InP-based polarisation independent wavelength demultiplexers

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30 2. PHASAR demultiplexers: a review amplifiers and switches, in more complex devices for use in multi-wavelength networks. In this section a number of applications will be discussed further. 2.3.1 Wavelength routers A wavelength router is obtained by designing the input and the output side of a PHASAR symmetrically, i.e. with N input and N output ports. For the cyclical rotation of the input frequencies along the output ports (as described in section 1.2), it is essential that the frequency response is periodical (as shown in figure 1.3b), which implies that the FSR should equal N times the channel spacing. From equation 2.6 it can be seen that this is obtained by choosing: ΔL whereby Ng is the group index of the waveguide mode, N is the number of frequency channels and Δfch is the channel spacing. For this design, the procedure described in section 2.2.2 should be changed similarly to section 2.2.4 paragraph (ii). By fixing the incremental length according to equation 2.36 the divergence angle Δα is fixed through equation 2.3 and Ra through Ra = da ⁄ Δα (see figure 2.1b). With this choice of FSR, the non-uniformity L u is fixed and will be in the order of 3 dB, which can be explained as follows. Channels at a frequency Δf FSR /2 away from the central frequency will experience an excess loss L u of at least 3 dB, because the focal spot corresponding to this frequency will be equally divided among two orders, which focus symmetrically around the centre of the image plane. As in a periodical design the frequency spacing between the outer channels comes close to the FSR, the outer channels will experience an excess loss L u in the order of 3 dB. Wavelength routers have been applied in various configurations in add-drop multiplexers and wavelength selective switches [47,48,64,65,90,130,143-147] and in multi-wavelength networks [54]. In combination with a DFB laser used as a wavelength converter, a wavelength router has also been applied as a wavelength switch [130, 135]. 2.3.2 Multi-wavelength receivers A multi-wavelength receiver is obtained by integration of a demultiplexer with a photodiode array. The first PHASAR receiver (reported in 1993 by Amersfoort et al. [4,5]), applied a twinguide waveguide structure where the passive region was obtained by locally removing the absorbing top layer. Integrated receivers have also been used in buried waveguide structures [182] and in polarisation independent raised-strip waveguides [9,122]. A wavelength-flattened receiver module, hybridly integrated with a silicon bipolar front-end array, has been discussed by Steenbergen et al. [138,139,140]. Recently, a low-loss (3 dB on-chip loss) 8-channel WDM receiver with 10 GHz bandwidth per channel has been reported [141]. 2.3.3 Multi-wavelength lasers c = ---------------------- N gNΔ f (2.36) ch Today’s WDM systems use wavelength-selected or tunable lasers as sources. Multiplexing of a number of wavelengths into one fibre is done by using a power combiner or a wavelength multiplexer. Integrated multi-wavelength lasers have been produced by combining a DFB-laser array (with a linear frequency spacing) with a power combiner on a single chip [12,173,174].
2.3 Applications 31 Using a power combiner for multiplexing the different wavelengths into a single fibre is a very tolerant method, but it introduces a loss of 10 ⋅ logN dB, with N being the number of wavelength channels. The combination loss can be reduced by applying a wavelength multiplexer, at the cost, however, of more stringent requirements on control of the laser wavelengths. An elegant solution to this problem is combining a broadband optical amplifier array with a multiplexer into a Fabry-Perot cavity as depicted in figure 2.14. This principle was first demonstrated in the MAGIC-laser [169] in a hybridly integrated form. If one of the Semiconductor Optical Amplifiers (SOAs) is excited, the device will start lasing at the passband maximum of the multiplexer channel to which the SOA is connected. In principle all SOAs can be operated and (intensity) modulated simultaneously. An important advantage of this component is that the wavelength channels are automatically tuned to the passbands of the multiplexer and coupled to the single output port with low loss. Mirror SOA SOA SOA SOA MUX Figure 2.14 Integrated multi-wavelength laser. Mirror Zirngibl and Joyner reported the first multi-wavelength lasers based on integration of a SOAarray with a PHASAR [68,178,179] and demonstrated it with a 9 × 200 Mb/s transmission experiment [180]. Despite their long cavity length, these lasers show single mode operation in a wide range of operating conditions [181]. Direct modulation speeds in excess of 1 Gb/s were reported recently [184]. Power coupled into a fibre is still low. Highest power reported so far is 0.15 mW [132,136]. Joyner et al. [70] reported integration of a MW-laser with an electro-absorption modulator. They used the power, radiated into an adjacent order of the phased array, to couple light out of the cavity into the modulator. A problem in MW-lasers with a small FSR is that the laser may start lasing in a wrong order and, consequently, at a wrong frequency. Doerr et al. [43] proposed and demonstrated a method to suppress the transmission for undesired orders by chirping the incremental length ΔL in the array. Tachikawa et al. [147] reported a 32-channel discretely tunable laser based on a PHASAR with SOAs, with one reflecting mirror connected to both the 4 input and the 8 output ports. The 32 wavelengths are generated by powering the proper SOA pairs. 2.3.4 Wavelength-selective switches and add-drop multiplexers Add-drop multiplexers (ADMs) form a special class of wavelength selective switches. They are used for coupling one or more wavelength signals from a main input port into one or more drop ports by operating the corresponding switches. The other signals are simultaneously routed into the main output port, together with the signals applied at the proper add ports. Figure 2.15a shows the configuration as produced by Tachikawa et al. [144,146,149]. The fibre 4 × 8
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Page 5: Aan Renée
Page 8 and 9: viii Contents 3 Polarisation conver
Page 10 and 11: x Contents Summary 161 Samenvatting
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98 6. MMI-based components coupler
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100 6. MMI-based components Table 6
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106 6. MMI-based components whereby
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108 6. MMI-based components Transmi
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114 6. MMI-based components Polaris
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124 6. MMI-based components Accurat
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128 7. Discussion and conclusions s
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130 Appendix A. Overview of phased-
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132 Appendix A. Overview of phased-
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134 Appendix B. Dispersion of the p
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136 Appendix C. Waveguide mode effe
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138 Appendix D. Cross talk penalty
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140 Appendix D. Cross talk penalty
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142 Appendix E. Excitation coeffici
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144 Appendix F. Phase correction in
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146 References M.A. Koza, R. Bhat,
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148 References Shah, L. Curtis, R.J
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150 References with multimode inter
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152 References 1991. [109] Lord Ray
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154 References der Tol, P. Demeeste
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156 References [169] I.H. White, K.
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158 List of symbols and acronyms fc
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160 List of symbols and acronyms MM
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162 Summary PHASAR demultiplexer. B
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164 Samenvatting eerste maakt gebru
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166 Dankwoord experimentele data vo
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InP-based polarisation independent wavelength demultiplexers

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