Brittany Lynn

A survey on recent advances in optical communications

Recent advances in optical communications not only increase the capacities of communication system but also improve the system dynamicity and survivability. Various new technologies are invented to increase the bandwidth of individual wavelength channels and the number of wavelengths transmitted per fiber. Multiple access technologies have also been developed to support various emerging applications, including real-time, on-demand and high data-rate applications, in a flexible, cost effective and energy efficient manner. In this paper, we overview recent research in optical communications and focus on the topics of modulation, switching, add-drop multiplexer, coding schemes, detection schemes, orthogonal frequency-division multiplexing, system analysis, cross-layer design, control and management, free space optics, and optics in data center networks. The primary purpose of this paper is to refresh the knowledge and broaden the understanding of advances in optical communications, and to encourage further research in this area and the deployment of new technologies in production networks.

Design and Preliminary Implementation of an N

We have demonstrated a diffraction-based nonblocking, scalable N × N optical switch employing a digital micromirror display (DMD) with 12 μs switching speed, performing 100 times faster than the currently available technology. The distributed nature of diffraction makes this switch more robust than one-to-one reflective systems where a single mirror failure incapacitates an entire connection. We thereby address a key bottleneck in data centers and optical aggregation networks by decreasing circuit-switching speed and allowing for facile port count scalability.

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We have recently demonstrated several improvements in material properties and optical design to increase the resolution, size, brightness, and color range of updatable holograms using photorefractive materials. A compact system has been developed that is capable of producing holograms with brightness in excess of 2,500 cd/m2 using less than 20mW of CW laser power. The size of the hologram has been increased to 300mm × 150mm with a writing time of less than 8 seconds using a 50 Hz pulse laser. Optical improvements have been implemented to reduce the hogel size to less than 200 μm. We have optimized the color gamut to extend beyond the NTSC CIE color space through a combination of spatial and polarization multiplexing. Further improvements could bring applications in telemedicine, prototyping, advertising, updatable 3D maps and entertainment.

N Diffractive All-Optical Fiber Optic Switch

Organic photorefractive polymer composites can be made to exhibit near 100% diffraction efficiency and fast writing times, though large external slants are needed to project the applied field onto the grating vector. We show here that the use of interdigitated electrodes on a single plane provides similar performance to these standard devices and geometries but without a external slant angle. This new devices structure also greatly improves the diffraction efficiency and sensitivity compared to less slanted standard devices necessary for some real applications, such as holographic displays, optical coherence imaging, and in-plane switching.

Recent advancements in photorefractive holographic imaging

In this paper, we analyze the optical performance of a commonly used 1-step recording geometry for stereograms and compare it to the fully fringe rendered case, taking a published optical model of the human eye into account. We compare our results to a model that conserves wavefront curvature. We then demonstrate how to optimize hogel sampling parameters as a function of image depth, size, and viewer distance. We conclude that stereograms suffer from little degradation from a viewing distance larger than 2 meters, but that nearer field images can significantly benefit by adding a second order phase correction.

Interdigitated coplanar electrodes for enhanced sensitivity in a photorefractive polymer

The development of a single mode fiber-based pulsed laser with variable pulse duration, energy, and repetition rate has enabled the characterization of photorefractive polymer (PRP) in a previously inaccessible regime located between millisecond and microsecond single pulse illumination. With the addition of CW and nanosecond pulse lasers, four wave mixing measurements covering 9 orders of magnitudes in pulse duration are reported. Reciprocity failure of the diffraction efficiency according to the pulse duration for a constant energy density is observed and attributed to multiple excitation, transport and trapping events of the charge carriers. However, for pulses shorter than 30 μs, the efficiency reaches a plateau where an increase in energy density no longer affects the efficiency. This plateau is due to the saturation of the charge generation at high peak power given the limited number of sensitizer sites. The same behavior is observed in two different types of devices composed of the same material but with or without a buffer layer covering one electrode, which confirm the origin of these mechanisms. This new type of measurement is especially important to optimize PRP for applications using short pulse duration.

