Room-temperature pulsed laser action was obtained in chromium-activated forsterite (Cr:Mg2SiO4) for both 532 and 1064 nm pumping. Free running laser emission in both cases is centered at 1235 nm and has a bandwidth of approximately 30 nm. Slope efficiency as high as 22 percent was measured. Using different sets of output mirrors and a single birefrigent plate as the intracavity wavelength selecting element tunability over the 1167 to 1268 nm spectral range was demonstrated. Continuous wave laser operation at room temperature was obtained for 1064 nm pumping from a CW Nd:YAG laser. The output power slope efficiency is 6.8 percent. The gain cross section is estimated to be 1.1 x 10 to the 19th sq cm. Spectroscopic studies suggest that the laser action is due to a center other than the trivalent chromium (Cr 3+), presumably the tetravalent chromium (Cr 4+) in a tetrahedrally coordinated site

Alfano, R. R.

Gayen, S. K.

Petricevic, V.

English

NASA Technical Reports Server

NONRADIATIVE RELAXATION AND LASER ACTION 
- IN TUNABLE SOLID STATE LASER CRYSTALS 
1 (NAG -1 -810) 
FINAL TECHNICAL REPORT 
submitted to the 
National Aeronautics and Space Administration 
For the period covering September 1, 1987 to December 31, 1988 
V. Petricevic, S. K. Gayen and R. R. Alfano 
Institute for Ultrafast spectroscopy and Lasers 
Departments of Physics and Electrical Engineering 
The City College of New York, New York 10031 
ibASa-CP-182620) N G P E A C I A l l V E  FELAJATZUN b189-19539 
AbG LASEIG ACTICE I I  T G E I B L E  S t I IC S'IATB 
lASE5 CbYSZALS E i o a l  ' I echn icz l  Iaport, 1 
Sep, 1S&7 - 2 1  Cec, 1988 ( C i t y  Call. of tbe Unclas 
C i t y  Goiv .  of HEW Icrt . )  5 F CSCL 206 G3/36 0195593 
https://ntrs.nasa.gov/search.jsp?R=19890010168 2020-03-20T04:10:25+00:00Z
A b s t r a c t  
Room-temperat’Ire pulsed l a s e r  act ion has been obtained i n  
chromium-activated f o r s t e r i t e  (Cr:Mg,SiO,) f o r  both 532-nm and 1064-nm 
pumping.  Free running l a s e r  emission i n  both cases  is centered a t  
1235 nm, and has bandwidth of - 30 nm. Slope eff ic iency a s  h igh  a s  
2 2 %  has been measured. Using  d i f f e ren t  s e t s  of output  mirrors and a 
s i n g 1  e birefr ingent  p l  a t  e as the i n  t r aca v i  t y w a vel en g t  h-se I ec  t ing  
element t unab i l i t y  over the  1167-1268 nm s p e c t r a l  range has been 
demonstrated. Continuous-wave laser. operat ion a t  room temperature 
has been obtained for  1064-nm pumping from a cw Nd:YAC l a s e r .  The 
output power s lope  e f f ic iency  is 6 . 8 % .  The gain cross  sec t ion  is 
est imated t o  be 1 . 1 ~ 1 0 ” ~  cm2. Spectroscopic s tud ie s  suggest t ha t  the 
l a s e r  ac t ion  is due t o  a ‘center ’  o ther  than the  t r i v a l e n t  chromium 
( C r  3t), presumably the  t e t r a v a l e n t  chromium (Cr*’) i n  a t e t r a h e d r a l l y  
coordinated s i t e .  
2. INTRODUCTION 
Recently, we have r epor t ed  room-temperature pulsed l a s e r  action 
i n  chromium-doped f o r s t e r i t e  (Cr:Mg,SiO,) fo r  both 532-nm and 1064-nm 
axci  t a t  ion. Laser emission is continuously tunable  over the 
1167-1268 nm s p e c t r a l  range, w i t h  a po ten t ia l  for  extending the 
t u n i n g  range beyond 1300 nm. T h i s  wavelength range is of grea t  
importance i n  op t i ca l  communication and eye-safe ranging. The l a rge  
bandwidth of t he  l a s e r  emission promises generation of u l t r a s h o r t  
pulses  through mode-locked operation. Large c r y s t a l s  of chromium- 
doped f o r s t e r i t e  can be grown by Czochralski method, which means t h a t  
t h i s  c r y s t a l  has a po ten t ia l  t o  be used a s  an amplifier medium i n  the  
near in f ra red .  Furthermore, the success fu l  cw l a s e r  operation of C r  
f o r s t e r i t e  has the  p rac t i ca l  implication t h a t  various types of l a s e r  
and ampl i f ie r  designs a r e  possible.  
