Free space optics are heavily affected by the atmospheric conditions you mention, especially at longer distances. In your environment you also have to take into account such things as exhaust stacks and flares. These also cause atmospheric distortions that can disrupt optical transmissions.
My suggestions in order of preference:
1)    Call your local service provider, (Sprint, Time Warner, etc,) and ask about direct IP connectivity. The larger carriers are offering high bandwidth IP services now. You may be able to get a business class Cable or DSL circuit and do a VPN connection between offices.
2)    Look into point to point services, (T1’s, etc.). You can get a couple point to point T1’a, bind the channels on the router to load share, and it offers some redundancy for T1 failure.
3)    Point to point wireless. Motorola offers a Canopy backhaul radio that will do 20 Mbit full duplex, built in security, and will operate over 10 miles. We use these in several applications and they work well. There are many unlicensed, and licensed alternatives for short haul, high data rate communications.

For redundancy, I would recommend a combination of methods. In several locations we use the common carrier, (Sprint, Verizon, etc.) for primary, with a wireless backhaul as backup. At other locations we use the wireless as primary with older in-plant cabling as backup.

Part of the problem is that the new office building is actually part of an older plant's campus of buildings. That plant, a phosphate mine that hasn't been in use for years has wiring from the late sixties and is located a little off the beaten track for cable --DSL might be an option if the phone company didn't charge for rewiring all the way out (I fear it is POTs all the way).

Having 2 separate T1s _is_ the alternative FSOptics is being compared to. The cost of the FSO link would go to near zero after a spcified number of payments, where doubling the cost of the T1s is permanent. It would also be true that on those rare occasions when the T is down, it would be down on both sides so that one office would be unable to print/phone/email the other side even though our LAN was up and feeling fine.

The FSO option we are looking at does spec in some redundancy in the form of 10Mps RF connection, but we could probably achieve that level or higher with a couple of cantennaes and wireless routers to hop the 300 meters of not-ours-land between the two properties. We can if need be, run fiber out to a shack at the edge of these properties and have a shorter space to link --that is part of what we are studying in this problem.

Thanks for your input. Anyone else want to pipe in that use a Canonbeam or other free space optical unit?

Scott Stanek
Arr-maz Custom Chemicals dBA

Scott, I would try wireless 2.4 Ghz. It is less sensitive to atmospheric conditions that the 5 Ghz stuff. Optical is going to be a nightmare.
At the new location with it's old wiring, I would try the LRE solution that Cisco sells, it's like DSL, and runs on single pair copper.

Why not work with a local civil engineer and have them contract a boring underneath the railroad tracks for your fiber run?  You might can also lease out space in your conduit to other companies as well.

Just a thought.

There are quite a few decent options for
line-of-sight communications.   Investigate
microwave and infrared links - telco's and
pipelines used microwave extensively as
transport (and still do, to some extent).
Infra-red is relatively newer, and a lot
of vendors are appearing.  Either technology
can be engineered for reliable service.
There are other options (satellite, lower
radio bands, visible light bands, etc.),
but one of those two seems more appropriate
to your needs.

If you do not require in-house equipment,
your local communications provider (telephone
company) can probably offer a range of options
such as DSL, OC1/OC3, ATM, or frame relay.

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