This is not a perfect explanation, but I hope it helps.
When something is frequency modulated there are 2 frequencies, I would not define one as "normal".
There is a frequency that is a digital 1, there is a frequency that is a digital 0.
So, when you are receiving, lets look at a conceptually simple detector. You have a bandpass for each frequency, and on the output of these, you have a rectifier which measures your relative power.
When you are receiving, you need to know the bitrate, or you need a shared clock (which requires a second connection to clock the data, or a second communication channel that cycles between its two frequencies to send the data). Often you read off of the two rectified signals as if you were reading a UART line.
Normally your data rate is significantly slower than your modulation frequency. Normally by a factor of infinity, which in engineering we can get by as approximating as 10.
Please let me know if there is something I can expand on to make this make more sense.
That would really depend on where the harmonic distortion is occurring.
If it is downstream of the modulator, then it is the first - ie, the modulation spread is multiplied. In the easy-to-image case where the harmonic distortion were coming from an RF power amplifier this is what you would get.
If it is upstream such that the harmonics are present going into the modulator, then the second, only the center frequency is multiplied.
In simple cases, the FM modulation is applied to the source oscillator, but it is also possible to apply it afterwards, for example by mixing with a modulated local oscillator signal. However, there are likely to be filters following the mixer which may substantially suppress the harmonic content.
Best Answer
Solidstate lasers have internal capacitance (typically junction) and on times. Modulating something quickly with an internal capacitance will tend to just result with a smaller amplitude or not coming on at all. I don't know much about the diodes in a DVD drive, I'd guess it depends on the drive, however, there are plenty available online that do switch faster: https://www.arrow.com/en/products/plt5520-b1-2-3/osram-opto-semiconductors?gclid=Cj0KCQjw-Mr0BRDyARIsAKEFbef1wvwnZJ7LD2As3XJQe2lQ_xlWibDgCYdb348_OCXGmHE6qhXRzKYaArtDEALw_wcB
If you don't already know Digikey and Mouser are the big box stores of choice for this sort of thing, but if you need something special and are willing to pay out your nose, thor labs probably has it.