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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 22 — Nov. 1, 2004
  • pp: 5457–5468
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High reflectivity superstructured FBG for coherent optical code generation and recognition

Xu Wang, Koji Matsushima, Akihiko Nishiki, Naoya Wada, and Ken-ichi Kitayama  »View Author Affiliations


Optics Express, Vol. 12, Issue 22, pp. 5457-5468 (2004)
http://dx.doi.org/10.1364/OPEX.12.005457


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Abstract

The performance of the phase-shifted superstructured fiber Bragg grating (SSFBG) for optical code (OC) recognition was investigated with different reflectivity as well as input pulse width. The auto-correlation peak (PA) and the ratios of PA to the maximum wing level (P/W) and cross-correlation level (P/C) were used to quantitatively evaluate the OC recognition performance. There is a conflict between obtaining high PA and high P/W and P/C ratios in high reflectivity regime. The approach of applying apodization technique to improve the performance in high reflectivity regime is proposed. The comparative experimental investigations with 127-chip 160-Gchip/s SSFBG are carried out to confirm the effectiveness of the proposed approach. Error-free transmission with multiplexing of two active users has been successfully achieved by the apodized SSFBG at a data rate of 1.25 Gbit/s.

© 2004 Optical Society of America

1. Introduction

Optical code (OC) generation and recognition techniques have been primarily investigated for application in the optical code division multiple access (OCDMA) system to process the OC in the optical domain [1

1. P. R. Prucnal, M. A. Santoro, and T. R. Fan, “Spread spectrum fiber-optic local area network using optical processing,” J. Lightwave Technol. 4, 547–554 (1986). [CrossRef]

3

3. D. D. Sampson, G. J. Pendock, and R. A. Griffin, “Photonic code-division multiple-access communications,” Fiber Integer. Opt. 16, 129–157 (1997). [CrossRef]

]. It has recently been noticed that OC labels have good prospect in versatile applications for OCDMA, optical burst switching (OBS) and optical packet switching (OPS) systems and will be able to play an important role in future Internet protocol (IP) over photonic networks [4

4. K. Kitayama, “Code division multiplexing lightwave networks based upon optical code conversion,” IEEE J. Selec. Areas Commun. 16, 1209–1319 (1998). [CrossRef]

6

6. K. Kitayama and N. Wada, “Photonic IP routing,” IEEE Photonic Technol. Lett. 11, 1689–1691 (1999) [CrossRef]

]. There are a number of different OC generation/recognition schemes, which can be classified according to their operation principle as incoherent or coherent schemes, or according to their processing dimensions as one-dimensional (1-D), or two-dimensional (2-D) schemes. Figure 1 shows an illustration of these classifications.

The incoherent techniques, which work on an optical-power-intensity basis, process OCs in a unipolar (0,1) manner, which results in disadvantages such as small code size, low optical power and bandwidth efficiency, and poor correlation property [1

1. P. R. Prucnal, M. A. Santoro, and T. R. Fan, “Spread spectrum fiber-optic local area network using optical processing,” J. Lightwave Technol. 4, 547–554 (1986). [CrossRef]

16

16. R. A. Griffin, D. D. Sampson, and D. A. Jackson, “Coherence coding for photonic code-division-multiple access networks,” J. Lightwave Technol. 13, 1826–1837 (1995). [CrossRef]

]. In contrast, coherent techniques, which work on a field-amplitude basis, process OCs in a bipolar (-1, +1) manner all optically; thus, they are superior to incoherent techniques in the overall performance [3

3. D. D. Sampson, G. J. Pendock, and R. A. Griffin, “Photonic code-division multiple-access communications,” Fiber Integer. Opt. 16, 129–157 (1997). [CrossRef]

7

7. X. Wang and K. Kitayama, “Analysis of beat noise in coherent and incoherent time-spreading OCDMA,” J. Lightwave Technol. 22, 2226–2235 (2004). [CrossRef]

, 17

17. M. E. Maric, “Coherent optical CDMA networks,” J. Lightwave Technol. 11, 854–864 (1993). [CrossRef]

25

25. X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota, and K. Kitayama, “Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,” in European Cnference of Optical Communication (ECOC’04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]

]. On the other hand, OCs could be processed in either the time domain [1

1. P. R. Prucnal, M. A. Santoro, and T. R. Fan, “Spread spectrum fiber-optic local area network using optical processing,” J. Lightwave Technol. 4, 547–554 (1986). [CrossRef]

8

8. G. E. Town, K. Chan, and G. Yoffe, “Design and performance of high-speed optical pulse-code generators using optical fiber Bragg gratings,” IEEE J. Select. Quantum Electron. 5, 1325–1331 (1999). [CrossRef]

, 17

17. M. E. Maric, “Coherent optical CDMA networks,” J. Lightwave Technol. 11, 854–864 (1993). [CrossRef]

19

19. J. A. Salehi, A. M. Weiner, and J. P. Heritage, “Coherent ultrashort light pulse code-division multiple access communication systems,” J. Lightwave Technol. 8, 478–491 (1990). [CrossRef]

] or the frequency domain [3

3. D. D. Sampson, G. J. Pendock, and R. A. Griffin, “Photonic code-division multiple-access communications,” Fiber Integer. Opt. 16, 129–157 (1997). [CrossRef]

6

6. K. Kitayama and N. Wada, “Photonic IP routing,” IEEE Photonic Technol. Lett. 11, 1689–1691 (1999) [CrossRef]

, 9

9. H. Tsuda, H. Takenouchi, T. Ishii, K. Okamoto, T. Goh, K. Sato, A. Hirano, T. Kurokawa, and C. Amano, “Spectral encoding and decoding of 10 Gbit/s femtosecond pulses using high resolution arrayed-waveguide grating,” Electron. Lett. , 35, 1186–1187 (1999). [CrossRef]

11

11. S. Yegnanarayanan, A. S. Bhshan, and B. Jalali, “Fast wavelength-hopping time-spreading encoding/decoding for optical CDMA,” IEEE Photon. Technol. Lett. 12, 573–575 (2000). [CrossRef]

, 20

20. C. C. Chang, H. P. Sardesai, and A. M. Weiner, “Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5 Km fiber link,” IEEE, Photon. Technol. Lett. 10, 171–173 (1998). [CrossRef]

25

25. X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota, and K. Kitayama, “Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,” in European Cnference of Optical Communication (ECOC’04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]

] in 1-D schemes, and in the frequency and time domains simultaneously [12

12. K. Yum, J. Shin, and N. Park, “Wavelength-time spreading optical CDMA system using wavelength multiplexers and mirrors fiber delay lines,” IEEE, Photon. Technol. Lett. 12, 1278–1280 (2000). [CrossRef]

16

16. R. A. Griffin, D. D. Sampson, and D. A. Jackson, “Coherence coding for photonic code-division-multiple access networks,” J. Lightwave Technol. 13, 1826–1837 (1995). [CrossRef]

] in 2-D schemes.

