WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --Practical Examples of Parallel Alignment Automation

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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --PI’s Fast, Multi-Channel Photonics Alignment (FMPA)
technology is a set of firmware-level commands built into its
highest-performance digital nanopositioning and hexapod
controllers.
These commands allow fast coupling
optimization between photonic and other optical devices andassemblies, including optimization across multiple degrees-offreedom, inputs and outputs, elements and channels.
Importantly, these optimizations can often be performed inparallel, even if the individual optimizations interact.
Examples where significant process savings can be achieved
span the spectrum from multichannel Silicon Photonics
devices to LIDAR sensors to smartphone camera assemblies.

1 Serial versus parallelalignments
For example, in the short waveguides increasingly utilized in
Silicon Photonics (SiP) devices, the input and output couplings
can steer each other. As one side is optimized, the othershifts slightly and needs re-optimization. Formerly, thisnecessitated a time-consuming, serial sequence of back-andforth adjustments of the input, then the output, repeating
until a global consensus alignment was eventually achieved.
Similarly, when optimizing an angle, the transverse alignment
would be impacted and would conventionally need to be reoptimized, again in a time-consuming serial loop.
But with FMPA, these interacting alignments can often be
optimized simultaneously, in parallel. This allows a global
consensus alignment to be achieved in one go. Tracking andcontinuous optimization of all the alignments is also possible
in many circumstances, allowing compensation of drift, curing
stresses, and so on.
The results are much higher production throughput and often
dramatically lower costs. As devices become more complex
and precise, and as their production and test requirements
grow more demanding, this parallelism is increasingly criticalto process economics.

Fig. 1

Aligning the inputs and outputs of waveguide devices at anindustrial pace requires parallel optimisation and nanoscaleaccuracy.

1.1

Look for the loops

Fully exploiting this capability for maximum overall costsavings can require some different thinking than what one
might be used-to with classical alignment hardware. In
general, one looks for loops of sequential alignments, forwhich simultaneous optimizations can usually be substituted.
This article reviews a few sample applications and discusses
implementation issues to illustrate how this remarkable newcapability can be utilized to maximize productivity in test andpackaging.

2 Background of FMPA operation
The device alignment should be broken down into discretealignment processes. For example, probing a waveguide with
one input and one output using lensed fibers typicallyinvolves four alignment processes:
1.
2.
3.
4.

transverse optimization routine, input
transverse optimization routine, output
Z optimization routine, input (beam waist seek)
Z optimization routine, output (beam waist seek).

If the device has one or more additional inputs or outputs,
add as necessary:
5.
6.

theta-Z optimization routine, input
theta-Z optimization routine, output.

If the device requires optimization in theta-X and theta-Y,
add:
7.
8.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
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gimbaling optimization routine, input
gimbaling optimization routine, output.

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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --And so on. Dividing the overall alignment task into these subprocesses is key to identifying which processes can be
performed simultaneously.
With FMPA, you take the list of alignment routines and define
these directly into the controller. This only needs to be done
once (and can be changed or updated any time). Once aroutine is defined, it can be executed repeatedly. You can
execute more than one routine at a time —this is theparallelism!
Defining a process means instructing the controller whichaxes are involved in the process, which analog input presents
the quantity to be optimized (optical power, MTF…), andvarious process options. Give each process a name… thenumbers in the list we just made are perfect for that.

Fig. 3

Dividing a task like waveguide I/O coupling into sub-tasks like “1”and “2” shown will illuminate opportunities for parallel
execution. Here, the two processes can proceed in parallel eventhough they interact, especially in the case of short waveguides
where inputs and outputs steer each other. Similarly, processes
related geometrically (such as a transverse and Z optimization insituations such as shown, with an angled beam) can be
performed in parallel.

Fig. 4

NanoCube®, piezo-based, high dynamic, 3-axis scannerwith 100 µm travel range. Besides its nanoscale resolution
and blazing speeds, this flexure-based subsystem canperform continuous tracking without wear.

3.1

Long travel options

Routines are executed by calling the Fast Routine Start
command, FRS. Referring to the list we just constructed,
FRS 1 would commence the transverse optimization on the
input. FRS 2 would commence the transverse optimizationon the output. And FRS 1 2 would do both at once!

3 Types of alignment routines
For each side of the device, independent alignment engine
hardware is of course necessary. Any number of alignmentengines can be used; most common configurations utilize one
or two, but three or more will be increasingly common as SiP
technology matures.
Most often, each alignment engine is constructed of a multiaxis long-travel assembly and a shorter-travel, high-speed,
high-resolution piezoelectric multi-axis nanopositioning stage.
The modularity of the approach is a key benefit. Someapplications don’t require the long travel mechanism; some
applications don’t require the speed, resolution or continuous
tracking capability of the nanopositioning stage. In any case,
all FMPA algorithms and processes are virtually identicalregardless of the type of motion system involved; only thecapabilities will differ.

