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10 GHz electro-optical OR/NOR directed logic device based on silicon micro-ring resonators ARTICLE in OPTICS LETTERS · APRIL 2014 Impact Factor: 3.18 · DOI: 10.1364/OL.39.001937 · Source: PubMed
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April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
1937
10 GHz electro-optical OR/NOR directed logic device based on silicon micro-ring resonators Ping Zhou, Lei Zhang, Yonghui Tian, and Lin Yang* State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China *Corresponding author: [email protected] Received January 7, 2014; revised February 9, 2014; accepted February 18, 2014; posted February 21, 2014 (Doc. ID 204376); published March 24, 2014 We report the demonstration of an OR/NOR directed logic device, which consists of two cascaded micro-ring resonators (MRRs) modulated through electric-field-induced carrier depletion in reverse-biased PN junctions embedded in the ring waveguides. The resonance wavelength mismatch between the two nominally identical MRRs, caused by fabrication error, is compensated by thermal tuning. Two high-speed electrical signals applied to the PN junctions act as the operands, and the logical operation results are represented by the optical powers detected at the two output ports of the device. The working parameters of the device, including the working wavelength, the voltage applied to the micro-heater, and the voltages applied to the two PN junctions are extracted from the static response spectra of the device. Dynamic experimental results show that the OR/NOR logical operations can be achieved at the speed of 10 GHz. © 2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.3750) Optical logic devices; (230.4555) Coupled resonators; (250.5300) Photonic integrated circuits. http://dx.doi.org/10.1364/OL.39.001937
Directed logic is an innovative optics-inspired architecture proposed to implement Boolean functions [1,2]. In directed logic structure, the operation is performed by a network of elements, each of which implements a switching operation [3–9]. It takes a finite amount of time for each switch to respond to a change in its controlling electrical signal and all of the switches operate simultaneously, so their switching delays do not accumulate, thus resulting in a high-speed logic operation. The directed logic structure constructed using a silicon-based micro-ring resonator (MRR), is promising, owing to its low power consumption, high compactness, and compatibility with CMOS fabrication process [5,10,11]. Recently, many directed logic devices based on MRRs have been proposed and demonstrated [5,12–16]. We have demonstrated a thermo-optic OR/NOR and AND/NAND logic circuit [13], and an electro-optic AND/ NAND logic circuit employing electric field-induced carrier injection at the p-type intrinsic n-type (PIN) junctions [14]. Those circuits have rather low working speeds since the thermo-optic and carrier injection modulation schemes are employed. In this Letter, we report a directed logic circuit consisting of two cascaded MRRs, modulated by electric field-induced carrier depletion in reversebiased PN junctions, which can perform OR/NOR functions at a speed of 10 GHz. The schematic of the proposed OR/NOR directed logic device is shown in Fig. 1(a), which is composed of two add–drop MRRs. The two MRRs are designed with identical physical parameters, so they nominally have the same resonance wavelength. Since the device fabrication accuracy is limited, there exist tiny differences between the actual resonance wavelengths of the two MRRs. To compensate for this difference, two titanium nitride (TiN) micro-heaters are fabricated on the top of both MRRs. Furthermore, PN diodes are embedded around the ring waveguides to achieve the carrier depletion modulation. 0146-9592/14/071937-04$15.00/0
The four ports of the circuit are denoted as Input, Through, Drop, and Add, which have been marked in Fig. 1(a). A beam of monochromatic light with the wavelength of λw and a constant power is coupled into the Input port. Each MRR is driven by an electrical pulse signal, which is regarded as the operand. When the PN junction is reverse-biased by an electrical pulse signal, the resonance wavelength of the corresponding MRR shifts to a larger value because of the plasma dispersion effect. The high and low levels of the electrical pulse signal driving the PN diode represent logic 1 and 0, respectively, while the larger and smaller optical output powers at the output ports represent logic 1 and 0, respectively. The two MRRs are defined to be on-resonance at working wavelength λw , when the driving voltage is at a high level. According to the above definitions, when the driving voltages applied to the two resonators are both low (X 0, Y 0), the MRRs are both off-resonance at
Fig. 1. (a) Schematic and (b) micrograph of the OR/NOR directed logic device. CML, continuous monochromatic light; EPS, electrical pulse signal; and MRR, micro-ring resonator. © 2014 Optical Society of America
1938
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
