Nam Quoc Ngo

Nanyang Technological University, Singapore, Singapore

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Publications (153)231.48 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A Sagnac loop filter with two pieces of high birefringence fiber having equal lengths, and spliced together at a fixed angle of 30o displacement between the two principle axes, is proposed in this letter. Gain equalization of erbium-doped fiber amplifiers (EDFAs) is implemented by tuning only the polarization controller in the loop filter. Experimental result shows that there remains a deviation of \pm 1 dB in the region of the flattened profile with the useful bandwidth of about 23 nm, thereby demonstrating the effectiveness of the method. An effective mathematical model and mechanism is also given for further explanation.
    Chinese Optics Letters 06/2012; 10(7):070604-070604. · 1.07 Impact Factor
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    Thomas Y. L. Ang, Nam Quoc Ngo
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    ABSTRACT: We utilize the contra-propagating cavity modes that arise from the evanescent coupling of both the resonators to the bus waveguide in a twin coupled traveling-wave microresonators (MRs) system to generate flat-band slow light (SL). The contra-propagating cavity modes will generate multipeaks in the resonance spectra. Flat-band SL can be generated if such multipeaks become undistinguishable and merge into one single broadened peak that is maximally flat when the inter-resonator coupling strength is optimized relative to the resonators-to-bus-waveguide coupling strengths. The bandwidth and the group delay can be tuned by adjusting the coupling strengths. It is shown that the delay-bandwidth products of the output light at the through (reflection) port are 3- to 12-fold (6- to 24-fold) higher than that of conventional MR-based SL systems. Fabrication tolerance and cavity losses analyses have also revealed that the proposed scheme is rather robust to the fabrication errors and limitations of current state-of-the-art semiconductor processing technology.
    Journal of the Optical Society of America B 05/2012; 29(5):924-. · 1.81 Impact Factor
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    Thomas Y. L. Ang, Nam Quoc Ngo
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    ABSTRACT: We look into the use of the intracavity backscattering in twin-coupled traveling wave microresonators (TWMRs) to generate enhanced coupled-resonator-induced transparency (CRIT) and optical Fano resonance (OFR). Intracavity backscattering makes it possible to either generate a single CRIT peak or a pair of CRIT peaks within one free spectral range in the transmission spectrum. The distance between the twin-CRIT peaks can be tuned by controlling the intracavity backscattering strength. Also, the use of intracavity backscattering allows the simultaneous production of both fast and slow light effects. In addition, it is found that the symmetric CRIT peaks can be reshaped into asymmetric OFR line shapes either by using TWMRs with different intracavity backscattering strengths when one input is launched into the circuit or by modulating the phase/amplitude difference between the dual contrapropagating inputs, which are launched into the circuit in the presence of intracavity backscattering. These allow switching between CRIT and OFR to be realized in the absence of gain or phase tuning elements in the cavities, unlike conventional twin-coupled TWMR systems.
    Journal of the Optical Society of America B 05/2012; 29(5):1094-. · 1.81 Impact Factor
  • Nam Quoc Ngo
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    ABSTRACT: This paper presents an overview of recent advances and applications of temporal optical differentiators (ODs) and temporal optical integrators (OIs). The existing applications of ODs include ultrawideband communications (UWB), optical metrology, dispersion-managed soliton pulse generation for ultrahigh-speed soliton transmission systems, ultrashort flat-top pulse generation for the demultiplexing of 640 Gbit/s data in ultrahigh-speed optical communication systems, and optical computing. The existing applications of OIs include optical computing, optical memory and optical analog-to-digital converter (OADC). Two novel applications of an OD are also suggested here and they are laser induced breakdown spectroscopy (LIBS) and non-return-to-zero to pseudo-return-to-zero (NRZ-to-PRZ) data conversion for optical clock recovery in optical communications. The focus here is on amplitude-based rather than intensity-based ODs and OIs.
    Information Science, Signal Processing and their Applications (ISSPA), 2012 11th International Conference on; 01/2012
  • Thomas Y. L. Ang, Nam Quoc Ngo
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    ABSTRACT: We analytically characterize the temperature dependence of Raman amplification, Raman attenuation, and parametric Raman wavelength conversion (PRWC) in submicrometer silicon waveguides (WGs) over the temperature range of 100 to 500 K , near the O-band and C-band wavelengths of 1.33 and 1.55 μm . The efficiencies of Raman amplification/attenuation and PRWC are studied in the context of how the interplay among the Raman gain, two-photon absorption, free-carrier absorption, sidewall roughness, pump-signal-input intensity ratio, and phase matching condition influences the wave propagation in the submicrometer WG at different temperatures. Our results show that the effects of temperature variation can be harnessed to enhance and tune the Raman amplification/attenuation and PRWC. This offers a more dynamic control of the Raman performances of submicrometer silicon WG devices as compared to conventional silicon Raman WG devices operating at a fixed (room) temperature.
