JP3507566B2 - Laser pulse generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser pulse generator used for various optical radars and the like.

[0002]

2. Description of the Related Art A laser pulse generator used in various optical radars such as a berth speedometer has a large technology to generate a laser pulse with a narrow width in order to improve the detection resolution of distance and approach speed. Has become a problem. Conventionally, as a device for generating a laser pulse having a narrow width, a device utilizing an instantaneous breakdown phenomenon of a transistor is disclosed in Japanese Patent Laid-Open No. 117382/1989.

In the above laser pulse generator, as shown in FIG. 5, the positive electrode and the cathode of a capacitor C, which is charged by a positive DC power source via a resistor R and a bias voltage supply terminal DC, are connected to each other. Between the grounded anode of the laser diode LD to be driven, m diode-connected transistors (four in this example, Q1 to Q4) in which the emitter terminal and the base terminal are DC short-circuited are connected in series. , N driving units D (in this example, configured to supply a trigger signal from the input terminal IN to the base terminal of the final-stage transistor via the transformer T connected between the base terminal and the emitter terminal). 8) Parallel connection. The bias supply terminal DC has 5
A positive bias voltage of about 00volt is supplied.

This bias voltage causes the collector-emitter bias voltage of 125 volt, which is equally divided, to be generated in each of the four transistors Q1 to Q4 connected in series in each drive unit D. The collector-emitter bias voltage is set to a value close to the breakdown voltage V _BEC of each of the transistors Q1 to Q4.
The trigger voltage supplied to the base terminal of the final-stage transistor Q4 connected in series triggers breakdown between the collectors and emitters of the transistors connected in series, resulting in a momentary short-circuit state. Along with this, the positive charges accumulated in the capacitor C are discharged through the transistors Q1 to Q4 that are momentarily short-circuited and the low resistance laser diode LD, and a rapid discharge current flows through the laser diode LD. This rapid discharge current causes the laser diode LD to generate a sharp light pulse.

When the collector-emitter breakdown voltage of each of the m transistors connected in series is V _BEC , the voltage V applied to the capacitor C in each drive circuit D is V≈mV _BEC (volt). ... (1). The capacitance of the capacitor C in each drive circuit D is c
(Farads), the charge charge amount q is q≈mV _BEC c (Coulomb) ... (2) The total charge amount Q of the capacitors of all n driving circuits connected in parallel is Q = nq ≒ mncV _BEC (Coulomb) ··· (3).

Since the resistance value of the resistor R is set to a relatively large value, it flows into the laser diode LD through the resistor R and the transistors Q1 to Q4 connected in series during the discharging period of the capacitor C. The current has a small value that can be ignored as compared with the discharge current from the capacitor C. Therefore, if the current flowing through the laser diode is i (t), then ∫i (t) dt = Q≈mncV _BEC (coulomb) (4).

Referring to the equation (4), in order to increase the peak value of the current flowing through the laser diode LD, that is, the peak value of the light emission amount, a large number of transistors having a breakdown voltage as high as possible are connected in series / parallel. At the same time, it turns out that it is better to cause breakdowns at the same time as much as possible. If the capacitance value c of the capacitor C is increased, the accumulated charge amount Q is increased, but since the discharge time constant (cr) is increased, there is a limit to increase the capacitance value. The laser pulse generator shown in FIG. 5 can generate a laser pulse having a peak light amount of 100 W and a half width of about 5 nanoseconds (nsec).

The breakdown of each transistor lasts only for a very short period of time, and the total amount of charges flowing in each transistor is limited by the amount of charges charged in the capacitor. Therefore, thermal breakdown of each transistor is prevented. Effectively avoided. That is, when the discharge of the capacitor in each drive circuit is stopped after an extremely short time, the breakdown state of each transistor Q1 to Q4 in each drive circuit cannot be maintained, and the bias state just before the breakdown, that is, before the trigger pulse is input. Return to the initial state of.

[0009]

Recently, with the improvement in performance required for laser radar devices such as berth speedometers,
Narrower width, eg peak light intensity 100W, half width 1.5 n
There is a need for a device that can generate laser pulses of the order of sec. However, the conventional laser pulse generator shown in FIG. 5 has a problem that a laser pulse having such a narrow width cannot be generated.

Therefore, it is an object of the present invention to provide a laser pulse generator capable of generating a laser pulse having a narrow width. Another object of the present invention is to provide a small-sized and inexpensive laser pulse generator.

[0011]

According to the laser pulse generator of the present invention, in each driving transistor, one main electrode terminal is connected to the ground point, and the other main electrode terminal is a direct current via a resistor. It is connected to a voltage source and each control electrode terminal is connected to a common trigger pulse input terminal. In each drive transistor, the main electrodes are short-circuited for a short time by the trigger pulse received by the control electrode terminal. A driving capacitor is connected between one of the main electrode terminals of each driving transistor and the other electrode terminal of the laser diode whose one electrode terminal is grounded. According to a preferred embodiment of the present invention, it further comprises an accelerating capacitor connected in parallel with each of the driving transistors.

