The discovery of thermal poling in 1991 opened up the possibility of electro-optic mod- ulation and frequency conversion in glass fibres and waveguides. Together with funda- mental studies aimed at enhancing the value of the second-order nonlinearity, the first working devices were demonstrated. Since the nonlinearity is located a few microns below the anodic surface, the challenge from the technological point of view was the fabrication of fibres and waveguides having the core close to that surface. In 1989,
even before the discovery of thermal poling, Li and Payne induced a permanent linear electro-optic coefficient in a fused silica fibre subjected to strong electric field by means of internal electrodes [116]. The fibre had two holes running parallel to the core that were filled with gallium liquid electrodes at room temperature. A 65 cm long fibre modulator was fabricated by exposing the core to 4000 kV/cm. The induced linear electro-optic coefficient wasr '2χ(2)/n4 = 2×10−3pm/V. In 1994 a D-shape fibre, where the core is
placed close to the flat side of the fibre, was used for the first time and aχ(2) as large as
0.2 pm/V was obtained after poling [117]. The reproducibility of the results was greatly improved by poling in vacuum in order to circumvent problems caused by partial electric breakdown in air during poling [118]. Also in 1994, the research group in Albuquerque, fabricated a poled fibre electro-optic modulator using a D-shape fibre [119]. HF etching of the fibre allows the reduction of the thickness of the cladding above the core to only 6µm. The reported linear electro-optic coefficient was 0.05 pm/V. The major issue while poling D-shape fibre is that breakdown may occur between the flat electrode and the ground. Further improvement of the packaging using polyimide as insulator resulted in r = 0.3 pm/V and the half-way driving voltage of the fibre modulator was 75 V at 633 nm and for a 12 cm long device. A planar device was first obtained in 1996 by the NTT Opto-electronics laboratories. 2x2 electro-optic switching was demonstrated in an integrated Mach-Zehnder interferometer constructed with thermally poled GeO2-doped
silica based channel waveguides on a Si substrate. The switching voltage was 1700 V in the 1.55µm wavelength region for a 36 cm long device [120]. Fascinating is also the demonstration of poling of holey fibres by Faccio and co-workers [121]. A maximum linear electro-optic coefficient of' 0.02 pm/V was obtained after 40 minutes poling at 4 kV at 280◦C. This work demonstrates that the air-holes structure of micro-structured
fibres is compatible with poling. More than for electro-optic modulators, this work opens new possibilities for efficient and broadband frequency conversion owing to the endlessly single mode operation and tight mode confinement of micro-structured fibres [122]. A conspicuous amount of work on the fabrication of electro-optic modulators has been carried out by the research group at Sydney University. A powerful tool they have been using is a Mach-Zehnder interferometer employed for a real-time monitoring of the evolu- tion of the E-O coefficient during poling of twin-hole fibres [123, 124]. The experimental set-up enabled the measurement of both amplitude and direction of the frozen-in field Edc and also of theχ(3)in poled and unpoled fibres [123]. On the basis of the experimen-
tal observations, including a different behavior for positive and negative poling [125] and direct visualization of the depletion region in fibres [114], they proposed a model for the thermal poling in fibre based on the competition between three fields, the applied field, a shielding field created by the diffusion of charges which opposes to the applied field, and an ionizing field originated by charge emission and glass ionization as the depletion region builds up [114, 126]. The depletion region was found to be formed all around the hole containing the anode electrode. A novel design of twin-hole fibre, having a ring of either boron or erbium to act as a trap, or donor, of electrons respectively, was
fabricated in order to improve the poling performance [112]. Indeed the B-ring doped twin-hole fibre exhibited longer stability, in the range of few years [127] compared to hundreds of days in the undoped fibres used by them [128]. It should be mentioned that several groups observed an enhancement ofχ(3) in the fibre after thermal poling [127].
This enhancement was measured to be'2 by the group in Sydney and was found to be 1.98 by Garcia et al. after poling of a channel waveguide [13]. A tentative explanation was given by Kashyap [129] but overall the enhancement ofχ(3) is still questionable and
not all groups could observe it [14]. A very promising technology has been developed by Fokine et al. [130]. The holes of a twin-hole fibre have been filled with a AuSn alloy in the liquid state at 300◦C under pressure. Subsequent poling has been carried out
at 250◦C with the alloy in the solid state. A wide wedge shaped nonlinear region was
revealed around the anodic hole, by etching, in agreement with simulations considering the air surrounding the fibre to be at the same potential as the cathode electrode [14]. From the point of view of an electro-optic modulator or 2×2 switch, the good contact between the electrodes and the walls of the holes ensured by this technology, makes it possible to use the electrodes in order to apply the driving field. An half-way voltage,Vπ, of 1.37 kV at 1550 nm for a 20 cm device was demonstrated. Although filling the holes with alloys enables over a meter long electrodes, in this first experiment, the total length of the devices was limited by the loss caused by the interaction of the light modes with the metallic interfaces. Improvements in the fibre design are expected to lead towards long>1 m electro-optic modulators with a switching voltage as low as 50 V [14].