United States Patent c191
`Beckmann et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,794,249
`Dec. 27, 1988
`
`[54] OPTICAL TIME DOMAIN
`REFLECTOMETER WITH HETERODYNE
`RECEPTION
`
`(75]
`
`Inventors: Friedrich-Karl Beckmann, Pinneberg;
`Wolfgang Hoppe, Norderstedt;
`Reinhard Kniichel, Elmshom; Jiirgen
`Kordts, Wedel, all of Fed. Rep. of
`Germany
`
`[73] Assignee: U.S. Philips Corporation, New York,
`N.Y.
`[21] Appl. No.: 26,797
`
`Mar. 17, 1987
`(22] Filed:
`Foreign Application Priority Data
`[30]
`Mar. 20, 1986 [DE] Fed. Rep. of Germany ....... 3609371
`Int. 0.4 ................................................ HOlJ 5/16
`(51]
`[52] U.S. CI. .................................... 250/227; 356/73.1
`[58] Field of Search ................ 250/227; 455/610-612,
`455/609; 356/73.1
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,708,471 11/1987 Beckmann et al ................. 356/73.1
`Primary Examiner-David C. Nelms
`Assistant Examiner-K.haled Shami
`Attorney, Agent, or Firm-Paul R. Miller
`
`[57]
`ABSTRACT
`The invention relates to an optical time domain reflec(cid:173)
`tometer (OTDR) with heterodyne reception for deter(cid:173)
`mining the attentuation of an optical waveguide (mea(cid:173)
`suring waveguide) by measuring the back-scattered
`portion of light pulses sent into the measuring wave(cid:173)
`guide. This structure is comprised of a modulated laser
`light source sending a send beam into the measuring
`waveguide and a laser light source which constitutes a
`local oscillator and transmits continuous light, on
`whose light of a wavelength differing by an intermedi(cid:173)
`ate-frequency from the back-scattered light from the
`transmission light source is superposed and is applied to
`a photodetector having an intermediate-frequency elec(cid:173)
`tric output signal which is filtered and evaluated. To
`improve the signal-to-noise ratio it is provided that the
`transmission light source is a transmission laser 1 whose
`light is influenced in consecutive time intervals (t1 to t2)
`such that the light frequency varies between two cut-off
`frequencies fLt and fL2 and that the light source forming
`the local oscillator is a laser transmitting light at a fre(cid:173)
`quency fLo, which is located outside the range between
`the cut-off frequencies fLt and fL2 and differs from a
`frequency value (fLM), located between the frequencies
`fLt and fL2, by the intermediate frequency (FzF).
`
`18 Oaims, 2 Drawing Sheets
`
`XMSN
`LASER
`
`..fl.
`
`MODULATION GENERATOR
`
`MEASURING
`WAVEGUIDE
`4
`
`OPTICAL _____ __...,.
`~-:;::;i:=::::::--"....,-O_P_Tl-CAL FIBER COUPLER
`ISOLATOR
`OPTICAL ISOLATOR
`
`OPTICAL
`DETECTOR
`
`BANDPASS
`FILTER
`
`AMPLIFIER
`
`6
`
`B
`
`LOCAL OSCILLATOR
`CLO> LASER
`
`IF DETECTOR
`11
`
`10
`
`EVALUATION CKT
`
`HALLIBURTON, Exh. 1006, p. 0001
`
`

`

`U.S. Patent Dec. 27, 1988
`
`Sheet 1 of2
`
`4,794,249
`
`XMSN
`LASER
`
`OPTICAL
`ISOLATOR
`
`OPTICAL
`DETECTOR
`
`BANDPASS
`FILTER
`
`AMPLIFIER
`
`MODULATION GENERATOR
`
`MEASURING
`WAVEGUIDE
`4
`
`OPTICAL FIBER COUPLER
`OPTICAL ISOLATOR
`
`6
`
`LOCAL OSCILLATOR
`CLO> LASER
`
`IF DETECTOR
`11
`
`EVALUATION CKT
`
`5
`
`·~
`
`8
`
`9
`
`C>
`
`FI G.1
`
`-A'f ZF .
