Electro-optic modulation in an anisotropic artificial Kerr medium

A scheme using one intensity modulator and two phase modulators driven directly by sinusoidal waveform to generate an optical-frequency comb (OFC) is experimentally demonstrated. It is relatively simple, where Bragg grating or specially tailored waveforms are not used. By setting the ratio of direct-current bias to half-wave voltage and phase shifts between sinusoidal waveform applied on the intensity and two phase modulators at appropriate values, 29 comb lines with spectral power variation less than 1.5 dB at 10 GHz are obtained. And the scheme has adjustability; a flat-top OFC with frequency spacing of 9.5-10 GHz is achieved.

A new type of all-silicon surface-normal optical intensity modulator at 1.3 mu m is reported. It can be easily butt-coupled with a cleaved single-mode fiber. The device utilizes free-carrier effects in silicon to achieve phase modulation and a built-in Fabry-Perot cavity to convert the phase modulation into intensity modulation. A 10% modulation depth with a driving current density as low as 6*10/sup 3/ A/cm/sup 2/ was demonstrated. Because it can be easily coupled with single-mode fiber, and at the same time it is compatible with silicon technology, this device can provide an interface between silicon electronic circuitry and fiber optics in applications such as the fiber-to-home return link where system cost is a deciding figure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A simple and low-cost method for simultaneously realizing phase-to-intensity modulation conversion and efficiency improvement of fiber-optic links is proposed and demonstrated by utilizing a fiber Bragg grating (FBG). Optical phase modulation (PM) is converted to intensity modulation (IM) by reflecting the phase-modulated light at the slope of the FBG reflection spectrum. As the optical carrier is suppressed by the FBG, the modulation depth can be enhanced at the same time. Experimental results show that the effective optical PM-IM conversion is realized and an additional 5.5 dB electrical signal gain can be obtained with appropriate suppression of the optical carrier.

We propose and experimentally demonstrate the concurrent generation of an optical code label and payload data by use of off-the-shelf optical intensity and phase modulators in a tandem structure. This technique is capable of reconfigurable generation of an arbitrarily long optical code label of phase-shift keying concurrently with the generation of payload data, without the need for a special class of optical encoder. This technique will be powerful for all-optical packet switching networks, based on scalability and compactness, for optical code labels.

A chromatic dispersion (CD) compensation scheme for analog optical link is introduced and experimentally demonstrated based on parallel modulators module, which composes of electro-optic phase and intensity modulators. The dispersion-induced power fading is adequately compensated over operational bandwidth of 0-18 GHz using our scheme. CD-tolerant transmission over 24.45-km single mode fiber has been achieved. Crucial parameters such as gain and spur free dynamic range (SDFR) are also analyzed to evaluate the CD effect on the analog optical links. Compared with conventional links based on double-sideband (DSB) intensity modulation, the proposed scheme with hybrid modulation improves the link gain and SFDR up to 35 dB and 33 dB· Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> , respectively.

We review the evolution of modulation formats designed for direct detection, from on-off keying intensity modulation (IM) of a single polarization to three-dimensional formats where the intensity of both polarizations and the interpolarization phase are all independently modulated. We present the transmitter and receiver architectures capable of modulating an increasing number of dimensions, all orthogonal to each other. We detail the digital signal processing for Stokes vector receivers capable of recovering all three dimensions of a lightwave via self-beating direct detection, performing simultaneously polarization derotation and mitigation of intersymbol interference. We also analyze two different transmitter architectures modulating all three dimensions where the interpolarization phase is modulated over two levels using either a direct phase modulator (DPM) or a binary phase modulator, also called a BPSK modulator. We study the impact of increasing the bandwidth efficiency from 4 to 5 b/symbol by adding two level modulation of the interpolarization phase of a dual-polarization 4-level pulse amplitude modulation (PAM4) signal on the signal quality of the PAM4 IM on the polarization bearing the phase modulator by modeling the optical fields generated by both architectures. Assuming equal quality of the driving signals to the PAM4 and phase modulator, we show that a DPM minimizes the impact on PAM4 for short reach direct detect systems operating close to the zero dispersion wavelength where chromatic dispersion is negligible.

