HT-7

A Nd:glass laser oscillator using rotating beam on the HT-7 Thomson Scatterin

g Apparatus

 L.Yang, J.Y.Zhao, Y.D.Li, Z.S.Fang, J.S.Mao,

V.Sannikov* A. Gorshkov*

 

ASIPP, Hefei, Anhui, 230031, P.R.China.

* PRC Kurchatov Institute, Moscow 123182. RUSSIA

HT-7Abstract

• Thomson scattering diagnostic has been an important and standard method for measuring electron temperature and density profiles on all modern tokamaks. When designing a Thomson Scattering system, the principal problems are getting sufficient scattering signal, suppression of stray light and suppression of plasma background light. Increasing the laser energy is clearly one of the methods which help to resolve these problems. In Thomson scattering diagnostic on our HT-7 Tokamak the solid state Nd:glass laser is used because it can be produced in large sizes to store high energy per unit volume. To usual solid state laser, the “thermal lensing” effect can’t be neglected. Many methods to resolve this problem have been reported. The most special way has obvious advantages by using rotating optical beam in the oscillator. In our ring cavity master oscillator, by using Dove prism placed in some angle, we can get the rotating optical beam. Comparing with other methods, this method is more simple and efficient. Further more, due to the dove prism, the resonant cavity of the master oscillator can pump and extract energy from as much of the laser medium as possible and keep reasonable beam quality.

HT-7

• 1 Introduction

• 2 The working principles of the usual ring cavity and our ring oscillator.

• 3 The stability of the resonator

• 4 The analysis of output of the laser beam

• 5 Summary

Contents

HT-7Introduction

• For Thomson scattering diagnostic of high temporal evolution, we should study the thermal-lensing effect in our Nd:glass laser.

• The reason of arousing thermal-lensing effect in solid state rod.

• Different methods to reduce the “thermal lensing” effect in solid state laser rod have been reported. Rotating the cross-section of the oscillator beam in resonator is the unique one.

Reference:

1 G.N.Steinberg; Phys.Rev.(A), 1971, 4, 1182

2 R.L.Carman et al.; Appl.Phys.Lett., 1969, 14, No.4, 136

3 Deng ximing et al.; Chinese laser, 1982, 10(5), 274-276

HT-7The analysis of the thermo-lensing effect

flashlamps pInput beam

s prodInput beam►electrostrictive self-focusing effect

►To solid state rod laser, the refractive index of the rod will distribute in the form of parabola in poloidal direction when pumping exist.

►With the energy of laser beam arousing, the thermal-lensing effect will produce.

Ref: Deng ximing et al.; Chinese laser, 1982, 10(5),274-276

rod

2f 2f

The different opinions about the reason for Thermal-lensing effect in solid state rod oscillator

HT-7Unique method to reduce thermal-lensing effect

rod

Reflective mirror

Rotating axis

Rotating mirror

Reflective mirror

Input beam Output beam

Main meaning: By making the input beam rotate along the laser axis, the energy distribution of the beam in the cross-section will become even, which can help to reduce the thermal-lensing effect.

Experiment have shown at lest two features of this kind of cavity:► It will arise the damage threshold of the laser.► The brightness of the output laser will increase obviously.

◆ It is effectively used in laser with 5-10ms in duration.

HT-7

M1

z

L

θ

γ

M4(R1)

0

M3(R,S)

Due to the dove prism, the cross-section of the oscillating beam will rotate in the cavity which can help to reduce the thermo-lensing effect of the solid state rod.

M2

With different R1, asymmetric output will produce.

According to our master oscillator, the largest output is about two times of the least output.

When the oscillating beam is traveling in the cavity, it will deflect in the horizontal surface due to the structure of the ring cavity

The self-deflection effect of the beam in the ring cavity

The schematic of the ring cavity

The working principle of the oscillator(one)

HT-7

Y’

P

X’

γ

Due to the dove prism, the oscillating beam will deflect in vertical direction.When the beam is shed on the surface of the dove prism, the output laser beam will rotate by angle 2γ. So, when the beam is rotating in the oscillator, the energy distribution on the polidal direction of the rod will become even, which help to reduce the thermal-lensing effect in the rod.Further, the polarization of the beam is decided by γ.

1

0

p

B

The working principle of the oscillator(two)

HT-7

~00 ~30--50 ~50<~100

The spots of the output laser in different situations

With , the cavity is just like the usual ring cavity and one little bright laser spot wil

l produce because only the population inversion in near field of the center in the rod wil

l drop to the metastable level. With a little increasing of the , more and more ions are

pumped to the high energy level and produce the laser pulse. So, the laser spot become

larger and bright . When the angle is too large, a null hole will produce in the center o

f the laser spot because some of the deflecting effect of the dove prism.

0

HT-7Analysis of the stability of the resonator

The optical matrix of the ring cavity is following :

Two kinds of stabilities of the cavity

* *0)1(

)1(

fd

qd 0)1(

0 fd

dW

a

c

b

d

a

c

b

d

condition of strict thermal stability condition of weak thermal stability

The cavity don’t fit for the condition of strict thermal stability. Due to the weak

condition of which, the solution to the equation exist. So, the mode pattern and

divergence of the beam in far field don’t sensitive to the thermal-lensing effect.

Ref: Cao qing et al.;Acta physica sinica,1997,46(1)

HT-7The analysis of the output energy of the laser

• When setting R1=0 and R1=1, we can see that the output energy of the laser(y0) will vary with different s. Considering , R=0.6

We can see that the output y0 with R1=1 is about two times the y0 with R1=0

during the whole range of s and the deviation of y0 with R1=0 is small.

500

LgG

1 2 3 4 5 6 7 8 9 10x 10

-3

-3

-2

-1

0

1

2

3

4

5

6

7x 10

-3

s

dyo/ds

1 2 3 4 5 6 7 8 9 10x 10

-3

60

70

80

90

100

110

120

s

y0

Ref: Lin fucheng et al.; Chinese laser,1985,14(2),70-72

HT-7Summary

• The master oscillator in our laser system consists of a ring resonator.

Due to the dove prism placed in the cavity in some angle, the cross-secti

on of the oscillating beam will rotate which will help to reduce the them

ostresses accumulation during the operation. When the reflection of the

mirror M4 is changed, asymmetric output is obtained. Considering the

use of laser in our diagnostic, we set . The high power laser pulse in o

ne direction will produce during the burst.

• Further, the Q-swich is down by plate of LiF-2 crystal with colored ce

nters. When the angle between the axis of the optical beam and the bottom surface of the prism is changed, different output will produce.

• The cavity is fit for the condition of weak thermal stability. The mode pattern and the divergence of the beam in far field don’t sensitive to the thermal-lensing effect.

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