Spectral phase encoding of ultra-short optical pulse in time domain for OCDMA application

Abstract: We propose a novel reconfigurable time domain spectral phase encoding (SPE) scheme for coherent optical code-division-multiple-access application. In the proposed scheme, the ultra-short optical pulse is stretched by dispersive device and the SPE is done in time domain using high speed phase modulator. The time domain SPE scheme is robust to wavelength drift of the light source and is very flexible and compatible with the fiber optical system. Proof-of-principle experiments of encoding with 16-chip, 20 GHz/chip binary-phase-shift-keying codes and 1.25 Gbps data transmission have been successfully demonstrated together with an arrayed-wave-guide decoder.


Introduction
Optical code division multiple access (OCDMA), where users share the same transmission medium by assigning unique pseudo-random optical code (OC), is attractive for next generation broadband access networks due to its features of allowing fully asynchronous transmission with low latency access, soft capacity on demand, protocol transparency, simplified network management as well as increased flexibility of QoS control [1~3]. In addition, since the data are encoded into pseudo-random OCs during transmission, it also has the potential to enhance the confidentiality in the network [4~6]. Figure 1 illustrates a basic architecture and working principle of an OCDMA passive optical network (PON) network. In the OCDMA-PON network, the data are encoded into pseudo random OC by the OCDMA encoder at the transmitter and multiple users share the same transmission media by assigning   At the receiver, the OCDMA decoder recognizes the OCs by  performing matched filtering, where the auto-correlation for target OC produces high level  output, while the cross-correlation for undesired OC produces low level output. Finally, the  original data can be recovered after electrical thresholding. Recently, coherent OCDMA technique with ultra-short optical pulses is receiving much attention for the overall superior performance over incoherent OCDMA and the development of compact and reliable en/decoders (E/D) [7~15]. In coherent OCDMA, encoding and decoding are performed either in time domain or in spectral domain based on the phase and amplitude of optical field instead of its intensity. Figure 2(a) shows the working principle of the coherent time spreading (TS) OCDMA, where the encoding/decoding are performed in time domain. In such a system, the encoding is to spread a short optical pulse in time with a phase shift pattern representing a specific OC. The decoding is to perform the convolution to the incoming OC using a decoder, which has an inversed phase shift pattern as the encoder and generates high level auto-correlation and low level cross-correlations. Planar lightwave circuit (PLC) [7], superstructured fiber Bragg grating (SSFBG) [13~15], and multi-port E/D with waveguide grating configuration [15] can be used as the en/decoder for this scheme. High capacity coherent TS-OCDMA have been demonstrated by different en/decoders [17~18]. One drawback is that the auto-correlation has not only a high level peak but also low level sidelobes as shown in Fig. 2(a) [15].

Peak Peak
Wing Wing  On the other hand, the spectrally phase encoding time spread (SPECTS) OCDMA system performs the en/decoding in spectral domain [8~12]. Figure 2(b) shows the working principle of the SPECTS-OCDMA. In such a system, the encoding is done to the broad spectrum of the ultra-short optical pulse and different spectral component has different phase shift. This also results the spreading of the short pulse in time domain. The decoding is to give inverted phase changes to different spectral components by the proper decoder with a complementary phase shift pattern compared to encoder. Therefore, the auto-correlation for the proper code can recover the spectrum as well as the temporal shape of the original pulse, while the cross-correlation still spread in time similar as the encoded signals. The spectral phase encoding (SPE) scheme can also be applied in pulse shaping as well for applications such as high-repetition-rate pulse burst generation [19~20], RZ-to-NRZ converting [21] and etc. The SPECTS en/decoders include spatial lightwave phase modulator (SLPM) [9~10], high resolution phase encoder/decoder [8], micro-ring-resonator (MRR) [11] or monolithically integrated en/decoders including AWG and phase shifters [12]. Generally, in the SPECTS-OCDMA system, high spectral resolution is always desirable for the sake of the spectral efficiency [8]. As a limit, the resolution of each chip should equal to the mode spacing, or the repetition rate of the optical pulses [8,21]. Therefore, the wavelength stability requirement for laser source and encoder/decoder is very stringent, which appears an issue for this system. Another issue is the compatibility of the encoder/decoders with the fiber optic system because the most of them are based on free-space optics. The third issue is the relatively high loss with these encoder/decoders.

