We demonstrate all-optical 4x10 Gbps NAND gate with an individual input data rate of 10 Gbps using single mode Fabry-Pérot laser diode. The proposed scheme is based on the principle of multi-input injection locking. All-optical NAND gate is one of the universal logic gates which can be used for realizing all other logic gates for optical communication and networks. The output performance of the proposed all-optical multi-input NAND gate is verified with output spectrum domain results, waveforms, rising-falling time, and eye diagram measurement at 10 Gbps input. The ON/OFF contrast ratio of 41 dB is measured at the spectrum domain output when all four input beams are logic high. In all other combinations of four inputs, maximum ON/OFF contrast ratio of 1.5 dB ON/OFF is measured. Clear output waveform, output eye diagram with an extinction ratio of about 11 dB, and rising-falling time of about 35 ps are obtained. The BER measurement is carried out and we found the power penalty of about 1.7 dB at BER of 10−9.
© 2015 Optical Society of America
All-optical signal processing is getting lot of attention due to its advantages over electronics owing to high bandwidth capability, data transparency and less electro-magnetic interference [1–3]. Various optical units such as logic gates, data format conversion, switches, flip-flops, data erasing function, and other optical control units have been demonstrated using optical technology [4–9]. Among different optical units, all-optical NAND gate is the basic unit of optical computing, switching, and optical signal processing such as addressing, multiplexing, regeneration, and label swapping . Among many other active optical components, semiconductor optical amplifier (SOA) is one of the most widely used active device for optical units since they are easy to integrate and compact in size, and possess higher bandwidth capability [10–12]. However, SOA requires high biasing current of about 200 mA and is expensive . Besides SOAs, Fabry-Pérot laser diodes (FP-LDs) based on injection locking [13–17] are also widely used in demonstration of various optical units, network elements, and passive optical network. Several works using FP-LDs have been proposed for realizing various optical components and signal processing like optical logic gates , wavelength converters , optical switch , optical flip-flops , data format transformer  and bit error monitoring . Commercially available FP-LDs are multi-mode in nature and we coin it as multi-mode FP-LD (MMFP-LD) henceforth. The side mode suppression ratio (SMSR) of MMFP-LD is only about 3 dB under normal biasing condition without any external beam injection. As a result, MMFP-LDs require external probe beam for the operation as in SOAs, requiring additional components such as polarizer and coupler. Therefore commercial FP-LDs are modified to provide a single dominant mode wavelength output, i.e. self-injected mode, with a SMSR of about 36 dB, which is known as single mode Fabry-Perot laser diode (SMFP-LD) . Hence, SMFP-LDs possess significant advantage of requiring fewer number of components for signal processing.
In this paper, we propose and successfully demonstrate multi-input optical logic NAND gate using SMFP-LD for the first time. We observed the required power for injection locking of SMFP-LDs with and without the suppression of the dominant mode of the SMFP-LDs. The required power difference between injection locking with and without suppression of the dominant mode makes the multiple injection locking possible in SMFP-LDs  and suppress the dominant mode by only desired combination of input injected beams. Based on multi-input injection locking and analysis of power requirement for different wavelength detuning and injected mode of input beam, we demonstrate four inputs optical NAND gate. In this case, SMFP-LD is configured in such a way that the output, which is the self-injected mode of SMFP-LD, is suppressed providing low logic output only when all four injected beams are logic high else the self-injected mode is not suppressed resulting in logic high output. The proposed four inputs optical NAND gate is demonstrated with individual input data rate of 10 Gbps.
The proposed all-optical 4x10 Gbps input optical NAND logic gate can be used for decision making circuits, and control circuits in all-optical networks. Also, a decision making circuit based on multi-input injection can be implemented with desired combination of input beams. The rest of the paper is organized as follows. Section II provides basic operating principle to realize all-optical multi-input NAND logic gate. In section III, we present the experimental setup and results and finally section IV concludes with the discussion of the proposed work.
