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Technology Stocks : Lightpath Technologies: LPTH New WDM player -- Ignore unavailable to you. Want to Upgrade?

To: R. Allan Choiniere who wrote (377)	2/17/1998 9:01:00 PM
From: Tim Bagwell	Read Replies (2) \| Respond to of 1219

Allan, I'll take a stab and Craig can fill in the gaps. DWDM is indeed shooting many colors of light down a fiber. In long haul networks the light is in the infrared but that's not important. The main problems don't have so much to do with harmonics as with dispersion. To put digital data on the light you just turn the laser on and off to form a 1 or 0. As the bit rate moves up the transitions from 1 to 0 become increasingly distorted due to fiber dispersion. This is because the transition contains a lot of high frequency energy which travels at a slightly different velocity than the lower frequency energy. Now you have components of the same signal travelling at slightly different velocities and that is dispersion. These components add up at the receiver with different delays and distortion results. To detect different colors of light one simply needs to build a filter for each channel of the WDM system. Then you put a broadband detector behind the filter and it only responds to the modulation in that channel. Depending on how good the filters are there can be crosstalk between channels and this is another source of distortion. As channels get crowded closer together it becomes increasingly challenging to build optical filters that can do a good job of limiting crosstalk. Also, as the bit rate increases some energy begins to spill over into the adjacent channel leading to a similar result. Both of these effects lead to "bit errors" that corrupt the data being sent. Long haul networks typically strive to have less than 1 error in 10E13 bits sent so you can imagine that everything has to be finely tuned. Hope that helps.