- Email: [email protected]
Effects of MPI noise on various modulation formats in distributed Raman amplified system
Optics Communications 255 (2005) 41–45 www.elsevier.com/locate/optcom
Effects of MPI noise on various modulation formats in distributed Raman amplified system S.B. Jun *, E.S. Son, H.Y. Choi, K.H. Han, Y.C. Chung Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-Dong, Yuseong-Gu, Daejeon 305701, Republic of Korea Received 3 February 2005; received in revised form 15 May 2005; accepted 31 May 2005
Abstract We evaluated the effects of MPI noise on various modulation formats in a distributed Raman amplified system. The results show that RZ-DPSK is the most tolerant modulation format to MPI noise. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Multi-path interference; Modulation format; Raman amplifier
1. Introduction Recently, there have been substantial interests in the distributed Raman amplifiers (DRAÕs) for the development of high-capacity WDM systems [1,2]. This is mainly because DRA can improve the optical signal-to-noise ratio and reduce the impact of fiber nonlinearity. However, as the gain of DRA increases, it could generate significant multipath interference (MPI) noise and limit the sys* Corresponding author. Tel.: +82 42 869 5456; fax: +82 42 869 3410. E-mail addresses: [email protected] (S.B. Jun), ychung@ ee.kaist.ac.kr (Y.C. Chung).
temÕs performance. Thus, for the optimization of DRA gain, it would be necessary to quantify the effect of MPI noise. Previously, there have been several reports on the effects of MPI noise in the distributed Raman amplified system [3–5]. These reports evaluated the effects of MPI noise in the presence of amplified spontaneous emission (ASE) noise on various modulation formats such as non-return-to-zero (NRZ), return-to-zero (RZ), differential phase-shift keying (DPSK), return-to-zero differential phase-shift keying (RZDPSK), return-to-zero alternate-mark-inversion (RZ-AMI), and filtered phase-shaped binary transmission (PSBT). However, all these reports estimated the effects of MPI noise in a simulated
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.05.037
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
experimental condition (i.e., the MPI noise was not generated in a real Raman environment). This was mainly to have the capability of adjusting the relative amounts of MPI and ASE noises independently. In this paper, we investigated the effects of MPI noise on various modulation formats of 40-Gb/s signals (such as NRZ, RZ, DPSK, RZDPSK, RZ-AMI, and filtered PSBT) experimentally in a Raman amplified system. As a result, we could measure the system impairments caused by MPI and ASE noises while adjusting the Raman gain. The results show that the maximum improvement of Q-factor, achievable by using DRA, is different for various modulation formats by 1 dB.
2. Experiments and results Fig. 1(a) shows the principle of measuring the MPI noise of modulated signal. To calibrate the MPI noise in the presence of other noises, we first generated the MPI noise by using a self-homodyne interferometer [6]. This optically generated MPI noise was measured in the frequency region below the lowest frequency component of the modulated signal (i.e., at the frequency region below 80 MHz for the 40 Gb/s signal with pattern length of 271) by using a RF spectrum analyzer. We then inte-
Signal
Bit rate
grated the electrical spectrum in the range of 4– 35 MHz, while varying the amount of the optically generated MPI noise. The background noises, caused by ASE noise, thermal noise, and shot noise, were subtracted from the integrated value. As a result, a linear relation between the measured value and the optically generated MPI noise was obtained, as shown in Fig. 1(b). Using this linear relation, we could estimate the MPI noise of the modulated signal in our experiment. Fig. 2 shows the experimental setup used to evaluate the effects of MPI and ASE noises on various modulation formats. We modulated a DFB laser, operating at 1559.3 nm, with 40-Gb/s signal obtained by electronically multiplexing four copies of 10-Gb/s signals. The pattern length of 10-Gb/s signal was set to be 271 for the MPI measurement, and 2231 for the Q-factor measurement. The modulated signal was then sent to the transmission link consisted of 100-km long medium-dispersion fiber (as = 0.22 dB/km, ap = 0.26 dB/km, D = 8.5 ps/km/ nm, Aeff = 71 lm2). The input signal power was set to be 2 dBm. This input power was low enough to neglect the deleterious nonlinear effects [7,8]. The fiber loss was compensated by using a Raman pump laser operating at 1465 nm. The maximum output power of this pump laser was 600 mW. For the measurement of MPI noise, we used an Erbium-doped fiber amplifier (EDFA) in conjunction
Noise
I n t e g r a t e d E l e c t r i c a l N o is e ( d B m )
42
frequency
Noises measuring point a
-60 -65 -70 -75 -80 -85 -90
MPI Calibration Data Linear Fit
-95 -100
-50
-40
-30
-20
MPI (dB) b
Fig. 1. (a) Principle of measuring the MPI noise of the modulated signal and the measured electrical spectrum of MPI noise (inset). (b) Integrated electrical noise versus MPI noise.
