Spatial coherence at the output of multimode optical fibers

Area of Science:

• Optical physics
• Fiber optics
• Coherence theory

Background:

• The spatial coherence of light emerging from optical fibers is crucial for various applications.
• Understanding how modal dispersion affects coherence in multimode fibers is an ongoing challenge.

Purpose of the Study:

• To directly measure the modulus of the complex degree of coherence at the output of a step-index multimode optical fiber.
• To investigate the influence of monochromatic and broadband pumping on the output coherence.
• To develop and experimentally verify a model for residual coherence.

Main Methods:

• Utilizing a lateral-sheering, delay-dithering Mach-Zehnder interferometer for direct measurement.
• Employing monochromatic and broadband light sources for pumping the optical fiber.
• Analyzing the relationship between coherence properties and fiber/source parameters.

Main Results:

• Monochromatic pumping consistently yields spatially coherent output.
• Broadband pumping leads to partially coherent output due to modal dispersion.
• The coherence radius depends solely on the numerical aperture.
• Residual coherence is influenced by the number of modes and modal dispersion relative to source coherence time.

Conclusions:

• Modal dispersion in multimode fibers significantly impacts spatial coherence for broadband sources.
• A simple model accurately describes the observed residual coherence.
• Experimental verification confirms the model's predictions regarding coherence properties.