A transmission scheme is proposed which loads differential phase shift keying (DPSK) information and circular polarization shift keying (CPolSK) information on the same optical carrier for free space optical links through an atmospheric turbulent channel. Theoretical models of modulation and demodulation are derived and a design is specified to detect and recover the information with two mutually independent branches in order to meet the requirement for a high real-time transmission capacity and to entirely remove modulation-induced cross talk. Simulation results show that the CPolSK/DPSK system has a high tolerance to intensity scintillation and phase noise caused by atmospheric turbulence. A bit error rate performance comparison of a CPolSK system, a DPSK system, and a CPolSK/DPSK system is also presented to comprehensively demonstrate that the CPolSK/DPSK system outperforms the other two modulation techniques in terms of a high real-time transmission capacity.

High peak-to-average power ratio (PAPR) of the orthogonal frequency division multiplexing (OFDM) signal is one of the limitations to the transmission performance of an optical OFDM system. Many PAPR reduction techniques have been proposed in previous works. However, most of them only consider a single scheme while overlooking the complementary features of these techniques. We propose a twofold PAPR reduction technique called Hadamard transform combined with a partial transmit sequence (PTS) (HTCP). The proposed HTCP scheme combines the merits of two complementary techniques, i.e., Hadamard transform and PTS, to improve the performance of the optical OFDM system in terms of PAPR and bit error rate (BER). Furthermore, the side information generated in PTS is transmitted by pilot sequences which increase the utilization of subcarriers. Least square estimation is used to estimate the pilot signal’s phase to recover the side information. The HTCP scheme is theoretically analyzed in a direct-detection optical OFDM system. Simulation results show that the HTCP scheme has a better performance with regards to PAPR and BER compared with the case of applying only the Hadamard transform or PTS technique.

Traditional cable television (CATV) network configuration is mainly a branch topology. When fibers break, there are always disconnections of CATV and Internet access. Meanwhile, network service providers usually cannot be instantly informed of network breakage; network problems are noticed only after the users call for services. Network breakage location detection and repair are, in general, costly and time-consuming. This paper proposes a new framework to protect CATV fiber optic networks, where a HUB with multiple optical protection switching (OPS) is constructed between the head end and the fiber nodes. The first layer ranging from the head end to the HUB forms a 1+1 protection path ring structure. The second layer ranging from the HUB to the fiber nodes forms multiple 1+1 protection path ring structures. The framework can form a four fiber optic path protection architecture with multiple ∞-shapes between the head end and the fiber nodes. When the CATV network system experiences a malfunction, such as fiber optic cable breakage or poor signal quality, OPS can immediately switch the fiber transmission path in order to maintain uninterrupted CATV network traffic. After simulation tests on fiber path switching time automatic protection switching, frame loss, packet jitter, latency, bit errors, and system traffic flow monitoring by manually switching OPS and pulling broken fiber optic cable, results show that the fiber path protection switching mechanism can react immediately, and test data results show that network traffic is unaffected, so the proposed framework can effectively enhance the survivability and quality of service of CATV network systems.

A dual-parameter optical fiber sensor is proposed and demonstrated. It is based on an intermodal interferometer (IMI) with an inline embedded fiber Bragg grating (FBG). The IMI is formed by cascading a taper structure and a spherical-shaped structure through a segment of a single-mode fiber. Due to the different wavelength shifts of the IMI and FBG to temperature and liquid level, simultaneous measurement can be achieved. Experimental results indicate a good linear relation between the wavelength shift and external parameters (temperature and liquid level). The sensitivities of 0.066  nm/°C and −0.133  nm/mm are achieved experimentally for temperature and liquid level, respectively. The interesting properties of the sensor include good operation linearity, compact size, and high sensitivity.

Acousto-optically Q-switched operation of a Ho:SSO laser is reported for the first time. An average power of 2.23 W was obtained at a pulse repetition frequency from 5 to 20 kHz, with a corresponding to optical-to-optical conversion efficiency of 14.4% and a slope efficiency of 20%. The highest energy per pulse of 0.44 mJ in 28 ns was achieved at 2112 nm with a repetition rate of 5 kHz and peak power of 15 kW. The lifetime of Ho³+ in the Sc₂SiO₅ crystal host was also measured.

