FR2716041A1 - Compact, pulsed laser diode control with high pulse repetition frequency and high power. - Google Patents

Abstract

A high power pulsed laser diode drive, maintaining high efficiency, low weight and compactness, providing energy storage capacity having microstrip structure with low impedance. The semiconductor laser controller includes a control unit for controlling an input control signal, an electrical power and pulse load conditioning unit for receiving said controlled control signal in said control unit and thereby generating energy electrical, an energy storage unit for receiving said electrical energy from said electrical power and pulse load conditioning unit and for storing the received electrical energy, a trigger light source and control unit for generating a low power optical laser light when the electric energy has been stored in the energy storage unit, an optically activated semiconductor switching unit to convert the capacitively charged electric energy into a high current pulse, and a high power laser array for converting said high current pulse received from said communication unit semiconductor operation optically activated into a high power optical pulse to be emitted. The energy storage unit has an energy storage capacitor having a non-uniform microstrip structure with very low impedance.

Description

p

  BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a laser diode control and more particularly to a pulsed laser diode control , with high pulse repetition frequency and high power.

Description of the prior art:

  In general, laser commands are mainly classified as commands for gas lasers, for monolithic lasers and for semiconductor lasers. Although generating a high output power, gas lasers and monolithic lasers are bulky, heavy and expensive. In addition, they show a deterioration in yield. Furthermore, semiconductor lasers are compact, light, inexpensive and very efficient. By virtue of such advantages, the use of semiconductor lasers has recently experienced an increasing evolution.

  The conventional high power pulsed laser control has a very high circuit impedance.

  Due to the significant impedance mismatch between the control circuit (high impedance) and the laser network (extremely low impedance), a large part of the electrical energy is lost in the form of heat instead of being used for operate the laser network. However, for the operation of the high power semiconductor laser, it must be supplied with a current greater than the threshold level. Therefore, the energy lost in the form of heat must be compensated by increasing the pulse bias voltage.

  Therefore, the conventional high power pulsed control is designed with very high maximum power. This requires an even higher power semiconductor switch.

  In conventional high power pulsed control, when the energy lost is increased, the maximum power required of the semiconductor switch increases sharply. As a result, the performance of the laser control such as rise and fall times, pulse width, and pulse repetition frequency (PRF) of the output laser pulse deteriorates rapidly, while greatly increase the size and weight of the order.

  A critical parameter in the operation of the semiconductor laser diode is the level of current supplied. At low current levels, i.e., below the threshold current level, the laser diodes generate some spontaneous emission without laser output (laser light). When the current level increases, the laser diodes cross a threshold beyond which the population in the middle of the laser diode reverses and the laser effect begins.

  Therefore, below the threshold current, very little laser light is emitted and its emission efficiency is very low. Once the current level crosses the threshold, the light output increases strongly.

  High power laser diodes, called laser diode strips or laser arrays, are produced by manufacturing a large number of laser diodes on a single substrate. The laser output power level is proportional to the number of laser diodes in the laser array. Obvious advantages of this manufacturing technique are low manufacturing cost, mass production, miniaturization, and high reliability. The disadvantage is an extremely low device resistance in the on state (much less than 1 ohm).

  Because laser arrays are manufactured by connecting many direct polarized pn junction devices, i.e., laser diodes, in parallel, the on-state resistance of laser arrays decreases as the number of laser diodes in the network increases. Typically, the on-state resistance of high power laser arrays is in a range from a few ohms to less than 0.01 ohm. Meanwhile, when the output power level of the laser arrays increases (the number of laser diodes in the array becomes larger), the threshold current level for these lasers increases sharply.

  The modulation method for the operation of a high power pulsed laser is direct modulation in which the laser light is modulated by controlling the flow of current in the laser array. For the operation of a pulsed laser, with high pulse repetition frequency and high power, a very high current pulse with high pulse repetition frequency must be generated by the laser control and delivered to a load of extremely low impedance (laser network).

  The performance of conventional high power pulsed laser controls is essentially dependent on the capabilities of high power semiconductor switches (such as silicon controlled rectifier (SCR), power field effect transistor (power FET), insulated gate bipolar transistor (IGBT) and the bipolar power transistor).

