A laboratory 4 x 4 transparent node for the space frequency domain has been set up in which optical frequency division multiplexed (OFDM) signals at different ...
TRANSPARENT SWITCHING NODE FOR OPTICAL FREQUENCY DIVISION MULTIPLEXED SIGNALS
R.-P. Braun, C. Caspar, H.-M. Foisel, K. Hejmes, B. Strebel. N. Keil a n d H. H. Y a o Indexing terms. Optical transmission, Optical communication, Optical switchinq A laboratory 4 x 4 transparent node for the space frequency domain has been set up in which optical frequency division multiplexed (OFDM) signals at different bit rates and modulation schemes can be switched simultaneously. The feasibility, signal path architecture, supervision, control, and performance of this node are reported.
frequencies pass the output filters whereas all unwanted spectral components are rejected [4]. In this setup the optical frequency convertors are used as routing elements, i.e. input signals at both frequencies f, or f, are routed to the fixed output frequency of the respective convertors. Two by two signals at different frequencies are then combined in a 3 d B fibre coupler or a polymer Y-branch. Finally, the output signals leave the switching node at the desired frequencies f , andf, and the desired output fibres. System management: A system management section supervises, stabilises, and controls the optical spectra, elements and switching functions of the node. It contains a 16 channel heterodyne spectrometer and a 16 channel filter control unit, shown in the upper and lower part in Fig. 2, respectively. fibre directional
Introduction: The optical frequency division multiplex (OFDM) technique offers an improved exploitation of the wide fibre transmission bandwidth and a flexible installation of new services in existing O F D M networks. For the expansion from distribution to dialogue services, transparent optical switching in the space frequency domain has been proposed [l, 23. Using this technique, it should he possible to switch signals at different bit rates and modulation schemes simultaneously through a switching node; for that to occur signals have to remain in the optical domain passing no optoelectronic interfaces. Such a transparent optical 4 x 4 switching node was built, and we report the feasibility, signal path architecture, supervision, control, and performance of this node.
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Signal path architecture: The signal path architecture of the laboratory switching node shown in Fig. 1 was chosen for a simple 4 x 4 node out of a variety of possible structures. system munugement
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Fig. 1 Architecture oJ laboratory transparent 4 x 4 O F D M switching
node Two input fibres each carrying two signals at the optical frequenciesf, andf, are connected to the node. Based on a non-blocking structure the signals are processed in four parallel paths. To achieve the desired distribution or switching functions the signals are switched through a space and a frequency stage. In the space stage the switching is performed by fibre bend switches together with passive distribution networks (fibre directional couplers). The active switching components of the frequency stage are fibre Mach-Zehnder filters which select individual input signals. The filters are bistably switched to the desired frequency f i or f 2 by optoelectronic feedback. The selected signals are routed through optical frequency convertors to fixed output frequencies fl and f i , depending on the used paths. Out of a pool of possible frequencies the output frequencies are chosen to be equal to the input frequencies in order to demonstrate the worst case with increased requirements for the optical filtering. The optical frequency convertors consist of DFB lasers which are injection locked by pump lasers [3] followed by fibre Mach-Zehnder filers. Only signals at the fixed output 91 2
dyne spectrometer (upper part) and a 16 channel filter control unit (lower part)for the simultaneous monitoring, stabibsation, and control of thefrequencies of the optical carriers andfilters, respectively The 16 channel heterodyne spectrometer monitors the optical spectra at different points of the switching network and controls the carrier frequencies. Its sweep laser works at the wavelength of the transmission signals at 1550nm. The sweep laser of the 16 channel filter control unit emits outside of the transmission band at 1553 nm in order to prevent a disturbance of the data transmission. It is swept over the periodic characteristics of all filters, monitoring and stabilising their frequency positions individually by thermal tuning. Both circuits are driven by the same saw-tooth generator and they are connected and controlled by a reference filter. This results in simultaneous monitoring and control of the carrier frequencies and filter curves with a matched optical frequency relation. This transparent 4 x 4 OFDM switching node was presented during the ECOC'92 at the Heinrich-Hertz-Institut in Berlin. Within this week, stable operation of the system was demonstrated with fluctuations of the optical frequency of about f 500 MHz.
