one example of a suitable component, the local controller 11 comprises a Microchip 32-bit PIC with built-in hardware peripherals including serial communications ports, analogue to digital conversion ports and digital input/output ports. The PIC may also be supported by additional power supply and noise filtering circuitry. Examples of suitable components include Power Supply IC’s, Resistors, Capacitors and Inductors.

In an embodiment, the command supplied to the hardware controller 13 by the local controller 11 comprises a “SetPoint” voltage signal which is directly proportional to the desired drive current though the circuit. The hardware controller 13 is configured to replicate this voltage signal via the MOSFET switches.

As such, the hardware controller 13 is arranged so as to receive current and voltage feedback from the laser diode circuit. In the embodiment of Figure 1, this is achieved by a current sense signal 15 and a voltage sense signal 17 which are directed to the hardware controller 13. The voltage is measured at the bottom end of the MOSFETS and fed into a comparator circuit whereby the detected voltage is compared to the “SetPoint” value that is being provided by the local controller 11. This comparison enables control of the current through the circuit, the significance of which will become apparent below. Note that, in this embodiment, there is no direct measurement of voltage or current through the circuit. This avoids valuable space been taken up on the printed circuit board. Other embodiments may employ direct current and voltage sensors.

In a further embodiment, the local controller 11 also generates a capacitor set point voltage signal which it provides to the power supply 1 (connection not shown in Figure 1) which is directly proportional to the desired voltage of the storage capacitor 3.

Characterisation and testing may be employed to verify that the capacitor and voltage set points set by the local controller 11 are suitable for their intended purpose.

The process of charging the storage capacitor for firing of the laser will now be described in detail. In an embodiment, two-step charging of the capacitor 3 may be performed for firing of the laser.

The first step of the two step charging process comprises charging the storage capacitor 3 via the power supply 1 to a pre-set voltage, referred to henceforth as the “ARMED” voltage. This pre-set voltage comprises a state in which the storage capacitor 3 is charged so that it stores a significant but non-optimal voltage according to the requirements of the system. In the embodiment of Figure 1, where the laser diode 5 is configured to pump a laser gain medium, therefore, the ARMED voltage is close to, but not quite sufficient to ensure firing of the optically pumped laser across the system’s full operational environment. Note that, in some laser arrangements, the armed voltage may be sufficient to ensure firing under certain conditions.

When the command to fire is received by the local controller 11, the second phase of charging is carried out. This comprises a short period of time in which additional charge is applied to the storage capacitor 3. The voltage therefore increases to the so-called “FIRE”, or optimal, voltage. This voltage is sufficient for firing of the optically pumped laser. The capacitor voltage setpoint may be increased slightly to this “optimal” value in a very short period of time, for example on the order of 10 ms. Once the FIRE voltage has been reached, the hardware controller 13 switches the MOSFETS 7 in response to a setpoint voltage signal received from the local controller 13. The storage capacitor 3 is then discharged over the laser diode 5 until lasing from the optically pumped laser occurs. The discharge from the storage capacitor 3 during this process is sufficient to pulse the laser diode 5 which, after some time, causes the laser to fire; the stored charge released from storage capacitor 3 is thus greater than the charge required to achieve the lasing threshold.

The skilled person will appreciate that single step charging, where the storage capacitor is charged to the necessary voltage for firing the laser in a single step, is also suitable for use in accordance with embodiments. However, charging of the storage capacitor in two-steps as described above is efficient and known to extend the lifetime of storage capacitors.

The two-step charging process described above is shown schematically in Figure 2 which shows a graph of the capacitor voltage VcaP with time. When in the ARMED state, the storage capacitor 3 is maintained at the capacitor storage voltage Varm 23. When the capacitor transitions to the FIRE state, additional charge is fed to the capacitor until it reaches a voltage 25 (Vfjre or Vboost) sufficient to fire the laser. Once the storage capacitor 3 reaches FIRE voltage 25 at time 21, the MOSFET switches 7 are switched to allow discharge of the capacitor.

On switching of the MOSFET switches at time 21, current begins to flow through the driver circuit and the voltage of the capacitor drops instantaneously by VESr due to electrical series resistance within the circuit, as described above. The value of VESr is a function of the known current and the ESR. This drop is followed by a gradual decrease in voltage between times 21 and 29 as energy discharges from the capacitor. At the end of the pulse, at time 29, the MOSFET switches are switched so as to prevent current flow through the circuit and the discharge ends. The voltage level then rises by VEsr as this ESR is no longer present in the circuit due to zero current flow.

