Here is the original link to the info about VCA, FET and Opto modes in Logic's compressor:
I've copied here for easy reference:
Optical isolators â€¢
The optical isolator section of compressors uses a light bulb (or an LED) to glow brighter or dimmer in response to incoming audio. A photocell (or phototransistor) is used to track the varying brightness of the bulb and change gain accordingly. This is a good example of an average responding detector. The inherent lag time that the bulb/photocell has in response to audio is factored into the attack and release time performance. The Teletronix LA-2 and the transistorized UREI LA-3 leveling amps are examples of this type of compressor. Compressors using this method are used a lot for bass guitar, vocals, program mix and drums. These compressors offer simple, natural sounding control (unless pressed hard).
Field Effect Transistor compressors use a special transistor to vary gain. FETs were the first transistor to emulate tubes in the way they worked internally. Inherently a high-impedance device, the FET compressor sounds like no other box, and not many examples exist because of the expense of the extra attendant circuitry required. FET compressors are extremely fast, clean and reliable. I like to use these on vocals where a good amount of compression sounds good or better than the same amount with another unit, and on drums for room mics or individual drum mics. UREI's 1176LN peak limiter and LA Audio's Classic II stereo compressor/limiter are examples of FET-based compressors.
Voltage Controlled Amplifier compressors are the most versatile of all and so are the greatest in number. The VCA can quickly change gain in response to many different detectors looking at the same signal. VCA compressors are for the really tough cases where you want strict control over level and dynamics. However, they can be as gentle as any other compressor or anywhere in between. I like a good VCA compressor on vocals, drums, guitars, synths, bass, mix--basically anytime I need a compressor.
Vari-gain compressors â€¢
Variable Gain compressors include all units that incorporate discrete circuitry other than VCAs, FETs or opto-isolators. I put the Manley Variable-Mu tube unit in this category, and I like using these types for vocals, drums and stereo mix.
Computer based/digital compressors â€¢ Computer-based or digital compressors are now more prevalent, offering the ability to process audio in the digital domain under precise computer control. Digital audio stored in a computer has the advantage of being "predictively" processed. You can have zero attack time gain reduction as well as nearly infinite control of all parameters of compression on a moment-to-moment basis. The Waves L1-UltraMaximizer TDM plug-in for Pro Tools is one example of a software-based compressor. TC Electronic makes the Finalizer Express hardware unit for studio mastering. This is a single-rackspace, stand-alone, digital multiband compressor.