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Integrated Holographics Overview
Photons are not electrons. An obvious fact, yet its implications have not been thoroughly incorporated into current visions of future photonic integrated circuits, which center on wire-like channel waveguides and crystal-like structural analogs. Electrons interact strongly with each other and their environment - photons do not. Electronic interaction leads to electronic bands and gaps crucial in electronic device operation. Electronic interaction leads to the need for non-intersecting wire-like links between active electronic devices - a need which substantially defines allowed electronic integration formats. Left to their own devices, photons do not interact and are happy to ignore one another, leading to the possibility of circuits wherein photonic signal lines are delocalized and overlapping - allowing for great freedom in the placement of active devices.
To enable photonic circuits with free signal overlap, a photonic transport fabric is needed that provides signal-specific guiding action. Photonic fabrics comprised of overlaid webs of computer-designed, nanofabricated, slab-waveguide, holographic structures uniquely support delocalized, overlapping photonic circuits.
Developments in this field may literally revolutionize thinking about how best to integrate photonic function. Integrated slab-waveguide holographic structures easily provide signal-selective spatial routing. This means that individual structures interact only with wavelength or wavefront differentiated signal streams - even if other signals are simultaneously present.
General holographic structures, photolithographically scribed or nanoprinted (see Figures 1 and 2), have only recently become practical due to the near simultaneous development of quarter-micron and finer DUV photolithographic and nanoprinting resolution, centimeter-scale spatial coherence of scribed structures, and slab waveguide materials and fabrication methods that are robust and stable.
Nanofabricated integrated holographic structures comprise a platform technology with a multitude of specific applications ranging from the general photonic transport fabrics described above to multiplexers for wave division multiplexing in local, access, and long-haul communication networks, spectral filters for the recognition of complex spectral signatures in military and commercial applications, signal coders for use in optical code division multiplexing, and cross-correlation-based optical processors for use in all-optical signal processing and control as needed, for example, in optical header recognition. In all cases, multiple functions, spectral signatures, optical codes, etc. can be integrated onto single device die and through use of simple 1xN switches be individually selected - providing a functional equivalency to real-time programmability. Alternatively, electro-optically active materials may be implemented to directly provide real-time programming of device function.
LightSmyth is developing lithographically scribed holography from the platform perspective - committing extensive effort to understanding process strengths and limits. LightSmyth has a very strong intellectual property position relative to lithographically scribed holographic structures ranging from the basic concept, to real-time programming, a variety of critical structural and process enhancements crucial to effective implementation, and to specific applications of the technology. LightSmyth was the first to recognize the general power of photolithographically scribed or nanoimprinted holographic structures applied to guided wave applications. Its pioneering role combined with its current company-wide commitment to development of the technology make LightSmyth uniquely qualified to move the technology forward.
Integrated waveguide holographics is applicable to a large number of specific product areas. We describe here some basic devices as implemented in slab or channel waveguides and point out some of their principal features.
Holographic Bragg Reflectors (HBRs)
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| Figure 3. Single channel holographic bragg reflector spectral |
In Figure 3, a simple integrated holographic slab-waveguide device, a holographic Bragg reflector (HBR), is shown. This device is comprised of an array of etched contours. The contours collectively diffract light propagating in a 2D slab waveguide so that it is retro-reflected with its wavefront optimally matched to enter the output port and its spectral content filtered according to the HBR design function. Very general spectral transfer functions can be implemented by control of the relative diffractive amplitude and phase of the various diffractive contours. LightSmyth has developed and patented unique approaches to the phase and amplitude apodization of the HBR to produce a wide range of spectral transfer functions. The HBR filter possesses uniquely flexible spectral programming so that it can be used in applications involving the recognition of complex spectral signatures or in communications applications requiring filtering with square or otherwise controlled spectral bandpass.
The diffractive contours of Figure 3 comprise a volume hologram. Contours may be thought of as interference fringes created by input and output signal beams matched to respective I/O port parameters. The diffractive contours individually act like optimized aspheric imaging devices that act to convert the input signal wavefront to match that of the desired output signal. The diffractive contours are not created by actual interference between optical beams. Desired input/output beams are computer-simulated. A volume holographic structure is numerically computed from those beams and output as a data file for a lithographic fabrication tool. The computer design and lithographic fabrication approach allows full control of every detailed element of the structure on a pixel-by-pixel basis. Such unprecedented control allows for practical, low-cost implementation of photonic signal control concepts that designers could only dream of previously.
It should be noted that each diffractive contour schematically shown in Figure 3, interacts very weakly with the input signal beam (or any other beam). It is only the cooperative, coherent interaction of diffractive contours comprising the entire holographic structure that allows efficient coupling of the input and output beams. The holographic structure is constructed so that it will coherently interact exclusively with input beams of specific spatial wavefront, direction, and wavelength. It is this characteristic feature of volume holographic structures that allows for the support of individually controlled yet freely crossing and overlapping photonic signal streams.
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| Figure 4. Eight channel holographic bragg reflector multiplexer |
The simple single-channel HBR can be used as a building block for multiplexing devices. In Figure 4, eight individual HBR units are written on a single slab waveguide chip to form an 8-channel mux/demux unit. HBR muxes may be implemented for coarse or dense WDM applications. The layout shown is exemplary only. Typical optimized designs feature spatial overlap of individual HBR structures and various additional optimization elements. Each HBR can be designed to provide an arbitrary spectral bandpass profile including desired flat-top profile without insertion loss penalty. Since each HBR is independent, channels may have arbitrary spectral separation and bandpass design. A single silica-on-silicon die of 0.3x1 cm size is capable of supporting a self-contained, athermal, 16-channel CWDM mux. More discussion of multi-HBR devices are given in the publications listed elsewhere on this website.
Integrated holographic slab waveguide devices need not be restricted to retro-reflective geometries. Holographic structures have broad power to transform signal directions and wavefronts. Integrated holographic structures support photonic transport layers that may be implemented on electronic chips and backplanes. The transport layer serves to route optical signals from multiple source points within the electronic processing layers to multiple receivers without regard for signal stream overlap or interference.
Channel Waveguide Gratings
A channel waveguide grating may loosely be thought of as an integrated fiber Bragg grating. With Lightsmyth's lithographic fabrication approach, it is possible to design and implement essentially any grating functionality consistent with grating physics. Retroreflecting channel waveguide gratings, having tailored spectral transfer functions and fully consistent with powerful photonic integration, are easily produced. An array of channel waveguides on a small chip can be programmed with a set of transfer functions for redundancy, yield enhancement and post-fabrication fine-tuning of device response.
Outlook
In early years of photonic integrated circuits, designs followed electronic example. In the future, the tools of nanofabrication, the principles of volume holography, and the refinement of slab waveguide fabrication methods will provide pathways to exploiting the unique and characteristic properties of photonic systems. Future photonic devices and circuits will exploit architectures and methods entirely beyond and unavailable to their electronic forebearers.