Fibre-optic interferometer

• US 4,759,627 A
• Filed: 03/05/1986
• Issued: 07/26/1988
• Est. Priority Date: 04/04/1985
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Andersson, Per-Ola, Thylen, Lars H., Persson, Sven A. R.
• Primary Examiner(s): Willis, Davis L.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US06/836,395
• Time in Patent Office: 874 Days
• Field of Search: 356/345, 250/227
• US Class Current: 356/477
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices

Abstract

The invention relates to a fibre-optic Mach-Zehnder Interferometer for measuring such as change in length. Two light conducting fibres (1,2) are connected to a coherent light source (5). A measurement converter (6), actuated by a measurement quantity (P) changes the length of the optical path and thereby the phase angle for the light wave in one fibre (2). The phase angles for the light waves in the fibres (1, 2) are compared and a corresponding output signal (I) is fed back to compensate for the phase angle change in the measurement converter (6). To make the comparison result (I) independent of the light intensities in the light conducting fibres (1,2) these are connected to an opto-electronic directional coupler (7). This is selected such that the output signal (I) varies as the phase displacement φ, according to the relationship: I=2×A×B×sin φ, where A and B are the amplitudes of the light waves in the fibres (1,2). By using the directional coupler (7), the interferometer can be made simple and accurate. The opto-electronic coupler ( 7) allows the use of polarized light, whereby accuracy may be further improved.

Claims

1. A fibre-optic interferometer for measuring a change in quantity and comprising a common coherent light source, two-light-conducting fibres, a first directional coupler connecting said light source to said fibres, a measurement converter connected to one of said fibres for changing the length of the optical path, and thus the phase angle, for a light wave passing through said one fibre in response to said change in the quantity, comparing means for comparing the phase angles of the light waves in the fibres to yield a comparison value, compensating means for compensating said change in phase angle by a compensation value, the compensation value constituting a measurement value, and opto-electronic coupling means for making the comparison value substantially independent of light intensity in the light-conducting fibres, said opto-electronic coupling means including a second directional coupler which has two inputs and two outputs and light conductors connecting the inputs and outputs via an interaction area which is disposed such that the light power is distributed equally between the outputs and, when a light wave from the light source is applied to either or both inputs has a phase different (φ

) deviating from a given phase difference (φ

.sub. o), the outgoing intensities are affected by a factor depending on the phase difference (φ

) essentially according to relationships;

space="preserve" listing-type="equation">U.sub.1 =(A.sup.2 +B.sup.2)/2+A B sin (φ

-φ

.sub.o)

space="preserve" listing-type="equation">U.sub.2 =(A.sup.2 +B.sup.2)/2-A B sin (φ

-φ

.sub.o)

where A and B are the amplitudes of the light waves at the inputs, φ

is their mutual phase difference, φ

is said given phase difference and U₁ and U₂ are the intensities of the light waves at the outputs so that the difference value between these intensities will be essentially independent of variation in light intensities in the fibres on compensation of the phase difference (φ

) to the given value (φ

_o), said first directional coupler having inputs and outputs for said two light conducting fibres and light conductors connecting said inputs and outputs via an interaction area to adjust the light power at said outputs, said one light conducting fibre connected to one of said outputs of said first directional coupler being connected, to said measurement converter which in turn is connected to one of said inputs of said second directional coupler, said interferometer further comprising a wafer of optoelectronic material, said first and second directional couplers being on said wafer, a control circuit connected to said comparing means for producing said measurement value, and an electrode on said wafer connected to said control circuit for adjusting said first and second directional couplers.

2. A fibre-optic interferometer according to claim 1 wherein said compensating means further comprises detectors respectively connected to the outputs of said second directional coupler, said comparing means comprising a difference circuit connected to said detectors and to said control circuit.