TUE zyxwvu 4 8 zyxwvutsrqponmlkjihgfe 16 zyxwvutsrqponmlkjihgf CTuK17 Table 1. One Typical Solution of the Phase adjustment Factors for N = 2 to 16 zyxwvu N zyxwvutsrqponmlkj IRelstive phase factors for each group m zyxwvutsrqponmlkji 2 Irn 421 (0, 0, n. 0) {O, zyxwvutsrqponmlkjihgfed I[, -n12, n. n12.- n 12, - 1112, - zyxwvutsrq n12) (0, zyxwvutsrqponmlk -x. n 12,- x 12, n 12, n 12,-n 12, -x 12.- n , 0, n 12, -n 12, -n 12. -x 12, -I[ 12, -n 12) N IRelstive phase facton for each group m 3 l(O,O, 2 094) 5 1(0,-21115,2513, -21[15,0) CTuK17 Fig. 3. A numerical example with N = 8. Dash line: original pulse shape; Solid line: pulse shape after repetition rate multiplication. Some typical solutions are listed in Table 1 for N = 2 to 16. It is interesting to note that solutions can be found for any given N and when N is an integer power of 2, the required phase-shifts have only four discrete values. For practical implementation of our scheme, we propose to use cascade side- coupled ring resonators3 as shown in Fig. 2(a). An ideal single side-coupled ring resonator is an all-pass fdter with a phase spectrum as shown in Fig. 2(b). By carefully adjusting the equivalent path length of the ring resonators, one can achieve the required phase adjust- ments specified in Table 1 for each group. The number of required resonators is at most N - 1, and in practice it can be much less. (For example, for N = 4, only one resonator is needed.) Figure 3 shows a numerical example for N = 8 to prove the validity of previous analyses. In summary, we have proposed and ana- lyzed a novel scheme for losslessly increasing the intensity repetition rate of a steady pulse train by using optical all-pass filtering. The scheme should be useful in generating a highly repetitive optical pulse train with a repetition rate of tens and hundreds of GHz. 1. E. Yamada, etal., Electron. Lett. 31,1324- 1325, 1995. 2. T. Kuri and K. Kitayama, Electron. Lett. 3. C.K. Madsen and G. Lenz, IEEE Photon. Technol. Lett., Vol. 10, No. 7, pp. 994- 996,1998. 32,2158-2159,1996. CTuKl8 Optical clock repetition rate multiplier T. Papakyriakopoulos, K. Vlachos, A. Hatziefremidis, H. Avramopoulos, Department of Electrical and Computer Engineering, National Technical University of Athens, 157 73 Zographou, Athens, Greece; E-mail: tpapak@softlab.ece. ntua.gr The rapidly maturing technologies of high per- formance, active and passive opto-electronic devices has helped to spurn an intense interest in several research groups for the development of ultra-high speed, all-optical logic circuits.' One essential subsystem for ultra high speed, optical logic circuits is a high repetition fre- quency optical clock source. A number of short pulse, high repetition rate laser sources have been demonstrated for this purpose,*.3 but they almost always have to rely on high frequency microwave sources to provide the drive signal or narrow frequency linewidth and stabilised DFB laser sources and sophisticated compression techniques. In the present com- munication, we report a short pulse, high rep- etition rate laser source that is capable of pro- ducing 15 ps pulse trains, at a repetition frequency of up to 34.68 GHz. The principle of its operation relies on a master-slave oscillator arrangement. In this instance the master oscil- lator is provided by a 5.78 GHz gain switched DFB, to which a fiber ring laser is synchro- nised. This arrangement of obtaining the high repetition rate optical clock signal presents two significant advantages. (a) The high repetition frequency optical clock requires a low fre- quence and therefore less expensive rf signal generator. (b) The low repetition frequency rf and optical signal may be used as the universal reference signal of all the high repetition fre- quency optical clocks in the optical logic cir- cuit. In this way data may be introduced into the logic circuit at a low rate, so that it is compatible with commercially available modulators. The basis of the concept on which repeti- tion rate multiplication is achieved in our laser source relies on two key observations. The first is that the fast saturation of a semiconductor optical amplifier (SOA) by an externally intro- duced picosecond optical pulse, may be used for gain modulation in a fiber ring laser and the generation of stable picosecond pulse trains SDAY AFTERNOON / CLE0'99 / 121 Tmdk F&cr EDFA Spthrnirr CTuK18 Fig. 1. Experimental setup. from such a s y ~ t e m . ~ The second observation is that by using this technique to mode-lock a fiber ring laser, it is possible to tune the fre- quency fext, of the externally introduced pulse train to fext = (N + l/n)Sfring, and to obtain an output pulse train at a frequency nfext. In this equation N and Sfring is the order of harmonic mode-locking and fundamental frequency of the ring laser and n is an integer number. This method for repetition rate multiplication is only possible, if the gain modulation in the laser cavity is provided by a saturable amplifier so as not to present loss at any time. Figure 1 shows the experimental layout. All the components used in the cavity are pigtailed with standard single mode fiber. Gain was pro- vided from a 500 p m InGaAsP/InP ridge waveguide SOA. The waveguide facets were angled at 10" and were antireflection coated. The SOA had a peak gain at 1535nm and could provide 23 dB small signal gain at 250 mA dc drive current. Faraday isolators were used at the input and output of the SOA to ensure unidirectional oscillation in the ring cavity. After the SOA, a 3 dB fused optical fiber cou- pler was used to insert the externally intro- duced signal and to obtain the output from the source. A tunable filter with 5 nm bandwidth was used for wavelength selection. As the SOA exhibited a 2 dB gain dependence, a polariza- tion controller was introduced at its input port. The total length of the ring cavity was 14.6m corresponding to 13.9MHzfundamen- tal frequency. The external signal was provided from a gain switched DFB laser at 1548.9 nm, which was compressed in dispersion compensat- ing fiber to produce 12 ps pulses at 5.78 GHz. The output of the DFB laser was amplified in an EDFA and its polarization state was con- trolled for optimum performance before entry into the ring. With the synthesizer source of the DFB ad- justed to a harmonic of the fundamental of the ring cavity (approx. 5.7 GHz) and the EDFA adjusted to provide 800 p W into the cavity, the ring laser breaks into stable, mode-locked op- eration at this frequency. By changing the driv- ing frequency in the synthesizer source by 13.9/n MHz and n varying from 2 to 6, the laser produces pulse trains at 11.56 GHz, 17.34 GHz, 23.12 GHz, 28.9 GHz and 34.68 GHz. Figure 2 shows the output pulse trains at (a) 17.34 GHz, (b) 23.12 GHz, (c) 28.9 GHz and (d) 34.68 GHz monitored on a 40 GHz sampling oscilloscope. Figure 3 shows the corresponding second har- monic autocorrelation traces obtained at (a) 17.34 GHz (b) 34.68 GHz. The pulse widths obtained from the fiber ring laser were ap- proximately 15 ps for all repetition frequencies and the output power was about 60 kW. The process by which the ring oscillator mode-locks in the presence of the external pulsed signal relies on the fast saturation ofthe SOA. The mode-locked pulse experiences a