[Laser] Daytime optical contacts

[Tim Toast]

Hi Stuart,
 
Very exciting work you and the others have been doing. I still find it amazing how 
well an LED can work as both transmitter and receiver even while not perfectly 
matched as to RX and TX wavelengths. Apparently it's not holding you guys back at 
all. The signal penalty for using subcarriers (if there is one) is not either. And doing it 
in full daylight is like icing on the cake.
 
The wavelength issue has been brought up in here before. I think with diode lasers, 
due to the bandwidth being so much narrower, the effect is much more pronounced, 
sadly. It would be nice to be able to make a laser version that could at least work as 
well as LED's. To be able to use single optics easily is definitely worth the lack of full 
duplex for ordinary voice links.
There may be a way to have full duplex anyway by multiplexing and synchronizing 
the transceivers with some sort of outboard switching box. Switching from TX 
to RX while the other end switches from RX to TX at some rate above 10 hz or so. 
I realize you are using subcarriers, so switching noise in the baseband would not 
affect that i'm guessing - unless a harmonic of the switching rate were near the subcarrier 
frequency - close enough to give an audio beat.I wonder if there is some intrinsical 
signal level penalty with this kind of multiplexing? It might be done without having to 
rely on GPS - TV sets do it at much faster rates without needing GPS by using a sync 
pulse. At any rate, it may not be worth the added effort and cost. 
 
Good luck with all the experimenting and keep us updated!
 
-toast
 
 
 
> Message: 1
> Date: Wed, 10 Aug 2011 07:18:13 -0400
> From: Stuart <stuart.wisher at talktalk.net>
> Subject: [Laser] Daytime optical contacts
> To: laser at mailman.qth.net
> Message-ID: <8CE25665421DF55-13F0-771F at FRR5-L19.sysops.aol.com>
> Content-Type: text/plain; charset="us-ascii"
> 
> 
> Hello Tim,
> 
> 
> 
> I have not been on for a while so this is a bit of a catch-up on some UK 
> activity.
> 
> 
> Barry, G8AGN has started a Yahoo group, "UKNanowaves where there are a lot 
> of messages re current developments, files and photos of gear etc.
> 
> 
> The name sprang from a microwave round table we were both attending. Barry and 
> group refer to "lightwave communication, and the group to which I belong 
> refer to "optical communication". During a talk on microwaves, Peter, 
> G3PHO was kind enough to add in comments on our work referring to 
> "Nanowaves", and we decided to adopt the term. We went on to 
> demonstrate nanowave communication over a short distance in full sunlight.
> 
> 
> Our UK groups have been focussing on daylight operations during the summer. We 
> have found that red light can be made to work in full sunshine and distances up 
> to 40-odd km have been successfully worked. Stuart and group have recently 
> departed to near infrared (NIR), and after making a start on 940nm we 
> successfully made a 6.3km contact in full sunshine. 
> 
> 
> We have since built new rigs on 850nm after correspondents on this reflector 
> kindly pointed me in this direction as atmospheric attenuation is likely to be 
> less.
> Yesterday we made our first contacts on 850nm using LED transceivers (the same 
> LED used on both receive and transmit). The text below is a report of this 
> activity and the results gained:
> 
> 
> 
> Two successes on 850nm
> 
> 
> Yesterday afternoon, two contacts were made on the 850nm Near Infra Red (NIR) 
> band, following on from the recent 6.3km contact on 940nm.
> 
> 
> Stuart, G8CYW operated from Currock Hill IO94BW93, some 800 feet asl starting 
> just before 3pm today, 9th August. The first contact was with Gordon,G8PNN using 
> his rig located at IO95CA64 with Brian, G8KPD (rig still stowed in his car, such 
> confidence!) and Peter, G8POG were present. This was a 10km distance and Stuart 
> lined up his exteme beacon ,  itself co-aligned with a carrier on 850nm, brought 
> to bear on to Gordon's location. The beacon was seen immediately and Gordon 
> lined up his transceiver in short order. Of course since both the rigs were LED 
> transceivers, we were both now aligned on each other and FM signals were  
> immediately exchanged at end-stopping S-meter indications both ways. 
> 
> 
> Brian, Gordon and Peter then departed for IO95DC27, Saltwick, at 21.5km distance 
> and 340 feet asl. This time both Brian's and Gordon's (identical) rigs 
> were deployed. Peter spotted Stuart's location and used his Fresnel-equipped 
> Xenon strobe to mark the location for Stuart who replied with the same exteme 
> beacon and co-aligned 850nm signal. The same lining up procedure as before 
> rapidly brought about contacts for all stations on FM (all segments of the FT817 
> S-meter illuminated) and SSB signals were at S+ as recorded by the HF rig. The 
> FM signals were fully quieting as would be expected. It seems odd that nothing 
> visible passes between stations once the beacon is turned off. The hardest thing 
> was to find each other's location in the daylight!
> 
> 
> It is interesting to note that even in the afternoon sunshine, the noise level 
> on the FT-817 was S1 to 2 on SSB and S6 on FM. As stated before, it seems that 
> the 850nm LEDs used for both receive and transmit have the property of not 
> responding much to the daylight, very handy.
> 
> 
> Given these signal levels, it is clear there is much potential on this band with 
> these rigs for a greater range, and in due course, we will revisit some of the 
> sites we have used in the past on 630nm (visible red light) to attempt greater 
> distances.
> 
> 
> Regards to all,
> Stuart
>