3. Resonant SEMZM
3.3. Bandwidth Enhancement by Frequency Equalization
The resonance of the SEMZM is exploited to enhance the bandwidth of the entire electro-optical system: the modulator itself, along with the electronics employed to drive it, e.g. data sources, DACs, and driver amplifiers. As introduced in section 3.1, the target application is the transmission of a 56G OOK signal using driving electronics whose 3 dB cut-off frequency is lower than half the symbol rate. The resonance peak of the SEMZM is thus designed to be at 28 GHz. The quality factor of the resonance is ideally such as to exactly compensate for the losses of the surrounding components. Since these losses are unknown, a quality factor appropriate for a flat transfer function at the electronics’ cut-off frequency, here half the baud rate, is selected.
When the segment is driven in a 50 Ω environment, each modulator’s arm is loaded with half the source resistance. An additional 5 Ω must be added in order to account for the additional series resistance of the SEMZM. The total damping resistance is thus 30 Ω. With such a load, the region of interest in LC space regions of interest are pushed to areas with small capacitive loads of about 100 fF (Figure 3-25-a).
As discussed in Chapter 3.2.1, this capacitance is a function of the segment’s length and therefore connected to the modulation efficiency. A larger CMZ is thus advisable in order to lower the segment’s drive voltage. For this reason, a high-frequency resistor Rp is placed in parallel with the source resistance, at the segment’s input. The effect of this resistor is to lower the resonator loading resistance. Therefore, the region of interest is shifted towards the right, i.e. towards regions associated with larger capacitance values (Figure 3-25-b).
With known target RLC parameters, the development of the SEMZM continues as already discussed. A segmented modulator with 16 identical sections of length 225 µm is designed and fabricated. The number of segments is maximized in order to achieve lowest possible switching voltage. DC characterization is performed and the fiber-to-fiber insertion loss is measured to be 8.5 dB at 1550 nm. When all the segments are connected together, a Vπ = 1 V is measured, resulting in a modulation efficiency of 0.36 V·cm and a (Vπ · IL) product as low as 11.6 V·dB at the best biasing condition. The working SEMZMs are assembled on a PCB where four segments are provided the necessary additional inductance by means of 175 µm long gold bondwires. Supplementary 50 Ω tapers are designed in order to fit the 39 Ω RF resistors and match the pitch of the available 4 x SG RF probe (Figure 3-26). The number of assembled segments is equal to four only because that was the number of RF data lines that could be provided to the modulator with the correct time delay.
__________ 3 Resonant SEMZM.
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resonant frequency, GHz
inductance, pH
capacitance, fF
(a) R = 30 Ω
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500 50
40 30 20 10 0
resonant frequency, GHz
inductance, pH
capacitance, fF
(b) R = (50||39) / 2+5 Ω
Figure 3-25: Resonators in LC space for two different loading resistances: (a) 30 Ω and (b) 16 Ω.
The areas of interest for the 56G OOK bandwidth enhancing application are highlighted in magenta.
Figure 3-26: Bandwidth enhancing SEMZM with 4 assembled segments. The segments are 225 µm long and are provided with the required additional inductance with a pair of 175 µm long gold wires. Small-size RF resistors are employed to lower the source loading impedance.
The equivalent circuit of the assembled single-ended segment’s branch with a RF resistor in parallel is depicted in Figure 3-27-a. The small-signal EO parameters are measured for each of the four segments in the same setup as shown in Figure 3-21. The comparison between the simulated transfer function and the measured one shows good agreement (Figure 3-27-b). The measured resonance frequency is 32 GHz with a quality factor Q = 4.5 dB. The difference from the desired value is attributed to difficulties in controlling the bondwire inductance value and an imprecise estimation of the R and C parameters.
Despite this, the obtained transfer function serves well for the foreseen application, since the peak at 28 GHz is not far from 3 dB.
Large-signal measurements are performed by simultaneously driving three segments of the SEMZM (Figure 3-29). Only three segments are driven because of the limited availability of RF filters. A 56 GBd OOK PRBS signal is generated with a four-channel bit pattern generator (BPG) whose outputs are fed to high bandwidth SHF drivers through the Bessel-Thomson filters. The drivers are then connected to the segments of the modulator.