The OC en/decoders are the most crucial components in these optical implementations. Techniques that have been adopted include fiber-optic delay lines (FODL) [1

1. P. R. Prucnal, M. A. Santoro, and T. R. Fan, “Spread spectrum fiber-optic local area network using optical processing,” J. Lightwave Technol. 4, 547–554 (1986). [CrossRef]

3

3. D. D. Sampson, G. J. Pendock, and R. A. Griffin, “Photonic code-division multiple-access communications,” Fiber Integer. Opt. 16, 129–157 (1997). [CrossRef]

, 17

17. M. E. Maric, “Coherent optical CDMA networks,” J. Lightwave Technol. 11, 854–864 (1993). [CrossRef]

18

18. N. Wada and K. Kitayama, “A 10 Gb/s optical code division multiplexing using 8-chip optical bipolar code and coherent detection,” J. Lightwave Technol. 17, 1758–1765 (1999). [CrossRef]

], planar lightwave circuits (PLC) [4

4. K. Kitayama, “Code division multiplexing lightwave networks based upon optical code conversion,” IEEE J. Selec. Areas Commun. 16, 1209–1319 (1998). [CrossRef]

6

6. K. Kitayama and N. Wada, “Photonic IP routing,” IEEE Photonic Technol. Lett. 11, 1689–1691 (1999) [CrossRef]

, 19

19. J. A. Salehi, A. M. Weiner, and J. P. Heritage, “Coherent ultrashort light pulse code-division multiple access communication systems,” J. Lightwave Technol. 8, 478–491 (1990). [CrossRef]

], spatial light modulators (SLM) [9

9. H. Tsuda, H. Takenouchi, T. Ishii, K. Okamoto, T. Goh, K. Sato, A. Hirano, T. Kurokawa, and C. Amano, “Spectral encoding and decoding of 10 Gbit/s femtosecond pulses using high resolution arrayed-waveguide grating,” Electron. Lett. , 35, 1186–1187 (1999). [CrossRef]

, 20

20. C. C. Chang, H. P. Sardesai, and A. M. Weiner, “Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5 Km fiber link,” IEEE, Photon. Technol. Lett. 10, 171–173 (1998). [CrossRef]

21

21. A. Grunnet-Jepsen, A. E. Johnson, E. S. Maniloff, T. W. Mossberg, M. J. Munroe, and J. N. Sweetser, “Fiber Bragg grating based spectral encoder/decoder for lightwave CDMA,” Electron. Lett. 35, 1096–1097 (1999). [CrossRef]

], arrayed-waveguide grating (AWG) [10

10. Z. Wei, H. M. H. Shalaby, and H. Ghafouri-Shiraz, “Modified Quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems,” J. Lightwave Technol. 19, 1274–1281 (2001). [CrossRef]

, 12

12. K. Yum, J. Shin, and N. Park, “Wavelength-time spreading optical CDMA system using wavelength multiplexers and mirrors fiber delay lines,” IEEE, Photon. Technol. Lett. 12, 1278–1280 (2000). [CrossRef]

13

13. H. Fathallah, L. A. Rusch, and S. LaRochelle, “Passive optical fast frequency-hop CDMA communications system,” J. Lightwave Technol. 17, 397–405 (1999). [CrossRef]

], fiber Bragg gratings (FBG) [8

8. G. E. Town, K. Chan, and G. Yoffe, “Design and performance of high-speed optical pulse-code generators using optical fiber Bragg gratings,” IEEE J. Select. Quantum Electron. 5, 1325–1331 (1999). [CrossRef]

, 11

11. S. Yegnanarayanan, A. S. Bhshan, and B. Jalali, “Fast wavelength-hopping time-spreading encoding/decoding for optical CDMA,” IEEE Photon. Technol. Lett. 12, 573–575 (2000). [CrossRef]

, 14

14. X. Wang and K. T. Chan, “A sequentially self-seeded Fabry-Perot laser for two-dimensional encoding/decoding of optical pulses,” IEEE J. Quantum Electron. 39, 83–90 (2003). [CrossRef]

16

16. R. A. Griffin, D. D. Sampson, and D. A. Jackson, “Coherence coding for photonic code-division-multiple access networks,” J. Lightwave Technol. 13, 1826–1837 (1995). [CrossRef]

, 22

22. P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,” J. Lightwave Technol. 9,1352–1365 (2001).

25

25. X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota, and K. Kitayama, “Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,” in European Cnference of Optical Communication (ECOC’04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]

], holographic device [26

26. T. W. Mossberg, “Planar holographic optical processing devices,” Optics Lett. 26, 414–416 (2001). [CrossRef]

], and micro-electromechanical systems (MEMS) [27

27. P. Ebrahimi, M. Kargar, M. Hamer, A. E. Willner, K. Yu, and O. Solgaard, “A 10-ms-tuning MEMS-actuated Gires-Tournois filter for use as a tunable wavelength demultiplexer and a tunable OCDMA encoder/decoder,” Optical Fiber Communication Conference (OFC’04) (Optical Society of America, Washington, D.C., 2004), ThQ2.

]. Figure 1 shows their applications for different schemes.

