Recently a new mechanism for passive Q-switching in fiber lasers based on cooperative dynamics of linear Rayleigh backscattering (RS) and Stimulated Brillouin Scattering (SBS) has been reported in Yb- and Er-doped fiber lasers with high pump powers (greater than 2 W). At such high pump levels, the intensity of the light generated inside the fiber laser cavity exceeds considerably the SBS threshold, so that the conditions for passive Q-switching due to nonlinear backscattering are easily achieved and, for this reason, no special optimization of the laser configuration is required. Here, we report on experiments in Er-doped fiber lasers with low pump power levels. The Q-switching operational mode of Er-doped fiber lasers was observed at pump power levels in the range of 30 - 100 mW. Throughout the experiments, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. In general, the process was slower in comparison with previous experiments at high pump power level. The output pulse duration was in the range of 10 - 20 ns and the peak power of the pulses was less than approximately 100 - 200 W. For this reason, all nonlinear processes except SBS did not influence the pulse formation process. The experimental results are well explained by a theoretical model based on RS-SBS dynamics.

Erbium-doped superfluorescent fiber sources (SFS) in the standard forward, backward, and double-pass configurations exhibit significant dependencies of the output mean wavelength on the pump state of polarization. This is due largely to polarization-dependent gain (PDG) and gives rise to long-term drifts of the source output mean wavelength through changes in the fiber birefringence. By replacing the mirror in a double- pass source with a Faraday rotator mirror (FRM), the magnitude of the SFS mean wavelength variations due to pump polarization changes is reduced by a factor of 5 - 40, depending on source design parameters. Simulation results as well as experimental demonstration of the Faraday double-pass SFS are discussed in this paper. We measured 10 parts per million for the polarization-related mean-wavelength variations in this source, approaching the requirement for high-accuracy fiber gyros.

Generation of picosecond pulses at two distinct wavelengths is interesting for wavelength-division-multiplexing, fiber communication and sensing. For this purpose, we achieved harmonic active mode locking simultaneously at two wavelengths separated by about 15 m in an Erbium-doped fiber laser. Dual- wavelength lasing was obtained with two wide-bandwidth (greater than 1 nm) nonchirped high-reflectivity fiber Bragg gratings inserted in the laser cavity. The fiber Bragg gratings were written with 275-nm light from an Ar laser in hydrogen-loaded fibers. Optical path lengths and losses were carefully adjusted at each wavelength to obtain perfect mode locking at both wavelengths. Total cavity dispersion was set in the anomalous dispersion regime and optimized at each wavelength independently to generate solitons. Pulses at 3-GHz repetition rate were obtained at two wavelengths simultaneously with pulse widths of 16 ps and 13 ps, at 1547 nm and 1562 nm respectively. Time-bandwidth products of 0.37 and 0.34 respectively confirmed that the pulses were nearly transform-limited at each wavelength.

Long period fiber grating devices are finding a broad range of applications as wavelength selective filters and spectrally selective sensors. In this paper I describe the theory and fabrication of long period fiber gratings, including mask considerations for UV fabrication, and give an overview of applications. Two applications are described in detail: an embeddable point sensor for strain and temperature in a structure, and an all-fiber, coupler-free Mach-Zehnder interferometer for WDM filtering applications.

We have fabricated long-period fiber grating by focused irradiation of femtosecond laser pulses at 800 nm. The examined glass fiber is a non-H₂-loaded standard single mode fiber for telecommunication. We have found that, after focused irradiation of femtosecond pulses, refractive index at the focal point increases. We examine a thermal stability of the fabricated fiber gratings. Values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500 degrees Celsius are the same as initial values before heat treatment. The fiber gratings fabricated by focused irradiation of femtosecond pulses at 800 nm have higher resistance to a thermal decay compared with those of UV-induced long-period fiber gratings. We discuss the mechanism of index change. We have proposed that this technique is useful for fabrication of long-period fiber gratings with a superior aging characteristic.

We placed silica optical fibers into hydrogen environments of up to 30,000 psi (2,041 atm) and characterized their photosensitivity and UV-induced optical loss. We detail the relationship between the hydrogen content in the fiber and the UV-induced index change for a delivered radiation dose. We also observed high UV-induced optical loss (greater than 30 dB/cm) in these fibers and studied the cause of this loss.

We present an analysis of the transmission properties of index-shifted Bragg gratings. Theoretical predictions of the transmission properties of these gratings are obtained using a generalized transfer matrix method. The predictions show that a precise modification of the refractive index profile of Bragg gratings can lead to the generation of narrowband transmission windows within a low transmission band.

