Design of Integrated Bragg Grating-Based Filters for Optical Communications

 

Project Staff: Thomas Murphy, J. Todd Hastings, M. Jalal Khan, Michael H. Lim, Joseph Huang, Professor Hermann Haus, and Professor Henry I. Smith.

Sponsor: Air Force Office of Scientific Research Contract F49620-96-1-0126


We have developed a set of new lithographic techniques specifically tailored to meet the needs of integrated Bragg gratings. As a vehicle for demonstrating these techniques, we are in the process of developing two novel devices which could play an important role in future optical networks.

The first device we are developing, depicted in Figure 39, is based upon quarter-wave-shifted Bragg gratings. When a quarter-wave shift is introduced in an otherwise uniform Bragg grating, the resultant structure behaves as an optical resonator, similar to a Fabry-Perot cavity or a ring resonator. The structure is designed such that only one wavelength channel from a multi-wavelength system will excite the resonator. The device therefore acts as an add/drop filter, enabling the addition or extraction of a channel from the bus waveguide, while leaving all other channels unaffected. The second resonator, located below the bus, ensures that there is no appreciable reflection of the resonant channel into the input port of the device.

Figure 39: A schematic diagram of the resonator-based add/drop filter. Several independent data channels, each at a different wavelength, travel along the bus waveguide. One channel excites the quarter-wave-shifted Bragg resonator, and is tapped off in the upper port of the device. This device may be operated in reverse, allowing one to selectively add a channel to the bus.



The device depicted in Figure 39 is a first-order filter, which has the characteristic Lorentzian bandpass response expected for a single-pole resonator. By cascading multiple resonators, it is possible to achieve more complicated higher-order filters. To address the complex design challenges of these filters, we have developed an equivalent-circuit model that maps the Bragg-grating-based waveguides onto equivalent electrical circuits consisting of resistors, inductors, and capacitors. Once this association has been made, the spectral response of the filter may be engineered using standard circuit tables. For example, we have used the equivalent circuit technique to design third-order Butterworth filters. Once we have mapped the electrical parameters to their corresponding optical parameters, we use computer simulations to calculate the physical dimensions of the waveguides and gratings that yield the desired values for these optical parameters. This dual approach of using analytic techniques and computer simulations to design devices enables us to generate detailed design tables which take into account, and allow for, unpredictable variations in the manufacturing sequence.

The second device that we are developing, depicted in Figure 40, is a simpler Bragg-grating filter. The gratings in this device are long, structures without quarter-wave shifts. In this implementation, each of the Bragg gratings acts like a wavelength-selective reflector. The two identical Bragg gratings are integrated in a Mach-Zehnder interferometer, which separates the signals reflected from the gratings from the input signal. Light is launched in the upper left port of the device, and split equally by the coupler. A portion of the light is reflected by the identical Bragg gratings located in the arms of the interferometer. Provided the arm lengths are matched, these reflected signals recombine and emerge in the lower left port of the device.

Figure 40: A schematic diagram of Bragg gratings integrated in a Mach-Zehnder interferometer. The identical gratings located in the opposite arms are designed to reflect a portion of the incident light. The filtered signal emerges in the lower left port.


Depending upon the characteristics of the Bragg grating, the filter can be configured to perform many different functions. For example, by appropriately selecting the length and depth of the Bragg grating, the reflection spectral response can be made to have a bandpass shape. The spectral response is typically apodized by slowly varying the strength of the grating along its length. With this configuration, the device performs as an add/drop filter: one wavelength channel is reflected by the gratings, while all other channels pass-through unaffected. The same channel may be simultaneously added by launching it in the upper right port.

We are also investigating the integration of Bragg gratings with Silicon-On-Insuljmator (SOI) ridge waveguides. These waveguides are clad by SiO2 below and by air above. By carefully selecting the silicon thickness, ridge height, and ridge width, one can maintain single-mode operation while providing a large waveguide cross-section for efficient fiber coupling. A typical waveguide cross-section is shown in Figure 41a. Figure 41b shows the experimentally measured filter response of a long uniform grating etched into the waveguide ridge. SOI is commercially available and can be processed with standard Si fabrication techniques. In addition, no cladding overgrowth is required. Despite these advantages, the difficulties of minimizing loss and optimizing fiber coupling while maintaining adequate grating-strengths make SOI a challenging material system.

The devices described here illustrate the rich variety of optical filters that can be constructed using integrated Bragg gratings in various materials systems. We are currently in the process of building and testing these devices.

Figure 41: (a) A typical cross section of a Silicon-on-Insulator ridge waveguide. (b) The experimentally measured transmission response of a 4mm long uniform grating etched 150nm into the waveguide ridge. Both TE and TM responses are shown.