On the o ther  hand, spectroscopy of chromium-doped f o r s t e r i t e  is 
very unusual and intriguing. I t  is the  f irst  chromium-activated l a s e r  
c r y s t a l ,  t o  our knowledge, where the t e t r a v a l e n t  chromium ion (Cr”) 
i n  a t e t r a h e d r a l  s i t e  is presumably responsible  f o r  l a s e r  act ion i n  
the near in f ra red .  
In  t h i s  paper we review the las ing  and spectroscopic  propert ies  
of t h i s  new and important tunable  s o l i d - s t a t e  l a s e r  c r y s t a l .  
- 2 -  
11. ACCOMPLISHMENTS 
C R Y S T A L  P R O P E R T I E S  A N D  SPECTROSCOPY 
F o r s t e r i t e  is a member of t h e  ol ivine f a m i l y  of c r y s t a l s .  A uni t  
c e l l  of f o r s t e r i t e  has four formula u n i t s  in an orthorhombic s t r u c t u r e  
of t h e  space group Pbnm. The uni t  c e l l ’ s  dimensions-are: a=4.76!, 
b=l0.221,  and ~35.991. The C r 3 +  ion s u b s t i t u t e s  f o r  the Mgz+ ion i n  
two d i s t i n c t  oc t ahedra l ly  coordinated s i t e s :  one ( M l )  w i t h  inversion 
symrnetpy (Ci) and the o t h e r  (M2) wi th  mirror symmetry ( C s ) .  The 
occupation r a t i o  of the two s i t e s  by t he  C r 3 +  ion is Ml:M2=3:2. 
Both the  s i n g l e  c r y s t a l s  of Cr:Mg,Si04 used fo r  spectroscopic  and 
l a s e r  experiments x e r e  grown by the Czochralski method a t  t h e  
Electronic  Mater ia ls  Research Laboratory of the M i t s u i  Mining and 
Smelting Co., Ltd. ,  Japan. The first c r y s t a l ,  r e f e r r e d  t o  a s  sample 1 
h e r e a f t e r ,  is a 9mmx9mmx4.5mm rectangular  paral le lepiped w i t h  t h P  
t h r e e  m u t u a l l y  orthogonal axes or iented along t h e  b ,  c ,  and a 
c rys t a l log raph ic  axes of the c r y s t a l .  It contains 0.04 a t  . d, of C r  
ions which is equivalent t o  a chromium ion concentration of 6 . 9 ~ 1 0 ’ ~  
ions/cm ’. The second c r y s t a l  (sample 2 )  is a 6mmx6mmx30mrn 
r ec t angu la r  pa ra l l e l ep iped  with t h e  t h r e e  mutually orthogonal axes 
or iented along t h e  a, b ,  c ,  c rys t a l log raph ic  axss of the c r y s t a l .  It 
contains 2 . 8 ~ 1 0 ”  chromium ions per cm’. The 6mmx6mm fztces of the 
c r y s t a l  were broad-band an t i - r e f l ec t ion  coated, such t h a t  the 
r e f l e c t i v i t y  over the 1050-1250 nm s p e c t r a l  range is l e s s  than 0.55.  
The room-temperature fluorescence and absorption s p e c t r a  of 
Cr:Mg,Si04 taken w i t h  sample 1 f o r  E( ( b  axis a r e  shown in Fig. 1 .  
2.4 7
1 ABSORPTION i 
I .8 
W 
g 1.2 
0 
cn 
m a 
0.6 
0.0 
W 
0 
2 i  
LL 
0 
300 900 1500 
WAVELENGTH (nm) 
Fig. 1. Absorption and f luorescence spectra of Cr:MgzSi04 a t  room 
temperature.  Both t h e  s p e c t r a  were taken f o r  El ( b  axis  and exc i t a t ion  
along a axis. The thickness  of the  sample along a ax i s . i s  4.5 mm. 
The room-temperature f luorescence is a broad band covering t h e  
wavelength range from 680-1400 nm. The absorption spectrum is 
character ized by two broad bands centered a t  740 nm and 460 nm,  
a t t r i b u t e d  t o  the * A z  -* ‘Tz and ‘A, + ‘T, t r a n s i t i o n s ,  r e spec t ive ly ,  of 
the  C r 3 +  ion. The broad, weak absorption band spanning the 850-1200 
nm range is s imi l a r  t o  t h e  one observed in chromium-doped CSCC’ and is 
a t t r i b u t e d  t o  t r a n s i t i o n s  between s t a t e s  in another ‘ cen te r ’ ,  
presumably t h e  t e t r a v a l e n t  C r ”  ion in a t e t r a h e d r a l  site. Th i s  
- 3 -  
a b s o r p t i o n  band o v e r l a p s  a s i g n i f i c a n t  p o r t i o n  o f  Cr:Mg2SiOs e m i s s i o n ,  
and  i n h i b i t s  l aser  a c t i o n  i n  t h a t  r e g i o n .  
