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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 11 — May. 21, 2012
  • pp: 11605–11614

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Electro-optic directed logic circuit based on microring resonators for XOR/XNOR operations

Lei Zhang, Jianfeng Ding, Yonghui Tian, Ruiqiang Ji, Lin Yang, Hongtao Chen, Ping Zhou, Yangyang Lu, Weiwei Zhu, and Rui Min  »View Author Affiliations


Optics Express, Vol. 20, Issue 11, pp. 11605-11614 (2012)
http://dx.doi.org/10.1364/OE.20.011605


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Abstract

We report the implementation of the XOR and XNOR operations using an electro-optic directed logic circuit based on two cascaded silicon microring resonators (MRRs), which are both modulated through the plasma dispersion effect. PIN diodes are embedded around the MRRs to achieve the carrier-injection modulation. The inherent resonance wavelength mismatch between the two nominally identical MRRs caused by fabrication errors is compensated by two local microheaters above each MRR through the thermo-optic effect. Two electrical modulating signals applied to the MRRs represent the two operands of the two operations. Simultaneous bitwise XOR and XNOR operations at 100 Mbit/s are demonstrated.

© 2012 OSA

1. Introduction

With the shrinking of transistors and metal interconnects, the power-consumption and transmission-bandwidth limitations of silicon electronics have been approached [1

M. J. Kobrinsky, B. A. Block, J. F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Roberston, S. List, I. Young, and K. Cadien, “On-chip optical interconnects,” Intel Technol. J. 8, 129–141 (2004).

–3

G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, and J. Bowers, “III-V/silicon photonics for on-chip and inter-chip optical interconnects,” Laser Photon. Rev. 4(6), 751–779 (2010). [CrossRef]

]. The convergence of optics and electronics at the chip level has become a necessity for the next generation processors. Recently, the field of silicon photonics is gaining more and more attention due to the potential of fabricating optical devices using standard semiconductor fabrication techniques and integrating them with microelectronic chips [4

M. Lipson, “Guiding, modulating, and emitting light on Silicon-challenges and opportunities,” J. Lightwave Technol. 23(12), 4222–4238 (2005). [CrossRef]

–11

R. Q. Ji, L. Yang, L. Zhang, Y. H. Tian, J. F. Ding, H. T. Chen, Y. Y. Lu, P. Zhou, and W. W. Zhu, “Five-port optical router for photonic networks-on-chip,” Opt. Express 19(21), 20258–20268 (2011). [CrossRef] [PubMed]

].

On the one hand, it has been widely regarded as a means for keeping on track with Moore’s Law by using optical interconnects to provide faster data transfer both between and within microchips [1

M. J. Kobrinsky, B. A. Block, J. F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Roberston, S. List, I. Young, and K. Cadien, “On-chip optical interconnects,” Intel Technol. J. 8, 129–141 (2004).

–3

G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, and J. Bowers, “III-V/silicon photonics for on-chip and inter-chip optical interconnects,” Laser Photon. Rev. 4(6), 751–779 (2010). [CrossRef]

]. For this purpose, various optical components have been developed such as the laser sources [7

D. Liang and J. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]

], electro-optic modulators [8

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]

], photodetectors [9

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics 4(8), 527–534 (2010). [CrossRef]

], as well as the optical routers for networks-on-chips (NoC) [10

N. Sherwood-Droz, H. Wang, L. Chen, B. G. Lee, A. Biberman, K. Bergman, and M. Lipson, “Optical 4x4 hitless slicon router for optical networks-on-chip (NoC),” Opt. Express 16(20), 15915–15922 (2008). [CrossRef] [PubMed]

, 11

R. Q. Ji, L. Yang, L. Zhang, Y. H. Tian, J. F. Ding, H. T. Chen, Y. Y. Lu, P. Zhou, and W. W. Zhu, “Five-port optical router for photonic networks-on-chip,” Opt. Express 19(21), 20258–20268 (2011). [CrossRef] [PubMed]

]. On the other hand, the advanced fabrication techniques and abundant silicon photonic device library have been leveraged to develop devices and subsystems to assist in high-speed signal processing [12

J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4(8), 535–544 (2010). [CrossRef]

–16

A. Khilo, S. J. Spector, M. E. Grein, A. H. Nejadmalayeri, C. W. Holzwarth, M. Y. Sander, M. S. Dahlem, M. Y. Peng, M. W. Geis, N. A. DiLello, J. U. Yoon, A. Motamedi, J. S. Orcutt, J. P. Wang, C. M. Sorace-Agaskar, M. A. Popović, J. Sun, G.-R. Zhou, H. Byun, J. Chen, J. L. Hoyt, H. I. Smith, R. J. Ram, M. Perrott, T. M. Lyszczarz, E. P. Ippen, and F. X. Kärtner, “Photonic ADC: overcoming the bottleneck of electronic jitter,” Opt. Express 20(4), 4454–4469 (2012). [CrossRef] [PubMed]

]. It’s an attractive and promising solution to handle with high-bandwidth signals compared with traditional ways in terms of size, cost and power-dissipation.

