When focusing a free-space laser beam into an optical fiber, submicron positioning accuracy (using high-precision and expensive mechanical mirror mounts and fiber mounts) is typically required to optimize fiber-coupling efficiency. An alternative from TEM Messtechnik (Hannover, Germany) called FiberLock instead uses a single-mirror piezoelectric actuator to not only optimize initial alignment, but to actively compensate for mechanical and thermal drifts that occur over time.
The FiberLock feedback setup places the piezo-driven mirror between the laser and coupling lens; an output mirror taps a portion of the beam with a photodetector. Mirror tip/tilt (the scanning range of the actuator is usually two orders of magnitude larger than the fiber core diameter) enables a coarse or "blind" scan that provides a 3D visualization of the coupling efficiency. Once locked into a coarse position, FiberLock adds small modulations to the x and y positions of the beam waist that disappear at the photodetector when coupling is optimized, actively compensating for any drifts in the fiber tip position or changes in the incident beam. Since the device only stabilizes two degrees of freedom, manual adjustments of the focus lens and fiber position are still necessary to optimize alignment; however, these adjustments are minimal compared to six-degree-of-freedom adjustments required for passive alignment setups. Furthermore, FiberLock can be used to compensate for intensity noise and pointing fluctuations of the laser, at the expense of some optical power. Contact Lee McCrumb at [email protected].