Gas lasers, Media transmisyjne 2
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11
Other Gas Lasers
K.M. Abramski, E.F. Plinski
CONTENTS
11.1 Introduction ............................................................................................................ 495
11.2 He–Ne Lasers .......................................................................................................... 496
11.2.1 Constructions and Technology .................................................................. 496
11.2.2 Physics of He–Ne Laser ............................................................................. 498
11.2.3 Resonator in He–Ne Lasers: Mode Structure and Spectrum of
Radiation ................................................................................................... 499
11.2.4 Frequency Stabilization of He–Ne Lasers.................................................. 502
11.3 Ion Lasers ............................................................................................................... 505
11.3.1 Construction and Supply System of Ion Lasers......................................... 506
11.3.2 Physics of Ion Lasers—Ar Ion Laser ......................................................... 508
11.3.3 Kr Ion Laser .............................................................................................. 514
11.3.4 White Ar–Kr Ion Laser.............................................................................. 515
11.3.5 Applications of Ion Lasers ......................................................................... 516
11.4 FIR Laser................................................................................................................ 517
11.4.1 FIR Molecules ........................................................................................... 517
11.4.2 Line Assignment......................................................................................... 520
11.4.3 FIR Laser Radiation.................................................................................. 522
11.4.4 Representative FIR Transitions ................................................................. 524
11.4.5 FIR Laser Constructions ........................................................................... 525
11.4.6 Applications of FIR Lasers........................................................................ 529
11.5 The Submillimeter HCN Laser ............................................................................... 529
11.6 Xe Laser .................................................................................................................. 531
11.7 The N
2
Laser........................................................................................................... 533
References ......................................................................................................................... 535
11.1 INTRODUCTION
This chapter deals with gas lasers other than those discussed earlier in this book, which also
play an important role in physics and technology, but are not in the forefront of scientific and
commercial development now.
In most cases, the reason is simple—their technology had been developed in the past and it
reached a level where these lasers are mainly developed commercially (like helium–neon,
argon–krypton ion lasers). The development of some other gas lasers has stopped after some
period of their intense investigations (FIR, HCN, xenon, nitrogen lasers). This chapter reviews
six types of lasers:
Atomic lasers (helium–neon, He–Ne)—Section 11.2.
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Gas Lasers
Probably the most popular gas lasers with the best-established technology. They are very well
known to all students in academic laboratories using visible (red, green, yellow, infrared) radi-
ation in elementary experiments with a coherent light.
Ion lasers (argon ion)—Section 11.3.
These are quite useful lasers operating in the green–blue and ultraviolet (UV) regions as well.
These lasers, with relatively high power in a visible region, are particularly popular in light-show
enterprises and holography or medicine as well.
FIR lasers (far infrared radiation)—Section 11.4.
These are lasers operating in millimeter and submillimeter wavelengths joining two electromag-
netic opto- and radioregions. They are based on optically pumped multiatom molecular gas
media. The FIR lasers still wait for their undoubtedly great future.
Submillimeter hydrogen cyanide (HCN) lasers—Section 11.5.
These are the submillimeter lasers that are not pumped optically. They are pumped by discharge
like many gas lasers. We introduce them rather like an exotic laser that was invented in the past.
IR xenon (Xe) lasers—Section 11.6.
Xe as a high gain medium can be applied as an ion laser but it is probably more popular as an
atomic laser giving radiation in the IR region. Different excitation techniques (DC, pulsed, RF,
e-beam, x-ray, nuclear) can be demonstrated with this laser medium.
Molecular UV (nitrogen [N
2
] lasers)—Section 11.7.
It used to be quite an exciting ‘‘Do It Yourself ’’ device for amateurs. The air, rich in N
2
, can be
easily applied to the laser medium without any vacuum technology. Hence, many ideas and
constructions have been presented for pulsed lasing in the past and present (see web sites).
This is not the end of the list and some future development of the gas lasers can be expected.
11.2 HE–NE LASERS
He–Ne lasers are probably the most popular gas lasers in many university laboratories. Most
students passing elementary courses in physics, optics, photonics, or optoelectronics are quite
familiar with these lasers. Their nice red, green, orange, yellow beams (or some IR, as well)
are applied to many elementary experiments—interferometers, modulators, holograms, scan-
ners, and the like.
11.2.1 C
ONSTRUCTIONS AND
T
ECHNOLOGY
We will start from the description of the He–Ne laser constructions and technology. Taking
into account the contemporary worldwide laser market, it is difficult to imagine today that each
laboratory designs and elaborates He–Ne lasers, as it happened 30 to 40 years ago in most
laboratories. The technology of a He–Ne laser has become perfect during the last four decades.
Many He–Ne lasers have operated for over 20 to 30 years without any visible degradation.
Their practical lifetime overcomes that of the laser diodes. Development of technology of
He–Ne lasers has established two basic constructions of laser tubes presented in Figure 11.1.
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497
Brewster windows
Capilare
Anode
Output mirror
Hollow
cathode
Total reflecting
mirror
(a)
Output
laser beam
Hollow cathode
Glass-metal
sealing
High reflectivity
planar mirror
(
)
(+)
Power
supply
Output mirror
Precision
capilare
Anode
(b)
Gas mixture
FIGURE 11.1 Typical structures of helium–neon discharge tubes and resonators: (a) with Brewster
windows and external mirrors; (b) compact with internal mirrors.
In both cases, the main element of the laser tube is a capillary in which the laser plasma is
formed. The internal diameter of the capillary is determined by two conditions:
– It should select the basic TEM
00
mode and it should damp the higher-order transverse
modes.
