Single-mode fiber coupler
Single-mode fiber coupler
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
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.
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.
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