Semiconductor laser modulating circuit

By using two or more series connected silicon controlled rectifiers a laser odulator is created which provides higher current, faster rise time and narrower pulses than can be provided by existing single or multiple silicon controlled rectifier circuits. In an example circuit using two such rectifiers, the first is gate triggered and the second is turned on by a rapid voltage increase across its anode and cathode.

BACKGROUND OF THE INVENTION 
This invention relates to modulated circuits. In particular it relates to 
laser modulators of laser diodes; and, even more specifically it relates 
to laser modulators of laser diodes capable of providing high current, 
fast rise time, narrow pulses. 
Previous circuits that were used to provide these results were limited by 
the fact that with the use of silicon controlled rectifiers, SCRs, the 
fastest SCRs available had low breakdown voltages. To increase the peak 
pulse current of such a circuit without increasing the rise time or pulse 
width required increasing the supply voltage. However, supply voltage 
could not be increased beyond the breakdown voltage of the SCR. Alternate 
circuits which could avoid the difficulty of breakdown voltage ran into 
the problem that by using multiple SCRs the turn on time was limited by 
the turn on time of the slower SCR. The overall turn on time and rise time 
were slower than single SCR circuits which had breakdown voltage 
limitations. 
The need for high current is so the laser diode is able to provide a 
stronger signal. The need for narrow pulses is to prevent over heating. 
The requirement for fast rise time is to allow electronic range gating by 
use of a laser diode. Without a fast rise time it is impossible to use 
laser diodes for electronic ranging of targets. Electronic ranging as used 
here refers to using the time of return of the leading edge of a pulse 
from a target to determine the range to the target, as in radar systems. 
SUMMARY OF THE INVENTION 
A relatively high voltage modulating circuit is developed to provide 
controlled current pulses to a given circuit. For purposes of illustration 
the circuit is shown and described driving a laser diode. It will 
hereafter be referred to as a laser modulator, however, it is equally 
applicable to any similarly modulated circuit. 
A laser modulator circuit using two or more SCRs avoids breakdown voltage 
problems encountered in single SCR triggered circuits. However, only one 
of the SCRs is triggered by a timing pulse. The second SCR is triggered or 
turned on by the breakdown voltage induced across its terminals when the 
first SCR is turned on by the triggering pulse. The voltage rapidly 
applied across the cathode and anode of the second SCR results in a large 
rate of change of voltage with respect to time, dv/dt. This causes the 
turn on time of the second SCR to be very fast. 
An object of the invention is to provide a laser modulator circuit for 
laser diodes which provides high current, fast rise time, and narrow 
pulses so that the laser diode can be used for electronic range gating.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a common circuit used for modulating laser diodes. Voltage source 
10, denoted by the B+ mark, provides a current flow through resistor 12, 
labeled recharge resistor. When SCR 14 is off, capacitor 16, which is tied 
to ground through diode 18, charges up. When capacitor 16 fully charged, 
current flow ceases and the circuit is ready to discharge. When a trigger 
signal is provided across terminals 20, SCR 14 is turned on and acts as a 
short in the circuit. Capacitor 16 can now discharge along the path 
indicated by arrows 22. This path passes through laser diode 24, and the 
current flow causes light to be emitted as indicated by arrow 26. In order 
to increase the peak pulse current of this circuit, larger voltages at 
point 10 are required. Larger voltages cause a breakdown of SCR 14. Thus 
the amount of signal that can be generated through laser diode 24 is 
limited. 
An attempt to solve this problem is shown in FIG. 2. Once again the input 
voltage 10 is indicated by the B+ point and once again a recharge resistor 
12 is shown with a capacitor 16. Components having the same reference 
numerals in different figures serve the same function each figure. The 
difference is that there are 2 SCRs labeled 28 and matching resistors 
labeled 30 connected in the circuit. The purpose of resistors 30 is to 
divide the input voltage 10 evenly across SCRs 28. Thus 1/2 B+ appears 
across each one. 
The circuit is triggered by applying a positive pulse to trigger-1 across 
points 20 and simultaneously another positive pulse to trigger-2 across 
points 32. These trigger pulses can be supplied by a transformer. Since 
the SCRs are in series, the circuit is not fully on until both SCRs are 
fully on. The circuit cannot turn fully on until the slower of the two 
SCRs turns fully on. Since the SCRs were triggered at the same time, the 
impedance of the faster SCR has not dropped to its lowest possible value 
when the slower SCR is turning on. This causes the slower SCR to turn on 
even slower. In other words, a residual moderate impedance in the faster 
SCR will increase the RC time constant of the discharge circuit, which 
will slow down the slower SCR even more. Thus it is difficult to provide a 
fast rise time in this circuit. The flow rise time in the circuit will 
keep laser diode 24 from being used as an electronic range gating device. 
