We performed encoding-decoding and associated transmission experiments at 40 Gb/s. A trace of the encoded pulse, shown in Figure 5(b), is obtained using a fast photodiode and a sampling oscilloscope (detection bandwidth: ϳ55 GHz). Obviously, the phase characteristics of the encoded trace cannot be detected through square-law detection; however, reshaping of the initial pulses [Fig. 5(a)] to a pattern of similar duration to that of the design code is clearly observed. The encoded pattern was decoded using the matched SSFBG. The corresponding oscilloscope trace for the decoded pulse is presented in Figure 5(c), which clearly shows that short, distinct pulses are reformed. This result confirms that good code-recognition quality can be obtained by using the SSFBGs. Figure 5(d) shows the trace for the decoded pulse under unmatched case Q 2 :Q* 1 ; short pulses are not reformed.We examined the decoded pulses using the SHG intensity autocorrelator (Ͻ100-fs resolution), in order to assess the duration of the resultant correlation signal. The results of the SHG autocorrelation measurements of the pattern-recognition pulse are shown in Figure 6. The trace we obtained is compared to the theoretical intensity-autocorrelation function of the decoded signal under single-channel operation. The agreement between the calculated and measured traces for the single-channel encoding/decoding process Q 1 :Q* 1 is seen to be excellent. We confirmed that the actual width of the peak for the process Q 1 :Q* 1 was 5.2 ps, which is in good agreement with our theoretical calculation of 4.9 ps.The results of Figures 5 and 6 validate the effectivity of the quaternary-phase coding and decoding of short pulse and illuminate that high-speed all-optical encoding and decoding at 40 Gb/s can be achieved by using three-chip, 240-Gchip/s SSFBGs. CONCLUSIONIn conclusion, we have designed and fabricated a three-chip, 240-Gchip/s encoder/decoder. The encoder/decoder consists of a superstructured fiber Bragg grating (SSFBG). Also, we have experimentally demonstrated 40-Gb/s quaternary-phase encoding and decoding using SSFBGs. The experimental results, building on the effectivity of high-rate all-optical encoder/decoder, show that SSFBG technology represents a promising technology for optical-pulse processing. The high-quality quaternary-pulse encoding and decoding may find use in a variety of all-optical network implementations, including both OCDM and packet-switched networks. Any practical system will, of course, require longer codes in order to accommodate more users, and we are currently actively investigating this issue. ACKNOWLEDGMENT INTRODUCTIONRecently, several terabit optical-transmission systems (including that in [1]) have been demonstrated, yet they are not extraordinary issues any more. This is because, as the transmission speed and capacity have increased, the required signal-processing speed at each switching node has exceeded the physical limit of the electronic components. In order to overcome such an obstacle, alloptical packet switch...
A new construction of optimal binary sequences, identical to the well known family of Gold sequences in terms of maximum nontrivial correlation magnitude and family size, but having larger linear span is presented. The distribution of correlation values is determined. For every odd integer r 2 3, the construction provides a family that contains 2' + 1 cyclically distinct sequences, each of period 2'-1. The maximum nontrivial correlation magnitude equals 2(r+1)/2+ 1. With one exception, each of the sequences in the family has linear span at least (rZr) / 2 (compared to 2r for Gold sequences). The sequences are easily implemented using a quaternary shift register followed by a simple feedforward nonlinearity. Index Tenns-Binary sequences, Gold sequences, sequences with low correlation, large linear span.
The Levenshtein bound on aperiodic correlation, which is a function of the weight vector, is tighter than the Welch bound for sequence sets over the complex roots of unity when M ≥ 4 and n ≥ 2, where M denotes the set size and n the sequence length. Although it is known that the tightest Levenshtein bound is equal to the Welch bound for M ∈ {1, 2}, it is unknown whether the Levenshtein bound can be tightened for M = 3, and Levenshtein, in his 1999 paper, postulated that the answer may be negative. A new weight vector is proposed in this paper which leads to a tighter Levenshtein bound for M = 3, n ≥ 3 and M ≥ 4, n ≥ 2. In addition, the explicit form of the weight vector (which is derived by relating the quadratic minimization to the Chebyshev polynomials of the second kind) in Levenshtein's paper is given. Interestingly, this weight vector also yields a tighter Levenshtein bound for M = 3, n ≥ 3 and M ≥ 4, n ≥ √ M , a fact not noticed by Levenshtein.
Compared with the perfect complementary sequence sets, quasicomplementary sequence sets (QCSSs) can support more users to work in multicarrier CDMA communications. A near-optimal periodic QCSS is constructed in this paper by using an optimal quaternary sequence set and an almost difference set. With the change of the values of parameters in the almost difference set, the near-optimal QCSS can become asymptotically optimal and the number of users supported by the subcarrier channels in CDMA system has an exponential growth.
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