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Side bands on diode laser (unexpected wavlength shift.)

Discussion in 'Electronic Design' started by George Herold, Nov 20, 2012.

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  1. 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.

    George H.
  2. Guest

    Is the diode self-tuned or does it depend on the external F-P
    resonator for tuning?

    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.

    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?

    (caveat- I'm still on my first cup of coffee)

    Mark L. Fergerson
  3. Well self tuned I guess. It's got an external grating that does
    feedback into the diode
    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.
    Well not to *any* precision :^)
    But I've got 'typical' graphs of how the wavelength changes with
    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.
  4. Hmm, That's an interesting idea. So the RF goes in via an sma right
    on the laser head, and the 'DC' current drive is in a separate box,
    connected by a 6 foot computer cable with DB9 connectors on the ends.
    The current drive is mostly copied from Jan Hall, RSI '93. There's a
    big inductor before the real DC part. But there's this opamp ~1MHz
    current modulation circuit that's on the laser side of the inductor.
    I've no idea how that behaves with 100 MHz leaking in from the back

    Oh! I've also got these ESD protection diodes across the laser diode
    in the laser head. One Schottky in reverse, and three pn's in series
    in the forward direction.... (?)

    Thanks, I've got a few things to look at.

    George H.

    George H.
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