Understanding FOG Light Sources: SLDs vs Laser Diodes in Fiber Optic Gyros

The light source is one of the most critical components in a fiber optic gyro. It determines the stability, noise performance, and overall accuracy of the interferometric measurement that enables rotation sensing. While mechanical gyros depend on moving parts, FOG systems rely entirely on the behaviour and quality of light. For this reason, choosing the correct light source is essential for delivering consistent heading accuracy in marine navigation.

This page explains the differences between Superluminescent Diodes (SLDs) and laser diodes, the two most common optical sources used in FOG systems. It also describes how these choices influence performance, reliability, and suitability for marine applications.

Why the Light Source Matters in Fiber Optic Gyros

The performance of a fiber optic gyro begins with the characteristics of its optical source. Light quality influences the phase shift measurement, the stability of the interference pattern, and the accuracy of the final heading output.

Impact on accuracy

The accuracy of a FOG is directly affected by:

Wavelength stability

Power consistency

Coherence properties

Sensitivity to back reflections

If the light source fluctuates, the interference pattern becomes noisy. This reduces the gyro’s ability to detect small phase differences and introduces errors into the navigation output.

Signal stability

Marine vessels operate in environments with:

Temperature fluctuations

Continuous vibration

Electrical noise

Shock events

A stable light source ensures that these environmental factors do not disturb the optical signal. Stability is essential to maintain performance during long voyages and in rough sea conditions.

What is a Superluminescent Diode (SLD)

Superluminescent Diodes combine characteristics of both LEDs and lasers, producing stable light, broadband, and low noise. For many marine-grade FOG systems, SLDs have become the preferred optical source.

How it works

An SLD emits light through spontaneous emission, which is then amplified internally. This produces a beam that has:

High output power

Broad spectral width

Low coherence length

The low coherence length is important because it suppresses interference effects from parts of the system that are not meant to influence the measurement.

Technical advantages

SLDs offer several key benefits in fiber optic gyros:

Very low coherence noise

Reduced susceptibility to back reflections

High stability over time

Low phase noise

Consistent output power

Minimal interference with environmental changes

These characteristics lead to improved bias stability and more reliable long-term accuracy, which makes SLD-based FOGs highly attractive for marine operations.

What is a Laser Diode

Laser diodes emit highly coherent, narrow-linewidth light. They are commonly used in communication systems, sensors, and optical devices that require strong directional beams.

Overview

Laser diodes produce light through stimulated emission, which creates a beam that is:

Highly coherent

Narrow in spectral width

Directional and intense

These features make laser diodes extremely efficient but also sensitive to environmental disturbances.

Performance characteristics

Laser diodes offer:

Higher intensity

Longer coherence length

Precise wavelength control

However, the coherence length and optical intensity can introduce drawbacks in a FOG application, particularly when dealing with:

Back-reflected light

Coherent noise

Mode hopping

Temperature-induced wavelength shifts

For these reasons, laser diodes require more complex stabilisation and control when used in interferometric sensors.

SLD vs Laser: Which is Better for FOG

Both Superluminescent Diodes and laser diodes can be used in fiber optic gyroscopes, but their behaviour under operational conditions determines which is more suitable for marine navigation. The comparison below explains why most modern FOG systems use SLDs as the primary light source.

Coherence

Coherence describes the uniformity of the light wave. It affects how light interacts with reflections, imperfections, and environmental disturbances.

Laser diodes have very high coherence. This increases the sensitivity of the gyro but also increases noise caused by back reflections and fiber imperfections.

SLDs have lower coherence. This reduces unwanted interference and leads to a cleaner and more stable interference pattern.

Lower coherence results in better long-term bias performance, which is critical for heading accuracy.

Noise

Laser diodes can exhibit:

Mode hopping

Relative intensity noise

Coherent backscatter

These noise types directly affect the gyro output and can cause instability in measurement.

SLDs produce broadband light with significantly lower noise, which improves the reliability of the FOG output in real-world marine conditions.

Bias drift

Bias drift refers to the gradual deviation of the gyro output over time.

Laser-based FOG systems may experience drift due to wavelength instability and temperature variation.

SLD-based systems show superior bias stability, maintaining accurate measurements across long voyages and varying environmental conditions.

This stability is one of the main reasons SLDs are preferred for marine-grade optical gyros.

Environmental stability

Marine environments challenge sensors with vibration, temperature fluctuation, and humidity.

Laser diodes are sensitive to temperature changes, which can shift the wavelength and affect accuracy. Additional control electronics are often required to manage these environmental influences.

SLDs provide greater resilience because:

They have lower sensitivity to temperature

They exhibit reduced nonlinear behaviour

They maintain stable output across a wider thermal range

This leads to more predictable performance in the unpredictable maritime environment.

How EDGE Engineers Choose the Right Light Source

Selecting the correct light source for a fiber optic gyro requires a balance of optical performance, environmental resilience, and overall system stability. EDGE applies marine-grade engineering principles to ensure that its FOG systems operate reliably on all vessel types.

Marine grade design considerations

EDGE evaluates each light source using criteria that include:

Coherence properties suitable for stable interference

Low noise characteristics for clear signal processing

Long lifespan for reduced maintenance

Consistent performance under vibration

Stable behaviour across temperature variations

Resistance to back reflection and optical interference

These considerations ensure that the final gyro unit meets the operational expectations of bridge officers, fleet managers, and technical teams.

Product implementation in UdhaK SX 1

The UDHAK SX 1, EDGE’s flagship fiber optic gyro compass, incorporates an SLD-based design to ensure:

Stable heading performance

Low drift

High noise immunity

Fast settling time

Compatibility with marine navigation systems

By using a carefully selected SLD source, the UDHAK SX 1 achieves high accuracy and reliability while maintaining the robustness required for continuous maritime operations.

Conclusion

The choice of light source plays a fundamental role in the performance of a fibre optic gyro. While laser diodes are powerful and efficient, their high coherence and sensitivity to environmental change can introduce instability in an interferometric sensing system. Superluminescent diodes, on the other hand, offer broadband, low-noise light that supports stable, high-precision measurement in demanding marine conditions.

For this reason, most modern marine-grade FOG systems use SLDs as their optical source. Their stability, resilience, and low noise characteristics make them ideal for continuous heading reference applications on commercial vessels, offshore units, defence craft, and autonomous platforms.

EDGE’s engineering team incorporates these advantages into the UDHAK SX 1, ensuring that each FOG unit performs reliably across global maritime environments.