Technology

Light has four fundamental characteristics associated with it: wavelength, amplitude, polarization and phase. The first three of these properties are relatively easy to control using for example filters, beamsplitters and interferometers. However, it is harder to control the phase of light.


What is meant by the “phase”?

In the natural world, we often see the phenomenon of synchronization. For example, these could be dolphins jumping above the sea or a school of fish swimming together and quickly changing direction. However, the light does not ordinarily have a synchronized phase and there are a number of applications that would be extremely beneficial if light was made to have such a property. By only being able to control the phase of the light without any change in the intensity and rotation of polarization state, the Hamamatsu range of Liquid Crystal on Silicon Spatial Light Modulators (LCOS-SLM) have become an extremely powerful optical tool for finely controlling laser beams and removing wavefront distortions in applications ranging from Laser Micromachining and Beam Shaping to Optical Tweezing and Microscopy studies.


There are many different kinds of Spatial Light Modulators (SLM) and they differ markedly in their construction and operation. However that said, all SLMs are adaptive and programmable optical devices that manipulate (modulate) light that is either transmitted through or reflected by them. This manipulation can be with respect to the amplitude, phase or polarization of the incident light. SLMs are composed of a number of movable micromirror arrays or Liquid Crystal based microdisplays that can be controlled or addressed by simply connecting the SLM to a computer which is used as an external monitor. Moreover, there are several ways in which the addressing can be achieved ranging from optically or electrically addressed or through a CMOS backplane. What is so powerful about SLMs is that they control the corresponding spatial positions independently.


However, there are disadvantages associated with micromechanical SLMs that use deformable micromirror arrays compared to Electro-Optical SLMs. Micromechanical SLMs are larger, require both higher drive voltages and more complicated computational requirements, suffer strong interactions and are less robust (and more fragile) than their Electro-Optical SLM counterparts.


Within the Electro-Optical, Liquid Crystal (LC) based SLM family, there are again differences in their construction and operation. The first distinction is if they work by either translucent or reflective means.  Although easier to integrate into an optical system, translucent SLMs have a significantly lower Optical Fill Factors and overall have lower light utilization efficiencies than reflective SLMs which limit their practical usefulness.


On the other hand, Reflective LC based SLMs use Liquid Crystal on Silicon (LCOS) devices that have Optical Fill Factors and Efficiencies of > 90% in some cases. The two most common types of LC materials used in their construction are either ferroelectric or nematic types. SLMs based on ferroelectric LCs technology enable just two different molecular orientations and are used for display applications that require high speed switching frequencies in the kHz region. This contrasts with Nematic LC based SLMs in which the molecules can be orientated parallel, vertically aligned or as a helical “twisted” structure. Hamamatsu’s LCOS-SLM uses parallel aligned nematic LC and uses a CMOS backplane for the addressing. Using this construction in part allows pure phase modulation to be achieved which is so important for the applications described above.


Hamamatsu Photonics has been developing SLM technology for over 30 years now and have much experience in their design and construction. Early models range from optically and electrically addressed versions, using ferroelectric and twisted nematic LC and now Hamamatsu Photonics design and manufacture a wide range of LCOS-SLMs in-house at the factory allowing each LCOS-SLM to be optimized to the user’s laser and application.


How does the Hamamatsu LCOS-SLM operate?


Please see below for the configuration of the LCOS-SLM.


An active matrix circuit is formed on the silicon substrate for applying voltages to the pixel electrodes. The phase is modulated by the parallel-aligned liquid crystal layer and the amount of phase modulation varies according to the applied voltage level. The dielectric mirror will increase the light utilization efficiency to > 90%.


Software is used to convert a phase image into intensity values (0 to 255, 8 bit) for each pixel using a Look Up Table (LUT) which enables an LCOS-SLM image to be created that consists of a 2D phase distribution. This is then consequently transmitted to the LCOS-SLM itself.



What kind of applications can the LCOS-SLM be used for?


There are many different kinds of applications that LCOS-SLMs have been used in and these can be summarized into 4 product areas:


Laser Material Processing

Adaptive Optics & Opthalmology

Optical Tweezers & Micromanipulation

Structured Illumination, Laser Beam Shaping & Pulse Shaping


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