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Estimate of diode laser bandwidth

Started by George Herold April 10, 2015
Hi all, after tribulations with HV opamps and leaky inductors, 
my diode laser seems to be happy again.
Here's a scan of the laser through a Fabry-Perot confocal cavity.
(F-P FSR ~ 380 MHz, finesse > 100.)  
https://www.dropbox.com/s/6zpgdfs6cqus7ig/TEK0015.BMP?dl=0

I put 10 MHz side bands on the laser to give a scale.
Looking at the 'scope shot would I be safe in claiming
1 MHz (or better) of laser bandwidth?  

Thanks... this is mostly a question of estimating noise 
from a 'scope shot.

George H.
On 04/10/2015 10:39 AM, George Herold wrote:
> Hi all, after tribulations with HV opamps and leaky inductors, > my diode laser seems to be happy again. > Here's a scan of the laser through a Fabry-Perot confocal cavity. > (F-P FSR ~ 380 MHz, finesse > 100.) > https://www.dropbox.com/s/6zpgdfs6cqus7ig/TEK0015.BMP?dl=0 > > I put 10 MHz side bands on the laser to give a scale. > Looking at the 'scope shot would I be safe in claiming > 1 MHz (or better) of laser bandwidth? > > Thanks... this is mostly a question of estimating noise > from a 'scope shot. > > George H. >
It looks a bit wider than that, right around one division FWHM, which would make it about 2.8 MHz wide. That's about what you'd expect from that cavity, so it looks like the real line width is unresolved. You might try making a Mach-Zehnder with a path difference of a few metres and look at the FM-AM conversion. That'll demodulate the noise pretty well, and you can compute the line width from the noise spectrum. What are the peaks out at +- 10 MHz? 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 net http://electrooptical.net
I dont understand the scope shot.

It looks like a scope with the x axis is the time domain.
The op is asking as if this was a spectrum analyzer
shot where x would be the frequency domain.

