Side bands on diode laser (unexpected wavlength shift.)

[George Herold]

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.

Thanks,
George H.

 

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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?

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

 

[George Herold]

  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.

 

[George Herold]

At a guess, the RF is confusing the DC bias supply.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -

- Show quoted text -

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
end.


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.