Robert C. Youngquist

Multiplexing fiber-optic sensors by frequency modulation: cross-term considerations.

The cross-term problem in a multiplexed fiber-optic sensor system using a frequency-modulated laser source is described, and two proposed solutions are presented. One is to eliminate cross terms by locating them at different frequencies from the signal frequencies. The other is to separate the sensors by several source coherence lengths so that their outputs do not interfere. The latter technique is demonstrated experimentally, resulting in only two peaks corresponding to the two fiber interferometers at frequencies of 200 Hz and 1 kHz.

Two-mode fiber interferometer/amplitude modulator

Single-mode fiber used at a wavelength below its cutoff wavelength guides second-order modes which travel at different phase velocities than the fundamental modes. Such two-transverse-mode fiber forms a single-strand interferometer, and studies have shown that the output may be set at quadrature by using all-fiber polarization controllers. Furthermore, modulation of the two-mode fiber with a piezoelectric stretcher produces amplitude modulation. Two-amplitude modulators have been constructed and studied operating at 633 and 830 nm, and a modulation depth of 97% has been observed at 23.4 kHz for an excess loss of ~6 dB.

Coherence length and output power of GaAlAs single-mode diode lasers: a rule-of-thumb relationship

An approximation is presented stating that the coherence length in meters of a GaAlAs laser is equal to the output power given in milliwatts. (AIP)

An Analysis Of The Photothermal Drive Of A Quartz Force Sensor

This paper investigates the feasibility of running a quartz force sensor with an optical drive. The general theory behind optical (strictly speaking, photothermal) driving of a resonant structure is presented and this is used in conjunction with empirical results obtained from an existing system to clarify problem areas and to suggest improvements. A straightforward method for optical reading is briefly described.

Polarimetric coherence multiplexing using high-birefringence optical-fiber sensors and short coherence sources

We describe a polarimetric coherence multiplexed system that uses a source with a short coherence length to reduce the effect of cross talk and of phase-induced intensity noise. The system is constructed with sections of highbirefringence optical fibers, which allow the differential path lengths to be matched within micrometer accuracy. A minimum detectable phase sensitivity of 3 microrad/ radicalHz has been demonstrated.

Selective interferometric sensing by the use of coherence synthesis

A technique is discussed and demonstrated whereby the sensitivity of an imbalanced fiber-optic interferometric sensor is changed by as much as 70 dB by modifying the power spectrum and thus the coherence function of the input optical source. This phenomenon is then extended and used to monitor selectively single sensors in a small fiber-optic ladder array by synthesizing coherence functions that result in a high sensitivity for the sensor to be observed but a low sensitivity for all the others. Limitations of the demonstrated implementation are discussed and possible future designs presented.

Optical-fiber systems with nonreciprocal couplers

The effectively nonreciprocal behavior of a single-mode to multimode optical-fiber coupler is discussed and results from polished couplers given. Two different applications in local-area networks and recirculating rings are investigated and the results presented.

Optical Fibre Resonator Rotation Sensor Using a Low Coherence Source

A novel optical fibre resonator rotation sensor using a low coherence source is described. The sensitivity of the system is compared to the conventional phase modulated Sagnac interferometer and a higher sensitivity appears feasible.

Polarisation-Insensitive Optical Fibre Phase Modulator

An acoustic fibre phase modulator, operating up to a few MHz, is described which employs a piezoelectric plate in a squeezing action. A novel construction using two machined brass v-grooves results in a much reduced sensitivity to optical polarisation and the optimum groove angle is independent of the level of friction on the walls of each groove. A normalised birefringence of 0.04 was achieved at 1.5MHz, with a modulation efficiency of approximately 0.23 rad/Vrms., for a 2cm long device.