A monolithically integrated all-optical exclusive-OR (XOR) logic gate is experimentally demonstrated based on a Michelson interferometer (MI) gating device in InGaAs/AlAsSb coupled double quantum wells (CDQWs). The MI arms can convert the pump data with return-to-zero ON-OFF keying (RZ OOK) to binary phase-shift keying (BPSK) format, then two BPSK signals can interfere with each other for realizing a desired logical operation. All-optical format conversion from the RZ OOK to BPSK is based on the cross-phase modulation to the transverse electric (TE) probe wave, which is caused by the intersubband transition excited by the transverse magnetic (TM) pump light. Bit error rate measurements show that error free operation for both BPSK format conversion and XOR logical operation can be achieved.
© 2014 Optical Society of America
Ultrafast all-optical signal processing devices are essential for future communications networks , where an important aspect is all-optical logical operation. Exclusive OR (XOR) gate is one of the most important logic functions in digital computation and communication. It is indispensable in encoding and decoding schemes, and can be used in many occasions such as half adder, parity checking, and pseudo-random bit sequence (PRBS) generators , etc. All-optical XOR operation for binary phase shift keying (BPSK) signal  has been successfully demonstrated in a number of platforms such as highly nonlinear silica fiber (HNLF) , periodically poled lithium niobate (PPLN) , and semiconductor optical amplifiers (SOAs) . However, these platforms suffer from some drawbacks such as fiber dispersion in HNLF, requirement for precise temperature control in PPLN, and free-carrier induced patterning effects in SOAs that limit the operation speed. All-optical logic gates have also been realized in many monolithically integrated devices, such as chalcogenide (As2S3) planar waveguide , silicon nanowire , and photonic crystal . Most of these logic devices are based on the four-wave mixing (FWM), while utilizing an idler at a different wavelength, increasing the complexity of the experimental system. Therefore, alternative platforms are of significant interest for a different operation mechanism without any change of the system bandwidth capacity.
InGaAs/AlAsSb coupled double quantum wells (CDQWs) are a good candidate owing to its distinctive nonlinear cross-phase modulation (XPM) mechanism  with only a few picoseconds relaxation time , which is free from the patterning effect beyond 100-Gb/s operation. For such CDQWs, with the intersubband transition (ISBT) caused by the transverse magnetic (TM) pump light, XPM can be induced to the transverse electric (TE) probe light, which excites interband transition and is immune to ISBT absorption . It has been used for all-optical signal processing experiments such as demultiplexing , wavelength conversion , and non-return-to-zero (NRZ) to return-to-zero (RZ) format conversion . This ISBT-induced XPM has already been applied in many monolithically integrated high-speed all-optical switch devices [16–18], which could be used for delivering real-time video signals of a 172-Gb/s optical-time-division-multiplexed (OTDM) system . In this paper, we expand its application scopes and demonstrate for the first time a RZ on-off keying (OOK) to BPSK format conversion and an all-optical XOR logical operation. The RZ OOK to BPSK format conversion can be achieved by an ISBT wire waveguide, and we can use two BPSK signals to interact with each other for realizing a desired logical operation. In the following, the BPSK format conversion is introduced using a straight wire waveguide. Then an all-optical XOR operation is realized with a monolithic Michelson interferometer (MI) gating device. Bit error rate (BER) measurements are performed and error free operation for both format conversion and logical operation can be achieved.