Particularly, in the coherent time -spreading (TS) scheme, the OC is generated by spreading the coherent optical pulses in time, and the phase of the optical carrier is changed according to a specific bipolar signature pattern. It is similar to that in the electrical field, which has been studied quite extensively for radar and wireless CDMA communication applications. In this scheme, controlling the optical paths in the order fractions of optical wavelength is required to guarantee the optical pulses are coherently interfering with each other during the signal processing [3

3. D. D. Sampson, G. J. Pendock, and R. A. Griffin, “Photonic code-division multiple-access communications,” Fiber Integer. Opt. 16, 129–157 (1997). [CrossRef]

4

4. K. Kitayama, “Code division multiplexing lightwave networks based upon optical code conversion,” IEEE J. Selec. Areas Commun. 16, 1209–1319 (1998). [CrossRef]

]. PLC that can satisfy this critical requirement has been applied as en/decoder [4

4. K. Kitayama, “Code division multiplexing lightwave networks based upon optical code conversion,” IEEE J. Selec. Areas Commun. 16, 1209–1319 (1998). [CrossRef]

6

6. K. Kitayama and N. Wada, “Photonic IP routing,” IEEE Photonic Technol. Lett. 11, 1689–1691 (1999) [CrossRef]

]. However, PLC can not easily generate OC in the orders of a hundred-chips order long, because of the physical constraint of the silica substrate and design complexity, which will result in an inability to generate a large number of OCs, leading to poor system scalability. Given these fact, the FBG might be a powerful alternative.

Fig. 1. Classifications of OC generation/recognition techniques.

The FBG has been developed into a critical component for many applications in fiber-optic communication and sensor systems [28

28. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15, 1277–1294 (1997). [CrossRef]

29

29. C. R. Giles, “Lightwave applications of fiber Bragg grating,” J. Lightwave Technol. 15, 1391–1404 (1997). [CrossRef]

]. A “superstructured” FBG (SSFBG) is defined as an FBG with a slowly varying refractive-index modulation profile imposed along its length [22

22. P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,” J. Lightwave Technol. 9,1352–1365 (2001).

, 30

30. B. J. Eggleton, P. A. Krug, L. Poladian, and F. Ouellette, “Long periodic superstructure Bragg gratings in optical fibers,” Electron. Lett. 30, 1620–1622 (1994). [CrossRef]

]. Furthermore, a full complex refractive-index modulation profile can be realized in an SSFBG by inserting phase shifts between different segments, as shown in Fig. 2(a). With an injection of a short optical pulse, this phase-shifted SSFBG can generate a series of coherent short optical pulses whose phases are determined by the pattern of the phase shifts in the SSFBG. If the lengths of all segments are all the same as Lchip and the refractive-index modulation is constant along the whole grating, the light can penetrate the whole grating length, and the individual segments of the grating contribute more or less equally to the reflected response. The phase-shifted SSFBG thus works as an optical transversal filter to generate a binary-phase-shift-key (BPSK) [22

22. P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,” J. Lightwave Technol. 9,1352–1365 (2001).

] or a quaternary-phase-shift-key (QPSK) [23

23. P. C. Teh, M. Ibsen, J. H. Lee, P. Petropoulos, and D. J. Richardson, “Demonstration of a four-channel WDM/OCDMA system using 255-chip 320-Gchip/s Quarternary phase coding grating,” IEEE Photonic Technol. Lett. 14, 227–229 (2002). [CrossRef]

] OC from its impulse response, and it can also perform correlation for OC recognition. Figures 2(b) and (c) illustrate the principles of BPSK OC generation and recognition respectively. This sort of phase-shifted SSFBG can be fabricated with a single short phase mask of length Lchip by continuous grating writing [22

22. P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,” J. Lightwave Technol. 9,1352–1365 (2001).

] or holographic techniques [25

25. X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota, and K. Kitayama, “Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,” in European Cnference of Optical Communication (ECOC’04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]

]. These techniques provide a high flexibility in producing different ultra-long optical code. High-precis ion phase control can be achieved as well for BPSK, QPSK or even more multiple phase level OC.

Fig. 2. Superstructured FBG with phase shifts for OC generation/recognition (a) Configuration, working principle for BPSK OC generation (b) and recognition (c).

2. Phase-shifted SSFBG and OC selection

The phase-shifted SSFBG OC en/decoder in Fig. 2(a) contains Nchip segments with length Lchip, where Nchip is the number of chips of the represented BPSK OC {ai: ai ∈(0,1), 0< i≤Nchip} and Lchip=Tchip/(2nc), Tchip is the temporal interval duration of the OC, n is the refractive index of the optical fiber, and c is velocity of light in vacuum. It is noteworthy that the SSFBG can’t discriminate a BPSK OC initiated with “1” from its complement code initialed with “-1” as shown in the bottom of Fig. 2(b). This would be a problem in electrical-signal processing such as wireless CDMA, where both the code processing and signal detection are based on the complex-form electrical field amplitude. However, as the optical signal detection is based on signal intensity instead of field amplitude, therefore, neglecting this OC representation degeneracy in the SSFBG en/decoder will not have any detrimental effect.

Various code sets used for wireless CDMA communications such as M sequence, Gold codes, Kasami code [32

32. E. H. Dinan and B. Jabbari, “Spreading codes for direct sequence CDMA and wideband CDMA cellular networks,” IEEE Commun. Mag. 36, 48–54 (1998). [CrossRef]

], have been adopted as the OCs [22

22. P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,” J. Lightwave Technol. 9,1352–1365 (2001).

25

25. X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota, and K. Kitayama, “Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,” in European Cnference of Optical Communication (ECOC’04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]

]. In electrical field, the decoding process is to periodically correlate the encoded signal:

C(k)=i=1NChipaibi+k{a=b,Autocorrelationab,Crosscorrelation
(1)

where {ai} and {bi} are two codes in a code set, ai=ai+NChip. Therefore, these code sets are designed to have good periodic auto/cross-correlation properties. However, in the coherent OC generation/recognition with SSFBG, the OC recognition process does not perform the periodic correlation because the adjacent optical pulses are not coherent pulses. Therefore, the desired code sets should have good aperiodic correlation property instead of periodic correlation property, which is given by ai=0, if i>NChip in Eq. (1). It is still difficult to find a large number of available codes that have good aperiodic correlation property. A simple approach for our application is to select several code subsets from the original code set (e.g., Gold code), in which some specific aperiodic auto/cross-correlation criteria can be guaranteed. To quantitatively measure the aperiodic correlation property of an OC in coherent TS schemes, we introduce two parameters in this paper: one is the ratio of auto-correlation intensity peak over the maximum wing level (P/W ratio) as the measurement of the auto-correlation property, and another is the ratio of the peak over the maximum cross-correlation level (P/C ratio) for measuring the cross-correlation property. Table 1 lists several subsets of 127- and 511-chip Gold codes for different correlation criteria to illustrate this approach. To obtain the numbers in this table, we have calculated all the P/C and P/W ratios with all the codes in the code sets.

