Influence of co - and counter-propagating light on the phase-mismatch effect in semiconductor optical amplifiers

peer-reviewedThe phase-mismatch effect, caused by the difference in confinement factor between transverse electric (TE) and transverse magnetic (TM) modes, has not been observed in semiconductor optical amplifiers (SOAs) and is not typically accounted for. In this work, we investigate the characteristics of the phase-mismatch four wave mixing (FWM) effect, which occurs in SOA devices. Our results reveal a sinc-like response in the intensity of co- and counter-FWM conjugates as a function of the detuning shift between interacting beams. It is demonstrated that the coherence lengths between the phase match/mismatch cycles differ between co- and counter-propagating nondegenerate FWM experiments and depend on the amplified TE/TM polarizations of interacting waves aligned with respect to the birefringent axes of SOAs. The coherence lengths between each phase match/mismatch cycle in co-propagation experiments are found to be 1600 and infinite GHz, respectively, compared to 800 and 400 GHz found in counter-propagation experiments.ACCEPTEDpeer-reviewe

Low threshold linear cavity mode-locked fiber laser using microfiber-based carbon nanotube saturable absorber

In this work, we demonstrate a linear cavity mode-locked erbium-doped fiber laser in C-band wavelength region. The passive mode-locking is achieved using a microfiber-based carbon nanotube saturable absorber. The carbon nanotube saturable absorber has low saturation fluence of 0.98 µJ/cm2. Together with the linear cavity architecture, the fiber laser starts to produce soliton pulses at low pump power of 22.6 mW. The proposed fiber laser generates fundamental soliton pulses with a center wavelength, pulse width, and repetition rate of 1557.1 nm, 820 fs, and 5.41 MHz, respectively. This mode-locked laser scheme presents a viable option in the development of low threshold ultrashort pulse system for deployment as a seed laser

WDM for Multi-user Indoor VLC Systems with SCM

A system that employs wavelength division multiplexing (WDM) in conjunction with subcarrier multiplexing (SCM) tones is proposed to realize high data rate multi-user indoor visible light communication (VLC). The SCM tones, which are unmodulated signals, are used to identify each light unit, to find the optimum light unit for each user and to calculate the level of the co-channel interference (CCI). WDM is utilized to attain a high data rate for each user. In this paper, multicolour (four colours) laser diodes (LDs) are utilized as sources of lighting and data communication. One of the WDM colours is used to convey the SCM tones at the beginning of the connection to set up the connection among receivers and light units (to find the optimum light unit for each user). To evaluate the performance of our VLC system, we propose two types of receivers: an array of non-imaging receivers (NI-R) and an array of non-imaging angle diversity receivers (NI-ADR). In this paper, we consider the effects of diffuse reflections, CCI and mobility on the system performance

Application of MFI-UF fouling index with NOM fouling under various operating conditions

Fouling research with polymeric membranes has demonstrated that various natural organic matter (NOM) fractions contribute differentially to membrane fouling behavior. However, limited studies exist analyzing the sensitivity of the MFI-UF to be used as a tool to differentiate NOM fouling components. The results here indicate that MFI-UF is a suitable tool for assessing NOM fouling. Specifically, NOM fouling potential was in the order of organic proteins (as BSA), polymers (alginate), and humic acid, respectively. Further, a mixed solution containing BSA, alginate and humic acid fouled similarly to the BSA solution indicating the high fouling potential of organic proteins in membrane systems. The MFI-UF value was found to increase by > 30% with increasing pressure (1–3 bar) and decreasing temperature (35°C–5°C). The filtered water volume was found to correlate with the MFI-UF values indicating the dependency of the method on testing conditions. Incorporating water viscosity and pressure values against normalized conditions (20°C and 2 bar) with the standard MFI-UF equation was found to be useful to estimate MFI-UF values at variable operating conditions, thus, enhances the potential application range of MFI-UF as a fouling index for NOM

Effects of feed water temperature on irreversible fouling of ceramic ultrafiltration membranes

Temperature is known to influence the filtration performance of membrane systems through its direct impact on water viscosity. This research demonstrates that the changes in natural organic matter (NOM) fouling behavior with temperature are over and beyond simple viscosity changes in water. Constant flux experiments were performed in a tubular ceramic ultrafiltration (UF) system at 5, 20, and 35 °C. The unified membrane fouling index (UMFI) was used to identify NOM reversible and irreversible fouling mechanisms; while the modified UF fouling index (MFI-UF) was used to predict the fouling potential of NOM. The results showed that after correcting for viscosity to standard 20 °C compared to 5 °C, UMFI values were higher than expected and reflected the higher fouling irreversibility observed at the lower temperature. The lower water temperature resulted in an increase in NOM retention along with decrease in backwash and chemical cleaning effectiveness as determined by the UMFI and FEEM analyses. However, increased water temperature did not adversely impact existing backwash or chemical cleaning protocols. In addition, The MFI-UF exhibited the same trend as UMFI for establishing NOM fouling and retention, and therefore, the MFI-UF method is suitable for use as fouling predictor with ceramic membrane systems