The orthogonal frequency division multiplexing (OFDM) modulation technique has been used widely in visible light communications (VLC) systems to combat intersymbol interference. At the same time, the inherent drawback of OFDM with a high peak-to-average power ratio (PAPR) is brought into OFDM visible light communications (VLC-OFDM). Furthermore, considering the limited dynamic range characteristics of light-emitting diodes, the performance degradation caused by a high PAPR is more serious in VLC-OFDM. In this paper, we propose a partial transmit sequence (PTS) technique based on the combination of a genetic algorithm (GA) and a hill-climbing algorithm (GH-PTS) to solve the problem of high PAPR. GH-PTS is a modified PTS technique based on GA-PTS. Essentially, GH-PTS is a local optimization of GA-PTS. Simulation results show that the optimized technique is able to reduce PAPR more effectively without any loss of bit error rate performance than the GA-PTS technique in VLC-OFDM system.

The gain flatness of an L-band erbium-doped fiber amplifier (EDFA) is still an open issue. We improve the conventional single-pass two-stage configuration by placing a fiber Bragg grating (FBG) with the center wavelength at 1529.4 nm. Then, a weak lasing resonance cavity is formed by this FBG and a fiber isolator, which is able to increase the pump conversion efficiency by reflecting the residual amplified spontaneous emission and compressing the raised gain around 1570 nm based on the gain-clamped effect. The experimental results show that by adopting the proposed scheme, the gain un-flatness of the L-band EDFA is controlled within ±0.4  dB in the range of 1570 to 1610 nm, and an ∼29  dB average gain is simultaneously maintained.

We propose and experimentally demonstrate a hybrid CATV/16-QAM-Digital CATV/16-QAM-OFDM in-building network over a 40-km single-mode fiber and graded index-plastic optical fiber/10-m visible light communication. The application of external injection technology with the addition of an optoelectronic feedback that raises the resonance frequency of the laser diode results in increased system transmission capability. Good performances of carrier-to-noise ratio, composite second-order, and composite triple-beat are obtained for the CATV signal. A low bit-error-rate value is achieved for the 16-QAM-Digital CATV/16-QAM-OFDM signal.

Contradictions among receiver optical power, bandwidth of photodiode, and transmission distance are ubiquitous in visible light communication (VLC) systems. In order to moderate such contradictions and improve the performance of VLC systems, we proposed a space diversity receiver for a light-emitting diode (LED)-based VLC system and experimentally demonstrated the highest data rate for an RGB LED with a 1.4  Gbit/s data rate employing two detectors located in different space positions at the same time. The maximal ratio combining algorithm was used in the VLC system for the first time. As a result, the bit error ratios of the receivers can be reduced by 48.08 dB (red LED), 12.04 dB (green LED), and 27.48 dB (blue LED) compared with any separate detector. The performance of the VLC system was significantly improved. It turned out that it was highly desirable to use multiantennas in the VLC system.

Molybdenum oxide (MoO_x) and nickel oxide (NiO_x) thin films were deposited by reactive biased target ion beam deposition. MoO_x thin film resistivity varied from 3 to 2000  Ω·cm with a temperature coefficient of resistance (TCR) from −1.7% to −3.2%/K, and NiO_x thin film resistivity varied from 1 to 300  Ω·cm with a TCR from −2.2% to −3.3%/K, both easily controlled by varying the oxygen partial pressure. Biased target ion beam deposited high TCR MoO_x and NiO_x thin films are polycrystalline semiconductors and have good stability in air. Compared with commonly used vanadium oxide thin films, MoO_x or NiO_x thin films offer improved process control for resistive temperature sensors.

