Re: [time-nuts] Using lasers for data transmission
On 1 May 2016 15:02, "Bruce Griffiths" wrote: > > The solution with high power is to use a beam expander so that the unaided eye cannot collect a power greater than the safe limit. Using near IR beams also helps. > Bruce It is a long time since I worked with lasers, so my knowledge is both out of date and my memory not perfect. But I was a long while ago regularly using and sometimes aligning a picosecond pulsed laser at about 800 nm. The average output power was 1 W, so a very dangerous class IV laser. It certainly hurt if one had the beam on ones skin, but it was not sufficiently powerful to noticeably burn the skin if one did the obvious thing and moves ones hand away. But I believe people need to be particularly careful using IR lasers. The lens in the eye will not focus Iinfra red on the retina, so that is probably why you say IR is safer. However one of the protective methods the eye has is a "blink response". One blinks if one perceives a light source as bright. Blinking offers some protection to the eye. But since one does not see an IR laser source, one does not blink, so IR lasers disable one of the eyes protective mechanisms. Hence there are various complications that arrive when discussing laser safety issues. I don't have the knowledge to advise on what is or not safe, but will warn there are several effects which are important, and many people don't realise this. Laser safety is a nontrivial subject. Dave. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
Thank you, I personally were talking about 1400nm 1mw lasers, however. Supplying just above the threshold current is not a problem. does the raising time can be reduced if using lower current/voltage raises or falls? I mean: how's calculated the raise time, full-scale pulse or for a mW/mA or so amount? Ilia. Il 01/05/2016 18:48, Mark Sims ha scritto: There are "eye-safe" wavelengths that some laser diodes can operate at (generally greater than 1300 nm). These are much less prone to damage eyes. Basically your eyeball juice blocks the wavelength. Still, there is some potential for cornea and lens damage at higher powers. http://www.laserfocusworld.com/articles/2008/03/photonic-frontiers-eye-safe-lasers-retina-safe-wavelengths-benefit-open-air-applications.html ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Ilia Platone via Ferrara 54 47841 Cattolica (RN), Italy Cell +39 349 1075999 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
On 5/1/16 9:48 AM, Mark Sims wrote: There are "eye-safe" wavelengths that some laser diodes can operate at (generally greater than 1300 nm). These are much less prone to damage eyes. Basically your eyeball juice blocks the wavelength. Still, there is some potential for cornea and lens damage at higher powers. http://www.laserfocusworld.com/articles/2008/03/photonic-frontiers-eye-safe-lasers-retina-safe-wavelengths-benefit-open-air-applications.html If they eye blocks (as in absorbs) the power, fine, you don't get a retina burn, but you get cataracts or other damage instead (more like "looking into the waveguide with your remaining good eye"). It's all about energy deposition and there's copious literature on what safe levels are (on Wikipedia even) This kind of thing is fairly easy to do safely, you just have to work through the scenarios and recognize that doing it outside isn't like doing it the relatively controlled environment of a lab. With reference to my previous comment about binoculars and telescopes, I don't think you have to worry about someone at the intended receiver site looking back at the transmitter (any beam divergence at all will make the energy density so low at the receiver, it's probably not an issue). It's someone who's a lot closer than you expected turning around and looking back at the transmitter or an unexpected specular reflection off something. BTW those geophysics measurements with red visible lasers across California were mostly done with HeNe lasers, because that's what was available back then. They also have a nice long cavity (The 1mW SpectraPhysics 155 that I had back in 1978 had a 30cm-ish cavity), so the spectral purity is quite good and the beam divergence is low. The HeNe are also a lot brighter visually than the red diode laser pointers for the same optical power. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
On 5/1/16 3:22 AM, Bruce Griffiths wrote: The solution with high power is to use a beam expander so that the unaided eye cannot collect a power greater than the safe limit. Using near IR beams also helps. Bruce IR is a problem for eye safety, because IR doesn't trigger the blink reflex, so you can inadvertently "stare into the laser with the remaining good eye". If you want to stay below, say, 1 mW/square cm, and you're running a 20 mW laser, you'd want 20 square cm of aperture. That's about 5cm diameter. You should bear in mind that if you're doing "mountain top to mountain top" type applications there might be someone looking at you with binoculars or a telescope, which makes their "light gathering aperture" much larger, and increases the risk of injury. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
