WO1992004656A1 - Impulsion optique et procede et appareil de traitement de ladite impulsion - Google Patents

Impulsion optique et procede et appareil de traitement de ladite impulsion Download PDF

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Publication number
WO1992004656A1
WO1992004656A1 PCT/GB1991/001468 GB9101468W WO9204656A1 WO 1992004656 A1 WO1992004656 A1 WO 1992004656A1 GB 9101468 W GB9101468 W GB 9101468W WO 9204656 A1 WO9204656 A1 WO 9204656A1
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WO
WIPO (PCT)
Prior art keywords
pulse
phase
optical
modified
squeezed
Prior art date
Application number
PCT/GB1991/001468
Other languages
English (en)
Inventor
Keith James Blow
Original Assignee
British Telecommunications Public Limited Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by British Telecommunications Public Limited Company filed Critical British Telecommunications Public Limited Company
Priority to JP3515497A priority Critical patent/JPH06504381A/ja
Publication of WO1992004656A1 publication Critical patent/WO1992004656A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3511—Self-focusing or self-trapping of light; Light-induced birefringence; Induced optical Kerr-effect
    • G02F1/3513—Soliton propagation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping

Definitions

  • This invention relates to optical pulses and in particular to methods and apparatus for processing a coherent optical pulse.
  • the invention finds particular application to optical systems employing coherent optical detection schemes.
  • optical refers to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
  • Coherent detection schemes provide a significant improvement over direct intensity detection systems in which the optical detector responds only to the power of a received optical signal.
  • the optical detector responds only to the power of a received optical signal.
  • the conversion gain provided by a square law photodetector results in a receiver sensitivity that is limited by the shot noise of the local oscillator or incoming signal.
  • Coherent detection has therefore been seen as a route for realising a sensitivity almost equal to the quantum shot noise limit.
  • Such detection schemes have found use in a wide range of applications, particularly optical communications systems, but also in optical sensors and for making very high resolution optical measurements.
  • the fundamental quantum mechanical fluctuations in the optical fields of an optical signal can be conveniently expressed in terms of operators X and Y which are the quadrature components of the electric field operator.
  • operators X and Y which are the quadrature components of the electric field operator.
  • the uncertainties or fluctuations in each quadrature are equal, as shown in Figure 1, and these define the well known shot noise limit (SNL) which determines the minimum noise level in such optical detection schemes.
  • SNL shot noise limit
  • the measured noise for a squeezed state which is now phase dependent with reference to a local oscillator, can be reduced well below the normal shot noise level and therefore the use of squeezed states of light yields a quantum limited detection sensitivity well above the standard SNL for a variety of optical coherent detection schemes.
  • phase noise squeezing has been applied to continuous wave (CW) optical signals only.
  • CW continuous wave
  • Such a phase-squeezed signal can then be used as a carrier signal subsequently modulated in phase or frequency in correspondence with information to be transmitted or as a local oscillator signal in a coherent detector such as an unbalanced heterodyne detector where the signal to the detector is very small, or together in a single system, to obtain signal detection below the usual quantum noise limit.
  • phase squeezing is to propagate a CW optical signal of sufficiently high intensity along an optical waveguide of a material exhibiting an optical non-linearity so that self-phase modulation (SPM) results in the desired phase squeezing.
  • SPM self-phase modulation
  • Such a phase-squeezed CW carrier has constant phase and so can be used with another constant phase signal in a coherent detector - as a local oscillator to be combined with an optical carrier that has been modulated by an information signal or as an information signal to be combined with a local oscillator signal.
  • a phase-squeezed optical pulse of substantially constant phase is provided.
  • the phase of a pulse according to the present invention is substantially independent of time and will reduce the shot noise, albeit by different amounts, throughout the signal.
  • Such optical pulses can be used in a coherent detection system, as the information carrier or the local oscillator, or both, to obtain sub-SNL detection, for example.
  • GAWBS can also be suppressed as the optical signal is in the form of a pulse so providing a larger bandwidth signal.
