F Ahmad - Academia.edu (original) (raw)

Papers by F Ahmad

Microwave and Optical Technology Letters, 2015

The aim of this article is to report the fabrication and characterization of dispersed multiwalle... more The aim of this article is to report the fabrication and characterization of dispersed multiwalled carbon nanotubes (MWCNTs) polymer composites and use them as passive SAs in Q-switched Erbiumdoped fiber (EDF) for ultrafast laser generation at 1.5-mm region. CNT polymer composites were fabricated into thin films by homogenizing different concentrations (1.8, 4.8, and 10.1 wt. %) of MWCNTs with host polymers polyethylene oxide. The fabricated thin film was then deposited between two fiber connectors as passive SA and incorporated into fiber laser's ring oscillator. The EDFL exhibited a central wavelength of 1560.5 nm when pump power is fixed at 42 mW. The Q-switching operation starts at the lowest threshold of 39.3 mW for all the three different SA concentrations. For 1.8 wt. % MWCNTs concentration, the pulse repetition rate of the Q-switched EDFL is started from 35.6 kHz, while for 4.8 wt. % and 10.1 wt. %, the pulse repetition rate started at 78.5 and 66.1 kHz, respectively. At the MWCNTs concentration of 1.8 wt. %, the pulse energy of the Q-switched EDFL increases from 7.9 to 24.7 nJ as the pump power increases from 39.3 to 58.8 mW. V

Chinese Optics Letters, 2015

We propose and demonstrate a passively mode-locked fiber laser operating at 1951.8 nm using a com... more We propose and demonstrate a passively mode-locked fiber laser operating at 1951.8 nm using a commercial thulium-doped fiber (TDF) laser, a homemade double-clad thulium-ytterbium co-doped fiber (TYDF) as the gain media, and a multi-walled carbon nanotube (MWCNT) based saturable absorber (SA). We prepare the MWCNT composite by mixing a homogeneous solution of MWCNTs with a diluted polyvinyl alcohol (PVA) polymer solution and then drying it at room temperature to form a film. The film is placed between two fiber connectors as a SA before it is integrated into a laser ring cavity. The cavity consists of a 2 m long TDF pumped by a 800 nm laser diode and a 15 m long homemade TYDF pumped by a 905 nm multimode laser diode. A stable mode-locking pulse with a repetition rate of 34.6 MHz and a pulse width of 10.79 ps is obtained when the 905 nm multimode pump power reaches 1.8-2.2 W, while the single-mode 800 nm pump power is fixed at 141.5 mW at all times. To the best of our knowledge, this is the first reported mode-locked fiber laser using a MWCNT-based SA. OCIS codes: 060.3510, 320.7090.

Journal of Chromatography B: Biomedical Sciences and Applications, 1992

A chromatographic method for the specific determination of glutathione in malignant cell lines is... more A chromatographic method for the specific determination of glutathione in malignant cell lines is described. The method is based on the ability of glutathione-S-transferase to specifically and quantitatively conjugate glutathione to 1-chloro-2,4-dinitrobenzene and chromatographic quantitation of the resultant conjugate, dinitrophenyl-S-glutathione, by reversed-phase liquid chromatography. The assay can be performed on 20,000 g supernatants of cell homogenates without acid extraction. 2-Mercaptoethanol, a sulfhydryl compound often used as a thiol-protective agent to preserve enzymatic activities of a number of enzymes, did not interfere with glutathione determination by this method. The dinitrophenyl-S-glutathione isolated from either standard glutathione samples or from cell homogenates was shown to be identical to authentic dinitrophenyl-S-glutathione using mass spectrometry. Recovery of glutathione in standard samples by the current method was identical to that determined using 5,5'-dithiobis(2-nitrobenzoic acid). Exogenous glutathione added to supernatants of cell homogenate in the presence or absence of 2-mercaptoethanol was also completely recovered.

