Silicon p+-i-n+ diode Mach-Zehnder electrooptic modulators having an ultra-compact length of 100 to 200 µm are presented. These devices exhibit high modulation efficiency, with a Vπ·L figure of merit of 0.36 V-mm. Optical modulation at data rates up to 10 Gb/s is demonstrated with low RF power consumption of only 5 pJ/bit.
© 2007 Optical Society of America
A primary factor driving the high level of research interest in the silicon photonics platform originates from this system’s intrinsic compatibility with CMOS electronics. The ability to leverage the mature manufacturing infrastructure behind this technology creates the potential for integrating advanced silicon-based waveguide networks as high-bandwidth on-chip optical interconnects for high performance computing applications [1, 2]. One of the principal components required within any optical communication link is the electrooptic modulator, which serves the function of encoding an optical carrier wave with a high speed electronic data signal.
Silicon modulators based upon free carrier electrooptic effects  have continued to evolve over the past two decades. Beginning with epitaxial silicon-on-silicon substrates, in which a lightly doped Si layer on a heavily doped Si substrate served as the optical waveguide , rib waveguide Mach-Zehnder interferometers  and electroabsorption modulators  using embedded p+-i-n+ diodes for charge injection were demonstrated. Modulator development subsequently progressed toward the use of silicon-on-insulator (SOI) substrates [7, 8], where high index contrast optical confinement has enabled the use of compact low-loss waveguide bends (radius <5 µm) , ultimately resulting in the deeply scaled nanophotonic wire devices frequently used today.
A recent example of the extent to which SOI optical modulators have been scaled is illustrated by devices based upon the micro-ring resonator. Charge injection based p+-i-n+ diode modulators with ultra-compact footprint (approximately 0.0002 mm2) have been realized using high quality factor optical micro-resonators [10–12]. The compact nature of these devices is made possible on account of the enhancement of modulation efficiency near sharp spectral resonances. However, it is precisely this same resonant enhancement which simultaneously makes these structures highly sensitive to small variations in bias conditions, operating temperature, and fabrication tolerances.
In contrast, the familiar Mach-Zehnder modulator (MZM) geometry possesses the advantage of broadband spectral operation and improved tolerance to environmental and process fluctuations. Recent studies of high-performance SOI MZMs have included metal-oxide-semiconductor (MOS) capacitor [13, 14] and p-n diode [15, 16] driven devices. However, the large footprint occupied by these modulators, having active lengths on the order of several millimeters, restricts their use within highly integrated ultra-compact on-chip interconnect systems, where it is likely that hundreds if not thousands of modulators will be required on a single die. An ultra-compact MZM device would therefore be an indispensable choice for many on-chip applications, including filtering and switching functions in addition to optical modulation.
In this report, we demonstrate several ultra-compact (100 to 200 µm length), low RF power (5 pJ/bit), high speed SOI MZM devices, capable of operating at data rates up to 10 Gb/s . Despite the conventionally held view that a non-resonant modulator such as the MZM must naturally be designed with a long optical interaction length, the devices reported here demonstrate that it is indeed possible to design a high-performance ultra-compact MZM component with micron-scale dimensions, suitable for on-chip optical interconnect applications.
2. Design and fabrication
The ultra-compact MZM devices demonstrated here were designed using nanophotonic rib waveguides with embedded p+-i-n+ diodes as charge injection-based electrooptic phase shifters. A cross-sectional scanning electron microscope (SEM) image of the active waveguide structure is shown in Fig. 1(a). These rib waveguides were nominally 550 nm wide and 220 nm in height, with a 35 nm silicon slab providing a conductive path between the anode and cathode, for the injection of free carriers into the intrinsic waveguide core.
In comparison with previously demonstrated SOI MZM devices, where much larger rib waveguides having cross-sectional areas on the order of 1 µm2 or larger were employed, the ultra-compact cross-section of nanophotonic rib waveguides simultaneously confines both the injected free carriers and the fundamental TE00 mode profile to an effective area of approximately 0.12 µm2. Localization of both photons and free carriers within such a small region produces two effects beneficial for efficient modulator performance. First, during each ON/OFF modulation cycle, the diode injection current is forced to flow through a cross-section only 220 nm in height, producing large changes in the free carrier density within the intrinsic rib waveguide core. Second, the ultra-compact modal cross-section assures nearly complete free carrier/optical mode overlap, resulting in large modulation of the modal effective index. Taken together, these two characteristics of p+-i-n+ diode nanophotonic rib waveguides lead to highly efficient operation as electrooptic phase shifters, enabling MZM devices requiring greatly reduced active length, as will be illustrated below.
Furthermore, while rib waveguides with larger dimensions require large radius bends (R>20 µm) in order to avoid excessive bend losses, the nanophotonic rib waveguide yields ultra-compact low-loss bends with radii <5 µm , as well as ultra-compact Y-junction splitters . Therefore, the total footprint of MZM devices designed with nanophotonic rib waveguides is limited only by the length of the phase shifter section LMZM, rather than the bend radius.
