Fiber Optics

Fiber Optics

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.

First, by way of 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 sensor for measuring beams coming out of a fiber:
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.
Detailed technical information for measurements of diverging beams, tutorials, and product information, are available here.

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.)

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