High-speed electro-optic switch based on nonlinear polymer-clad waveguide incorporated with quasi-in-plane coplanar waveguide electrodes

This work is part of a continuing project that is investigating the use of integrated optical (IO) waveguides and optical fibers coated with nonlinear optical (NLO) polymers as electric field sensing devices. The IO waveguides were fabricated using three different techniques: (1) in the NLO polymer itself (on glass and silicon substrates), (2) in glass microscope slides using thin film silver diffusion and silver ion exchange methods, and (3) in thin film optical epoxies. The Ag+ glass and UV cured optical epoxy waveguides were subsequently coated with NLO polymers; the 3-D patterned waveguides provided lateral confinement of the optical wave that propagated in the polymer. A low index buffer layer of polysiloxane glass resin (Owens Illinois GR650E) was needed on the silicon substrates to decrease the propagation loss. The NLO polymers were dip coated or spun on these substrates and were poled with a corona poling technique with fields of the order of 3 MV/cm. Interferometric and polarization rotation methods were used to measure the effects of applied electric fields on the NLO devices at wavelengths of 633, 780, and 830 nm. Electrooptic coefficients were determined and compared with similar coefficients obtained using second harmonic generation methods on the same NLO polymers.

A Mach–Zehnder interferometer (MZI) electro-optic (E-O) switch based on a guest–host material DR1/PC was designed and fabricated. The optical properties and long-term stability of the material were studied. By optimizing the concentration of DR1, the DR1/PC material exhibited an E-O coefficient of 12.8 pm/V at 1310 nm. The characteristic parameters of the waveguide were calculated, and the fabrication process was strictly controlled. The MZI switch on-off time was found to be about 50 ns. The extinction ratio was measured to be about 13.4 dB. At room temperature, the switch could keep operating for at least 150 days.

This paper presents a novel class of nonlinear optical (NLO) polymer for second harmonic generation (SHG). This new class of NLO polymer consists of the linear polyurethane, T-polymer, of tolylene 2,4-diisocyanate (TDI) with 3-[(2-hydroxyethyl)amino]-5-(hydroxymethyl)-4‘-nitroazobenzene (T-AZODIOL), whose dipole moment is aligned transverse to the main chain backbone. For comparison, the linear polyurethane L-polymer, whose NLO chromophore is incorporated into the main chain, is synthesized from TDI with 4-[N-(2-hydroxyethyl)-N-methylamino]-3‘-(hydroxymethyl)azobenzene (AZODIOL). T-polymer poled at optimum conditions of a corona poling voltage 8.0 kV, a temperature of 95 °C, and a time of 60 min shows a large second-order nonlinearity of d33 = 1.6 × 10-7 esu (67 pm/V) with good thermal stability at room temperature. The oriented NLO dipole moments for T-polymer do not show significant relaxation at ambient conditions after 60 days except for small depression a few days after poling, whereas, the SHG activity of L-polymer is largely decayed at room temperature. The better thermal stability of this new class of T-polymer is related to the smaller free volume in T-polymer.

Response speed and power consumption are improved for a Mach–Zehnder interferometer (MZI) thermo-optic (TO) switch, by using a hybrid silica/polymer waveguide structure and optimizing both the thickness of the silica under cladding and that of the PMMA-GMA upper cladding. Fabrication techniques, including chemical vapor deposition (CVD), spin-coating and wet-etching, are adopted to develop the switch sample. Under 1550 nm wavelength, the driving powers under ON and OFF states are measured to be 0 and 13 mW, respectively, indicating a switching power of 13 mW. The fiber-to-fiber insertion loss under the ON state is 15 dB, the extinction ratio between the ON state and the OFF state is 18.3 dB, and the rise time and fall time are 73.5 and 96.5 µs, respectively. Compared with the TO switches based on Si/SiO2 or all-polymer waveguide structure, the proposed device possesses both low power consumption and fast response speed, by virtue of the large TO coefficient of the polymer core, thin upper/under claddings and the large thermal conductivity of silica.

Nonlinear optical (NLO) polymers have been considered promising materials for wavelength conversion at a low pump power. However, they have not been readily adopted to practical applications due to their high absorption coefficients, especially at a shorter interacting wavelength. Our theoretical analysis proves that the influence of absorption coefficients can be mitigated significantly in cascaded wavelength conversion (CWC) processes. According to our example study, maximum conversion efficiencies for CWC can compare even with those for second-harmonic generation in many NLO polymers. Thus CWC can become a pertinent application of NLO polymers. However, to obtain such efficient CWC, several realistic problems should be resolved in practical devices.

