Optical Intensity Modulation Research Articles

Simple optical single-sideband modulator for fibre-radio applications

A simple scheme for optical single-sideband (OSSB) modulation based on the combination of a commercial optical intensity modulator and passive fibre optic components is proposed. In addition to dispersion compensation, this OSSB modulator has experimentally demonstrated enhancement of the optical modulation depth and receiver sensitivity of a Ku-band antenna remoting link.

광변조기의 양방향 변조를 이용한 chirped fiber grating의 시간지연 측정법

광변조기의 양방향 변조를 이용한 chirped fiber grating의 시간지연 측정법

Optical fiber chromatic dispersion measurement using bidirectional modulation of an optical intensity modulator

We have proposed a simple method for measuring the chromatic dispersion and the time delay of optical fiber by using bidirectional modulation of a Mach-Zehnder modulator. To demonstrate the performance of the proposed method, we have measured the chromatic dispersion of optical fiber with length of 2.235 km by using this method over wavelength of 1500-1580 nm. The difference between the measured and the given chromatic dispersion is less than 0.19 ps/nm/km over the measured wavelength range.

KrF excimer laser trenching of <bold>X</bold>-cut <inline-formula><math display="inline" overflow="scroll"><msub><mi>LiNbO</mi><mrow><mn>3</mn></mrow></msub></math></inline-formula> for realization of optimized optical modulator electrode structures

An investigation of the micromachining of trenches in lithium niobate (LiNbO3) using direct imaged 248 nm KrF excimer laser ablation is presented. Trenches 20 μm wide and 0.5-7.5 μm deep have been produced. These trenches are assessed and are deemed suitable for the machining of integrated electro-optic structures. Based on the characterized ablation conditions, trenches of high quality have been successfully fabricated on an X-cut optical modulator which is potentially important for the realization of efficient broadband optical intensity modulators.

The Synthesis and Characterisation of Nematic NLO Polymer Networks

Ultra-fast optical intensity modulators are required for high speed telecommunications and organic electro-optical materials offer the advantages of low cost and fast switching capability. We propose a new method to fabricate electro-optical waveguide modulators using low temperature processing and poling with low electric field strength. Our method uses nematic photoreactive mesogens with large second-order polarisability and low viscosity. They can be poled with an electric field of low strength and simultaneously photopolymerised to maintain polar order. A nematic mixture with a melting point of 30°C was successfully crosslinked when aligned by an electric field. New tolane based chromophores show second order polarisabilities up to 180 × 10−30 esu, ten times the value of para-nitroaniline.

IF signal transmission at 60 GHz-band using direct modulation of a two-mode locked Fabry-Perot slave laser

We demonstrate the fiber transmission of an IF signal at 60 GHz-band using a two-mode injection-locked Fabry-Perot slave laser for fiber radio systems. The IF signal can be generated by the CW modulation of the slave laser without using an optical intensity-modulator. We confirm the fast response of the modulated slave laser and a 77 dB dynamic range for signal modulation against third-order distortion. The IF signals power deviation caused by fiber dispersion is also investigated.

LiNbO3 optical intensity modulator packaged with monitor photodiode

The assembly technique to simply install a monitor photodiode (PD) with a LiNbO/sub 3/ (LN) Mach-Zehnder modulator in the same package is proposed. A light radiated from the Mach-Zehnder Y-branch toward the LN output facet is lead into a glass bead, which is bonded to the LN facet as a fixture of the optical fiber. The end of the glass bead is cut to have an angled face, and the radiated light is reflected normal to the LN waveguide axis and toward the PD fixed on the side of package. The curved surface of the glass bead is effective to roughly focus the light onto the PD. We demonstrate that such monitor PD outputs an intensity modulation exactly reverse to the LN output modulation with a sensitivity higher than 0.03 A/W per optical output power of LN, indicating a usability of the PD output to monitor the drift of LN modulator.

