Lasers

Chiseling out The Chip!

This post may be a bit redundant with the info I added in the other place, but I am excited, so I felt the need to rewrite some of it here.

Le Chip! This work took a while. To celebrate, I thought it deserves a few words in the blogs. During the past year or so, I was/have-been/will-continue-to-be working on an image sensor ADC testchip. It was finally taped out yesterday! What’s left now is some additional gastronomical work on the tapeout cake and the drainage of a rusty bottle of champagne.

The chip in all its ugly majesty with all these redundant power pads and LVDS pairs.

The chip in all its ugly majesty with all these redundant power pads and LVDS pairs.

The core of the testchip is a fast 12-bit column-parallel ramp ADC at 5u pitch, utilizing some special counting schemes to achieve the desired 1us ramp time at slow clock rates. Alongside, to be able to fully verify the pipelined CDS functionality and crosstalk, I’ve built a pixel array in line-scan configuration, some fast LVDS drivers, clock receivers, references, state machines, a few 8-bit iDACs, bond pads, ESD, and some other array-related stuff, all from scratch! The chip has a horizontal resolution of 1024 and 128 lines with RGBW color filters and microlenses.

On the top-left corner there are some experimental silicon photomultipliers and SPAD diodes. These I plan to measure for fun and I promise to post the results in any of the two blogs.

Unfortunately, this chip wouldn’t yield tons of publicaiton work, apart from the core ADC architecture and comparator. To test the ADC one needs a whole bunch of other fast readout blocks, which in the end are not something novel, but yet, one needs them and designing these takes time. Finishing up this test system was a lot of work and I realize that it might be a bit risky and ambitious to be doing this as part of a doctorate. What if it fails to work because a state machine had an inverted signal somewhere? Or the home-made ESD and pads suffer from latch-up? Or the LVDS driver CMFB is unstable and I cannot readout data out? Or there is a current spike erasing the content of the SRAM? Or, or, or ?

We university people don’t have the corporate power to tapeout metal fixes twice a month until we’re there. I probably have another two or three chip runs for my whole doctorate. It may therefore be better (and more fun) to stick with small but esoteric modules, which one can verify separately and have time to analyze in detail. But hey, I’ll quote a colleague here: “It is what it is, let’s think how we can improve things.”

Finally, I have added this little fella who I hope will be my lucky charm.

Le Duck!

Mr Le Duck!

With his 15um of height, could he compete in the annual “smallest duck on the planet” contest? Cheers!

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VIP pass to a holograpahy lab

Greetings! I am proud to present our brand new labs at the Institute of Optical Materials and Technologies! After a whole year of construction work, repairs and various emotions, the renovated labs are finally ready for action. And for our (many…) readers exclusively, I will present our optical arsenal:

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Overview of the CW lasers lab

Our CW lasers lab is home of Coherent’s mighty Verdi laser – a DPSS at 532 nm wavelength and a maximum output power of 12 W. Right next to it is the Japanese hero from Kimmon – a He-Cd laser at 441,6 nm and 0,18 W output.

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He-Cd laser and a huge mirror

We also got two laser diode systems from BWtek at 780 and 635 nm and a few more systems from Cobolt and Coherent – no need for an extensive description of everything for now, hopefully you’ll meet again with some of these lasers in a future post that will be more specific. The important thing is we got the reds, the greens and the blues covered. So a multicolour hologram, maybe, someday? It will be very exciting!

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Lots of optomechanics as well…

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These tools are never enough… And never organised

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Preparation for a holographic recording setup… You can see how huge the laser spot is and we will make it even bigger in order to “capture” the object

Apart from the CW lab, we also got a separate pulse lab, where our two Nd:YAG lasers rest for now. They are around 1 μm (I always forget the exact value) but they are mainly used for second harmonic generation. Third and fourth harmonics are also possible although weak.

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This guy is from Stuttgart (or so it says on the sticker)

Hmm, this post turned out shorter than I imagined… Sorry! I hope you liked this sneak peek of the labs and I will be glad to show you some *real* work soon. In the meantime, here’s an abstract-spectrum-thing painting I made for our new office rooms at the Institute – if my colleagues like it, we may even hang it on the wall:

The hyperfine structure of cesium

Today in the lab of atomic spectra we built a setup for observing the hyperfine structure of cesium. It is a big deal for atomic clocks as they use this transition to “tick”. It is also a very very tiny and fine effect (as its name suggests) so it’s pretty exciting that we can see it with such simple setup.