Mark
On 04/10/2015 11:44 AM, makolber@yahoo.com wrote:
> I dont understand the scope shot. > > It looks like a scope with the x axis is the time domain. > The op is asking as if this was a spectrum analyzer > shot where x would be the frequency domain. > > Mark >
Looks like the laser is being current-tuned across the resonator line width. (A millisecond per division is a bit on the quick side for your average piezo-controlled Fabry-Perot.) The plot is basically like a spectrum analyzer but with a linear vertical scale. 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 net http://electrooptical.net
On Friday, April 10, 2015 at 11:20:25 AM UTC-4, Phil Hobbs wrote:
> On 04/10/2015 10:39 AM, George Herold wrote: > > Hi all, after tribulations with HV opamps and leaky inductors, > > my diode laser seems to be happy again. > > Here's a scan of the laser through a Fabry-Perot confocal cavity. > > (F-P FSR ~ 380 MHz, finesse > 100.) > > https://www.dropbox.com/s/6zpgdfs6cqus7ig/TEK0015.BMP?dl=0 > > > > I put 10 MHz side bands on the laser to give a scale. > > Looking at the 'scope shot would I be safe in claiming > > 1 MHz (or better) of laser bandwidth? > > > > Thanks... this is mostly a question of estimating noise > > from a 'scope shot. > > > > George H. > > > > It looks a bit wider than that, right around one division FWHM, which > would make it about 2.8 MHz wide. That's about what you'd expect from > that cavity, so it looks like the real line width is unresolved.
Yeah, I was hoping that the noise on the sides would tell me something about the noise/ BW of the laser.
> > You might try making a Mach-Zehnder with a path difference of a few > metres and look at the FM-AM conversion. That'll demodulate the noise > pretty well, and you can compute the line width from the noise spectrum.
Hmm so beat the laser with itself.. but ~3m = 100nS later. I'll need a reasonable fast detector. Way back when I beat two free running lasers together and looked at the linewidth.. (laser wavelength set ~ 100 MHz apart.) But I only had my 'scope FFT to measure the spectrum and... well I was pushing the limits of the 'scope and so a fairly uncertain measurement. Will the Mach Zender trick move the signal down to DC?
> > What are the peaks out at +- 10 MHz?
I'm RF modulating the laser current at 10MHz to put side bands on the laser. They are just there to set the scale of the sweep. George H.
> > 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 net > http://electrooptical.net
On Friday, April 10, 2015 at 11:44:38 AM UTC-4, mako...@yahoo.com wrote:
> I dont understand the scope shot. > > It looks like a scope with the x axis is the time domain. > The op is asking as if this was a spectrum analyzer > shot where x would be the frequency domain. > > Mark
Hi Mark, It's an external cavity diode laser, And I'm slowly sweeping the laser frequency by changing the grating angle. I'm sending the beam through a Fabry-Perot cavity which has a "width" of about 3-4 MHz.. So the time axis is also the laser frequency axis. Calibration is supplied by the 10 MHz side bands. The F-P cavity is the highest resolution "thing" that I have, I was hoping noise would tell me something about the laser bandwidth. (But as usual there may be some error in my thinking.) George H. Oh the F-P cavity does not have scanable end mirrors. (Which is not typical) The end mirrors are fixed. Which we get away with because we can scan the laser.
On 04/10/2015 12:19 PM, George Herold wrote:
> On Friday, April 10, 2015 at 11:20:25 AM UTC-4, Phil Hobbs wrote: >> On 04/10/2015 10:39 AM, George Herold wrote: >>> Hi all, after tribulations with HV opamps and leaky inductors, my >>> diode laser seems to be happy again. Here's a scan of the laser >>> through a Fabry-Perot confocal cavity. (F-P FSR ~ 380 MHz, >>> finesse > 100.) >>> https://www.dropbox.com/s/6zpgdfs6cqus7ig/TEK0015.BMP?dl=0 >>> >>> I put 10 MHz side bands on the laser to give a scale. Looking at >>> the 'scope shot would I be safe in claiming 1 MHz (or better) of >>> laser bandwidth? >>> >>> Thanks... this is mostly a question of estimating noise from a >>> 'scope shot. >>> >>> George H. >>> >> >> It looks a bit wider than that, right around one division FWHM, >> which would make it about 2.8 MHz wide. That's about what you'd >> expect from that cavity, so it looks like the real line width is >> unresolved. > > Yeah, I was hoping that the noise on the sides would tell me > something about the noise/ BW of the laser. >> >> You might try making a Mach-Zehnder with a path difference of a >> few metres and look at the FM-AM conversion. That'll demodulate >> the noise pretty well, and you can compute the line width from the >> noise spectrum. > Hmm so beat the laser with itself.. but ~3m = 100nS later. I'll need > a reasonable fast detector. Way back when I beat two free running > lasers together and looked at the linewidth.. (laser wavelength set ~ > 100 MHz apart.) But I only had my 'scope FFT to measure the spectrum > and... well I was pushing the limits of the 'scope and so a fairly > uncertain measurement.
The bandwidth you need is only a couple of times the laser linewidth, so it shouldn't be hard to do. If you don't have a baseband spectrum analyzer, it's a little more difficult. I forget--is this a visible or IR laser?
> Will the Mach Zender trick move the signal down to DC?
It will if you use it to measure the coherence length, which is where the fringes wash out, but that might be pretty long.
>> >> What are the peaks out at +- 10 MHz? > > I'm RF modulating the laser current at 10MHz to put side bands on > the laser. They are just there to set the scale of the sweep.
Right. Missed that the first time round, thanks. 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 net http://electrooptical.net
On Friday, April 10, 2015 at 12:30:18 PM UTC-4, Phil Hobbs wrote:
> On 04/10/2015 12:19 PM, George Herold wrote: > > On Friday, April 10, 2015 at 11:20:25 AM UTC-4, Phil Hobbs wrote: > >> On 04/10/2015 10:39 AM, George Herold wrote: > >>> Hi all, after tribulations with HV opamps and leaky inductors, my > >>> diode laser seems to be happy again. Here's a scan of the laser > >>> through a Fabry-Perot confocal cavity. (F-P FSR ~ 380 MHz, > >>> finesse > 100.) > >>> https://www.dropbox.com/s/6zpgdfs6cqus7ig/TEK0015.BMP?dl=0 > >>> > >>> I put 10 MHz side bands on the laser to give a scale. Looking at > >>> the 'scope shot would I be safe in claiming 1 MHz (or better) of > >>> laser bandwidth? > >>> > >>> Thanks... this is mostly a question of estimating noise from a > >>> 'scope shot. > >>> > >>> George H. > >>> > >> > >> It looks a bit wider than that, right around one division FWHM, > >> which would make it about 2.8 MHz wide. That's about what you'd > >> expect from that cavity, so it looks like the real line width is > >> unresolved. > > > > Yeah, I was hoping that the noise on the sides would tell me > > something about the noise/ BW of the laser. > >> > >> You might try making a Mach-Zehnder with a path difference of a > >> few metres and look at the FM-AM conversion. That'll demodulate > >> the noise pretty well, and you can compute the line width from the > >> noise spectrum. > > Hmm so beat the laser with itself.. but ~3m = 100nS later. I'll need > > a reasonable fast detector. Way back when I beat two free running > > lasers together and looked at the linewidth.. (laser wavelength set ~ > > 100 MHz apart.) But I only had my 'scope FFT to measure the spectrum > > and... well I was pushing the limits of the 'scope and so a fairly > > uncertain measurement. > > The bandwidth you need is only a couple of times the laser linewidth, so > it shouldn't be hard to do. If you don't have a baseband spectrum > analyzer, it's a little more difficult.
Well an SRS (720?) but only to 100kHz. I've been drooling over one of those cheaper Rigol SA's
> > I forget--is this a visible or IR laser?
NIR (780 nm)
> > > Will the Mach Zender trick move the signal down to DC? > > It will if you use it to measure the coherence length, which is where > the fringes wash out, but that might be pretty long.
So what am I expected to see with the Mach Zender? I should set the path length so that I'm in the "middle" of a fringe. Then a wavelength change gives the maximum amplitude change... I'll have to work out the math. Hmm I could do exactly the same thing with an unequal arm Michelson? This is really only going to measure the short time linewidth of the laser. When I beat two lasers together I measured a fairly narrow bandwidth ~100kHz or so.. But the whole thing had "jitter" and would bounce around by ~+/- 1 MHz or so on a ~second time scale. George H.
> > >> > >> What are the peaks out at +- 10 MHz? > > > > I'm RF modulating the laser current at 10MHz to put side bands on > > the laser. They are just there to set the scale of the sweep. > > Right. Missed that the first time round, thanks. > > 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 net > http://electrooptical.net
Ok, i understand now.
Thank you for the reply.

