DVD laser and photodiode

I got some samples of a plastic-packaged Osram IR laser that puts out about 10 watts peak, and managed to get 1 ns pulses out of it driving with a gaasfet. You can also get the lasers that have just the chip soldered to the edge of a little square brass block with a mounting hole in the middle, which is probably optically superior to the plastic package. Either one goes for a few dollars in quantity.

The little 850 nm fiber-coupled VCSELS have risetimes around 100 ps... hard to measure, since the good fast detectors are blind at 850. The Optek parts are available stock from Newark and are pretty good.

Probably an avalanche transistor driver is easiest to make to do lidar type stuff. ECL, maybe eclips lite, has enough swing to drive a laser.

John

Hi,

This article says that a typical DVD 16X writer requires 230mW of optical power, and the wavelength of the laser is around 650nm, red.

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Also for a 16X DVD the datarate is up to 418 Mbps, so the laser and photodiode have very fast rise/fall times.

Does anyone know of a site where the DVD optics have been "experimented" with? :) Is it feasible to use the laser and photodiode for other purposes than as in a DVD, ie. long distance optical datalink or laser rangefinding (time of flight and/or phaseshift measurement)

cheers, Jamie

Jamie, The fast signals require significant power, a few nA won't give 500MHz, and neither do just a few photons. Mreaning long distance, whatever this means, requires some expensive optics. The narrower the laser beam, the more it widens over the distance, thus you're quickly at 10cm for a mile or two.

Yes, I think an optical link is doable. How fast over which distance require some experiments.

Rene

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The DVD reader I took apart has two lasers (one infrared for CD and one red light for DVD) with 3pins each, I guess one pin is the modulation signal and the other two pins are power and ground? Here is a pic from what I was able to salvage from the DVD player:

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Sorry for the poor image quality, I am still using a sub 1-megapixel camera with no optical zoom :)

bottom left is the photodetector, it is a 10pin IC with a clear top on it to let the light in, any idea on where to find a datasheet and/or how to hook this up to detect light? I think it has multiple photodiodes inside it. I think the chip that interfaces to this photodetector is the Toshiba TA1323.

the black chunk above the photodetector is the DVD laser diode I think.

Then there are 4 lenses from the DVD optics on the right of the picture.

I couldn't remove the suspected CD laser as it was glued in pretty tightly.

Any tips on hooking this stuff up? :) I am most interested in the photodiode as I haven't been able to find any other high speed red light sensitive photodiodes.

cheers, Jamie

Hi,

I hooked up the red laser diode and it seems to work fine, from reading on the net, the three pins on it are 1. ground, 2. photodiode cathode for laser intensity feedback, 3. laser diode cathode.

The ground pin is easy to see as it is hooked up to the case, I used a ~4Volt (lithium battery) supply with a 2k resistor and this gave a red light coming out of the laser diode, the light is not focused at all and scatters quickly, without a lens. I went up to an 820ohm resistor and the red light still looked the same. Be careful if you do this! I put some cotton in front of the laser and watched as it glowed red. I don't know how much of the spectrum of light is red, and if there is any infrared, so I am wary of doing any "open air" tests with these laser diodes, my vision is too valuable :) Also the red light is close to infra red already, so it probably is brighter, and more dangerous than it appears.

Here is the hookup I used:

4V ----> 2k resistor ---> laser cathode 0V ---> ground pin (connected to the diode case)

Now to try the photodiode IC.. (need help on this one!)

cheers, Jamie

Jamie Morken wrote:

Not so much the material as the size, because high capacitance is a killer. That part's 4pF is a serious problem, up to 10 times higher than the detector I used measuring the Hitachi's 1200MHz response.

--
 Thanks,
    - Win

I have a possible application for lidar, but it wont stand the $200,000 price tag of commercial stuff. The idea is to measure doppler shift on particles in the air to get the local wind speed. Any info on cheap lasers, optics and detectors capable of getting a range of the order of

200 metres backscatter in air is welcome.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

BTW the 664nm red DVD laser was an Hitachi HL6504FM and my detector an Optek OPF480 PIN diode, biased at -100V, measured into a 25 ohm load (double-end termination) with an Agilent network analyzer.

--
 Thanks,
    - Win

LIDAR means measuring backscatter from usually pulsed lasers. The mentioned pricetag is not just because these guys are expensive. Consider a backscatter coefficient of say 10^-4, of which by means of a 30cm reflective mirror you again get only a fraction. Throw in the r^2 attenuation and you quickly reach 120dB of dynamic range extending far into the noise. The only way to get the signal out of the noise is to start with enormous peak power at excitation. The response is a modulation of the r^2 attenuation, giving a distance profile of scattering.

You could make the setup far simpler by measuring forward attenuation, eg between some highrise buildings, in case you're not interested on a distance profile.

