Laser Measurement - PC Interfacing and Data Logging Laser Power and Energy
Introduction
to systems that can attach to a computer or even log and store
data onboard the meter itself.
Data logging involves the sequential gathering data in order to
learn more about a process or a system. This information can
provide increased knowledge about how a system works.
Some common uses of data logging capabilities include:
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Quality Assurance – to retain and share data that
documents how a laser is functioning; for example,
outgoing or incoming inspection, or during a laser service
event.
Experiments – to gather and analyze data to assess
results of experiments or to optimize a process.
Process Control – to serve as a feedback control loop,
whether performed periodically or real-time.
We see a greater demand for analysis and archiving of laser
energy and laser power measurement data than in the past.
This has to do with more stringent quality systems, as well as
the use of lasers in ever widening fields and more complicated
and demanding tasks. As these tasks get more complicated,
people find themselves needing to perform process
optimizations, or even real-time feedback to control the laser
output, within ever shrinking process windows. To respond to
this trend, manufacturers of laser measurement equipment
include additional data logging capabilities into their newer
products.
This paper begins by summarizing the typical components of a
laser power and energy system, and then covers the typical
data logging methods available on the market, and concludes
by listing several special considerations that need to be
considered for certain types of applications such as pulse
energy measurement.
Components of a Laser Measurement System
A typical laser measurement system includes two separate
components: a sensor and a meter. The sensor is placed in the
path of a laser beam and heats up as it is exposed to laser
energy resulting in a raw voltage or current output. The sensor
must be attached to a meter to convert the raw output into a
useful measurement in terms of Joules or Watts.
Manufacturers of laser measurement products typically have
numerous different sensors due to the variety of lasers on the
market. They also typically have a selection of several
different meters that perform various tasks ranging from
simple power measurement with no data logging capabilities,
Examples of typical laser measurement systems
What are my options for saving the data?
There are two main methods of collecting data with laser
power and energy meters that are commonly on the market
today.
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You can attach a meter to a computer and log the data
directly to the PC via a variety of PC interfacing options.
Some meters allow you to log data onto a stand-alone
meter itself and move the data to a PC later.
Gathering Data with PC Attached
One way to gather laser power and energy data is to attach a
meter directly to a PC. Meters will commonly have a USB
connection, and some will also have RS-232 or IEEE-488
GPIB interfaces. Application software, usually included with
the meter purchase, can be installed onto the PC and then used
to communicate with the meter. Software from various
manufacturers have different types of user interfaces, and most
software will allow you to perform basic or advanced remote
control of the meter, display batch statistics and a trend chart,
and allow you to save the data into a file on the computer hard
drive.
Another way to gather data with a PC is to write your own
piece of software, often using drivers provided with the meter
software installation CD. In this case, the software sends
remote control host commands to the meter via the PC
interface. The meter will typically have commands that allow
measurement data (sometimes just raw data and statistics data)
to be sent to the remote application.
The most advanced meters offered by some laser measurement
manufacturers provide full remote control of the meter,
allowing it to be installed inside a laser processing system,
laser burn-in station, or a scientific experiment. Measurement
can then be controlled remotely by control software without
1 data that is captured with a thermopile averages out the
variation in individual pulse energy over a period of time,
typically on the order of one or more seconds depending upon
the power level.
any human intervention.
LabMax PC applications software
Gather Data Onboard Meter
More recent meters, such as
Coherent’s LabMax family of
meters, also allow data to be
collected onboard the meter and
transferred to a computer later,
either through a USB flash drive
or a PC interface. This is convenient if the laser measurement
is taking place some distance from a computer, or if you want
the flexibility of saving data on the fly without the
requirement of having a PC nearby.
These types of meters will allow you to save multiple files
onto the meter or even directly onto a USB flash drive. Some
meters will also allow a customized name to be entered for
each data set. In the case of LabMax, the meter can apply a
date code and even auto increment file names so that data is
not overwritten during a series of tests. These files can then be
moved to a computer and analyzed later. Data is often saved in
a convenient format, such as comma delimited files, that allow
them to be easily imported automatically into a spreadsheet
like Microsoft Excel.
Are there any special considerations to make when
logging laser measurement data?
You can be faced with some unique challenges when trying to
capture the energy of certain pulsed lasers. Typically in laser
measurement, if a laser is pulsed and its repetition rate is much
faster than 10 to 20 kHz, it is very difficult to capture the pulse
energy of every single pulse. In these types of applications,
people often switch to a measurement device that measures the
average power of the laser with a thermopile or optical sensor.
This alleviates any challenges associated with measuring and
capturing the measurement data from fast lasers. However, the
Another challenge associated with kilohertz pulsed lasers, if
you are measuring the individual pulse energy, is that a large
amount of data can be gathered very quickly. Some meters
will gather a data point every single laser pulse or every other
laser pulse, depending upon how fast the laser is firing and
how fast the data interface and PC software can keep up. At
these data rates the file can add up to many megabytes in a
matter of seconds. This can make data analysis difficult in
programs like Microsoft Excel, in which the spreadsheet can
limit the amount of data that can be analyzed.
Managing the quantity of data gathered can be handled in a
couple of different ways. If a very large amount of data must
absolutely be gathered, you may want to consider gathering
and analyzing the data directly on a PC using a National
Instruments LabView program. LabView is able to open and
calculate or plot large data sets. Another option, available on
some meters, is to decimate the data set by entering a
sampling factor, such as one data point per second, or one per
hour, etc. This allows you to manage the batch size while still
monitoring a fast laser over a long period of time.
Summary
As requirements for process stability and control increase,
whether to improve yield or reduce downtime or some other
reason, the trend is that routine monitoring and analysis of
laser output is becoming necessary and commonplace in many
applications.
Laser measurement equipment manufacturers are keeping
pace with this trend, and newer meters on the market often
have very useful features that offer customers several options
related to gathering, analyzing, and saving data. For example,
data can be captured and saved in real-time by attaching the
meter to the PC and running a software application, or data
can be gathered and stored inside the meter or stored to a USB
flash drive and analyzed on a PC later. Additionally, some
meters, such as Coherent’s FieldMaxII and LabMax laser
power and energy meters, can be remotely controlled through
host commands, enabling remote laser monitoring and
feedback loop controls without any human intervention.
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