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Getting the Best Optical Power Measurement
National Instruments provides
hardware and software tools that enhance results and cut costs.
Over the last few years, the demand for optical networks
has exploded. An increasing number of communications manufacturers
are designing products to leverage the high bandwidth and
smaller size offered by optical components over their copper
counterparts. In fact, some industry experts project that
the optical network component industry will grow at 40 percent
for the next three years. This growing opportunity has optical
component manufacturers in many sectors of the industry elated
at demand. At the same time, they are searching for faster,
more cost-effective ways to automate the manufacturing process.
Generally, companies must verify and calibrate optical components
for manufacturing, quality assurance, research, production
tests, etc. Although the types of tests vary depending on
the components being manufactured, optical power measurement
is common across all optical devices and procedures.
Optical power measurement is the most common measurement
in the optical industry. There are several optical power sensors,
which convert light to an electrical signal. The most common
sensor uses photodiodes that output current. This low-level
current passes through a transimpedance amplifier that applies
a gain and converts the signal to voltage. Sensors on the
market are tailored to general and specific applications,
depending on their specifications, materials, and design.
The main factors to consider when selecting a sensor are detectable
wavelength (nm), power range (W), measurable area (inches
squared), and response (rise) time. If an optical sensor that
meets your application criteria cannot be located, you can
simply choose from the thousands of base-line detector devices
such as photodiodes and phototransistors, and use an appropriate
amplifier to create a conditioned signal readable by a plug-in
data acquisition (DAQ) board or oscilloscope.
A DAQ system, or a voltmeter, reads the voltage returned
by the sensor and converts it to an optical power response.
Both the voltage reading and the energy response are sensor-dependent
and often depend on the wavelength being analyzed. The measures
of power are generally expressed in watts (W) or decibel milliwatts
(dBm), while tests application values, like insertion loss,
are measured in decibels (dB).
Measuring the optical loss or insertion loss of an optical
component is also a very common test. In it, the device under
test is placed between a light source and an optical power
meter. The source emits a signal into the DUT. These sources
(examples include tunable lasers from New Focus and continuous-wave
lasers from EXFO) are generally specialized devices to output
signals with wavelengths in the optical transport bands. The
power meter measures the intensity of the light as it enters
and exits the DUT. By measuring the light both before and
after the DUT, the amount of power lost inside the DUT is
calculated.
The optical power meter, the instrument traditionally used
to measure optical power, is a standalone device with one
or two input channels. OPMs convert a sensor input to a power
response based on the sensor's response table. A computer-based
system using a plug-in data acquisition board can perform
the same optical power measurements using an optical sensor
with lower cost, faster performance, better flexibility, and
better system integration. In addition, a 16-bit DAQ board
is more than 0.008 percent accurate, three orders of magnitude
better than the average sensor.
The DAQ system is the least expensive of the three configurations.
When using all the channels, the DAQ system with eight channels
costs $10,000, which is more than $10,000 less expensive than
a stack of optical power meters and about $2000 less expensive
than the optical switch configuration.
By implementing a DAQ system with an optical sensor, you
can measure and test your units in a fraction of the time
needed by standalone GPIB instruments or optical switches.
Some switches take 180 milliseconds to route a signal. In
the same time the PCI-6052E can make 7492 optical measurements,
provided you have a fast enough optical sensor. Another benefit
of NI hardware is the real-time system integration bus (RTSI).
The NI timing bus directly connects DAQ, IMAQ and motion boards
for precise synchronization of functions. With RTSI, the boards
can use the same clock and triggers. Measurements, movements,
and pictures are synchronized every time they are taken.
In addition to providing improved performance, the DAQ system
is flexible and easy to update. The DAQ boards reads voltages
and converts the voltage readings to power values through
software. Therefore it is compatible with a variety of photodetectors
from many vendors within the voltage range of operation of
the DAQ boards. It requires only the conversion factor to
translate the sensor's output to real power values. In addition,
you can also use the same DAQ board to measure other signal
types such as thermocouples, signal waveforms, circuitry responses,
etc.
For example, you can set up the eight channels on the PCI-6052E'to
read any combination of these signals. Moreover, it provides
two analog output channels to output waveforms or control
voltages, eight bidirectional digital lines to turn on/off
equipment, and two general-purpose counter/timers to generate
TTL pulses or measure time precisely. More importantly, when
used in a production environment, these PC-based systems can
be adapted and modified as the process model changes.
By using an integrated platform, you can easily use DAQ,
motion control, GPIB, serial, and image acquisition components
from NI. You can take advantage of programming examples in
LabVIEW, Measurement Studio, and TestStand. One example of
an application is optical component precision alignment. With
a PC-based system, you can obtain up to 40 times faster alignment
with a much smaller footprint. You can synchronize optical
power measurements, control motion axes, and analyze video
inputs to achieve the best hardware performance. Using an
integrated platform, you can decrease development time, perform
faster measurements, increase throughput, and maintain a smaller
support staff.
