ultra-wideband FM
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From: Jan Panteltje on 13 Jul 2010 12:12 On a sunny day (Tue, 13 Jul 2010 08:59:09 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <k33p36hk4s26tua3bk8j4v8ufg5ol4tlcf(a)4ax.com>: >On Tue, 13 Jul 2010 08:43:13 -0700, Tim Wescott <tim(a)seemywebsite.com> >wrote: > >>On 07/13/2010 08:29 AM, John Larkin wrote: >>> >>> >>> Hi, >>> >>> One of the nasty things about cheap fiber-coupled lasers is that they >>> have terrible amplitude stability and linearity, full of mode jumps >>> and such. Given that, sending a signal over a fiberoptic link using >>> amplitude modulation is usually done with a stable CW laser feeding a >>> lithium-niobate modulator. The modulator itself is nonlinear and >>> expensive and a nuisance to drive and bias. >>> >>> Digitizing and sending samples is OK, up to a point. It gets messy at >>> some point from a sheer speed standpoint. >>> >>> So the idea of using FM pops up. If my baseband analog signal were, >>> say, DC to 150 MHz, and I picked the highest carrier center frequency >>> that's reasonably easy to work with, say 1 GHz, it could maybe be >>> done. The laser driver and receiver aren't too difficult. The issues >>> are the modulator, the demodulator, and the pure signal theory >>> necessary to turn the time-domain behavior of the link into classic >>> measures like s/n and distortion of the recovered baseband signal. >>> Asymmetrically bandlimiting an FM signal is computationally messy. >>> Would not they use QAM maybe up to 64 constallation and a nice error correction scheme? The data throughput would be much higher then with simpe FM / PM whatever.
From: John Larkin on 13 Jul 2010 12:28 On Tue, 13 Jul 2010 16:12:58 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Tue, 13 Jul 2010 08:59:09 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><k33p36hk4s26tua3bk8j4v8ufg5ol4tlcf(a)4ax.com>: > >>On Tue, 13 Jul 2010 08:43:13 -0700, Tim Wescott <tim(a)seemywebsite.com> >>wrote: >> >>>On 07/13/2010 08:29 AM, John Larkin wrote: >>>> >>>> >>>> Hi, >>>> >>>> One of the nasty things about cheap fiber-coupled lasers is that they >>>> have terrible amplitude stability and linearity, full of mode jumps >>>> and such. Given that, sending a signal over a fiberoptic link using >>>> amplitude modulation is usually done with a stable CW laser feeding a >>>> lithium-niobate modulator. The modulator itself is nonlinear and >>>> expensive and a nuisance to drive and bias. >>>> >>>> Digitizing and sending samples is OK, up to a point. It gets messy at >>>> some point from a sheer speed standpoint. >>>> >>>> So the idea of using FM pops up. If my baseband analog signal were, >>>> say, DC to 150 MHz, and I picked the highest carrier center frequency >>>> that's reasonably easy to work with, say 1 GHz, it could maybe be >>>> done. The laser driver and receiver aren't too difficult. The issues >>>> are the modulator, the demodulator, and the pure signal theory >>>> necessary to turn the time-domain behavior of the link into classic >>>> measures like s/n and distortion of the recovered baseband signal. >>>> Asymmetrically bandlimiting an FM signal is computationally messy. >>>> > >Would not they use QAM maybe up to 64 constallation and a nice error correction scheme? >The data throughput would be much higher then with simpe FM / PM whatever. That would require digitizing the baseband signal on one end and DACing it on the other. That remains a possibility, but the numbers are intimidating. But as I mentioned, affordable fiber-coupled VCSEL or similar lasers are horribly nonlinear. Constellation-type modulations need a lot of linearity in the channel, and generally rely on error correction to push the channel limits. Lasers are usually used on/off, simple NRZ data. John