`
`fLQ
`f L2
`L
`A t Z f .,.....- -~_.__
`r
`tu
`
`A f ( 500 MHz •.. 16 Hz)
`
`tl
`
`t2
`
`1:s
`
`FI G.2
`
`HALLIBURTON, Exh. 1006, p. 0002
`
`

`

`U.S. Patent
`
`Dec. 27, 1988
`
`Sheet 2 of2
`
`4,794,249
`
`Jl MODULATION GEN
`.__,_-* 2
`
`OPTICAL FIBER
`COUPLER
`13
`
`Ls
`
`L~L OSC LASER
`OPTICAL FIBER
`COUPLER
`
`14
`
`24
`
`Lo
`
`10
`
`MEASURING
`WAVEGUIDE
`4
`
`19 OPTICAL
`__
`DET
`
`AMPL
`
`MIXER
`BANDPASS FILTER
`
`fs
`
`FIG. 3
`
`HALLIBURTON, Exh. 1006, p. 0003
`
`

`

`1
`
`4,794,249
`
`OPTICAL 1ThlE DOMAIN REFLECTOMETER
`WITH HETERODYNE RECEPTION
`
`2
`Anyway, the cut-off frequencies fLt and f£2 of the
`transmission laser must be coarsely stabilized. The cut(cid:173)
`off frequencies can be sufficiently spaced from a fre(cid:173)
`quency range located between them, which is used to
`The invention relates to an optical time domain re- 5 form evaluatable back-scatter pulses. The transmission
`flectometer (OTDR) with heterodyne reception for
`laser then always passes through a frequency range of a
`determining the attenuation of an optical waveguide
`bandwidth AfZF, which, after the frequency fLo of the
`local oscillator has been superimposed on it, results in
`(measuring waveguide) by measuring the back-scat-
`tered portions of light pulses transmitted in the optical
`an intermediate-frequency output signal at the bandpass
`waveguide, comprised of a modulated laser light source 10 filter.
`The frequency of the transmission laser beam can be
`sending a send beam into the optical waveguide and a
`laser light source which constitutes a local oscillator
`varied versus time in any optional manner. A preferred,
`and transmits continuous light, on whose light of a
`particularly single solution, however, is the solution in
`wavelength, differing by an intermediate frequency, the
`which the control current of the transmission laser is
`back-scattered light of the transmission light source is 15 monotonicly changed for each back-scatter pulse be-
`superposed and is applied to a photodetector whose
`tween two time intervals ti and t1. Then the laser prop-
`intermediate-frequency electric output signal is filtered
`erty is, (which as such is an unwanted property), such
`that its transmission frequency depends to a low extent,
`and evaluated.
`In an arrangement of this type, disclosed in ECOC
`but for the present invention to an adequate extent, on
`83, "9th European Conference on Optical Communica- 20 the electric control current. Customary laser diodes
`tion", pages 177 to 180, a single laser-light source is used
`have useful "tuning
`transconductances" of 100
`from whose light beam a partial beam is tapped for MHz/mA to 3 GHz/mA.
`forming the local oscillator. The residual beam is sent
`The variation versus time of a "chirp" of the trans-
`with a time shift into the measuring optical waveguide
`mission laser which can be accomplished by, for exam-
`by an acousto-optical modulator (AOM) at a frequency 25 ple, a corresponding variation versus time of the elec-
`which is shifted by an acoustic intermediate frequency.
`tric control current, must increase or decrease mono-
`As the laser simultaneously produces the local oscillator
`tonicly, so that, during a finite period of time a send
`beam it must transmit continuous light. With lasers of
`energy is passed into the measuring waveguide whose
`this type it is however only possible to obtain light
`back-scatter portions form, after the Lo-frequency fLo
`outputs which are less than can be obtained with pulsed 30 has been superimposed on them, an intermediate-fre-
`lasers since they can be used for directly detecting ar-
`quency portion which can be removed by the bandpass
`rangements without heterodyne reception. The im-
`filter.
