OPTICAL TIME DOMAIN REFLECTOMETRY BY PHOTON COUNTING

MHz signal generated by a crystal oscillator having 10'6frequency stability.

Results: Fig. 2 shows an example of the mixer output spec-

trum. The spike at 40 MHz shows an induction due to spuriouscoupling between the measuring circuit and the 40 MHz power

amplifier driving the modulator. The original spectral spreadcan be estimated from the mixer output spectrum around/=40 MHz, but excluding this spike. In this process weassume that the distribution is Lorentzian.5dB ,r40 80frequency,MHz1307/21Fig. 2 Example of mixer output spectrum401 20Fig.0 02 0-4 0-6normalised bias current,(l-I(h)/Ith|3O7/3|3 Measured 3 dB spectral width as a function of bias currentI Measured TheoreticalFig. 3 shows the measured 3 dB spectral width Sf as a func-tion of the bias current /. The abscissa is scaled by (/ — I,h)/I,h,where I,h denotes the threshold current of the laser. It is foundthat (5/approaches 8 MHz as the current increases.Discussion: Theoretically, the 3 dB spectral width of a laserabove threshold is given as2PoN2N.2 -(1)where/denotes the oscillation frequency, A/the passive resona-tor linewidth, Po the power emitted by the lasing material andN2 and Nt the numbers of atoms at upper and lower levels,respectively.For the laser used in the experiment, we estimate thatAf^ 150 GHz, N2/(N2 - Ni) ~ 1, and Po is nearly equal tothe emitted light power, which is 5 mW at the right-hand sideof Fig. 3. These values lead to <5/= 3 MHz.The measured value of df is in good agreement with theabove crude theoretical estimation. On the other hand, theinversely proportional relation found in Fig. 3 (the brokencurve) substantiates eqn. 1 qualitatively, because Po is nearlyproportional to (/ - llh)H,h.When the spectrum analyser is used in a fine resolutionmode, the spectrum of the mixer:output is found to have manyspike-like components. This is probably due to the 'long-lineeffect' of the fibre, which affects the spectral structure of theoscillator.Acknowledgment: We thank T. Tsukada of Hitachi Ltd. forsupplying the laser diodes, and N. Inagaki and K. Okamoto ofIbaraki ECL for supplying the fibre used as the delay line.

T. OKOSHI 19th June 1980K. KIKUCHIA. NAKAYAMA

Department of Electronic EngineeringUniversity of TokyoHongo, Bunkyo-ku, Tokyo 113, Japan

References1 HINKLEY, E. E., and FREED, c: 'Direct observation of the Lorentzianline shape as limited by quantum phase noise in a laser abovethreshold', Phy. Rev. Lett., 1969, 23, pp. 277-2802 YARIV, A.: 'introduction to optical electronics' (Holt, Reinhart and

Winston, New York, 1971), see eq. (10.6-13)

0013-5194/80/160630-02$! .50/0

OPTICAL TIME DOMAINREFLECTOMETRY BY PHOTON COUNTING

Indexing terms: Optics, Optical fibresA photon counting technique has been used to extend greatlythe range of optical time domain reflectometry or 'back-scatter' for fault location in optical fibre systems. A range ofmore than 40 dB of one-way fibre loss has been achieved evenwhen the break was index-matched to eliminate anyreflection.

Introduction: Optical time domain reflectometry or 'back-scatter' is a useful technique for locating faults in optical fibresystems. Essentially the same apparatus can be used either toprobe the attenuation of a fibre along its length or to pinpointa discrete fault by observing the associated reflection. A perfectfibre end reflects about 4% of the power incident upon it butany degradation in end quality or cleanliness can reduce thisfigure dramatically. Consequently, a break can only be locatedwith complete certainty by observing the cessation of the back-scattered signal. The difficulty of making this observation risesvery rapidly as one attempts to penetrate further into a fibreand it becomes increasingly important to maximise the inputpower and the ability of the receiver to recover the signal. Sincethe need for portability precludes the use of high power sourcessuch as Nd:y.a.g. lasers,1 most effort has been concentrated onreceiver design. Commercial backscatter apparatus typicallyhas a range (measured in one-way loss of telecommunications-type fibre) of about 15 dB. Using a GaAs laser source andanalogue signal recovery, a range of 25 dB has been achieved2

but even this is inadequate when set against repeater sectionattenuations of up to 60 dB. A new approach is needed.We report here a major increase in range—to over 40 dB—obtained by using a photon counting technique. A furtheradvantage is that digital methods greatly simplify the asso-ciated electronics.