OPTICAL TIME DOMAIN REFLECTOMETRY BY PHOTON COUNTING
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40 Indexing terms: Optics, Optical fibres A photon counting technique has been used to extend greatly the range of optical time domain reflectometry or 'backscatter' for fault location in optical fibre systems. A range of more than 40 dB of one-way fibre loss has been achieved even when the break was index-matched to eliminate any reflection.
631
T. OKOSHI
K. KIKUCHI A. NAKAYAMA
19th June 1980
Department of Electronic Engineering University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan
References
HINKLEY, E. E., and FREED, c : 'Direct observation of theபைடு நூலகம்Lorentzian line shape as limited by quantum phase noise in a laser above threshold', Phy. Rev. Lett., 1969, 23, pp. 277-280 2 YARIV, A.: 'introduction to optical electronics' (Holt, Reinhart and Winston, New York, 1971), see eq. (10.6-13) 1
MHz signal generated by a crystal oscillator having 10' 6 frequency stability. Results: Fig. 2 shows an example of the mixer output spectrum. The spike at 40 MHz shows an induction due to spurious coupling between the measuring circuit and the 40 MHz power amplifier driving the modulator. The original spectral spread can be estimated from the mixer output spectrum around / = 4 0 MHz, but excluding this spike. In this process we assume that the distribution is Lorentzian.
5dB ,r
0013-5194/80/160630-02$! .50/0
40 frequency,MHz
Fig. 2 Example of mixer output spectrum
80 1307/21
OPTICAL TIME DOMAIN REFLECTOMETRY BY PHOTON COUNTING
Po
N2 N.2 -
(1)
where/denotes the oscillation frequency, A/the passive resonator linewidth, Po the power emitted by the lasing material and N2 and Nt the numbers of atoms at upper and lower levels, respectively. For the laser used in the experiment, we estimate that Af^ 150 GHz, N2/(N2 - Ni) ~ 1, and P o is nearly equal to the emitted light power, which is 5 mW at the right-hand side of Fig. 3. These values lead to <5/= 3 MHz. The measured value of df is in good agreement with the above crude theoretical estimation. On the other hand, the inversely proportional relation found in Fig. 3 (the broken curve) substantiates eqn. 1 qualitatively, because Po is nearly proportional to (/ - llh)H,h.
ELECTRONICS LETTERS 31st July 1980 Vol. 16 No. 16
Introduction: Optical time domain reflectometry or 'backscatter' is a useful technique for locating faults in optical fibre systems. Essentially the same apparatus can be used either to probe the attenuation of a fibre along its length or to pinpoint a discrete fault by observing the associated reflection. A perfect fibre end reflects about 4% of the power incident upon it but any degradation in end quality or cleanliness can reduce this figure dramatically. Consequently, a break can only be located with complete certainty by observing the cessation of the backscattered signal. The difficulty of making this observation rises very rapidly as one attempts to penetrate further into a fibre and it becomes increasingly important to maximise the input power and the ability of the receiver to recover the signal. Since the need for portability precludes the use of high power sources such as Nd:y.a.g. lasers,1 most effort has been concentrated on receiver design. Commercial backscatter apparatus typically has a range (measured in one-way loss of telecommunicationstype fibre) of about 15 dB. Using a GaAs laser source and analogue signal recovery, a range of 25 dB has been achieved2 but even this is inadequate when set against repeater section attenuations 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 further advantage is that digital methods greatly simplify the associated electronics. Photon counting technique: When an avalanche photodiode is operated in a 'Geiger tube' breakdown mode, it is biased above its normal operating voltage and breakdown may be initiated by a single photon.3 5 In our case, for example, an RCA C30902E has a normal room temperature operating bias of 203 V. The avalanche gain over the active area of the diode is not uniform and gradually increasing the bias beyond 203 V causes
When the spectrum analyser is used in a fine resolution mode, the spectrum of the mixer:output is found to have many spike-like components. This is probably due to the 'long-line effect' of the fibre, which affects the spectral structure of the oscillator. Acknowledgment: We thank T. Tsukada of Hitachi Ltd. for supplying the laser diodes, and N. Inagaki and K. Okamoto of Ibaraki ECL for supplying the fibre used as the delay line.
