Are there RF lasers? - Page 1
Forum section: electronics / general

author: bob808
author_rank: Frequent Contributor
timestamp: March 08, 2017, 11:57:05 pm
content: As the title says, I was just thinking about this and had to ask.Are there lasers that emit way down of visible light? Like MHZ/GHZ frequencies.I think it would be wonderful as an applicationI know there are RF excited tubes, but they emit close to visible spectrum.Hope I didn't ask something stupid

author: wraper
author_rank: Unknown
timestamp: March 09, 2017, 12:04:42 am
content: That won't be a laser but antenna. For example, 1 Ghz has a wavelength of 30cm, 100 MHz - 3m.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 09, 2017, 12:13:50 am
content: Yes. Masers......rhymes with Lasers.......took a class in 1976 that reviewed both and used them at Bell labs...ya I'm that old!https://en.m.wikipedia.org/wiki/Maser

author: bob808
author_rank: Frequent Contributor
timestamp: March 09, 2017, 01:04:21 am
content: Quote from: wraper on March 09, 2017, 12:04:42 amI presumed that. I find it interesting for long range communication (line of sight) with small amounts of power. I don't know yet if they are more or less efficient than lasers.Quote from: grouchobyte on March 09, 2017, 12:13:50 amGreat, thanks! I'll do a read on it.Any modern applications/devices?p.s. this stuff is amazing!

author: Nusa
author_rank: Super Contributor
timestamp: March 09, 2017, 01:52:57 am
content: Line of sight? How far do you need?I once hooked up an outbuilding over 2 miles away to 2.4 Ghz wifi with highly directional Yagi antennas. Rooftop to rooftop, to avoid ground clutter.

author: bob808
author_rank: Frequent Contributor
timestamp: March 09, 2017, 01:58:41 am
content: Not much, but my interest with this is in low power communication. I guess this is down to the efficiency of the maser.Also as I've read this tech is used in satellite com.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2017, 02:29:30 am
content: Lasers are defined by high (spacial and temporal) coherency and luminosity.We've got temporal coherency in spades: oscillators can be disciplined to various high stability sources, enough to have astronomical coherency (i.e., we can record communications from the Voyager probe, using synchronized planet-wide radiotelescopes; also, this is receiving not transmitting, but reciprocity says they're equivalent challenges).Spacial coherency is good too, because a small antenna is a point source. Well, that's easy, most radio waves are bigger than their transducers! Arrays and parabolic reflectors are bigger, and so have to deal with coherency, but they're quite good since the tolerances are relatively easy to control and any error would make directivity worse (an overly spread out, or speckled, beam).As for luminosity, that tends to be the biggest miss, but the pattern of an antenna is what it is, it's just a matter of cranking up the power.However, it's not Light Amplification by Stimulated Emission of Radiation, so the term isn't applicable, though you can produce a beam with similar properties.Tim

author: PointyOintment
author_rank: Frequent Contributor
timestamp: March 09, 2017, 02:34:06 am
content: I don't know much about masers, but from what I've read (just the Wikipedia article, and that was a while ago), they seemed to be more of RF energy sources that you need to attach waveguides and stuff to (like magnetrons) rather than self-sufficient emitters of coherent radiation (like typical lasers). Am I wrong (or am I misunderstanding something)?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 09, 2017, 04:26:27 pm
content: Google and Wikipedia are your friends on this kind of stuff. I was just checking my memory before replying, and confirmed that masers preceded lasers.The defining characteristic of both is not coherency, those are results of the fundamental mechanism first posited by Einstein in 1917, stimulated emission. Spatial coherency, temporal coherency and luminosity are useful attributes of employing this effect.https://en.wikipedia.org/wiki/Maser

author: Cerebus
author_rank: Super Contributor
timestamp: March 09, 2017, 04:32:30 pm
content: Quote from: PointyOintment on March 09, 2017, 02:34:06 amL.A.S.E.R. = Light Amplification Through Stimulated Emission of RadiationM.A.S.E.R. = Microwave Amplification Through Stimulated Emission of Radiationi.e. They're exactly (as in mode of action, not necessarily precise mechanics) the same thing but at different frequencies. Masers, IIRC, came first and prompted a search for doing the same thing at higher (i.e. optical) frequencies. Yup, just checked, the first designation for lasers was "optical maser".

