Control of laser diode

[Guest]

I wondered if there are any electronic experts out there with some information about laser diodes. I am planning to build a Bath Interferometer in order to test a mirror which is already in progress. At the end of this post is a link to an excellent Utube video in which Ed Jones describes how to make one.  The interferometer uses a laser diode but Ed Jones puts in a switchable resistor in series with the power supply in order to dim the laser. Apparently it's easier to set up with the laser diode on full power but for actual measurements it's better to have it dimmed. Ed Jones says that in the dimmed mode the laser is producing coherent light but not actually lasing which seems a little odd. I though that lasers produced coherent light by lasing. There are laser diodes available from maplin which are described as 3V diodes so presumably putting a resistor in series reduces the voltage across the diode. Does anyone know anything about this? I've already built a Foucault tester which I'm using so this Bath Interferometer is just out of interest.

https://www.youtube.com/watch?v=6KQAjmUC7KE

 

[saac]

woodblock, I built a Bath Interferometer a while back, initially as part of a mirror grinding project and then subsequently used in the classroom to cover the physics of interference.  I use both a laser diode and ultra bright led as the light source.  The laser, as expected was more effective and it was not controlled (dimable) as you have described with a resistor.  The interferograms it produced were well defined and bright enough to be analysed using Open Fringe.  My advice is to build it without the resistive control (you can easily add it latter if needed). In any respect, I doubt you would get the diode to fire if you dropped the voltage by any sizeable degree.  Coincidentally I used the green 3v diodes from Maplin and it worked fine.  The setup is shown below testing a commercially produced 70mm diameter mirror.  Good luck with it, it's an interesting project, I'm looking to modify mine using a better XY table to give better positioning control.

 

Jim

 

[Guest]

Thanks Jim,

I'm slightly confused by the terminology. I looked up control circuits for Laser Diodes and some of them were quite complicated, basically current limiters. But I've also seen reference to laser diode modules which I suspect has a control circuit built in so all you have to do is connect a supply. Was the one you used like that?

With the platform I decided to invest in an XYZ platform from China via Amazon which is £109. I think I can also use that for my Foucault tester which at the moment has a rather clumsy arrangement using a dial gauge for the measurement.

Steve

[saac]

Steve, I would say that the control circuit is part of the diode arrangement, certainly the diode laser supplied by Maplin and those more commonly available elsewhere have the control circuit as an integral part of the assembly.  All you need to do is connect the two power leads to the positive and negative terminals of a suitable source.  I used a small psu again from Maplin, the kind that has a range of voltage outputs.  Word of warning, be very mindful of the voltage it is very easy to blow the diode.    I've used both greeen and red laser diode but green produces the brightest interferograms.  I'll post some more details of the diode and psu that I used if it would be of any help  when I get home.

ps - just to explain the photograph of my setup - the diode is actually contained inside a section of 15mm copper pipe, you can see the top end of the diode itself peeking out of the pipe.  I made a number of these , including ones with the different coloured LEDs.  Having the diode protected by the pipe just allowed easier swapp out the laser/led when we were using it in school - its use is really cosmetic.  The greater diameter also meant I could hold the diode better in the little "ring tubes".

Jim

[rl]

Laser diodes have a threshold current below which the output is monochromatic but not coherent...just like a standard LED. it's generally about 30% of the design current. The safest drive circuit is a constant current source which can be made with less than £5 worth of electronics if you can solder. Constant voltage drive is virtually guaranteed to end in tears..... the current needs to be limited to a safe value.

Semiconductor lasers are quite static-sensitive (moreso than a lot of semiconductors) so really need careful handling but this goes away if they're permanently wired to a current drive source. This used to be a major financial disaster but  nowadays they are cheap enough to replace but it's still an inconvenience.

RL

[Guest]

Thanks RL, I've found a number of simple current limiter circuits suitable for a laser diode. The thing that I wasn't sure about was that a number of interferometer designs I found just connect a supply directly to the laser diode module so I thought maybe some of them have control circuits built in. Some of the designs use standards laser pointer devices as used for presentations and in fact I have two laser devices already one is my collimator and the other is a distance measuring device.

Thanks again Jim, I did wonder about the copper pipe. Perhaps you could also explain something about the way the Bath interferometer works. The test beam goes through a positive lens which spreads the laser beam into a cone of light which covers the mirror. The mirror reflects that back to the centre of curvature of the mirror via the diagonal mirror to the camera. So that cone will have an angle according to the radius of curvature and diameter of the mirror. In general that cone angle will be small assuming a fairly long focal length of the mirror.