From: John Larkin on 13 Jul 2010 12:34 On Tue, 13 Jul 2010 19:08:42 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Tue, 13 Jul 2010 08:29:45 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>Digitizing and sending samples is OK, up to a point. It gets messy at >>some point from a sheer speed standpoint. >> >>So the idea of using FM pops up. If my baseband analog signal were, >>say, DC to 150 MHz, and I picked the highest carrier center frequency >>that's reasonably easy to work with, say 1 GHz, it could maybe be >>done. The laser driver and receiver aren't too difficult. > >Have you considered PWM (or power position modulation) ? > >Should be easier to modulate than high modulation index FM. That might be worth looking at. It would certainly be easy to generate and detect; it becomes, essentially, AM. We'd have to experiment with the optic links to get an estimate of how the lasers and TIAs behave in PWM mode. One possibility is a transmitter-side feedback loop, like the ones people use with analog optocoupler links... the only catch being that the receiver may be many miles and many dB away. John
From: Vladimir Vassilevsky on 13 Jul 2010 12:45 John Larkin wrote: > So the idea of using FM pops up. If my baseband analog signal were, > say, DC to 150 MHz, and I picked the highest carrier center frequency > that's reasonably easy to work with, say 1 GHz, Then, the optimal FM modulation index is going to be ~1. Which makes FM about as sensitive to noise and distortion as AM. No free lunch. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Phil Hobbs on 13 Jul 2010 12:56
John Larkin wrote: > > Hi, > > One of the nasty things about cheap fiber-coupled lasers is that they > have terrible amplitude stability and linearity, full of mode jumps > and such. Given that, sending a signal over a fiberoptic link using > amplitude modulation is usually done with a stable CW laser feeding a > lithium-niobate modulator. The modulator itself is nonlinear and > expensive and a nuisance to drive and bias. > > Digitizing and sending samples is OK, up to a point. It gets messy at > some point from a sheer speed standpoint. > > So the idea of using FM pops up. If my baseband analog signal were, > say, DC to 150 MHz, and I picked the highest carrier center frequency > that's reasonably easy to work with, say 1 GHz, it could maybe be > done. The laser driver and receiver aren't too difficult. The issues > are the modulator, the demodulator, and the pure signal theory > necessary to turn the time-domain behavior of the link into classic > measures like s/n and distortion of the recovered baseband signal. > Asymmetrically bandlimiting an FM signal is computationally messy. > > I'd expect that commercial VCOs wouldn't have anything like this sort > of fractional modulation bandwidth. And if they did, a varicap > modulating an LC oscillator would probably distort like mad. (Faint > echoes of the capacitor charge debate?) The modulator may have to be > some EclipsLite version of a 555 on steroids. Or a multi-GHz VCO > heterodyned down. Yuk: sounds like RF. > > On the theory side, does anyone know of (or have?) one of the high-end > math tools that could do a quantitative signal-quality analysis of > such a link, given, say, approximate experimental data on the > time-domain behavior of the laser link? Hiring a consultant to do this > would be a desirable alternate to getting and learning this stuff > ourselves. > > Any thoughts? > > John > > Most Fabry-Perot type diode lasers will current-tune by a wave number or two (30-60 GHz) between mode hops, and can be modulated up to about 1 GHz. They're very sensitive to back-reflections--to be sure of good behaviour, you need at least 60 dB of optical isolation. Choosing the right temperature will help keep the mode hops at bay, but you'll have to keep tweaking T as the laser ages and the phase of the inevitable fibre feedback changes. DFB lasers, as used in telecom, are much better behaved, but modern ones hardly current-tune at all (like 100-300 MHz/mA). This is so that you can AM them in a dense WDM network without the resulting chirp scribbling all over the neighbouring channels. AM is a better idea with these guys--like a class B amplifier, they're nice and linear once you get above threshold, so some AC-coupled scheme should work fine. For current tuning, your best bet would be a FP laser in the 750-830 nm band, with some automatic scheme for avoiding mode hops. How are you planning on demodulating the FM? This usually needs an interferometer. Cheers Phil Hobbs (Stuck in Atlanta due to weather in NYC) -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |