Position Sensing Detector Interfacing

[VinayKumarPabha]

I want to build the circuit attached with this post to measure the position of the laser.
Please clarify my doubts regarding it
1. What is the current range produced by each quadrant in the photo detector.
2. Can the gain of the transimpedance amplifiers and sum and difference affect the position information
3. Can the temperature effect the operation of the circuit.
4. The X and Y outputs obtained are in the form of voltage. How to read the position from them. Are they needed to processed further. If so then tell me what tools are needed to process them.

Thank You........

 

[IRstuff]

Since you neglected to include the ENTIRE datasheet, most of your questions cannot be answered. However, in general, responsivity is on the order of tenths of amps per watt. Note that a quad detector requires that the laser spot be a substantial fraction of the entire area of the detector to produce good results. You may want to look at quadrilateral PSDs, which are similar, but do not have actual gaps in detector.

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[VinayKumarPabha]

Thank you for your reply..
I want to use the 4 quadrant position sensing detector of Hamamatsu_s4349. I've pasted the link below which leads to the datasheet of the photo diode. I've known that each quadrant of the photo detector provides a current in the range of nA. I first want to build the pre-amplifier stage which must convert nA into mV and later sum and differential amplifier stage. Could you please suggest me regarding the selection of component values for biasing the op-amps.

http://www.hamamatsu.com/resources/pdf/ssd/s4349_kmpd1007e02.pdf

 

[zappedagain]

1. You answered that with the datasheet.
2. Yes, temperature can affect the measurement. Mainly temperature drift in the gain resistors, and then op-amp variation with temperature.
3. Yes. See #2.
4. A multichannel A/D converter is your best bet. You can get hardware that will plug into a PC for further processing.

Unless you have the time to learn a lot about electronics on this project, I suggest you purchase one of these:

http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=4400

Z

 

[MikeHalloran]

The Old School way to track a laser beam is to not worry too much about the specific response function of your quadrature detector, but to hang the detector on the pen carrier of a pen plotter, set up the detector electronics to drive the pen to the center of the laser spot, and move the laser away from the plotter, say 10 meters, to provide a substantial baseline distance and to make the pen plot readable. Drop the pen, turn on the laser, and watch the beam dance.



Mike Halloran
Pembroke Pines, FL, USA

 

[VinayKumarPabha]

Thank you all..

I want to analyze the three outputs X,Y,SUM by using labVIEW. Can anyone suggest me a cost effective 3-channel data logger with PC connectivity for my project.

Vinay Kumar

 

[IRstuff]

Shouldn't you therefore pick a logger that's LV compatible?

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[VinayKumarPabha]

I may have to. The thing is I am new to all these things. I thought that the data from the data logger can be read into the labVIEW if the data logger is connected to PC. So please suggest me a cost effective data logger whose data can be read into the LV and which is suitable for my application.

Thank you...

 

[analogkid2digitalman]

Why not just purchase a data aquisition card (DAQ) from National Instruments?
You have not mentioned your desired sampling rate or other critical items (voltage range, resolution, etc).

-AK2DM

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"It's the questions that drive us"
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[MikeHalloran]

Uh, VKP, you may have missed a step.

You can't derive a laser spot position signal from the X,Y outputs of a quadrant detector. ... because there isn't a simple transfer function to infer where the centroid of the beam is, relative to the axes of the quadrature detector. The detector only indicates the direction in which the spot deviates from the detector center.

You have to _move_ the quadrant detector, physically, in two axes, to 'zero' the X and Y outputs of the quadrature detector and set up a servo loop to keep them zeroed. Then you can get X and Y position signals, from the motion system, not from the detector.

Alternatively, you can use the motion system to 'scan' the quadrant detector over the full extent of the beam, and use the detector SUM output and the motion system X,Y position signals to 'map' the beam.


OR, you could buy the Thorlabs Position Sensing Detector (not the Thorlabs Quadrant Detector) recommended by Zappedagain, which does have X and Y position outputs and shouldn't need to be physically moved unless your beam wanders quite a lot.

We're kind of guessing what exactly you're trying to do there.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Perhaps, you should explain that problem you are trying to solve, and not get so hung up on the details.