Fig. 2

F-712.MA2 high precision fiber alignment system.

For situations requiring no angular optimizations or arrayalignments, a stack of linear stages is sufficient. Otherwise ahexapod is required— not only in situations where full sixdegree-of-freedom positioning and optimization is necessary
but also in simpler situations, as the hexapod allows therotational centerpoint of even a single angular optimization
to be placed on the optical axis, at the beam waist, etc. Thisis vital for reducing parasitic geometric errors, another key toimproved overall productivity. Sometimes very long travel isnecessary in one or two axes for loading operations, and inthese cases the hexapod can be mounted on a long-travel
motorized stage. (The hexapod controller accommodates
two additional DC-servomotor axes. Alternatively, forcesensor elements can be integrated.)

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --|ε(θ)|=∇I=(I_min-I_max)/I_min
Equation 1: The observed gradient serves as a measure of alignment error.

From the observed modulation you can mathematicallydeduce the local gradient via a very simple calculation such as
Equation 1. Note that the gradient ∇I falls to zero at
optimum.

Fig. 5Single-sided fiber alignment system.

Any axes in an FMPA system can perform any of these typesof alignments (subject to the physical capabilities of the axes,of course). So, areal scans can be performed with motorizedstage axes, which can be very handy for finding first light.
Gradient searches are most familiar from transverse
optimization but they can also be performed (for example) ina single linear axis, which is ideal for localizing the beam waist
in a lensed coupling, or in a gimbaling fashion to optimize anangular orientation. There are many possibilities! These arehighly general-purpose algorithms suitable for all kinds of
optimizations, including bulk optic, cavity and pinholealignments.

4 The alignment processes
There are two types of processes: areal scans intended to
localize a peak within a defined region, and gradient searches
intended to efficiently optimize coupling (and optionally track
it to mitigate drift processes, disturbances, etc.).

4.1

Gradient searches

Gradient searches perform a small circular dither motion of
one device versus the other, which modulates the coupling.
The amount of modulation of the figure-of-merit being
optimized (for example, optical power or MTF) is a measureof the local gradient of the coupling. The modulation falls tozero at optimum (Fig. 6).

Fig. 6

Fig. 7

Optical power distribution

In general, a unique feature of FMPA is that different gradientsearches can be performed in parallel.
Transverse
optimizations tend to be the most sensitive and also the mostaffected by other alignments. So, transverse routines tend to
be relegated to high-speed, high-resolution piezoelectric
stages such as PI’s P-616 NanoCube. The high speed andcontinuous tracking capability of the NanoCube allows
transverse optimization to be maintained during Z andangular optimizations that would ordinarily require the timeconsuming, looping sequential approach.

Graphical depiction of gradient determination via a circulardither, which modulates the coupled power (or other quantity)
observed. The phase of the modulation with respect to thedither indicates the direction towards maximum while its
amplitude falls to 0 at optimum.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --4.2

Areal scans

Scanning an area to determine the approximate location of
the highest coupling peak is useful for a variety of tasks:First-light seeking.
Profiling for dimensional characterization of a coupling.
This can be an important process-control step.
Localizing the main mode of a coupling for subsequent
optimization by a gradient search. This hybrid approachhelps prevent locking-onto a local maximum and is very
powerful.

In addition to reducing the areal scan to a single command,
FMPA controllers have automatic curve-fitting capabilities
built in, plus a data recorder that can capture the profile onthe-fly for later retrieval, analysis or databasing. FMPA arealscans are very fast, 300msec or so for typical NanoCube
applications and loads. The curve-fitting capability can fit aGaussian to a fairly sparse scan (meaning an especially fastscan), allowing good localization of the optimum coupling
point without taking a lot of time to do a really fine scan.
Another capability is finding the centroid of a flat-top (“tophat”) coupling, such as seen when probing a deposited
photodetector with a single-mode fiber. This allows the scan
to terminate with the fiber at the geometric center of a flat ortilted top-hat coupling.

Fig. 8

Optical power distribution

Fig. 9Sinusoidal area scanFig. 10

Spiral area scanFig. 11

Uniquely, PI FMPA controllers can perform a fast areal scan andautomatically calculate and align robustly to the centroid
position of upright and tilted top-hat couplings.

