- Email: [email protected]
All-optical binary half-adder
1 November 1998
Optics Communications 156 Ž1998. 22–26
All-optical binary half-adder A.J. Poustie ) , K.J. Blow, A.E. Kelly, R.J. Manning BT Laboratories, Martlesham Heath, Ipswich IP5 3RE, UK Received 15 April 1998; accepted 22 June 1998
Abstract We demonstrate an all-optical binary half-adder which simultaneously generates the bit-serial SUM and CARRY bits from two binary modulated input pulse trains. Additionally, the circuit can implement the XOR function of the two input data streams. q 1998 Elsevier Science B.V. All rights reserved.
In future photonic networks, all-optical processing functions will be required to add functionality whilst minimising network latency. To achieve digital optical logic ŽDOL. processing of high speed digitally modulated pulse sequences, certain key functions must be performed on the optical data. We have previously demonstrated the function of ‘memory’ in an all-optical regenerative design which not only provides extremely stable storage w1x but also has the ability to restore the optical logic level w2x. In addition, we have shown how optical pulses can be both written-in and read-out from this type of memory architecture w3x. Here we demonstrate another important function in DOL, the ability to perform modulo two addition of two input data sequences with an all-optical half-adder. The serial half-adder is a well known function in digital electronics and is the basis for a variety of more complex processing circuits such as a full adder and binary counters w4,5x. Our all-optical design uses three TOAD w6x all-optical switching gates to perform the half-adder function and has two outputs: one for the SUM bits and one for the CARRY bits. If only the SUM output is considered, then the circuit also achieves the XOR of the two input data sequences. XOR is another important logical function in DOL and has previously been demonstrated in all-fibre nonlinear loop mirrors w7,8x. However, in switching gates based on nonlinear loop mirrors with semiconductor opti-
)
E-mail: [email protected]
cal amplifiers ŽSOA. such as the TOAD, the XOR is more difficult to achieve in a single gate owing to the gain saturation properties of the SOA w9x. The approach adopted here to obtain XOR is to logically combine AND and OR operations Ž A [ B s Ž A q B .Ž AB.. with the benefit that the combined operation should be scaleable in speed to the switching speed of the individual TOAD gates Ž) 40 Gbitrs w10–12x.. The electronic logic diagram of the serial half-adder is shown in Fig. 1 using the conventional electronic gate symbols. Fig. 2 shows the all-optical implementation of this logical circuit. Each TOAD is a type of all-optical nonlinear loop mirror comprising a 50:50 fused fibre coupler, a second fibre coupler to introduce the switching pulses, polarisation controllers to bias the loop and a SOA as the nonlinear element. The SOA is placed slightly asymmetrically in the loop to create a temporal switching window w6x, typically about 50 ps in this experiment. Each optical pulse train is derived from a jitter-suppressed, gain-switched distributed feedback ŽDFB. laser giving ; 12 ps pulses at a repetition rate of ; 1 GHz w1–3x. The two input data sequences A and B are created by modulating two of the pulse sources using electroabsorption modulators ŽEAM. driven by an electronic pattern generator ŽHP80000.. Adjustable optical delay lines ŽSantec ODL320. and lengths of optical fibre are used to bring the pulses into the correct synchronism for switching in the TOADs. The switching pulses are obtained by amplifying the pulses in erbium doped fibre amplifiers ŽEDFA. and the typical switching energy was ; 100 fJ per pulse.
0030-4018r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 3 4 9 - 6
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
Fig. 1. Electronic logic diagram of a serial binary half-adder.