    Journal of the Optical Society of America B 05/2011; 28(6):1556-1565. · 1.81 Impact Factor
  • Source
    Nam Quoc Ngo, Yufeng Song
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    ABSTRACT: We show analytically and numerically that a practically realizable first-order optical Hilbert transformer (OHT) can simultaneously function as a negative/positive first-order temporal optical differentiator (OD) in the stopband and a broadband ±90° phase shifter in the passbands. An integrated-optic OHT based on a four-tap finite impulse response filter is designed using the Remez iteration algorithm and is numerically verified.
    Optics Letters 03/2011; 36(6):915-7. · 3.39 Impact Factor
  • Nam Quoc Ngo, Yufeng Song, Bo Lin
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    ABSTRACT: We present the design and analysis of a wideband and tunable optical Hilbert transformer (OHT) using a tunable waveguide-based finite-impulse response (FIR) filter structure by using the digital filter design method and the Remez algorithm. The tunable Nth-order waveguide-based FIR filter, which simply consists of N delay lines, N tunable couplers, N tunable phase shifters and a combiner, can be tuned, by thermally adjusting the tunable couplers and tunable phase shifters, to tune the bandwidth of an OHT using silica-based planar lightwave circuit (PLC) technology. To demonstrate the effectiveness of the method, the simulation results have an excellent agreement with the theoretical predictions. The tunable OHT can function as a wideband and tunable 90° phase shifter and thus has many potential applications. The two unique features of wideband characteristic (up to ~ 2 THz) and tunable bandwidth (THz tuning range) of the proposed OHT cannot be obtained from the existing OHTs.
    Optics Communications 02/2011; 284(3):787-794. · 1.54 Impact Factor
  • Thomas Y. L. Ang, Nam Quoc Ngo
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    ABSTRACT: We present a theoretical investigation on the interplay between the intra-cavity backscattering and the losses out of the cavity when dual contra-propagating inputs are launched into a traveling wave microresonator, which has random surface defects (or backscatters) that are intentionally introduced. By adjusting the amplitude and/or phase differences between the dual inputs, the interaction of the cavity modes with the backscatters can be controlled. Consequently, the transmission and dispersion of the output light can be easily manipulated. This feature makes the dual input scheme highly attractive for continuously tunable fast and slow light applications, particularly if active tuning elements, such as p - i - n diode and heater, are absent in the cavity. Continuous tunability in the group delay of the fast and slow light is also demonstrated at the C-band wavelength of 1.55 μ m .
    Journal of the Optical Society of America B 11/2010; 27(12):2774-2783. · 1.81 Impact Factor
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    ABSTRACT: A simple ultra-wideband (UWB) impulse radio transmitter is experimentally demonstrated using an electrooptic phase modulator (PM) and a delay interferometer (DI). By applying an electrical nonreturn-to-zero (NRZ) baseband signal to the PM, a pair of optical Gaussian pulses with opposite polarities is generated at the two output ports of the DI. By properly setting the delay between these two polarity-reversed optical Gaussian pulses using an optical delay line (ODL) component, an optical UWB monocycle pulse is generated. Pulse amplitude modulation of the UWB monocycle pulse can also be achieved by driving the PM with an NRZ baseband signal at a flexible bit rate. Experimental results verify that the proposed scheme can be used to simultaneously generate and amplitude modulate the UWB monocycle pulse. Furthermore, both the shape and polarity of the generated UWB monocycle pulse can be easily controlled by appropriately adjusting the delay of the ODL. Pulse polarity modulation of the generated UWB monocycle can also be realized by electrically switching the bias voltage of the DI.
    IEEE Photonics Technology Letters 10/2010; 22(20):1479-1481. · 2.18 Impact Factor
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    ABSTRACT: Inverse-Gaussian apodized fiber Bragg gratings (IGAFBGs) are numerically studied using the transfer matrix method and fabricated by the commonly used phase-mask scanning technique in a single-step scanning process. The IGAFBG can serve as a dual-wavelength passband filter, whose wavelength spacing can be continuously tuned by introducing a tunable chirp through applying a strain gradient in principle. Also, an IGAFBG with identical dual passbands having 0.144nm wavelength spacing is experimentally achieved. We also show that an IGAFBG can act as a multipassband filter with varied free spectral ranges (FSRs), and the largest FSR variation of this IGAFBG is nearly seven times more than that in a comparable FBG pair filter. An IGAFBG with varied FSRs of approximately 16.125, approximately 12.25, approximately 8.5, and approximately 6.375GHz is fabricated. This multipassband varying-FSR IGAFBG filter can find applications in step-tunable microwave generations.
