The measurement of the power output of laser diodes or fibers is a daily routine in the field of telecommunication components testing. Optical power meters using a bare detector claim high sensitivity but at a cost of potential measurement inaccuracies caused by the effects of polarisation, local saturation, signal 'bounce-back' and beam misalignment. A welcome alternative is the integrating sphere. Through multiple internal reflections, the sphere offers high attenuation for high power measurements. Also through multiple internal reflections, measurement errors caused by polarization effects with flat-field detectors do not occur. The sphere port diameter can be enlarged by increasing the sphere diameter which allows measurement of larger diameter beams. The radiation output of connected or bare fibers can be measured using fiber optic connectors or adapters mounted onto the front of the sphere. The sphere geometry will nullify any errors caused by misalignment or polarization effects. Because of the small source size and the limited power levels in these applications the sphere diameter can be small. This reduces cost while increasing system sensitivity. To reduce measurement error, particular attention must be placed on the design of the shadow screen (baffle) in front of the detector. This screen must ensure that no direct radiation from the laser source is detected. Only reflected signals should reach the detector.

Lasers with collimated (parallel) beams are typically measured with a flat-field detector, whose active size is larger than the laser beam diameter. Because of the high power of lasers, the responsivity of the detector may have to be reduced by an attenuation filter. But there is a risk of measurement errors due to polarization effects, surface reflections from optical surfaces in the light path and misalignment of the beam on the detector.

Lasers with uncollimated (divergent) beams cannot be measured with a flat-field detector because of the different and wide angles of incident radiation. The power output of these lasers is typically measured with detectors mounted to an integrating sphere to collect all incoming radiation independent of the angle of incidence. The integrating sphere approach has other advantages. Due to multiple internal reflections inside the sphere, it is a good attenuator, ideal for high power measurements. Maximum power capability is only limited by the sphere’s upper operating temperature limit. Also, through multiple internal reflections, measurement errors caused by polarization effects with flat-field detectors are limited. The sphere port diameter can be enlarged by increasing the sphere diameter which allows measurement of larger diameter beams creating greater adaptability of measurement for different lasers and beam sources.
 
Information for these applications can be found under:

 Products section: • Laser Power / Laser Stray-Light Detectors
                           • X9₁₀: Power Meter for Telecommunication Testing
                           • X9₃: Laser Power and Laser Stray-light Meter 
                           • PT-9610: Laser Qualification Meter 

 Tutorials section: • Measurement of Light with integral Detectors (III.1.b./III.2.a) 
                                 • VI.8. Lasers Measurements  


 Download section: Light_Detectors.pdf (1.2 MB)

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