The invention is a method and circuit for shaping laser output pulses from a laser resonator. A Q-switch and polarizer act in combination to control the loss to the resonator and provide the laser output representative of such loss. An optical diode senses the level of the output pulse and provides an output signal that when amplified is used to control voltage from a supply to provide a control signal which is applied to the Q-switch to control the shape of the output pulse by adjusting its length

Barnes, Norman P.

English

NASA Technical Reports Server

P R I N T  F I G .  2 
N O T I C E  
c 
41 
T h e  i n v e n t i o n  d i sc losed  i n  t h i s  d o c u m e n t  r e s u l t e d  from 
research i n  a e r o n a u t i c a l  a n d  space a c t i v i t i e s  performed u n d e r  
p r o g r a m s  of t h e  N a t i o n a l  A e r o n a u t i c s  a n d  Space A d m i n i s t r a t i o n .  
The i n v e n t i o n  is owned by NASA a n d  is ,  therefore ,  a v a i l a b l e  for 
l i c e n s i n g  i n  a c c o r d a n c e  w i t h  t h e  NASA P a t e n t  L i c e n s i n g  
R e g u l a t i o n  ( 1 4  Code of Federal  R e g u l a t i o n s  1 2 4 5 . 2 ) .  
T o  e n c o u r a g e  commercial u t i l i z a t i o n  of NASA-owned i n v e n t i o n s ,  
i t  is  NASA p o l i c y  t o  g r a n t  l i c e n s e s  t o  commercial c o n c e r n s .  
A l t h o u g h  NASA e n c o u r a g e s  n o n e x c l u s i v e  l i c e n s i n g  t o  promote 
c o m p e t i t i o n  a n d  a c h i e v e  t h e  w i d e s t  p o s s i b l e  u t i l i z a t i o n ,  NASA 
w i l l  c o n s i d e r  t h e  g r a n t i n g  of a l i m i t e d  e x c l u s i v e  l i c e n s e ,  
p u r s u a n t  t o  t h e  NASA P a t e n t  L i c e n s i n g  R e g u l a t i o n s ,  when s u c h  a 
l i c e n s e  w i l l  p r o v i d e  t h e  n e c e s s a r y  i n c e n t i v e  t o  t h e  l i c e n s e e  t o  
a c h i e v e  ear ly  prac t i ca l  a p p l i c a t i o n  of t h e  i n v e n t i o n .  
A d d r e s s  i n q u i r i e s  a n d  a l l  a p p l i c a t i o n s  for  l i c e n s e  for  t h i s  
i n v e n t i o n  t o  NASA P a t e n t  C o u n s e l ,  L a n g l e y  Research C e n t e r ,  Mail 
Code 279 ,  L a n g l e y  S t a t i o n ,  Harnpton, VA 23665.  Approved  NASA 
forms f o r  a p p l i c a t i o n  f o r  n o n e x c l u s i v e  o r  e x c l u s i v e  l i c e n s e  a re  . 
a v a i l a b l e  from t h e  a b o v e  address .  
- 
L a R C  
--_ . __ -- ~ 
(NASA-Case-LAR-14203-1) YCTHOO A’4O C I R C U I T  N BY-2 a ti 17 
FOR S H A P I N G  LASFR OUTPUT PUCSFS Patent 
Appl ication (NASA.  L d n y l e y  Qest‘arch 
C e n t e r )  9 p C I i C l  ZUE Uncl as  
6 3 / 3 6  0?2Qt347 
. .  
https://ntrs.nasa.gov/search.jsp?R=19890019446 2020-03-11T17:48:51+00:00Z
AWARDS ABSTRACT 
NASA CASE NO. LAR 14203-1 
METHOD AND CIRCUIT FOR SHAPING LASER OUTPUT PULSES 
The invention is a novel method and circuit for shaping laser output pulses from a 
laser resonator by sensing and utilizing a portion of a laser output pulse to produce 
a control signal used to control the length and thus shape of the resulting laser 
output pulse by varying the loss of a Q-switch in the laser resonator. The invention 
is expected to find governmental and other commercial application. 
Inventor: Norman P. Barnes 
Address: 103 Stone Lake Court 
Tabb, VA 23602 
SS: 
Employer: NASA Langley Research Center 
Initial Evaluator: Anthony Jalink 
Serial No. : 071359,459 
Filed: May 31, 1989 
0
, 
I 
- 
I 
e
'
 
