Effect of spatial coherence on damage occurrence in multimode optical fibers

Area of Science:

• Optics and Photonics
• Materials Science
• Laser Physics

Background:

• High-fluence delivery of ultraviolet (UV) laser pulses in multimode optical fibers is challenging due to optical damage.
• Damage often initiates below the fiber facet threshold, caused by localized "hot spots" from constructive interference.
• Reducing spatial coherence is crucial for mitigating such damage.

Purpose of the Study:

• To investigate the impact of spatial coherence on optical damage in multimode fibers for UV laser delivery.
• To establish methods for controlling spatial coherence to enable high-energy transmission.
• To achieve record energy delivery levels in the UV spectrum.

Main Methods:

• Utilizing a large-diameter core fiber (1.5 mm) as a mode scrambler to reduce spatial coherence.
• Employing different lengths of the mode scrambler fiber to control coherence.
• Observing damage occurrence in a smaller core diameter fiber (0.6 mm).
• Performing numerical simulations using the beam-propagation method.

Main Results:

• A direct correlation was found between the degree of spatial coherence and optical damage occurrence.
• Damage was typically observed near the fiber facet, linked to spatial coherence.
• Numerical simulations confirmed that high-order mode excitation degrades spatial coherence.
• A new record of over 100 mJ UV energy delivery was achieved using a 1.5 mm core fiber.

Conclusions:

• Degrading spatial coherence is an effective strategy to prevent optical damage in multimode fibers.
• This method allows for significantly higher energy delivery of nanosecond-pulsed UV laser radiation.
• The findings pave the way for advanced applications requiring high-power UV laser transmission through optical fibers.