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
40G
LD
For MPI Measurement 40G CLK
Pump DPSK
OSA
1465 nm
RZ-DPSK 0.5 nm
40G
LD
RF Spectrum Analyzer
PD
MDF
40G CLK
Phase MOD
0.5 nm
100
NRZ RZ
Intensity MOD 40G
LD
43
RZ-AMI
Phase MOD Delay-line Interferometer
Rx
DCF
Filtered PSBT
Pre-Amp.
For Q-factor Measurement
0.3 nm
Decision Circuit
Fig. 2. Experimental setup to evaluate the effects of MPI and ASE noises on various modulation formats.
with the Raman amplifier. This was necessary to maintain the optical power incident on the photodiode at a constant level while varying the Raman gain. The ASE–ASE beat noise was suppressed by using an optical bandpass filter (bandwidth: 0.5 nm) placed in front of the photodiode. Under this condition, we measured the electrical spectra while varying the Raman gain for NRZ signal, as shown in Fig. 3(a). These results were used together with the linear relation in Fig. 1(b) for the estimation of the MPI noise in the transmission link. Fig. 3(b) shows the optical signal-to-noise ratios measured against MPI and ASE noises as a function of the Raman gain. In this figure, OSNRASE and OSNRMPI represent the optical signalto-ASE noise ratio and optical signal-to-MPI noise ratio, respectively. The measured data agreed well with the calculated values.
Unlike ASE noise, MPI noise would have different effects for various modulation formats. This is because the spectral distributions of MPI and ASE noises are different from each other. While the MPI noise is a replica of the signal spectrum, the ASE noise is spread over the amplifierÕs gainbandwidth. As a result, if the same receiver were used, the effect of ASE noise should be identical regardless of the modulation formats. However, in case of MPI noise, the effect could be different for various modulation formats due to their different optical bandwidths (i.e., when we used a modulation format with a broader optical bandwidth, a larger amount of MPI noise could result in the outside of the receiverÕs bandwidth). To evaluate the effects of MPI noise on various modulation formats, we measured the Q-factors while varying the Raman gain. For this measurement, the
-50
55
-55 -60 -65 -70
OSNRASE OSNRMPI
50 45 40 35
-75 -80
a
0 dB 5 dB 10 dB 15 dB 20 dB 22 dB
O S N R (dB )
Electrical Power (dBm)
Raman Gain
0
10
20
Frequency (MHz)
30
30 b
0
2
4
6
8
10 12 14 16 18 20 22
Raman Gain (dB)
Fig. 3. (a) Measured electrical spectra while varying the Raman gain. (b) The measured optical signal-to-noise ratios (for MPI or ASE noises) as a function of Raman gain. The solid and dashed lines are the calculated values limited by ASE and MPI noises, respectively.
