{"id":2318,"date":"2026-07-28T09:04:06","date_gmt":"2026-07-28T09:04:06","guid":{"rendered":"https:\/\/photo-detector.com\/?p=2318"},"modified":"2026-07-28T09:04:12","modified_gmt":"2026-07-28T09:04:12","slug":"aktive-grose-der-lichtempfindlichen-flache-photodiode","status":"publish","type":"post","link":"https:\/\/photo-detector.com\/de\/photosensitive-area-active-size-photodiode\/","title":{"rendered":"Die Auswirkungen der Spezifikationen der aktiven Gr\u00f6\u00dfe der fotosensitiven Fl\u00e4che auf den Nachf\u00fchrbereich des Galvanometerspiegels"},"content":{"rendered":"\n

If you have spent hours on an optical bench trying to figure out why your galvo mirror position feedback starts getting wildly non-linear at the edges of your scan angle, you are not alone. Most structural optical engineers jump straight to blaming mirror inertia, driver amplifier tuning, or mechanical flexure. But more often than not, the real culprit is sitting right on the feedback board: a fundamental mismatch between your optical beam geometry and your photosensitive area active size photodiode<\/strong> specifications.<\/p>\n\n\n\n

When designing high-speed optical scanning heads for laser marking, additive manufacturing, or medical imaging, getting the optical feedback loop right is everything. You need crisp response times, minimal phase lag, and rock-solid linear signal reproduction across the entire mechanical deflection range.<\/p>\n\n\n\n

In this article, we are going to dive deep into how photodiode geometry impacts your galvo mirror tracking range. We will walk through the exact optical geometry math, examine real-world chip dimensions, evaluate spot size interaction, and look at actual engineering trade-offs so you can size your photodetector correctly on the first spin of your PCB design.<\/p>\n\n\n\n

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Photodiode Chip Dimension Outline vs. Active Photosensitive Area<\/h2>\n\n\n\n

One of the most common mistakes in early-stage optomechanical packaging is confusing the physical die size with the light-sensitive surface area. When reading a detector datasheet, you will see two very distinct dimensional callouts: the photodiode chip dimension outline<\/strong> (the total physical footprint of the silicon die) and the actual active area dimensions.<\/p>\n\n\n\n

The physical chip outline includes guard rings, passivation layer borders, bond pad real estate, and dicing street tolerances. The photosensitive active area, on the other hand, is strictly the p-n or p-i-n junction region exposed to incident photons where electron-hole pairs are efficiently collected to generate photocurrent.<\/p>\n\n\n\n

Why does this matter so much for galvo tracking? Because your mechanical tolerance budget depends on total chip size for placement, but your optical tracking geometry cares exclusively about the active photosensitive boundary.<\/p>\n\n\n\n

Model \/ Sensor Category<\/th>Physical Chip Outline<\/th>Active Photosensitive Area<\/th>Junction Capacitance (VR = 5V)<\/th>Typical Galvo Tracking Setup<\/th><\/tr><\/thead>
Single-Element PIN (Compact)<\/td>3.2 x 3.2 mm<\/td>2.9 x 2.9 mm<\/td>~25 pF<\/td>Short-path high-speed feedback<\/td><\/tr>
Standard NIR Si PIN<\/td>3.5 x 3.5 mm<\/td>3.0 x 3.0 mm<\/td>~30 pF<\/td>General industrial laser steering<\/td><\/tr>
Two-Element Segmented PIN<\/td>3.4 x 3.2 mm<\/td>Dual (1.6 x 3.2 mm each)<\/td>~18 pF per element<\/td>High-precision differential sensing<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

When you specify a sensor like the PDC-C2928-NIR-B 940nm photodiode chip<\/a><\/strong>, you are looking at a tailored silicon layout designed to maximize active collection while keeping parasitical capacitance low enough for multi-kilohertz galvo feedback loops.<\/p>\n\n\n\n

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The Optical Geometry Math: Connecting Beam Deflection to Active Area<\/h2>\n\n\n\n

Let’s break down how a galvo mirror’s mechanical angle translates into physical spatial displacement across the photodetector face.<\/p>\n\n\n\n

When a galvo mirror rotates by a mechanical angle Theta (measured in degrees or radians), the reflected optical beam deflects by twice that angle, or 2 * Theta. If the photodiode is positioned at an optical lever arm distance L from the center of rotation of the galvo mirror, the linear displacement Delta_X of the laser spot centroid across the photodiode plane is calculated as:<\/p>\n\n\n\n

Delta_X = L * tan(2 * Theta)<\/p>\n\n\n\n

For small angles, you can approximate this using the paraxial approximation:<\/p>\n\n\n\n

Delta_X = 2 * L * Theta (where Theta is in radians)<\/p>\n\n\n\n

However, the laser beam is not an infinitely small point source; it has a finite beam diameter or laser spot size photodetector<\/strong> footprint, typically defined at the 1\/e^2 intensity threshold (d_s).<\/p>\n\n\n\n

To maintain a linear response, the entire laser spot (or at least a fixed percentage of its energy distribution, such as 99%) must remain completely within the boundaries of the active photosensitive zone. Once the outer tail of the Gaussian beam profile spills over the edge of the active region\u2014a phenomenon known as beam clipping\u2014the photodiode output signal drops non-linearly relative to the mirror angle.<\/p>\n\n\n\n

Therefore, the maximum unclipped linear beam displacement Delta_X_max allowed by your photosensitive area active size photodiode<\/strong> is:<\/p>\n\n\n\n

Delta_X_max = (A_x – d_s) \/ 2<\/p>\n\n\n\n

Where:<\/p>\n\n\n\n