In this paper, the optical and electrical pulse responses of an optimized high-speed organic light-emitting diode (OLED) is investigated as a function of the pulse duration. The original design is based on the coplanar waveguide electrodes that are implemented on a transparent and conductive oxide thin film. The designed electrodes with reduced serial resistances (13 $\Omega $ ) allow to maximize the pulse energy delivery and to feed the organic device with short electrical pulses of few nanoseconds for the first time. This is achieved by designing the electrodes to have 50- $\Omega $ characteristic impedance, thanks to microwave techniques, and by implementing small values of capacitance (4.5 pF). The experimental results demonstrate $\boldsymbol \mu $ -OLED electrical and optical responses to electrical excitation pulses as short as 2.5 ns, for the first time, and state-of-the-art current densities above 1.7 kA/cm2. Moreover, and rather unexpectedly, the developed characterization procedure to extract the reported results demonstrated the first direct measurement of the electroluminescence lifetime of organic materials in the nanosecond time scale.

We have synthesized and evaluated nonlinear optical (NLO) properties of azo dye attached novel NLO polymers with high optical nonlinearity and/or high thermal stability. An azo-dye attached NLO polymer with a furan derivative as an acceptor was synthesized. The second order NLO susceptibility (χ(2) 33) at 1.3 μm fundamental wavelength was more than two times larger than that of the NLO polymer with dicyanovinyl (DCV) acceptor and the same azobenzene structure. A χ (2) 33 of 43pm/V was obtained. Polymethacrylic acid reacted with a typical azo dye, Disperse Red 1 dye (DR-1), in its side-chain was also synthesized using a new approach, i.e., via intermediate reaction of the acid and N,N'- dicyclohexylcarbodiimide. Glass transition temperatures Tg of 170°C were obtained for the dye attached polymethacrylic acid with the DR- 1 contents of 29 mol%. A χ (2) 33 of 53pm/V at 1.3 μm was obtained. This polymer can be further improved to have more stablility by heat treatment. Through this treatment, the methacryl imide structure was formed and the polymer has Tg of 165°C. This imidized polymer exhibited a χ (2) 33 of 45 pm/V at 1.3 μm and maintained about 90% of its initial value after 230 hours or more at 100°C. Novel NLO polyarylethers where electron-withdrawing groups for NLO activity were introduced into the polymers by post-functionalization were developed. These NLO polyarylethers have high Tgs, typically higher than 200°C. A 100 mol% DR-1 dye containing NLO polymer with the fluorene structure has very low optical propagation loss of 0.42dB/cm at 1.3 μm wavelength and shows low polarized light dependence of the refractive indices. The polymer has χ (2) 33 of about 57pm/V at 1.3|um fundamental wavelength and the nonlinearity was maintained at 100°C for more than 200 hrs Key words: second order nonlinear optics, nonlinear optical polymers, thermal stability, furan derivative, polymethacrylic acid, azo dye, imidization, polyarylether

A novel fiber type traveling-wave modulator is developed. The finite element method (FEM) has been used to analyze the performance of the proposed modulator with coplanar waveguide (CPW) electrode structures. The optimized structures of the traveling-wave modulator are obtained. The results show that the novel modulator has a 3dB optical bandwidth of 112GHz, a half driving voltage of 2.7V, and characteristic impedance of 51.2Ω at 1.55μm wavelength.

The investigation of dipole relaxation processes in nonlinear optical (NLO) polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or α relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function ε̃(T)=ε′(T)−iε″(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time τα(t) is numerically determined from the dielectric loss ε″(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong α and the weak β relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a γ-relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the β relaxation.

A compact and easy-to-use experimental set-up for pyroelectric investigations of poled nonlinear optical (NLO) polymers is reported. The arrangement allows for the investigation of the NLO dipole relaxation under isothermal or nonisothermal (e.g., linear heating of the polymer) conditions. Furthermore, polarization patterns in the plane of a polymer film and polarization profiles in its thickness direction can be measured in the same experimental set-up. In waveguide structures, it is possible to determine the thicknesses of an active guiding layer and of the claddings, if their thermal parameters are known. The isothermal pyroelectric relaxation data are discussed within the framework of phenomenological relaxation functions. It is found that they cannot be fitted with the stretched exponential function, a result which is in agreement with broadband dielectric-relaxation measurements. Pyroelectric experiments are the only method for investigations of all these problems in one and the same easy-to-use set-up.