Directed logic is a logic paradigm proposed by Hardy and Shamir [17

J. Hardy and J. Shamir, “Optics inspired logic architecture,” Opt. Express 15(1), 150–165 (2007). [CrossRef] [PubMed]

], which takes advantage of the propagation of light to carry out Boolean functions [17

J. Hardy and J. Shamir, “Optics inspired logic architecture,” Opt. Express 15(1), 150–165 (2007). [CrossRef] [PubMed]

–20

R. Soref, “Reconfigurable integrated optoelectronics,” Adv. Optoelectron. 2011, 627802 (2011). [CrossRef]

]. In essence, the directed logic circuit is a network of switching elements which control the propagation direction of light passing through the network. Unlike traditional implementation of optical logic, the directed logic is specially adapted to the features and promises of optics since it depends on the propagation of light other than the nonlinear interactions between light and materials [21

V. Van, T. A. Ibrahim, P. P. Absil, F. G. Johnson, R. Grover, and P.-T. Ho, “Optical signal processing using nonlinear semiconductor microring resonators,” IEEE J. Sel. Top. Quantum Electron. 8(3), 705–713 (2002). [CrossRef]

–23

Q. F. Xu and M. Lipson, “All-optical logic based on silicon micro-ring resonators,” Opt. Express 15(3), 924–929 (2007). [CrossRef] [PubMed]

]. Compared to traditional digital logic circuit, directed logic circuit has markedly less state delay because the operands that determine the states of the switching elements do not pass through the preceding elements in the circuits. Therefore, all elements can perform their switching functions simultaneously and the results are given instantaneously.

In this paper we report an electro-optic directed logic circuit for XOR/XNOR operations. Compared to the previous demonstration presented in [24

L. Zhang, R. Q. Ji, L. X. Jia, L. Yang, P. Zhou, Y. H. Tian, P. Chen, Y. Y. Lu, Z. Y. Jiang, Y. L. Liu, Q. Fang, and M. B. Yu, “Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators,” Opt. Lett. 35(10), 1620–1622 (2010). [CrossRef] [PubMed]

, 25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

], the plasma dispersion effect is utilized to modulate the microring-resonator-based optical switches. Degeneration of the spectra of the device shows up again as in [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

]. The XOR/XNOR operations are carried out correctly at 100 Mbit/s in both the degenerate and non-degenerate regions.

2. Design and fabrication

The schematic of the XOR/XNOR directed logic circuit is shown in Fig. 1(a) . The two MRRs function as 1 × 2 and 2 × 2 optical switches in the circuit, respectively. The four ports of each MRR are denoted as input, through, add and drop according to their functions. Monochromatic light with the wavelength of λ coupled into the input and add ports will be directed to the drop and through ports, respectively, if the MRR is on-resonance at λ. And if the MRR is off-resonance at λ, light coupled into the input and add ports will be guided to the through and drop ports, respectively (i.e. bypass the MRR).

Fig. 1 (a) Schematic and (b) micrograph of the electro-optic XOR/XNOR directed logic circuit based on two cascaded microring resonators (CW: continuous wave, MRR: microring resonator, EPT: electrical pulse train, OPT: optical pulse train).

Two electrical logic signals of X and Y are used to control the resonant states of the two MRRs, respectively. It’s assumed that the MRRs are on-resonance at λ if the applied electrical logic signals X and Y are at low levels (representing ‘0s’) and off-resonance at λ if X and Y are at high levels (representing ‘1s’). According to these rules, two logic outputs of 'X·Y+ X¯· Y¯' and 'X· Y¯+ X¯·Y' can be obtained at the drop and through ports of MRR2, respectively. Those two signals are just the results of 'X⊙Y' and 'X⊕Y' operations, where the symbols '⊙' and '⊕' represent the XNOR and XOR operators, respectively. Details on the principle are presented in [24

L. Zhang, R. Q. Ji, L. X. Jia, L. Yang, P. Zhou, Y. H. Tian, P. Chen, Y. Y. Lu, Z. Y. Jiang, Y. L. Liu, Q. Fang, and M. B. Yu, “Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators,” Opt. Lett. 35(10), 1620–1622 (2010). [CrossRef] [PubMed]

]. Those two output ports of the circuit are called drop port and through port hereinafter for simplicity. It can be noted in Fig. 1(a) that there are two arched segments in the waveguides connecting the four coupling areas of the two MRRs. Such two arched waveguides are designed on purpose to adjust the length difference between the two arms connecting the MRRs, which has a distinct impact on the response spectra of the device [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

].

