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):