Terahertz modulator using photonic crystals

In this letter, a novel terahertz wave modulator based on a silicon oxide/polyaniline photonic crystal is proposed. The modulation mechanism of the novel modulator is based on a dynamic shift of the photonic band gap by the applied external electric field. Its performances were investigated with the finite-difference time-domain method. The novel modulator has 3 dB modulation bandwidth of 10 kHz, a size as small as 20 mm and its extinction ratio larger than 30 dB at the frequency of 1 THz.     

Impact of extinction ratio of single arm sin LiNbO3 Mach-Zehnder modulator on the performance of 10 and 20 Gb/s NRZ optical communication system

We investigate the impact of extinction ratio of single arm sin² LiNbO₃ Mach-Zehnder (MZ) amplitude modulator on the performance of 10 and 20 Gb/s single-channel optical communication system. For different fiber lengths, the system performance has been analyzed with the increase in the extinction ratio. The effect of variation in dispersion parameter has also been illustrated. The impact of extinction ratio (ζ), dispersion parameter and length of the fiber has been further optimized with minimum bit error rate (BER) at optimal decision threshold (10⁻⁹) for 10 and 20 Gb/s bit rate. It is found that the system gives optimum performance at extinction ratio (ζ) value 20 dB. The increase in the transmission distance from 468 km for 10 Gb/s to 532 km for 20 Gb/s has been reported, and 8 dB improvement in the Q value has been observed as the value of ζ is increased from 10 to 20 dB. At 20 Gb/s, the system gives optimum performance for dispersion parameter value only up to 4 ps/nm km; however, at 10 Gb/s the system can operate for dispersion values up to 14.3 ps/nm km. Further we investigate the self-phase modulation (SPM) effect for the increase in the input power. It is observed that the SPM effect is negligible below 3 dB m input power and it increases at higher power levels.     

Frequency modulated and polarization maintaining fiber laser with narrow linewidth

A novel configuration of fiber laser with frequency modulation is presented. Frequency modulation, stable polarization state and narrow linewidth are realized by using the waveguide phase modulator, polarization maintaining devices and saturable absorber. It is shown that the laser output reaches 23 mW, linewidth is less than 1 kHz, polarization extinction ratio is higher than 20 dB and maximum value of frequency deviation can reach 7.5 MHz.     

Skin-effect analysis of a shielded polymer Y-fed coupler electro-optic modulator over a wide bandwidth of 94 GHz

A novel technique and related formulations are proposed for analyzing the influences of the skin-effect on the performances of a polymer Y-fed coupler electro-optic modulator with shielded push-pull micro-strip electrodes. Using the extended point-matching method, coupled mode theory and electro-optic modulation theory, thorough design and optimization are performed. By introducing the effective applied voltage, formulas of the high-frequency response under skin-effect are deduced, and characteristics under skin-effect are analyzed. Under the central wavelength of 1550 nm, the half-wave voltage is as low as 1.097 V for the device with an active region length about 8.884 mm. Considering the influences of the skin-effect, the 3-dB modulating bandwidth of the microwave signal is up to 94 GHz. A high extinction ratio of more than 20 dB and a low insertion loss of less than 4.18 dB are achieved when the microwave frequency is below 68 GHz under skin-effect. This design technique is proven to be accurate by the comparison with the beam propagation method (BPM).     

Highly birefringent photonic crystal fiber polarization splitter made of soft glass

A soft glass dual core polarization splitter based on highly birefringent photonic crystal fiber (PCF) is proposed and the full vector finite element method (FEM) is employed to analyze the impacts of structural parameters on birefringence and the coupling length, and simulation results show that high birefringence on the order of 10⁻² can be obtained at 1.55 μm, moreover, hole size, hole pitch and elliptic ratio all affect birefringence and the coupling length. Based on these results, the PCF's structure is optimized to realize a polarization splitter of 282 μm whose largest extinction ratio is around −45.42 dB at 1.55 μm. Meanwhile, the bandwidth at the extinction ratio of −10 dB is about 90 nm, and around 32 nm at −20 dB.     

A very short vertical directional coupler with polarization-insensitive high extinction ratios

We propose a very short vertical directional coupler with polarization-insensitive high extinction ratios. The structure of the coupler has two deep-ridge waveguide structures in the upper and lower parts which can be implemented using a double-sided wafer process. The coupling length and extinction ratios of the vertical directional coupler with symmetric structures decrease as the thicknesses of the inner cladding layer and core layer decrease. The extinction ratio is improved using a slight asymmetry in the refractive indices of two core layers. As the thicknesses of the inner cladding layer and core layer decrease, the length of the vertical directional coupler for extinction ratios greater than 30 dB of both TE and TM modes decreases. A vertical directional coupler with a very short length less than 100 μm and polarization-insensitive high extinction ratios greater than 30 dB can be implemented using the structure proposed in this paper.     

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.     

