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Technology Stocks : Lightpath Technologies: LPTH New WDM player
LPTH 6.735-1.3%Nov 19 3:59 PM EST
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To: Duane L. Olson who wrote (282)12/23/1997 1:42:00 AM
From: Tim Bagwell  Read Replies (1) of 1219
 
Duane,

Thanks for the feedback. I'm always happy to have someone keep me honest because I don't want to miss something important. If a new laser technology is found then we want to be on top of it. But keep in mind that DFB's laser have been in development for over 30 years and they will be hard to replace. They have been perfected to a fine art.

I didn't see, for example, a claim that they reduced fiber dispersion, but rather the signal distortion caused by fiber dispersion

Of course, if I wasn't clear earlier I was refering to signal distortion caused by fiber dispersion. The claim by the laser vendor was that their lower linewidth would help reduce signal distortion caused by fiber dispersion. This is true for low bit rates.

I don't believe that a lower linewidth will have a significant effect on the signal distortion due to dispersion for high bit rates. DFB lasers already have low linewidth which is on the order of a few hundred kHz. Since this is already low compared to the bit rate it has little effect on signal distortion. Reducing it further wont have much additional benefit.

Now the claim about low noise is interesting. I believe DFB's run about -145 dBc/Hz and so are about 25 dB worse. However, this is not usually a problem in WDM systems which use filters. The filter helps to suppress the noise between channels. So carrier to noise in a WDM system is usually dominated by the very broadband EDFA noise. Laser noise is only a problem in cases where the modulation is very weak. This may be more of a consideration in CATV applications.

Another point to keep in mind is laser lifetime and drift. High power lasers tend to live shorter lives. This is a distinct disadvantage in the telecom world. Also with channel spacing decreasing the lasers must be extremely stable which may also not be consistent with high power.

Believe me, I'd love to find a company with a lock on a new laser technology that is important. Maybe it's worth digging up the patent reprints to look at this further...just to make sure I'm not missing something.
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