  Conventional high power pulsed laser control, using a circuit topology in which the high voltage capacitance is pulsed polarized and the electrical energy is then discharged by switching the power semiconductor switch, has a very high circuit impedance. Due to the significant mismatch between the control circuit (high impedance) and the laser network (extremely low impedance), a large part of the electrical energy is lost in the form of heat instead of being used to operate the laser network. The amounts of heat generated are so large that a fan must be installed to remove this heat from the control. In addition, (since the operation of a laser network requires a certain level of current), the lost energy must be compensated by increasing the bias voltage.

  As the maximum allowable power of the conventional laser control increases, other performance (such as rise and fall times, pulse width, and pulse repetition frequency of the output laser pulse) laser control deteriorate rapidly, while the size and weight of the control greatly increase.

  As a result, conventional high-power pulsed commands are heavy and very bulky, compared to laser networks, and their performance such as rise time, fall time and pulse repetition frequency are severely limited.

  Besides the maximum peak laser output power, other important and critical parameters of the pulsed laser control are the modulation speed (high pulse repetition frequency), pulse width, efficiency, weight and compactness. . There is no pulsed laser control available producing a high peak power light pulse at a high pulse repetition frequency while maintaining high efficiency, light weight and compactness.

  SUMMARY OF THE INVENTION An object of the invention is therefore to provide a high power pulsed laser diode control maintaining high efficiency, low weight and compactness by implementing an energy storage capacity of a structure. non-uniform microstrip with low impedance.

  According to the present invention, this object can be achieved by a semiconductor laser control comprising control means for controlling an input control signal, electrical power and pulse charge conditioning means for receiving said control signal controlled in said control means and thereby generate electrical energy, energy storage means for receiving said electrical energy from said electrical power conditioning and pulse charge means and for storing the electrical energy received, source means trigger and control light for generating low power optical laser light when the electrical energy has been stored in the energy storage means, semiconductor switching means optically activated to convert the electrically charged capacitance energy into a high current pulse, and a rese a high power laser for converting said high current pulse received from said optically activated semiconductor switching means into a high power optical pulse to be emitted, wherein said energy storage means comprises a storage capacity of energy with very low impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

  Other objects and aspects of the invention will appear in the light of the description of embodiments given below with reference to the appended drawings in which: - Figure 1 is a block diagram of a laser diode control according to the present invention; - Figures 2a and 2b are respectively a plan view and a sectional view illustrating an energy storage capacity having a non-uniform microstrip structure in the form of a fan; - Figures 3a and 3b are respectively a plan view and a sectional view illustrating an energy storage capacity having a non-uniform microstrip structure in the form of concentric bands; - Figure 4 is a block diagram illustrating cooperation between the energy storage capacity according to the present invention and a laser network; and - Figures 5a to 5c are respectively graphs illustrating sequences of the flow of energy in this command.

  DETAILED DESCRIPTION OF THE EMBODIMENTS

PREFERRED

  FIG. 1 is a block diagram of a compact, high repetition frequency and high power pulsed laser diode control according to the invention, as shown in FIG. 1. The pulsed laser diode control comprises a control unit control 100, an electrical power and impulse load conditioning unit 200, an energy storage unit 300, a light source and control unit 6 - trigger 400, an optically activated semiconductor switching unit 500 , and a high power laser network 600.

  The control unit 100 monitors a signal input from an input stage and sends the monitored signal to the electrical power and impulse load conditioning unit 200. The control unit 100 applies a signal to the 400 trigger light source and control unit.

  The electrical power and impulse load conditioning unit 200 conditions the primary electrical power coming either from an alternating power line or from a battery, by means of the signal from the control unit 100 and then sends a certain energy to the energy storage unit 300.

  The electrical power and impulse load conditioning unit 200 also sends some energy to the trigger light source and control unit 400.

  The energy storage unit 300 comprises a low impedance capacitor having a non-uniform microstrip structure and stores the energy received from the electrical power and pulse charge conditioning unit 200.

  The trigger light source and control unit 400 includes a trigger light source for emitting low (or medium) power optical laser light while being controlled upon receipt of the control signal from the unit 100 when the energy storage in the energy storage unit 300 is completed.