The a b s o r p t i o n  and f l u o r 9 s c e n c e  spectra of t h e  ' c e n t e r '  i n  t h e  
near i n f r a r e d  s p e c t r a l  r e g i o n  a r e  shown i n  Fig. 2. In? worn- 
t e m p e r a t u r e  a b s o r p t i o n  s p e c t r u m  is a double-humped band c o v e r i n g  t h e  
850-1 200 nm w a v e l e n g t h  r ange .  The room-temperature f l u o r e s c e n c e  
s p e c t r u m ,  e x c i t e d  by 1064-nm r a d i s t i o n  from a c w  Nd:YAC laser e x t e n d s  
from 1000-1400 nm and peaks a t  1 1 4 0  nm. A t  l i q u i d  n i t r o g e n  
t e m p e r a t u r e  b o t h  t h e  s p e c t r a  show a s h a r p  zero-phonon l i n e  a t  1093 nm 
fol lowed by e l a b o r a t e l y  s t r u c t u r e d  s idebands .  The f l u o r e s c e n c e  
l ifetime is 15 p s  a t  r o o m - t e m p e r a t u r e  and  20 us a t  l i q u i d  n i t r o g e n  
temperature. 
m. 
r cn 
C 
t 
W :-FLUORESCENCE 
W 
0 
0 
Z - 
WAVELENGTH (nm) 
Fig. 2. Near i n f r a r e d  a b s o r p t i o n  and f l u o r e s c e n c e  spec t r a  of Cr:Ng,SiO, 
f o r  1064-nm e x c i t a t i o n  a t  room temperatu-9 ( s o l i d  l i n e )  and l i q u i d -  
n i t r o g e n  t e m p e r a t u r e  ( b r o k e n  l i n e )  f o r  E l  Ib  a x i s .  
' PULSED LASER ACTION 
The first laser  e x p e r i m e n t s  were c o n d u c t e d  w i t h  s a m p l e  1 i n  a 
s t a b l e  c a v i t y .  Details of t h e  c a v i t y  a r r a n g e m e n t  have been descr ibed  
elsewhere. The f u n d a m e n t a l  and t h e  s e c o n d  ha rmon ic  emissions from a 
Q-switched Nd:YAG laser o p e r a t i n g  a t  a 10-Hz r e p e t i t i o n  r a t e  were used  
for e x c i t a t i o n  of t h e  n e a r - i n f r a r e d  a n d  t h e  v i s i b l e  bands,  
r e s p e c t i v e l y .  P u l s e d  l aser  a c t i o n  was o b s e r v e d  fo r  bo th  t h e  1064-nm 
and t h e  532-nm pumping a t  or above t h e  r e s p e c t i v e  t h r e s h o l d s  of 
1.25 mJ and 1.37 m J  of absorbed ene rgy .  The a m p l i t u d e  and d u r a t i o n  of 
t h e  Cr:Mg2SiOs laser  p u l s e ,  as well a s  its d e l a y  v i t h  respect t o  t h e  
pump p u l s e ,  v a r i e d  with t h e  pUlSe-tO-pUlSe e n e r g y  f l u c t u a t i o n  of t h e  
pump p u l s e s .  However, f o r  s i r n i l a r  l e v e l  of e x c i t a t i o n  and w i t h i n  t h e  
time r e s o l u t i o n  of t h e  e x p e r i m e n t ,  t he re  was no appreciable  d i f f e r e n c e  
i n  t h e  d e l a y  be tween t h e  pump p u l s e  and t h e  o u t p u t  l aser  p u l s e  f o r  
t h e  two pump w a v e l e n g t h s .  The spectra of t h e  f r e e - r u n n i n g  
Crtforsteri te laser  f o r  both 1064-nm and  532-nm pumping peaked a t  
1235 nm and had FWHM of 30 nm and 27 nm, r e s p e c t i v e l y .  These f a c t s  
c l e a r l y  i n d i c a t e  t h a t  t h e  same ' C e n t e r '  is a c t i v e  i n  laser  a c t i o n  f o r  
b o t h  t h e  532-nm and 1064-nm e x c i t a t i o n s .  For  532-nrn pumping there is 
a fas t  t r a n s f e r  of e x c i t a t i o n  from the  l e v e l s  d i r e c t l y  pumped t o  t h e  
l a s i n g  l e v e l .  In  case of 1064-nm pumping, t h e  l a s i n g  l e v e l  is 
d i r e c t l y  p o p u l a t e d .  The o u t p u t  power s l o p e  e f f i c i e n c i e s  were 1.8 % 
fo r  1064-nm pumping and 1 . 4 5  f o r  532-nm pumping,  indicating h igh  
l o s s e s  in  the  cavity.  
