22 Ruby laser and Q- Switching

Devendra Mohan

RUBY LASER

The first laser emission using ruby crystal was demonstrated by T.H. Maiman in 1960. Ruby is known as sapphire with a small percentage (0.05%) of Cr3+ replacing Al3+ in aluminum oxide (Al2O3). The chromium ions impart pink color to the ruby and are responsible for the emission of light by ruby.

The working of a ruby laser is described as below:

The active material is a cylindrical ruby rod which is ~ 0.8 cm in diameter and ~ 15 cm in length. The ends are flat to better than λ/10. The cylindrical surface of the rod is grounded to prevent total internal reflection.

The Optical pumping mechanism is used for creating population inversion. It is in the form of helical xenon discharge tube and the ruby rod is placed at the axis. The little part of pump energy is used to excite the atoms and the rest generates heat and hence cooling is required for which circulating water arrangement is made.

Resonant Cavity is made by fully reflecting plate at the left and partially reflecting at the right. The ruby laser is commercially available in both pulsed and continuous wave (CW) laser depending upon the optical source of pumping mechanism.

In ruby laser, energy levels are those of Cr³⁺ and it has two main pump bands ⁴F1 and ⁴F2 centered at wavelength of ~0.55μm(green) and 0.42μm(violet),respectively. Each pump band is ~1000Å width. The metastable levels split up into two sublevels ⁴F1 with a separation of ΔE=29 cm^-1.

Xenon flash lamp provides a flash light of 5600Å, while other flash lamps give 0.42μm to 0.56μm. Energy level diagram of ruby laser is as below

E₁ and E₂ are pump bands, M is a metastable band and G is the ground level. The life time of pump band is of ~10-9sec and that of metastable band is of the order of 10-³ s. The ruby rod is placed inside the xenon flash lamp. Flash lamp and ruby rod coincides with the focal line of cylindrical reflector. The energy from flash lamp raises the atoms to the pump bands and these decay to metastable band via non-radiative transition. The population inversion is achieved in a very short time as the metastable state gets highly populated and the atoms from metastable band decay to ground level emitting the radiation of wavelength ~6943 Å and ~6929 Å. The lines are separated about ~14 Å.

The ruby laser is a three level laser system. The photons travelling along the axis of cavity are reflected back and forth and pass many times through the amplifying medium and it is worth noticing that the photons travelling in any other direction would be lost after a few reflections. The output power of commercially available ruby laser is ~ 100 joules/pulse and a pulse lasts for ~10^-7 sec.

Production of Giant Pulse: Q Switching

The flash lamp operation gives rise to pushed output of laser emission. During the time flash lamp does not operate, population of upper level is depleted at a very fast rate and the lasing action does not occur till the arrival of next flash from the lamp. However, the output consists of high intensity spikes of ~0.1μs to ~1.0μs duration. This phenomenon of spiking is due to laser transitions at a faster rate at threshold flash lamp power. In this way the laser operation stops and a start depending upon the reinversion of population inversion at next flash lamp power. Hence a series of pulses is produced, while we require that the maximum energy ios concentrated in a single pulse of very short duration.

The technique of producing a short intense pulse of light is known as Q-switching.It is called Q-switching because the technique involves switiching the Q factor from a low to very a high value of the resonator/cavity. It is closing of the optical cavity by any means until the population inversion is built up much above threshold and suddenly opening it to permit large fraction of energy to come out in a single giant short pulse.

As mentioned above the output of a solid state laser (Here Ruby laser) consists of ~1 msecs. long bursts of spikes, each lasting for 550ns and average spacing between spikes is about few μsecs. If a shutter is introduced in front of one of the mirrors, it will prevent oscillations to take place and active medium is continuously pumped, then population inversion is raised much above threshold level and then the shutter is suddenly opened, the energy stored comes out in the form of a giant pulse. The process changes the Q value of the cavity from very low value before opening to a very high value after opening. The output consists of a giant pulse if the shutter is opened in a shorter time than required for building of laser oscillations. Obviously, if shutter opening is slow then output consists of series of pulses with less peak power.

