Laser Power Measurement Integrating Spheres

Labsphere LPM Laser Power Integrating Spheres

Labsphere LPM laser power integrating spheres are purpose designed for measuring lasers and other challenging light sources and excel in those applications where a thermal laser power meter would lack sensitivity, or a photodiode detector would not be able to capture the full beam. When combined with a photodetector or spectrometer, an integrating sphere acts as a wide-angle, directionally-insensitive collector of light and is ideal for measuring lasers and other “difficult-to-measure” light sources characterised by large area beams, high divergence or high power levels. We also offer complete radiometer systems based upon the LPM sphere – LPMS radiometers.

Key Features

• Purpose designed integrating spheres for measuring the radiant flux (power) of lasers and other “difficult-to-measure” light sources
• Novel, non-radial detector port location renders the sphere insensitive to beam direction, divergence or dynamic displacement
• Sphere sizes: 2, 4 or 6” diameter
• Choose from 3 coatings: Spectraflect, Spectralon and Infragold
• Optional: attach a fibre optic spectrometer to your sphere

Solves the Problem of Measuring “Difficult-to-Measure” Lasers

An integrating sphere radiometer provides a simple solution to measuring the output power of divergent as well as collimated lasers sources, for example high power laser diode bars and arrays or fibre-coupled diodes. The beam size, divergence and power from such lasers can be difficult (if not impossible) to capture with a regular laser power meter. Using an integrating sphere combined with a fast-response photodetector simplifies the measurement of “difficult to measure” lasers and greatly reduces alignment sensitivity compared with regular optical power meters.

A correctly designed integrating sphere will provide accurate and highly repeatable measurements of laser power regardless of the beam size, divergence, polarisation, beam pointing and any dynamic beam displacement. The inherent optical attenuation exhibited by an integrating sphere also reduces the flux reaching the photodetector, allowing the use of fast response photodiode type detectors with high power lasers. Without a sphere, high power lasers have to be measured with thermal detectors which respond much more slowly to changes in the laser power level.

The Labsphere LPM Laser Power Integrating Sphere Design

Labsphere’s LPM laser power integrating spheres are suitable for measuring both CW and pulsed laser powers from nanowatts to hundreds of Watts in the 350-1700nm spectral range, depending upon the sphere size and coating chosen and the type of photodetector used. The special integrating sphere design positions the recessed detector port in a non-radial position, rendering it insensitive to changes in laser beam pointing or divergence for angles up to +/- 40° from normal. To put it another way, with a Labsphere LPM sphere, you get power measurements you can trust regardless of the laser beam diameter, divergence and static or dynamic beam pointing direction.

Why not just use a general purpose type of integrating sphere? Integrating sphere theory tells us that light entering an integrating sphere creates a uniform radiance distribution over the entire interior surface of the sphere after the first reflection from the sphere wall. In reality, this is not really the case, and many integrating spheres suffer from some degree of alignment sensitivity or radiance nonuniformity. In other words, the radiance distribution in many spheres can depend upon the exact alignment of the light source or beam of light into the sphere. This is highly undesirable, and although the effect lessens with increasing sphere size, it is common for integrating spheres to feature baffles that screen the extended light source from the detector.

The case of collimated or semi-collimated sources, such as the beams from lasers and laser diodes is slightly different. Here, the laser beam creates a “hot spot” (localised zone of disproportionately high radiance) where the beam impinges upon the sphere wall. Integrating spheres designed for use with collimated sources often employ a baffle to screen the detector from the hot spot. This lessens the dependence of the detector signal on the alignment of the laser beam into the sphere. However, crucially, it doesn’t eliminate the effect.

It is however possible to design a sphere for laser power measurements that doesn’t employ baffles and still give readings that are not affected by the beam direction or divergence over a wide range of angles. This is the approach taken with the Labsphere LPM laser power integrating spheres. The LPM spheres are designed with the detector port placed in a non-radial position which forces the photodetector to view the radiance of a small patch of sphere wall directly above the input port of the sphere. By limiting the field of view of the detector, we avoid the detector seeing the first strike “hot spot” from the laser beam, which greatly reduces the alignment sensitivity of the detector signal with variations in the direction of the laser beam. The LPM integrating sphere design is more-or-less alignment insensitive over +/- 40° horizontally and vertically with respect to the normal to the plane of the input port.

Integrating Sphere Coatings

Labsphere LPM laser power integrating spheres are available with three coatings. Choose Spectraflect, a barium sulphate-based coating for the 250-2500nm spectral range. For higher performance, consider a machined Spectralon sphere, a thermoplastic with the highest reflectance in the 250-2500nm band which promotes higher throughput, greater radiance uniformity and a much higher level of environmental stability and durability. For the infrared band from 800nm to 15+μm, the electro-plated Infragold coating is available.

Sphere Sizes & Ports

The LPM laser power integrating spheres are offered in 2, 4 and 6-inch diameters, and with input port diameters of 0.5, 1 and 2-inches respectively. Each sphere is fitted with two detector ports, one of which would normally be used for a photodiode detector, and the other available for coupling to a fibre optic spectrometer using an optional fibre optic port adaptor. When not in use, the spectrometer port is sealed with a port plug. A 1/4-20 threaded mounting boss is fitted at the sphere’s south pole position and each sphere is supplied complete with a post mount, post mount holder and sturdy base plate.