OD-RISS 303 R HIGH REPETITION RATE AND MODE-LOCKED PHOSPHATE GLOSS 1/1LRSER(U) FOREIGN TECHNOLOGY DIV URIGHT-PRTTERSON AFO ON

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A HIGH REPETITION RATE AND MODE-LOCKED PHOSPHATE GLASS LASER

by

He Huijuan, Lu Guoxian, et al.

Approved for public release;Distribution unlimited. 2

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FTD-ID(RS)T-0003-86 4 April 1986

A HIGH REPETITION RATE AND MODE-LOCKED PHOSPHATEGLASS LASER

By: He Huijuan, Lu Guoxian, et al.

English pages: 9

Source: Zhong Guo Jiguang, Vol. 11, Nr. 7, 1984,pp. 385-388

Country of origin: ChinaTranslated by: SCITRAN

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79.%.•7 Wm %1* 7 7 ."

, Vol. 11 Chinese Journal of Lasers No. 7

A HIGH REPETITION RATE AND FODE-LOCKED PHOSPHATE GLASS LASER

• " by

He Huijuan, Lu Guoxian, Li YongchunQian Linxin, Gu Shenru, and Zhao Longxin

Shanghai Institute of Optics and Fine MechanicsAcademia Sinica

Abstract: High repetition rate operation of a picosecond

g lass laser up to 10 Hz was achieved by using a new kind of

onosphate glass. The pulse duration is 24 ps, the spectral width

is 0.84 A and the total energy of the pulse train is 6 mJ.

The key to the operation of glass materials at high

reoetition rates lies not only in an improvement of their thermal

conductivity, but also in an avoidance or elimination of the

undesired ontical pumping induced thermal effects, such as the

optical oath change due to change in the index of refraction with

dntem.Peratu re, and the linear expansion coefficient, c . For

silicate glass, both -n and c are positive. The new-type of"-" dna,

"h.->nate glass adopted in our laboratory exhibits negative and

..ioe o behavior, so that chanje!- in the optical path resujl-'

toth of them can be compensated 4 each other. As a result,

such a chance c -.n reach a minimum value each time when light

travels back ant, forth in a laser cavity.

The relation between chanDe in the total ortica2 Dath and

tem-eraturE is .

Received July 2n, 19F-.

I

W1(Ar)r,- Wi iT (r) (i1) .:.

1 d( )

where c=' -, r is the distance from the center of the rod, and thedn•-"

therm -optical coefficient, W= + (n-1) .

''hen L and o( have different signs, Ap(r) decreases dramatic-dt

ally. This is the reason why glass lasers can be operated at high

repetition rates without the optical pumping induced thermal effect.

Table 1 lists the properties of the two phosphate glasses

m used in this investigation. it is shown from the table that bothdn

glasses have negative u-t values. Since W values for these two

lar.ec are 8. x10-7/OC and 4X10-7/oC, respectively, AD(r) in eqn.

(1) almost tends toward zero.

For a satisfactory laser operation to occur at repetition

rates, it is necessary to design an optimum. thermostable cavity.

1We discussed this kind of ca'"i in a mode-locked laser,

specially under the general conditions with a*O, where a is the

distance between the center of the rod and the output lens. Char7e

in the stability of the laser output light beam due to a small

chan,:e in the focal lengtn of the inner lens depends on the

icr vati;, of the facu ' size, W1 with respect to the focal length,

~., d/-- . When the G factor of the resorar.ce cavity satisfies

G,- G,trr G- . . - (: )

dW1 df , --.- t9, where sutscripts 1 an i refcr to the quantity cf

tr-e output lead an, that of the back surface, resnectively. The "

facula sizes on lens 1, lens 2, and the laser rod are

Cz). . . . (4)

7( .

