Opto Speed (OS), an innovative company in the field of optical and opto-electronic components for fiber based optical communication, includes in its product portfolio photodiodes, semiconductor lasers, optical amplifiers, and optical receivers. The components for future 40 Gb/s systems (OC-768/STM-256) are under development.
An optical receiver transforms optical signals into electrical ones and conditions them for further processing.
Blockdiagram of the optical receiver
The photodiode at the input of the receiver converts the incoming light into an electrical current which is transformed into a voltage by the transimpedance amplifier. A threshold control circuit, together with an external regulation electronic, generates symmetric differential signals. The limiting differential post amplifier conditions thesignal levels for the proceeding clock and data recovery circuit.
The goal of this CTI project was to develop next generation optical receivers. We therefore designed and laid out the 40 Gb/s circuits at IME and Opto Speed provided the high speed transistor and packaging technologies.
In order to meet the high speed requirements, not a traditional silicon technology but an Indium Phosphide (InP) based technology was used to realize the circuits. Besides to excellent high frequency characteristics of this semiconductor, this technology offers the possibility to monolithically integrate the photodiode and the transistors on one chip. Therefore, the length of the critical connection between photodiode and electronic circuit is minimized. Furthermore, the packaging of the circuit is simplified.
Since the receiver operates at the full data rate of 40 Gb/s, all circuits have to be carefully optimized with respect to speed. A crucial issue is the layout, i.e. the location and interconnects of the circuit components (transistors, resistors, etc.) on the chip. Line lengths of only 10 µm already influence the characteristics of the circuit.
Chip photo of a 40 Gb/s receiver chip
In order that the receiver can be deployed in a system, it has to be mounted in a high frequency package. A critical aspect is the coupling of the fiber to the circuit. The fiber has to be aligned with tolerances of a few micro meters in order to focus the whole light beam to the photodiode. Moreover, the connection of the electrical 40 Gb/s signals is very demanding.
40 Gb/s receiver module
The optical receiver was characterized using a Lightwave Component Analyzer. Bandwidths between 33 and 40 GHz and optical-electrical gains between 200 and 800 V/W were reached. The eye diagram shows the correct functionality at 40 Gb/s
40 Gb/s eye diagram
Within the scope of this two year CTI project (5286.1. FHS), we developed, fabricated, and characterized 40 Gb/s optical receiver front-ends. The fabrication, the development of the transistor and packaging technology as well as the qualification was done by OS. Circuit design and layout were the main tasks of the IME. The major focus was on optimization of the high frequency characteristics. Furthermore, the characterization of the chips and modules were partially carried out by the IME. The total project volume was 1’237 kCHF, whereof a federal contribution of 380 kCHF was paid to the IME.
By the contribution of the CTI, a close collaboration between University of Applied Sciences and the industry was made possible. OS could push the development of new products due to more human resources. The IME extended and directly employ its know-how in high speed circuits design as well as in semiconductor technologies. This is fundamental for the IME in order to be an attractive and well qualified partner for the industry in the field of high-speed IC design.
Contact : Dr. Alex Huber, IME