The circuit described is capable of generating high‐current (2–50 A), fast‐rise‐time (10 nsec), square‐wave pulses into a 50‐Ω load. This circuit may be used for driving injection lasers at high repetition rates (up to 1.5 kHz) when connected to coaxial cables

Armstrong, D. R.

Katzir, A.

Yariv, A.

English

Caltech Authors - Main

Downloaded 24 May 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp
1127 Notes 
FIG. 2. Top end plate of pump show-
ing installation of deflector plate (A) 
and impeller bar (B). 
approximate plot of flow rate as a function of head rise at an 
operating voltage of 4.5 V de. The flow rate and maximum 
head rise appear to be directly proportional to the square 
of the applied voltage. The unit has been tested successfully 
to an upper limit of 12 V de but is currently being operated 
at 4.5 V de to reduce waste when filling small cold traps. 
The unit described here has a simple and compact design. 
The end plates for the pump are made from available 
printed circuit board material (G-10 epoxy glass). These 
were cut into 3.81-cm-diam circles using metal shears. The 
dimensions given in Fig. 2 are for a motor sold by Radio 
Shack (catalog No. 273-310). The motor is rated 30 000 rpm 
at 12 V de. It is approximately 2.5 em long and 2.5 em in 
diameter. The bearing material is Oilite. If a substitute 
motor is used, particular attention must be given to the 
brushes, as these must carry about five times more current 
than required in normal usage. The only part that requires 
particular care is the manufacture of the aluminum impeller 
bar (2.22X0.63SX0.317 em). The shaft hole should be 
drilled with a good # 4 7 bit in a drill press. This will ensure a 
good press fit on the motor shaft. The hole should be care-
fully located to ensure proper balance. The outer pump 
casing and the deflector plate were cut from a duplicator 
fluid can. Thin copper or brass may also be used. A 1.587 
X 10.149 em strip, fabricated into a cylinder of 3.180 em 
diam with 0.159 em overlap at the ends, constituted the 
pump casing. The deflector plate consisted of a 0.952 
XS.319 em strip bent to 1.27 em radius. The motor is 
pressed into the upper end plate and soft soldered. Using 
FIG. 3. Configuration of pump 
end plates (omit 0.952-cm hole 
for the lower plate). 
1.2 CM DIAM 
0.6 CM 
1127 
0.952 CM DIAM 
a vise as a press, the impeller is then pressed onto the motor 
shaft. The deflector plate and the pump casing are soldered 
to the upper end plate (as shown in Fig. 3). The lower end 
plate is then soldered in place, taking care to align the inlet 
hole with the motor shaft. The delivery tube is now inserted 
into the outlet and glued with low-temperature epoxy. Our 
model operates on 4-6 V de, drawing approximately 5 A in 
normal operation. A thin-walled, stainless-steel delivery 
tube with the power wires wound around it should decrease 
the heat flow. This alteration should allow the pump to 
remain submerged with little loss of LN2. This would also 
reduce the strain on the motor due to continual exposure to 
extreme temperature changes. Cavitation appears to occur 
at all normal submergences, the flow rate increasing almost 
linearly with the depth of submergence. The unit will, how-
ever, continue to operate as long as the pump housing 
remains submerged. 
The pump has proven to be both reliable and effective. 
Prolonged operation in a glass or other high-vacuum Dewar 
that has been emptied could lead to damage to the pump 
and Dewar due to excessive heating. This should not be a 
factor in metal Dewars that rely on low temperature to 
maintain their insulating properties. It is expected that this 
device will have other applications. While the unit described 
here works quite well for LN2, it is not recommended for 
use in liquid air or oxygen. The sparks produced at the com-
mutator would then constitute an explosion hazard. 
*Work supported by an East Texas State University research grant. 
Fast, high current, high repetition rate pulse generator for 
injection lasers 
D. R. Armstrong, A. Katzir, and A. Yariv 
California Institute of Technology, Pasadena, California 91125 
(Received 10 March 1975) 
The circuit described is capable of generating high-current (2-50 A), fast-rise-time (10 
nsec), square-wave pulses into a 50-!l load. This circuit may be used for driving injection 
lasers at high repetition rates (up to 1.5 kHz) when connected to coaxial cables. 
The commercially available injection laser sources may be 
operated efficiently at room temperature by high-current 
pulses (1Q-100 A) of perfect square-wave shape and short 
Rev. Sci. lnstrum., Vol. 46, No. 8, August 1975 
duration (1Q-200 nsec).1 There are applications where such 
pulses are to be delivered to injection lasers which are con-
