Laser specs:
- Yb Laser
- Wavelength=1070 nm
- Mode of operation : cw/modulated
- Maximum average power=400 W
- Modulation frequency= 0-50 kHz
Usually are laser on/off switched. So long as ton / toff is constant, the mean-power is constant. Laser modulationis possible up to severel GHz, depending on the internal carrier lifetime. The laser driver cicuit ist the most limiting part.
I will limit my answer to semiconductor lasers. The depth of modulation of the optical output of a semiconductor laser is a function of modulation frequency due to a number of effects. Multimode lasers resonate at high frequency due to carrier-photon interactions. This resonance shows up in the FM response at a frequency that is power dependent since the more photons in the cavity, the 'stiffer' the feedback and the higher the resonance frequency. Typical resonant frequencies appear between 1 and 10GHz. There is also a drop in FM response due to parasitic effects like capacitance. This effect combines with the resonance. In single mode lasers, spatial holeburning can contribute to the FM response as well. Here are a few early references on the subject of semiconductor modulation: R. Olshansky, P. Hill, V. Lanzisera and W. Powazinik, "Frequency response of 1.3µm InGaAsP high speed semiconductor lasers," in IEEE Journal of Quantum Electronics, vol. 23, no. 9, pp. 1410-1418, September 1987; R. S. Tucker, "High-speed modulation of semiconductor lasers," in IEEE Transactions on Electron Devices, vol. 32, no. 12, pp. 2572-2584, Dec. 1985; P. A. Morton et al., "25 GHz bandwidth 1.55 mu m GaInAsP p-doped strained multiquantum-well lasers," in Electronics Letters, vol. 28, no. 23, pp. 2156-2157, 5 Nov. 1992.
Fundamentally, PM/FM modulates the angle of a periodic signal. So, it does not affect the total power of the signal. It just distributes the initial carrier power to sidebands depending on the value of modulation index. More details can be easily found from standard literature on angle modulated signals.
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