We present different glass ion-exchanged waveguide polarizer configurations using an electro-optic polymer (conjugated PMMA-DR1) overlay. The polarizer function is accomplished by modifying the propagation constant of a guided mode by changing the birefringence induced in the polymer by a poling process. A TE pass polarizer with an extinction ratio of 39 dB, has been realized using a corona poling. Further developments require a better control of the poling process. A TE polarizer structure using poling in the vertical direction have also been tested. A TM polarizer with planar metallic electrodes for poling is presented. Design considerations and fabrication results are presented.

High density integrated optics on the scale of VLSI is of interest as it allows complicated optical interconnect circuitry to be mass produced. In this paper we present micron-sized high Q resonant cavity structures based on silicon on insulator devices. These resonant cavities may be used in channel dropping filters and modulators. Because of their small size, they have high packing densities on the order of one million devices per square centimeter. This technology has the added advantage in that it can utilize the embedded VLSI electronics manufacturing capacity. In previous work, we studied silicon on oxide photonic band gap (PBG) devices and demonstrated devices with a 400 nm stop band and with a defect which had a Q of 265 centered at a wavelength of 1560 nm. In addition, we fabricated 3 to 5 micrometer radii micro-rings with Qs of approximately 250 and free spectral widths of over 20 nm. In this work, we report results on micro-racetracks, which are oval shaped resonators, with resonances that are approximately 16 nm apart and Qs of about 1000. These racetracks incorporate a vertical coupling technology in which the bus waveguides and the ring are on separate planes. This vertical coupling scheme allows for independent control of the Q of the ring via the distance between the ring and the bus. We demonstrate higher order multi-resonator filters with similar Q and free spectral range to the single resonator filters. The individual resonators in each filter have slightly different resonant frequencies from each other resulting in multi-peaked resonances and lower drop efficiencies. Finally, we show that it is possible to thermally tune the resonances by 1 nm leading to a 10:1 contrast ratio.

Polarization dependent loss (PDL) is an important parameter for optical passive devices used in high speed optical fiber transmission line. In this paper, the polarization dependent losses of some commonly used optical passive devices such as isolators, attenuators and connectors are studied theoretically and experimentally. A simple way for evaluating PDL values for some bulk passive devices such as isolators, attenuators and connectors is presented and the calculated values match well with the measurement results. We believe that this study is helpful for the design of passive devices and the management of optical fiber communication systems.

Optical switching devices that can perform at picosecond to femtosecond speeds are on demand because of interest to develop multi-gigabit, multi-user, optical networks. Two fundamental design issues are the choice of nonlinear material as the active medium for the switch and the switching architecture. Wave guide based switches designed with silica fiber have demonstrated ultra-fast switching up to femtosecond speeds. Figure 1 shows the switching speed versus power-length product for different types of materials. At the two extremes are rare-earth doped, and silica. Rare-earth doped materials have the smallest power-length product of about 10 W-cm, but also have the least switching speed. At the other extreme, silica has the fastest switching speed of 10¹³ Hz but also has the largest power-length product of 6 kW-cm. Semiconductor-doped glasses (SDG) and metal-doped glasses appear to have a good compromise between switching speed and power-length product. There is therefore interest to research such optical materials, and novel switching architectures that can simultaneously down-scale device geometry, and power requirements for switching.

Silica glass plays a key role in photonic systems because of its excellent optical properties, such as low loss, low fabrication cost and high photo-refractive damage threshold. Unfortunately, silica, being centrosymmetric, has no intrinsic linear electro-optic (LEO) coefficient or second-order nonlinearity (SON). However, thermal poling has been demonstrated to produce a LEO coefficient and SON of approximately 1 pm/V in silica glass and fiber. It is necessary to understand the mechanism of thermal poling in order to achieve a larger, stable and reliable LEO effect. A series of thermal poling experiments on silicate fiber was carried out. The in situ measurements of the total LEO coefficients (the sum of the poling field induced LEO coefficient and the thermal poling induced residual LEO coefficient) suggest movement of charges. Thermal poling induced residual LEO coefficients are measured in situ during prolonged negative thermal poling. Both the shielding field and the ionization field are frozen-in at room temperature and lead to LEO effect. The time evolution of the residual LEO coefficients shows that the competition between the shielding and ionization fields is a linear process. Using this new understanding, a specialty optical fiber was developed for the production of thermally poled optical fiber devices.

