need help

HELLO, we r in great trouble for my senior design project in school, I will ask for some advice. my project is to make a device to find polarization axis of a lens. we are passing laser light(using laser pointer) through the lens of which axis needs to find and a analyzer lens. if 2 lens's axis are perpendicular thn they block light. as we know the axis of the analyzer, we will rotate the lens with a stepper motor unless all the light is blocked. We r trying to detect the light by using a photodiode. built atransimpedence amplifier without any Cf using lm741. now we see theoutput is very noisy. gives very very different datas at differnettime . i took data from the TIA and tried to find fft of data andfound all signal is in DC and noises on higher freqs. but spec saysoutput will be 100 level. i dont know y i am getting output at DC. Ihave built a filter also, cutoff at 50 Hz. Is filtering with this willhelp?? we need to detect measurable voltage change in TIA with every 1degree rotation of the lens by the stepper. we gonna use ADC to changethis voltage to use in microcontroller. my question is whether i should use filter or need to use lock in amp.Is modulating the signal or demodulating is easy?? i need to finishthis part in 1 week. as i am a student dnt have tht much idea and timeat all. I still have to work on the rest of the project by april. So, what will be less time consuming ?? money is not a factor. if Ican buy good less noisy transimpedence amp, filter or lock in amp itwill work too. All i need less time consuming direction. If anybody give me some suggestion I will be obliged. will eagerlywait for reply.

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you need to modulate the laser with a set frequency and use a decoder or pass filer in the op-amp stage. the photo detector should be set up to detect sudden changes of light and not the DC component. i guess you could do that a couple of ways. Capacitive decoupling or common mode input to both + & - of the op-amp having one input using a bypass cap to remove the DC. also you will need to use the Nulling inputs of the 741 since the Cmr (common mode rejection) isn't really that good and the offset is a little off.

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Next time get an earlier start.

Good Luck! Rich

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Rich Grise

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