`provement in the signal-to-noise ratio (SIN) obtainable
`A particularly simple solution using a pulsed trans-
`by means of the heterodyne principle is partly lost be-
`. mission laser is obtained when the control current of the
`cause of the fact that on the one hand a portion of the 35 transmission laser is controlled from "zero" to a maxi-
`transmission intensity must be tapped-off for the local
`mum value between the instants ti and t1.
`oscillator beam and that on the other hand lasers which
`Uniformly spaced consecutive send pulses are ob-
`transmit continuous light have relatively low optical
`tained because of the fact that the control current of the
`transmission laser is amplitude-modulated. Then, both
`output powers.
`The invention has for its object to improve the signal- 40 when the control current increases or decreases during
`passing through the frequency range AfZF, evaluatable
`to-noise ratio in an arrangement of the type defined in
`pulses are produced. The minimum value of the energiz-
`the opening paragraph.
`This object is accomplished in that the first light
`ing current must be so small that at this value, compared
`source is a transmission laser 1 whose light is influenced
`with the maximum energization, a substantially disre-
`such that in consecutive time intervals (ti to t1) the light 45 gardable thermal load of the laser is obtained.
`A particularly advantageous embodiment of the in-
`frequency varies between two cut-off frequencies fLt
`and f£2, and that the light source forming the local
`vention is characterized in that the light beam of the
`oscillator is a laser transmitting light at a frequency fLo
`transmission laser is guided into the measuring wave-
`which is located outside the range between the cut-off
`guide via a first optical isolator and through a first di-
`frequencies fLt and f£2 and differs from a frequency 50 rect path of an optical fiber coupler, and that the light
`beam of the Lo-laser in the return direction is guided via
`value located between the frequencies f£1 and fL2 by an
`amount equal to the intermediate frequency.
`a second optical isolator and through the second direct
`Since the light required for the local oscillator is
`path of the first optical fiber coupler.
`produced by its own laser, the light power of the trans-
`When one wants to avoid the use of optical isolators
`mission laser sent into the measuring waveguide is not 55 such as they must be used in unbound-mode radiation
`reduced by the power of the local oscillator. In addi-
`techniques, a preferred embodiment of the invention is
`tion, a pulsed-mode operation is possible for the trans-
`possible which is characterized in that the light beam of
`mission laser. Consequently, it is possible to transmit
`the transmission laser is guided into the test-wave guide
`pulses of a significantly higher intensity than is possible
`via a direct path of a first fiber coupler, and that a por-
`with continuous wave lasers.
`60 tion of the optical back-scatter signal derived from the
`For the "chirp" (frequency variation versus time) of
`first fiber coupler is guided, combined with the light
`the transmission laser, which "chirp" is provided in
`beam of the Lo-laser, to the optical detector via a sec-
`accordance with the invention, it is not necessary to
`ond fiber coupler. In this situation, it is advantageously
`stabilize the frequency of the transmission laser and
`possible to recover in a simple manner synchronizing
`more specifically the differential frequency for the local 65 pulses for the evaluation of the back-scatter signal by
`oscillator (Lo) laser, so that the considerable cost and
`providing that a partial beam, tapped from the light
`design effort for control are not necessary. Only the
`beam of the transmission laser, has superposed on it a
`frequency fLo of the Lo-laser is to be stabilized.
`portion of the Lo-laser light beam and is applied to a
`
`HALLIBURTON, Exh. 1006, p. 0004
`
`

`

`4,794,249
`
`4
`3
`The mode of operation of the invention will now be
`second photodetector whose electric output signals
`described in greater detail with reference to the varia-
`trigger the circuit which evaluates the back-scatter
`tion of the frequency f of the transmission laser 1 in the
`pulses.
