- Email: [email protected]
All-optical burst-mode clock extraction based on the thresholding operation of a modified terahertz optical asymmetric demultiplexer
15 February 1999
Optics Communications 160 Ž1999. 225–229
All-optical burst-mode clock extraction based on the thresholding operation of a modified terahertz optical asymmetric demultiplexer Hyuek Jae Lee ) , Kwangjoon Kim, Hae Geun Kim Photonic Switching Section, Electronics and Telecommunications Research Institute (ETRI), 161 Kajong-Dong, Yusong-Gu, Taejon 305-350, South Korea Received 14 October 1998; accepted 21 December 1998
Abstract To implement all-optical burst-mode clock extraction we adopt a modified terahertz optical asymmetric demultiplexer ŽMTOAD.. The transmittance and reflectance of the MTOAD depend on the input intensity. For the MTOAD, two levels of pulse intensity can be chosen in such a way that while the pulses with similar intensity are reflected for both strong and weak pulses, only the strong pulse transmits. The device is useful, for example, for bit-level clock extraction from a packet, where strong and weak intensity pulses are assigned to ‘1’ and ‘0’, respectively. When the input optical signal power is fixed to y1.6 dBm and the intensity ratio between ‘1’ and ‘0’ is varied in the range of 0.2–0.5, the extinction ratio ŽER. at the transmitted port is more than 10 dB and a clock amplitude jitter ŽCAJ. of the bit-level clock at the reflected port is less than 14%. Inversely, when the input power is varied in the range of y6–y 1 dBm with fixed intensity ratio of 0.3, more than 11 dB of ER and less than 15% of CAJ are obtained. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 42.79.S; 42.65.P Keywords: Optical communications; All-optical clock extraction; Semiconductor optical amplifier; Terahertz optical asymmetric demultiplexer
1. Introduction Recently, several all-optical switching techniques using nonlinear optical components such as dispersion sifted fiber ŽDSF. or a semiconductor optical amplifier ŽSOA. have been actively investigated w1–7x. There are two kinds of all-optical switching devices, i.e., optical AND logic and optical intensity dependent switch. While the former is widely used for all-optical demultiplexing of optical time division multiplexing ŽOTDM. data w2x and all-optical header detection w3x, the latter is used for extinction ratio
)
Corresponding author. E-mail: [email protected]
ŽER. enhancement w4x and packet-level clock pulse extraction w5x. Deng et al. w5x have proposed a modified TOAD ŽMTOAD. by removing the control input port and applied it to ‘packet-level’ clock pulse extraction. Here, the MTOAD plays the role of an all-optical intensity dependent self-routing switch: the strong intensity input pulse is transmitted to an output port, while the weak intensity input pulse is totally reflected to the other output port. In this Letter, we propose an all-optical burst-mode ‘bit-level’ clock extraction scheme based on the fact that the transmittance and reflectance of the MTOAD depend on the input intensity. For a MTOAD, two levels of pulse intensity can be chosen in such a way that while the reflected pulse intensity of both strong and weak pulses are comparable, only the strong pulse transmits. We experi-
0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 6 8 0 - 4
226
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
mentally investigated the scheme by measuring the ER and the clock amplitude jitter ŽCAJ. at 2.5 Gbps and demonstrated its ‘bit-level’ clock pulse extraction capability. Also, amplitude regulation of various amplitude packets using a MTOAD is demonstrated. The proposed scheme may play an essential role in real-world all-optical networks, where packet-base burst-mode ‘bit-level’ clock extraction and intensity regulation of packets from different paths are inevitable.