    Applied Optics 09/2010; 49(25):4715-22. · 1.69 Impact Factor
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    ABSTRACT: A optical filter based on Sagnac interferometer was proposed to be acted as a comb filter with equal frequency spacing and good signal to noise ratio (SNR), which was composed of an 8.14 m stress-induced Hi-Bi (high-birefringence) PM (polarization-maintaining) fiber. Using this multi-wavelength Sagnac comb filter and a gain flattening Sagnac filter that made the output spectra flattening at different pump powers, a 25-channel multi-wavelength all-fiber source were successfully generated with channel spacing of 0.8 nm with respect to the center wavelength at 1550 nm and flattened gain about ±1 dB peak deviation. The channel spacing can be further reduced to 0.4 nm to produce a DWDM (dense wavelength division multiplexing) source, simply by increasing the Hi-Bi fiber to be 16.28 m. It can be used in many applications such as WDM (wavelength division multiplexing), optical amplifiers with a high SNR, narrow band filters and optical sensors.
    Laser Physics 07/2010; 20(7):1625-1628. · 1.03 Impact Factor
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    ABSTRACT: An all-optical scheme for simultaneously realizing OR and AND logic gates based on three-input four-wave mixing (FWM) arising in a single semiconductor optical amplifier (SOA) is proposed and demonstrated. It has the ability to process not only conventional non-return-to-zero-ON–OFF-keying (NRZ-OOK) and return-to-zero-OOK (RZ-OOK) formats but also carrier-suppressed return-to-zero-OOK (CSRZ-OOK) format signals. Firstly, the performance of 40Gb/s logic operation is numerically evaluated by a comprehensive dynamic SOA model considering three input signal induced FWM effect. Then, 10Gb/s experimental demonstrations with clear waveforms and high extinction ratios (ERs) further verify the logic integrity of this scheme. Thus, the OR and AND logic gates simultaneously achieved within a single logic unit is compact and cost-effective for future optical signal processing applications.
    Optics Communications 02/2010; 283(3):349-354. · 1.54 Impact Factor
  • Thomas Y. L. Ang, Nam Quoc Ngo
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    ABSTRACT: We present an analytical study on the effects of coupler-induced localized backscattering (CILB) on the nature of superluminal and subliminal propagations in a traveling wave microresonator (TWMR). It is found that in the weak CILB regime, the flexibility to control the light propagation velocity is improved, and fast and slow light with high output power can be generated for a TWMR with a small net optical gain.
    Journal of the Optical Society of America B 01/2010; · 1.81 Impact Factor
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    ABSTRACT: A 40 Gb/s multifunction optical format conversion module based on nondegenerate four-wave mixing (ND-FWM) arising in a single semiconductor optical amplifier (SOA) is proposed and demonstrated. It has the ability to achieve NRZ-OOK/DPSK/DQPSK to CSRZ-OOK/DPSK/DQPSK, RZ-OOK/DPSK/DQPSK to CSRZ-OOK/DPSK/DQPSK, and CSRZ-OOK/DPSK/DQPSK to RZ-OOK/DPSK/DQPSK format conversions. A comprehensive dynamic model to investigate twelve FWM-related waves in SOA is presented. Using this model, the proposed module with multiple format conversions is numerically investigated. We find that the SOA based scheme can be used for high-speed all-optical multifunction format conversion, because one of the FWM participators also serves as an assist light to accelerate SOA gain recovery. Meanwhile three identical converted signals with different wavelengths can be obtained simultaneously for all-optical wavelength multicast application. Finally, the bit-error-rate (BER) measurements of some 10 Gb/s operations are demonstrated in order to further verify the proposed multifunction format conversion module. The power penalty is measured to be less than 1 dB at the BER level of 10<sup>-9</sup>.
    Journal of Lightwave Technology 11/2009; · 2.86 Impact Factor
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    ABSTRACT: We report a new type of silica-based all-solid fiber with a 2-D nanostructure core. The nanostructure core fiber (NCF) is formed by a 2-D array of high-index rods of sub-wavelength dimensions. We theoretically study the birefringence property of such fibers over a large wavelength range. Large-mode-area (LMA) structure with a typical high birefringence in the order of 10-4 can be easily realized. The attenuation of the fabricated NCF is as low as 3.5 dB/km at 1550 nm. Higher macro- and micro-bending losses compared with those of the single-mode fiber (SMF) due to the reduced index difference have been observed experimentally, which suggests that the NCF is potentially useful for curvature and strain sensing applications. A fiber Bragg grating (FBG) inscribed in such a novel fiber is side-polished to make use of its evanescent field for refractive index sensing. The refractive index sensitivity obtained is one order of magnitude higher than that of the side-polished FBG in SMF, while the temperature and strain performances are comparable with those of the SMF-based FBG.
