GB2248990A - Testing optical fiber using optical heterodyne technique - Google Patents

Abstract

In apparatus for testing an optical fiber by using a pulsed light beam, a coherent light beam emitted from a light source (1A) is branched into a signal light beam and a reference light beam. The signal light beam is shifted in frequency by a predetermined amount and pulsed (at 1C), to be subsequently input to an optical fiber (10) under test through a directional optical coupler (1E), which then receives return light beam from the fiber which is mixed with the reference light beam. A polarized light beam splitter (2) separates the light beams from the directional optical coupler into first and second orthogonally polarized light components, which are then converted into first and second electric signals, hetorodyning occurring at the detectors (4A, 4B). A phase corrector (5) matches the first electric signal in phase with the second signal. An adder (6) adds together the output of the phase corrector with the second electric signal. Effects of polarisation are reduced thereby. <IMAGE>

Description

APPARATUS FOR TESTING OPTICAL FIBER BY USING OPTICAL HETERODYNE TECHNIQUE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to an optical heterodyne type optical fiber testing apparatus for locating a possible fault such as breakage in an optical fiber and/or measuring a loss involved in transmission of a light beam by inputting a coherent light beam to the optical fiber at one end thereof and detecting a return light beam b using an optical heterodyne detection. More particularly, the present invention is concerned with an improved optical fiber testing apparatus which is capable of correcting or compensating for drifts attributable to variance in the polarized state of the return light beam.

Description of Related Art For a better understanding of the background of the invention, description will be made of an optical fiber testing apparatus knon heretofore by reference to Fig. 3 of the accompanying drawings.

In Fig. 3, a reference symbol 1A denotes a coherent light source, 1B denotes an optical distributor (beam splitter) for branching a coherent light beam emitted by the light source 1A into a signal light beam 11 and a reference light beam 12, a symbol 1C denotes an optical frequency shifter, 1D denotes a timing pulse generator, 1E denotes a directional optical coupler, 3A denotes an optical heterodyne type photodetection circuit, 4A denotes an electric signal detection circuit, 7 denotes a signal processing circuit, 8 denotes a display unit, and 10 denotes an optical fiber under test.

In operation, a coherent light beam having a frequency f and emanated from the light source 1A is inputted to the optical distributor 2 to be separated into the signal light beam 11 and the reference light beam 12.

The frequency f of the coherent light beam may be 200 THz, b way of example.

The signal light beam 11 is introduced into the optical frequency shifter 1C which is so designed as to shift the frequency of the input light beam 11 by a predetermined amount. Further, the optical frequency shifter 1C is repetitively turned on and off under the timing of the timing pulse generator lD, whereby a pulsed light beam which has a frequency shifted by Af from that of the light source 1A is produced from the output of the optical frequency shifter 1C. The pulsed light beam thus derived is inputted into the optical fiber 10 under test through the medium of the directional optical coupler 1E.

The amount of the shift in the frequency (i.e. Af) brought about by the optical frequency shifter 1C is assumed to be 100 MHz, by way of example.

Return light 13 from the optical fiber 10 which results from the Fresnel reflection, backward scattering and other is introduced into the directional optical coupler 1E to be thereby coupled with the reference light beam 12. The output of the directional optical coupler 1E is then inputted to the optical heterodyne type photodetection circuit 3A. As the result of coupling or synthcsization of the return light beam 13 and the reference light beam 12 in the directional optical coupler 1E, the output light beam thereof contains frequency components representing a sum and a difference in frequency between the return light beam 13 and the reference light beam 12, respectively.The optical heterodyne photodetection circuit 3A is designed to detect the difference frequency component through optical heterodyning and output a corresponding electric signal. In other words, the photodetection circuit 3A outputs the useful electric signal having a frequency of df, which signal is then supplied to the signal processing circuit 7 by way of the detection circuit 4A which may be composed of a diode, filter, amplifier and others. In the processing circuit 7, the electric signal of the frequency df undergoes signal processings such as analogue-to-digital (AID) conversion, arithmetic processing for detecting or locating faults in the optical fiber, determination of transmission loss and others. The result of such processing is displayed on the display unit 8.