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[VinayKumarPabha]

My main intention is to track the laser beam position by using the quadrant PSD with the help of labVIEW. I,ve finished the sum and differential part and I am going to try to amplify the output of the PSD. I've known by googling that we first make the X and Y outputs zero by centering the beam onto the PSD . And we move the beam from the center of the quadrant PSD and continuously record the outputs X,Y,SUM to track the beam. What is this servo loop you wrote about, MikeHalloran. We don't always have to keep them zeroed, do we? Let me know if I am going in the wrong way. And I need a three channel cost effective data data logger with PC connectivity to convert the three analog outputs into digital and analyze them in labVIEW to track the laser position.

Thank you.....

V

 

[IRstuff]

A 4-quadrant detector works best nulling the outputs, physically moving the detector to accomplish the nulling, and using the positioning system to tell you how much you moved. As I mentioned initially, if you do not move the quad detector, you need to use a quadrilateral detector. The quad detector's off-null outputs are not necessarily reliable for accurately measuring position.

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[VinayKumarPabha]

I don't think I quite get it. You are saying that I have to move the quadrant detector. 1.Keeping the laser beam steady and moving quadrant detector for centering the beam and 2. Moving the beam to the center of the detector. Aren't both of them the same. Why to move the detector while nulling and why to move the beam while measuring the movement. Could you explain me the procedure clearly. I have searched online but I could not get much information about quadrilateral PSDs. Anyhow I already have the quadrant detector, so no thoughts about other detectors.

Thank you...

V

 

[MikeHalloran]

Most of us have assumed, I think, that you are trying to measure and record the laser's beam pointing stability.

Perhaps you were unaware of this, but laser beams tend to wobble with time. At least gas lasers do; I think it has to do with local depletion of active ions within the laser cavity, and I know that it's related to minute changes in the thermal environment of the laser tube and mechanical strains associated with thermal deflection of the tube's support. I assume that similar influences affect solid state and diode lasers. For gas lasers, the deviation is not great, a few milliradians or tens of milliradians, but it can make a huge difference in performance of the sort of applications where lasers are used.

Again, you need to state clearly, exactly what problem you are trying to solve.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Perhaps, rather than keeping us guessing, you could accurately describe what problem you are trying to solve, rather than getting into the weeds of what solution you thought might solve your problem.

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[VinayKumarPabha]

Although there many other applications of PSDs with the laser beam, first of all I want to track the movement of the laser beam to get onto the track. The datasheet of the PSD I am using and the schematic I want to build are available in the links I posted earlier. Instead of using the dividers as shown in the schematic I want to give these three outputs to the labview for plotting a graph "deflection vs voltage", so that I can measure how much distance did the laser beam move. To convert these three analog voltages into digital and to log them into the PC for plotting a graph in the labview I need a cost effective data logger, a 3-channel one coz I need to convert three analog voltages into digital. The process I am going, is it right?

V

 

[MikeHalloran]

Uh, no.

Your proposed circuit plus LabView allows you to plot 'voltage'.

You have not mentioned how you are going to control the laser beam position or how you are going to measure random position changes, so you don't have a way to plot 'deflection'.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Almost enough information. What is actually moving the laser? What is the "track?" Where is the quad to be located? Are you attempting to measure true position or angle?

However, I reiterate, a quad detector is just not that accurate for position MEASUREMENT; it's really for position SENSING, i.e., it will tell you roughly where the spot is. Additionally, most laser beams tend to be very small, and therefore produces large amounts of position noise on a quad, particularly if the beam is straddling two quadrants. You are better off with either a quadrilateral device, which has no gaps, or just a camera, which has much higher precision and resolution.

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[VinayKumarPabha]

By tracking I meant that I first want to measure the position of the laser beam so that I can get used with using the equipment and with the process. I have some mount to place the photodiode on and a linear stage to move the laser beam which somewhat looks like the one in the image in the link below. And I also have mount to place the laser on the stage.

IRstuff, I think understood why the quad detector can only be used for sensing the spot of the laser beam. Can you explain me what did you mean by position measuring.

Can data loggers continuously record and store the information. What does the sampling rate of a data logger mean.