The following switching systems are used for Q-switching:

(a) Mechanical shutter:

A fully reflecting mirror is made to rotate rapidly about an axis perpendicular to the axis of resonator so as to make it instantaneously parallel to the output reflector. The mirror is rotated with a synchronous motor and flash lamp is timed to fire so that mirror is parallel to the other reflector when population inversion is maximum.

(b) Electro-optical shutter: The function of electro optical shutter is based on Kerr effect or Pockel effect.

Kerr Effect

The degrees of alignment of some materials, when placed in an electric field, the dipoles are aligned along the direction of electric field and also depend upon the strength the electric field. But there are some materials in which the molecules are not symmetrical then the molecules are anisotropic and show birefringent character. When the electric field induced birefringence in isotropic solids is, the phenomenon is said to be Kerr electro-optic effect or simply the Kerr effect. Mostly, the Kerr effect is observed in certain liquids and glasses. So far, nitrobenzene is known to be the ideal liquid which behaves exactly as birefringent. In the presence of electric field of constant strength, Birefringence (n0-ne) is proportional to the square of the field and to the wavelength. Therefore, the optical path difference between ordinary and extraordinary rays in a cell of thickness‘t’ is

Δn=KλE²

Here K is referred to Kerr constant. The value of K is 2.4×10^-10 cm. V^-2 for nitrobenzene.

The liquid is kept between two flat parallel plates spacing several millimeters. The potential difference applied between the plates is of the order to ~10-20 kV. Now the cell containing the liquid is located between crossed polarizes, it acts as a fast shutter known as electro-optic shutter.

The direction of electric field is kept at 45⁰ to the direction of polarizer axis. When there is no electric field, light is not transmitted. In the presence of electric field, the light passes such that

(n0-ne)l=λ/4

And the Kerr cell acts as half wave plate in double pass and rotates the plane of polarization by π/2. The voltage necessary for this action is the half wave voltage.

Peak Power Emitted During the Pulse:

The estimate of the Total Energy emitted during the pulse of a Q-switched laser and in peak power is obtained as follows:

If N₁ and N₂ are the populations of the two levels, respectively, the total number N₀ per unit volume being given by

N₀=N₁+N_2,

Then normalization gives n1=N₁/N₀ & n 2=N₂/N₀

n₁+n₂=1

The single-pass gain of the laser rod is given by G=exp [σ0 (n₂-n₁) L]

Here, σ0 is the absorption cross-section and the quantity n2-n1 is the normalized population inversion.

Suppose, the normalized population inversion is ni (before the shutter is switched on),and at the end of the pulse it is nf. Then the total energy emitted during the pulse is

E=1/2( n_i – n_f)N0Vħω

Here V is the volume of the active material.

The factor ½ has a significance that the population difference changes by two units every time a quantum is emitted.

The duration of the Q-switched pulse is estimated from the decay of the pulse in a cavity. If L is the length of the cavity the time taken by the pulse to make a round trip is given by

t₁=2L/C

Each time the pulse strikes the mirror, it loses (1-r) of its energy, as r is the reflection coefficient of the output mirror,.

The fraction of the energy lost in unit time, is (1-r)/t₁ . Hence the cavity life time t_c is given by

t_c = t₁/1-r

This is the delay time of the Q-switched pulse which has a full width of about 2t_c.

For a nearly triangular shape of the pulse, one can estimate the peak power of the pulse using the relation

P =E/2tc

you can view video on Ruby laser and Q- Switching

References:

Springer Series in Optical Sciences, 1977.
Springer Series in Optical Sciences, 1984.
Principles of Lasers, 2010.
Young, Matt. “Electromagnetic and Polarization Effects”, Springer Series in Optical Sciences, 1986.
Springer Series in Optical Sciences, 1986.
Principles of Lasers, 1998.
M. Zahn. “Transform relationship between Kerreffect
Optical phase shift and nonuniform electric field distributions”, IEEE Transactions on Dielectrics and Electrical Insulation, 4/1994