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Grazing incidence dye lasers with and

without intracavity lenses:

a comparative study

A. G. Yodh Y. Bai J. E. Golub and T. W. Mossberg

Harvard University, Physics Department, Cambridge,

Ma ssachusetts 02138.

Received 7 Septembe r 1984.

0003-6935/84/132040-03$02.00/0.

1984 Optical Society of America.

Tunable pulse dye laser technology has undergone rapid

development since its introduction.

1

15

Lisboaet al.

11

have

recently reported that insertion of a lens in the cavity of a

grazing-incidence pulsed dye lase r

12-15

results in output pulses

of higher power and narrower spectral width. They did not,

however, indicate the e xtent to which the lens should improve

dye laser performance or present systematic measurements

of actual observed improvement. We present here results of

an analytical and experimental study of this problem.

We restrict our atte ntion to the dye laser configuration of

Fig. 1(a), in which the cavity end mirror serves as the outp ut

coupler. Output taken through the end mirror has the ad

vantage of being relatively free of broad band spectral ba ck

ground at the expense of being relatively low in power

13

when

compared to laser output taken from the zeroth order of the

grating.

Generally speaking, th e linew idth of a dye laser is given by

an expression of the form

where is the linewidth in centimeters, is the angle be

tween the grating normal and light incident from the dye

cell,

Fig. 1. (a) Schematic of the grazing-incidencedye laser studied. (b)

Amplified broadband endwindowreflection propagating toward the

grating and the path (dashed lines) of single returning frequency

component. (c) Same as (b) except that the intracavity lens is in

place. Light at

frequency

which does not p ss

through

the dye

cell

is shownfor clarity. Thelens ispositioned approximatelyonefocal

length from the dyecelland s closeto the grating spossible.

isthe ang ular dispersion of the light returning from the g rating

when the grating operates in first order,

16

a is the groove

spacing on the grating, and

R

is the effective angular spread

of light returning from the gra ting which is amplified in the

dye cell. We discuss limiting expressions for

R

with and

without an intracavity lens.

Consider the case in which there is no intracavity lens.

When the grating is not filled with light, a single spectral

componen t of the light incident on the grating with divergence

I

[see Fig. 1(b)] return s toward t he dye cell with its diver

gence essentially unchanged, i.e.,

Herew

2

is the beam diameter just before the grating. In the

opposite limit, inwhichthegr tingacts as the limiting aper

tur e (i.e., if l cos

w

1

W

2

whereW

s the beam diameter

at th e dye cell), the divergence of an isochromatic beam re

turning toward th e dye cell will increase so tha t

Here and throughout this analysis we assume tha t the grat

ing-tuning mirror distance can be neglected.

Now consider the case in which an intracavity lens is

presen t. Assuming tha t the light propaga ting toward the

grating from the dye cell is diffraction limited, the lens (lo

cated one focal length from the dye cell) reduces the diver

gence of the bea m by th e factor w

1

/w

2

[see Fig.

1(c)].

If the

grating is not filled, we have

When th e grating is the limiting aperture of the system lcos

w

2

),we have

We see tha t grating-limited linewidths of = ) / 2 l)

[Eq. 2(b) or 3(b) in Eq. (1)] are in principle obtainable with

2040 APPLIED OPTICS / Vol . 23 No. 13 / July 1984

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or wi tho ut the in t racavi ty lens . No te, however , tha t th is

l imi t ing l inewidth i s ob tained at smal ler angles of incidence

when the in t racavi ty lens i s p resen t . This resu l t i s importa n t

since grating efficiencies fall off drama tically as ap p ro ach es

90 .

In fact, as described below, our dye laser would not even

lase at large enough angles for the gra t ing to become the l im

i t ing ape r ture . W hen the grat ing i s no t a l imi t ing aper tu re,

t h e d y e l ase r l i n ewid th wi th t h e len s i s g iven b y

wh ere i s t h e d y e l ase r l i n ewid th wi th o u t t h e l en s .

Th e in t rac avi ty lens is usefu l in anoth er way.

11

As shown

in Fig. 1(b), without an intracavity lens, l ight returning to the

dye cell has a large spatial extent. Spreading occurs in both

ver t ical and hor izontal d i rect ions, and hence only a f ract ion

of the re turnin g light is amplified. W ith the lens [see Fig. 1(c)]

essen t ial ly al l the avai lab le l igh t wi th in th e effective accep

tance angle

(

R

is conc entrate d in to the dye ampl i f icat ion re

g ion . Thi s refocusing effect leads to h igher dye laser ou tpu t

powers .