Tatie 1. Froperties of the Phosphate Glasses

0- NX i

"a 2. 1S0Z1.06*0.1

3~' 3.n .7

a(1O12 .~ LO2 130.3

'in i# >3 -68.5d t I

i 1W;O- 7 Z'-"4

1 "c4 1532

t 1- ',4 1054

1-para. eter; i-n.r.-liear index of refraction; 3-emission - .

cross section; 4-srn.,; 5-thermal expansion coefficient;

a-variation of' index of refraction with temperature;

7-tnermo-ontical coefficient; 8-index of refraction;

9-pm; 13-wave length of laser; 11-fluoresence lifetime;

and 12-,microsecond.

and _2 G,2 2-- -38 1- 1.[

e,- (I 2 , 2- 2,8U, +,6+-l+ ( ) _

where and a and b are the distance between the inner lens and

the front cavity lens 1 and that between the inner lens and back

cavity lens 2, respectively. When the disposition of the parts ir.

t changes, the thermostable state of thethe cavity changes, i .e. , cags .1

cavity also changes. Fig. 1 presents the G- stability curve-fcr

the most insensitive cavity of the laser rod thermal lens with r"7?

-- **.(

a~ - -~~.a - -a- * !a'.A~ ** .- -.-- - . - .. -. .- -'- .--

different values ofp parameter. According to the practical

dimensions of the optical elements now available, we selected P=-/7 IF

and calculated the fa.-!o size, W1 as a function of thermal focal

length, f, as shown in Fig. 2. It can be easily inferred from this

figure that when f reaches 1.5m, the cavity is deviated from the

thermostable region and with f_41.1m, the cavity is in the unstable

region. Changes in the thermal focal length with pulse reDctition

2 -

3-0

.7 'Y

Go, " -

Fi. 1. The G;1 2 stability curves of the thermally insensitive

cavity

0

I •

i2-

F2:. ?. Variation cf the facula size, W1 with thermal focal ler.g+h.

Crdinatc: a''a-upic; and abscissa: reo(tition rate.1-7.i ; nrd "-.

r-i-4-2

i< , . ._2 +. , ,_.- . ++ + i+-.+v - .< + .:,+ . . .+ -++.. ..+ . -.•.. - .i.+. .- -".". ..-i , .*- i -.. -

rate were measured for phosphate glass N21, a silicate glass and

Nd:YA'. The results are shown in Fig. 3. Below the dot line, the

cavity is within the unstable region. It appears that Nd:YAG can

be operated at tens Hz, the phosphate glass at frequencies higher

than 10 Hz, and the silicate glass only at frequencies lower than I?

5 Hz. In cases where the silicate glass is operated at 5 Hz, the

cavity is already within the unstable region.

The active-passive locking mode was adopted to the laser

which had a 1.5m long cavity consisting of a 7m complete reflection

concave rirror and a tapered plate mirror with R=88%. A 1mm trick

dve cell was connected directly to the concave mirror. The dye is

pentamethyldyne dissolved in 1,2-ethyl di-chloride. The back and

forth transmissivitv of small signals is 0.84; the rod dimension,

6mmx1C,., ; the total width of the nulse train at the half peak

h io ht, 150 an:-econd (Fig. 4), h total output energy of the

Fig. 3. Changes in the thermal

focal length with pulse rcre-

tition rate for Nd:T.3 N,N d .GI Iand a silicate glass.

4 N. Ordin te: focal lengthr

abscissa: repetition rate.SinK- 2.

"/-silicate ilass; '-unstaV" 7

region; 3-Hz; and 4-m.. -

---5-

-5 I<

train, 6 rn.J. Tnt pulse width was! measured, ucinE double-ph, tcn

fluorornetry. Fig. 5 shows the densimeter scanning, curve with a

purse width of 24 ps. The spectrum was run using an 1-m~ gratinF I-

srectrograph. Its densimeter scanning curve wh~ere

Fi.4. The moce-locked phosphate glass Dulse train

oscillograph. Time scale: 50 ns/cm.

Fig. . Tne aensimeter scannin ; curve of the double-

Dhoton fluorometry. i-ps.