author: coppice
author_rank: Super Contributor
timestamp: March 09, 2017, 05:12:57 pm
content: Quote from: Cerebus on March 09, 2017, 04:32:30 pmMost LASERs are actually LOSERs - Light Oscillator through Stimulated Emission of Radiation. Most MASERs really are amplifiers. LASERs which are actual amplifiers do exist. Chains of them are used to create coherent light beams of extreme intensity.

author: rfeecs
author_rank: Frequent Contributor
timestamp: March 09, 2017, 06:41:17 pm
content: Quote from: bob808 on March 09, 2017, 01:04:21 amYou don't need a MASER to transmit a long distance at low power. You need good antennas. An example is the Voyager spacecrafts. They are still communicating from more than 7 billion (7,000,000,000) miles away using a 20 Watt transmitter:https://fhastros.files.wordpress.com/2015/09/descanso4-voyager_new.pdfNo MASER involved. They use a TWTA (Traveling wave tube amplifier). Nowadays they could certainly use a solid state amplifier.The MASER was indeed invented before the LASER, by Charles Townes who died only a couple years ago at age 99. Townes also claimed to have invented the LASER. He lost a long and drawn out patent fight. The patents are long expired now.http://news.berkeley.edu/2015/01/27/nobel-laureate-and-laser-inventor-charles-townes-dies-at-99/

author: coppice
author_rank: Super Contributor
timestamp: March 09, 2017, 06:53:16 pm
content: Quote from: rfeecs on March 09, 2017, 06:41:17 pmWhilst a sold sate amp is certainly possible these days, the last time I looked satellites still used TWTs.

author: Codebird
author_rank: Regular Contributor
timestamp: March 09, 2017, 07:00:28 pm
content: QuoteThey make very handy in-line amplifiers for undersea fiberoptic cables. Erbium doped fiber, a regular 'LOSER' to pump those ions, a little opti-magic to broaden the bandwidth, and you have an optical amplifier.The only use I've heard of for MASERs in modern times is in timekeeping. An atomic clock is basially a specialised MASER.

author: rfeecs
author_rank: Frequent Contributor
timestamp: March 09, 2017, 07:01:52 pm
content: Quote from: coppice on March 09, 2017, 06:53:16 pmScanning through this reporthttps://fhastros.files.wordpress.com/2015/09/descanso4-voyager_new.pdfThe S-band transmitter did actually have both an SSA and a TWTA.I'm sure TWTAs are still used in satellites. It probably all depends on power level, bandwidth, weight trade-offs, etc.I also noticed in the report that the ground station did use MASER low noise amplifiers, but they were eventually replaced with HEMT amplifiers for better noise figure.

author: Gyro
author_rank: Super Contributor
timestamp: March 09, 2017, 07:41:24 pm
content: Quote from: rfeecs on March 09, 2017, 06:41:17 pmThat says impressive things about its cathode emission for the past 40 years, not to mention still being able to source the heater current after all that time.

author: coppice
author_rank: Super Contributor
timestamp: March 09, 2017, 07:47:29 pm
content: Quote from: Gyro on March 09, 2017, 07:41:24 pmAt least they don't have to worry about the vacuum going soft.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2017, 07:49:05 pm
content: Quote from: Gyro on March 09, 2017, 07:41:24 pmIndeed. I think thermionic emission is a long-since solved problem; wonderful things are possible with materials science and semiconductor-level chemical purity.This was known, back in the heydays of the tube, but it wasn't reasonable given the expense and QC and materials involved to achieve it. Undersea cables used tubes for signal boosting, and achieved tube-millennia of continuous, fault-free operation!Tim