/- 
I I I I I I 
6 W F4 
I 
-5
-i 
I 
I I I 
l
'
h
 
I I 
qp I I 
- dl I I I 
LAR 14203-1 -1 - PATENT APPLICATION 
METHOD AND CIRCUIT FOR SHAPING LASER OUTPUT PULSES 
Cross-Reference to Related Applications 
5 This application is related to copending application serial number 9 
filed 
Evolution Time Interval of a Laser Output Pulse" (NASA Case 
, 1989 entitled "Method and Circuit for Controlling the 
NO. LAR 13772-1). 
10 Background of the Invention 
- 1. Field of the Invention 
The invention is a method and circuit for shaping laser output pulses and 
more particularly from a Q-switched laker device. 
- 2. Description of the Prior Art 
It is known to produce a high energy laser output having a long 
pulselength using a low energy laser oscillator that undergoes relaxation 
oscillations. While this method works, it is not very efficient. Essentially, only 
the energy in one of the relaxation oscillations is utilized while the rest is 
discarded. Other known methods such as the use of a conventional normal 
mode laser and an electro-optic switching system to slice out only a portion of 
the resulting pulse have proven to be even less efficient and less repeatable. 
15 
20 
25 Ob-iect and Summary of the Invention 
An object of the invention is to overcome the limitations of the prior art by 
providing a novel method and circuit for producing laser energy more 
efficiently and with a shaped or controlled output pulse length. 
30 
LAR 14203-1 -2- PATENT APPLICATION 
Brief Description of the Drawinas 
FIG. 1 illustrates a prior art method and circuit for achieving a long 
pulselength in a laser pumping device including a low energy oscillator which 
5 undergoes relaxation oscillations; and 
FIG. 2 illustrates a schematic of a preferred embodiment of the invention 
including an electro-optic element for producing long laser output pulses from 
a Q-switched laser device. 
10 Brief Description of the Invention 
Referring to the drawing, FIG. 1 represents a prior art laser resonator as 
generally designated by reference numeral 10 as including a totally reflecting 
mirror 1 1, an active or laser medium 12 such as to provide gain, a polarizer 
13, an electro-optic switch 14, and a second partial reflector or mirror 15. In 
operation while the active laser medium 12 is being excited, the electro-optic 
switch or Q-switch 14 is adjusted so that any radiation entering the portion of 
the laser resonator which contains the Q-switch will undergo a change from 
linear polarization to circular polarization on the first pass. On the return 
pass, the circular polarization is converted to orthogonally polarized radiation 
that is reflected by the polarizer 13. Thus, with the switch 20 closed there is 
no feedback and laser oscillation (voltage 14 applied) will not take place. 
In the operation of conventional prior art resonator 10, a pump source 
such as a flash lamp 16 excites the active laser medium 12. In this excited 
condition any photon simultaneously emitted along the axis 17 may cause 
laser action. However, in this configuration the polarizer 13 and Q-switch 14 
acting in combination prevent laser action permitting only radiation polarized 
along a particular axis to pass through the polarizer 13. The resulting linearly 
polarized radiation that passes through the polarizer 13 is converted to 
circularly polarized radiation in passing through the Q-switch 14 when 
activated by a suitable voltage 19 by closing switch 20. Some of this 
circularly polarized radiation is reflected by the partial reflector mirror 15 back 
15 
20 
25 
30 
LAR 14203-1 -3- PATENT APPLICATION 
through the Q-switch 14 in the reverse direction and converted to orthogonally 
polarized linear radiation which is rejected by the polarizer 13. Thus, no 
feedback results and no laser oscillation or action occurs. 
By deactivating the Q-switch 14 (removing the voltage 19 by opening 
switch 20) the described conversion of the polarization in the electro-optic 
switch 14 no longer occurs and laser oscillation or action starts and the 
resulting laser output pulse 18 is extracted through the partial mirror 15. The 
length of the resulting output pulse 17 is determined essentially by the photon 
lifetime of the resonator 10 and the number of times over threshold as is 
5 
10 known. 
In accordance with a preferred embodiment of the invention as shown in 
FIG. 2, the action of a Q-switch 14’ is controlled to provide a laser output 17’ 
having a selectively variable output pulse length. As in the prior art circuit, 
during excitation of an active laser medium 12’ there is no feedback of optical 
radiation in the resonator 10’ formed by a total mirror 11’ and a total mirror 