Table 1. Selection of OC subsets from 127-/511-chip Gold codes.

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3. Performance of SSFBG en/decoder in different refractive -index regime and the improvement by apodization

Figure 3(a) is the calculated reflection spectrum of a weak SSFBG (peak reflectivity <-30 dB) with a 127-chip BPSK Gold code. Here, Lchip=0.64 mm, corresponding chip duration Tchip is about 6.4 ps, and maximum index modulation Δn0=1.0×10-6. The code is shown at the top of the figure as well. The transfer matrix method (TMM) is used here to model the phase-shifted SSFBG, while for the non-uniform gratings discussed below, piecewise uniform approach is used as well [28

28. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15, 1277–1294 (1997). [CrossRef]

]. The input optical pulse is assumed to be transform-limited with Gaussian shape, whose spectrum is also plotted in Fig. 3(a). However, the variation of input pulsewidth will result in different correlation performance of the SSFBG en/decoder [33

33. S. Kutsuzawa, S. Oshiba, A. Nishiki, S. Kobayashi, and H. Iwamura, “Phase-coding OCDM using fiber-Bragg-grating with enlarged signal pulse width,” in OSA Trends in Optics and Photonics (TOPS) Vol.86, Optical Fiber Communications Conference, Tech. Dig. (Optical Society of America, Washington, DC, 2003), pp.136–137.

]. Figure 3(b) shows the calculated P/W and P/C ratios vs. the input pulsewidth for the 127-chip SSFBG. The P/C ratio is the average with 10 codes. The target ratios (approx. 37) are calculated directly by using the OC sequences. With shorter pulsewidth, the P/W and P/C ratios are closer to the target value, while the increase of the pulsewidth results in decrease of both the ratios. The shorter optical pulse therefore is more suitable for obtaining better correlation properties. However, the broad spectrum width will lower the frequency efficiency of the wavelength res ource. We need to make a compromise. In the calculations hereafter, we assume that the pulsewidth is approximately equal to Tchip, that is, 6.4 ps.

Fig. 3. Spectrum and performance of SSFBG with different input pulse width (a) Spectrum; (b) P/W and P/C ratios vs. input pulse width.

Figure 4 shows the calculated waveforms of the generated OC from 127-chip SSFBG encoders with various values of Δn0. The intensity of the generated OC is quite uniform with low Δn0, while with increasing Δn0, the waveform becomes more and more uneven and decays with time, and its tails (indicated by the arrows in the figures) become clearer. These waveform degradations are due to two factors: one is the increased loss because the encoded signal has to penetrate through the whole grating, and the other is the stronger multiple reflections between different grating segments.

Fig. 4. Waveforms of the generated OC with different Δn0.

Fig. 5. Performance of SSFBG en/decoder with different Δn0 (a) Power reflectivity, (b) P/W, P/C ratios and the auto-correlation peak value vs. Δn0..

Table 2. Performance of SSFBG in different Δn0 regime.

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We previously proposed using a non-uniform profile of the refractive index along the whole grating length to mitigate the low reflectivity issue of SSFBG en/decoder [24

24. K. Matsushima, X. Wang, S. Kutsuzawa, A. Nishiki, S. Oshiba, N. Wada, and K.I. Kitayama, “Experimental demonstration of performance improvement of 127-Chip SSFBG en/decoder using apodization technique,” IEEE Photonic Technol. Lett. 16, 2192–2194 (2004). [CrossRef]

, 31

31. X. Wang, A. Nishiki, and K. Kitayama, “Improvement of the coding performance of SSFBG en/decoder by apodization technique,” Microwave and Optical Techno. Lett. 43, 247–250 (2004). [CrossRef]

]. When the light signal is reflected by an SSFBG, the signal reflected from the far-end of the grating will experience more loss after penetrating through the whole grating than that from the near-end. Accordingly, the idea of using a non-uniform refractive index profile along the grating length is to compensate this loss properly. Here, we refer “apodization” to such non-uniform index profile. The simplest method is to balance the intensity of the reflected signals from the individual segments, it is similar as that in incoherent TS FBG encoders [8

8. G. E. Town, K. Chan, and G. Yoffe, “Design and performance of high-speed optical pulse-code generators using optical fiber Bragg gratings,” IEEE J. Select. Quantum Electron. 5, 1325–1331 (1999). [CrossRef]

]. In Fig. 6, the monotonically increased profile marked as “AP-I” is one of the apodization profiles determined by this method. As indicated in Fig. 5, comparing to the uniform SSFBG with the same refractive index, the SSFBG with the AP-I profile has slightly improved P/W and P/C ratios and same value of PA. However, this method is effective only in low-Δn0 regime because the multiple reflections induced tails and, most importantly, the coherent coupling between the adjacent FBG segments are ignored in the calculation. Actually, it will completely fail if Δn0 is higher (in regime II) [31

31. X. Wang, A. Nishiki, and K. Kitayama, “Improvement of the coding performance of SSFBG en/decoder by apodization technique,” Microwave and Optical Techno. Lett. 43, 247–250 (2004). [CrossRef]

]. In Fig. 6, the profile indicated by AP-II was obtained by a try-and-error method [31

31. X. Wang, A. Nishiki, and K. Kitayama, “Improvement of the coding performance of SSFBG en/decoder by apodization technique,” Microwave and Optical Techno. Lett. 43, 247–250 (2004). [CrossRef]

]. With this apodization profile, the SSFBG exhibits high P/W as well as high PA values by maintaining P/C ratio as indicated in Fig. 5, while the average Δn0 is in higher part of regime II. Note that the improved P/W ratio and PA value even surpass the limits that a uniform SSFBG can attain, thus showing the significant improvement that the apoization technique can achieve.

