LiNbO3 Optical Modulator Research Articles

Comprehensive Study of Z-Cut Highly Integrated LiNbO3 Optical Modulator with Adjustable Chirp Parameters

AbstractIn this paper, the domain inversion is used in a simple fashion to improve the performance of a Z-cut highly integrated LiNbO

X-Cut LiNbO3 Optical Modulators Using Gap-Embedded Patch-Antennas for Wireless-Over-Fiber Systems

We propose an x-cut LiNbO3 optical modulator using gap-embedded patch-antennas for wireless-over-fiber systems. The proposed device is composed of an array of narrow-gap-embedded patch-antennas and an optical waveguide located at the center of the gap without a buffer layer. The modulation efficiency of the proposed x-cut LiNbO3 optical modulators was enhanced by 6 dB compared to the z-cut LiTaO3-based devices.

Modeling and Analysis of LiNbO3 Optical Modulator with Two Step Back Slot Structure

A numerical model of x-cut LiNbO3 optical modulator with a two step back slot structure to satisfy the velocity matching condition without the layer of silicon dioxide is presented. The analysis is based on the method of transverse technique. An equivalent circuit is deduced to represent the structure. Generalized trial quantities are introduced as virtual adjustable sources in the equivalent network representation of boundary conditions. The lossy conductor of a planar transmission line is represented by a particular two port network. Thus, metallic losses can be evaluated for any metallization thickness without restricting the conductor modeling to a simple surface impedance approximation. The dependence of the effective microwave index, the characteristic impedance and the product of the drive voltage and the electrode interaction length on the thickness of substrate near the ground electrode has been investigated. The optical response of the modulator is also evaluated. This modulator is sufficient to 40 Gb/s optical transmission systems.

Development of Electro-Optic Phase Modulator for 94 GHz Imaging System

This paper presents the latest improvements in design and fabrication of ridged LiNbO3 optical phase modulators operating in the W-band for millimeter-wave imaging applications at 94 GHz. It describes the design conditions for effective index matching between optical carrier at 1550 nm and millimeter-wave signal, as well as impedance matching and propagation losses reduction. The impacts of different geometric parameters on the device performance are discussed. In addition, the paper reports the diverse material processing techniques for device fabrication. Characterization of the fabricated devices shows conversion efficiency as high as 1.045 W- 1 at 94 GHz with a 7 V dc half-wave voltage and a 3.7 dB optical insertion loss.

An optical link for microwave clock distribution using optical carrier suppression and DC drift compensation

AbstractBias electrode DC drift of LiNbO3 optical modulators becomes a critical problem when these devices operate over a long time period. This article presents a novel system of DC drift compensation for optical microwave transmissions using optical carrier suppression technique. In our case, such a system is dedicated to the optical distribution of high spectral purity microwave signals in future satellite payloads. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1634–1637, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22544

Modelling and experimental validation of an x-cut four phase modulators structure

A general model to describe the operation of an x-cut LiNbO3 optical modulator comprised of four phase modulators in parallel, originating two inner Mach–Zehnder modulators (MZMs) integrated in one outer MZM is proposed. This model considers the application of arbitrary electrical signals to the electrode inputs, instead of electrical tones as in previous models. Such improvement allows the accurate modelling of a wide range of applications to the optical modulator. Moreover, an experimental method to characterise the steady state of each inner MZM and outer MZM, without the interference of the other MZMs, is proposed. This characterisation provides the three bias voltages required to apply to each MZM according to the desired output modulation. Validation of the proposed model for dynamic operation is done using transmission of non-return to zero signals along a dispersive fibre with chirped and chirp-free signals at the output of the modulator. Excellent agreement between experimental and theoretical results is obtained in both steady-state and dynamic regimes.

10Gbit/s FSK transmission over 95km SMF using a LiNbO3 modulator

We demonstrate high-speed optical frequency-shift-keying (FSK) transmission over a 95km single-mode fiber (SMF) at 10Gbit/s using a LiNbO3 optical FSK modulator and an optical interleaver. The measured eye diagrams, bit error rate (BER) performances and optical spectrum indicate the FSK modulation is similar to zero chirp non-return-to-zero (NRZ) on-off-keying (OOK) transmission. Therefore, in view of dispersion tolerance, the FSK modulation can be treated as well as NRZ OOK.

