Y-branch Waveguide Research Articles

Highly efficient linear waveguide modulator based on domain-inverted electro-optic polymers

A Y-branch waveguide directional coupler modulator based on domain-inverted electro-optic polymers is demonstrated for highly ef- ficient analog modulation. A detailed theoretical analysis is given to show its working principles based on coupled-mode theory. Domain-inverted electro-optical polymers are obtained by using a high-voltage pulsed pol- ing technique. The demonstrated domain-inverted polymers represent a promising technology for creating a new class of electro-optic modula- tors with superior performance over conventional counterparts. By using domain inversion in a Y-branch waveguide directional coupler, the poly- meric electro-optic waveguide modulator provides a highly linear modu- lation transfer curve that shows a ;100% modulation efficiency with an intermodulation distortion less than 230 dB. The demonstrated modula- tor also features a switching voltage of 10 V and an extinction ratio of 228 dB at operation wavelength of 1.32 mm. A linear modulation transfer curve and a 3-dB operating point are obtained without a bias voltage or an asymmetric device structure through a single uniform electrode.

Crosstalk-, loss-, and length-reduced digital optical Y-branch switches using a double-etch waveguide structure

We demonstrate a novel Y-branch digital switch with a double-etch waveguide structure to greatly enhance the loss and crosstalk performance. The double-etch design increases the coupling between the two tapered Y-branch waveguides, which allows for efficient switching while decreasing the radiation loss that arises from the branching angle. This combined advantage also allows us to reduce the switch length, which in turn results in an efficient high-speed design. The measured crosstalk and loss are -19 and 3 dB, respectively, for a 3-mm-long switch. Further crosstalk reduction is expected from the simulation of a more optimized design.

Polymeric waveguide polarization splitter with a buried birefringent polymer

A waveguide polarization splitter is demonstrated based on a low-loss polymer waveguide and a birefringent polyimide. Crosslinkable fluorinated polymers with an excellent stability and a low absorption loss are utilized for the device. The polyimide is buried under one branch of the Y-branch waveguide to enhance the birefringence between the TE and TM modes. By the adiabatic mode evolution, the TE mode is coupled to the branch with the polyimide strip, while the TM mode propagates through the other branch without the polyimide. For the device with a branch angle of 1/400 rad, we obtained a crosstalk less than -20 dB and a fiber-to-fiber insertion loss of 3.8 dB.

Optimizing the bias and modulation voltages of MQW Mach-Zehnder modulators for 10 Gb/s transmission on nondispersion shifted fiber

The chirp and optical extinction ratio of a multiple quantum-well (MQW) Mach-Zehnder modulator depend on the device design and on the voltage waveforms applied to the arm electrodes. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for a conventional modulator and a /spl pi/-phase-shift modulator. Measured attenuation and phase constants for an optical signal propagating in the modulator waveguide are used to accurately model the Mach-Zehnder modulators. The influence of asymmetric Y-branch waveguides in the modulators is examined taking into consideration group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity. When the modulators are operated with maximum optical extinction ratio, the dispersion limited transmission distance depends on the device design (phase-shift and Y-branch splitting ratio) and modulation format (dual drive or single drive). Optimization of the bias and modulation voltages reduces this dependence significantly, while also increasing the dispersion limited transmission distance.

Optimal control theory for optical waveguide design: application to Y-branch structures

A recently introduced optimal control theory method for optical waveguide design is applied to Y-branch waveguides and Mach-Zehnder modulators. The method simultaneously optimizes many parameters in a chosen design scheme; computational effort scales mildly with the number of parameters considered. Significant improvement in guiding efficiency relative to intuitively reasonable initial parameter choices is obtained in all cases.

Feedback controlled variable optical attenuatorfor channel power regulation in WDM systems

A variable optical attenuator is described which uses an asymmetric polymeric Y-branch waveguide. It incorporates an optical monitoring tap to regulate the optical output regardless of variations in input light power and polarisation. The dynamic attenuation range was > 18 dB at 1.55 µm. The attenuator was successfully used to regulate channel powers within 0.5 dB in a WDM system.

