more power
Nov. 16th, 2008 08:07 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
nibotOne of our current efforts at LIGO is to increase the laser power.
The sensitivity of our gravitational wave detector is ultimately limited by the granularity of light*. The light coming out of our detector, like any light, is packaged up in little bundles of energy, photons. In any given length of time, the number of photons coming out of the detector is an integer. We can't measure half a photon. It's the same problem as doing a survey where you don't survey enough people. If you ask only ten people who's going to win the election, your survey results can never be better than 10%. The solution is to survey more people. To measure more photons. As we put more laser power into our machine, the granularity of light matters less and less.
The noise caused by the granularity of light is called "shot noise." The name sticks because it makes us think of shot, like the light is a spray of BB's out of a shotgun. But it's really named after Walter Schottky.
We've replaced our ~ 8 Watt laser with a new 35 Watt one. But putting more light into the machine is not so simple as getting a more powerful laser, for at least two reasons.
* at frequencies above ~ 100 Hz
1. Things heat up
Our optics are very, very good. Made out of fused silica, which is essentially synthetic quartz, they absorb only a few millionths of the power passing through them.
Eight watts or 35 watts might not sound like much—any incandescent light bulb emits this much power—but we use resonant cavities to trap the light. Inside the machine, the light levels build up to several kilowatts of laser power. Advanced LIGO will have megawatts circulating inside.
The optics, of course, absorb some of this power, which makes them warm up, just a tiny bit, in the center of the optic where the light is passing through. The temperature gradient changes the effective curvature of the optic, which affects the behavior of the machine. Signals change and control systems becomes unstable. The algorithms designed to keep the machine at its operating point can no longer keep up--we "lose lock."
Just as the SR-71 was designed to fit together properly only when hot, so were LIGO's optics. However, because the absorption of the optics could not be predicted precisely in advance and changes over time; and especially now that we are increasing the laser power, the heating of the optics doesn't necessary create the "right" curvature change.
To correct this, we have a "thermal compensation system" (TCS). This consists of two extra lasers, 30 Watt carbon dioxide lasers. We use special conical lenses to make a beam that's ring shaped, an annulus like the cross-section of a donut, which is projected onto the optics to heat them around the central beam. This annular heating is used to make the temperature of the optics more uniform, to remove the temperature gradient that is created by the main beam.
The beam from the carbon dioxide laser is a different wavelength (10.6 microns) than the main laser beam (1 micron). While the bulk of the optics is almost perfectly transparent to the main laser (as opposed to the reflective coatings on the surface of the bulk), it is almost perfectly opaque to the carbon dioxide laser beam. The light from the CO2 beam is absorbed, heating the optics.
Unlike our main laser, the carbon dioxide lasers are bought commercially. They are used industrially for welding. Here's what it looks like when you aim one at a brick.
So one activity these days (I mean, nights) is to tune the powers of the TCS lasers to best compensate for the main laser power.
(You might think that for really low noise operation, we'd want our optics to be cold, and you'd be right. The prototype machine CLIO, located 1000 meters under Japan, is testing cryogenic techniques. A Large-scale Cryogenic Gravitational-wave Telescope is proposed.)
2. Light pushes on things
It's true, light pushes. A photon carries energy and momentum. When a photon bounces off of a mirror, the photon's momentum changes directions. Where it had momentum +p before hitting the mirror, it has momentum -p afterwards, a difference of -2p. Conservation of momentum tells us that something else has to get an extra +2p momentum to compensate; the mirror gets this momentum. This is called radiation pressure.
You don't feel it because the force of light hitting you is extremely tiny. But our instrument is very sensitive, and it does feel the radiation pressure. Each of our mirrors is hung from two steel wires, forming a pendulum. Pairs of mirrors face each other, forming optical cavities, in which the light power builds up tremendously. The light actually pushes the mirrors apart. It's almost like the mirrors are connected by a spring, whose springiness depends on the light power. We call it the optical spring.
More insidiously, if the light doesn't hit the mirrors exactly in their centers, the mirrors feel a torque and are rotated very slightly. As if our optics are connected by springs, they begin bouncing around. Worse, after some threshold in power is crossed, these radiation pressure effects can become unstable, meaning that once it starts, it will only get worse. The mirrors get pushed apart in such a way that the light can no longer resonate between them, and, again, we lose lock.
To prevent this, again, we have a compensation system. We measure the angles of the mirrors and the alignment of the laser beam, and we compensate for any errors by pushing on the mirrors with magnets. But while our system for doing this has worked so far, it is not yet up to the task of handling the power levels we intend to put into the machine. And that will keep several graduate students and scientists up at night for some months to come.
--
For more, see![[livejournal.com profile]](https://www.dreamwidth.org/img/external/lj-userinfo.gif)
nibot_lab.
On Friday night we put 12 W into the machine for 12 minutes.
The sensitivity of our gravitational wave detector is ultimately limited by the granularity of light*. The light coming out of our detector, like any light, is packaged up in little bundles of energy, photons. In any given length of time, the number of photons coming out of the detector is an integer. We can't measure half a photon. It's the same problem as doing a survey where you don't survey enough people. If you ask only ten people who's going to win the election, your survey results can never be better than 10%. The solution is to survey more people. To measure more photons. As we put more laser power into our machine, the granularity of light matters less and less.
The noise caused by the granularity of light is called "shot noise." The name sticks because it makes us think of shot, like the light is a spray of BB's out of a shotgun. But it's really named after Walter Schottky.
We've replaced our ~ 8 Watt laser with a new 35 Watt one. But putting more light into the machine is not so simple as getting a more powerful laser, for at least two reasons.
* at frequencies above ~ 100 Hz
1. Things heat up
Our optics are very, very good. Made out of fused silica, which is essentially synthetic quartz, they absorb only a few millionths of the power passing through them.
