References D., CHAUVET, M., VIALLET, J.E., and CHAWKI, M.J.: 'First tunable narrowband 1 . 5 5 optical ~ drop filter using a dynamic photorefractive grating in iron doped indium phosphide', Electron. Lett., 1994, 30, (22), pp. 1883-1884 HERVE, D., CHAUVET, M., VIALLET, J.E., and COQUILLE, R : 'Polarization independent tunable narrowband 1 . 5 5 grating ~ filter in photorefractive iron doped indium phosphide'. Proceedings of IEEE Lasers and Electro-Optics Society Annual Meeting, San Francisco, CA, 1995 YAMADA, M., and SAKUDA, K.: 'Analysis of almost-periodic distributed feedback slab waveguides via fundamental matrix approach', Appl. Opt., 1987, 26, (16), pp. 34743478 French patent 951 1469 DELORME, F., SLEMPKES, s., ALIBERT, G., ROSE, B., and BRANDON, J.: 'Butt-jointed DBR laser with 15nm tunability grown in three MOVPE steps', Electron. Lett., 1995, 31, (15), pp. 1241c1245 HERVE, Optical subcarrier multiplexed system using microwave-optical mixing Lu Chao Indexing terms: Subcarrier multiplexing, Optical communication ~ Optical SCM systems using microwave-optical mixing are proposed in the Letter. Narrowband optical resonant receivers instead of wideband optical receivers employed in conventional SCM systems can be used in this scheme. Performance evaluations for both intensity modulated and external modulated SCM systemsusing microwave optical mixing are discussed. Introduction: In a conventional optical subcarrier multiplexing system (SCM), a receiver with a bandwidth greater than the highest subcarrier frequency is used. This requires a wideband photodiode and a wideband preamplifier with low equivalent input noise current, which is difficult to design especially when the subcarrier is in the tens of gigahertz range. To overcome this problem several schemes have been proposed. These include a coherent SCM system [I] and an optical prefdtering SCM system [2]. However, complicated control schemes are needed to offset lock two optical carriers or to lock the optical fdter passband to the selected optical-microwave frequency. An optical-microwave mixer based on a Mach-Zehnder modulator was proposed in [3] for sensor applications to increase the dynamic range. It offers a good way to convert high frequcucy microwave signals down to lower frequency signals using optical means. This optical-microwave mixer can be applied to SCM systems as proposed in this Letter. To evaluate the performance of the systems, both direct modulated and external modulated SCM systems with microwave-optical mixing are discussed in this Letter. a laser source M-2 modulator 'ne,,' local osci Ilator Direct modulation; b external modulation Theory: The proposed SCM system is shown in Fig. 1. At the transmitting end subcarrier multiplexed multichannel signals can either be used to directly modulate the injecting current of a laser diode, or a Mach-Zehnder external modulator can be used to change the laser diode output power according to the SCM multi- ELECTRONICS LETTERS 25th April 1996 N where Le is tb: excess loss caused by optical fibre and connectors, a(t)is an electrical modulating signal, Pra,yer is the laser diode output power, mi is the modulation index for each subcarrier channel, N is the number of subcarrier channels andf; are subcarrier frequencies. Oth'Er parameters follow the same definition as in [3]. We can expand eqns. I and 2 in a series of Bessel functions. Following ;I similar method as given in [l], the carriednoise ratio at the output of the bandpass filter is given by C -~ R ( $ R P ~ T D L mi , . J~ ( El2 2e(~lZP~,,TDL,)Bn~i,,)2L~BPS(a~~~~~L,)2RINBBP (3) for the direct imodulation system, or C -R ( E P ~ T L, ; J~ ) .r1( (E 2) J," ( 2)) 2 e ($ RPL, ?'D Le)BB P BB &$ RPh??T D - RINBB F b % z d (4) for the extern(x1modulated system, where R is the responsivity of the photodiode, <icy> is the equivalent input noise current density of the optical receiver, BBpis the bandpass filter bandwidth, RIN is the relative intensity noise, and a$ is the mixing product term generated by JfLo and 3rd-order intcrmodulation product terms of the first external modulator, while 02, is the mixing product terms generated by ?fL,and higher-order product terms generated by the frst external modulator. We have a?zd - (5) where K3 is the number of IMPS and is bounded by 3 P / 8 [l]. By careful examination we can see that 02, = 0 if 2hF2 f,,, - f,,,, where fmSxand f,,, are the maximum and minimum frequency of the subcarrier channels and f i F = f; -Lo. This requirement is necessary in order to avoid the image frequency problem. Numerical results and discussion: To evaluate the performance of demodulator Fig. 1 SCM system using optical-microwave mixer a plexed signals. At the receiving end, a microwave-optical mixer is used to mix the incoming signal with a microwave tunable LO. This will convert the required channel frequency to a predetermined IF frequency. It can be followed by an optional fibre amplifier to boost the received signal further before a narrowband resonant optical receiver is used to convert the optical signal to an electrical signal. An electrical bandpass filter can then be used to reject unwanted signals and a demodulator will be used to recover the original transmitted signal. The signals at the outputs of the Mach-Zehnder mixer for direct and external modulated SCM are given by: Vol. 32 the proposed systems, the output carrierhoke ratio (CNR) of a typical system is studied. A solid-state laser with Pi,,, = 15dBm is used for the external modulated system, while a DFB laser with Pi,,,, = 3dBm is used for the direct modulated system. The modulator