Session RF Photonics for CATV and HFC Systems (28-30 July) Reduction of 3 rd Order Intermod

Summer Topicals ’99CONTRIBUTED PAPER FORMPaper Title:Reduction of 3rd Order Intermodulation of a Semiconductor Laser by an Adaptive Low-Cost Predistortion Circuit at 1.8 GHzAbstract: A low-cost adaptive predistortion circuit to reduce 3rd order intermodulation of semiconductor lasers has been designed. Using a microcontroller to adjust thecontrol voltages the intermodulation has been reduced by 20 dB over thefrequency range of 1750 to 1870 MHz.Author: Guido SteinerAffiliation: Swiss Federal Institute of Technology Zurich, Laboratory for Electromagnetic Fields and Microwave ElectronicsAddress:Gloriastrasse 35City:ZurichZip:CH-8092Country:SwitzerlandPhone:+41 1 632 66 81Fax:+41 1 632 11 98E-mail:steiner@ifh.ee.ethz.chSession: RF Photonics for CATV and HFC Systems (28-30 July)Reduction of 3rd Order Intermodulation of a Semiconductor Laser by an Adaptive Low-Cost Predistortion Circuit at 1.8 GHzGuido Steiner, Stephan Hunziker* and Werner BaechtoldSwiss Federal Institute of Technology (ETH) ZurichLaboratory for Electromagnetic Fields and Microwave ElectronicsCH-8092 Zurich, Switzerland, e-mail: steiner@ifh.ee.ethz.ch* Contraves Space, CH-8052 Zurich, Switzerland, e-mail: czhs@mail.wee.ocag.chAbstract- A low-cost adaptive predistortion circuit to reduce 3rd order intermodulation of semi-conductor lasers has been designed. Using a microcontroller to adjust the control voltages the intermodulation has been reduced by 20 dB over the frequency range of 1750 to 1870 MHz.I. INTRODUCTIONAnalog fiber communications systems require high linearity. In fiber optic links the laser diodes (LDs) are the most important sources of nonlinearities [1]. By decreasing the 3rd order intermodulation (IM3) the spurious free dynamic range (SFDR) of the overall link improves. There are two major methods to reduce intermodulation: feedforward compen-sation [2] and predistortion [1], [3]. Feedforward compensation also reduces the laser intensity noise, but requires a greater number of components such as an additional LD, a photodiode and an optical coupler resulting in higher costs for the total system.A simpler approach is the predistortion technique. Predistortion can reduce the IM3 of the LD by generating an IM3 with the same amplitude but inverse phase. Unlike a feedforward system, the predistortion technique does not reduce noise. A common structure of a predistortion circuit is reported in [3]. The circuit consists of two paths: A linear path and a 3rd order distortion path with a cubic law device, an adjustable phase shifter and an adjustable attenuator.Using such a predistortion circuit a LD IM3-reduction of 23 dB over 25 MHz has already been reported [1]. The bandwidth and the IM3-reduction of the predistortion circuit described in [1] is limited due to the amplitude and phase deviations between the linear and the 3rd order distortion path. In order to reduce IM3 by 20 dB the amplitude deviation must be less than 60.5 dB and the phase deviation must be less than 658. Mainly because of the amplitude deviation of the phase shifter theserequirements are hard to meet.II. NEW TOPOLOGY AND EXPERIMENTALRESULTSAdding a phase shifter and an attenuator to the linearpath improves the symmetry of the two paths withrespect to time delay, amplitude and phase deviation.This leads to an increase of bandwidth and IM3-reduction. An additional improvement can beachieved by placing the phase shifter and theattenuator in front of the cubic law device. Thisrelaxes the bandwidth requirement of the phaseshifter and the attenuator to one third, because theIM at 2f16f2 and 2f26f1 (f1, f2: input frequencies) is generated after this devices. In such a configurationit must be guaranteed that the phase shifters and theattenuators generate much less IM3 than the cubiclaw device. A schematic diagram of the predistortioncircuit is shown in Fig. 1.Fig. 1:Schematic diagram of the predistortioncircuit.The proposed predistortion circuit has been designedin microstrip technology. A 1808-hybrid (Fig. 2, a) delivers the input signal for the linear and the nonlinear path. The signal in the linear path passes through a phase shifter (b) which is kept at a fix bias voltage where the amplitude deviation shows aminimum over the desired frequency range. The phase shifter in the nonlinear path (c), providing a phase shift of more than 3608, ensures an exact phase difference of 1808 between the two paths. In the ideal case, this phase shifter would be biased at the same voltage as the phase shifter in the linear path, resulting in equal amplitude transfer functions. An adjustable attenuator (d) sets the correct amplitude of the predistorted signal. Placing an identical attenuator (biased at a fix voltage) also in the linear path (e) maintains high symmetry. An amplifier (f) ensures that the cubic law device (g), consisting of a 1808-hybrid and two antiparallel diodes [1], operates in its nonlinear region. Completing the linear path with an amplifier (h) and a delay line (j) keeps the symmetry at a high level. The delay line has been designed to have the same electrical length as the cubic law device. AThe output signal of the predistortion circuit is fed to a 1.3 µm Fabry-Perot MQW-LD (Hitachi HL 1326CF). Fig. 3 shows the measured output signals without (top left) and with predistortion circuit (left), where the control voltages were set manually. The input signals at 1790 and 1830 MHz generate IM3 at 1750 and 1870 MHz, respectively. Setting the appropriate phase and amplitude of the nonlinear path reduces the IM3 by more than 25 dB over a bandwidth of 120 MHz. The suppression of IM3 at higher power levels is limited by the occurrence ofthe 5th order intermodulation, which cannot be reduced by the presented predistortion circuit.III. ADAPTIVE PREDISTORTIONUsing the predistortion circuit for real world optical transmission links, the control voltages for the phase shifter and the attenuator have to be readjusted due to laser aging effects [1]. Downconverting and lowpass-filtering of the IM3 generates a mixing signal proportional to the IM3 in the transmission band. After the A/D-conversion, a cheap micro-controller tracks the phase and the amplitude for maximum IM3-reduction. This adaptive process reduces the intermodulation by 20 dB over a frequency range of 120 MHz (Fig. 3, top right) which is - to our knowledge - the largest bandwidth for an adaptive predistortion circuit operating atFig. 3: Measurements ofIM3 without (top left),adaptive (top right) andmanually adjustedpredistortion circuit(left).REFERENCES[1]S. Hunziker, Analysis and Optimization of fiber opticSCM-Links, Ph.D. thesis (in German) ETH Zurich, Nr.12604, 1998.[2]L. Fock et. al., "Reduction of semiconductor laserintensity noise by feedforward compensation:experiment and theory, J. Lightwave Technol., vol. 10,no. 12, pp. 1919-1925, Dec. 1992.[3]J. Namiki, "An Automatically Controlled Predistorterfor Multilevel Quadrature Amplitude Modulation,IEEE Trans Comm., vol. 31, no. 5, pp. 707-712, May1983.。