DE19503611A1 - Driver stage for pulsed semiconductor lasers - Google Patents

Abstract

A high power, pulsed laser diode driver uses an energy storage capacitor of a non-uniform stripline structure (figures 2 and 3) with a very low impedance. The semiconductor laser driver includes a control unit 100 controlling an input drive signal, electric power conditioning and pulse charging unit 200 receiving the controlled drive signal and generating electrical energy, and an energy storage unit 300, containing the energy storage capacitor, which stores the electrical energy. A triggering light source and drive unit 400 generates low power laser light when electrical energy has been stored in the energy storage unit 300, and the laser light triggers optically activated semiconductor switch unit 500 which connects stored electrical energy to high power laser diode array 600 providing a high current impulse which produces a high power, optical pulse. <IMAGE>

Description

The present invention relates to a driver stage for pulsed semiconductor lasers, especially for those with high Pulse repetition frequency, pulsed high-power laser diodes.

Laser drivers are essentially divided into gas lasers, Solid state lasers and semiconductor lasers. Although they are high Delivering output powers are gas and solid-state lasers bulky, heavy and expensive. In addition, the Efficiency of these lasers is very poor. In contrast to that Semiconductor lasers compact, light, cheap and their efficiency is much higher. Because of these benefits Semiconductor lasers have been used increasingly in recent years found.

A critical parameter when operating a Semiconductor laser diode is the current supplied. A certain one Current threshold must be exceeded before Laser activity begins. At low, below this Currents are no laser light sent out and only spontaneous, uncorrelated occurs Emission on. With increasing current values a threshold becomes exceeded, above which the excited states in Laser medium are more populated than the basic state. At this cast inversion takes the place of the spontaneous Emission stimulates emission on and light is in the form of Laser light emitted.

There is practically no laser light below this threshold emitted and the emission efficiency is very low. If but the current has exceeded the threshold, the increases Power of the emitted light strongly.

Modern high-performance semiconductor lasers do not consist of one single laser diode, but have numerous laser diodes on that in a row or a matrix-like grid (Array) are arranged. In their manufacture, one  large number of laser diodes produced on a single substrate. The output power of the laser is proportional to the number of Laser diodes on the laser array. Great advantages of this Manufacturing processes are in the minor Manufacturing costs, the possibility of mass production, the Miniaturization and high reliability. The disadvantage is as explained below, the extremely low resistance the laser diode arrays in the forward direction, which is often clear is less than 1 ohm.

The reason for this is that the arrays are out Semiconductor laser diodes are made by a large number components operated in the forward direction with p-n transition, that is the laser diodes, connected in parallel become. As the number of laser diodes in the array increases, the ON resistance of the entire laser array. Typical values for the resistance of a high power laser array in Forward direction is in the range of a few ohms to less than 0.01 ohms.

In contrast, the driver circuit of the conventional pulsed high-power laser a very large one Impedance. As a result, there occurs a considerable Impedance mismatch between the driver circuit (high Impedance) and the laser array (extremely low impedance). Because of this mismatch, only a small portion of the electrical energy used to operate the laser array, while most of it is lost in the form of heat.

To exceed the laser threshold, the high-performance Semiconductor laser can be supplied with a current above this Threshold is. The current supplied by the driver must therefore on the one hand be so high that it compensates for the heat losses can. However, since the necessary electricity to reach the Laser threshold with increasing number of diodes in the laser array also increases sharply, on the other hand is an additional one Driver current increase required.  

In order to generate pulsed laser light, high-performance Semiconductor laser the current flow in the laser array according to the desired modulation scheme of the laser light directly controlled. The shape of the light pulse generated corresponds essentially that of the current pulse in the array. For one pulsed laser operation at high power and high Pulse repetition frequency (IFF) must be strong from the driver stage Current pulses with high IFF are generated and sent to a load with extreme low impedance, namely the laser array.

The conventional pulsed high performance driver is therefore on designed for very high performance. This applies in particular to the switch that controls the power supply to the laser array. Today, a switch is usually a high-performance Semiconductor switch used.

Because with the high required operating current also the Loss energy is very high, the semiconductor switch for very high performance. This worsens but the characteristics of the laser driver regarding Rise and fall times of the pulses, the attainable minimum pulse width and the pulse repetition frequency (IFF). At the same time, such a driver becomes large and heavy.

The performance of a conventional driver stage for a pulsed high power laser depends mainly on Performance of the high-performance semiconductor switches. When such switches are mainly controlled silicon Rectifiers (SCR), power field effect transistors (Power FET), IGBT (insulated bipolar transistor Gate) and bipolar power transistors are used.