In this paper we propose a silicon optoelectronic modulator based on the Plasma Dispersion Effect used in conjunction with a Distributed Bragg Reflector which converts the phase shift induced by the free carriers injected by a P-i-N diode into variation of its reflectivity. The device is integrated in a low-loss Silicon-On-Insulator waveguide. With respect to a previous paper [A. Cutolo et al., Appl. Phys. Lett., 71 (2), 1997] we suggest an inversion of the driving scheme; we propose to tune the Bragg mirror in the ON state, when the light passes through the mirror unchanged, and then to inject the electron-hole plasma to put the mirror in the total reflection configuration and therefore to switch the light off. In this case the two plasma-induced optical effects, that is the variation of refractive index and absorption coefficient, are concurrent and both contributing to an efficient switching of the light. This new driving scheme allows to reach better performances in terms of modulation depth efficiency; in fact we show how, in this case, a 100% modulation depth is achieved. An exhaustive description of the optical structure and its guiding properties, together with the analysis of the electrical behavior of the modulator are given. Moreover, we show how dissipated power is also reduced allowing us to design a highly optimized version of the modulator. Finally, a comparison with other interferometric structures is given and it is shown how this kind of modulator results very attractive in terms of dissipated power and reduced area occupied on chip.

To fabricate a low-driving voltage, high-speed x-cut lithium niobate modulator with 50-ohm electrode impedance, we introduce a novel design approach, which employs a patterned SiO₂ buffer layer. Experimental results showed that the partial removal of SiO₂ buffer layer was effective in both lowering driving voltage and suppressing dc-drift of the modulators.

An optoelectronic switching network with 2-D optical fiber bundle arrays I/O access device is presented in this paper. An optoelectronic recirculating Banyan network based on CMOS/SEED smart pixel device is used in this configuration. Thirty-two X two single-mode fiber bundle array and 32 X 2 multi- mode fiber bundle array are fabricated respectively based on the features of high density, high precision and array permutation of the CMOS/SEED optoelectronic integrated devices. The measuring results show that the center to center spacing between adjacent optical fibers in the same layer of the fiber array is 125 micrometer, and the spacing between adjacent layers is 500 micrometer. Displacing tolerance of the fiber bundle arrays is less than 2 micrometer and the angular tilt error is less than 0.02 degree.

Optical interconnects offer advantages over electrical interconnects in terms of clock skew, crosstalk, and RC delay for ULSI (Ultra Large Scale Integrated-Circuit) silicon technology. Optical interconnects are also applicable in optical communications where compact optical devices are fabricated and incorporated in an on-chip integrated optical system. Polycrystalline silicon (polySi)/SiO₂ is an attractive waveguiding system that offers significant advantages in both applications with its compact size and compatibility with multilevel CMOS processing. Based on the process optimization that led to a low-loss polySi material, we have fabricated compact waveguide bends and splitters that were microns in size. To study the modal behavior in bending and splitting, we compared multi-mode and single-mode waveguides that were used in fabricating bends and splitters. Two waveguide cross-section dimensions, 0.5 micron X 0.2 micron and 2 microns X 0.2 microns, were used for single- mode waveguide and multi-mode waveguide, respectively. Micron- sized bending was realized with a low loss of a few dBs. Single-mode bends showed less than 3 dB loss for a bending radius of 3 microns, which was lower than that for multi-mode bends. Two different types of splitters, single-mode Y- splitters and multi-mode Y-splitters were fabricated and characterized in terms of their splitting uniformity. One X four and 1 X 16 optical power distribution systems were built based on different splitting schemes and their output power uniformity was compared. Due to the high dielectric contrast of our polySi/SiO₂ waveguide system, the smallest 1 X 16 optical power distribution was realized in an area smaller than 0.0001 cm².

A micromechanically wavelength-tunable optical filter with an integrated pin-photodetector for the wavelength band around 1.55 micrometer is demonstrated. The micromechanical modification of the resonator length realized by either thermal or electrostatic actuation of micromachined Bragg reflectors is used as tuning principle. The maximum tuning of the filter can be determined by its free spectral range (FSR) in the order of 40 nm, according to a resonator length around 30 micrometer. The required micromechanical displacement of the movable Bragg mirror of 800 nm is observed for an actuation voltage of 32 V utilizing capacitive actuators, while a heating power of 1.3 mW is required for electrothermally actuated membranes. Epitaxial (InAlGaAs/InAlAs) as well as dielectric (SiO₂/Si₃N₄) material compositions are used for the Bragg reflectors to meet the mechanical and optical demands of the filter. The experimental full width at half maximum (FWHM) of the tunable wavelength division multiplexing (WDM) filter is 0.24 nm corresponding to a finesse of F equals 180. The insertion loss at resonance wavelength is 2.8 dB, whereas the contrast between maximum and minimum transmission exceeds 40 dB. The integration of an InGaAs/InP photodiode and a bulk- micromachined Bragg mirror allows the realization of a wavelength-selective pin photodetector. We report on bulk- micromachined thermally actuated highly selective photodetectors with a maximum tuning range of 35 nm, a FWHM around 0.4 nm and a tuning sensitivity of 20 nm mW^-1. The technology discussed in this paper will be compatible to opto-electronic integrated circuit (OEIC) fabrication processing based on the InP-material system and therefore will enable the realization of receiver front ends with higher functionality for future dynamic WDM systems.

