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XPost: sci.electronics.design
From: gherold@teachspin.com
On Nov 20, 3:00 pm, "n...@bid.nes" wrote:
> On Nov 20, 8:39 am, George Herold wrote:
>
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> > So I was putting ~100MHz sidebands on a diode laser (modulating the
> > current.) to get a frequency 'measure' of the length of a (~20cm)
> > Fabry-Perot cavity (confocal). Wavlength ~780nm, FSR ~380 MHz, a
> > source at 190 MHz would be perfect, but I don't have anything that
> > high. So I cranked up the power with 95 MHz and got the second order
> > sidebands at the frequency I wanted. (RF voltage of about 1 Vp-p into
> > 50 ohms so modulating the current by maybe +/- 10mA, it's not clear
> > how much of the RF gets into the diode.)
>
> > What was weird (unexpected) was that when I cranked up the RF power
> > the laser frequency shifted... but I had to *increase* the laser
> > current to get it back to the wavlength where it started. The
> > increase in the DC current was a few mA out of ~50mA.
>
> > Anyone know what's going on? I expected that adding RF would heat up
> > the diode and thus lead to a lower DC current.
>
> Is the diode self-tuned or does it depend on the external F-P
> resonator for tuning?
Well self tuned I guess. It's got an external grating that does
feedback into the diode
>
> Did the wavelength shift up or down, and did it shift smoothly or
> are you looking at mode hopping? If it's internally tuned I'd expect
> heating to lengthen the diode's resonator so that you can hop from fa
> to fb then back to fa as the resonator length changes by increments of
> a half-wavelength.
Smooth... (Hmm up or down, I forgot.) So I had to increase the DC
current to get back to the original wavelength.. Increasing the (DC)
current is much the same as increasing the temperature. Both tend to
make the diode go to shorter wavlengths.
>
> If it's externally stabilized the front face of the diode's internal
> resonator may be forming another resonator with the F-P's input side
> which changes length as the diode expands with heating.
>
> Can you monitor the diode's temperature change with wavelength
> change to any precision?
Well not to *any* precision :^)
But I've got 'typical' graphs of how the wavelength changes with
temperature.
>
> (caveat- I'm still on my first cup of coffee)
At 3:00 PM? An all-nighter?
I've not done a lot of measurements with RF injected currents. Next
time I look at this I'll see what happens for lower frequencies. (Say
0.1 to 1MHz.)
George H.
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> Mark L. Fergerson- Hide quoted text -
>
> - Show quoted text -
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