The conventional driver stage for a pulsed Semiconductor laser uses a circuit in which a High voltage capacitor with a pulse bias charged and the stored electrical energy through Turning on the semiconductor switch is discharged. As above already mentioned goes because of the bad  Impedance matching between the high impedance of the Driver circuit and the extremely low impedance of the laser Arrays lost a lot of electrical energy as heat. Doing so generates so much heat that the driver stage usually with a fan must be cooled. The energy loss must be above by a higher bias of the capacitor be compensated.

Will a conventional laser driver stage for ever higher Services are designed, other properties, such as Example rise and fall times, pulse width and IFF of the emitted laser pulses) of the driver getting worse while at the same time the size and weight of the driver increase significantly.

As a result, the conventional driver stages for pulsed high power lasers heavy and bulky, especially compared to the laser array itself, and her Performance in terms of rise and fall times and Pulse repetition rates are severely restricted.

In addition to the achievable output power of the laser, there are others important and critical parameters of a pulsed laser driver the modulation speed of the pulse train, the pulse width, the efficiency, the weight and the compactness. There is currently no pulsed laser driver, the light pulses with high Top performance and short pulse duration with a high one Can generate pulse repetition frequency and at the same time one high efficiency, low weight and compactness guaranteed.

The object of the present invention is therefore to provide a driver stage for a pulsed high-power laser diode for the generation of short-term light pulses with high Top performance, which is lightweight and large Compactness has a high efficiency.

This object is achieved by the invention Driver stage for semiconductor lasers with a control unit for  Regulation of a control signal, a charge pulse generator, the Means for processing the electrical energy includes the can receive the control signal and thereby electrical energy generate an energy store that stores this electrical energy stores, a trigger light source with driver means for Generation of an optical trigger signal after the electrical energy was stored in the energy store, one optically activated semiconductor switch to the capacitive stored electrical energy in a strong current pulse convert and with a high power laser array to the through the optically activated semiconductor switch to the array conducted current pulse into a high power optical pulse convert that can be broadcast, the Energy storage a capacitor with very low impedance for energy storage.

Due to the improved impedance matching between Driver circuit and laser diode array are the heat losses effectively reduced and a larger proportion of im Energy stored in the capacitor is a current pulse in the Laser diode array sent. The pulse duration of the generated Laser pulses are essentially based on the duration of the Trigger light pulse, the optically activated Semiconductor switch opens a certain time, determined.

To generate laser pulses at the output of the High power laser that is significantly shorter than the duration of the Trigger pulse, the capacitor is for Energy storage preferably not as a stripline uniform impedance.

The stripline advantageously has a fan-like or circular disk-like shape.

A pulsed laser from is preferably used as the trigger light source low or medium power used.  

The present invention also relates to a method for Generation of short laser light pulses with high output power using the driver stage according to the invention, in which a control unit a control signal to a Charge pulse generator sends, the charge pulse generator one generated pulse-shaped bias in an energy storage, so that electrostatic energy is stored capacitively after A control signal is reached when a certain bias is reached triggered, which is sent to a trigger light source, so that the trigger light source generates a short light pulse, this light pulse on the active surface of an optical activatable semiconductor switch is passed so that the Switch during the duration of the trigger light pulse electrically conductive connection between the energy store and manufactures a high power laser array so that the capacitive stored energy in the form of a current pulse from Energy storage flows to the high-performance laser array, the Current pulse in a high-performance laser array into one Laser light pulse is converted, and this laser light pulse can be broadcast.

Preferred embodiments of the Driver stage for semiconductor lasers according to the invention based on the attached drawings explained in more detail.

It shows

Fig. 1 is a block diagram of the laser diode driver according to the invention;

Fig. 2 is a plan view of a fan-like formed as a strip conductor energy storage capacitor of the invention;

Fig. 3 is a longitudinal section through the condenser of Fig. 2;

Fig. 4 shows another embodiment of the energy storage capacitor of the invention, wherein the capacitor is formed as a circular ring-like strip conductor;

FIG. 5 shows a section through the center of the circle of the capacitor of FIG. 4;

Fig. 6 is a block diagram showing the path of the current pulse of the inventive energy storage capacitor in the high-power laser array;

Fig. 7 three graphs that illustrate the time course of the energy flow in the driver stage.

Fig. 1 is a block diagram of a driver stage according to the invention for a compact, pulsed semiconductor lasers with high power and high pulse repetition frequency. The pulsed laser driver comprises a control unit 10 , a charge pulse generator 20 , which includes means for power conditioning or conversion, an energy store 30 , a trigger light source 40 with associated driver unit, an optically activatable semiconductor switch 50 and a high-power laser array 60 .