We propose a WDM system of sech or Gaussian type of optical pulses using a set of fixed- or sliding-frequency notch filters for channel-channel isolation. We have first demonstrate that a soliton trapped in a channel between two notches is very robust. We have also predicted optimum ratios between the channel separation and the soliton's spectral width for both the sech and Gaussian pulses. We find that the Gaussian pulses admit approximately 70% more channels than sech pulses for the same bandwidth. We investigated suppression of effects of interchannel collisions. We demonstrate that the effects can be largely eliminated by notch filters, while requiring an additional gain approximately 100%, and only approximately 5%, respectively for the sech and Gaussian pulses, of the basic gain for compensating the fiber loss.

All-optical demultiplexing of a 40 Gb/s signal to four channels at 10 Gb/s each has been successfully demonstrated using optical four-wave mixing in a travelling wave semiconductor optical amplifier. It is also expected that any set of data bits up to 1 Tbit/s can be retrieved using the four-wave mixing process.

Surface gratings with period ranging from 4 to 1.4 micrometer are UV-imprinted in an acrylate-rich hybrid sol-gel glass. The method of fabrication does not involve any etching step. We investigate structural changes occurring in the material upon exposure to light. Large amplitude modulation, smooth surface and high diffraction efficiencies are obtained. Preliminary tests show that the gratings are robust to temperature and solvent exposure.

A bit error rate (BER) analysis for the Non-linear Optical Loop Mirror (NOLM) demultiplexer is presented using a combined optical receiver and NOLM model. The control pulse used in the NOLM model is assumed to be of soliton shape in order to obtain a flat-top switching window to reduce the effect of timing jitter between the control and signal pulses. The NOLM model is incorporated into an optical receiver model for a bit error rate (BER) analysis. It is found that noise due to timing jitter, cross-talk and BER are strongly dependent on the walk-off time between the control and signal pulses in the NOLM demultiplexer. Results show that the power penalty of a NOLM demultiplexer using a soliton control pulse can be minimized with an optimum value of walk-off time. In addition the cross-talk of the Terahertz Optical Asymmetric Demultiplexer (TOAD) has been investigated using a TOAD model. Simulation results show that the cross-talk of a TOAD increases as the asymmetry time is increased. A comparison of the two devices for 100 Gb/s demultiplexing shows that the cross-talk of a TOAD (with an optical amplifier recovery time of 300 ps) is at least 3 dB lower than that of NOLM for maximum demultiplexed output.

We report experimental results when pumping with up to three different wavelengths a fluorozirconate TM³⁺-doped fiber. The wavelengths involved are the Nd:YAG line at 1.06 micrometer and two Stokes-components at around 1.117 micrometer, and 1.17 micrometer. Side light fluorescence (SLF) and end light fluorescence (ELF) in the blue, red, and NIR are analyzed when different proportions among the pumping bands are used. It is demonstrated that blue fluorescence emission is more efficient by pumping simultaneously with two Stokes components instead of pumping by only one.

The ability to engineer glass properties through the selection and adjustment of chemical composition continues to make glass a leading material in both active and passive applications. The development of optimal glass compositions for integrated optical applications requires a number of considerations that are often at variance with one another. Of critical importance is that the glass offers compatibility with standard ion exchange technologies, allowing fabrication of guided wave structures. In addition, for application as an active material, the resultant structures must be characterized by absence of inclusions and low absorption at the lasing wavelength, putting demands on both the selection and identity of the raw materials used to prepare the glass. We report on the development of an optimized glass composition for integrated optic applications that combines good laser properties with good chemical durability allowing for a wide range of chemical processing steps to be employed without substrate deterioration. In addition, care was taken during the development of this glass to insure that the selected composition was consistent with manufacturing technology for producing high optical quality glass. We present the properties of the resultant glasses, including results of detailed chemical and laser properties, for use in the design and modeling of active waveguides prepared with these glasses.