[0012]

Since all the driving transistors are connected in parallel with each other, an extremely low internal impedance state is realized during breakdown. As a result, the discharge time constant determined by the product CR of the capacitance value of the driving capacitor and the internal resistance value of the driving transistor is reduced, a steep discharge current flows through the laser diode, and a steep pulsed laser beam is generated. Light is generated. Also, because one of the main electrode terminals of the driving transistor such as the emitter is grounded, the transformer that was inserted between the main electrode terminal and the control electrode terminal in the conventional device is not required, and the entire device is compact. become. Along with this, the influence of the propagation delay time in the device is reduced, and high speed operation becomes possible. Hereinafter, the present invention will be described in more detail with reference to Examples.

[0013]

1 is a circuit diagram showing the configuration of a laser pulse generator according to an embodiment of the present invention. The laser pulse generator of this embodiment is a drive unit DR including a single drive bipolar transistor Q, a drive capacitor Cd, an acceleration capacitor Cs and a series resistor R.
However, 30 pieces are connected in parallel between the bias voltage supply terminal DC and the laser diode LD to be driven.

The laser diode LD to be driven has its anode terminal grounded and its cathode terminal connected to each drive unit DR. In each drive unit DR, the emitter terminal of the single drive bipolar transistor Q is grounded, and the base terminal is connected to the common trigger pulse supply terminal IN. Further, the collector terminal of the driving transistor Q is connected to the bias voltage supply terminal DC through the resistor R in terms of direct current, and is connected to the cathode of the laser diode through the driving capacitor Cd in terms of alternating current and is accelerated. It is grounded through the capacitor Cs.

The bias voltage supply terminal DC has 125 volt
A positive bias voltage of the order of magnitude is supplied, and this is applied as the collector-emitter bias voltage of each driving transistor Q. The collector-emitter bias voltage is set to a value on the verge of the breakdown voltage V _BEC of each driving transistor Q. This breakdown typically occurs due to an avalanche multiplication phenomenon of charge carriers such as electrons, but it may also occur due to other causes. Each driving transistor Q momentarily breaks down between the collector and the emitter all at once by the trigger pulse supplied from the trigger pulse generating circuit TPG to the respective base terminals via the common trigger pulse input terminal IN. It becomes a low resistance state close to the short circuit state. This low resistance state is simply referred to as “short-circuit state” in this specification.

Along with this, the positive charge accumulated in the positive electrode of the driving capacitor Cd flows to the ground point through the driving transistor Q which is momentarily short-circuited, and at the same time, is accumulated in the negative electrode. negative charges disappear is neutralized by the positive charge flowing through the laser diode LD from the ground. With the discharge of the driving capacitor Cd, a rapid discharge current flows in the laser diode LD, and the laser diode LD generates sharp pulsed laser light.

Since the emitter terminal of each driving transistor Q is grounded, it is possible to directly supply the trigger pulse to the base terminal without interposing a transformer between the emitter terminal and the base terminal. As a result, the overall size of the device is reduced and the effect of propagation delay time in the device is reduced,
High-speed operation becomes possible. Further, it is possible to avoid the deviation of the breakdown start time of each driving transistor due to the variation of the winding length of each transformer, and hence the expansion of the emission pulse width. In the circuit diagram of FIG. 1, the drive units DR are arranged one-dimensionally. However, in practice, in order to reduce the distance between the trigger pulse supply terminal IN and each drive unit DR as much as possible and remove the influence of the delay time, each drive unit DR is centered on the trigger pulse supply terminal IN. Two-dimensionally or three-dimensionally arranged closely.

The laser pulse generator of FIG. 1 in which the number of drive circuits DR was set to 30 was actually prototyped, and the current waveform flowing in the laser diode LD was observed with a sampling oscilloscope. Next, an equivalent circuit as shown in FIG. 2 was created so as to match the observed current waveform. However, the driving bipolar transistor Q, the driving capacitor Cd, and the accelerating capacitor Cs in FIG. 2 are each a single driving bipolar transistor, a driving capacitor, and an accelerating capacitor corresponding to 30 parallel-connected driving bipolar transistors. It is expressed by an element.

FIG. 3 shows the result of computer simulation of the current waveform i (t) flowing through the laser diode LD and the collector voltage waveform V (t) of the driving transistor Q in the equivalent circuit of FIG. . In this computer simulation, each element constant in the equivalent circuit of FIG. 2 is set to the following value. Voltage of constant voltage source Vo = 130 volt, inductance of coil L1 = 0.5 nH (nano Henry), resistance value of resistor r1 = 0.2
Ω, capacitance value of driving capacitor Cd = 150 PF, capacitance value of acceleration capacitor Cs = 300 PF, inductance of coil L2 = 1.5 nH, capacitance value of capacitor C2 = 100 PF, resistor r2 Resistance value = 1 Ω, resistance value of resistor r3 = 0.5 Ω, resistance value of resistor r4 = 10 Ω.