2. Operating details
The basic operating principle of four inputs optical NAND gate is based on the multi-input injection locking, which is obtained by the power management of input injected beams. The output beam, which is the dominant mode of the SMFP-LD is suppressed only when all input injected beams are logic high. In multi-input injection locking, the individual power of input beams are arranged in such a way that the self-injected mode will be suppressed providing logic “0” output only when all input beams are logic “1”. In this paper, we coin the term “injection locking without suppression” when the injected mode of SMFP-LD gains the power of −12 dBm or more and also the dominant mode is not suppressed more than 3 dB. Beyond the suppression of 3 dB of the dominant mode, the dominant mode starts to suppress abruptly, which can be seen from Fig. 1(b) and 1(c). The abrupt suppression of the dominant mode violates the condition of the injection locking without suppression of the dominant mode. In order to meet this condition of 3 dB suppression, the power of the injected mode should be greater or equal to −12 dBm, which is sufficient to be considered as logic “1”. Similarly, the term “injection locking with suppression” is used when input beam is injected to any of the modes of SMFP-LDs and the self-injected mode is suppressed giving ON/OFF contrast ratio of 30 dB or more. The amount of suppression of dominant mode is dependent on the total amount of power of input beams used for injection locking in the SMFP-LD. The required total power for injection locking and suppressing the dominant mode is dependent on wavelength detuning. Lower detuning require lower injected power for injection locking and higher detuning needs higher input power. However, too small detuning may cause unstable locking [13, 26].
Figure 1(a) shows basic experimental set up for examining the effect of wavelength detuning on the required power of input injected beam for injection locking. From our experiment, we observed that the minimum wavelength detuning can be set to 0.04 nm for stable operation with low input power. These observations suggest that the required input beam power for injection locking and suppressing the dominant mode is directly proportional to wavelength detuning. For constant wavelength detuning, we obtain the amount of power, which is sufficient to injection-lock any of the modes of the SMFP-LD but is not enough to suppress the dominant mode. The difference in required power for injection locking with and without suppression of the dominant mode makes it possible to inject number of input beams in a SMFP-LD. Generally on injection locking, wavelength detuning is maintained less than half of the mode separation ( = 1.16 nm) for stable operation, which is less than 0.58 nm (1.16 nm/2) for the SMFP-LD used in this experiment.
Figure 1(b) shows the required power for injection locking with suppression of the dominant mode with different wavelength detuning. From Fig. 1(b), we observe that the power required for suppressing the dominant mode increases with increase in the wavelength detuning. Hence, the threshold value of input injected beam power is shifted to right with increase in the wavelength detuning. Also, the width of hysteresis increases with increase in wavelength detuning, which is consistent with the research work presented in . The effect of input injected mode on the suppression of the dominant mode is shown in Fig. 1(c). The dominant mode of SMFP-LD is considered as 0th mode and modes at the right sides are considered as 1st, 2nd, 3rd, and so on. The required power for injection locking increases when the input beam is injected to the farther mode of the SMFP-LD, however the difference in power is less than 3 dBm, which is due to the small power difference of different modes of SMFP-LDs under normal biasing condition. Hence, based on wavelength detuning and mode of SMFP-LD at which the input beam is injected, the required power for injection locking with and without the suppression of the dominant mode can be varied for demonstrating multi-input injection locking. Figure 1(d) shows the effect of multi-input beam injection on the SMFP-LD in which threshold value for the suppression of the dominant mode increases with increase in number of injected input beams.
The power management of input injected beams for multi-input injection locking is shown in Fig. 2. In Fig. 2, red color beams show the spectrum of self-injected mode of SMFP-LDs, whereas others are input injected beams. λ1 to λn represent wavelength of input injected beams and P1 to Pn are their corresponding minimum injected beam power required for considering injection locking without the suppression of the self-injected mode of SMFP-LD. Pinj refers for power required for input injected beam to be injection locked in any mode of SMFP-LD without suppression of the dominant mode of SMFP-LD. The individual power of each injected beam is equal or greater than Pinj but less than that of the Ps (power of the beam, which is sufficient to suppress the self-injected mode). In Fig. 2(a), injected beam A alone cannot suppress the self-injected beam, λ0, since the power of beam A is less than Ps, whereas in Fig. 1(b), beam λ0 is suppressed because the combined power of beam A and beam B is more than Ps. Similarly, we can configure input injected beams in such a way that number of input beams can be injection locked without suppressing λ0 as shown in Fig. 2(c) unless all input injected beam are logic high as shown in Fig. 2(d). Hence, the power arrangement of input beam for multi-input NAND gate can be written as
In multi-input injection locking with four inputs NAND gate, each of all four input beams should have the power of more than Pinj but should be less than Ps. Also, with any of the combination of four input injected beams except for all are logic high, the total power injected should be less than Ps. When all four input beams are logic high, total power injected to SMFP-LD will be greater than Ps and hence, the dominant mode will be suppressed. This arrangement provides optical NAND gate output since the output of SMFP-LD will be logic low only when all four input beams are logic high.