Fig. 6. Refractive index apodization profiles of SSFBG.

4. Experimental investigation with 127-chip SSFBG

In this investigation, we prepared three groups of 127-chip 160-chip/s SSFBGs for comparison: the apodized SSFBG (AP), uniform LR and HR SSFBGs. Their refractive-index profiles are shown in Fig. 6. The AP-I type apodization was tested to show the feasibility and the effectiveness of the apodization technique. Figure 7 shows the measured spectra of the SSFBGs along with the calculation result. Two different 127-chip BPSK Gold code sequences, represented by GC-A and GC-B, were implemented in the prepared SSFBG. The patterns of these codes are shown at the top of Fig. 7.

Fig. 7. GC patterns and the measured (solid lines) and calculated (dashed lines) reflectivity spectrum (a) uniform LR samples, (b) uniform HR samples, and (c) AP samples.

For the LR SSFBG, Fig. 8(a) shows the close-up of the auto-correlation waveform measured by optical sampling oscilloscope (dots in the figure) as well as the calculated waveform (solid line). The measured pulsewidth is about 5.6 ps (input pulse width is about 2.3 ps), while the P/W ratio is about 13.3, which are in a good agreement with the theoretical predictions. The wavelength-deviation tolerance of the grating was evaluated in the experiment as well. The SSFBGs’ central wavelength was changed by electrically tuning the temperature of the package. The measured peak values of the auto-/cross-correlations with different central wavelength deviation between OC encoder and decoder are shown in Fig. 8(b) together with the simulation results. The measured temperature tolerance range is about ±0.3 °C, and the temperature stability of the package (±0.1 °C) is within this range.

Fig. 8. Performance of the uniform LR SSFBG en/decoder (a) auto-correlation waveform (dots: measured by optical sampling oscilloscope; solid line: calculated) (b) peak intensity of the auto-/cross-correlation (AC/CX) vs. temperature drift of the SSFBG OC decoder.

Table 3 summarizes the measured peak reflectivities Rf as well as the peak powers of the reflected signals Pen. The AP sample yields the highest reflectivity and optical power. The uniformity of Rf in the AP group is about 0.1 dB, whereas in the LR and HR groups, the uniformity is about 3dB and 0.2 dB, respectively. The improvement of the uniformity of the AP group is due to the fact that with the minimum controllable illumination, the UV-induced index change is smaller in the high-index-change regime (resulting in a higher precision of controlling the index change than that in the low-index-change regime) [34

34. D. Johlen, H. Renner, A. Ewald, and E. Brinkmeyer, “Fiber Bragg grating Fabry-Perot measurement of the UV-induced index change,” in European Conference of Optical Communication (ECOC’98) (Madrid, Spain, 1998), pp. 393–394.

].

Table 3. Peak reflectivity of the different SSFBG samples.

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The measured waveforms of the input pulse and the reflected signals from the OC encoders are shown in Fig. 9. As for the input pulse’s waveform, the ripples on the left ends of the waveforms, indicated by 1, are presumably caused by some reflections in the measurement equipment. The undershooting and overshooting after the auto-correlation peak, indicated by 2 and 3, respectively, are due to the detector’s impulse response. From Figs. 9(b) to (d), all results are measured under the same conditions and are comparable.

The encoded signal from the LR encoder has a uniform waveform with low intensity, while the waveform of the HR sample exhibits a decay from the beginning of the code, and higher intensity. The waveform from the AP sample shows an improved uniformity compared with the HR one, and has the highest intensity level among them.

Fig. 9. Waveforms of (a) input pulse, and generated OC-A signals from (b) LR sample, (c) HR sample, and (d) AP sample.

The measured auto- and cross-correlation waveforms are shown in Fig. 10. For the purpose of comparison, the intensity is normalized by the auto-correlation peaks. The maximum wings in the auto-correlation waveforms, which are indicated by 4 in the figures, are located in front of the auto-correlation peaks. It is difficult to estimate the exact P/W and P/C ratios in this experiment as the signal is detected by photodetector with 30-GHz bandwidth, which corresponds to 30 ps time resolution, and this precision is not good enough. However, qualitative comparison between them shows that t he P/W and P/C ratios are lower in the case of the HR samp le than in the case of the LR sample, while in the case of the AP sample, these ratios as well as the PA value are improved.

Fig. 10. The auto-/cross-correlations waveforms of (a) LR, (b) HR and (c) AP samples.

A transmission experiment was carried out to evaluate the overall BER performance of the SSFBG OC decoders. The experimental setup is shown in Fig. 11. The mode-locked fiber ring laser (MLFL) generates the 2.8 ps optical pulses at a repetition rate of 10 GHz with a central wavelength of 1549 nm. This signal is then modulated by the first electro-absorption modulator (EAM) into 1.25 Gb/s for transmission. The second EAM further modulates the signal by 223-1 pseudo-random binary sequence (PRBS). The data signal is then split into two arms, each with a different time delay, and encoded into GC-A and GC-B with OC encoders A and B via circulators, respectively. The time -delay difference between the two arms is set to be larger than the coherent length of the light source thus the signals from the two arms can be regarded as from different light sources. These signals are then combined and transmitted through a short length of optical fiber. At the receiver, the multiplexed signals are recognized by the OC decoder A and, finally, measured by the BER tester.

Fig. 11. Experimental setup of the BER measuring experiment with 2 MUX users (MLFL: mode locked fiber laser; EAM: electro-absorption modulator; LPF: low pass filter; ATT: Attenuator).

The measured BERs for the three groups of samples with and without multiplexing are shown in Fig. 12. In the case of single-user transmission (open marks in the figure), the AP group has the best receiver sensitivity, which is 1.7 dB less than the LR group and 3 dB less than the HR group for error-free transmission (BER<10-9). In the case of two-user multiplexing (filled marks in the figure), only the AP group can achieve error-free transmission. The power penalty to the single-user case is 3.8 dB, which is presumably due to the beat noise [7

7. X. Wang and K. Kitayama, “Analysis of beat noise in coherent and incoherent time-spreading OCDMA,” J. Lightwave Technol. 22, 2226–2235 (2004). [CrossRef]

]. It is therefore concluded that, the AP group SSFBG en/decoder has better overall BER performance compared to the LR and HR group samples, which shows the effectiveness of the apodization technique.

