Two Dimensional Silicon PSDs

A position sensitive detector (PSD) works like a normal photodiode. The laser that falls upon the active area generates a photo current that flows in the direction of the p and n areas. Unlike a photodiode, a PSD possesses several electrical contacts. This results in a distribution of the photo current among the contacts, depending on the position of the laser spot.

The position can be calculated by the difference in current of two opposing contacts. When the total current is normalized, the position signal no longer depends on the incoming laser intensity. Unlike pixel elements (CCD), the resolution of position sensitive detectors is not restricted by the pixel size nor is an extensive computer analysis necessary. Segmented position detectors such as differential and quadrant detectors can only measure distances up to the size of the beam diameter.

A PSD, however, is suited for measuring larger distances across the whole active surface. Form, size, and intensity distribution of the laser spot are to a large extent inconsequential. The power center of the laser spot determines its position.

A two dimensional PSD makes continuous detection of the position of a laser spot along two axes possible. It possesses four contacts.

The position is calculated using the following formula:

X = LX / 2 x (X1-X2 / X1+X2)

Y = LY / 2 x (Y1-Y2 / Y1+Y2)

In addition to the standard PSD with a side length from 2 x 2 mm to 45 x 45 mm, UV and YAG optimized detectors and PSDs with increased spectral sensitivity (ES series) are available.

Not only that but position sensitive detectors can be equipped with an anti-reflection coating. The manufacturer can provide calibration data for all PSDs, which contains the linearity of the detector.

Our Swedish partner SiTek is the market leader in high quality position sensitive detectors (PSDs). In addition to unrivaled quality regarding linearity and speed, SiTek is also setting new standards regard size.

The 2L45_SU24 measures 45 mm x 45 mm, making it the largest two-dimensional PSD worldwide. With a position linearity of typically +/- 0.3% and a rise time of only 7 µs this PSD is optimally suited for fast positioning and alignment tasks in the spectral range from 400 nm to 1100 nm.