We have s tud ied the effect o f an in t racavi ty lens on the

output power and spect ral l inewidth of a home-bui l t g raz-

ing-incidence dye laser. Th e laser consisted of [see Fig. 1(a)]

of a4 reflectivity wedge output coupler

EW,

a flow-through

dye cell

DC,

an l

=

5-cm holographic grat ing

G

with 2400

l ines/mm, and a p lane f ron t -surface tun ing mirror M. Two

intracavi ty lenses having focal lengths of 8 and 15 cm were

successively s tud ied . Eac h lens was p laced ap proxim ately

one focal length from the dye cell and as close as possible to

the grat ing . Only the d is tance betwe en the dye cel l and lens

was var ied as the lenses were in terchanged; i .e . , o ther com

pon ent separa t ions were held f ixed. Excim er laser pu lses of

~5 -n sec d u ra t i o n , ~5 -m J en erg y , an d 3 08 -n m wav e l en g th

were used to excite a 1.4 X 10

- 3

-M so lu t ion of rhodamine B

dye in ethanol . Th e dye laser ou tp ut pu lses , tuned to the pea k

of the dye gain prof ile (~60 0 nm ), had a dur at ion of ~ 2 nse c.

The depth of the exci ted reg ion in the dye was measured to

b e 25 0 - 30 0 m .

The resu l ts o f our relat ive power measurements are shown

in Fig. 2. We plot I

L

/I

NL

, wh ereI

L

I

NL

) is th e dye laser in

tens i ty wi th (wi thout) the in t racav i ty lens as a funct ion of .

As ex p ec t ed

I

L

is> I

NL

for all 0. It is significan t to no te tha t

I

N L

itself decre ased by the large factor of ~ 1 0

3

as i n c reased

from 87 to 89.

Our l inew idth results are in close agree men t with the simp le

mode l pres ente d ear l ier . Using w

1

= 3 00 m an d assu m in g

1

is d i f f raction l im i ted , we expect

w

2

1.5w

1

(

w

2

2w

1

)

in

th e case of th e 8-cm (15-cm) focal len gth lens. In our con

figuration, the grating becomes the l imiting aperture at angles

somew hat larger tha n 89. Consequen t ly , we expect , a t least

for lower values of 0 , th at th e lenses should redu ce th e dye

laser l inewidth [see Eq . (4)] by ~3 0 and 50 in the case of the

8- and 15-cm lenses , respect ively . Th ese est ima tes are nec

essar i ly crude because of the complex beam prof i les that ac

tual ly ex is t in the dye laser . In pract ice on ly the 15-cm lens

consis ten t ly narrowed the laser 's l inewidth (see Fig . 3 ) , in

which case we observed l inew idth reduc t ions of 20-40 . At

0 =89, the 15-cm lens stil l narrow s the laser l inewidth despite

the fact that i t has essen t ial ly reached our crude est imate of

th e g ra t in g - l im i ted m in im u m l in ewid th . Th e o b serv ed r e

duct ion at 0 =89 is in agreem ent , however , wi th our expe c

t a t i o n t h a t t h e g ra ti n g is n o t y e t t h e l im i ti n g ap er tu re . Th e

dye laser would not lase, and we were hence unable to make

linewidth measurements in the grating-limited high-0 regime.

Two exper imental d rawbacks associated wi th the in t racavi ty

lens were the long-cavi ty lengths required for l inewidth re-

Fig. 2. Relative dye laser outp ut power as a function of grating angle

Fig. 3. Dye laser linewidth as a function of grating angle 0.

duct ion at exci tat ion dep ths of 300 m and the co mpl icat ion

of an addi t ional lens mo unt in the cav i ty . S t ra y ref lect ions

off the lens d id no t cause no t iceable problems.

In summary , we note that a dye laser 's l inewidth i s min i

mized by minimizing

d / d )

R

.

Th e b as i c L i t tm an l ase r

design minimizes d / d ) by working at large grat ing angles

bu t has no provision for minimizing

R

. This approach works

wel l except that g rat ings tend to be very ineff icien t at the

ext remely large angles of incidence necessary to ach ieve

g ra t i n g - l im i ted l i n ewid th s . Th e b as ic Han s ch d es ig n m in i

mizes R by using a high-quality high-magnification telescope

bu t works at g rat ing angles where eff iciency a nd

d / d )

are

relatively large. Grating -limited performan ce is reach ed only

when expensive qual i ty telescopes are employ ed . In the

present modif ied Li t tman design , addi t ion of the s imple in

t racavi ty lens provides a smal l reduct ion in

R

so th a t so m e

wh at smal ler more eff icient g rat ing angles can be used .

One of us , A.G.Y. , g ratefu l ly acknow ledges the sup por t o f

Arm y fel lowship DAA G29-83-G-000 8. Th is work i s sup

por ted f inancial ly by the Jo in t Serv ice Elect ronics Program

(contract N00014-75-C-0648) .

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2042 APPLIED OPTICS / Vo l. 23 No. 13 / July 1984