Laser fundamentals III
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00:00
VideoLaserOpticsSingle (music)Audio frequencyFundamental frequencyMaterialQuality (business)Ground station
00:20
Optical cavitySingle (music)Audio frequencyMode of transportSelectivity (electronic)Short circuitLaserCartridge (firearms)Meeting/Interview
01:07
Optical cavityVisible spectrumIonRear-view mirrorLightInterferometrySensorHood (vehicle)Beam splitterLaser
02:24
Visible spectrumOptical cavitySeparation processAmplitudeAudio frequencyRemotely operated underwater vehicleOrder and disorder (physics)Longitudinal waveSingle (music)Mode of transportLaserInterferometrySpectrum analyzerNegativer WiderstandSeries and parallel circuitsAstronomisches FensterRear-view mirrorGlassReflexionskoeffizientLaserDiagram
05:17
SolidCar tuningGlassField strengthAirliner
06:03
LaserSingle (music)Audio frequencyOptical cavityVisible spectrumFACTS (newspaper)Remotely operated underwater vehicleSwitchDiagram
07:13
Single (music)Audio frequencyRail transport operationsLaceLaserOrder and disorder (physics)Mode of transportVisible spectrumScale (map)Intensity (physics)Optical cavityTelescopic sightCylinder headFACTS (newspaper)Diagram
10:48
LaserOptical cavityAudio frequencySingle (music)MultiplizitätVisible spectrumSemiconductor device fabricationDiagram
Transcript: English(auto-generated)
00:00
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00:21
PROFESSOR OSCAR BLANCHETT GABRIEL KUHN. In this next demonstration, we're going to show how one can get a laser to oscillate at only one frequency, when normally the laser oscillates at many frequencies associated with the longitudinal modes. In this case, we're going to look at an argon laser, which
00:42
is over here. And we're going to do the single frequency selection by placing an ethelon inside the cavity. Now, an ethelon is a short Fabry-Porot cavity, which then we'll place inside the argon laser cavity.
01:02
And we'll be able to select single frequency. Now we're ready to look at the spectrum of the light from an argon ion laser. Here's the argon ion laser. And the output is coming out from the other end of the laser over here.
01:21
We're going to reflect it by mirrors onto this mirror here. And then reflected light from this mirror will go on to this mirror and is then reflected into this scanning Fabry-Porot interferometer. The free spectral range of this cavity is 15 gigahertz. And here is the detector.
01:41
And we also put a hood over the path between the cavity and detector to prevent room light from reaching the detector. At the same time, we have a beam splitter here. We're going to reflect a little bit of the light into another scanning Fabry-Porot interferometer over here. This is a longer cavity, has a free spectral range
02:02
of 1 and 1.5 gigahertz. Again, the detector for this cavity is over here. First, we're going to look at the spectrum of the laser light with the 15 gigahertz free spectral range cavity. So now let's go over to the scope and look at the output spectrum.
02:24
Now we can see the output of the 15 gigahertz free spectral range scanning Fabry-Porot cavity on the oscilloscope. What we see here is the free spectral range, which is, again, 15 gigahertz. That's the separation between these two peaks.
02:43
And the spectrum of the laser looks like it's about a few gigahertz wide. The reason for this is that the gain curve in an argon laser is pretty broad. It's of the order of 10 to 15 gigahertz. And lots of longitudinal modes oscillate.
03:03
And they compete with each other. And right now, they're blending to give you this big blob of several gigahertz spectrum. Now for many applications, this broad spectrum is not of much use, for example, in applications
03:23
using interferometry. For such applications, you need to make the laser oscillate at a single frequency. The popular way of generating single frequency, these big lasers, is to use an etalon and put it inside the laser cavity
03:41
to select out the single frequency. So when we come back, we'll put in an etalon inside the laser and observe single frequency output behavior. Now I'm going to put an etalon inside the laser cavity.
04:03
Here's the etalon in a holder. It's a very simple thing. It's a piece of glass, parallel piece of glass, 1 centimeter thick, and has a reflectivity of about 35% on each surface.
04:21
And it's held in this mount here, so I can then adjust it within the laser cavity. So now I'm going to place the etalon inside the laser cavity. There's a little space here between the Brewster window and one of the mirrors.
04:43
So here we are. Here's the etalon in place. And all I have to do now is adjust the alignment. And here we are now. We get lasing.
05:05
We have lasing now. And now we're ready then to go and look at the output of the spectrum analyzer with this etalon in place. Since the etalon is a solid piece of glass,
05:20
I cannot change its length very easily, but I can effectively change its length by misaligning it. And this way, then, I can get a tuning of the etalon by simply rotating the etalon.
06:04
Now that we have the etalon inside the cavity, the spectrum is single frequency. And again, the free spectrum range is 15 gigahertz, but the output now is single frequency. And in fact, by adjusting the etalon,
06:22
I can tune this frequency across the gain curve of the argon laser, which is right now about a few gigahertz. Let me do it again over here.
06:56
The finesse of the cavity, this 15 gigahertz cavity,
07:01
is not very high. So what we'll do, we'll switch to the other cavity, the one that has a 1 and 1-1-2 gigahertz free spectrum range, has a much higher finesse, and we'll be able to see some interesting behavior of this single frequency operation of the laser.
07:21
On the scope now, we have the output of the 1 and 1-2 gigahertz Fabry-Perot cavity. As you can see, the spacing between the modes here is 1 and 1-2 gigahertz, and the finesse is pretty high, probably of the order of 300 or so.
07:44
As we can see, the output of the laser is single frequency. And now what I'm going to do, I'm going to misalign the etalon and see what happens.
08:05
Now, what you notice is that because I'm tuning the etalon, I'm also going to be tuning the laser frequency. But the laser frequency is not tuning smoothly. It's tuning in jumps. So let me do it again.
08:34
Here we are. Jumps, or so-called mode hops, of the order
08:44
of the free spectrum range of the laser cavity, which is of the order of 150 or so megahertz, because the laser cavity is about a meter long. So again, let me show you the hops again.
09:09
Now let me try to do it with the other knob on the etalon, since it's a less sensitive knob. Now you can see the hops much better.
09:23
Here we are. There's two hops there, one, two. In fact, if we expand the scale, then we can make the hops even larger.
09:41
Here the scale is expanded. Let me try again. You can see the hops now much more dramatically. Again, it's about 1.5, 150 megahertz or so per mode hop.
10:10
In fact, the intensity is supposed to, as I tune the etalon, the intensity is supposed to go down and then hop, then go up, down, and then hop. Let's do it one last time.
10:21
One hop, and another hop, another hop, and so on. So when we use the etalon inside the cavity, then we should be expecting these mode hops when the etalon gets misaligned,
10:42
or its length changes by a small amount. Now when one uses an etalon like this inside a laser cavity, there's one thing that one should never do, and that is align the etalon perfectly normal with respect
11:01
to the axis of the cavity or the laser beam inside the cavity. Because if you do, then you're going to get all sorts of multiple cavities taking place, and the spectrum goes absolutely haywire. So now I'm going to demonstrate that. I'm going to now align the etalon
11:25
to be normal to the laser cavity, and we see that the spectrum goes absolutely haywire. And all we have to do is just tilt away, and here we get quiet single frequency behavior.
11:42
And we go back and bring it to normal alignment, and here we see the spectrum going absolutely haywire. So again, the no-no with the etalons is not to place them normal to the beam.
12:04
Just have to tilt them away just a little bit, and you get nice quiet single frequency behavior.