3. Experimental setup and results
Figure 3 shows the experimental setup for the proposed all-optical four inputs NAND logic gate using a single SMFP-LD. The SMFP-LD used in the experiment is specially designed FP-LD with an external cavity, which is formed by eliminating an inclination from the coupling fiber in the conventional MMFP-LD that has an InGaAsP multi quantum well structure with 300 µm cavity length . These specially designed SMFP-LDs have a self-locking dominant mode. The refractive index of the active region changes with change in temperature. As a result, there is a change in optical path length in the laser diode, providing optimal mode-matching condition for single-mode oscillation. The dominant mode of SMFP-LD can be changed to a range of wavelength of 10 nm by varying the operating temperature. The side modes present in SMFP-LD have the power difference of more than 35 dB to the self-injected mode providing high SMSR under normal biasing condition. The side modes present in SMFP-LD can be injection locked with the help of external beams. With the application of external beams, the dominant mode of the SMFP-LD can be suppressed, providing high ON/OFF contrast ratio.
The SMFP-LD used in the experiment is biased with a driving current of 21 mA (threshold current of 13 mA) and is operated at a temperature of 24°C for demonstrating three input NAND gate with three tunable lasers (TLs) in Fig. 3 and four TLs with same SMFP-LD is used to demonstrate four inputs NAND gate as shown in Fig. 3. The operating temperature is changed to 27°C to show the tunability of the dominant mode of the SMFP-LD in the case of four inputs NAND gate. Under these operating condition of 24°C and 27°C, SMFP-LD has self-injected dominant mode at the wavelength of 1544.41 nm with the power of −6.63 dBm and 1545.98 nm with the power of −5.46 dBm, respectively. The operating temperature is controlled by using thermo-electric controller (TEC) which has the stability of 0.1°C. Tunable lasers, TL1, TL2, TL3, and TL4 are injected at the wavelength of 1550.34 nm, 1551.49 nm, 1553.82 nm, and 1552.67 nm, respectively. During our experiment, we have observed that input beams can be injection-locked to any of the side modes of the SMFP-LD. The side modes of SMFP-LDs are observed in the range of 1535 nm to 1555 nm. Hence, the input beams can be injection locked to any of the modes within the range of 1535 nm to 1555nm with proper amount of wavelength detuning and power of injected input beam. Input beams from TLs are modulated with 10 Gbps Non Return to Zero signal and pseudo random bit sequence of 231-1 to measure the output waveforms and eyes diagrams, respectively. Polarization controllers, PC1, PC2, PC3, and PC4 are used to minimize the losses in the Mach-Zehnder modulator, which are polarization dependent and PC5, PC6, PC7, and PC8 are used to allow only TE polarized light for the injection locking of SMFP-LD. Coupler, CO is used to combine all the input injected beams that are injected to SMFP-LD. Band pass filter (BPF) is used to filter out the dominant mode of the SMFP-LD, which is output of the proposed multi-input optical NAND gate.
To demonstrate multi-input optical NAND gate, we set the wavelength detuning of 0.04 nm to all four input injected beams and control the power injected to the SMFP-LD. Then, the power of individual beam is controlled in such a way that the power of individual beam is greater than the power required for injection locking in the corresponding mode but less than the power required for suppressing the self-injected mode. In this paper, we have demonstrated spectrum result for three and four inputs optical NAND gate using a single SMFP-LD. In both cases, the power of all injected beams are managed in such a way that with only all input injected beams with logic high, the self-injected mode of SMFP-LD is suppressed providing logic “0” else the self-injected mode is not suppressed and gives logic “1” which is equivalent to NAND operation.