The device is fabricated on a silicon-on-insulator (SOI) wafer with 220-nm-thick top silicon and 2-μm-thick buried oxide layer. Rib waveguides with a height of 220 nm, a width of 400 nm and a slab thickness of 70 nm are used to construct the circuit, which only supports quasi-TE fundamental mode [24

L. Zhang, R. Q. Ji, L. X. Jia, L. Yang, P. Zhou, Y. H. Tian, P. Chen, Y. Y. Lu, Z. Y. Jiang, Y. L. Liu, Q. Fang, and M. B. Yu, “Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators,” Opt. Lett. 35(10), 1620–1622 (2010). [CrossRef] [PubMed]

, 25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

]. The gaps between ring and straight waveguides are chosen to be 330 nm to achieve a balance between the extinction ratios of the drop and through ports of each MRR. The radii of the ring waveguides are both 10 μm. An elliptical structure (long axis = 6.25 μm, and short axis = 1.5 μm) is adopted to reduce the scattering at the crossing of the waveguides [26

T. Fukazawa, T. Hirano, F. Ohno, and T. Baba, “Low loss intersection of Si photonic wire waveguides,” Jpn. J. Appl. Phys. 43(2), 646–647 (2004). [CrossRef]

]. 248-nm deep ultraviolet (UV) photolithography is used to define the device pattern. Inductively coupled plasma etching process is used to etch the top Si layer (Figs. 2(a) and 2(b)). Spot size converters (SSCs) are integrated on the input and output terminals of the waveguides to enhance the coupling between the waveguides and the fibers. The SSC is a 200-µm-long linearly inversed taper with 180-nm-wide tip [27

M. M. Geng, L. X. Jia, L. Zhang, L. Yang, P. Chen, T. Wang, and Y. L. Liu, “Four-channel reconfigurable optical add-drop multiplexer based on photonic wire waveguide,” Opt. Express 17(7), 5502–5516 (2009). [CrossRef] [PubMed]

]. After the waveguide is etched, two PIN diodes are formed with the two ring waveguides as the intrinsic regions. The p- and n-doping concentrations are both 5.5 × 1020 cm−3, with both doped regions located 500 nm away from the sidewall of the ring waveguide ridge (Figs. 2(c) and 2(d)). After the doping, a 1500-nm-thick silica layer is deposited on the Si layer as the separate layer (SL) by plasma enhanced chemical vapor deposition (PECVD). Then a 150-nm-thick titanium nitride (TiN) layer is sputtered on the SL and two microheaters are fabricated by deep UV photolithography and dry etching (Fig. 2(e)) [28

P. Dong, R. Shafiiha, S. Liao, H. Liang, N.-N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Wavelength-tunable silicon microring modulator,” Opt. Express 18(11), 10941–10946 (2010). [CrossRef] [PubMed]

]. Another silica layer of 300 nm is deposited by PECVD on the TiN heaters. Via holes to the PIN diodes and microheaters are etched on the silica layer in two steps (Figs. 2(f) and 2(g)). Then a 1000-nm-thick aluminum layer is sputtered and etched to be wires and pads connected to the microheaters and PIN diodes (Fig. 2(h)). Finally, the end-face of the SSC is exposed by a 110-µm-deep etching process as the world-to-chip interface (Fig. 2(i)) [27

M. M. Geng, L. X. Jia, L. Zhang, L. Yang, P. Chen, T. Wang, and Y. L. Liu, “Four-channel reconfigurable optical add-drop multiplexer based on photonic wire waveguide,” Opt. Express 17(7), 5502–5516 (2009). [CrossRef] [PubMed]

]. The micrograph of the device is shown in Fig. 1(b). The 200-µm-long SSCs are not included in this micrograph. The side lengths of the two square pads located at the upper-left and upper-right of the micrograph are both 100 µm. The effective area of the device including the SSCs is about 1.2 × 0.4 mm2.

Fig. 2 Process flow of the device: (a) and (b) etching of the top Si layer by 150 nm and 70 nm, respectively, (c) and (d) p- and n-doping and through boron and phosphorus implantation, (e) deposition and etching of the TiN layer to form the microheater, (f) and (g) etching of the SiO2 layer to form the via holes to the PIN diodes and microheaters, (h) deposition and etching of the Al layer to form the wires and pads, (i) deep etching to form the end-face of the SSCs.

3. Experimental results

The fabricated device is characterized by an amplified spontaneous emission (ASE) source, an optical spectrum analyzer (OSA) and a tunable voltage source. The broadband light is coupled into the device through a lensed fiber. The output light is collected by another lensed fiber and fed into the OSA. The tunable voltage source is used to drive the microheater above the MRR with shorter resonance wavelengths. When this MRR is heated up, the effective refractive index of the optical mode in the ring waveguide increases and it will resonate at the same wavelengths as the other MRR.

The response spectra at the two output ports of the device are shown in Fig. 3 , with MRR2 being tuned by a heating voltage of 2.92 V to align the resonance wavelengths of the two MRRs. As the two arms connecting the two MRRs have the same lengths, the first and the third resonant regions in Fig. 3 are degenerate, which has been shown and explained in [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

]. The spectra of the drop ports at these two degenerate resonant regions should be flat due to the constructive interference between two light beams from two different paths [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

]. Shallow dips still appear at these two regions due to the difference of the two nominally identical MRRs and arms caused by fabrication errors.

