LASER VÀ

ỨNG DỤNG
TS. Nguyễn Thanh Phương
Bộ

môn Quang học và

Quang điện tử
Chương IV:

Các loại laser và

ứng dụng
17/04/2011 3
Chương IV: Các loại laser và

ứng dụng
Nhắc lại: những yếu tố cấu thành laser
•

tương tác giữa ánh sáng và vật chất
•

đảo mật độ tích lũy
•

môi trường khuếch đại thích hợp
•

buồng cộng hưởng quang học


of ruby at room temperature
IV.1. Laser rắn
17/04/2011 10
Neodymium Lasers
6.3.2. Neodymium LasersIV.1. Laser rắn
17/04/2011 11
•

crystal
-

Nd:YAG

is the most important material used for solid state laser systems.
YAG

stands for

Yttrium-Aluminum-Garnet, Y
3

Al
2

O
12

, a colourless, isotropic
crystal. For a Nd:YAG laser rod ~1% of the Y

"strongest" laser transition at
1064.1 nm
-

lasing is mainly supported by the R
2

sub-level of the
4
F
3/2

level. At room
temperature ~40% of
4
F
3/2

atoms
are in R
2

(Ê Boltzmann).
-

lower laser level is
4
I
11/2



material processing (cw

and pulsed lasers)
welding (Schweißen), marking, writing, drilling (Bohren) (sizes of few µm
possible), cutting
-

illumination and ranging (military)
-

medical, especially ophthalmology (Augenheilkunde)
-

pumping of other lasers

(e.g. frequency doubled Nd:YAG for pumping of
Ti:Sa lasers) and non-linear optics

(e.g. frequency doubling [532 nm],
tripling [355 nm], quadrupling [256 nm], parametric conversion).
-

Nd:glass lasers and corresponding amplifiers are also used for laser fusion

experiments
IV.1. Laser rắn
17/04/2011 14
•


-

discharge lamps (cw)
… discharge pumping continued
-

discharge/filament tube
is mounted inside a flow
tube

which carries the
coolant (liquid).
-

typical electrical power: ~ 1…10 kW,
~ (100V, 50 A)
-

lifetimes few 10h … ~ 1000 h
-

arc length ~ 50mm
arc lamps
filament lamps
-

typical electrical power: ~ 1kW,
-

lifetimes ~100h


of the crystal

is a serious, power-limiting

issue.
Absorption of pump plight outside the pump band, and heating due to non-

unity quantum efficiency
•

will induce thermal lensing

through temperature dependence of the
index of refraction. This modifies the resonator geometry dynamically!
•

thermal stress

causes

birefringence

and can even lead to damage of
the crystal.
Reduction of problems arising from thermal loading requires
•

uniform pumping
•


different slab geometries exist
… discharge pumping continued
multiple flash

lamp

design
single

(dual) flash

lamp

design
IV.1. Laser rắn
17/04/2011 22
-

disk geometry
beneficial for ultra-high power pulsed solid-state laser systems like those
used for laser fusion (at Lawrence Livermore National Laboratory):
… discharge pumping continued
-

better cooling,
-

larger aperture (∅



operation simplified: reduced cooling requirements, no high voltage
"spikes", no UV-light which degrades crystal, optics and coolant.
-

a single diode laser

can provide a few W cw-power

(typically not
fundamental mode). Single transverse mode laser diodes with ~0.1

W up to
1 W output power exist. Sometimes broad stripe diode lasers, 1D-arrays

("bars") or 2D-arrays

can be used.
IV.1. Laser rắn
17/04/2011 26
… diode pumping continued
-

there are several geometries for optical pumping with laser diodes
•

end pumped systems
(single and double)
-


Isolated Single-mode End-pumped
Ring), or alternatively, an NPRO

(Non-Planar Ring-Oscillator):
the crystal itself constitutes the
amplifier, optical resonator, and
optical diode

to enforce uni-

directional oscillation.
T. J. Kane

and R. L. Byer, Opt. Lett. 10 (2), 65 (1985) ;
I. Freitag et al., Opt. Commun. 115, 511 (1995)
-

physical, optical, thermal properties
of Nd:YAG
IV.1. Laser rắn
17/04/2011 30
•

other solid state lasers
IV.1. Laser rắn
17/04/2011 31
Tuning range for various transition metal solid state lasers
large tuning range
of Ti:Sa is basis
for ultra-short

lower temperature, but for some gases and transitions the lower laying
rotational vibrational

states re-thermalize

faster than some excited
rotational vibrational

state: transient inversion

between rotational-

vibrational

states is generated.
-

optical pumping

(with another laser)
-

most common type is based on a continuous or pulsed discharge
•

general features
-

gas lasers are among the most powerful