The circuit operates initially by using 50:50 couplers to split A and B to the TOADs. TOAD1 is used to generate the logical AND of inputs A and B. Here we use two different wavelengths for A Ž1538 nm. and B Ž1533 nm. so that they can be distinguished; however, polarisation diversity w13x or directionality w14x can also be used for same wavelength inputs. Generally, it is fully acceptable to use different wavelength inputs in DOL circuits since the data is always processed locally and never propagated over large distances where dispersion could be a problem. TOAD1 is biased to reflection and checks for the condition where A and B are both logical ONE, which requires a CARRY bit in the binary addition. In this case the transmitted output of TOAD1 is ONE and generates the CARRY bit, otherwise the output is ZERO. The CARRY bit is wavelength converted to 1551 nm in TOAD3 w1x so that it can be used as a switching pulse in TOAD2 and be distinguished in wavelength from the input pulses. The logical OR of inputs A and B is simply generated by a 50:50 coupler w15x before the input to TOAD2. This combines the binary patterns in the correct bit-serial synchronism by delaying one pattern relative to the other before the coupler to account for any differential path lengths for
23
the two patterns. Even though the optical pulses in the combined OR pattern are overlapped, there is no interference when A s B s 1 since the wavelengths are different and the larger optical energy for this case is also unimportant since these combined pulses can still be fully switched in TOAD2 as described below. The SUM output of the half-adder is completed by appropriately delaying the OR result Žto account for the additional optical path length through TOAD1rTOAD3. and inputting this into TOAD2. This delay ensures that the correct bit-serial addition of the pulses is obtained. TOAD2 acts as the CARRY inverter and final AND gate in the half-adder circuit. TOAD2 is biased to transmission in the absence of a switching pulse and therefore if A s 1, B s 0 or A s 0, B s 1 then the TOAD2 output is ONE. If AB s 1 then the output of TOAD1rTOAD3 is used to switch TOAD2 back to reflection. This simultaneously redirects both the input optical pulses from A and B back to TOAD2 input and hence generates a ZERO at TOAD2 output. In general, the TOAD can switch several input wavelengths simultaneously upon the application of a single switching pulse w16x. This then generates the bit-serial modulo 2 addition of inputs A and B and completes the all-optical binary half-adder with access to both the SUM and CARRY bits. The experimental logical pattern for A was 110110010110101100101 and for B it was 101110101001101001001 and these were chosen to be relatively short in length so that the half-adder logical outputs could be examined in detail. On each figure, the data patterns used for A and B are shown as the binary sequences and the patterns are arranged vertically in bit-level synchronisation with the output of the oscilloscope. Fig. 3 shows the two input sequences to the half-adder circuit. Note that any
Fig. 2. Schematic diagram of the all-optical half-adder. The adjustable optical delays for bit synchronisation are not shown. IrP, input; OrP, output.
24
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
Fig. 4. CARRY output Ž1551 nm. Ž5 nsrdiv..
TOAD1 retains the undesired amplitude modulation of input B. However, the output from TOAD3 is relatively equalised through the cosine interferometric switching response of the TOAD w2x. Figs. 5 and 6 show the SUM output independently resolved at wavelengths 1538 and 1533 nm, respectively, with a bandpass optical filter. Fig. 7 shows these two measured patterns combined electroni-
Fig. 3. Experimental oscilloscope traces Ž5 nsrdiv. of Župper. input data pattern A Ž1538 nm. and Žlower. input data pattern B Ž1533 nm..
undesired amplitude modulation of the input data patterns Ži.e. of the ONEs. arises mainly from the imperfect electrical response of the RF amplifiers and EAMs used to create the patterns. Fig. 4 shows the CARRY output for these two binary inputs and corresponds to ONE when AB s 1. The trace shown in Fig. 4 is actually the wavelength filtered switching pulses Ž1551 nm. measured at the transmitted output of TOAD2 and so the contrast ratio appears degraded due to the amplified spontaneous emission ŽASE. of EDFAa4 and of the SOA in TOAD2. Extinction ratios of ) 20 dB were typically measured for the CARRY output from TOAD3, indicating that there was minimal degradation of the switching by amplitude modulation w17x. Generally, it is advantageous to derive the CARRY output from TOAD3 rather than TOAD1 since the output of
Fig. 5. SUM output Ž1538 nm. Ž5 nsrdiv..
