Fiber and Photonics Alignment Algorithms

Precision motion mechanics, such as Aerotech’s FiberMax_HP, are a critical component for high-quality results with efficient throughput in modern fiber alignment and silicon photonics alignment processes. However, the hardware for your precision motion equipment is not the only part of the system to consider for alignment applications. The control software that optimizes the motion is just as critical as the hardware that is optimized for performing the alignments. Faster, more effective alignments mean improved alignment quality and higher throughput, resulting in more efficient production.

Aerotech’s A3200 controller has built-in alignment algorithms that can be used out-of-the-box for a variety of fiber and silicon photonics alignment needs. Aerotech’s fiber alignment routines are used in a variety of motion systems and for a variety of fiber aligning applications.

First-Light Alignment Algorithms

Aerotech’s first-light alignment algorithms are used to perform rough positioning before running a fine alignment algorithm to fully optimize the fiber alignment. FIBER SPROUGH: The FIBER SPROUGH command, which stands for “Spiral Rough,” executes a spiral motion pattern, sampling the power during the profile, with the origin of the spiral being the axes coordinates at the time the command is executed.

The algorithm will sample power at a specified rate until one of the following conditions is reached:

1. SRThreshold: A user-specified Spiral Align 1power threshold that, upon being reached or exceeded, will cause the fiber alignment to terminate
2. SRMaxRadius: A user-specified maximum search radius where the alignment algorithm will terminate when the spiral has reached this radius.

Other FIBER SPROUGH parameters include spiral spacing and motion type, such as stepped sampling or, in most cases, continuous sampling. The FIBER SPROUGH algorithm is used as a first-light alignment to perform rough alignments before using a fine alignment algorithm to optimize the final placement of the fiber.

FIBER GEOCENTER: FIBER GEOCENTER is Aerotech’s second algorithm for finding first light in a fiber alignment. FIBER GEOCENTER executes a raster pattern of a specific size based on the GCScanSize parameter, and records each time the pattern passes through a specified threshold, GCEdgeValue. These records are made whether the path is going from high power to low or vice versa. Once the raster scan is completed, the algorithm calculates the geometric Geocentercenter of all of the GCEdgeValue points and uses that as the ideal rough alignment location.

For example, in the animation to the right, each time the algorithm crosses the higher power level (the light blue circle), a GCEdgeValue was recorded (yellow data points). The optimized rough alignment is then calculated as the green X.

Power Optimization Alignment Algorithms

Aerotech’s power optimization alignment algorithms are designed to fully optimize a fiber alignment to the maximum power position. Some of these require first light before they can operate, while others do not need first light.

FIBER SPFINE

The FIBER SPFINE Spiral Align 2algorithm works in a very similar way to the FIBER SPROUGH, except that it does not include a threshold parameter. The FIBER SPFINE command will run until it reaches the limit parameter SFEndRadius, which is the user-defined maximum radius of the spiral. Upon completion of the spiral path, the algorithm will finish by relocating the axes to the location with the highest power reading. Like FIBER SPROUGH, the spiral size, spacing, and number of samples can all be user-defined.

FIBER FASTALIGN (Requires First Light)

The FIBER FASTALIGN command uses an iterative process to explore an area and locate the optimized power location. The starting point must already be in the power distribution area. The user defines offsets for each axis (FAOffsetAxis1 and FAOffsetAxis2) to perform the iterative search, and then FASTALIGN will take a power reading offset from the origin in each axis direction. The algorithm will analyze which axes of motion caused the power level to rise, and will relocate the origin based on this information and then reiterate the process. The user sets a threshold as the FATermTolerance parameter, and once power reading differences are within this parameter, the search stops. The search will also end if the number of iterations exceeds a user-defined FAMaxNumiterations parameter. In either case, the axes will end up at the location where the best power reading is found. FIBER FASTALIGN3D, 4D, 5D, and 6D are also offered, and work with similar algorithms as FASTALIGN but are optimized for more than two dimensions.

FIBER CENTROID (Requires First Light)

The FIBER CENTROID Centroidalgorithm is a fiber alignment method that is particularly useful when the power peak is a plateau or has multiple peaks. FIBER CENTROID will move in one direction until it finds a power level reduction defined by the parameter CEdgeValue, and then it will reverse direction until it finds a similar drop in the opposite direction. Once this is complete, the bisection of these edges is defined as the centroid coordinate for that axis, and the process is reiterated for another axis. This process is effective at finding the center of a plateau-shaped power peak or the center of a power peak region with multiple, uneven peaks.

FIBER HILLCLIMB

The FIBER HILLCLIMB routine is used to search in a positive or negative direction along one axis at a time for a local power peak. If the peak is not identified in the first direction, the direction is reversed and the rest of the axis is explored. Once a peak is found, the algorithm returns to this position. Parameters such as maximum distance, sample increment, power threshold, and maximum displacement can all be defined by the user.

Which Algorithm Do I Use?

Aerotech’s fiber and photonics alignment algorithms can all be used in different types of fiber alignment applications. Depending on the Square and Gaussian Plotobjective of the application, the power distribution profile of the fiber, throughput requirements, and other considerations, different algorithms can perform the alignment better than others. For example, if the fiber’s power distribution profile is Gaussian (shown in blue to the right), as it is in many cases, the FastAlign algorithm will find the peak power location more rapidly than the other algorithms. However, in the event that the power is distributed in a flat top profile (shown in red to the right), FastAlign may not be the best algorithm to use. It will find a local maximum and possibly the absolute maximum, but given the power distribution profile, this maximum might be an edge of the power plateau rather than the center. This is because the power at the edge of the flat top profile is theoretically the same as the power in the middle. Often, the fiber will need to be aligned in the center of the flat top, rather than the edge. In this case, the centroid algorithm would prove to be the most effective at finding the center of the flat top.

Similar application-specific considerations are important when deciding whether to use the SPROUGH algorithm or GEOCENTER algorithm when doing a first-light alignment. While GEOCENTER will guarantee that the fiber is alignedSpiral Align 2 in the center of the defined power area, it takes significantly longer to execute a GEOCENTER algorithm than a SPROUGH. For most applications, SPROUGH will provide a good enough first-light starting point to execute a power optimization algorithm. In some scenarios, however, a GEOCENTER algorithm may be the preferred solution. Often, the best solution for an alignment process is to use a combination of algorithms. For example, in the animation to the right, a SPROUGH command is used to find the first light of the signal, and a SPFINE command is used to optimize the position. Application requirements define which algorithm or combination of algorithms to use, which is why Aerotech’s fiber and silicon photonics alignment solutions offer users the full toolbox of algorithms to achieve their specific alignment goal.

Aerotech’s photonics alignment algorithms can be used with both standard components and custom mechanical systems. Whatever your alignment requirements, Aerotech has the hardware and software to assist you in your application.

Please contact your local Aerotech Applications Engineer for further information about Aerotech’s alignment algorithms or any of our other product offerings.