2. Operational detail Fig. 1Ža. shows packet-level clock extraction using a MTOAD proposed by Deng et al. w5x. Their packet is composed of a strong clock pulse at the first bit position followed by weak data pulses. At a receiving node, the strong clock pulse and weak data pulses are separated as shown in Fig. 1Ža.. Fig. 1Žb. shows the bit-level clock extraction scheme proposed in this Letter. The strong and weak pulses of the packet correspond to data ‘1’ and ‘0’, respectively. If we properly choose the intensity levels of the pulses, the bit-level clock can be extracted from the packet data as shown in Fig. 1Žb.. Practically, the intensity ranges of the strong and weak pulses are set by adjusting the current to the SOA, the coupling coefficient of tunable directional coupler ŽTDC., and the arrival time difference to the SOA between the split pulses. The operation of Fig. 1Žb. is different from that of Fig. 1Ža.: while the latter totally suppresses the reflection from strong pulse, the former makes the reflected pulse intensity from both strong and weak pulses equal. Fig. 2 shows a schematic for a MTOAD. The MTOAD is constructed by connecting output ports 3 and 4 of a TDC through the SOA. When a pulse enters port 1, it is split into two pulses; the one from port 3 goes in the clockwise ŽCW. direction and the other from port 4 goes in the counterclockwise ŽCCW. direction. SOA is placed with the displacement D xr2 from the midpoint of the fiber loop. The CCW pulse arrives at the SOA before the
Fig. 1. The schematic illustration of the operation for Ža. an intensity-dependent self-routing switch and Žb. all-optical clock extraction.
Fig. 2. The schematic of a MTOAD. SOA: semiconductor optical amplifier.
CW pulse. While the recovery time t of the SOA is shorter than the pulse period, D x is short enough to guarantee the arrival time difference between CCW and CW pulses, D xrc, is much shorter than t . Here, c is the light velocity in the optical fiber. The intensity of the transmitted and reflected signals by the MTOAD are given by w5,7x Ir s Iin a Ž 1 y a . Ž Gcw q Gccw .
Ž
'
q2 a Ž 1 y a . Gcw Gccw cos udiff ,
.
Ž1.
y2 a Ž 1 y a . Gcw Gccw cos udiff ,
Ž2.
2
It s Iin a 2 Gcw q Ž 1 y a . Gcw
ž
'
/
where Iin is the input intensity, Ir is the reflected intensity, It is the transmitted intensity, and a is the coupling coefficient of the TDC. Gcw and Gccw denote gains of the SOA for the CW and the CCW pulses and udiff is the phase difference between them. The phase difference and the gains are coupled by w7x a udiff s ucw y uccw s y ln Ž GcwrGccw . , Ž3. 2 where a is the linewidth enhancement factor. For simplicity, we set the TDC coupling coefficient a to 0.5. First of all, let us consider a weak input pulse. Because it induces negligible perturbations to the SOA, Gcw and Gccw are equal and udiff f 0. Therefore, it becomes Ir f IinGcw and It f 0. The MTOAD acts as an ordinary fiber loop mirror. On the other hand, when a strong intensity pulse enters the MTOAD, the CCW pulse arrives at the SOA and obtains high gain. This affects the gain and phase of the following CW pulse. So, it becomes Gcw ) Gccw and udiff is no more negligible. From Eqs. Ž1. and Ž2., we get Ir f 0.25IinŽ A q B . and It f 0.25IinŽ A y B .. Here, A s Gcw q Gccw and B s 2Ž GcwGccw .1r2
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
227
cosŽ udiff .. As udiff gradually increases to yp , B ™ y2Ž GcwGccw .1r2 . Hence, Ir is no longer proportional to Iin . By properly adjusting Gcw and Gccw which are functions of the SOA current, the displacement D x, and the TDC coupling coefficient a, Ir can be made to saturate to a certain intensity. Namely, with proper adjustment of the above parameters, we can manipulate the input intensity dependence of the transmittance and reflectance of the MTOAD. Especially, the TDC coupling coefficient a plays the main role. Practically, if a is in the range of 0.35 - a - 0.5, the reflected pulses for strong or weak input pulses are comparable.
3. Experimental results
Fig. 4. ERs and CAJs as a function of the intensity ratio b when the input optical mean power is y1.66 dBm.
