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We introduce the concept of dual-illuminated photodetectors for high-power applications. Illuminating the photodetector on both sides doubles the number of optical channels, boosting DC and RF power handling capability. This concept is demonstrated utilizing multiple-stage dual-illuminated traveling wave photodetector circuits in silicon photonics, showing a maximum DC photocurrent of 112 mA and a 3-dB bandwidth of 40 GHz at 0.3 mA. Peak continuous-wave RF power is generated up to 12.3 dBm at 2 GHz and 5.3 dBm at 40 GHz, at a DC photocurrent of 55 mA. High speed broadband data signals are detected with eye amplitudes of 2.2 V and 1.3 V at 10 Gb/s and 40 Gb/s, respectively. A theoretical analysis is presented illustrating design tradeoffs for the multiple-stage photodetector circuits based on the bandwidth and power requirements.
© 2015 Optical Society of America
1. Introduction
Silicon photonics has recently received considerable attention in a wide range of research areas such as optical communications [1], optical interconnects [2], optical sensing [3,4], microwave photonics [5], and particle accelerators [6], because of its complex functionalities, high yield, and compatibility with electronic CMOS fabrication process for low cost. One of the key building blocks in silicon photonic integrated circuits (PICs) is monolithically integrated germanium (Ge) photodetector. Many Ge-based photoreceivers have been demonstrated in silicon PICs for optical communications [1]. In addition to highly sensitive photodetection, detectors with high-power and high-speed capability are particularly important for another wide range of applications that require high gain, excellent linearity and low noise floor, such as next-generation optical communications, analog optical links, and photonic microwave applications [7,8].
To achieve high-speed detection under high optical power, the design of photodetectors needs to overcome the fundamental physical constraints, such as space charge effect and thermal failure [9]. These constrains can be removed with careful designs in both device structure and circuit architecture. The former focuses on the optimization of high-speed device structure to handle high optical power in photodiodes, such as InP-based uni-traveling carrier photodetector (UTC-PD) and evanescently coupled photodetectors [10–15]. These UTC-PD chips can be integrated on silicon photonic platform using the bonding technique [12,13]. Although the InP-based UTC-PDs possess superior power handling capability [10–13], depending on the number of bonded photodetectors, the fabrication process could be more challenging than the monolithically grown photodetectors, especially on 8” or 12” silicon photonic wafers having the commonly used 220 nm device silicon layer. Monolithic Ge-based UTC-PDs have also been demonstrated with a maximum RF output power of −11.7 dBm at 30 GHz and a 3-dB bandwidth of 30 GHz [14]. On the other hand, large evanescently coupled Ge photodetectors were demonstrated with a saturation DC photocurrent of 120 mA at −8 V bias and a 3-dB bandwidth of 4.5 GHz [15]. The low 3-dB bandwidth is limited by the junction capacitance due to the large device area.
Another approach to improve power handling is to design distributed circuit architectures to spread optical power among many photodetectors, such as photodiode arrays and traveling-wave photodetectors [13,16–21]. This approach is particularly suitable for silicon PICs because the silicon PIC process facilitates the design of complex circuit architectures connected by low loss optical waveguides. Traveling wave Ge photodetector arrays were demonstrated in [21], showing a saturation DC photocurrent of 65 mA at −4 V with a bandwidth of 20 GHz, measured at low optical power level.
Previous high-power photodetector designs, to the best of our knowledge, are illuminated on one side [10–21]. According to Beer’s law for photon absorption, the optical power in the waveguide photodetector theoretically decays to 36% after propagating one characteristic absorption length 1/Γα, where Γ is the confinement factor and α is the absorption coefficient. The region within one characteristic absorption length generates most electron-hole pairs into the electrical current, whereas the region beyond this length contributes less to convert the optical power, and it instead reduces the device bandwidth by increasing parasitic loading. Such a situation is even more critical for high-power operation, as the increased photocurrent would cause further saturation from the space charge and the thermal effects within the characteristic absorption length of the devices [9,15], while the rest of the photodetector can still generate photocurrents. Effective use of the unsaturated region, such as feeding each photodetector on two sides, can significantly improve the conversion efficiency and performance of the high-power photodetectors.