The control unit 10 monitors the operating signals arriving from an input stage and sends control signals to the charge pulse generator 20 and the associated power converter device. In addition, the control unit 10 sends signals to the driver unit of the trigger light source 40 .

The power converter of the charging pulse generator 20 suitably processes the electrical energy originating from a primary energy source and sends charging pulses to the energy store 30 . A certain amount of electrical power is also sent from the unit 20 to the driver unit of the trigger light source 40 . Primary energy sources can be, for example, an AC power source fed by the network and suitable transformers, or a battery.

The energy store 30 comprises a low impedance capacitor which, in a preferred embodiment, is constructed as an uneven strip conductor. The energy store 30 stores the energy supplied by the pulse generator 20 .

The trigger light source 40 and its associated driver stage have a light source which, in a preferred embodiment, emits laser light with low or medium power. The trigger light is advantageously emitted as a light pulse when the driver of the trigger light source 40 has received a corresponding signal from the control unit 10 . Such a control signal is sent out when the energy storage in the storage unit 30 is completed.

Variety, known per se optically activated switches can be used, such as thyristors, PIN switch as optically aktiverbarer semiconductor switch 50, massive GaAs switch, solid Si-switch, solid InP switches etc. The switch 50 is controlled by that of the Laser light received by the trigger light source is switched on and thus serves to convert the electrostatic energy capacitively stored in the storage unit 30 into a pulse of high current intensity.

Since the optically activatable semiconductor switch is only switched on for the duration of the trigger light pulse, the generated current pulse can only have a maximum of this time period. The current pulse generated in this way is passed to the high-power laser diode array 60 , where a laser light pulse with high power is produced. If this process is repeated regularly, the laser diode array emits the desired pulsed laser light with high power.

The pulsed high-performance driver circuit according to the invention uses innovative components and the operation of the Semiconductor laser differs extremely from the known  Mode of operation. Instead of the conventional High voltage capacitors and high performance semiconductor switches for modulating the current conducted into the laser diode array According to the invention, strip conductors are not used uniform impedance as energy storage capacitors used. The high efficiency of the circuit achieved in this way leads to light and compact semiconductor lasers and enables greater flexibility in construction. In particular, under Using the stripline structure Energy storage capacitor with very low impedance easily getting produced. Another advantage is that that by using the optically activatable Semiconductor switch, which is preferred by a Trigger light source serving laser diode is switched, the known critical limitations of high performance Semiconductor switches, such as slow rise and Fall times, low pulse repetition frequency and large Pulse width, easily overcome. The beneficial Combination of an energy storage capacitor with lower Impedance and that of a diode laser with low or medium power triggered semiconductor switch leads to a compact driver stage for pulsed high-performance Semiconductor laser, which is able to transmit light pulses with high Top performance and short pulse duration with high To generate pulse repetition frequency.

The energy store 30 according to the invention is designed as a strip conductor with a non-uniform impedance. A fan-shaped first embodiment is shown in FIGS. 2 and 3, while FIGS. 3 and 4 show a further embodiment in which the strip conductor has an annular shape. Basically, the structure of the stripline corresponds to a capacitor, ie two metallic electrodes 33 are separated from one another by a dielectric 34 . The strip conductors of FIGS. 2 to 4 have a different impedance over their length, which is achieved by a special geometric design. An outer end 32 and an inner end 31 of the lines can be distinguished. The inner end of the stripline is connected to the semiconductor switch. The capacitance and the characteristic impedance of a stripline are known to be as follows:

C are the capacitance, A the electrode area of the capacitor, ε₀ the influence constant, t the thickness of the dielectric medium, ε _r the dielectric constant of the dielectric medium, Z₀ the characteristic impedance of the stripline and w the width of the electrode of the stripline.

By utilizing the dependence of the strip conductor's impedance Z₀ on its geometric dimensions, an energy storage capacitor with a very low impedance can be developed. This takes advantage of the fact that the impedance Z _{in of} the inner end 31 and the impedance Z _{out of} the outer end 32 of the line can be different.

But if electrical energy from the capacitor with lower Impedance can be transmitted to the high-performance laser array most of this energy is used to operate the laser array be used. Because of the better impedance matching the low impedance capacitor reduces the Energy loss when transferring energy from the capacitor to Laser array considerably.

The main function of the energy store 30 is the brief storage of the electrical energy in the form of electrostatic energy. The energy is stored there because the switch 50 arranged between the storage unit 30 and the laser array 60 is open when the energy store is being charged and thus prevents the charge from flowing away. However, when the switch is turned on, the boundary condition at the connection area of the energy storage capacitor with the switch changes from the open to the closed state. As soon as the boundary conditions are changed in this way, the construction of the capacitor as a strip conductor comes into play and the energy stored electrostatically in the capacitor flows as a traveling wave towards the load through the switch.