2. All-optical RZ OOK to BPSK format conversion
Experimental setup for the RZ OOK to BPSK format conversion is illustrated in Fig. 1. A TM-polarized RZ OOK data signal at 1559 nm is combined with a TE-polarized continuous-wave (cw) light at 1545 nm by a 3-dB coupler, and they are launched into the ISBT waveguide by a lensed fiber. The RZ OOK data signal is generated via a lithium niobate Mach-Zehnder intensity modulator driven by a 10-Gb/s PRBS of length 27-1 with 50% duty-cycle. The light source of OOK signal is a mode-locked fiber laser. For the ISBT wafer with InGaAs/AlAsSb CDQWs, it was grown on a semi-insulating InP substrate by molecular beam epitaxy method. The CDQWs is designed to have high XPM efficiency with an expense of high signal propagation loss . After the growth of an InP buffer, a 0.55-μm-thick CDQWs layer and a 1-μm-thick InP upper cladding layer were grown. The CDQWs were highly Si-doped to activate intersubband absorption. To make a trade-off between signal loss and XPM efficiency, subsequent phosphorus ion implantation and rapid thermal annealing (RTA) at 700 °C for 60 s was carried out based on the quantum well intermixing . The ISBT waveguides are 2.5-μm-deep and 1.3-μm-wide straight mesa structures, fabricated by electron beam lithography and subsequent RIE, ICP etching processes. The mesa was covered with benzocyclobutene (BCB) polymer and cleaved into 1-mm length, while both facets were anti-reflection coated. Through 75-μm long taper, the waveguide is gradually increased to 25-μm long, 2.5-μm wide input/output ports, for better optical coupling with a lensed fiber.
The measured XPM efficiency of the fabricated 1-mm-long ISBT waveguide is around 0.8 rad/pJ, which requires almost 4-pJ TM pump energy to induce π phase shift to the TE probe signal at a wavelength of 1545 nm. For the XPM efficiency measurement, the TM pump has a 10-GHz repetition rate without carrying any RZ OOK data. Similar to the setup in Fig. 1, a system using a delay-line interferometer (DLI) with a 25-ps delay time was applied to convert the phase shift to intensity variation . Phase bias of the DLI was adjusted to π/2, and the phase shift was deduced from Δϕ = sin–1(ΔI/I), where I and ΔI were the corresponding intensity and its variation in the time domain recorded by an optical sampling oscilloscope (OSO), respectively. Nonlinear phase shift increases almost linearly with the TM pump energy. And XPM efficiency can be fitted out from the phase slope with varying pump energy. A propagation loss of about 2.6 dB/mm is measured for TE wave caused by the interband absorption and waveguide edge roughness, while TM light has a more than 40 dB high absorption mostly due to the intersubband absorption, which is beneficial for the ISBT-induced XPM.
After traveling through the ISBT waveguide, BPSK signal on TE wave can be generated with phase modulation corresponding to the input TM RZ OOK data. The converted BPSK signal is extracted using a tunable optical band pass filter (OBPF) and demodulated by a DLI with a 1-bit delay (100 ps). The destructive interference condition for the BPSK demodulator was used in our experiment. An erbium-doped optical fiber amplifier (EDFA) was used at the DLI output port to increase the optical intensity of the demodulated signal. The spectra of the probe light, the converted BPSK signal, and the demodulated wave are shown in Fig. 2(a), recorded by an optical spectrum analyzer (OSA) at a 0.01-nm resolution bandwidth. We can see that the spectrum of the converted BPSK signal after the ISBT waveguide is significantly modulated by the pump pulse. The demodulated signal has a complementary peak and valley values compared to the original BPSK signal, which is due to the destructive interference condition of the BPSK demodulator with an additional π phase bias.
The corresponding waveforms are measured by an OSO with a time resolution of 0.5 ps. Figure 2(b) shows that of the input pump signal, the phase-modulated BPSK output after the ISBT waveguide, and the demodulated signal after delay line interferometer, respectively. The converted BPSK signal has slight amplitude fluctuations, corresponding to the weak pump-induced probe signal absorption for the ISBT-induced-XPM . The demodulated wave has a pulse shape of almost 1.5-ps wider than the input pump signal (with full-width at half-maximum of about 2 ps). This pulse width increase is associated with the short relaxation time of the internal nonlinear mechanism and related with the modulated phase . Nevertheless, the stable format conversion operation and clear eye open can be observed.