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
25
that the correct binary half-adder operation has been completed on the optical data. We also measured the all-optical half-adder with shorter and longer input patterns and obtained similar performance to the results presented here. Typical extinction ratios measured at the SUM output for the 1533 nm Ž1538 nm. pulses were 13 dB Ž11.5 dB. and were probably limited by the relatively poor gain-switched pulses of the 1538 nm source. If further cascading of the TOADs was required, then any accumulated reduction in the TOAD contrast could be removed by using all-optical regeneration with optical thresholding to restore the optical logic levels w2x. In conclusion, we have successfully demonstrated an all-optical binary half-adder and logically combined XOR gate. The bit-serial optical design should allow the circuit to process arbitrary length data streams and should be scaleable in bit-rate to the fastest switching speed of the all-optical gates.
Fig. 6. SUM output Ž1533 nm. Ž5 nsrdiv..
Acknowledgements cally onto a single trace. It is actually this combination of these two sequences that constitutes the true binary SUM output Žor also A XOR B . and although we did not simultaneously optically resolve the two wavelengths Ž1533r1538 nm. at TOAD2 output in this experiment, this could easily be achieved with wavelength division multiplexing ŽWDM. couplers which transmit several wavelengths at once. From careful examination it can be seen
The authors would like to thank Colin Ford and Dave Moodie at BT Laboratories for the supply of packaged SOAs and EA modulators. We also thank Maurice Wilkes for useful discussions.
References
Fig. 7. Combined SUM output Ž1538q1533 nm. Ž5 nsrdiv..
w1x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 140 Ž1997. 184. w2x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 146 Ž1998. 262. w3x A.J. Poustie, A.E. Kelly, K.J. Blow, R.J. Manning, in Nonlinear Guided Waves and their Applications, Optical Society of America, Washington, DC, 1998, paper NThB3. w4x M.V. Wilkes, Automatic Digital Computers, Methuen, London, 1956. w5x A.F. Benner, J. Bowman, T. Erkkila, R.J. Feuerstein, V.P. Heuring, H.F. Jordan, J. Sauer, T. Soukup, Appl. Optics 30 Ž1991. 4179. w6x J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, IEEE Phot. Technol. Lett. 5 Ž1993. 787. w7x M. Jinno, T. Matsumoto, Optics Lett. 16 Ž1991. 220. w8x K.L. Hall, K.A. Rauschenbach, Electron. Lett. 32 Ž1996. 1214. w9x R.J. Manning, A.D. Ellis, A.J. Poustie, K.J. Blow, J. Opt. Soc. Am. B 14 Ž1997. 3204. w10x A.D. Ellis, D.A.O. Davies, A. Kelly, W.A. Pender, Electron. Lett. 31 Ž1995. 1245. w11x N.S. Patel, K.A. Rauschenbach, K.A. Hall, IEEE Phot. Technol. Lett. 8 Ž1996. 1695.
26
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
w12x R.J. Manning, G. Sherlock, Electron. Lett. 31 Ž1995. 307. w13x A.J. Poustie, R.J. Manning, K.J. Blow, Electron. Lett. 32 Ž1996. 1215. w14x E. Jahn, N. Agrawal, M. Arbert, H.J. Ehrke, D. Franke, R. Ludwig, W. Peiper, H.G. Weber, C.M. Weinert, Electron. Lett. 31 Ž1995. 1857.
w15x V.P. Heuring, H.F. Jordan, J.P. Pratt, Appl. Optics 31 Ž1992. 3213. w16x S. Diez, R. Ludwig, H.G. Weber, Electron. Lett. 34 Ž1998. 803. w17x R.J. Manning, A.E. Kelly, A.J. Poustie, K.J. Blow, Electron. Lett. 34 Ž1998. 916.