Rene

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There are plenty of fast photodiodes around. Their responsivity is given by the material. Eg the AEPX65 is recommended.

Rene

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It was just a quick lash up.

.. bias-tee laser .. ________| |______,---------, .. ________|-||-+--|______(-50R-|>|-' .. 50-ohm |____X__| \\\\ mirror PD bias .. coax | \\\\| optics 100V .. 50R //| & etc | .. | // __|____ .. laser // ________| X |___50-ohm .. current ,-|>|----+-)_______|--+-||-|____ coax .. supply +-||-50R-' | coax |_______| term. .. '----------' bias-tee

Components were 1206 SMT hand-soldered with zero-distance spacing. Bias-tees were 12GHz-bandwidth Picosecond Pulse Labs. A first attempt at a PCB stripline replacement only goes to 600MHz so far.

One should always be careful of course, but in this case the laser assembly included optics to spread the beam to 1/4" substantially lowering its intensity down to a veritable dull glow. :>)

--
 Thanks,
    - Win

Could you give more details of your circuit that you used? I can visualize it several different ways :) Weren't you a bit scared of the laser light too? :)

cheers, Jamie

The responsivity was meant for the spectral color of the light. You're right, the speed is done with the size. Unfortunately that calls for some aligned optics unless plenty of light is available.

Rene

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Thanks for the thoughts, but the app requires the source and receiver at the same location ( moving vehicle ). And we cant have high power, the laser must be eye-safe. Guess its one of those expensive solutions, but I would still like to know what sources and detectors are around.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

AFAIK, the pulsed lasers are still considered eye save when the rep rate is sufficiently low, say 1 Hz or lower. I've seen a handheld Q switched YAG issuing single pulses of say 20kW with a pulsetime of 1ns or below. At least at that time, the regulations didn't cover it. Such a laser is still doable in the size of two fists. The optics is much bigger. I'd suggest a 20 to 30cm parabol mirror for the receiver. I'd don't know the angular margin of the backscatter, but it could be sufficiently narrow, especially on short range such that you have to use the same mirror for the sending. Your receiving electronics then would have to be fast enough to get the 1ns pulse and its distance square decay. There are wonderful logamps that make the exponential appear linear.

Rene

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Cool! That looks like a neat circuit, I am buying that same photodiode from digikey (part#: 365-1029-ND) and will try to emulate your circuit with my antique 10MHz scope to start with as I don't have the Ghz+ network analyzer! :)

Are you going to feed the output of that circuit into an RF amplifer for your application? I was thinking the max2611 or max2650 LNA amps might work well for this, although they start to lose gain at around 500MHz.

I will make my own bias tee's, and I don't mind about getting RF in my powersupply at this point, so they should be rather simple with maybe a couple series inductor to the bias supply, and for the AC coupling caps would 0.1uF be ok? I checked some of the "Picosecond Pulse Labs" bias tee datasheets, and they seem to have a range of different capacitor values. I guess the actual value of the AC coupling capacitor isn't that important as long as it is big enough so that it doesn't charge up more than ~10% from the AC input?

What are the cap and resistor for across the photodiode? I would guess it is an RC snubber but I wouldn't think that would be desired for this circuit unless it is a very small cap value?

I have been reading that red light appears much less bright than similiar power green or blue light would appear. I don't know if this makes red light potentially more dangerous than green/blue light though?

cheers, Jamie

Yes, at very high frequencies like 1GHz, it's best not to attempt to fully turn the laser on and off. At 300MHz or under, that's OK, but may still not be the best approach.

Not my next step - one of our scientists will be using it to evaluate nanowire FETs.

The circuit I posted (and you wrote up) doesn't look much like a typical laser driver or optical receiver circuit, certainly not like most of the ones I design. The primary goal throughout, which you didn't show or analyze on your little writeup, it to maintain a very clean 50-ohm impedance at all points for the drive signal and for the received signal, and double terminated as well. As I mentioned, when one of us attempted to translate the lashup to a PCB, the performance dropped from 1200MHz to 600MHz despite being very careful, indicating the important of avoiding any impedance discontinuities.

--
 Thanks,
    - Win

Hi,

I posted a conversion of the ascii art circuit to eagle cad pdf: "

's%20circuit.pdf"

Is the laser current supply used to keep the laser always lasing, and then a small signal modulation is fed to the laser through the coax? Does the laser need to always be lasing to be able to achieve high frequency operation? I was thinking of turning the laser on and off to send 0's and 1's (but this is probably slower than just changing the lasing brightness. I don't know what Win's application is, but the next step seems to be getting this signal into digital logic :)

cheers, Jamie

There are laserdiode drivers at Analog Devices doing 1.25 to 10GBit. Yes, at some point, impedance drops towards the laser.

Rene

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