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A new scheme of cross-gain modulation wavelength converter with good performance on extinction ratio
15 March 2001
Optics Communications 189 (2001) 235±239
www.elsevier.com/locate/optcom
A new scheme of cross-gain modulation wavelength converter with good performance on extinction ratio q Nan Chi a,*, Shuqiang Chen a, Lin Xu b, Jiang Qi a, Yaojun Qiao a, Yuan Zheng a a
Optical Communication Center, Beijing University of Posts and Telecommunications, Beijing 100876, People's Republic of China b International Engineering Department, China Telecom, Beijing 100032, People's Republic of China Received 9 September 2000; received in revised form 11 December 2000; accepted 15 December 2000
Abstract A new scheme is presented in this article which, by using a dispersion-imbalanced ®ber loop mirror, gives a better solution to improve the extinction ratio degradation of wavelength converter based on cross-gain modulation. The maximum improvement of the extinction ratio is observed to be 3.5 dB in the wavelength conversion experiment with a clock signal at 2.5 GHz. Ó 2001 Published by Elsevier Science B.V. Keywords: Cross-gain modulation; Wavelength conversion; Dispersion-imbalanced ®ber loop mirror
1. Introduction Wavelength conversion is most likely to be the key technology for the implementation of future WDM network [1]. Among several possible techniques used in wavelength converters such technologies based on semiconductor optical ampli®ers (SOAs) as cross-gain modulation (XGM) and cross-phase modulation (XPM) have attracted considerable attention, since they appear with relatively high conversion eciency and polarization insensitivity. The simplest con®guration as it has, XGMs relatively large frequency chirping and extinction ratio degradation limit its transmission q This work was supported by National Science Foundation of China (no. 69772034), Key foundation of Ministry of Posts and Telecommunications and Fok Yingtung Educational Foundation. * Corresponding author. E-mail address: [email protected] (N. Chi).
and cascadability. Recently, a new approach has shown extinction ratio improvement by cascading a SOA and a nonlinear optical loop mirror (NOLM), latter one is used as a wavelength conversion medium [2]. To the best of our knowledge, here for the ®rst time, we demonstrate a more simple structure of a XGM wavelength converter which, including a SOA and a dispersion-imbalanced ®ber loop mirror (DILM), really performs very well on extinction ratio. The unique self-switching properties of the DILM usually apply to the OTDM system for the generation of high-quality optical pulses or for interchannel cross-talk reduction after demultiplexing [3]. However, in our scheme the chirping of converted signal is skillfully harnessed to obtain and control phase dierence in two counterpropagating direction in DILM. Therefore we can bene®t in two aspects at same time: one is the chirping decrease and another is improvement of extinction ratio. Extinction ratio is observed to be
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 0 0 8 - 2
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N. Chi et al. / Optics Communications 189 (2001) 235±239
increased by 3.5 dB during wavelength conversion experiment with a 2.5 GHz clock signal. 2. Principle and theoretical simulation The setup of our experiment is shown in Fig. 1. The XGM eect in SOA can be accurately described through a kind of multisection rate equation model as mentioned in Ref. [4]. The signal beam carrying the information modulates the gain of SOA by depleting the carriers. It causes changes in the refractive index and the phase of the optical beams in the cavity. The probe beam encounters the modulated gain and refractive index and thus the probe amplitude and phase are changed by the input signal. In order that the carrier dynamics can be studied along the length of the ampli®er, the SOA is segmented into a number of small sections. The multisection dynamic rate equation for the carrier density is used: X gp ;i IP ;i gASE;i PASE;i dni J ni m m 1 ds qV se;i m1;2 Epm EASE r Ii Ii
1
egi Li 1 gi L i
2
Index i corresponds to the dierent ampli®er sections and index m refers to the dierent optical input beams. ni is carrier density, J is the bias current density, q is the electronic charge, and r is the active layer area. se is the carrier recombination lifetime, gmi is the material gain, Ii represent the average light intensity for input beam m inside segment i. The gain coecient is a cubic formula with empirically-determined constants