Methods for measuring the RF half-wave voltage of LiNbO3 optical modulators

We present methods to measure the RF half-wave voltage Vπ(f) using a photodetector and an RF spectrum analyzer based on (i) the nonlinear response and (ii) the two-point technique. The results obtained from these methods for an LiNbO3 Mach–Zehnder optical modulator at 6.3 GHz are compared and good agreement is found between all the methods. Also, measurement methods based on optical-spectrum techniques, namely, the optical response and two-tone methods, are analyzed. An error analysis shows that the new and simpler techniques proposed offer similar levels of measurement uncertainty. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 440–443, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21011

Effects of parasitic modes in high-speed LiNbO3 optical modulators

The characteristics of RF parasitic modes and the methods to suppress leakage phenomena in LiNbO(3) optical modulators were studied. The high frequency RF power transmission characteristics were simulated and experimented in the respects of LiNbO(3) wafer thickness, the kind of material contacting the back surface of the modulator chip, the gap and width of the CPW (co-planar waveguide) electrodes. An appropriate RF electrode geometry, to minimize coupling efficiency between co-planar waveguide and substrate mode, is proposed. Experimental results prove that the approaches made in this work are effective for broadening of modulation bandwidth.

Low-driving-voltage band-operation LiNbO 3 modulator with lightwave reflection and double-stub structure

A reflection-type Mach-Zehnder LiNbO3 optical modulator with a double-stub structure that can modulate both forward and backward lightwave to achieve low-driving-voltage optical modulation for band-operation is developed. Using optical reflection the modulation efficiency is doubled. The halfwave-voltage of the fabricated modulator was 2.9 V at 7 GHz.

LiNbO 3 resonant-type optical modulatorwith double-stub structure

A resonant-type LiNbO3 optical modulator with symmetric stubs and modulating electrodes was investigated. Modulation efficiency was enhanced by the resonance of the modulating electrode. The enhancement factor of the fabricated modulator was 3.41 at 10 GHz.

X-cut lithium niobate optical single-sideband modulator

A hermetically packaged x-cut LiNbO3 optical single-sideband modulator is presented, which successfully performs a carrier-suppression ratio over 25 dB with an RF driving voltage of 8 Vp-p at 10 GHz.

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

Estimation of frequency response for high-speed LiNbO3 optical modulators

A method of directly measuring the optical sideband in modulated output light by using an optical spectrum analyser is described for quantitatively estimating frequency responses of LiNbO/sub 3/ optical modulators in a practical large-signal operation. The conventional methods of estimating microwave power from photo-detectors using an electrical network analyser or an electrical sampling oscilloscope are theoretically compared with the method of using an optical spectrum analyser. The frequency responses measured with the method that uses an optical spectrum analyser are shown to correspond consistently to the small-signal modulation for any value of modulation index in measurements and to be useful in estimating precisely the frequency response of ultra-high-speed optical modulators. The responses estimated with the conventional methods mentioned above depend on the modulation index at a larger-signal modulation of m/sub 0/>/spl sim/0.1. In particular, the responses when an electrical sampling oscilloscope is used vary remarkably with the value of m/sub 0/. These analyses make possible accurate estimation of the LiNbO/sub 3/ modulator's frequency response, and thus aid in its practical use in large-signal operation of pulse modulation.

Quasistatic analysis of coupled coplanar waveguide traveling-wave electrodes for electro-optic modulators

In this letter, an LiNbO3 optical modulator using a coupled coplanar waveguide (CPW) with an SiO2 buffer layer is analyzed accurately by incorporating the traveling-wave electrode thickness effect. The effect of coupled ground planes, electrode, and buffer layer thickness on the effective dielectric constant and characteristic impedance of coupled CPW electrode is discussed. Numerical computations show that increasing the width of the coupled ground planes can slightly improve the phase velocity mismatch between electric and optical waves. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 20: 284–286, 1999.

