1. Introduction

Optical packet switching (OPS) can efficiently cope with the bursty nature of traffic patterns in data-centric networks. In the future, Data in the Optical Domain Network (DOD-N) routers may be needed to provide the capability of switching data at hundreds of Tb/s. Optical label switching (OLS) operates directly in the optical layer by encapsulating payload data with a label at the ingress node and swapping the label in routing at the intermediate nodes. All-optical orthogonal labeling has attracted much attention recently [1, 2] as a promising approach for optical label switched networks. In this scheme, the label information is carried by the phase of the optical carrier, while the payload is encoded into a low extinction ratio (ER) intensity modulated signal. Hence label insertion and removal (swapping) does not affect the packet length. Orthogonal labeling has the advantage of transparent operation allowing it to be compatible with different network standards and upgradeability since changes in label length do not affect the overall packet duration. Its advantages also include simultaneous detection and recovery of label and payload when compared with the bit-serial labeling [3], more efficient use of bandwidth, less stringent requirement on wavelength accuracy and notch filters when compared with subcarrier multiplex (SCM) labeling [3, 4] and optical carrier suppression and separation (OCSS) labeling [5]. Orthogonal labeling is simpler to implement than optical code division multiplexing labeling (OCDM), which needs a large number of coders and decoders at each node [6].

In orthogonal labeling, however, precise adjustment of the ER between the amplitude shift keying (ASK) and differential phase shift keying (DPSK) modulation signals is required. This increases the system complexity since the ER of optical packets should be maintained at each intermediate node. Besides, for the integrity of the DPSK label, a limited ER of the ASK payload is required. There is a tradeoff as an increase of the ER will give a better transmission performance of the ASK payload but cause a detrimental effect on the DPSK label and vice versa [7]. Recently, we have proposed and demonstrated [7, 8] how to remove the transmission penalty due to excess frequency-chirp associated with the π phase change at the bit boundaries of the DPSK label impressed onto the ASK payload. Here, we propose and demonstrate an orthogonal labeling scheme based on a half-bit delayed dark return-to-zero (HBDDRZ) payload with DPSK label. In this proposed scheme, precise payload ER adjustment at source nodes and ER maintenance at each intermediate node is not necessary. The high ER of both payload and label improves the receiver margin during their detections. Besides, the HBDDRZ payload is particular suited for use with a DPSK label due to the RZ-like nature of the demodulated DPSK [9], resulting in minimal effect on the DPSK label. The proposed scheme employs a nonlinear optical loop mirror (NOLM) [10, 11] to generate the HBDDRZ payload, which is then encoded with a DPSK label. Label swapping is performed by birefringence switching in semiconductor optical amplifier (SOA) [12, 13], which removes the old label whilst preserving HBDDRZ payload.

2. Principle of HBDDRZ and DPSK orthogonal labeling

In the proposed orthogonal labeling scheme, the payload is carried by a dark RZ signal at bit-rate R and period T=1/R. If Δt is the full-width half maximum (FWHM) of the dark RZ pulse, there is a time interval T - Δt between one bit and the following bit that has only a small variation in optical power from its maximum value. The use of this time interval for phase label encoding ensures the integrity for the label. A phase modulated DPSK label at bit-rate R can be added to the dark pulse sequence in this time interval. The dark RZ payload is half-bit delayed with respected to the label, so that negligible crosstalk between the payload and label is observed even both of them have high ER. The HBDDRZ is thus particularly suited for use with DPSK due to the RZ-like nature of the demodulated DPSK signal.

3. Experiment

Figure 1 shows the experimental setup of the optical label encoding and swapping of orthogonal signal based on HBDDRZ payload and DPSK label. In the source node of the proposed optical label switched networks, the payload information will be encoded as a dark RZ signal, while the label is encoded via DPSK. Upon arrival of optical packet in each intermediate node, its power is split. One part goes to the label extraction unit (LEU), where a Mach-Zehnder interferometer (MZI) demodulates the DPSK label, which is then detected by a PIN photo-diode (PD). The electrical signal may be processed for label clock recovery, label information retrieval and new label generation. The other part of the input packet is fed into a label removal unit (LRU), where birefringence switching is performed using a single SOA. The old DPSK label is erased and the dark RZ payload is preserved in a new wavelength due to the intensity sensitive birefringence switching of SOA. New label information intended for the next routing node will be inserted orthogonally onto the wavelength converted optical packet in the label insertion unit (LIU).