g a1 n
a2 k
kN 2 a3 k
kN 3
3
kN is the peak gain wavelength. The nonlinear phase change, arising from carrier density induced changes in refractive index, is given by 2pLi dN 2pLi C ni n0 dN NR Cnp /i dn k dn k 4 Li is the cavity length, n0 is the transparency carrier concentration, k is the beam wavelength, NR is the guide refractive index and np is the value of carrier density of zero input power. dN=dn is the rate of change of active region refractive index with carrier density. The values for the parameters used in the calculations are listed in Table 1. Fig. 2 shows the converted signal power and frequency chirp. The converted parts are blue shifted on the leading and red shifted on the trailing edges giving rise to fast pulse broadening for transmission over SMF ®ber, and the degradation of extinction ratio can be obviously observed. After transmitting through a 500 lm SOA the extinction ratio of converted signal is lower than 6.5 dB. The DILM is made up of a DCF ®ber and a DSF ®ber. Coming out of the coupler the converted signal split into two beams which continuously transmit in two opposite direction. These
Table 1 List of parameter values J L w d a1 a2 a3
Bias current SOA length Active layer width Active layer thickness Material gain constant Material gain constant Material gain constant
n0
Carrier density at transparency Carrier density of zero input power Section length Refractive index Change in refractive index with carrier density Con®nement factor
np Li NR dN=dn Fig. 1. Experiment setup. GSL: gain switch laser, TL: tunable laser, PC: polarization controller.
n0
C
200 mA 500 lm 1.2 lm 0.15 lm 2:5 10 20 m2 0.074 cm 1 nm 3:155 10 4 cm 1 nm 3 1:1 1024 m 3 2 1024 m 10 3.5 1:2 10 0.31
2
3
26
m
3
N. Chi et al. / Optics Communications 189 (2001) 235±239
237
larger SPM eect when transmitting through the second hop which will induce a larger phase shift. The phase dierence can be divided into two parts, one is induced in DCF ®ber, the other is induced in DSF ®ber. The peak power of the two beams is nearly the same when they inject into the DCF ®ber from two direction. Thus the phase shift of clockwise-propagating beam is same with the contra-direction beam in DCF ®ber. However, the peak power of two direction beams in DSF ®ber is dierent, hence the phase dierence between these two beams occurs in DSF ®ber, as follows
Fig. 2. Simulation of light power (above) and frequency chirping (bottom) of converted signal output of XGM wavelength converter.
two beams then are combined by the coupler after propagating through the loop and output from DILM. The output signal intensity of DILM is given by [5] Pout jA1 L; t
2
A2 L; tj
1
cos D/ Pin 2
5
where A1 , A2 are amplitude of clockwise-propagating pulses, anticlockwise-propagating pulses, respectively. D/ is the phase dierence between A1 and A2 . The largest output power can be obtained when D/ is equal to p. Assuming the GVD eect can be neglected, the phase shift is mainly induced by SPM eect which is given by [6] /max Zeff cP0
D/ Zeff c P01 P02 p 1 C 2 Zeff cP0
1
7
where P01 is the peak power of clockwise-propagating pulses, P02 is peak power of another pulses. C is frequency chirping. With presumption that D/ come up to p and that DCF ®ber length is tantamount with DSF ®ber length, the variation of peak power and frequency chirping of input signal versus ®ber length is demonstrated by Fig. 3. The required peak power decreases when frequency chirping increases, stated dierently, enhancing the peak power of input signal will reduce the required frequency chirping. We simulated the transmission of the chirped converted signal in the DILM. Figs. 4 and 5 show the dependence of output peak power and extinction ratio on DCF and DSF ®ber length. When DCF length increases the output peak power will
6
c is the nonlinear coecient, P0 is the peak power, Zeff is the eective ®ber length. The beam which passes DCF ®ber part (assumed in clockwise direction) will ®rstly experience a pulse compression due to frequency chirping and its peak power will increase. On the other hand the beam which goes in the opposite direction cannot get a improved peak power. Comparing with the anticlockwise-propagating beam, the clockwisepropagating beam has a larger peak power before coming into the DSF ®ber, therefore it will have a
Fig. 3. Theoretical variation of peak power and frequency chirping of input signal with ®ber length.
