Frank Bucholtz

Analysis of the response of long period fiber gratings to external index of refraction

This paper demonstrates that the change in wavelength of a long period fiber grating attenuation band with changes in external index of refraction can be enhanced by proper selection of the grating period. We calculate and experimentally verify that the wavelength shift caused by changing the external index from n=1 to n=1.44 of the attenuation band which appears in the 1400-1600 nm region in a 200-/spl mu/m period grating is four times that in a 350-/spl mu/m period grating. Changes in the spectrum over a wavelength range from 1100 to 1600 nm and 1<n<1.72 index range are also presented, and implications for grating design when enhanced or reduced index sensitivity is desired are discussed. Finally, we demonstrate the use of a highly index-sensitive long period fiber grating as a chemical concentration sensor.

Long-Haul Analog Photonics

A paper on long-haul analog photonics is presented using theory and experimental results. Various analog fiber-optic modulation formats are reviewed, including intensity modulation with direct detection, phase modulation with interferometric demodulation, and suppressed-carried techniques. Modulation linearization methods are reviewed. The limitations of and requirements for photodiode detectors are described.

Phase Modulation With Interferometric Detection as an Alternative to Intensity Modulation With Direct Detection for Analog-Photonic Links

We analyze the performance of analog-photonic links employing phase modulation with interferometric demodulation and compare it to intensity-modulation direct-detection links. We derive expressions for RF gain, RF noise figure, compression dynamic range, and spurious-free dynamic range for both architectures. We demonstrate theoretically and experimentally that phase-modulated links can outperform intensity-modulated links over substantial frequency ranges.

Wide-band QAM-over-fiber using phase modulation and interferometric demodulation

Transmission of microwave quadrature amplitude modulation over an all-Raman 105-km fiber-optic link is demonstrated. The link employs optical phase modulation and single-port interferometric demodulation to deliver up to 6 Gb/s in a single channel.

Beating Nyquist with light: a compressively sampled photonic link

We report the successful demonstration of a compressively sampled photonic link. The system takes advantage of recent theoretical developments in compressive sampling to enable signal recovery beyond the Nyquist limit of the digitizer. This rather remarkable result requires that (1) the signal being recovered has a sparse (low-dimensional) representation and (2) the digitized samples be incoherent with this representation. We describe an all-photonic system architecture that meets these requirements and then show that 1 GHz harmonic signals can be faithfully reconstructed even when digitizing at 500 MS/s, well below the Nyquist rate.

Real-time hyperspectral detection and cuing

The Dark HORSE 1 (Hyperspectral Overhead Reconnais- sance and Surveillance Experiment 1) flight test has demonstrated au- tonomous, real-time visible hyperspectral detection of military ground tar- gets with real-time cuing of a high-resolution framing camera. An overview of the Dark HORSE 1 hyperspectral sensor system is pre- sented. The system hardware components are described in detail, with an emphasis on the visible hyperspectral sensor and the real-time pro- cessor. Descriptions of system software and processing methods are also provided. The recent field experiment in which the Dark HORSE 1 system was employed is described in detail along with an analysis of the collected data. The results evince per-pixel false-alarm rates on the or- der of 10 25 /km 2 , and demonstrate the improved performance obtained

The performance of analog photonic links employing highly compressed erbium-doped fiber amplifiers

We present the tradeoffs of employing highly compressed erbium-doped fiber amplifiers (EDFAs) in analog microwave photonic links. We employ the newly developed concept of noise penalty and introduce the concept of generalized relative intensity noise to facilitate system design. Theoretical and experimental results demonstrate that a highly compressed EDFA: 1) can achieve near shot-noise-limited performance, even with large optical noise figure and 2) can simultaneously increase the spurious-free dynamic range, increase the compression dynamic range, increase the analog gain, and decrease the analog noise figure, as compared to an unamplified link with identical components.

Object detection by using "whitening/dewhitening" to transform target signatures in multitemporal hyperspectral and multispectral imagery

Changes in atmosphere, ground conditions, scene temperature, solar illumination, and sensor response can significantly affect the detected multispectral and hyperspectral data. Using uncorrected spectral target signatures in spectral matched filter searches therefore results in target detection with concomitant high false-alarm rates due to changes in multispectral and hyperspectral images. This letter introduces the use of the whitening/dewhitening (WD) transform to help correct target spectral signatures under varying conditions. An important feature of this transform is that it does not require subpixel registration between images collected at two distinct times. The transform was tested on images taken from two very different data collects using different sensors, targets, and backgrounds. In one dataset, the transform was applied to hyperspectral images taken from airborne longwave infrared sensor binned to 30 bands and the other data collect used images of a variety of tanks, trucks, calibration panels that were collected using bore-sighted broadband visible, shortwave infrared, midwave infrared, and longwave infrared staring array sensors. Target spectral signatures were transformed using imagery of spatially overlapping regions from datasets collected at different times and processed using the whitening and then dewhitening transform (inverse of a whitening transform). Use of the WD transform yielded a large target-to-clutter ratio (TCR) and was compared to the TCR derived from other transforms that approximated the cross-covariance matrix. In addition, the WD-transformed signatures applied in a matched filter search found targets (some concealed behind vegetative foliage or underneath camouflage) with low false-alarm rates as shown in a receiver operator characteristic curve. This letter demonstrates that the WD transform enhances searches for concealed targets in multisensor and hyperspectral data.

Wide-band predistortion linearization for externally modulated long-haul analog fiber-optic links

We analyze the performance of a custom wide-band predistortion linearizer designed to operate in conjunction with an externally modulated analog photonic link. In addition, we propose an extended spurious-free dynamic-range (SFDR) specification for analog photonic links employing linearization techniques. The benefits of predistortion linearization for analog fiber links limited by optical amplifier noise are demonstrated and a 6-12-GHz 20-km analog link is shown to achieve a SFDR of 108.4 dB/spl middot/Hz/sup 2/3/, a 6-dB increase relative to the uncompensated link.

Analysis of an Analog Fiber-Optic Link Employing a Low-Biased Mach–Zehnder Modulator Followed by an Erbium-Doped Fiber Amplifier

In this paper, a complete analysis of an analog fiber-optic link employing a low-biased Mach-Zehnder modulator followed by optical amplification with an erbium-doped fiber amplifier is presented. The expressions for RF gain, RF noise figure, second- and third-order intercept points, and spurious-free dynamic range are derived. Experimental data are employed to support the analytical results, and the optimal bias points for the metrics are demonstrated.