author: coppice
author_rank: Super Contributor
timestamp: March 09, 2017, 08:10:49 pm
content: Quote from: T3sl4co1l on March 09, 2017, 07:49:05 pmSome cathodes achieve superb life times...... and yet military vacuum tube computers didn't skimp on cost or QC, and had serious problems with up-time due to cathode failures.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2017, 08:29:18 pm
content: Quote from: coppice on March 09, 2017, 08:10:49 pmAs I recall, most tellings of e.g. ENIAC were that they used lots of 12AX7s, neons and relays. (Hrm, and maybe thyratrons to drive the relays?) That's not very specific though.I know they made "Computor" tubes at some point; I have a few myself. (The first issue was cathode poisioning under cutoff conditions, or something like that.) It doesn't seem like they went very hard on it, though. It was certainly possible to get useful reliability (perhaps weeks or months of up-time, rather than hours or days). Probably they calculated that time lost due to failures wasn't enough to justify more than nominal improvements in the tubes.Tim

author: coppice
author_rank: Super Contributor
timestamp: March 09, 2017, 08:37:38 pm
content: Quote from: T3sl4co1l on March 09, 2017, 08:29:18 pmPeople I knew who worked on one early military computer said a day was a good up time before one of their 15,000 vacuum tubes died. They used custom made tubes, because cheap commercial devices like the 12AX7 failed too often.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 09, 2017, 08:44:57 pm
content: Laws of large numbers. With 15,000 tubes, if they had a 15,000 hour MTBF you would pop one an hour once you reached steady state. Since they presumably started with new tubes you would do better than that once you got the infant mortality out during the early life of the computer. (Maybe they burned the tubes in for a few hours on a test stand before using them in the computer). I don't have tube MTBF numbers at hand, but 15000 hours seems like it would be ball park of the real figure. (Plus or minus an order of magnitude).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2017, 09:28:02 pm
content: In contrast, we're talking about (re: space TWTs, undersea cables) MTBFs orders of magnitude higher -- years and decades. Figures that were achieved through hard work, and AFAIK, never reduced to consumer scale mass production. Which, kind of a chicken and egg problem there, of course (high-rel is expensive, {so | because} it's low production).But also, semiconductors wholly superseded any need of that kind of quantity of tubes, let alone high-rel ones. And, curiously, semiconductors demand even greater purity just to work at all, so as it's turned out, we have industrial scale processes that are more than good enough to do it now; they're just working on something even better.Tim

author: rfeecs
author_rank: Frequent Contributor
timestamp: March 09, 2017, 11:15:53 pm
content: Looking at the report a little more, each spacecraft has three TWTAs and one SSPA (Solid State Power Amp). There are two redundant TWTAs for the X band transmitter. One of these failed in 1987 on Voyager 1, and one failed in 1998 on Voyager 2. They each switched over to the remaining X band TWTA when the failure occurred. So one failed after 10 years, the other after 21 years.From what I can tell, a typical lifetime of a satellite TWTA is about 100,000 hrs, which is 11 years.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: March 10, 2017, 06:24:44 am
content: Wikipedia says, in part:Quote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2017, 07:19:16 am
content: AFAIK, the usual industrial/HQ tubes were largely in the same vein as the Computor tubes: enhanced lifetime and robustness*, with somewhat better process control, formulation, etc. But nothing overboard, because who would pay for that?*Although sometimes, the life was merely specsmanship: some industrial tubes are rated below their commercial counterparts, so, of course they'll last longer within ratings... 7027 is an example of this, compared with 6L6GC. Though not an example if compared to 6L6GB (and actually an anti-example if compared to 7027A, so make of that what you will...) I don't remember any better examples offhand unfortunately.Most industrial tubes have more reinforcing bars, plate ribs, mica (or even ceramic) sheets, etc., making them more suitable for high vibration conditions. Example: 12AU7(A) vs. 5814 (ruggedized) vs 5963 (hey, a computor type).The super quality ones were constructed differently than commercial types, because of differences in material properties, strict mechanical requirements, and chemical purity.Tim