Chinese Optics Letters, 2014

We propose a Q-switched Er-doped fiber laser (EDFL) with a threshold pumping power as low as 7.4 ... more We propose a Q-switched Er-doped fiber laser (EDFL) with a threshold pumping power as low as 7.4 mW, and demonstrate using graphene polyvinyl alcohol (PVA) thin film as a passive saturable absorber (SA). The SA is fabricated from graphene flakes, which is synthesized by electrochemical exfoliation of graphite at room temperature in 1% sodium dodecyl sulfate aqueous solution. The flakes are mixed with PVA solution to produce a thin film, which is then sandwiched between two ferrules to form a SA and integrated in the EDFL ring cavity to generate a stable Q-switched pulse train. The pulse train operates at 1560 nm with a threshold pump power of 7.4 mW. At maximum 1480 nm pump power of 33.0 mW, the EDFL generates an optical pulse train with a repetition rate of 27.0 kHz and pulse width of 3.56 μs. The maximum pulse energy of 39.4 nJ is obtained at a pump power of 14.9 mW. This laser can be used as a simple and low-cost light source for metrology, environmental sensing, and biomedical diagnostics.

A Q-switched ytterbium-doped fiber laser (YDFL) is proposed and demonstrated using a newly develo... more A Q-switched ytterbium-doped fiber laser (YDFL) is proposed and demonstrated using a newly developed multi-walled carbon nanotubes polyethylene oxide (MWCNTs-PEO) film as a passive saturable absorber (SA). The saturable absorber is prepared by mixing the MWCNTs homogeneous solution into a dilute PEO polymer solution before it is left to dry at room temperature to produce thin film. Then the film is sandwiched between two FC/PC fiber connectors and integrated into the laser cavity for Q-switching pulse generation. The laser generates a stable pulse operating at wavelength of 1060.2 nm with a threshold pump power of 53.43 mW. The YDFL generates a stable pulse train with repetition rates ranging from 7.92 to 24.27 kHz by varying 980-nm pump power from 53.42 to 65.72 mW. At 59.55-mW pump power, the lowest pulse width and the highest pulse energy are obtained at 12.18 µs and 143.5 nJ, respectively.

IET Optoelectronics, 2015

Chinese Optics Letters, 2013

We report a ring cavity passively harmonic mode-locked fiber laser using a newly developed thuliu... more We report a ring cavity passively harmonic mode-locked fiber laser using a newly developed thuliumbismuth co-doped fiber (TBF) as a gain medium in conjunction with a carbon nanotube (CNT)-based saturable absorber. The TBF laser generates a third harmonic mode-locked soliton pulse train with a high repetition rate of 50 MHz and a pulse duration of 1.86 ps. The laser operates at 1 901.6 nm with an average power of 6.6 mW, corresponding to a pulse energy of 0.132 nJ, at a 1 552 nm pump power of 723.3 mW. OCIS codes: 320.0320, 140.0140.

2012 International Conference on Open Source Systems and Technologies, 2012

Archives of Biochemistry and Biophysics, 1962

A study has been made of the acid pyrophosphatase, carbamoylphosphate ornithinecarbamoyl(CPOC) tr... more A study has been made of the acid pyrophosphatase, carbamoylphosphate ornithinecarbamoyl(CPOC) transferase, invertase, and malic dehydrogenase activities of Saccharomyces cerevisiae grown in a defined medium containing either an optimal or a suboptimal concentration of n-biotin; the effect of supplementing the biotin-deficient medium with L-aspartic acid and/or oleic acid was also examined. Acid pyrophosphatase, invertase, and malic dehydrogenase activities were frequently higher in yeast grown under conditions of varying biotin deficiency than in biotin-optimal yeast. It is concluded that synthesis of these enzymes is not specifically biotin dependent and that the different activities in biotin-deficient yeast are probably the result of an alteration in the normal induction and repression mechanisms and possibly also of a decreased synthesis of repressor substances. CPOC transferase activity was always low in yeast grown in biotin-deficient media as compared with biotinoptimal yeast, and it is suggested that synthesis of this enzyme may be specifically biotin dependent. Yeast grown in biotin-deficient medium, with or without oleic acid, was shown to be more fragile than yeast grown in biotin-optimal medium or in biotindeficient medium supplemented with aspartic acid; these observations are discussed in relation to changes in the cell wall metabolism of yeast accompanying biotin deficiency.