To improve the l a s e r  performance by reducing the l o s s e s  an t i -  
r e f l ec t ion -coa ted  sample 2 was used, and ca r?  vas taken t o  ove r l ap  
t h e  pump beam and cavi ty  mode accura t e ly .  The sample was 
longi tudinal ly  pumped by t h e  fundamental 1064-nm. 10-ns  pulses  from a 
Q-switched Md:YAC l a s e r  i n  a cavi ty  s imi l a r  t o  t h a t  used e a r l i e r .  An 
output  power s lope  eff ic iency of 226, was obtained u s i n g  an  output  
coupler  having 87 % r e f l e c t i v i t y  over the l a s ing  region. 
, TUNABLE O P E R A T I O N  OF FORSTERITE LASER 
Tunable operation of C r : f o r s t e r i t e  l a s e r  has been obtained over 
t he  1 1  67-1 268 nrn s p e c t r a l  range. A s i n g l e  birefringent c r y s t a l l i n e  
qua r t z  p l a t e  was in se r t ed  i n  t he  cavi ty  a t  S r sws te r ’ s  angle  w i t h  
r e s p e c t  t o  the cavi ty  ax i s  a s  the in t r acav i ty  wavelength-selective 
element. Smooth t u n i n g  aver t he  1167-1268 nm s p e c t r a l  rang9 was 
obtained by r o t a t i n g  the  t i l t e d  p l a t e  about an axis perpendicular t o  
its s u r f a c e  and t h e  r e s u l t  is displayed in F i z .  3. 
a 
W 
W 
> 
1160 1200 1240 1280 
WAVELENGTH (nm) 
Fig. 3. The r a t i o  of C r : f o r s t e r i t e  laser output (EL) t o  t h e  absorbed 
pump energy ( E p )  as a funct ion of wavelength. The curve t o  t h e  r igh t  
was taken w i t h  an output  coupler having 98% r e f l e c t i v i t y  f o r  
1200-1300 nm range, and one t o  t h e  l e f t  w i t h  an output  coupler  w i t h  
r e f l e c t i v i t y  t h a t  varied from 9 9 %  a t  1150 nm t o  87% a t  1200 nm. 
A t  t h e  peak of t h e  tuning curve a t  1220 nm, t h e  output  laser energy is - 7 pJ/pulse f o r  an absorbed pump energy of 0.9 m J / p u l s e .  Similar 
t unab le  operat ion has been obtained f o r  532-nm pumping a s  well. 
. CH LASER OPERATION 
To obtain cw l a s e r  act ion i n  C r : f o r s t e r i t e ,  sample 2 was placed 
a t  t h e  cen te r  of a n e a r l y  concentr ic  c a v i t y  formed by two 5-cm radius  
mirrors .  The output  mirror had - 1 % transmission f o r  t h e  1175-1250 
nm range. The 1064-nm rad ia t ion  from a cw Nd:YAC l a s e r  was focused 
- 5  - 
by a 75-mm f o c a l  l e n g t h  l e n s  t o  l o n g i t u d i n a l l y  pump t h e  s a m p l e  a l g n q  
t h e  30;mm p a t h  l e n g t h .  The pump beam propaga ted  a l o n g  t h e  c a x i s  and 
was l i n e a r l y  p o l s r i z e d  a l o n g  t h e  b axis  of t h e  c r y s t a l .  The pump 
beam was chopped a t  a d u t y  f a c t o r  of 9: l  t o  reduce h e a t i n g  e f f?c ts .  
When t h e  pump beam was n o t  chopped,  t h e  C r : f o r s t e r i t e  l a s e r  o p e r a t e d  
a t  40% r educed  power,  i n d i c a t i n g  t h e  e f f e c t  d u e  t o  l o c a l  h e a t i n g .  
Quasi-cw laser o p e r a t i o n  was r e a d i l y  o b t a i n e d  f o r  pumping above 
t h e  l a s i n g  t h r e s h o l d  of 1.25 I./ o f  a b s o r b e d  power. The s p e c t r u m  of 
t h e  fnee - runn ing  cw C r : f o r s t e r i t e  laser  peaks a t  1244 nm and has  a 
b a n d x i d t h  o f  1 2  n n .  
