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Laser fundamentals I

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Laser fundamentals I
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Spectrum of laser light
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49
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CC Attribution - NonCommercial - ShareAlike 4.0 International:
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This resource contains demonstrations used to illustrate the theory and applications of lasers and optics. A detailed listing of the topics can be found below. Lasers today are being used in an ever-increasing number of applications. In fact, there is hardly a field that has not been touched by the laser. Lasers are playing key roles in the home, office, hospital, factory, outdoors, and theater, as well as in the laboratory. To learn about lasers and related optics, one usually takes a course or two, or acquires the necessary information from books and journal articles. To make this learning more vivid and more exciting, and, one hopes, more understandable, one needs to see some of the basic phenomena involved. To fill this need, Professor Ezekiel has videotaped 48 demonstrations that illustrate most of the fundamental phenomena relating to lasers and physical optics. By using split-screen inserts and a wide range of video-recording capabilities, it is possible to show real-time effects in lasers and optics with the simultaneous manipulation of the components that cause these effects. In this way, one can see effects in close up that would be difficult, if not impossible, to display in front of an audience or in the classroom. These video demonstrations are designed for: The individual student of lasers and optics who wants to observe the various phenomena covered in theoretical treatments in courses, books, and technical papers. The Instructor in lasers and optics in a company, university, college, or high school who wants to illustrate, in class, many of the fundamental phenomena in optics and lasers.
LaserVideoOpticsVisible spectrumFundamental frequencyMaterialGround stationQuality (business)
Visible spectrumLightLaserNeon lampFACTS (newspaper)Audio frequencySpectrometerElectromagnetic spectrumLaserRear-view mirrorLecture/Conference
LaserOptical cavityAudio frequencyAM-Herculis-SternMode of transportSpectrum analyzerRear-view mirrorVisible spectrumSpectrometerFACTS (newspaper)
LightAM-Herculis-SternTransmission (mechanics)Plane (tool)LaserAudio frequencyFACTS (newspaper)
Visible spectrumTemperatureEffects unitCurrent densityAtmosphere of EarthOptical cavityAudio frequencyTelescopic sightFACTS (newspaper)Separation processActive laser medium
Telescopic sightOptical cavityAudio frequencyBandwidth (signal processing)Active laser mediumPower (physics)HyperbelnavigationLaserVideoFACTS (newspaper)
Mode of transportOptical cavityLaserLecture/Conference
AM-Herculis-SternMode of transportLaser
LaceLaser
Visible spectrumLaserRear-view mirrorLecture/ConferenceMeeting/Interview
Rear-view mirrorOptical cavityLaser
ElektronentheorieScanning probe microscopyLecture/Conference
Crystal twinningOptical cavityAM-Herculis-SternMode of transportOrder and disorder (physics)CouchLaserSemiconductor device fabricationLongitudinal waveMolding (process)
LaserAM-Herculis-SternVisible spectrumBrewster's angleMode of transportRail transport operationsTelescopic sightRear-view mirrorPlane (tool)FACTS (newspaper)Optical cavityAudio frequencySingle (music)Transmission (mechanics)Angle of attackMeeting/Interview
Single (music)AM-Herculis-SternLaserMode of transportAtmosphere of EarthAudio frequencyOptical cavityRail transport operationsCartridge (firearms)FACTS (newspaper)
Spectral linewidthOptical cavityLaserVideo
VideoLaserFACTS (newspaper)Active laser mediumRemotely operated underwater vehicleOptical cavityAM-Herculis-SternJitter-EffektMode of transport
LaserMode of transport
LaserAM-Herculis-SternMode of transportFACTS (newspaper)
Mode of transportLaserBrewster's angleAM-Herculis-SternCartridge (firearms)Astronomisches FensterAngle of attackLecture/ConferenceMeeting/Interview
AM-Herculis-SternLaserMode of transportGas-filled tubeLecture/Conference
Transcript: English(auto-generated)
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Now we're all set to look at the spectrum of laser light. For example, is the spectrum, is it a single frequency or is it multiple frequency or what have you? In fact, we're going to look at the spectrum of light from two lasers, two helium neon lasers.
We have this laser here with external mirrors and the one over here with internal mirrors. And the way we're going to look at the spectrum is by using the optical spectrum analyzer here. So now let me turn on this laser with external mirrors.
And the light from the laser then is reflected by this mirror and this mirror. And here it is going right into this optical spectrum analyzer, which is a scanning Fabry-Perot cavity. The output of this spectrum analyzer
then goes to an oscilloscope over there. As we can see on the oscilloscope, we have more than one frequency. In fact, we have several frequencies, sometimes three
and sometimes even four. So the spacing of the modes here is about 270 megahertz, which is consistent with the length of the laser cavity of 56 centimeters.
Now, the first thing I'm going to do is see whether the polarization of all these laser frequencies is the same or not. So what I'm going to do is insert the polarizer in the beam and then rotate the polarizer.
In fact, let me up the gain a little bit here. And let me rotate the polarizer or the transmission axis of the polarizer to see whether the polarization is
the same for all of them. And as you can see, I can extinguish all of them when the polarization is horizontal and bring them all up when the polarization is vertical.
And remember, this is the light that was plane polarized. So now we've shown that indeed all the frequencies that come out from this laser, all of them have the same polarization in the vertical plane.