Fig. 3 Response spectra obtained at the drop and through ports of the fabricated device with MRR2 being tuned by a heating voltage of 2.92 V to make it resonate at the same wavelengths as MRR1.

The working wavelength can be chosen in either the non-degenerate or the degenerate regions, which will be shown in the next two subsections, respectively. Simultaneous operations of XOR and XNOR operations are achieved in both operating modes.

3.1 The first operating mode: working in the non-degenerate region

The working wavelength is determined from the spectra when neither of the two PINs is actuated (Fig. 3). According to the aforementioned principle, a maximum (representing a ‘1’) and a minimum (representing a ‘0’) should be obtained at the drop port and the through port, respectively, when the two applied electrical signals are both at low level (representing two ‘0s’). We choose 1556.38 nm in the second non-degenerate region as the working wavelength (Figs. 4(a) and 4(e)).

Fig. 4 Response spectra obtained at (a-d) the drop port and (e-h) the through port of the device. Voltages applied to the PIN diodes of MRR1 and MRR2 are both 0 V in (a) and (e), 1 V and 0 V in (b) and (f), 0 V and 1 V in (c) and (g), and both 1 V in (d) and (h). The dashed arrow indicates the location of the working wavelength. MRR2 has a heating voltage of 2.92 V.

After the working wavelength has been chosen, the static operating principle is validated. As shown in Figs. 4(a)-4(d), a maximum is obtained at the drop port when the two applied electrical signals are both at low levels or high levels, and a minimum is obtained otherwise. As shown in Figs. 4(e)-4(h), a minimum is obtained at the through port when the two applied electrical signals are both at low levels or high levels, and a maximum is obtained otherwise. Therefore, the XNOR and XOR operations are performed correctly at the drop and through ports of the device, respectively. The diminishing of the extinction ratio of the activated MRRs is related to the loss induced by the injection of carriers to the ring waveguides. Such a phenomenon has no negative effect on the operation of the device since the optical signal circulating in the circuit will bypass the MRR with carriers injected [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

].

After the static operating principle is validated, the analog voltage representing logic ‘0’ and ‘1’ for each MRR should be determined. Firstly, only an electrical signal at 100 Mbit/s is applied to the PIN diode of MRR1. The output optical signals at the drop and through ports are converted to electrical signals by a high-speed photodetector and observed with a real-time oscilloscope. Waveforms with the best extinction ratio are obtained when the applied signal has an amplitude of 350 mV with an offset of 405 mV. So the voltages representing logic ‘0’ and ‘1’ for MRR1 are 230 mV and 580 mV, respectively. Secondly, only an electrical signal at 100 Mbit/s is applied to the PIN diode of MRR2. The voltages representing logic ‘0’ and ‘1’ for MRR2 are found to be 356 mV and 544 mV, respectively. In both tests, MRR2 is always tuned by a heating voltage of 2.92 V.

A monochromatic light at 1556.38 nm from a tunable laser is coupled into the device and the output light at the through and drop ports of the circuit is fed into a high-speed photodiode. Two pseudo-random binary sequence (PRBS) non-return-to-zero (NRZ) signals at 100 Mbit/s with the aforementioned magnitudes are applied to the PINs of the two MRRs. The electrical signals converted by the photodetector and the two electrical signals applied to the two MMRs are fed into a four-channel real-time oscilloscope for waveform observation. The dynamic operation results are shown in Fig. 5 . It can be found that the XOR and XNOR operations are carried out correctly at the through and drop ports simultaneously.

Fig. 5 Signals applied to two MRRs and detected at the through and drop ports in the first operating mode. Signals at 100 Mbit/s applied to (a) MRR1 and (b) MRR2. Results of (c) XOR operation result at the through port and (d) XNOR operation result at the drop port.

As shown in Figs. 5(c) and 5(d), there are positive spikes between two consecutive outputs of ‘0s’, and negative spikes between two consecutive ‘1s’, which also appear and are explained in [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

]. The duration times of those spikes, as well as the rising and falling times of the output signals, which limit the working speed of the device, are determined by the diffusion and recombination time of the free carriers in the PIN diode.

The speed of the device can be greatly improved by engineering the NRZ driving signal to be the so-called pre-emphasized type [29

Q. F. Xu, S. Manipatruni, B. Schmidt, J. Shakya, and M. Lipson, “12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators,” Opt. Express 15(2), 430–436 (2007). [CrossRef] [PubMed]

]. However, the carrier-depletion modulation mode has shown a much faster working speed over the carrier-injection modulation mode, and no pre-emphasized signal is required [30

J. C. Rosenberg, W. M. Green, S. Assefa, T. Barwicz, M. Yang, S. M. Shank, and Y. A. Vlasov, “Low-Power 30 Gbps Silicon Microring Modulator,” in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB9.