In this paper, we introduce, for the first time, the concept of the dual-illuminated photodetector to double the power handling capability of a single photodetector. The power handling can be further improved by combining the proposed dual-illumination concept with the circuit architectural features of multiple-stage photodiode arrays that use a traveling wave design. This idea is universal and can be applied to any type of photodetector (e.g. PIN, MSM or UTC-PD) and photonic platforms (e.g. silicon or III-V) for high power applications. To illustrate this concept, dual-illuminated periodically-loaded Ge traveling wave photodetectors (TWPDs) were demonstrated to distribute optical power into photodetectors with multiple optical input ports using the standard process in the silicon photonics foundry [22,23]. As a result, these TWPDs substantially increase the saturation DC photocurrent to 112 mA (limited by coupling loss and EDFA) at −3V with a 3-dB bandwidth of 40 GHz at 0.3 mA. Without changing the standard process in the silicon photonic foundry, our high-power photodetector circuit can be easily integrated with other photonic building blocks, such as waveguide modulators and passive components, enabling the design of integrated photonic systems with more complex functionalities.
2. Design and fabrication
Figure 1 shows the circuit architecture of our n-stage dual-illuminated periodic traveling wave photodetectors. Each photodetector is illuminated on both sides by splitting the optical power from the common source. This power-splitting architecture offers two advantages over the conventional one-side illuminated architecture. First, it doubles the power handling capability of each photodetector by effectively using the entire absorbing region of the photodetector. Second, it can distribute the device temperature more uniformly, enlarging the maximum optical input power prior to thermal failure.
The power handling capability can be further improved by connecting these dual-illuminated photodetectors into photodetector arrays. This improvement is however traded off with a reduction in bandwidth resulting from an increase in the total capacitance of the photodetector arrays. This bandwidth constraint can be removed by using the n-stage periodic-loaded traveling wave configuration, in which the lumped device problem is converted into electrical transmission line design that requires velocity matching. Consequently, the n-stage dual-illuminated TWPD is fed by 2n input optical channels, where every two optical channels form a pair that propagates with identical optical delay into the photodetector (dual illumination). The optical and electrical velocity matching, facilitated by the optical delays, determines the distance between each photodetector. The optical signals in each pair are then simultaneously fed into both sides of the corresponding photodetector, generating photocurrents that are summed up in phase due to velocity matching. In practice, an n-stage dual-illuminated TWPD could be constructed by n photodetectors, 2n-1 1 × 2 waveguide splitters, and 2n-2 optical delay lines relative to the path of PD1 (see Fig. 1).
We designed two dual-illuminated TWPDs: a 4-stage and an 8-stage PIC. The 4-stage PIC contains four PIN photodetectors, seven 1 × 2 waveguide splitters, six optical delay lines, and a surface-normal grating coupler. Furthermore, the 8-stage PIC is composed of 38 integrated optical components. By considering the periodic capacitive loading of the photodiodes, we designed the electrical transmission lines based on impedance matching and velocity matching. A microwave transmission line composed of coplanar strips was designed with a simulated characteristic impedance of 50 Ω and an electrical velocity of 4.5x107 m/s. To match the optical group velocity that was simulated to be about 7.5x107 m/s, we designed the optical delay line so that the optical path length was about 67% longer than the electrical path length. For high-speed testing, the TWPD is terminated with an on-chip 50 Ω resistor to minimize the reflected traveling wave. The TWPDs were fabricated on an 8” silicon-on-insulator wafer with a device silicon layer of 220 nm [23]. The chip size is 1 mm x 0.7 mm including the probing pads, the input coupling waveguide and the grating coupler. The effective area of the TWPD is much smaller.
3. Experimental results
We tested the photodetectors via surface-normal grating couplers, which have about 4-4.5 dB of coupling loss for the TE polarization. The waveguide loss and the splitter loss were estimated to be about 2.5 dB/cm and 0.2-0.3 dB, respectively. Three configurations were measured to compare the high-power characteristics: single photodetector, single photodetector with dual illumination, and TWPD with dual illumination.
3.1 DC characteristics
We first characterize the DC performance of the photodetectors, including the dark current and the responsivity. At −3 V, the dark current is measured to be about 3.5 μA for the single photodetector, while the TWPD shows a larger current of 59.2 mA, mainly due to the current flow through the 50 Ω termination. This termination current is calibrated out in our photocurrent measurements. To test the DC photocurrents, the laser wavelength and power were set to be 1550 nm and 5 dBm, respectively. Different optical power levels were then generated using an EDFA and monitored by a reference photodetector. Figure 2 shows the DC photocurrents as a function of input optical power at the device for three configurations, with the coupling loss and the splitter loss being calibrated out. The responsivity at −3V was extracted to be 0.80, 0.79, 0.76 A/W for the three configurations, respectively. It follows that the single photodetector with dual illumination can handle more optical power than that with single illumination. The photocurrent increases from about 17 mA to 43 mA. The dual-illuminated TWPD can further boost the power handling capability, resulting in a DC photocurrent of 112 mA at 153 mW input power. Note that this TWPD has not reached its maximum power-handling point; instead, we are limited by the coupling loss, setup loss and EDFA maximum output power. This limitation can be removed by reducing the system loss (e.g. improved coupling) and by using higher power EDFA.