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Fig. 2 System management of the node including a I6 channel hetero-
Experimental results and conclusion: In the 4 x 4 switching node, all laser components such as transmitters, pumps, local lasers, sweep lasers, and frequency convertors were DFB lasers with natural linewidths of 30 MHz. The frequency spacingf, -f, was set to 12.5GHz. The other optical components such as couplers, polarisation controllers, attenuators, switches, and filters were realised in fibre technology. The polarisation adjustment and the optical power levelling were performed by Lefevre type manual polarisation controllers and bend type manual fibre attenuators. The space and frequency switching were carried out by the fibre switches and the optical filters with switching speeds of lOms and 1 s, respectively [4]. Because circuit switching is performed in the space and frequency domain, only the arrangement of the transmission channels depends on the switching speed, but
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ELECTRONICS LETTERS
13rh M a y 1993
Vol. 2 9
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not the bit rates of the transmitted signals. However, these bistable switches are very useful for laboratory work. For future applications they can be replaced by fast switches of other technologies. In particular, optically addressed switches should be used with regard to optical signalling and selfrouting. The signal paths through the node including frequency conversion are transparent and can be defined as transmission filters with an insertion loss of -15dB, a bandwidth of 6.5 GHz and the frequency characteristic as shown in Fig. 3.
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STREBEL, B., BACHUS, E.-J.,and VATHKE, 1.: ‘Switching in coherent multi-carrier systems’. Globecom, Dallas, 1989, Conf. Record, Vol. I, pp. 32-36 BRAUN, R.-P., BACHUS, E.-]., CASPAR, C., FOISEL, H.-M., and STREBEL, E.: ‘Transparent all optical coherent-multi-carrier 4 x 2 switching node’. Proc. ECOC, 1991, Paris, Vol. 3, pp. 92-95 BRAUN, R.-P., CASPAR, c., FOISEL, H.-M., and STREBEL, 8.: ‘Optical components of a multi-camer switching system’. Proc. ECOC, 1992, Berlin, Vol. 1, pp. 409-412 CHENG, Y. H., MARCAnLI, E. A. J., and OKOSHI, 1.: ‘Phase-noisecancellation heterodyne receivers for coherent optical communication systems’. OFC, Houston, 1989, Tech. Dig., p. 41 BACHUS, E. I., BRAUN, R.-P., CASPAR, c., MISEL, H.-M., and STREBEL, B.: ‘Phase noise cancellation of directly frequency modulated optical signals’. Proc. ECOC, 1989, Gothenburg Vol. 3, pp. 17-20 KEIL, N., STREBEL, B., YAO, H. H., and KRAUSER, J.: ‘Applications of optical polymer waveguide devices on future optical communication and signal processing’. Proc. SPIE, 1991, San Diego, Vol. 1559. pp. 278-287
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Fig. 3 Frequency characteristic of an optical transparent channel through the switching node including optical filters and frequency conversion
The main part of the insertion loss is given by four 3 d B directional couplers inserted in a path. The final 3 dB of insertion loss arises from three optical filters and the frequency convertor. Within the transmission bandwidth, modulated carriers with arbitrary bit rates and modulation schemes can be transmitted through the node which was demonstrated by two transmission experiments. The receiver sensitivities at a bit error rate of were measured to be -45 and - 38 dBm for a 38 Mbit/s NRZ coded signal using a phase noise cancellation scheme [ 5 , 61 and a 140 Mbit/s CMI coded signal using an FSK modulation scheme with a frequency deviation of 1.8 GHz, respectively. These poor sensitivities were mainly given by the nonoptimised transmitter and receiver electronics. Other influences were the spectral broadening by tbe frequency conversion and the signal distortion by the optical filters. The spectral broadening can be eliminated using pump lasers [3] with small linewidths, but signal distortion is still a problem of optical filtering, particularly in cascaded filters. A major part of the node is the system management section which is necessary for monitoring, stabilisation, and control in order to achieve the desired switching function and stable operation. An upscaling to larger nodes is in principle possible, using schemes from a variety of possible architectures; then switching structures and the system management will concomitantly be more complex. Future practical applications will need optical integration with the optoelectronic integrated circuits performing very complex functions. The polymer technology may be a promising candidate. As a first trial a 2 x 1 power combiner (Fig. 1) was replaced by a singlemode polymer Y-branch [7] which shows fibre to fibre insertion losses in the two arms of 10 and ll.5dB. However, the purpose of this work is to demonstrate the feasibility of a transparent O F D M switching and to study and solve the arising problems.
