optical thing

We got a sample of a TO-can laser, red, in a flat-window package. Our fast photodetectors are all fiber optic, so the problem is to get some of the light into the fiber so we can see the optical waveform. Sticking a fiber up against the laser window produced nothing measurable, as a little math will demonstrate. The area of a 9 micron single-mode fiber, or even a 60 micron multimode, is really small. The laser makes maybe 50 milliwatts or so and our fast photodetector needs maybe a milliwatt, so we need to capture at least 2% of the light.

So, the logical thing to do is ask Phil. He pointed us to some stuff at Thorlabs, and we wound up with this:

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Our driver board is pulsing the laser on the left, and the laser is inside the expensive collimator on one side of the Thorlabs thing. That turns the sloppy laser light into a tight collimated beam, and the expensive thing on the right focusses that into the fiber.

Each end has 6 adjustments (roll, pitch, yaw pointing, X and Y position, focal length) so the probability of finding a usable bunch of settings is essentially zero. There are tricks, in Phil's book and a Youtube video, so we eventually got it to work.

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John Larkin                  Highland Technology Inc 
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John Larkin
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Align it with a visible laser to get the gross errors out then switch to IR. What WL are you working at? 1550? That's a nice piece of hardware.

tm

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tm

The laser we're testing is 630 or something, bright red.

When we first turned the rig on, the photodiode saw nothing but there was a tiny visible red dot at the far end of the fiber. I wonder how much optical power it takes to be visible there; it must be a small fraction of a microwatt.

One trick is to work backwards, push light into the fiber and hit a card on the left side, to sort of begin to align the right side of the system. We did that. Then we used a sensitive photodetector, instead of the fast one, to walk things in.

When it worked, the formerly tiny red spec at the fiber exit became a blaze of light, not safe to look at. You can aim it at a screen and see a complex, squirmy blob of bright light (this is multimode.)

The object now is to test a bunch of cheap level-type free-space lasers, intended to be CW, and see if any can make good fast pulses.

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John Larkin                  Highland Technology Inc 
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John Larkin

If the laser is attached to the board, you might have some issues with the (plexi/lexan?) base and metal plate creeping around. If so, grab a cheap granite surface plate. If not, nevermind my blathering.

Someday when I find my spare time I've got to experiment with ball-end-lensing on fibers. I've got at least one splicer that will probably never splice again, but could possibly be the arc source for that sort of thing.

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Reply to
Ecnerwal

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Steering a laser into a fiber is always a bit of an issue. I don't do it v ery often, but I have notes somewhere. I think at some point in the proces s I look at the reflected light.. but it sounds like you've got it running. One issue you sometimes see with diode lasers is that the light reflected from the fiber end gets back into the diode. (You've got this external 'ca vity' - from diode to front face of fiber.) which can make the diode 'wig out'. It tends to happen at 'perfect' coupling.

George H.

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George Herold

The nice thing about those U-frame things is that you can put bulk-optical things like waveplates and beamsplitters in the middle of a mostly-fibre system.

I always found those particular collimator gizmos to be a bit hairy to use, because to get them to be mechanically stable you have to have the screws fighting each other pretty hard, which makes them strip easily.

The Oz Optics ones that Thor copied have the same problem.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

The laser is clamped inside the expensive collimator thing on the left, and its leads plug into pin sockets on our board, so the plastic base doesn't matter. As time goes on, I expect we'll have to acquire a proper optical table and more expensive gadgets, like mounts and mirrors and things.

It seemed naiively to me that all you'd need to couple the laser to the fiber would be one lens, but I guess the angles or NA or something are wrong. A ball lens is the usual technique, but with a windowed laser I guess we can't get close enough to the chip to make that work.

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John Larkin                  Highland Technology Inc 
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John Larkin

What's the point of using a cheap laser if you need a mui-expensivo chunk of optics to couple it to a fiber?

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

It's because you're coupling an oscillator straight to an antenna via a feedline, and when your antenna or feedline shows a strong resonance then it messes up your oscillator tank. You need to use modern radio practice and couple that oscillator to a nice amplifier that has good output to input isolation, _then_ couple that out to the feedline.

Lasers are basically like amateur radio transmitters from 1929. It's pretty amazing to see what the high-tech power oscillator station looked like, not to mention the enthusiasm (and puzzlement) that greeted MOPA (master-oscillator, power amplifier) transmitters.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

We're just learning. The fiber coupling is so that we can get signal from a free-space laser into our super-expensive NewFocus 40 GHz o/e converter and then into a sampling scope. The ultimate application, if there is one, would be free-space.

What we want to do is sell laser drivers. One way to do that is to document their behavior with available lasers. We've seen some strange and maybe useful things already.

Lasers are weird. Not only the semiconductor physics, but the packaging and wire bonding and such is all over the place. If they are good, it's usually by accident. Data sheets are generally pitiful, and the manufacturers may not be able to furnish basic stuff like v-i curves or capacitance.

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John Larkin                  Highland Technology Inc 
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John Larkin

Except that there aren't any. Optical amplifiers have no reverse isolation--you have to put that in yourself, using e.g. circular polarization or Faraday rotation.

MOPA works with lasers too, but you have to be really careful that the amplifier doesn't lase. Also since the second stage has no resonator to narrow its bandwidth, you wind up with all the wideband spontaneous emission from the amplifier.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Phil Hobbs

Hmm, Interesting analogy Tim. So there are several tricks to get around the feedback. One can attenuate the beam. (the reflected light gets attenuated twice.) (like adding 50 ohm pads?)

And then you can buy expensive optical isolators. (two linear polarizers with 45(?) degrees of Faraday rotation in between.) (maybe that's like a four port circulator?)

But my fav cheap trick is to put a linear polarizer and 1/4 waveplate right before the offending reflection. They use to make AR films to go in front of monitors which did that. (I can't think of an analogous electrical circuit.)

George H.

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George Herold

I know. I was purposely pulling the metaphor to the breaking point. Think of it as metaphor taffy.

There are a lot of interesting parallels between lasers and early 20th century radio practice, though.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

If you have an amateur radio license you need to collect at least one grid square with your rig. Just because.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

That's very true. I keep wanting to find an application for an optical superregen, but although I've found a few candidates, the laser rushbox never seems to make the final cut, mainly because of its noise bandwidth.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Generally, you use a sphere lens to couple lasers to fibers. They do a poor job, but are a LOT better than nothing.

Jon

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Jon Elson

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The interposition of a window shouldn't make much difference, since 
the divergence before the window or after the window - assuming an 
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John Fields

The edge-emitting lasers here have an oval pattern about 5 x 20 degrees, so they sure don't need any more diverging.

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I think this Thorlabs thing has one positive lens on each end.

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John Larkin         Highland Technology, Inc 

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John Larkin

[snip]

There is another trick to be aware of. If one laterally displaces the laser slightly to the left of the lens' optical axis, and the receiving fiber slightly to the right, the light falling on the fiber entrance face will be slightly off angle, and the reflection will not be able to get back into the laser chip. For the optical constraints, look up the Scheimpflug Principle.

Joe Gwinn

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Joe Gwinn

Didn't Phil suggest that you bust the window out of the package? He admitted to such things in his book. Even told how to do it without destroying the device.

Joe Gwinn

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Joe Gwinn

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