Through four-wave mixing (FWM) process in highly-nonlinear fiber (HNLF) or semiconductor device, the phase-modulation depth of one converted FWM component could be doubled compared with that of the original input signal. Therefore, with a multilevel phase-modulated signal and a CW light as inputs, after FWM process in a nonlinear media, phase pattern (0, π) carried in the input multilevel phase-modulated signal will not be transferred to one converted FWM component, which could be referred to as an optical phase erasure process. We experimentally demonstrated format conversion from 320-Gb/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) to 160-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) through the proposed all-optical phase erasure scheme. The phase information carried in the converted binary RZ-DPSK is logically equal to the input Q-component, or a logical XOR operation result between I and Q components of the input RZ-DQPSK, which correspond to a serial or parallel DQPSK transmitter for the input RZ-DQPSK signal. It can be applied to erase a binary tributary from a multilevel modulation format.
©2009 Optical Society of America
Various advanced phase-modulated formats, such as differential binary phase-shift keying (DPSK) and differential quadrature phase shift-keying (DQPSK), have attracted lots of research attention for their potential to enhance the transmission capacity and spectral-efficiency in optical transmission systems. Different modulation formats are expected to be deployed in different networks to accommodate different requirements. Therefore, format conversion among different modulation formats is desirable to achieve transparent interconnections among different networks. Recently, format conversion schemes that combine several low-speed binary formats to a single high-speed multilevel format have been widely investigated [1–4]. They could support transparent traffic grooming from low-speed edge networks to high-speed core networks.
Considering a multilevel modulation format as a multiplexed signal in phase and intensity domains, conversion from a multilevel modulation format to a binary one would be useful for enabling the all-optical cross-connection from core networks to edge networks. We have proposed an optical phase erasure technology that enables format conversion from a multilevel modulation format such as DQPSK to a binary modulation format like DPSK . It can be applied to erase a binary tributary from a high-speed trunk in core networks, for further transfer to edge networks. In this paper, we experimentally demonstrate a conversion scheme from 320-Gb/s return-to-zero (RZ) DQPSK to 160-Gb/s RZ-DPSK, for the first time, through an all-optical phase erasure scheme based on four-wave mixing (FWM) in highly-nonlinear fiber (HNLF).
2. Operation principle
The operation principle of the proposed optical phase erasure is based on the FWM process in HNLF. As shown in Fig. 1, after power amplifications, CW light at ω 1 and an RZ-DQPSK signal at ω 2 are combined and fed into a length of HNLF. After the HNLF, a converted signal at ω 221 is obtained with the electrical field, E 221, given by:
where ωi, Ai, and θi (i∈[1,2]) are the angular frequency and the corresponding input field amplitude and phase of amplified CW light and input DQPSK, respectively, and k is a proportional constant related to FWM efficiency. As shown in Fig. 1, because the phase-modulation depth is doubled in the resultant phase at ω 221, i.e. Δθ 221=2 Δθ 2-Δθ 1, the phase state (0, π) carried in the input DQPSK signal is erased, and finally only a binary-coded DPSK signal is obtained at the converted signal at ω 221. Therefore, DQPSK can be successfully converted to DPSK through the proposed all-optical phase erasure. It is not necessary to lock the phase of input CW and RZ-DQPSK in the process. Note that, as the modulation depth of the phase noise carried in the input DQPSK is also doubled in this process. To avoid additional degradation, it is supposed that the proposed scheme would be applied before the deployment of signal regeneration in a gateway among different networks in optical transparent networks.
The format conversion scheme is demonstrated with a generated DQPSK signal using a serially-cascaded DQPSK transmitter in this paper. The binary data carried in the converted DPSK is logically equal to the Q-component of the input DQPSK. Note that the proposed scheme also could be simply applied to the case with an input DQPSK generated using a parallel in-phase/quadrature (IQ) modulator . Using the generated DQPSK through a parallel IQ modulator as input signal, logically, the converted binary DPSK signal is the XOR (or XNOR) logical operation result between I and Q components of the input DQPSK. Table 1 summarizes the logic and phase mapping between input and output signals in these two cases. The discussion is under the assumption that a conventional DQPSK pre-coder, for serial or parallel transmitter, is employed in the transmitter side. With an input DQPSK generated using a serially-cascaded DQPSK modulator, the obtained binary DPSK is logically equal to the Q-component in the transmitter side. No additional post-coder is required to recover the original data. If a parallel DQPSK transmitter is employed to generated RZ-DQPSK, it is hard to recover the original I and Q components. However, it may be potentially useful for the optical label processing or recognition when applying DQPSK format in optical label switching (OLS) networks. Through the current proposed scheme, only the original Q-component or the XOR operation result between input I and Q component could be extracted. Additional process is required to arbitrarily extract one of the components carried in the input DQPSK.
