Displacement Phase Modulator (DPM™)
Endless Applications with Phase Modulation
Increases in computing processing and new techniques for holographic control as well as other technological advancements such as increased laser power have made phase modulators more usable and more desirable for applications ranging from material processing to sensing to medical. The thin-film MEMS technology that enables the high-speed phase elements of Silicon Light Machine’s Light Valve technology is uniquely suited for creating ultra-fast phase modulators with exceptional power handling. Ribbon-based modulators 1D and piston-based 2D devices currently support UV-VIS modulation, while NIR versions up to 1550 nm are in development.
Phase Modulation for Beam Forming or Holographic Control
Principle
An extremely simple method of phase modulation.
Silicon Light Machines’ MEMS phase modulators operate on an extremely simple method of phase modulation: piston. The 1D phase modulators are composed of electrostatically actuated ribbons while the 2D phase modulators faceplates on electrostatically actuated flexures.
In either case, a reflective surface is deflected by a voltage, causing incident light on that pixel to travel further, creating a phase delay in reference to the quiescent state.
Up to one half wavelength is required to create a 2 modulation at normal incident, at which point “phase-wrap” allows for full holographic control suitable for most applications.
Note that additional deflection is needed at a non-normal angle of incidence because the axial phase modulation is reduced by the cosine of the angle of incidence.
A = Active Ribbon
1D Phase Modulation with electrostatically actuated ribbons
2D Phase-only faceplates on electrostatically actuated flexures
Advantages
Speed, power, and precision for demanding applications.
High Speed
Due to their low mass, high tension and short stroke, the 1D phase micro-ribbons can switch in less than 300 ns. This is more than ten times faster than the DMD tilt mirrors and one thousand times faster than liquid crystal spatial light modulators. The GLV®’s high-switching speed facilitates high-line refresh rates typically between 250–500 kHz. The piston-based 2D phase pixel is bulkier than its ribbon cousin, but switching speeds greater than 200 kHz are still possible.
High Power Handling
The 1D phase modulator is made using the same robust silicon-nitride comprising the GLV® that endows it with unparalleled power handling capability. GLV®s have been used for years in harsh industrial applications with incident powers of 80W per device at power densities as high as 10kW/cm2. The 2D phase modulator also has robust piston faceplates based on the same technology used in our very high-power laser processing optical heads.
Analog Phase Modulation
The basic piston modulation of the MEMS devices leads to natural analog phase modulation. Phase resolution is limited only by the bit-depth of the electronic driver. Drivers may be configured to offer a linearized phase-DAC response.
Non-contact, High Reliability
Unlike other MEMS spatial light modulators, both the ribbon and Displacement Phase Modulator (DPM™) are non-contact devices and require no lubricants inside the package. This attribute has proven to be a significant form of reliability, especially in high-fluence UV applications.
Broad Wavelength Possibilities
While a single wavelength should be used at a time, the simple piston method of phase modulation means that wavelength range of a device is only limited by maximum deflection of the MEMS reflector. Current devices support UV through green, while red through NIR – 1550nm devices are in development.
Imaging & Optics
Flexible modulation techniques for precision beam shaping and wavefront control.
The phase modulator may be used in many different configurations depending on the application e.g., as a programmable filter in a 4F system or as a holographic projector in a Fraunhoffer or Fourier set-up.
The illumination of the 2D phase modulator is therefore dependent on the use-case and may be as simple as collimating a fiber output to match the modulator aperture to using a custom lens for purely uniform illumination.
The 1D phase modulator does have a uniquely high aspect ratio, requiring line illumination. In both the 1D or 2D case, a minimal angle of incident of less than 15 degrees is recommended.
Line illumination for the 1D phase modulator can be generated from laser sources by a variety of methods including a Powell lens, a DOE (Diffractive Optical Element), or a cylindrical lens. For illustration purposes, a Powell lens is shown in the figure. The circular beam from the laser is dispersed uniformly in angle in one axis.
A “slow axis” collimator is then used to collimate the rays into a “top-hat” profile of uniform intensity along the phase modulating long axis. Next, an orthogonal “fast axis” cylindrical lens is used to focus the beam onto the device.
Typical numerical apertures for the fast axis focus are between 0.01 and 0.1. The full width of the line illumination should be less than one third of the ribbon length (50–75 µm). A smaller waist (~25 µm) is often used to minimize lateral positioning precision at the expense of axial positioning.
Example voltage-deflection curves may be provided with a modulator. These curves may be manipulated for desired phase modulation of differing wavelengths and angle of incidences by referring to the equation θ = 4π d sin(φ)/ λ; the phase delay θ is equal to 4 times pi times the deflection d times the cosine of the incident angle φ divided by the wavelength λ.
Otherwise in situ calibration can be achieved by directly measuring the wavefront response using a device such as a Shack–Hartmann wavefront sensor or deflection using a confocal microscope or by measuring the intensity response of a rolling diffraction grating pattern after 4F filtering, as shown.
Pixels
Resources
Further information and application notes
Beamforming for Optical Communications
A presentation on the use of high-speed MEMS phased arrays for beamforming in Free Space Optical (FSO) and Light Fidelity (LiFi) communication systems.
Controlling the Grating Light Valve™ in Real-Time Applications
Technical discussion on the methods and systems for controlling GLV®, PLV™, and DPM™ devices in applications that require real-time adjustments and feedback.
MEMS Displacement Phase Modulator for Quantum Computing
Silicon Light Machines’ high-speed MEMS Displacement Phase Modulator (DPM™) is a breakthrough technology enabling scalable, precise, and ultra-fast optical control for neutral atom quantum computing applications.