The 1, 2, and 3 of Light Measurements with DAQ

Overview

To measure light, engineers have traditionally used optical power meters -- stand-alone boxes that integrate detector, amplifier, analog/digital conversion, display, and/or export functions. Today, computer-based optical power meter systems -- low-cost systems which only contain an amplified photodetector and a data acquisition board -- offer all the features of stand-alone box optical power meters in addition to scalability to higher number of channels, advanced data analysis, and rapid collection of optical power measurements. Engineers assemble these computer-based systems by choosing the best products for (1) converting the optical power to an electrical signal, (2) converting the analog signal to a digital signal, and (3) analyzing and displaying the results. We discuss these three phases of light measurement.

1. Optical to Electrical Conversion

When measuring optical power you first select the proper photodetector for your application. The photodetector converts light into a proportional voltage or current. See equation 1. Photodetectors with amplification output voltage signals are composed of a photodiode and a transimpedance amplifier. The current generated by the photodiode is proportional to the amount of optical power and responsivity of the photodiode to the specific wavelength.

Equation 1. Current_out(Amps)=Responsivity_nm(Amps/Watts)*Power_in(Watts)

Equation 2 shows that the current output of the photodiode is converted into a voltage via the transimpedance amplifier for a plug-in A/D board or oscilloscope to digitize the optical signal.

Equation 2. Voltage_out(Volts)=Transimpedance_gain(Volts/Amps)*Current_out(Amps)

Thus, choosing the correct photodetector for an optical power measurement depends on the characteristics of the incident light, photodiode characteristics, and amplifier gains. For the remainder of this paper, we assume that we are using photodetectors with built-in amplification.

Incident Light
Light is an electromagnetic wave. The visible light that we see occupies a narrow band of the electromagnetic spectrum surrounded by other waves, such as radio frequency, microwave, and radar. Visible light varies from 400 to 750 nm in wavelength. Infrared light, which is used in communications and temperature applications, represents wavelengths larger than 750 nm. Ultraviolet light, used in life sciences, is defined by wavelengths below 400 nm.

Photodiodes that sense light are typically made of semiconductor materials such as silicon and germanium. When light is incident on photodiodes, the diodes react based on the incident wavelength and intensity, and generate a small current. Different materials react distinctly depending on the incoming light wavelength. Therefore, the construction material of the photodiode dictates the detectable wavelength or best range in which the photodiode operates. For example, if the light source is a common laser pointer that emits a laser with a wavelength of 650 nm, photodetectors made of Si, Ge, and GaAs are preferable. If the light source is in the C-band (wavelengths around 1530 and 1565 nm), the choice is Ge, InGaAs, and InGaAsP. Figure 1 below lists some common semiconductor materials and the wavelengths to which they best react.

Responsivity Curve
The amount of current generated from incident light on a photodetector is associated with the photodiode responsivity. Responsivity is expressed as current/optical power (Amps/Watts) and is provided to you by the manufacturer of the photodetector. Generally, the photodetector electrical response to different wavelengths is not linear. Figure 2 plots non-linearity of wavelength versus responsivity. For example, a light source that has a wavelength of 1,000 nm would generate a responsivity of 0.5 A/W in the photodetector. A light source that has a wavelength of 1,500 nm would have a response factor of 0.9 A/W.

Power Range
Optical power, an energy measurement, is defined as the intensity of incident light over the photodetector and expressed in Watts and dBm (0 dBm is defined as 1 mW). The minimum amount of optical energy that you can measure is a factor of the accuracy and signal conditioning provided by the photodetector and the external noise introduced in the system. Therefore, photodetector manufacturers specify gains and accuracy of the manufactured models. Another factor to consider when working with energy sources is the maximum amount of energy that you can measure without damaging the photodetector. Remember this depends on the type of material used in the photodetector construction. All detectors have a damage threshold that you should respect.

Detectable Bandwidth
A common dimension used to identify photodetectors is detector response time or rise time. How fast can the photodetector can respond to varying light intensities? For example, to accurately read a light signal modulating at 200 kHz, you must use a photodetector that can respond at least that fast.

2. Electrical to Digital Conversion

A computer-based system using a plug-in data acquisition board can digitize the signal from a photodetector and perform the measurement. By implementing a DAQ system with a photodector, you can measure and test optoelectronics components in a fraction of the time needed with stand-alone GPIB or serial instruments. When testing components such as optical switches, the increased speed of computer-based optical power measurements and the greatly reduced cost of high-channel applications far surpass the benefits of proprietary stand-alone instrumentation and its costs. Computer-based optical power measurement systems are also ideal if you need to synchronize optical power measurements with motion and vision. For instance, you can decrease alignment and assembly time while increasing production yields for precision alignment applications.

Computer-Based Power Meter Specifications
You can use the NI 6052E and other NI data acquisition boards in conjunction with photodetectors available from companies such as New Focus, Thorlabs, and Thermo Oriel. For example, the NI 6052E data acquisition board for PCI and PXI has a 333 kS/s sampling rate, 16 analog inputs, and 16-bit resolution. You can connect it to a photodetector using the BNC-2120 terminal block. For most applications, you should choose a data acquisition board with 16-bit or higher resolution to take full advantage of the dynamic range of the photodetector.

