The phase delay (phase modulation) provided by a reflective liquid crystal on silicon spatial light modulator (SLM) depends on a number of things, including the applied control voltage, ambient conditions, and the characteristics of the incident light. When a manufacturer calibrates the SLM’s phase delay as a function of the applied voltage, the calibration is typically performed under specific operating conditions and for collimated light with a particular wavelength and angle of incidence. Since an application’s setup and operating conditions can differ significantly from the manufacturer's, it is often beneficial to recalibrate the SLM’s phase delay for each application.
This Video Insight demonstrates an interferometric method for calibrating the phase delay of reflective SLMs that are designed to modulate the phase without affecting the linear polarization state of the incident light. The calibration setup requires only a collimated, linearly polarized input beam and a camera, in addition to the SLM.
It is common to represent phase delay patterns using grayscale values, in which each shade of gray represents a specific phase delay. Calibration determines the specific phase delay provided for each grayscale value, under the current operating conditions. The calibration approach applies a uniform phase delay to one half of the SLM's panel. This side provides a reflected beam, as a mirror would. A binary phase grating is applied to the other half, which provides a diffracted beam. The reflected and negative first order diffracted beams overlap and create interference fringes, which are monitored during the calibration procedure. The voltage applied to the mirror side of the SLM is varied across the full range, and the fringes shift each time the applied voltage changes. The fringe shift is directly related to the change in phase delay.
A second video ( • Polarization Dependenc... ) made to accompany this Video Insight provides additional background information about the optical properties of the liquid crystal layer. The effects of the applied voltage on the liquid crystal layer’s refractive index are illustrated, as well as the effects of the layer on linearly polarized light with different polarization orientations. Note that the SLM provides unexpected results when the linearly polarized light is not aligned parallel with what is often called the SLM's optic axis, which is actually the rotation plane of the molecules in the liquid crystal layer.
00:00 Introduction
01:30 Phase Delays & Grayscale Values
04:28 SLM Calibration Approach
06:22 Linearly Polarize & Align Input
07:48 Placing the Camera
08:27 Applying Calibration Pattern
08:50 Half-Wave Plate Purpose & Use
09:34 Optimize Fringe Properties
11:25 Measurement & Analysis
Components used in this Demonstration Include:
- EXULUS-HD1/M Spatial Light Modulator: www.thorlabs.com/newgrouppage...
- CS165MU1/M Scientific Camera: www.thorlabs.com/newgrouppage...
- DJ532-10 Laser Diode: www.thorlabs.com/newgrouppage...
- LDM56 Laser Diode Mount: www.thorlabs.com/newgrouppage...
- ITC4005 Laser Diode Controller and TEC: www.thorlabs.com/newgrouppage...
- S130C Power Sensor: www.thorlabs.com/newgrouppage...
- PM400 Power Meter: www.thorlabs.com/newgrouppage...
- BE20M-A 20X Beam Expander: www.thorlabs.com/thorproduct....
- RSP1C Rotation Mount: www.thorlabs.com/newgrouppage...
- PRM1 High-Precision Rotation Mount: www.thorlabs.com/newgrouppage...
- LPVIS100-MP2 Linear Polarizer: www.thorlabs.com/newgrouppage...
- Half-Wave Plate : www.thorlabs.com/navigation.c...
- SM1M10 Lens Tube: www.thorlabs.com/newgrouppage...
- Universal Post Holders: www.thorlabs.com/newgrouppage...
- Optical Posts, Ø1/2” and Ø12 mm: www.thorlabs.com/newgrouppage...
- BA2F Flexure Clamping Base: www.thorlabs.com/newgrouppage...
- CF125 Clamping Fork: www.thorlabs.com/newgrouppage...
- CS1 Cable Strap: www.thorlabs.com/newgrouppage...
- TC4 Balldriver Caddy: www.thorlabs.com/newgrouppage...
For more photonics how-to videos, visit www.thorlabs.com/newgrouppage...