Comparisons of NOM fouling and cleaning of ceramic and polymeric membranes during water treatment

This research examines the effect of various NOM fractions on ceramic and polymeric UF membranes performance in terms of fouling and cleaning. Fouling experiments were performed using five model solutions, humic acid, protein as bovine serum albumin (BSA), alginate with and without calcium, and a combined NOM mixture. Two chemical agents were selected: an oxidant (NaOCl) and caustic (NaOH). Fouling and cleaning behavior were assessed using the resistance in series (RIS) model, membrane permeability, carbon mass balance, and fluorescence excitation and emission matrix (FEEM) analysis. The results demonstrated that NOM fouling order of the ceramic UF was similar to polymeric UF with the following trend: NOM mixture ≈BSA > alginate ±Ca+2 > humic acid. However, the backwash efficiency was 1.5×–2× higher for the ceramic UF in comparison to the polymeric UF, indicating a much higher hydraulic reversibility for the ceramic UF. A carbon mass balance in compliment with FEEM plots determined that NOM removal by the ceramic UF was ≈ 10% higher than the polymeric UF. Chemical cleaning was found to be effective for both membrane types. Thus, it was not possible to conclude, that the ceramic membrane demonstrated an advantage for chemical cleaning under the conditions studied

Chemical cleaning of ceramic ultrafiltration membranes – Ozone versus conventional cleaning chemicals

This study investigates chemical cleaning mechanisms of a tubular ceramic UF membrane. The effect of cleaner type (ozone (O 3 ), sodium hypochlorite (NaOCl) and sodium hydroxide (NaOH)), clean in place (CIP) pH (11 vs. 12), and cleaning sequence on the removal of irreversible fouling of hydrophobic (humic acids) and hydrophilic (alginate with and without calcium (alginate + Ca +2 and alginate - Ca +2 , respectively)) NOM fractions were investigated. Results showed that different NOM types responded differently to chemical cleaning. Alginate-Ca +2 and humic acids were equivalently removed by NaOCl or NaOH whereas a lower cleaning efficiency of alginate + Ca +2 was observed. Increasing the pH of NaOCl and NaOH CIP increased the removal of the chemically reversible fouling index (UMFI cr ). The efficiency of NaOCl was always lower than that of NaOH at the same pH, which was attributed to surface tension (λ) differences in the CIP water and potential differences in cleaning mechanism. The ceramic UF CIP cleaning using O 3 (0.50 mg O 3 /mgC) for 1 h demonstrated higher cleaning efficiency for humic acids and alginate ± Ca +2 , (%UMFI cr > 98%), than NaOCl or NaOH alone (%UMFI cr >80%). The O 3 CIP was as effective as 4 h cleaning using a sequential NaOH/NaOCl or combined NaOCl + NaOH CIP

Influence of co - and counter-propagating light on the phase-mismatch effect in semiconductor optical amplifiers

The phase-mismatch effect, caused by the difference in confinement factor between transverse electric (TE) and transverse magnetic (TM) modes, has not been observed in semiconductor optical amplifiers (SOAs) and is not typically accounted for. In this work, we investigate the characteristics of the phase-mismatch four wave mixing (FWM) effect, which occurs in SOA devices. Our results reveal a sinc-like response in the intensity of co- and counter-FWM conjugates as a function of the detuning shift between interacting beams. It is demonstrated that the coherence lengths between the phase match/mismatch cycles differ between co- and counter-propagating nondegenerate FWM experiments and depend on the amplified TE/TM polarizations of interacting waves aligned with respect to the birefringent axes of SOAs. The coherence lengths between each phase match/mismatch cycle in co-propagation experiments are found to be 1600 and infinite GHz, respectively, compared to 800 and 400 GHz found in counter-propagation experiments

Phase-mismatch dependence of the four-wave mixing effect in semiconductor optical amplifiers

The phase-mismatch effect due to polarization-dependent mode confinement factor has been shown to be not a crucial problem in semiconductor optical amplifiers (SOAs) and is usually not accounted for. The phase-mismatch four-wave mixing (FWM) process in SOA devices is experimentally reported. The results reveal a sinc-like behavior in the intensity of FWM conjugate as a function of wavelength separation between transverse electric (TE)/transverse magnetic (TM) pumps due to induced confinement factors difference. Efficient FWM occurred for a detuning shift of about 500 GHz, limited by phase-mismatch conditions and determined by coherence length required for low and high frequencies to complete a full phase-match cycle. Phase-match FWM with an infinity coherence length can be fulfilled by proper alignments of co-polarized TE/TM modes of input waves with respect to the birefringent axes of the device structure