Stefan Schidl

Optical Wireless Communication With Adaptive Focus and MEMS-Based Beam Steering

An optical wireless communication system for an operation with wavelengths detectable by silicon optoelectronic integrated circuits is described. We use direct modulated vertical cavity surface emitting lasers as a transmitter. The field of view of the laser beam is adjusted with an adaptive optical system and aligned with a micro-electro-mechanical system based mirror for beam steering. To receive the modulated laser beam, we develop a receiver chip in 0.35 μm BiCMOS technology. The experimental system shows a 3 Gb/s wireless transmission over a distance of 7 m with a bit-error rate <;10-9 without cost intensive optical components and complex adjustment procedure.

PIN Photodiode Optoelectronic Integrated Receiver Used for 3-Gb/s Free-Space Optical Communication

In this paper, an optoelectronic integrated receiver chip including five PIN photodiodes will be presented. A large 200-μm diameter photodiode connected to a high-speed transimpedance amplifier works as a 3-Gb/s receiver for optical wireless communication. Four surrounding photodiodes allow for the adjustment of the incoming laser ray. The complete chip was realized in a silicon 0.35-μm BiCMOS technology to benefit from the available intrinsic zone in this technology. Due to this intrinsic zone and an antireflection coating, the responsivity reaches a value of more than 0.5 A/W for wavelengths from 630 to 760 nm. Furthermore, the capacitance of the center photodiode is less than 0.6 pF for reverse bias voltages larger than 3 V. For proof of concept, a steerable and adjustable light source was built based on a micro-electro-mechanical system mirror, on a focusing unit, and on a direct modulated vertical cavity surface emitting laser with a wavelength of 680 nm. The complete system is capable of establishing a 3-Gb/s data transfer over a distance of 19 m at a BER of <;10-9, and over a distance of 18 m at a BER of <;10-12.

Linear Mode Avalanche Photodiode With High Responsivity Integrated in High-Voltage CMOS

A fully integrable avalanche photodiode fabricated in a 0.35-μm standard high-voltage CMOS process is presented. The device achieves a high unamplified responsivity (M = 1) of 0.41 A/W and a maximum responsivity (M = 6.6 · 10⁴) of 2.7 · 10⁴ A/W for 5-nW optical power using a 670-nm laser source. The maximum bandwidth of 850 MHz was measured at 500-nW and 5-μW optical power which results in a responsivity bandwidth product of 17.4-GHz · A/W for a gain of M = 50.

Correction of the temperature induced error of the illumination source in a time-of-flight distance measurement setup

A systematic investigation of the combination of a reference path and a reference pixel as correction method for a systematic error induced by the light source of a time-of-flight (TOF) distance measurement sensor is presented. A change of the bandwidth of the light source, e.g. caused by drifting temperature of the used LEDs results in a bandwidth dependent distance error. The presented method allows reducing this error over a large operating range by ~97 %, i.e. to 3 %.

CMOS chip with multi junction photo detector for sensing biomedical signals

Optical biomedical signals are of great interest for diagnostic purposes because of their non-invasiveness. Conventionally a single photo detector is used to acquire the optical signal of a single wavelength at a time. In this work we propose a sensor chip which employs a multi junction sensor with three vertically stacked photo diodes with different wavelength dependent responsivities. This enables simultaneous registration of multiple optical biosignals at different wavelengths, e.g., as required for the monitoring of blood oxygenation, a vital biomedical parameter. Thus energy consumption and time of the monitoring can be reduced. The detector and an on chip amplifier are realized in a standard CMOS process (0.35μm, triple well) without any process modifications. The on chip amplifier has a logarithmic transfer characteristic to cope with a high dynamic range of the optical input signals which leave the transilluminated skin region. The realization of this sensor chip is discussed and the resulting photoplethysmograms are demonstrated.

Vertical triple-junction RGB optical sensor with signal processing based on the determination of the space-charge region borders

A triple-junction RGB optical sensor with vertically stacked photodiodes and signal processing that provides precise values of the currents generated by blue, green, and red light is presented. The signal processing is based on the determination of the border depths of the space-charge regions of all three photodiodes. A current-mode implementation using current conveyors and variable-gain current amplifiers is introduced. The responsivities of all three photodiodes calculated using the proposed approach are in very good agreement with the measured results.

Corrections to “Optical Wireless Communication With Adaptive Focus and MEMS-Based Beam Steering” [Aug 1 2013 1428-1431]

In the above letter [1] , it was assumed that the concomitant angular displacement of the laser beam is equal to the deviation angle of the MEMS mirror. However, according to the reflection law, the angular displacement of the reflected beam is twice the angle through which the mirror has rotated [2] , see Fig. 1 . Therefore, the field of view (FOV)—the sum of the maximum positive and negative deviation angle of the laser ray,—is twice as large as the previously published numbers and the following corrections should be made.

Advances in triple junction photo diodes

Triple junction photo diodes are a type of photo diodes where three pn-junctions are vertically stacked on top of each other. Each of these pn-junctions has a distinctive spectral responsivity. Therefore, this type of photo diode is very useful for attaining a color sensitive photo diode for sensor applications. These diodes can be implemented in different standard semiconductor processes. In this work we compare three different implementations in BiCMOS (0.6 μm) and CMOS (90 nm and 0.35 μm) processes.