'155-619
`
`6117186
`
`EX
`AU 263
`OR
`4,596,052
`
`United States Patent c191
`Wright et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,596,052
`Jun. 17, 1986
`
`[73] Assignee:
`
`[54] COHERENT OPTICAL RECEIVER
`[75]
`Inventors: Stephen Wright, Finsbury Park,
`England; Philip R. Couch, Roanoke,
`Va.
`International Standard Electric
`Corporation, New York, N.Y.
`[21] Appl. No.: 496,747
`[22] Filed:
`May 20, 1983
`[30]
`Foreign Application Priority Data
`May 20, 1982 [GB] United Kingdom ................. 8214733
`Int. 0.' ............................................... H04B 9/00
`[51]
`[52] U.S. a ..................................................... 455/619
`[58] Field of Search ................ 455/617, 619; 356/345,
`356/358
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,694,656 9/1972 Henning .............................. 350/370
`
`4,063,084 12/1977 Goodwin et al .................... 455/619
`
`OTHER PUBLICATIONS
`The American Heritage Dictionary, Second College Edi(cid:173)
`tion, 1982, p. 1120.
`Primary Examiner-Joseph A. Orsino, Jr.
`Attorney, Agent, or Firm-John T. O'Halloran
`[57]
`ABSTRACT
`A coherent optical receiver for an input optical signal
`including a local oscillator source of coherent optical
`signal a single beam splitter/combiner to which both
`the input and local oscillator signals are applied to pro(cid:173)
`duce two substantially identical combined optical sig(cid:173)
`nals, a pair of substantially identical photodetectors
`electrically connected in series, each photodetector
`receiving one of the combined optical signals, and an
`output amplifier the input of which is connected to the
`series connection between the photodetectors.
`
`7 Claims, 2 Drawing Figures
`
`LOCAL
`osc.
`
`~ 13
`
`V+
`()
`
`PL
`~ .... ~
`
`T
`INPU
`SIGN AL
`p s
`
`PL+P5
`2
`
`.....-11
`
`DET.
`
`I 1
`
`"'
`
`Iz
`
`I 1- I 2
`
`DET.
`
`r14
`
`[>
`
`PL+Ps
`2
`
`12 .....
`
`v-
`
`HALLIBURTON, Exh. 1010, p. 0001
`
`

`

`U.S. Patent
`
`UT
`INP
`SIGN AL
`p s
`
`.
`
`• PL
`
`10,
`
`PL+Ps
`2
`
`"'
`"' -
`
`4,596,052
`
`Jun 17, 1986
`Fig 1 ..
`~ 13
`
`V+
`
`LOCAL
`DSC.
`
`I~
`
`l../" 11
`OET .
`
`I1
`
`Ir
`
`I 1- I 2
`
`(14
`
`[>
`
`-;
`
`PL +Ps .
`2
`
`12 .._
`
`v- .::.
`
`DET.
`
`I2
`
`-
`
`Fig.2
`J1/2 SIGNAL
`
`(al
`
`I1
`
`DC
`OFFSET
`
`t
`
`I2 ~
`
`t
`
`(bl
`
`(cl
`
`HALLIBURTON, Exh. 1010, p. 0002
`
`