This can also cause over-heating problems as the current is present for a 
longer time in laser diode 24. 
In both FIG. 1 and FIG. 2 recharging of capacitor 16 is achieved through 
the turning off of the SCRs shown by the current flow dropping to a very 
small value. What happens is that when capacitor 16 is fully discharged, 
the only current source left is due to voltage 10. The function of 
recharge resistor 12 is to limit the current to a value less than the 
holding current needed to keep the SCRs on. After SCRs 28 turn off, 
current provided by voltage source 10 begins recharging capacitor 16. 
FIG. 3 is a circuit diagram of the present invention which avoids the 
problems present in FIGS. 1 and 2. Once again the voltage source 10 is 
shown with a recharge resistor 12. Voltage source 10 and capacitor 16 will 
be of pre-determined values dependent on the pulse current desired. 
Recharge resistor 12 functions as a current control circuit for voltage 
source 10. The simplest current control circuit is provided by simply 
making resistor 12 as large as necessary to current below the holding 
current necessary for the SCRs as previously discussed. Another version of 
current control circuits which are possible in this position will be shown 
later. 
Resistors 30 again divide the voltage 10 evenly across the two SCRs shown. 
The SCRs in this circuit are identified by numbers 34 to distinguish that 
these SCRs are wired differently from those previously shown. The anode of 
SCR-1 is connected to the cathode and to the gate of SCR-2. The first SCR 
or SCR-1 is again turned on by a trigger pulse 20 as shown. When SCR-1 is 
turned on it now acts as a short. Point 36 becomes effectively tied to 
ground. Since SCR-1 is triggered alone, it turns fully on and its 
impedance drops to its lowest possible value before SCR-2 can react. Thus, 
the entire voltage B+ is applied across SCR-2 since point 36 is shorted to 
ground. The large change of voltage with respect to time or dv/dt is 
sufficient to provide an alternative mode for turning on SCR-2. The actual 
voltage at which the SCR turns on in this mode can be less than the 
breakdown voltage. However, the total voltage, B+, will be higher as its 
level will be determined by the current desired. The faster turn on time 
in this mode is due to the high value of dv/dt which takes a very short 
time to build up to the desired voltage level. 
It should be noted that SCRs have 2 trigger modes. The first is a positive 
signal to the gate, which is what was used in the previous circuitry 
shown. The second is a rapid increase in voltage across the cathode and 
anode, which is a significantly faster mode of turn-on for an SCR. Also 
note that by tying the gate and cathode of SCR-2 together, SCR-2 cannot be 
gate triggered by noise. SCR-2 still allows current to flow out of the 
gate after the SCR has been dv/dt triggered. This is the fastest possible 
configuration for turning on SCR-2. It should be noted that what in the 
circuit of FIG. 1 was a limitation, i.e. a breakdown voltage problem, is 
now resolved in FIG. 3 to be an advantage of the method of triggering 
SCR-2. By using SCR-2 in the faster turn on mode, only one trigger pulse 
is necessary to initiate the discharge of capacitor 16. 
FIG. 4 repeats the circuitry shown in FIG. 3 and is therefore labeled with 
the same numbers wherever appropriate. This circuit adds a transistor 38 
which is attached to a pulse source 40 which is synchronized to trigger 
source 20. In addition it should be noted that what has previously been 
identified as recharge resistor 12 is now resistor 42. This is another way 
of having current control circuitry between the voltage source 10 and SCRs 
34. Transistor 38 can be driven by a pulse 40 synchronous with trigger 
pulse 20 so it acts as an on-off switch between voltage source 10 and the 
rest of the circuitry. Pulse source 40 turns transistor 38 off during the 
trigger period and for an adequate time after the trigger period so the 
SCRs have time to turn off. Thus, by no longer having a minimum current 
always flowing from voltage source 10, voltage source 10 can in effect be 
disconnected from the circuit insuring turn off by SCRs 34 when capacitor 
16 current drops below the holding current. This addition of transistor 38 
can provide a faster recharge time on capacitor 16. This is because 
resistor 42 can now be a significantly smaller resistance than recharge 
resistor 12 which was used in the other embodiment. Thus once the SCRs 34 
are turned off, the lower resistance 42 provides a larger current into 
capacitor 16, which permits it to charge up faster. Decreasing the 
recharge time makes it possible to pulse laser diode 24 at a faster rate.