Glass Additive and Subtractive Manufacturing of a Fiber Optic Rotary Encoder for Downhole Pressure Sensing

This paper presents a fiber optic rotary encoder-based pressure sensor for downhole applications. The rotary encoder, which is composed of an encoding pad and an all-glass optical fiber sensing head, converts the rotation angles of a pressure transducer to digital codes. The optical fiber only serves as the telemetry channel to directly transmit the data in digital format, such that the environmental temperature influence and the long-time drift are minimized. The glass additive and subtractive manufacturing techniques are used to embed the multi-channel optical fibers into a bulk-fused silica glass substrate. Because the fused silica glass shares a similar material property to the optical fiber (e.g., thermal expansion coefficient), the optical fiber sensing head is well sealed and fixed even at an elevated temperature. The picosecond laser blackening technique is used to fabricate the high reflectivity contrast encoding pad with high accuracy. The proposed pressure sensor was manufactured and experimentally verified to have a signal-to-noise ratio of 23 dB and a linear pressure response with a root-mean-square error of 0.37. During the 400-hour 250°C long-term stability test, there was no digital code variation. In addition, a mathematical model to study the relationships between the sensor's performances and design parameters were established. According to the model, a 12-bit encoder (0.02% pressure sensitivity) can be achieved with a radius of 20 mm.