David J. F. Cooper

Time-division multiplexing of large serial fiber-optic Bragg grating sensor arrays

Time-division multiplexing is a promising method for the interrogation of fiber-optic Bragg grating sensors arrays for measurement of strain and temperature. We examine the performance of these systems to determine the parameters for high-sensitivity, low-cross-talk operation. It is shown that the performance can be greatly improved by use of a short time resolution in the demultiplexing process. We propose a new method of demultiplexing with an electro-optic modulator to read out the sensor pulses by gating the signal with 400-ps resolution. The system is demonstrated experimentally to provide 0.15-microepsilon/square root(Hz) strain resolution in a 50-Hz bandwidth within a full-scale range of 8000 microepsilon. The system parameters are capable of handling at least 50 time-addressed sensors on a single fiber.

Transmission filters with multiple flattened passbands based on chirped moire gratings

In-fiber chirped moire gratings (CMGs) are attractive as wavelength selective elements, and in particular as multiple passband transmission filters, for optical communications. However, as with single phase-shifted gratings, the passbands exhibit nonideal filter characteristics such as a Lorentzian shape with narrow transmission peaks, and rounded, possibly broad bottoms, and are not suitable for system design. We show that we can flatten the passbands by introducing regions of constant refractive index within the CMG. By choosing the number, lengths, and location of the regions with constant refractive index, passbands with steep slopes and ripples less than 0.5 dB can be achieved.

Simple and highly sensitive method for wavelength measurement of low-power time-multiplexed signals using optical amplifiers

We describe a simple method for the wavelength measurement of optical signals that is easily capable of measuring a 1-nW average power optical signal with a wavelength resolution of 0.1 pm/Hz/sup 1/2/ while maintaining a large measurement range in excess of 12 nm. The system uses an erbium-doped fiber amplifier to increase the signal level before being measured with a wide-band edge filter. This technique is well suited to the measurement of low duty cycle time-multiplexed signals such as those in multiplexed fiber sensor systems. We show that the measurement of the amplified signal is improved despite the broadband nature of the amplified spontaneous emission noise. We show for the first time that the addition of an amplifier can increase the detection capabilities of the edge filter method beyond the shot noise limit of an unamplified measurement.

Simple high-performance method for large-scale time division multiplexing of fibre Bragg grating sensors

A new architecture for large-scale time multiplexing of fibre Bragg grating sensors (BGSs) is presented. Multiplexing of fibre BGSs often requires a trade-off between measurement performance and the number of sensors per fibre. Our technique utilizes optical amplifiers to increase the available system power for making high-sensitivity sensor measurements on a serial BGS array containing a large number of sensors. The optical amplifiers allow the system to be interrogated with a much lower-power source and simple measurement method. The design process for a variety of sensing conditions is illustrated and the performance is experimentally verified with a 100-sensor system.

Transmission edge filters for power equalization of EDFA's

We propose and demonstrate the use of transmission edge filters based on apodized linearly chirped fiber Bragg gratings for providing power equalization among different wavelength channels in an erbium-doped fiber amplifier module. The filters we fabricated provide a dynamic range of /spl ap/13 dB, an amplitude variation of /spl plusmn/0.5 dB from an ideal linear edge, and negligible group delay in transmission. Using the grating filters, we demonstrate power equalization, with no observed variation in signal levels after compensation, for three WDM signals having >6-dB difference in power levels before amplification. Our technique can also be extended to provide active power equalization by incorporating a feedback loop.