Optics Communications 156 Ž1998. 22–26
All-optical binary half-adder A.J. Poustie ) , K.J. Blow, A.E. Kelly, R.J. Manning BT Laboratories, Martlesham Heath, Ipswich IP5 3RE, UK Received 15 April 1998; accepted 22 June 1998
Abstract We demonstrate an all-optical binary half-adder which simultaneously generates the bit-serial SUM and CARRY bits from two binary modulated input pulse trains. Additionally, the circuit can implement the XOR function of the two input data streams. q 1998 Elsevier Science B.V. All rights reserved.
In future photonic networks, all-optical processing functions will be required to add functionality whilst minimising network latency. To achieve digital optical logic ŽDOL. processing of high speed digitally modulated pulse sequences, certain key functions must be performed on the optical data. We have previously demonstrated the function of ‘memory’ in an all-optical regenerative design which not only provides extremely stable storage w1x but also has the ability to restore the optical logic level w2x. In addition, we have shown how optical pulses can be both written-in and read-out from this type of memory architecture w3x. Here we demonstrate another important function in DOL, the ability to perform modulo two addition of two input data sequences with an all-optical half-adder. The serial half-adder is a well known function in digital electronics and is the basis for a variety of more complex processing circuits such as a full adder and binary counters w4,5x. Our all-optical design uses three TOAD w6x all-optical switching gates to perform the half-adder function and has two outputs: one for the SUM bits and one for the CARRY bits. If only the SUM output is considered, then the circuit also achieves the XOR of the two input data sequences. XOR is another important logical function in DOL and has previously been demonstrated in all-fibre nonlinear loop mirrors w7,8x. However, in switching gates based on nonlinear loop mirrors with semiconductor opti-
)
E-mail: [email protected]
cal amplifiers ŽSOA. such as the TOAD, the XOR is more difficult to achieve in a single gate owing to the gain saturation properties of the SOA w9x. The approach adopted here to obtain XOR is to logically combine AND and OR operations Ž A [ B s Ž A q B .Ž AB.. with the benefit that the combined operation should be scaleable in speed to the switching speed of the individual TOAD gates Ž) 40 Gbitrs w10–12x.. The electronic logic diagram of the serial half-adder is shown in Fig. 1 using the conventional electronic gate symbols. Fig. 2 shows the all-optical implementation of this logical circuit. Each TOAD is a type of all-optical nonlinear loop mirror comprising a 50:50 fused fibre coupler, a second fibre coupler to introduce the switching pulses, polarisation controllers to bias the loop and a SOA as the nonlinear element. The SOA is placed slightly asymmetrically in the loop to create a temporal switching window w6x, typically about 50 ps in this experiment. Each optical pulse train is derived from a jitter-suppressed, gain-switched distributed feedback ŽDFB. laser giving ; 12 ps pulses at a repetition rate of ; 1 GHz w1–3x. The two input data sequences A and B are created by modulating two of the pulse sources using electroabsorption modulators ŽEAM. driven by an electronic pattern generator ŽHP80000.. Adjustable optical delay lines ŽSantec ODL320. and lengths of optical fibre are used to bring the pulses into the correct synchronism for switching in the TOADs. The switching pulses are obtained by amplifying the pulses in erbium doped fibre amplifiers ŽEDFA. and the typical switching energy was ; 100 fJ per pulse.
0030-4018r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 3 4 9 - 6
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
Fig. 1. Electronic logic diagram of a serial binary half-adder.
