Probably 1/4 wave plate and linear polarizer, at least that's what I use as "poor mans" optical isolator. You can tune the optical thickness by tilting the 1/4 wave plate around either the fast or slow optical axis. (with 45 degree input light.)
George H.
VCSELs have very short cavities, so they're less vulnerable to backreflections causing mode hopping. On the other hand, they're generally much more prone to running multimode spatially. One high speed VCSEL IBM used to use for datacom ran in a 6th order mode with 12 peaks--it looked like a chrysanthemum.
Cheers
Phil Hobbs
Your average CW diode laser needs about 60 dB (optical) isolation to avoid mode hopping. In a fibre system that means a 2-stage Faraday isolator. The quarter wave plate / polarizing beamsplitter is good for
40 dB optical with great care, but not more, so you need quite a bit of path loss to be safe.VCSELs in general are less vulnerable.
Cheers
Phil Hobbs
That seems about right.. though I've never measured the attenuation. I'm guessing I don't get 40 dB, cheap plastic waveplate and polarizer... and I'm using the polarizer near the edge of it's useful wavelength range, about 1% crossed transmission, rather than 10^-3 or better in the 'good' region. Still with enough futzing one can make it work.
George H.
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