Just to be picky, this isn't optical FM because that would involve modulating the frequency of light which is possible with a tunable laser.
You need a 100kHz VCO but the problem is getting sufficient modulation at that frequency sinewave isn't easy. You could build a triangle VCO and use a wave shaper to get a sine output. Another option is a beat frequency oscillator. Build two RF oscillators, one with a fixed frequency of 1MHz and a VCO with a centre frequency of 1.1MHz and a range and put them through a mixer. The output from the mixer will be 100kHz with no signal and you'll only need to change the frequency of the VCO by +/-20kHz to give a sweep of 80kHz to 120kHz.