If I have two switches that are about 400m apart and I want to connect them through a optical fibre connection, whats the best way to do this? I am thinking of using media converters that take a GbE connection on the switch side and convert this to multi-mode optical transmission signals that can be re-converted on the other side. These converters can either perform WDM for full-duplex over a single optical fibre or I could use a two optical fibres for Rx and Tx. Still new to optical data transmission and am just curious to know if there are better ways.
Second the postings of the other gents. I would avoid media converters for a single run fiber between devices. Another point of failure from the school of hard knocks. Most GigE to fiber converters are generally inexpensive devices that if they go offline on the fiber side can't report the link down unless it supports snmp polling or traps. Of all of the CWDM and DWDM devices I worked with all of the wave lengths for Tx go down one fiber and all of the Rx go down the other. The only time I use single strand fiber is for video transmission gear which is the normal for SDI protocols.
Stick with switches that have sfp slots and try and purchase sfp's and switches that support DOM "digital optical monitoring" so you can get power level stats. DOM can be a lifesaver troubleshooting tool.
Media converters are fine if your just going from a copper switch port out to an LED street sign that doesn't get updated often or isn't mission critical.
Every customer that I have worked with that had media converters eventually pulled them out over troubleshooting lacking information.
I think the easiest way is going to depend on the capabilities of your switches. If they have SFP ports then the easiest way would be to install the correct fiber module and connect the new fiber port in your switch to your existing fiber with a patch cable (pay close attention to connectors). If your switches are void of such technology and you don't have the budget to procure new ones then you are relegated to media converters. (Keep in mind, media converters are a stop gap measure.)
Single fiber optics I believe are not very prevalent, so sticking with your desire for easy I would not pursue them unless you are low on available fiber and desire to preserve what is left.
You may find that a pair of gigabit switches with SFP slots is, in fact, cheaper (or the same price, or nearly) as a pair of media converters. Media converters tend to be overpriced, as the majority of fiber installations don't use any, so they are relatively low-volume as compared to switches with SFP slots and copper ports.
While the default logic has long been pushing multi-mode for "relatively short links" 400m is a fairly long multi-mode link, especially if you might ever upgrade to 10GbE. It may be long enough that you will find (as I did in 2010) that single-mode fiber is actually cheaper by enough to pay for slightly higher priced SFPs. At that time single-mode cable was about half the cost of OM3/4, and SFPs were about 35% more expensive for single-mode (new - used is different, see below.)
In general, if installing fiber you don't want to get locked into BX/singlefiber SFPs - 12 fiber cable is barely more expensive than 1 fiber cable (at least it is if you are shopping the right vendors) as the cable itself costs a good deal more than the fiber in it. You'll never be sad to have 10 "extra" fibers later on, either. BX (WDM) SFPs are for when you have limited fibers and you want to get more data on the same fiber without installing any more.
If you are willing to shop used items, single-mode SFPs that do gigabit ethernet quite happily but were made to do (now horribly outmoded) 4 Gb fiberchannel are available used for dirt cheap. So are Gigabit SFP switches. If not willing to shop used, prices are a bit stiffer.