`time interval between ti and t2. For the formation of a
`To reduce the pulse period of the transmission laser it
`is advantageous for deviations of the chirp range be- 5 back-scatter pulse the transmission laser 1 is in the
`switched-on state during the interval t1 to ti. This may
`tween the cut-off frequencies fL1and/or fL2ofthe trans-
`mission laser to be reduced by a control circuit. Then
`be accompanied by a corresponding control of the con-
`the total overall pulse duration of the transmission laser
`trol current of the transmission laser 1 by a variation of
`needs only to be a little longer than the desired duration
`the transmission frequency f in accordance with the
`of the transmission of evaluatable frequency compo- 10 characteristic curve shown, which increases monoton-
`icly from the cut-off frequency fLt to the cut-off fre-
`nents.
`An advantageously implementable embodiment of
`quency f£2. A range fL2-fL1=500 MHz to 1 GHz is
`the invention is characterized in that the difference
`preferred. The frequencies fLt and fL2 are smaller than
`between the cut-off frequencies fLt and f12 has a value
`the optical frequency fLo of the Lo-Laser 6 and encom-
`between 300 MHz and 2 GHz. In addition, it is advanta- 15 pass a frequency range of a width ll.fZF around the fre-
`quency fLM which is smaller than the Lo-frequency fLo
`geous that the intermediate frequency fZF has a value
`between 0.5 and 15 GHz.
`by an amount equal to the transmission frequency of the
`If so required, the bandwidth of the bandpass filter at
`bandpass filter 8, namely the intermediate frequency
`fZF. Only the frequencies within the frequency band
`the receiver end, which determines the transmission
`period of evaluatable transmission frequencies, can be 20 ll.fZF form during the time interval Ts back-scatter sig-
`nals of this type, which after the Lo-frequency fLo has
`reduced because of the fact that it is in the form of a
`tracking filter.
`been superimposed on them, result in evaluatable output
`The invention and its advantages will now be de-
`signals of the bandpass filter 8. The spacing between the
`scribed in greater detail with reference to advantageous
`cut-off frequencies fLt and fL2 can be chosen to be so
`25 great that the frequency range ll.fZF is sufficiently far
`embodiments shown in the accompanying drawings.
`FIG. 1 shows an embodiment of the invention.
`removed from the cut-off frequency fLt and f£2, so that
`FIG. 2 shows the "chirp" variation of a transmission
`temperature and/or tolerance-determined fluctuations
`of the cut-off frequencies may be permitted, if possible
`laser.
`FIG. 3 shows a modified embodiment of the inven-
`without a design effort and cost for a control. On the
`tion without optical isolators and with additional drive 30 other hand the time difference t2-t1 must not be chosen
`and control elements.
`such that they are unnecessarily much greater than the
`The frequency of the transmission laser 1 is temporar-
`useful transmission period Ts to avoid unnecessary load-
`ily controlled by the modulation generator 2, while the
`ing of the transmission laser 1.
`energizing current of the laser 1 is correspondingly
`The arrangement shown in FIG. 3 which is a modifi-
`changed. The laser 1 can, for example, be controlled 35 cation of the arrangement shown in FIG. 1 does not
`require optical isolators. Consequently it is not neces-
`from the off-state to the on-state between the instants t1
`and t2. It can however alternatively be switched to and
`sary to connect any optical elements radiating in the
`fro or be continuously modulated between two operat-
`unbound-mode. The optical fiber couplers 13 and 14 are
`ing modes with a predetermined time variation of the
`inserted such that light from the transmission laser 1
`control current. In each case the laser 1 has between the 40 cannot read the Lo-laser 6 nor can light from the Lo-
`cut-off instants ti and t2 a monotonic frequency "chirp"
`laser 6 reach the transmission laser.
