M. Janos

Signal-induced refractive index changes in erbium-doped fiber amplifiers

This paper reports the measurement and analysis of signal power-induced changes in the refractive index spectrum of an aluminosilicate erbium-doped fiber amplifier (EDFA). For a constant 980 nm pump power of 11 mW, a 1553 nm signal power of 4.6 mW caused a peak-to-trough change in refractive index of (3.8/spl plusmn/0.4)/spl times/10/sup -8/ across the EDFA gain spectrum. A Kramers-Kronig transform of the signal-induced change in absorption coefficient agreed well with the measured change in refractive index. This result conflicts with the behavior of the system predicted by a homogeneously broadened two-level model and suggests limitations in the representation of the Er/sup 3+/:glass system by a discrete set of homogeneously broadened transitions.

Growth and decay of the electrooptic effect in thermally poled B/Ge codoped fiber

Using an in situ technique for measuring the induced electrooptic effect during poling, we have studied the growth and decay characteristics of thermally poled twin hole B/Ge codoped fiber devices. The decay characteristic measured at elevated temperatures were best fitted with a stretched exponential function, indicating a distribution of relaxation times is present in this material. Using the Arrhenius relation, we calculate an activation energy for the stability of the electrooptic effect with this material and poling geometry in the range from 25 to 28 kJ/mol (0.28-0.31 eV), corresponding to a lifetime at 298 K of approximately 45 days.

All-fiber acoustooptic phase modulators using chemical vapor deposition zinc oxide films

For the first time we report an all-fiber acoustooptic phase modulator using a zinc oxide (ZnO) film deposited by modified single source chemical vapor deposition. This technique allows deposition over the full 360/spl deg/ fiber surface without the need for sample rotation, greatly simplifying the manufacturing process. The maximum phase shift measured for our 6 mm long devices was 3.5 rads for a drive power of 580 mW. Unlike devices fabricated using sputtered ZnO films the maximum attainable phase shift is not significantly limited by thermal and mechanical loss effects at higher driving powers which we attribute to the excellent chemical composition of our chemical vapor deposition (CVD) grown films. A maximum efficiency of 0.28 rad//spl radic/(mW)/cm of device length was measured which may be attributed to the relatively thin films used in this experiment (0.4-0.9 /spl mu/m).

Rayleigh longitudinal profiling of optical resonances within waveguide grating structures using sidescattered light

The resonant characteristics of Bragg gratings in fibers are characterized by measurement of the wavelength dependence of the Rayleigh scattering of light propagating in the core along the grating length. This technique is found to give information on the standing-wave intensity profile in the grating that cannot be deduced from the standard transmission or reflection spectra.

All Optical Switching In Rare-earth Doped Fibers

The nonlinear response seen in rare-earth doped fibers is due to the change in absorption induced by optical pumping of the resonant transitions of the rare-earth ions. Since many rare-earth ions exhibit long-lived metastable states, these changes in absorption, and hence, the associated changes in refractive index, can be large for relatively modest absorbed pump powers. For example, typical values for the induced index change are of the order 1 part in lo6 for absorbed pump powers of a few milliwatts, and all-optical switching has been observed for pump powers as low as 1 mW. Since these rare-earths can also be used to form efficient fiber lasers it has been possible to use the lasing dynamics of the fibers to: 1) demonstrate a digital nonlinear behavior, by utilizing the clamping of the population inversion at threshold,’ and 2) improve the response times by using Q-switching induced stimulated depopulation of the excited state? A further advantage of this type of nonlinearity is that in-fiber Bragg reflection gratings can be readily incorporated into the nonlinear medium and can be used to give highly wavelength selective modulation of the transmitted signal. This has been demonstrated in two ways using the pump induced index change. Firstly by side pumping of a doped fiber grating in order to introduce a phase shift part of the way along the grating, which creates a transmission notch in the reflection band of the grating;3 and secondly by axial pumping of the doped fiber grating causing a shift in its reflection band such that a propagating signal positioned near the band-edge of the grating experiences a greater or lesser transmi~sivity.~ It is commonly assumed that to make use of these resonant nonlinearities, it is necessary to operate within, or close to, the rare-earth absorptions, which tends to introduce high losses at the signal wavelength. However, it has recently been demonstrated that the induced index change seen in rare-earth doped fibers is due not only to the localized changes in absorption of the optically pumped transitions themselves, but also to changes in absorption of all transitions that involve FRIDAY MORNING 1 CLEO/PACIFIC RIM / 265

Resonantly enhanced nonlinearaties in rare-earth-doped fibers and waveguides

The use of rare-earth dopants for all-optical switching is of interest because of the very low pump powers required to achieve full switching. In addition, the ease with which rare-earth ions can be incorporated into silica based fibers and waveguides makes them ideally suited for the fabrication of fiber compatible components. The principal disadvantages of this type of nonlinearity are that the relaxation times are characteristically slow, and relatively long interaction lengths are required to achieve phase changes of the order of (pi) . In this presentation, an overview of the work being carried out in Australia will be given, concentrating on the techniques developed to minimize the relaxation times and power length products of these nonlinearities.

Time Evolution of Electro-optic Effect in Fibre During Thermal Poling

Poled silica fibre with electro-optic effect expands the photonic functionality of the fibre. The understanding of its origin, however, is in a similar way to that of fibre Bragg grating formation, still not very clear. More experimental data is needed in order to clarify the process. In this paper, we present the measurement of the time evolution of the electro-optic effect in silica fibre during thermal poling. The new observation suggests that the current model of thermal poling of silica glass need to be modified.

Recent results with thermal poling of fiber

Recent work on the thermal poling of silicate optical fiber is presented. This paper includes the background on thermal poling, optimization of poling conditions, lifetime of the electro-optic effect as well as discussions on the change in (chi) (3) after poling and the validity of the frozen-in field model for thermal poling.