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Wavelength Division Multiplexing



andy_con

ClioSport Club Member
  clio 182
So… this is a project I undertook last year, im sure a number of people know I love to play with lasers.

The technique is called wavelength division multiplexing, in simple terms its how you get lots of data down a fibre optic cable. So you combine (overlay) different frequencies of light into a single beam and fire them down a fibre. If you fire ten different frequencies of light down one fibre you have 10x times the amount of data flowing down it. So rather than dig up streets to lay more fibre cables, you increase the amount of frequencies down a single fibre.

huXARv.jpg




I have no interest in fibre optic cables.

In the world of laser shows red is always an issue, not enough power or too big a beam. So what if you could use wavelength division multiplexing to combine more than one beam into a single beam? At the moment there are two options, use a polarizing beam splitter (PBS) to combine (overlay) two beams into one. Or use lots of small beams side by side to make a powerful but fatter single beam. With a PBS you are limited to two beams only and with the second option you end up with a big beam. So neither are great.

If you think about shining white light / the sun into a prism you get a rainbow out.

7JjF3c.jpg




what they do with wavelength division multiplexing is shine different frequencies (colours) of light into a prism and combine them into a single beam. So if you could get different frequencies of red light and shine them into a prism you could get a single beam out, sounds simple!! Yeah sure…

in order for it to be worthwhile you wouldn’t want any less than 5 beams to be combined, otherwise your not going to get any decent power. Spec sheets say that with a temperature change from -10 to 60 degrees the frequency will be between 632nm and 642nm.




so in order to combine 5 diodes each would need to be 2nm apart.

In order to be able to measure the frequency of the lasers I found a second hand wavemeter on ebay in the USA. I took a massive gamble and paid £500 for it, this isn’t my site –

http://hololaser.kwaoo.me/electronics/WA2000.html



it took many hours with the help of some friends to align and get some kind of sensible reading from, but it was working. So it wasn’t £500 down the drain. This is mine in action –




next I needed a way of launching a beam down a fibre and into the wavemeter, so I made a free space fibre coupler. On the front is a brass barrel holding a lens, this enables you to focus the laser beam onto the tip of the fire optic cable. The thread on the lens is M9x0.5

j8a4KQ.jpg


the fibre plugs into the back behind the lens and this part has XYZ adjustment screws.

ZHORkG.jpg


Then all of that was on springs and adjusters so I could move it up/down and left/right.

ti6lvA.jpg


L81shZ.jpg


SdYzEF.jpg



All the screws/adjusters were purchased from Thorlabs and were fine pitch at M3x0.2, I had to buy the matching tap ☹

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1202



once that was all sorted I then had to build a laser rig where I could control the temperature of each laser beam and steer the beams.

So started by machining up some diode holders.
On the bottom was a slot for a small tec/peltier.
On the top was a hole to insert a temperature probe so I knew the temp of each block.
on the front was an M9x0.5 threaded hole for the adjustable focusing lens
on the back was a hole to insert the diode, which is 5.6mm dia. 4x M2 holes around it for a retainer to hold the diode in place and the hole at the bottom was for another temperature probe to give feedback to the TEC drivers.

WVeM8c.jpg




I purchased 4x tec/peltier drivers from here –

https://www.live-lasersystems.at/d/diode-tec-drivers.html


I had a few diode drivers laying around so that wasn’t an issue and I purchased 4x of these diodes –

https://sites.google.com/site/dtrslasershop/home/diodes/sharp-gh0631ia2g-185mw



I then made a rig to set it all up

so a few different power supplies on the right

bottom left is a screen that can display 4x temperature inputs

in front of the screen is the 4x tec controllers

in front of the tec controllers are the 4x diodes

to the right of the diodes is the control board for the screen that reads the temperatures.

yp0C5R.jpg



This is everything wired up
Diodes wired to the drivers
Probe on top of each diode holder wired up to display each temp on the blue screen
Tec/peltier under each aluminium diode holder wired to the driver and the probe on the bottom rear also wired to the driver so it knows whats what.

WdrYD9.jpg



I purchased some cheap £5 glass prisms off ebay and did some playing.

It became clear very quickly that setting the temperature below 5 degrees caused a lot of condensation build up, to the point where I would get little puddles. But if I set one diode to 5 degrees, put it through the prism and put a mark on the wall at the end of my workshop. If I then turned the temperature up to 40 degrees you could watch the beam drift across the wall maybe 5-10mm.

The wavemeter turned out to not be the right tool for what I wanted. It was unbelievable sensitive, just walking across the floor made the reading change. I also discovered it liked a really nice clean round laser beam fired into it, not a distorted messy beam from a 40 degree diode. A spectrometer probably would have been better.

So the conclusion of maybe 3 months acquiring and making parts, 2 weeks of building and 2 weeks of testing was that yes in theory you could use wavelength division multiplexing to combine more than one red beam into a single beam. But it would be very difficult to put into real work use, EG. in a laser projector for show use. It would probably end up being a pretty large setup and very difficult to keep stable (aligned).

