Single-mode fiber coupler
 

   Aligning LASER light into the core of a single-mode fiber requires submicron resolution and a high level of stability. For this demanding task, most of today's fiber couplers are either flexure designs or motorized and automated alignment systems. Both configurations are expensive.  The SM-CFTx fiber coupler replaces these costly systems with a simple concept that uses the optical lever-arm effect, or "light fulcrum", to increase the resolution of the system (see attached applications note). 

   An asphere with essentially infinite magnification is used to maximize single-mode coupling efficiency - fiber-coupling aspheres focus to a diffraction-limited spot.  In our system, the ratio of the focal lengths is approximately 14:1, so a 20 degree rotation of the adjustment lens translates the focused spot by 1 micron.  This makes it easier to center the focused spot on the single mode fiber end face by minimizing adjustment overshoot common to single mode adjustments.

   This compact fiber translator uses our vibration-resistant OTX.5 series multi-axis optic positioners, which can take a 3G jolt without moving, as the basis for the design.  100 TPI hex adjustment screws provide accurate sub-micron translation.  The Y-axis knob uses a unique capture screw design to ensure no drift due to overloading in that axis.  For added stability after alignment, we have added non-influencing axis locks to maintain stability in challenging environments.

   The SM-CFTx uses an asphere to couple to the fiber, not a microscope objective.  Even a very high quality 100X objective, for example, will only take a 1 mm beam down to a 10 micron diameter, and a commonly used 20X to a 50 micron focused spot.  Since single-mode fibers have 5 -10 micron core diameters, most of the energy remains uncoupled. 

   Coupling efficiency is maximized when the NA of the fiber is closely approximated by the NA of the coupling optics, a technique known as " mode matching ".  Dry high-power microscope objectives, in general, have numerical apertures in the 0.8 range.  Lower-power objectives, with closely matching NAs, have focused spots much larger than the core diameter of single-mode fibers.  The asphere we use has an NA of 0.15 - much closer to the low NA characteristic of most single-mode fibers. 

   SM-CFTx fiber translators are compatible with our CFC ( connectorized fiber ) and BFC ( bare fiber ) fiber chucks.  They are compatible with most commonly used connectors.  A solid brass body with finger cutouts is used to facilitate the attachment of the connector to the chuck.  The graduated dial is marked 360° in 15° increments - great for polarization-maintaining fibers.