One 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 nibot_lab.


On Friday night we put 12 W into the machine for 12 minutes.

category: lab life category: science category: grad school pubDate: Mon, 17 Nov 2008 03:02:00 GMT

This afternoon at the coffee shop, while paging through my notebook, I came across an old annotation: "paella," as in "try making that delicious spanish saffron-flavored rice dish at home." Coincidentally, cherryspecial had just invited me to a vegan potluck, and so my mission was obvious.

olive oil	generous splash
vegetable stock	1 cup	(omit?)
water	1 cup	boiled
white wine	1 cup	yellow tail pinot grigio
garlic	4 cloves	smashed with a fork
yellow onion	1 big one
rice	2 cups	"Arborio" recommended
tomatoes	5	roma, cubed
bell peppers	1 orange, 1 yellow	diced
artichoke hearts	1 can (14 oz/8.5 oz dry)	drained
baby corn	1 can (15 oz/7 oz dry)	drained
mushrooms	1 box (8 oz)	champignon, quartered
saffron strands	a pinch	let steep in the boiled water
smoked paprika	couple tbsps	i love this stuff

I took a look at a bunch of recipes at epicurious.com and then did my own thing.

One of the exicting things about paella is that it uses saffron, famously the world's most expensive spice. I bought a bottle that contained a paper envelope that contained a cellophane envelope that contained 1.7 grams of saffron for $8 at Albertsons (unit price: $133/oz). Fortunately, a little goes a long way. The flavor of saffron is potent yet subtle. (I first came to love it in the form of saffron gelato at the little ice cream shop in Lund, Sweden.)

The other spice, smoked paprika, is exciting too (and much cheaper). Paprika (powdered dried red bell pepper) is totally useless, as it is completely flavorless. But this stuff is powdered smoked red pepper and has a delightful, pungent smoky taste, in addition to its amazing brick-red color.

First I boiled a cup of water and added a pinch of saffron. Then, in large sauce pan, I sauteed the onions in olive oil, then added the bell pepper, garlic, and other vegetables. Once all this was hot and bubbly, I added the other liquids: the saffron-water, the vegetable stock, and the white wine. Once that began to boil, I turned the burner down to minimum, added the rice, and let it cook as usual.

Delicious!

The big chunks of mushroom work really well. The artichoke hearts and baby corn are good too. These guys are good because they provide interesting textures. Really, it's hard to resist adding shrimp, and I might have to add shrimp to my leftovers. For vegetable stock, I used vegetable bullion, which I think was a mistake because there is that unmistakable MSG aftertaste. I'm not sure the wine does anything, although it is delicious by itself. Maybe omit the bullion so that the wine stands out better; or add water in place of the wine and serve the wine as a beverage.

The potluck was fun, too, though I arrived late, and the other food was delicious. This is a weekly Sunday thing so I might make it a habit.

category: food pubDate: Mon, 03 Nov 2008 06:07:03 GMT

My housemate, G, made fun of me for never having set a mousetrap before. He told me of his boyhood home, beside a cane field, and how after the sugar harvest the fields are burned, and how all manner of rats and snakes and other wildlife would seek refuge in their home.

G and I do not see each other much. He goes to sleep at 10. and I rarely get home from work by 10. He goes to work at seven and I aim for noon. We see each other at weekends, but even then, somehow, not so much.

When I first moved in I could tell he was disappointed by my lack of enthusiasm about a possible field trip to "Rooms To Go" to outfit our new abode.

For the first time we went out together today, to Lowe's to pick up some brackets for shelving he was installing in his room. He crashed his truck some months ago and can't drive anymore.

Strolling through Lowe's, he turned to me and asked me, emphatically, and in a way that conveyed it had been on his mind for some time,

"Tobin, how come you don't have anything?"

Ultimately I could only say, "Well, I kind of like it this way." With only a mattress, a suitcase, and a pile of books.

Later I mentioned that I had been thinking about getting a desk.

pubDate: Sun, 02 Nov 2008 08:13:19 GMT

3. Buying the house. The co-op is certainly fulfilling a role in (y)our lives. It's a fun place to live, and living there is a significant experience, something every member will remember and learn from and cherish. The co-op is fulfilling its social role. By buying and preparing food in bulk, it is partially fulfilling its financial role.

