Principal Investigator:
Jingming Xu, PhD, Professor
School of Engineering
Brown University
Providence, RI
Brief Description:
Gratings are optical devices used to achieve wavelength-dependent characteristics by means of optical interference effects. These wavelength-dependent optical characteristics can serve to reflect light of a specific wavelength while transmitting or refracting light at all other wavelengths. Such characteristics are useful in a wide range of situations including the extraction of individual wavelength-channels in wavelength division multiplexed (WDM) optical communication systems, or providing wavelength-specific feedback for tunable or multi-wavelength semiconductor lasers.
Gratings are broadly categorized as multi-wavelength grating or single-wavelength periodic grating; both have multiple subtypes, but each has major limitations and drawbacks in varying applications. For example, gratings require a considerable amount of space in terms of current integration standards and miniaturized devices; if multiple single-wavelength gratings are required, a significant loss of space occurs. Hence, it is desirable to have a single device capable of processing several wavelengths in a space-efficient manner. In optical transmission, optical networks must correct for dispersion, which can directly impede operation. Some forms of dispersion can be corrected for with single-wavelength gratings, while others cannot.
Other challenges with various types of gratings depending upon application include: difficult implementation for any number of reasons; lack of flexibility in location and relative strength of reflective peaks; inefficient due to large fraction of grating-free space; inability to tune; lack of high resolution; excessive group-delay ripple or incomplete dispersion compensation; incident light issues; extreme manufacturing constraints; vast amounts of semiconductor real estate required; intermodulation; issues with multi-peak gratings vs. control over individual peak shape; lack of capability to generate flat-top channels or near-arbitrary reflectance spectra; computationally difficult and inefficient synthesis methods; limited applicability; and traditional grating systems are not amenable to setting optical characteristics for a continuous range of wavelengths. Therefore, compelling reasons exist for the development of improved methods for detection of optical wavelengths and in new optical devices.
The novel and flexible technology offered here addresses the limitations of conventional gratings and grating systems through its numerous embodiments. This invention is an optical component that includes at least one optical supergrating. The optical supergrating includes a quantized refractive index profile adapted to exhibit a finite plurality of refractive indexes, which in turn are adapted to generate a reflectance spectrum in at least one spectral band corresponding to a Fourier transformed analog refractive index profile.
The unique supergrating has three defining and differentiating properties from those of other grating technologies. First, the supergrating relies on a discrete number, historically 2 (but any suitable number), of effective refractive index levels, enabling the form of a binary grating - the binary supergrating (BSG). Different values of effective refractive index may be attained by varying the real refractive index in any part or in the vicinity of the supergrating, or by any other method that varies the effective refractive index experienced by propagating light. Second, the supergrating resembles a sampled structure characterized by a set of sample points, each associated with a sample region, which may take a variety of shapes, and are often referred to as refractive index pixels. The effective refractive index is substantially fixed within each pixel. Transitions between the grating’s index levels cannot occur at arbitrary positions, but rather must occur at boundaries of regions defined by the sample points. The BSG can be described by a series of (often binary) digits, indicating the refractive index setting at each sample point. Third, an optical wave-front incident on the grating experiences multiple interactions with the grating features, and the supergrating operates in the Bragg diffraction regime.
Applications are numerous for the production of or, improved functionality in: fiber-optic systems, optical couplers, and lambda routers for high speed telecommunications cable/networks; WDM optical communication systems; tunable or multi-wavelength semiconductor lasers for advanced analytical instruments such as spectroscopy; optical dispersion controllers for use in polarimeters/lasers; optical spatial separators; add/drop filters; and wavelength equalizers; among others for commercial/industrial and scientific R&D purposes. Markets include: advanced materials and/or scientific analytical instruments; lasers; advanced microelectronic components; telecommunications, semiconductor and/or fiber-optic equipment; scientific R&D research tools.
Information:
US patent 7,356,224 is issued (3/9/2006)
US patent 7,373,045 is issued (5/13/2008)
US patent 7,496,257 is issued (2/24/2009)
US patent 8,041,163 is issued (10/18/2011)
Corresponding foreign applications are issued/pending