Introduction Erbium-doped integrated optical amplifiers play an important role in fiber-optic telecommunication networks operating in the third window around 1.55 μm, due to their wide gain band and excellent noise behaviour. Applications can be found in e.g. amplifiers for loss compensation[l], lasers for signal generation in WDM-based systems (2] and cross-phase modulators for all-optical switching[3]. Driven by the rapid development of erbium-doped fiber amplifiers, a lot of research has been done on planar erbium-doped amplifiers, in a number of different host materials. Good results have been demonstrated in e.g. silica [4,5] and LiNb03 [6], where high amplification values of 10-15d.B have been demonstrated.

For the first time a polymeric 8/spl times/8 optical switch matrix based on 1/spl times/2 digital optical switches has been realized. The first prototypes exhibit an average insertion loss of 10.7 dB and an average extinction of 30.3 dB at 1.536 /spl mu/m with polarization independent digital switch characteristics.

Recently developed photobleachable polymers show a loss of < 0.1 dB/cm at 1300 nm and < 0.15 dB/cm at 1550 nm. Nonchromophore containing polymers show film waveguide losses of < dB/cm at 1300 nm and 1550 nm. Refractive indices in these materials can be tuned within a range of 0.05 by changing the polymer composition. Multilayers of cross-linked (solvent resistant) layers, each 2-10 microns have been deposited by multiple spinning steps. Using these multilayers, fully embedded, fiber-compatible strip waveguide structures have been created by masked bleaching of multilayers with chromophore containing corelayers. The lateral refractive index contrast is thereby tuned by changing the chromophore content of the corepolymer. Poling-induced loss has been investigated by wavelength and polarization dependent measurements of losses in films. The results indicate that this loss is due to increased scattering. Bleached channel waveguides in a poled (at 125 V/micrometers ) nonlinear optical polymer have been made showing losses of < dB/cm at 1300 nm. Rapid photodegradation at 1300 nm has been observed in stilbene containing channel waveguides. In a nitrogen atmosphere no degradation was seen. The same is true for waveguides in air at 1550 nm. This suggests the attack of the stilbene chromophores by singlet oxygen. Therefore a new generation of low-loss, linear, and nonlinear optical polymers based on singlet oxygen resistant molecules has been developed. The linear optical polymers are used for the realization of low-insertion loss (< 2 dB), digital (switch voltage 3-6 V) and efficient (switching power < mW, cross talk - 20 dB) pigtailed and packaged 1 X 2 switches. They utilize the strong thermo-optic effect in polymers. Their switching time is therefore limited to 1 ms whereas their polarization dependence is < 0.3 dB.

A new integrated optic wavelength multi/demultiplexer based on a bent asymmetric Y-branch is proposed and investigated. The broadband multi/demultiplexer is demonstrated to combine/split 660 nm pump light and 1535 nm signal light with coupling efficiencies of about 85% and 95% respectively, which is useful for erbium doped integrated optic lasers. The bent Y-branch multi/demultiplexer is simple to fabricate and rather insensitive to processing variations.< >

Erbium-doped Y2O3 integrated optical amplifiers are designed for low-threshold operation and 3 dB amplification. The most important design parameter for minimal threshold, the erbium concentration, is found to have an optimum value of 0.35 at% for a given waveguide structure with 1.0 dB cm-1 background loss. The corresponding threshold power is 7 mW. The pump power to obtain 3 dB gain is found to be 22 mW for an amplifier with an optimum erbium concentration of 0.6 at% and 2.8 cm length. At 30 mW pump power the maximum gain is shown to be 5 dB.

The intensity dependent refractive index and absorption coefficient are third order optically nonlinear effects which offer the possibility to realise all-optical switching devices. The refractive index change Δn due to the applied field E is expressed as Δn = n2 · |E|2. Important material parameters are the value and relaxation time of the nonlinear coefficient n2, the maximum achievable refractive index change Δnsat and the linear absorption α0 [1]. Rare earth-doped materials are well known for their use in optical amplification but may also be of importance for all-optical switching. By implantation of the rare earth atoms in a (poly)crystalline material it is possible to obtain sharp absorption peaks which are accompanied by a relatively broad dispersive change of the refractive index. This property may be used in a device where a pump beam tuned at the absorption peak changes the refractive index which influences the behaviour of a signal beam tuned just outside the absorption region. The n2 relaxation time of this type of materials is in the order of ms but can be drastically reduced by injection of a short turn off pulse [2]. The Δnsat scales proportional to the rear earth doping concentration and may be increased to about 10-4.

A signal enhancement of 10 dB at 1535 nm with a low launched pump power of 1 mW at 1480 nm is achieved for sputtered Er3+:Y2O3 channel waveguides on oxidised silicon substrates. With the developed reactive co-sputtering process using sputterguns the erbium concentration can be varied easily and reproducibly.

Self-induced phase modulation in erbium-doped polycrystalline Y(2)O(3) integrated-optic waveguides is studied by use of a Mach-Zehnder interferometric setup. We determined the wavelength dispersion of the nonlinear refractive index near the (4)F(9/2) absorption band and show that it corresponds qualitatively with a Kramers-Krönig analysis. The nonlinear coefficient at 670 nm was determined to be 1.4 x 10(-14) m(2)/W and has an electronic origin with a slow relaxation time of 6 ms.

Erbium-doped Y2O3 planar optical waveguides have been fabricated by spray-pyrolysis deposition. The attenuation spectrum of the waveguide shows peaks that are due to absorption of the erbium ions. The as-deposited layers also show photoluminescence sharply peaking at 1540 nm with additional Stark splitting. The thin layers of Er3+:Y2O3 obtained are promising for the realization of integrated-optic amplifiers and lasers.

The second- and third-order nonlinear susceptibilities gC(2) and gC(3) of a polymer thin film are determined for three wavelengths (543.Snm, 632.8nm and 1523nm) using a method based on the linear and quadratic electro-optic effect. The frequency dependences of gC(2) and gC(3), deduced from a simple two-level electron-spring model, are consistent with our experimental results. Using this frequency dependence the intensitydependent refractive index, determined by nonlinear grating coupling, is transformed to a gC(3) value for the quadratic electro-optic effect. This value is in good agreement with our experiments. The layer structure used is simple and consists basically of a polymer layer sandwiched between thin metal electrodes.