Fig. 2 DC photocurrent as a function of input optical power at the device, (a) Comparison of three different configurations at −3 V, (b) Four-stage dual-illuminated traveling wave photodetector.
3.2 RF characteristics: bandwidth and RF output power
To test high-speed performance of the TWPDs, we first measured the electrical-electrical (E/E) S-parameters of the TWPDs (see Fig. 3) using the Keysight N4373D lightwave component analyzer (LCA). This measurement allows us to verify the transmission line design, including the characteristic impedance, RF loss and the electrical velocity of the traveling wave structure. What follows demonstrates that our design matches 50 Ω and the targeted electrical velocity 4.5x107 m/s well. For comparison, we also plot our S-parameter simulations that include the circuit model of the photodetector extracted from the single photodetector measurement. The measurements and simulations show good agreement for the 4-stage TWPD, while the S11 measurement is slightly different from the simulation for the 8-stage TWPD due to aggregation of uncertainties in the transmission line model and individual device characterization. The impact on RF return loss is not significant, as the resulting performance is better than −10 dB over the frequency band of interest.
Fig. 3 E/E S-parameters measurement and ADS modeling of the 4-stage and 8-stage TWPDs, (a) Magnitude of S11 at −3 V, (b) Magnitude of S21 at −3 V.
To investigate the effect of the traveling-wave architecture on the RF performance, we also simulated the 4-stage and 8-stage PD arrays without transmission line elements between the diodes, hence as a lumped element array as opposed to a traveling wave array. Both simulated S11 and S21 show worse performance than the traveling wave designs, particularly in the high frequency range. Due to the impedance mismatch between the lumped PD arrays and the system impedance, the S11 results for this PD array is worse than −10 dB, which would results in RF mismatch loss and thus ripples in the transmission function, versus frequency. Moreover, due to the large capacitance summed up by the each photodiode, the E/E simulated bandwidth of the PD arrays drops significantly (more than 10 GHz) compared to the traveling wave cases, indicating that a traveling wave design is necessary for the PD arrays.
We then tested the optical-electrical (O/E) bandwidth of the TWPDs by using the same setup, in which an EDFA was used to adjust the input power levels. Figure 4(a) shows the O/E frequency response of the TWPDs under different optical input power. At a bias of −3 V, we obtain a 3-dB bandwidth of about 40 GHz and 32.5 GHz for the 4-stage and 8-stage TWPDs, respectively, at 0.3 mA DC photocurrent. Due to the nonlinear properties of the photodetector [24], the bandwidth drops slightly when the optical power increases, which shows a 3-dB bandwidth of 35 GHz and 25 GHz for the 4-stage and 8-stage TWPDs, respectively, at 13 mA DC photocurrent.
Fig. 4 Normalized O/E frequency responses of the 4-stage and 8-stage TWPDs at −3V using (a) Lightwave component analyzer, (b) Optical heterodyne setup.
To further characterize the photodetector circuits, we tested the circuits using an optical heterodyne setup [24]. This measurement technique offers several advantages that complement the LCA method well, such as the ability to measure absolute RF power and the flexibility to reduce optical coupling loss due to the limitation of our LCA setup. After optimizing the beat RF signals by tuning the laser power and the polarization, the modulated optical signals at different beat frequencies (detected by a reference photodiode) were sent into the photodetector circuit under test. The generated photocurrents were then collected by a calibrated RF power meter. Figure 4(b) shows the large-signal frequency response of the TWPDs under different optical input power. The frequency response based on the heterodyne measurement matches that based on the LCA measurement well, showing a variation of less than 0.5 dB in RF response. At a bias of −3 V, we obtain 3-dB bandwidths of about 38 GHz and 31.6 GHz for the 4-stage and 8-stage TWPDs, respectively, at 2 mA photocurrent. When the optical power increases to a DC photocurrent of about 55 mA, the bandwidth reduces to about 9.5 GHz and 15 GHz for the 4-stage and 8-stage TWPD, respectively. The bandwidth dependence of the photocurrent is mainly due to the space charge effect. The generated electron-hole pairs that are not quickly swept by the applied electric field create an internal electric field that is against the applied field. The transit time for the generated electron-hole pairs increase, thus reducing the bandwidth at high input power levels.