Uniquely, FMPA’s areal scan options include single-frequency
sinusoid and spiral scans. These are much faster than
traditional raster or serpentine scans since they are truly
continuous and avoid the settling requirements of the stopand-start motions used in the traditional scans, and thefrequency can be selected to avoid exciting structural
resonances. A constant-velocity spiral scan may also be
selected, allowing data to be acquired with constant density
across the spiral.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --a.

5 Example 1: Wafer probing of
angle-insensitive devices
Even in the simplest case of a short waveguide device withjust one input and one output, the steering interaction
mentioned above can present a frustrating process
bottleneck. Add the additional alignments necessary forangle-sensitive couplings and array devices, and the situation
would grow complex and time-consuming very quickly.
Parallelism mitigates all this and makes short work of thetask.
For this example, let us first consider a planar waveguidedevice with a single input and single output, both accessible
for probing via diffractive couplers. Many thousands of suchdevices are commonly fabricated on large wafers, so
throughput is very important. The diffractive couplers
typically project the waveguide’s input and output out of thewafer at a typically 10-25° angle from the vertical. Often,
lensed probe fibers are used, so there is a fairly distinct
optimum separation along the optical axis. Wafer probers ofgood quality provide wafer placement accuracies much lessthan the 100µm × 100µm × 100µm field of view of the
NanoCube, so first light seeks are generally not needed on a
per-device basis in probing applications.

b.

To accommodate steering effects, loop:i.

Align one side to maximize throughput

ii.

Align the other side to maximize throughput

Move in Z and evaluate if the move direction
improved coupling

2. Repeat the above loops until optimized.
Overall time required is often many tens of seconds!
Using FMPA, the process is much simpler and can be two or
more orders of magnitude faster. Fundamentally, one
defines gradient searches 1-4 from the list (again, this only
needs to be done once, though any process can be modified
or re-defined freely) and then for each device:Issue the Fast Routine Start command: FRS 1 2 3 4

Execution is typically complete in a few hundred msec.
A single E-712 controller supports up to four P-616
NanoCubes, which can even be deployed on different
workstations— they don’t all need to be processing the samedevice.

Note that the optical Z axis is at an angle to the mechanical Z
axis for normal mounting of the stage stacks:
Z_optical ∦ Z_mechanical

Usually it is not desirable to tilt the motion assembly toaccommodate the angled optical beam since the mechanical
XY plane should remain parallel to the wafer to avoid
collisions. Consequently optimization motions in mechanicalZ must be accompanied by compensating motions in X and Y
to keep everything aligned.

This is an ideal case for parallelism! From the generic list ofalignment routines we compiled above, the first four apply:
1.
2.
3.
4.

transverse optimization routine, input
transverse optimization routine, output
Z optimization routine, input (beam waist seek)
Z optimization routine, output (beam waist seek).

Using traditional, non-parallel alignment technology, theconventional approach would be:

1. To accommodate Z_optical ∦ Z_mechanical, loop:

Fig. 12E-712 digital piezo controller.

5.1

Tracking and the completion criterion

A signature advantage of the gradient search is that it cannot
only optimize but also track its optimum. If you have several
gradient searches operating on a device, all can track
simultaneously. Alternatively, for your application you may
wish to align and then stop and hold position at the optimum.
This criterion—whether to align-and-stop or keep tracking—is
an example of a parameter you can adjust in the process
definition to fine-tune the process’ behavior to meet yourapplication goals; there are many such options. Since thePhysik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --process depends on the instantaneous gradient of thecoupling, it is natural to define its stop point in terms of thegradient. We call this the Minimum Level, or ML. Setting theprocess’ ML parameter to 0 means it should never be satisfied
and should track until commanded to stop. This is very usefulfor accommodating drift processes, such as in elevatedtemperature testing. Setting ML to a small but non-zerovalue causes the gradient search to terminate at the observed
optimum position. Note that ML=0 tracking should only be
performed with flexure-guided mechanisms due to thepotential for mechanical wear in mechanical mechanisms.

6 Example 2: Wafer probing of
angle-sensitive or array devices
Building on Example 1, you can accommodate angle-sensitive
devices and array devices by using a hexapod rather than an
XYZ stack of stages for the long travel motion. For many
applications a hexapod will provide sufficient resolution andspeed, otherwise (or when continuous tracking is needed) a
NanoCube can be attached. Again, the modularity of the
FMPA architecture yields considerable flexibility.
Alignment hexapods have many advantages over a stack ofconventional linear stages and angular positioners such as
goniometers. First, hexapods are full six-degree-of-freedom
devices, and their rotational centerpoint is freely settable
anywhere in space. This means you can rotate about a fiber
tip, a beam waist, a waveguide axis or any other opticallydesirable point in space.
PI hexapods present a sensible Cartesian coordinate system
to the user (X, Y, Z, θX, θY, θZ) and allow you to easily cast androtate that coordinate system as desired. Among otherthings, this means you can mount a hexapod on an angle
bracket to minimize overall footprint while still having an XY
scan plane parallel to a wafer or other important datum
surface.
Again the FMPA areal and gradient search capabilities (forlinear and angular axes), data recorder and profile fitting
functionality is built-into the controllers, yet costs are
typically less than a stage stack of equivalent resolution andmotion performance.

