Dr. Carlos Rivera, Executive Director of Waptel
One of the basic building blocks of any communications system is the modulator, whose functionality may be directly integrated into the source or performed externally. In general, wideband optical communications systems employ external electro-optical modulators to enhance their performance. One reason is that direct modulation assumes that the information to be transmitted, encoded in an electrical signal, acts on the source directly, so that the modulation process itself involves changes in the refractive index of the active material as a consequence of dynamic variations in charge density and gain, resulting in spectral broadening associated with the dynamic shift in the laser wavelength itself. This phenomenon is coupled with other distortions introduced by the interaction processes between the photon and electron populations involved in the generation of the modulated laser signal, such as relaxation oscillations, which result in limiting the maximum transmission rate for this operating scheme. On the other hand, external electro-optical modulators possess intrinsic capabilities that make them useful as individual components for developing applications in various fields including electromagnetic sensors, optoelectronic oscillators, and, in general, advanced photonic processing and computing systems.
Typically, electro-optical modulators are implemented in integrated platforms to increase the interaction of light with the active medium. At the same time, this integration favours the design of more complex structures and circuits with added functionality. However, the very practical design of waveguides and integrated structures leads to difficulties in realising efficient coupling with the outside, especially with fibre optics, both due to geometrical differences and modal mismatch. Although techniques are known to improve coupling efficiency, there is an associated cost that may make their adoption difficult in some applications. Regardless of the above, electro-optical modulators can be based on refractive phenomena or on changes in absorption due to the application of an electric field. Each technological solution has its advantages and disadvantages. In particular, III-V semiconductors have advantageous properties for the development of electroabsorption-based modulators, being the best technological option overall and at present for this type of structures. The published results point in this case to bandwidths above 50GHz using compact waveguide structures (≤100µm) with operating voltages of a few volts compatible with the radio frequency signals generated by the controllers, and even bandwidths of the order of 100GHz for distributed waveguide structures with sizes in the 200µm range. Given the large market and the complexity of device manufacturing, commercial products have been oriented to cover mainly the markets with the highest demand in the area of consumer transceivers. The integrated implementation with edge coupling is the selected option.
Figure 1. Basic device designs to realise electro-absorption modulators for vertical operation
However, there are other applications where a natural vertical coupling solution is more suitable, mainly for the realisation of wireless optical links or for some types of photonic processing. Layered structures with vertical operation also have a similar potential in terms of bandwidth as compact waveguide structures with concentrated parameters. WAPTEL has developed a proprietary technology for the fabrication of electroabsorption modulators based on vertically operating quantum well structures. This technology is available in InP and GaAs platforms for use in different spectral bands covering 850nm to 1600nm, and there is also a working line with a GaN platform for operating wavelength bands in the visible and near-ultraviolet. Depending on the final application, WAPTEL offers different device designs (see some examples in Figure 1), optimised for operation in harsh environments or for different bandwidth to power ratios. Figure 2 shows some first-generation large area electro-absorption modulators manufactured by WAPTEL for general purpose asymmetric communications.
Figure 2. Large-area electroabsorption modulators manufactured
The basic structures of the devices are complemented to implement the interconnections with the outside and passivate the surfaces, and coplanar type technologies can be used for high-frequency operation.