Theoretical study of polarization dependence of carrier-induced refractive index change of quantum dot.

The influences of dot material component, barrier material component, aspect ratio and carrier density on the refractive index changes of TE mode and TM mode of columnar quantum dot are analyzed, and a multiparameter adjustment method is proposed to realize low polarization dependence of refractive index change. Then the quantum dots with low polarization dependence of refractive index change (<1.5%) within C-band (1530 nm - 1565 nm) are designed, and it shows that quantum dots with different material parameters are anticipated to have similar characteristics of low polarization dependence.

[Spectral Study on the Effects of Angle-Tuned Filter Wedge Angle Parameter to Reflecting Characteristics].

Three-port tunable optical filter is a key device in the all-optic intelligent switching network and dense wavelength division multiplexing system. The characteristics of the reflecting spectrum, especially the reflectivity and the isolation degree are very important to the three-port filter. Angle-tuned thin film filter is widely used as a three-port tunable filter for its high rectangular degree and good temperature stability. The characteristics of the reflecting spectrum are greatly influenced not only by the incident angle, but also by the wedge angle parameter of the non-paralleled wedge thin film filter. In the present paper, the influences of the wedge angle parameter to the reflectivity and the half bandwidth are analyzed, and the reflecting spectrum characterstics are simulationed in different wedge angle parameter and polarity. The wedge angle-tuned thin film filter with 0.8° wedge angle parameter is fabricated. The experimental results show that keeping the wedge angle the same orientation to the incident angle will worsen the reflectivity and the rectangular degree of the reflecting spectrum. However, keeping the wedge angle orientation reverse to the incident angle will enhance the reflectivity and decrease the bandwidth, which will give higher reflectivity and isolation degree to the three-port filter than that of high parallel degree angle-tuned thin film filter.

Ultra-wide band signal generation using a coupling-tunable silicon microring resonator.

Ultra-wide band signal generation using a silicon microring resonator tuned to an NRZ-DPSK modulated optical carrier is proposed and demonstrated. The scheme is shown to enable the generation of UWB signals with switchable polarity and tunable bandwidth by simply tuning the coupling regions of the microring resonator. Monocycle pulses with both negative and positive polarities are successfully synthesized experimentally.

Hybrid coding method of multiple orbital angular momentum states based on the inherent orthogonality.

We put forward a hybrid coding method of multiple orbital angular momentum (OAM) states based on the inherent orthogonality. Theoretical analysis shows this new coding method can tremendously improve the coding efficiency compared to the conventional way, and the OAM mode can be extended to all the transverse orthographic modes such as Bessel beams or Laguerre-Gauss modes. We also present a typical scheme to achieve the encoding of hybrid OAM states based on a concentric ring resonator array. We further suggest a digital device and a phase hologram pattern to decode the hybrid OAM states.

Theoretical study of polarization insensitivity of carrier-induced refractive index change of multiple quantum well.

Characteristics of polarization insensitivity of carrier-induced refractive index change of 1.55 µm tensile-strained multiple quantum well (MQW) are theoretically investigated. A comprehensive MQW model is proposed to effectively extend the application range of previous models. The model considers the temperature variation as well as the nonuniform distribution of injected carrier in MQW. Tensile-strained MQW is expected to achieve polarization insensitivity of carrier-induced refractive index change over a wide wavelength range as temperature varies from 0°C to 40°C, while the magnitude of refractive index change keeps a large value (more than 3 × 10-3). And that the polarization insensitivity of refractive index change can maintain for a wide range of carrier concentration. Multiple quantum well with different material and structure parameters is anticipated to have the similar polarization insensitivity of refractive index change, which shows the design flexibility.

[Research on the transmittance spectrum of wedge thin film filter with oblique incidence].

Angle-tuned thin film filter is widely used in the DWDM system for its broad tunable wavelength range and high rectangular degree. The transmissivity and the half bandwidth is greatly influenced not only by the incident angle, but also by the wedge angle of the non-paralleled thin film filter. In the present paper, the influences of the wedge angle on the transmissivity and the half bandwidth were detailedly analyzed. The proper wedge angle and the orientation can greatly improve the characteristics of the transmittance spectrum. The angle-tuned thin film filter with 0.8 degrees wedge angle was also fabricated. The experimental results show that keeping the wedge angle with the same orientation to the incident angle will worsen the transmissivity and the rectangular degree of the transmittance spectrum. However, keeping the wedge angle orientation reverse to the incident angle will greatly enhance the transmissivity and the rectangular degree of the filter and its tunable wavelength range will broaden by 10 nm.

Comparison analysis of optical frequency comb generation with nonlinear effects in highly nonlinear fibers.

We compare several schemes for broadband optical frequency comb (OFC) generation based on several nonlinear effects in highly nonlinear fiber (HNLF). Cascaded four wave mixing (CFWM) and self-phase modulation (SPM) processes in HNLF are proved to be effective ways for spectrum broadening. We investigate some parameters affecting the performance of the output OFC in detail. When only CFWM occurs in the HNLF, broadband OFC can be generated with poor power flatness. When only SPM occurs in the HNLF, we obtain a 10 GHz OFC of 103 comb lines within 5-dB power deviation. When both CFWM and SPM simultaneously occur in the HNLF, we obtain a 10 GHz OFC of 143 comb lines within 4.5-dB power deviation. All the OFC generation schemes have the advantages of tunability of central wavelength and repetition frequency.