Fig. 3 shows an experimental setup to demonstrate the operation of all-optical burst-mode ‘bit-level’ clock extraction using an MTOAD. The SOA is 1000 mm long and of polarization insensitive type. An optical clock of 2.5 GHz with ; 60 ps FWHM is generated by gain-switching the DFB-LD from a pulse pattern generator ŽPPG. clock signal. The Mach–Zehnder modulators, the MZ-mod1 and the MZ-mod2, generate optical packets as shown in Fig. 3. Let b be the intensity ratio between strong and weak pulses. By properly adjusting the polarization controller PC1 and
bias voltage, the ER of data after the MZ-mod1 can be varied, i.e., we can give b an arbitrary value. To investigate the characteristics of the burst-mode bit level clock extraction, we have generated data with high and low intensity by applying MZ-mod1 to ‘101010 PPP ’ and MZmod2 to ‘111111 PPP ’. When the input optical power of the MTOAD is y1.66 dBm, the ER of the output at port 1 and the CAJ of the output at port 2 as a function of intensity ratio b are shown in Fig. 4. The SOA current is fixed to 100 mA, a is varied within 0.35–0.5, and D xrc
Fig. 3. Experimental setup.
228
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
Fig. 5. ERs and CAJs as a function of the input optical mean power when the intensity ratio b is fixed to 0.3.
is adjusted in 30–80 ps. It seems that the asymmetry in the range of a is mainly due to the insertion loss of the optical tunable delay line ŽOTDL., which is more than ; 1 dB. To measure the quality of the extracted bit-level clock, we defined the CAJ as s CAJ s = 100 Ž % . Ž4. M where s and M denote the standard deviation and the mean obtained from the intensity histogram at the midpoint of the bit-level clock, respectively. As is shown in Fig. 4, for 0.2 - b - 0.5, the ER is more than 10 dB and the CAJ is less than 14%. Fig. 5 shows the ER and the CAJ as a function of the input power, when b is fixed to 0.3. We obtained more than 11 dB ER and less than 15% CAJ in the range of input power y6–y 1 dBm. Using the MZ-mod1 and the MZ-mod2, we have generated two packets, ‘1011101’ and ‘10100001’, at 2.5 Gbps as shown in Fig. 6Ža.. Fig. 6Žb. and Žc. show the packet data and the bit-level clock extracted from the packet of Fig. 6Ža., respectively. Here, the SOA current is set to 100 mA, a is adjusted to 0.45, and D xrc is ; 50 ps. The proposed scheme can be used as an intensity regulator. For example, when packets of different origin
Fig. 6. Experimental result of the data and the bit-level clock extraction from the burst-mode packet using a MTOAD. Ža. Burst-mode input packet. Žb. Extracted data. Žc. Extracted bit-level clock.
Fig. 7. Experimental result of amplitude regulation for the burstmode packet with different amplitudes using a MTOAD. Ža. Burst-mode input packet with different amplitudes. Žb. Amplitude-regulated output packet.
arrive at a point, the intensity of each packet could be quite different. In that case, if we set the thresholding level of our scheme to the intensity of the weakest packet or lower, all the packets will have similar intensities after using the scheme. The experimental result for amplitude regulation of the packet is shown in Fig. 7. The packets used in the experiment are shown in Fig. 7Ža., which are generated by the scheme shown in Fig. 3. The intensity ratio between two packets is 0.2. Nevertheless, the amplitude regulation is successfully conducted. Here, the SOA is driven at 80 mA, a is 0.4, and D xr2 c is ; 45 ps. It is noteworthy that the packet format used in this intensity regulation does not contain weak ‘0’ pulses for bit-level clock extraction.
