Steven A. Newton

Real-time long range complementary correlation optical time domain reflectometer

The improved performance of an optical time-domain reflectometer (OTDR) through the application of a correlation technique using codes with complementary autocorrelation properties is discussed. The theoretical foundations of the method are presented along with experimental results exhibiting the best one-way range reported to date for a practical long-haul, long-wavelength OTDR system. >

Spectral analysis of optical mixing measurements

A general rigorous theory of optical heterodyne and homodyne measurements is presented. The power spectrum of the photocurrent resulting from two uncorrelated optical beams mixing on a photodetector is derived. In particular, a rigorous analysis is presented for the delayed self-homodyne method which is used to characterize laser source linewidth by a Mach-Zehnder interferometer with a delay exceeding the source coherence length. Existing treatments are generalized to address non-Lorentzian laser sources of arbitrary lineshape. The analysis is further generalized to cover the case of modulated nonstationary sources. An example of the application of this theory is given. It is shown how the theory may be used to interpret an experimental result obtained using the gated delayed self-homodyne technique for characterizing the frequency chirp of laser sources under modulation. >

Coherent FMCW reflectometry using a temperature tuned Nd:YAG ring laser

A long-coherence-length, diode-pumped, monolithic Nd:YAG laser operating at 1.32 mu m is used to perform coherent FMCW (frequency-modulated continuous wave) reflectometry measurements. Compared with semiconductor-based lasers, the low phase noise of the Nd:YAG source offers greatly increased distance range combined with increased sensitivity to optical reflections. Measurement of Rayleigh backscatter from 50 km of fiber is demonstrated with over 60 dB of two-way optical dynamic range. Two-point spatial resolution is demonstrated to be better than 10 cm. This source and coherent measurement technique may have potential for both short and long-haul reflectometry applications.<<ETX>>

Characterization of human scalp hairs by optical low-coherence reflectometry

Optical low-coherence reflectometry is used to investigate the internal structure and optical properties of human scalp hair. Regardless of hair color, the refractive index of the cortical region remains within the range of 1.56-1.59. The amplitude of the backscattered infrared light coupled into different-colored hair confirms the relative melanin content. Discontinuities in the refractive index permit identification of distinct structural layers within the hair shaft.

High-frequency photodiode characterization using a filtered intensity noise technique

Optical filtering of amplified spontaneous emission improves measurement dynamic range for frequency response measurements of optoelectronic receivers. For high bandwidth receivers, a novel periodically filtered intensity noise technique is proposed. Response measurements using these techniques on a 1 GHz and 30 GHz receiver are demonstrated.<<ETX>>

Low-power ultrashort optical pulse characterization using linear dispersion

We demonstrate a new technique to characterize short optical pulses, which is both simple and capable of yielding pulse characteristics in near-real time (greater than once a second). The optical pulse to be characterized is spectrally dispersed until it is long enough to be easily measured by a fast detector and electronic oscilloscope. The phase of this dispersed pulse is then determined, in our case by means of an optical delay-line discriminator. Once the phase and intensity of the dispersed pulse is known, and knowing the linear dispersion, one can use a simple linear transform to obtain the phase and intensity of the original input pulse. We have applied this technique to the characterization of subpicosecond and picosecond pulses from various fiber lasers. In one set of experiments a fiber ring laser provided roughly transform-limited pulses of 0.63 ps width at an 11-MHz repetition rate. The pulses were attenuated to 10 /spl mu/W, to eliminate any nonlinear effects, and coupled into 6 km of standard telecommunications fiber. The output of the 6 km fiber was coupled into the discriminator and the output detected using a fast detector and oscilloscope.

5-mm-resolution optical-frequency-domain reflectometry using a coded phase-reversal modulator

Network analysis optical-frequency-domain reflectometry is demonstrated with what is to our knowledge the highest resolution reported to date (5 mm in fiber) and a dynamic range of 73 dB (electrical). The system uses a novel 13-bit Barker-coded phase-reversal optical modulator, a high-speed photodiode, and a commercial microwave vector network analyzer.

Novel approaches to optical reflectometry

The author reviews the fundamentals of optical reflectometry, including the standard optical time-domain reflectometry (OTDR) measurement. Some novel reflectometry schemes are described, including a spread-spectrum approach to long-range OTDR, as well as new optical frequency-domain reflectometry (OFDR) techniques that have proven useful for high-resolution measurements. These novel reflectometry techniques are suitable for testing both fiber links and small components. The spread-spectrum technique allows improved dynamic range without sacrificing resolution. High-speed modulators and detectors now allow the extension of OFDR techniques to high frequencies. Resulting measurements show a large dynamic range and resolution better than 4 mm.<<ETX>>

Single-frequency output from a broadband-tunable external fiber-cavity laser

Single-frequency operation is demonstrated by using a semiconductor laser coupled to a tunable fiber-optic reflective filter. This external fiber-cavity laser is stepwise tunable over a 66-nm range centered around 1300 nm. Short-term laser linewidths are measured to be less than 50 kHz.

Photocurrent amplification in Schottky photodiodes

Up to 20 dB of photocurrent amplification with a frequency response of 2 GHz has been observed in GaAs Schottky photodiodes. The amplification may be caused by a reduction in Schottky built‐in potential under illumination. The device may find application in improving the sensitivity of optical receivers.