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Why are most of the VCSEL light holes of 50G PAM4 not "round"?

Why are many manufacturers starting to design non-circular light exit holes in 50G PAM4 VCSEL products? The reason is to reduce RIN.

RIN is relative intensity noise, that is, noise relative to the strength of the signal. Noise is defined as uncertainty in magnitude of deviation from ideal. For example, we want a flame to stabilize at a height of 1cm, which is the ideal amplitude. However, the flame is beating, sometimes 0.9cm, and sometimes 1.2cm. This deviation based on the ideal level is noise.

Similarly, VCSEL is a multi-mode laser. If the output of multiple modes (spots) has a power value jump, it is the noise of the relative intensity (target output power) (the power value fluctuates up and down, and there is a slight jump).

This fluctuating change, if you accumulate 100 or 500 signals, the eye pattern will be very hairy and cannot be opened, and PAM4 is more sensitive to this uncertainty. I hope the noise is as small as possible. NRZ has only two signal amplitudes and is more tolerant to noise.

It just so happens that the encoding format of the optical module, 25Gbps VCSEL, in Ethernet is NRZ, and the encoding format of 50Gbps VCSEL in Ethernet becomes PAM4, which is an industrial node associated with 50G and PAM4.

Let's zoom in on the light hole. If these modes are distributed in a circular light hole, it will be like a flashlight. If it is round, it will roll, and the output light will change back and forth like a stage spotlight. stable.

Turning the light hole into an ellipse, a semicircle, or a tooth shape can make the energy distribution of light and electricity not have this "wheel" instability. Relatively speaking, if the light spot is kept as far as possible, the noise It's small. Suitable for PAM4 modulation format which is more sensitive to noise.

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