1 Department of Photonics Engineering, Technical University of Denmark2 Systems, Department of Photonics Engineering, Technical University of Denmark3 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark
This thesis concerns the applications of semiconductor components, primarily electroabsorption modulators (EAMs), in optical signal processing and labelling for future all optical communication networks. An introduction to electroabsorption modulators is given and several mechanisms that form the basis of electroabsorption are briefly discussed including Franz Keldysh effect, Quantum-Confined Stark Effect (QCSE) and Quantum-Confined Franz-Keldysh effect. QCSE is found to be more effective for absorption modulation than FKE at room temperature due to the quantum confinement of electrons and holes. Experimental investigations on electrical-to optical (e/o) modulation of the EAM are presented. From the measured power transfer curves, static extinction ratios larger than 20 dB were obtained for wavelengths in the C-band. It is also shown that the insertion loss and static extinction ratio decrease with the signal wavelength, indicating that an optimum wavelength can be found as a trade-off between the on-off ratio and the signal-to-noise ratio. The chirp property and the small signal bandwidth for electrical-to-optical modulation of the EAM are investigated. It is found that the measured chirp α–parameter ranges from –0.4 to 0.8 depending on the reverse bias; the higher the bias, the smaller the chirp becomes. Negative chirp may be achieved by sacrificing the extinction ratio and the output power. The small signal bandwidth was measured to be as large as 24 GHz. Cross absorption modulation (XAM) in an EAM is discussed including an introduction to the carrier effects and a simple model that simulates the carrier dynamics. Based on this model the static characteristics of an EAM under optical excitation are investigated theoretically; the results demonstrate the capability of an EAM used for wavelength conversion and 2R regeneration. The optical-to optical (o/o) modulation bandwidth and frequency chirp are experimentally investigated. It is found that the o/o modulation bandwidth drastically depends on the quantum well depth while the e/o modulation bandwidth is mainly influenced by the electrical bonding pad size. A device having a small pad and shallow wells shows 24 GHz bandwidth for both e/o and o/o modulation. In the o/o chirp measurements very small chirp α–parameters are obtained. Depending on the operating wavelength and the bias the chirp α–parameter ranges from –0.6 to 0.2. It is also found that higher bias voltages and shorter wavelengths are preferred to obtain a small or negative chirp α–parameter. The principle of EAM-XAM based wavelength conversion is discussed and a wavelength conversion experiment at 40 Gb/s is presented. The influence of some operation parameters, including pump light power, reverse bias of the converter and probe light wavelength, is experimentally investigated for the wavelength-converted light, including its chirp performance. As a result of this investigation, a higher pump power (up to 20 dBm) and a relatively larger reverse bias (-2.5 V) are preferred for obtaining both larger extinction ratio and lower chirp of the converted signal. A multi-wavelength conversion scheme (8 × 40Gb/s) is demonstrated, where the receiver sensitivity for the back-to-back case is –33 dBm and the average power penalty for the eight converted channels is 9.2 dB. The best channel at 1555.7 nm has a power penalty of 8 dB. The wavelength dependence of the power penalty is explained by studying the impact of the extinction ratio and the average power of the converted signal on the Q parameter. Physical explanations for the optimum pump power and device length is given by considering impacts on the extinction ratio, average power and pulse width of the wavelength-converted signal. Other wavelength conversion schemes such as fibre-based cross phase modulation (XPM) and optical filtering, fibre-based Kerr switch, fibre based four-wave-mixing (FWM) and semiconductor optical amplifier (SOA)-based cross gain modulation (XGM), are briefly discussed. As a result of the comparison, it is suggested that fibre-based solutions have relatively lower power penalties and have great potential for ultra-high speed operation while single semiconductor devices can so far operate at 40 Gb/s and are more attractive in terms of compactness, stability