`from fLt to fL2 or also in the opposite direction. During
`The detection and evaluation of the back-scattered
`the interval from t1 to t2 the transmission laser 1 sends
`signals is basically effected as in the arrangement shown
`in FIG. 1. The electric output signal of the detector 5 is
`light power into the measuring waveguide 4 via the
`optical isolator 7 and the fiber coupler 3, which prefera- 45 conveyed via the amplifier 25 to the bandpass filter 15
`whose intermediate-frequency output signal is detected
`bly is a 3 db coupler. Each energy portion ll.P/ll.fpro-
`duces independently a back-scatter signal. Portions of
`and evaluated.
`the signals back-scattered during the interval from ti to
`To reduce the bandwidth of the bandpass filter 15,
`t2 are applied to the optical detector 5 (photo diode) via
`more specifically at high values of the intermediate
`the fiber coupler 3 together with the local oscillator 50 frequency fZFthis filter is in a form of a tracking filter.
`By means of the IF-oscillator 16 and the mixer 17 a
`beam transmitted by the Lo-laser 6 via the optical isola-
`tor 12. The optical isolator 7 prevents interfering light
`further frequency conversion is effected into a range in
`portions of the Lo-laser from reaching the transmission
`which the bandpass 15 can easily be implemented with
`laser via the coupler 3. Inversely, the optical isolator 12
`a narrow bandwidth.
`In the practical example shown in FIG. 3 further
`prevents light from the transmission laser from reaching 55
`switching circuits are provided which, if so required,
`the Lo-laser.
`The electric output signal of the photodetector 5
`enable advantageous additional drive and control func-
`contain intermediate-frequency portions. As soon as the
`tions and which can be provided according to the re-
`quirements. The photodetector 19 receives a portion of
`frequency thereof becomes located within the tuned
`passbandwidth of the bandpass filter 8 during a time 60 the light of the Lo-laser 6 from the optical fiber cou-
`piers 13and14, on which light a portion of the transmis-
`interval Ts located between the instants t1 and t2, a signal
`appears at the output of the bandpass filter 8, which is
`sion light of the transmission laser 1 is superimposed by
`amplified in the amplifier 9 and applied to an intermedi-
`the coupler 18. The intermediate-frequency output sig-
`ate-frequency detector 10 whose output signal contains
`nal detected by the electric detector 22 via the amplifier
`information about the local variation of the attenuation 65 20 and the bandpass filter 21 is applied to the evaluation
`of the measuring waveguide 4 and is evaluated and
`circuit 11. The bandpass filter 21 is tuned to the same
`indicated in a customary manner by the evaluation cir-
`intermediate frequency and bandwidth as the bandpass
`filter 15. Its bandwidth is also narrowed by the addition
`cuit 11.
`
`HALLIBURTON, Exh. 1006, p. 0005
`
`

`

`4,794,249
`
`5
`6
`continuous light through a second direct path of said
`of the intermediate frequency, or IF, oscillator 16 via
`the mixer 26.
`multiple light paths of said optical fiber coupler in said
`The output signal of the electric detector 22 appears
`second opposite direction.
`5. An arrangement according to claim 4, wherein said
`earlier and whithout ambiguity than the output signal of
`the detector 10 which is delayed by the back-scatter S control means are disposed to reduce deviations of a
`"chirp" range between said cutoff frequencies, fLI and
`time difference and is consequently suitable for trigger-
`fL2.
`ing the evaluation circuit 11.
`This is particularly important when the instant at
`6. An arrangement according to claim 4, wherein said
`cutoff frequencies fLI and fL2 have a difference value
`which the intermediate-frequency back-scatter signal
`10 between 300 MHz and 2 GHz.
`appears is not accurately defined.
`In the event of any deviation from a nominal fre-
`7. An arrangement according to claim 4, wherein said
`quency fsit is possible to readjust the chirp-range of the
`frequency fZF has a value between 0.5 and 15 GHz.
`transmission laser 1 via the electric line 24, using a con-
`8. An arrangement according to claim 4, wherein said
`trol circuit 23.
`circuit means includes bandpass filters with at least one
`What is claimed is:
`IS of said bandpass filters being a tracking filter.