MZM devices with various p+-i-n+ diode phase shifter lengths LMZI (as shown in Fig. 1(b) inset) were fabricated on a SOI substrate (p-type, resistivity 10 Ω-cm) having a 2 µm thick buried oxide layer. A series of aligned electron beam lithographic exposures was used to define rib waveguides, anode (boron, p+~1×1020 cm-3) and cathode (phosphorus, n+~1×1020 cm-3) implants, and ohmic contact pads using nickel silicide. The distance between the heavily doped regions (hatched areas in Fig. 1(a)) and the edges of the rib waveguide core was nominally 400 nm. However, the nickel silicide contact pads (highlighted in gold) were formed closer to the waveguide edges than intended, resulting in additional on-chip optical loss as will be described below. Finally, polymer spotsize converters were formed over SOI waveguide inverted tapers  for efficient coupling to tapered optical fibers, and device chips were cleaved for testing.
3. Experimental results and discussion
An electrical I-V curve obtained for a MZM with a 100 µm long active p+-i-n+ phase shifter region is shown in Fig. 1(b). Under typical high speed operating conditions, large current densities flow across the nanophotonic rib waveguide cross-section, as the p+-i-n+ diode is modulated between forward (ON) and reverse biased (OFF) states. Despite channeling of this current through the thin 35 nm silicon slab at the anode/cathode terminals, the forward differential resistance of 49 Ω illustrated in the figure indicates that low electrical impedance can be achieved, as necessary for matching to 50 Ω drive electronics and ensuring low power consumption. Furthermore, this measurement serves to emphasize that the high impedance observed previously (on the order of 10 kΩ) for comparable nanophotonic rib diode structures [10, 11] is not a fundamental limitation of the waveguide structure itself, but rather depends upon the dopant profile and contact resistance. In addition, the measured device exhibited a reverse leakage current of less than 10 nA, indicating that the reverse bias potential generates a large electric field localized across the depleted intrinsic region of the waveguide core, resulting in fast, efficient sweep-out of injected carriers. Finally, a reverse bias capacitance of 200 fF was measured, suggesting that this device has a high-frequency RC cutoff of approximately 16 GHz.
The spectral transmission characteristics of the MZM devices near λ=1550 nm were characterized using a broadband LED light source polarized to excite the TE guided mode, and an optical spectrum analyzer. Interference fringes with a free spectral range (FSR) of 7.3 nm were observed, corresponding to a built-in 80 µm path length difference between the two branches of the MZM, as shown in the inset schematic of Fig. 1(b). A balanced MZM would be desirable to maximize the optical bandwidth of the device. However, for illustration purposes in this work, the presence of interference fringes assisted in distinguishing between thermooptic and electrooptic effects under low-frequency modulation, and in the measurement of extinction ratios.
A measured on-chip loss of 12 dB and relatively low extinction ratio of 6–10 dB originated primarily from metal losses caused by unintentional placement of the nickel silicide contact pads approximately 200 nm away from the edges of the rib waveguide, as shown in Fig. 1(a). However, these losses do not present a fundamental limitation. Increasing the distance between the lossy contact pads and the waveguide without moving the highly doped p+ and n+ regions would result in nearly identical electrical performance, while significantly decreasing on-chip loss and improving extinction.
Measurements of the figure of merit Vπ·L and data transmission, described below, were carried out by coupling light from a TE-polarized tunable laser into the MZM device and tuning to a quadrature point near λ=1550 nm. The modulated output was amplified by an erbium doped fiber amplifier (EDFA), and detected using a 15 GHz optical receiver connected to a digital communications analyzer (DCA) electrical module with 20 GHz bandwidth.
Measurement of Vπ under AC conditions permits observation of the intrinsic free carrier electrooptic efficiency of the MZM, which under DC operating conditions can be masked to a great extent by slow thermooptic effects. The voltage Vπ required to produce a carrier injection-induced π phase shift was measured by applying a 9 MHz sinusoidal electrical drive signal, as shown in Fig. 2(a), to a LMZM=200 µm long modulator, under a forward bias conditions (Vbias=1.8 V). The drive amplitude was increased until the slope of the modulated optical signal changed sign at the peaks/troughs of the drive waveform, as illustrated in Fig. 2(b). A complete half-period of optical modulation (between the two arrows in Fig. 2(b), for example) was observed for a peak-to-peak applied voltage of Vπ=1.8 V, leading to a MZM figure of merit of Vπ·L=0.36 V-mm. This value of Vπ resulted from a change of Δn~4×10-3 in the modal effective index within the 200 µm long phase shifter, corresponding to injection of a free electron-hole pair density of approximately 1.5×1018 cm-3. In comparison with recently published silicon Mach-Zehnder modulators [13–16], the measured Vπ·L figure of merit represents a 100x improvement in the electrooptic modulation efficiency. Furthermore, while previous MZM devices have required lengths on the order of several millimeters to achieve a significant phase shift, the present device demonstrates a π phase shift with an active section approximately 20x to 50x shorter. As discussed above, the observed improvement in performance originates from simultaneous confinement of both the injected free carriers and the mode profile within the ultra-compact 0.12 µm2 cross-section of the SOI nanophotonic rib waveguide structure.
