18 Adaptive Electronic Linearization of a Coherent Heterodyne Optical Receiver J. Basak and B. Jalali University of California, Los Angeles, Department of Electrical Engineering, Los Angeles, CA 90095, USA Abstract - We demonstrate the electronic linearization of a coherent heterodyne receiver in a phase modulated optical link. Third order nonlinearity suppression of 7.2 dB and spurious free dynamic range (SFDR3) improvement of 2.4 dB is obtained with an adaptive CMOS linearizer circuit. Index Terms - optical communications, electro-optic modulation, phase detection, optical receivers, heterodyne. I. INTRODUCTION High linearity and large dynamic range are the primary requirements for analog fiber optic links deployed in military as well as commercial applications such as antenna remoting, phased array antennas, wireless communications, and CATV [1, 2]. Externally modulated links using the linear electro- optic effect are an appropriate choice for such applications. However, it is well known that the intensity modulated links using this effect suffer from nonlinearities in the electrical-to- optical transfer function, thus limiting the achievable Spurious Free Dynamic Range (SFDR) at the transmitter end [3]. Transmitter linearization by electronic feed-forward [4] and pre-distortion [5, 6, 7] methods have been successfully used to improve the dynamic range of analog optical links to a certain extent. In contrast to the intensity modulated links, phase modulated links using the electro-optic effect, are very linear at the transmitter end. In such links, detection of optical phase is achieved by introducing interference between two optical paths. The most popular optical phase detection method is the Coherent Heterodyne (or Homodyne) detection. A simplified model of a coherent detection link is shown in Fig. 1. For linear operation of the link, a constant 900 phase shift needs to be maintained between the signal and the LO laser. This is achieved by the Optical Phase Locked Loop (OPLL) shown in Fig. 1. The OPLL is primarily used for frequency synchronization of the LO laser with the signal laser. This results in significant phase noise reduction in the system and ensures high fidelity of the phase modulated data. The OPLL can also be designed to maintain a constant phase difference between the LO and the signal laser. The analysis and design of an OPLL have been discussed in detail in several earlier works [8, 9]. In recent times, the availability of fiber lasers with linewidths as low as 1-3 kHz [10, 11] have made the design of an OPLL much easier by relaxing the need for large frequency bandwidth of the required feedback electronics, and that for a short loop-propagation-delay. This has resulted in ________________ Transmitter Signal Phase Laser Modulator l d Photoetector Detected Receiver Fig. 1. Block diagram of a phase modulated optical link. phase modulated (PM) links becoming less complex to design and hence a more viable option. However, despite the linearity of the transmitter, these links are limited by nonlinearity at the receiver end [12, 13]. A brief analysis of the PM link is presented here to emphasize the effect of coherent detection scheme on its linearity. The electric fields of the modulated signal and the LO lasers, respectively, are given by Es = ->-P exp k0s t + v9m (t) + f9n,S )] ( l) ELO 2PLO exp[j(wLOt + pnfL0)] (2) where Ps (PLO), coS (WLO) and (Pn,S (qJn,LO) are the optical power, frequency and the phase noise of the signal (LO) electric fields, and ym(t) is the phase modulating signal. Taking into account the 90° phase difference introduced between the signal and LO lasers, and the 3 dB loss in the coupler, the output of the photodetector can be written as: pd = (PS +LO (3) + 7S7 PsLO Sin(l(WFt + 9 m(t) +9 n,S f n,LO) where, (9JF= S-WLO and il is the responsivity of the photodetector. Assuming that the OPLL is appropriately locked, (Pn,S-(Pn,LO << (pm(t) << 1. Expanding (3) under these assumptions gives the photocurrent to be: Ipd = (PS+PLO) + 17 P5 PLO [cos(JIFt) sin(Ym (t)) + sin(wJFt) CoS(Ym (t))] 2 (PS + PLO) (4) &0C3 (t) 2 +t) + '17 P-s 'PLO r sCOS(WFt)(/m (t) + COS(wJ-Ft) 3! nc-I )(i +! s 2(wFt 0-7803-9542-5/06/$20.00 ©2006 IEEE