Path Imbalance Research Articles

Reduction of semiconductor laser diode phase and amplitude noise in interferometric fiber optic sensors

An optical configuration employing two conventional Michelson interferometers and a fiber Fabry-Perot interferometer connected in parallel is used to demonstrate the principle of common mode rejection of both the amplitude and frequency noise of a semiconductor laser. Common mode noise rejection is maximized when the outputs of the two interferometers with matched path imbalance, fringe visibility and amplitude are differentially combined. One interferometer is used as a reference, and the other as a sensing interferometer. The fiber Fabry-Perot interferometer is used as the sensing interferometer and is demonstrated as a miniature acoustic sensing element.

Generation of microwave signals from a laser diode coupled to an external cavity

AbstractThis article describes a technique by which a laser diode coupled with an external cavity is used in an interferometric system resulting in a direct envelope modulation of the optical carrier. The laser output is fed to a Mach‐Zehnder interferometer in which the time delay associated with the path imbalance is adjusted to half the time period Tm of the modulating sawtooth current. The two signals are heterodyned to simulate an envelope modulated optical signal.

Multiplexing systems for interferometric sensors with short imbalances

A novel method is proposed for multiplexing interferometric sensors with short path imbalances, i.e., integrated-optic, two-mode, or polarimetric fiber sensors. Here sensors are connected in parallel with time delays different from an additional common reference arm. The use of sawtooth frequency modulated source results in different beat frequency carriers for different sensors. An allocation scheme for frequency division multiplexing is presented. The optical signal power and the maximum number of sensors are estimated.

Frequency-division multiplexing of optical-fibre sensors using a frequency-modulated source

Three basic configurations, namely, (i) parallel with a common reference, (ii) parallel with separate references and (iii) series, are studied for a multiplexed system of optical-fibre sensors. With a frequency-modulated laser source, each sensor of a different predetermined path imbalance results in a different beat frequency at the detector and signals are multiplexed in the frequency domain. The cross-term problem arising from undesired interference associated with more than one sensor is described. A series configuration has more cross terms than a parallel configuration if there are four sensors or more and, consequently, suffers more signal power reduction if there are seven sensors or more. Several schemes are presented, which ensures the separation of cross terms from desired signals. In a parallel system with a common reference, signals and cross terms are allocated in the lower and in the higher region, respectively, or to every other line in the frequency domain. In a parallel system with separate references, sensors are made incoherent to each other to produce no cross terms. In a series system, more elaborate schemes are necessary. The estimated maximum number of sensors indicates that a parallel configuration should be used if there are 10 sensors or more to be multiplexed.

Resonant cavity sensor for integrated optics

The design of interferometric sensors, fabricated on single integrated optic chips for use in a multiplexed system, is considered. A multiple-pass interferometer using a resonant cavity appears extremely attractive, since it has a large path imbalance which may easily be varied. Sensors may therefore be fabricated that are highly sensitive and easily individualized. The resonant cavity sensor is analyzed, and response curves are presented under a variety of conditions. Experimental results are presented for temperature sensors fabricated in LiNbO 3 .