4. Conclusions In this Letter we have experimentally demonstrated all-optical burst-mode ‘bit-level’ clock extraction using a MTOAD which is useful for ultrafast all-optical OTDM networks. Although the experiment is conducted at 2.5 Gbps, 100 Gbps or higher-range operations will be possible with the introduction of a proper carrier recovery time reduction scheme such as a CW-holding beam as in Ref. w8x. Due to concurrent transmission of data and bit clock, the bit-level clock extraction based on all-optical thresholding alleviates the time jitter problem that may sometimes occurs. Also, for burst-mode packets with different amplitudes, the same scheme can be used to amplitude regulation. A similar operation such as extinction ratio enhancement using a nonlinear optical loop mirror ŽNOLM. composed of a DSF in the fiber loop w4x, can be constructed. Although the NOLM scheme has ultrafast switching capability, it is very bulky Žusually several kilometers of DSF are used. and requires high switching energy of several tens of picojoules. On the other hand, a terahertz optical asymmetric demultiplexer ŽTOAD. w6x or a semiconductor laser amplifier in a loop mirror ŽSLALOM. w7x has the
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
advantage to be integrated within compact size and required the switching energy of only a few hundred fJ. w3x
Acknowledgements
w4x
This work has been supported by the Ministry of Information and Communication, South Korea.
w5x w6x
References w1x N.J. Doran, D. Wood, Opt. Lett. 13 Ž1988. 56. w2x P.A. Andrekson, N.A. Olsson, J.R. Simpson, D.J. Digiovanni,
w7x w8x
229
P.A. Morton, T. Tanbun-Ek, R.A. Logan, K.W. Wecht, IEEE Photon. Technol. Lett. 4 Ž1992. 644. K.H. Ahn, X.D. Cao, Y. Liang, B.C. Barnett, S. Chaikamnerd, M.N. Islam, J. Opt. Soc. Am. 14 Ž1997. 1228. B.-E. Olsson, P.A. Andrekson, IEEE Photon. Technol. Lett. 7 Ž1995. 120. K.-L. Deng, I. Glesk, K.I. Kang, P.R. Prucnal, IEEE Photon. Technol. Lett. 9 Ž1997. 830. J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, IEEE Photon. Technol. Lett. 5 Ž1993. 787. M. Eiselt, W. Pieper, H.G. Weber, J. Lightwave Technol. 13 Ž1995. 2099. R.J. Manning, G. Sherlock, Electron. Lett. 31 Ž1995. 307.
Optics Communications 160 Ž1999. 225–229
All-optical burst-mode clock extraction based on the thresholding operation of a modified terahertz optical asymmetric demultiplexer Hyuek Jae Lee ) , Kwangjoon Kim, Hae Geun Kim Photonic Switching Section, Electronics and Telecommunications Research Institute (ETRI), 161 Kajong-Dong, Yusong-Gu, Taejon 305-350, South Korea Received 14 October 1998; accepted 21 December 1998
Abstract To implement all-optical burst-mode clock extraction we adopt a modified terahertz optical asymmetric demultiplexer ŽMTOAD.. The transmittance and reflectance of the MTOAD depend on the input intensity. For the MTOAD, two levels of pulse intensity can be chosen in such a way that while the pulses with similar intensity are reflected for both strong and weak pulses, only the strong pulse transmits. The device is useful, for example, for bit-level clock extraction from a packet, where strong and weak intensity pulses are assigned to ‘1’ and ‘0’, respectively. When the input optical signal power is fixed to y1.6 dBm and the intensity ratio between ‘1’ and ‘0’ is varied in the range of 0.2–0.5, the extinction ratio ŽER. at the transmitted port is more than 10 dB and a clock amplitude jitter ŽCAJ. of the bit-level clock at the reflected port is less than 14%. Inversely, when the input power is varied in the range of y6–y 1 dBm with fixed intensity ratio of 0.3, more than 11 dB of ER and less than 15% of CAJ are obtained. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 42.79.S; 42.65.P Keywords: Optical communications; All-optical clock extraction; Semiconductor optical amplifier; Terahertz optical asymmetric demultiplexer