–32

X. Xiao, H. Xu, X. Y. Li, Y. T. Hu, K. Xiong, Z. Y. Li, T. Chu, Y. D. Yu, and J. Z. Yu, “25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions,” Opt. Express 20(3), 2507–2515 (2012). [CrossRef] [PubMed]

]. The improvement of the speed performance using the carrier-depletion mode is left for future work.

3.2 The second operating mode: working in the degenerate region

In the last operating mode, the working wavelength of 1556.38 nm is chosen from the fourth resonant region of the MRRs (Fig. 3). Actually, the working wavelength can also been chosen from the degenerate regions, i.e. the first and the third resonant regions according to the principle of the device [24

L. Zhang, R. Q. Ji, L. X. Jia, L. Yang, P. Zhou, Y. H. Tian, P. Chen, Y. Y. Lu, Z. Y. Jiang, Y. L. Liu, Q. Fang, and M. B. Yu, “Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators,” Opt. Lett. 35(10), 1620–1622 (2010). [CrossRef] [PubMed]

,25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

]. To verify this point, the resonance wavelength of 1546.70 nm from the third resonant regions is chosen to be the working wavelength in the second operating mode (Figs. 6(a) and 6(e)).

Fig. 6 Response spectra obtained at (a-d) the drop port and (e-h) the through port of the device. Voltages applied to the PIN diodes of MRR1 and MRR2 are both 0 V in (a) and (e), 1 V and 0 V in (b) and (f), 0 V and 1 V in (c) and (g), and both 1 V in (d) and (h). The dashed arrow indicates the location of the working wavelength. MRR2 has a heating voltage of 2.92 V.

After the working wavelength has been chosen, the static operating principle is validated. As shown in Figs. 6(a)-6(d), a maximum is obtained at the drop port when the two applied electrical signals are both at low levels or high levels, and a minimum is obtained otherwise. As shown in Figs. 6(e)-6(h), a minimum is obtained at the through port when the two applied electrical signals are both at low levels or high levels, and a maximum is obtained otherwise. Therefore, the XNOR and XOR operations are performed correctly at the drop and through ports of the device, respectively. The diminishing of the extinction ratio of the activated MRRs caused by the injected-carrier-induced loss also appears in Fig. 6, which does not hinder the operation of the device.

The characterization of the dynamic operation in the second operating mode is as same as the steps in the first operating mode except for the different working wavelength. Two PRBS NRZ signals at 100 Mbit/s are applied to the two MRRs. The modulating and result electrical signals are observed in a four-channel real-time oscilloscope. The results are shown in Fig. 7 . The XOR and XNOR operations are carried out correctly at the through and drop ports simultaneously. Spikes still show up in Fig. 7 as in Fig. 5 due to the speed-limited transitions of two different tuning statuses of the MRRs [25

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

].

Fig. 7 Signals applied to two MRRs and detected at the through and drop ports in the second operating mode. Signals at 100 Mbit/s applied to (a) MRR1 and (b) MRR2. Results of (c) XOR operation result at the through port and (d) XNOR operation result at the drop port.

The quality factors (Q factor) of the two MRRs are both about 7,000 (with the 3-dB bandwidths of about 0.21 nm). For carrier-injection modulation, high Q factors are desirable for low-voltage and low-power operation. Higher Q factor means smaller 3-dB bandwidth, so smaller carrier concentration change can result in enough extinction. However, a too high Q factor will make a MRR be too sensitive to the environmental temperature change. So a moderate Q factor of 7,000 is suitable for the device shown in this paper. While for the MRR utilizing the carrier-depletion modulation or other advanced modulation schemes, the Q factor should not be too high for an extra reason. Higher Q factor means longer photon lifetime, which will further limit the working speed of the MRR in addition to the limitations imposed by the carrier dynamic process. This is also left for discussion in our future work utilizing more advanced modulation schemes to achieve faster operation.

4. Conclusion

We implement simultaneous XOR and XNOR operations using an electro-optic directed logic circuit based on two cascaded microring resonators. Bitwise operations at 100 Mbit/s are demonstrated in two different operating modes employing carrier-injection modulation. The carrier-induced loss in the modulating process does not impede the operation of the device. Further improvement of the working speed of the device relies on the utilization of more advanced modulation schemes, which is left for future work.

Acknowledgments

This work has been supported by the National Natural Science Foundation of China (NSFC) under grants 60977037 and 60907001 and the Beijing Municipal Natural Science Foundation under grant 4112059.

References and links

1.

M. J. Kobrinsky, B. A. Block, J. F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Roberston, S. List, I. Young, and K. Cadien, “On-chip optical interconnects,” Intel Technol. J. 8, 129–141 (2004).

2.

D. A. B. Miller, “Rationale and challenges for optical interconnects to electronic chips,” Proc. IEEE 88(6), 728–749 (2000). [CrossRef]

3.

G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, and J. Bowers, “III-V/silicon photonics for on-chip and inter-chip optical interconnects,” Laser Photon. Rev. 4(6), 751–779 (2010). [CrossRef]

4.