Fig. 12. Comparison of BER performance with and without MUX.

5. Conclusion

The phase-shifted SSFBG is a promising device for coherent TS OC generation/recognition in OCDMA as well as OC-label based photonic MPLS [5

5. K. Kitayama and M. Murata, “Versatile optical code-based MPLS for circuit, burst, and packet switchings,” J. Lightwave Technol. 21, 2753–2764 (2003) [CrossRef]

] networks. The code sets used for wireless CDMA could be adopted as the OC. However, as the OC is required to have good aperiodic (instead of periodic) correlation properties, we need to further select subsets from the above code sets accordingly.

We use the P/W and P/C ratios and PA to quantitatively measure the OC recognition performance of the SSFBG in different reflectivity regimes. The relationship between Rf, L in, P/W and P/C ratios as well as PA vs. Δn 0 show the limitation of uniform SSFBG in high reflectivity regime. This is due to the loss and multiple reflections in the grating. We proposed an approach of adopting apodization technique to this issue. The proposed apodization profile significantly improves the OC recognition performance in terms of obtaining high P/W and P/C ratio as well as high PA simultaneously. This improvement means that the SSFBG can operate even with high reflectivity.

We carried out a comparative experimental investigation with 127-chip 160-Gchip/s SSFBGs : with apodization vs. without apodization. The apodized SSFBGs exhibited higher reflectivity as well as better correlation performance simultaneously. The apodized SSFBGs also had better BER performance in the transmission experiment with multiplexing of two active users. These performance improvements confirmed the effectiveness of the proposed approach. The wavelength deviation between the OC encoder and decoder was investigated as well. The temperature tolerance range is measured to be about ±0.3 °C agreeing with the theoretical calculation.

Acknowledg ments

The authors would like to thank for F. Kubota of NICT for his encouragement and Y. Tomiyama for his technical support.

References and links

1.

P. R. Prucnal, M. A. Santoro, and T. R. Fan, “Spread spectrum fiber-optic local area network using optical processing,” J. Lightwave Technol. 4, 547–554 (1986). [CrossRef]

2.

J. A. Salehi, “Code division multiple-access techniques in optical fiber networks, Part I: fundamental principles,” IEEE Trans. Commun. 37, 824–842 (1989). [CrossRef]

3.

D. D. Sampson, G. J. Pendock, and R. A. Griffin, “Photonic code-division multiple-access communications,” Fiber Integer. Opt. 16, 129–157 (1997). [CrossRef]

4.

K. Kitayama, “Code division multiplexing lightwave networks based upon optical code conversion,” IEEE J. Selec. Areas Commun. 16, 1209–1319 (1998). [CrossRef]

5.

K. Kitayama and M. Murata, “Versatile optical code-based MPLS for circuit, burst, and packet switchings,” J. Lightwave Technol. 21, 2753–2764 (2003) [CrossRef]

6.

K. Kitayama and N. Wada, “Photonic IP routing,” IEEE Photonic Technol. Lett. 11, 1689–1691 (1999) [CrossRef]

7.

X. Wang and K. Kitayama, “Analysis of beat noise in coherent and incoherent time-spreading OCDMA,” J. Lightwave Technol. 22, 2226–2235 (2004). [CrossRef]

8.

G. E. Town, K. Chan, and G. Yoffe, “Design and performance of high-speed optical pulse-code generators using optical fiber Bragg gratings,” IEEE J. Select. Quantum Electron. 5, 1325–1331 (1999). [CrossRef]

9.

H. Tsuda, H. Takenouchi, T. Ishii, K. Okamoto, T. Goh, K. Sato, A. Hirano, T. Kurokawa, and C. Amano, “Spectral encoding and decoding of 10 Gbit/s femtosecond pulses using high resolution arrayed-waveguide grating,” Electron. Lett. , 35, 1186–1187 (1999). [CrossRef]

10.

Z. Wei, H. M. H. Shalaby, and H. Ghafouri-Shiraz, “Modified Quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems,” J. Lightwave Technol. 19, 1274–1281 (2001). [CrossRef]

11.

S. Yegnanarayanan, A. S. Bhshan, and B. Jalali, “Fast wavelength-hopping time-spreading encoding/decoding for optical CDMA,” IEEE Photon. Technol. Lett. 12, 573–575 (2000). [CrossRef]

12.

K. Yum, J. Shin, and N. Park, “Wavelength-time spreading optical CDMA system using wavelength multiplexers and mirrors fiber delay lines,” IEEE, Photon. Technol. Lett. 12, 1278–1280 (2000). [CrossRef]

13.

H. Fathallah, L. A. Rusch, and S. LaRochelle, “Passive optical fast frequency-hop CDMA communications system,” J. Lightwave Technol. 17, 397–405 (1999). [CrossRef]

14.

X. Wang and K. T. Chan, “A sequentially self-seeded Fabry-Perot laser for two-dimensional encoding/decoding of optical pulses,” IEEE J. Quantum Electron. 39, 83–90 (2003). [CrossRef]

15.

N. Wada, H. Sotobayashi, and K. Kitayama, “2.5 Gbit/s time-spread/wavelength-hop optical code division multiplexing using fibre Bragg grating with super continuum light source,” Electron. Lett. 36, 815–817 (2000). [CrossRef]

16.

R. A. Griffin, D. D. Sampson, and D. A. Jackson, “Coherence coding for photonic code-division-multiple access networks,” J. Lightwave Technol. 13, 1826–1837 (1995). [CrossRef]

17.

M. E. Maric, “Coherent optical CDMA networks,” J. Lightwave Technol. 11, 854–864 (1993). [CrossRef]

18.

N. Wada and K. Kitayama, “A 10 Gb/s optical code division multiplexing using 8-chip optical bipolar code and coherent detection,” J. Lightwave Technol. 17, 1758–1765 (1999). [CrossRef]

19.

J. A. Salehi, A. M. Weiner, and J. P. Heritage, “Coherent ultrashort light pulse code-division multiple access communication systems,” J. Lightwave Technol. 8, 478–491 (1990). [CrossRef]

20.

