Relative intensity noise (RIN) of selective oxidized multimode VCSELs has been investigated. The VCSELs show an excellent noise performance with a RIN <= -140 dB/Hz at 500 MHz and only one resonance peak in the RIN spectrum, as long as the total light power is detected. The situation deteriorates if the transmission link contains mode-selective losses. Introducing external filtering of the emitted light by an aperture or a polarizer, one may observe an increase of RIN by several orders of magnitude and multiple-resonance peaks in the RIN spectrum. However, even in spite of a very restrictive external filtering a RIN of -120 dB/Hz can still be achieved, which enables a 1.25 Gb/s transmission. The multiple-resonance peaks in the filtered RIN spectra do not describe the different relaxation resonance frequencies of transverse modes. We believe there is only one relaxation resonance frequency,which is proportional to the square root of the photon number in the active layer, irrespective of the number of existing modes. The other peaks in the RIN spectrum can be considered as"mode partition frequencies,"which result from the carrier interchanges between the modes. Using a simplified numerical model, which takes two modes into account, analytical expressions describing the RIN of each mode, as well as of the total power have been derived. It has been found that the larger the overlap between the modes, the smaller the"mode partition frequency"and the larger is the maximum mode partition noise (MPN).
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