Accuracy Measurements
You have three options when considering accuracy. First, you can use NIST-traceable instruments. On average, photodetector accuracy error varies from 2.5 to 20 percent, while DAQ board accuracy errors are fractions of a percent (0.008 percent in the case of the NI 6052E). That means the error of an optical power measurement system that uses a DAQ board and a photodetector mostly results from the photodetector.

We ship all National Instruments measurement products with a Certificate of Conformance for traceability to the National Institute of Standards and Technology (NIST) standards. If you create a measurement system with a calibrated sensor, such as those made by Thermo Oriel (thermo-oriel.com) and NI measurement products, you can calculate the total uncertainty of the system and create a NIST traceable optical power measurement. To calculate the accuracy of the whole system, you take into account the individual components that comprise the system. For example, if you have a board with 0.008 percent uncertainty and a photodetector with a 5 percent uncertainty, then the total uncertainty of the system is (0.008%2+5%2) = 5.0000064 percent.

Calibration with a Precise Power Meter as Reference
Your second option for improving the accuracy of your measurements is using a precise power meter as your reference point. You can then compare your results to this reference point. In this case, the photodetector and DAQ board system would measure the same source measured by the precise power meter. Using NI-DAQ driver software, you can calibrate your system to the same results. You can complete this calibration using "scales" in an easy-to-use graphical interface called Measurement & Automation Explorer (MAX) as presented in Figure 3. With "scales," you can alter the display of your measurements from one unit to another. For instance, if you are measuring optical power, you can have the DAQ output of 3 V display as -1 dBm on your program.

Relative Measurements
Your third option for determining the accuracy of your measurements is relative measurements. Several optical power measurements are relative by nature. For example, the objective of alignment tasks is to maximize optical power passing through the components being aligned. If you are concerned about the difference in light levels from one position to the next, absolute NIST calibrated optical power measurements often are not necessary.

For more information on accuracy measurements, read the Insights series, "Understanding Accuracy Specifications and Computer-Based Instruments".

3. Analysis and Display

National Instruments LabVIEW is a graphical programming language used extensively in the research, manufacturing, and testing of optical components. With LabVIEW, you can control the data acquisition board to take optical power measurements. Later, you can retrieve the data for the purposes of analyzing, displaying, and storing it. For analyzing the data, LabVIEW has several functions that can calculate statistical values, apply curve fitting, and find data trends. LabVIEW also can display the data on charts and graphs, store the data in a spreadsheet or a database application, and publish the results on Web.

See Also:
Learn about the latest version of LabVIEW 6.1

Example Application

Another benefit of computer-based optical power measurement systems is advanced synchronization. This is important when you want to perform characterization tests of passive optical components. For example, a common Fiber Bragg Grating (FBG) test is to send a broad range of wavelengths into the grating and see if the FBG has attenuated the proper wavelengths. To complete this test, you need to sweep the wavelengths and synchronize the measurement with each new wavelength. A New Focus tunable laser can generate a trigger signal that you can connect to a NI plug-in DAQ board. This trigger signal tells the National Instruments DAQ board to digitize the optical power measurement with each wavelength generated. Figure 4 describes the system setup. Using this method, you can provide complete spectrum measurements in less than a second with resolutions close to 1 picometer. For more information, please see the New Focus Application Note 10 (http://www.newfocus.com/Online_Catalog/literature/apnote10.pdf).


Alignment, passive component characterization, and laser diode testing are just some of the required tasks that engineers in the optoelectronics (OE) industry perform every day. Facing the increasing challenges of improving yields, implementing frequent process changes, and increasing throughput, you can benefit from automating your processes with the development of a single, easy-to-use automated system for such OE applications. By combining (1) a properly selected photodetector, (2) a National Instruments data acquisition board, and (3) the LabVIEW graphical programming language, you can easily assemble a system that meets your needs.

See Also:
New Focus Application Note 10 -- Swept Wavelength Testing
NI Communications Information

Reader Comments | Submit a comment »

Whoa! Power is Not Energy
your statement; "Optical power, an energy measurement, is defined as the intensity of incident light over the photodetector and expressed in Watts ..." Power is not energy but rather energy per unit time, a rate. Doesn't effect the remainder of the article but for completeness sake ...
- Ron Scott, Naval Air Warfare Center. Ronald.scott@navy.mil - Jan 12, 2007

PMT's as detectors in flow-cytometry
It should be more exaustive to consider detectors like the PMT's and the related problems about the "base line" restoring technique applying a transimpedance amplifier and connecting it to a NI-DAQ PCI board. These problems are always to be focused especially when people must detect weak fluorescence pulses, like in Flow-Cytometry.
- umberto martinelli, scientific devices & S.Te.Bi . umartincyto@libero.it - Feb 20, 2002