A Wavelength Converter Integrated with a Discretely Tunable Laser ...

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116 8. Wavelength Converter integrated with a discretely tunable laser by swapping the optical connections to the two probe ports. However, since one of the probe ports of the converter with integrated laser, is already connected to the laser, co- or counterpropagation operation is fixed by design. Therefore, two devices were realized on the chip from Fig. 8.2: a co-propagating version consisting of MWL1 + AOWC1 and a counter propagation version consisting of MWL2 + AOWC2. The co-propagating version is expected to have the highest bandwidth, but it has the disadvantage that filtering is needed to separate the control and the converted wavelengths. The dimensions of the chip are 3.5x7.5 mm¦ . The SOAs can be identified by the relatively thick black lines. All outputs of the converter are looped back to one facet of the chip in order to enable fiber-chip coupling with a tapered lensed fiber array. On the converter side of the chip a four-layer anti-reflection (AR) coating was applied, which reduced the residual reflection of TE and TM polarization to dB, according to calculations. 8.3 Fabrication The MWL-WLC chip in this chapter was fabricated in InGaAsP/InP by applying a three step Low-Pressure Metal-Organic Vapor Phase Epitaxy (LP-MOVPE) process. In the first deposition step a 500 nm thick InP n-type buffer layer was grown before a 190 nm Q(1.25) layer, a 120 nm Q(1.55) layer, a 190 nm Q(1.25) layer, and a 200 nm thick p-InP layer. In the second step, the passive regions were defined by etching away the active layer. The active regions are 20 wide rectangular stripes parallel to the long flat [ ¤¤ ] of the wafer (Fig. 8.2). In the selective area growth that followed, the active regions were butt-joined to a passive Q(1.25) waveguide layer and a not intentionally doped (nid) InP layer [91]. In the third epitaxy step, a 1300 nm thick p-InP cladding layer and a 550 nm p-InGaAs contacting layer were grown. The waveguide layout was defined in a 100 nm thick SiN mask using photo-lithography. The ridge waveguides were dry-etched up to 100 nm into the film layer using a reactive ion etcher (RIE). At a specific point during the RIE etch, the SiN mask layer was removed from the passive waveguide regions in order to etch away the InGaAs contact layer. This prevents optical leakage from the waveguides to the contact layer. After the waveguide etch, all SiN was removed and a new 380 nm SiN passivation layer was deposited by PE-CVD. On top of the SOAs, the SiN was removed for electrical contacts. The metal contacts were defined using a lift-off process with negative photo resist and deposited using a sputter process. Following the lift off, the chip was annealed for Ohmic contacts. Finally, the chip was cleaved, coated and soldered on a copper chuck for characterization. More detail on the fabrication and the waveguide structure can be found in Chap. 5. 8.4 Static measurements The MWL-WLC was first characterized in static operation to find an operation point with a high ER in the converted output. The measurements were done with the set-up that is shown The order in which is coated and soldered is of importance. The AR-coating did not hold on the facet if InP solder was present on the bottom side of the chip. In that case the solder must be removed by polishing the chip.
8.4 Static measurements 117 Control DFB EDFA PLC ATT SOA mwl SOApre CW SOA 1 SOA 2 InP chip α Fiber Array Figure 8.3: Measurement set-up for static measurements. OSA Probe Out in Fig. 8.3. The control signal is generated with a DFB laser and amplified by an EDFA. Then the control signal passes an attenuator (ATT) and a polarization controller (PLC), before it is fed into the chip. The probe output from the chip is fed into an optical spectrum analyzer (OSA), which was set to a 1 nm resolution bandwidth. The output contains both the probe and the control signal. Electrical contacts to the SOAs were realized with probe needles. The temperature of the chip was controlled with a Peltier element at 23 C. Operating the MWL-WLC chip requires the simultaneous biasing of 4 SOAs: one of the laser SOAs (SOA ), two SOAs in the converter arms (SOA and SOA ¦ ), and pre-amplifier SOA . A suitable injection current of SOA was determined by monitoring its LI-curve at the converter output port. In order to get laser power at the probe output, SOA was injected at ¤ mA. SOA ¦ was switched off. The LI-curves for all channels are shown in Fig. 8.4a). It can be seen that the threshold current is around 90 mA for all channels. The curves are rather irregular above 110 mA, which is due to back reflections into the laser from the output facet, as is explained in Sec. 8.6. The laser current was set at ¤¨ mA in the wavelength conversion measurements. At this current the LI-curves were still relatively stable. Using a higher current increases the likeliness of heating problems and it does not necessarily increase the laser power, as can be seen from the LI-curve of channel 4. Furthermore, the over 1 mW laser output power at this injection current was sufficient for conversion experiments The laser ¤¨ spectra at mA are shown in Fig. 8.4b. The shoulders that can be seen in channel 3 and 4 are due to back reflections into the laser. The electrical switching curve in Fig. 8.5a shows the MZI transmission of channel 1 as a ¦ function of . Current mA was kept ¤ constant. It can be seen that optimal destructive interference is ¦ ¤ found at mA. For all other channels, a similar minimum was ¦ found for within mA of this value. The noisy behavior of the curve is due to back reflections into the laser (Sec. 8.6). The above described injection currents were used in the non-inverting conversion measurements that are described below.
