Method in the signal processing of a received train of pulses, a

Details Method in the signal processing of a received train of pulses, a Method in the signal processing of a received train of pulses, a

1984-04-02

1986-07-15

Orsino, Jr., Joseph A.

Optics: measuring and testing

For optical fiber or waveguide inspection

307352, 328151, 455619, G01N 2100, H04B 900

Patent

active

046003037

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method in detuned sampling of a train of pulses with a stable, high repetition frequency, f.sub.r, received by a noise generating detector circuit, in particular for the measurement of pulse broadening through an optical fiber whose output end is coupled to the input of the detector.
The invention is particularly useful for the measurement of pulse broadening in a multimode optical fiber. Pulse broadening is also called dispersion and covers the phenomenon that the pulse width increases when a light pulse is propagated in the optical fiber, which is due to the fact that the various types of modes, which in combination form the signal, are propagated at slightly different velocities through the fiber because in practice the fiber cannot be made with ideal parameters. The pulse broadening substantially determines the transmission capacity of the fiber for tele-communication purposes in that increasing pulse broadening corresponds to decreasing bandwidth. It is therefore important that this measurement can be made in situ because it is not possible in practice to estimate the bandwidth of an optical fiber comprising several splices on the basis of knowing the bandwidth of each individual part of the fiber.
In principle, there are two different pulse broadening measuring methods. (See e.g. NBS special publication 597: Technical Digest Symposium on optical fiber measurements, 1980, p. 49-54). One method is called frequency sweep, where a signal of periodically varying frequency is applied to the input of the fiber. The frequency of the transmitted signal is analyzed on the output of the fiber, and information about the pulse broadening can be obtained by computation. Even though phase difference cannot be measured in practice by this process, it has been the predominant method of making measurements over great distances. This is so because the other method, called the pulse method, is vitiated by the drawback that it is not reliable when the attenuation in the optical fiber stretch is large, and this process is therefore mostly used for laboratory measurements where the fiber length and thus the attenuation is relatively small. In this method, attenuation is a problem because the noise of the detector is relatively great so that the signal
oise ratio is poor. The noise of the detector cannot be reduced right away since the detector must have such a large band width that its pulse response is small compared with the pulses transmitted through the optical fiber during the measurement. Therefore, in the known pulse method devices it was attempted to remedy the poor signal
oise ratio by using high power lasers as senders, but high power lasers can only operate at a significantly lower repetition frequency and at other wavelengths than the low power lasers which are used in practice for communication purposes; therefore this known measurement method will not completely reflect the fiber characteristics which are relevant in normal communication operations.
The object of the invention is to provide a method in the signal processing of a train of pulses, enabling the bandwidth of an optical fiber to be measured in accordance with the said pulse method even in those cases where the optical fiber has such a great attenuation that the previously known pulse measuring technique is inadequate when a laser of the same type as is used in practice for telecommunication is to be used in the pulse measuring sender.
This object is achieved in that the detuned frequency, f.sub.s, at which the received signal is sampled, is given by f.sub.r, and that the sampling signal is generated by means of a pulse generator whose repetition frequency is controlled by means of a phase-detecting circuit, to which there are applied partly a reference signal indicative of the displacement frequency .DELTA.f, partly a low frequency signal derived from the signal received and sampled at the frequency f.sub.s and which has a sufficient bandwidth to contain essentially all the information in the signal received. When the

REFERENCES:
patent: 3869680 (1975-03-01), Fletcher et al.
patent: 3944811 (1976-03-01), Midwinter
patent: 4496937 (1985-01-01), Kitagawa et al.
Presby, H. M. et al, "Novel Bandwidth Measurement System for Short-Fiber Lengths", Rev. Sci. Instrum. 52(11), Nov. 1981, pp. 1660-1663.

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