44
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
Raman-amplified signal was passed through a dispersion-compensating fiber (DCF) module, and then sent to a pre-amplified receiver. The 3-dB bandwidth of this receiver was 32 GHz. Fig. 4 shows the Q-factor improvement achieved by Raman gain
Q-factor Improvement (dB)
7 Filtered PSBT NRZ DPSK RZ-AMI RZ RZ-DPSK
6 5 4 3 2 1 0
0
2
4
6
8
10
12
14
16
18
20
22
Raman Gain (dB) Fig. 4. Q-factor improvement achieved by Raman gain for various modulation formats such as NRZ, RZ, DPSK, RZDPSK, RZ-AMI, and filtered PSBT. The error bars were obtained from the 5 sets of independent measurements. The measured data agreed well with the calculated values (solid curves).
for various modulation formats (such as NRZ, RZ, DPSK, RZ-DPSK, RZ-AMI, and filtered PSBT). The baseline Q-factor was 15.56 dB. The measured data agreed well with the calculated values. In this calculation, Q-factor was obtained by using the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi equation, Q ¼ C= 1=OSNRASE þ j=OSNRMPI , where C is constant and j is the parameter indicating the relative impacts of ASE and MPI noises [4]. However, we noted that the parameter, j could be different for each modulation format. Thus, we measured the optical spectra of various modulation formats as shown in Fig. 5 and determined j by using the ratio of the optical power within the receiverÕs bandwidth (i.e., shaded regions in Fig. 5) to the total optical power. As the Raman gain increased, the improvement of Q-factor achievable by DRA became different for various modulation formats by about 1 dB. This was because the relative impact of MPI noise was increased with Raman gain, as expected from Fig. 3(b). The results also confirmed that RZ formats (RZ, RZ-DPSK, and RZ-AMI) had more tolerance to MPI noise due to their wider signal bandwidths [3]. In particular, RZ-DPSK was the most tolerant modulation format to MPI noise.
Optical Power (dBm)
0
Filtered PSBT
NRZ
-10
DPSK
-20 -30 -40 -50 -60 1558
2 x Be 1559
1560
1561 1558
Wavelengh (nm)
1559
1560
1561 1558
1559
1560
1561
Wavelengh (nm)
Wavelength (nm)
0
Optical Power (dBm)
RZ
RZ-DPSK
RZ-AMI
-10 -20 -30 -40 -50 -60 1558
1559
1560
Wavelengh (nm)
15611558
1559
1560
Wavelength (nm)
1561 1558
1559
1560
1561
Wavelength (nm)
Fig. 5. Measured optical spectra of various modulation formats (resolution BW: 0.02 nm). The shaded area indicates the optical power residing within the receiverÕs bandwidth (32 GHz).
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
3. Summary We evaluated the effects of MPI noise on various modulation formats such as NRZ, RZ, DPSK, RZDPSK, RZ-AMI, and filter PSBT. Because of MPI noise, the Q-factor improvement achievable by DRA was different for various modulation formats by 1 dB. The results show that RZ-DPSK was the most tolerant modulation format to MPI noise.
References [1] H. Kidorf, K. Rottwitt, M. Nissov, M. Ma, E. Rabarijaona, IEEE Photon. Technol. Lett. 11 (May) (1999) 530.
45
[2] D. Mahgerefteh, H.-Y. Yu, D.L. Butler, J. Goldhar, D. Wang, E. Golovchenko, A.N. Pilipetskii, C.R. Menyuk, L.J. Joneckis, Effect of randomly varying birefringence on the Raman gain in optical fibers, CLEOÕ97, paper CThW5, May 1997. [3] C.R.S. Fludger, Y. Zhu, V. Handerek, R.J. Mears, IEEE Electron. Lett. 37 (July) (2001) 970. [4] J. Bromage, L.E. Nelson, C.H. Kim, P.J. Winzer, R.J. Essiambre, R.M. Jopson, Tech. Digest OFCÕ2002 TuR3 (2002). [5] C. Martinelli, G. Charlet, L. Pierre, J. Antona, D. Bayart, Tech. Digest OFCÕ2003 FE3 (2003). [6] C.R.S. Fludger, R.J. Mears, IEEE/OSA J. Lightwave Technol. 19 (Aug) (2001) 536. [7] R.-J. Essiambre, P. Winzer, J. Bromage, Chul Han Kim, IEEE Photon. Technol. Lett 14 (July) (2002) 914– 916. [8] M. Daikoku, N. Yoshikane, I. Morita, Tech. Digest OFCÕ2005 OFN2 (2005).