A new method of detecting phase modulation, based on using an “external” optical waveguide interferometer, is considered. This “detection” is weakly dependent on the length, position, and external conditions of the propagation channel for the phase-modulated light and, in many cases may prove more efficient than conventional methods of constructing phasemodulated fiber-optic systems.

We report the design and realization of a high-performance InGaAsP/lnP multiple quantum-well waveguide phase modulator based on the quantum confined Stark effect. Use of the quaternary InGaAsP for the well material enabled a high phase-modulation efficiency to be obtained with a low intensity modulation throughout the 1.5-1.55 μm fiber band. A 180° phase modulation of TE polarized light was obtained in a 4 mm long device under 5 V bias with just 0.6 V at 1.49 μm wavelength and 0.9 V at 1.55μm wavelength. The phase shift increased quadratically with applied bias, in accordance with theory, giving a total phase shift of 1100° (TE) and 830° (TM) at 1.4 μm at 6 V, with 500° (TE) and 400° (TM) at 1.55μm. The refractive index change associated with the measured phase shift was found to have an approximate inverse-square dependence on the energy detuning from the exciton resonance. The intensity modulation associated with the phase modulation was very low. At 1.49 μm, there was just 0.7 cm-1 measured excess loss for TE light, with 1.6 cm-1 for TM light. This excess loss decreased at longer wavelengths and vanished at around 1.54 μm. No optical intensity dependence was discernible up to the maximum cavity power of 10? W/m2. Device speed was limited to −1 GHzby RC charging times.

The performance of optical inter‐satellite communications using microwave photonics technologies employing intensity modulation with direct detection (IMDD) and phase modulation (PM) with interferometric detection are investigated and demonstrated with a numerical example. The theoretical model considering the saturated gain and amplifier spontaneous emission noise of optical booster amplifier and optical preamplifier is constructed for both architectures. Furthermore, the authors compare the performance of PM with interferometric detection to optimum biased IMDD in terms of radio frequency (RF) gain, RF noise figure and compression dynamic range (CDR). The numerical results show that the PM with interferometric detection can achieve better RF gain, RF noise figure and CDR than the optimum biased IMDD link as the laser output power is lower than 14.34, 12.45 and 4.2 dBm, respectively.

We propose a dual modulation scheme that integrates phase and intensity modulation, enabled by a directly modulated laser (DML) and an electro-absorption modulator (EAM). Unlike conventional intensity modulation schemes, we employ the frequency chirp of the DML for phase modulation through the time integral. The EAM is utilized for intensity modulation due to its advantages in frequency chirp and extinction ratio performance. To address the phase distortion induced by chromatic dispersion (CD) and bandwidth constraints, we derive a phase damage formula and propose a modified equalization algorithm based on the second-order Volterra equalizer (VE), which can effectively mitigate the differential phase interference. We experimentally validate the feasibility of generating, transmitting, and recovering the dual-modulation signals with high spectral efficiency (SE). Results indicate that compared with the traditional single-stage modulation schemes that employ only a DML or an electro-absorption modulated laser (EML), the dual modulation scheme achieves the maximum transmission rate of 84 Gb/s. Moreover, after applying the second-order VE, the system exhibits strong robustness to variations in DML chirp values, eliminating the need for precise measurement of DML chirp parameters. Additionally, the bit error rate (BER) for BtB and 25-km SSMF transmission exhibits negligible differences. The proposed dual modulation scheme enhances the transmission rate and SE and ensures high compatibility with existing passive optical networks (PONs), providing a cost-effective solution for next-generation optical access networks.

High-speed PAM-4 is a promising way for future low-cost short-reach fibre communications. However, for 1550 nm transmission with the lowest attenuation for standard single-mode fibre (SSMF), the fibre dispersion-induced inter-symbol interference (ISI) becomes non-linear in direct detection. Dispersion pre-compensation at the transmitter side is necessary to avoid such non-linear ISI. Different from previous methods to generate the vector signal from dispersion pre-compensation using IQ modulators, in this study a digital dispersion pre-compensation (DDPC) scheme based on an intensity modulator (IM) plus a phase modulator (PM) is studied. Taking into account the digital-to-analogue converter (DAC) bandwidth limit on the vector signal generation, anti-aliasing filtering on both the magnitude signal and the phase signal should be applied. The error rate performance of the IM-PM based DDPC scheme is analysed in detail using numerical simulations. For 50 Gb/s PAM-4 transmission at 1550 nm over SSMF, over 50 km fibre transmission can be supported if the two DACs have enough bandwidth larger than 17.5 GHz. If the DAC bandwidth is limited to 15 GHz, it is also possible to transmit over 20 km SSMF. Meanwhile, it is verified that practical 5-bit DAC resolution is enough for the IM-PM based DDPC scheme to work for a wide range of SSMF lengths.

