Interferometer Puns

Does anyone have any suggestions for cheap interferometers or deflectometers for small flat surfaces?

I'm trying to give a recommendation for someone who wants to measure flats and prisms <10mm and fairly loose Irregularity/flattness requirements for its size.

What are your dirt cheap recommendations for interferometers? Any vendors known for budget interferometers, get something off ebay?

Also are there any commercially available deflectometers suited for precision optics measurements? I assume there might be some cheap options there. The QED deflectometer for mrf spot measurement comes to mind.

I am experimenting with a laser diode (temperature and current stabilized) with an external cavity for wavelength-selective feedback (external cavity laser diode, ECDL). My goal is to make a nominally multi-mode laser diode (single transverse mode, multiple longitudinal modes) to operate in a single longitudinal mode to increase its coherence length for interferometric measurements.

In my interferometer setup, I divide the ECDL output into REF and OBJ arms by a 50:50 beamsplitter (BS). The REF beam is directed to camera sensor with a pair of mirrors and a second BS. The OBJ beam is phase-stepped by a piezoelectric actuator and aimed onto a diffuse object surface. A lens images a part of the scattered light onto the sensor, where it overlaps with the REF beam, producing an interference pattern.

The ECDL setup uses the classical Littrow configuration [https://www.rp-photonics.com/external_cavity_diode_lasers.html ]. With the help of an alignment disk, I have adjusted the external cavity (grating+mirror mounted onto same xy-tilt mirror) so that the diffracted beam enters back to the laser diode. I'm able to get the diode to lase slightly below the threshold by optimizing the grating angle.

The attached video shows the spectrum of the ECDL output, recorded using a makeshift spectrum analyzer. In the beginning, the grating is purposely de-tuned so that there is no feedback. I then tune the grating closer to Littrow angle. This leads to imperfect feedback that causes chaotic mode-hopping. When the feedback angle is optimized, the laser "locks" into a single mode (I believe). By tuning grating angle, a desired mode can be selected.

I have tuned the ECDL setup to the best of my abilities, and believe that it operates in single-longitudinal mode. However, the beam seems not to interfere properly, as the fringe contrast is low, and moving one of the interferometer arms by a piezo actuator causes barely any intensity modulation. This is puzzling, as the same laser diode interferes much more strongly when operated without any external feedback. Also, in the mode-hopping regime, the fringe contrast appears much higher, although the fringe pattern changes rapidly due to continuously varying set of lasing modes.

I have checked the beam polarizations at the sensor plane. They are within 10˚ from one another. I am now wondering whether any of the following issues could explain the poor interferome

If we built a 1km radio telescope on the far side of the moon, for example, could we link it up to the ones on earth well enough to create a massive interferometer? If it is possible what are the problems with it?

With the James Webb Space Telescope reaching its launch site in South America yesterday, I was wondering if it was possible to use it and the Hubble Space Telescope together as an interferometer? If it is possible, are there any plans to position them at opposite sides of Earth and use the telescopes as an interferometer?

Basically what im asking is if we had two telescopes at Earth L4 and L5 would they need to have such precise instruments that they are able to perfectly redirect the collected light to focus it at a point between them(or at one of the two telescopes). Or can they collected the data, transfer it electronically to Earth where it can be combined in a computer without the need for high precision instruments?

In two different textbooks, there are two different formulas with different derivation styles for the "No Fringe Formation" Condition.

In approach (a), they use an amalgamation of bright and dark for 2 wavelengths having very minute difference in the following manner:

2dcostheta=n*λ(1) -------- (1)

2dcostheta= (n+1/2)*λ(2) ----------- (2)

Subtracting both the equations we get, n=λ(2)/(2(∆λ))

Now using this value of 'n' for small angles in (1) we get d= λ(1)λ(2)/(4∆λ)

This is one formula.

In the other textbook they have said that fringe pattern may disappear for a bright of nth order and another bright for (n+1)th order. So proceeding in similar fashion as above (for small angles etc.)

2d=nλ(1)=n'λ(2) where n'=n+1

Thus n=λ(2)/{(λ(1)-λ(2)}

Using above value of n, we eventually get

d= λ(1)λ(2)/(2∆λ).

This is the other formula.

Now I would greatly appreciate if someone would help me understand which is the correct one because I used both of them and they apply for different questions which are of the type- "find separation of mirrors for which there is no fringe observed".

Both have a distinction of 1/2.

