New trend for optical signal-to-noise ratio of disturbed Raman fiber amplifier

In a distributed Raman fiber amplifier (DRFA), Raman amplification allows a lower signal launch powers to transverse the span above the noise floor while still increasing the optical signal-to-noise ratio (OSNR). It improves the noise figure and reduces the nonlinear penalty of fiber systems. In this paper, we demonstrate a new trend of OSNR at different pump configurations: forward, backward and bidirectional pumping for DRFAs as a function of fiber length. We also present the variation of OSNR with both input pump power and input signal power. It is found that forward pumping provides the highest OSNR, reaching its maximum value of 37 dB. However, backward pumping provides the smallest OSNR that has its maximum of 22 dB and the bidirectional pumping provides the moderate OSNR between the others having its peak of 26 dB.     

Splitting-on-demand optical power splitters using multimode interference (MMI) waveguide with programmed modulations

Reconfigurable multi-channel optical power splitter is proposed and its optical properties are calculated. The device can dynamically reconfigure the number of splitting channels by providing programmed refractive index modulations on a multimode interference (MMI) waveguide. A reconfigurable 3-channel optical power splitter is designed to work as 1 × 1, 1 × 2 or 1 × 3 optical power splitter depending on the state of the heat electrodes using thermo-optic modulation, and the input light can be distributed to three output channels with sequential orders. The device can work in the whole C-band (1530-1565 nm) with extinction ratio better than −29.0 dB, excess loss better than −0.45 dB, imbalance better than 0.08 dB and polarization dependent loss (PDL) better than 0.14 dB. The design conception is scalable to a multi-channel splitting-on-demand optical power splitter which can divide input light to 1, 2, …, N output channels equally by using the 3-channel reconfigurable optical power splitter as a building block.     

Simple tunable multiwavelength Brillouin/Erbium fiber laser utilizing short-length photonic crystal fiber

We have demonstrated a simple ring cavity tunable multiwavelength Brillouin/Erbium fiber laser (MWBEFL), in which 70 m highly nonlinear photonic crystal fiber (HNL-PCF) is used as the Brillouin gain medium. The fiber laser utilizes recycling mechanism to enhance stimulated Brillouin scattering (SBS). The configuration that consists of only 3 optical components is easy to be integrated and improves the practicality. At the maximum 1480 nm pump power of 110 mW and the Brillouin pump power of 3 dBm, 10 stable output channels with more than 10 dB optical signal to noise ratio (OSNR) and 0.078 nm channel spacing could achieve 10 nm tuning ranges.     

Boost up of four wave mixing signal in semiconductor optical amplifier for 40 Gb/s optical frequency conversion

The 40 Gb/s optical frequency converter for non-return to zero differential phase shift keying (NRZ-DPSK) signal by using four wave mixing in semiconductor optical amplifier (SOA) have achieved sucessfully. The optimized signal-to-pump ratio for NRZ-DPSK by using optimized SOA structure with enhanced FWM effect is also evaluated. The optimum signal-to-pump ratio is 12 dB and 10 dB with Q factor penalty of 0.685 dB and 0.663 dB. The dependence of four wave mixing efficiency and converted signal power on signal input power is studied and it is evaluated that four wave mixing efficiency decreases with increase in the input power. The impact of pump power, signal-to-pump ratio, and SOA parameters with Q factor penalty for 40 Gb/s has been illustrated. It has shown that converted signal power increases up to the saturation power of semiconductor optical amplifier, then decreases. It is observed that for the optimum pump power, OSNR of converted signal varies little with signal input power.     

Investigation of four wave mixing effect with different number of input channels at various channel spacing

In this paper the design, implementation and performance analysis of four wave mixing (FWM) in optical communication system for different number of input channels is presented using various values of channel spacing. Here, all the input channels have been spaced evenly at various values like 6.25 GHz, 12.5 GHz, 25 GHz, 40 GHz, 50 GHz with the different number of channels at the input i.e. with 2, 4, 6, 8, 12 input channels. The simulation results reveal that the four wave mixing is minimum when the channel spacing is maximum i.e. 50 GHz with minimum number of channels i.e. 2 input channels. It is observed that on increasing the channel spacing, the interference between the input frequencies decreases and hence the four wave mixing also decreases. Also, on increasing the number of input channels/users, the interference between the input frequencies increases and thus, the four wave mixing also increases.     

