VCO

So I think it’s time somebody posted to the physics blog…where has everyone been all July?

I am currently 6 weeks into my REU, with 4 more to go. Up to now I’ve been working on assembling and testing a voltage controlled oscillator (VCO). The VCO is part of the laser frequency stabilization control loop here at LIGO. The interferometer requires a very stable laser beam because fluctuations in the laser’s frequency that is input to the interferometer are indistinguishable from a change in the lengths of the interferometer arms (the thing we’re trying to measure). The laser that we currently use is far too noisy out of the box. (Laser noise by-the-way comes from things like electrical jitter in power supplies and control circuits, thermal noise acting on the laser cavity, and the optical pumping that makes the laser lase). To correct for noise, we take the beam before it is injected into the main interferometer and pass it through several (active and passive) stages that eliminate excess noise. The VCO device that I am working on is a single electronic (rack mount format) component that is part of the stage that eliminates high frequency noise (in a range of ~10kHz to 100kHz). The VCO takes as input a DC error signal that tells how far the laser is from a target/reference frequency. It turns this DC signal into an AC (RF) signal who’s frequency is modulated proportional to the DC voltage. The AC signal is used to drive an acousto-optic modulator (AOM) that shifts the frequency of the laser’s light by the frequency that it is driven with, thereby “locking” the laser to the reference frequency.

We anticipate that we will need to modulate the laser’s frequency by plus-minus 1MHz. What makes this complicated is that any VCO that can be purchased commercially (they’re essentially diodes that have a voltage dependent capacitance) has way too much intrinsic frequency noise–we’d end up introducing a second noise source into the laser rather than eliminating existing noise. To get around this, we start with a VCO that operates at much higher frequencies than we need (this typically means a higher signal-to-noise ratio) and divide the signal down in frequency, thereby dividing down the noise by the same amount. In our setup we start with a 1GHz signal that we can modulate plus-minus 130MHz and divide it down to an 8 plus-minus 1MHz signal (our target frequency gain). Since the AOM needs ~80MHz to operate, we then mix this signal with an extremely stable reference signal from a crystal oscillator to obtain an 80 plus-minus 1MHz signal with low frequency noise–at least in theory!

The VCO is in the prototype stage so I’ve spent the last several weeks working out bugs in the design. It was initially laid out entirely on paper (CAD in reality) and specifications for custom parts were sent off to manufacturers. We’ve had several issues with the physical layout of components (you can’t fit two parts in the same space in real life) and I’ve spent some time correcting resistor values on this chain of op-amps. All things considered, I’m quite impressed that my advisor made so few design errors, given the complexity of the device.

I have also been testing for, characterizing and eliminating noise sources in the circuitry. I’ve replaced a bunch of noisy (probably blown) op-amps and have consequently become fairly good at delicate soldering. (I replaced a ridiculously small chip the other day…about 1/4 inch square, with 10 pins on two sides.)

As of now, I’m waiting on some parts that have yet to arrive before I can do any more meaningful tests. I have a project report due next week, so it’s actually nice to have a little downtime right about now. Once the parts arrive, I’ll be able to start assembling a second VCO (LIGO has two interferometers after all) and I’ll be able to do more extensive noise tests and in addition to side-by-side comparisons between the two identical (hopefully) boxes.

I have to admit that eastern Washington is not quite as fun as it sounds like Germany is, but the local farmer’s market is quite nice and we’re a short drive from the mountains (where I spent a weekend camping out, see picture). I’m thinking I’ll spend a weekend at Hells Canyon sometime in the future when it’s not too hot.

The VCO:

Northeast Oregon (panorama shot):

LIGO is crazy, and other stuff

Greetings from the deserts of the Pacific Northwest! Yes, I said deserts. Washington State has the reputation of being a very wet and rainy place, so you wouldn’t think that there’d be any deserts anywhere near it, much less in the state itself. Eastern Washington however, is very dry. As they say here, the Seattle area magically steals all the rain…but it’s no magic, just mountains and a bit of thermodynamics!