Fresnel light modulator with an array of channel waveguides

One-dimensional focusing and optical modulation have been achieved using a Fresnel light modulator (FLM) based on a Fresnel lens consisting of Ti-diffused channel waveguides on LiNbO₃. By this device, the height of diffraction pattern near the focal point is modulated with an applied voltage while the focusing effect is almost kept. The experimental results obtained from the FLM with 13 zones at 633 nm are as follows: the focal length is 12.4 cm, the full width of beam at half power 47 μm, the half-wave voltage 28 V, the extinction ratio 10.9 dB and the insertion loss 11.6 dB.     

Fully packaged high-speed Ti:LiNbO3, waveguide mach-zehnder intensity modulator

A fully packaged high-speed Ti:LiNbO₃, Mach-Zehnder intensity modulator at a 1.5μm wavelength has been designed and fabricated. The design has been considered with respect to optical insertion loss, switching voltage, and modulation bandwidth. Re fiber-pigtailed device has a 6.7 dB fiber-to-fiber loss and an optical modulation bandwidth of 8 GHz. The voltage required to switch the device from the on to off state is measured to be 7.3 V, with an on-off extinction ratio of 21 dB.     

Hybrid photonic crystal 1.31/1.55 μm wavelength division multiplexer based on coupled line defect channels

The objective of this paper is to investigate the implementation of a hybrid photonic crystal (PhC) 1.31/1.55 μm wavelength division multiplexer (WDM) and wavelength channel interleaver with channel spacing of roughly 0.8 nm between the operating wavelengths of 1.54-1.56 μm. It is based on 1-D photonic crystal (PhC) structure connected with an output 2-D PhC structure. The power transfer efficiency of the hybrid PhC WDM at 1.31 μm and 1.55 μm were computed by eigen-mode expansion (EME) method to be about 88% at both the wavelengths. The extinction ratios obtained for the 1.31 μm and 1.55 μm wavelengths are − 25.8 dB and − 22.9 dB respectively.     

A photonic wire-based directional coupler based on SOI

A photonic wire-based directional coupler based on SOI was fabricated by e-beam lithography (EBL) and the inductively coupled plasma (ICP) etching method. The size of the sub-micron waveguide is 0.34 μm × 0.34 μm, and the length in the coupling region and the separation between the two parallel waveguides are 410 and 0.8 μm, respectively. The measurement results are in good agreement with the results simulated by 3D finite-difference time-domain method. The transmission power from two output ports changed reciprocally with about 23 nm wavelength spacing between the coupled and direct ports. The extinction ratio of the device was between 5 and 10 dB, and the insertion loss of the device in the wavelength range 1520-1610 nm was between 22 and 24 dB, which included an about 18.4 ± 0.4 dB coupling loss between the taper fibers and the polished sides of the device.     

A novel optical mm-wave generation scheme based on three parallel Mach-Zehnder modulators

We propose a novel optical mm-wave generation scheme based on three parallel Mach-Zehnder modulators (MZMs) for the first time. First, the scheme is investigated theoretically, which suggests that it can be used for sextupling, 12-tupling, and 18-tupling mm-wave generation. Then simulation results are given, 60 GHz mm-wave is generated from 5 GHz, or 10 GHz RF oscillator based on frequency 12-tupling or sextupling, and 90 GHz mm-wave is generated from 5 GHz RF oscillator based on frequency 18-tupling. The optical sideband suppression ratio (OSSR) and the radio frequency spurious suppression ratio (RFSSR) are analyzed by simulation, in which several non-ideal factors are taken into consideration. Results indicate that all the three mm-wave generation methods are practical and very good performance can be obtained when the extinction ratio of the MZM is 30 dB, even if the extinction ratio of the MZM is 20 dB, the performance is still good, especially for the sextupling mm-wave generation method, whose performance is excellent and insensitive to the extinction ratio of MZM, the non-ideal RF driving voltage and the non-ideal DC bias. At last, we set up a RoF system by simulation to verify the transmission performance of the scheme. The BER performance and eye diagrams are given.     

Operation of low driving voltage cascaded modulator over non-cascaded modulator for 4 dB extinction ratio at 100 giga bit per second (Gbps)

This paper describes improved results when comparing cascaded traveling wave electro absorption modulator (TWEAM) to non-cascaded TWEAM by simulation. Large signal modeling is used for both types of modulators to achieve 4 and 10 dB extinction ratios (ERs) with flat frequency response for applications in short distance as well as long distance optical fiber communication. To obtain 4 and 10 dB ERs with 110 GHz 3 dB bandwidth, a cascaded TWEAM requires 0.4 V peak to peak (V_P-P) and 1 V_P-P input driving voltages respectively. A non-cascaded TWEAM requires about two times the input driving voltage compared to the cascaded modulator to achieve the same values of ER and 3 dB bandwidth. Both modulators have been simulated with the same bias and also use the same circuit parameters except for the total active segment lengths (1 and 0.5 mm for cascaded and non-cascaded modulator respectively) and microstrip lengths to obtain the same ERs and 3 dB bandwidths.     