Time-domain optical response of an electrooptic modulator using FDTD

A time-domain analysis of an LiNbO/sub 3/ electrooptic modulator using the finite-difference time-domain (FDTD) technique is performed. This allows for the calculation of optical modulation and the time-domain optical response of an electrooptic modulator. The electromagnetic fields computed by FDTD are coupled to standard electrooptic relations that characterize electrooptic interactions inside the embedded Ti diffused LiNbO/sub 3/ optical waveguides. The electric field-dependent change in the index of refraction inside these optical waveguides and resulting minute phase shifts imparted to optical signals propagating along the device are determined in time, allowing for the simulation of optical intensity modulation. This novel approach to LiNbO/sub 3/ electrooptic modulators using a coupled FDTD technique allows for previously unattainable investigations into device operating bandwidth and data transmission speed.

Self-aligned coupled cavity GaAs/AlGaAs midinfrared quantum-cascade laser

A monolithic GaAs/AlGaAs quantum-cascade laser with self-aligned focused ion beam-cut coupled cavities, emitting in the range of λ=9.4 μm is demonstrated. Separate pulsing of two optically coupled laser sections enables control of the lasing in single-mode and in multimode regimes. Side mode suppression ratios better than 25 dB are shown. Mode control over two single modes spaced by 3.6 cm−1 is presented. An optical output intensity modulation in a range of 20 dB is achieved. The laser exhibits a peak output power in the range of 180 mW at cryogenic temperatures. The observed mode spacing is in a good agreement with the calculation.

Modeling of broad-band traveling-wave optical-intensity modulators

In this paper, an accurate simulation tool for the electrical and optical response of broad-band traveling-wave optical intensity modulators is presented, which takes into account multisectional electrical transmission lines. This model is applied to analyze a high-speed fully packaged LiNbO/sub 3/ Mach-Zehnder interferometer.

Optical intensity modulator based on a novel electrooptic polymer incorporating high /spl mu//spl beta/ chromophore

We have synthesized a novel electrooptic (EO) polymer based on a high /spl mu//spl beta/ chromophore incorporating tricyanobutadiene acceptors. A crosslinked polyurethane network was also adopted to enhance its thermal stability. In order to find the optimum poling condition for the polymer, the influence of the electric poling profile on optical characteristics such as the EO effect, thermal stability, and damage was investigated. Then a high-speed intensity modulator using the EO polymer was designed and fabricated. The measured half-wave voltage V/sub /spl pi// was 4.5 V at the wavelength of 1.31 /spl mu/m. Accordingly, the achieved EO coefficient r/sub 33/ was as high as 25 pm/V, and the thermal stability of the poled polymer was as high as 95/spl deg/C. Finally, the modulator was successfully operated up to 40 GHz.

Activation energy of DC-drift of x-cut LiNbO3 optical intensity modulators

The DC-bias-induced drift phenomenon in LiNbO/sub 3/ optical intensity modulators is a main cause of device wearout failure. In order to estimate the device lifetime, an activation energy value Ea of the drift is needed, and Ea=1.0 eV is already known for z-cut LiNbO/sub 3/ modulators. However, Ea of x-cut LiNbO/sub 3/ modulators is not known even though there is a possibility that the Ea depends on the crystal orientation. Here, Ea=1.4 eV is obtained experimentally for the x-cut LiNbO/sub 3/ modulator with a SiO/sub 2/ buffer layer from their drift measurements between 50/spl deg/C-140/spl deg/C.

Initial bias dependency in dc drift of z-cut LiNbO<sub>3</sub> optical intensity modulators

Estimation of a long-term dc drift behavior is essential for the application of LiNbO3 optical intensity modulators to fiber communication systems. Therefore temperatureaccelerated tests are carried out using a feedback-biascontrol operation system, and the results are converted into a duration at ordinary device operation temperatures, e.g., 50°C. The activation energy Ea for the data conversion was reported to be about 1 eV for z-cut LiNbO3 modulators. 1–3 In the feedback-bias-controlled operation, a certain dc voltage is applied to the ac-driven modulator sample as the initial dc bias, and this applied dc voltage is varied continuously to keep the state of the optical output modulation at the initial state. Furthermore, the initial bias voltage set to be the same for all samples is effective in simplifying the device qualification process. However, there is a possibility that the initial dc bias will change, sample by sample, because the bias voltage required to adjust the initial output modulation state depends not only on a device design but also on an initial temperature for the system operation. The output modulation states are strongly affected by mechanical fluctuation introduced into devices during the fabrication, and a complete equalization of them is difficult. To provide practical information on the longterm drift performance of z-cut LiNbO3 modulators, we carried out experiments to show a relationship between the initial dc bias and the resulting dc drift. Because the z-cut

Very-high-speed All-optical Code-division Multiplexing Systems Using a 2^n Prime Code

We demonstrate experimental all-optical code-division multiplexing (AO-CDM) systems using 64-ps optical pulses and a 2" prime code of n = 3. A distinguishing feature of this experiment is that the modulation of an ultrashort optical clock stream by electrical data is realized without using any optical intensity modulator at each transmitter. Moreover, only low-cost optical 2 x 2 couplers and fiber delay lines are employed to implement all-serial encoders and decoders for a 2n prime code.

