Acousto-Optics

Wafers

Acousto-Optics

Acousto-Optic: The diffraction of an incident optical beam by acoustic waves.

Acoustic Power: The power generated inside the crystal by the transducer. This will be generally be lower than the   RF power for the electro-mechanical conversion ratio is less than 1.

Active Aperture: Aperture over which performance specs apply.

Bragg Angle: Depending on the wavelength on and the RF frequency, it is the angle of incidence that provides the efficient diffraction into a single diffracted order.

Collimated Beam: Light beam that is composed of parallel rays.

Contrast Ratio: The ratio between the maximum and minimum light intensity in the first order light beam when the acoustic waves are on and off.

Diffraction Efficiency: The percent of the zero order light beam that can be diffracted to the first order beam. It is determined by the ratio of the power of the diffracted light beam to the incident power of the beam.

First Order Beam: The diffracted beam that is generated when the light interacts with the acoustic waves.

Maximum Deflection Angle: The angle from which the first order beam will scan when the RF frequency is varied across the full RF bandwidth.

Modulation Bandwidth: The maximum frequency that allows light beam to be amplitude modulated. It is related to rise time and can be increased by reducing the beam size.

Optical Acceptance Cone: Degrees of Optical divergence in the incident beam.

Resolution: As defined by the Rayleigh Criterion, it is the maximum number of resolvable spots that a deflector can generate corresponding to the maximum number of separate position of the diffracted light beam.

RF Power: Electric power that can be delivered by the driver.

Rise Time: The time it takes for the laser beam to respond to a change in the RF signal. It is proportional to the time it takes for the laser beam to be crossed by the acoustic waves. Rise time can generally by decreased by reducing beam size.

Separation Angle: The angle between the zero order and the first order beams.

Zero Order Beam: The beam directly transmitted through the cell.

Wafers

Surface Acoustic Wave (SAW): A technology which converts frequency signals into acoustic signals and confined them within a small substrate made from Crystalline materials like Lithium Niobate.

Mask: A flat piece of glass that contains the patterns to be produced on a wafer.

Warp: The difference between the highest and lowest point on the wafer surface measured normal to the selected focal plane in the free state.

Bow: The difference between the selected focal plane and the surface of the wafer at the center in the free state. This value can be positive or negative.

Taper: The lack of parallelism between the back surface of the wafer and the selected focal plane (best fit plane). The value reported is the amount of rise in the focal plane, not the slope of the surface, and is therefore expressed in microns for the entire given diameter.

Total Thickness Variation (TTV): The difference between the highest and the lowest point on the top surface of the wafer.

Local Thickness variation (LTV): A stepper exposure simulation measure the difference between the highest point and lowest point on the top surface of the wafer for each exposure site. Results can be reported as the maximum value found for each wafer or the percentage exposure sites under a given LTV value. Common site areas used are 5x5mm and 10x10mm.

Focal Plane Deviation (FPD): The greatest distance above or below the selected focal plane. Measurements are reported as positive or negative, whichever is greater.

Total Indicated Reading (TIR): The difference between the highest and lowest point on the wafer surface measured normal to the selected focal plane. This essentially removes overall taper of the wafer from the measurements.

Optical Grade: Grade of material used in optical applications such as high speed modulators and polarization-controlling devices. Optical material is low strain with no inclusions.

Refractive Grade: Grade of material used for lower demanding optical applications such as Wollaston prism for DVD heads and blur-filters for digital cameras. Strains are tolerable but inclusions are not.

SAW Grade: Most commonly produced grade and is used for making surface acoustic wave devices. This is for non-optical applications and the least demanding in terms of impurities and crystalline imperfections.