Dear Ilia On Sun, May 1, 2016 at 6:40 PM, Ilia Platone wrote: > The problem would be modulating a 10GBASE-T signals into a single laser > beam, and demodulating it using (I think) an APD. > The White Rabbit cards use SFP (small form-factor pluggable) lasers that plug into the card. These incorporate both the laser(s) (there can be an uplink and downlink laser) and APD. Modulation in these devices is simply by varying the current. This puts a frequency chirp on the laser which exacerbates dispersive effects. You can do better with an electro-optic device like a Mach-Zehnder intensity modulator - but this isn't necessary. > except the one depending on light travel, that shouldn't be a problem if > using White Rabbit, there could be some problem with the modulating and > transmitter/receiver delay response times. I remember reading that the SFPs are a source of jitter but nonetheless, sub-ns timing is achievable. > I mean that lasers offer 100ps rise time, and the APDs I found offer 5ps > rise time, these must be multiplied by all the wires that GigE needs, which > should be 4 pairs if I remember correctly, and some are bi-directional, plus > the modulation process. > > Ilia. > Cheers Michael ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
The solution with high power is to use a beam expander so that the unaided eye cannot collect a power greater than the safe limit. Using near IR beams also helps. Bruce On Sunday, 1 May 2016 9:00 PM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote: > Hi, > Several (many?) years ago National Geographic magazine show a picture taken here in southern California of the state government sending red laser signals between different mountain tops to keep track what was going on near fault lines > There were no technical details on what was taking place. So it can be done. > At a hamfest a few years ago I bought both a red and green 35 mW laser pen for about $15 each. They do shine a long, long way. 35 mW is certainly unsafe to the eyes, so be very careful. There maybe legal issues about doing this. > Whether these are powerful enough, or can be properly modulated for what is needed, I have no idea. You can pretty much modulate any laser diode. There are two important currents to know about * Threshold current I_th - below which it will not lase. * Maximum operating current I_max - above which the device will be destroyed. You can AM modulate them by applying a DC current I_th + (I_max - I_th)/2. Then superimpose the modulation which has a peak value of (I_max - I_th)/2. Those currents ensure that the laser is always lasing, and gets you theoretically 100% modulation. For best lifetime, run at lower levels of peak modulation current. Watch out for transient currents - lasers make transistors look like antisurge fuses! For point to point contact you want a beam which diverges as little as possible. IIRC the divergence is something like inversely proportion to the cavity length. For this reason a diode laser with its short cavity is not optimal. But of course they are cheap. A veey long time ago I used to know quite a lot about lasers, but not using them for years I realise that I have forgotten an awful lot! FWIW, at university we had a 10 W argon ion laser. I think it took about 50 kW to produce those 10 W. When it was disposed of, it was sold to a company that put on light shows. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
Threshold current should not be a problem because if there's no data the laser could go into "power saving mode". As am modulation a simple buffer/r2r network DAC should do the job. The signals to transmit are three: Tx, and two bidirectional. Ilia. Il 01/05/2016 10:27, Dr. David Kirkby (Kirkby Microwave Ltd) ha scritto: Hi, Several (many?) years ago National Geographic magazine show a picture taken here in southern California of the state government sending red laser signals between different mountain tops to keep track what was going on near fault lines There were no technical details on what was taking place. So it can be done. At a hamfest a few years ago I bought both a red and green 35 mW laser pen for about $15 each. They do shine a long, long way. 35 mW is certainly unsafe to the eyes, so be very careful. There maybe legal issues about doing this. Whether these are powerful enough, or can be properly modulated for what is needed, I have no idea. You can