M. Lipson, “Guiding, modulating, and emitting light on Silicon-challenges and opportunities,” J. Lightwave Technol. 23(12), 4222–4238 (2005). [CrossRef]

5.

R. A. Soref, “The past, present and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]

6.

B. Jalali, M. Paniccia, and G. Reed, “Silicon photonics,” IEEE Microw. Mag. 7(3), 58–68 (2006). [CrossRef]

7.

D. Liang and J. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]

8.

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]

9.

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics 4(8), 527–534 (2010). [CrossRef]

10.

N. Sherwood-Droz, H. Wang, L. Chen, B. G. Lee, A. Biberman, K. Bergman, and M. Lipson, “Optical 4x4 hitless slicon router for optical networks-on-chip (NoC),” Opt. Express 16(20), 15915–15922 (2008). [CrossRef] [PubMed]

11.

R. Q. Ji, L. Yang, L. Zhang, Y. H. Tian, J. F. Ding, H. T. Chen, Y. Y. Lu, P. Zhou, and W. W. Zhu, “Five-port optical router for photonic networks-on-chip,” Opt. Express 19(21), 20258–20268 (2011). [CrossRef] [PubMed]

12.

J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4(8), 535–544 (2010). [CrossRef]

13.

M. H. Khan, H. Shen, Y. Xuan, L. Zhao, S. J. Xiao, D. E. Leaird, A. M. Weiner, and M. Qi, “Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,” Nat. Photonics 4(2), 117–122 (2010). [CrossRef]

14.

S. Fathpour and N. A. Riza, “Silicon-photonics-based wideband radar beamforming: basic design,” Opt. Eng. 49(1), 018201 (2010). [CrossRef]

15.

S. Lin, Y. Ishikawa, and K. Wada, “Demonstration of optical computing logics based on binary decision diagram,” Opt. Express 20(2), 1378–1384 (2012). [CrossRef] [PubMed]

16.

A. Khilo, S. J. Spector, M. E. Grein, A. H. Nejadmalayeri, C. W. Holzwarth, M. Y. Sander, M. S. Dahlem, M. Y. Peng, M. W. Geis, N. A. DiLello, J. U. Yoon, A. Motamedi, J. S. Orcutt, J. P. Wang, C. M. Sorace-Agaskar, M. A. Popović, J. Sun, G.-R. Zhou, H. Byun, J. Chen, J. L. Hoyt, H. I. Smith, R. J. Ram, M. Perrott, T. M. Lyszczarz, E. P. Ippen, and F. X. Kärtner, “Photonic ADC: overcoming the bottleneck of electronic jitter,” Opt. Express 20(4), 4454–4469 (2012). [CrossRef] [PubMed]

17.

J. Hardy and J. Shamir, “Optics inspired logic architecture,” Opt. Express 15(1), 150–165 (2007). [CrossRef] [PubMed]

18.

H. J. Caulfield, R. A. Soref, and C. S. Vikram, “Universal reconfigurable optical logic with silicon-on-insulator resonant structures,” Photon. Nanostr. Fundam. Appl. 5(1), 14–20 (2007). [CrossRef]

19.

Q. F. Xu and R. A. Soref, “Reconfigurable optical directed-logic circuits using microresonator-based optical switches,” Opt. Express 19(6), 5244–5259 (2011). [CrossRef] [PubMed]

20.

R. Soref, “Reconfigurable integrated optoelectronics,” Adv. Optoelectron. 2011, 627802 (2011). [CrossRef]

21.

V. Van, T. A. Ibrahim, P. P. Absil, F. G. Johnson, R. Grover, and P.-T. Ho, “Optical signal processing using nonlinear semiconductor microring resonators,” IEEE J. Sel. Top. Quantum Electron. 8(3), 705–713 (2002). [CrossRef]

22.

X. Zhang, Y. Wang, J. Q. Sun, D. M. Liu, and D. X. Huang, “All-optical AND gate at 10 Gbit/s based on cascaded single-port-couple SOAs,” Opt. Express 12(3), 361–366 (2004). [CrossRef] [PubMed]

23.

Q. F. Xu and M. Lipson, “All-optical logic based on silicon micro-ring resonators,” Opt. Express 15(3), 924–929 (2007). [CrossRef] [PubMed]

24.

L. Zhang, R. Q. Ji, L. X. Jia, L. Yang, P. Zhou, Y. H. Tian, P. Chen, Y. Y. Lu, Z. Y. Jiang, Y. L. Liu, Q. Fang, and M. B. Yu, “Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators,” Opt. Lett. 35(10), 1620–1622 (2010). [CrossRef] [PubMed]

25.

L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express 19(7), 6524–6540 (2011). [CrossRef] [PubMed]

26.

T. Fukazawa, T. Hirano, F. Ohno, and T. Baba, “Low loss intersection of Si photonic wire waveguides,” Jpn. J. Appl. Phys. 43(2), 646–647 (2004). [CrossRef]

27.