  The optically activated semiconductor switching unit 500 belongs to the types of optically activated switches such as thyristors, p-i-n switches, large AsGa switches, large Si switches, large InP switches, etc.

  The switching unit 500 is switched to a mode passing through the light received from the trigger light source and control unit 400 to convert the capacitively charged electrostatic energy into the high current pulse.

  When the optical light from the optical trigger source and control unit 400 is introduced into the optically activated semiconductor switching unit 500, the capacitively charged electrostatic energy is converted into a pulse high current. Then the high power laser array 600 produces high power optical pulses.

  The proposed high-power pulsed control circuit uses innovative circuit elements and a radically different operating principle. Rather than using high-voltage capacitance and a high-power semiconductor switch to modulate laser arrays, the non-uniform microstrip structure used as energy storage capacity provides design flexibility, high circuit efficiency , low weight and compactness. In particular, with this microstrip structure, the design of an energy storage capacity having a very low circuit impedance becomes very easy. When using the laser diode triggered semiconductor switch, the critical limitations of power semiconductor switches such as slow rise and fall times, low pulse repetition frequency and large pulse width are easily overcome. Successful combination of low impedance energy storage capacity and low (or medium) power laser diode triggered semiconductor switch results in compact pulsed laser diode control capable of generating peak power light pulses high with narrow pulse width at high pulse repetition frequency.

  The energy storage unit 300 is made of non-uniform impedance microstrip structures such as fan-shaped or concentric stripe structures, as shown in Figures 2a and 2b, and 3a and 3b, respectively. . The capacitance and the characteristic impedance of the microstrip are given by the following relationships: C =% o & A / t, Farad (F) Zo = (377 xt) / (xW), ohms (Q) - 8 - o, respectively, C is the capacitance, A is the electrode surface of the capacitance, cO is the permittivity of the vacuum, t is the thickness of the dielectric medium, & is the dielectric constant of the dielectric medium, Zo is the characteristic impedance of the microstrip and w is the width of the electrode of the microstrip.

  By exploiting the geometric effects of the non-uniform microstrip structure, an energy storage capacity is achieved with a very low impedance.

  When electrical energy is transferred from the low impedance capacitance to the high power laser network, most of this energy is used to operate the laser network. Therefore, the low impedance capacitance greatly reduces the energy loss during the energy transfer from the capacitor to the laser array.

  The main function of the energy storage unit 300 is to temporarily store electrical energy in the form of electrostatic energy.

  However, when the energy blocking switch is switched to on mode, the boundary condition of the energy storage capacity on the contact area with the switch is changed from the open state to the closed state. As soon as the boundary condition is changed, the electrostatic energy stored in the capacitor becomes a traveling wave and begins to flow to the charge through the switch.

  In particular, when the energy storage capacity is connected in the appropriate load impedance, as shown in Figure 4, the latter acts as a transmission line. Instead of having discharged waveforms with an RC time constant decay curve, current pulses are produced with short rise and fall times. The width of these pulses is approximately the round-trip wave transit time of the energy storage capacity. And the amplitude of the current pulse is larger than the current amplitude obtained from the uniform impedance microstrip because of the gain factor associated with the impedance transformation.

  The amplitude of the current generated is given by the following relation: - 9 - I = {(g / (Ron + Rm)} xV {(Ron + Rm) / (Zin + Ron + Rm)}, Ampere (A) o , respectively, g is the gain factor due to the impedance transformation between the characteristic interior and exterior impedances of the non-uniform microstrip structure (the value of the coefficient g is in a range between 1 and 2), Rm is l adapted external impedance including the impedance in the on state of the laser network), V is the pulse polarized voltage, Zin is the internal characteristic impedance of the microstrip, and Ron is the impedance on the state of the semi switch -driver. In an ideal case of suitable impedance, in which Ron is negligible and Zin and Rm are identical, the current pulses generated thus become: I = (g / Rm) x (V / 2), Ampere (A) This is almost equivalent to the well-defined case of a suitable uniform microstrip, except for the gain factor g. The gain factor g provides a further improvement in circuit efficiency. Therefore, the use of the microband structure of low impedance, non-uniform, as energy storage capacity, leads to an extremely high efficiency of the circuit.