    Journal of Lightwave Technology 06/2009; 27(11):1548-1555. · 2.86 Impact Factor
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    ABSTRACT: We propose and experimentally demonstrate a novel fiber-optic edge filter based on modulating the chirp rate of a π-phase-shifted fiber Bragg grating (FBG) operating in transmission mode. The phase shift induced passband in the transmission spectrum is utilized as the edge filter. The dependence of the π-phase-shifted FBG’s transmitted spectral response on the chirp rate has been numerically studied in detail and experimentally confirmed, to the best of our knowledge, for the first time. The linear wavelength range of the proposed edge filter can be tuned by changing the chirp rate of FBG. The edge filter is further tested as a wavelength interrogator, and the experimental results are in good agreement with numerical results. The proposed fiber-optic edge filter has several unique advantages which include simple structure, cost effectiveness, high sensitivity, flexible tunability, and optical circulator is not required, and thus has interesting potential applications, especially as a wavelength interrogator in FBG foot sensors, FBG ultrasound and vibration sensors, and FBG distributed sensors, where the required wavelength ranges are very small (<0.4 nm).
    Optics Communications 04/2009; · 1.54 Impact Factor
  • D. Liu, N. Q. Ngo, H. Liu
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    ABSTRACT: A stable dual-wavelength erbium-doped fiber laser with a linear cavity is formed by a polarization-maintaining uniform fiber Bragg grating (PM-FBG) and a polarization-maintaining linearly chirped fiber Bragg grating (PM-LCFBG), both of which were fabricated on a high-birefringence (Hi-Bi) fiber. Experimental results show stable dual-wavelength lasing operation with a wavelength separation of ∼0.22nm, which can be tuned down to as small as 0.05nm and a large optical signal-to-noise ratio (OSNR) of over 40dB under room-temperature. Microwave signal at frequency of 9.41, 18.03 and 27.46GHz is achieved by heterodyned the output lasing wavelengths on a photodetector.
    Optics Communications 04/2009; 282(8):1611-1614. · 1.54 Impact Factor
  • D. Liu, N. Q. Ngo, H. Liu
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    ABSTRACT: 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 3dB power level variation and with a wavelength spacing of ∼0.8nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40dB.
    Optics Communications 04/2009; 282(8):1598-1601. · 1.54 Impact Factor
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    ABSTRACT: We propose and experimentally demonstrate a novel tunable fiber optic edge filter based on modulating the chirp rate of a pi-phase-shifted fiber Bragg grating (FBG) operated in reflection mode. The phase shift induced notch in the reflection spectrum is utilized as the edge filter. The dependence of the pi-phase-shifted FBG's spectral response on the chirp rate has been numerically studied in detail and experimentally confirmed for the first time. The linear wavelength range of this edge filter can be tuned by changing the chirp rate of FBG. A fiber optic edge filter is further obtained experimentally and tested as a wavelength interrogator, which is in a good agreement with numerical results. The proposed edge filter has advantages of simple-structure, cost-effectiveness, high sensitivity, and flexible tunable, thus opening up some applications, especially as wavelength interrogator in small wavelength range.
    PhotonicsGlobal@Singapore, 2008. IPGC 2008. IEEE; 01/2009
  • Source
    Xia Li, P. Shum, Nam Quoc Ngo, Teehiang Cheng
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    ABSTRACT: Phase sampling and phase shift techniques are introduced in a uniform fiber Bragg grating (FBG) structure to achieve a multi-bandpass filter with ultranarrow transmission bandwidth. A pair of dual-bandpass transmission windows are opened at 1542.32, 1542.36, and 1542.66, 1542.71 nm, respectively. The channel spacing between the pair can be designed by the phase sampling period, as well as the wavelength spacing in the dual-bandpass can be controlled through the phase shift length. This new kind of bandpass structure can be applied in the ultranarrow filtering in multi-channel, multi-wavelength distributed feedback Bragg lasing, photonic generation of microwave signal etc.
    Proc SPIE 11/2008;

Publication Stats

1k Citations
231.48 Total Impact Points


  • 2002–2011
    • Nanyang Technological University
      • • School of Electrical and Electronic Engineering
      • • Photonics Research Centre (PhRC)
      Singapore, Singapore
  • 2008–2009
    • Beijing University of Posts and Telecommunications
      • State Key Laboratory of Information Photonics and Optical Communications
      Beijing, Beijing Shi, China
  • 2007
    • Ningbo University
      Ning-po, Zhejiang Sheng, China
  • 1994–2007
    • Monash University (Australia)
      • Department of Electrical and Computer Systems Engineering, Clayton
      Melbourne, Victoria, Australia
  • 2004
    • Zhejiang University
      • Department of Information Science and Electronic Engineering
      Hang-hsien, Zhejiang Sheng, China
  • 2003–2004
    • Xiangtan University
      Siangtan, Hunan, China