λw , the optical signal bypasses the two resonators successively and is then directed to the Through port, so the optical powers at the Drop port (Z 1 ) and Through port (Z 2 ) are at a low level and a high level, respectively (Z 1 0, Z 2 1). When the voltages applied to MRR1 and MRR2 are high and low, respectively (X 1, Y 0), MRR1 is on-resonance at λw , the optical signal is downloaded by MRR1, and the optical powers at Z 1 and Z 2 are at high and low levels, respectively (Z 1 1, Z 2 0). Similarly, the Drop and Through ports can obtain the same optical level results (Z 1 1, Z 2 0) when MRR1 is applied with low-level voltage and MRR2 is applied with high-level voltage (X 0, Y 1), because the optical power is downloaded by MRR2. When MRR1 and MRR2 are both driven by high-level voltages (X 1, Y 1), the monochromatic wave is downloaded to the Drop port, and the optical power is at a high level at the Drop port and a low level at the Through port (Z 1 1, Z 2 0). According to the above analysis about the four different working states, the truth table and corresponding resonance states of the two MRRs are summarized in Table 1. As can be seen from Table 1, the proposed circuit can realize logical OR and NOR operations at the Drop and Through ports, respectively. The device is fabricated on an 8 in. (0.2 m) silicon-oninsulator (SOI) wafer with a 220-nm-thick top silicon layer and a 2-μm-thick buried SiO2 layer. 180-nm-deep UV photolithography is used to define the pattern, and an inductively coupled plasma etching process is employed to etch the top silicon layer. A rib waveguide with 400 nm in width, 220 nm in height and 70 nm in slab thickness is employed, which only supports the fundamental quasi-TE mode [15]. The radii of the ring waveguides are both 10 μm, which is big enough such that the radiation loss of the ring resonator is not so high and the Q factor is acceptable. The center-to-center distance between two MRRs is 400 μm, which is big enough to avoid thermal crosstalk. According to the presented structure parameters and the scattering matrix model, the coupling efficiency can be deduced to be about 0.2. After the silicon waveguides are formed, the p-type and n-type doping regions, with doped concentrations of 1 × 1018 cm−3 and 8 × 1017 cm−3 , respectively, are formed around the two ring waveguides. The width of overlapping part between the p-doping region and the rib waveguide is 220 nm, and the width of the overlapping part between the n-doping region and the rib waveguide is 180 nm. The reason why the doping region is asymmetrically designed is because the hole is more efficient to change the refractive index than the electron. The p+ and n+ doping concentrations are 5.5 × 1020 cm−3 , and the edge-to-edge distance from the heavy doping region
to the ring waveguide is 800 nm. A layer of about 1.5-μm-thick SiO2 is deposited as a separate area, and then two TiN micro-heaters, with thicknesses of 150 nm, are fabricated on the top of the MRRs to compensate for the detuning resonances of the MRRs, which are mainly induced by the limited fabricating accuracy. At last, aluminum traces are formed to connect the micro-heaters, PN diodes, and the pads. The main difference between the proposed device and our previous work is the modulation scheme. In the current work, the MRRs are modulated by carrier depletion in reverse-biased PN junctions, whereas in the previous work, the MRRs are modulated by carrier injection in forward-biased PN junctions. Besides that, the gaps between the straight waveguides and the ring waveguides increased from 325 to 400 nm so that the Q factor increased to 12,000, which is vital for the carrier depletion modulation, since the tuning efficiency is low. The micrograph of the device is shown in Fig. 1(b). The waveguides are sheltered by the pads of PN diodes, so dashed lines are employed in Fig. 1(b) to represent the location of waveguides. An amplified spontaneous emission broadband optical source, an optical spectrum analyzer (OSA), and three adjustable voltage sources are employed to measure the static spectral response of the device. The broadband optical source is coupled to the Input port of the device by a tapered lensed fiber, and the output light signals from the Through port and Drop port of the device are successively collected by another lensed fiber, which is connected to the OSA. The spectral responses at the Though port and Drop port of the device are illustrated in Fig. 2. In practice, the resonance wavelengths of the two MRRs are slightly different because of the inevitable fabrication error. Hence, one voltage source is used to drive the micro-heater above the MRR, which has a shorter resonant wavelength. All three voltage sources are coupled into the device by high frequency probes. Unlike the previous work using thermo-optic effects [13] and PIN diodes [14], in which the working wavelength is simply chosen at one resonance peak, we determine the working parameters by analyzing the response spectra of four working states. As shown in Fig. 2, the
Table 1. Truth Table for Structure and Resonance States of MRRs at Working Wavelength λw X
Y
0 1 0 1
0 0 1 1
Z1 Z2 Resonance Resonance (Drop) (Through) State of MRR1 State of MRR2 0 1 1 1
1 0 0 0
Off On Off On
Off Off On On
Fig. 2. Spectral responses at the Through and Drop ports of the device without voltage applied to the micro-heater nor voltages applied to the PN junctions.
