Limits of Pulse Frequency..?

[journeyman]

So I was toying with this idea...

The definition of a meter is approx. the amount of distance light travels in 1/300,000,000 of a second.


So, if 300MHz at 100% DC is a solid line, one cycle equalling one meter. So if you pulse the laser at 50% DC, then in theory, instead of a solid beam you would get dashes, a half meter long, separated by gaps a half meter long. Correct me if I'm wrong!

However that is pretty fast, can diodes switch that fast? I'm sure a driver is possible if my phone is running at 1.2 GHz...

 

[Cyparagon]

That is correct. There are some lasers that have a pulse so short, the beam is a single wavelength long.

Standard laser diodes should have no problem getting to 300MHz. A 12x bluray burner operates at 432Mbit/s which corresponds to 432MHz. What would be the point of modulating it this fast outside of optical storage though?

 

[Benm]

Measuring light speed if you have a fast enough photodiode at hand?

 

[LSRFAQ]

I work on a tunable 780 nm to 1100 lab laser with 160 picosecond pulses at a 76 Mhz rep rate. (ti-saph) It modulates itself, the crystal self pulses.

Light travels about .299 mm in one picosecond, so even having air flowing across the optical table can cause me problems. I need 2 160 picosecond pulses from two different lasers to hit the same place within 10 picoseconds..
Its a headache some times, but it can be done...


Thats 1996 technology. We need a new one, so our groups existing one can be yours for a mere 150,000$.

You can modulate most standard can diodes to about 1 Ghz if you apply some DC and leave it above threshold. Modulate does not always mean 100% on-off at those levels.

Much above that and you need a optimized package with impedance matching to the 50 ohm cable used at those frequencies, and a longer diode die to avoid resonances.

BTW, you can measure the speed of light or distances with a 40 Mhz oscilloscope and a cheap laser, you only need a little modulation at a low frequency.

There are kits sold for this, and you put a bicycle reflector down at the end of a football field, and look at the phase shift when the light bounces back.


Steve

 

[Benm]

Hmm.. at 40 MHz you only need a 4 meter long track to get a phase inverted reflection on a scope, so you could easily demonstrate that indoors.

Modulating laser diodes partly can indeed be done a great speed if you don't need it to turn off completely, and you can rely on the driver to deliver average current only. You could probably ac couple a fm radio signal into a laser diodes output and recover it at the other end with little effort.

 

[Sigurthr]

You could probably ac couple a fm radio signal into a laser diodes output and recover it at the other end with little effort.

See: "AM Voice Laser Transmitter/Transciever" or "Free-Space optical communications". That is exactly how it is done - you set a constant current driver to deliver the quiescent current level and then couple an AC audio signal over that to modulate the laser diode. The audio level and quiescent current levels must be set to avoid clipping or laser diode destruction.

 

[Benm]

I dont mean coupling the ac signal, but a 100 MHz-ish FM signal into the output. You could even do stereo and RDS that way

 

[Sigurthr]

Oh, I thought you meant the audio from a FM radio, not frequency modulated radio signals themselves.

You can't FM modulate a laser's signal. It's frequency is hard locked (ignoring temperature) in production by the gain medium (its frequency is the wavelength). You can AM modulate it or PM modulate it, but that is it. There is no heterodyning of laser emissions (aside from SFG, but that is different). You could convert the FM signal into an envelope and then couple that to the laser or alternate between two current levels based on the FM signal's frequency, but those are both actually forms of AM modulation.

In other words you can alter the amplitude of a laser (AM Modulation) or the phase angle (PM Modulation) of its emission, but you can't alter it's frequency.

 

[Hiemal]

Well, if you changed the frequency you'd be changing the color...Pretty sure that's what tuneable lasers are!

I'm pretty sure when it comes down to modulating laser diodes it's just junction capacitance and inductance that really limits how fast you can do it. That and you'll need to find suitable driving electronics to do it as well... fun stuff.