125 7
0 1 2 3 4 5  
ABSORBED PUMP POWER (W) 
Fig. 4. 9 u t p u t  power of t he  cw C r : f o r s t e r i t e  l a s e r  as a f u n c t i o n  o f  
a b s o r b e d  pump power. 
The c w  o u t p u t  power of t h e  Cr:fors ter i te  laser  as a f u n c t i o n  of 
t h e  a b s o r b e d  pump power is d i s p l a y e d  i n  Fig. 4 .  The measured s l o p e  
e f f i c i e n c y  is 6 . 8 % .  The e x p e r i m e n t a l l y  o b t a i n e d  v a l u e s  of t h e  
a b s o r b e d  pump power a t  t h e  t h r e s h o l d  and  the  s l o p e  e f f i c i e n c y  
t o g e t h e r  w i t h  known m i r r o r  r e f l e c t i v i t i e s  have been used t o  estimate a 
number of key laser p a r a m e t e r s .  The r o u n d - t r i p  c a v i t y  l o s s  is 
estimated t o  be 1 2 . 7 5 .  The e f f e c t i v e  e m i s s i o n  c r o s s  s e c t i o n  is - 
l.1x10"gcm2, and  t h r e s h o l d  i n v e r s i o n  d e n s i t y  is 2 ~ 1 O ' ' c m - ~ .  Impor t an t  
laser and s p e c t r o s c o p i c  p r o p e r t i e s  of  C r : f o r s t e r i t e  a r e  summarized i n  
T a b l e  1. 
- 6 -  
TABLE -1 Spectroscopic a n d  13ser propert ies  of C r : f o r s t e r i t e  
Pro pert y -------------------------------------------------------- Val ue 
Major pump bands 
Fluorescence band 
Room temper a t  u r  e f 1 uor escence l i f e t i m e  
Cr-ion concentrat ion 
Lasing wavelength ( c e n t e r )  
Spe c t  r a1 bandxidt h 
Slope eff ic iency 
Tuning range 
Effect ive emission c ross  sec t ion  
850-1200 nm, 600-850 nm, 
and 350-550 nm 
680-11100 nm 
15 PS 
- 7x10'*ions/cm3 
1235 nm (pulsed)  
1 2 4 4  nm ( c w )  
- 30 nm ( p u l s e d )  
- 1 2  nm ( c w )  
- 7 2 5  (pulsed)  
6 .85 ! c w )  
1 1 6 7 - i 2 6 8  nn 
1 . 1  x i  0 - 1  9 c m z  
. 
I11 P u b l i c a t i o n s  a n d  P r e s e n t a t . i o n s  
T h e  p r e s e n t . a t . i o n s  o f  r e s e a r c h  f i n (  i n g s  i n  i n t e r n a t . i o n a 1  
c o n f e r e n c e s  a n d  p a p e r s  p u b l i s h e d  d u r i n g  t . h i s  r e p o r t i n g  p e r i o d  a r e  
l i s k e d  b e l o w :  
1. 
2. 
3 .  
4 .  
5.  
*. 
6. 
7 .  
8. 
9 .  
Spec t. r oscop  i c  P rope r t. i e s  o f T r i v a  l e n t  C h  r omi uin-Doped 
F o r s t e r i t e :  A N e w  P o t . e n t i a 1  T u n a b l e  Laser C r y s t . a l " ,  V .  
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M e e t i n g  o n  T u n a b l e  S o l i d  S t a t e  Laser T e c h n i c a l  D i g e s t .  S e r i e s ,  
Vol .  2 0 ,  ( O p k i c a l  S o c i e t y  of America,  W a h s i n g t o n ,  D.C. 1 9 8 7 )  p p .  
PD8-1-PD8-4. P r e s e n t e d  at. t h e  T o p i c a l  Mee t . i ng  on T u n a b l e  S o l i d  
St .ate L a s e r s ,  26-28 O c t o b e r ,  1 9 8 7 ,  W i l l i a m s b u r g ,  V i r g i n i a .  
"Laser A c t . i o n  i n  Chromium-Doped F o c s t e r i t e " ,  V. P e t r i E e v i d ,  S.K. 
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t h e  I n t . e r n a t i o n a 1  C o n f e r e n c e  on Lasers  ' 8 7 ,  7 - 1 1  December ,  1 9 8 7 ,  
L a k e  T a h o e ,  N e v a d a .  
" L a s e r  A c t . i o n  i n  Chromium-Doped F o r s t e r i t . e " ,  V .  P e t . r i E e v i d ,  S.K. 
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Nonradiative relaxation and laser action in tunable solid state laser crystals

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