Effects of MPI noise on various modulation formats in distributed Raman amplified system S.B. Jun *, E.S. Son, H.Y. Choi, K.H. Han, Y.C. Chung Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-Dong, Yuseong-Gu, Daejeon 305701, Republic of Korea Received 3 February 2005; received in revised form 15 May 2005; accepted 31 May 2005
Abstract We evaluated the effects of MPI noise on various modulation formats in a distributed Raman amplified system. The results show that RZ-DPSK is the most tolerant modulation format to MPI noise. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Multi-path interference; Modulation format; Raman amplifier
1. Introduction Recently, there have been substantial interests in the distributed Raman amplifiers (DRAÕs) for the development of high-capacity WDM systems [1,2]. This is mainly because DRA can improve the optical signal-to-noise ratio and reduce the impact of fiber nonlinearity. However, as the gain of DRA increases, it could generate significant multipath interference (MPI) noise and limit the sys* Corresponding author. Tel.: +82 42 869 5456; fax: +82 42 869 3410. E-mail addresses: [email protected] (S.B. Jun), ychung@ ee.kaist.ac.kr (Y.C. Chung).
temÕs performance. Thus, for the optimization of DRA gain, it would be necessary to quantify the effect of MPI noise. Previously, there have been several reports on the effects of MPI noise in the distributed Raman amplified system [3–5]. These reports evaluated the effects of MPI noise in the presence of amplified spontaneous emission (ASE) noise on various modulation formats such as non-return-to-zero (NRZ), return-to-zero (RZ), differential phase-shift keying (DPSK), return-to-zero differential phase-shift keying (RZDPSK), return-to-zero alternate-mark-inversion (RZ-AMI), and filtered phase-shaped binary transmission (PSBT). However, all these reports estimated the effects of MPI noise in a simulated
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.05.037
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
experimental condition (i.e., the MPI noise was not generated in a real Raman environment). This was mainly to have the capability of adjusting the relative amounts of MPI and ASE noises independently. In this paper, we investigated the effects of MPI noise on various modulation formats of 40-Gb/s signals (such as NRZ, RZ, DPSK, RZDPSK, RZ-AMI, and filtered PSBT) experimentally in a Raman amplified system. As a result, we could measure the system impairments caused by MPI and ASE noises while adjusting the Raman gain. The results show that the maximum improvement of Q-factor, achievable by using DRA, is different for various modulation formats by 1 dB.
2. Experiments and results Fig. 1(a) shows the principle of measuring the MPI noise of modulated signal. To calibrate the MPI noise in the presence of other noises, we first generated the MPI noise by using a self-homodyne interferometer [6]. This optically generated MPI noise was measured in the frequency region below the lowest frequency component of the modulated signal (i.e., at the frequency region below 80 MHz for the 40 Gb/s signal with pattern length of 271) by using a RF spectrum analyzer. We then inte-
Signal
Bit rate
grated the electrical spectrum in the range of 4– 35 MHz, while varying the amount of the optically generated MPI noise. The background noises, caused by ASE noise, thermal noise, and shot noise, were subtracted from the integrated value. As a result, a linear relation between the measured value and the optically generated MPI noise was obtained, as shown in Fig. 1(b). Using this linear relation, we could estimate the MPI noise of the modulated signal in our experiment. Fig. 2 shows the experimental setup used to evaluate the effects of MPI and ASE noises on various modulation formats. We modulated a DFB laser, operating at 1559.3 nm, with 40-Gb/s signal obtained by electronically multiplexing four copies of 10-Gb/s signals. The pattern length of 10-Gb/s signal was set to be 271 for the MPI measurement, and 2231 for the Q-factor measurement. The modulated signal was then sent to the transmission link consisted of 100-km long medium-dispersion fiber (as = 0.22 dB/km, ap = 0.26 dB/km, D = 8.5 ps/km/ nm, Aeff = 71 lm2). The input signal power was set to be 2 dBm. This input power was low enough to neglect the deleterious nonlinear effects [7,8]. The fiber loss was compensated by using a Raman pump laser operating at 1465 nm. The maximum output power of this pump laser was 600 mW. For the measurement of MPI noise, we used an Erbium-doped fiber amplifier (EDFA) in conjunction
Noise
I n t e g r a t e d E l e c t r i c a l N o is e ( d B m )
42
frequency
Noises measuring point a
-60 -65 -70 -75 -80 -85 -90
MPI Calibration Data Linear Fit
-95 -100
-50
-40
-30
-20
MPI (dB) b
Fig. 1. (a) Principle of measuring the MPI noise of the modulated signal and the measured electrical spectrum of MPI noise (inset). (b) Integrated electrical noise versus MPI noise.
