Demonstration of a directed optical encoder usingmicroring-resonator-based optical switches

Yonghui Tian,1 Lei Zhang,1 Ruiqiang Ji,1 Lin Yang,1,* and Qianfan Xu2

1Optoelectronic System Laboratory, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China2Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, MS-366, Houston, Texas 77005, USA

*Corresponding author: oip@semi.ac.cn

Received July 13, 2011; revised August 24, 2011; accepted August 27, 2011;posted August 29, 2011 (Doc. ID 150990); published September 22, 2011

We propose and demonstrate a directed optical logic circuit that performs the encoding function from a 4 bitelectrical signal to a 2 bit optical signal based on cascaded microring switches. The four logic input signals controlthe states of the switches, while the two logic outputs are given by the optical power at the output waveguides.For proof of concept, a thermo-optic switching effect is used with an operation speed of 10kbps. © 2011 OpticalSociety of AmericaOCIS codes: 130.3120, 130.3750, 230.4555, 250.5300.

Directed logic is a novel logical paradigm that employsthe optical switch networks to perform the logical opera-tion. Compared to the traditional logic, the directed logichas many potential advantages such as higher operationspeed, lower latency, etc. [1–6]. A microring resonator(MRR) based on a silicon-on-insulator (SOI) is an attrac-tive structure owing to its outstanding performances,such as compact size, ultralow power consumption, anda complementary metal oxide semiconductor (CMOS)-compatible process. Therefore, the directed logic usingthe MRR-based optical switching networks is easy to rea-lize large-scale integration and low-cost manufacture in ahigh-volume CMOS-photonics foundry.Recently, many directed logic circuits based on MRRs

have been proposed [6–10], and some of them have beendemonstrated including XOR/XNOR [8], OR/NOR, AND/NAND [9] gates, and an optical decoder [10]. In this Let-ter, we propose and demonstrate a directed logic circuitbased on cascaded microring switches, which performsthe encoding function from a 4 bit electrical signal to a 2bit optical signal.The proposed circuit composed of three tunable MRRs

and three waveguides is shown in Fig. 1(a). Monochro-matic continuous optical wave with the working wave-length of λw is modulated by the electrical pulsesequences I1, I2, I3, and I4 applied to MRR1, MRR2,MRR3, and MRR4, respectively. We use the logical 1and 0 to represent the high and low levels of the voltageapplied to the MRRs. The optical power at the output portY 1 or Y 2 defines the logic output. Logical 1 is obtainedwhen the signal light appears at the optical output portY 1 or Y 2, and logical 0 is obtained when the signal light isabsent at the optical output port Y 1 or Y 2. Note that theproposed encoder is not a priority encoder but a commonencoder. It is active when and only when one of the inputelectrical signals (I1, I2, I3, and I4) is at the high level.Therefore, only four input combinations (1 0 0 0, 0 1 00, 0 0 1 0, and 0 0 0 1) are possible. No matter whetherMRR1 exists or not, the input electrical signal combina-tion of 1 0 0 0 corresponds to the output optical signalcombination of 0 0. That is to say, whether MRR1 existsor not, the device can work well. Therefore, we regardMRR1 as a dummy device.

Each MRR acts as an optical switch and the status ofthe switch is controlled by the voltage applied to it, asdefined below. The MRR is on-resonance at λw when theapplied voltage is at the high level. The light is directed tothe drop port. The MRR is off-resonance at λw when theapplied voltage is at the low level and the light passesthrough the MRR without disturbance and appears at thethrough port. According to the above definition, theworking principle of the device is as follows: the opticalpower is at the low level at the output ports Y 1 and Y 2(Y 1 ¼ 0, Y 2 ¼ 0) when I1 ¼ 1, I2 ¼ 0, I3 ¼ 0, and I4 ¼ 0;the optical power is at the high level at the Y 2 port and atthe low level at the Y 1 port (Y 1 ¼ 0, Y 2 ¼ 1) when I1 ¼ 0,I2 ¼ 1, I3 ¼ 0, and I4 ¼ 0; the optical power is at the highlevel at the Y 1 port and at the low level at the Y 2 port(Y 1 ¼ 1, Y 2 ¼ 0) when I1 ¼ 0, I2 ¼ 0, I3 ¼ 1, andI4 ¼ 0; the optical power is at the high level at both theY 1 and Y 2 ports (Y 1 ¼ 1, Y 2 ¼ 1) when I1 ¼ 0, I2 ¼ 0,I3 ¼ 0, and I4 ¼ 1. In order to illuminate the principleof the device clearly, the logical truth table of the deviceis summarized in Table. 1. We find that the proposedarchitecture can perform the encoding function from a4 bit electrical signal to a 2 bit optical signal.

