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Laser Cavity Modes All lasers produce light over rangeof frequencies, known as the gainbandwidth . For example, aHeNe laser has again bandwidth of approximately

1.5 GHz (a wavelength range ofabout 0.002nm at a centralwavelength of 633 nm), whereas aTi:Sapphire has a bandwidth of about128 THz (a 300 nm wavelengthrange centered around 800 nm).

http://en.wikipedia.org/wiki/Helium-neon_laserhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Ti-sapphire_laserhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Ti-sapphire_laserhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Helium-neon_laser

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Mode Locking In lasers with only a few oscillating modes acting independently,interference between the modes can causebeating effects in the laseroutput, leading to random fluctuations in intensity. In lasers with many thousands of modes, these interference effects tendto average to a near-constant output intensity, and the laser operation is

known as a c.w. or continuous wave . If each mode operates with a fixed phase between it and the othermodes, the modes of the laser will periodically all constructively interferewith one another, producing an intense burst or pulse of light.

the minimum possible pulse duration

t is given by

http://en.wikipedia.org/wiki/Beat_(acoustics)http://en.wikipedia.org/wiki/Beat_(acoustics)

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Concepts of Mode Locking

Out of phase

RANDOM phase for all the laser modesIrradiance vs. Time

Time Time

Out of phase Out of phaseIn phase

LOCKE

D phases for all the laser modes

Mode locking is a method to obtain ultrafast pulses from lasers, which are thencalled mode-locked lasers mode

chrome://newtabhttp//www.youtube.com/watch?v=efxFduO2Yl8

http://chrome//newtabhttp//www.youtube.com/watch?v=efxFduO2Yl8http://chrome//newtabhttp//www.youtube.com/watch?v=efxFduO2Yl8

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Basic principles of ultrafast lasersBandwidth vs Pulsewidth

narrow spectrum

continuous wave (CW)

broader spectrum

pulses (mode-locked)

broadest spectrum

shortest pulses

bandwidth

duration

= const.

For the HeNe laser with a 1.5 GHz spectral width, the shortest Gaussian pulse consistentwith this spectral width would be around 300 picoseconds; for the 128 THz bandwidthTi:sapphire laser, this spectral width would be only 3.4 femtoseconds.

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Active Mode Locking

When driven with an electrical signal, AOM produces a sinusoidalamplitudemodulation of the light in the cavity. The amplitude modulator acts as a weakshutter to the light bouncing betweenthe mirrors of the cavity, attenuating the light when it is "closed", and letting itthrough when it is "open". Considering this in the frequency domain, if a mode has optical frequency ,and is amplitude-modulated at a frequencyf , the resulting signal hassidebands at

optical frequencies f and + f . If the modulation rate f is synchronised to the cavity round-trip time , then

these sidebands correspond to the two cavity modes adjacent to the original mod Since the sidebands are driven in-phase, the central mode and the adjacentmodes will be phase-locked together. As a result, a single pulse of light will bounce back and forth in the cavity.

www.olympusmicro.com/primer/java/lasers/ndylf/index.html

http://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Sidebandhttp://chrome//newtabhttp//www.olympusmicro.com/primer/java/lasers/ndylf/index.htmlhttp://chrome//newtabhttp//www.olympusmicro.com/primer/java/lasers/ndylf/index.htmlhttp://en.wikipedia.org/wiki/Sidebandhttp://en.wikipedia.org/wiki/Amplitude_modulationhttp://en.wikipedia.org/wiki/Amplitude_modulation

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P o w e r

Time

P o w e r

Time

cw cw ML

P o w e r

Time

Q-switch P o w e r

Time

Q-sw.ML

Types of Laser Output