Are stereograms holograms? A human perception analysis of sampled perspective holography

Photorefractive composites derived from conducting polymers offer the advantage of dynamically recording holograms without the need for processing of any kind. Thus, they are the material of choice for many cutting edge applications, such as updatable three-dimensional (3D) displays and 3D telepresence. Using photorefractive polymers, 3D images or holograms can be seen with the unassisted eye and are very similar to how humans see the actual environment surrounding them. Absence of a large-area and dynamically updatable holographic recording medium has prevented realization of the concept. The development of a novel nonlinear optical chromophore doped photoconductive polymer composite as the recording medium for a refreshable holographic display is discussed. Further improvements in the polymer composites could bring applications in telemedicine, advertising, updatable 3D maps and entertainment.

Diffraction response of photorefractive polymers over nine orders of magnitude of pulse duration.

After a brief historical introduction about photorefractive materials, this chapter provides an extensive overview of the mathematical modeling of the photorefractive effect in organic compounds. The theories of charge photo-generation, transport and trapping, as well as chromophore orientation in the space-charge field are detailed. We then discuss the different molecular species providing the respective functionalities to the PR effect: electroconductive matrices, nonlinear chromophores, photo-sensitizers, and plasticizers, along with the recent developments in the search for more effective materials. Several electrode geometries for different types of devices are described before a section on material characterization. This later include measurement techniques of the molecular properties such as energy levels, photoconduction, and index change, followed by the holographic setups such as four-wave mixing and two-beam coupling, along with the theory to extract the important parameters out of the measured quantities.

Recent advances in photorefractive polymers

We measured the diffraction efficiency response of two photorefractive polymer devices according to the duration of the single laser pulse used to record the hologram. The pulse duration was varied from 6 nanoseconds to 1 second, while the pulse energy density was maintained constant at 30 mJ/cm2. This changed the peak power from 5 ×109 mW to 30 mW. We observed a strong reciprocity failure of the efficiency according to the pulse duration, with a reduction as large as a factor 35 between 1 second and 30 μs pulse duration. At even lower pulse duration (< 30 μs), the efficiency leveled out and remained constant down to the nanosecond exposure time. The same behavior was observed for samples composed of the same material but with and without buffer layers deposited on the electrodes, and different voltages applied during the holographic recording. We explained these experimental results based on the charge transport mechanism involved in the photorefractive process. The plateau is attributed to the single excitation of the charge carriers by short pulses (τp < 30 μs). The increase of efficiency for longer pulse duration (τp > 30 μs) is explained by multiple excitations of the charge carriers that allows longer distance to be traveled from the excitation sites. This longer separation distance between the carriers increases the amplitude of the space-charge field, and improves the index modulation. The understanding of the response of the diffraction efficiency according to the pulse duration is particularly important for the optimization of photorefractive materials to be used at high refresh rate such as in videorate 3D display.

Introduction to the Photorefractive Effect in Polymers

Photorefractive (PR) polymers change their index of refraction upon illumination through a series of electronic phenomena that makes these materials one of the most complex organic systems known. The refractive index change is dynamic and fully reversible, making PR materials very interesting for a large variety of applications such as holography and 3D display. In order to improve the recording speed and achieve videorate for our stereographic display application, we have introduced a new type of electrode geometry where the anode and cathode are in the same plane and are shaped as interpenetrating combs. This type of electrode geometry does not require the sample to be tilted with respect to the writing beams to record the hologram, which is a significant advantage. To monitor the highly non-homogeneous field resulting from this configuration, we used a multiphoton microscope to directly observe the chromophore orientation in situ upon the application of an electric field. Most recently, we developed a fast repetition rate laser (10kHz) where the pulse width can be adjusted from microseconds to milliseconds so that, in conjunction with a ns Q-switched Nd:YAG laser and an externally chopped CW laser, the diffraction efficiency of the material could be measured over 9 orders of magnitude. This measurement helps us better understand the mechanism of grating buildup inside photorefractive polymers.