Nowadays, Radio over Fibre technology (RoF), which can integrate both advantages of fiber optic networks and wireless has been studied intensively, due to numerous applications in broadband wireless communications (e.g. WLAN, GSM, UMTS). In contrast to other transmission media, optical fiber has superior properties, such as light weight, low attenuation loss, small size, and especially the large bandwidth and insensitivity to electromagnetic radiation. These advantages make it the optimal solution for efficiently transporting radio signals in wireless network.

Fig. 1: Radio over Fiber System [1]

A typical RoF system is shown in Figure 1. Generally, direct modulation of semiconductor laser, e.g. distributed-feedback laser diode (DFB LD) is used to achieve the most cost effective solution in comparison to others, such as Electro-absorption Modulator (EAM), Mach-Zehnder modulator (MZM).  In an RoF system, both modulation and photodetection devices can contribute to the link distortion, which degrades the linearity of optical links [2]. Therefore, it is essential to suppress nonlinear distortion to improve the performance of the system.

When two frequencies are presented simultaneously to a DFB LD, harmonic distortion and intermodulation are generated.  Although both harmonic and intermodulation distortions arise from the same underlying mechanism, second-order intermodulation and third-order intermodulation are normally larger than second and third harmonics, respectively. Fourth order and higher orders terms typically are considered as negligible because of low power levels, or they fall well outside the operational bands. Here, we target third intermodulation compensation due to its strong influence on the system.

Fig. 2: Block Diagram of Predistortion Circuit in Radio over Fiber System [3].

Among various linearization techniques, two general methods: feedforward [4, 5] compensation and predistortion [6, 7], are adopted to suppress the nonlinear distortion generated by lasers to meet such stringent requirements for linearity. The feedforward technique usually requires a greater number of components such as additional laser diodes, photodiodes, and optical couplers, resulting in higher costs and complexity of the system. Comparing with feedforward techniques, predistortion is superior since it is cost-saving and easy to implement. The predistortion approach inserts an inverse transfer function nonlinear component in front of the nonlinear laser, as illustrated in Figure 2. The cascaded stage realizes improved linearity performance than the laser itself.

A typical analog laser predistorter was introduced in [6]. In this work, the RF signal is separated into two paths: one linear path consists of a time delay line and the other nonlinear path for quadratic-law and cubic-law generation. The magnitude and phase of the distortion generation path can be adjusted through phase shifters and Variable Gain Amplifier (VGA). Finally the linear path and the correction signal are recombined and fed to the laser. In this predistorter prototype, linearity is achieved at the expense of increasing cost and system complexity since phase shifters, VGA, and extra power splitters are used. Predistortion circuit also can be designed using standard CMOS technology [7]. Multiple tank cells and tunable gain cells involved in this predistorter design lead to additional power dissipation, and its operation bandwidth is only 300MHz.

In this work, we are developing a low-cost, high efficiency, broadband analog CMOS predistortion circuit to reduce third-order intermodulation of a DFB laser, which can be widely used in multiservice RoF industrial systems. Such a design can be easily integrated with other components. Power dissipation, noise, and bandwidth are optimized in this design. In the future, this design approach can extended to suppress higher order distortions in universal strategy of compensation. A prototype design in 90 nm CMOS is planned.

[1] Ng’oma, “Radio-over-Fibre Technology for Broadband Wireless Communication Systems,” Ph.D. thesis, Electrical Engineering, Eindhoven Univ., 2005, page 19.

[2] Charles H. Cox, Edition III, Analog Optical Links Theory and Practice, Chapter 6, Cambridge University Press, 2004.

[3] X. Fernando, “Radio over Fiber-An Optical Technique for Wireless Access,” Ryerson communications lab, Toronto, Canada [Online]. Available:
 http://cms.comsoc.org/SiteGen/Uploads/Public/Docs_Globecom_2009/XavierROFT16.pdf, Page 10.

[4] T. Ismal, C. Liu, J. Mitchell, A.J. Seeds, “High-Dynamic-Range Wireless-Over-Fiber Link Using Feedforward Linearization”, IEEE, J. Lightwave Technology, vol. 25, no. 11, Nov. 2007.

[5]H.K. Cheung, I.D. Robertson, V. Postoyalko,  S. Iezekiel, “Nested Loop Feedforward Linearization of
Directly Modulated Laser Diode”, IEEE Topic Meeting on Microwave Photonics, Oct. 2009.

[6] L.Roselli, V. Borgioni, F. Zepparelli, F. Ambrosi, M Comez, P. Faccin, A, Casini, “Analog Laser Predistortion for Multiservice Radio-Over-Fiber System”, IEEE, J. Lightwave Technology, vol. 21, no. 5, May 2003

[7] Z. Xu, L. MacEachern, “A Predistortion Circuit Design Technique for High Performance Analogue Optical Transmission”, IEEE, Microsystems and Nanoelectronics Research Conference, 2008.