So now let me take the polarizer out and readjust the gain on the scope. So back now to the three frequencies. You can see that they move around because the cavity is drifting in length due to the air currents or temperature effects or what have you.
And even, in fact, as I speak or as I tap on the cavity, you can see that I can create a mess of the spectrum just by simply tapping. If I lean on the table, you can see again
that I can make the frequencies wander around. Now, the fact that I have more than one frequency means there is enough gain for several frequencies to oscillate due to the fact that the gain medium has some bandwidth.
Not very huge, but some bandwidth. Now, I can make the laser go at one frequency by introducing loss, by simply misaligning the cavity to introduce loss. So you can see here I've got only two frequencies. And in fact, I'm going to up the sensitivity of the scope because the power goes down when I misalign.
You can see here I have essentially two frequencies. And if I add more loss, I can have only one frequency. So in a way, I can run this laser at one frequency,
but it's difficult to keep the other one out. And then you can see that as I lean on the cavity, I can make this frequency move around. Now, generally, that's not a good way of getting single frequency, and we have other techniques for getting single frequency, which
we'll discuss later. For this laser, it's best to align it for the highest power out, and this way will automatically get more than one frequency. And for a lot of applications, this is fine, all right?
So to summarize then, this laser that is 56 centimeters long, or the cavity is 56 centimeters long, then gives us about three modes. And the spacing between each mode is about 270 megahertz.
And all the modes have the same polarization. Now, we're ready to look at the other laser, the laser that has internal mirrors and also shorter in length. So when we come back, then we look at that laser.
Now, we're ready to look at the spectrum of this laser here with internal mirrors. The setup for looking at the spectrum is the same as before, but let me just remind you of it. Here's the output of the laser reflected by this mirror, this mirror, into the scanning Fabry-Perot
cavity. The output of the cavity then goes onto the oscilloscope over there. Now, as you can see on the scope, and let me adjust the center of the scan. As you can see, we have two big modes,
and these are spaced by 680 megahertz, which is consistent with the length of the cavity, of the laser cavity of 22 centimeters. The 680 megahertz is given by C over 2L, the spacing between longitudinal modes.
So these are two longitudinal modes of the laser. This little fellow here is a third mode that's coming in at an odd position because the free spectral range of the scanning Fabry-Perot cavity is not quite large enough, so we're getting a wraparound from another order.
So let's not worry about this one too much. So let's look at, essentially, the two main longitudinal modes of this laser. Now, let's look at now the polarization of these modes.
So now what I'm going to do is take the polarizer, and as we did with the other laser, let's take the polarizer and look at the spectrum. Now, if we look at the scope after I
make a slight adjustment of the gain because of the loss in the polarizer, now what I'm going to do is look at the spectrum of the laser light on the scope as a function of polarization. So first, you can see with the polarization
set at this angle, you can see, essentially, we have predominantly one mode. And then if I rotate the transmission axis of the polarizer, I can extinguish this mode and bring up the other one.
Now, let me just center the mode on the scope so I can bring up the other one. You can see, indeed, I have single frequency operation on just one mode. So I either have this mode here, or if I rotate the polarizer, I can bring up the other one.
So again, let me go back to the first one and then to the one over here. Now, this is different from what we had with the external mirrors and the Brewster windows, where we found that all the modes were
of the same polarization. Now, here, we find that one mode is one polarization, and the other one is polarized orthogonal to the first one. And in fact, this explains why the output of the laser wasn't plane polarized, as in the external mirror cavity.
Now, this is very interesting that because we have internal mirrors, the modes have orthogonal polarization. At least adjacent modes have orthogonal polarization. And in fact, this is a very easy way
of selecting single frequency operation by simply placing a polarizer in the beam and then selecting one frequency. Now, let's look at this single frequency behavior. If I want to tune the laser frequency,
I simply blow some air onto the laser to the eye blew too much. Do it again. So you can see that I can scan the laser frequency
by simply changing the length of the cavity. In this case, I'm cooling off the cavity. Now, let me also point out that the line width that you see, the line width that you see here, is not the laser line width at all. The line width that you see here is determined by the scanning
Fabry-Perot cavity. The line width, the true line of the laser, is very narrow. In fact, in principle, it's a fraction of a millihertz. But because laser jitters and what have you, you'll get a little broadening, but certainly nowhere
near as wide as what you see over here. And again, let me bring up the other mode and the other polarization. And again, this one will also tune
across the gain curve of the medium of the laser. So in summary then, we've seen that for this laser here
that's about 22 centimeters apart, that we get longitudinal modes of 680 megahertz apart. And the polarization of each mode is different. In fact, adjacent modes have orthogonal polarization. While with this longer laser of length 56 centimeters,
we found that the mode spacing was 270 megahertz. Again, it's consistent with the length of the laser and that the polarization of all the modes was identical. Now I want to leave you with this puzzle.
Why? In this case here, the laser with internal mirrors, the polarizations of the modes were different, while the one here, the polarization was the same. Well, to answer this one here is easy because of the Brewster windows and the only polarization that can
lay is the polarization set by the angle of the Brewster windows. But in this case here, we don't have any windows to set the polarization. We only have just two mirrors sealed onto the discharge tube.
And we saw that adjacent modes have orthogonal polarizations. So here's a nice little puzzle for you to think about. Now we're not done yet with laser properties. There are several other experiments that we have prepared for you. So when we come back, we'll show you
other aspects of laser behavior.