Cross Cross--correlation correlation
Recently, spectral shaping to be done in time domain by stretching short pulse with dispersive components has also been proposed and demonstrated [22~24]. This time domain spectral shaping technique can be much flexible and compatible with fiber optic system. In this paper, we propose and demonstrate a novel reconfigurable time domain SPE scheme for coherent OCDMA application. The dispersive components can use single mode fibers (SMF) and dispersion compensation fibers (DCF), or long chirped fiber Bragg gratings (LCFBG). Conventional high speed PM can generate OC at 40 Gchip/s chip-rate. The potential advantages of the proposed SPE scheme include: 1. Good flexibility. This scheme offers fast (bit-by-bit) OC reconfigurable capability, which is unique compared to conventional techniques. 2. Relaxed wavelength alignment requirement. The wavelength alignment for laser and encoder is done in time domain, the requirement is much lower compared to conventional schemes. 3. Simple configuration and good compatible with fiber optic system. This scheme can even be simplified by combining the SPE and data modulation with a single modulator. 4. Low loss and low cost. One probable issue of this scheme is that the data rate of the system is limited by the speed of the PM and the length of the OC. For a system with data-rate higher than 1 Gbps, the length of OC will have to be lower than 40-chip. But it should be enough for many OCDMA applications. The stretched signal has temporal profile same as its spectrum, each narrow pulse in the stretched signal corresponds to one longitudinal mode. The total dispersion of the RDF is -312.5 ps/nm, therefore one bit duration of 800 ps covers 2.56 nm (320 GHz) spectral range. By properly adjusting the delay of the input signal, the BPSK code patterns can be precisely modulated onto this spectral range to perform the SPE. After PM, a span of SMF with +312.5 ps/nm dispersion was used for pulse compressing and finally generated the SPE signal. The proposed scheme is very robust to the wavelength drift of the light source because every 1 GHz optical frequency change can be compensated by accordingly changing the delay for 2.5 ps, which is very easy to be achieved either optically or electrically. The requirement for the synchronization between optical and electrical signals is also not very strict, there was no obvious change of the en/decoded signal in the experiment if the optical and electrical signal shifted several pico-second with each other.

Encoding/decoding experiments
We have not investigated the chose of suitable codes at this stage. In the experiment, we have tried with five different BPSK code patterns to demonstrate the feasibility of the proposed scheme: two of them are from 15-chip M-sequences and other 3 are randomly selected. The   Figure 6 shows the spectra and waveforms of the SPE pulses with these codes. Finally, we used an AWG spectral phase controller as the spectral phase decoder. The AWG device has 32 channels, which matches to the ITU grid, centered at 1552.51  Figure 7(a) shows the spectrum of this device. It can control the phase, as well as the amplitude, of each channel individually [20] to perform spectral phase decoding. Figure 7(b) shows the measured auto-(thick dark line) and cross-correlations (thin color lines). The peak ratios between auto-/cross-correlation is around 2, the discrimination of different codes can be done by proper thresholding [7~10]. However, peak ratio is not very high so far. This is mainly due to the existing of notches in the spectral response of the AWG decoder, which could be improved by using decoders with much smooth spectral response [19].

Transmission experiment
We have also carried out transmission experiment with the proposed scheme. The setup is shown in Fig. 8(a). The 1.25 Gbps, 2 23-1 pseudo random bit sequence (PRBS) was modulated by an IM before SPE. Two different code patterns (C1&2) were used for SPE. The dispersion of the transmission system was managed globally: the 19.5 km transmission fiber functions for pulse compression as well and ~545 m RDF was used in the receiver to compensate dispersion mismatch. The AWG decoder was also configured to decode the two codes, the decoded signals were detected by a photo-detector (PD) followed by a 2.5 GHz low-pass-filter (LPF). The measured bit-error-rate (BER) performances were shown in Fig. 8(b) with the open marks together with the eye diagrams. Error free has been reached in both cases. With this setup, the data modulation was done before the SPE. To simplify the system, both data modulation and SPE can be done with a single modulator-either a hybrid modulator for OOK or a single PM for DPSK modulation. To investigate this, we moved the IM after the PM. The BERs were shown in Fig. 8 the power penalties are 5.9 and 5.3 dB respectively for the two patterns. As it can be seen from the eye diagrams, this degradation is mainly due to that the duration of the stretched SPE signal exceeds one bit duration, resulting in the non-ideal modulation. Figure 8(c) shows the spectrum of the optical signal in 5 nm span. Only the central part with about 2.6 nm bandwidth is correctly modulated by the data, the rest part outside this range, which is not negligible, will result the performance degradation. This issue can be further solved by using a proper optical filter to block the unwanted spectral part or enlarging the spectral chip to reduce the total dispersion requirement.

Conclusion
In conclusion, we have proposed a reconfigurable time domain SPE scheme for coherent OCDMA application. The proof-of-principle experiments were demonstrated with 5 different 16-chip, 20 GHz/chip BPSK codes and 1.25 Gbps data rate. Compared to conventional SPE schemes, the wavelength alignment requirement for laser source and encoder can be relaxed in the proposed scheme, so it is very robust to the wavelength drift of the light source. It is very flexible and compatible with the fiber optical system. It also has the potential to be simplified by combining the SPE and data modulation with a single modulator.