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N. Chi et al. / Optics Communications 189 (2001) 235±239
Fig. 4. Theoretical variation of output power with DCF ®ber length and DSF ®ber length.
saturation output, the gain dierence between TE mode and TM mode is 0.5 dB. A polarization independent isolator made by E-TEK is used for unidirectional operation, and preventing any feedback. A gain switch laser operating at 2.5 GHz provides the input signal, namely pump beam. Probe beam is a tunable laser (Photonetics TUNICS-1550) with a wavelength tunable range of 1480±1580 nm. The converted signal is ®ltered by a Santec OTF-610 ®lter with 3 dB bandwidth of 0.4 nm and an operation range of 1530±1565 nm. The length of DSF ®ber is 2.2 km and DCF is 1.84 km. In order to enhance the nonlinear eect the converted signal is ampli®ed to 17 dBm before entering into DILM by an EDFA. Fig. 6(a) is the eye diagram of original signal and Fig. 6(b) is for converted signal immediately out of SOA. The extinction ratio of original signal is no less than 15 dB and that of the converted one is 6.02 dB. The arrow indicate the zero optical power level. Adjusting the polarization controller in DILM can achieve the optimum output as shown in Fig. 7. The relation between the output
Fig. 5. Theoretical variation of extinction ratio with DCF ®ber length and DSF ®ber length.
®rstly increase and then become ¯at. The improvement of extinction ratio for an average of 12 dB is clearly shown in Fig. 5 with a DCF length of 2 and 5 km, however, if the DCF is 10 km, the ER will signi®cantly decrease when the DSF length increase. The results indicate that an enhancement of extinction ratio of chirped converted signal will possibly be achieved through such asymmetric DILM.
Fig. 6. The waveform of (a) original input signal and (b) converted signal output of SOA.
3. Experimental result The 1.5 lm SOA is a 500 lm-long Alcatel 1901 with typical ®ber to ®ber 25 dB gain and 9 dBm
Fig. 7. The optimum waveform of output through asymmetric DILM.
N. Chi et al. / Optics Communications 189 (2001) 235±239
239
wavelength conversion experiment with a 2.5 GHz clock signal demonstrate a maximum improvement of extinction ratio of 3.5 dB. It is expected theoretically that the ratio improvement shall be able to reach 12 dB by properly choosing the ®ber length of DCF and DSF.
References
Fig. 8. The measured output power versus the extinction ratio of converted signal after DILM.
power and extinction ratio we got from our experiment is shown in Fig. 8. The extinction ratio is increased by 3.5 dB which means the optimum ratio reaches 9.5 dB. 4. Conclusion For the ®rst time we successfully achieved the signi®cant enhancement of extinction ratio occurred in XGM wavelength converter by cascading a SOA and a DILM. The results of the
[1] T. Durhuus, B. Mikkelsen, C. Joergensen, S. Danielsen, K. Stubkjaer, All-optical wavelength conversion by semiconductor optical ampli®ers, IEEE J. Lightwave Technol. 14 (1996) 942±954. [2] J. Yu, A. Clausen, H. Poulsen, X. Zheng, C. Peucheret, P. Jeppesen, 40 Gbit/s wavelength conversion in cascade of SOA and NOLM and demonstration of extinction ratio improvement, Electron. Lett. 36 (2000) 963±964. [3] I. Khrushchev, I. Phillps, A. Ellis, R. Manning, D. Nesset, D. Moodie, R. Penty, I. White, OTDM applications of dispersion-imbalanced ®ber loop mirror, Electron. Lett. 35 (1999) 1183±1185. [4] M. Asghari, I.H. White, R.V. Penty, Wavelength conversion using semiconductor optical ampli®ers, IEEE J. Lightwave Technol. 15 (1997) 1181±1190. [5] N. Chi, S. Chen, K. Guan, All-optical wavelength conversion based on a nonlinear optical loop mirror, APCC/ OECC'99, Beijing, China, pp. 1396±1398. [6] G.P. Agarwal, Nonlinear Fiber Optics, second ed., The Institute of Optics, University of Rochester, Rochester, Academic Press, New York, 1995, pp. 90±92.
