Silicon Mach-Zehnder modulators with reduced series resistance in lateral PN junction rib-waveguide phase shifters for enhanced high-speed response are fabricated and characterized. Extinction ratio higher than 10dB is obtained at 10.3-11.7 Gbps with mask margins of 27% (10.3-Gbps 10GBE), 16% (10.7-Gbps STM-64/OC-192) and 10% (11.3-Gbps STM-64/OC-192) in eye-diagram measurements incorporating mask tests using a RF cut-off filter. In unfiltered eye-diagram measurements without mask tests, extinction ratio higher than 13 dB is obtained at 10.0-12.5 Gbps. The silicon modulators reveal high-speed performance comparable with that of lithium-niobate modulators in high-speed optical fiber telecommunications.
© 2011 OSA
Mach-Zehnder (MZ) modulators are promising for high-speed data transmission in optical-fiber telecommunications in dense wavelength-division multiplexing (DWDM). They allow signal modulation without frequency chirping under push-pull operation, and hence eliminate pulse and spectral broadening in a broad spectral range . Frequency chirping has been characterized as α-parameter for optical modulator. It can be shown that α-parameter is inversely proportional to 10ER/20, in which ER denotes extinction ratio. High extinction ratio in MZ modulators is thereby essential for the high-speed optical-fiber telecommunications. Lithium niobate MZ modulators have been extensively used for the high-speed data transmission. Extinction ratio 13 dB or higher was reported in eye-diagram measurements without mask tests at 10 Gbps .
Silicon MZ modulators are widely interested because of their potential in small footprints for integration and low costs for fabrication. Extinction ratio in eye-diagram measurements incorporating mask tests using a RF cut-off filter has been reported for Si MZ modulators as 8.9 dB with 10.3-Gbps 10GBE mask margin of 56% and 8.0 dB with 28-Gbps 100GBE mask margin of 57% [3,4]. The data on the 10.3-Gbps eye-diagram measurements were cited elsewhere . In eye-diagram measurements without mask tests, 10-dB extinction ratio has been reported for Si MZ modulators with 15-dB optical loss at 40 Gbps . Further increase in extinction ratio without increase in optical loss is desired for Si MZ modulators for the high-speed optical-fiber telecommunications. One of major factors that limit high-speed response and extinction ratio in Si MZ modulators is resistance-capacitance coupling in waveguide phase shifters. Phase modulation in Si MZ modulators is based on refractive index modulation due to carrier-plasma dispersion in carrier depletion and recovery in PN junction waveguide phase shifters . The coupling limits response time in carrier depletion and recovery.
Here, we report extinction ratio higher than 10 dB in eye-diagram measurements incorporating masks tests using a RF cut-off filter and 13 dB in unfiltered higher-bandwidth eye-diagram measurements without mask tests at 10.0-12.5 Gbps for Si MZ nonreturn-to-zero (NRZ) modulators having lateral PN junction rib-waveguide phase shifters. Fiber-to-fiber loss of the modulators is lower than 10.5dB. To realize such high extinction ratio and clean eye diagrams without increase in optical loss, series resistance coupled with junction capacitance in the phase shifters has been reduced by increasing thickness of Si side slabs for faster carrier depletion and recovery.
2. Setup for eye-diagram measurements and mask tests
Electrical modulation signals in 231-1 NRZ pseudo-random bit stream (PRBS) were supplied from a pulse pattern generator (Anritsu MP1800A, MU181020A and MU181000A). The signals were passed to an electrical modulation driver and input as RF data as shown in Fig. 1 . The RF data were input to one of the phase-shifter arms of a Si MZ modulator through a bias tee with DC reverse bias. The input RF data and the DC reverse bias were applied on a contact pad of a signal line of traveling-wave coplanar electrodes (not shown in Fig. 1). The reverse bias was about −5 V.