and integration. An important advantage of the EAM-based wavelength conversion scheme is that the frequency chirp of the converted signal is very small, which is desirable for long distance transmission and optical labelling systems. An all-optical demultiplexing experiment from 160 Gb/s to 10 Gb/s using a single EAM with a very simple waveguide structure is presented. All 16 demultiplexed tributary channels are error free with an average receiver sensitivity of –25.3 dBm. An improvement of up to 6 dB in the receiver sensitivity by regeneration of the demultiplexed channel by an additional EAM acting as a saturable absorber is also demonstrated. A discussion of 2R regeneration based on a non-linear intensity transfer function is given. It is reiterated that a 2R regenerator can not reduce the BER but can inhibit its accumulation. The non-linear transfer function of an EAM is frequency dependent and the main improvement from an EAM-based regenerator is the enhancement of the ER and the suppression of the noise in a space bit. Applications of EAMs in optical label processing using various orthogonal labelling schemes are discussed. Through EAM-based wavelength conversion label encoding and recognition are realised for two-level labelled signals consisting of a 10 Gb/s Amplitude Shift Keyed (ASK) payload and a 2.5 Gb/s Differential Phase Shift Keyed (DPSK) label. The receiver sensitivities for the payload/label in back-to-back case and after label encoding are –25.6/-28.1 dBm and –23.7/-21 dBm, respectively. Using an EAM for optical label insertion and a MZ-SOA for optical label erasure and payload regeneration in the ASK(10 Gb/s)/ Frequency Shift Keying (312 Mb/s) orthogonal modulation format, the complete functionality of a network node including two-hop transmission and all-optical label swapping is experimentally demonstrated. The cascaded transmission and label swapping result in 1.9 dB power penalty for the payload and 1.8 dB penalty for the label. Operating as external modulators, two EAMs are used to encode and erase the optical label in the return-to-zero (RZ)-DPSK/ASK and non-return-to-zero (NRZ)-DPSK/ASK format. We experimentally demonstrated label encoding, transmission over a 50 km SMF link, and label erasure of a 40 Gb/s RZDPSK modulated payload with an orthogonal 2.5 Gb/s ASK label. The penalties for the payload and label due to labelling and transmission are 4.5 dB and 2 dB, respectively. A similar experiment was carried out for a NRZDPSK/ASK labelled signal. Compared to the RZDPSK/ASK scheme, NRZDPSK/ASK has a smaller spectrum width and the labelling penalty for the payload is 2.7dB larger while the label performance is almost the same. The modulation cross talk between the ASK payload and the DPSK label is theoretically analysed. As a result it is found that for a noiseless ASK/DPSK system with an infinite ASK extinction ratio, error free detection of the label can be obtained when the payload bit rate is at least 73 times larger than that of the label in case of balanced detection. For the single-ended DPSK receiver an even larger bit rate ratio (~130) is needed. Since real DPSK systems work at a relatively high bit rate this condition is normally not met. To solve this problem, instead of using a moderate ASK extinction ratio, we introduced a base band coding scheme named mark-insertion coding for the ASK payload and using this coding scheme we realised label swapping based on a RZASK payload at 40 Gb/s and a DPSK label at 622 Mb/s using an EAM as the label swapper. A new polarization modulation scheme is proposed and various signal-processing functions based on Polarization Shift Keying (PolSK) modulation format are demonstrated. Polarization modulation is implemented by a normal Mach Zednder Modulator operating in a special but simple way. Detection and erasure of polarization information are realised by a device that is comprised of a polarization controller and a polarization beam splitter. A new orthogonal labelling scheme based on a 40 Gb/s DPSK payload and a 2.5 Gb/s PolSK label is proposed and experimentally demonstrated. The most striking feature of this new labelling format is that there is no modulation crosstalk between the payload and the label, in contrast with all previous orthogonal modulation formats. Swapping penalties are 0.15 dB and 0.6 dB for the payload and the label respectively. Penalties due to swapping and 40 km SMF transmission are 2.2 dB and 2.9 dB for the payload and the label respectively.