Optically based semiconductors are far from mainstream and most people would associate this technology with devices such as gallium arsenide (GaAs) or gallium nitride (GaN) based LEDs – or possibly the CMOS image sensors that are rising in popularity very rapidly, especially in security and automotive applications. Recently, micro electro-mechanical systems (MEMS) have become more popular, integrating mechanical components with semiconductor technology. Now, leading manufacturers are integrating optical elements, meaning that a new sector – micro-opto electro-mechanical systems (MOEMS) – has emerged.

 

Integrating CMOS signal-processing elements into micro-machined solutions has provided switching and control solutions to the data centre and telecom industries for some time. Precision alignment of optical fibres to facilitate connections via ferrules or grooves that were etched on the surface of a silicon substrate was one of the earliest examples of this technology being used in the telecoms industry. Precision etching can also be used to create selective interference, which is a key requirement for filtering and separating channels for wavelength division multiplexing (WDM). 

 

One significant benefit if silicon based micro assemblies is that they can be tuned, meaning that such filters can be tuned. A displacement in the order of tens of nanometres is sufficient to tune a filter for fibre-optic wavelengths. The most common approach is to split the filter grating into fixed and moving elements with the movable element being moved electrostatically in response to changing charge levels in closely located circuitry. These moving parts are often based upon a cantilever structure and this technology has also been used in optical switches where micro-mirrors are moves to reflect light from a source into one of many receiver channels.

 

One of the more sophisticated MOEMS implementations is Texas Instruments’ (TI) digital light projector (DLP) products that have formed the basis of projectors and other devices for the past ten years or so. The DLP array is based on a Cartesian grid and consists of a huge number of cantilevered micro mirrors that swivel based upon the presence (or absence) of electrostatic energy. A fixed light source is projected onto the mirror array and, by changing the charge, mirror angles can be adjusted so that small beams of light can either be reflected onto a target surface, or towards a light absorbing area, effectively turning ‘off’ the pixel associated with that mirror. Rapidly moving the mirror so that the light flicks from one area to the other can be used to adjust the brightness levels, in proportion to the mark/space ratio – the more time spent pointing to the absorbing area, the darker the pixel is, and vice-versa.

 

DLP is behind a significant amount of modern projection of audio visual content and is prevalent in cinemas and becoming increasingly so in our homes as projection TVs become more popular. However, innovative designers are now finding new applications for DLP in our homes – especially as many devices become smaller and lack the space for a sophisticated display. By combining DLP with short-throw lenses, control panels for home technology such as a thermostat or alarm system can use any flat surface as a human-machine interface (HMI). By integrating a vision sensor or small camera, gesture recognition software can detect the user’s inputs and ensure that the system responds accordingly. This approach gives a better user experience as small, fiddly buttons can be avoided, and home devices can be smaller and more aesthetically pleasing.

 

The applications for this technology are virtually unlimited and, as well as acting as the HMI, these DLP-based systems can also provide notifications or alarms. For example, if a door or window is open when an alarm is being set this can be shown. Instructions for tasks, such as cooking, can be displayed right in the work area, to avoid having to refer back to a printed book, which may not be conveniently placed. Any device can display its status on any flat surface – the floor, a wall, and a work surface, even the ceiling. Not only can devices become smaller as front panel space is no longer needed for an HMI, they also become more reliable as there are no switches or keypad that can wear out or fail prematurely. The display can always be turned on or off by a simple, non-contact, wave of the hand or other gesture.

 

The third dimension 

The growth of 3D printing could also benefit from DLP technology, using it to direct the lasers that form the 3D objects. Virtual Reality (VR) or Augmented Reality (AR) can also benefit, as DLP is both high resolution and low latency, key requirements of these kinds of systems. By projecting an image onto the glass windscreen in vehicles, DLP can also be used to create a sophisticated head-up display (HUD) which improves the driver experience, and road safety.

 

DLP technology from TI is now available in resolutions from 640 x 360 to 1920 x 1080 to address applications from simple HMIs through to high-end smart home requirements. Additionally, IntelliBright (an application that can auto-adjust brightness and contrast to match different types of projection surface and ambient light conditions) is often directly supported.

 

While MOEMS remains far from significant in the context of the overall optoelectronics market, its importance in controlling light to be useful in modern applications has been recognised. Smart homes are already putting it to use in many applications and it is expected 

to find significant uses in wearable devices, healthcare and Internet-of-Things (IoT) applications in the short to medium future. Clearly the potential of MOEMS is only just starting to be understood.

 

About the Author:

Mark Patrick is the Supplier & Technical Marketing Manager EMEA of Mouser Electronics. Mark joined Mouser Electronics in July 2014 having previously held senior marketing roles at RS Components. Prior to RS, Mark spent 8 years at Texas Instruments in Applications Support and Technical Sales roles and holds a first class Honours Degree in Electronic Engineering from Coventry University. For details, contact Helen Chung, Asia PR Specialist of Publitek, on email: helen.chung@publitek.com