Laser fundamentals I: Light inside and light outside laser
This is a modal window.
The media could not be loaded, either because the server or network failed or because the format is not supported.
Formal Metadata
Title |
| |
Title of Series | ||
Number of Parts | 49 | |
Author | ||
License | CC Attribution - NonCommercial - ShareAlike 4.0 International: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal and non-commercial purpose as long as the work is attributed to the author in the manner specified by the author or licensor and the work or content is shared also in adapted form only under the conditions of this | |
Identifiers | 10.5446/42716 (DOI) | |
Publisher | ||
Release Date | ||
Language |
Content Metadata
Subject Area | ||
Genre | ||
Abstract |
|
23
24
25
26
36
38
39
40
41
42
43
44
45
46
47
48
49
00:00
LaserVideoOpticsLaserElectric lightFundamental frequencyMaterialGround stationQuality (business)
00:20
LaserLaserValve amplifierRear-view mirrorTransmission (mechanics)Optical cavityLecture/Conference
00:59
KardierenLaserOptical cavity
01:13
SensorLaserOptical cavityLecture/Conference
01:30
Laser
01:37
GlassLaserOptical cavityMapLecture/Conference
02:12
Spare partAngle of attackStagecoachMarker penElectric lightAngeregter ZustandScreen printingLaserRotationAM-Herculis-SternGlassReflexionskoeffizientBrewster's angleMapBooster (rocketry)
03:38
GlassLaser
03:47
Cartridge (firearms)Marker penElectric lightStagecoachGlassLecture/Conference
04:17
LaserOptical cavityAngle of attackGlassLecture/Conference
04:36
Optical cavityGlassScreen printingAngle of attack
04:58
Lecture/Conference
05:04
GlassFACTS (newspaper)Optical cavityBrightnessScreen printingComputer animation
05:36
LaserElectric lightRear-view mirrorOptical cavityIntensity (physics)Lecture/ConferenceMeeting/Interview
06:20
LaserFACTS (newspaper)Optical cavityPhotonTheodoliteShort circuitPhotonicsMeeting/Interview
Transcript: English(auto-generated)
00:00
The following content is provided under a Creative Commons license. Your support will help MIT OpenCourseWare continue to offer high quality educational resources for free. To make a donation or view additional materials from hundreds of MIT courses, visit MIT OpenCourseWare at ocw.mit.edu.
00:21
This demonstration is for those of you who would like to dig a little more deeply into laser behavior. As we know, a laser like this one here consists of two mirrors and an amplifier. And if the gain of the amplifier is bigger
00:40
than the losses in the cavity, then we can get laser action. And if, let's say, this mirror has a little bit of transmission, then we can get laser light coming out that we can use. So let me turn then this laser on and show you here on this card the output of the laser.
01:07
Now in this demonstration, we'd like to show that the light inside the laser cavity that's going backwards and forwards between the two mirrors inside the laser cavity is much bigger than the light that's coming outside.
01:22
The question is, how do we do this? Because if we try to place a detector inside the laser cavity, like here, we would interrupt the laser action and the laser quits. So obviously, that's not a smart way of doing this.
01:40
So what we're going to do, we're going to use a simple method like this. We're going to take a piece of glass, and depending on the orientation of the piece of glass, we're going to sample the light both outside the cavity and inside the cavity. And hopefully, the loss that we will place inside
02:02
the laser cavity is so small that we will not interrupt the laser action, at least by too much. So here we go. So first, we have to show that we can sample some of the light with this piece of glass. So here we are.
02:22
Here's this piece of glass. Now, let me tell you what's happening. This piece of glass is placed at this orientation, and then I can rotate it like so using this rotation stage.
02:43
Now, if you look at the reflection from piece of glass on this little screen, you can see that the light gets extinguished when I am at the Brewster angle. Remember the polarization of this laser in the vertical plane so that I can achieve Brewster angle
03:03
condition using this orientation of the glass. So now, when I have Brewster's angle, it's when these two markers, one on the rotating part of the stage and the other one on the fixed part of the rotation stage, when they line up, we're at Brewster's angle. And when I move away from Brewster's angle like so,
03:23
I can increase the amount of light sampled by the piece of glass. So let's say if I set it this position here, then I have a small percentage of light that's reflected by the piece of glass onto the screen. Now, what I'm going to do is take a similar piece of glass
03:42
and put it again in the beam like so. And again, the markers are the same as in the other stage. So when the markers are lined up,
04:00
then I get no light reflected, which is means Brewster's condition. And when I'm over here, I get pretty much the same amount of light as in this case. So now I have calibrated these two pieces of glass in terms of angle.
04:23
Now what I'm going to do, I'm going to take one of them. And without changing the angle, place it inside the laser cavity. And hopefully, the laser won't go out on me. So as I put it in here, without changing the angle,
04:43
you can see I'm going to sample some of the light and you'll see it on this screen. And note that the word inside here, that means that this piece of glass is inside the laser cavity. And then one outside the laser cavity is labeled, again, by the word outside.
05:06
And now I would like you to look at the spot reflected by the piece of glass outside the cavity and the spot reflected by the piece of glass inside the cavity. So if we have now the split screen in position,
05:21
you can see that the one inside is much brighter than the one outside. In fact, it's brighter in this case by about a factor of 50 or so. The question I want to leave with you is why the light coming out of the laser
05:43
is much weaker than the light inside the laser. And what determines that ratio? So that's a nice little puzzle for you. Now, sometimes one would like to increase the light coming
06:00
out of the laser. Now, how can you do it? Well, you notice that since there's a lot of light inside the cavity, one way of quickly increasing the laser intensity, even though for a very short time, is by removing one of the mirrors. And for example, if I take this mirror out from the laser
06:25
very quickly, then I can essentially dump out all the photons that are stored in the laser cavity. In fact, this is called cavity dumping. And what this gives you, this gives you a huge increase in the number of photons
06:41
coming out from the laser, but only for a very short time. In fact, it just takes all the photons that are stored in the laser cavity and just dumps them out, and for, of course, for a very short time. In fact, the time is just twice the transit time of the cavity.
07:01
So the fact that one knows something about the light inside the laser is bigger than the light outside the laser, then one can come up with this concept of cavity dumping, which can be useful for some applications.