The lower limit of the voltage 27 at the end of the critical pulse must at least be greater than the voltage required for operation of the diode. However, the precise voltage 25 to which the capacitor must be charged in order to fire the laser will depend not only on the laser diode bandgap voltage but must also take account of resistance in the driver system and the capacitance of the capacitor. The charge voltage required to discharge a pulse of time t over the laser diode can be calculated using the following formula:

(1) where V0(T) is the laser diode bandgap voltage; I(T) is the desired current as a function of temperature; T is the temperature; RT is the combined resistance including the minimum FET resistance, the sense resistance, the diode resistance and any stray driver losses; t is the desired pulse length, C is the capacitance of the capacitor; and Resr is the ESR.

As can be seen from equation (1) and figure 2, an increase in the ESR causes an increase in the charging of the capacitor required to fire the laser.

In an embodiment, the capacitor is configured to discharge a subcritical pulse in order to measure the ESR in driver circuit. In this embodiment, the capacitor is charged and then discharged over the laser diode but the current and duration of the pulse are insufficient to fire the laser, i.e. the pulse does not result in sufficient emission from the laser diode to excite the optically pumped laser above the lasing threshold.

The duration and current of the pulse will therefore vary depending on the particular laser architecture employed. The key feature of the pulse however, is that it has a shorter length and a lower current than the firing pulse. This has the effect of reducing the overall optical energy emitted by the diodes and hence the energy absorbed by the laser rod relative to the firing pulse.

To discharge the subcritical pulse according to this embodiment, the capacitor is discharged over the laser diode 5. The duration of the pulse is controlled by the hardware controller 13, by controlling the MOSFET switches 7, as in the manner of normal pulse for firing the laser, as described above. In this embodiment, as in the case of the firing pulse, the local controller 11 (PIC) informs the hardware controller 13 (FPGA) and provides a setpoint signal to the hardware control. The setpoint signal in the case of the subcritical pulse differs from that of the firing pulse as the voltage requirement is smaller. In an embodiment, the local controller is configured such that there is separation of the critical pulse and subcritical pulse trigger signals. This is done using an l2C communications interface protocol between the components which necessitates the use of separate commands for each trigger method. Separation of the trigger signals in this way reduces the chance of the incorrect pulse being emitted and accidental lasing.

The variation in capacitor voltage before, during and after the discharge of the subcritical pulse according to one embodiment is shown in Figure 3. As in Figure 2, the capacitor is charged to the ARMED state with a voltage of Vann. In this case, however, there is no additional charging of the capacitor to the fire voltage. Instead, at time 36, the capacitor is discharged from voltage Varm to a voltage 31 via a pulse with a known current and a known duration.

Again, an instantaneous voltage drop due to ESR occurs at time 36, at the start of the pulse and a corresponding voltage jump occurs at the end of the pulse at time 37. As in Figure 2, the lower limit of voltage 31 which is reached by the capacitor is equal to or above V0(T), the laser diode bandgap voltage. The voltage must remain above the diode bandgap voltage in order to maintain current flow through the circuit. Note that Figures 2 and 3 are not to scale.

The capacitor voltage is sampled just before and just after the end of the pulse at time 37. For example, samples may be taken at time 33 and time 35 as shown in Figure 4. The capacitor voltage is sampled as described above.

In an embodiment, measurement of the voltage of the capacitor before and after discharge of the subcritical pulse is done by taking a number of samples, n. There is no limit to the number of samples that may be taken; the higher the number of samples that can be obtained, the more accurate the measurement. In an embodiment, 5 samples are measured before and after the end of the pulse. The samples are averaged to obtain a pre- and post- end of pulse value of the voltage, pre and

Vout post, respectively. These are calculated using the following formula

(2) where Vsample is the value of the voltage measured during each sample. This averaging means that any effects due to noise can be minimised.

From these measurements, the ESR voltage drop can then be calculated using the following formula:

(3) where Voul fnxl is the voltage of the capacitor measured after the end of the pulse (i.e. at time 35) and Vout pre is the voltage of the capacitor measured before the end of the pulse (i.e. at time 33). Because the current of the pulse is known, the value of VEsr can be used as a measure of the ESR in the driver circuit. Repeated emission of subcritical pulses over time allows the variation of ESR in the system with time to be determined.

Note that whilst averaging of samples reduces noise variation, a quantifiable error in the VESR calculation may still be present. However, tuning of the sampling frequency and size can reduce this error to below a significant threshold.

The skilled person will appreciate that the most accurate calculation of ESR will be obtained when the samples employed to calculate Vout ^are taken as close as possible to the end of the pulse. However, the precise timing of the sampling will depend on the sampling speed of the chosen implementation, limited by, for example, the precise nature of the equipment employed.