First, here is a brief description of atomic structure notation. The source of the text and picture is http://webphysics.davidson.edu/Alumni/JoCowan/honors/section1/THEORY.htm

The electrostatic attraction between the electron and the nucleus could be described by the principal quantum number, n. The combination of l and s gives an electron’s total angular momentum, J. Magnetic coupling between the electron’s orbit and spin causes an energy splitting between levels with different J called the fine structure. The fine structure is split again into the hyperfine structure denoted by the letter F. The hyperfine structure is due to a magnetic coupling between the electron’s total angular momentum, J, and the nuclear spin, I.

In order to observe the hyperfine structure, i. e. the two distinct energy levels at F=3 and F=4, and to measure the frequency difference between them, we carried out the following experiment.

The concept is to set the laser generation exactly at the wavelengths of absorption of cesium (they are two known peaks around 895 nm). For this we use an IR laser diode and we can modulate its wavelength by changing the temperature and the supplying (triangular) current. We are thus “scanning” the laser. In order to ensure the scan is smooth – that is, the laser doesn’t “jump” from one mode to another, we add an interferometer Fabri-Perot (IFP) which also serves as an etalon for the frequencies.

scheme

So we have the laser diode, connected to a thermoregulator and powered with triangular current, which is monitored on the oscilloscope screen. The beamsplitter sends the laser beam through the interferometer and the output is detected by a photodiode, hooked to the oscilloscope. The other part of the beam is reflected by a mirror in such manner that it passes through a glass case with cesium inside. The output is again recorded with a photodiode and displayed on the oscilloscope screen.

Finally, at the right adjustment of the scan, we get our lovely hyperfine structure:

Screen Capture

Green signal is the current, yellow is the interferometer signal and blue is cesium signal with the energy level splitting

The only thing left is to process the data we recorded with the oscilloscope and plot it. Now, to find out what is the frequency difference, we’ll need two things: the FSR (free spectral range) of the interferometer and the number of peaks of the IFP signal between the two cesium split levels. The FSR = c/4L, where L is the length of the IFP and in our case 0,2 m. Thus, the FSR is 375 MHz.

Graph-cesium

As you see above, I have counted the number of peaks and they are 22. So for our final result, we multiply them and get frequency difference of 8,25 GHz which is close enough to the real one of ~9 GHz, considering how imprecisely the experiment was made and the fact that our IFP was pretty bad. Well, it is possible that I messed up somewhere, I’ll find out in a week. 🙂

Laser rays, beams and divergence

I guess no one says “laser rays” in English, but in Bulgarian the two words for ray (infinitely narrow) and beam (as a shaft or bundle) of light are frequently interchanged. This purely linguistic difference could actually teach us a bit how divergence works.

One of the most well-known and fascinating properties of laser light is that it’s collimated. The beam stays narrow as it propagates in space, unlike the light emitted from the Sun, the lamps at home or any other typical source of light. A truly unique and very useful quality that I will write about in detail some other time.

Now, there is no such thing as perfect collimation and sooner or later every laser beam starts to diverge due to diffraction of light. It is very important to know the divergence of a laser and what is needed for certain applications. This is because, obviously, you cannot have minimal divergence without sacrificing something else. And here comes the ray/beam difference.

divergence

We have a collimated light beam that passes through an aperture with width D. Imagine that’s where the beam leaves the resonator. When it’s free to go, it would slowly start spreading out (divergence) and we could estimate the angle alfa (half the angle of the spread) as seen above. In this case k is a positive constant with value near 1, so we could ignore it. So let’s look at D. If it was indeed a laser ray, that implies that D is zero. But as D approaches zero, alfa approaches infinity which means we get a huuuuge divergence and lasers are supposed to have small(er) divergence angles. That’s why it’s called a laser beam with finite width and the bigger D, the smaller the angle, the better the collimation. But we get a big laser spot. So it’s either a narrowly focused beam with big divergence, or a wide beam with small divergence angle.

Also note that alfa is proportional to the wavelength lambda. Then, if we have a constant width, the laser with the shorter wavelength will have weaker divergence. That’s why there’s an interest in blue lasers: with them more information could be recorded.

More in-depth info awaits in my future post on Gaussian beams.