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State: Solid Lasers in in I of in Light

The document summarizes solid state lasers, specifically ruby lasers. It describes how ruby lasers work, with chromium ions in ruby crystal absorbing green light from a flash tube and emitting red light. Ruby crystals have mirrors coated on the ends to form an optical resonator. Another solid state laser described is neodymium-doped yttrium aluminum garnet, which is pumped by a lamp and emits infrared light. Glassy solid state lasers are also discussed as having advantages like uniform doping but lower thermal conductivity.

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Nurul Fahmi Arief
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56 views1 page

State: Solid Lasers in in I of in Light

The document summarizes solid state lasers, specifically ruby lasers. It describes how ruby lasers work, with chromium ions in ruby crystal absorbing green light from a flash tube and emitting red light. Ruby crystals have mirrors coated on the ends to form an optical resonator. Another solid state laser described is neodymium-doped yttrium aluminum garnet, which is pumped by a lamp and emits infrared light. Glassy solid state lasers are also discussed as having advantages like uniform doping but lower thermal conductivity.

Uploaded by

Nurul Fahmi Arief
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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294

Lasers realizations could easily take up all our time. I shall be able to do no more than describe a few of the better known lasers.

12.6.'l

Solid state lasers

The first laser constructed in 1960 was a ruby laser. The energy-level diagram for the transitions in ruby (Cr ions in an Al2O3 lattice) is given in Fig. 12.3. I
remarked above that ruby owed its characteristic red colour, to absorptionbands of the complementary colour, green. This absorption is used in the pumping process. A typical arrangement is sketched in Fig. 12.4, which shows how the light from a xenon discharge flash tube 'pumps'the ruby to an excited state. Now the emission process is sornewhat different here from that which I sketched previously for three-level systems. The atoms go from level 3 into level 2 by giving up their energy to the lattice in the form of heat. They spend a long enough time* in level 2 to permit the population there to become greater than that of level l. So laser action may now take place between levels 2 and l,

* Energy levels

in rvhich

atoms can

pause for a fairly long tirne (a lew rnilliseconds in thc present case) are
called 'metastable'.

giving out red iight.


The ruby itself is an artificially grown single crystal that is usually a cylinder, with its ends polished optically flat. The ends have dielectric (or metal) mirrors evaporated on to them. Thus. as envisaged in the previous section, the resonator comprises ttvo reflectors. Some power is certainly lost by diffraction, but these losses are small provided the dimensions of the mirror are much larger than the r.vavelength. It needs to be noted that one of the mirrors must be imperfect in order to get the power out. Another notable representatir,e 9f solid-state crystalline lasers is Nd3+ : YAG, that is neodymium ions in an yttrium-aluminium gamet. It is a four-levei laser radiating at a rvavelength of 1.06 pr.m pumped bv a tungsten or mercury lamp. Laser operation at the same frequencv ma)' be achieved by putting the neodymium ions into a g1a.ss host materiai. Glasses have several advantages

Pump

Non-radiative transitions

levels

o
;1/w+
l..l\(r
tr3n\ltton

CrounJ

\rrte

t
I

Fig. 12.3
Energy levels of the Cri+ ion in rub-v The pump levels are broad bands in the green and blue, which efficiently absorb the flash tube light. Level 2 is really a doublet (rwo lines very close to each other) so that the laser light consists of the two red lines of wavelengths 694.3 and 692.9nm.

comparison lvith crystals: they are isotropic. they can be doped at high concentrations with excellent uniformity, they can be fabricated by a variefy of processes (driliing, drawing. fusion, cladding), they can have indices of reflaction in a fairly r.vide range, and last but not least, they are considerably cheaper than crvstalline materials. Their disadvantage is low thermal conductivitv. which makes glass lasers unsuitable for high average

in

porver applications.

Fig.12.4
General arrangement ofa ruby laser. The ruby and the flash tube are mounted along the foci of the elliptic cylinder ret-lector lor maximum translerence of pumP light.

Elliptic cylinder reflecting container

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