The high frequency performance of charge injection-based silicon electrooptic modulators is limited by the slow recombination dynamics of minority carriers injected into the p+-i-n+ diode junction under forward bias , while the depletion of injected carriers in reverse bias can occur on much shorter time scales . The large asymmetry between the rise and fall time of the carrier concentration within the nanophotonic rib waveguide waveguide, and accordingly the free carrier induced phase shift, results in significant intersymbol interference (ISI) which limits the maximum data rate to approximately 1 Gb/s . However, pre-emphasis is a commonly used technique for mitigating signal degradation in systems suffering from ISI [22, 23], and was used to demonstrate high speed operation of the ultra-compact MZM devices described here. A NRZ electrical drive signal from a pseudo-random bit sequence (PRBS) generator was pre-emphasized in similar manner to that described by Xu et. al. . For all experiments discussed below, the modulator drive signal was composed of a 1.2 V peak-to-peak NRZ pattern with pre-emphasis pulses having 3.5 V peak amplitude occurring at each transition between a 1-bit and a 0-bit. A bit sequence from a 5 Gb/s pre-emphasized drive signal is shown in Fig. 3(a), while Fig. 3(b) illustrates the corresponding modulated optical output when the drive signal was applied to a 100 µm long MZM device.
Figure 4(a) contains an eyeline diagram of the modulated optical signal at the output of a 100 µm long MZM operating at 5 Gb/s (27-1 PRBS). Under these conditions, the device was biased at Vbias=0.3 V, and drew a DC current of 2.17 mA. Given the p+-i-n+ diode forward resistance of 49 Ω (Fig. 1(b)), this corresponds to a DC power consumption of 230 µW. An additional RF average power of 41 mW was required for data transmission, calculated assuming a 49 Ω load driven by the 5 Gb/s pre-emphasized electrical waveform. Operation at 10 Gb/s was demonstrated using the increased optical modulation efficiency afforded by a longer 200 µm MZM device, as illustrated in Fig. 4(b). In this case, the modulator device was unbiased, drew a DC current of 2.69 mA under operating conditions, and consumed DC and RF power of 287 µW and 51 mW, respectively. The high-speed 10 Gb/s performance of the 200 µm long MZM represents a 25x improvement in demonstrated data rate in comparison with recent work in which a MZM of similar length (250 µm) and waveguide design was reported [24, 25].
As the above measurements illustrate, the RF power is by far the dominant contribution to the power dissipation. While this is true of most SOI optical amplitude modulators, the RF power is seldom reported in the literature, as shown in the device comparison compiled in Table 1. Most frequently, only the bias power, or the power required to switch the optical transmission from ON to OFF under DC conditions (i.e. no data transmission) is disclosed. In general, this figure severely underestimates the power required for high speed operation, and does not present a realistic picture for link power budgeting purposes. For example, in order to compose metrics useful for comparing various high-bandwidth on-chip interconnect architectures (i.e. optical versus electrical), such as the energy required to transmit a single bit, a full tally of all power consumed by the modulator is needed. In comparison with the only report of RF power consumption of which the authors are aware [24, 25] (100 to 200 mW for a 10 GHz tone, also no data transmission), the 51 mW power consumption reported here for 10 Gb/s data transmission is superior, corresponding to a required energy per bit of approximately 5 pJ/bit. It is possible that a small fraction of this power is reflected from the p+-i-n+ diode due to impedance mismatch at high frequencies. Design of optimized on-chip termination loads may permit the energy per bit to be reduced further.
It is worthwhile noting that an optimized 100 µm long MZM of the present design may occupy a miniscule silicon footprint as small as 100 um×10 um=0.001 mm2, this being only ~5x larger than the footprint occupied by a typical microring resonator modulator [10, 11]. In applications where the broadband optical performance and improved tolerance to on-chip temperature variations and fabrication process fluctuations are critical, the electrooptic MZM design presented here illustrates that suitable broadband SOI nanophotonic modulators can be realized without paying the prohibitively large footprint penalty conventionally associated with comparing non-resonant versus resonant modulator designs.
We have demonstrated SOI Mach-Zehnder electrooptic modulators having ultra-compact 100 to 200 µm long p+-i-n+ diode active regions, suitable for on-chip optical interconnect applications. The measured Vπ·L figure of merit was 0.36 V-mm, this being a factor of 100 times smaller than previously reported for SOI MZM devices. This enhanced electrooptic efficiency originated from nearly complete free carrier/optical mode overlap in high-confinement nanophotonic rib waveguides. Using electrical pre-emphasis, NRZ optical modulation at data rates up to 10 Gb/s has been shown, with low RF power consumption of 51 mW or 5 pJ/bit.
The authors would like to thank Alexander Rylyakov, Clint Schow, and Dan Kuchta for providing instrumentation and valuable advice for pre-emphasis equalization measurements. This work was supported in part under DARPA contract N00014-07-C-0105.
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