11 Laser Amplifiers

8/14/2019 11 Laser Amplifiers

http://slidepdf.com/reader/full/11-laser-amplifiers 1/21

Laser Amplifiers

Most optical amplifiers are laser amplifiers, where the amplification is based on stimulatedemission.Here, the gain medium contains some atoms,

ions or molecules in an excited state, whichcan be stimulated by the signal light to emitmore light into the same radiation modes.Such gain media are either insulators doped

with some laser-active ions, osemiconductors, which can be electrically oroptically pumped.



In addition to stimulated emission, there alsoexist other physical mechanisms for opticalamplification, which are based on various types ooptical nonlinearities.

Optical parametric amplifiers are usually basedon a medium with χ (2) nonlinearity,

But there are also parametric fiber devices usingthe χ (3) nonlinearity of a fiber.

Other types of nonlinear amplifiers are Ramanamplifiers and Brillouin amplifiers, exploiting thedelayed nonlinear response of a medium.



An important difference betweenlaser amplifiers and amplifiers based on nonlinearities is that

laser amplifiers can store someamount of energy. whereas nonlinear amplifier

provide gain only as long as thepump light is present.



Multi pass Arrangements, Regenerative Amplifiers, and Amplifier Chains

A bulk-optical laser amplifier often providesonly a moderate amount of gain.

Typically only few decibels.

This applies particularly to ultra-short pulseamplifiers.

The effective gain may then be increasedeither by arranging for multiple passes of the

radiation through the same amplifiermedium, or by using several amplifiers in asequence (amplifier chains).





Gain Saturation

For high values of the input light intensity, theamplification factor of a gain medium saturates.

This is a natural consequence of the fact that anamplifier 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 beextracted within a very short time.

Therefore, during some short time interval theoutput power can exceed the pump power by many orders of magnitude.



Detrimental Effects

For high gain, weak parasitic reflections cancause parasitic lasing, i.e., oscillation

without an input signal, or additional outputcomponents 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 amplifiersalso add some excess noise to theoutput.This applies not only to laser

amplifiers, where excess noise canpartly be explained as the effect of

spontaneous emission.But also to nonlinear amplifiers.



Ultrafast Amplifiers

Amplifiers of different kind may also be usedfor amplifying ultra short pulses.

In some cases, a high repetition rate pulse

train is amplified, leading to a high averagepower while the pulse energy remainsmoderate.

In other cases, a much higher gain is applied

to pulses at lower repetition rates, leading tohigh pulse energies and correspondingly huge peak powers.





There are two power measurements for apulsed laser: peak power and averagepower.The average power is simply a

measurement of the average rate at whichenergy flows from the laser during anentire cycle.

For example, if a laser produces a singlehalf joule pulse per second, its averagepower is 0.5 W.



The peak power is a measurement of the rate at which energy comes outduring the pulse.

If the same laser produces its half jouleoutput which is microsecond long pulse,then the peak power is 500,000 W (0.5J/10-6 s = 500,000 J/s).



PULSE REPETITION FREQUENCY (PRF)

It is a measurement of the number of pulsesthe laser emits per second.

The period of a pulsed laser is the amount o

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 thatthe laser is producing output, the pulseduration divided by the period.



For example, let's consider a flash-pumped,Q-switched Nd:YAG laser that produces 100mJ, 20 ns pulses at a prf of 10 Hz.

The average power is equal to the pulseenergy 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 pulseenergy divided by the pulse duration:

Ppeak

= (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 asgreat as the average power.









NEED FOR VARIOUS Q - SWITCHES

Placing a beam block in front of the laser mirror is astraightforward approach to Q-switching a laser, butit isn't very practical.

Getting the beam blocked quickly is difficultprocess.

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 greaterthan the speed of sound—which is not very easy todo.



FOUR TYPES Q - SWITCHES

Mechanical Q-switches actually move a mirrorto switch the resonator Q.

Acousto-optic (A-O) Q-switches diffract part of

the light passing through them to reducefeedback from a resonator mirror.

The polarized lights passing through anelectro-optic (E-O) Q-switch can be rotated sothat a polarizer prevents light from returningfrom a mirror.



A dye Q-switch absorbs lighttraveling toward the mirror until the

intensity of the light becomes sogreat that it bleaches the dye,

The bleached dye allows

subsequent light to pass through theQ-switch and reach the mirror.

Date post:	30-May-2018
Category:	Documents
Upload:	kaushik4208
View:	216 times
Download:	0 times

11 Laser Amplifiers

Documents