Investigation of tunable trap filter utilizing intense signal four wave mixing model in highly non-linear fiber

Tunable trap filter based on highly non-linear fiber (HNLF) utilizing degenerate four wave mixing (FWM) model has been proposed and investigated. The output powers in a FWM configuration are calculated as a function of the input signal power, with the result that new-wavelength is efficiently generated, leading to about −26 dB signal depletion. The signal loss spectra are also simulated under the condition of different initial signal powers, injected wavelengths, and fiber lengths.     

A bandwidth-efficient channel allocation scheme for mitigating FWM in ultra-dense WDM-PON

In this paper, we investigate four-wave mixing (FWM) effects in the ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) system and propose an efficient channel allocation scheme to mitigate the FWM impact. This scheme is formed by grouping signal channels into several blocks with different channel spacing. Through numerical analysis and simulations, our proposed scheme is verified to be able to reduce the FWM effects and have higher bandwidth efficiency than the traditional unequal channel allocation scheme. The simulation results also demonstrate that our proposed scheme can achieve nearly 4 dB increases in optical power budget of the UDWDM-PON systems at the BER of 1e−3, in comparison with the equal spaced channel scheme.     

光网络节点设备信噪比恶化的测量

从理论与实验两方面分析了光网络节点设备光信噪比（OSNR）恶化量的测量,导出了单节点设备输出,输入侧光信噪比之差的公式。理论分析表明：节点设备输出,输入侧光信噪比之差值在一般测试条件下不能反映节点设备对光信号的影响,能作为节点设备光信噪比的恶化量来衡量设备对光信号的影响;为使测量结果能反映光节点参数对光信号的综合影响,测试系统应当满足文中给出的增益条件C＝1。实验结果支持了理论分析,实验结果表明,对于C＝1时光信噪比恶化量为4.5dB的光点设备,当C从3变化与0.16时,其输入、输出侧光信噪比之差从1．8dB变化到11．3dB.     

Duty-cycle division multiplexing (DCDM)

A multiplexing technique, which is based on duty-cycle division, is proposed. The channel multiplexing and demultiplexing are performed electrically at the single user bit rate, which is very economic. In a three-user system (3×10 Gb/s), the simulation results show that the best receiver sensitivity value achieved is −30.1 dBm with an optical signal-to-noise ratio (OSNR) of 22.3 dB, while the chromatic dispersion tolerance ranges from 192 to 280 ps/nm. Migration from 30 to 120 Gb/s is achieved with the penalty of 6.4 and 5.2 dB in the receiver sensitivity and OSNR, respectively, for the worst user.     

Influence of degenerate four-wave mixing on the performance of supercontinuum-based multiwavelength optical source

The influence of degenerate four-wave mixing (FWM) on the performance of supercontinuum-based multiwavelength optical source has been investigated in detail experimentally and theoretically. Numerical simulation results show that the degenerate FWM effect has a deteriorative influence on the spectral uniformity and the optical signal-to-noise ratio (OSNR) of supercontinuum-based optical source, and by suppressing degenerate FWM effect the performance enhancement of the supercontinuum can be achieved successfully. These results are also confirmed by our experiments. Experimentally, by suppressing degenerate FWM the crosstalk of adjacent channels to the filtered channel can be reduced by as much as 15 dB, and consequently the measured receiver sensitivity at 10 Gbit/s for the filtered optical source is improved from $-1.7$ to $-17.8$~dBm.     