Mask tests, which are commonly executed for commercialized optical modulators, require a standardized RF cut-off filter to eliminate data variation due to individual performance difference in measurement apparatus in eye-diagram measurements. Mask margin was measured in the mask tests. In unfiltered eye-diagram measurements without the RF cut-off filter, on the other hand, high-speed performances of optical modulators can be characterized in a wider RF frequency bandwidth up to the limitation of measurement apparatus, although mask tests are unavailable. In this paper, filtered and unfiltered eye-diagram measurements were executed. In filtered eye-diagram measurements, an optical sampling head (Agilent 86105B), in which Bessel-Thomson (BT) filter is installed as a RF cut-off filter, was utilized. Another optical sampling head (Agilent 86116C-040 and 86107A) was utilized for RF bandwidth higher than 40 GHz in unfiltered eye-diagram measurements. The output eye diagram in Fig. 1 was acquired in an unfiltered eye-diagram measurement.
Light from single-mode continuous-wave laser was focused into the Si modulator, and output from the modulator was taken using lensed PANDA fibers as illustrated in Fig. 1. Laser wavelength and power were 1585.52 nm and 14 dBm, respectively. Input laser light to and output light from Si MZ modulators were in transverse-electric polarization.
3. Silicon MZ modulator with reduced series resistance
Silicon MZ modulators studied here were fabricated by CMOS-compatible fabrication processes using 8-inch silicon-on-insulator (SOI) wafers having 2-μm thick buried oxide layer. The Si MZ modulators consist of MZ waveguide interferometers, which include 4-mm length Si rib-waveguide phase shifters in interferometer arms and channel waveguides connected to these phase shifters. The rib and channel waveguides were patterned in 220 nm-thick SOI layer. Lateral PN junction was formed in a Si rib-waveguide phase shifter . In Fig. 1, top-view photograph of an asymmetric Si MZ waveguide interferometer is presented with an illustrated cross-section of a Si rib-waveguide phase shifter. The photograph was taken before fabrication of RF traveling-wave coplanar electrodes on the waveguide interferometer. Width w and thickness tR of the central rib part are 600 nm and 220 nm, respectively. Thickness tS of a Si slab on each side of the core is 95 nm.
The slab thickness has a significant effect on the resistance-capacitance coupling. To estimate coupling time constant, an equivalent circuit in Fig. 1 is adopted to the rib-waveguide phase shifter. Lateral PN junction is a capacitor. Junction resistance is neglected because of low leakage current at applied reverse bias ~5 V. A Si slab on each side of the central rib is a series resistor. Carrier depletion and recovery corresponds to discharging and recharging in the capacitor. Compared to a modulator with a slab thickness of 60 nm , the series resistance of a modulator with a slab thickness of 95 nm was extracted to be reduced from 8.2 Ω to 5.7 Ω. Capacitance C is 1.6 pF for 4 mm-long phase shifter at the applied reverse bias, same as in the previous report, since rib width and dopant concentration are unchanged . Response time defined as 2RC with the series resistors on the both sides is estimated as 18ps in the present phase shifter, while as 26 ps for the phase shifter with 60-nm thickness Si slabs. This leads to reduction in each of rise and fall times by 16 ps in output optical waveforms.
Rise and fall times in output optical waveforms from the Si MZ modulators with 60-nm and 95-nm slab thickness were measured by applying a square-like periodic RF waveform as presented in Fig. 2 . Mean rise and fall times in the input RF waveform are 20.8 ps and 21.4 ps. In the output waveforms, mean rise and fall times are 39.8 ps and 29.0 ps for the modulator with 60-nm thickness, while 22.2 ps and 21.5 ps with 95-nm thickness. The reduction in the rise time is more than 17 ps in good agreement with the estimation based on the equivalent circuit. The fall-time reduction is 7.5 ps, which seems to be limited by the response time of the measurement apparatus.
Fiber-to-fiber loss in the present Si MZ modulators is lower than 10.5 dB over C and L-bands as the transmission spectrum in Fig. 3 . Out of this, optical loss in the phase-shifters account for 3.2 dB (0.8 dB/mm). The phase-shifter loss is same as that with 60-nm slabs . However, additional phase-shifter loss is generated if the slabs are thicker than 95 nm, since the optical mode is not sufficiently confined in a waveguide core and more extended towards side electrodes. The Si MZ modulators with 95-nm thick Si slabs allow enhancement of high-speed response without additional optical loss.