pretty much modulate any laser diode. There are two important currents to know about * Threshold current I_th - below which it will not lase. * Maximum operating current I_max - above which the device will be destroyed. You can AM modulate them by applying a DC current I_th + (I_max - I_th)/2. Then superimpose the modulation which has a peak value of (I_max - I_th)/2. Those currents ensure that the laser is always lasing, and gets you theoretically 100% modulation. For best lifetime, run at lower levels of peak modulation current. Watch out for transient currents - lasers make transistors look like antisurge fuses! For point to point contact you want a beam which diverges as little as possible. IIRC the divergence is something like inversely proportion to the cavity length. For this reason a diode laser with its short cavity is not optimal. But of course they are cheap. A veey long time ago I used to know quite a lot about lasers, but not using them for years I realise that I have forgotten an awful lot! FWIW, at university we had a 10 W argon ion laser. I think it took about 50 kW to produce those 10 W. When it was disposed of, it was sold to a company that put on light shows. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Ilia Platone via Ferrara 54 47841 Cattolica (RN), Italy Cell +39 349 1075999 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
> Hi, > Several (many?) years ago National Geographic magazine show a picture taken here in southern California of the state government sending red laser signals between different mountain tops to keep track what was going on near fault lines > There were no technical details on what was taking place. So it can be done. > At a hamfest a few years ago I bought both a red and green 35 mW laser pen for about $15 each. They do shine a long, long way. 35 mW is certainly unsafe to the eyes, so be very careful. There maybe legal issues about doing this. > Whether these are powerful enough, or can be properly modulated for what is needed, I have no idea. You can pretty much modulate any laser diode. There are two important currents to know about * Threshold current I_th - below which it will not lase. * Maximum operating current I_max - above which the device will be destroyed. You can AM modulate them by applying a DC current I_th + (I_max - I_th)/2. Then superimpose the modulation which has a peak value of (I_max - I_th)/2. Those currents ensure that the laser is always lasing, and gets you theoretically 100% modulation. For best lifetime, run at lower levels of peak modulation current. Watch out for transient currents - lasers make transistors look like antisurge fuses! For point to point contact you want a beam which diverges as little as possible. IIRC the divergence is something like inversely proportion to the cavity length. For this reason a diode laser with its short cavity is not optimal. But of course they are cheap. A veey long time ago I used to know quite a lot about lasers, but not using them for years I realise that I have forgotten an awful lot! FWIW, at university we had a 10 W argon ion laser. I think it took about 50 kW to produce those 10 W. When it was disposed of, it was sold to a company that put on light shows. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Using lasers for data transmission
The problem would be modulating a 10GBASE-T signals into a single laser beam, and demodulating it using (I think) an APD. except the one depending on light travel, that shouldn't be a problem if using White Rabbit, there could be some problem with the modulating and transmitter/receiver delay response times. I mean that lasers offer 100ps rise time, and the APDs I found offer 5ps rise time, these must be multiplied by all the wires that GigE needs, which should be 4 pairs if I remember correctly, and some are bi-directional, plus the modulation process. Ilia. Il 01/05/2016 07:13, Perry Sandeen via time-nuts ha scritto: Hi, Several (many?) years ago National Geographic magazine show a picture taken here in southern California of the state government sending red laser signals between different mountain tops to keep track what was going on near fault lines. There were no technical details on what was taking place. So it can be done. At a hamfest a few years ago I bought both a red and green 35 mW laser pen for about $15 each. They do shine a long, long way. Whether these are powerful enough, or can be properly modulated for what is needed, I have no idea. On line I saw at least one China site that had much larger outputs available. The prices were modest. FWIW. Regards, Perrier ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Ilia Platone via Ferrara 54 47841 Cattolica (RN), Italy Cell +39 349 1075999 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