The reference beam goes to the mirror where it appears as a dot. That is reflected back still as a narrow beam and then back through the positive lens where it is spread out into a cone with an angle according to the focal length of the positive lens. That will generally be a much wider angle than the one of the test beam because the focal length of the positive lens will usually be much shorter than that of the mirror under test. 

So coming into the camera are two cones of light, a wide angle cone which is the reference beam and a narrow cone of light which is the test beam. Does that matter? Also does it matter that the reference beam only appears on the mirror as a dot. Are you still testing the whole mirror?

Cheers

Steve

 

[saac]

Steve here's a photograph of the diode removed from the copper pipe.  You can see at the base of the assembly the diode driver (control circuit); it's the part wrapped in black heat-shrink.  You do not need to attach anything else to the diode other than a 3v supply  - the driver circuit is self contained and non adjustable.  The diode from Maplin draws approx 300mA when operating normally.  I've read through some other descriptions of DIY interferometers where, as you have mentioned, they drop the voltage supply to take the diode out of lasing mode.  They appear to do this to help on the initial setup to locate the beams - the brightness of the diode when lasing apparently making it difficult to do the initial setup.   All I can say is that I have not experienced this problem, but that is not to say that there may be an advantage in doing so, but from my experience, with the diode and mirror set I used, it was not required.  Given that the driver circuit is already part of the diode assembly you certainly do not need to add anything in order to fire it up.  Again my advice would be keep it simple on your build just use the diode as supplied and take it from there.

Your description of the operation of the interferometer is sound.  The interference pattern is produced because the two separate beams experience a path difference (between the expanded beam and the reference beam), the path difference results in a change in phase between the two beams. A path difference equal to a whole number of wavelengths (n λ ) produces constructive interference (bright fringes) while a path difference of n+1/2 ( λ )  produces dark fringes or destructive interference.  You can then analyse the resulting interference pattern with a fast Fourier Transform (FFT) to generate the surface profile of the mirror.  There are a number of software applications available to do the FFT analysis.  I would download the open source software "Open Fringe" if you have not already done so, its easy to use and fairly intuitive.

I've also included in the photograph the psu (maplin) I used and the XY stage I'm looking to use to upgrade my setup.  The interferometer can be tricky to set up initially - because you are measuring to nm scale it is touch/vibration sensitive - a solid/steady setup is essential.  The micrometer controlled xy stage is a good idea.   Have you thought about how you will control the vertical position and also how you will mount your mirror.  An adjustable height mirror stand can also be useful. 

I've included a link to a you tube tutorial on the actual setup of the interferometer.  This one is one of the better ones I've found on the net, again easy to understand.

The starryridge explanation of the Bath Interferometer is also worth a read and it contains some useful links (Open Fringe etc)

Re centre beam on the mirror - it is the expanded beam that will illuminate the whole mirror surface. So no it does not matter. Hopefully teh YouTube video will make it clear, I found it really useful in understanding teh setup. 

 

Just one final note - get ready to do a victory dance when you get your interference pattern for the first time - it's the same feeling as seeing Saturn first time round!  Videoing your victory dance for upload to SGL is optional, but it would be well received.  -

Interferometer Setup - Youtube

Starryridge bath Interferometer Description

 

Jim

[Guest]

Thanks Jim,

I've already downloaded open fringe and I've also ordered all the parts so I'm just waiting now.  I've ordered some bits from a place in the US called SurplusShed which seems really good for odds and ends of optical stuff.  The platform I've ordered is an XYZ platform so it will do the up and down adjustment. It was the most expensive part of the setup but I'm sure I can use it for other things so I saw it as an investment.

I have to say that I get as much fun out of all this as I do out of actual observing of which there's been precious little this year.

I'll let you know how I get on.

Steve

 

[saac]

Steve,

SurplusShed are really good for this type of project, they supplied my beam splitter and flat mirror with mount.  I know what you mean about the price of the linear stages I was shocked.    I only bought the XY stage a few months ago.  It's only a small one with 12 mm travel but it was £60 or so.  I think they are a good investment though - I keep telling myself that anyway. :)  If you haven't already got a small commercial finished mirror to use as a test to prove your setup you can pick something up easily on AstroBoot.  Good luck with the project , like I said it really is exciting when you get the interference pattern to appear. 

Jim