Figure 4 (a) shows the spectrum domain result of three input NAND gate. For three inputs optical NAND gate, the power required for injection locking with and without the suppression of the dominant mode is about −9.2 dBm and −19 dBm, respectively. TL4 in Fig. 3 is not used for three input NAND gate. Other input beams: Beam A, B, and C have individual power of −12.8 dBm, −14.6 dBm, and −13.2 dBm, respectively and hence, total power injected to the SMFP-LD with presence of all beam is about −8.7 dBm, which is greater than the power required for the suppression of the dominant mode. With absence of any of three beams, the maximum total power injected to the SMFP-LD is about −9.98 dBm, which is less than Ps (−9.3 dBm). Hence, the self-injected mode is not suppressed with the absence of any of three injected beams. The change in the self-injected mode power with any of the combination of three beams except for all injected beams are logic “1”, is about 1.2 dB, whereas with all three input beams injected (all logic “1”), the self- injected beam is suppressed to −54.94 dBm, which gives the ON/OFF contrast ratio of about 50 dB.
Similarly, Fig. 4(b) shows the spectrum domain result for four input optical NAND gate using a single SMFP-LD. Four input beams A, B, C, and D are injected to a SMFP-LD with the power of −12.54 dBm, −16.27 dBm, −15.8 dBm, and −15.17 dBm, respectively. When all four input beams are logic “1”, the total power injected to SMFP-LD is −8.65 dBm, which is sufficient to suppress the dominant mode. In this case, the power of the dominant mode is measured as −49.87 dBm, providing ON/OFF contrast ratio of about 44.47 dB. When any of the beam is logic “0”, the total input injected power is less than −9.4 dBm, which is less than −8.9 dBm (power required for suppressing the dominant mode when four input beams are injected). Hence, with any of beams absent, the dominant mode is not suppressed. We measured the maximum suppression of about 1.5 dB of the dominant mode with all other combination except for all four beams as logic “1”, which proves the proposed four input optical NAND gate using SMFP-LD.
Oscilloscope traces of input beams (A, B, C, and D) with 16 bit NRZ pulse trains and the output of 4x10 Gbps optical NAND gate (Y) using SMFP-LD is shown in Fig. 5 with eye diagram. All input beams are 16 bit NRZ with different pulse train. We can observe from Fig. 5 that the output of four input NAND gate is only logic “0” when all input pulse trains are logic “1”. Rising and falling time of 35 ps and 31.7 ps are measured. Eye diagrams are measured with 10 Gbps PRBS 231-1. The extinction ratio of 11.81 dB is measured for the output of proposed all-optical four input NAND gate. BER measurement is performed for the proposed all-optical NAND gate and we found the power penalty of about 1.7 dB at the BER of 10−9, which is shown in Fig. 6. The clear waveforms, less rising falling time, eye diagram, and BER measurement verify the proposed all-optical 4x10 Gbps input NAND gate using SMFP-LD.
In this paper, we proposed multi-lambda injection locking and performed experimental analysis with effect of input beam injection on different modes and wavelength detuning which are important factors for multi-lambda injection locking. From our analysis, we found that the power required for injection locking with suppression of self-injected mode in SMFP-LD is dependent on the mode of SMFP-LD where the beam is injected. The nearer mode requires less input power for injection locking with suppression of the dominant mode. We present the injection locking of three and four inputs in SMFP-LD and verify multi-input NAND gate in spectrum domain. Clear output waveforms, rising-falling time of about 35 ps and, eye diagram with an extinction ratio of about 11 dB, and power penalty of 1.7 dB at the BER of 10−9 is measured for four input NAND gate. Our analysis and results confirm the feasibility of SMFP-LD for multi-lambda injection locking system, which can be further used for realizing various multi-input logic units, multi-input controlled switches, multi-decision making circuits, and others that are important in optical communication and networks.
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