15’ which replaces the partial mirror 15 in FIG. 1. In the preferred 
configuration shown in FIG. 2, a quarterwave plate 1 8  also acts like a 
constantly activated Q-switch such as Q-switch 14’. Thus, when laser 
oscillation is desired a selected variable switching voltage 19’ applied to 
Q-switch 14’ negates part or all of the effect of the quarterwave plate 18’ 
depending upon the amount of applied switching voltage 19’. Radiation 
rejected by the polarizer 13’ passes through reflector 21’ as a laser output 
pulse 17’. This is in contrast to FIG. 1 in which the laser output pulse 17 
comes from the partial mirror 15. 
While the conventional Q-switch 14 is activated or fully switched on .or off 
by a full quarterwave voltage, a full quartetwave voltage 19’ is not applied to 
the Q-switch 14’ initially in accordance with the invention. Instead, it is 
applied over the duration d of pulse 17’. 
switch 14’ to sustain laser oscillation at a low level. As the active laser 
medium 12’ is depleted and the optical gain decreased, the voltage 19’ 
applied to the Q-switch 14’ increases due to the action of amplifier 24’. This 
15 
20 
25 
In practicing the method, sufficient voltage 19’ is initially applied to the Q- 
30 
LAR 14203-1 -4- PATENT APPLICATION 
provides more feedback and laser oscillation continues at a reduced optical 
gain level. As the optical gain decreases further, the voltage 19’ is increased 
further and therefore the level of feedback output 23’ is also increased to 
maintain laser output pulse 17’ at a constant level. This action provides the 
desired controlled and longer pulse length d of the output pulse 17’ which 
vanes linearly with the power of that portion of the output pulse 17’ received 
by photodiode 22’’ the output 23’ of which is amplified by amplifier 24’. The 
gain of the amplifier 24’ is adjusted so that the negative output 26’ offsets the 
positive voltage 27’ providing a variable positive output pulse 19’ which 
activates Q-switch 14‘ and thus output pulse 17’. The 19’ varies over the 
duration of pulse 17’ to maintain a constant output pulse 17’. 
As shown in FIG. 2, the desired and controlled switching voltage 19’ for 
operating the Q-switch 14’ is applied from a conventional amplifier 21’ to 
initiate and maintain laser oscillation. As this laser action continues, part of 
the resulting laser output signal 17’ is detected by a suitable photodiode 22’, 
the output 23’ of which is amplified by a conventional amplifier 24’ the output 
26’ of which is applied to the negative pin of the differential amplifier 21’. The 
positive voltage 19’ required to initiate’laser oscillations is applied to the Q- 
switch 14’ through the differential amplifier is from a suitable battery 27’ 
connected to ground 28’ and to the positive pin of differential amplifier 21’ 
through a suitable control switch 29’. If the level of the laser output 17’ 
becomes too large, the positive voltage 19’ applied to the Q-switch 14’ to 
facilitate laser oscillation is negated by the amplified laser output 26’. By 
negating the applied positive voltage, the resonator provides less feedback 
which reduces the laser output 17’. As the optical gain decreases, the level 
of the laser output 17’ also tends to decrease. This, in turn, provides less 
negative feedback 26’, which tends to increase the optical feedback in the 
resonator. Thus the laser action tends to increase. Through proper 
adjustment of the operating parameters of the feedback system, such as the 
initial setting of voltage 27’ and the gain of the amplifier 24’’ the duration d or 
length of laser output pulse 17’. 
5 
10 
15 
20 
25 
30 
LAR 14203-1 -5- PATENT APPLICATION 
While a preferred and alternative embodiments have been described in 
detail, numerous changes and modifications may be made without departing 
from the invention which is to be limited only by the appended claims. 
What is claimed is: 
5 
LAR 14203-1 PATENT APPLICATION 
6 
-9( 
METHOD AND CIRCUIT FOR SHAPING LASER OUTPUT PULSES 
Abstract of the Disclosure 
5 The invention is a method and circuit for shaping laser pulses 17’ in which 
a laser medium 12’ in a laser resonator 10’ that includes a Q-switch 14’ and 
polarizer 13’ which act in combination to control the loss of the resonator 10’ 
and provide the laser output 17’ representative of such loss. An optical diode 
22’ senses the level of the output pulse 17’ and provides an output signal 23’ 
that when amplified is used with a control voltage from a supply 29’ to provide 
a control signal 19’ which is applied to Q-switch 14’ to control the shape of 
the output pulse 17’ by adjusting its length. 
10 


Method and circuit for shaping laser output pulses

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server