The same heterodyne setup also allows us to test the RF power handling capability. Figure 5 shows the RF output power as a function of RF frequencies at different optical power levels (biased at −3V). At a DC photocurrent of 55 mA, the 4-stage TWPD achieves a maximum RF power level of about 12.3 dBm at 2 GHz, 7.5 dBm at 20 GHz and 3.9 dBm at 40 GHz. On the other hand, the 8-stage TWPD reaches a maximum RF power level of about 12.3 dBm at 2 GHz, 8.8 dBm at 20 GHz and 5.3 dBm at 40 GHz. Note that the 8-stage TWPD has not reached its maximum RF power-handling point due to the coupling loss, setup loss and EDFA maximum output power. The RF compression is also plotted in Fig. 5, in which the results confirm that the 8-stage TWPD has better performance in terms of RF power handling.
Fig. 5 RF power handling capability of the (a) 4-stage and (b) 8-stage TWPDs at −3V. RF compression at different frequencies for the (c) 4-stage and (d) 8-stage TWPDs at −3V.
3.3 High-power high-speed data transmission
To characterize the TWPDs for data transmission, we tested the photodetector circuits with NRZ modulated signals (PRBS 231-1) at different optical power levels and different speeds. The resulting eye diagrams are shown in Fig. 6, in which at 10 Gb/s we achieve maximum eye amplitudes of 1.7 V and 2.2 V at 45 mA and 50 mA DC photocurrents for the 4-stage and 8-stage TWPDs, respectively. Also shown in Fig. 6 are data rates of 20 Gb/s, 30 Gb/s and 40 Gb/s. At these higher data rates, the 8-stage TWPD can still maintain good signal integrity and eye openings at high power levels because the bandwidth degradation due to large optical power is less severe.
4. Discussion
4.1 Design tradeoffs between bandwidth and number of stages
Figure 4 reveals that the bandwidth and power handling capability of the multiple-stage photodetector circuits mainly depends on the frequency response of each stage. This frequency response is a combination of the O/E response of each photodetector and the E/E response of the transmission line, which is related to the input optical power and the number of stages (length of the transmission line). At a low optical level where the nonlinear effect of the photodetector can be neglected, there exists a tradeoff between the bandwidth and the number of stages, which can be seen from the 3-dB bandwidths of 38 GHz and 31.6 GHz for the 4-stage and 8-stage TWPDs at 2 mA.
On the other hand, at a high power level where nonlinear effects and thermal effects play important roles, the bandwidth of each photodetector drops. In this scenario, the bandwidth of the TWPDs reduce less for the larger number of stages at the same input power because each photodetector is illuminated by less power, due to more optical power splitting before the PDs. This explains why the experiments yield eye openings at 45 mA and the 3-dB bandwidths of 9.5 GHz and 15 GHz for the 4-stage and 8-stage TWPDs at 55 mA, respectively. These results show that the design considerations for the number of stages should be determined by the bandwidth and the power requirements.
4.2 Maximum number of stages and loss mechanisms
Expanding the discussion in Section 4.1, TWPD with more stages can theoretically handle higher power, because large optical power can be split into n stages; that is, Popt/m ≤ PPD, max, where Popt is the optical input power, PPD, max is the maximum power that a single photodetector can handle either before entering the nonlinear region or before failure, m equals n for a single-illuminated TWPD and equals 2n for a dual-illuminated TWPD. Therefore, an infinite-stage TWPD circuit can ideally handle infinite optical power because each photodetector is always illuminated with the same optical power that is lower than its limitation. However from a practical standpoint, the bandwidth requirement and circuit losses limit the maximum number of stages in a high-power high-speed TWPD [25], which can be explained from optical and electrical perspectives as follows.
The maximum optical input power is limited by the optical losses of the silicon PICs, including the waveguide splitter loss and waveguide propagation loss. Note that we do not take into account the coupling loss in this discussion; therefore, Popt is the on-chip optical power after coupling into the PICs. Assuming each 1 × 2 splitter has a loss of αSP dB, a dual-illuminated n-stage TWPD requires 2n-1 splitters, resulting in a total splitter loss of (2n-1) × αSP dB. Given that the splitter loss in our silicon PICs is about 0.2 dB, the 4-stage, 8-stage, and 16-stage dual-illuminated TWPDs would possess total splitter losses of 1.4 dB, 3dB and 6.2 dB, respectively. The splitter loss becomes more significant with more stages, limiting the power that should be delivered to the photodetectors. One possible solution is to design a low loss 1 × m splitter instead of cascading many 1 × 2 splitters for large number of stages.
