Xref: utzoo sci.electronics:18260 sci.optics:119
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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics,sci.optics
Subject: Re: Laser diodes/ optics (esp. IR)
Summary: Techniques to improve distance
Keywords: Lasers,Infra-red,optics
Message-ID: <4779@kitty.UUCP>
Date: 7 Mar 91 04:19:32 GMT
References: <1991Mar5.164506.18567@gdt.bath.ac.uk>
Followup-To: sci.electronics,sci.optics
Organization: Recognition Research Corp., Clarence, NY
Lines: 25

In article <1991Mar5.164506.18567@gdt.bath.ac.uk> ee8kh@gdt.bath.ac.uk (K House) writes:
>I'm currently undertaking a small project using Infra-red LEDs to transmit 
>information over distances of about 50 metres (through air).
>However, the range I am currently achieving is not really high enough for the
>application I have in mind and so I am toying with the idea of using laser 
>diodes as emitters instead of the diodes I am using at present.

	I don't believe it is necessary to utilize laser diodes to achieve
greater range, which later in your article you state is 200 meters.  One
of the simplest techniques in extending range to 200 meters (which is about
the limit, btw) is to use multiple IR-emitting diodes in an array.  Since
multiple IR-emitting diodes cost less than an IR laser diode, this should
be an attractive alternative.

	A second technique is to use a suitable IR filter material for the
photodiode receiver.  Failure to filter light below 800 nm will result
in serious limitations to amplification of photodiode signals since its
input will be significantly loaded by ambient lighting - especially that
of fluorescent illumination.  Many fluorescent lighting systems produce
a really ugly 120 Hz signal with many higher frequency components because
it is not a sine wave due to ballast characteristics.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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