EU photonic wireless project develops state-of-the-art high speed photodiodes
There is currently a great deal of interest in photonic generation of carrier frequencies around the atmospheric windows at the E-band (60 GHz to 90 GHz) and the F-band (90 GHz to 140 GHz) for the development of high capacity wireless systems in future femto-base stations. The number of operators offering FD-LTE (Frequency division -Long term evolution) and WiMAX based femtocells is expected to grow from zero to 25% by 2012. This is fuelled by recent market reports suggesting that the demand for higher data rates will continue to increase, eventually pushing the carrier frequencies towards the F-band. Wireless transmission in the E-band based on optoelectronic technology has already demonstrated data rates up to 12.5 Gb/s with On-Off modulation schemes, and reaching up to 27 Gb/s with spectrally efficient modulation techniques.
One of the most promising carrier generation methods is photomixing two optical wavelengths on a high speed photodiode, which in combination with injection and phase locking techniques can generate tuneable, high power, narrow linewidth millimetre carrier wave signals. The photodiode is a key component in these systems, being the element where the optical signals are converted to the electrical domain. Uni-Traveling Carrier Photodiodes (UTC-PDs) have been the dominant type of photodiodes for these applications due to their wide bandwidth and high millimetre-wave output power levels.
Fig. 1: The 120GHz dual photodiode.
The research conducted by the consortium of the European project iPHOS – www.iphos-project.eu -has achieved significant advances in this area. The project’s objective is to develop wireless communication systems for the licensed 71-76 GHz E-band frequency spectrum as well as for the license-exempt frequency range around 120 GHz. Recent results report the development and integration of high speed photodiodes for both frequency bands.
Worldwide, regulatory bodies have opened-up the frequency ranges 71-76 GHz and 81-86 GHz for enabling licenced broadband wireless communications. For such E-band Radio-over-Fiber systems, the consortium has developed compact and packaged photonic transmitter modules. The photonic transmitters feature high-power 1.55µm InP double mushroom waveguide photodiodes (PDs). For low-cost packaging, the PDs are hybrid integrated with innovative RF laminate-based 70/80 GHz GCPW-to-WR12 transitions. The packaged modules feature a single mode fiber input, a rectangular waveguide (WR12) output for connecting the antenna as well as DC connectors for biasing the PDs.
Fig. 2: The 71-76GHz transmitter module.
Besides the high-output power levels of the PDs of 0 dBm (1mW) in the 70/80 GHz bands and a high linearity, the developed transmitters also exhibit an excellent frequency flatness and return loss of below +/-0.5 dB and below -24 dB, respectively. Due to the high-output power, short-range 1 Gb/s broadband wireless transmission has been achieved without using any RF amplifier, neither in the wireless transmitter nor in the wireless receiver. What’s more, the excellent frequency flatness of the transmitter allows multiple carrier operation. For any carrier frequency within the entire 71-76 GHz band, the maximum power penalty for error-free (BER<10-9) operation has been measured to be below 1.2 dB. The consortium is now striving for integrating 70/80 GHz GaAs HEMT amplifiers in order to also enable broadband wireless communications over longer medium-range distances.
The other band of interest, the F-band, has many potential applications in the short term, being a license exempt range. The consortium has demonstrated the feasibility of monolithic integration of evanescently coupled Uni-Traveling Carrier Photodiodes (UTC-PDs) having a bandwidth exceeding 100 GHz with Multimode Interference (MMI) couplers. The significance of this advance has been to produce photodiodes compatible with active-passive photonic integration technology, achieving a high 3-dB bandwidth of up to 110 GHz and a generated output power of more than 0 dBm (1 mW) at 120 GHz with a flat frequency response over the microwave F-band (90-140 GHz). To enable linewidth reduction schemes, we have designed dual photodiode structures in which one provides the output signal and the other provides the signal to a stabilization loop. This opens the road for the development of narrow linewidth millimeter-wave photomixing sources on photonic integrated circuits (PIC). The possibility of developing a wireless transmission system on a large-scale PIC based on Indium Phosphide represents a significant technological innovation that will decrease size, power consumption and cost while improving reliability.
More information on the iPHOS project is available at www.iphos-project.eu
Bio
Dr. Guillermo Carpintero is Faculty Member of Universidad Carlos III de Madrid, in the Electronics Engineering department – He is the coordinator of the iPHOS project and can be reached at guiller@ing.uc3m.es
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