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This study proposed a high-quality fiber ring laser combining four-subring resonators and a saturable absorber within a ring cavity. An un-pumped erbium-doped fiber is introduced as the saturable absorber (SA) within the ring structure. By employing the vernier effect, we arrange four subrings with different lengths to form the four-subring resonators, achieving a single-longitudinal-mode (SLM) operation. At a pump power of 100 mW, the laser output power and wavelength deviations during a one-hour test were measured to be 0.01 dB and 0.059 nm, respectively. Additionally, using the delayed self-heterodyne measurement system and Lorentzian fit analysis, we measured an ultra-narrow linewidth of 623.1 Hz.

Silicon-based photonic modulators are a primary choice for on-chip optical devices with CMOS fabrication compatibility. The phase shifter in a silicon photonics modulator plays a significant role in determining the efficiency of the modulator to meet the optical data communication's future demands. Obtaining high extinction ratio (ER) and with acceptable bit error rate (BER) at low voltage and low loss was kept as the primary objective for the proposed PIN phase shifter in an unbalanced silicon Mach-Zehnder modulator. The phase shifter length was kept at 2 mm, and the carrier doping region was reduced to decrease the carrier absorption loss. The concentration of P and N in the phase shifter was set to 7 × 1017cm−3and 5 × 1017cm−3, and the intrinsic gap was varied (50, 100, 150, 200, 250, 300, 350 and 450 nm) for the study to obtain the optimum gap to meet the objective. For 200 Gbps, the proposed modulator with intrinsic gap 150 nm obtained 18.68 dB ER having VπL = 0.8 V.cm. Insertion loss obtained for the phase shifter was 3.421 dB/cm. The proposed design is expected to enhance the performance of silicon optical modulators for commercial applications, and also other applications such as optical switches, delay lines, and optical interconnect.

Fiber Bragg gratings (FBGs) with various interrogation schemes to estimate the FBG’s spectrum shift have been widely used in fiber sensing systems. Wavelength swept laser (WSL) based interrogation architectures have been proposed to offer rapid and high-quality sensing performance. However, for getting higher sensing accuracy, the demands for high-performance WSL may push the system cost. Under these considerations, a WSL distribution architecture allowing multiple sensing processing units (SPUs) to share the WSL is studied in this Letter. A self-synchronization scheme is proposed to enable flexible SPU deployment with no concerns for the clock calibration. The proposed system is experimentally studied. Temperature estimation error of ∼2.5^∘C and ∼0.5^∘C with sensitivities of 0.13°C/ms and 0.14°C/ms, respectively, for the high and small temperature ranges are demonstrated.

A multi-point fiber sensing system formed from a linear cavity laser is proposed. Various optical sensing systems have been investigated, for example, using fiber Bragg grating (FBG) and Brillouin scattering for multi-point sensing. This paper focuses on a simple sensing system by using multi-wavelength lasing with parallel cavities and a semiconductor optical amplifier (SOA). First, optical nonlinearity in amplification of the SOA is discussed to clarify the effects of gain saturation and four-wave mixing on the proposed multi-channel sensing system. And then lasing conditions in the linear cavity laser consisting of an SOA, an arrayed waveguide grating (AWG), and FBGs are theoretically investigated. The multi-wavelength lasing power is found to be limited mainly by gain saturation in the SOA. The lasing power for the eight-channel system is evaluated to be −8.5  dBm when the total loss in the linear cavity is 10 dB. The lasing power can be increased by 3 dB when the channel number is decreased to four. Next, multi-wavelength lasing in the cavity consisting of an SOA, an AWG, a loop mirror, and fiber mirror reflectors is experimentally demonstrated up to eight channels. Finally, two-channel temperature sensing ranging from 13°C to 76°C is experimentally confirmed by using two FBGs as the sensing elements with an AWG having 100-GHz bandwidth.

Physical layer network coding (PNC) improves the throughput in wireless networks by enabling two nodes to exchange information using a minimum number of time slots. The PNC technique is proposed for two-way relay channel free space optical (TWR-FSO) communications with the aim of maximizing the utilization of network resources. The multipair TWR-FSO is considered in this paper, where a single antenna on each pair seeks to communicate via a common receiver aperture at the relay. Therefore, chip interleaving is adopted as a technique to separate the different transmitted signals at the relay node to perform PNC mapping. Accordingly, this scheme relies on the iterative multiuser technique for detection of users at the receiver. The bit error rate (BER) performance of the proposed system is examined under the combined influences of atmospheric loss, turbulence-induced channel fading, and pointing errors (PEs). By adopting the joint PNC mapping with interleaving and multiuser detection techniques, the BER results show that the proposed scheme can achieve a significant performance improvement against the degrading effects of turbulences and PEs. It is also demonstrated that a larger number of simultaneous users can be supported with this new scheme in establishing a communication link between multiple pairs of nodes in two time slots, thereby improving the channel capacity.