`

`1
`
`COHERENT OPTICAL RECEIVER
`
`4,596,052
`
`2
`lation of the undesirable noise components due to local
`oscillator power fluctuations and restores all of the
`wanted signal components, as shown in FIG. 2(c).
`There is thus no significant signal or local oscillator
`S power loss· through the splitter/combiner and the opti(cid:173)
`mum splitting fraction is always 50%. With the series
`photo detector connection, low noise photo detector
`biassing is simplified and large photocurrents due to
`local oscillator operation are always cancelled. Because
`of the efficient use of local oscillator and signal power a
`lower power local oscillator may be used. The scheme
`is amenable to integrated or fibre optic implementation.
`In the latter case, as an alternative to the use of a half-sil(cid:173)
`vered mirror beam splitter/combiner, a directional opti(cid:173)
`cal fibre coupler may be used.
`Also, instead of having a single amplifier connected
`to pbint X, the diode outputs may be separately ampli(cid:173)
`fied and then combined differentially.
`We claim:
`1. A coherent optical receiver for optical input signal
`at a basic frequency having modulation imposed
`thereon as frequency modulation, comprising a local
`oscillator source of a coherent optical local oscillator
`signal at a frequency substantially ·corresponding to the
`basic frequency of the input signal; mixing means to
`which both the input and local oscillator signals are
`applied to produce two coherently combined optical
`signals which both vary in intensity in dependence on
`the frequency modulation but differ from each other in
`phase in such a manner that those components of the
`combined optical signals which vary with the local
`oscillator signal power are in-phase while those compo(cid:173)
`nents which vary with the frequency modulation are in
`antiphase with one another; a pair of substantially iden(cid:173)
`tical photodetectors each having an input connected to
`a different one of two points of different electrical po(cid:173)
`tentials and an output, each photodetector receiving
`one of the combined optical signals and generating in
`response thereto an electrical output signal containing
`electrical equivalents of said components of the respec-
`tive combined optical signal, said electrical output sig(cid:173)
`nal appearing at said output of the respective photode(cid:173)
`tector; and means for providing a resultant output signal
`representative of the difference between said electrical
`output signals appearing at said outputs of said photode-
`tectors with attendant elimination of the in-phase com(cid:173)
`ponents from and addition of the antiphase components
`of the electrical output signals in said resultant output
`signal.
`2. A receiver according to claim 1 wherein the local
`oscillator source is a semiconductor laser.
`3. A receiver according to claim 1 wherein the photo(cid:173)
`detectors are avalanche photodiodes.
`4. A receiver according to claim 1 wherein the mix(cid:173)
`ing means is a half-silvered beam splitter/combiner.
`5. A receiver according to claim 1 wherein the mix(cid:173)
`ing means is a fibre-optic directional coupling.
`6. A receiver as defined in claim 1, further comprising
`a connecting line directly connecting said photodetec(cid:173)
`tors in series with one another; and wherein said detect(cid:173)
`ing means includes a tap line connected to said connect-
`ing line.
`7. A receiver according to claim 6, wherein said de(cid:173)
`tecting means includes amplifying means interposed in
`said tap line.
`
`This invention relates to a coherent optical receiver
`using balanced detectors.
`The general principles of coherent detection of mod(cid:173)
`ulated signals are well established. In the field of optical
`communication systems, coherent receivers using laser
`local oscillators and balanced detectors have been de(cid:173)
`scribed in "Laser Receivers" by Monte Ross, published 10
`by Wiley, N.Y. In that book, at pages 112-113, there is
`described a receiver in which an input signal plus a local
`oscillator reference signal is fed to one of a pair of basi(cid:173)
`cally identical detectors, whilst the input signal minus
`the local oscillator reference signal is fed to the other IS
`detector of the pair. The arrangement requires, in addi(cid:173)
`tion to a beam splitter, two half silvered mirrors, a 180°
`phase changer for the local oscillator signal, and a sub(cid:173)
`traction circuit for the two detector outputs. The sub(cid:173)
`tracted outputs from the two detectors form the output 20
`signal.
`According to the present invention there is provided
`a coherent optical receiver for an input optical signal Ps
`including a local oscillator source of coherent optical
`power PL at a frequency substantially that of the input 2S
`signal mixing means to which both the input and local
`oscillator signals are applied to produce two substan(cid:173)
`tially identical coherently combined signals, a pair of
`substantially identical photodetectors, each photodetec(cid:173)
`tor receiving one of the combined signals, and means 30
`for amplifying the two photodetector outputs differen(cid:173)
`tially.
`An embodiment of the invention will now be de(cid:173)
`scribed with reference to the accompanying drawings,
`in which:
`FIG. 1 illustrates schematically a coherent optical
`receiver using a pair of balanced detectors, and
`FIG. 2 illustrates waveforms associated with the
`receiver of FIG. 1.
`In the arrangement shown in FIG. 1 an input optical 40
`signal Psis directed to a half silvered mirror beam split(cid:173)
`ter/combiner 10. Half of the input signal P2 is directed
`to a first detector 11, e.g. an avalanche photodiode,
`whilst the other half of the input signal Psis directed to
`a second identical photodetector 12. Similarly a coher- 4S
`ent signal PL, generated by a local oscillator laser 13
`which may be a semiconductor laser, is also directed to
`the beam splitter/combiner 10 such that half the refer(cid:173)
`ence signal is directed to each photodetector. The local
`oscillator and input signals coherently combine and SO
`interfere. Each photodetector therefore receives a com(cid:173)
`bined signal which varies in intensity at the difference
`between local oscillator and input frequencies. The two
`avalanche diodes 11, 12 are electrically connected in
`series between bias terminals V + and V - . An output 55
`amplifier 14 has its input connected to the mid point X
`of the series connection between diodes 11 & 12.
`As shown in FIG. 2(a) the detector current I from
`diode 11 contains a large d.c. level offset due to the
`average local oscillator power, a noise component due 60
`to local oscillator power fluctuations, and a difference
`frequency component which will contain the input
`signal modulation. The same d.c. offset and noise com(cid:173)
`ponents are present in the complementary current Ii
`from diode 12, but the modulation component is in 65
`antiphase with that of I as shown in FIG. 2(b). Summing
`the two currents differentially at point X causes cancel-
`
`3S
`
`* * * * *
`
`HALLIBURTON, Exh. 1010, p. 0003
`
`