insertioin loss T, is 4dB. Other parameters are: BBp = 60MHz, R = 0.7, RIN = -155dB/Hz, <ieq> = 3pA/dHz, Le = 15dB and V, = V,,, = 9V. There are all together 16 subcarrier channels, whiish are equally spaced between 4 and 5.5GHz. j& is fmed at 1GHz. The CNFUmodulation index is shown in Fig. 2 for different excess loss levels. Here m, for external modulation is defined as nV/V,. For a direct modulation system, the CNR increases with the modulation index without considering clipping distortion. For an external modulation system there is an optimum modulation index which increases with excess loss levels. No. 9 839 Polarisation-independent all-optical demultiplexing up to 200Gbit/s using fourwave mixing in a semiconductor laser amplifier t U 0 .c _ T. Morioka, H. Takara, S. Kawanishi, K . Uchiyama a n d M. Saruwatari 0 L aJ .c 0 0 +- Indexing terms: Multiwave mixing, Demultiplexing, Semiconductor lasers L a, L Polarisation-independent all-optical time-division demultiplexing -3 -’0 Or is demonstrated up to 2OOGbitis with <0.5dB polarisation I 1 0-1 0-2 0.3 modulation index mi 1594/2) Fig. 2 CCNR against modulation index f o r different excess loss, Le 0 changes from OdB to 20dB with 2dB steps external modulation, ....... direct modulation ~ Beyond this optimum modulation index value the CNR decreases due to CT$. At large modulation index value CNR is decided by oTd irrespective of the received optical power level. The effect of local oscillator power is studied in Fig. 3. For both external and direct modulation systems CNR will increase first when local oscillator power is increased then it will follow the value change of a Bessel function, decrease first then increase again. With Le = 15dB, a carrierlnoise ratio of 17dB can be easily achieved with a local oscillator power of SdBm. dependency based on four-wave mixing in depolarised TE and TM modes in a travelling-wavesemiconductorlaser amplifier. Introduction: All-optical time-division demultiplexing is one of the most fundamental technologies in realising future Gbitls all-optical networks. Among the requirements for all-optical demultiplexers, polarisation-independent (PI) operation [1, 21 is a vital one because they usually handle optical signals whose polarisation varies rapidly after transmission of long fibres. In this Letter we propose a simple PI all-optical TDM demultipiexer that utilises the ultrafast four-wave mixing (FWM) process [3] of both TE and TM modes in a semiconductor laser amplifier (SLA) in which input pump and signal pulses are deliberately depolarised to suppress polarisation coupling in an SLA; as a result, stable error-free PI operation is achieved up to 200Gbitis with i0.5dB polarisation dependency. nm I I 100-200GbiVs 0 & -5- 1 t .I L . 0 -1 5 I I -10 -5 0 5 10 15 local oscillator power,dBm 20 [594/jl Time, ps external modulation. ....... direct modulation ~ Conclusion: The study in this Letter suggests that an optical mixer can be used for the SCM system. The main advantage is that a narrowband tuned optical receiver instead of wideband optical receiver can be used. Variation of CNR with modulation index and LO power level was studied. The results suggest that a good system performance can be achieved with practical system parameters. 0 IEE 1996 4 March I996 Electvonics Letters Online No: I9960562 LU Chao (School of Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263, Singapore) References and OLSHANSKY, R.: ‘Multichannel coherent FSK experiments using subcarrier multiplexing techniques’, J. Lightwave Technol., 1990, LT-8, pp. 406415 2 GREENHALGH, P.A., ABEL, R.D., and DAVIES, P.A.: ‘Optical prefiltering in subcarrier systems’, Electron Lett., 1992, 28, pp. 1850-1852 3 GOPALAKRISHNAN, G K., BURNS, w.K., and BULMER, c.H.: ‘Microwave-optical mixing in LiNbO, modulators’, IEEE Trans., 1993, MTT-41, pp. 2383-2391 1 GROSS, R., 840 Time, ps Fig. 1 Experimental setup of polarisation-independent all-optical demultiplexer with 100 and 200Gbit/s input signals MOD: modulator, SOP: state of polarisation, PI: polarisation-independent, PM: polarisation-maintaining, SLA: semiconductor laser Fig. 3 Carrierhoise ratio against LO power amplifier Experiment and results: Fig. 1 shows the experimental setup of the proposed PI all-optical demultiplexer. The pump and signal sources were both 6.3 GHz actively mode-locked Er3+-dopedfibre ring lasers (ML-EDFRL) outputting 1545.9nm, 4.2ps and 1552.7nm, 3.7ps transform-limited pulses with a time-bandwidth product of 0.38 and 0.37, respectively, for 100GbiUs operation. For 200GbiVs operation, the pump and signal sources were a supercontinuum (SC) pulse source [4] that generated 1543.5nm, 1.7ps and 1553.lnm, 1.7ps transform-limited pulses with timebandwidth products of 0.33 and 0.32, respectively. The 6.3GHz signal pulses were 16 x (or 32 X) time-division multiplexed to 100GbiVs (or 20OGbiUs) after modulation by a LiNbO, intensity modulator (2I5-l, PRBS). 100GbiUs and 200Gbith input signal waveforms observed with a newly developed optical sampling scope with a 0.5ps temporal resolution [5] are also shown in Fig. 1. Both pump and signal pulses were independently amplified and input to an SLA through a polarisation-maintaining(PM) WDM multiplexer with pump polarisation at an angle (normally 45 degrees) with respect to TE (TM) polarisation of the SLA to excite FWM with equal FWM efficiency in both modes. The input pump and signal average optical powers were 9.4dBm and 2.1 dBm for 100Gbitls operation, and 7.3dBm and 2.3dBm for 200GbiUs oper- ELECTRONICS LETTERS 25th April 1996 Vol. 32 No. 9
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