M. M. Geng, L. X. Jia, L. Zhang, L. Yang, P. Chen, T. Wang, and Y. L. Liu, “Four-channel reconfigurable optical add-drop multiplexer based on photonic wire waveguide,” Opt. Express 17(7), 5502–5516 (2009). [CrossRef] [PubMed]

28.

P. Dong, R. Shafiiha, S. Liao, H. Liang, N.-N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Wavelength-tunable silicon microring modulator,” Opt. Express 18(11), 10941–10946 (2010). [CrossRef] [PubMed]

29.

Q. F. Xu, S. Manipatruni, B. Schmidt, J. Shakya, and M. Lipson, “12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators,” Opt. Express 15(2), 430–436 (2007). [CrossRef] [PubMed]

30.

J. C. Rosenberg, W. M. Green, S. Assefa, T. Barwicz, M. Yang, S. M. Shank, and Y. A. Vlasov, “Low-Power 30 Gbps Silicon Microring Modulator,” in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB9.

31.

G. L. Li, X. Z. Zheng, J. Yao, H. Thacker, I. Shubin, Y. Luo, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, “25Gb/s 1V-driving CMOS ring modulator with integrated thermal tuning,” Opt. Express 19(21), 20435–20443 (2011). [CrossRef] [PubMed]

32.

X. Xiao, H. Xu, X. Y. Li, Y. T. Hu, K. Xiong, Z. Y. Li, T. Chu, Y. D. Yu, and J. Z. Yu, “25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions,” Opt. Express 20(3), 2507–2515 (2012). [CrossRef] [PubMed]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(130.3750) Integrated optics : Optical logic devices
(230.5750) Optical devices : Resonators
(250.5300) Optoelectronics : Photonic integrated circuits
(130.4815) Integrated optics : Optical switching devices

ToC Category:
Integrated Optics

History
Original Manuscript: February 10, 2012
Revised Manuscript: April 27, 2012
Manuscript Accepted: April 30, 2012
Published: May 7, 2012

Citation
Lei Zhang, Jianfeng Ding, Yonghui Tian, Ruiqiang Ji, Lin Yang, Hongtao Chen, Ping Zhou, Yangyang Lu, Weiwei Zhu, and Rui Min, "Electro-optic directed logic circuit based on microring resonators for XOR/XNOR operations," Opt. Express 20, 11605-11614 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-11-11605