We propose and fabricate an ultrafast and high extinction ratio 1×4 optical switch based on a cascade, dual-output Mach–Zehnder interferometer (MZI) on the lithium niobate platform. The optical switch consists of cascaded, identical, dual-output MZI structures, and each of them includes a Y-branch waveguide, two parallel interference arms (electrode action area), and a 3 dB directional coupler. Four output optical paths can be switched through three groups of electrode hierarchical modulation. The test results of the fabricated device show that the insertion loss of each port is less than 7.1 dB and that the maximum extinction ratio is higher than 39.3 dB. The optical switch can achieve a high extinction ratio greater than 20 dB within the 50 nm wavelength range. The measured half-wave voltage is less than 4.13 V, and the minimum switching rise and fall times reach 125 ps and 250 ps, respectively. This result shows that the proposed and fabricated 1×4 integrated optical switch on a lithium niobate platform has the advantages of a fast response rate, high extinction ratio, and low switching voltage.

The purpose of this work is to investigate the effect of corona poling on the optical functions of the nonlinear-optical (NLO) polymers. We measured the complex refractive indices of NLO polymers deposited on glasses before and after corona poling using spectroscopic ellipsometry at room temperature at various incidence angles. In addition, we observed the absorption spectral change using ultraviolet-visible spectroscopy after corona poling, which is attributed to the chromophore orientation. In order to make NLO polymers, we attached a NLO active dye and photocrosslinkable moieties as side chains of poly(4-hydroxystyrene), deposited the polymer thin films on glass substrates by spin-coating, and corona-poled it along the surface-normal direction to align the dipoles of the chromophores along the electric field direction. In detail, PSDR1-25 designates a poly(4-hydroxystyrene) where 25% of hydroxyl groups in the side chain are substituted by disperse red 1 (DR1) and PSDR1-50 means the same except for 50% substitution. PSDR1 has the second-order NLO property, which is expected to show an acentric ordering by poling electric field. We used a parametric optical constant model to estimate the complex refractive index of the polymers. We found that the poling induced the uniaxial property of the complex refractive index of the polymers by aligning the chromophores along the poling direction. With increased poling voltage, we found that the oscillator strength of the major peak near the 2.4eV (designated as Ea) peak decreases in the surface-parallel direction (x), whereas that of the surface-normal direction (z) increases. Moreover, a detailed analysis of the dielectric functions using the standard critical point model showed that the poling induced a new peak Ec (∼2.6eV) between dominant Ea (∼2.3eV) and weak Eb (∼2.9eV) peaks for PSDR1-50 polymers. With increased poling voltage, the Ec peak strength increased compared to those of Ea and Eb peaks. We tentatively attribute Ec to lower symmetry, possibly Stark effect, induced in the chromophores by the corona poling.

A nonblocking 3×3 polymer thermo-optic (TO) switch array based on the total-internal-reflection (TIR) effect is demonstrated. The switch array consists of three 2×2 TIR TO switch elements with a total device length of 15.5 mm, and achieves nine switching states with no more than one switch element working. The fabricated 3×3 TO switch array shows excellent switching performance with low crosstalk for all switching states of less than -20 dB. The power consumption of each switch element is below 53 mW, and the switching rise time and fall time are ∼450 and 550 μs at 1550 nm wavelength, respectively.

We have successfully fabricated a serially grafted waveguide combining a type of organic nonlinear optical (NLO) crystal, 4-(p-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST), and a transparent polymer. To utilize the large electrooptic coefficient of the DAST effectively, a waveguide structure is desirable. Since organic crystals with large NLO effects show large optical losses, a major part of this serially grafted waveguide was composed of the transparent polymer. The input/output optical beam has been effectively coupled to the DAST crystal waveguide through the transparent polymer waveguide. This serially grafted structure is a potential candidate for fabricating many types of practical devices with optical nonlinearity such as the Mach-Zehnder interferometer(MZI).

High speed semiconductor optical modulators are very important components for optical communication systems [1]. Characteristics such as low drive voltage, wide bandwidth, low insertion loss, low wavelength and temperature sensitivity, and low cost are essential for these optical modulators. Recently 40 GHz InP modulators with a drive voltage of only 2.3 V have been reported which use an n-i-n waveguide structure instead of the conventional p-i-n structure to reduce both the microwave and optical losses [2]. However InP-based modulators have the drawback of the wavelength dependence of the drive voltage. GaAs based modulators do not have this problem due to its larger bandgap energy. To date GaAs modulator has been demonstrated with a 35 GHz optical bandwidth and 5 Vcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</inf> [3]. In this paper, we report a new design for GaAs modulators based on an n-i-p-n structure. The travelling-wave coplanar waveguide (CPW) electrodes are employed to realize high speed operation. By optimization, an electrical 3-dB bandwidth of nearly 40 GHz (optical 3-dB bandwidth of 80GHz) and a V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</inf> around 6.6 V are predicted for a 5 mm long phase modulator.