Optical frequency comb (OFC) generated using cascaded intensity and phase modulators was experimentally demonstrated. Very flat OFC can be achieved by cascading intensity and phase modulators driven directly by sinusoidal waveform, where chirped fiber Bragg grating or specially tailored radio frequency waveforms are not required. It is found that the spectral flatness of OFC is related to direct current (DC) bias of intensity modulator and the optimum ratio of DC bias to half-wave voltage is 0.35. In the experiment, 15 comb lines within 1 dB spectral power variation are obtained at 10 GHz microwave frequency. The experimental results agree well with the simulation.

The use of subsequent MgO indiffusion to bury Ti indiffused waveguides in LiNbO 3 is shown to reduce the surface scatter contribution to waveguide propagation loss. Assessment of waveguide propagation loss foil- non-buried and buried waveguides at wavelengths 0.851μn, 1.06μm, 1.32tim and 1.52μm show a λ dependence for non-buried waveguides, indicating a surface scatter mechanism. Buried waveguide propagation loss exhibits no such dependence on wavelength, with a propagation loss of i0.25dB/cm over the wavelength range. Reduced excess loss of buried λ- junction waveguide structures, in Mach Zehnder intensity modulators, is demonstrated. This improvement is ascribed to increased light confinement and reduced scatter in the vicinity of the bifurcation region. Active performance of phase and intensity modulators, with non-buried and buried waveguides, indicate no significant change in required drive voltage. This is due to a choice of MgO conditions which lead to sufficient waveguide burial to reduce propagation loss, but to little change in Ti diffusion depth and hence optical/electrical field overlap characteristic.

Traditional phase generated carrier (PGC) schemes require that the carrier phase delay be kept zero and the phase modulation depth be maintained at a certain value; otherwise, serious nonlinear errors will be introduced into the phase demodulation of a sinusoidal phase modulation interferometer. Proposed herein is a robust carrier phase delay extraction method for nonlinear errors compensation of PGC demodulation that mitigates the effect of the interference phase shift. The carrier phase delay is directly extracted by adopting the first-, second-, and fourth-order in-phase and quadrature harmonic components generated by quadrature multi-harmonics frequency down conversion. Further, with the extracted phase delay, real-time compensations for the phase delay and phase demodulation depth are realized simultaneously in PGC demodulation. Theoretical analysis and derivation of the proposed method are given in detail. Simulation and displacement measurement experiments with different phase delays and phase modulation depths are performed to verify the effectiveness of the proposed method. The experimental results demonstrate that the proposed method is able to precisely extract the phase delay and efficiently eliminate the effects of the phase delay and modulation depth instability.

We report on the operation and performance of a novel electrooptic polarization scrambler that is capable of generating any desired combination of optical phase and polarization modulation. The scrambler comprises a dual-drive Mach-Zehnder-type intensity modulator on lithium niobate and an external microoptic polarization splitter which combines the two complementary outputs of the integrated-optic intensity modulator in orthogonal polarization states. The modulation indexes of the sinusoidal phase and polarization modulation in the output light can be varied independently of each other via the amplitudes and phases of the two drive voltages to the Mach-Zehnder modulator. The scrambler has a 3-dB modulation bandwidth of 14 GHz and operates in the 1.5-1.6-/spl mu/m wavelength range with an overall fiber-to-fiber insertion loss of less than 5 dB. At 5 GHz modulation frequency and electric drive powers between 390-2030 mW, we demonstrate phase-chirped and chirp-free depolarization with a residual degree of polarization of less than 0.02 and continuously variable phase modulation to 2.3-rad peak phase deviation.