 

Fig. 1. Experimental setup of orthogonal labeling based on HBDDRZ payload and DPSK label. DFB: distributed feedback laser, MOD: modulator, EDFA: erbium-doped fiber amplifier, FC: fiber coupler, DSF: dispersion shift fiber, PC: polarization controller, VOD: variable optical delay, PM: phase modulator, SMF: single mode fiber, SOA: semiconductor optical amplifier, MZI: Mach-Zehnder interferometer, LEU: label extraction unit, LRU: label removal unit and LIU: label insertion unit.

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In the source node, the dark RZ payload was generated by using NOLM. The control signal was a 10-GHz, 1551 nm (λ₀) gain-switched signal from a distributed feedback laser diode (DFB1) that generated 20 ps FWHM pulse train. The payload information was then encoded onto the pulse train by an optical modulator at pseudo-random binary sequence (PRBS) of 2²³-1. Its power was then amplified by an erbium-doped fiber amplifier (EDFA) to generate a signal of average optical power of 12.8 dBm. The CW signal (λ₁), at 1567 nm was generated by a DFB₂ with input power of 6 dBm. The NOLM consists of two 3-dB fiber couplers (FC₁ and FC₂) and 5 km of dispersion shift fiber (DSF). The CW signal is split at the FC₂ into two counterpropagating components which experience identical phase shifts as they travel around the loop. They interfere constructively at the FC₂, and exit back through the input port of the NOLM. The loop can be imbalanced by introducing a control RZ signal that copropagates with one of the two CW components, causing a nonlinear phase shift to the copropagating CW component. The control RZ was launched via FC₁. When the two counterpropagating CW components interfere at FC₂, the wavelength converted RZ will appear at the output port of the NOLM, while the corresponding dark RZ signal will appear at the output port of the circulator. An optical filter with 3-dB bandwidth of 0.65 nm selected the converted dark RZ signal. The DSF has a nonlinear index n₂=2.67×10^-20 m²/W, zero dispersion wavelength of 1559 nm. In order to minimize the walk-off between the input RZ and the CW inside the NOLM and maximize the switching efficiency, it is necessary to minimize the group delay difference between the two wavelengths. The two wavelengths were set equally on opposite sides of the zero dispersion wavelength of the DSF. The switching efficiency of the NOLM is given by [10]:

$P_0=\frac{1}{2}{P}_C\left[1-\cos \left(2\gamma P_{S}L\right)\right]$

where P₀ is the power of the wavelength converted signal, P_C and P_S are the power of the CW and peak power of signal pulses respectively, γ denotes the nonlinear coefficient of the DSF, and L is the loop length. The switching condition of the NOLM where the output power P₀ is maximized is:

$L=\frac{\pi }{2\gamma P_S}$

An EDFA at the output of the circulator compensated the loss during transmission and an optical filter with 3-dB bandwidth of 1.1 nm removed the out-of-band amplified spontaneous emission (ASE). The 10 Gb/s DPSK label was then encoded onto the dark RZ payload by the phase modulation (PM). The variable optical delay (VOD) provided a half bit delay for the dark RZ with respect to the label. The signal was transmitted over 40 km single mode fiber (SMF) with dispersion compensation. In the intermediate node, the label information of the orthogonal signal was detected by the LEU, as shown within the inset of Fig.1. It consisted of a MZI which demodulated the DPSK label for detection by a PIN PD. The old label removal was performed by birefringence switching inside the LRU, where the dark RZ payload was transferred to a new wavelength which had no existing label encoding. The orthogonal signal was at -3 dBm before being launched into the SOA, which had a gain peak at 1555 nm and was biased at 195 mA. CW light generated by DFB₃ at 1556 nm with power of -4 dBm was also coupled into the SOA though the 3-dB coupler FC₃. Initially, only the CW light was launched into the SOA with the polarization set at around 45° with respect to the TE axis of the SOA. The polarization controller (PC₆) at the output of the SOA was used to linearize the outgoing polarized light. The CW light, after passing the SOA was blocked by a polarizer. The orthogonal modulated signal was then injected together with the CW into the SOA with TE polarization. The dark RZ in the orthogonal modulated signal depletes the carrier in the SOA, causing refractive index changes, which introduces a change in birefringence for the CW beam in the SOA. The CW will be switched “ON” after the polarizer when the dark RZ modulates the effective refractive index of the TE mode of the CW with respect to the TM mode and the payload is thus transferred to the new wavelength of 1556 nm [12]. A tunable filter selected the converted dark RZ at 1556 nm, which was then encoded by a new label in DPSK modulation by a PM in the LIU.

