opinion of laser modulation technique

opinion about the article: http://dx.doi.org/10.1070/QEL16297

about the article

introduction

first to introduce the advantages of AO modulator:

small switching times,simplicity,ease of use, relatively low cost,great variety of modifications,optimisation for specific task, required wavelength,wide operating spectrum range

next to say the limit:

have to have the high radiation resistance, only leave quartz to choose, but quartz is not of high AO quality. So have to use high-frequency (HF) control signal of high power (up to 100 W), and lead to the need for water-cooling, which is complicated.

Then say what to do:

need to develop a modulator with high radiation resistance without water cooling system

last talk about KYW and introduce the following context

high radiation resistance,transparency for light in a wide spectral range (0.4–5 mm) good to control high power radiation, more efficient, next to compare

Structure and features of AO modulators made of KYW

introduce the mutual orientation of the axes of the coordinate systems of KYW

figure1

two geometries:

two polarisation

  • light direction: close to the second-order symmetry axis Y of the crystal
  • wave direction: in the direction of the Z^0^ axis(($\phi = 0°$))
  • ensure close and sufficiently large M2 values for light of both polarisations

figure2a

most effecient(the greatest value of the AO figure-of-merit M~2~)

  • propagation in the plane Z’Y, which is obtainedby the rotation of the plane Z^0^Y by the angle of –30°($\phi = -30°$) around the Y axis and light should be polarised along the Z^0^ axis
  • piezotransducer is applied are orientednormally to the optical axes

figure2b

other properties

the acoustic wave walk-off angle is small: 3.4° and 1.6°velocities of acoustic waves at j = –30° and 0 are equal to 4.56´105 and 4.79´105 cm s–1

figure3

Investigating the characteristics of modulators

first stage

use 632.8-nm helium–neon laser with circular polarisation
diffraction coefficient T: $ T = sin^2 [(\pi /2) \sqrt{P/P_0 }]$
diffraction efficiency of AO modulators: $ \omega = T / P|_{T<<1} = \pi^2 /4 P_0 $

figure4a figure4b

second stage

using the same optical scheme with the aim of comparison with the existing analogues
measure diffraction efficiency of the commercially available MZ-321
compare it with new KYW modulator

KYW exceed much (3.7 and 6.6 times) but the result may not be fully correct for the MZ-321 has much longer AO interaction length and makes the estimate obtained not completely correct and the ultrasonic converters are different

figure5 figure4b

when all condition same:
exceed 5.1 and 9.2 times, same with calculation

figure6 figure4b

compare with paratellurite (TeO2):
thought efficiency is smaller for 2-3 times but the radiation strength of TeO2 is much lower
breaking pulse threshold for paratellurite: 0.3 GW cm^–2^ at wavelength of 1 mm
breaking pulse threshold for KGd(WO4)2: 50GW cm^–2^ (KGW)
breaking pulse threshold for KYW: 0.8GW cm^–2^
for high power laser, KYW is better(though not very much)

Conclusions

experiment and calculation confirm:

  • exceeds the efficiency of a typical AO quartz modulator by approximately 10 times

  • modulators made of KYW provide approximately equal diffraction efficiencies for both polarisationcomponents of the light beam, which allows nonpolarisedradiation to be modulated without significant losses.(five times.)

  • do not require water cooling,

about what i learn

laser modulation can be categroied as follow:

1
2
3
4
5
6
7
8
9
10
11
12
modulation
|---- by position
|          |---- internal modulation
|          |---- external modulation
|---- by feature 
|          |---- laser as wave
|                  |---- amplitude modulation
|                  |---- phase modulation
|                  |---- frequency modulation
|          |---- laser as pulse
|                  |---- pulse modualtion
|                  |---- pulse coded modulation(digital modulation)