Generation of a superstable Lorentzian pulse train with a high repetition frequency based on a Fabry-Perot resonator integrated with an electro-optic phase modulator

A simple technique for converting a continuous-wave laser beam into a stable Lorentzian pulse train with a high repetition frequency is demonstrated experimentally. We generated transform-limited pulses of up to 40 GHz, which were composed of higher-order sidebands produced by a Fabry-Perot resonator integrated with an electro-optic phase modulator (EOM). The rf power supplied to the EOM determines the pulse width in the pulse train. This approach enables the pulse width to be continuously tuned from 2.1 to 7.0 ps at the same repetition frequency without any wavelength shift. Furthermore, we experimentally evaluated the stability of the pulse train's amplitude and obtained stable bit-error-free operation at 9.95 GHz.     

Radiation hardness of electro-optic asymmetric Fabry-Perot modulators

Analog optical links for LHC experiments are being developed, using electro-optical modulators as front-end transmitters. In the inner trackers, these devices will be subject to a harsh radiation environment, combining ionizing radiation and hadrons, up to 10 Mrads and 10¹⁴ (neutron-equivalent)/cm², over the experiment lifetime. The radiation hardness of the Asymmetric Fabry-Perot Modulator (AFPM) has been tested, with both kinds of radiations, up to LHC total doses and fluences. Results confirm a radiation hardness which qualifies the AFPM for application in the detector front ends.     

Diffraction light modulator based on transverse electro-optic effect in short-pitch ferroelectric liquid crystals

A diffraction modulator that exploits the transverse electro-optic effect in ferroelectric liquid crystals is proposed for applications in displays and in spatial light modulators. Experiments with a short-pitch ferroelectric liquid crystal aligned homeotropically show an achromatic contrast ratio of greater than 100:1 available with oblique readout. The sources of the contrast deterioration and the tolerance of the proposed scheme to this deterioration are analyzed. For selected directions of readout the light output obtains low sensitivity to the polarization of the readout light. Nonlaser light sources can be used in a practical display setup based on the proposed principle.     

Single-crystal modifications of the Mach-Zehnder electro-optic modulator

Two single-crystal modifications of the Mach-Zehnder electro-optic modulator are described. Preliminary theoretical estimates show that these variants are capable of ensuring the amplitude modulation of laser beams with elevated power (no less than several watts) at frequencies up to 4 GHz. The obtained modulation characteristics show that the modulation depth in the proposed devices reaches 50–80% and a half-wave voltage of these devices amounts to 100–160 V.     

Velocity matching of a GaAs electro-optic modulator

New designs for the velocity matching of a deep-etched semiconductor electro-optic modulator are presented. A tantalum pentoxide (Ta2O5) coating is considered here for achieving velocity matching between the microwave and the optical signals. The effects of the velocity mismatch, the conductor loss, the dielectric loss, and the impedance mismatch are studied in relation to the optical bandwidth of a high-speed semiconductor modulator. It is shown that both the dielectric loss and the impedance matching play key roles for velocity-matched high-speed modulators with low conductor loss. The effects of Ta2O5 thickness on the overall bandwidth and on the half-wave voltage-length product VpiL are also reported.     

Characterizing a fiber-based frequency comb with electro-optic modulator

We report on the characterization of a commercial- core fiber-based frequency comb equipped with an intracavity free-space electro-optic modulator (EOM). We investigate the relationship between the noise of the pump diode and the laser relative intensity noise (RIN) and demonstrate the use of a low-noise current supply to substantially reduce the laser RIN. By measuring several critical transfer functions, we evaluate the potential of the EOM for comb repetition rate stabilization. We also evaluate the coupling to other relevant parameters of the comb. From these measurements, we infer the capabilities of the femtosecond laser comb to generate very-low-phase-noise microwave signals when phase-locked to a high-spectral-purity ultra-stable laser.     

Calibration of laser frequency scan with an electro-optic modulator

An electro-optically modulated laser beam was used to excite the Yb1S0 (6s ) 2?3P1 (6 s 6 p ) transition. Fluorescence was recorded as the laser frequency was scanned across the transition. Each isotope generated multiple peaks in the spectrum separated by the modulation frequency that permitted the frequency scan to be calibrated. The resulting isotope shifts agree well with existing data obtained with an interferometer to monitor the change in laser frequency. Electro-optic modulators have the advantages of being relatively inexpensive and simpler to operate than interferometers whose length must typically be stabilized with a laser whose wavelength is locked to an atomic transition.     

