Fiber Optics Beam Measurement

There is often confusion when dealing with beams emerging from a fiber. The geometry is a bit different; we have a diverging beam coming out of a small aperture. The parameters usually used to specify a standard laser beam power measurement (such as diameter) are not used in the same way here, and other parameters are often specified (such as Numerical Aperture, core diameter) that are not relevant to standard measurement situations.

Technical Background

A beam of light coming out of a small aperture diverges; this is related to basic wave physics, and in fact this applies to any wave. The angle of divergence is a function of the aperture size. It is often defined by numerical aperture, which simply means the sine of the half angle. The fiber itself is often specified by, among other things, its core diameter (typically tens of um).

Choosing a Fiber Optic Sensor

If one uses our Sensor Finder to choose a sensor for measuring a beam of some watts of power, but where the Sensor Finder asks for “Diameter” one enters the fiber core diameter (remember, tens of um), chances are no results will come up. The core diameter is not the same as the beam diameter; choosing a sensor will involve knowing the beam diameter at the sensor surface. Coming out a fiber, if the sensor is even just a small distance away the beam may well have expanded enough (and reduced its power density enough) to enable regular sensors to work OK.

In some cases, a diverging beam – such as that coming out of a fiber - can be fully captured and measured by simply choosing a sensor having a large enough aperture. Not always is this practical, though. Often, an Integrating Sphere is used. Ophir has a wide range of Integrating Spheres, of various sizes, with and without built-in calibrated sensors.

Fiber Adapters

In most applications, fibers are terminated by standard connectors, such as FC, ST and others. Ophir offers a range of adapters to allow connecting these terminations to our sensors. Usually there is mounting bracket specific to a given sensor type (meaning mechanical configuration – which may include a number of sensor models), and then 1 or several fiber connectors can be connected to this bracket. For some sensors there is no need for a bracket; details are given in our catalog and on the relevant sensor product pages here on our website. Distance is built into the mechanical configuration of these fiber adapters; this means that, normally, by the time the beam reaches the sensor surface it has expanded as described above, and damage issues are thus avoided. (Note however, that this is not always a certainty. For example, when using a sensor from the PD300 series, especially with filter IN, the distance from the fiber tip to the sensor’s surface could be quite small. In cases where any one parameter seems like it is going to be near a limit, a quick sanity check is a good idea.)

Fiber Optics Resources

The NanoScan Beam Profiler For aligning fiber optic components
System Integration of Fiber Lasers and Its Effects on Laser Quality
Uncovering Cause of Bad Welds in Fiber Delivery System
Tech tip: Measuring Power Out of a Fiber
Measuring Really Small Laser Beams (< 10 microns)

Fiber Optics Measurement Products

Description	Features
PD300 10x10 mm Aperture 300 mW Silicon Photodiode Sensor	350 to 1100 nm silicon photodiode sensor 500 pW to 300 mW power measurement range Swivel mount for hard to measure places Comes with filter in / filter out options	PD300 10x10 mm Aperture 300 mW Silicon Photodiode Sensor
3A-IS Silicon 3W Divergent Beam 1.6 in. Integrating Sphere	1.6 inch ID integrating sphere for divergent beam Silicon photodiode for 350 to 1100 nm spectral range 1 µW to 3 W power measurement range 12 mm diameter aperture	3A-IS Silicon 3W Divergent Beam 1.6 in. Integrating Sphere
3A-IS-IRG InGaAs 3W Divergent Beam 1.6 in. Integrating Sphere	1.6 inch ID integrating sphere for divergent beam InGaAs photodiode for 800 to 1700 nm spectral range 1 µW to 3 W power measurement range 12 mm diameter aperture	3A-IS-IRG InGaAs 3W Divergent Beam 1.6 in. Integrating Sphere
IS6-D-VIS Silicon 30W Divergent 5.3 in. ID Ø26mm Aperture Integrating Sphere	5.3 in. inner diameter 4-port integrating sphere for divergent beams Silicon photodetector with filter for 400-1100 nm spectral range 20 µW to 30 W power measurement range Port plugs, covers, adapters and reducers available	IS6-D-VIS Silicon 30W Divergent 5.3 in. ID Ø26mm Aperture Integrating Sphere
SP920s USB 1624x1224 CCD 190-1100 nm Laser Beam Profiler	1624 x 1224 pixel resolution with a 4.4 µm pitch 14 fps at full resolution USB 3.0 communication interface BeamGage Standard or Professional software included	SP920s USB 1624x1224 CCD 190-1100 nm Laser Beam Profiler
SP920s-1550 USB 1624x1224 CCD 1440-1605 nm Laser Beam Profiler	1624 x 1224 pixel resolution with a 60 µm pitch 15 fps at full resolution USB 3.0 communication interface BeamGage Standard or Professional software included	SP920s-1550 USB 1624x1224 CCD 1440-1605 nm Laser Beam Profiler
BeamSquared^® SP204S CMOS M² Beam Propagation Analyzer	SP204S 2472 x 2064 CMOS camera covers 266 to 1100 nm Automatic ISO measurement within 1 minute Continuous usage in production line Unparalleled accuracy with patented calibration	BeamSquared^® SP204S CMOS M² Beam Propagation Analyzer

Product Comparison