Joint effects of aperture averaging and beam width on the performance of free-space optical communication links, under the impairments of atmospheric loss, turbulence, and pointing errors (PEs), are investigated from an information theory perspective. The propagation of a spatially partially coherent Gaussian-beam wave through a random turbulent medium is characterized, taking into account the diverging and focusing properties of the optical beam as well as the scintillation and beam wander effects. Results show that a noticeable improvement in the average channel capacity can be achieved with an enlarged receiver aperture in the moderate-to-strong turbulence regime, even without knowledge of the channel state information. In particular, it is observed that the optimum beam width can be reduced to improve the channel capacity, albeit the presence of scintillation and PEs, given that either one or both of these adverse effects are least dominant. We show that, under strong turbulence conditions, the beam width increases linearly with the Rytov variance for a relatively smaller PE loss but changes exponentially with steeper increments for higher PE losses. Our findings conclude that the optimal beam width is dependent on the combined effects of turbulence and PEs, and this parameter should be adjusted according to the varying atmospheric channel conditions. Therefore, we demonstrate that the maximum channel capacity is best achieved through the introduction of a larger receiver aperture and a beam-width optimization technique.

This study considered the application of fiber Bragg grating (FBG) to a composite material leaf spring as a vehicle loading sensing system. The loading of the vehicle by the leaf spring was calculated by the period change of the fiber grating caused by deformation of the composite material leaf spring under loading. From the experiment results, a good linear relationship between loading and Bragg wavelength shift for light. The R-squared value for the linear regression reached 0.99. The loading sensitivity is about 0.267 nm/N.

The thermal drift coefficient of the Bragg wavelength of fiber Bragg grating (FBG) is unacceptably high when the FBG is used as a wavelength reference or a wavelength-selective passive component, especially in a dense wavelength division multiplexing (DWDM) system. A light, small and robust carbon fiber reinforced polymeric composite structure is proposed to compensate for the temperature induced wavelength drift in FBG. It has been shown to be able to reduce the temperature induced wavelength shift of a fiber Bragg grating from 10 pm/°C to below 1 pm/°C. Stability in long term performance over a two month period has also been demonstrated. A fabrication route that leads to optimal compensation performance has also been proposed.

A reconfigurable wavelength-division-multiplexing (WDM) add/drop multiplexer (WADM) is constructed using optical switches (OSWS) and fibre Bragg gratings (FBGs). Selected one or two channel(s) can be added/dropped at the same multiplexer by properly controlling the optical switch pair and fibre Bragg grating arrangement. A␣four-channel system is experimentally demonstrated for a data rate of 2.5 Gb s−1 and a single-mode fibre (SMF) distance of 100km. A negligible maximum power penalty of only 0.3dB is observed when compared with back-to-back transmission.

By using a newly designed bidirectional erbium-doped fiber amplifier, we demonstrated a 2×2.5 Gb/s repeated bidirectional transmission over a 100-km single-mode fiber. Two tunable optical bandpass filters were used to suppress the Rayleigh backscattering and amplified spontaneous emission in the bidirectional transmission system. The bit error rate performances were measured for both directions. A negligible power penalty of 0.2 dB for the 2×2.5 Gb/s transmission due to backreflections was observed when comparing the bidirectional transmission with single-channel, unidirectional transmission.

A dynamically selective multiwavelength cross-connect for wavelength division multiplexed (WDM) networks based on fibre Bragg gratings (FBGs) and optical switches is reported. Dynamically single- or multi-channel cross-connect functionality can be realized according to control of the optical switches and the FBGs' arrangement. Bit-error-rate performance with negligible power penalty is achieved in a 2.5Gbs-1x3 WDM channels over 100km conventional single-mode fibre (SMF) network demonstration. The characteristics of low channel crosstalk, uniform channel loss, high scalability and cascadability, and low cost of this device could provide more reconfiguration flexibility and network survivability for WDM networks.