In one example of a suitable implementation, a subcritical pulse of 20 A is driven for 512ps. The first sample of Vout pre is taken at 496ps from the start of the pulse (i.e. 16 ps before the end of the pulse). Four subsequent samples are taken at time intervals of 3.2ps.

The skilled person will appreciate that different hardware implementations may permit higher or lower sampling rates.

Note that the timing of the samples taken after the end of the pulse is less critical than those taken during the pulse; it does not matter when the measurement is taken as long as the voltage has settled following the end of the subcritical pulse emission. The precise timing of this measurement will therefore depend on the equipment employed.

Note that because a voltage drop due to ESR is also observed at the beginning of the pulse, the ESR voltage drop could alternatively be calculated by measuring the voltage before emission of the pulse and towards the start of the pulse. However, sampling at the end of the pulse has been experimentally shown to be less affected by transients than the corresponding start of the pulse.

Further, because the duration and current of the pulse are known, the voltage drop can, in theory, be calculated from the value of the voltage of the capacitor at any time during the emission of the pulse. However, the skilled person will appreciate that the larger the time gap between the samples and the beginning or end of the pulse, the more difficult the calculation of VESR will become as the discharge profile of the capacitor will be affected by the parasitics of the circuit, the driver circuit resistances, diode resistances and ESR as well as the capacitance at which the system is operating.

In other embodiments, or laser assemblies employing single-step charging, the capacitor is not charged to the ARMED voltage in order to fire the subcritical pulse but is instead charged to some other level of voltage. The selection of the size of the subcritical pulse will depend on the requirements of the system. The pulse should be small enough that no pre-lasing occurs. The pulse should also be small enough to allow fast recharge of the voltage should a command to fire the laser be received. For example, for a capacitor subject to a constant recharge current of 2A, it would take 5ms to recharge the capacitor to its pre-pulse level following the emission of a 20A subcritical pulse for 500 microseconds. In an embodiment, the size of the subcritical pulse is selected to ensure that laser firing latencies are not significantly compromised. The skilled person will appreciate that requirements of the subcritical pulse can be achieved in a number of ways.

Note that the measurement of the ESR could, of course, also be performed during discharge of a critical pulse itself (resulting in firing of the laser, as described above) as, as shown in Figure 2, an ESR voltage drop is observed whenever the capacitor is discharged through the circuit. However, the advantage of firing subcritical pulses for ESR measurement is that action to compensate for ESR, such as will be described below, can be taken in advance of receiving a command to fire the laser.

From Equation (1) the relationship between and VESR is given by

(4) where Isp is the current of the subcritical pulse. In an embodiment, the FIRE voltage 25 to which the capacitor is charged in order to fire the laser is adjusted according to the measured VESR. Thus, the firing voltage of the capacitor Vcharge is set dynamically to compensate for varying ESR.

In one embodiment, when the measured VESR exceeds a predetermined threshold, heating of the capacitor is initiated in order to lower the ESR. This embodiment will be explained in more detail below.

In another embodiment, when the measured VESR exceeds a predetermined threshold, the system may register an ESR error. This may occur, for example, if the value of ESR measured exceeds the amount expected for the ambient temperature in question, indicating that ESR due to factors other than simply temperature are at play.

Systems according to the embodiments described above are able to perform automatic diagnosis of ESR problems in the laser driver circuit and to adapt the charging voltage accordingly so that no loss in performance of the laser diode occurs. This can all be done without requirement for additional components in the system.

Further, such systems are able to identify exactly when heating needs to be applied. Crucially, this is determined not from the temperature of the capacitor but by its ESR. ESR in capacitors increases with age and use. It also increases inversely with temperature, i.e. at low temperatures the ESR increases. Consequently, as a capacitor ages, the ESR at a certain temperature will continue to increase. Thus, the temperature at which ESR becomes a problem in such systems will increase as a capacitor ages and with use. Thus, the older a capacitor, the higher the temperature at which heating will be required - potentially limiting low temperature operation of the system. Because systems according to the above embodiments do not rely on temperature to determine when heating is to be applied, the approach is flexible and can adapt to a capacitor as it ages without manual resetting of the ambient temperature threshold for heating. For the same reasons, the system is also unaffected by any loss of temperature calibration. A flow chart of one implementation of ESR measurement using a subcritical pulse in accordance with one embodiment is shown in Figure 4.

In step S401, the ambient temperature of the system is checked. The checking of the temperature is repeated periodically. In step S403 the change in ambient temperature is compared with a threshold. If the change in temperature does not exceed the threshold, no action is taken and the system returns to S401. If the temperature has changed by more than the given threshold, the system proceeds to S405 and the system is flagged as not ready. In an embodiment these checks are performed by the local controller 11 and any commands to fire the laser are rejected if the system is flagged in this way.