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Acknowledgments: The authors would like to thank E.-J.
Bachus for many useful discussions and the Deutsche Bundepost Telekom for funding the work. 25th March IY93 C IEE 1993 R.-P. Braun, C . Caspar, H.-M.Foisel, K. Heimes, B. Strebel, N. Keil and H. H. Yao (Heinrich-Hertz-Instirut f i r Nachrichtentechnik Berlin GmbH, Einsteinufer 37, W-1000 Berlin 10, Germany)
References 1
KUROYANAGI, s., and NISHI, T.: ‘Wavelength-division photonic switching system’. OEC, Chiba, 1992, Final Program & Tech. Dig., pp. 242-243
ELECTRONICS LETTERS
13th May 1993 Vol. 29 No. 10
NEAR R O O M TEMPERATURE CONTINUOUS WAVE LASING CHARACTERISTICS OF GalnAsP/lnP SURFACE EMITTING LASER
T. Baba, Y. Yogo, K. Suzuki, F. K o y a m a a n d K. Iga Indexing terms: Semiconductor lusers, Lasers
The first near room temperature continuous wave lasing operation of a GaInAsP/InP surface emitting laser has been achieved by employing a buried heterostructure and a novel MgOjSi heatsink mirror. A dramatic reduction of threshold current at room temperature and a circular narrow output beam were demonstrated. GaInAsP/lnP vertical cavity surface emitting laser diodes (SELDs) are of great interest owing to their peculiar features acceptable for next generation optical communication systems and optical interconnects. Although room temperature pulsed operation of GaInAsP/lnP SELDs has been realised for a few years [l-41, room temperature continuous wave (CW) operation has not been achieved yet. The key issue for achieving room temperature CW operation is the reduction of threshold current, series resistance, and thermal resistance. In our previous work, we demonstrated the reduction of threshold current and series resistance by employing a circular planar buried heterostructure (CPBH) and a peculiar apertured contact [5, *]. As a result, CW operation up to -57°C was obtained, which was the highest temperature recorded at that time. Very recently, we achieved the first room temperature CW operation of GaInAsP/InP SELDs and briefly reported its preliminary results [SI. The significant improvement for this achievement was the realisation of an efficient heatsinking of the buried active region using a newly introduced thermally conductive MgOjSi dielectric multilayer mirror. In this Letter, we present the details of some of the CW lasing characteristics observed. The device structure and the fabrication process were very similar to those described previously [SI. The quaternary circular active region (bandgap wavelength of 1.37 pm, thickness of 0.7 pm and diameter 121m) was buried by p and n current blocking layers, p cladding layer and highly p-doped quaternary contact layer through a maskless CPBH regrowth process. The aperture of the contact layer just above the active region was 14pm in diameter. We used 8.5 pairs of MgOjSi multilayers and Au/Ni/Au metal as the p-side mirror and six pairs of SiOJSi as the n-side mirror. The devices were bonded p-side down on an Au-coated diamond heatsink with G a BABA, T., SUZUKI, K., YOGO, Y., IGA, K., and KOYAMA, F.: ‘Low threshold room temperature pulsed and -57”Ccw operations of 1.3pm
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