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
40G
LD
For MPI Measurement 40G CLK
Pump DPSK
OSA
1465 nm
RZ-DPSK 0.5 nm
40G
LD
RF Spectrum Analyzer
PD
MDF
40G CLK
Phase MOD
0.5 nm
100
NRZ RZ
Intensity MOD 40G
LD
43
RZ-AMI
Phase MOD Delay-line Interferometer
Rx
DCF
Filtered PSBT
Pre-Amp.
For Q-factor Measurement
0.3 nm
Decision Circuit
Fig. 2. Experimental setup to evaluate the effects of MPI and ASE noises on various modulation formats.
with the Raman amplifier. This was necessary to maintain the optical power incident on the photodiode at a constant level while varying the Raman gain. The ASE–ASE beat noise was suppressed by using an optical bandpass filter (bandwidth: 0.5 nm) placed in front of the photodiode. Under this condition, we measured the electrical spectra while varying the Raman gain for NRZ signal, as shown in Fig. 3(a). These results were used together with the linear relation in Fig. 1(b) for the estimation of the MPI noise in the transmission link. Fig. 3(b) shows the optical signal-to-noise ratios measured against MPI and ASE noises as a function of the Raman gain. In this figure, OSNRASE and OSNRMPI represent the optical signalto-ASE noise ratio and optical signal-to-MPI noise ratio, respectively. The measured data agreed well with the calculated values.
Unlike ASE noise, MPI noise would have different effects for various modulation formats. This is because the spectral distributions of MPI and ASE noises are different from each other. While the MPI noise is a replica of the signal spectrum, the ASE noise is spread over the amplifierÕs gainbandwidth. As a result, if the same receiver were used, the effect of ASE noise should be identical regardless of the modulation formats. However, in case of MPI noise, the effect could be different for various modulation formats due to their different optical bandwidths (i.e., when we used a modulation format with a broader optical bandwidth, a larger amount of MPI noise could result in the outside of the receiverÕs bandwidth). To evaluate the effects of MPI noise on various modulation formats, we measured the Q-factors while varying the Raman gain. For this measurement, the
-50
55
-55 -60 -65 -70
OSNRASE OSNRMPI
50 45 40 35
-75 -80
a
0 dB 5 dB 10 dB 15 dB 20 dB 22 dB
O S N R (dB )
Electrical Power (dBm)
Raman Gain
0
10
20
Frequency (MHz)
30
30 b
0
2
4
6
8
10 12 14 16 18 20 22
Raman Gain (dB)
Fig. 3. (a) Measured electrical spectra while varying the Raman gain. (b) The measured optical signal-to-noise ratios (for MPI or ASE noises) as a function of Raman gain. The solid and dashed lines are the calculated values limited by ASE and MPI noises, respectively.