In step S407, the subcritical pulse is discharged as described above. This includes performing the voltage sampling during and after the firing of the pulse. In step S409, the sampling results are used to calculate VESR. If the ESR is below a given threshold the system proceeds to S415. If the ESR exceeds a given threshold, then the system proceeds to S413 prior to S415. In S413, heating of the system is initiated in order to reduce the ESR in the system. This heating may be performed in a number of ways, as will be discussed in more detail below. In S415, Vcharge, described above, is recalculated using the obtained VESR and the charging voltage of the system in order to fire the laser is adjusted accordingly. This means that, when the capacitor receives an instruction to fire the laser, the correct amount of charge will be discharged over the diode in order to fire it.

In step S417, the system is flagged as ready and returns to S401.

Note that if heating is initiated in step S413 in order to reduce ESR, the system will begin to heat up and the process of figure 4 will ensure that the value of Vcharge is adjusted accordingly.

In the embodiments described above, any heating of the capacitor may performed by an external heater. Alternatively, heating may be performed by firing subcritical pulses from the capacitor. Embodiments in which subcritical pulses are employed to heat the laser diode driver circuit will now be described in detail.

In an embodiment, the system of Figure 1 is configured to emit subcritical heating pulses from the storage capacitor. These pulses have the same form as the ESR measurement pulses represented in Figure 3 and are emitted using the same process, with the exception that sampling of the voltage is not required.

Emission of a subcritical pulse of charge has the effect of heating the laser diode circuit directly from within. This is a fundamentally different process to the heating of such a system using conventional heaters which heat the system externally. The use of a subcritical pulse to heat the system is analogous to heating food using a microwave; the heat is generated within the components themselves.

The effect of heating the laser diode circuit in this way is to reduce the ESR of the driver circuit thereby improving operation of the laser at low temperatures. The heating pulse also has the additional effect of warming the laser resonator through the absorption of the subcritical pulse in the gain medium and heating of the diode itself. This improves laser efficiency when cold.

Because systems according to the above embodiments do not require external heating systems, the size, cost and weight of such systems can be reduced to a minimum. The internal heating also allows for a superior into action time for heating of the system.

In an embodiment, the discharge of the subcritical heating pulses is repeated periodically in order to maintain the temperature of the capacitor. In an embodiment, pulses are emitted at high frequencies.

In one embodiment, the heating pulses are initiated on the basis of measurements of ambient temperature or the temperature of the capacitor. In this embodiment, the pulses simply perform the function of heating the system, no calculation of VEsr is carried out nor required. When the temperature of the capacitor reaches the desired temperature, the capacitor ceases to emit subcritical pulses.

In another embodiment, the system is configured to emit subcritical ESR measuring pulses in addition to subcritical heating pulses. VEsr is calculated as described above using the measuring pulse. When the ESR reaches a threshold value, heating pulses are emitted in order to heat the capacitor and reduce ESR.

In yet another embodiment each pulse functions as both a heating and a measuring pulse. In this embodiment, sampling of the voltage is taken for every pulse and the pulses are repeated periodically. Thus, in this embodiment, there is continuous monitoring of the system ESR.

In a further embodiment, the frequency of the pulses is adjusted according to the value of Vesr. For example, when the measured value of VEsr is high, the subcritical pulses are emitted with high frequency. This heats the system, causing the value of VESr to drop. When VESr reaches an acceptable value, the frequency of subcritical pulses is lowered to a level sufficient to maintain the temperature (and therefore ESR). A flowchart showing a method of controlling the ESR error according to this embodiment is shown in Figure 5. In step S601, the subcritical pulse is discharged. This includes sampling of the voltage as described above. In step S603, the ESR error is calculated from the voltage measurements. In Step S605, the calculated error is compared with a threshold. If the ESR error is greater than the threshold, the system proceeds to step S609. In S609, the subcritical pulse frequency is set to a value sufficiently high as to heat the system. The system then returns to step S601.

If the ESR threshold is lower than the threshold then the process proceeds to step S611 from step S605 and the subcritical pulse frequency is set to a frequency sufficient to maintain the temperature (and therefore ESR) of the system. This frequency is lower than that set in step S609. The system then returns to S601.

In an embodiment, hysteresis is employed to prevent chatter on the threshold between acceptable and unacceptable ESR. This process is analogous to temperature control performed by classical thermostat bang-bang controllers.