Broadband and low driving-voltage LiNbO3 optical modulators

Travelling-wave type LiNbO3 (LN) optical modulator structures employing thicker coplanar waveguide (CPW) electrodes are presented, for achieving a lower driving-voltage and broader bandwidth characteristic with an impedance and velocity match condition. To design a high-performance device-structure, a figure of merit p is introduced for travelling-wave type optical modulators, which can make an excellent modulator with a smaller value of p. Microwave characteristics of the modulators with a ridged type LN-substrate are theoretically compared to those of planar-type modulators. As a result, the characteristic-impedance and the product of driving-voltage and interaction-length of the electrodes in modulators are shown to be almost determined only by the buffer-layer thickness for any modulator-structure under a velocity-matching condition. By utilising a wider gap and accordingly thicker electrodes with a ridged LN-substrate, LN modulator structures having a 40 GHz-band and far less than a 3 V driving-voltage have been clarified under a 50 Ω characteristic-impedance system.

Design and Performance of LiNbO 3 Optical Modulator with a Superconducting Electrode

The performance of a LiNbO3 optical modulator with a superconducting electrode as the transmission line of a traveling-wave signal has been studied theoretically as well as experimentally. In the case of velocity matching between signal and optical waves using a shielding plane on top of a coplanar waveguide, numerical calculations of the attenuation constants of both superconducting and normal-conducting transmission lines indicate that the performance of the optical modulator is far superior to that using normal metals with respect to the figure of merit of bandwidth/driving voltage. Microwave characteristics of a traveling-wave-type LiNbO3 optical modulator with a superconducting electrode (Nb) have been also studied experimentally in the temperature range from 300 K to 4.2 K. In the frequency range between dc and 26.5 GHz, it is shown that the obtained modulation depth is in good agreement with the theoretically expected one. The present results demonstrate the possible applications of superconducting electrodes to high-performance LiNbO3 optical modulators.

Frequency-dependent propagation characteristicsofcoplanar waveguide electrode on 100 GHz Ti:LiNbO 3 optical modulator

The microwave propagation characteristics of a coplanar waveguide electrode in the frequency range 45 MHz – 110 GHz were studied in order to evaluate the factors which limit the performance of a high-speed ridged Ti:LiNbO3 optical modulator. Since the effective microwave index is constant in the higher frequency range, neither structure nor material dispersions of the modulator are observed. In addition to the conductor loss, the dielectric loss also increases above 20 GHz. The dielectric loss is shown to originate not only in the LiNbO3 substrate, but also in the silicon oxide buffer layer.

Theoretical analysis of the sensitivity of electric field sensors using LiNbO3 optical modulator

The electric field sensor using an optical modulator makes possible accurate measurement without disturbing the electromagnetic field to be measured and is expected to have applications in EMC measurement. In this paper, a theoretical analysis of the sensitivity of the electric field sensor using an LiNbO3 modulator and the error factors are described. First, the electric field sensor consisting of a rod, an optical modulator, an optical source, and an optical detector is expressed by an equivalent circuit. Next, of the equivalent circuit parameters, those related to the sensor rod are derived by a numerical analysis using the method of moments. Those related to the optical modulator are derived by the theoretical analysis and measurement. A method is presented for determination by measurement in regard to the optical source and the detector. An electric field sensor is fabricated and its characteristics are evaluated. It is found that the difference between the measured value and the theoretical value of the sensitivity is about 8.2 dB. The causes of the difference are the input capacitance and the insertion loss of the optical modulator. These two parameters are measured beforehand and used in the calculation. Then, the sensitivity of the electric field sensor can be estimated with an accuracy of 1 dB. The analysis method used in this paper is found to be effective for sensitivity analysis of the electric field sensor. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80(12): 79–89, 1997

Characteristic of a broadband Ti:LiNbO3 optical modulator with buried electrodes and etched grooves in the buffer layer

Traveling wave Ti:LiNbO3 Mach-Zehnder optical modulators with buried electrodes and etched grooves in the SiO2 buffer layer are analyzed by the finite element method. The tradeoff between the bandwidth BW and the half-wave voltage Vπ is discussed. The value of BW/Vπ is used to weight the total performance of the modulator. Taking a thick buffer layer and etching deep grooves in the buffer layer are demonstrated as two effective methods to improve the performance of the modulator. A 3-dB optical bandwidth of 18 GHz with half-wave voltage 5V at a wavelength of 1.55 pm could be obtained even though the electrode is not very thick. When the requirement of half-wave voltage is not very critical, a bandwidth of more than 100 GHz can be obtained.