If, in particular, an impedance matching 55 is provided between the switch and the laser array, as shown in FIG. 6, the energy storage capacitor acts like a transmission line. Instead of the usual discharge curve known from capacitors, which essentially corresponds to an exponential drop determined by an RC time constant, a current pulse with a steep rising and falling edge is generated. The pulse duration essentially corresponds to the transit time of the wave for the outward and return travel in the stripline. The amplitude of the current pulse thus obtained is greater than the current amplitude of a strip line with a uniform impedance, because an amplification factor occurs due to the impedance transformation. For the maximum amplitude of the current, one obtains (see FIG. 6):

I = {(g / (R _on + R _m )} × V {(R _on + R _m ) / (Z _in + R _on + R _m )}, ampere (A)

Here: g is the gain factor due to the impedance transformation between the inner and outer characteristic impedance of the stripline with a non-uniform structure (where g usually applies 1 <g <2), R _{m is} the outer matching impedance (where R _{m is} also the impedance of the laser array in Forward direction includes), V the bias of the voltage pulse for charging the capacitor, Z _in the inner characteristic impedance of the strip line and R _on the impedance of the semiconductor switch in the forward direction. In the case of complete impedance matching, where R _{on is} negligible and Z _in and R _{m are the} same, the following relationship is obtained for the current pulse generated:

I = (g / R _m ) × (V / 2), ampere (A)

With the exception of the gain factor g, this corresponds to practically the known case of the adapted, uniform Stripline. The gain factor g leads to a further improve the efficiency of the circuit. Therefore, the use of a non-uniform leads Stripline with lower impedance than Energy storage capacitor for extremely high efficiency of the circle.

The mode of operation of the driver stage according to the invention for a semiconductor laser is described below with reference to FIGS. 1 and 6. First, an input unit of the control unit 10 transmits the operating signal. The control unit then activates the power converter and the charge pulse generator 20 . The primary power supplied via an AC mains cable or from a battery is suitably processed and the capacitor of the energy store 30 is charged with the aid of the charging pulse generator. At the point in time at which the pulse-shaped bias on the capacitor reaches its peak value, a control signal emitted by the control unit 10 activates the trigger light source and its associated driver unit.

The trigger light source generates optical pulses 41 with a fast rise time and a very high pulse repetition frequency and sends these pulses to an optically activatable semiconductor switch 50 .

To transmit the optical pulses 41 of the trigger light source 40 , these are preferably coupled into a fiber optic cable (not shown) and routed in the fiber optics to the semiconductor switch 50 , where they are coupled out again.

The optical trigger signal penetrates the active surface of the semiconductor switch. The photons of the trigger light generate a sufficient number of electron-hole pairs so that the state of the switch changes from completely open (not conductive) to completely closed (conductive, resistance R _on ).

As soon as the switch 50 is switched on, the electrostatic energy stored in the capacitor of the energy store 30 is discharged in the form of a narrow current pulse. The highest current values of this pulse are far higher than the threshold required for laser activity. The current pulse is applied to the laser array 60 and a laser light pulse 61 with high power and fast rise and fall time is generated.

A graphical representation of the energy flow in the driver stage according to the invention is shown in FIG. 7. Figure 7 a shows the time course of the voltage pulse that charges the energy storage unit 30 . Charging begins at time t₀ with a signal from control unit 10 . After reaching the maximum voltage on the capacitor, the trigger light source 40 receives the signal at time t 1 to emit an optical trigger pulse. The time course of the trigger pulse is shown schematically in Figure 7 b. The time (t₂'-t₁ ') in which the semiconductor switch illuminated by the trigger pulse is closed corresponds to the pulse duration pw of the trigger pulse. The electrostatically stored energy flows in the form of a narrow current pulse to the laser diode array; the time course of the driver current in the laser array is shown in Figure 7 c. The current pulse is converted into an optical pulse in the laser array 60 , the pulse duration of which is essentially determined by the pulse shape, in particular the pulse duration pw 'of the driver current pulse, apart from the fact that the rise time of the laser pulse is faster than the rise time of the driver current pulse. The pulse duration pw 'of the generated current pulse is in turn essentially determined by the pulse duration of the optical trigger pulse and the wave propagation time in the energy storage capacitor. The designations t₁, t₂, t₁ ', t₂', and t₁ '', t₂ '' are intended to make it clear that the corresponding times are not identical, but that runtime effects play a role, for example the current pulse in the laser array begins at a somewhat later time (Time t₁ '') as the switch is closed by the trigger light pulse (time t₁ '). However, the more compact the driver circuit, the smaller these differences become.

