Tasshi Dennis

Optical spectral amplitude CDMA communication

We report the first demonstration of bipolar coding techniques in the optical spectral domain for incoherent optical code division multiple access (CDMA) communication. Based on the modulation and detection principles that we have developed, the power spectrum of an erbium-doped superfluorescent fiber source was encoded using bipolar codes and decoded using an optical bipolar correlator. A CDMA testbed consisting of two encoders and one decoder was implemented with bulk optics and free-space transmission. Our measurements verify the correlations between the bipolar codewords and demonstrate the rejection of multiple access interference.

Absolute frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser

A Cr:forsterite laser-based frequency comb is stabilized simultaneously to two NIST frequency references. Several optical frequency reference frequencies are then measured from 1315 nm - 1620 nm, including methane lines near 1330 nm.

Vector Signal Characterization of High-Speed Optical Components by Use of Linear Optical Sampling With Milliradian Resolution

We demonstrate linear optical sampling measurements optimized for characterization of the signals produced by optical components. By sampling the optical electric field before and after the component, we isolate the full vector field (phase and amplitude) of the signal separate from the input laser drift. Synchronization of the low-jitter mode-locked sampling laser (e.g., frequency comb) with the modulation rate allows measurement of the phase with milliradian noise. As a demonstration, we measure 10-Gb/s differential phase-shift keying modulated data with several different lasers. The technique is readily scalable to systems of much higher bandwidth.

Optical implementation of bipolar codes

The physical implementation of a bipolar encoding scheme suitable for WDM fiber-optic networks is reported with both experimental and theoretical results. The power spectrum of an erbium-doped superfluorescent fiber source is encoded, the bipolar correlations of the codes are verified, and rejection of multiple-access interference is demonstrated in a fiber-based testbed. Simulations of the correlation process identify key optical parameters and physical characteristics important to the design of future systems. A modification to the experimental testbed, made according to the theoretical analysis, results in improved correlation performance.

High-accuracy photoreceiver frequency response measurements at 1.55 µm by use of a heterodyne phase-locked loop

We demonstrate a high-accuracy heterodyne measurement system for characterizing the magnitude of the frequency response of high-speed 1.55 µm photoreceivers from 2 MHz to greater than 50 GHz. At measurement frequencies below 2 GHz, we employ a phase-locked loop with a double-heterodyne detection scheme, which enables precise tuning of the heterodyne beat frequency with an RF synthesizer. At frequencies above 2 GHz the system is operated in free-run mode with thermal tuning of the laser beat frequency. We estimate the measurement uncertainties for the low frequency range and compare the measured high-frequency response of a photoreceiver to a measurement using electro-optic sampling.

Wavelength references for 1300-nm wavelength-division multiplexing

We have conducted a study of potential wavelength calibration references for use as both moderate-accuracy transfer standards and high-accuracy National Institute of Standards and Technology (NIST) internal references in the 1280-1320-nm wavelength-division-multiplexing region. We found that most atomic and molecular absorption lines in this region are not ideal for use as wavelength references owing to factors such as weak absorption, complex spectra, or special requirements (for example, frequency-doubling or excitation with an additional light or discharge source). We have demonstrated one of the simpler schemes consisting of a tunable diode laser stabilized to a Doppler-broadened methane absorption line. By conducting a beat-note comparison of this reference to a calcium-based optical frequency standard, we measured the methane line center with an expanded uncertainty (2/spl sigma/) of /spl plusmn/2.3 MHz. This methane-stabilized laser now serves as a NIST internal reference.

Measurements of BER performance for bipolar encoding of an SFS

This paper reports on the measurements of bit error rate (BER) performance for a broad-band optical communications scheme that encodes the power spectrum of an erbium-doped superfluorescent fiber source (SFS) with bipolar equivalent codes. The proposed scheme, like spectrum-sliced wavelength division multiplexing (WDM), suffers from the excess noise associated with the detection of a thermal-like source. BER calculations based on measured testbed characteristics and a simple model are compared with BER measurements for one and two active users. In addition, the performance of a single user in the presence of broad-band spectral interference is investigated, giving a preliminary estimate of multiuser capacity. This paper discusses various solutions for increasing network capacity and performance, relevant to the proposed scheme as well as to wavelength division multiplexing (WDM).

Achieving high absolute accuracy for Group-delay measurements using the Modulation phase-shift technique

We have developed a modulation phase-shift (MPS) system for measuring relative group delay (RGD) in optical components with high absolute accuracy and simultaneously high temporal and wavelength resolution. Our 200-MHz system has a 3.2-pm wavelength resolution and has demonstrated a group-delay resolution of 0.072 ps for repeated measurements of an artifact based on a hydrogen-cyanide gas cell. The expanded uncertainty (2/spl sigma/) is /spl plusmn/0.46 ps for a single spectral measurement (/spl sim/ 3.4-pm steps) of a narrow 20-ps group-delay feature of the artifact. To our knowledge, this is the first time that the sources of measurement uncertainty for this technique have been described and quantified. A method for predicting the group delay of the gas-cell artifact from measured absorption spectra is described, and an uncertainty analysis for the prediction method is also presented. The implementation required to achieve results of the highest accuracy for both measurements and predictions is discussed.

Spectroscopic phase-dispersion optical coherence tomography measurements of scattering phantoms

We demonstrate a novel technique to determine the size of Mie scatterers with high sensitivity. Our technique is based on spectral domain optical coherence tomography measurements of the dispersion that is induced by the scattering process. We use both Mie scattering predictions and dispersion measurements of phantoms to show that the scattering dispersion is very sensitive to small changes in the size and/or refractive index of the scatterer. We also show the light scattered from a single sphere is, in some cases, non-minimum phase, and therefore the phase of the scattered light is independent of the intensity. Phase dispersion measurements may have application to distinguishing the size and refractive index of scattering particles in biological tissue samples.

A Novel Solar Simulator Based on a Supercontinuum Laser for Solar Cell Device and Materials Characterization

The design, operation, and application of a novel solar simulator based on a high-power supercontinuum fiber laser are described. The simulator features a multisun irradiance with continuous spectral coverage from the visible to the infrared. By use of a prism-based spectral shaper, the simulator can be matched to any desired spectral profile, including the ASTM G-173-03 air-mass 1.5 reference spectrum. The simulator was used to measure the efficiency of gallium arsenide (GaAs), crystalline silicon (Si), amorphous Si, and copper-indium-gallium-selenide (CIGS) thin-film solar cells, showing agreement with independent measurements. The pulsed temporal characteristic of the simulator was studied and would appear to have a negligible influence on measured cell efficiency. The simulator light was focused to a spot of approximately 8 μm in diameter and used to create micrometer-scale spatial maps of full spectrum optical-beam-induced current. Microscopic details such as grid lines, damage spots, and material variations were selectively excited and resolved on GaAs and CIGS cells. The spectral shaping capabilities were used to create output spectra appropriate for selectively light-biasing multijunction cell layers. The simulator was used to create variable blue-rich and red-rich spectra that were applied to a GaInP/GaAs tandem solar cell to illustrate the current-limiting behavior.