A thermo-optic modulating scheme is adopted for theproof of concept since it demands a less complex device

Fig. 1. (Color online) (a) Architecture and (b) micrograph ofthe device (CW: continuous wave, EPS: electrical pulse trains).

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layer structure and consequently yields easier fabricationsteps. The device is fabricated on an 8 in SOI wafer witha 220 nm top silicon layer and a 2 μm buried SiO2 layer.The micrograph of the device is shown in Fig. 1(b).248 nm deep UV photolithography is used to define thedevice pattern and an inductively coupled plasma etchingprocess is used to etch the top silicon layer. The ribwaveguide has less sidewall area compared to the stripwaveguide of similar dimensions, which can reduce thescattering loss on the waveguide sidewall and thus thetransmission loss compared to the strip waveguide of si-milar dimensions and a similar fabrication process [11].Therefore, the bus waveguide and MRR are formed witha submicron rib waveguide with a width of 400 nm, aheight of 220 nm, and a slab thickness of 70 nm. Finiteelement method calculation shows that the waveguideonly supports a TE-like fundamental mode, which agreeswith the experimental results [9,10]. The radius of eachmicroring is 10 μm, and the gaps between the straightwaveguides and the ring waveguides are 450 nm. Depos-ited as a separate layer is 1:5 μm thick SiO2. After thewaveguides are fabricated, the titanium microheaterswith the thickness of 120 nm are fabricated on the top ofthe MRRs, which are employed to tune the MRRs throughthe thermo-optic effect, and the aluminum trances with50 μm in the width are formed to connect the pads andthe microheaters. Note that we do not fabricate dummyMRR1 and only fabricate the pad and microheater ofMRR1 where the logic signal I1 can be applied [Fig. 1(b)].We use an amplified spontaneous emitting source, an

optical spectrum analyzer (OSA), and four tunable vol-tage sources to test the static response of the device.Broadband light is coupled into the input port of thedevice through a lensed fiber, and the output light isfed into the OSA through another lensed fiber. Four tun-able voltage sources are used to drive the four microhea-ters above the MRRs, respectively. When the MRRs areheated up, the refractive index of the silicon increasesand the resonant wavelengths of the MRRs shift to thelonger wavelength.The static response spectra of the device at the output

ports Y 1 and Y 2 are shown in Figs. 2 and 3. In principle,any wavelength at the right side of the peak located at1550:88 nm can be chosen as the working wavelength.However, in order to achieve a sufficiently large ex-tinction ratio and the least power consumption, the work-ing wavelength is chosen to be at the 1552:15 nm. Theoptical power at both the output ports is very low there[Figs. 2(a) and 3(a)] when the voltage applied to MRR1 is0:5V and the voltages applied to other MRRs are 0V(I1 ¼ 1, I2 ¼ 0, I3 ¼ 0, and I4 ¼ 0.). Although the MRRsare designed to have the same structural parameters,they have slightly different resonant wavelengths,which is mainly due to the limited manufacturing

accuracy. Therefore, we observe two resonant peaks inFigs. 2(a) and 3(a). Clearly, the peak at 1548:84 nm isfrom MRR4, the peak at 1549:48 nm is from MRR3, andthe peak at 1550:88 nm is from MRR2. Figure 2(b) is si-milar to Fig. 2(a) and there is also a dip at λw in Fig. 2(b)(representing 0) when the voltage applied to MRR2 is1:68V and the voltages applied to other MRRs are 0V.Apparently, MRR2 does not affect the static responsespectra at the Y 1 port [see Fig. 1(a)]. The resonant peakof MRR3 shifts from 1549:48nm to λw (representing 1)when the voltage applied to MRR3 is 2:44V and thevoltages applied to other MRRs are 0V [Fig. 2(c)]. Theresonant peak of MRR4 shifts from 1548.84 to 1552:15 nm(representing 1) when the voltage applied to MRR4 is3:12V and the voltages applied to other MRRs are 0V[Fig. 2(d)].