– It should be thin enough to depopulate lower laser levels because of extra collisions of
excited Ne atoms with the capillary walls (see next chapter).
These two conditions establish a typical internal diameter of capillary at d 1–2 mm. It also
fulfills important practical condition for optimal pressure–diameter product [1,2]:
pd 3:6 [Torr cm]
(11:1)
The laser tube is filled with the mixture of the He and Ne in a typical proportion of about
10:1, respectively, at the total pressure p 2–5 Torr. The typical value of electric field–
pressure ratio in the laser tube is:
E
p
2:8
V
cm Torr
(11:2)
where electrons reach the temperature T
e
80,000 K in the discharge.
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The plasma in the capillary is obtained mostly by DC discharge (3–15 mA), although the
first He–Ne laser was excited by the inductively coupled RF discharge [3]. The highly
developed high-vacuum technology applied in the process of the laser tube manufacturing
is the main factor that established high-quality long-life laser discharge tubes. The laser tube
presented in Figure 11.1b shows a typical cross section of the laser with internal mirrors. The
central part of the laser is formed with a capillary, and the discharge current passes through
its internal part. Both the electrodes, aluminum cathodes and anode are naturally composed
of Kovar (nickel–iron–cobalt alloy) tubes, which form high-quality hard glass metal seals. It
is known that Kovar alloy and some special glasses (borosilicate) have similar thermal
expansion. Hence, such metal glass seals accept great changes of temperature very well
without damaging stresses. The external glass tube forms the reservoir for a gas mixture
sufficiently increasing the total ratio of mixture volume to plasma volume. A specially
inserted getter can neutralize some impurities, which can appear during discharge.
Considering the power supplies, it has to be mentioned that supply technology has also
reached the perfect level. Very efficient, small dimension power supplies are available for all
types of discharge tubes. Their operation is based on the converter, where DC voltage is
transported into the required high operation voltage. The starting voltage needed to switch on
the discharge (which depends mainly on the length of the tube and pressure) can reach the
value from 8 to 15 kV. After switching the discharge on, the voltage drops to the operation
value. The operation voltage depends on the tube parameters. Shorter tubes (20 cm long) have
a typical operation voltage U
op
1000–1500 V, when longer tubes (60 cm) need operation
4000–6000 V. Well-designed supply has a current ripple at the level below
0.3%. As usual, the ballast resistor (100–200 kV) has to be connected in series with the tube to
stabilize the discharge.
11.2.2 P
HYSICS OF
H
E
–N
E
L
ASER
The diagram of energy levels significant to the laser actions in He–Ne mixture is presented
in Figure 11.2. Laser actions are obtained from transitions of the Ne atoms. The He atoms
play an important role in transferring pumping energy. There are two excited levels of He (2
3
S
and 2
1
S around 20 eV) shown, which deliver energy to the upper laser level of Ne atom (3s
and 2s levels). The s-levels of Ne atoms are around the energies of 19.8 and 20.6 eV,
respectively. The He atom excited to levels 2
3
S or 2
1
S is not able to decay spontaneously
back to the ground state (forbidden radiation transitions). These levels have quite long
lifetimes (a few seconds).
Free electrons appear as a result of the ionization process of atoms. They are accelerated in
the DC electrical field. The free electrons that have high kinetic energy collide with He atoms
in the ground state and excite them to the metastable levels according to the equations:
e
kinetics energy
He
ground level
! He(2
1
S) e
kinetic energy
20:6 eV,
(11:3)
e
kinetics energy
He
ground level
! He(2
3
S) elastic energy:
(11:4)
Because He atoms dominate in the mixture, there is a high probability of their collisions with
neutral Ne atoms. Energetic coincidence of metastable levels with 3s and 2s levels of Ne
allows transferring energy from excited He atom to the Ne atoms as the effect of elastic
collisions. The upper levels (excited in transfer process) form the population inversion.
There are many lasing transitions that can be forced to operate when an optical resonator
with dielectric selective mirrors is applied. The He–Ne laser can operate at many
wavelengths in the visible and infrared range. Many of these wavelengths are well established
voltage of U
op
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499
Helium
Neon
21
[eV]
E
=
47.939e
3 eV
Laser
transition
3391.3 nm
2
1
S
2
3s
3p
4
5
1
4
Atomic
collisions
E
=
+
38.905e
3 eV
10
20
2
3
S
2
2s
3
Laser
transitions
4
5
632.8 nm
611.8 nm
593.9 nm
543.4 nm
“First” laser
transition
1.1523 nm
19
2p
1
4
Pumping
6
8
10
18
667.8 nm
Radiative
decay
609.6 nm
594.4 nm
1s
2
Collisions with
tube walls
4
5
17
Ground state
FIGURE 11.2 The helium–neon energy level diagram.
commercially. Table 11.1 lists most of the lasing transitions. The bold ones indicate lines that
are commercially developed and available [2].
As indicated in Figure 11.2, the lower levels have to be fast depopulated in order to keep
population inversion. Fortunately, decay times of 2p levels are short enough, and these levels
are depopulated by spontaneous emission to 1s levels. (This spontaneous emission determines
orange–red color of the discharge observed in the tube.) Depopulation of 1s levels is caused
by collisions with the glass walls of the discharge tube. That is why a discharge tube should be
thin enough to assure high probability of collision depopulation.
11.2.3 R
ESONATOR IN
H
E
–N
E
L
ASERS
:M
ODE
S
TRUCTURE AND
S
PECTRUM OF
R
ADIATION
The small gain of most He–Ne laser transitions determines particularly high-quality, low-
loss optics and a Gaussian configuration of optical resonators. Very high selectivity and
3
3
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