Figure 6 shows the change in pulse frequency with ESR error according to this embodiment. At high ESR error, towards the right hand side of the graph, the test pulses are discharged at a frequency 41 high enough to heat the driver circuit. As the circuit heats up, the ESR error decreases. Once the error falls below the threshold error value 45, the frequency of the pulses is decreased to a frequency 43 which is sufficient to maintain a constant temperature of the driver circuit.

If in contrast, if the system moves from low ESR error (left hand side of the diagram) to higher ESR error, such that the error increases above the threshold 47, the frequency of the sub-critical pulses is increased so as to actively heat the driver circuit. The thresholds 45 and 47 are not equal, thereby avoiding chatter on the threshold.

Thus, smart heating of the capacitor is employed to minimise ESR error and ensure efficient function even in low-temperature environments. The dual role of the subcritical pulses enables continuous diagnosis of the ESR during heating and therefore feedback in order to control the amount of heating power supplied.

Note that in the embodiments of Figures 5 and 6, the frequency of the test pulses is varied with ESR error. In other embodiments, the pulse voltage or the current may be altered instead. These embodiments allow power saving to be effected in the system because the power application is varied to achieve a given measured ESR rather than a given temperature.

In an embodiment, the ESR error shown in Figure 6 corresponds to the value of VESR calculated as described above. In another embodiment, the ESR error combines information relating to VESR with information relating to Vout pre. If the capacitor is discharged with a known current and for a known time, the value of Vout pre will vary according to the ESR in the system. Measurement of this value therefore gives a second indication of ESR and acts as a double check in case there is an error in the measurement of VESR.

In this embodiment the ESR error shown in Figure 6 is calculated according to the following equation:

(5) where threshold is a predetermined threshold of tolerance for VESR and limit is a limit of tolerance for Vout pre.

The trend in the error can also be calculated:

(6)

Where n and η-1 indicate sequential subcritical pulses.

One example of heating and holding thresholds which could be applied to systems according to this embodiment is given in the table below.

Table 1

Systems and methods according to the above described embodiments may be employed in laser target designators or laser range finders or in any diode pumped laser or pulsed power supply where sub-critical pulses are possible without triggering activation of the load.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims

CLAIMS:

1. A method of heating a driver circuit for supplying a pulsed power supply to a load, wherein said driver circuit comprises a capacitor, the method comprising: performing a charging step comprising charging the capacitor to a predetermined voltage; performing an emitting step after said charging step comprising emitting a pulse of charge from the capacitor with predetermined duration and predetermined current, wherein the charge comprised within said pulse is insufficient to activate a load supplied by the driver circuit; and repeating the charging and emitting steps with a predetermined frequency.

2. The method of claim 1, further comprising measuring the voltage of the capacitor before or after the emission of the pulse; measuring the voltage of the capacitor during the emission of the pulse; and calculating the difference between the voltage measured before or after emission of the pulse and the voltage measured during emission of the pulse so as to determine equivalent series resistance in the driver circuit.

3. The method of claim 2, wherein the voltage of the capacitor is measured after the emission of the pulse and wherein measuring the voltage of the capacitor during the emission of the pulse comprises measuring it substantially at the end of the pulse.

4. The method of claim 2, wherein the voltage of the capacitor is measured before the emission of the pulse, and wherein measuring the voltage of the capacitor during the emission of the pulse comprises measuring it substantially at the beginning of the pulse.

5. The method of any one of claims 1 to 4, wherein the load is a lasing medium and the charge comprised within said pulse is insufficient to cause lasing.

6. The method of any one of claims 1 to 4, wherein the load is an optically pumped laser.

7. The method of claim 2, further comprising altering the charging and emitting frequency according to the determined equivalent series resistance.

8. The method of claim 7, wherein the charging and emitting steps are repeated with a first predetermined frequency when equivalent series resistance in the capacitor is above a first threshold and with a second predetermined frequency when equivalent series resistance in the capacitor is below a second threshold, wherein said first frequency is higher than said second frequency and the equivalent series resistance at said second threshold is less than or equal to the equivalent series resistance at said first threshold.

9. The method of claim 8, wherein said second threshold is less than said first threshold and altering the charging and emitting frequency according to said determined equivalent series resistance comprises altering the frequency when the equivalent series resistance decreases below the second threshold or increases above the first threshold and maintaining the existing frequency when the equivalent series resistance decreases below the first threshold or increases above the second threshold.

10. The method of claim 2, further comprising altering the duration of the pulse of charge according to said determined equivalent series resistance.

11. The method of claim 2, further comprising altering the current of the pulse of charge according to said determined equivalent series resistance.

12. A system for driving a laser diode comprising a capacitor and configured to perform the method of any one of claims 2 to 11.