Microwave and Optical Technology Letters, 2015

The aim of this article is to report the fabrication and characterization of dispersed multiwalle... more The aim of this article is to report the fabrication and characterization of dispersed multiwalled carbon nanotubes (MWCNTs) polymer composites and use them as passive SAs in Q-switched Erbiumdoped fiber (EDF) for ultrafast laser generation at 1.5-mm region. CNT polymer composites were fabricated into thin films by homogenizing different concentrations (1.8, 4.8, and 10.1 wt. %) of MWCNTs with host polymers polyethylene oxide. The fabricated thin film was then deposited between two fiber connectors as passive SA and incorporated into fiber laser's ring oscillator. The EDFL exhibited a central wavelength of 1560.5 nm when pump power is fixed at 42 mW. The Q-switching operation starts at the lowest threshold of 39.3 mW for all the three different SA concentrations. For 1.8 wt. % MWCNTs concentration, the pulse repetition rate of the Q-switched EDFL is started from 35.6 kHz, while for 4.8 wt. % and 10.1 wt. %, the pulse repetition rate started at 78.5 and 66.1 kHz, respectively. At the MWCNTs concentration of 1.8 wt. %, the pulse energy of the Q-switched EDFL increases from 7.9 to 24.7 nJ as the pump power increases from 39.3 to 58.8 mW. V

Chinese Optics Letters, 2015

We propose and demonstrate a passively mode-locked fiber laser operating at 1951.8 nm using a com... more We propose and demonstrate a passively mode-locked fiber laser operating at 1951.8 nm using a commercial thulium-doped fiber (TDF) laser, a homemade double-clad thulium-ytterbium co-doped fiber (TYDF) as the gain media, and a multi-walled carbon nanotube (MWCNT) based saturable absorber (SA). We prepare the MWCNT composite by mixing a homogeneous solution of MWCNTs with a diluted polyvinyl alcohol (PVA) polymer solution and then drying it at room temperature to form a film. The film is placed between two fiber connectors as a SA before it is integrated into a laser ring cavity. The cavity consists of a 2 m long TDF pumped by a 800 nm laser diode and a 15 m long homemade TYDF pumped by a 905 nm multimode laser diode. A stable mode-locking pulse with a repetition rate of 34.6 MHz and a pulse width of 10.79 ps is obtained when the 905 nm multimode pump power reaches 1.8-2.2 W, while the single-mode 800 nm pump power is fixed at 141.5 mW at all times. To the best of our knowledge, this is the first reported mode-locked fiber laser using a MWCNT-based SA. OCIS codes: 060.3510, 320.7090.

Journal of Chromatography B: Biomedical Sciences and Applications, 1992

A chromatographic method for the specific determination of glutathione in malignant cell lines is... more A chromatographic method for the specific determination of glutathione in malignant cell lines is described. The method is based on the ability of glutathione-S-transferase to specifically and quantitatively conjugate glutathione to 1-chloro-2,4-dinitrobenzene and chromatographic quantitation of the resultant conjugate, dinitrophenyl-S-glutathione, by reversed-phase liquid chromatography. The assay can be performed on 20,000 g supernatants of cell homogenates without acid extraction. 2-Mercaptoethanol, a sulfhydryl compound often used as a thiol-protective agent to preserve enzymatic activities of a number of enzymes, did not interfere with glutathione determination by this method. The dinitrophenyl-S-glutathione isolated from either standard glutathione samples or from cell homogenates was shown to be identical to authentic dinitrophenyl-S-glutathione using mass spectrometry. Recovery of glutathione in standard samples by the current method was identical to that determined using 5,5'-dithiobis(2-nitrobenzoic acid). Exogenous glutathione added to supernatants of cell homogenate in the presence or absence of 2-mercaptoethanol was also completely recovered.

