Emission spectra and cross-section spectra of neodymium laser ... - Core

Optical

and Quantum

Electronics

24 (1992)

591-601

Emission spectra and cross-section spectra of neodymium laser glasses J.

FURTHNER,

A.

PENZKOFER

Naturwissenschaftliche

Faku/tat

W-8400

Regensburg,

Germany

Received

7 August;

Amplified

spontaneous

revised

II -

Physik,

15 November;

emission

Universitat

accepted

Regensburg,

3 December

spectra and light amplification

1991

spectra of

some

N d : g l a s s rods (silicate glass Schott LG680, phosphate glasses Schott LG760 and 3+

Hoya LHG5) are measured by pulsed flashlamp excitation. The spontaneous emission distribution,

the stimulated

emission

cross-section

spectra

and the excited

state

absorption cross-section spectra are extracted. Excited state absorption prevents laser action around 1320 nm for the F 2 - l 4

4

3 /

1.

1 3 /

2 transition of N d

3 +

in the investigated glasses.

Introduction

Nd

3 +

d o p e d crystals a n d glasses are w i d e l y a p p l i e d solid-state laser materials [1-6]. F r o m

4

the F of

3

3 / 2

(X «

upper laser level o f the 4f electrons o f the N d 4

the

I

manifold

y

with

J = 15/2

1.32/mi), / = 11/2 (A «

(transition

3 +

ions transitions occur to a l l levels

wavelength

X «

1.8 / m i ) , J = 13/2

1.06 /mi) a n d / = 9/2 (X « 0.88 / m i , g r o u n d state). A n

energy level d i a g r a m is i n c l u d e d i n F i g . 8 [2]. T h e N d : glass lasers generally operate 4

o n the F

4

3 / 2

- I

1 1 / 2

t r a n s i t i o n e m i t t i n g a r o u n d 1.06 / m i where they have great i m p o r t a n c e as

h i g h p o w e r a n d short pulse solid-state lasers. L a s e r a c t i o n o f N d : g l a s s lasers o n the 4

F

4

3 / 2

- I

1 3 / 2

4

t r a n s i t i o n [7-9] (flashlamp p u m p i n g at r o o m temperature) a n d o n the

F

4

3 / 2

- I

9 / 2

transition ( r o o m temperature laser p u m p i n g [10] a n d l o w temperature flashlamp p u m p i n g [8, 11, 12]) has been reported. T h e luminescence l i n e w i d t h s o f the N d : glass laser transitions are rather b r o a d (Av « to 200 c m

- 1

100

) due to S t a r k s p l i t t i n g o f the i n v o l v e d levels a n d i n h o m o g e n e o u s b r o a d e n i n g

i n the glass m a t r i x [2-6]. L a s e r wavelength t u n i n g across the luminescence l i n e w i d t h is readily achieved b y i n s e r t i o n o f a t u n i n g element i n the laser o s c i l l a t o r [13-16]. T h e possible laser transitions a n d wavelength t u n i n g ranges depend o n the effective amplification cross-section d i s t r i b u t i o n o&(X) = & (X) em

— o {X) where cr QX

em

is the stimulated 4

emission cross-section a n d p{K U ) the 2

x

feedback light is amplified i n the r o d (cr ffW > 0), w h i l e i n regions o f p(X, U ) < p(K U ) e

the feedback light is attenuated (o&{k)

4. T h e o r e t i c a l

2

x

< 0).

relations

T h e theoretical relations between amplified spontaneous emission, light a m p l i f i c a t i o n ,

WAVELENGTH

X

(nm)

Figure

3 Normalized amplified spontaneous emission (

(

) f o r p h o s p h a t e laser g l a s s S c h o t t L G 7 6 0 . P u m p v o l t a g e U = 8 0 0 V. T h e c u r v e s i n t h e left a n d r i g h t

) a n d normalized spontaneous emission spectra

f i g u r e are e x p a n d e d v e r t i c a l l y b y 1 0 * a n d 5 * , r e s p e c t i v e l y .

spontaneous emission, stimulated emission cross-section, a n d excited-state cross-section are derived i n the f o l l o w i n g .

absorption

4 . 1 . Relation between spontaneous emission and amplified spontaneous emission T h e r e l a t i o n between amplified spontaneous emission p o w e r d i s t r i b u t i o n P A S E W spontaneous emission p o w e r d i s t r i b u t i o n P (X) is given by [4, 27, 29]

a

n

d

sp

exp{[

-

^s (A> P

ex

_

~ 1

/1MAr J

3 +

In E q u a t i o n 1 it is assumed that the N d excitation is constant across the r o d diameter, i.e. N (r z) = N (z) where r is the r a d i a l r o d coordinate a n d z is the axial r o d coordinate. N is the upper laser level p o p u l a t i o n n u m b e r density averaged over the r o d length, i.e. N = ^ A T ( z ) d z / / . N depends o n the flashlamp p u m p power, the F spontaneous emission lifetime and the amplification o f the spontaneous emission [27,29]. A n accumulation o f p o p u l a t i o n i n the t e r m i n a l laser levels is neglected. T h e gain factor G o f light a m p l i f i c a t i o n is u

9

u

u

4

u

u

u

3 / 2

G(X)

=

exp{K (A) m

a (X)]NJ} ex

(2)

Insertion o f E q u a t i o n 2 into E q u a t i o n 1 a n d rearranging the terms gives P

V

W

=

PASEW^Z!

(3)

Figure

4 Normalized amplified spontaneous emission (

(

) f o r p h o s p h a t e laser g l a s s H o y a L H G 5 . P u m p v o l t a g e U = 7 0 0 V. T h e c u r v e s in t h e left a n d r i g h t f i g u r e

) a n d normalized spontaneous emission spectra

are e x p a n d e d b y 1 0 * a n d 5 x , r e s p e c t i v e l y .

F o r weak a m p l i f i c a t i o n G emission P

s p

1 (low flashlamp p u m p voltage o r s m a l l cr ) the spontaneous eff

becomes e q u a l to the a m p l i f i e d spontaneous e m i s s i o n

P EAS

4.2. Relation between spontaneous emission and stimulated emission crosssection T h e r e l a t i o n between

the

4

(integration over a l l F

3 / 2

spontaneous e m i s s i o n d i s t r i b u t i o n E(X) = -> %

P (X)/jP (X)dl sp

sp

transitions) a n d the stimulated emission cross-section

d i s t r i b u t i o n