Eight watts or 35 watts might not sound like much—any incandescent light bulb emits this much power—but we use resonant cavities to trap the light. Inside the machine, the light levels build up to several kilowatts of laser power. Advanced LIGO will have megawatts circulating inside.
The optics, of course, absorb some of this power, which makes them warm up, just a tiny bit, in the center of the optic where the light is passing through. The temperature gradient changes the effective curvature of the optic, which affects the behavior of the machine. Signals change and control systems becomes unstable. The algorithms designed to keep the machine at its operating point can no longer keep up--we "lose lock."
Just as the SR-71 was designed to fit together properly only when hot, so were LIGO's optics. However, because the absorption of the optics could not be predicted precisely in advance and changes over time; and especially now that we are increasing the laser power, the heating of the optics doesn't necessary create the "right" curvature change.
To correct this, we have a "thermal compensation system" (TCS). This consists of two extra lasers, 30 Watt carbon dioxide lasers. We use special conical lenses to make a beam that's ring shaped, an annulus like the cross-section of a donut, which is projected onto the optics to heat them around the central beam. This annular heating is used to make the temperature of the optics more uniform, to remove the temperature gradient that is created by the main beam.
The beam from the carbon dioxide laser is a different wavelength (10.6 microns) than the main laser beam (1 micron). While the bulk of the optics is almost perfectly transparent to the main laser (as opposed to the reflective coatings on the surface of the bulk), it is almost perfectly opaque to the carbon dioxide laser beam. The light from the CO2 beam is absorbed, heating the optics.
Unlike our main laser, the carbon dioxide lasers are bought commercially. They are used industrially for welding. Here's what it looks like when you aim one at a brick.
So one activity these days (I mean, nights) is to tune the powers of the TCS lasers to best compensate for the main laser power.
(You might think that for really low noise operation, we'd want our optics to be cold, and you'd be right. The prototype machine CLIO, located 1000 meters under Japan, is testing cryogenic techniques. A Large-scale Cryogenic Gravitational-wave Telescope is proposed.)
2. Light pushes on things
It's true, light pushes. A photon carries energy and momentum. When a photon bounces off of a mirror, the photon's momentum changes directions. Where it had momentum +p before hitting the mirror, it has momentum -p afterwards, a difference of -2p. Conservation of momentum tells us that something else has to get an extra +2p momentum to compensate; the mirror gets this momentum. This is called radiation pressure.
You don't feel it because the force of light hitting you is extremely tiny. But our instrument is very sensitive, and it does feel the radiation pressure. Each of our mirrors is hung from two steel wires, forming a pendulum. Pairs of mirrors face each other, forming optical cavities, in which the light power builds up tremendously. The light actually pushes the mirrors apart. It's almost like the mirrors are connected by a spring, whose springiness depends on the light power. We call it the optical spring.
More insidiously, if the light doesn't hit the mirrors exactly in their centers, the mirrors feel a torque and are rotated very slightly. As if our optics are connected by springs, they begin bouncing around. Worse, after some threshold in power is crossed, these radiation pressure effects can become unstable, meaning that once it starts, it will only get worse. The mirrors get pushed apart in such a way that the light can no longer resonate between them, and, again, we lose lock.
To prevent this, again, we have a compensation system. We measure the angles of the mirrors and the alignment of the laser beam, and we compensate for any errors by pushing on the mirrors with magnets. But while our system for doing this has worked so far, it is not yet up to the task of handling the power levels we intend to put into the machine. And that will keep several graduate students and scientists up at night for some months to come.
--
For more, see
nibot_lab.On Friday night we put 12 W into the machine for 12 minutes.
![[identity profile]](https://www.dreamwidth.org/img/silk/identity/openid.png){kind=small}
no subject
Date: 2008-11-17 03:45 am (UTC)(I want to call it not a telescope, but a teletact or something similarly strange.)
no subject
Date: 2008-11-17 03:53 am (UTC)The hanging of the mirrors is actually to reduce their coupling to ground motion. If you wiggle the top of pendulum at a frequency much greater than its resonant frequency, the coupling of that motion to the bottom of the pendulum is suppressed. But, of course, at the resonant frequency, it is amplified...
the bombing doesn't start in earnest until next year. But now we have this (http://ilog.ligo-la.caltech.edu/ilog/pub/ilog.cgi?group=detector&date_to_view=11/11/2008&anchor_to_scroll_to=2008:11:11:12:12:08-gtraylor):
no subject
Date: 2008-11-18 01:35 am (UTC)Or at any rate, YOU should.
no subject
Date: 2008-11-18 03:19 am (UTC)Clearing some trees is surely prudent. Falling trees did all the damage here in "upstate" Louisiana.
no subject
Date: 2008-11-17 10:51 am (UTC)So the light has no escape at all? Is that why it has such an effect?
LIGO is meant to measure gravity, right? I've seen a kind of pock-marked diagram of earth's gravity... but I guess what drew me to LIGO in the first place was a layman's sense of sarcasm about the odd gravity in New Orleans-- that it has to be heavier than other places, to account for things. I don't know if "heavier" is the correct way to call in in science, but it's definitely a piece of conversation going around. Is this what LIGO is meant for, in the midst of fenagling from loggers and mineral speculators?
no subject
Date: 2008-11-19 12:22 am (UTC)no subject
Date: 2008-11-19 03:04 am (UTC)In fact, here's an oscilloscope plot of a cavity emptying (http://nibot-lab.livejournal.com/75801.html) after we shut off the input power.
no subject
Date: 2008-11-24 01:01 am (UTC)Hmmm… “Do not look into huge fucking laser with remaining eye!”
Date: 2008-11-19 06:29 am (UTC)