11 Laser Amplifiers

Laser Amplifiers Most optical amplifiers are laser amplifiers,

where the amplification is based on stimulated emission. Here, the gain medium contains some atoms, ions or molecules in an excited state, which can be stimulated by the signal light to emit more light into the same radiation modes. Such gain media are either insulators doped with some laser-active ions, or semiconductors, which can be electrically or optically pumped.

In addition to stimulated emission, there also

exist other physical mechanisms for optical amplification, which are based on various types of optical nonlinearities. Optical parametric amplifiers are usually based on a medium with χ(2) nonlinearity, But there are also parametric fiber devices using the χ(3) nonlinearity of a fiber. Other types of nonlinear amplifiers are Raman amplifiers and Brillouin amplifiers, exploiting the delayed nonlinear response of a medium.

An important difference between

laser amplifiers and amplifiers based on nonlinearities is that laser amplifiers can store some amount of energy. whereas nonlinear amplifiers provide gain only as long as the pump light is present.

Multi pass Arrangements, Regenerative Amplifiers, and Amplifier Chains A bulk-optical laser amplifier often provides

only a moderate amount of gain. Typically only few decibels. This applies particularly to ultra-short pulse amplifiers. The effective gain may then be increased either by arranging for multiple passes of the radiation through the same amplifier medium, or by using several amplifiers in a sequence (amplifier chains).

Gain Saturation For high values of the input light intensity, the

amplification factor of a gain medium saturates. This is a natural consequence of the fact that an amplifier cannot add arbitrary levels of energy or power to an input signal. However, as laser amplifiers store some amount of energy in the gain medium, this energy can be extracted within a very short time. Therefore, during some short time interval the output power can exceed the pump power by many orders of magnitude.

Detrimental Effects For high gain, weak parasitic reflections can

cause parasitic lasing, i.e., oscillation without an input signal, or additional output components not caused by the input signal. This effect then limits the achievable gain. Even without any parasitic reflections, amplified spontaneous emission may extract a significant power from an amplifier.

A related effect is that amplifiers

also add some excess noise to the output. This applies not only to laser amplifiers, where excess noise can partly be explained as the effect of spontaneous emission. But also to nonlinear amplifiers.

Ultrafast Amplifiers Amplifiers of different kind may also be used

for amplifying ultra short pulses. In some cases, a high repetition rate pulse train is amplified, leading to a high average power while the pulse energy remains moderate. In other cases, a much higher gain is applied to pulses at lower repetition rates, leading to high pulse energies and correspondingly huge peak powers.

There are two power measurements for a

pulsed laser: peak power and average power. The average power is simply a measurement of the average rate at which energy flows from the laser during an entire cycle. For example, if a laser produces a single half joule pulse per second, its average power is 0.5 W.

The peak power is a measurement of

the rate at which energy comes out during the pulse. If the same laser produces its half joule output which is microsecond long pulse, then the peak power is 500,000 W (0.5 J/10-6 s = 500,000 J/s).

PULSE REPETITION FREQUENCY (PRF) It is a measurement of the number of pulses

the laser emits per second. The period of a pulsed laser is the amount of time from the beginning of one pulse to the beginning of the next. It is the reciprocal of the prf. The duty cycle of a laser is the fractional amount of time that the laser is producing output, the pulse duration divided by the period.

For example, let's consider a flash-pumped,

Q-switched Nd:YAG laser that produces 100 mJ, 20 ns pulses at a prf of 10 Hz. The average power is equal to the pulse energy divided by the pulse period: Pulse average = Energy / pulse) / Period = 10-1 J/ 10-1 s = 1 J/s = 1 W

The peak power is equal to the pulse

energy divided by the pulse duration: P peak = (Energy / pulse) / Pulse length = 10-1 J/ 2 x 10-8 s = 5 x 106 J/s = 5 MW The peak power is five million times as great as the average power.

NEED FOR VARIOUS Q - SWITCHES Placing a beam block in front of the laser mirror is a

straightforward approach to Q-switching a laser, but it isn't very practical. Getting the beam blocked quickly is difficult process. If the beam is 0.5 mm in diameter and the block must be pulled out in a few nanoseconds. The block must be jerked out with a velocity greater than the speed of sound—which is not very easy to do.

FOUR TYPES Q - SWITCHES Mechanical Q-switches actually move a mirror

to switch the resonator Q. Acousto-optic (A-O) Q-switches diffract part of the light passing through them to reduce feedback from a resonator mirror. The polarized lights passing through an electro-optic (E-O) Q-switch can be rotated so that a polarizer prevents light from returning from a mirror.

A

dye Q-switch absorbs light traveling toward the mirror until the intensity of the light becomes so great that it bleaches the dye, The bleached dye allows subsequent light to pass through the Q-switch and reach the mirror.