30 driving transistors Q connected in parallel
Since the resistance value of the internal resistance r1 is 0.2 Ω, the internal resistance of one driving transistor is 0.2 Ω × 30 = 6 Ω.
Becomes Therefore, in the conventional device, four driving transistors each having an internal resistance of 6Ω are connected in series, so that the total internal resistance is 6Ω × 4 = 24Ω. As a result, in the device of the present invention, the internal resistance of the driving transistor becomes so small that it hardly contributes to the discharge time constant. That is,
The discharge time constant is mainly determined by the driving capacitor Cd and the internal resistance of the laser diode (eg, the sum of the resistance values of the resistors r2 and r3 = 1.5Ω).

FIG. 4 shows the equivalent circuit of FIG. 2 in which only the accelerating capacitor Cs is removed in order to clarify the operation of the accelerating capacitor Cs constituting each drive circuit DR of FIG. A computer simulation of the current waveform i (t) flowing through the laser diode LD and the collector voltage waveform V (t) of the transistor Q when the same value as that of the computer simulation in the case of FIG. The results are shown.

Comparing the results of the computer simulations of FIGS. 3 and 4, the coil L1 of the equivalent circuit of FIG.
A series resonance occurs between the accelerating capacitor Cs and the accelerating capacitor Cs, and the collector voltage V (t) of the driving transistor Q inverts to a negative value accordingly. It turns out that the current waveform i (t) is obtained.

The structure in which the anode of the laser diode is grounded and the positive bias voltage is supplied to the driving npn transistor whose emitter is grounded has been described above. However, the cathode of the laser diode may be grounded and the negative bias voltage may be supplied to the driving pnp transistor whose emitter is grounded.

Further, the configuration in which the trigger pulse generation circuit TRG is directly connected to the trigger pulse supply terminal IN has been illustrated. However, a transformer in which one terminal of each of the primary winding and the secondary winding is grounded may be interposed between the pulse generation circuit TRG and the trigger pulse supply terminal IN.

[0025]

As described in detail above, in the laser pulse generator of the present invention, since all the driving transistors are connected in parallel with each other, an extremely low internal impedance state is realized at the time of breakdown, and the discharge is realized. It is possible to generate a steep pulsed laser beam due to the current.

Further, since one main electrode terminal of the driving transistor such as the emitter is grounded, as many transformations as the driving unit inserted between the main electrode terminal and the control electrode terminal in the conventional device are provided. This eliminates the need for a container and reduces the overall size of the device. Along with this, the influence of the propagation delay time in the device is reduced, and high speed operation becomes possible.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a laser pulse generator according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the laser pulse generator of FIG.

3 is a current waveform i (t) flowing in a laser diode and a collector voltage waveform V (t) of a driving transistor, which are obtained by computer simulation with respect to the equivalent circuit diagram of FIG.

4 is a current waveform i (t) flowing through a laser diode and a collector voltage waveform V (t) of a driving transistor obtained by another computer simulation with respect to the equivalent circuit diagram of FIG.

FIG. 5 is a circuit diagram showing a configuration of a conventional laser pulse generator.

[Explanation of symbols]

DR drive circuit Laser diode for LD drive Q drive bipolar transistor Cd drive capacitor Cs acceleration capacitor R resistor DC bias voltage supply terminal IN trigger pulse supply terminal TRG trigger signal generation circuit

Continuation of the front page (56) Reference JP-A-58-7941 (JP, A) JP-A-4-208581 (JP, A) Actually opened 62-34454 (JP, U) Actually opened 59-45952 (JP , U) Actual Kaihei 2-122462 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (3)

(57) [Claims] 1. A DC power supply, one main electrode terminal of each of which is connected to a ground point, each other main electrode terminal of which is connected to a DC voltage source via a resistor, and each control electrode terminal of which is connected. A plurality of driving transistors connected to a common trigger pulse supply terminal, which breaks down between the one and the other main electrode terminals by a trigger pulse received by the control electrode terminal, and shifts to a short-circuit state for a short time; A laser diode to be driven whose electrode terminal is grounded, and a plurality of driving capacitors connected between each of the other main electrode terminals of each of the driving transistors and the other electrode terminal of the laser diode, A trigger pulse supply circuit that supplies the trigger pulse to the common trigger pulse supply terminal. Ruth generator. 2. The laser pulse generator according to claim 1, further comprising an accelerating capacitor connected in parallel between the one and the other main electrode terminals of each of the driving transistors. 3. The laser pulse generator according to claim 1, wherein each of the driving transistors is arranged two-dimensionally in close contact with the common trigger pulse supply terminal as a center. .