Negative refractive-photonic crystal (NR-PC) lenses that can exceed the diffraction limit of focus resolution for imaging and target detection in the near field have gotten more and more special attention in recent years. Three flat lens groups with Ag defects based on NR-PC are designed, and the focusing imaging in the NR-PC three flat lens groups is concluded with the extension of Snell’s law, and the influence on the resolution for a target detection dynamic scanning scheme is simulated by using the finite difference time domain method. An optimal-doped structure with Ag defects is achieved by different simulation combinations. The refocusing resolution 0.18834λ is achieved in the optimal structure and there is approximately a 0.06806λ improvement in the refocusing resolution compared to those undoped with Ag (0.2564λ); it also possesses distinct smaller side-lobes than a single flat lens doped with Ag. This means the optimal detecting ability for the three NR-PC flat lens groups with Ag defects is more improved than that for a single undoped and doped with Ag. This is significant for the perfect imaging being achieved for a particle structure.

We present the design of a parabolic multimode waveguide used as a spot size width converter. The designed structure allows reducing the input spot size width an order of magnitude using a very short structure with respect to linear or conventional tapers. An illustrative example of spot width reduction from 10 microns to a Si wire of 1 micron with a small length in the order of 17 microns and a coupling efficiency of ∼90% is presented. Using a linear taper structure with a taper length >100  μm, the coupled power is still less than that obtained by the parabolic multimode interference (PMMI) guide. This means that the PMMI offers a length reduction of more than 5× with respect to the linear taper to achieve the same spot reduction.

In order to reduce the polarization-induced impact on the incoherent optical frequency domain reflectometry based single-mode fiber-optic distributed temperature sensor, a synchronous polarization scrambling technique with a low-speed electrically driven polarization controller (EPC) is presented. The polarization-induced error is derived by error analysis. By simulating the distribution of the states of polarization on the Poincare sphere, optimized EPC driven parameters are selected. The polarization scrambling process is synchronous with the frequency response measurement of the Raman backscattered light. Additionally, the scrambling period is set to be equal to the measurement time of each frequency response. Experimental results show that the polarization-induced error is ∼±3°C, and it is basically in accord with the result of a theoretical error analysis. By using the synchronous polarization scrambling technique, the polarization-induced fluctuation of the measured temperature distribution has been almost eliminated.

An efficient temperature measuring and monitoring method in an interferometric fiber-optic gyroscope (FOG) for space application is presented. Resource-saving digital temperature sensors and integrated optical-circuit temperature sensors are designed and separately implemented in a field programmable gate array and the second closed loops of FOG. A multipoint temperature measurement in FOG is conducted using both kinds of temperature sensors. An FOG status-monitoring strategy is proposed by defining a status index based on multipoint temperature information and heat-transmission model of our four-axis FOG structure. Temperature measurement and status-monitoring strategy are verified by experimental results.

The effect of an oxygen atmosphere on the expansion dynamics of a laser-produced vanadium-oxygen plasma has been investigated using a fast intensified charged-coupled device camera. We find regimes of the plasma plume expansion ranging from a free plume at vacuum and low oxygen pressures, through collisional and shock-wave-like hydrodynamic regimes at intermediate oxygen pressure, finally reaching a confined plume with subsequent thermalization of the plume particles at the highest pressure of the oxygen gas. Vanadium oxide nanostructures thin films were synthesized from this plasma and the resulting vanadium oxide phases studied as a function of the plume dynamics. We found monoclinic vanadium dioxide (VO₂) (M1) and VO₂ (B) nanoparticles in thin films deposited at 0.05 mbar. Pure phases of vanadium trioxide (V₂O₃) smooth and pentoxide (V₂O₅) nanorods thin films were detected at 0.01 and 0.1 to 0.2 mbar, respectively. Thin films containing VO₂ (M1) were found to have a reversible metal-to-insulator transition at 61°C. This work paves the way to VO phase control by judicious choice of laser and plasma conditions.

Doping titanium dioxide (TiO₂) with impurities has been extensively investigated for the optimal use of solar energy in order to improve photocatalyst performance. Density functional theory calculations are performed to investigate the effects of Tm doping on the energy band structures and optical properties of anatase TiO₂ with various doping concentrations. The results show that Tm doping can effectively reduce the band gap of pure anatase TiO₂ by introducing Tm 4f states above the top of the valence band, and thus results in a red shift in the optical absorption edges. Furthermore, the calculations of optical properties indicate that Tm doping TiO₂ at low levels presents the prominent optical absorbance in the low energy region. Our results have important implications for the understanding and further development of photocatalytic functionalities of anatase TiO₂ by introducing a dopant for effective band gap engineering.