Polymeric tunable optical attenuator with an optical monitoring tap for WDM transmission network

A polymeric tunable optical attenuator has been fabricated using asymmetric Y-branch waveguides integrated with an optical monitoring tap. One arm of the branch serves as the main output port, while the other arm helps remove the residual optical power resulting from attenuation. An optical power tap used for the monitoring port is introduced to take a fraction of the main port power. This monitoring port is useful for observing the main output continuously. Furthermore, it can be fed back to the electrical driver to maintain the attenuated output regardless of variations in input light polarization and power. By utilizing the modal evolution effect in asymmetric branches, the attenuator has enhanced fabrication tolerance and reduced wavelength sensitivity. Also, no electrical bias is needed to achieve maximum optical transmission. The attenuator exhibited a dynamic range of more than 20 dB at 1550 nm and a response time of about 1 ms. The polarization dependent loss was about 0.9 dB, and the wavelength uniformity was less than 1.2 dB over the range of 1530-1560 mm.

Integrated optical polarization splitter based on photobleaching-induced birefringence in azo dye polymers

An integrated optical polarization splitter has been fabricated by utilizing the photobleaching-induced birefringence in an azo dye polymer. It consists of a Y-branch waveguide formed by the reactive ion etching with one of the two arms photobleached. The refractive index of the photobleached arm is decreased for the TE mode and increased for the TM mode. The performance of the splitter was measured as a function of the energy of the photobleaching beam and compared to a wave propagation simulation of the device. The measured cross talks are less than -28 dB for the TM mode and -24 dB for the TE mode at a wavelength of 1310 nm. The measured excess losses for the TE and TM modes, which measure the effect of the Y branch and the photobleaching, are 0.3 and 0.4 dB, respectively. The insertion loss was 5 dB, which includes the input fiber to waveguide coupling loss.

Design of a Low-loss Y-branch Optical Waveguide

A low-loss Y-branch optical waveguide with a wide gap between two output waveguides was designed. Its performance was simulated by the beam propagation method. In spite of its wider gap, the excess loss was much less than that in the case of conventional Y-branches. The wider gap is desirable for better step coverage in buried waveguides (e.g. silica waveguides).

Symmetric three-branch optical power divider with a coupling cap

Proposed is a new fork-type three-branch optical power divider consisting of two different dielectric materials. The structure is composed of a symmetric Y-branch waveguide with a phase front retarder and a center branch located a coupling gap apart. The present power divider suffers minimum radiation losses as compared to previously reported fork-type three-branch devices consisting of two dielectric materials. Numerical simulations are carried out with a finite difference beam propagation analysis for representative three different sets of significant structural parameters as the refractive index and waveguide width. Numerical results also contain the power dividing characteristics for the case where sharp wedges of index distribution are blunted.

Segmented photorefractive waveguides in LiNbO 3 :Fe

We propose what is, to our knowledge, a novel technique for fabricating a segmented waveguide by optical irradiation in a photorefractive LiNbO3:Fe crystal. The waveguide consists of many localized high-refractive-index regions that are fabricated by illumination of a focused laser beam. We fabricate straight, curved, and Y-branch waveguides. In the straight waveguides the transmitted power of a guided beam as a function of the period of segmentation and the dark decay time are measured. The tolerance for fabrication errors is also investigated both experimentally and numerically. The fabricated waveguide can be optically modified. We demonstrate that a curved structure can be transformed into a Y-branch structure.

Microprism for Wide-Angle Low-Loss Y-Junction Waveguide

A wide-angle low-loss Y-branch waveguide with butterfly-shaped microprisms is designed. The microprisms not only compensate the phase difference but enhance the mode-conversion evolution in the branching region. Simulation results predict that a mode separation with low branching losses can be achieved in the wide-angle Y-branch. The transmitted power efficiency can be as high as 89 % for the branching angle up to 20 .

Longitudinal mode perturbation of laser diodes with dry etched total internal reflector

The spectral characteristics of rooftop reflector (RTR) laser diode and serpentine shaped laser diode (SSLD) are presented. Lasers are fabricated using two step Cl/sub 2/-based reactive ion beam etching (RIBE). It is found that the spectra of RTR laser and SSLD are very different from those of Fabry-Perot cavity lasers. Also a Y-branch optical waveguide with total internal reflector (TIR) mirror was fabricated to measure backward reflection from the TIR mirror. The results suggest that the spectre of lasers with TIR mirrors have a strong longitudinal mode selectivity due to the scattering and reflection at the TIR mirror.