Compact, flexible and versatile photonic differentiator using silicon Mach-Zehnder interferometers.

We propose and experimentally demonstrate the flexibility and versatility of photonic differentiators using a silicon-based Mach-Zehnder Interferometer (MZI) structure. Two differentiation schemes are investigated. In the first scheme, we demonstrate high-order photonic field differentiators using on-chip cascaded MZIs, including first-, second-, and third-order differentiators. For single Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.5%. In the second scheme, we demonstrate multifunctional differentiators, including intensity differentiator and field differentiator, using an on-chip single MZI structure. These different differentiator forms rely on the relative shift between the probe wavelength and the MZI resonant notch. Our schemes show the advantages of compact footprint, flexible functions and versatile differentiation forms. For example, high order field differentiators can be used to generate complex temporal waveforms, such as high order Hermite-Gaussian waveforms. And intensity differentiators are useful for ultra-wideband pulse generation.

High-order photonic differentiator employing on-chip cascaded microring resonators.

We propose and experimentally demonstrate a high-order photonic differentiator using on-chip complementary metal oxide semiconductor-compatible cascaded microring resonators, including first-, second-, and third-order differentiators. All the microring resonator units have a radius of 150 µm and a free spectral range of 80 GHz. The microring resonator can implement the first-order derivative of the optical field near its critical coupling region. Hence higher-order differentiation can be obtained by cascading more microring units on a single chip. For the periodical Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.2%. The differentiation of pseudo-random bit sequence signals at 5 Gbit/s is also demonstrated. Our scheme is a compact and low-power-consumption solution since the cascaded microring units are fabricated with compact size on the silicon-on-insulator substrate.

All-optical binary phase-coded UWB signal generation for multi-user UWB communications.

An all-optical incoherent scheme for generation of binary phase-coded ultra-wideband (UWB) signals is proposed and experimentally demonstrated. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PM-IM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating the phase-modulated light waves at the positive and negative slopes of the DI transmission spectra, binary phase encoded UWB codes (0 and π) are generated. We also experimentally demonstrate a bipolar UWB coding system with a code length of 4, operating at 1.25 Gb/s. And the decoding is analyzed as well. Our proposed system has potential application in future high-speed UWB impulse radio over optical fiber access networks.

Bandwidth and wavelength-tunable optical bandpass filter based on silicon microring-MZI structure.

A novel and simple bandwidth and wavelength-tunable optical bandpass filter based on silicon microrings in a Mach-Zehnder interferometer (MZI) structure is proposed and demonstrated. In this filter design, the drop transmissions of two microring resonators are combined to provide the desired tunability. A detailed analysis and the design of the device are presented. The shape factor and extinction ratio of the filter are optimized by thermally controlling the phase difference between the two arms of the MZI. Simultaneous bandwidth and wavelength tunability with in-band ripple control is demonstrated by thermally tuning the resonance offset between the two microring resonators.

Generation of a 640 Gbit/s NRZ OTDM signal using a silicon microring resonator.

A 640 Gbit/s NRZ OTDM signal has been successfully generated for the first time by format conversion of a 640 Gbit/s OTDM signal from RZ to NRZ. First, a coherent 640 Gbit/s OTDM RZ signal is generated by wavelength conversion of the original incoherent OTDM signal utilizing Kerr switching in a highly nonlinear fiber. Second, RZ-to-NRZ format conversion is achieved in a specially designed silicon microring resonator with FSR of 1280 GHz, Q value of 638, high extinction ratio and low coupling loss to optical fiber. A 640 Gbit/s NRZ OTDM signal with very clear eye-diagram and narrower bandwidth than both the original incoherent 640 Gbit/s and the wavelength converted coherent 640 Gbit/s RZ OTDM signals has been obtained. Bit error ratio measurements show error free (<10(-9)) performance at a received power of -30 dBm for all the OTDM channels of the 640 Gbit/s NRZ signal, with very low power penalty (<0.5 dB) and improved dispersion tolerance compared to the wavelength converted RZ case.

Multi-channel WDM RZ-to-NRZ format conversion at 50 Gbit/s based on single silicon microring resonator.

We comprehensively analyze multiple WDM channels RZ-to-NRZ format conversion using a single microring resonator. The scheme relies on simultaneous suppression of the first order harmonic components in the spectra of all the RZ channels. An optimized silicon microring resonator with free spectral range of 100 GHz and Q value of 7900 is designed and fabricated for this purpose. Multi-channel RZ-to-NRZ format conversion is demonstrated experimentally at 50 Gbit/s for WDM channels with 200 GHz channel spacing using the fabricated device. Bit error rate (BER) measurements show very good conversion performances for the scheme.

Pattern effect reduction in all-optical wavelength conversion using a two-electrode semiconductor optical amplifier.