  • a method of processing a coherent optical pulse of pulse shape ⁇ (t) comprises modifying the phase structure of the pulse so as to form a modified pulse having a modified phase structure and subsequently propagating the modified pulse along an optical waveguide of a material exhibiting an optical non-linearity, the modified phase structure being such that the pulse is phase-squeezed and has a substantially constant phase after propagation along the waveguide .
  • the necessary modified phase structure to obtain the required squeezing is calculated from the phase structure of the pulse that is to be processed as will be explained below. If the pulse to be processed has a substantially constant phase, as has a pulse produced by a transform limited laser for example, ⁇ (t) is readilly determined. If , however, the pulse does not have a constant phase it may be very difficult to determine its phase structure but having done so then again the required modified phase structure is readily calculated. Preferably, then, the optical pulse to be processed is one having a constant phase structure.
  • phase-squeezed pulse can be shown to be given by
  • ⁇ (t) ⁇ 0 - KX ⁇ (t) 2 + 1 ⁇ 2 tan -1 (1/KX ⁇ (t) 2 )
  • ⁇ 0 is an arbitrary constant
  • n 2 is the Kerr constant
  • ⁇ o is the angular frequency of the pulse
  • n eff is the effective refractive index of the mode
  • A is the mode area
  • N ⁇ R 4 rdr/ ( ⁇ R 2 rdr) 2
  • apparatus for processing a coherent optical pulse of pulse shape ⁇ (t) comprises modifying means for modifying the phase of the pulse to form a modified pulse having a modified phase structure, an optical waveguide of a material exhibiting an optical non-linearity and means for coupling the modified pulse to the optical waveguide, the modified phase structure being such that the pulse is phase-squeezed and has a substantially constant phase after propagation along the waveguide.
  • the optical waveguide along which the modified pulse is subsequently propagated may be any suitable optical waveguide exhibiting an optical non-linearity with suitably low loss.
  • a convenient choice would be a silica-based standard telecommunications single mode optical fibre if it intended to splice it to a similar fibre of an optical fibre communications systems though other waveguides exhibiting an optical non-linearity could be used.
  • the phase of the pulse may be modified in any convenient manner.
  • a spatial filtering technique may be used in which phase adjustment is achieved by adjusting the optical frequency components of the pulse once spatially dispersed within a temporally non-dispersive grating apparatus.
  • Such methods are described in articles entitled "High-Resolution Femtosecond Pulse Shaping" Journal of the Optical Society of America B/Optical Physics 5 No.8 (1988) ppl563-1572 by A.M. Weiner, J. P. Heritage and E. M. Kirschner and "Synthesis of Phase-Coherent, Picosecond Optical Square Pulses", Optics Letters II No.3 (1986) March pp 153-155 by A. M. Weiner.
  • An alternative method is to apply a time dependant phase change by transmitting the pulse through an appropriately driven phase modulator.
  • Other methods of modifying the phase of the pulse may, of course, be used to carry out the method of the present invention.
  • the means for coupling the modified pulse to the waveguide may comprise a lens, for example.
  • phase-squeezed optical pulse of substantially constant phase is able to be produced by methods or apparatus other than according to the second and third aspects of the present invention, such a pulse will still fall within the scope of the first aspect of the present invention.
  • Figure 1 is a quadrature representation of a shot-noise limited coherent state with amplitude / ⁇ / and for all phase angles;
  • Figures 2(a) and 2(b) show the quadrat representation of phase and amplitude squeezed sta respectively, for an optical field in which the uncertainty in one quadrature has been lessened at the expense of the other;
  • Figure 3 is a schematic diagram of apparatus according to the present invention.
  • an exemplary apparatus for producing constant-phase, phase-squeezed optical pulses has a temporally non-dispersive grating apparatus 2, constituting the pulse phase modifying means, coupled to a silica-based, single mode optical fibre 4.
  • a mode-locked Nd: YLF laser 5 produces approximately sech 2 constant phase optical pulses of 2.2 kw height 50ps width at a 76 MHz repetition rate.
  • the input pulses are incident on a first grating 6 of a pair of gratings 6,8 placed at the focal planes of a unit magnification confocal lens pair 10,12.
  • a spatially modulated mask 14 is inserted midway between the lenses 10 and 12.
  • These phase modified pulses are then coupled into the optical fibre 4 by means of a lens 16.
  • the optical fibre 4 exhibits an intensity dependent Kerr effect which causes self-phase modulation of the modified pulse as pulse propagates along it.
  • the phase structure of the modified pulse is such that the pulses emerging from the fibre 4 are phase-squeezed and have constant phase.
  • the mask pattern is readily determined by the methods described in the above referenced articles.