4. Eye-diagram measurements and mask tests
4.1 Filtered eye-diagram measurements with mask tests at 10.3-11.3 Gbps
Filtered eye-diagram measurements incorporating mask tests were executed for output optical data from a Si MZ modulator at 10.3-11.3 Gbps. The measured data are presented in Fig. 4 . At 10.3 Gbps, 10GBE mask for Ethernet protocol was inserted to the eye diagram. Mask margin is indicated as a dark-gray area surrounding a mask of a light-gray diamond-shaped or rectangular box in each eye diagram. A mask margin of 27% has been obtained. At 10.7 Gbps and 11.3 Gbps, STM-64/OC-192 mask for SDH/SONET protocol was inserted to the eye diagrams. Mask margin is 16% and 10%, respectively. Therefore, optical data in SDH/SONET and Ethernet protocols can be generated in NRZ formats at 10.3-11.7 Gbps by using the Si MZ modulators. These values in extinction ratio are higher than those reported for Si MZ modulators having silicon-insulator-silicon capacitor phase shifters [3,4]. In terms of mask margin, the highest value has been reported as 57% with masks for Ethernet protocols, whereas no attempts have been made with masks for SDH/SONET protocols . Extinction ratios and mask margins in the filtered eye-diagram measurements are listed in Table 1 .
4.2 Unfiltered eye-diagram measurements at 10.0-11.3 Gbps
Unfiltered eye diagrams without the RF low-pass filter were acquired at 10.0-11.3Gbps to measure extinction ratio of the Si MZ modulator in a higher measurement bandwidth. The eye diagrams are presented in Fig. 5 . Extinction ratio is 13.9 dB, 13.8 dB, 13.9 dB and 13.8 dB at 10.0 Gbps, 10.3 Gbps, 10.7 Gbps and 11.3 Gbps, respectively. About 14-dB extinction ratio has been achieved with the Si MZ modulators. This is almost as high as the extinction ratio reported for the LiNbO3 modulators at 10 Gbps . Peak-to-peak voltage of 4.5-5 V was applied as RF drive voltage to the Si modulators with 50-Ω impedance. For LiNbO3 modulators of 75-GHz bandwidth, 5-V drive voltage was reported . The Si modulators can be operated with RF drive voltage almost same as for LiNbO3 modulators.
4.3 Eye-diagram measurements at 12.5 Gbps
Eye diagrams have been acquired in filtered and unfiltered measurements at 12.5 Gbps. The data are presented with an eye diagram of input RF PRBS data in Fig. 6 . Extinction ratio in the filtered eye diagram (with BT filter) is 10.3 dB, while it is 13.2 dB in the unfiltered eye diagram (no BT filter). Extinction ratio which is higher than 10 dB and 13 dB in the filtered and the unfiltered eye-diagram measurements is verified at bit rates up to 12.5 Gbps, respectively.
High-speed response of Si MZ NRZ modulators having lateral PN junction rib-waveguide phase shifters has been enhanced by increasing side slab thickness from 60 nm to 95 nm and hence reducing series resistance from 8.2 Ω to 5.7 Ω. Fiber-to-fiber loss of the Si modulators with 95-nm slab thickness is lower than 10.5 dB. Additional optical loss is not generated with the increase in slab thickness. Extinction ratio higher than 10 dB at 10.3-11.7 Gbps has been verified in filtered eye-diagram measurements incorporating mask tests with mask margins of 27% (10.3-Gbps 10GBE), 16% (10.7-Gbps STM-64/OC-192) and 10% (11.3-Gbps STM-64/OC-192). In unfiltered eye-diagram measurements without mask tests, extinction ratio higher than 13 dB has been verified at 10.0-12.5 Gbps. The Si MZ modulators can be used to transmit optical data in SDH/SONET and Ethernet protocols in the high-speed optical-fiber telecommunications.
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