Fig. 13

Cascade Microtech‘s pioneering CM300 photonics-enabledengineering wafer prober integrates PI‘s Fast Multichannel
Photonics Alignment systems for high throughput, wafer-safe,
nano-precision optical probing of on-wafer Silicon Photonics
devices. Top: XYZ prober configuration. Bottom: 6-DOF hexapod
perober configuration. Courtesy Cascade Microtech div. ofFormfactor, Inc.

Consider the case of an on-wafer device with an array input,
output or both. On the sides with the arrays or other anglesensitive elements, the hexapod-based alignment engine ismounted.
Using traditional, non-parallel alignment technology, theconventional approach to optimizing this would utilize aneven more complex looping sequence:

For the first channel:−

To accommodate Zoptical ∦ Zmechanical, loop:

 To accommodate steering effects, loop:•
Align one side to maximize throughput•
Align the other side to maximize throughput−−
Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
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Move in Z and evaluate if the move direction
improved coupling
Repeat the above loops until optimized.
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WHITEPAPER – Practical Examples of Parallel Alignment Automation-- Scott Jordan --Increment in θZ.. It is unavoidable that the transversealignment will be degraded by this.
Repeat (A). Evaluate if the increment improved couplingin the Nth channel.
Repeat (A)-(C) until the both the first and Nth channels
are optimized. For most applications this will mean theentire array is optimized.

8 Author

Using FMPA, the process is again much simpler and vastly
faster. As in Example 1, one defines gradient searches 1-4
from the list, and then defines either a single-axis sinusoidal(“areal”) scan or a gradient search in θZ. Let’s call this process
5. (The gradient search requires some initial coupling, but theareal scan does not, so the choice of one or the otherdepends on the application, fixturing, device consistency, etc.
Also, setting soft limits to prevent a gradient search fromwalking away if optical power is cut is both prudent and easy.)
Then for each device:

Set the NanoCube to track continuously (ML=0 forprocesses (1)-(4)) for the first channel. Issue the Fast
Routine Start command: FRS 1 2 3 4With the NanoCube tracking the transverse and Z
couplings on both sides of the waveguide, issue the Fast
Routine Start command for the θZ process 5.

Scott Jordan is head of the photonics market segment in the
globally active PI Group, and a PI Fellow. He lives in Silicon
Valley and has been with PI for around 20 years; he wasactive as director of NanoAutomation technologies and madea decisive contribution to continued technologicaldevelopment of the company.
A physicist with an MBA in Finance/New Ventures, Scott iswell known in the community for his passion andengagement.

Execution is typically complete in less than a second.
Addressing gimbaling (θX/θY) alignment is similarly simple andfast with FMPA.

7 SummaryThis is intended to be an illustrative but not exhaustive
description. Similar sequences can be quickly devised for
packaging, chip-test and other applications. There are otherways of performing each of the examples we have presented,
and application considerations may recommend different
approaches or modifications. There are additional options
and parameters that should be considered in an actual
application. PI Applications Engineers can guide you, and weoffer effective on-site training and consultation services tospeed your implementation. But meanwhile we hope that thisoverview provides confidence that the productivity ofparallelism is accessible in your application.

9 About PI
Well known for the high quality of its products, PI (Physik
Instrumente) has been one of the leading players in the global
market for precision positioning technology for many years. PI
has been developing and manufacturing standard and OEM
products with piezo or motor drives for 40 years. All key
technologies are developed in-house. This allows thecompany to control every step of the process, from design toshipment: The precision mechanics and electronics as well as
position sensors.
By acquiring the majority shares in ACS Motion Control, aworldwide leading developer and manufacturer of modular
motion controllers for multi-axis and high-precision drive
systems, PI has made a major step forward in providing
complete systems for industrial applications with the highest
demand on precision and dynamics. In addition to four
locations in Germany, the PI Group is represented
internationally by fifteen sales and service subsidiaries.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

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WP4010E Practical Examples of Parallel Alignment Automation 04/2018 Subject to change. © Physik Instrumente (PI) GmbH & Co. KG 2018

Overall time required is typically multiple minutes.