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
1939
wavelength peaks around 1548 nm are chosen as the working area to realize the function of the device. In practice, MRR2’s resonance wavelength is 1547.44 nm, which is shorter than MRR1’s resonance wavelength, 1547.89 nm. A bias voltage of 1.78 V is applied to the micro-heater above MRR2, leading to the right-shift of MRR2’s resonance peak, after which the two MRRs’ resonance peaks coincided at around the wavelength of 1547.89 nm. This coincidence configuration is considered as the initial state of the device. At the initial state, the voltages applied to the PN junctions of the MRRs are both 0 V (X 0, Y 0). The corresponding spectral curve at the Drop port is illustrated in Fig. 3(a). Next, two DC voltage signals, denoted as V e1 and V e2 , are reversely applied to the MRR1 and MRR2, respectively, through the PN junctions. When only V e1 is applied with a certain value (X 1, Y 0), the spectral response at the Drop port is that shown in Fig. 3(b). Since the free carriers are depleted from MRR1, and the refractive index of MRR1 increases, its resonance peak will move toward the longer wavelength. Similarly, when only V e2 is applied (X 0, Y 1), spectral response at the Drop port is as shown in Fig. 3(c). When both MRR1 and MRR2 are driven by DC voltages (X 1, Y 1), their resonance peaks move together, as shown by the corresponding spectral response depicted in Fig. 3(d). We analyze the response spectra, as depicted in Fig. 3, to explain how to determine the working parameters, including λw , V e1 , and V e2 . Two factors are considered when determining the working wavelength. First, the extinction ratio between the optical power of output logic 1 and 0 should be large enough (>5 dB) so that the photodetector can distinguish them. Second, the output optical power of the high level should be big enough (insertion loss
10 GHz electro-optical OR/NOR directed logic device based on silicon micro-ring resonators ARTICLE in OPTICS LETTERS · APRIL 2014 Impact Factor: 3.18 · DOI: 10.1364/OL.39.001937 · Source: PubMed
DOWNLOADS
VIEWS
34
58
4 AUTHORS, INCLUDING: Lei Zhang
Yonghui Tian
Chinese Academy of Sciences
Chinese Academy of Sciences
961 PUBLICATIONS 16,597 CITATIONS
29 PUBLICATIONS 248 CITATIONS
SEE PROFILE
SEE PROFILE
Lin Yang Chinese Academy of Sciences 69 PUBLICATIONS 442 CITATIONS SEE PROFILE
Available from: Lin Yang Retrieved on: 25 June 2015
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
1937
10 GHz electro-optical OR/NOR directed logic device based on silicon micro-ring resonators Ping Zhou, Lei Zhang, Yonghui Tian, and Lin Yang* State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China *Corresponding author: [email protected] Received January 7, 2014; revised February 9, 2014; accepted February 18, 2014; posted February 21, 2014 (Doc. ID 204376); published March 24, 2014 We report the demonstration of an OR/NOR directed logic device, which consists of two cascaded micro-ring resonators (MRRs) modulated through electric-field-induced carrier depletion in reverse-biased PN junctions embedded in the ring waveguides. The resonance wavelength mismatch between the two nominally identical MRRs, caused by fabrication error, is compensated by thermal tuning. Two high-speed electrical signals applied to the PN junctions act as the operands, and the logical operation results are represented by the optical powers detected at the two output ports of the device. The working parameters of the device, including the working wavelength, the voltage applied to the micro-heater, and the voltages applied to the two PN junctions are extracted from the static response spectra of the device. Dynamic experimental results show that the OR/NOR logical operations can be achieved at the speed of 10 GHz. © 2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.3750) Optical logic devices; (230.4555) Coupled resonators; (250.5300) Photonic integrated circuits. http://dx.doi.org/10.1364/OL.39.001937
Directed logic is an innovative optics-inspired architecture proposed to implement Boolean functions [1,2]. In directed logic structure, the operation is performed by a network of elements, each of which implements a switching operation [3–9]. It takes a finite amount of time for each switch to respond to a change in its controlling electrical signal and all of the switches operate simultaneously, so their switching delays do not accumulate, thus resulting in a high-speed logic operation. The directed logic structure constructed using a silicon-based micro-ring resonator (MRR), is promising, owing to its low power consumption, high compactness, and compatibility with CMOS fabrication process [5,10,11]. Recently, many directed logic devices based on MRRs have been proposed and demonstrated [5,12–16]. We have demonstrated a thermo-optic OR/NOR and AND/NAND