Improving the performance of electrical duty-cycle division multiplexing with optimum signal level spacing

Performance optimization of 3 × 10 Gbps conventional electrical-duty-cycle division multiplexing (C-E-DCDM) technique is investigated. It is shown that controlling signal level spacing can optimize its performance. Two level spacing optimization techniques, one in electrical domain and another in optical domain are examined. In general, performance of the C-E-DCDM is improved significantly using both approaches. The results show by optimization, an improvement of around 5.5 dB can be achieved for the C-E-DCDM in terms of receiver sensitivity and optical signal-to-noise ratio using both electrical and optical methods. However, chromatic dispersion tolerance in one of the optimization approaches is degraded by around 34 ps/nm for negative dispersion, while the positive dispersion tolerance improved compared to the C-E-DCDM.     

Three-dimensional polymer optical waveguide interleaver with selectable channel spacing

A polymer optical waveguide interleaver with selectable channel spacing and adjustable flat top based on a three-dimensional structure is reported. This interleaver is made of two waveguides laid in two different but parallel layers. These two waveguides cross-over each other three times, with an electrode heater deposited on top of each crossing point to adjust the coupling ratio between the guides. This 3D structure has many advantages over the corresponding 2D counterpart such as the ease of fabrication and the adjustment of coupling ratio between the two waveguides. The channel spacing can be selected to be either 1.6 nm, 0.8 nm or 0.53 nm simply by changing the power supplied to each heater; also at the 1.6 nm channel spacing it is possible to create a flat-top spectral response (0.6 dB ripple over 0.5 nm), again by tuning the coupling ratios using the electrical heaters.     

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.     

Stable multiwavelength fiber ring laser with equalized power spectrum based on a semiconductor optical amplifier

We experimentally demonstrated a new structure of a multiwavelength semiconductor optical amplifier (SOA) ring laser based on a fiber Sagnac loop filter that can generate up to 25 stable output lasing wavelengths at room temperature. By varying the length of a polarization-maintaining (PM) fiber within the Sagnac loop filter, the wavelength spacing between the output lasing wavelengths can be changed to a desired value. By tuning a polarization controller (PC) within the Sagnac loop filter, stable multiwavelength 1550-nm operation with up to 17 lasing lines within 3 dB power level variation and with a wavelength spacing of ∼0.8 nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40 dB.     

Research of multiple wavelength light source based on super-continuous spectrum

In order to meet the requirements of the multi-wavelength light source of large-capacity, high-speed, long-distance optical communication system, we researched the multi-wavelength light source based on super-continuum (SC), analyzed the main factors in the SC generation, such as dispersion, nonlinear effects. The SC simulation and optimization around the input pulse width, peak power, fiber length, non-linear coefficient parameters for analysis. The optimized results: SC input achieved 25 GHz repetition rate, amplified by high power EDFA, input average power did not exceed 33 dBm. The output of the SC 3 dB bandwidth was greater than 70 nm, after AWG, output 320 wavelengths, wavelength spacing is 0.2 nm (25 GHz), the signal-to-noise ratio was greater than 30 dB.     

Multistage gain-flattened hybrid optical amplifier at reduced wavelength spacing

In this paper, two stage hybrid optical amplifier (HOA) composed of a single erbium doped fiber amplifier and Raman amplifier is proposed for dense wavelength division multiplexed (DWDM) system and investigate the impact of reduced channel spacing. The performance has been evaluated in the term of gain, gain flatness and noise figure. Also, using gain equalization technique, hybrid optical amplifier that has a gain flatness of 3 dB, and a noise figure of less than 7.4 dB is observed.     

Switchable dual-wavelength erbium-doped fiber-ring laser based on one polarization maintaining fiber Bragg grating in a Sagnac loop interferometer

A switchable erbium-doped fiber-ring laser providing dual-wavelength outputs with orthogonal polarizations when operating at room temperature is proposed. One polarization-maintaining fiber Bragg grating (PMFBG) in a Sagnac loop interferometer is used as the wavelength-selective filter. Due to the enhancement of the polarization hole burning (PHB) by the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). The optical signal-to-noise ratio (OSNR) is over 42 dB. The amplitude variation over 90 min is less than 0.6 dB for both wavelengths.     

0.16 nm spaced multi-wavelength Brillouin fiber laser in a figure-of-eight configuration

A stable and compact multi-wavelength Brillouin fiber laser (BFL) operating at room temperature is experimentally demonstrated using a 100 m long photonic crystal fiber (PCF) in conjunction with a figure-of-eight configuration. At a Brillouin pump (BP) level of 15.3 dBm, 7 simultaneous lines with 20 GHz or 0.16 nm line spacing is achieved by removing the odd-order Stokes lines. The anti-Stokes lines are also generated via four wave mixing process in the laser cavity. Compared with the Erbium-based multi-wavelength laser, this BFL has advantages in term of channel spacing and flexibility in the choice of operating wavelength. The output spectrum of the proposed BFL can be tuned by 80 nm, dependent on the availability of an appropriate BP source. The multi-wavelength BFL shows a good stability with power fluctuations of less than 0.5 dB over more than 3 h.     

