Why use fiber optic delivery?

Instrument builders in both biotechnology and semiconductor manufacturing are under constant pressure from their customers to create more functional, more reliable and higher-performing equipment.  In this pursuit, singlemode fiber optic delivery of the laser beam is increasingly being seen as a critical technology.

Fiber optic delivery offers smaller, neater instrument design.
The beauty of a fiber optic laser delivery system is in the flexibility it gives the instrument designer to route the laser beam.  It allows the laser source to be separated from the instrument front end, and, when used with a gas laser such as He-Ne or Argon ion, fiber optic delivery protects the instrument from the heat and vibration generated by the laser.

Fiber optic delivery improves the performance of the instrument.
When used with a diode laser, fiber optic delivery actually improves the quality of the beam by suppressing the higher order transverse modes and transmitting a single traverse mode of light that is almost Gaussian in profile.  This removes the need for additional spatial filters.

All types of lasers exhibit transverse jitter, or beam pointing instability caused by thermal and opto-mechanical instabilities in the laser cavity – typically in the region of 30 microradians per °C.  Use of a singlemode fiber optic delivery reduces this dramatically – Point Source designs offer a pointing beam stability of better than one microradian per °C regardless of the laser technology used.

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Dispelling the myths of fiber optic delivery

The core diameter of a singlemode fiber operating within the visible wavelengths range is typically about five microns.  Because of this, the fiber system is sometimes perceived as being inefficient, difficult to use and potentially unstable.

In fact, the technology has developed significantly in the last 15 years to overcome these concerns. 

Careful design of the input optics to mode-match the laser beam parameters to the fiber achieves excellent results – typically 70% coupling efficiency or better.

The latest systems are exceptionally easy to use and highly repeatable.  For example, the Point Source system takes about two minutes to align and suffers less than 0.5% loss over 100 repeat insertions. This has been achieved through a combination of sub-micron factory optical alignment that guarantees beam position and beam angle of less than 100 microns and 200 microradians respectively, and detailed kinematic design of the fiber-to-laser interface.

The last concern with singlemode fiber is that, because it has a circular cross-section, it degrades the polarization of the laser.  This is because when it is bent or coiled, the stress optic co-efficient of the glass induces birefringence.  The outcome is two modes of propagation, which recombine with an unpredictable phase relationship at the output, resulting in polarization drifting and fading.

This problem is avoided in modern systems by the use of polarization-preserving fiber which is not optically symmetrical and has strong internal birefringence cased by stress-applying sectors. The internal birefringence is significantly higher than normal bend-induced levels, so as long as the laser is correctly aligned to either of the two axes, polarization is preserved.

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