C. C. Chang, H. P. Sardesai, and A. M. Weiner, “Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5 Km fiber link,” IEEE, Photon. Technol. Lett. 10, 171–173 (1998). [CrossRef]

21.

A. Grunnet-Jepsen, A. E. Johnson, E. S. Maniloff, T. W. Mossberg, M. J. Munroe, and J. N. Sweetser, “Fiber Bragg grating based spectral encoder/decoder for lightwave CDMA,” Electron. Lett. 35, 1096–1097 (1999). [CrossRef]

22.

P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,” J. Lightwave Technol. 9,1352–1365 (2001).

23.

P. C. Teh, M. Ibsen, J. H. Lee, P. Petropoulos, and D. J. Richardson, “Demonstration of a four-channel WDM/OCDMA system using 255-chip 320-Gchip/s Quarternary phase coding grating,” IEEE Photonic Technol. Lett. 14, 227–229 (2002). [CrossRef]

24.

K. Matsushima, X. Wang, S. Kutsuzawa, A. Nishiki, S. Oshiba, N. Wada, and K.I. Kitayama, “Experimental demonstration of performance improvement of 127-Chip SSFBG en/decoder using apodization technique,” IEEE Photonic Technol. Lett. 16, 2192–2194 (2004). [CrossRef]

25.

X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota, and K. Kitayama, “Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,” in European Cnference of Optical Communication (ECOC’04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]

26.

T. W. Mossberg, “Planar holographic optical processing devices,” Optics Lett. 26, 414–416 (2001). [CrossRef]

27.

P. Ebrahimi, M. Kargar, M. Hamer, A. E. Willner, K. Yu, and O. Solgaard, “A 10-ms-tuning MEMS-actuated Gires-Tournois filter for use as a tunable wavelength demultiplexer and a tunable OCDMA encoder/decoder,” Optical Fiber Communication Conference (OFC’04) (Optical Society of America, Washington, D.C., 2004), ThQ2.

28.

T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15, 1277–1294 (1997). [CrossRef]

29.

C. R. Giles, “Lightwave applications of fiber Bragg grating,” J. Lightwave Technol. 15, 1391–1404 (1997). [CrossRef]

30.

B. J. Eggleton, P. A. Krug, L. Poladian, and F. Ouellette, “Long periodic superstructure Bragg gratings in optical fibers,” Electron. Lett. 30, 1620–1622 (1994). [CrossRef]

31.

X. Wang, A. Nishiki, and K. Kitayama, “Improvement of the coding performance of SSFBG en/decoder by apodization technique,” Microwave and Optical Techno. Lett. 43, 247–250 (2004). [CrossRef]

32.

E. H. Dinan and B. Jabbari, “Spreading codes for direct sequence CDMA and wideband CDMA cellular networks,” IEEE Commun. Mag. 36, 48–54 (1998). [CrossRef]

33.

S. Kutsuzawa, S. Oshiba, A. Nishiki, S. Kobayashi, and H. Iwamura, “Phase-coding OCDM using fiber-Bragg-grating with enlarged signal pulse width,” in OSA Trends in Optics and Photonics (TOPS) Vol.86, Optical Fiber Communications Conference, Tech. Dig. (Optical Society of America, Washington, DC, 2003), pp.136–137.

34.

D. Johlen, H. Renner, A. Ewald, and E. Brinkmeyer, “Fiber Bragg grating Fabry-Perot measurement of the UV-induced index change,” in European Conference of Optical Communication (ECOC’98) (Madrid, Spain, 1998), pp. 393–394.

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2340) Fiber optics and optical communications : Fiber optics components
(060.4230) Fiber optics and optical communications : Multiplexing
(060.4250) Fiber optics and optical communications : Networks

ToC Category:
Research Papers

History
Original Manuscript: October 1, 2004
Revised Manuscript: October 21, 2004
Published: November 1, 2004

Citation
Xu Wang, Koji Matsushima, Akihiko Nishiki, Naoya Wada, and Ken-ichi Kitayama, "High reflectivity superstructured FBG for coherent optical code generation and recognition," Opt. Express 12, 5457-5468 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-22-5457


