Alok K. Das

Automatic determination of the remaining cladding thickness of a single-mode fiber half-coupler

A method for the exact determination of the remaining cladding thickness (h) of a polished single-mode fiber, from the measurement of the throughput attenuation, is shown. Experimental loss variations with h are verified with the theoretical expression of the loss coefficient, which depends on the fiber parameters, source wavelength, and refractive index of the liquid placed over the polished fiber.

Laser direct writing polymeric single-mode waveguide devices with a rib structure

A focused argon-ion laser beam is used on a spin-coated polymeric thin-film deposited upon a SiO2/Si substrate to polymerize the core for fabrication of Gaussian profile optical channel waveguides. A rib structure that allows only the fundamental mode to propagate even with its higher dimension and high-index contrast between the core and the cladding was fabricated. When the thickness of the core-index region outside the rib section decreases, the waveguide produces higher-order modes at the output. The waveguide reported here has cross-sectional dimensions and numerical apertures that match the single-mode fibers for efficient coupling. I used a mixture of two intermiscible acrylate monomers for the cladding and the core of the waveguides. The polymerization process and its dependent dwell time or scan speed and the laser power intensity are shown. I present the operational characteristics of directional couplers using a rib waveguide structure with a core-cladding index difference.

Low-loss fusion splices of optical fibers

A low-loss splicing method, based on discharge fusion of optical fibers by a simple apparatus and by applying pressure between fibers before fusion, was developed. Average splice losses of about 0.07 and 0.15 dB for single-mode (SM) fibers having core diameters of 10 and 7μm, respectively, and 0.02 dB for 50-μm core diameter graded-index (GI) fibers are obtained. Fusion loss and fusion time are obtained minimum for better end preparation having low initial alignment losses at critical pressure and temperature. Mathematical expressions for the variation of fusion time and splice loss with effect of applied pressure between the fibers, for different practical axis alignment, showing the optimum condition to have minimum splice loss are made. Experimental fusion losses are analyzed in terms of residual misalignment of off axis, angular tilt of the fibers during aligning, and air gaps in the splicing zone. Optimum fusion time is obtained by considering the forces due to applied pressure, thermal expansion, and surface tension in the viscous melted glass of the fiber. Theoretical curves of fusion times and splicing losses versus applied pressure agree with the experimental results. The decrease of fusion time to about 1.3 times and splice loss to about two times were found when applied pressure is varried from low to its critical value of 20-25 g. The splice losses are found at a minimum for the operating temperature range of 1980°C to 2140°C for silica fibers. Experimental results of the histogram of bar chart of splice losses agree with the derived mathematical expressions assuming a statistical distribution function of splice losses.

A wide-angle X-junction polymeric thermooptic digital switch with low crosstalk

We present our recent design of a wide-angle asymmetric X-junction for the implementation of a digital-optical switch. The simulated crosstalk in the OFF and ON states are -28 and -26 dB, respectively, at an X-branching angle of 1/spl deg/. Polymeric material benzocyclobutene was used to fabricate our design and its switching property was demonstrated using the thermooptic effect. The measured crosstalk is approximately -24 dB in both states with rise-time and fall-time 140 and 210 /spl mu/s, respectively.

A wide-angle X-junction in polymer using truncated-structural branches (TSB)

We present a detailed design and characterization for a compact X-junction we proposed recently. Even at a wide full-branching angle of 1.4/spl deg/, simulation predicts the crosstalk and insertion loss to be -20.1 and 0.2 dB, respectively; the wavelength bandwidth is 230 nm (1390-1620 nm). Devices made in polymer realize -16.1-dB crosstalk and 0.3-dB loss.

Characteristics of polymeric optical passive single-mode waveguiding devices fabricated by an argon-ion laser

The fabrication of polymeric single-mode Gaussian profile optical waveguides is described. We used poly(methyl methacrylate) and a mixture of two intermiscible monomers for the cladding and the core, respectively, of the waveguides. The cores of the waveguides were fabricated by with an argon-ion laser beam. The waveguides had single-mode Gaussian refractive-index profiles. By using such waveguides, we fabricated directional couplers for power coupling to the adjacent waveguides and also parallel waveguide arrays for preventing power coupling to adjacent waveguides for use in interconnect chips. We analyzed the characteristics of these couplers by using the coupled-mode theory and compared the results with those obtained with the beam propagation method. Experimental results showed good correlation with the theoretical values. We designed optical bus arrays for interconnect chips, considering the variation of normalized frequency V, the power penalty, and the dimensions of the waveguides and the separation between them. The number of waveguides in the bus array increased with increasing V. For a known value of V, a waveguides density increases with a decrease of its dimensions. The theoretical maximum number of waveguides is ~1490/cm and ~846/cm for 2 mum x 2 mum and 4 mum x 4 mum single-mode waveguides, respectively, to satisfy a 1-dB power penalty criterion at bit-error rate of 10(-9). We fabricated interconnect bus arrays with fifteen 4 mum x 4 mum waveguides for a 3-cm coupling length, and the experimental results were verified to be in good agreement with the theoretical values.

Efficient method of coupling from a single-mode fiber to a thin-film waveguide

A new method of coupling from a single-mode fiber to a thin-film waveguide through a coupling fiber is reported. This method provides high coupling efficiency, whereby the coupling fiber helps to match the coupling conditions. Experimentally the maximum coupling efficiency of 78% was obtained by use of a polymeric thin-film waveguide, in which propagation loss of the film and other coupling means to measure characteristics of the polymeric films.

Minimization of Heating Power for Thermooptic Waveguide Type Devices

Thermal analysis of thermooptic device using planar optical waveguide structures is performed by using implicit finite difference method. Isothermal profiles of the medium having the core and cladding are shown from the computation results. We have proposed two new structures where lower heating power is required when compared to that of the conventional structure. A compromise between heating power and response time is made for different applications of thermooptic devices.

Precise control of the center wavelength and bandwidth of wavelength-selective single-mode fiber couplers

Low-loss asymmetrical single-mode fiber wavelength-selective directional couplers are fabricated at low cost. Proper control of the remaining cladding thickness and the interaction length of the half-couplers produces exactly the required bandwidth and center wavelength of the filter. Increasing the interaction length reduces the bandwidth and the sidelobes. Calculated values show good agreement with the experimental results.

Single-mode fiber–linearly tapered planar waveguide tunable coupler

We developed a simple system of tunable fiber-film coupler using a linearly tapered thin-film planar waveguide (PWG) evanescently coupled by a single-mode distributed fiber half-coupler. We investigate the characteristics of the coupler theoretically and experimentally taking into consideration the refractive index (n(f)) of nonuniform films, the magnitude of nonuniformity (m) of the films, and the source wavelength (lambda). The thickness variation of the nonuniform film is along the direction of propagation of optical power. Tapered and plano-concave thin films of a mix of oils as well as a plano-concave poly(methyl methacrylate) film were fabricated to serve as nonuniform PWGs. Similar to single-mode fiber with a uniform thickness PWG coupler, such a coupler also provides light modulation with a change of n(f). However, position shifting of a half-coupler in a tapered PWG structure along the direction of propagation exhibits the variation of fiber throughput power. This action serves as a simple system for a tunable fiber-film coupler. Wavelength-dependent throughput fiber power for such a coupler also behaves as a filter. The center wavelength can be controlled by shifting the position of the half-coupler. A coupling fiber as a half-coupler can be used for efficient coupling. We performed a theoretical analysis of the structure using Marcuses model and observed good agreement with the experimental results.