Optics Communications 189 (2001) 235±239
www.elsevier.com/locate/optcom
A new scheme of cross-gain modulation wavelength converter with good performance on extinction ratio q Nan Chi a,*, Shuqiang Chen a, Lin Xu b, Jiang Qi a, Yaojun Qiao a, Yuan Zheng a a
Optical Communication Center, Beijing University of Posts and Telecommunications, Beijing 100876, People's Republic of China b International Engineering Department, China Telecom, Beijing 100032, People's Republic of China Received 9 September 2000; received in revised form 11 December 2000; accepted 15 December 2000
Abstract A new scheme is presented in this article which, by using a dispersion-imbalanced ®ber loop mirror, gives a better solution to improve the extinction ratio degradation of wavelength converter based on cross-gain modulation. The maximum improvement of the extinction ratio is observed to be 3.5 dB in the wavelength conversion experiment with a clock signal at 2.5 GHz. Ó 2001 Published by Elsevier Science B.V. Keywords: Cross-gain modulation; Wavelength conversion; Dispersion-imbalanced ®ber loop mirror
1. Introduction Wavelength conversion is most likely to be the key technology for the implementation of future WDM network [1]. Among several possible techniques used in wavelength converters such technologies based on semiconductor optical ampli®ers (SOAs) as cross-gain modulation (XGM) and cross-phase modulation (XPM) have attracted considerable attention, since they appear with relatively high conversion eciency and polarization insensitivity. The simplest con®guration as it has, XGMs relatively large frequency chirping and extinction ratio degradation limit its transmission q This work was supported by National Science Foundation of China (no. 69772034), Key foundation of Ministry of Posts and Telecommunications and Fok Yingtung Educational Foundation. * Corresponding author. E-mail address: [email protected] (N. Chi).
and cascadability. Recently, a new approach has shown extinction ratio improvement by cascading a SOA and a nonlinear optical loop mirror (NOLM), latter one is used as a wavelength conversion medium [2]. To the best of our knowledge, here for the ®rst time, we demonstrate a more simple structure of a XGM wavelength converter which, including a SOA and a dispersion-imbalanced ®ber loop mirror (DILM), really performs very well on extinction ratio. The unique self-switching properties of the DILM usually apply to the OTDM system for the generation of high-quality optical pulses or for interchannel cross-talk reduction after demultiplexing [3]. However, in our scheme the chirping of converted signal is skillfully harnessed to obtain and control phase dierence in two counterpropagating direction in DILM. Therefore we can bene®t in two aspects at same time: one is the chirping decrease and another is improvement of extinction ratio. Extinction ratio is observed to be
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 0 0 8 - 2
236
N. Chi et al. / Optics Communications 189 (2001) 235±239
increased by 3.5 dB during wavelength conversion experiment with a 2.5 GHz clock signal. 2. Principle and theoretical simulation The setup of our experiment is shown in Fig. 1. The XGM eect in SOA can be accurately described through a kind of multisection rate equation model as mentioned in Ref. [4]. The signal beam carrying the information modulates the gain of SOA by depleting the carriers. It causes changes in the refractive index and the phase of the optical beams in the cavity. The probe beam encounters the modulated gain and refractive index and thus the probe amplitude and phase are changed by the input signal. In order that the carrier dynamics can be studied along the length of the ampli®er, the SOA is segmented into a number of small sections. The multisection dynamic rate equation for the carrier density is used: X gp ;i IP ;i gASE;i PASE;i dni J ni m m 1 ds qV se;i m1;2 Epm EASE r Ii Ii
1
egi Li 1 gi L i
2
Index i corresponds to the dierent ampli®er sections and index m refers to the dierent optical input beams. ni is carrier density, J is the bias current density, q is the electronic charge, and r is the active layer area. se is the carrier recombination lifetime, gmi is the material gain, Ii represent the average light intensity for input beam m inside segment i. The gain coecient is a cubic formula with empirically-determined constants
g a1 n
a2 k