Temperature stabilized optical waveguide modulator

A low drive-voltage optical modulator using a Ti-diffused UNbO₃ optical waveguide has been fabricated. Stabilization against ambient temperature change was realized by using a miniature halfwave plate. The halfwave voltage, 3 dB bandwidth, optical insertion loss and extinction ratio were 3·8 V (at 1·06m wavelength), 850 MHz, 10 dB and 13 dB, respectively. A reduction scheme for the optical absorption caused by metallic electrodes, and an analysis of the modulator high frequency response are also reported.     

Single-sideband modulated radio-over-fiber system based on long period fiber grating

We present a prototype for optical single-sideband (SSB) modulated radio-over-fiber (RoF) system by employing a long period fiber grating (LPFG). A LPFG with 13.78 nm base width of transmission spectrum and 0?23.2 dB of transmission depth was designed by using commercial software. Then it is used in RoF SSB modulation scheme. In the scheme, a Mach?Zehnder modulator modulates the light wave with millimeter-wave driving signals to realize optical double-sideband (ODSB) modulation, the generated ODSB modulation signals pass through a LPFG. Due to the negative slope in transmission spectrum, the lower sideband experiences higher attenuation than the upper sideband. Thus the conversion from ODSB to optical single sideband with carrier (OSSB + C) can be easily achieved by using only one LPFG. Also, the carrier to sideband ratio (CSR) can be reduced by using a LPFG, results show the CSR can be decreased from 12.49 dB to 1.1 dB.     

Splitting-on-demand optical power splitters using multimode interference (MMI) waveguide with programmed modulations

Reconfigurable multi-channel optical power splitter is proposed and its optical properties are calculated. The device can dynamically reconfigure the number of splitting channels by providing programmed refractive index modulations on a multimode interference (MMI) waveguide. A reconfigurable 3-channel optical power splitter is designed to work as 1 × 1, 1 × 2 or 1 × 3 optical power splitter depending on the state of the heat electrodes using thermo-optic modulation, and the input light can be distributed to three output channels with sequential orders. The device can work in the whole C-band (1530-1565 nm) with extinction ratio better than −29.0 dB, excess loss better than −0.45 dB, imbalance better than 0.08 dB and polarization dependent loss (PDL) better than 0.14 dB. The design conception is scalable to a multi-channel splitting-on-demand optical power splitter which can divide input light to 1, 2, …, N output channels equally by using the 3-channel reconfigurable optical power splitter as a building block.     

All-optical logic AND-NOR gate with three inputs based on cross-gain modulation in a semiconductor optical amplifier

We report the realization of a novel all-optical logic AND-NOR gate based on cross-gain modulation (XGM). The used scheme requires only one SOA to perform the logic gate with three input signals. A 8.5 dB dynamic extinction ratio with a switching time of about 650 ps for the rise time and 100 ps for the fall time.     

An efficient electrode arrangement for TWSOA based inline detector

An efficient electrode scheme is developed to enhance the inline detector performance of a 1.55 μm, InP–InGaAsP, Traveling Wave-Semiconductor Optical Amplifier (TW-SOA). A traveling wave approach is used to determine the voltage developed along the length of the TWSOA, accurately. A single electrode along the entire length of the device and a single, short length electrode kept at certain distance from the front facet are investigated, for input optical power levels ranging from −25 dBm to +5 dBm. Efficient position of the single electrode along the cavity length is determined for maximum detected voltage. Under this scheme, the inline detector with an electrode length of 100 μm positioned at 200 μm from the front facet, is found to provide a improvement of 6 dB and 8 dB in detected voltage for 40 mA and 50 mA bias respectively, at −10 dBm input, when compared with the maximum detected voltage reported in the literature.     

A tunable and reconfigurable microwave photonic filter based on two cascaded modulators and a dispersive medium

A novel tunable and reconfigurable microwave photonic filter based on two cascaded modulators and a dispersive medium is proposed, theoretically discussed and experimentally demonstrated. A single-wavelength LD and a modulator (either a Mach-Zehnder modulator or a phase modulator which can avoid the dc bias drift problem) are used to obtain a multi-wavelength optical source. By adjusting the modulation frequency and the bias voltage on the modulators, the wavelength spacing and the relative amplitude of the optical tones change, thus making the filter tunable and reconfigurable. The experimental results show that the proposed filters have good tunability and reconfigurability and the mainlobe-to-sidelobe ratio of around 25 dB is achieved.     

Polarization beam splitter with wide bandwidth in air-hole-based periodic dielectric waveguides

A polarization beam splitter with wide bandwidth and simple structure in air-hole-based periodic dielectric waveguides has been proposed and designed. Operation principle of the device is based on different directional coupling properties of beams in TE and TM polarizations in parallel periodic dielectric waveguides. Performances have been evaluated by a finite-difference time-domain simulation. Results show that the polarization beam splitter provides a wide bandwidth of 113 nm with both a high extinction ratio (higher than 21 dB) and a low insertion loss (less than 1.5 dB) for optical communication wavelengths at ∼1.55 μm. Moreover, the performances of the polarization beam splitter are insensitive to longitudinal alignment errors in the coupling region, which is desirable for device fabrication and practical application.