Tunable dual wavelength fiber laser incorporating AWG and optical channel selector by controlling the cavity loss

In this paper, we propose and demonstrated a dual wavelength fiber laser (DWFL) based on the use of an erbium doped fiber (EDF) gain medium as well as an 1 × 24 Arrayed Waveguide Grating (AWG) together with two optical channel selectors (OCS) to provide channel spacing tunability. The output power of the two wavelengths is equalized by controlling the cavity loss in the DWFL using two Programmable Optical Attenuators (POAs). The widest spacing obtained from the DWFL is 18.13 nm while the narrowest spacing is 0.8 nm. The DWFL has good stability with only minor power fluctuations of less than 1.5 dB and a Side Mode Suppression Ratio (SMSR) of approximately 69.1 dB with peak fluctuations of less than 2.3 dB.     

Analytical model, analysis and parameter optimization of a super linear electro-optic modulator (SFDR > 130 dB)

An analytical model of a super linear optical modulator with high spurious-free-dynamic-range (SFDR > 130 dB) is presented and analyzed. The linear modulator is referred to as IMPACC which stands for Interferometric Modulator with Phase-modulating And Cavity-modulating Components. The modulator is based on a unique combination of a RF-driven phase-modulator (PM) and a ring resonator (RR) within a Mach-Zehnder interferometer (MZI) configuration. Our analysis shows that our design can achieve SFDR values which are ~ 20 dB higher than the standard MZI modulator and 3-5 dB from the Ring Assisted Mach-Zehnder Interferometer (RAMZI) modulator. Both PM and RR in the IMPACC are simultaneously driven by a RF signal of the same frequency, but not necessarily the same amplitudes. The analytical model shows that the combination of these two optical elements, with the proper choice of RF-driving and device parameters, can lead to four important and compelling consequences. First, it offers a wholistic and elegant model in which the standard MZI modulator and the RAMZI modulator are just special cases of IMPACC. Second, the model offers an excellent parameter optimization methodology for fast parameter (internal and/or external) selection and performance evaluation. Third, it provides additional degree of control through the introduction of an external control parameter, the RF power split ratio (F). Lastly, it demonstrates one unique feature of IMPACC such as adaptive SFDR characteristics, where manufacturing tolerances in the transmission coefficient (τ) of the RR can be compensated with proper adjustments of the external parameter of the power split ratio (F).     

Multi parameter optimization of Raman Fiber Amplifier using genetic algorithm for S band dense wavelength division multiplexed system

A simple genetic algorithm is implemented to perform multi parameter optimization of Raman Fiber Amplifier for 100 channel S band dense wavelength division multiplexed system at 25 GHz interval. A cost effective system using single Raman pump is investigated aiming at maximum average gain. The single counter propagating pump is optimized to frequency of 211.528 THz and 652.93 mW power level with optimum Raman fiber length of 44.064 Km. There is evidence to show that the optimum solution presents a small gain variation (less than 3 dB) over an effective bandwidth covering 197–199.475 THz. The optimized configuration enabled an adequate system performance in terms of acceptable Q-factor (19.52 dB) and BER (1.46 × 10⁻²¹).     

Effects of low cut-off frequency of optical receiver on the performance of lightwave systems using pilot tones

We investigate the effects of low cut-off frequency of optical receiver on the performance of lightwave systems. The results show that we can reduce the tone-induced power penalty by ∼0.8 dB (tone frequency = 1 MHz) using a high-pass filter in the optical receiver. In addition, our calculation shows that the power penalty can be negligible (<0.1 dB) even when the low cut-off frequency of the 10 Gb/s optical receiver is increased up to ∼10 MHz.     

Switchable triple-wavelength erbium-doped fiber laser using a single fiber Bragg grating in polarization-maintaining fiber

A stable and narrow wavelength spacing multiwavelength erbium-doped fiber laser is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with a narrow wavelength spacing of less than 0.1 nm via using a single fiber Bragg gratings written in polarization-maintaining (PM) fiber. By adjusting polarization controller, the wavelength spacing of dual-wavelength lasing oscillations can be tuned to as small as 0.032 nm. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The room-temperature operation principle is based on the polarization hole burning and deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL). The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature.     

Performance enhanced down-stream signalling for next generation long-reach 10 Gbit/s passive optical networks

In this paper, we have analyzed the signal processing methods both in digital and optical domain to enhance the transmission performance of downstream signalling in long reach passive optical networks (LR-PONs). The impact of non-linear (NL) equalization through signal processing, i.e. Volterra Equalization (VE), Digital Backpropagation (BP) and Optical Phase Conjugation with Non-linearity Module (OPC-NM) is investigated, in 10 Gbit/s (XG) DP-QPSK long-reach wavelength division multiplexed (WDM) PONs without midspan repeaters over 120 km standard single mode fibre (SMF) link for down-stream signals. Due to the compensation of optical Kerr effects, the sensitivity penalty is reduced to 2 dB by BP algorithm, 1.5 dB by VE algorithm and 2.69 dB by OPC-NM. Moreover, with the implementation of NL equalization technique we are able to get the transmission distance of 126.6 km SMF for the 1:1024 split-ratio at 5 GHz channel spacing in the non-linear region. Furthermore, the concept of super passive optical network (S-PON) is also evaluated, which involves a repeater stage consisting of optical amplifiers, to study the feasibility for receiver side signal processing and simplification.     