I arrived here in Richland, WA (my REU town) after two weeks and one day on the road. I drove ~9,000 miles total and my truck really needs an oil change. I visited the Grand Canyon, Yellowstone, the Redwoods and Olympic National Park among other things. I took few pictures of the bigger tourist sights (eg old faithful, Grand Canyon)–they are some of the most photographed places in the world after all. I will mention what the pictures don’t tell you: old faithful smells like a giant fart because of the sulfur. It was overall, a really great trip. I really enjoyed the scenery in the Northwest, but Wyoming gets the overall top spot for favorite place.

I’m writing after my first full week here at LIGO. So far we’re working on settling in and getting the necessary paperwork done. I’ve been given a look at my project: mostly I will be building electronic equipment. I was handed a stack of circuit diagrams that I need to assemble. At this point I need to give a big thank-you to Dr. Goggin’s electronics class, without which I would be pretty lost right now.

The tumbleweeds around here are quite a sight. For the benefit of those of you who are, like me, from back east: a tumbleweed is a sagebrush plant that has died and been dried out. Eventually the stalk breaks in the wind and the whole plant blows across the desert, often for hundreds of miles. The sagebrush grow fairly round, facilitating their tumbling. They range from about the size of a softball to larger than a beachball. Cars on the highway that are traveling directly into the wind often collect a large pile of tumbelweeds on their grilles. Overall I must say that I find them quite comical–there’s just something about how they bounce across the desert…

It’s very dry out here. The sky one day was brown from blowing dust and they have signs along the road telling us that it’s illegal to throw burning material out of your car. We were notified one day over the PA system that there was a brush fire along the main road that runs between the site and the town, and that we all should take an alternate route home. This of course prompted stories from the staff of brush fires, which apparently turn tumbleweeds into hurtling balls of flame.

LIGO itself is pretty amazing. For those who need an introduction, LIGO is a giant michelson interferometer with 4km long arms. It was built as part of an attempt to detect gravity waves originating from distant, energetic cosmic events. A passing gravity wave moves the mirrors in the interferometer, causing a detectable change in the output light. The difficulty in the whole project is that we expect (based on the predictions of general relativity) that a passing gravitational wave will deform the distance between the mirrors in the 4km arms by about 10^-18 or 10^-19 meters depending on the strength of the source and its distance from earth. What is 10^-19 meters? well, it’s about a billion times smaller than the diameter of an atom or a thousand times smaller than the diameter of an atomic nucleus. Insane.

Most of the work done here at LIGO is focused on eliminating sources of noise in the interferometer. To a large degree the entire LIGO project is a massive exercise in control theory. The complexity of the systems used to keep out external effects is amazing. The equipment is first built on its own concrete slab that is physically separated from the main foundation and rest of the building. Things like the HVAC system are even located a good distance away from the interferometer to reduce vibrations. The optics are isolated from the ground by pendulums so that a vibration in the ground will be damped out. The pendulum isolation system is only effective for higher frequency motions (> ~1Hz). Even though LIGO is not trying to detect gravity waves with such low frequency, it is still necessary to counter low frequency motions so that the arms are maintained at a resonant length for the laser light inside (the interferometer is “locked”). Effects such as tidal stretching of the ground and thermal expansion during the day are some of the main causes of low frequency motion of the mirrors. In fact, the effect of tidal forces is about 200um over the 4km length of the arms–a big deal when you are trying to detect motion on the order of 10^-19m! Low frequency motions are compensated for by servo motors that move the mirrors to keep them at the correct distance.

Oh ya…we have really big lasers. LIGO is currently using a 30W 1064nm laser, which will be upgraded to a 200W laser as part of the Advanced LIGO project’s improvements. The quantum mechanical signal to noise ratio goes as N/Sqrt(N) where N is the number of photons, so increasing the power will decrease this type of noise. Since LIGO is a power recycled interferometer, the amount of power bouncing around in the arms can be in the neighborhood of 10,000-100,000W. This massive amount of light causes thermal heating of the mirrors, which deforms them and changes their optical properties. To keep the mirrors functional, a secondary laser system was built with the sole purpose of heating the mirrors in a pattern that will compensate for the thermal deformation by the main laser. For example, the second laser might by aimed into an annular pattern around the main beamspot. (in keeping with the philosophy that any problem caused by lasers can be solved with more lasers.)