logic circuit [13], and an electro-optic AND/ NAND logic circuit employing electric field-induced carrier injection at the p-type intrinsic n-type (PIN) junctions [14]. Those circuits have rather low working speeds since the thermo-optic and carrier injection modulation schemes are employed. In this Letter, we report a directed logic circuit consisting of two cascaded MRRs, modulated by electric field-induced carrier depletion in reversebiased PN junctions, which can perform OR/NOR functions at a speed of 10 GHz. The schematic of the proposed OR/NOR directed logic device is shown in Fig. 1(a), which is composed of two add–drop MRRs. The two MRRs are designed with identical physical parameters, so they nominally have the same resonance wavelength. Since the device fabrication accuracy is limited, there exist tiny differences between the actual resonance wavelengths of the two MRRs. To compensate for this difference, two titanium nitride (TiN) micro-heaters are fabricated on the top of both MRRs. Furthermore, PN diodes are embedded around the ring waveguides to achieve the carrier depletion modulation. 0146-9592/14/071937-04$15.00/0
The four ports of the circuit are denoted as Input, Through, Drop, and Add, which have been marked in Fig. 1(a). A beam of monochromatic light with the wavelength of λw and a constant power is coupled into the Input port. Each MRR is driven by an electrical pulse signal, which is regarded as the operand. When the PN junction is reverse-biased by an electrical pulse signal, the resonance wavelength of the corresponding MRR shifts to a larger value because of the plasma dispersion effect. The high and low levels of the electrical pulse signal driving the PN diode represent logic 1 and 0, respectively, while the larger and smaller optical output powers at the output ports represent logic 1 and 0, respectively. The two MRRs are defined to be on-resonance at working wavelength λw , when the driving voltage is at a high level. According to the above definitions, when the driving voltages applied to the two resonators are both low (X 0, Y 0), the MRRs are both off-resonance at
Fig. 1. (a) Schematic and (b) micrograph of the OR/NOR directed logic device. CML, continuous monochromatic light; EPS, electrical pulse signal; and MRR, micro-ring resonator. © 2014 Optical Society of America
1938
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
λw , the optical signal bypasses the two resonators successively and is then directed to the Through port, so the optical powers at the Drop port (Z 1 ) and Through port (Z 2 ) are at a low level and a high level, respectively (Z 1 0, Z 2 1). When the voltages applied to MRR1 and MRR2 are high and low, respectively (X 1, Y 0), MRR1 is on-resonance at λw , the optical signal is downloaded by MRR1, and the optical powers at Z 1 and Z 2 are at high and low levels, respectively (Z 1 1, Z 2 0). Similarly, the Drop and Through ports can obtain the same optical level results (Z 1 1, Z 2 0) when MRR1 is applied with low-level voltage and MRR2 is applied with high-level voltage (X 0, Y 1), because the optical power is downloaded by MRR2. When MRR1 and MRR2 are both driven by high-level voltages (X 1, Y 1), the monochromatic wave is downloaded to the Drop port, and the optical power is at a high level at the Drop port and a low level at the Through port (Z 1 1, Z 2 0). According to the above analysis about the four different working states, the truth table and corresponding resonance states of the two MRRs are summarized in Table 1. As can be seen from Table 1, the proposed circuit can realize logical OR and NOR operations at the Drop and Through ports, respectively. The device is fabricated on an 8 in. (0.2 m) silicon-oninsulator (SOI) wafer with a 220-nm-thick top silicon layer and a 2-μm-thick buried SiO2 layer. 180-nm-deep UV photolithography is used to define the pattern, and an inductively coupled plasma etching process is employed to etch the top silicon layer. A rib waveguide with 400 nm in width, 220 nm in height and 70 nm in slab thickness is employed, which only supports the fundamental quasi-TE mode [15]. The radii of the ring waveguides are both 10 μm, which is big enough such that the radiation loss of the ring resonator is not so high and the Q factor is acceptable. The center-to-center distance between two MRRs is 400 μm, which is big enough to avoid thermal crosstalk. According to the presented structure parameters and the scattering matrix model, the coupling efficiency can be deduced to be about 0.2. After the silicon waveguides are formed, the p-type and n-type doping regions, with doped concentrations of 1 × 1018 cm−3 and 8 × 1017 cm−3 , respectively, are formed around the two ring waveguides. The width of overlapping part between the p-doping region and the rib waveguide is 220 nm, and the width of the overlapping part between the n-doping region and the rib waveguide is 180 nm. The reason why the doping region is asymmetrically designed is because the hole is more efficient to change the refractive index than the electron. The p+ and n+ doping concentrations are 5.5 × 1020 cm−3 , and the edge-to-edge distance from the heavy doping region