But the economic realization of the co-op requires that the co-op buy its house. The co-op is supposed to "bring the benefits of home ownership to a transient population". The not-for-profit organization stands in as the owner, while the membership rotates through. The money the co-op saves by not paying rent gets poured into infrastructural improvements. The co-op has a lot of cool stuff now: gardens and hot tubs and a movie theatre. Think of how much more the co-op could accomplish if it had thousands of dollars in development money? It could buy itself double-paned windows and insulation, and then you wouldn't be so cold all winter, and would save even more money on heating. You could build a greenhouse. You could make the third floor habitable. You could start a cafe down the street. You could help new fledgling co-ops get off the ground. You could buy another house! But first you have to buy this one.

There are some well-founded worries, but they can be addressed.

Will the co-op be able to fill the responsibility of home ownership? Of saving enough money to replace the roof when the need arises? Of performing routine maintenance? I think so, but if you have doubts, you can hire NASCO to help you.

The price. It's irrelevant. We've spent $35,000 in rent while quibbling over whether the purchase price of the house should be $50,000 or $75,000. It's a steal at anything less than $100,000 anyway.

The house itself. You don't think the co-op is actually going to move, do you?

Imaginary legal problems. Are just that.

The only reason the co-op exists at all is because Chris & I agreed to rent nine units of housing from Dave and Dave despite only having two other members lined up. And those members lined up based on only our ranting about what an awesome co-op we'd have if they came and built it with us. With this faith, they packed up their moving trucks, and drove to Rochester. The membership began trickling in, and they stepped up to the challenge. (I don't even remember how Far and Heather turned up, but
one day they were there, clip-boards in hand, giving tours and preaching the co-op dream.) Soon we had to turn people away--we had a waiting list! It was a leap in the dark, but it worked. Precisely because of the "damn the torpedoes" attitude. It was a dream and leap and we knew we were going to make it work, and it did.

The co-op needs to make the next leap. Please buy the house.

category: co-op pubDate: Sun, 26 Oct 2008 21:18:55 GMT

category: louisiana category: trespassing pubDate: Fri, 24 Oct 2008 00:00:02 GMT

A crew of visitors from Hanford, Caltech, and U. Florida is here to work on upgrading certain components of our laser interferometer gravitational wave detector. In the evening we went out to dinner at Albasha's, a pretty good Greek/Lebanese restaurant. One of the topics of conversation was of visitors' accomodations when visiting the Observatory.

Our visitors all stay in hotels and drive to the site in rental cars.

But Rick regaled us with the description of an alternative scheme for hosting visitors, the way things are done at another Caltech-run astronomical observatory. Apparently the Owen's Valley Radio Observatory is outfitted with a small dorm featuring simple rooms—bed, dresser, etc—and a big and wonderful communal kitchen, and, apparently, living room with fireplace and movie-viewing capacities. Not only that, I'm told that OVRO has a cook who comes to the lab every day to fix lunch and to make brown-bag dinners for staff working late into the night.

Now that's the way a lab should be!

The idea of retiring to an on-site communal living room with fireplace (hot chocolate in hand) situated amongst the remarkably beautiful eastern Sierra Nevada after a long day of fiddling with radio telescopes sounds pretty great.

katworthy, how is it really?

category: ovro category: ligo category: us-395 category: eastern california category: lab life pubDate: Thu, 23 Oct 2008 07:30:27 GMT

Late nights at the lab. We barbecued chicken (yes, from Whole Foods—it's a vice I'm cultivating) for dinner.

At dinner, talking about the current economic meltdown, Valera took a drag on his cigarette and smiled.

"Oh, I've been through a crash before..."

He's Russian.

category: economics category: lab life pubDate: Wed, 22 Oct 2008 07:01:04 GMT

If it is a dark weekend night and you are all alone staffing a laboratory in the middle of a louisiana swamp, I suggest you listen to The Ghost of Bobby Dunbar on the control room speakers.

category: radio category: lab life category: louisiana pubDate: Mon, 20 Oct 2008 05:29:14 GMT

Work at the lab lately has been quite satisfying. Working with Stefan and Hartmut, our current visitors, has been quite a pleasure. I feel more confident, and have had more time to think about things on my own. Also, we've made a lot of progress.

