Fiber Optics Sensor + DAQ

[umie]

Dear all NI Experts,

I am still a newbie with Data Acquisition and LabVIEW.

A few months ago I was given a PCI-6036E DAQ card for me to perform data acquisition project with it. At that point too I was told about a fiber optic sensor by a friend and thought of using it as the sensor for my project without having a thorough knowledge of the sensor itself.

As I go further, I realized that the fiber optic sensor, Fiber Bragg Grating will only react by shifting the wavelength when there is a change of temperature or strain near the gratings. I haven't started incorporating the FBG with the DAQ card yet as I was still learning LabVIEW since it was my first encounter with that programming language. I was studying a DAQ system using just thermistor.

Later on I read this article in http://www.ni.com/opticalsensing/ and realized that in order to extract the information from the FBG sensor we need to interrogate it, and NI has provided an interrogator to actually make use of DAQ system with FBG.

I didn't have the fund to buy an interrogator, and I think performing data acquisition with just FBG, PCI-6036E and CB-68LP is possible, but only to realize there is also a need to use a photodetector in order to convert the light intensity to electrical power since PCI-6036E can only accept voltage or current source. (might have missed out the most important part of converting light intensity to electrical signals' knowledge previously).

So I was give a photodetector with an unknown details of its manufacturer and tested it to the reflected signal from FBG and I thought I might have gotten a voltage through my LabVIEW. My LabVIEW vi was only based on voltage measurements from the DAQ Assistant.

What I'm not sure of now is whether is it really a voltage I got from the photodetector is was it just a form of noise? I connected my photodetector with a voltage source of +5V from the DAQ card itself and placed a 100k ohm resistor in series at the other end of its leg and measured the voltage between the resistors' legs. 

Since the output of the FBG when it reacts with the change of temperature or strain lies in the shift of the wavelength, I can't really see much difference of it's performance directly by connecting the photodetector through the AIO port of the DAQ card. Changing the power intensity with every wavelength shift is a different study itself.

I may have chosen the wrong sensor to work with, being newbie to both LabVIEW and the whole DAQ process itself, as I am not really an expert in the complicated electrical circuit as well so I was just wondering is it possible to make this system work? What other information I needed to be cleared of before I can actually see the correct power intensity information out of the FBG.

[Christian Mergl]

The right DAQ Hardware to use would be the PXIe-4844 Optical Sensor Interrogator.

Christian

[umie]

Hi Christian,

Thank you for your reply. I don't seem to have enough fund to get the interrogator . It's okay, I guess I shall try to figure out some other method or try with a different sensors instead. 

[leventsirma]

With your current system, since you lack funds, I can only suggest you a limited number of methods. You can try sending a very basic signal and see if you can detect it using your photodiode. If you still think you’re getting any noise, try using a buffer to increase signal voltage before feeding it to the DAC. Also, you can set detection level for signals if you want to filter out the noise.

[nhnues]

Sadly, you cannot use the FBG as a sensor without some way of interrogating the optical wavelength and the easiest way to do that is by sweeping either the input to the FBG or the output from the FBG.  One possible cheaper option (instead of the NI interrogator) for you is to buy a scanning tunable filter (TF) from Micron Optics, but then you'd have to build or buy an electrical circuit that would sweep the voltage of the TF.  Once you can sweep the wavelength, the output of your photodetector will be zero except for when the TF passband overlaps the FBG wavelength.  Then, the sweep voltage tells you what the wavelength reflected by the FBG is and that gives you the temperature or strain.  If you want to pursue using the FBG as a sensor, I can send you a research paper describing the origins of this approach to strain/temperature measurement.