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References

  1. M. J. Kobrinsky, B. A. Block, J. F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Roberston, S. List, I. Young, and K. Cadien, “On-chip optical interconnects,” Intel Technol. J.8, 129–141 (2004).
  2. D. A. B. Miller, “Rationale and challenges for optical interconnects to electronic chips,” Proc. IEEE88(6), 728–749 (2000). [CrossRef]
  3. G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, and J. Bowers, “III-V/silicon photonics for on-chip and inter-chip optical interconnects,” Laser Photon. Rev.4(6), 751–779 (2010). [CrossRef]
  4. M. Lipson, “Guiding, modulating, and emitting light on Silicon-challenges and opportunities,” J. Lightwave Technol.23(12), 4222–4238 (2005). [CrossRef]
  5. R. A. Soref, “The past, present and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron.12(6), 1678–1687 (2006). [CrossRef]
  6. B. Jalali, M. Paniccia, and G. Reed, “Silicon photonics,” IEEE Microw. Mag.7(3), 58–68 (2006). [CrossRef]
  7. D. Liang and J. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics4(8), 511–517 (2010). [CrossRef]
  8. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010). [CrossRef]
  9. J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010). [CrossRef]
  10. N. Sherwood-Droz, H. Wang, L. Chen, B. G. Lee, A. Biberman, K. Bergman, and M. Lipson, “Optical 4x4 hitless slicon router for optical networks-on-chip (NoC),” Opt. Express16(20), 15915–15922 (2008). [CrossRef] [PubMed]
  11. R. Q. Ji, L. Yang, L. Zhang, Y. H. Tian, J. F. Ding, H. T. Chen, Y. Y. Lu, P. Zhou, and W. W. Zhu, “Five-port optical router for photonic networks-on-chip,” Opt. Express19(21), 20258–20268 (2011). [CrossRef] [PubMed]
  12. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics4(8), 535–544 (2010). [CrossRef]
  13. M. H. Khan, H. Shen, Y. Xuan, L. Zhao, S. J. Xiao, D. E. Leaird, A. M. Weiner, and M. Qi, “Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,” Nat. Photonics4(2), 117–122 (2010). [CrossRef]
  14. S. Fathpour and N. A. Riza, “Silicon-photonics-based wideband radar beamforming: basic design,” Opt. Eng.49(1), 018201 (2010). [CrossRef]
  15. S. Lin, Y. Ishikawa, and K. Wada, “Demonstration of optical computing logics based on binary decision diagram,” Opt. Express20(2), 1378–1384 (2012). [CrossRef] [PubMed]
  16. A. Khilo, S. J. Spector, M. E. Grein, A. H. Nejadmalayeri, C. W. Holzwarth, M. Y. Sander, M. S. Dahlem, M. Y. Peng, M. W. Geis, N. A. DiLello, J. U. Yoon, A. Motamedi, J. S. Orcutt, J. P. Wang, C. M. Sorace-Agaskar, M. A. Popović, J. Sun, G.-R. Zhou, H. Byun, J. Chen, J. L. Hoyt, H. I. Smith, R. J. Ram, M. Perrott, T. M. Lyszczarz, E. P. Ippen, and F. X. Kärtner, “Photonic ADC: overcoming the bottleneck of electronic jitter,” Opt. Express20(4), 4454–4469 (2012). [CrossRef] [PubMed]
  17. J. Hardy and J. Shamir, “Optics inspired logic architecture,” Opt. Express15(1), 150–165 (2007). [CrossRef] [PubMed]
  18. H. J. Caulfield, R. A. Soref, and C. S. Vikram, “Universal reconfigurable optical logic with silicon-on-insulator resonant structures,” Photon. Nanostr. Fundam. Appl.5(1), 14–20 (2007). [CrossRef]
  19. Q. F. Xu and R. A. Soref, “Reconfigurable optical directed-logic circuits using microresonator-based optical switches,” Opt. Express19(6), 5244–5259 (2011). [CrossRef] [PubMed]
  20. R. Soref, “Reconfigurable integrated optoelectronics,” Adv. Optoelectron.2011, 627802 (2011). [CrossRef]
  21. V. Van, T. A. Ibrahim, P. P. Absil, F. G. Johnson, R. Grover, and P.-T. Ho, “Optical signal processing using nonlinear semiconductor microring resonators,” IEEE J. Sel. Top. Quantum Electron.8(3), 705–713 (2002). [CrossRef]
  22. X. Zhang, Y. Wang, J. Q. Sun, D. M. Liu, and D. X. Huang, “All-optical AND gate at 10 Gbit/s based on cascaded single-port-couple SOAs,” Opt. Express12(3), 361–366 (2004). [CrossRef] [PubMed]
  23. Q. F. Xu and M. Lipson, “All-optical logic based on silicon micro-ring resonators,” Opt. Express15(3), 924–929 (2007). [CrossRef] [PubMed]
  24. L. Zhang, R. Q. Ji, L. X. Jia, L. Yang, P. Zhou, Y. H. Tian, P. Chen, Y. Y. Lu, Z. Y. Jiang, Y. L. Liu, Q. Fang, and M. B. Yu, “Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators,” Opt. Lett.35(10), 1620–1622 (2010). [CrossRef] [PubMed]
  25. L. Zhang, R. Q. Ji, Y. H. Tian, L. Yang, P. Zhou, Y. Y. Lu, W. W. Zhu, Y. L. Liu, L. X. Jia, Q. Fang, and M. B. Yu, “Simultaneous implementation of XOR and XNOR operations using a directed logic circuit based on two microring resonators,” Opt. Express19(7), 6524–6540 (2011). [CrossRef] [PubMed]
  26. T. Fukazawa, T. Hirano, F. Ohno, and T. Baba, “Low loss intersection of Si photonic wire waveguides,” Jpn. J. Appl. Phys.43(2), 646–647 (2004). [CrossRef]
  27. M. M. Geng, L. X. Jia, L. Zhang, L. Yang, P. Chen, T. Wang, and Y. L. Liu, “Four-channel reconfigurable optical add-drop multiplexer based on photonic wire waveguide,” Opt. Express17(7), 5502–5516 (2009). [CrossRef] [PubMed]
  28. P. Dong, R. Shafiiha, S. Liao, H. Liang, N.-N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Wavelength-tunable silicon microring modulator,” Opt. Express18(11), 10941–10946 (2010). [CrossRef] [PubMed]
  29. Q. F. Xu, S. Manipatruni, B. Schmidt, J. Shakya, and M. Lipson, “12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators,” Opt. Express15(2), 430–436 (2007). [CrossRef] [PubMed]
  30. J. C. Rosenberg, W. M. Green, S. Assefa, T. Barwicz, M. Yang, S. M. Shank, and Y. A. Vlasov, “Low-Power 30 Gbps Silicon Microring Modulator,” in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB9.
  31. G. L. Li, X. Z. Zheng, J. Yao, H. Thacker, I. Shubin, Y. Luo, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, “25Gb/s 1V-driving CMOS ring modulator with integrated thermal tuning,” Opt. Express19(21), 20435–20443 (2011). [CrossRef] [PubMed]
  32. X. Xiao, H. Xu, X. Y. Li, Y. T. Hu, K. Xiong, Z. Y. Li, T. Chu, Y. D. Yu, and J. Z. Yu, “25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions,” Opt. Express20(3), 2507–2515 (2012). [CrossRef] [PubMed]

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