44
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
Raman-amplified signal was passed through a dispersion-compensating fiber (DCF) module, and then sent to a pre-amplified receiver. The 3-dB bandwidth of this receiver was 32 GHz. Fig. 4 shows the Q-factor improvement achieved by Raman gain
Q-factor Improvement (dB)
7 Filtered PSBT NRZ DPSK RZ-AMI RZ RZ-DPSK
6 5 4 3 2 1 0
0
2
4
6
8
10
12
14
16
18
20
22
Raman Gain (dB) Fig. 4. Q-factor improvement achieved by Raman gain for various modulation formats such as NRZ, RZ, DPSK, RZDPSK, RZ-AMI, and filtered PSBT. The error bars were obtained from the 5 sets of independent measurements. The measured data agreed well with the calculated values (solid curves).
for various modulation formats (such as NRZ, RZ, DPSK, RZ-DPSK, RZ-AMI, and filtered PSBT). The baseline Q-factor was 15.56 dB. The measured data agreed well with the calculated values. In this calculation, Q-factor was obtained by using the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi equation, Q ¼ C= 1=OSNRASE þ j=OSNRMPI , where C is constant and j is the parameter indicating the relative impacts of ASE and MPI noises [4]. However, we noted that the parameter, j could be different for each modulation format. Thus, we measured the optical spectra of various modulation formats as shown in Fig. 5 and determined j by using the ratio of the optical power within the receiverÕs bandwidth (i.e., shaded regions in Fig. 5) to the total optical power. As the Raman gain increased, the improvement of Q-factor achievable by DRA became different for various modulation formats by about 1 dB. This was because the relative impact of MPI noise was increased with Raman gain, as expected from Fig. 3(b). The results also confirmed that RZ formats (RZ, RZ-DPSK, and RZ-AMI) had more tolerance to MPI noise due to their wider signal bandwidths [3]. In particular, RZ-DPSK was the most tolerant modulation format to MPI noise.
Optical Power (dBm)
0
Filtered PSBT
NRZ
-10
DPSK
-20 -30 -40 -50 -60 1558
2 x Be 1559
1560
1561 1558
Wavelengh (nm)
1559
1560
1561 1558
1559
1560
1561
Wavelengh (nm)
Wavelength (nm)
0
Optical Power (dBm)
RZ
RZ-DPSK
RZ-AMI
-10 -20 -30 -40 -50 -60 1558
1559
1560
Wavelengh (nm)
15611558
1559
1560
Wavelength (nm)
1561 1558
1559
1560
1561
Wavelength (nm)
Fig. 5. Measured optical spectra of various modulation formats (resolution BW: 0.02 nm). The shaded area indicates the optical power residing within the receiverÕs bandwidth (32 GHz).
S.B. Jun et al. / Optics Communications 255 (2005) 41–45
3. Summary We evaluated the effects of MPI noise on various modulation formats such as NRZ, RZ, DPSK, RZDPSK, RZ-AMI, and filter PSBT. Because of MPI noise, the Q-factor improvement achievable by DRA was different for various modulation formats by 1 dB. The results show that RZ-DPSK was the most tolerant modulation format to MPI noise.
References [1] H. Kidorf, K. Rottwitt, M. Nissov, M. Ma, E. Rabarijaona, IEEE Photon. Technol. Lett. 11 (May) (1999) 530.
45
[2] D. Mahgerefteh, H.-Y. Yu, D.L. Butler, J. Goldhar, D. Wang, E. Golovchenko, A.N. Pilipetskii, C.R. Menyuk, L.J. Joneckis, Effect of randomly varying birefringence on the Raman gain in optical fibers, CLEOÕ97, paper CThW5, May 1997. [3] C.R.S. Fludger, Y. Zhu, V. Handerek, R.J. Mears, IEEE Electron. Lett. 37 (July) (2001) 970. [4] J. Bromage, L.E. Nelson, C.H. Kim, P.J. Winzer, R.J. Essiambre, R.M. Jopson, Tech. Digest OFCÕ2002 TuR3 (2002). [5] C. Martinelli, G. Charlet, L. Pierre, J. Antona, D. Bayart, Tech. Digest OFCÕ2003 FE3 (2003). [6] C.R.S. Fludger, R.J. Mears, IEEE/OSA J. Lightwave Technol. 19 (Aug) (2001) 536. [7] R.-J. Essiambre, P. Winzer, J. Bromage, Chul Han Kim, IEEE Photon. Technol. Lett 14 (July) (2002) 914– 916. [8] M. Daikoku, N. Yoshikane, I. Morita, Tech. Digest OFCÕ2005 OFN2 (2005).