The prior art optical fiber testing apparatus suffers from a problem that the level of the electrical signal detected by the optical heterodyne type photodetection circuit fluctuates under the influence of variance in the polarized state of the return light beam, as a result of which it becomes difficult or even impossible to perform the test or measurement of the optical fiber with satisfactory accuracy.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical heterodyne type optical fiber testing apparatus which can overcome the problem of the prior art testing apparatus.

In view of the above and other objects which will become apparent as description proceeds, it is taught by the present invention in its broadest sense that the return light beam and the reference light beam are separated into orthogonal (X- and Y-) polarized light components which are then detected through optical heterodying, whereon the electrical signals obtained thereby are matched in phase by a phase corrector so that the phase difference between the orthogonal components of the polarized beams are canceled out. Subsequently, the phase corrected electric signals are added together to provide a utility electric signal for further processing to evaluate the optical fiber.Thus, variation in the phase difference between the orthogonal polarized light components can satisfactorily be suppressed, whereby the test and measurement of the optical fiber can be carried out with high accuracy and reliability.

Thus, according to an aspect of the invention, there is provided an apparatus for testing an optical fiber by using a pulsed light beam which comprises a light source for emanating a coherent light beam, optical branching mean for branching the coherent light beam into a signal light beam and a reference light beam, an optical frequency shifter for shifting a frequency of the signal light beam by a predetermined amount, pulse generating means for generating pulses for driving the optical frequency shifter to thereby generate a frequency-shifted and pulsed signal light beam, a directional optical coupler for inputting the pulsed signal light beam to an optical fiber under test and receiving a return light beam from the optical fiber to thereby synthesize the return light beam with the reference light beam, a polarized light beam splitter for separating a light beam outputted from the directional optical coupler into first and second orthogonal polarized light components, first photodetector means for converting the first polarized light component into a first electric signal, second photodetector means for converting the second polarized light component into a second electric signal, phase correcting means for making the phase of the first electric signal coincide with that of the second electric signal, adder means for adding the output signal of the phase correcting means with either one of the first or second electric signal, and signal processing means connected to the output of adder means for processing the output signal of the adder means to thereby evaluate the optical fiber under test in respect to performance thereof.

The above and other objects, features and advantages of the present invention will be better understood from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a general arrangement of the optical fiber testing apparatus according to an embodiment of the present invention; Fig. 2 is a view for graphically illustrating polarized states, respectively, of a reference light beam and a return light beam sent back from an optical fiber under test; and Fig. 3 is a block diagram showing a prior art optIcal fiber testing apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS o, the present invention will be described in detail in conjunction with preferred or exemplary embodiments thereof by reference to the drawings.

Fig. 1 is a block diagram showing generally an arrangement of an optical fiber testing apparatus according to an embodiment of the present invention.

As will be seen in Fig. 1, the optical fiber testing apparatus shown therein differs from the one described hereinbefore in conjunction with Fig. 3 in that there are additionally provided a polarized light beam splitter 2, an optical heterodyning photoelectric detection circuit 3B, an electric signal detection circuit 4B, a phase correcting circuit 5 and an adder circuit 6. Accordingly, in Fig. 1, equivalent or same parts as those shown in Fig. 3 are denoted by like reference symbols and repeated description thereof will be omitted in the following description.

The synthesized light beam outputted from the directional optical coupler 1E is separated into orthogonal polarized light components by means of the polarized light beam splitter 2.

At this juncture, reference is made to Fig. 2, hich illustrates typical polarized states of the return light beam 13 and the reference light beam 12 on a plane perpendicular to the direction in hich the light beam outputted from the directional optical coupler 1E travels.

In Fig. 2, X- and Y-axes are determined rather arbitrarily.