1. Introduction Recently, several all-optical switching techniques using nonlinear optical components such as dispersion sifted fiber ŽDSF. or a semiconductor optical amplifier ŽSOA. have been actively investigated w1–7x. There are two kinds of all-optical switching devices, i.e., optical AND logic and optical intensity dependent switch. While the former is widely used for all-optical demultiplexing of optical time division multiplexing ŽOTDM. data w2x and all-optical header detection w3x, the latter is used for extinction ratio
)
Corresponding author. E-mail: [email protected]
ŽER. enhancement w4x and packet-level clock pulse extraction w5x. Deng et al. w5x have proposed a modified TOAD ŽMTOAD. by removing the control input port and applied it to ‘packet-level’ clock pulse extraction. Here, the MTOAD plays the role of an all-optical intensity dependent self-routing switch: the strong intensity input pulse is transmitted to an output port, while the weak intensity input pulse is totally reflected to the other output port. In this Letter, we propose an all-optical burst-mode ‘bit-level’ clock extraction scheme based on the fact that the transmittance and reflectance of the MTOAD depend on the input intensity. For a MTOAD, two levels of pulse intensity can be chosen in such a way that while the reflected pulse intensity of both strong and weak pulses are comparable, only the strong pulse transmits. We experi-
0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 6 8 0 - 4
226
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
mentally investigated the scheme by measuring the ER and the clock amplitude jitter ŽCAJ. at 2.5 Gbps and demonstrated its ‘bit-level’ clock pulse extraction capability. Also, amplitude regulation of various amplitude packets using a MTOAD is demonstrated. The proposed scheme may play an essential role in real-world all-optical networks, where packet-base burst-mode ‘bit-level’ clock extraction and intensity regulation of packets from different paths are inevitable.
2. Operational detail Fig. 1Ža. shows packet-level clock extraction using a MTOAD proposed by Deng et al. w5x. Their packet is composed of a strong clock pulse at the first bit position followed by weak data pulses. At a receiving node, the strong clock pulse and weak data pulses are separated as shown in Fig. 1Ža.. Fig. 1Žb. shows the bit-level clock extraction scheme proposed in this Letter. The strong and weak pulses of the packet correspond to data ‘1’ and ‘0’, respectively. If we properly choose the intensity levels of the pulses, the bit-level clock can be extracted from the packet data as shown in Fig. 1Žb.. Practically, the intensity ranges of the strong and weak pulses are set by adjusting the current to the SOA, the coupling coefficient of tunable directional coupler ŽTDC., and the arrival time difference to the SOA between the split pulses. The operation of Fig. 1Žb. is different from that of Fig. 1Ža.: while the latter totally suppresses the reflection from strong pulse, the former makes the reflected pulse intensity from both strong and weak pulses equal. Fig. 2 shows a schematic for a MTOAD. The MTOAD is constructed by connecting output ports 3 and 4 of a TDC through the SOA. When a pulse enters port 1, it is split into two pulses; the one from port 3 goes in the clockwise ŽCW. direction and the other from port 4 goes in the counterclockwise ŽCCW. direction. SOA is placed with the displacement D xr2 from the midpoint of the fiber loop. The CCW pulse arrives at the SOA before the
Fig. 1. The schematic illustration of the operation for Ža. an intensity-dependent self-routing switch and Žb. all-optical clock extraction.
Fig. 2. The schematic of a MTOAD. SOA: semiconductor optical amplifier.
CW pulse. While the recovery time t of the SOA is shorter than the pulse period, D x is short enough to guarantee the arrival time difference between CCW and CW pulses, D xrc, is much shorter than t . Here, c is the light velocity in the optical fiber. The intensity of the transmitted and reflected signals by the MTOAD are given by w5,7x Ir s Iin a Ž 1 y a . Ž Gcw q Gccw .
Ž
'
q2 a Ž 1 y a . Gcw Gccw cos udiff ,
.
Ž1.
y2 a Ž 1 y a . Gcw Gccw cos udiff ,
Ž2.