A two-electrode semiconductor optical amplifier that has two sections and two separated electrodes is proposed for an all-optical wavelength converter. Pattern effect reduction based on dynamic gain compensation is theoretically investigated using a time-domain model that takes into account the carrier diffusion process between the two sections, and the Q factor of the converted light is calculated to evaluate pattern effects. Simulation results show that the Q factor is greatly improved when the section lengths and the currents are appropriately selected.

Design of temperature-independent arrayed waveguide gratings based on the combination of multiple types of waveguide.

We develop a design theory for a temperature-independent arrayed waveguide grating (TI-AWG) based on the combination of multiple types of waveguide. Each type of waveguide has a path-length difference between adjacent arrayed waveguides, and the path-length difference ratio is introduced as tuning parameter. A TI-AWG with Si wire and slot waveguides is given as an example. The thermal spectra shift of the TI-AWG can be tuned from redshift to blueshift in an ultralarge range, and the modified interference order can be reduced or enhanced. The device size is about one-fifth that of the narrow-wide-wire design that uses a combination of narrow and wide Si wire waveguides. The results are verified by the simulation of prototype devices via a two-dimensional finite-difference time-domain program.

Photonic generation of ultra-wideband signals by direct current modulation on SOA section of an SOA-integrated SGDBR laser.

Photonic ultra-wideband (UWB) pulses are generated by direct current modulation of a semiconductor optical amplifier (SOA) section of an SOA-integrated sampled grating distributed Bragg reflector (SGDBR) laser. Modulation responses of the SOA section of the laser are first simulated with a microwave equivalent circuit model. Simulated results show a resonance behavior indicating the possibility to generate UWB signals with complex shapes in the time domain. The UWB pulse generation is then experimentally demonstrated for different selected wavelength channels with an SOA-integrated SGDBR laser.

Ultrahigh-Q microwave photonic filter with Vernier effect and wavelength conversion in a cascaded pair of active loops.

A new cascaded microwave photonic filter that can realize a high Q value is presented. It consists of two infinite impulse response filters based on two active loops. Owing to wavelength conversion employing cross-gain modulation of the amplified spontaneous emission spectrum of the semiconductor optical amplifier in one loop, the interference between the modulated optical signals of different taps from two loops can be avoided, and stable transmission characteristics of the filter can then be achieved. Using this cascaded structure, the free spectral range and Q value can be increased significantly, and tunability can also be realized. Measured results of a high Q of 3338 and a rejection ratio of about 40 dB are achieved.

Active microring optical integrator associated with electroabsorption modulators for high speed low light power loadable and erasable optical memory unit.

We propose and analyze a novel loadable and erasable optical memory unit based on an active microring optical integrator associated with electroabsorption modulators (EAM) on III-V material system. The gain of the active microring is characterized by the two energy band model with amplified spontaneous emission noises taken into account. Based on the light field propagation equation in the active microring waveguide and the transfer function of the EAM-MZI switch, the step function performances of the optical memory under the gain matching condition are discussed for different injection light powers. After that, the memory operation of the novel optical memory unit is analyzed in detail. Simulations show that, the step function response and memory performances are affected by the carrier consumption. However, such influence will be released, and the memory operates well for the low light power injection case. The novel optical memory unit is promising to be cascaded connected and densely integrated for high speed low power optical data stream storage and buffer.

A proposal for two-input arbitrary Boolean logic gates using single semiconductor optical amplifier by picosecond pulse injection.

We propose theoretically two-input arbitrary Boolean logic (NOR, OR, AND, XOR, XNOR, NAND) using single semiconductor optical amplifier (SOA) assisted by several detuning optical filters. The probe spectrum is broadened by picosecond pulse injection in the SOA, and four consequent optical Gaussian filters are used to select different frequency components to acquire logic NOR, OR, AND, XOR, respectively. Then two additional logic gates, XNOR and NAND, are realized by combining two logic channels. The power penalty, Q-factor, and extinction ratio are measured for all logic gates. It is shown that the output logic with dark return-to-zero (RZ) format has a large power penalty. The Q-factor is larger than 6 and the extinction ratio is larger than 6.3 dB for all logic gates within 16 nm wavelength range.

Simultaneous multiple DWDM channel NRZ-to-RZ regenerative format conversion at 10 and 20 Gb/s.

We propose and demonstrate simultaneous optoelectronic NRZ-to-RZ regenerative format conversion for multiple DWDM channels using a single phase modulator (PM) and a fibre delay-interferometer (DI). In order to accommodate multiple DWDM channels, the DI is designed to have a free spectral range (FSR) equal to the channel spacing. This thus extracts the chirp induced by the phase modulation for all the channels at the same time. Since the original carriers are suppressed to some extent, the NRZ-to-RZ conversions can be achieved with regeneration. Multi-channel format conversion is successfully demonstrated for 16 channels at 10 Gb/s and 8 channels at 20 Gb/s, with a channel spacing of 100 GHz. Bit error ratio (BER) measurements at 10 Gb/s show 3.5 and 4.2 dB penalty improvements for 50 and 75 km transmission without dispersion compensation, respectively. Significant extinction ratio (ER) improvement and timing jitter reduction is observed for the converted channels.