1 20
0
02 0-4 0-6 normalised bias current,(l-I (h )/I th
|3O7/3|
Fig. 3 Measured 3 dB spectral width as a function of bias current I Measured Theoretical
Fig. 3 shows the measured 3 dB spectral width Sf as a function 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 found that (5/approaches 8 MHz as the current increases. Discussion: Theoretically, the 3 dB spectral width of a laser above threshold is given as2
631
T. OKOSHI
K. KIKUCHI A. NAKAYAMA
19th June 1980
Department of Electronic Engineering University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan
References
HINKLEY, E. E., and FREED, c : 'Direct observation of theபைடு நூலகம்Lorentzian line shape as limited by quantum phase noise in a laser above threshold', Phy. Rev. Lett., 1969, 23, pp. 277-280 2 YARIV, A.: 'introduction to optical electronics' (Holt, Reinhart and Winston, New York, 1971), see eq. (10.6-13) 1
MHz signal generated by a crystal oscillator having 10' 6 frequency stability. Results: Fig. 2 shows an example of the mixer output spectrum. The spike at 40 MHz shows an induction due to spurious coupling between the measuring circuit and the 40 MHz power amplifier driving the modulator. The original spectral spread can be estimated from the mixer output spectrum around / = 4 0 MHz, but excluding this spike. In this process we assume that the distribution is Lorentzian.
5dB ,r
0013-5194/80/160630-02$! .50/0
40 frequency,MHz
Fig. 2 Example of mixer output spectrum
80 1307/21
OPTICAL TIME DOMAIN REFLECTOMETRY BY PHOTON COUNTING
Po
N2 N.2 -
(1)
where/denotes the oscillation frequency, A/the passive resonator linewidth, Po the power emitted by the lasing material and N2 and Nt the numbers of atoms at upper and lower levels, respectively. For the laser used in the experiment, we estimate that Af^ 150 GHz, N2/(N2 - Ni) ~ 1, and P o is nearly equal to the emitted light power, which is 5 mW at the right-hand side of Fig. 3. These values lead to <5/= 3 MHz. The measured value of df is in good agreement with the above crude theoretical estimation. On the other hand, the inversely proportional relation found in Fig. 3 (the broken curve) substantiates eqn. 1 qualitatively, because Po is nearly proportional to (/ - llh)H,h.
ELECTRONICS LETTERS 31st July 1980 Vol. 16 No. 16
Introduction: Optical time domain reflectometry or 'backscatter' is a useful technique for locating faults in optical fibre systems. Essentially the same apparatus can be used either to probe the attenuation of a fibre along its length or to pinpoint a discrete fault by observing the associated reflection. A perfect fibre end reflects about 4% of the power incident upon it but any degradation in end quality or cleanliness can reduce this figure dramatically. Consequently, a break can only be located with complete certainty by observing the cessation of the backscattered signal. The difficulty of making this observation rises very rapidly as one attempts to penetrate further into a fibre and it becomes increasingly important to maximise the input power and the ability of the receiver to recover the signal. Since the need for portability precludes the use of high power sources such as Nd:y.a.g. lasers,1 most effort has been concentrated on receiver design. Commercial backscatter apparatus typically has a range (measured in one-way loss of telecommunicationstype fibre) of about 15 dB. Using a GaAs laser source and analogue signal recovery, a range of 25 dB has been achieved2 but even this is inadequate when set against repeater section attenuations 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 further advantage is that digital methods greatly simplify the associated electronics. Photon counting technique: When an avalanche photodiode is operated in a 'Geiger tube' breakdown mode, it is biased above its normal operating voltage and breakdown may be initiated by a single photon.3 5 In our case, for example, an RCA C30902E has a normal room temperature operating bias of 203 V. The avalanche gain over the active area of the diode is not uniform and gradually increasing the bias beyond 203 V causes
When the spectrum analyser is used in a fine resolution mode, the spectrum of the mixer:output is found to have many spike-like components. This is probably due to the 'long-line effect' of the fibre, which affects the spectral structure of the oscillator. Acknowledgment: We thank T. Tsukada of Hitachi Ltd. for supplying the laser diodes, and N. Inagaki and K. Okamoto of Ibaraki ECL for supplying the fibre used as the delay line.
1 20
0
02 0-4 0-6 normalised bias current,(l-I (h )/I th
|3O7/3|
Fig. 3 Measured 3 dB spectral width as a function of bias current I Measured Theoretical
Fig. 3 shows the measured 3 dB spectral width Sf as a function 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 found that (5/approaches 8 MHz as the current increases. Discussion: Theoretically, the 3 dB spectral width of a laser above threshold is given as2