Another optical loss mechanism in the silicon PICs is the waveguide propagation loss. Assuming the waveguide propagation loss is αWG neper per unit length, the nth stage photodetector has an additional loss factor exp(-αWG × (n-1) × ΔLWG) relative to the first stage photodetector, where ΔLWG is the optical waveguide length difference between the adjacent photodetector. Given that αWG is about 2.5 dB/cm and ΔLWG is about 130 μm in our silicon PICs, the additional waveguide losses for the last stage with respect to the first stage photodetector are about 0.1 dB, 0.2 dB and 0.5 dB for the 4-stage, 8-stage, and 16-stage dual-illuminated TWPDs, respectively. This result indicates that the waveguide propagation loss can be neglected compared to the splitter loss in our silicon PICs. Finally, when the optical power continues to increase, the optical power would be eventually limited by the nonlinear effects of the silicon material, such as two-photon absorption and free carrier absorption.
The RF loss of the periodically loaded transmission line sets constraint on the bandwidth of the TWPDs. Figure 7(a) shows the Keysight Advanced Design System (ADS) simulation of the RF loss per unit length. To simplify the analysis, we first evaluate the capacitively-loaded transmission line by neglecting the ohmic loss of the metal (aluminum) and assuming a lossless capacitive model of the photodetector. This artificial transmission line, unlike the ideal transmission line, can be treated as a microwave low-pass filter, with a simulated roll-off frequency beyond 160 GHz. Such a roll-off frequency has a negligible effect on the bandwidth of the TWPDs. Next, the simulation results show that the ohmic loss of the aluminum transmission line has a minor effect on the RF loss, slightly increasing the per unit length loss by about 0.2 dB.
Fig. 7 (a) Simulated RF loss per unit length for the TWPDs. The lossy PD model is shown in the inset, (b) Simulated E/E S21 for the 4-stage, 8-stage, 16-stage and 32-stage TWPDs.
The RF loss of the TWPDs is dominated by the parasitics of the PIN photodetectors, particularly the serial resistance RS that is caused by p doping and n doping. The serial resistance of our photodetectors is extracted to be about 16.5 Ω, dramatically increasing the RF loss when this lossy circuit model is included, as illustrated in Fig. 7(a). This result indicates that the signal integrity is determined by the photocurrent generated by the first photodetector because this photocurrent has to propagate the longest distance to reach the probing pads, which is confirmed by the S21 simulations in Fig. 7(b) for different number of stages. Furthermore, the frequency dependence of the RF loss results in larger attenuation for the high frequency components, degrading the signal integrity for broadband applications in high-speed data transmission.
5. Conclusions
We introduce the concept of multiple-stage dual-illuminated traveling wave photodetector circuits for high-power and high-speed. This concept is demonstrated using 4-stage and 8-stage photodetector circuits in silicon photonics. Dual-illumination of the photodetectors offers several important advantages resulting in improved high-power operation: the number of optical channels is doubled, the absorbing region is more efficiently used, and the device temperature is more uniformly distributed. Based on this design, a maximum DC photocurrent of 112 mA is obtained at −3 V. The traveling wave design ensures high-speed operation: a 3-dB bandwidth of 40 GHz at −3 V and 0.3 mA. We also show that our TWPDs can generate a maximum RF power of approximately 12.3 dBm at 2 GHz and 5.3 dBm at 40 GHz, at a DC photocurrent of 55 mA. Data transmission experiments further show that our TWPDs can achieve large eye amplitudes of 2.2 V and 1.3 V at 10 Gb/s and 40 Gb/s, respectively, indicating that our dual-illuminated TWPDs can be very useful for high-power and high-speed applications, such as microwave photonics, optical sensing and optical communication. Finally, we theoretically analyze the design tradeoffs of the multiple-stage photodetector circuits based on the bandwidth and power requirements.
Acknowledgments
Part of this project is funded by Intelligence Advanced Research Projects Agency (IARPA) under the SPAWAR contract number N66001-12-C-2011. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. We acknowledge support of Drs. Carl McCants and Dennis Polla at IARPA, Drs. T. -Y. Liow and Patrick G. -Q. Lo of the Institute of Microelectronics, Singapore on fabrication, and support of M. Zirngibl at Bell Laboratories.
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