Publicaciones: GaN-based waveguide devices for long-wavelength optical communications

In order to make full use of the wide bandwidth provided by optical fibers, more and more wavelength channels have been used in wavelength division multiplexed ~WDM! optical networks. Among others, WDM optical demultiplexers, optical switches, and wavelength routers are fundamental devices in multiwavelength, dynamic optical networks. With the rapid advancement of photonic integrated circuits (PIC), silica-based array waveguide grating (AWG)has become a popular approach for WDM multiplexing and demultiplexing.1 Since silica is not a semiconductor material and hence purely passive, the transfer function of a silicabased AWG is usually not tunable, or can only be slowly tuned by thermal effect.2,3 InP has been another popular material to make planar waveguide PICs. As a semiconductor material, InP-based PIC can potentially be made fast tunable with carrier injection,4 however, due to its high refractive index, high temperature sensitivity, and high insertion loss,5 InP-based AWG devices have so far not become commercially competitive.

III-nitride wide-band-gap semiconductor materials have attracted much attention in recent years.6,7 In addition to emitting in UV/blue wavelength region, III-nitride optoelectronic devices are able to operate at high temperatures and high power levels due to their mechanical hardness and larger band offsets. Research in III nitrides has so far been focused on their applications in blue/UV wavelength regions. Their optical characteristics and potential applications in infrared for optical communications remain largely unknown. In this letter, we propose to make functional optical waveguide devices using GaN/AlGaN semiconductor materials and explore their potential applications in infrared wavelength regions for fiber-optic communications.

In order to design guided-wave optical devices, the knowledge of material refractive indices in the operating wavelength region is essential. Due to the unavailability of experimental data in infrared, we have conducted the refractive index measurements for AlxGa1-xN with different Al molar fractions. In order to perform this measurement, a number of samples of AlxGa1-xN films were grown by metmetalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The films thickness range from 1.1 to 1.5 mm and Al molar fractions range from x50.1 to x50.7.8 To evaluate the refractive index of each film, optical transmission spectra were measured. Due to the Fabry–Perot ~FP! interference caused by the two facets of the film ~one facet is between AlxGa1-xN and the air and the other facet is formed between AlxGa1-xN and sapphire!, optical transmission efficiency is wavelength dependent. With the knowledge of the film thickness, the film refractive index can be obtained by best fitting the measured optical transmission spectrum to a well-known FP transmission equation.

Figure 1(a) shows the measured refractive indices of AlxGa1-xN versus wavelength for several different Al molar fractions. The continuous curves in the same figure were numerical fittings by using the first order Sellmeier dispersion formula:

The coefficients for best fitting are displayed in Fig. 1~a! and their variations versus Al molar fraction x are shown in Fig. 1(b). Since we are mostly interested in the refractive indices in 1550 nm wavelength window, this information can be collected from Fig. 1 and the following polynomial expression is obtained for the Al molar fraction ~x! dependence of the refractive index at 1550 nm wavelength:

The monotonic decrease of AlxGa12xN refractive index with the increase of Al molar fraction x makes the design of single-mode optical waveguide devices straightforward.

We used beam-propagation method (BPM) simulation tools to design single-mode optical waveguide devices. Figure 2 shows schematically the cross section of the designed single-mode waveguide based on GaN core and AlxGa1-xN cladding. Several different waveguide configurations have been designed, including straight waveguides, and 232 waveguide couplers. To verify the design, a number of waveguide samples were prepared. In the fabrication process, a 4-mm-thick epitaxial film of AlxGa1-xN was grown on a sapphire substrate and a 3-mm-thick GaN film was deposited on top of the AlxGa12xN layer and then the optical waveguide structures were formed by photolithographic patterning and inductively coupled plasma (ICP) dry etching.9 According to the design, the etching depth is controlled at ;2.8 mm and the Al molar fraction x is about 3%. As an example,Fig. 3 shows a typical 232 waveguide coupler fabricated with this process. The power splitting ratio of this particular coupler was designed to be 3 dB.