Photonics - Raster Scanning using a Piezo Stage

Table of Contents

Introduction
Traditional Raster Scan
Raster Scan with Constant Velocity Ramp
Spiral Scan

Introduction

The nPoint LC.400 series of controllers possess a number of internal functions for use with raster scanning. It is possible to generate a traditional raster scan through the simple Raster Scanning GUI used in the traditional raster scan below.

Additional raster scanning patterns can be produced via the arbitrary scan capability of the controller. This provides complete flexibility for the type of scanning pattern, reducing the time needed to complete a scan.

In addition, the nPoint LC.400 controllers can simultaneously record up to 8 streams of internal data, including voltage measurement from the BNC analog inputs. In such cases, the analog inputs can be disabled for position command functionality and the recording capability can be retained.

This is useful in applications that benefit from correlating stage sensor readings with measurements such as fiber alignment intensity. The internal recording function is capable of sampling every 24 µs. For a given buffer size, the rate can be modified to record for extended periods of time. Sensor and BNC analog input values are recorded with 20 bit resolution.

In the following sections, the various raster scanning modes are demonstrated using the NPXY60Z20-257 stage.

Traditional Raster Scan

Constant velocity motion in the fast axis and stepping in the slow axis.

Pros

  • It is very easy to construct an image from the sampled data
  • Simple pattern to synchronize with external data acquisition

Cons

  • Data at the edges of the fast axis may have to be discarded due to tracking error when motion changes direction
  • The overall mechanical assembly may be disturbed by high frequency motion profile content and acceleration, resulting in noisy data acquisition

Traditional Raster Scan – Scanning this area with 50 lines gives a step size of 1.02 microns in the line axis (50 microns/49 steps). 5 ms dwell time is used at the beginning of each line to let the stage settle. The raster scan completes in less than 1 second.

Traditional raster scan pattern. Lines represent X vs. Y sensor data for a 40 X 50 micron section of the 40 Hz triangle raster scan.

Raster Scan with Constant Velocity Ramp

Sine wave is employed in the fast axis and single constant velocity ramp for the slow axis.

Pros

  • Smooth motion profile prevents noisy data acquisition by reducing mechanical disturbance
  • The entire data set can be used without discarding data when motion changes direction

Cons

  • Line spacing at the edges of the fast axis is twice the line spacing at the center

Raster Scan with Constant Velocity Ramp – With a typical 100 mA piezo drive controller (more current capability is optional), this stage is able to scan a 50 micron by 50 micron area with a 40 Hz sine waveform for the fast axis. Since this mode of scanning does not require dwell time at the end of each line the overall scan time is reduced. With 50 lines this scan completes less than 700 milliseconds.

Raster Scan with Constant Velocity Ramp Pattern. The blue line represent X vs. Y sensor data for a 50 x 50 micron section of the 40 Hz sine wave scan.

Spiral Scan

Pros

  • Constant line spacing
  • Very smooth motion

Cons

  • Scan covers less area for a given fast axis frequency and amplitude

Spiral Scan – With a typical 100 mA piezo drive controller (more current capability is optional), this stage is able to scan a 50 micron diameter area at 40 Hz. With 1 micron spacing, this scan completes in less than 700 milliseconds.

Spiral Scan Pattern. The line represents X vs. Y sensor data for a 50 micron diameter 40 Hz spiral scan.

With the same piezo stage and greater 200 mA current option, the system is capable of scanning at 80 Hz, covering an area of 50 µm diameter in less than 350 ms. This is the fastest scan accomplished with this system under the conditions tested.

This information has been sourced, reviewed and adapted from materials provided by nPoint | nanopositioning and motion control.

For more information on this source, please visit nPoint | nanopositioning and motion control.

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