Pulsed Laser Oscillator

The Neodynium-YAG laser consists of a rod of the material which can be pumped by a flash lamp at a rate of about 15 Hz. The output is Q-switched and mode-locked with the use of a saturable absorber and an acoustooptical modulator. The output consists of an envelope of pulses which can be tuned for optimization by adjusting the mirrors, adjusting the prisms to change optical pathlength, adjusting the crystal in the acoustooptic modulator, and adjusting the frequency of the modulator.

Q-Switching

Q-switching for a laser refers to techniques for obtaining brief, high-energy pulses rather than continuous wave operation. This helps the operation of a pulsed laser oscillator. The basic idea is that for only a brief time is the beam allowed to pass back and forth between the mirrors to achieve the laser action, but the pumping action is continuous so that a large population inversion is waiting when the lasing condition is satisfied. The Q-switching is typically accomplished with an acoustooptic coupler or an electrooptical device.

The Q of a resonant circuit is the resonant frequency divided by the bandwidth of the resonance, and is a measure of the sharpness of the resonance. When laser action occurs, the Q is very high because you get optical amplification only for an extremely narrow range of frequencies by multiple passes between the mirrors of the laser system. If the light is prevented from making these multiple passes by any mechanism, then the Q is extremely low with the loss of laser amplification. Hence "Q-switching" to turn the laser on and off and help achieve mode locking.

Mode Locking for Lasers

A laser cavity will have a large number of longitudional standing wave modes within the spectral envelope of the laser transition. Even though the laser is in fact nearly monochromatic, the modes are so close together that there can be many thousands of modes within that narrow frequency range. If there is no influence to choose modes, then random quantum "noise" will trigger modes randomly and laser action can occur essentially continuously and with random phases with respect to other longitudional modes. If there is some influence which Q-switches the laser to interrupt the laser action and then lets through a brief burst of light, then that light pulse will make use of the population inversion which has accumulated in the interval and produce a stronger pulse. This helps with the operation of a pulsed laser oscillator.

Once initiated, a "locked" pulse can cycle the cavity every t=2L/c in time, and if it is allowed passage by the Q-switcher, the cycle of one pulse every 2L/c seconds will be repetitive.

Mode locking contributes to producing short pulses. At the time of initiation of the pulse when the Q-switcher "opens the gate", the modes are locked together with the same phase at a given time. Because the different modes oscillate at different frequency, they lose phase coherence and effectively lose coherence and cancel in a time which is the inverse of their overall frequency difference. If there is a wider frequency difference, the pulse produced is shorter. Dye lasers with extremely wide bandwidths have been used to produce ultrashort pulses.

Acousto-Optic Modulator

Light traveling through a quartz crystal can be diverted from its path by an acoustic wave. Light can be scattered off the areas of altered density caused by the acoustic wave in a process called Brillouin scattering. The analogy is made to Bragg scattering in that the sound waves produce effective planes for scattering the incident waves. The light reaching the point directly along the incident light path will be modulated by the presence of the acoustic wave. If that path is part of the amplification path to produce laser action, then the acoustic wave will "Q-switch" the laser on and off.

An acoustooptic coupler accomplishes Q-switching by scattering the incoming light from acoustic waves in a crystal. The periodic scattering of the light reduces the "quality" of the cavity and prevents laser action. Only during brief intervals when there is no scattering is laser action possible, so pulses are produced at these times when there is no density change along the light path through the crystal.

Moller reports the frequencies applied to the crystal for the production of the acoustic standing waves to be a few Hertz to 50 kHz for the acoustooptical modulator and that the resultant laser pulses are on the order of 100-500 picoseconds.

Electrooptical Q-switching

An electrooptical Q-switcher uses the Pockels effect. A Pockels cell between crossed polarizers normally transmits no light. A voltage pulse which rotates the plane of polarization by 90° will allow the light to pass and this shutter effect transmits a brief pulse of light enabling laser action during that brief interval. KDP is a common material for the Pockels cell - it can be used for modulation up into the GHz range.

Dye Cell Shutter

A dye such as Eastman Kodak 9860 polymethine dye dissolved in dichloroethane serves as a saturable absorber. Subjected to a 30 ps pulse, the dye may saturate and become transparent and then become opaque again, acting as a shutter of approximately 10 ps duration. This saturable absorber is used to both Q-switch and mode lock the laser. The result may be a train of 10 ps pulses some 1 microsecond in duration. Such a saturable absorber is an important component of a laser oscillator.