Chinese Optics Letters, 2014

We propose a Q-switched Er-doped fiber laser (EDFL) with a threshold pumping power as low as 7.4 ... more We propose a Q-switched Er-doped fiber laser (EDFL) with a threshold pumping power as low as 7.4 mW, and demonstrate using graphene polyvinyl alcohol (PVA) thin film as a passive saturable absorber (SA). The SA is fabricated from graphene flakes, which is synthesized by electrochemical exfoliation of graphite at room temperature in 1% sodium dodecyl sulfate aqueous solution. The flakes are mixed with PVA solution to produce a thin film, which is then sandwiched between two ferrules to form a SA and integrated in the EDFL ring cavity to generate a stable Q-switched pulse train. The pulse train operates at 1560 nm with a threshold pump power of 7.4 mW. At maximum 1480 nm pump power of 33.0 mW, the EDFL generates an optical pulse train with a repetition rate of 27.0 kHz and pulse width of 3.56 μs. The maximum pulse energy of 39.4 nJ is obtained at a pump power of 14.9 mW. This laser can be used as a simple and low-cost light source for metrology, environmental sensing, and biomedical diagnostics.

A Q-switched ytterbium-doped fiber laser (YDFL) is proposed and demonstrated using a newly develo... more A Q-switched ytterbium-doped fiber laser (YDFL) is proposed and demonstrated using a newly developed multi-walled carbon nanotubes polyethylene oxide (MWCNTs-PEO) film as a passive saturable absorber (SA). The saturable absorber is prepared by mixing the MWCNTs homogeneous solution into a dilute PEO polymer solution before it is left to dry at room temperature to produce thin film. Then the film is sandwiched between two FC/PC fiber connectors and integrated into the laser cavity for Q-switching pulse generation. The laser generates a stable pulse operating at wavelength of 1060.2 nm with a threshold pump power of 53.43 mW. The YDFL generates a stable pulse train with repetition rates ranging from 7.92 to 24.27 kHz by varying 980-nm pump power from 53.42 to 65.72 mW. At 59.55-mW pump power, the lowest pulse width and the highest pulse energy are obtained at 12.18 µs and 143.5 nJ, respectively.

IET Optoelectronics, 2015

Chinese Optics Letters, 2013

We report a ring cavity passively harmonic mode-locked fiber laser using a newly developed thuliu... more We report a ring cavity passively harmonic mode-locked fiber laser using a newly developed thuliumbismuth co-doped fiber (TBF) as a gain medium in conjunction with a carbon nanotube (CNT)-based saturable absorber. The TBF laser generates a third harmonic mode-locked soliton pulse train with a high repetition rate of 50 MHz and a pulse duration of 1.86 ps. The laser operates at 1 901.6 nm with an average power of 6.6 mW, corresponding to a pulse energy of 0.132 nJ, at a 1 552 nm pump power of 723.3 mW. OCIS codes: 320.0320, 140.0140.

2012 International Conference on Open Source Systems and Technologies, 2012

Archives of Biochemistry and Biophysics, 1962

A study has been made of the acid pyrophosphatase, carbamoylphosphate ornithinecarbamoyl(CPOC) tr... more A study has been made of the acid pyrophosphatase, carbamoylphosphate ornithinecarbamoyl(CPOC) transferase, invertase, and malic dehydrogenase activities of Saccharomyces cerevisiae grown in a defined medium containing either an optimal or a suboptimal concentration of n-biotin; the effect of supplementing the biotin-deficient medium with L-aspartic acid and/or oleic acid was also examined. Acid pyrophosphatase, invertase, and malic dehydrogenase activities were frequently higher in yeast grown under conditions of varying biotin deficiency than in biotin-optimal yeast. It is concluded that synthesis of these enzymes is not specifically biotin dependent and that the different activities in biotin-deficient yeast are probably the result of an alteration in the normal induction and repression mechanisms and possibly also of a decreased synthesis of repressor substances. CPOC transferase activity was always low in yeast grown in biotin-deficient media as compared with biotinoptimal yeast, and it is suggested that synthesis of this enzyme may be specifically biotin dependent. Yeast grown in biotin-deficient medium, with or without oleic acid, was shown to be more fragile than yeast grown in biotin-optimal medium or in biotindeficient medium supplemented with aspartic acid; these observations are discussed in relation to changes in the cell wall metabolism of yeast accompanying biotin deficiency.