3 Resonant SEMZM __________ .
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(a)
5 10 15 20 25 30 35 40
-4 -2 0 2 4 6
normalized S21, dB
frequency, GHz
SEMZM - measured SEMZM - simulated
(b)
Figure 3-27: (a) Equivalent lumped model of the assembled segment’s branch including the parallel resistor RP and (b) comparison of the small-signal measured and simulated EO parameters’ amplitude.
As the three sections are independently driven, the signal path is repeated three times.
Electrical delay lines are also included before the modulator to synchronize the signal on the different segments with the optical wave. The optical path starts with a 1550.1 nm ECL whose output is fed directly into the SEMZM through a lensed fiber. The optical output of the MZM is then amplified with an erbium-doped fiber amplifier (EDFA) and sampled with an optical oscilloscope.
Resonant MZM 20 GHz
filter
ECL
Network EO Analyzer SHF810
1 2 -12 5 10 15 20 25 30 35 40
-9 -6 -3 0 3
normalized S 21, dB
frequency, GHz resonant segment LiNbO3 reference
Figure 3-28: Small-signal measurement setup for the bandwidth enhancing SEMZM. Slow electronics is emulated using a Bessel-Thomson filter with 3 dB bandwidth of 20 GHz. The normalized S21 amplitude shows the 7 GHz bandwidth gain with respect to a LiNbO3 MZM reference.
The obtained 56 GBd eye diagram shows a clear opening at the measured data rate despite the bandwidth limitation of the driving electronics (Figure 3-30). The measured ER is 8.6 dB, limited by the short modulation length equal to only 675 µm. Driving a higher number of segments, a higher ER is to be obtained. As a comparison, the resonant SEMZM is replaced once again with the LiNbO3 reference and an open, but noisier eye diagram with ER = 10.7 dB is obtained.
__________ 3 Resonant SEMZM.
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DUT Scope
ECL EDFA
BPG 20 GHz FILTERs
SHF DRIVERs DELAY LINEs
Figure 3-29: Large-signal measurements setup for the 56 GBd bandwidth enhancement experiment.
Three segments are simultaneously driven with a properly timed OOK signal.
(a) (b)
Figure 3-30: Measured 56 GBd OOK optical eye diagrams for (a) the resonant SEMZM and (b) the Sumitomo LiNbO3 MZM reference when driven by 22 GHz bandwidth electronics.
Bit error ratio is measured over channel-induced dispersion. The dispersion is introduced in the setup with a tuneable dispersion compensator, inserted before the optical receiver.
The BER curves are measured for both the InP SEMZM and the LiNbO3 MZM (Figure 3-31).
The InP resonant modulator transmits the 56 GBd OOK signal with a BER lower than 10-9 over the equivalent of more than 2 km of fiber. The measured BER is below the hard-decision forward error correction (HD-FEC) threshold, i.e. 3.8 · 10-3 with 7% overhead, up to 4.5 km of channel length, assuming a dispersion per kilometre of single mode fiber of 17 ps/nm [57]. The BER curves are also measured versus optical signal-to-noise ratio (OSNR) to evaluate the impact of noise over the modulated signal. The OSNR is varied with the usage of an EDFA and a variable optical attenuator (VOA) inserted directly after the modulator. The measured BER is below the HD-FEC threshold for OSNR larger than 27 dB, whereas the LiNbO3 reference system never reaches this threshold. The higher noise-floor visible in the BER vs OSNR measurements is to be attributed to the different setup including the extra noise loading stage. In both comparisons, the resonating SEMZM performs better since the reference system does not have the bandwidth to transmit the signal error-free.
3 Resonant SEMZM __________ .
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-Log(BER)
Dispersion, ps/nm LiNbO3 SEMZM
HD-FEC
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-Log(BER)
OSNR, dB/0.1nm
LiNbO3 SEMZM
HD-FEC
Figure 3-31: 56 GBd BER measurements for the resonating InP SEMZM and the LiNbO3 reference when driven by 20 GHz bandwidth electronics. Scatter points represent the measured values.