Four-wave mixing in dual wavelength fiber laser utilizing SOA for wavelength conversion

In this paper, a novel configuration of a wavelength converter is set forth by utilizing a semiconductor optical amplifier (SOA) as a nonlinear gain medium to generate a four-wave mixing (FWM) effect by using a dual wavelength bi-erbium-doped fiber laser that uses an Arrayed Waveguide Grating (AWG) together with two optical channel selector (OSC) as selective elements to function as a dual wavelength switchable pump power. The four-wave mixing (FWM) is produced with a wavelength detuning of 7 nm from the pump and signal which used is as the converted signal at wavelength 1532.8 nm or 1534.5 nm for transferring data from the input signal at wavelength 1547.0 nm. Thus, even though the conversion efficiency is as low as −43 dB, it is still possible for applications as a wavelength converter.     

Amplitude Regenerative Characteristics of RZ-DPSK Wavelength Converter Based on Four-Wave Mixing in SOA

We investigate the phase-preserving amplitude regenerative characteristics of the return-to-zero （RZ） differential- phase-shift-keying （DPSK） wavelength conversion based on four-wave m/xing （FWM） in a semiconductor optical amplifier （SOA）. The Q-factor and the optical signal-to-noise ratio （OSNR） before and after conversion are experimentally obtained and analysed in different input noise power levels. In both the continuous-wave and synchronous clock pumping cases, we find that there is amplitude clamping in the FWM conversion due to the gain saturation of SOA, which can suppress the amplitude fluctuation of the converted DPSK signal before and after demodulation. We have achieved 2-dB Q penalty improvement in our experiment demonstration of lOGbit/s RZ-DPSK signal with OSNR lower than 19dB.     

An ultra compact photonic crystal wavelength division demultiplexer using resonance cavities in a modified Y-branch structure

An ultra small size 4-channel wavelength division demultiplexer based on 2D photonic crystal modified Y-Branch, suitable for integration, is proposed in this paper. The output wavelengths of designed structure can be tuned for communication applications (around 1550 nm) by choosing suitable defect parameters in the corner of each resonance cavity and output waveguides. The cross section of the structure is 313.28 μm² (17.8 μm × 17.6 μm) and desirable for integration based on popular planar technology. The bandwidth of each channel is near to 1 nm and the channel spacing is approximately 3.5 nm and wavelengths of demultiplexer channels are 1548.8 nm, 1551.9 nm, 1555.4 nm and 1559.3 nm respectively. Also, the crosstalk is between −33.1855 dB and −10.4947 dB. Furthermore, the mean values of the crosstalk and quality factor are −22.54 dB and 1496.7 respectively.     

Simulative analysis of pre- and post-compensation using CRZ format in WDM optical transmission link

This paper reports the effects of pre- and post-compensation using CRZ modulation format in long-haul WDM optical transmission link using wavelengths in three bandwidths viz. 1537.4; 1550; 1562.6 nm at per channel bit rates of 10 Gbit/s. It has been investigated here that optimization of dispersion map results in improved management of nonlinear effects in long-haul light wave systems operating in the quasi-linear regime. In addition, pre- and post-dispersion compensation was applied at the transmitter and receiver depending on the signal wavelength, which resulted in improvement of performance metrics viz. Q² (dB), BER and OSNR over longer transmission distances. It is reported here that optimum values of Q² dB of 17.1 dB, BER of 8.4933e⁻⁰¹⁵ and OSNR of 30.1 dB are obtained at 1550 nm at a transmission distance of 7360 km with pre- and post-compensation using CRZ modulation format.     

Liquid crystal enabled multi-channel power equalizer and stabilizer

Automatic power equalization and stabilization of a liquid crystal enabled channel-selectable optical demultiplexer is demonstrated. The variation among output signals from different channels was reduced from ∼10 dB to 0.15 dB with feedback control. The minimum ripple level of a single channel is ∼0.05 dB. As a variable optical attenuator (VOA), the attenuation range of the device is ∼12.6 dB.     

Bit-pattern-dependent non-degenerate four-wave-mixing crosstalk in dispersion-managed DWDM systems including walk-off

In this work, through a detailed derivation process, we obtain a semi-analytic model to deterministically calculate the variance of non-degenerate four-wave-mixing (FWM) noise for return-to-zero (RZ) dispersion and loss-managed system with lumped amplifiers including walk-off between channels. We show that, in general, when the value of duty cycle becomes smaller, the values of variance are more influenced by many factors, such as channel separation, dispersion management scheme and especially relative initial time delays of all interacting channels. For a 17-channel such system, the variance of non-degenerate FWM noise falling on the central channel is given.     