Part of today's excitement was in replacing a photodiode.



Our gravitational wave detector consists mainly of vacuum, enclosed by a steel tube and bounded by a collection of mirrors, and full of photons. The photons, little packets of light, carry all of the information. When the photons come out of the machine, we detect them with photodiodes, which turn streams of photons into streams of electrons, which feed through electrical circuits.

We use many photodiodes, several at each place where light comes out of the machine. A photodiode itself is a tiny piece of semiconductor, carefully mounted and connected via tiny wires to some bigger wires that poke out of its little can, and connect the photodiode to a collection of electronic circuits who are sensitive to variations in the intensity of the light at radio frequency, who gather this information, and send it out over a wire to more electronics, who feed the signal into computers, and servo systems, and data analysis algorithms. Servo systems that hold all our mirrors in exactly the right places. Data analysis algorithms that will tell us when we see a gravitational wave.

I don't know how much our photodiodes cost, or where we get them, but it is clear that they are very precious.



One of our photodiodes went bad recently, and today we replaced it. The new photodiode came as all new photodiodes do, mounted in a little metal can with a glass cover on top.

The glass cover protects the delicate photodiode and its delicate wires. But for us it is a problem; the small amount of light that gets reflected by the glass window instead of going through it corrupts our measurements. We have to take off the window, exposing the delicate photodiode.

This is done with a tool known as the "can opener," because it is used for opening photodiode cans. It is from ThorLabs, an company that makes scientific optical equipment, and as far as I can tell, it is specifically for this purpose.

I strongly appreciated Stefan's tutelage in this matter; he described how to perform the delicate operation, and then promptly left the room, leaving me to perform it. Trust leads to confidence. He's soloed an airplane; maybe the philosophy comes from there.

With the new photodiode, our gravitational wave detector is able to see 8.5 megaparsecs into space.

category: physics category: ligo category: lab life pubDate: Sat, 18 Oct 2008 09:23:54 GMT

This Friday the Oscar Wilde House in Berkeley is celebrating ten years of existence.

I wish I could go!

category: oscar wilde house category: co-op category: wilde category: berkeley pubDate: Mon, 13 Oct 2008 06:59:13 GMT

What do you eat?

200+ riders on critical mass tonight, and a sweet afterparty in the grass field on ivanhoe street, complete with a live band (the Tellers), an art show, and someone spinning fire poi. BTR is not a bad town.

Apparently the currently-ongoing LIGO collaboration meeting in Amsterdam is being held at a church.

This is just bizarre!

 
 

I ate scrambled eggs for the first time

I drove six hours through the empty darkness of Mississippi; now I'm in Oxford MS at Micah's house. Micah's an old housemate from when I first moved to Rochester. What an odd place for a reunion! Ryan is here, with his friend Matthias. Micah's house even looks like our old house in Rochester; he even has some of the furniture. Ryan's like a regular German now, having secret little conversations with Matthias. I can understand a lot of what he says, though. Last night sometime after 2am we went to Graceland Too (google it). The best part was when someone asked meekly to the other tourists, "is this the part where we get killed?" Right now Micah's making breakfast. Today's entertainment is to be the Ole Miss - Vanderbilt game tailgate party. I'm told it will be surreal.

I waded out into the flood in Old Mandeville yesterday. This is taken from Gerard Street in Mandeville, about a block from Lake Ponchartrain... Despite the wild surf on the lake, the floodwaters on the Mandeville city streets were calm, and folks took to them in various modes of transportation:



Everyone seemed very calm.



There were lots of enormous grasshoppers too:

Houston is going to get wallopped.

10:14 Wow-water crept up silently in the night, over the sea wall and to 80 paces from Pooh's house. Met more neighbors; nobody seems worried, so we'll stay put.

10:23 Hiccups in the power; sounds like transformers are popping.

10:31 Heavy rain comes and goes, but we're cozy inside and drinking tea.

14:17 i think the water is subsiding

14:43 People are canoeing down the street. Some other folks rode by in the scoop of a bulldozer. Apparently this always happens. Some Houston evacuees here.

I'm at Pooh's house in Mandeville... Pretty quiet here, but her neighborhood is behind police baracades. Peak storm surge expected here tonight.