Optical power measurement systems can be calibrated with
plug-in DAQ boards and optical sensors to provide NIST-traceable
values. Many plug-in products are shipped with a certificate
of conformance and calibration, which provides the documentation
to satisfy ISO-9000 requirements and provide traceability
to NIST. Calibrated optical sensors have uncertainties that
can be used with the calibrated specs of the DAQ boards to
calculate the total uncertainty of the system.
Many of NI's multifunction data acquisition devices can be
used to make these power measurements. NI delivers accurate
measurements, high resolution, long reliability, and minimum
noise. For example, the NI-6052E DAQ board is high-channel,
high-accuracy (16-bit) and high-speed (333,000 samples per
second). The 6052E directly connects with the BNC-2120, providing
eight BNC analog input channels. This is an ideal connection
for most of these photoreceivers, which connect to BNC cables.
NI can also supply cable (SH68-68-EP) and a terminal block
(BNC-2120).
Many vendors make light sensors suited for different wavelengths.
In the example below, the sensor is New Focus's Model 2001,
which performs visible light measurements (400-1060 nm). It
has a power range of 1 microwatt to 10 milliwatts, and uses
standard coaxial connections. The sensors can also be calibrated
using standard DAQ calibration tools in LabVIEW.
LabVIEW is used extensively for research and development,
manufacturing, production and test of optical components.
NI developed applications free of charge to measure optical
power and power loss. The following examples were designed
to read one and two sensors, respectively. They can, however,
read several sensors with simple modifications.
With LabVIEW, you can drive optical power measurements. LabVIEW
is a powerful package that integrates buses like GPIB and
serial RS232/485, and also equips the user with the power
of Excel, SQL, and the Internet. LabVIEW has statistical analysis
tools, such as average, max/min value, standard deviation,
data chart, data trend, curve fitting, data logging to disk,
and database.
This example of LabView optical power measurement front panel
can be downloaded from http://www.ni.com/telecom
and Opto Electronics may be selected. The example is divided
into three sections: acquiring the sensor data, converting
voltage to power, and presenting the results.
In section 1, the VI acquires the voltage data from the sensor,
using a data acquisition function: AI sample channel.vi. This
function reads the data from the sensor. You may also select
an option to average the input to improve accuracy. The example
can run without a DAQ board or any hardware in "Demo Mode,"
in which users may select an average voltage and the deviation
for the simulated input.
The Calibration Curve VI, in section 2, converts the voltage
acquired in section 1 to a power value. The user selects gains,
wavelength, and model of the sensor. With the sensor setups,
the conversion VI converts the voltage to an optical power
value in watts. Section 3 presents the values read on the
optical power display. A selection of units is available (watts,
milliwatts, microwatts, nanowatts, picowatts, and dBm). The
example also has a historical chart that helps the user to
identify trends and compare data.
NI also developed a LabVIEW example for optical power loss.
Using this example, you can compare the optical power of two
sensors. The example has the same structure as the optical
power measurement example, with three section: acquiring the
sensor data, converting voltage to power, and presenting the
results. The two main differences are in the sections for
acquiring data and presenting the results. In the first, two
channels of data are measured from two sensors instead of
only one in the former. In addition, the results displayed
is the difference between the two measurements representing
the power loss. As the demand for optical components continues
to grow, optimizing design validation and manufacturing test
of these components is crucial. Using open-standard PC-based
systems is not only an economical solution, but also a tool
that can be used to save manufacturing space, reduce development
time, increase manufacturing speed, and enhance hardware integration.
Synchronizing production hardware for optical power measurements,
motion control, vision and test brings improved performance
to the measurement and automation world. In addition, impressive
cost savings and performance enhancements are realized.
National Instruments provides hardware and software tools
that leverage computer technologies to create high-performance
measurement and automation systems. Using advanced plug-in
data acquisition boards in combination with high-speed photodetectors
and transimpedance amplifiers, fast and accurate optical power
measurements can be realized. Large optoelectronics companies
saw significant improvements in production cycles of tenfold
to fifteen-fold using NI plug-in DAQ boards as well as synchronized
motion and vision boards. By leveraging an open-industry standard,
such as PXI or CompactPCI, you can build a flexible and scalable
solution for optical power measurement, component alignment,
and production test.
For further information, please contact Jon Pafk at National
Instruments Inc., 11500 N. Mopac Expwy, Austin, TX 78759;
(512) 683-6868; fax: (512) 683-5759; E-mail: jon.pafk@ni.com.
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