Analysis of electro-optic crystal-based Fabry-Perot etalons for high-speed spatial light modulators

The use of electro-optic (EO) crystal-based Fabry-Perot modulators (FPMs) as high-speed spatial light modulators is proposed. The FPMs operate with an extremely low drive voltage and a high extinction ratio. It is revealed by analysis of both the linear EO effect and the inverse piezoelectric effect of various EO crystals that three kinds of crystal configuration are suitable as FPMs. One of these is applicable to isotropic crystals, point groups 23 and (-)43m, and the others are better suited for uniaxial EO crystals, point groups (-)42m and 3m. Typical EO crystals suitable as FPMs are ferroelectric crystals such as LiNbO3, LiTaO3, and LiIO3 and sillenite compounds such as Bi12SiO20 and Bi12GeO20 as well as compound semiconductors such as GaAs and GaP.     

High-finesse Fabry-Perot electro-optic sensors with enhanced sensitivity and high spatial resolution

We present a high-finesse optical cavity containing a LiTaO(3) electro-optic crystal, devoted to free-space electric field characterization. Theoretical considerations will show that the modulation depth is directly related to the transversal components of the field to be measured, thus opening the way to vectorial mapping of the electric field using a single electro-optic crystal. Also, a discussion about noise and sensitivity will be given. As the latter increases with the effective cavity length, and bandwidth decreases, a trade-off is realized, allowing us to measure an electric field of 60 mV/m/ sqrt[Hz] in a 110 MHz bandwidth. Cavity dimensions are less than 8 mm(3), giving an inner-crystal transverse spatial resolution of 70 microm and allowing pigtailed systems to integrate.     

Proposal for electro-optic multiplier based on dual transverse electro-optic Kerr effect

A novel electro-optic multiplier is proposed, which can perform voltage multiplication operation by use of the Kerr medium exhibiting dual transverse electro-optic Kerr effect. In this kind of Kerr medium, electro-optic phase retardation is proportional to the square of its applied electric field, and orientations of the field-induced birefringent axes are only related to the direction of the field. Based on this effect, we can design an electro-optic multiplier by selecting the crystals of 6/mmm, 432, and m3m classes and isotropic Kerr media such as glass. Simple calculation demonstrates that a kind of glass-ceramic material with a large Kerr constant can be used for the design of the proposed electro-optic multiplier.     

Benzofuran-terminated infrared dyes and their electro-optic properties in guest-host polymers

Molecular first hyperpolarizabilities and dipole moments of new benzo-fused dyes were calculated using quantum-mechanical density functional theory (DFT). Two new infrared dyes were synthesized: each one was terminated with 6-diethylaminobenzo[b]furan on one end, and with CF₃-tricyanodihydrofuran on the other end. The midsection of the π-electron framework for one dye contained a morpholino-substituted cyclohexenylene unit, and the other dye contained an ethoxysiloxane-substituted cyclohexenylene unit. Guest-host films were deposited on ITO-glass and contact poled. Electro-optic coefficients (r₃₃) were measured at a wavelength of 1550 nm by the attenuated total reflection method and by a modified simple Teng-Man reflection method. The measured values of r₃₃ were compared with values estimated from a well-known model that employs the molecular properties of the dyes and the film poling parameters. Thermal stability and electronic absorption spectra of the dyes were measured.     

Improved interferometer studies of linear electro-optic effects of dye-doped polymers

The basic Mach-Zehnder interferometer was modified for use in in situ temperature-dependent linear electro-optic (LEO) measurements of thin films of 4-dimethylamino-4?-nitrostilbene (DANS) doped into poly(methyl methacrylate) (PMMA). Optimum interferometer phase stability was possible because of an incorporated electronic feedback system. Film thickness variation was compensated for to obtain more accurate LEO coefficient measurements and thus the second-order susceptibility. Moreover, both the alpha relaxation associated with the glass transition, T(g), and beta relaxation associated with the secondary transition occurring below T(g) of PMMA + 2 wt.% DANS was obtained.     

Synthesis and characterization of binary chromophore polymers for electro-optic application

We prepared the side-chain electro-optic polymers by the active bonding of cyanate- and hydroxy-functional groups, and employed these side-chain polymers as hosts for the bi-chromophore system to enhance the electro-optic coefficient (r₃₃). In singular chromophore polymer (in PMMA host), the dependence of EO response on loading concentration was examined, r₃₃ about 26 pm/V was obtained. In order to further incorporate the chromophore, the loading behavior in binary chromphore systems was investigated. The optimal r₃₃ more than 40 pm/V was achieved, which was almost the sum of two individual polymers. Furthermore, the temporal stability at room temperature and 85 °C, respectively, over 500 h was evaluated for the potential application on electro-optic devices.     

