Geoffrey L. Burdge

Optical clock recovery from a data stream of an arbitrary bit rate by use of stimulated Brillouin scattering

Using a fiber-optic stimulated-Brillouin-scattering amplifier as an active filter, we have demonstrated optical clock recovery from 5-Gbit/s return-to-zero-format optical data. Definite patterns and pseudorandom bit sequences were tested. This scheme requires no prior knowledge of the clock frequency and is well suited for operation at higher data rates.

Optical bit-pattern recognition by use of dynamic gratings in erbium-doped fiber

We present a novel optical bit-pattern-recognition technique that uses erbium-doped fiber at room temperature. Counterpropagating beams write a patterned gain-depletion grating in pumped erbium-doped fiber. This grating, recorded in the erbium gain medium, can be used for correlation with other optical bit patterns. We have demonstrated correlation of arbitrary return-to-zero bit patterns of as many as 8 bits at 1 Gbit/s . Theory suggests that scaling to higher bit rates is feasible.

High-resolution reflection holography using DuPont photopolymer holographic recording film

We report an experimental study of the use of DuPont photopolymer Holographic Recording Film (HRF) to record high resolution reflection holograms. The goals of this work are to investigate techniques for recording images of the 3D surfaces of objects, such as integrated circuit chips, without the use of imaging optics and step and repeat techniques, and to investigate techniques for reconstructing these images through a conventional or slightly modified optical microscope. We use the Denisyuk reflection geometry to record single wavelength and multiple wavelength contact-copy and near-contact holograms of an integrated circuit chip. We present photographs of the holographic reconstructions as generated and viewed through a conventional microscope. The images demonstrate color contrast and clearly resolved features of less than 1 micron. In addition, we present an experimental investigation and theoretical modeling of the grating formation process in the DuPont photopolymer HRF. Experiments are described which measure the real-time growth and accompanying real-time shrinkage of reflection gratings. A theory is outlined that includes coupling of the writing beams with the real time growth and shrinkage of nonuniform gratings throughout the thickness of the recording emulsion. Results are presented for three types of DuPont photopolymer HRF.

Active compensation for horizontal-line-of-sight turbulence over near-ground paths

Atmospheric turbulence over long horizontal paths perturbs phase in the pupil of an optical communications receiver, and also can cause severe intensity scintillations. We describe a real time wavefront compensation system using PC technology to perform all wavefront control tasks. This system uses a modal correction scheme, and we report the first measurements of residual wavefront taken approximately 1 meter above ground level at 1 km range. The effects of turbulence, scintillations and control bandwidth on the correction are all examined.

A high-performance semiconductor optical amplifier/fiber Bragg grating wavelength converter

Summary form only given. We have demonstrated a novel (semiconductor optical amplifier) SOA/fiber-grating wavelength converter with relatively high extinction ratio (>30) using record low switching power (<-13 dBm) at 10 Gbit/s. This simple filtering technique provides a cost-effective alternative to existing wavelength conversion methods.

Characterization of an optical wireless communication channel

We describe four measurement techniques to characterize the performance of an optical wireless channel. The characteristics of each technique are illustrated through experimental measurements. These four techniques include: (1) bit-error rate measurements, (2) measurements of the mean and variance of the received optical power from which the bit- error rate can be estimated, (3) measurements of the fade rate and fade duration histograms of the optical carrier, (4) measurements of the intensity and phase distributions of atmospheric propagated laser beams using a Shack-Hartmann wavefront sensor.

Schack-Hartmann wavefront sensing measurements of near-ground propagated laser beams

Adaptive optics systems could be used to maintain the quality of a communication laser beam propagated near ground over a few kilometers of turbulent atmosphere. Such an adaptive optics system may incorporate a Shack- Hartmann sensor to measure the wavefront of an arriving beacon laser beam before precompensating that of the communication laser beam through a deformable mirror. We present experimental measurements of the wavefront of a laser beam propagated over distances of 0.94 and 2.4 km acquired using a 1200-subaperture Shack-Hartmann sensor. These data were acquired at 1 kllz frame rate during 2 second time intervals over the span of several days. We acquired and analyzed 41 sets of data. Our analysis shows that the atmosphere is not stationary over these 2second intervals, that the statistics of the wavefront may look very different for the same measured value of the atmosphere structure constant (C) and does not always follow the theoretical predictions based on Kolmogorov turbulence, and that the subtraction of the first 11 Zernike polynomials (first-order correction) improves the wavefront significantly unlike the subtraction of next 1 1 polynomials (third-order correction).

Performance update of a terrestrial laser-communication link with quadrature amplitude modulation

In this paper we describe recent tests of a unidirectional laser communication link operating in strong atmospheric turbulence conditions, near ground level, over horizontal path distances up to 2.4 km. The source is a 1.3 micrometers Nd:YAG, diode-pumped laser which is fiber-coupled to an electro-optic modulator. The transmitter includes an active tip-tilt system which maintains rough link alignment and compensates for slow pointing drift. The transmitted light is collected by a 7 cm diameter receiver aperture and focused onto a InGaAs p-i-n photodetector. The receiver includes a CCD camera for coarse alignment of the link. The optical carrier is modulated by a 160 Mbps digital signal using quadrature amplitude modulation (QAM) at a subcarrier frequency of 140 MHz or by a 678 Mbps digital on-off keyed (OOK) signal. The link receiver includes an electronic automatic gain control circuit to compensate for atmospheric induced signal fading. The electronic test equipment at the transmitter and the receiver is connected through an RF ethernet link to enable automated measurements of symbol error rate as a function of optical power, modulation format, and energy-per-bit to noise-density ratio. We report on the measurement of bit error rates and demonstrate error- free operation of the communication link using 160 Mbps QAM over 0.9 km and 678 Mbps OOK modulation over 2.4 km under certain atmospheric conditions.

High-resolution H1-H2 single-beam reflection holography using DuPont photopolymer holographic recording film

We report an experimental study of the use of DuPont photopolymer Holographic Recording Film to record high resolution reflection holograms of an integrated circuit chip with an initial film-to-object separation of 1.5 millimeters. A two-step H1-H2 recording sequence is used to transfer H1 images to the H2 film plane. Because of emulsion shrinkage, the optimum H2 recording wavelength is approximately 10 nm shorter than the H1 recording wavelength. The H2 reflection holograms, which are incoherently illuminated and viewed through a conventional microscope, reconstruct high resolution images with clearly resolved micron sized features.

Terrestrial laser communication link at 1.3 μm with quadrature amplitude modulation

In this paper, we describe the architecture and performance of a unidirectional, terrestrial laser communication link designed to operate over a range of 100 m to 1 km. The optical carrier is provided by a 1.3 micrometer laser which is modulated using an electro-optic modulator. Quadrature amplitude modulation (QAM) is used to modulate 150 Mbps input binary data at a subcarrier frequency of 140 MHz. At the receive side of the link, the optical signal is detected with a InGaAs p-i-n photodetector. Because the pointing of our low-cost link is passive, atmospheric fluctuations will induce amplitude fading of the optical carrier. The bit error rate performance of the QAM format is particularly sensitive to amplitude fading. Therefore, the performance of our link is sensitive to the effects of atmospheric turbulence. We discuss how these effects can be minimized by an appropriate choice of the beam diameter with respect to the receiver diameter and by incorporating an automatic gain control circuit. We present experimental results which demonstrate the operation of the link with 16-QAM and 256-QAM over a 100 m path with an atmospheric structure constant (Cn2) of 3 multiplied by 10-13 m-2/3.