In recent years, considerable research has been carried out relative to the electromagnetic (EM) propagation and refraction characteristics in metamaterials with emphasis on the origins of negative refractive index. Negative refractive index may be introduced in metamaterials via different methods; one such is the condition whereby the Poynting vector of the EM wave is in opposition to the group velocity in the material. Alternatively, negative refractive index also occurs when the group and phase velocities in the medium are in opposition. The latter phenomenon has been extensively investigated in the literature, including recent work involving chiral metamaterials with material dispersion up to the first order. This paper examines the possible emergence of negative refractive index in dispersive chiral metamaterials with material dispersion up to the second order. The motivation is to determine if using second- as opposed to first-order dispersion may lead to more practical negative index behavior. A spectral approach combined with a slowly time-varying phasor analysis is applied, leading to the analytic derivation of EM phase and group velocities, and the resulting phase and group velocities and the corresponding phase and group indices are evaluated by selecting somewhat arbitrary dispersive parameters. The results indicate the emergence of negative index (via negative phase indices along with positive group indices, as reported in the literature) or negative index material (NIM) behavior over information bandwidths in the low RF range. The second-order results are not significantly better than those for first-order results based on the theoretical analysis; however, greater parametric flexibility exists for the second-order system leading to the higher likelihood of achieving NIM over practical frequency bands. The velocities and indices computed using the Lorentzian and Condon models yield an NIM bandwidth around 200−400  Mrad/sec, about 2 orders of magnitude higher than that for the parametric approach; more importantly, NIM is found not to occur in the first order when using practical models.

The phase-only liquid crystal on silicon (LCOS) spatial light modulator has been used as a wavefront generator and reconstruction device for display applications due to its phase-only modulating property of incident light. In order to achieve the best reconstructed phase modulated wavefront, the properties of LCOS have to be fully known. The intensity of reflected light from LCOS decreases under normal incidence by going through a beam splitter. In order to improve the intensity of the reflected light beam, the oblique incidence is preferred during many applications. The oblique incidence characterization of a parallel aligned LCOS is investigated at a working wavelength of 532 nm on the basis of double-hole interferometric method. Through experiments, the phase modulation characterization of LCOS under the oblique incidence is obtained. In addition, an image postprocessing method is proposed to overcome the effect of flicker and coherent noise by simplifying the computation and increasing the measurement accuracy. The comparison of experimental results for different incident angles indicates that it plays an important role in the performance of phase modulation of LCOS, where the phase modulation decreases with the increasing angle of incidence.

This paper reports on the investigation of the electrically controlled multifocus, multi-imaging characteristics of an on-axis holographic polymer-dispersed liquid-crystal (H-PDLC) Fresnel lens. The Fresnel lens is examined within a PDLC cell through the analysis of interference fringes generated by on-axis plane and spherical waves. Experiments are conducted to investigate the multifocus and multi-imaging phenomena of the H-PDLC Fresnel lens, and a corresponding geometrical optical analysis is also provided. It is then demonstrated that the H-PDLC Fresnel lens is a plane-surface diffractive optical device which modulates the phase of incident light through a periodic change of refractive index and forms multiple symmetrical images. Its diffraction properties, which can be controlled electrically, have further potential applications in this field.

A comprehensive study has been performed to achieve all-angle self-collimation in basic two-dimensional square array photonic crystals with cylindrical scatterers. Based on plane wave expansion and finite difference time domain analysis for both rod- and hole-type structures, we report on all-angle self-collimation (SC) in the first band of the structure, which results in loss suppression due to out-of-plane scatterings. A lower threshold for index contrast has been obtained to achieve all-angle SC, which offers more design flexibility regarding structural parameters. Furthermore, it has been shown that a minimum and maximum coupling efficiency enhancement of ∼40% and 80% can be achieved for the proposed structure, respectively, by introducing a row of scatterers with proper radius at the input and the output air/photonic crystal interfaces.