Characteristics of low noise hybrid fiber amplifier

The characteristics of hybrid fiber amplifier (HFA) are investigated. HFA is composed of three stages: short-length EDFA pre-stage, DCF Raman amplifier, and power boosting EDFA. HFA has low noise figure, high output power, and also wide input power dynamic range. Gain control method of HFA is presented experimentally, and the transient gain excursion is suppressed to less than 0.5 dB at 3 dB channel add-drop. HFA can be used as line amplifier in optical transmission link even combined with distributed Raman amplifier due to wide input power dynamic range. The transmission performance of HFA is better than EDFA by more than 1.0 dB of Q-factor in 720 km SMF transmission.     

Performance analysis of NRZ, RZ, CRZ and CSRZ data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD

This paper presents the performance analysis of non-return-to-zero (NRZ), return-to-zero (RZ), chirped return-to-zero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in optical soliton transmission link under the impact of chirp and third-order dispersion (TOD). The performance of these data formats has been analyzed on the basis of certain performance metrics, viz, bit error rate (BER), Q² (dB), OSNR, eye opening, etc. It has been reported here that the performance of CRZ and CSRZ modulation format is better as compared to NRZ and RZ in a soliton transmission link. Further, CSRZ modulation format has been found to deliver optimum performance on the basis of performance evaluation metrics reported in this paper. In case of NRZ and CSRZ, comparatively narrow power spectrum has been observed. Best eye opening, highest value of Q² (dB) of 18 dB and lowest value of BER of the order of 10⁻¹⁶ has been reported in case of CSRZ among the considered data formats. The results have been obtained by varying noise figure from 3.0 to 9.0. No considerable effect of noise was observed. It was observed that at very narrow and ultra short pulse width, OSNR value suffers heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The results were obtained by varying chirp factor from −0.6 to +0.6. Negative chirp resulted in improved OSNR as compared to positive chirp. RZ data format yielded a broader optical spectrum, comparatively low spectral efficiency and poor OSNR thus it was found that RZ format is not suitable for optical soliton transmission under the impact of chirp and TOD.     

Analysis and experiment of all-optical time-interleaved multi-channel regeneration based on higher-order four-wave mixing in a fiber

Simultaneous all-optical multi-channel regeneration based on second-order four-wave mixing (FWM) in a single highly nonlinear fiber (HNLF) is studied. Interchannel crosstalk, especially cross gain saturation and generation of interchannel FWM, is avoided by properly time-interleaved channels. Preliminary experiment of 2 ch × 10 Gbit/s operation shows simultaneous noise reduction of the two channels and good agreement with numerical analysis. Numerical investigation on 2 ch × 40 Gbit/s operation using the same parameters as the experiment shows that the regeneration can improve qualities of both channels. For more number of channels and higher speed of operation, several parameters need to be adjusted to avoid interchannel crosstalks. Time synchronization techniques for the input channels are also discussed.     

Investigation of the optical gain and noise figure for multi-channel amplification in EDFA under optimized pump condition

We present the results of an investigation of optical gain and noise figure for simultaneous multi-channel amplification of an erbium doped fibre amplifier (EDFA) under optimized pump condition. Different pump configurations with varying input signal levels show interesting features on gain flatness. In the experiment, population inversion along the fibre length which determines the gain-spectra and noise characteristics of the amplifier is adjusted through optimized fibre length and injected pump power in order to minimize the gain-tilt at C-band. It is observed that bi-directional pumping manifests the best combination of low noise and high gain of EDFA which are useful as in-line repeaters in WDM network. We obtain 30 ± 1.5 dB intrinsically flat small signal gain from 1538 nm to 1558 nm band of wavelength with noise figure <4 dB for 16-channel simultaneous amplification in a single stage EDFA without gain flattening filter.