 

[Sigurthr]

Aye, Jared (Quack) is exactly right, though I have heard there are some odd semiconductor physics issues at sufficiently high frequencies which come in to play.

Btw, there aren't many chromatically tunable lasers (Ti:Saph comes to mind) around and none of them are in diode form.

 

[AUS]

Sorry a bit off topic... @ Suguthr, I just had a thought about this. From my recolection of modulation theory, you can't change the phase of a signal without instantaneously changing the frequency. Mind you that was a lot of year ago and I could be wrong.

It makes sense that modulating the current of a laser diode could alter its frequency slightly but finding a photodiode or something to measure this would be a nightmare.

A few years back we wanted to set up an audio feed between two buildings in Melbourne 700 meters apart. My boss wanted to use an 850MHz link but that was discounted because that frequency is used for 3G phones in Australia and I could see about 9 cell towers from there. The link guys wanted to use 2.4Ghz - also a bad idea in the centre of a capital city. I wanted to use a pulse modulated IR laser (although I couldn't remember how IR went in fog or smoke). We ended up merging companies and the links never went in.

I have AM modulated 5mW laser pointers in the past and demodulated them with a photodiode which is fairly easy.

You could probably modulate a laser with a fairly high RF frequency and see the standing waves on the beam, that would be cool.

 

[Sigurthr]

Sorry a bit off topic... @ Suguthr, I just had a thought about this. From my recolection of modulation theory, you can't change the phase of a signal without instantaneously changing the frequency. Mind you that was a lot of year ago and I could be wrong.

While true with electric or radio signals, it works a bit differently with lasers and light. The speed of light is determined by the medium through which it passes. By altering the medium you can alter the phase by delaying the light slightly in reference to an unaltered light source. The frequency never changes from the outside perspective.

It makes sense that modulating the current of a laser diode could alter its frequency slightly but finding a photodiode or something to measure this would be a nightmare.

When using gas lasers or when phase modulating any laser you can't modulate by altering the laser's driving method, instead you have to add the modulation to the beam after the laser with an AOM (AM Modulation only) or an EOM (AM or PM Modulation).

See:
Electro-optic modulator - Wikipedia, the free encyclopedia

The current level does not change the frequency of the diode aside from thermal effects. If you perfectly cool a laser diode it will be the same frequency at 1% power as it is at 100% power.

 

[Cyparagon]

Ben means the frequency at which the laser is modulated, not the frequency of the photons themselves.

 

[Benm]

^^ indeed!

Obviously i wasnt plant to change the color of the laser, but just to amplitude it with a 100 MHz a carrier wave, which is FM modulated.

If I find the time i'll build somehting like this as a proof of concept, but at lower frequencies (1 MHz or so) so i can use standard 4046 pll's to do the singal generation and demodulation.

For the radio enthousiast it may be nice to modulate the laser at 455 kHz or 10.7 MHz since very good bandbass filters are commonly available for those frequencies.

 

[Sigurthr]

Ahh, I see, sorry hehe. Misunderstandings like this are inevitable some times. =)

 

[Hiemal]

Aye, Jared (Quack) is exactly right, though I have heard there are some odd semiconductor physics issues at sufficiently high frequencies which come in to play.

Btw, there aren't many chromatically tunable lasers (Ti:Saph comes to mind) around and none of them are in diode form.

Yeah, the actual semiconductor starts to become a problem... it's why silicon diodes have reverse recovery times. I imagine laser diodes aren't any different.

I think the reason why is it's due to electron carriers being "slow". They take time to reset, in a way. If you try to push them faster than that they start to run in a very very undesireable mode and will produce loads of heat from resistance.

Basically it'll start acting like a resistor instead of a diode at high frequencies.

I kind of wonder why we don't have tuneable laser diodes; is it just due to the physics of how a laser diode works? I mean, if you think about it, laser diodes and gas lasers operate in a very similar manner... mirrors. I know that some laser diodes don't emit just one color, either. I think I read that a blue was actually giving off a little bit of yellow incoherent light....