Optical Component
Light waves are modified in response to an applied electric field through a device that utilizes the electro-optic effect. In the context of paper manufacturing, an electro-optic phase modulator is used within laser-based sensing systems to monitor web thickness and surface uniformity at high production speeds. It allows for the rapid tuning of signal phases without the use of moving mechanical parts.
Refractive Modulation
Crystal properties inside the device change the refractive index when a voltage is applied, which delays or advances the light wave passing through it. This change is nearly instantaneous, allowing for megahertz-range switching speeds that are necessary for real-time data acquisition on a moving paper machine. By controlling the phase shift, the system can perform precise interferometric measurements of the coating weight or the moisture content of the substrate.
The integration of such high speed electronics into the sensor head enables the detection of minute variations in fiber distribution that would be invisible to slower mechanical sensors.
Sensing Accuracy
High stability and low insertion loss are required to ensure that the measurement remains reliable over long production runs. Any drift in the modulation response can lead to errors in the thickness reading, potentially resulting in rolls that do not meet the customer specification. These devices are usually housed in protective enclosures to prevent dust and heat from the mill environment from interfering with the sensitive optical crystals.