2
It s Iin a 2 Gcw q Ž 1 y a . Gcw
ž
'
/
where Iin is the input intensity, Ir is the reflected intensity, It is the transmitted intensity, and a is the coupling coefficient of the TDC. Gcw and Gccw denote gains of the SOA for the CW and the CCW pulses and udiff is the phase difference between them. The phase difference and the gains are coupled by w7x a udiff s ucw y uccw s y ln Ž GcwrGccw . , Ž3. 2 where a is the linewidth enhancement factor. For simplicity, we set the TDC coupling coefficient a to 0.5. First of all, let us consider a weak input pulse. Because it induces negligible perturbations to the SOA, Gcw and Gccw are equal and udiff f 0. Therefore, it becomes Ir f IinGcw and It f 0. The MTOAD acts as an ordinary fiber loop mirror. On the other hand, when a strong intensity pulse enters the MTOAD, the CCW pulse arrives at the SOA and obtains high gain. This affects the gain and phase of the following CW pulse. So, it becomes Gcw ) Gccw and udiff is no more negligible. From Eqs. Ž1. and Ž2., we get Ir f 0.25IinŽ A q B . and It f 0.25IinŽ A y B .. Here, A s Gcw q Gccw and B s 2Ž GcwGccw .1r2
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
227
cosŽ udiff .. As udiff gradually increases to yp , B ™ y2Ž GcwGccw .1r2 . Hence, Ir is no longer proportional to Iin . By properly adjusting Gcw and Gccw which are functions of the SOA current, the displacement D x, and the TDC coupling coefficient a, Ir can be made to saturate to a certain intensity. Namely, with proper adjustment of the above parameters, we can manipulate the input intensity dependence of the transmittance and reflectance of the MTOAD. Especially, the TDC coupling coefficient a plays the main role. Practically, if a is in the range of 0.35 - a - 0.5, the reflected pulses for strong or weak input pulses are comparable.
3. Experimental results
Fig. 4. ERs and CAJs as a function of the intensity ratio b when the input optical mean power is y1.66 dBm.
Fig. 3 shows an experimental setup to demonstrate the operation of all-optical burst-mode ‘bit-level’ clock extraction using an MTOAD. The SOA is 1000 mm long and of polarization insensitive type. An optical clock of 2.5 GHz with ; 60 ps FWHM is generated by gain-switching the DFB-LD from a pulse pattern generator ŽPPG. clock signal. The Mach–Zehnder modulators, the MZ-mod1 and the MZ-mod2, generate optical packets as shown in Fig. 3. Let b be the intensity ratio between strong and weak pulses. By properly adjusting the polarization controller PC1 and
bias voltage, the ER of data after the MZ-mod1 can be varied, i.e., we can give b an arbitrary value. To investigate the characteristics of the burst-mode bit level clock extraction, we have generated data with high and low intensity by applying MZ-mod1 to ‘101010 PPP ’ and MZmod2 to ‘111111 PPP ’. When the input optical power of the MTOAD is y1.66 dBm, the ER of the output at port 1 and the CAJ of the output at port 2 as a function of intensity ratio b are shown in Fig. 4. The SOA current is fixed to 100 mA, a is varied within 0.35–0.5, and D xrc
Fig. 3. Experimental setup.
228
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
Fig. 5. ERs and CAJs as a function of the input optical mean power when the intensity ratio b is fixed to 0.3.
is adjusted in 30–80 ps. It seems that the asymmetry in the range of a is mainly due to the insertion loss of the optical tunable delay line ŽOTDL., which is more than ; 1 dB. To measure the quality of the extracted bit-level clock, we defined the CAJ as s CAJ s = 100 Ž % . Ž4. M where s and M denote the standard deviation and the mean obtained from the intensity histogram at the midpoint of the bit-level clock, respectively. As is shown in Fig. 4, for 0.2 - b - 0.5, the ER is more than 10 dB and the CAJ is less than 14%. Fig. 5 shows the ER and the CAJ as a function of the input power, when b is fixed to 0.3. We obtained more than 11 dB ER and less than 15% CAJ in the range of input power y6–y 1 dBm. Using the MZ-mod1 and the MZ-mod2, we have generated two packets, ‘1011101’ and ‘10100001’, at 2.5 Gbps as shown in Fig. 6Ža.. Fig. 6Žb. and Žc. show the packet data and the bit-level clock extracted from the packet of Fig. 6Ža., respectively. Here, the SOA current is set to 100 mA, a is adjusted to 0.45, and D xrc is ; 50 ps. The proposed scheme can be used as an intensity regulator. For example, when packets of different origin
Fig. 6. Experimental result of the data and the bit-level clock extraction from the burst-mode packet using a MTOAD. Ža. Burst-mode input packet. Žb. Extracted data. Žc. Extracted bit-level clock.