3. XOR logic operation using MI switch
Since the interference of two BPSK signals can be used to realize a desired logical operation, and an OOK to BPSK format conversion can be achieved by a straight ISBT waveguide, we can adopt here, an MI structure for an ISBT-based logic gate. To proof our concept, a monolithic MI gating switch was fabricated and the experimental setup for an XOR operation is shown in Fig. 3. The gating device consists of a multi-mode interference (MMI) 3-dB coupler with a total length of 1 mm. The wafer was also phosphorus ion implanted but underwent RTA at 760 °C for 3 min to reduce the propagation loss to around 2 dB/mm, while the XPM efficiency slightly decreasing to 0.71 rad/pJ for a 1-mm-long straight waveguide with both facets anti-reflection coated . The left facet was anti-reflection coated for the input/output of TE probe signal, whereas that on the right was half-reflection coated for both TM pump light input and TE probe signal reflection. The reflected TE BPSK signals from two arms on the right of the MMI coupler can interfere with each other as an MI configuration. After fabrication of the photonic integrated circuit, the sample was planarized by BCB polymer and a resistive heater (80-nm-thick Ti and 10-nm-thick Au) was then attached to one arm for a static phase bias control.
As similar to that of the BPSK format conversion experiment, a 10-Gb/s RZ OOK data signal at 1559 nm is first generated and separated into two data branches through a 3-dB coupler. An optical delay line (Δτ) adjusts the relative time delay between the two data streams (here 21 bits data delay in the experiment). Variable optical attenuators for both data signals guarantee the input pump pulse intensity balance. Both data signals are adjusted to TM polarization and marked as route A and B. The average pump power was 44 mW for each route, due to the half-reflection coating. The cw probe (TE polarized) was centered at 1545 nm with an incident power of 15 mW. To simplify the experimental setup, we use the same waveguide port for both probe signal input and output. The destructive interference condition was used for the logic operation in our experiments. Through adjusting the applied voltage to the heater for phase bias control, a minimum signal output can be obtained, which corresponds to the XOR function of route A and B (A⊕B). A tunable OBPF and a low-noise EDFA are used to extract the desired logic output.
For the static MI gating performance, a 27-dB extinction ratio can be obtained and on-off switch can be realized with a 1.85-V heater voltage. With two pump data input to the gate, corresponding optimized eye-diagram of the 10 Gb/s XOR logic output signal is measured by an electric sampling oscilloscope (ESO), as shown in Fig. 4(a). The diagram shows a clear eye open, highlighting the effective all-optical XOR operation. The output spectra were also measured by an OSA, as shown in Fig. 4(b), for the probe signal without any pump data, with only one pump A (or B), and with both pump applied (for the XOR logical output), respectively. We can see from the blue line that the output spectrum is significantly modulated by the pump data, thus a switch gating can be fulfilled with either pump A or B applied. For the XOR operation with both data streams applied, the sharp peaks related to 10 GHz components disappear, which is due to the finite Fourier transform of the XOR results.
Figure 4(c) presents the time dependent waveforms of the output probe signal modulated by pump data stream A and B, respectively, along with the XOR logical operation with both data streams applied (all measured by an ESO as illustrated in Fig. 3). The logical relationship as well as the expected data patterns is demonstrated. The waveform peak corresponds to logic “1” with zero intensity to logic “0”. Top of Fig. 4(c) corresponds to the data of “1001101101011011110” (for Pump A) and middle is the data “0111110100001110001” (for Pump B), while the bottom output data is “1110011001010101111” (for A⊕B). It can be confirmed that the MI gate output accurately reflects the XOR logic operation of two input data signals.
4. Performance discussion
BER measurements are performed as a function of received power, in order to evaluate the system performance for BPSK format conversion and XOR operation, as shown in Fig. 5. Received power is defined as an input power to the EDFA located in front of Photo Detector. For the BPSK format conversion using a straight wire waveguide, the system power penalty can be improved with the increase of the probe TE signal intensity, as can be seen from the BER lines in Fig. 5(a) for the TE probe of 5, 10, and 15 mW, respectively, while the average TM pump OOK signal is kept at 20 mW. The power penalty is 12.2 dB for a 5-mW input probe while it is 7.5 dB for a 15-mW probe, at BER of 10−9 relative to the input pump OOK data. For the pump signal power dependence, the minimum power penalty is obtained for a π phase modulation, which corresponds to a 20-mW pump input. Too strong phase modulation (such as pump power of 26 mW) or too weak phase modulation (pump power of 14 mW) would deteriorate the system performance, due to the decreased output extinction ratio from the DLI. All the BPSK format conversion related BER results show a relative high power penalty, which is mainly caused by the poor DLI quality with an intrinsic low extinction ratio in our experiment. Nevertheless, the BER measurement shows that error free BPSK format conversion can be realized successfully.
