would there be any problem with using SFP+longwave (X6604A in FAS8300) at a distance of 20~30m?
I don't believe so. It supports up to 10km. I had to do a little digging from HWU (https://hwu.netapp.com/Cable/Index?typeId=1#none) and then Google took me to this page: https://www.finisar.com/sites/default/files/downloads/finisar_ftlx1471d3bcl_rohs-6_compliant_10gbs_10km_single_mode_datacom_sfp_transceiver_product_sp...
Since the Tx and Rx power values are the same (+0.5 dBm), there really isn't a minimum.
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