I attended Underwater Intervention in New Orleans and talked to several vendors about Fiber Optic tethers. I asked the WHOI presenter about Cameron's vehicle and they indicated that neither the Lander nor the Sub was tethered to the surface, although I believe the Landers went to Scripps. The WHOI Nereus uses bare fiber that is dispensed in both directions and then jettisoned. I think they are able to get continuous fiber in lengths of at least 20KM. I saw a nice Kevlar coated fiber cable with a break strength of 300 lbs/ 136 KG and a dry weight of 3.5 lbs 1000 feet which should be able to carry its own weight and a small ROV to depths of the 4 kM maximum continuous length. It sells for around $4 per foot or meter I forget which. Very high quality optical rotary joints necessary on the reel end were quite expensive, in excess of $1500. Chinese ones are cheaper and there is always the guy on the side of the boat that the Japanese used to dispense the tether to their Sub for interactive video. They used bare fiber and jettisoned it after the broadcasts. I didn't ask the price of a fiber optic penetrator, but several manufacturer's make them. You should be able to put together a 100 meter fiber tether fairly easily with off the shelf components. It might cost as much or more than your ROV using best quality components, but I think could be done for less. I'm trying to find out he costs for bare fiber and learn more about dispensing systems and see if the costs could be lowered. If you are going to jettison your tether you might want to consider some sort of recovery system for your ROV. Off the shelf GPS + Iridium systems potted and packaged are available, but if have to ask you can't afford them. I also assume but don't know, one could substitute the TP-Link WDM Ethernet to Fiber converters which cost about $50 and are good for 20kM to drive the system, space and power permitting, for the 2 wire Tenma interface currently used. For a 100 meter ROV this is all probably overkill as indicated below. I personally think stripping Ethernet cable and using one of the pairs is pretty clever.