to the ring waveguide is 800 nm. A layer of about 1.5-μm-thick SiO2 is deposited as a separate area, and then two TiN micro-heaters, with thicknesses of 150 nm, are fabricated on the top of the MRRs to compensate for the detuning resonances of the MRRs, which are mainly induced by the limited fabricating accuracy. At last, aluminum traces are formed to connect the micro-heaters, PN diodes, and the pads. The main difference between the proposed device and our previous work is the modulation scheme. In the current work, the MRRs are modulated by carrier depletion in reverse-biased PN junctions, whereas in the previous work, the MRRs are modulated by carrier injection in forward-biased PN junctions. Besides that, the gaps between the straight waveguides and the ring waveguides increased from 325 to 400 nm so that the Q factor increased to 12,000, which is vital for the carrier depletion modulation, since the tuning efficiency is low. The micrograph of the device is shown in Fig. 1(b). The waveguides are sheltered by the pads of PN diodes, so dashed lines are employed in Fig. 1(b) to represent the location of waveguides. An amplified spontaneous emission broadband optical source, an optical spectrum analyzer (OSA), and three adjustable voltage sources are employed to measure the static spectral response of the device. The broadband optical source is coupled to the Input port of the device by a tapered lensed fiber, and the output light signals from the Through port and Drop port of the device are successively collected by another lensed fiber, which is connected to the OSA. The spectral responses at the Though port and Drop port of the device are illustrated in Fig. 2. In practice, the resonance wavelengths of the two MRRs are slightly different because of the inevitable fabrication error. Hence, one voltage source is used to drive the micro-heater above the MRR, which has a shorter resonant wavelength. All three voltage sources are coupled into the device by high frequency probes. Unlike the previous work using thermo-optic effects [13] and PIN diodes [14], in which the working wavelength is simply chosen at one resonance peak, we determine the working parameters by analyzing the response spectra of four working states. As shown in Fig. 2, the
Table 1. Truth Table for Structure and Resonance States of MRRs at Working Wavelength λw X
Y
0 1 0 1
0 0 1 1
Z1 Z2 Resonance Resonance (Drop) (Through) State of MRR1 State of MRR2 0 1 1 1
1 0 0 0
Off On Off On
Off Off On On
Fig. 2. Spectral responses at the Through and Drop ports of the device without voltage applied to the micro-heater nor voltages applied to the PN junctions.
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
1939
wavelength peaks around 1548 nm are chosen as the working area to realize the function of the device. In practice, MRR2’s resonance wavelength is 1547.44 nm, which is shorter than MRR1’s resonance wavelength, 1547.89 nm. A bias voltage of 1.78 V is applied to the micro-heater above MRR2, leading to the right-shift of MRR2’s resonance peak, after which the two MRRs’ resonance peaks coincided at around the wavelength of 1547.89 nm. This coincidence configuration is considered as the initial state of the device. At the initial state, the voltages applied to the PN junctions of the MRRs are both 0 V (X 0, Y 0). The corresponding spectral curve at the Drop port is illustrated in Fig. 3(a). Next, two DC voltage signals, denoted as V e1 and V e2 , are reversely applied to the MRR1 and MRR2, respectively, through the PN junctions. When only V e1 is applied with a certain value (X 1, Y 0), the spectral response at the Drop port is that shown in Fig. 3(b). Since the free carriers are depleted from MRR1, and the refractive index of MRR1 increases, its resonance peak will move toward the longer wavelength. Similarly, when only V e2 is applied (X 0, Y 1), spectral response at the Drop port is as shown in Fig. 3(c). When both MRR1 and MRR2 are driven by DC voltages (X 1, Y 1), their resonance peaks move together, as shown by the corresponding spectral response depicted in Fig. 3(d). We analyze the response spectra, as depicted in Fig. 3, to explain how to determine the working parameters, including λw , V e1 , and V e2 . Two factors are considered when determining the working wavelength. First, the extinction ratio between the optical power of output logic 1 and 0 should be large enough (>5 dB) so that the photodetector can distinguish them. Second, the output optical power of the high level should be big enough (insertion loss