A tapered fiber with a depressed-index outer ring is fabricated and dispersion engineered to generate a widely tunable (1250-1650 nm) fundamental-mode leakage loss with a high cutoff slope (−1.2 dB/nm) and a high attenuation for stop band (>50 dB) by modification of both waveguide and material dispersions. The higher cutoff slope is achieved with a larger cross angle between the two refractive index dispersion curves of the tapered fiber and surrounding optical liquids through the use of depressed-index outer ring structures in double-cladding fibers.

A single-longitudinal-mode erbium-doped fiber laser with a passive multiple-ring cavity (MRC) is proposed for the first time to the authors’ knowledge. The laser is fundamentally structured by insertion of three different short ring cavities, which serve as mode filters, into the main cavity. When it is combined with a mode-restricting intracavity fiber Bragg grating, the MRC resonator ensures single-longitundinal-mode laser oscillation. The laser can successfully suppress side-mode frequencies of as much as 1??GHz and provide an output power of 23??mW with a side-mode suppression ratio of 51??dB at 1533??nm. The short-term linewidth of the laser output measured is ?2 kHz. The ability of this fiber laser to act as an AM transmitter source is also demonstrated.

By inserting a bidirectional erbium-doped fiber amplifier (EDFA) in between an optical circulator and a fiber Bragg grating (FBG), we realize an FBG-integrated optical limiting amplifier (OLA) with high dynamic range. The dual-pass OLA has a wide dynamic range of over 40 dB and a saturation signal output power of about 13.0 dBm. The performance of dual-pass OLA has no obvious degradation due to back reflection of the amplified signal. A negligible power penalty of about 0.3 dB is observed when compared with other conventional configurations. The FBG-integrated OLA configuration has potential application in wavelength division multiplexing (WDM) systems where high saturated power is needed for multichannel transmission. © 1998 Society of Photo-Optical Instrumentation Engineers.

A dynamically selective wavelength add/drop multiplexer (WADM) and a multiwavelength cross-connect (M-XC) configuration for dense wavelength division multiplexed networks are proposed. Multiple channels add/drop and/or cross-connect can be realised according to the control of the optical switches and fibre Bragg gratings arrangement. The selective WADM and M-XC devices could provide more reconfigurable flexibility and survivability in WDM networks.

Two erbium-doped fiber ring laser configurations are proposed and experimentally demonstrated. Both configurations employ array-waveguide-grating (AWG)-based cavities to select the operation wavelengths. The first experiment can achieve discrete wavelength tuning in a range of 31.8 nm with 2.0-dBm output power for each channel. The second experiment yields four wavelength lasing with −10-dBm output power for all signals after power equalization. The linewidth of either AWG laser is as small as 0.015 nm and the side-mode suppression ratio is over 55 dB. © 2003 Society of Photo-Optical Instrumentation Engineers.

The authors propose and demonstrate an eight-channel reconfigurable bidirectional wavelength division multiplexed (WDM) add-drop multiplexer in which all channels can be added/dropped independently in either direction. The performance of the bidirectional WDM add/drop multiplexer is experimentally studied for a data rate of 10 Gbit/s per channel, providing an overall capacity of 80 Gbit/s. It is found that the performance of the add/drop multiplexer is not degraded by a backward propagating signal.

Semiconductor laser amplifiers cause both distortion and crosstalk in WDM systems due to gain saturation. A method using light injection is experimentally demonstrated to reduce distortion and crosstalk. In a two-channel experiment with a data rate of 2.5 Gbit/s, it is found that light injection can eliminate the BER floor due to distortion and crosstalk.

We have proposed and demonstrated a novel signal-pump double-pass distributed Raman fiber amplifier (RFA) by using a three-port optical circulator as a reflective mirror. This unique RFA architecture not only doubles the amount of dispersion compensation, but also enhances the pumping efficiency. In comparison with the conventional signal-pump single-pass scheme, the proposed RFA has a 2.0-dB gain improvement and a 3.7-dB equivalent noise figure suppression, measured at 1598 nm. The proposed RFA may find wide applications in long-haul transmission and optical networks, where dispersion compensation is of great concern.

We succeeded in fabricating precision side-polished fibers embedded in silicon V-grooves and then polished six fibers using a home-made polisher. The control of the polishing depth of the side-polished fibers was as accurate as ±0.25 µ m. Another merit of our polished samples was their negligible losses even though the effective interaction lengths were long. Experimental calibrations of our samples agreed with theoretical results.