Looking for Help with Laser Interferometry and Collimating Lenses?

[kelvin5885]

Hi all

Im doing work on laser interferometry for my final project,just wondering if there are any experts or experienced people in this field here,thanks much!

 

[dlgoff]

Welcome to PF kelvin5885. I hope your stay here is fun.

Can you be more specific about your project? What do you find interesting about the subject?

 

[kelvin5885]

Hi dlgoff

Thanks for the reply!
My project revolves around utilizing the principle of laser interferometry to read tracks off vinyl records, instead of using a needle and piezoelement. I chose this project, even though I major in power engineering, because I've always been enthusiastic and passionate about sound quality, and the idea of using a laser to read analog signals is just so exciting!
Anyways, for the first step of my project, I am just trying to build a setup to read the outer track of the vinyl record,just to get some sort of result before worrying about how I can move my laser inwards. I'm going to base my set up on the Michelson set up for starters,and hopefully I can get some sort of result.

Any ideas,hints,or things I have to watch out for?
Cheers.

Kelvin

 

[elect_eng]

Hi dlgoff

Thanks for the reply!
My project revolves around utilizing the principle of laser interferometry to read tracks off vinyl records, instead of using a needle and piezoelement. I chose this project, even though I major in power engineering, because I've always been enthusiastic and passionate about sound quality, and the idea of using a laser to read analog signals is just so exciting!
Anyways, for the first step of my project, I am just trying to build a setup to read the outer track of the vinyl record,just to get some sort of result before worrying about how I can move my laser inwards. I'm going to base my set up on the Michelson set up for starters,and hopefully I can get some sort of result.

Any ideas,hints,or things I have to watch out for?
Cheers.

Kelvin

This sounds like an interesting idea. It should be a good, although probably difficult, project. I don't consider myself an expert in interferometry, but I've done some basic things like holograms and simple distance measurements with interferometers. Much of my work in this area, was based on optical fiber technology, but some of it was bulk optics based. Anyway, please share any results you get. I think this will be interesting to members here, and you will allow people to comment and help you out.

I can make some initial (very speculative) guesses as follows.

I don't know much about the properties of the vinyl used in records, but I would be worried that they are black and would tend not to reflect much light. It would be interesting to see if infrared wavelengths are better suited. There are some records that are not black, so you could track some of those down if you run into trouble. Ideally, it would be better if you could find or make a record out of a metal like gold or silver. This has a better chance of being usable in an interferometer.

Another issue is the coupling of light in the interferometer. The analog signal, via the groove depth, implies a reflective surface that is always changing its angle. This may direct the beam out of the interferometer, which might confuse the measurements. However, perhaps this effect will be too small to be an issue.

I also don't know about the depth range (modulation) of the record grooves. You will want to compare the distance excursion in the grooves with the wavelength of light you are using. If the modulation is much less than one wavelength (which i doubt), then you will have linear response, but poor sensitivity. If the modulation is comparable to the wavelength then you will have good sensitivity, but nonlinear analog response. If the modulation is much greater than the wavelength, you can count optical interference fringes in a digital fashion. This will make reception easier, but you will have a quantized (digital) encoding of the analog signal. If you can find out the typical range of distance in the grooves and post the result, perhaps I can say more.

 

[kelvin5885]

Thanks elect_eng for your input.

You've raised quite a few valid points I have to take into consideration. I have found the needle size to be 0.2mil x 0.7mil. I've actually shortlisted a couple of laser diodes to be used here, but having presented these to my supervisor, he suggested cutting cost and just to use a simple laser pointer. So first thing's first, I have to figure out how I can focus the laser from the laser pointer to fit the size of the groove, like a stylus. Moving the mechanical arm inwards is the least of my concern right now, as I will just try to read the outer groove of the record, just to get some results. Any ideas on what lenses I should use and how I can concentrate the laser to be 0.2mil x 0.7mil? and also, any input on what laser intensity I would want?
Thanks much.

Kelvin

 

[elect_eng]

I would look into small collimating lenses designed to focus laser diodes into optical fibers. Typical single mode fibers have 10 micrometer core diameters, while mutimode fibers have 50 or 100 micrometer core diameters. This indicates that lenses suitable for fiber can collimate to your requirement.

As far as power, if you are talking about laser pointer diodes use as much power as possible but consider that highly focused light might melt black vinyl.

 

[elect_eng]

By the way, use caution with lasers. They are an eye hazzard. While laser pointers are generally considered safe, any newly engineered system poses risks. You may drive the laser beyond the rated power levels or induce high energy pulses inadvertantly.

 

[MATLABdude]

Interesting idea... I remember reading about a scanner-based record player demo from a while ago, and you may find it enlightening (har har):
http://www.wired.com/entertainment/music/news/2003/02/57769

The link to the commercial product mentioned is broken, but it's the ELP Laser Turntable:
http://www.elpj.com/main.html

I echo elect_eng's warning about lasers. A laser pointer pointed directly at your eye probably isn't going to do a whole lot of damage (for a few moments, at least). A laser pointer, focused down to its minimum spot size, just might.
http://en.wikipedia.org/wiki/Gaussian_beam

However, I don't know what the coherence length of your standard laser pointer happens to be. Probably not too long, and long coherence length is something you need for interferometry to happen, of course. You might be better off with something from a CD/DVD-ROM (although these will be higher powered).

EDIT: I should mention that a low-powered RED laser pointer in the eye probably isn't going to do a lot of damage (for a few moments, at least). Green or blue is significantly more damaging, and may be accompanied by even more power in the IR, depending on the construction. Long story short, don't point laser pointers into (remaining) eye!

 

[kelvin5885]

I've decided to go with the TIM-209-1 at 1mW with wavelength at 650nm, I would expect this to be sufficient. I am just having issues on how to get collimating lenses to focus the laser into the grooves of the record, any ideas on where I should start? I like the idea of focusing the laser into an optical fiber, which would serve not to cause stress on the mechanical arm of the player, but whereabouts should I start looking for it? and what photodiodes as sensors would you recommend?