The circuit operates initially by using 50:50 couplers to split A and B to the TOADs. TOAD1 is used to generate the logical AND of inputs A and B. Here we use two different wavelengths for A Ž1538 nm. and B Ž1533 nm. so that they can be distinguished; however, polarisation diversity w13x or directionality w14x can also be used for same wavelength inputs. Generally, it is fully acceptable to use different wavelength inputs in DOL circuits since the data is always processed locally and never propagated over large distances where dispersion could be a problem. TOAD1 is biased to reflection and checks for the condition where A and B are both logical ONE, which requires a CARRY bit in the binary addition. In this case the transmitted output of TOAD1 is ONE and generates the CARRY bit, otherwise the output is ZERO. The CARRY bit is wavelength converted to 1551 nm in TOAD3 w1x so that it can be used as a switching pulse in TOAD2 and be distinguished in wavelength from the input pulses. The logical OR of inputs A and B is simply generated by a 50:50 coupler w15x before the input to TOAD2. This combines the binary patterns in the correct bit-serial synchronism by delaying one pattern relative to the other before the coupler to account for any differential path lengths for
23
the two patterns. Even though the optical pulses in the combined OR pattern are overlapped, there is no interference when A s B s 1 since the wavelengths are different and the larger optical energy for this case is also unimportant since these combined pulses can still be fully switched in TOAD2 as described below. The SUM output of the half-adder is completed by appropriately delaying the OR result Žto account for the additional optical path length through TOAD1rTOAD3. and inputting this into TOAD2. This delay ensures that the correct bit-serial addition of the pulses is obtained. TOAD2 acts as the CARRY inverter and final AND gate in the half-adder circuit. TOAD2 is biased to transmission in the absence of a switching pulse and therefore if A s 1, B s 0 or A s 0, B s 1 then the TOAD2 output is ONE. If AB s 1 then the output of TOAD1rTOAD3 is used to switch TOAD2 back to reflection. This simultaneously redirects both the input optical pulses from A and B back to TOAD2 input and hence generates a ZERO at TOAD2 output. In general, the TOAD can switch several input wavelengths simultaneously upon the application of a single switching pulse w16x. This then generates the bit-serial modulo 2 addition of inputs A and B and completes the all-optical binary half-adder with access to both the SUM and CARRY bits. The experimental logical pattern for A was 110110010110101100101 and for B it was 101110101001101001001 and these were chosen to be relatively short in length so that the half-adder logical outputs could be examined in detail. On each figure, the data patterns used for A and B are shown as the binary sequences and the patterns are arranged vertically in bit-level synchronisation with the output of the oscilloscope. Fig. 3 shows the two input sequences to the half-adder circuit. Note that any
Fig. 2. Schematic diagram of the all-optical half-adder. The adjustable optical delays for bit synchronisation are not shown. IrP, input; OrP, output.
24
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
Fig. 4. CARRY output Ž1551 nm. Ž5 nsrdiv..
TOAD1 retains the undesired amplitude modulation of input B. However, the output from TOAD3 is relatively equalised through the cosine interferometric switching response of the TOAD w2x. Figs. 5 and 6 show the SUM output independently resolved at wavelengths 1538 and 1533 nm, respectively, with a bandpass optical filter. Fig. 7 shows these two measured patterns combined electroni-
Fig. 3. Experimental oscilloscope traces Ž5 nsrdiv. of Župper. input data pattern A Ž1538 nm. and Žlower. input data pattern B Ž1533 nm..
undesired amplitude modulation of the input data patterns Ži.e. of the ONEs. arises mainly from the imperfect electrical response of the RF amplifiers and EAMs used to create the patterns. Fig. 4 shows the CARRY output for these two binary inputs and corresponds to ONE when AB s 1. The trace shown in Fig. 4 is actually the wavelength filtered switching pulses Ž1551 nm. measured at the transmitted output of TOAD2 and so the contrast ratio appears degraded due to the amplified spontaneous emission ŽASE. of EDFAa4 and of the SOA in TOAD2. Extinction ratios of ) 20 dB were typically measured for the CARRY output from TOAD3, indicating that there was minimal degradation of the switching by amplitude modulation w17x. Generally, it is advantageous to derive the CARRY output from TOAD3 rather than TOAD1 since the output of
Fig. 5. SUM output Ž1538 nm. Ž5 nsrdiv..