I think I’ve rambled enough, sorry for the length of this post! I do have to give a big thanks to Ian Noble at ISU who was nice enough to give me a place to sleep and a free meal when I drove through Pocatello, ID. Also Ian, I passed a sign when I got out here that said I was entering the most potato producingest county in America, and I’ve been told that the area here produces more potatoes than even Idaho…HA!

Here’s some pics from the latter part of my road trip: (thumbnailed this time!)

The Grand Canyon:

Hoover Dam:

The Olympic Mountains in Washington state:

The Oregon Coast:

The snowblower was very colorful:

Mountains in southern Colorado:

Mt St. Helens:

A buffalo on the road in Yellowstone National Park:

Greetings from the Rockies!

I’m writing to you from the town of Leadville, CO where there is public WIFI access (what an awesome town!). This is my 5th day on the road and it’s been wonderful. I have 5 states worth of mud on my truck (see picture) and I refuse to wash it until I get home (with the exception of doing the windows while filling up on gas–I need to see the road!).

So far I’ve camped out in central Nebraska, the Black Hills of South Dakota, the Big Horn Mountains in northern Wyoming, the Medicine Bow Mountains in southern Wyoming and the Rockies near Denver. I’m traveling in my small pickup which has a camper shell over the back that I can sleep in. As an impoverished college student, I’ve been subsisting on ham-sandwiches and ramennoodles, camping on public lands and going to state park campgrounds to shower, etc, etc. Public lands here out west are pretty nice, it’s really quiet and you can count on being undisturbed (with the exception of being woken up by a cow sniffing at the window of the truck). For the record, I did get stuck in a snowbank yesterday (it’s June). I carry appropriate tools and getting out wasn’t much hassle–it was quite fun actually.

The weather was fairly lousy the first three days and I didn’t take many pictures because most of the scenery was obscured by rain and fog. Since it’s cleared up I’ve taken more pics. Most of what I’ve posted below is from the last two days.

I have about 10 days left on the road before I have to be in Washington State doing physics. I think I’ll be heading southwest toward the Grand Canyon…

Pictures
The Big Horn Mountains in the Clouds:

Devils Tower on a rainy day:

Rain in the Great Divide Basin of Wyoming:

I camped here:

Rain west of Laramie, WY:

My truck!

Wind River Canyon, WY:

Mountains in the Wind River Range:

Hey all,

Since this is my first post here, I believe a bit of an introduction is in order. My name is Isaac Angert, I’m originally from St. Louis, MO and I’ll be entering my 4th (and final) year of the physics major at Truman State in the fall. I’ll be working this summer with the Laser Interferometer Gravitational-Wave Observatory (LIGO) project at their detector site in southwest Washington State near Richland, WA. Details of LIGO in general: Wikipedia LIGO

LIGO is fundamentally an astrophysics experiment, but the complexity of the apparatus and the difficulties of detecting gravity waves leads to a lot of other interesting research and technological development. Many of the summer REU projects offered in association with LIGO aren’t directly related to gravity waves, but deal instead with the hardware and methods that support the functioning of the interferometer. My project is one of these. I will be working with laser locking, particularly the Pound-Drever-Hall technique. According to my advisor, the work will be mostly hands-on-experimental. I will be working to characterize and test electronics, and other equipment as well as techniques that may potentially be useful for the detector sensativity upgrades that will be done as part of the Advanced LIGO project.

REU’s in general are great experiences, and great fun too. I did an REU last summer at Hope College in Michigan. It was a chance to get a feel for how research works, and it helped seal the deal on my interest in physics as a career. (I can’t fail to mention here that summers in Michigan are really nice, I think it got to 90 degrees just once). Most REU projects pay a stipend and provide room and board, though the exact logistics differ. My housing last summer was subsidized, but we had to buy our own food. Housing this summer has to be paid out of my stipend and again I’ll have to buy my own food. It looks like I’ll have about $4000 (a fortune to a college student) at the end of the program, not counting what I spend on food (and knowing me, this will be a lot).

My REU starts fairly late (the middle of June), so I’m currently visiting my parents in St. Louis, catching up with some old high school buddies, doing some summer reading and preping for a road trip that I’m really looking forward to. I’m planning to drive from Missouri to Washington State, taking about two weeks to drive around the western USA and stop and see the sights.

Isaac