As can be seen in this figure, the return light beam 13 is linearly polarized at an angle 9 relative to the X-axis, while the reference light beam 12 is polarized at an angle of 45 relative to the X-axis.

The return light beam 13 and the reference light beam 12 having the polarized states illustrated in Fig. 2, respectively, are separated into polarized light components of X- and Y-directions, respectively, by means of the polarized light beam splitter 2.

The X- and Y-polarized light components of the return light beam 13 synthesized with the X- and Y-polarized light components of the reference light beam 12, respectively, are detected through optical heterodying by the photodetection circuits 3A and 3B, as a result of which each of the signals outputted from the photodetection circuits 3A and 3B has a frequency which is equal to a difference Af in frequency between the return light beam 13 and the reference light beam 12.

The output signal from the photodetection circuit 3A is applied to one input 5A of the phase correcting circuit 5 via the electric signal detection circuit 4A, while that of the photodetection circuit 3B is applied to the other input 5B of the phase correcting circuit 5 via the electric signal detection circuit 4B, whereby the phases of the input signal 5B is corrected so as to coincide with that of the other input signal 5A. As a result of this, there is outputted from the phase correcting circuit 5 a phase-matched signal, which is then applied to one input of the adder circuit 6 having the other input to which the electric signal 5A outputted from the detection circuit 4A is applied.In this manner, there is obtained from the output of the adder an electric signal which is substantially immune to difference in the polarized state between the return light beam 13.

In general, there exists a difference in phase between the X- and Y-components of a polarized light beam.

This phase difference will undergo variation on the way of traveling through the optical fiber. Such variations in the phase difference will make appearance straightforwardly as corresponding fluctuation in the level of the electric signal as detected.

With the arrangement of the apparatus shown in Fig. 1, however, the influence of variations in the phase difference between the X- and Y-components of the polarized return light beam can be suppressed because the outputs of the photodetection circuits 3A and 3B as detected by the detectors 4A and 4B are matched in phase by the phase correcting circuit 5 and then added together by the adder 6.

As will now be appreciated from the foregoing description, the return light beam and the reference light beam are separated into orthogonal (X- and Y-) polarized light components which are then detected through optical heterodying, whereon the electrical signals obtained are matched in phase by the phase corrector so that the phase difference between the orthogonal components of the polarized beams are canceled out. Subsequently, the phase corrected electric signals are added together to provide a utility electric signal for further processing to evaluate the optical fiber. Thus, it is apparent that variation in the phase difference between the orthogonal polaried light components can satisfactorily be cancelled out, whereby the test and measurement of the optical fiber can be carried out with high accuracy and reliability.

Many features and advantages of the present invention are apparent from the detailed description and thus it is intended by the appended claims to cover all such features and advantages of the system which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims (1)

1. An apparatus for testing an optical fiber by using a pulsed light beam, comprising: a light source for emanating a coherent light beam; optical branching mean for branching said coherent light beam into a signal light beam and a reference light beam; an optical frequency shifter for shifting a frequency of said signal light beam by a predetermined amount; pulse generating means for generating pulses for driving said optical frequency shifter to thereby generate a frequency-shifted and pulsed signal light beam; a directional optical coupler for inputting said pulsed signal light beam to an optical fiber under test and receiving a return light beam from said fiber to thereby synthesize said return light beam with said reference light beam;; a polarized light beam splitter for separating a light beam outputted from said directional optical coupler into first and second orthogonal polarized light components; first photodetector means for converting said first polarized light component into a first electric signal; second photodetector means for converting said second photodetector means for converting said second polarized light component into a second electric signal; phase correcting means for making the phase of said first electric signal coincide with that of the second electric signal; adding means for adding the output signal of said phase correcting means with either one of said first or second electric signal; and signal processing means connected to the output of said adding means for processing the output signal of said adding means to thereby evaluate said optical fiber under test in respect to performance thereof.