In the case of good impedance matching, the pulse duration corresponds to pw ′ of the current pulse and thus the pulse duration of the emerging Laser light about the wave travel time for the return journey in Energy storage capacitor. The following relationship makes this Connection clearly:

The difference (r _out -r _in ) corresponds essentially to the length of the strip line, which determines the transit time of the wave on the line. In Figs. 2 and 4, these variables are shown for the two preferred embodiments, the energy storage capacitor to the invention; c₀ is the speed of light in a vacuum; r and r are _{out in} in cm words, it is c₀ = 3 × 10¹⁰ cm / sec to give the pulse duration pw 'in seconds. In particular, with good impedance matching, pw '<(t₂''-t₁'').

In the case of a very poor impedance matching, this corresponds to Pulse duration of the emerging high-power laser pulse in the essentially equal to the pulse duration of the optical Trigger pulse. The main reason for this is that of poor impedance matching Multiple reflections of the traveling wave. Be natural in this case the heat losses increase again and the The power of the emerging laser pulse will be reduced.

As already mentioned, a conventional Semiconductor switch the improved ability to handle high currents to be able to switch through worsened rise and Falling times of the current impulses bought and the switching times are getting longer. Therefore, the pulse repetition frequency becomes one  Power semiconductor switch decrease very quickly while the increase in switchable power is very slow increases.

Instead of current pulses with the help of a high-performance To produce semiconductor switches are invented optical impulses with fast rise time with the help of a Laser diode of low or medium power generated and this Pulses used as trigger light. The invention Semiconductor laser drivers can therefore generate strong current pulses with a lot of power generate faster rise time.

The geometric effect of a non-uniform Stripline structure allows the construction of a compact Energy storage capacitor with very low impedance. The low impedance of the capacitor reduces the Energy loss in the transfer of energy from the capacitor to the laser array. It should therefore be emphasized in particular that in In case of good impedance matching, the shape and duration of the emerging high-power laser pulses already through the Geometry of the stripline capacitor and not by the Switching times of the semiconductor switch is determined.

In contrast to the conventional driver stages for pulsed The pulsed laser according to the invention requires high-power lasers not high performance due to its high efficiency power supply and no fan for cooling.

With the driver stage for pulsed semiconductor lasers according to the invention, laser pulses of high power with a pulse duration down to a few nanoseconds can be generated. The smaller the difference between r _in and r _out , that is to say the length of the stripline, is selected, the shorter laser pulses can be generated.

Claims (5)

1. Driver stage for semiconductor lasers with a control unit ( 10 ) for regulating control signals, a charge pulse generator ( 20 ) which comprises means for processing the electrical energy, which can receive the control signals of the control unit ( 10 ) and thereby generate electrical energy, an energy store ( 30 ), which stores this electrical energy, a trigger light source ( 40 ) with driver means for generating an optical trigger signal ( 41 ) after the electrical energy has been stored in the energy store ( 30 ), an optically activatable semiconductor switch ( 50 ) for the capacitively stored electrical energy convert into a strong current pulse and with a high power laser array ( 60 ) to convert the current pulse passed through the optically activatable semiconductor switch ( 50 ) to the array ( 60 ) into a high power optical pulse ( 61 ) which can be emitted, the Energy storage ( 30 ) a capacitor mi t includes very low impedance for energy storage. 2. Driver stage according to claim 1, characterized in that the capacitor for energy storage as a stripline is formed with non-uniform impedance. 3. Driver stage according to claim 2, characterized in that the stripline is a fan-like or circular disk-like Has shape. 4. Driver stage according to one of claims 1 to 3, characterized in that the trigger light source ( 40 ) is a pulsed laser diode of low or medium power. 5. A method for generating short laser light pulses with high output power using a driver stage for semiconductor lasers according to one of claims 1 to 4, in which

• a control unit sends a control signal to a charge pulse generator,
• the charging pulse generator applies a pulse-shaped bias to an energy store, so that electrostatic energy is stored capacitively,
• after a certain bias voltage has been reached, a control signal is triggered which is sent to a trigger light source so that the trigger light source generates a short light pulse,
• - This light pulse is passed to the active surface of an optically activated semiconductor switch, so that the switch creates an electrically conductive connection between the energy store and a high-power laser array during the duration of the trigger light pulse, so that the capacitively stored energy in the form of a current pulse from the energy store to High power laser array flows,
• - The current pulse in the high-performance laser array is converted into a laser light pulse of high power, and this laser light pulse can be emitted.