`1. An optical time domain reflectometer arrangement
`9. An arrangement according to claim 1, claim 2, or
`having heterodyne reception for determining attenua-
`claim 3, further comprising first optical means for con-
`tion of an optical measuring waveguide comprising
`veying said laser light pulses into said optical wave-
`guide means by a direct path of a first optical fiber
`first laser means for transmitting laser light pulses,
`said laser light pulses passing in consecutive time 20 coupler, wherein a portion of an optical back-scattered
`signal from said first optical fiber coupler is conveyed
`intervals, ti to t2, with a light frequency varying
`between two cutoff frequencies, fLi and fL2,
`together with said continuous light of said second laser
`second laser means for transmitting continuous light
`at a frequency fL<>. said frequency fLobeing outside
`to said at least one detector means by a second optical
`a range between said cutoff frequencies, fL1 and fL2, 2s fiber coupler.
`and said frequency fLo differing from a frequency
`10. An arrangement according to claim 9, wherein
`value fLM between said cutoff frequencies, fLI and
`said first laser means provides a partial light beam, said
`f12, by an amount equal to a frequency fZF,
`partial light beam being superposed by a portion of said
`control means for monotonically varying control
`continuous light, and wherein said partial light beam
`current for said first laser means, said control cur- 30 and said portion are applied to a second detector means,
`rent being monotonically varied for each back-
`said detector means including a photodetector having
`scattered light pulse between said time intervals ti
`electrical output signals triggering said circuit means,
`and t2,
`said circuit means evaluating said backscattered signal.
`11. An arrangement according to claim 9, wherein
`optical waveguide means for receiving at least por-
`tions of said laser light pulses,
`3S said control· means are disposed to reduce deviations of
`at least one detector means receiving at least said
`a "chirp" range between said cutoff frequencies, fLI and
`fL2·
`continuous light for providing electrical signals,
`and
`U. An arrangement according to claim 9, wherein
`circuit means for filtering and evaluating intermedi-
`said cutoff frequencies, fL1 and fL2, have a difference
`ate ones of said electrical signals.
`40 value between 300 MHz and 2 GHz.
`2. An arrangement according to claim 1, wherein said
`.13. An arrangement according to claim 9, wherein
`control current is varied from zero to a maximum value
`S3.ld frequency fZFhas a value between 0.5 and 15 GHz.
`between said time intervals t1 and t2.
`14. An arrangement according to claim 9, wherein
`3. An arrangement according to claim 1, wherein said
`said circuit means includes bandpass filters with at least
`4S one of said bandpass filters being a tracking filter.
`control current is amplitude modulated.
`4. An arrangement according to claim 1, claim 2 or
`15. An arrangement according to claim 1, wherein
`claim 3, further comprising first optical means for con-
`said control means are disposed to reduce deviations of
`veying said laser light pulses in a first direction into said
`a "chirp" range between said cutoff frequencies, fLt and
`optical waveguide means, and second optical means for
`fL2·
`conveying said continuous light in a second opposite so
`16. An arrangement according to claim 1, wherein
`direction to said first direction into said at least one
`said cutoff frequencies, fLI and fL2, have a difference
`detector means, wherein said first optical means in-
`value between 300 MHz and 2 GHz.
`17. An arrangement according to claim 1, wherein
`eludes a first optical isolator and an optical fiber coupler
`said frequency fZFhas a value between 0.5 and 15 GHz.
`having multiple light paths, said laser light pulses being
`18. An arrangement according to claim 1, wherein
`conveyed through a first direct path of said multiple SS
`light paths of said optical fiber coupler to said optical
`said circuit means includes bandpass filters with at least
`waveguide means, and wherein said second optical
`one of said bandpass filters being a tracking filter.
`means includes a second optical isolator conveying said
`•
`•
`•
`•
`•
`
`60
`
`6S
`
`HALLIBURTON, Exh. 1006, p. 0006
`
`