Edwin Jan Klein - Universiteit Twente

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Chapter 6 6.4 A wavelength selective optical switch based on microring resonators The vertically coupled microring resonator building block with a radius of 50 µm has been used to implement a number of different devices. The simplest of these devices is the wavelength selective optical switch. As was shown in Figure 6.1 this switch can for instance be used as part of a channel selection array in a transceiver. The switch is based on the first order resonator cascade filter comprised of two resonators that was discussed in Chapter 2.6. The first resonator of the switch, which is shown in Figure 6.33a, is used to select a certain wavelength channel from a WDM input signal and drop it to the waveguide between the resonators. If the resonant wavelength of the second resonator matches that of the first, the signal on the channel will be transported to the drop port of the switch. The switch is then in the “on” state. However, if the resonance frequencies are shifted with respect to each other then less power will be dropped. The least power is dropped when the shift between the resonance frequencies is equal to half the FSR of the resonator. The switch is then in the “off” state. The “on”, “off” responses of the switch as well the response for an intermediate state, where the shift ∆λ between the resonances is ≈1.46, are given in Figure 6.33b. Power dropped by the switch (dB) -30 1547 1548 1549 1550 1551 a) b) Figure 6.33. Schematic drawing of an MR based switch a) and simulated responses for this switch b). 6.4.1 Switch design 0 -5 -10 -15 -20 -25 154 Switch "on" Almost off Switch "off" Wavelength (nm) The filter rejection ratio SRR which was discussed in Chapter 2.5 is an important parameter for the switch since the difference between the “on” and “off” state of the switch is equal to the SRR of the first resonator in the switch. Increasing this parameter, for instance by lowering the coupling coefficients, will therefore improve the “on/off” attenuation of the switch. Lowering the coupling will also increase the steepness of the response. This is beneficial to the power consumption of the device since it reduces the shift in the resonance wavelength that is required to reach a certain “on/off” attenuation. There is, however, a trade-off to be made since lower coupling will also reduce the bandwidth of the switch. In addition it has to be taken into account that the bandwidth of two cascaded resonators combined will be approximately ≈0.7 [149] of the bandwidth of a single resonator.
155 Microring-resonator building blocks In Figure 3.34 the filter bandwidth is given as a function of the “on/off” attenuation for a number of effective radii (R.ng). The values were calculated using Equation 2.31 and multiplying the results with ≈0.7. Switch bandwidth (GHz) 250 225 200 175 150 125 100 75 50 25 0 5 10 15 20 25 30 35 40 "On/Off" switch attenuation (dB) 20 µm 50 µm 75 µm 100 µm Figure 6.34. Maximum achievable bandwidth as a function of the switch “On/Off” attenuation for a number of effective radii (simulated for λ0=1550nm) For the design of the MR based switch the goal was to have a -3 dB band width of 50 GHz. The switch was based on the “standard” building block with the parameters as described in Table 6.1. The resonator in the building block has an effective radius of ≈87 µm. Therefore the maximum “on/off” attenuation is ≈12 dB. If it is assumed that the bend loss in the resonator is 10 dB/cm, then field coupling coefficients of ≈0.6 are required. 6.4.2 Characterization The switch was fabricated using the “Vertical I” fabrication process that was discussed in Chapter 5. A photograph of the realized switch is shown in Figure 6.35a. The size of the switch is about 200 x 200 µm excluding the heaters. The measured responses for the “on” and “off” state of the switch are given in Figure 6.35b. To switch the device off, 225 mW was dissipated in the heater of the first resonator which was used for the tuning. For the observed shift in the resonance frequency of 1.46 nm this implies a tuning of 6.5 pm/mW, which is close to the value predicted for these resonators. A comparison with the simulated curve in Figure 6.33b which was also shifted by 1.46 nm shows that the simulated and measured values look very similar apart from a difference in the insertion loss. The responses have been normalized to the measured power in the through port while the switch was in the “off” state. The responses therefore also show the drop-port insertion loss. The “on/off” attenuation of the switch is 12 dB. When the switch is “on” the crosstalk with the adjacent channels is ≈-20 dB (channel spacing of 0.8 nm). The on chip insertion loss of the switch is around 5 dB. Using the DropZone tool the bend loss and coupling coefficients were determined to be in the order of 44 dB/cm and 0.54 respectively. For these parameters the insertion loss of a single resonator can be calculated to be about 3.2 dB which fits quite well with the 6 dB measured for the two resonators combined.
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DENSELY INTEGRATED MICRORING- RESON
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DENSELY INTEGRATED MICRORING- RESON
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Chapter 1 Introduction The devices
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Page 217 and 218: List of Symbols Q - Quality Factor
Page 219 and 220: Bibliography [1] Dan Schiller, “E
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Regions,” IEEE Photonics Technolo
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[66] B. E. Little, S. T. Chu, J. V.
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[97] H. Haeiwa, T. Naganawa and Y.
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[135] A. Driessen, D. H. Geuzebroek
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[165] T. Barwicz, M. R. Watts, M. A
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Publication List Patents (pending)
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R. Dekker, L.T.H.Hilderink, M.B.J.
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Networks (BB Photonics),” Interna
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Dankwoord/Acknowledgements Zo, dit
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Edwin Jan Klein - Universiteit Twente

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