ultra-wideband FM

Frequency modulate what?

I assume you're going to frequency modulate the 1GHz carrier, then amplitude modulate the laser with that -- correct?

I think the most important "high-end" math tool in this case is the consultant's brain -- Scilab you can get off the web for free and go buy a nice car with the money you would have spent to buy Matlab. With either Scilab or Matlab you still need a nice squishy pile of neurons that knows how to feed in the questions the right way and interpret the results.

A carrier. With the baseband signal. That's how FM is usually done.

Yup. The laser would actually run on/off at the (modulated) carrier frequency.

Hence the option to have a consultant, a real RF signals guy familiar with the tools, furnish the neurons.

John

1. Bandwidth and throughput can be traded in many different ways, but there is no free lunch.
2. The bandlimiting, amplitude and phase distortion in the FM channel results in the nonlinear distortion of the modulating signal. This represents intractable problem except for the simple cases like a pure sine wave input. However the worst case estimates can be drawn.
3. Math tools: all that needed is a C compiler (or PowerBasic, if you like).

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Have you considered PWM (or power position modulation) ?

Should be easier to modulate than high modulation index FM.

On a sunny day (Tue, 13 Jul 2010 08:59:09 -0700) it happened John Larkin wrote in :

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.

scheme?

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

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

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

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)

On a sunny day (Tue, 13 Jul 2010 09:28:01 -0700) it happened John Larkin wrote in :

correction scheme?

OK, google gives lots of info, for example this on FM and its detection:

rrection scheme?

er.

sending an RF signal over fiber as intensity modulated light is nothing new....CATV does that...

FM modulating an RF carrier is nothing new..

FM modulating an RF carrier and sending that carrier over a fiber is a good way to reduce the effects of laser non-linearity ..

but its nothing new...

Mark

correction scheme?

That isn't quite my experience. We have used VCSELs in a rather amplitude-sensitive acquisition scheme and about the only thing I didn't like about them was their phase noise footprint (line width, we needed well under a MHz), probably caused by polarization hops due do how good we were coupled. We could make that change with fiber rotation and wiggling. It's been a while, but AFAIR we used a DFB in the end despite the fact that we couldn't tune the wavelength as much as we'd liked to.

But of course that all depends on the quantitative levels in the word "horribly" :-)

What is that loop supposed to do, provided you had super-fast threshold detectors? The only time I used a loop with laser diodes was when we had to set a certain amplitude or hang on to a not so well pronounced Bragg grating.

What kind of granularity (dynamic range) do you need here?

I wasn't planning to modulate the frequency of the light. I was thinking of driving the laser on/off at around 1 GHz, add FMing that.

Rob thinks I should just digitize the signal and ship it 8b/10b.

John

If 10 bits is enough you could also just PWM, as someone had already suggested.

Yes it is. I just wanted you to confirm that you weren't thinking of modulating the color of the laser and calling it "FM". It's theoretically possible, although it would probably be insanity to attempt with today's lasers.

It all sounds interesting. I assume (well, gather from other posts in the thread) the ultimate goal is to be able to send nice clean analog signals while dodging any need to digitize?

IFAIK this is done: there are (or used to be) off-the-shelf fiber-optic links that you can stick analog video into and get analog video out. I vaguely remember a hand-waving explanation that included mention of a high-frequency FM signal that then amplitude modulated the light.

Are you trying to replicate one of these, only cheaper? Ditto, only better? Reinventing the wheel because it feels good?

Rob is right.

For ~10 bit accuracy, the PWM rate must be ~20 times higher then the highest frequency of the signal.

Generating 10-bit linear ramp at 2 GHz is nontrivial.

(This accounts for negative feedback in transmitter. Without NFB, the results are going to be several times worse).

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Guilty. Done it, modulated the wavelength. But not at anywhere close to the speed John would need although I wouldn't see what would have prevented that if using a fat RF BJT or an LDMOS.

I think Paul's suggestion to do PWM was right on. John, IMHO math tools are less important here. What I feel is more important is to find the parts that can do the job with enough margins. The only tool I used on my last laser project was an HP11C. And that wasn't even mine, technically it belongs to my wife, I just confiscated it around 15 years ago :-)

I've seen FM ones but those are 10MHz BW, or less, usually.

It depends. John didn't post any requirements except bandwidth.

Math is very important there, as it will clearly illustrate the limitations of PWM or FM.

Besides the parts nonideality, there are fundamental problems.

Well, analog VCRs record FM signal to tape (THD ~ 1%, SNR ~40dB). You only have to do x50 frequency upscaled version :-)

VLV