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References

  1. P. R. Prucnal, M. A. Santoro, and T. R. Fan, �??Spread spectrum fiber-optic local area network using optical processing,�?? J. Lightwave Technol. 4, 547-554 (1986). [CrossRef]
  2. J. A. Salehi, �??Code division multiple-access techniques in optical fiber networks, Part I: fundamental principles,�?? IEEE Trans. Commun. 37, 824-842 (1989). [CrossRef]
  3. D. D. Sampson, G. J. Pendock, and R. A. Griffin, �??Photonic code-division multiple-access communications,�?? Fiber Integer. Opt. 16, 129-157 (1997). [CrossRef]
  4. K. Kitayama, �??Code division multiplexing lightwave networks based upon optical code conversion,�?? IEEE J. Selec. Areas Commun. 16, 1209-1319 (1998). [CrossRef]
  5. K. Kitayama and M. Murata, �??Versatile optical code-based MPLS for circuit, burst, and packet switchings,�?? J. Lightwave Technol. 21, 2753-2764 (2003 [CrossRef]
  6. K. Kitayama and N. Wada, �??Photonic IP routing,�?? IEEE Photonic Technol. Lett. 11, 1689-1691 (1999) [CrossRef]
  7. X. Wang and K. Kitayama, �??Analysis of beat noise in coherent and incoherent time-spreading OCDMA,�?? J. Lightwave Technol. 22, 2226-2235 (2004). [CrossRef]
  8. G. E. Town, K. Chan, and G. Yoffe, �??Design and performance of high-speed optical pulse-code generators using optical fiber Bragg gratings,�?? IEEE J. Select. Quantum Electron. 5, 1325-1331 (1999). [CrossRef]
  9. H. Tsuda, H. Takenouchi, T. Ishii, K. Okamoto, T. Goh, K. Sato, A. Hirano, T. Kurokawa and C. Amano, �??Spectral encoding and decoding of 10 Gbit/s femtosecond pulses using high resolution arrayed-waveguide grating,�?? Electron. Lett., 35, 1186 �??1187 (1999). [CrossRef]
  10. Z. Wei, H. M. H. Shalaby, and H. Ghafouri-Shiraz, �??Modified Quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems,�?? J. Lightwave Technol. 19, 1274-1281 (2001). [CrossRef]
  11. S. Yegnanarayanan, A. S. Bhshan, and B. Jalali, "Fast wavelength-hopping time-spreading encoding/decoding for optical CDMA," IEEE Photon. Technol. Lett. 12, 573-575 (2000). [CrossRef]
  12. K. Yum, J. Shin, and N. Park, �??Wavelength-time spreading optical CDMA system using wavelength multiplexers and mirrors fiber delay lines,�?? IEEE, Photon. Technol. Lett. 12, 1278-1280 (2000). [CrossRef]
  13. H. Fathallah, L. A. Rusch, and S. LaRochelle, �??Passive optical fast frequency-hop CDMA communications system,�?? J. Lightwave Technol. 17, 397-405 (1999). [CrossRef]
  14. X. Wang and K. T. Chan, �??A sequentially self-seeded Fabry-Perot laser for two-dimensional encoding/decoding of optical pulses,�?? IEEE J. Quantum Electron. 39, 83-90 (2003). [CrossRef]
  15. N. Wada, H. Sotobayashi, and K. Kitayama, �??2.5 Gbit/s time-spread/wavelength-hop optical code division multiplexing using fibre Bragg grating with super continuum light source,�?? Electron. Lett. 36, 815-817 (2000). [CrossRef]
  16. R. A. Griffin, D. D. Sampson, and D. A. Jackson, �??Coherence coding for photonic code-division-multiple access networks,�?? J. Lightwave Technol. 13, 1826-1837 (1995). [CrossRef]
  17. M. E. Maric, �??Coherent optical CDMA networks,�?? J. Lightwave Technol. 11, 854-864 (1993). [CrossRef]
  18. N. Wada, and K. Kitayama, �??A 10 Gb/s optical code division multiplexing using 8-chip optical bipolar code and coherent detection,�?? J. Lightwave Technol. 17, 1758-1765 (1999). [CrossRef]
  19. J. A. Salehi, A. M. Weiner, and J. P. Heritage, �??Coherent ultrashort light pulse code-division multiple access communication systems,�?? J. Lightwave Technol. 8, 478-491 (1990). [CrossRef]
  20. C. C. Chang, H. P. Sardesai, and A. M. Weiner, �??Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5 Km fiber link,�?? IEEE, Photon. Technol. Lett. 10, 171-173 (1998). [CrossRef]
  21. A. Grunnet-Jepsen, A. E. Johnson, E. S. Maniloff, T. W. Mossberg, M. J. Munroe, and J. N. Sweetser, �??Fiber Bragg grating based spectral encoder/decoder for lightwave CDMA,�?? Electron. Lett. 35, 1096-1097 (1999). [CrossRef]
  22. P. C. Teh, P. Petropoulos, M. Ibsen and D. J. Richardson, �??A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings,�?? J. Lightwave Technol. 9, 1352-1365 (2001).
  23. P. C. Teh, M. Ibsen, J. H. Lee, P. Petropoulos and D. J. Richardson, �??Demonstration of a four-channel WDM/OCDMA system using 255-chip 320-Gchip/s Quarternary phase coding grating,�?? IEEE Photonic Technol. Lett. 14, 227-229 (2002). [CrossRef]
  24. K. Matsushima, X. Wang, S. Kutsuzawa, A. Nishiki, S. Oshiba, N. Wada and K.I. Kitayama, �??Experimental demonstration of performance improvement of 127-Chip SSFBG en/decoder using apodization technique,�?? IEEE Photonic Technol. Lett. 16, 2192-2194 (2004). [CrossRef]
  25. X. Wang, K. Matsushima, A. Nishiki, N. Wada, F. Kubota and K. Kitayama, �??Experimental demonstration of 511-chip 640Gchip/s superstructured FBG for high performance optical code processing,�?? in European Cnference of Optical Communication (ECOC�??04) (Stockholm, Sweden, 2004), Tu1.3.7. [PubMed]
  26. T. W. Mossberg, �??Planar holographic optical processing devices,�?? Optics Lett. 26, 414-416 (2001). [CrossRef]
  27. P. Ebrahimi, M. Kargar, M. Hamer, A. E. Willner, K. Yu and O. Solgaard, �??A 10-µs-tuning MEMS-actuated Gires-Tournois filter for use as a tunable wavelength demultiplexer and a tunable OCDMA encoder/decoder,�?? Optical Fiber Communication Conference (OFC�??04) (Optical Society of America, Washington, D.C., 2004), ThQ2.
  28. T. Erdogan, �??Fiber grating spectra,�?? J. Lightwave Technol. 15, 1277-1294 (1997). [CrossRef]
  29. C. R. Giles, �??Lightwave applications of fiber Bragg grating,�?? J. Lightwave Technol. 15, 1391-1404 (1997). [CrossRef]
  30. B. J. Eggleton, P. A. Krug, L. Poladian, and F. Ouellette, �??Long periodic superstructure Bragg gratings in optical fibers,�?? Electron. Lett. 30, 1620-1622 (1994). [CrossRef]
  31. X. Wang, A. Nishiki and K. Kitayama, �??Improvement of the coding performance of SSFBG en/decoder by apodization technique,�?? Microwave and Optical Techno. Lett. 43, 247-250 (2004). [CrossRef]
  32. E. H. Dinan and B. Jabbari, �??Spreading codes for direct sequence CDMA and wideband CDMA cellular networks,�?? IEEE Commun. Mag. 36, 48-54 (1998). [CrossRef]
  33. S. Kutsuzawa, S. Oshiba, A. Nishiki, S. Kobayashi, and H. Iwamura, �??Phase-coding OCDM using fiber-Bragg-grating with enlarged signal pulse width,�?? in OSA Trends in Optics and Photonics (TOPS) Vol.86, Optical Fiber Communications Conference, Tech. Dig. (Optical Society of America, Washington, DC, 2003), pp.136-137.
  34. D. Johlen, H. Renner, A. Ewald, and E. Brinkmeyer, �??Fiber Bragg grating Fabry-Perot measurement of the UV-induced index change,�?? in European Conference of Optical Communication (ECOC�??98) (Madrid, Spain, 1998), pp. 393-394.

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