Synthesis, optical nonlinearity and fabrication of a waveguide device on urethane–urea copolymers

Four kinds of polyurethane and urethane–urea copolymer were prepared as optical nonlinear materials. These polymers included the same optical nonlinear chromophores and had different main chain structures. The physical properties and optical nonlinearities of the polymers were measured. Among them, a urethane–urea copolymer with a diphenylmethane backbone showed excellent optical nonlinearity (140 pm/V), long relaxation time (above 1000 hr) for the decay of nonlinearity at 100 °C and high molecular weight (high viscosity). This polymer was selected for device fabrication as a well-balanced polymer. In order to define a waveguide, a photobleaching process was examined using this polymer. Single-mode optical propagations were attained at 830 nm and at 1330 nm using the defined waveguides, respectively. Furthermore, multilayerd electro-optic intensity modulators were fabricated using the urethane–urea copolymer, and then the optical intensity modulations caused by applied voltage were demonstrated. A half-wave voltage of 18 V was obtained at 1 kHz modulation. Copyright © 1999 John Wiley & Sons, Ltd.

Experiments on high-speed all-optical code-division multiplexing systems using all-serial encoders and decoders for 2/sup n/ prime code

In this paper, we demonstrate experimental all-optical code-division multiplexing (AO-CDM) systems using 64-ps optical pulses and a 2/sup n/ prime code of n=3. A distinguishing feature of this experiment is that the modulation of ultrashort optical clock stream by electrical data is realized without using any optical intensity modulator at each transmitter. Moreover, only low-cost standard optical 2/spl times/2 couplers and fiber delay lines are employed to implement all-serial encoders and decoders for a 2/sup n/ prime code. As a result, this new system is more cost- and power-effective than a conventional AO-CDM system. Furthermore, the use of AO-CDM systems can offer parallel communications over a common fiber channel, which in turn can support real-time computer interconnections for image and data communications.

Optical slow-wave resonant modulation in electro-optic GaAs/AlGaAs modulators.

A new type of resonant optical intensity modulator has been developed by incorporating an optical slow-wave structure into a GaAs/AlGaAs Mach-Zehnder waveguide modulator. The first experimental results demonstrate improvements in the efficiency by factors of up to 2.9 at the resonant wavelength. The lowest switching voltage, V/sub /spl pi//, is 4.5 V.

Dynamic behavior and applications of a three-contact vertical-cavity surface-emitting laser

We have developed a novel three-contact vertical-cavity surface-emitting laser (VCSEL) with an intracavity absorber for use in high-speed data communications. We demonstrate two distinct designs for our VCSEL and discuss the device physics underlying the behaviors of the two designs. Furthermore, we demonstrate a wide range of applications for these VCSELs, including optical intensity modulation, phase and amplitude modulation of a microwave optical subcarrier, optical bistability, self-pulsation, and optical disk readout.

Activation Energy for Dc-drift in X-cut LiNbO_3 Optical Intensity Modulators

Existence of a wafer axial dependency in the activation energy (Ea) for the dc-drift of LiNbO3 modulators has been experimentally found. The Ea for the x-cut modulators is derived to be 0.2~0.5 eV, and 1 eV for the z-cut ones.

Dc drift reduction in <inline-formula><roman>LiNbO</roman><sub><roman>3</roman></sub></inline-formula> optical modulators by decreasing the water content of vacuum evaporation deposited <inline-formula><roman>SiO</roman><sub><roman>2</roman></sub></inline-formula> buffer layers

With increasing demand for LiNbO3 ~LN! optical intensity modulators for fiber communication systems, fine process control of LN device fabrication has become important to increase the fabrication yield and to supply low-priced modulators.