Identify this electronic component from DVD drive
This definitely is the optical receiver in the pickup head. Which other IC would be that naked?

In the center, you can already see black stripes, forming an array of photo diodes.

I have found this page with the following microscopic image of such a sensor, showing a 2x2 array in the center and four stripes above and below:



There may be other, in the past more simple setups, but this is the most common today. The arrangement of photo diodes purposes focus and alignment:

Alignment

Simply speaking, the projection of the tracks runs horizontally over the sensor in my picture, and only the 2x2 cells should see light and shadow from the pits and lands, wile the outer stripes should see a constant light level from the area between the tracks. If the stripes start to see pits and lands, too, the pickup head is misaligned and needs to be moved fore or back.

Focus

Well, this is tricky. 

The projection of a dot from a spherical lens is small circle. If the system is out of focus, the projection is a larger (blurry) circle, and doesn't give a hint into which direction the lens has to be moved.

The projection of a dot from cylindrical lens is a stripe, which becomes wider (and blurred) when out of focus.

An elliptically shaped lens (imagine a spherical lens stretched in one direction) has two focal planes, one for the higher and one for the lower curvature. As result, the projection of a dot is a "standing" ellipse in the one focal plane, a "lying" ellipse in the other plane, and a circle in between.

The pickup head uses such an elliptical lens rotated by 45° to focus the pits and lands onto the 2x2 cells. When the sensor is in the middle between the focal planes, a dot gives a circle with equal amount of light on each of the 2x2 cells. If the sensor is near to one of the focal planes, the projection becomes an ellipse with the large axis angled at +45° or -45°. I.e. two diagonally opposed cells of the 2x2 array get more light than the other two. Depending on which cells get more light, the system knows into which direction the lens has to be moved.



By the way, the webpage linked above has a picture showing a sketch of the entire system, including laser and disc. But they got the orientation of the sensor wrong. It can't detect alignment this way, and has to be rotated by 90°.

Ah, and you don't really get an image projection of the CD/DVD on the sensor, instead, it's an interference pattern. But the principle behind is the same.