Harnessing light with semiconductor Spatial Light Modulators
bertrand.noharet@acreo.se
Quantum well infrared photodetectors (QWIP) are image detectors produced at Acreo to detect infrared light radiations. Based on a similar technology, ultra-fast quantum well light modulators are now being developed to modulate and control light beams for a wide range of new applications.
During this past information technology decade, optics has often been associated with the detection, transport, storage or processing of information, and this fast-expanding field has recently been coined “Information Optics”. Looking at the coming decade and beyond, it is anticipated that Information Optics will continue to grow steadily as a research and development area for the telecommunication, datacommunication and machine vision industries.
SLMs for Information Optics
Spatial Light Modulators (SLMs) enable spatial modulation of light beams and are therefore key components in most free-space Information Optics systems. SLMs are large 2-D arrays of densely packed light modulators. Typical numbers of individual modulators, or pixels, are ranging from one thousand (1,000) to more than one million (1,000,000) within dozens of mm2. SLMs are usually electrically addressed: light modulation, be it amplitude, phase or polarisation, is controlled by independently applying electrical signals to every modulator cell. As programmable diffractive masks, SLMs can steer and transform light beams. As arrays of parallel and independent modulators, SLMs can convert electrical communication channels into parallel optical communication channels.
Several SLM technologies have been developed, from liquid crystals to magneto-optic materials. Intense efforts have recently been focused to the development of ferroelectrical liquid crystal and micro-mechanical mirror SLMs. These candidate techniques offer very good modulation properties, but their shortest response times are typically of the order of 10-100 µs. While this speed limitation may not be critical for most conventional applications, it restricts nevertheless the interest of these techniques for new high-speed applications.
Semiconductor SLMs
In order to reach higher modulation speeds, resonant electroabsorption effects in semiconductor materials can be advantageously used. The principle of resonant electroabsorption modulation is based on the rapid variation of absorption spectra of quantum well structures when a field is applied transversally to the wells. Applying a 5V voltage to such a structure can for example turn a material from opaque to transparent at a specific wavelength, thus achieving amplitude modulation of light within 1 ns (see figure ”Principle electroabsorption modulator”).
Semiconductor SLMs do not require current injection, and are very low power consumption devices. Semiconductor modulator arrays are commonly driven and synchronised by integrated Si CMOS drivers (figure right above). Semiconductor SLMs are therefore the results of hybridisation by flip-chip bonding of the semiconductor modulator chip with the Si ASIC driver chip (figure right below.)
Harnessing light
Owing to their very high speed, semiconductor SLMs are unique and could be useful for a wide range of applications:
- Programmable laser beam steering
- I/O multiplexing device in fast-access holographic memories
- Fast switching, for example for optical packet switching in telecom networks
- Pointing and tracking, for example to establish and maintain wireless optical communication links
- Free-space communication
- Long distance (up to several kilometres): fast transversal modulator for the transfer of information in a retromodulation configuration
- Short distances (below 10 cm): dense array of communication channels between parallel processors
- Optical information processing
- Optical pattern recognition: automatic comparison of the content of input images with a large database of reference images, for example for automatic personal identification based on biometrical parameters.
- Microwave photonics: filtering and correlation of analogue microwave signals, optical control phased-array antennas
- Pulse-shaping: modulation of individual spectral components of each light pulses for encryption and optical code-division multiple-access applications.
ACREO’s MQW SLM
For the last 6 years, ACREO has been involved in the field of optical information processing and beam steering. During this period, it has clearly appeared that these fields are to a very large extend relying on the availability of high-speed SLM devices, and that semiconductor SLMs may today be the most promising technology when speed is an critical issue.
At ACREO, we are currently conducting a 5-year research project on the development of multiple quantum well SLMs. First prototype SLMs, whose performances have been specified to compete with today’s state-of-the-art liquid crystal solutions, are to be evaluated during June 2001, with the following expected performances:
- 128x128 pixels
- 38 µm pixel pitch, 80% fill factor
- 3 modulation levels
- 10 000 frames per second
Next SLM prototypes will be specified together with interested industrial partners.