A numerical simulation was performed to verify the proposed scheme. As shown in Fig. 2, the phase patterns of input DQPSK and converted binary DPSK were represented in (a) and (b), respectively. Note that absolute phase was represented in the figure. The relative phase shift between two adjacent symbols in the converted binary DPSK is twice that in the input DQPSK. Four phase states were effectively converted to binary phase states after the conversion. After differential phase detections, the phase constellation diagrams of input RZ-DQPSK and converted RZ-DPSK were measured and shown in Fig. 3. It is verified that the format conversion from DQPSK to DPSK could be achieved through the proposed optical phase erasure scheme.
3. Experiment and results
Format conversion from 320-Gb/s DQPSK to 160-Gb/s DPSK was experimentally demonstrated to verify the proposed format conversion scheme using optical phase erasure technique. As shown in Fig. 4, to generate a 320-Gb/s RZ-DQPSK signal, a pulse train with a repetition rate of 10 GHz and a pulse-width of around 1.3 ps was generated from a semiconductor mode-locked laser diode (MLLD) at 1551 nm, and then phase-modulated by an integrated DQPSK modulator to generate a 20-Gb/s RZ-DQPSK signal. The 320-Gb/s RZ-DQPSK signal was obtained after multiplexing through a four-stage PLC-based multiplexer. After individual power amplification, CW light at 1561 nm was combined with the 320-Gb/s RZ-DQPSK signal and fed into a 500-m length of HNLF having a zero dispersion wavelength (ZDW), λ 0, of 1549 nm, a dispersion slope of 0.02 ps/nm2/km, and a nonlinear coefficient, γ, of 17 W-1km-1. The launched powers of the CW and RZ-DQPSK signals into HNLF were around 10.3 dBm and 19.9 dBm, respectively.
As shown in Fig. 5, after the HNLF, a FWM component coded as a 160-Gb/s RZ-DPSK signal was generated at around 1541 nm. Around -20-dB FWM efficiency was observed with reference to the launched power of the CW light. The clock tones observed for the CW light around 1561 nm should be mainly attributed to the cross-phase modulation caused by the adjacent RZ-DQPSK. A 6-nm bandpass filter was used to filter out the converted 160-Gb/s RZ-DPSK after the HNLF. The waveforms of the input 320-Gb/s RZ-DQPSK signal and the converted 160-Gb/s RZ-DPSK signal before phase de-modulation are shown in Fig. 6 (a) and (b). A pulse width of around 1.5 ps was observed for the converted 160-Gb/s RZ-DPSK signal. A slight increase in amplitude jitter was observed after conversion for the converted 160-Gb/s RZ-DPSK, which could be attributed to the introduced phase or intensity noise in the FWM process. The obtained 160-Gb/s RZ-DPSK was then de-multiplexed into a 10-Gb/s RZ-DPSK stream by a 160-Gb/s-to-10-Gb/s de-multiplexer based on two cascaded electroabsorption modulators (EAMs). A set of Mach-Zehnder delay interferometer (MZDI) and balanced photo-detector was employed for phase demodulation. Fig. 7 and Fig. 8 show the de-modulated eye diagrams of the 16 de-multiplexed tributaries for the input 320-Gb/s RZ-DQPSK signal (Q-component) and the converted 160-Gb/s RZ-DPSK signal. Clear eye openings were observed for both the input and output signals. The corresponding optical spectra of the input 320-Gb/s RZ-DQPSK and the converted 160-Gb/s RZ-DPSK are shown in Fig. 9(a), and Fig. 9(b), respectively.
Bit-error rates were measured at a bit rate of 10 Gb/s after de-multiplexing, as shown in Fig. 10(a). For the input RZ-DPSK signal, less than 1-dB receiver sensitivity difference was obtained between the de-modulated I and Q components for a tributary. Therefore, only the Q-component’s BER results for RZ-DQPSK are shown here. Fig. 10(b) shows the receiver sensitivity of sixteen tributaries of the input RZ-DQPSK and converted RZ-DPSK signals after de-multiplexing. Uniform performance was observed for the de-multiplexed tributaries. Thanks to the reduced symbol distance after conversion from RZ-DQPSK to RZ-DPSK, around 2.5-dB sensitivity improvement was obtained. In an ideal case , at least 5-dB sensitivity difference is expected. This indicates that a penalty of around 2 dB was introduced after conversion, which could be attributed to the accumulated phase and intensity noise in the FWM process, as well as the imperfect drivers for the DQPSK transmitter.
We have experimentally demonstrated format conversion from 320-Gb/s RZ-DQPSK to 160-Gb/s RZ-DPSK through an all-optical phase erasure scheme based on the FWM effect in HNLF. Thanks to the doubling effect of phase-modulation depth in FWM process, phase pattern (0, π) carried in the input DQPSK could be erased. Finally, a binary-coded DPSK was obtained from the input DQPSK.
References and links
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