WHITE PAPER Most optical networks have many fiber couplings and even minor losses at these junctions will produce significant signal losses that cause problems in data transmission. Precise fiber alignment at the optical couplings in a network is therefore a prerequisite for accurate and reliable optical data transmission since it produces the least signal loss before assembly or packaging of an optical system. Minimal signal loss also results in the lowest optical power requirements which, in turn, means fewer repeaters, lower capital costs and reduced incidence of failure. Alignment Parameters and Procedures Effective fiber alignment requires the precise adjustment of a precision motion control device and a suitable search algorithm that has been optimized for use in the alignment application. Figure 1 shows a typical search operation along with the positional parameters that are associated with optical fiber alignment. In the search procedure, the intensity of a well-characterized optical input beam (the laser diode in Figure 1) is compared against the output signal of the optical fiber being aligned. Positional/Rotational Parameters Motion controllers are employed that use a coordinate system in which an object is considered to have six degrees of freedom: three linear position parameters, along the X, Y, and Z-axes in a Cartesian co-ordinate system and three rotational parameters around those axes (see Figure 1(b)). All movements are defined in terms of translations along and/or rotations about the Cartesian axes. The fiber position is moved through a raster scan to detect first light - when the laser beam first enters the optical fiber (Figure 1(a)). Once first light is detected, the lateral, longitudinal, and angular Figure 1. The operations and positional parameters of optical fiber alignment; (a) scan operations; (b) positional parameters for the optical fiber alignment. coordinates of the fiber are incrementally adjusted to maximize the intensity of the optical signal output from the fiber. In the simplest case, only lateral (X, Y) adjustments are necessary, while in multi-channel cases, adjustments to all six degrees of freedom (X, Y, Z, 0x,0y, and 0z) may be required (Figure 1(b)). Motion Control Parameters Linear or rotary motion stages produce the controlled motions and trajectories that move objects during optical fiber alignment. The following parameters must be considered when selecting a motion system for optical fiber alignment: • Minimum Incremental Motion (MIM) is the smallest increment of motion that a device can consistently and reliably deliver. It is the actual physical performance of the motion controller (as opposed to Resolution which is a theoretical capability and not a practical parameter) and can range from 100 nm to 1 nm. Smaller MIM comes at significant costs in terms of alignment speed and beam power increments. MKS Instruments’ XMS linear stages are capable of 1 nm MIM and 300 mm/s speed.

an MKS Instruments linear motion stage 250 ms after movement. The stage exhibits less than 20 nm variation in position stability after settling. • Other motion parameters include: axis alignment, location of the gimbal point, system stiffness, pitch/ yaw, thermal considerations, fixture design, Abbe error, etc. Figure 2. 1 nm MIM of an XMS linear stage; Insert – MKS Instruments’ XMS50-S Linear Motor Stage. Repeatability is the ability to repeatably position an object. It can be unidirectional (always approaching the target position from the same direction) or bidirectional (approaching the target...

• Centroid Search moves along one axis and finds a peak then moves along a second axis to find the final peak. Centroid searches are useful with top-hat or multi-peak profiles. • Dichotomy Search explores one axis at a time in large increments until a peak is identified. Within this peak, another search cycle is performed using finer steps to find the peak maximum. Motion Control Systems Different kinds of motion control systems can be employed in fiber alignment, ranging from simple manual stages suitable for small scale and R&D applications to fully automated production systems with high precision...

• Hexapods can encounter difficulties in scanning applications that require a specific linear, rotational or arc path to be followed. Figure 5(b), shows the motion of a standard hexapod when commanded to move from one point to another in the X-axis (blue line). The deviation from a straight line in the path can be up to a millimeter. MKS Instruments’ hexapods use RightPath Trajectory Control to minimize the run-out to a couple of microns, enabling the hexapod to more precisely follow specified linear, rotational or arc trajectories. • exaViz simulation − HexaViz is free, downloadable H simulation...

Table 1. MKS Instruments Components for Fiber Alignment Systems. Conclusion Fast, accurate, and precise optical fiber alignment is critically important to the efficient operation of optical communication networks. Poorly aligned junctions between fibers and between fibers and optical devices result in excessive signal losses in a network which, in turn, results in higher equipment costs to avoid excessive incidence of failure. MKS Instruments provides a suite of motion control systems, search software, and ancillary system components that are ideal for use in optical fiber alignment applications....

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