Optimized half-wave voltage and insertion loss in a strip-loaded waveguide electro-optic polymer modulator

A strip-loaded waveguide, electro-optic modulator was designed and analyzed in terms of single mode conditions, optical loss due to the metal electrodes, modulation efficiency, and mode size. Two designs were compared: Design 1 optimized the half-wave voltage (V(π)=1.1 V) with a nearly symmetric waveguide by maximizing modulation efficiency and minimizing the overall thickness of the waveguide; Design 2 optimized the insertion loss by reducing coupling loss by 4.6 dB via a strongly asymmetric waveguide that maximizes the overall mode size to most efficiently overlap with a single mode fiber. Design 2 also has a favorable half-wave voltage (V(π)=1.75 V). Some general guidelines in the selection of cladding layers in a detailed design of a poled-polymer electro-optic modulator incorporating a strip-loaded waveguide structure are suggested.     

Theoretical and experimental studies of polarization mode dispersion of an electro-optic Mach-Zehnder modulator

A theoretical model is developed to study the polarization mode dispersion effect in an electro-optic Mach-Zehnder interferometric (MZI) modulator. The Stokes parameters and differential group delay (DGD) of the output light of a MZI modulator can be analytically derived with the proposed model, which is based on a three-dimensional Maxwell's wave equation approach. The theoretical model is validated to the extent possible by comparing the theoretical results of the Stokes parameters and DGD with experimental measurements that are based on the wavelength-scanning method and the Jones matrix eigenanalysis method.     

Optical sensor based on Fabry-Perot resonance modes

An oscillating wave sensor based on Fabry-Perot resonance modes has been developed. Different from the surface plasmon resonance sensors and the waveguide mode sensors in which the sample is located in the evanescent field region, the proposed device contains the sample in the core region that supports the oscillating field. Owing to the strong concentration of the electromagnetic field in the sensing medium, the proposed device exhibits unusual sensitivity enhancement, which has never been exploited in any other devices.     

Response of a Fabry-Perot optical cavity to phase modulation sidebands for use in electro-optic control systems

The worldwide endeavor to build long baseline laser interferometers to detect and study gravitational radiation is well under way. In the German-British GEO600 project, it is proposed to pass the sidebands induced on the light by an electro-optic phase modulator through a Fabry-Perot optical cavity used in transmission, called a mode cleaner. This can be achieved when the phase modulation frequency is matched to the first longitudinal-mode frequency of the mode cleaner cavity so that both carrier and sidebands are transmitted. The primary function of the mode cleaner is to reduce the geometry fluctuations associated with the light, and thus any such noise induced by the modulation process is also suppressed. We present the results of an experiment that investigates the feasibility of passing modulation sidebands through an optical cavity and the factors limiting its success. In particular, we show that it is possible to avoid introducing excess noise associated with the transmitted sidebands, provided that certain experimental criteria are satisfied. The research was carried out on a prototype mode cleaner cavity built and tested at Glasgow University but which is similar to the equivalent apparatus planned for GEO600.     

Controllable Fabry-Perot interferometer based on dynamic volume holograms

We present experimental results for a high-resolution interferometer based on dynamic volume holography in photorefractive materials. The proposed interferometer is able to detect small angular and phase deviations separately. It has no free spectral range limitations and is comparatively insensitive to light intensity fluctuations.     

Terahertz polarization real-time imaging based on balanced electro-optic detection

A terahertz (THz) polarization real-time imaging system that can effectively reduce experimental time consumption for acquiring a sample's polarization information is achieved. An alternative THz polarization measurement method is proposed. In this method, a <110> zinc-blende crystal is used as the sensor, and the probe polarization is adjusted to detect THz electric fields on the two orthogonal polarization components. The relative sensitivity of the imaging system to the THz polarization angle is estimated to be less than 0.5°. To illustrate the ability of the system, two samples are designed and measured by using the system. From their THz polarization real-time images, each region of these samples can be precisely presented. Experimental results clearly show the special influences of different materials on the THz polarization. This work effectively extends the information content obtained by THz real-time imaging and improves the feasibility of the imaging technique.