Giancarlo C. Righini - Integrated Optics: Modeling, material platforms and fabrication techniques

The search for an effective mean of transmitting information over long distances without being hampered by the environmental conditions, first of all by rain and water vapour, had started well before the discovery of laser. Thus, in the 1950s there was an emerging interest in looking at the possibility of using a microwave waveguide system for long-haul communication in place of the coaxial cable or radio relay systems used at that time. An early paper by Barlow []. It is interesting to note that the authors only had a doubt that such enormous communication capacities would ever be required!

The first demonstration of a laser source in 1960 by Maiman []. Starting from the formula expressing the minimum thickness of the slab symmetrical waveguide to support the propagation of the mode of order m

a four-mode waveguide was implemented by using two optically flat BK-7 glass plates spaced 32-m apart and 77-mm long, and by filling the inner space with a mixture of benzene and chlorobenzene. Transverse magnetic (TM) modes were excited by a HeNe laser with polarization perpendicular to the slab plane, and their number was controlled through temperature control, acting on the refractive index of the mixture.

Thus, the year 1963 may perhaps be considered the real birthyear of integrated optics; as a matter of fact, a few more papers were published in that year concerning the presence of a dielectric waveguide effect in another important optoelectronic structure: the laser diode pn junction [], comparing the results with the parameters of GaAs laser diodes.

The experimental work reported by Kaplan [ shows the images, taken at a microscope focused on the exit edge, of the modes supported by solid-core liquid-cladding waveguides constituted by fused quartz plates immersed in cineole (C10H18O; nD = 1.457@20C). The photo on the left shows the single mode (TE0) of an Ultrasil quartz plate 0.06-mm thick, whereas on the right, the photo of the TE03 modes supported by a fused quartz plate 0.13-mm thick, polished on purpose by Dell Optics Co.

Figure 1.1 Photographs of some guided modes in an Ultrasil quartz plate (core) immersed in cineole (cladding): TE0 mode (left) and TE03 (right). Adapted from Schineller et al. []

In the same period, namely January to November 1964 [ shows the experimental apparatus to test these dielectric-slab waveguides, consisting of two 77-mm plates of Schlieren quality borosilicate crown glass (k = 2.2952), suspended in chlorobenzene; the inside surfaces of the plates were optically flat. The spacing between the plates was adjusted by means of a differential screw, in the range of 3547 m. The refractive index of the liquid core could be adjusted by varying the temperature. Much care was taken to guarantee the parallelism of the plates: after adjustment, the plates were parallel to within 0.01 mrad.

Figure 1.2 Sketch of the experimental apparatus to test liquid-core solid-cladding optical waveguides. The container, with glass windows, is filled with the liquid that acts as waveguide core. The thickness of the waveguide, i.e. the spacing between the two glass flats, is adjusted by the differential screw (one turn of the screw changes the spacing by 10 m), whereas the refractive index of the core may be varied by changing the temperature of the liquid (the heather is not shown in the figure). The smaller differential screws are used to adjust the parallelism of the cladding glass plates. Reproduced from Schineller et al. []

Various components, including directional couplers, modulators, laser and detector, were also studied and experimentally demonstrated. For instance, using nitrobenzene as the liquid core, experimental tests of amplitude, phase and polarization modulators were performed [].

Figure 1.3 Sketch of the proposed solid-core liquid-cladding laser. The case is divided into two sections, separated by a glass window; water is circulated through the sealed upper half where the flashtube is located, whereas the open bottom half, containing the Nd-doped core, is immersed in a temperature-controlled container of dichlorobenzene. Reproduced from Schineller et al. []