Mark
On 04/10/2015 01:47 PM, George Herold wrote:
> On Friday, April 10, 2015 at 12:30:18 PM UTC-4, Phil Hobbs wrote: >> On 04/10/2015 12:19 PM, George Herold wrote: >>> On Friday, April 10, 2015 at 11:20:25 AM UTC-4, Phil Hobbs >>> wrote: >>>> On 04/10/2015 10:39 AM, George Herold wrote: >>>>> Hi all, after tribulations with HV opamps and leaky >>>>> inductors, my diode laser seems to be happy again. Here's a >>>>> scan of the laser through a Fabry-Perot confocal cavity. (F-P >>>>> FSR ~ 380 MHz, finesse > 100.) >>>>> https://www.dropbox.com/s/6zpgdfs6cqus7ig/TEK0015.BMP?dl=0 >>>>> >>>>> I put 10 MHz side bands on the laser to give a scale. Looking >>>>> at the 'scope shot would I be safe in claiming 1 MHz (or >>>>> better) of laser bandwidth? >>>>> >>>>> Thanks... this is mostly a question of estimating noise from >>>>> a 'scope shot. >>>>> >>>>> George H. >>>>> >>>> >>>> It looks a bit wider than that, right around one division >>>> FWHM, which would make it about 2.8 MHz wide. That's about >>>> what you'd expect from that cavity, so it looks like the real >>>> line width is unresolved. >>> >>> Yeah, I was hoping that the noise on the sides would tell me >>> something about the noise/ BW of the laser. >>>> >>>> You might try making a Mach-Zehnder with a path difference of >>>> a few metres and look at the FM-AM conversion. That'll >>>> demodulate the noise pretty well, and you can compute the line >>>> width from the noise spectrum. >>> Hmm so beat the laser with itself.. but ~3m = 100nS later. I'll >>> need a reasonable fast detector. Way back when I beat two free >>> running lasers together and looked at the linewidth.. (laser >>> wavelength set ~ 100 MHz apart.) But I only had my 'scope FFT to >>> measure the spectrum and... well I was pushing the limits of the >>> 'scope and so a fairly uncertain measurement. >> >> The bandwidth you need is only a couple of times the laser >> linewidth, so it shouldn't be hard to do. If you don't have a >> baseband spectrum analyzer, it's a little more difficult. > Well an SRS (720?) but only to 100kHz. I've been drooling over one > of those cheaper Rigol SA's >> >> I forget--is this a visible or IR laser? > > NIR (780 nm) >> >>> Will the Mach Zender trick move the signal down to DC? >> >> It will if you use it to measure the coherence length, which is >> where the fringes wash out, but that might be pretty long. > > So what am I expected to see with the Mach Zender? I should set the > path length so that I'm in the "middle" of a fringe. Then a > wavelength change gives the maximum amplitude change... I'll have to > work out the math. Hmm I could do exactly the same thing with an > unequal arm Michelson? > > > This is really only going to measure the short time linewidth of the > laser. When I beat two lasers together I measured a fairly narrow > bandwidth ~100kHz or so.. But the whole thing had "jitter" and would > bounce around by ~+/- 1 MHz or so on a ~second time scale.
The Mach-Zehnder works as a delay discriminator, which gives you the FM noise spectrum of the laser directly, assuming the AM noise is lowish. You integrate the FM spectrum (i.e. multiply by 1/omega) to get the PM spectrum, which is what you want. In order not to lose sensitivity too horribly, you want the delay to be some reasonable fraction of the coherence time, like 1/10 to 1/4, which is why I suggested a few metres' path difference. 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 net http://electrooptical.net