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A Wavelength Converter Integrated w
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Contents v 4.3 Waveguide components
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Chapter 1 Introduction 1.1 Optical
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1.2 Exploiting optical fiber capaci
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1.3 Wavelength conversion in WDM ne
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Chapter 2 Wavelength converter over
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2.2 Optical gating 9 λcontrol λpr
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2.2 Optical gating 11 powers (in th
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2.2 Optical gating 13 Michelson Int
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2.3 Coherent wave mixing (FWM, DFG)
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2.5 Summary 17 2.5 Summary The main
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Chapter 3 Semiconductor optical amp
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3.2 Amplification in SOAs 21 Table
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3.3 Extended Cavity Lasers and exte
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Chapter 4 Integration of waveguide
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4.3 Waveguide components 41 Table 4
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4.3 Waveguide components 43 4.3.2 P
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4.4 Integration trade-offs 45 the a
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4.4 Integration trade-offs 47 The c
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4.4 Integration trade-offs 49 speed
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4.4 Integration trade-offs 51 resis
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4.4 Integration trade-offs 53 activ
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4.4 Integration trade-offs 55 a) 60
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4.4 Integration trade-offs 57 due t
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4.4 Integration trade-offs 59 The
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4.5 Conclusions 61 4.5 Conclusions
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Chapter 5 Technology 5.1 Introducti
Page 71 and 72: 5.2 Epitaxy 65 of the stack Q(1.25)
Page 73 and 74: 5.3 Waveguide processing 67 InP Q(1
Page 75 and 76: 5.3 Waveguide processing 69 h l h S
Page 77 and 78: 5.4 Contact openings and metalizati
Page 79 and 80: 5.4 Contact openings and metalizati
Page 81 and 82: Chapter 6 MWL with absolute wavelen
Page 83 and 84: 6.3 MWL with absolute wavelength co
Page 85 and 86: 6.3 MWL with absolute wavelength co
Page 87 and 88: 6.5 Measurements 81 facet M 1 O4 O3
Page 89 and 90: 6.5 Measurements 83 127 mA, which e
Page 91 and 92: Chapter 7 MZI wavelength converter
Page 93 and 94: 7.2 Operating principle of the MZI
Page 95 and 96: 7.3 Converter performance 89 Note t
Page 97 and 98: 7.3 Converter performance 91 Chirp
Page 99 and 100: 7.4 Wavelength converter design 93
Page 101 and 102: 7.5 Fabrication 95 1 2 3 4 5 6 7 8
Page 103 and 104: 7.6 Measurements 97 Power out [dBm]
Page 105 and 106: 7.6 Measurements 99 mental and firs
Page 107 and 108: 7.6 Measurements 101 fiber-to-fiber
Page 109 and 110: 7.6 Measurements 103 P out [dBm] P
Page 111 and 112: 7.6 Measurements 105 Power [dBm] -1
Page 113 and 114: 7.6 Measurements 107 Substituting E
Page 115 and 116: 7.6 Measurements 109 7.6.4 BER meas
Page 117 and 118: 7.7 Conclusion 111 output extinctio
Page 119 and 120: Chapter 8 Wavelength Converter inte
Page 121: 8.2 Design 115 MWL1 M1 O1 O2 O3 O4
Page 125 and 126: 8.4 Static measurements 119 The LI-
Page 127 and 128: 8.5 Dynamic operation 121 BER 2.5 G
Page 129 and 130: 8.6 Discussion 123 Wavelength [μm]
Page 131 and 132: 8.6 Discussion 125 lower then the l
Page 133 and 134: 8.7 Conclusions 127 8.7 Conclusions
Page 135 and 136: Appendix A Mask plates The mask pla
Page 137 and 138: References [1] H. Takara, E. Yamada
Page 139 and 140: References 133 [24] R.G. Broeke, J.
Page 141 and 142: References 135 [51] C.Q. Xu, K. Shi
Page 143 and 144: References 137 [88] J.J.M. Binsma,
Page 145 and 146: Summary A wavelength converter inte
Page 147 and 148: Samenvatting Een golflengte omzette
Page 149 and 150: Dankwoord Een dankwoord hoort vol m
Page 151 and 152: echoduo een glas komen claimen, en
Page 153 and 154: List of Abbreviations AOWC All Opti
Page 155 and 156: Curriculum Vitae Ronald Broeke was
Page 157 and 158: List of publications 1. R.G. Broeke
Page 159: 18. R.G. Broeke A Chirped phased-ar
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