Semiconductor sources with integrated electroabsorption modulators and distributed feedback lasers (EML) are important for high speed (2.5 Gb/s and higher) transmission systems. These devices have very low spectral width under modulation which reduces the effect of fiber chromatic dispersion. The low spectral width is due to low linewidth enhancement factor ((alpha) -factor) of these devices. We report measurement of (alpha) as a function of bias for different EML devices. For EML devices with large isolation resistance between the laser and the modulator, (alpha) varies with the modulator voltage. For some bias level measured (alpha) is close to zero. The value of (alpha) has also been calculated using the band structure parameter of the active region. The measurement and calculation agree. For EML devices with low isolation resistance, the measured (alpha) is found to be nearly independent of modulator voltage. This is explained by considering the effect of leakage current between the laser and the modulator.

Strong pump absorption in Er/Yb codoped fiber grating lasers was found to cause large optical frequency fluctuation besides lasing wavelength shift. The temperature rise due to the pump induced heating within the Er/Yb fiber cavity was theoretically investigated, which caused the lasing wavelength shift with the increase of 980 nm pump power. An experimental setup was also built to estimate the optical frequency fluctuation under a fixed pump power. The observed 1.7 GHz optical frequency fluctuation was reduced to less than 90 MHz through proper thermal dissipation.

In many practical wavelength-division multiplexed (WDM) systems, erbium-doped fiber amplifiers (EDFA) must maintain a flat gain spectrum over a broad bandwidth for a wide temperature range. When the flatness specification is tight, the temperature dependence of the erbium ion spectra is too great to allow flatness over a wide range of temperatures. To date, all observed temperature variation data has been for specific EDFA designs and no simple rule has been developed for extending the observations to a wide range of EDFAs. We show that the gain and absorption parameters of an erbium- doped fiber follow easily fit dependencies on temperature. Consequently, the change in an EDFA spectrum with temperature can be easily predicted by use of a computer model or sometimes by direct computations using the known dependencies. Physical insight into the meaning of the dependencies is presented and predictions for typical EDFAs are shown.

Erbium doped fiber amplifiers are often used in all-optical networks. When the optical network uses linear regime of the pulse propagation, the goal of design optimization is gain flattening. This work presents the results of the study of nonlinear regime of propagation in optical fiber amplifier when pulses can propagate in soliton regime. Due to their remarkable stability and distortionless the solitons can be used as information bits in long-distance and high-bit-rate optical fiber communication systems. To compensate the strong attenuation suffered by soliton that has passed a long distance between neighboring reshaping points, one should apply a strong amplification. However, the soliton strongly amplified by the device with flattened gain is not longer a soliton, but a lump, the decay of which produces much radiation and, perhaps additional solitons. We developed new method that enabled us to choose the spatial dependence of the gain coefficient so that the amplified solitary wave remains an exact soliton at each point of fiber amplifier. The theory of optimal control of distributed media is applied for optimization of soliton propagation in nonlinear media. The goal of optimization is to eliminate nonsoliton radiation. An analytical solution of the optimal control problem is obtained.

Stimulated Raman scattering is analyzed as a potential mechanism for ultrafast switching and amplification in a nonlinear loop mirror (NOLM). The control pulse power requirements for the Raman switch are calculated to be practically the same as for the Kerr switch. The real and imaginary parts of the Raman susceptibility in a germanium- doped fiber for different GeO2 concentrations are derived. The stimulated Raman scattering is shown to dominate over the nonlinear Kerr effect with an increase of germanium concentration, resulting in the linear (at low Raman gain) and exponential (at greater Raman gain) transfer functions of the NOLM. An important advantage of this optical transistor is a possibility to obtain all-optical switching and amplification in a single device. We have demonstrated experimentally an optical switch with the amplification factor of 10 using 120 ps pulses in 20 mol.% germanium-doped fiber.

Furnace-melt, multi-component glasses are used to produce dense gain media for waveguide and micro-chip lasers. The compositional flexibility is often accompanied by elevated water contents, which can lead to hydroxyl (OH) quenching. OH quenching can significantly shorten excited state lifetimes, even at low pump powers. It therefore becomes important to know and control the OH content of laser glasses. While a simple relation between infrared vibration spectra and OH contents exists for vitreous silica, we show that this relation does not apply to multicomponent glasses. Instead, we present a self-consistent calculation to determine an order- of-magnitude estimate of the number of quenched rare-earth (RE) ions in multi-component glasses. Infra-red absorption spectra and fluorescence lifetimes are required. This method gives an accurate prediction of quench-shortened fluorescent lifetimes in a wide variety of host glasses.