kN 2 a3 k
kN 3
3
kN is the peak gain wavelength. The nonlinear phase change, arising from carrier density induced changes in refractive index, is given by 2pLi dN 2pLi C ni n0 dN NR Cnp /i dn k dn k 4 Li is the cavity length, n0 is the transparency carrier concentration, k is the beam wavelength, NR is the guide refractive index and np is the value of carrier density of zero input power. dN=dn is the rate of change of active region refractive index with carrier density. The values for the parameters used in the calculations are listed in Table 1. Fig. 2 shows the converted signal power and frequency chirp. The converted parts are blue shifted on the leading and red shifted on the trailing edges giving rise to fast pulse broadening for transmission over SMF ®ber, and the degradation of extinction ratio can be obviously observed. After transmitting through a 500 lm SOA the extinction ratio of converted signal is lower than 6.5 dB. The DILM is made up of a DCF ®ber and a DSF ®ber. Coming out of the coupler the converted signal split into two beams which continuously transmit in two opposite direction. These
Table 1 List of parameter values J L w d a1 a2 a3
Bias current SOA length Active layer width Active layer thickness Material gain constant Material gain constant Material gain constant
n0
Carrier density at transparency Carrier density of zero input power Section length Refractive index Change in refractive index with carrier density Con®nement factor
np Li NR dN=dn Fig. 1. Experiment setup. GSL: gain switch laser, TL: tunable laser, PC: polarization controller.
n0
C
200 mA 500 lm 1.2 lm 0.15 lm 2:5 10 20 m2 0.074 cm 1 nm 3:155 10 4 cm 1 nm 3 1:1 1024 m 3 2 1024 m 10 3.5 1:2 10 0.31
2
3
26
m
3
N. Chi et al. / Optics Communications 189 (2001) 235±239
237
larger SPM eect when transmitting through the second hop which will induce a larger phase shift. The phase dierence can be divided into two parts, one is induced in DCF ®ber, the other is induced in DSF ®ber. The peak power of the two beams is nearly the same when they inject into the DCF ®ber from two direction. Thus the phase shift of clockwise-propagating beam is same with the contra-direction beam in DCF ®ber. However, the peak power of two direction beams in DSF ®ber is dierent, hence the phase dierence between these two beams occurs in DSF ®ber, as follows
Fig. 2. Simulation of light power (above) and frequency chirping (bottom) of converted signal output of XGM wavelength converter.
two beams then are combined by the coupler after propagating through the loop and output from DILM. The output signal intensity of DILM is given by [5] Pout jA1 L; t
2
A2 L; tj
1
cos D/ Pin 2
5
where A1 , A2 are amplitude of clockwise-propagating pulses, anticlockwise-propagating pulses, respectively. D/ is the phase dierence between A1 and A2 . The largest output power can be obtained when D/ is equal to p. Assuming the GVD eect can be neglected, the phase shift is mainly induced by SPM eect which is given by [6] /max Zeff cP0
D/ Zeff c P01 P02 p 1 C 2 Zeff cP0
1
7
where P01 is the peak power of clockwise-propagating pulses, P02 is peak power of another pulses. C is frequency chirping. With presumption that D/ come up to p and that DCF ®ber length is tantamount with DSF ®ber length, the variation of peak power and frequency chirping of input signal versus ®ber length is demonstrated by Fig. 3. The required peak power decreases when frequency chirping increases, stated dierently, enhancing the peak power of input signal will reduce the required frequency chirping. We simulated the transmission of the chirped converted signal in the DILM. Figs. 4 and 5 show the dependence of output peak power and extinction ratio on DCF and DSF ®ber length. When DCF length increases the output peak power will
6
c is the nonlinear coecient, P0 is the peak power, Zeff is the eective ®ber length. The beam which passes DCF ®ber part (assumed in clockwise direction) will ®rstly experience a pulse compression due to frequency chirping and its peak power will increase. On the other hand the beam which goes in the opposite direction cannot get a improved peak power. Comparing with the anticlockwise-propagating beam, the clockwisepropagating beam has a larger peak power before coming into the DSF ®ber, therefore it will have a
Fig. 3. Theoretical variation of peak power and frequency chirping of input signal with ®ber length.