:~~'i ~ C! F_. T'0 cc.~t e r scarnnir curvt Df a Fsrectr-U7

i C t 110

iL.

.:-e spectrum width is 0.84 A is shown in Fig. 6. When the rene-

tition rate of the laser changed from 1, 5, up to 10 Hz, the out-[

put pulse train of the oscillograph was stable and generally the

output energy was also stable. This is similar to the output con-

ditions of an Nd:YAG laser. However, the mode-locked silicate

glass lasers can only be operated at 1 Hz under the same conditions.

When the repetition rate reached 5 Hz, the output quickly drorred

to zero. These results appear cor-lsten, with the analyses of the

therrostable cavity and thermal lens.

Because of the absorption of the pumped light and the tem-

perature gradients in the rod resltin from cooling of the rod

su rface, mechanical stresses were developed in the rod. Reference

2 gives the stress ecuation for a cylinder,

ac(r) - QS (29- r(

V.,'r) - QIZ _8r 2- )(

a r. e , .(W' - ")QS \'2r' - '2) ( )::

zcns. (61-(E) describe the radial stress, Lr, tang-ential stress, --

and axial stress, O in an isotropic rod of infinite length where

ThE Young's modulus, E, the thermal conductivity, K, and the Poisson

ratio, 1) are all the characteristic parameters of the material. -

Swe have P

wEre r0 i the radius of the rod; Lthe rod length; and lthc

internal friction Dower of the rod. The stress distribution is a

Parabola of r. Usin eqns.(t)(8) and substituting" thc materia2 '-

characteristic parameters, with the pump energy of 8 Ow, stress as

-7-

L

function of the radius of the phosphate glass rod was calculated,

as shown in Fig. 7. These curves indicate that at the rod surface,

the radial component of the stress approaches zero; at the center of

the rod, all the three components are negative, so that the center

is in comnression; and again at the surface, both the tancential andaxial ocrrponents are positive and the surface is in tension. The

>iouo stre:s appeared at the center and at the surface of the rod.

e..h. increase in the power dissination in the rod, teicr a' E

vurfacE increased. Addition of vectors 17# and r,, in eqns. (7) a:oc

(c) results in 0 max: %/ -) E P.-' KJ1-v) L (! ).:

We reasured tre diSI natio!, ener.y which caused runture of the r-d,

I

Fig. 7 Liscri u .... of t., intE '!.... .. ' .

a s rod. Oircinate: strec ,, ::,. n --, -':_

normal irec radius. I-K/,'

I-

_ :L LJ L "2 ii', '.' .-i :, d .2: x-f-"- h',',x;', ,, , ,..-- " -,_ _ . ... ,. , .:.,_ .. ....v.

T :,.- ounc tnat thc( thermal expansion haopened when N21 was &V'~w

a n4 N- 4 , ?9nOw. irom this, the tensile strengths were estimater!

1-" kc/cm7 for '121 and 230 kig/cm2 for N 2 4 . In fact, the actua

i-L~-_r strenftr. of a laser rod strongly depends on rod surface

fin~is', conditions. :t was reported by the Soviets3 tho.t the

measured ruinture strengthn of a phosphate glass is 205. kg/cm 2,

ex7,anF4,:n coefficient, )qx,-C-jC with the same tensile streno-th

as iven in this raoer.

The actu'_al onerat'on enrvof the mode-locked nhoshat F-

as asErs is 54 Jules. The thermal exc.ansion data show the

* rssbiitesof operatin6 these lasers at over 10 Hz, with the

To o)r o cr'sace s e 4n~ t h ermaI ex Pan s ion. A well polished rod

F ' r'aoe has no irrcer'actions and no stress, and can, therefore,

y~~.ca,. increasc_ ora loading.

:.assistar.:c - Of 5 lan Zhonchonc, Znarno J';nzhou and Ch-en

n-- - -rvioz tc glass ma.:ris ancn rarar.-ct(crs is

acr=n:wledce: witn tnarnzs.

M 'A; k: G : pr~

546

. . . .. . . . . .