There is a dip at λw when the voltage applied to MRR1is 0:5V and the voltages applied to other MRRs are 0V[Fig. 3(a)]. The resonant peak of MRR2 shifts from1550:88 nm to λw when the voltage applied to MRR2 is1:68V and the voltages applied to other MRRs are 0V

Table 1. Logical Truth Table of the Device

I1 I2 I3 I4 Y 1 Y 2

1 0 0 0 0 00 1 0 0 0 10 0 1 0 1 00 0 0 1 1 1

Fig. 2. (Color online) Response spectra of the device at theoutput port Y 1 with the voltages applied to MRR1, MRR2,MRR3, and MRR4 being (a) 0.5, 0, 0, and 0V, (b) 0, 1.68, 0,and 0V, (c) 0, 0, 2.44, and 0V and (d) 0, 0, 0, and 3:12V.

Fig. 3. (Color online) Response spectra of the device at theoutput port Y 2 with the voltages applied to MRR1, MRR2,MRR3, and MRR4 being (a) 0.5, 0, 0, and 0V, (b) 0, 1.68, 0,and 0V, (c) 0, 0, 2.44, and 0V and (d) 0, 0, 0, and 3:12V.

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[Fig. 3(b)]. There is also a dip at λw when the voltage ap-plied to MRR3 is 2:44V and the voltages applied to otherMRRs are 0V [Fig. 3(c)]. Obviously, MRR3 does not affectthe static response spectra at the Y 2 port [Fig. 1(a)]. Theresonant peak of MRR4 shifts from 1548:84 nm to λwwhen the voltage applied to MRR4 is 3:12V and the vol-tages applied to other MRRs are 0V [Fig. 3(d)].Hereto, we have analyzed all eight static response

spectra for four logical combinations. The results indi-cate that the device can implement the encoding functionfrom a 4 bit electrical signal to a 2 bit optical signal.The dynamic performance of the device is further char-

acterized (Fig. 4). Monochromatic light with the workingwavelength of 1552:15 nm is coupled into the input portof the device. Four binary sequence non-return-to-zerosignals at 10 kbps are converted to four electrical analogsignals first and then applied to the four MRRs, respec-tively (The high levels are 0.50, 1.68, 2.44, and 3:12V forMRR1, MRR2, MRR3, and MRR4, respectively, and thelow levels are 0V for all MRRs). The light at the outputports is fed into a detector. The electrical signals trans-formed by the detector and the four electrical signalsapplied to the four MRRs are fed into an oscilloscopefor waveform observation. Clearly, the device performsthe encoding function from a 4 bit electrical signal to a 2bit optical signal correctly (Fig. 4). Note that the powerlevel is the same for logical 0 but different forlogical 1 in different cases, which mainly results fromthe different functions of MRR4 in different cases. When

MRR4 is off-resonance at λw (I4 ¼ 0), all the input lightpasses through MRR4 and is further directed either to theoutput port Y 1 (Y 1 ¼ 1) by MRR3 (I3 ¼ 1) or to the out-put port Y 2 (Y 2 ¼ 1) by MRR2 (I2 ¼ 1). When MMR4 ison-resonance at λw (I4 ¼ 1), MRR4 behaves as a powersplitter. Half of the input light is directed to the outputport Y 1 (Y 1 ¼ 1) and the left half is directed to the outputport Y 2 (Y 2 ¼ 1). Therefore, the power level for logical 1in the second and third logical combinations is twicethat in the fourth logical combination. The similar phe-nomena also can be observed in the static response spec-tra (Figs. 2 and 3), where the power level for logical 1 is−14 and −11:5 dB at the output ports Y 1 and Y 2 whenMRR4 is on-resonance, respectively, and the power levelfor logical 1 is −9:7 and −9:3 dB at the output ports Y 1 andY 2 when MRR4 is off-resonance, respectively.

In conclusion, we have proposed and demonstrated adirected logical circuit that can implement the encodingfunction from a 4 bit electrical signal to a 2 bit opticalsignal.

This work has been supported by the National NaturalScience Foundation of China (NSFC) under grant60977037 and the National High Technology Researchand Development Program of China under grant2009AA03Z416.

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Fig. 4. (Color online) Signals applied to (a) MRR1, (b) MRR2,(c) MRR3, and (d) MRR4, (e) the result at the output port Y 1,and (f) the result at the output port Y 2 of the device.

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