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.     

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.     

Design and fabrication of arrayed waveguide grating using fluoropolymer PFS-co-GMA

A 33×33 polymer arrayed waveguide grating (AWG) multiplexer is optimized and fabricated. This device is made of polymeric materials named 2,3,4,5,6-pentafluorostyrene-co-glycidylmethacrylate (PFS-co-GMA). The central wavelength and wavelength spacing are designed to be 1550.918 and 0.8 nm, respectively. The calculated results are: the 3-dB bandwidth is about 0.24 nm, insertion loss is about 8.4 dB and crosstalk is −33.7 dB. The corresponding measured results are: the center wavelength is about 1550.85 nm, wavelength channel spacing is about 0.81 nm, 3-dB bandwidth is about 0.35 nm, crosstalk is about −20 dB, insertion loss is between 10.4 dB for the central port and 11.9 dB for the edge ports.     

1 × N add-drop filter structures using one dense wavelength division multiplexing thin-film filter

This paper proposes 1 × N add-drop filter structures in which only one thin-film filter (TF) is used. Our key idea is based on a combination of an angle-multiplexing concept and the flexibility of the optical fiber to allow a multiwavelength optical beam hit the TF several times, each time at a different angle but same position. Due to the TF angle sensitivity, the desired wavelength optical beam corresponding to the incident angle is therefore spatially isolated from the main optical beam. Our first TF-based 1 × N add-drop filter structure is arranged in a reflective design in which N wavelength optical beams can be dropped out from the main channel. For our transmissive architecture, N − 2 channels are directed to their associated output terminals while the remaining λ_N−1 and λ_N wavelength optical beams are sent out at the same port. Experimental proof of concept for our reflective TF-based 1 × 3 add-drop filter using one off-the-shelf TF, a triple fiber-optic collimator, and an optical circulator separates two wavelength optical beams with their channel spacing of 0.8 nm from the main channel. In this case, measured optical losses of 0.67 dB, 1.66 dB, and 2.59 dB are obtained for the first, the second, and the remaining dropped wavelength optical beams, respectively. Optical crosstalk and polarization dependent loss of <−18 dB and <0.08 dB are also investigated, respectively.     

Priority-based parameter optimization strategy for reducing the effects of four-wave mixing on WDM system

In order to meet the ultra high speed and ultra long-haul transmission distance in wavelength division multiplexing (WDM) systems, the nonlinear impairment affecting the overall spectral efficiency and system performance should be minimized. This paper proposes a strategy to mitigate the four-wave mixing (FWM) effect in WDM system. The strategy determines the effect of both single and combined effects of second, third, and fourth optimization priority parameters such as fiber length, input power, dispersion, channel spacing, and effective area on FWM power. A comparison study was made under different types of optical fiber such as single-mode fiber (SMF), dispersion shifted fiber, non-zero dispersion fiber, and non-zero dispersion shifted fiber. In addition, the system performance in term of bit-error-rate was calculated in the case of single priority (impact of effective area) and combined priority (impact of effective area, input power, fiber length and channel spacing). The results show that the FWM effect was reduced based on the transmission parameters order of optimization, i.e., priority selection proposed. Moreover, the results indicated that increasing sequentially the effective area, fiber length; channel spacing and decreasing the input power provide the most significant sequence in suppressing the effects of FWM. This priority sequence brought the suppression ratio to approximately 26.3% in SMF, which suppressed the FWM effects up to −50 dBm. In term of BER; the combined priority introduces improvement in BER of 2.31 × 10⁻²⁵ in comparison with single priority that has value of BER 4 × 10⁻¹⁴. Finally, this work suggests that the proposed priority-based parameter optimization strategy is an ideal solution for optimum performance of WDM system.     

Tunable 19 × 10 GHz L-band FP-SOA based multi-wavelength mode-locked fiber laser

We present a multi-wavelength mode-locked fiber ring laser incorporating a semiconductor optical amplifier (SOA) and a Fabry-Perot semiconductor optical amplifier (FP-SOA). Because the gain of the SOA is depleted by an external injection optical signal, the SOA acts as a loss modulator. The FP-SOA serves as a tunable comb filter. The presented laser source can generate 19 synchronized wavelength channels with the extinction ratio of about 21 dB, each mode-locked at 10 GHz, and mode-locked pulse width is about 40 ps. Oscillation wavelengths band can be tuned by adjusting the bias current of the SOA, and wavelength spacing also can be changed by using a tunable optical delay line (ODL) or a temperature controller. The polarization-insensitive devices ensure that the output power is rather stable. This fiber laser has potential applications in longer waveband (L-band) within the low-attenuation window.