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
25
that the correct binary half-adder operation has been completed on the optical data. We also measured the all-optical half-adder with shorter and longer input patterns and obtained similar performance to the results presented here. Typical extinction ratios measured at the SUM output for the 1533 nm Ž1538 nm. pulses were 13 dB Ž11.5 dB. and were probably limited by the relatively poor gain-switched pulses of the 1538 nm source. If further cascading of the TOADs was required, then any accumulated reduction in the TOAD contrast could be removed by using all-optical regeneration with optical thresholding to restore the optical logic levels w2x. In conclusion, we have successfully demonstrated an all-optical binary half-adder and logically combined XOR gate. The bit-serial optical design should allow the circuit to process arbitrary length data streams and should be scaleable in bit-rate to the fastest switching speed of the all-optical gates.
Fig. 6. SUM output Ž1533 nm. Ž5 nsrdiv..
Acknowledgements cally onto a single trace. It is actually this combination of these two sequences that constitutes the true binary SUM output Žor also A XOR B . and although we did not simultaneously optically resolve the two wavelengths Ž1533r1538 nm. at TOAD2 output in this experiment, this could easily be achieved with wavelength division multiplexing ŽWDM. couplers which transmit several wavelengths at once. From careful examination it can be seen
The authors would like to thank Colin Ford and Dave Moodie at BT Laboratories for the supply of packaged SOAs and EA modulators. We also thank Maurice Wilkes for useful discussions.
References
Fig. 7. Combined SUM output Ž1538q1533 nm. Ž5 nsrdiv..
w1x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 140 Ž1997. 184. w2x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 146 Ž1998. 262. w3x A.J. Poustie, A.E. Kelly, K.J. Blow, R.J. Manning, in Nonlinear Guided Waves and their Applications, Optical Society of America, Washington, DC, 1998, paper NThB3. w4x M.V. Wilkes, Automatic Digital Computers, Methuen, London, 1956. w5x A.F. Benner, J. Bowman, T. Erkkila, R.J. Feuerstein, V.P. Heuring, H.F. Jordan, J. Sauer, T. Soukup, Appl. Optics 30 Ž1991. 4179. w6x J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, IEEE Phot. Technol. Lett. 5 Ž1993. 787. w7x M. Jinno, T. Matsumoto, Optics Lett. 16 Ž1991. 220. w8x K.L. Hall, K.A. Rauschenbach, Electron. Lett. 32 Ž1996. 1214. w9x R.J. Manning, A.D. Ellis, A.J. Poustie, K.J. Blow, J. Opt. Soc. Am. B 14 Ž1997. 3204. w10x A.D. Ellis, D.A.O. Davies, A. Kelly, W.A. Pender, Electron. Lett. 31 Ž1995. 1245. w11x N.S. Patel, K.A. Rauschenbach, K.A. Hall, IEEE Phot. Technol. Lett. 8 Ž1996. 1695.
26
A.J. Poustie et al.r Optics Communications 156 (1998) 22–26
w12x R.J. Manning, G. Sherlock, Electron. Lett. 31 Ž1995. 307. w13x A.J. Poustie, R.J. Manning, K.J. Blow, Electron. Lett. 32 Ž1996. 1215. w14x E. Jahn, N. Agrawal, M. Arbert, H.J. Ehrke, D. Franke, R. Ludwig, W. Peiper, H.G. Weber, C.M. Weinert, Electron. Lett. 31 Ž1995. 1857.
w15x V.P. Heuring, H.F. Jordan, J.P. Pratt, Appl. Optics 31 Ž1992. 3213. w16x S. Diez, R. Ludwig, H.G. Weber, Electron. Lett. 34 Ž1998. 803. w17x R.J. Manning, A.E. Kelly, A.J. Poustie, K.J. Blow, Electron. Lett. 34 Ž1998. 916.