For the XOR logic operation, we measure the BER with only one pump data stream applied (A/B) and the XOR output with both pump data applied (A⊕B), as shown in Fig. 5(b). For the gate with only pump route B applied, it has a better BER performance than that with only route A. This is mainly due to the non-optimum 3-dB coupler of the fabricated MMI. And the heater may be a little close to the top surface of the waveguide for route A, which would have some absorption on the signal, since the photonic circuit is deeply etched (around 2.5 μm) and the covered BCB layer may be not enough thick. Due to the imbalance performance of route A and B, the performance for the final XOR logic operation is further deteriorated. However, error-free XOR operation can still be expected.
It should be noted that TE probe light travels through the ISBT waveguide twice in the MI gate, and it has a total propagation loss of about 4 dB, with an additional 3-dB half-reflection loss at the pump-input port. Thus the XOR performance of the device is more sensitive to the anti-reflection coating on the waveguide facet for the probe input and output. If the coating is not optimum and even a slight amount of input TE light is reflected, which would become more comparable with the XOR output signal and interfere with each other, thus the device performance will be further deteriorated. We could anticipate a significant reduction in the system power penalty by the improvement of the coupling and propagation losses as well as the fine optimization of MMI 3-dB coupler structure. However, a high signal transmittance usually corresponds to a weak XPM , so there is a trade-off between the signal propagation loss and XPM efficiency for an optimum system power penalty.
Due to the ultra-short ISBT relaxation time, the presented BPSK conversion or XOR logic device can also work at a higher signal bit rate beyond 10 Gb/s. The output signal pulse width increases with the modulated phase, where a higher pump intensity or material XPM efficiency would cause a wider pulse . And this limits the maximum data rate due to the interference of adjacent pulses at high signal repetition rate. For example, the output pulse width of about 3.5 ps for BPSK conversion can allow a 280 Gb/s maximum data rate. However, the CDQWs waveguide facet would break down at high input pump intensity, and the empirical intensity threshold is about 120 mW for a waveguide facet with anti-reflection coating at a 10 GHz repetition rate without carrying any data. Higher pump repetition rate or signal duty-cycle will induce more heat and make the waveguide facet much easier to break down. Thus the maximum working rate for BPSK conversion is only 60 Gb/s for a PRBS data with a 4-pJ pump pulse necessary for π phase shift. However, higher working data rate is also possible, with the expense of inadequate phase modulation and reduced system performance.
For the working bandwidth of the XOR logic gate, it is limited by the XPM efficiency, which depends more on the probe and pump wavelength, and thus the working condition will change for different working wavelength. For example, if decrease the probe wavelength, a high XPM efficiency can be obtained, at the expense of high signal loss . Nevertheless, compared with electric logic device, a wideband operation can still be expected and this ISBT logic gate can work at a high bit rate due to the intrinsic ultrafast nonlinear response with a low energy consumption of only sub 10 pJ. Furthermore, it should be noted that other type logic gate such as NOT or OR operation can also be realized by the current device, but with a different phase bias control.
In summary, we demonstrate an all-optical BPSK format conversion using a straight wire waveguide and an XOR logical operation with a monolithic MI switch, on a 10 Gb/s PRBS data stream. The BPSK format conversion is based on an ISBT-induced XPM mechanism for InGaAs/AlAsSb CDQWs, where TE probe light is immune to ISBT absorption and there is no idle wavelength needed as in FWM. BER measurements are performed and error-free operation clearly demonstrates the effectiveness of the phase modulation and XOR operation. The ISBT-based MI logic gate device can work at even higher data bit rates due to the intrinsic ultrafast modulation mechanism. This ISBT device has the advantages such as low-cost, low energy consumption, and wide working bandwidth, which would have broad application prospects for signal processing photonic devices in optical communications.
Jijun Feng acknowledges support from the Japan Society for the Promotion of Science.
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