238
N. Chi et al. / Optics Communications 189 (2001) 235±239
Fig. 4. Theoretical variation of output power with DCF ®ber length and DSF ®ber length.
saturation output, the gain dierence between TE mode and TM mode is 0.5 dB. A polarization independent isolator made by E-TEK is used for unidirectional operation, and preventing any feedback. A gain switch laser operating at 2.5 GHz provides the input signal, namely pump beam. Probe beam is a tunable laser (Photonetics TUNICS-1550) with a wavelength tunable range of 1480±1580 nm. The converted signal is ®ltered by a Santec OTF-610 ®lter with 3 dB bandwidth of 0.4 nm and an operation range of 1530±1565 nm. The length of DSF ®ber is 2.2 km and DCF is 1.84 km. In order to enhance the nonlinear eect the converted signal is ampli®ed to 17 dBm before entering into DILM by an EDFA. Fig. 6(a) is the eye diagram of original signal and Fig. 6(b) is for converted signal immediately out of SOA. The extinction ratio of original signal is no less than 15 dB and that of the converted one is 6.02 dB. The arrow indicate the zero optical power level. Adjusting the polarization controller in DILM can achieve the optimum output as shown in Fig. 7. The relation between the output
Fig. 5. Theoretical variation of extinction ratio with DCF ®ber length and DSF ®ber length.
®rstly increase and then become ¯at. The improvement of extinction ratio for an average of 12 dB is clearly shown in Fig. 5 with a DCF length of 2 and 5 km, however, if the DCF is 10 km, the ER will signi®cantly decrease when the DSF length increase. The results indicate that an enhancement of extinction ratio of chirped converted signal will possibly be achieved through such asymmetric DILM.
Fig. 6. The waveform of (a) original input signal and (b) converted signal output of SOA.
3. Experimental result The 1.5 lm SOA is a 500 lm-long Alcatel 1901 with typical ®ber to ®ber 25 dB gain and 9 dBm
Fig. 7. The optimum waveform of output through asymmetric DILM.
N. Chi et al. / Optics Communications 189 (2001) 235±239
239
wavelength conversion experiment with a 2.5 GHz clock signal demonstrate a maximum improvement of extinction ratio of 3.5 dB. It is expected theoretically that the ratio improvement shall be able to reach 12 dB by properly choosing the ®ber length of DCF and DSF.
References
Fig. 8. The measured output power versus the extinction ratio of converted signal after DILM.
power and extinction ratio we got from our experiment is shown in Fig. 8. The extinction ratio is increased by 3.5 dB which means the optimum ratio reaches 9.5 dB. 4. Conclusion For the ®rst time we successfully achieved the signi®cant enhancement of extinction ratio occurred in XGM wavelength converter by cascading a SOA and a DILM. The results of the
[1] T. Durhuus, B. Mikkelsen, C. Joergensen, S. Danielsen, K. Stubkjaer, All-optical wavelength conversion by semiconductor optical ampli®ers, IEEE J. Lightwave Technol. 14 (1996) 942±954. [2] J. Yu, A. Clausen, H. Poulsen, X. Zheng, C. Peucheret, P. Jeppesen, 40 Gbit/s wavelength conversion in cascade of SOA and NOLM and demonstration of extinction ratio improvement, Electron. Lett. 36 (2000) 963±964. [3] I. Khrushchev, I. Phillps, A. Ellis, R. Manning, D. Nesset, D. Moodie, R. Penty, I. White, OTDM applications of dispersion-imbalanced ®ber loop mirror, Electron. Lett. 35 (1999) 1183±1185. [4] M. Asghari, I.H. White, R.V. Penty, Wavelength conversion using semiconductor optical ampli®ers, IEEE J. Lightwave Technol. 15 (1997) 1181±1190. [5] N. Chi, S. Chen, K. Guan, All-optical wavelength conversion based on a nonlinear optical loop mirror, APCC/ OECC'99, Beijing, China, pp. 1396±1398. [6] G.P. Agarwal, Nonlinear Fiber Optics, second ed., The Institute of Optics, University of Rochester, Rochester, Academic Press, New York, 1995, pp. 90±92.