Hi,
I've been searching for a while on the Web but all manufacturers(Analog,Texas,Maxim,...) offer only digital solutions or low freq(tens of Khz) analog modulation. Does anyone know a LD Driver IC with analog modulation capability up to 2Ghz ? I want to assembly an small demo board on FR4 with some TO-can Laser diodes but I dont find any solution to provide a high freq analog modulation.
Thanks a lot. Regards, Rafael
The only way I've seen this done is by directly injecting the RF into the diode. (I've only done it up to several hundred MHz.... the limit of my frequency generator.) AC couple into the DC current line, then a 50 ohm 'termniation' resistor in parallel with an inductor (to allow the DC current to flow) and this feeds the diode.
George H.
How much current do you need? What kind of laser?
An analog laser diode driver is as simple as a signal and a resistor.
If you need gain, there are tons of cheap MMICs that have 6-12 GHz bandwidth. Just stick the laser diode in series with the output pullup resistor, or AC couple the MMIC output into the laser and provide a separate DC bias path. Check out the MiniCircuits parts.
Most lasers don't modulate very well. They tend to have mode jumps, "kinks", in their current-to-light curve. The high-end optical RF links use CW lasers and lithium niobate modulators.
John
VCSELs can be fairly benign in that respect but they do have mild polarization mode hops.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Doing 2 GHz with a TO-can is no joke, even if the diode chip itself is fast enough, which it probably isn't. The problems are both electrical and optical. The main optical reason is that the level populations in the upper and lower laser states often don't respond fast enough. Ordinary cleaved-cavity Fabry-Perot lasers IME have relaxation peaks near 1 GHz, so they don't respond terribly well at 2 GHz. You'll get some response, for sure, but it's very hard to get nice looking edges or nice open eye diagrams at 2 GHz with those lasers. (Even 1.25 Gb/s is a bit of a trick.)
There are VCSELs and drivers available commercially that work well up to
10 Gb/s, with higher frequencies in the works.On the other hand, if you just want to get 10% sinusoidal modulation at
2 GHz, you can probably do that by wiring a transformer secondary in series with the LD, with a nice big bypass cap on the other end of the secondary. That has the additional advantage of providing excellent ESD protection.Cheers
Phil Hobbs
-- 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
Strong modulation should in itself reduce or eliminate mode hops (see Phil Hobbs' book). AIUI the "modes" then don't have time to get going. Needs to be a few hundred MHz or above.
-- John Devereux
It's like going over a washboard road. Above a certain speed the rattling in the car is "gone". But the car's handling gets iffy and it is much more prone to unauthorized departures from the road :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
p r t l
I've always wanted to try this up at higher frequencies. We put sidebands on a Sanyo 74DL201 (?) and observe it with a confocal Fabry- Perot. (It's an easy way to 'put a yard stick' on the spectrum as I'm sure you know.) The diodes have relaxation oscillations right about
6.8X GHz. You might be able to pull it into the Rb 87 hyperfine transition.George H.
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Using a transformer to inject a little AC on a DC signal is an old trick for doing high frequency PSRR testing. Often RF generators can't supple the DC to run the DUT, so you roll your own transfer. minicircuits has nothing above 1.4GHz.
Hi Rafael, You would be smart to think about suggestions of the others also. I haven't worked up above 200MHz and... So the transformer idea is also a good one. How much modulation do you want?
The impedance of the diode (At DC) is pretty easy to estimate... It's
25mV (at room temperature) divided by the DC bias current... So for the currents I use this is some fraction of an ohm and I ignore it. The AC impedance is not someting I've thought about. You might imagine that the lead inductance would be an issue.George H.
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Opps that number is a DL-7140-201. (Which are about to go out of production.. sigh.)
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Bias tees are fine too. Note that the impedance of the LD chip will be very very low at 2 GHz--the total impedance will be dominated by the lead inductance. Another approach is wiring an RF transistor in parallel with the diode and driving that. It does require some attention to prevent oscillations though.
Cheers
Phil Hobbs
-- 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
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