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Optical modules with open-loop and closed-loop operation

Fiber and communications infrastructure knowledge brief Published 02 Aug 2023

Prepared for preliminary research by the ROGER FIBRE knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

Open-loop optical modules operate without feedback, offering speed and simplicity, while closed-loop modules use feedback to achieve higher accuracy and stability.

Overview

Optical modules, such as those used in adaptive optics (AO), fiber optic gyroscopes (FOGs), or Hall effect sensors, can be designed as open-loop or closed-loop systems. The distinction lies in whether the system continuously monitors its output to adjust its input: Open-loop systems operate without feedback. The system applies a predetermined correction or measurement without verifying the result.

Closed-loop systems continuously measure the output and adjust the input to minimize errors, maintaining precise control over the optical signal.

Adaptive Optics Applications

In adaptive optics, open-loop and closed-loop configurations affect how wavefront aberrations are corrected: Open-loop AO: The system applies corrections based on a model or pre-calibrated commands without real-time feedback. It is faster and can respond quickly to changes, making it suitable for good seeing conditions or when rapid phase compensation is needed. However, it is less stable over time and may accumulate errors if the system drifts or misaligns.

Closed-loop A

O: The system continuously measures the wavefront using a sensor and adjusts the deformable mirror to correct aberrations. This provides high stability and accuracy, especially under variable atmospheric conditions, but the response is generally slower than open-loop systems.

Fiber Optic Gyroscopes

For FO

Gs, the distinction is similar: Open-loop FOGs: Measure the interference pattern directly. They are compact, cost-effective, and suitable for medium-accuracy applications (1–10 °/h), such as UAVs or industrial robots. Their response can be slightly nonlinear at high rotation speeds.

Closed-loop FO

Gs: Use feedback to nullify the detected phase shift, achieving ultra-high precision (0. 001–0. 1 °/h).

They are essential for submarines, missiles, and aerospace systems, where accuracy and stability are critical.

Hall Effect Sensors

In Hall effect-based optical current or voltage sensors: Open-loop sensors: Measure the magnetic flux directly with a Hall device. They are small, lightweight, low-cost, and low-power, suitable for high-current measurements but less precise. Closed-loop sensors: Include a compensating coil and feedback circuit to balance the magnetic flux, providing higher accuracy and sensitivity for DC, AC, and complex waveforms.

Key Trade-offsFeatureOpen-LoopClosed-LoopFeedbackNoneContinuousAccuracyModerateHighSpeedFastSlower due to feedback processingStabilityCan drift over timeStable and self-correctingComplexitySimple, cost-effectiveMore complex, higher costTypical ApplicationsUAVs, robots, fast AO correction, low-cost sensorsAerospace, submarines, high-precision AO, sensitive measurementsSummaryOpen-loop optical modules are ideal for applications requiring speed, simplicity, and moderate accuracy, while closed-loop modules are preferred when precision, stability, and error correction are critical. The choice depends on the operational environment, required accuracy, and system complexity.

In adaptive optics, open-loop systems excel in fast phase compensation, whereas closed-loop systems ensure long-term stability and high-quality imaging. In gyroscopes and sensors, closed-loop designs provide superior precision at the cost of increased complexity and size.

Optical modules with open-loop and closed-loop operation - ROGER FIBRE ATELIER [PDF]

Optical modules with open-loop and closed-loop operation

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This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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