  The operation of the command begins with the sending of the operating instruction to the control unit 100 which undertakes a series of actions. First, the electrical power conditioning and impulse charge unit 200 is activated. The primary power coming either from an alternating power line or from a battery, is conditioned by the electrical power conditioning unit and impulse charge 200 and used to impulse charge the capacity of the storage unit of energy 300. At the instant when the pulse polarization voltage of the capacitance reaches the peak voltage, the triggering light source and control unit 400 is activated by the control signal coming from the control unit 100.

  - 10 - Trigger light source and control unit 400 generates an optical pulse with fast rise time at a very high pulse repetition frequency and sends it to the optically activated semiconductor switching unit 500.

  The optical pulse generated from the trigger light source and control unit 400 is coupled into the fiber optic strand, delivered through the optical fiber strand, and used to activate the semiconductor switching unit 500.

  After that, the trigger optical light passes through the active area of the semiconductor switch.

  Once the optical trigger light has entered the active area of the semiconductor switch, it produces a sufficient number of electron-hole pairs generated per photon so that the state of the switch changes from a fully open state (not conductor) to a completely closed state (conductor).

  When the switch is switched to on mode, the electrostatic energy stored in the capacity of the energy storage unit 300 is discharged in the form of narrow current pulses. As the current pulses, well above the threshold level, are flowing over the laser array 600, pulses of high power laser light are generated with rapid rise and fall times.

  The graphic sequences of the energy flow of this command are given in FIGS. 5a, 5b and 5c.

  When the semiconductor switch is triggered, the optical pulse shape generated is very similar to the pulse shape of the control current, except that the rise time of the output laser pulse is faster than the rise time of the control current pulse. However, the width of the current pulses generated is determined by the width of the optical trigger pulses as well as by the wave transit time in the energy storage capacity.

  In the case of a well-adapted impedance, the pulse width of the output laser light will be approximately equal to the round-trip wave transit time in the energy storage capacity, as expressed in the equation. following: - il - po = 2 x eY x (Yout-Yin) / (3 x 1010) seconds In the case of a large impedance mismatch, the pulse width of the output laser light will be very similar to the pulse width of the optical trigger pulse. This is mainly due to the multiple reflections of the traveling wave, caused by the highly mismatched impedance.

  For the power semiconductor switch, as the maximum power of the power semiconductor increases, both the rise time and the fall time of the switch slow down and the switching time in the on state increases. As a result, the pulse repetition frequency performance of the power semiconductor switch decreases rapidly while the maximum power of these switches increases slowly.

  Rather than producing a current pulse by means of the power semiconductor switch by generating an optical pulse with fast rise time from the low (or medium) power laser diode and using it as a trigger light, the semiconductor laser controller is capable of generating high current pulses with rapid rise times.

  Consequently, the geometric effect of the non-uniform microstrip structure allows the design of a compact energy storage capacity having a very low impedance. This very low impedance capacity significantly reduces energy loss during energy transfer from the capacity to the laser network.

  Unlike the conventional high power pulsed laser control, the resulting high efficiency pulsed laser control eliminates the need for a high power power supply and a heat exhaust fan.

  As shown in the description above, the present invention provides a high power pulsed laser drive having an energy storage capacity having a non-uniform microstrip structure with very low impedance. As a result, it is possible to significantly reduce the loss of electrical energy from the laser array while maintaining high efficiency, low weight and compactness.

  Although the preferred embodiments of the invention have been disclosed for purposes of illustration, those skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit of the. invention as disclosed in the appended claims.

- 13 -

Claims (2)

  1. Semiconductor laser control comprising control means for controlling an input control signal, means for conditioning electrical power and impulse load to receive said controlled control signal in said control means and thus generate a electrical energy, energy storage means for receiving said electrical energy from said electrical power and pulse charge conditioning means and storing the electrical energy received, trigger and control light source means for generating laser light low power optics when the electrical energy has been stored in the energy storage means, optically activated semiconductor switching means for converting the capacitively charged electrical energy into a high current pulse, and a laser array of great power to convert said co pulse a high value received from said optically activated semiconductor switching means into a high power optical pulse to be emitted, wherein said energy storage means comprises an energy storage capacity having a very low impedance.   2. The semiconductor laser controller of claim 1, wherein said energy storage capacity has a non-uniform impedance microstrip structure.