So I sold the wavemeter and manged to get every penny back I paid for it and put the laser rig on the shelf.



The end.
 
I love your craziness, good work!

How does a number of same-colour light beams get separated out at the other end? Is it simply the case that a prism at the receiving end will act in exactly the reverse of the prism at the sending end?
 

Rystar

ClioSport Club Member
  2003 Clio 172
Working with WDM in a commercial setting, this is seriously intriguing to me that this is even somewhat achievable on a DIY basis!!

We use it for networking at either 1310nm or 1550nm, sometimes over hundreds /thousands of miles. It's amazing how much the units can cost. Even if you're in at a maximum of a grand, you're still at 10% of a commercial unit built for purpose. Keep up the efforts! Following this
 

Rystar

ClioSport Club Member
  2003 Clio 172
Also just thinking more about this...

Not that i'm familiar with wavemeters at all but it sounds like it's expecting light from a Single Mode fibre source, which as you describe is a single beam fired down the centre. The FC/PC Fibre you're using looks to be single mode also judging by the yellow exterior. Using lasers in the method you are is very similar to Multi Mode Fibre in networking. This can (at its most basic level) be achieved by LED's, but is normally done by lasers in the 800-900nm range. It uses a really wide aperture, with no significant 'centre beam' if you like. I've not yet come across a technology (at my place of work, which is networking oriented mind!) that actually uses multimode for WDM, always single mode
 

andy_con

ClioSport Club Member
  clio 182
Working with WDM in a commercial setting, this is seriously intriguing to me that this is even somewhat achievable on a DIY basis!!

We use it for networking at either 1310nm or 1550nm, sometimes over hundreds /thousands of miles. It's amazing how much the units can cost. Even if you're in at a maximum of a grand, you're still at 10% of a commercial unit built for purpose. Keep up the efforts! Following this
you use this stuff for a living, cool!

having two very different frequencies to start with makes a big difference.

i really need to get a spectrometer and get two diode 1-2nm apart and then see if i can get them aligned through a prism.
 

andy_con

ClioSport Club Member
  clio 182
Also just thinking more about this...

Not that i'm familiar with wavemeters at all but it sounds like it's expecting light from a Single Mode fibre source, which as you describe is a single beam fired down the centre. The FC/PC Fibre you're using looks to be single mode also judging by the yellow exterior. Using lasers in the method you are is very similar to Multi Mode Fibre in networking. This can (at its most basic level) be achieved by LED's, but is normally done by lasers in the 800-900nm range. It uses a really wide aperture, with no significant 'centre beam' if you like. I've not yet come across a technology (at my place of work, which is networking oriented mind!) that actually uses multimode for WDM, always single mode
correct, the burleigh wavemeter wants a nice clean single mode laser beam to give an accurate reading. the fiber cable is a single mode one, the wavemeter does not work at all with anything multi modal.

although im using single mode red laser diodes, im using really short focal length lens (2mm). so if the lens and diode are 0.1mm off centre you get a dirty beam out, sadly my cnc machine is 25 years old and not as accurate as modern machines. plus when heating the diode to change its frequency the beam gets even messier.

so a spectrometer would be the best option for measuring the wavelength.
 

R3k1355

ClioSport Club Member
120mW laser diode and you're firing it onto the garage wall??

LSO at work would have a fit if that happened.
 

Touring_Rob

ClioSport Club Member
Awesome work, I wonder if you could use a high power 650nm with a converging collimating lens to focus a larger beam? I know that you can get 3-5w C-mount 650nm diodes.

Might also be useful for you to look into industrial laser cutting machines. They use various laser modules I believe of the same wavelength, combine them and focus into a very tight 'dot'.... I suspect the hard part is to get each input beam to be in phase so as to not get destructive cancellation of each input source.

I first encountered CS 10 or so years ago, I was thinking of buying a 182 and came across the site, I didn't join because it seemed to be full of mouthy teenagers, much like passion ford at the time which I was a member of so couldn't be arsed with two forums full of turds - It's really nice that a lot of these forums here and M3cutters (in the E46 area anyway) now seem to have a good proportion of non oiks!
 
Full eye sight at the moment
Apart from that one burnt out bit of retina? lol

Awesome work, I wonder if you could use a high power 650nm with a converging collimating lens to focus a larger beam? I know that you can get 3-5w C-mount 650nm diodes.

Might also be useful for you to look into industrial laser cutting machines. They use various laser modules I believe of the same wavelength, combine them and focus into a very tight 'dot'.... I suspect the hard part is to get each input beam to be in phase so as to not get destructive cancellation of each input source.

I first encountered CS 10 or so years ago, I was thinking of buying a 182 and came across the site, I didn't join because it seemed to be full of mouthy teenagers, much like passion ford at the time which I was a member of so couldn't be arsed with two forums full of turds - It's really nice that a lot of these forums here and M3cutters (in the E46 area anyway) now seem to have a good proportion of non oiks!
I remember PassionFord! :LOL:
 


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