4. Results and discussion

Figure 2 shows the bit-error rate (BER) measurements for both the payload and label at different nodes of the experiment. The power penalty of the HBDDRZ payload is 2 dB after label swapping. The power penalty may be cause by the inexact 90° polarization rotation in the birefringence switching. Negligible power penalty is observed for the DPSK label during swapping. The BER measurement of a NRZ label, employing in the previously approach of orthogonal labeling [7, 8] using ER of 2 dB is also included for reference. An improvement of 3-dB receiver margin is observed.

 

Fig. 2. BER measurements for the HBDDRZ payload and DPSK label at different nodes of the experiment. The BER of NRZ signal with extinction ratio of 2dB is included.

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The corresponding eye-diagrams are shown in Fig. 3(a)–3(d). The HBDDRZ payload is particular suited for use with DPSK labeling because of the RZ-like nature of the demodulated DPSK signal. The ER of the DPSK label and HBDDRZ payload are 13 dB and 8 dB respectively. Figure 4 shows the optical spectra measured at the output port of the circulator after the NOLM by an optical spectrum analyzer with resolution of 0.01 nm. We measured the spectral broadening of the CW signal increases from 0.06 nm (20-dB spectral width) to 0.16 nm due to cross-phase modulation (XPM). The inset of Fig. 4 shows the optical spectra of the CW signal at different input RZ signal powers. The spectrum of the input RZ signal is also broadened from 0.73 nm (20-dB spectral width) to 1.19 nm due to self-phase modulation (SPM) caused by DSF.

 

Fig. 3. Eye diagrams of HBDDRZ payload (a) after the source node and (b) after the LRU in the intermediate node; DPSK label after (c) the source node and (d) the LIU.

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Fig. 4. Optical spectra measured at the output port of the circulator after the NOLM. The spectral broadening of the CW signal increases from 0.06 nm (20-dB spectral width) to 0.16 nm when the input RZ is injected into the NOLM. Inset: Spectra of the CW signal at difference input RZ powers.

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Label removal was performed by birefringence switching inside a SOA. Figure 5(a) shows the optical spectrum of the CW signal generated by DFB₃ at the initial condition, which, in the absence of the input dark RZ signal in the SOA, was minimized by the polarizer at a power level of -36.28 dBm. The polarization state of the CW from the SOA changed when the dark RZ was injected into the SOA and the CW thus switched “ON” to a power of -12.89 dBm after the polarizer, as shown in Fig. 5(b).

 

Fig. 5. Optical spectra at output of polarizer (a) with no input dark RZ in the SOA and (b) with input dark RZ in the SOA.

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As the silica fiber based NOLM has potential for Tb/s switching operation due to its ultrafast optical nonlinearity, and birefringence switching using the SOA can operate at higher speeds than 10 Gb/s [12], we think that the proposed HBDDRZ-DPSK orthogonal labeling scheme can potentially operate at speeds beyond the 10 Gb/s shown in this paper. Although the NOLM and the birefringence switching demonstrated here are polarization dependent, polarization insensitive operation of NOLM based on twisted fiber; and birefringence switching in SOA using polarization diversity loop [13] may be implemented. Also, higher ER of dark RZ can be produced by higher nonlinear phase shift inside the NOLM if higher power EDFA or longer length of DSF is used.

5. Conclusion

Optical orthogonal label is a promising candidate for future optical label switched networks. In orthogonal labeling, precise adjustment of the ER between the intensity modulated payload and the phase modulated label is required. Here, we proposed and demonstrated an orthogonal labeling scheme based on the use of a HBDDRZ payload with a DPSK label. Precise payload ER adjustment at source nodes and ER maintenance at each intermediate node is not necessary. The high ER of both payload and label improves the receiver margin during signal detections. Also the HBDDRZ payload causes negligible effect on the RZ-like DPSK label. A NOLM was used to generate the HBDDRZ payload, which was then encoded with the DPSK label. Label swapping was performed based on birefringence switching inside a SOA. Experimental demonstration at 10 Gb/s was performed. Higher speed operation should be possible for the proposed scheme.