Monolithic integration of an InGaAsP-InP MQW laser/waveguide using a twin-guide structure with a mode selection layer

We demonstrate a monolithically integrated 1.55-μm wavelength InGaAsP-InP multiple-quantum-well (MQW) laser with a passive Y-branch waveguide in a vertical twin-waveguide structure. To reduce the sensitivity of the device performance characteristics to laser cavity length and variations in the layer structure, we introduce an In/sub 0.53/Ga/sub 0.47/As absorption, or "loss" layer. This layer eliminates the propagation of the even mode, while having minimal effect on the odd mode. The threshold current densities and differential efficiencies of the devices are unaffected by the loss layer. A record high coupling efficiency of 45% from the laser to the external passive waveguide is obtained.

Finite element beam propagation method for three-dimensional optical waveguide structures

A beam propagation method (BPM) based on the finite element method (FEM) is described for longitudinally varying three-dimensional (3-D) optical waveguides. In order to avoid nonphysical reflections from the computational window edges, the transparent boundary condition is introduced. The present algorithm using the Pade approximation is, to our knowledge, the first wide-angle finite element beam propagation method for 3-D waveguide structures. To show the validity and usefulness of this approach, numerical results are shown for Gaussian-beam excitation of a straight rib waveguide and guided-mode propagation in a Y-branching rib waveguide.

Wide-angle low-loss waveguide branching for integrated optics

A new wide-angle, low-loss, symmetrical Y-branch waveguide is proposed. The waveguide configuration utilizes ribs for lateral confinement in the planar guiding region underneath. This Y-branch structure can be fabricated easily without an additional process step. Together with a utilization of the multimode interference effect, a local decrease of the waveguide ridge in the wedge part of the Y-branch reduces the radiation loss. When proper@ designed, the proposed Y-branch has a radiation loss as low as 2.2 dB at a branch angle of 6° with the index difference (Δn/n) as small as 7.1 × 10−4 at a wavelength of 870 nm in the TE fundamental mode as compared to 12.6 dB for a conventional Y-branch. The proposed method yields corresponding advantages for waveguide designs with a higher Δn/n ratio and can also be adapted in combination with S-branch designs.

A novel Y-branch waveguide for power dividing and mode splitting

A novel Y-branch waveguide for variable-ratio power dividing and transverse electrical-transverse magnetic (TE-TM) mode splitting depending on the applied voltage is presented. The Y-branch waveguide is formed by two closely coupled waveguides tabricated by nickel indiffusion (NI) and by magnesium-oxide induced lithium outdiffusion (MILO) in a y-cut lithium niobate (LiNbO3) substrate. The TE component of the randomly polarized light is tuned between these two waveguides, such that the device can be either a power divider or a mode splitter depending on the applied voltage. The measured TE mode extinction ratio is about 20 dB.

Integrated optic Y-branching waveguides withan asymmetric branching ratio

Integrated optic Y-branching waveguides with an asymmetric branching ratio are proposed by using a novel and simple waveguide configuration. Realisation of a controllable wide branching ratio, simply by shifting the centre axes of the input taper waveguide and the branching waveguide group is clarified both theoretically and experimentally. 1 × 2 Y-branch splitters with an asymmetric branching ratio of 50.0 ~ 75.6%, a low excess loss of <0.2 dB and based on planar lightwave circuits are demonstrated.

Polymeric waveguide polarisation splitterbased on poling-induced birefringence

Guided-wave polarisation splitters are fabricated in electro-optic polymers by exploiting the poling-induced birefringence. One arm of the Y-branch type waveguide is poled to induce the birefringence selectively. By the mode evolution process, TM and TE modes are split between the poled and the unpoled arms.

Novel design of low-loss wide-angle symmetric Y-branch waveguides

Novel design of low-loss wide-angle symmetric Y-branch waveguides