The bottom of this picture on the Wikipedia page for ‘Delayed-choice quantum eraser’ shows an MZI experiment with a single photon. The photons that exit to the right exhibit constructive interference and the ones that exit out the top exhibit destructive interference. The image near the constructive interference photon looks like the wavelength has shortened, but the formulas indicate 2λ + k, which I think implies the wavelength has gotten longer?

As a follow up, a photon’s energy is inversely proportional to its wavelength. Where does the energy go to/come from as the photon’s wavelength is changed by constructive interference?

Say we wanted to build an absurdly large interferometer telescope in space. Maybe the baselines would be as large as low orbit, or maybe as large as an AU, or heck, maybe distributed throughout the solar system with baselines tens of AU long. I realize this would be impractical since interferometers only replicate the resolution of large telescopes not the sensitivity (so even if we had the resolving power to, for example, see details on the scale of kilometers on an exoplanet, we would never collect enough photons to actually detect anything), but I'm nonetheless interested in the engineering requirements.

I guess you would probably need extremely precise locations for each of the interferometer elements. How precise? Could some kind of lidar system handle it? Would you also need extremely accurate clocks for each element? Would atomic clocks be sufficient? Finally, what about the pointing of each individual element? I can imagine a ridiculously large interferometer which should approach say, pico-arcsecond resolution, but then would each individual element need to have pico-arcsecond orientation control, or would this requirement not be so stringent since the primary beam of each individual element would be quite large? For example, if we imagine an interferometer for 1 mm radiation composed of lots of 10 meter sized dishes separated by 10 AU, then the theoretical resolution of the interferometer would be about 1 mm/10 AU = 7e-16 radians = 1.4e-10 arcseconds. It would be really difficult to make sure each dish was pointed within 1.4e-10 arcseconds of the same direction. The primary beams of each element, however, would be 1e-4 radians = 20 arcseconds. It would be easy to aim all the elements within 20 arcseconds of the right direction.

Anyone know what the state-of-the-art techniques are for setting up a stable optical Mach-Zhender-Interferometer?

I'm looking to set up a Mach-Zehnder-Interferometer that is as phase stable as possible. One possible thing I could do is set up a bunch of beamsplitters and work with two frequencies & use frequency filters to keep the light separate. Then I can use a piezo on a mirror in one of the two paths to do some form of PID control with one of the two frequencies to keep the distance between the two paths constant. One issue that I can see is that piezo's typically expand asymmetrically and this causes a bit of a shift in angle on the mirror, which would decrease the interference visibility of the interferometer.

Also in a perfect world I would have some sort of optics that can't wobble very much at the micrometer/nanometer level. I'm not sure if I can buy some commercial mirrors/mounts that have minimal wobble.

Can someone point me in the right direction? Phase stability is my most important factor, and I plan to put some things in the beam path so doing everything free space is ideal.

I am about to measure the radius of curvature of beam divergence from a collimator (for QA purposes). I am not so very experienced with this measurement method, so I would like to ask if I apply the equation in the right way (as shown in the image)?

And, if anyone has experience with this, if you have any tips and tricks using this kind of interferometer please share if possible. Thank you very much :D

https://www.thorlabs.com/thorproduct.cfm?partnumber=SI050#ad-image-0

https://preview.redd.it/00f72dczjvd71.png?width=1215&format=png&auto=webp&s=df1f8b7945554480f6807b392eaf5ef6d4b59944

Recently, LIGO and Virgo detected rare mergers of black holes with neutron stars, for the first time. It is all over the news. Upon reading on it I do not seem to understand how these interferometers can obtain so much information about those events. Like they estimate the mass of each object in one of the events: a Black Hole of 9 solar masses and a Neutron Star of 1.9. Where the hell do they get that from?

I really like astronomy but only have superficial knowledge about it, so I get a bit of how those interferometers work but obviously nowhere near enough. My understanding is that they detect gravitational disturbances by analyzing the interference in laser measurements shot through each of their arms, then comparing the LIGO and the Virgo measurements to exclude any external interference – broadly speaking.

But I think they would only get data of the gravitational wave such as frequency, wavelength, etc. Not how far it came from, what type of object originated it or even less the mass of the objects that merged to create it!

Anyway, how do they figure that out?

Recently I made a Michelson interferometer for my physics research project. Everything is done with high quality and, in my opinion, is correct, the mirrors are installed on 3-axis adjustable mounts. However, I only have been able to get interference pattern in the form of stripes but not circles, how I expected it would be. I think that the problem is in the focus of the laser, but no matter how I change it, the picture remains the same. I would be glad to attach detailed photos here, but I really don't know how I could do it. If you could give me any advice, I would be extremely grateful.