nected to the end of a SO-n coaxial cable. The laser source 
Copyright © 1975 by the American Institute of Phvslcs 
Downloaded 24 May 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp
1128 Notes 1128 
+0 toiOOV +200V 
03 
VG20 
SCR1 
2N4442 
CHARGE 
Dz TRIGGER Rt 
IK IN4005 
Rz 
390 
43 
may be driven by a pulse generator which consists of a 
switch and a transmission line charged to a high voltage. 
When the line is discharged it produces a square-current 
pulse whose duration is proportional to the length of 
the line. Pulse generators which incorporate SCRs as 
switches2 are not fast enough, while those incorporating 
avalanche transistors3 •4 cannot hold the high voltages re-
quired. Instead, mechanical switches were tried, but their 
use was limited to low repetition rates. 1•5 
The Krytron (EG & G type KN-22) is a valuable device 
for the generation of short, high-current pulses. It is 
particularly useful where the pulses need to be transmitted 
over standard transmission lines because of its voltage 
rating, which is high enough to permit current levels of 
40-50 A into a SO-n load. 
For low repetition rates, it is sufficient to charge a trans-
mission line through a resistor, and discharge it through the 
Krytron. 6 However, a problem arises as the resistance is 
decreased in order to increase the repetition rate, because 
if the rate of rise of anode voltage is too high, the gas in the 
Krytron does not have time to de-ionize after a pulse, and 
the device remains in a state of continuous conduction, 
which rapidly destroys the cathode. 
A simple means of avoiding this difficulty is to charge 
the transmission line resonantly through a high-voltage pulse 
transformer, as shown in Fig. 1. With an appropriate choice 
of capacitor (C 1) and transformer (Tr), the line (TLr) may 
be charged to the rated voltage of the Krytron in a period 
of a few tens of microseconds, upon applying a trigger pulse 
to the silicon controlled rectifier (SCRr). The voltage to 
which the line is charged (and thus the amplitude of the 
current pulse) is proportional to the supply voltage of this 
stage, which need have a maximum value of less than 100 V. 
A similar circuit, with lower stored energy, may be used 
to trigger the Krytron. After the discharge, it is necessary 
to delay the charging of the transmission line for several 
hundred microseconds to allow the Krytron to de-ionize. 
With this type of circuit, it has been found possible to 
achieve repetition rates of up to 1.5 kHz. It is necessary, 
however, to keep the average current down to a level that 
will not overheat the Krytron. 
Some attention has to be paid, in designing the SCR 
circuits, to making sure that they will operate at the re-
Rev. Sci. lnstrum., Vol. 46, No. 8, August 1975 
II~ 
Ds 
IN4005 
SCRz 
2N4442 
Rs 
IK 
DISCHARGE 
TRIGGER 
FIG. 1. Krytron pulse generator. Manufacturers: 
V1, T1, T2-EG & G, Inc.; D,, D4 -Varo, Inc. 
FIG. 2. Current through a laser diode connected across a SO-Il coaxial 
cable. Horizontal scale-10 nsec/div; Vertical scale--10 A/div (10 sec 
exposure at 1.5 kHz). 
quired repetition rate without the SCRs remaining in the 
conducting state and, in effect, the circuit transfers this 
problem from the Krytron to the charging SCR, where it is 
more easily solved, and where the economic penalty is 
less severe. 
If the supply voltage for the Krystron triggering stage is 
adequate (about 200 V), the keep-alive electrode may also 
be operated from it. 
Typically we used a length of 5 m of RG-58 cable as a 
charging line (TL1), and the current pulse obtained at 1.5 
kHz is shown in Fig. 2. It is clear that the rise time is short, 
there is very little voltage reversal at the end of the pulse, 
and the jittering is rather low. When operating the circuit 
continuously at the highest rating (40 A and 1.5 kHz), the 
jittering becomes noticeable after about 45 min, and the 
rise time increases, but one can operate the circuit for 5-6 h 
before the Krytron burns out. 
Where higher currents are required, lower impedance 
transmission lines, such as two or more SO-n cables connected 
in parallel, may be used, with a correspondingly lower load 
resistance. 
1R. L. Carroll, Electronics 44 (No. 26), 59 (1971): 
•w. F. DeVilbiss and S. L. Klunk, RCA ApplicatiOn Note AN-4741 
(1971). 
3J R Andrews Rev. Sci. lnstrum. 45, 22 (1974). 
•J: V~nderwa!l,'W. V. Hattery, and Z. G. Sztanky, IEEE]. Quantum 
Electron. QE-10, 570 (1974). 
•EG & G Data Sheet KSSOO B-1 (1973). 


Fast, high current, high repetition rate pulse generator for

injection lasers

https://authors.library.caltech.edu/32696/1/ARMrsi75a.pdf

Fast, high current, high repetition rate pulse generator for injection lasers

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main