Fig. 7. Experimental result of amplitude regulation for the burstmode packet with different amplitudes using a MTOAD. Ža. Burst-mode input packet with different amplitudes. Žb. Amplitude-regulated output packet.
arrive at a point, the intensity of each packet could be quite different. In that case, if we set the thresholding level of our scheme to the intensity of the weakest packet or lower, all the packets will have similar intensities after using the scheme. The experimental result for amplitude regulation of the packet is shown in Fig. 7. The packets used in the experiment are shown in Fig. 7Ža., which are generated by the scheme shown in Fig. 3. The intensity ratio between two packets is 0.2. Nevertheless, the amplitude regulation is successfully conducted. Here, the SOA is driven at 80 mA, a is 0.4, and D xr2 c is ; 45 ps. It is noteworthy that the packet format used in this intensity regulation does not contain weak ‘0’ pulses for bit-level clock extraction.
4. Conclusions In this Letter we have experimentally demonstrated all-optical burst-mode ‘bit-level’ clock extraction using a MTOAD which is useful for ultrafast all-optical OTDM networks. Although the experiment is conducted at 2.5 Gbps, 100 Gbps or higher-range operations will be possible with the introduction of a proper carrier recovery time reduction scheme such as a CW-holding beam as in Ref. w8x. Due to concurrent transmission of data and bit clock, the bit-level clock extraction based on all-optical thresholding alleviates the time jitter problem that may sometimes occurs. Also, for burst-mode packets with different amplitudes, the same scheme can be used to amplitude regulation. A similar operation such as extinction ratio enhancement using a nonlinear optical loop mirror ŽNOLM. composed of a DSF in the fiber loop w4x, can be constructed. Although the NOLM scheme has ultrafast switching capability, it is very bulky Žusually several kilometers of DSF are used. and requires high switching energy of several tens of picojoules. On the other hand, a terahertz optical asymmetric demultiplexer ŽTOAD. w6x or a semiconductor laser amplifier in a loop mirror ŽSLALOM. w7x has the
H.J. Lee et al.r Optics Communications 160 (1999) 225–229
advantage to be integrated within compact size and required the switching energy of only a few hundred fJ. w3x
Acknowledgements
w4x
This work has been supported by the Ministry of Information and Communication, South Korea.
w5x w6x
References w1x N.J. Doran, D. Wood, Opt. Lett. 13 Ž1988. 56. w2x P.A. Andrekson, N.A. Olsson, J.R. Simpson, D.J. Digiovanni,
w7x w8x
229
P.A. Morton, T. Tanbun-Ek, R.A. Logan, K.W. Wecht, IEEE Photon. Technol. Lett. 4 Ž1992. 644. K.H. Ahn, X.D. Cao, Y. Liang, B.C. Barnett, S. Chaikamnerd, M.N. Islam, J. Opt. Soc. Am. 14 Ž1997. 1228. B.-E. Olsson, P.A. Andrekson, IEEE Photon. Technol. Lett. 7 Ž1995. 120. K.-L. Deng, I. Glesk, K.I. Kang, P.R. Prucnal, IEEE Photon. Technol. Lett. 9 Ž1997. 830. J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, IEEE Photon. Technol. Lett. 5 Ž1993. 787. M. Eiselt, W. Pieper, H.G. Weber, J. Lightwave Technol. 13 Ž1995. 2099. R.J. Manning, G. Sherlock, Electron. Lett. 31 Ž1995. 307.