Landscapes

  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Optics & Photonics (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optical Communication System (AREA)

Abstract

Appareil destiné à traiter une impulsion optique de laser (5) et comprenant un filtre spatial (6, 10, 14, 12, 8) destiné à modifier la structure de phase de l'impulsion de telle manière que lorsqu'il est couplé par une lentille (16) dans une fibre optique de silice monomodale (4), la modulation autoadaptable produit une impulsion à phase régulée, de phase constante. Ledit appareil trouve une application en tant qu'oscillateur local dans un détecteur cohérent, par exemple.
PCT/GB1991/001468 1990-08-31 1991-08-30 Impulsion optique et procede et appareil de traitement de ladite impulsion WO1992004656A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
JP3515497A JPH06504381A (ja) 1990-08-31 1991-08-30 光パルスおよびそれを処理する方法および装置

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB909019011A GB9019011D0 (en) 1990-08-31 1990-08-31 Methods and apparatus for processing an optical pulse
GB9019011.7 1990-08-31

Publications (1)

Publication Number Publication Date
WO1992004656A1 true WO1992004656A1 (fr) 1992-03-19

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1991/001468 WO1992004656A1 (fr) 1990-08-31 1991-08-30 Impulsion optique et procede et appareil de traitement de ladite impulsion

Country Status (4)

Country Link
JP (1) JPH06504381A (fr)
AU (1) AU8527191A (fr)
GB (1) GB9019011D0 (fr)
WO (1) WO1992004656A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0737852A2 (fr) * 1995-04-13 1996-10-16 CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. Procédé et dispositif de mesure du coéfficient Kerr de non-linéarité dans un fibre optique monomode

Non-Patent Citations (6)

* Cited by examiner, † Cited by third party
Title
J.OPT.COC.AM.B vol. 5, no. 8, August 1988, OPTICAL SOCIETY OF AMERICA pages 1563 - 1572; A.M.WEINER ET AL: 'High-resolution femtosecond pulse shaping' cited in the application see abstract *
J.OPT.SOC.AM.B vol. 4, no. 10, October 1987, OPTICAL SOCIETY OF AMERICA pages 1476 - 1489; G.J.MILBURN ET AL.: 'Optical-fiber media for squeezed-state generation' see abstract *
J.OPT.SOC.AM.B vol. 4, no. 10, October 1987, OPTICAL SOCIETY OF AMERICA pages 1565 - 1573; P.D.DRUMMOND ET AL.: 'Quantum-field theory of squeezing in solitons' see abstract *
J.OPT.SOC.AM.B vol. 7, no. 3, March 1990, OPTICAL SOCIETY OF AMERICA pages 386 - 392; H.A.HAUS ET AL.: 'quantum theory of soliton sqeezing: a linearized approch' see abstract *
OPTICS LETTERS vol. 11, no. 3, March 1986, OPTICAL SOCIETY OF AMERICA pages 153 - 155; A.M.WEINER ET AL.: 'synthesis of phase-coherent,picosecond optical square pulses' cited in the application see abstract *
OPTICS LETTERS vol. 14, no. 7, 1 April 1989, OPTICAL SOCIETY OF AMERICA pages 373 - 375; P.D.DRUMMOND ET AL.: 'Time dependence of quantum fluctuations in solitons' see abstract *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0737852A2 (fr) * 1995-04-13 1996-10-16 CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. Procédé et dispositif de mesure du coéfficient Kerr de non-linéarité dans un fibre optique monomode
EP0737852A3 (fr) * 1995-04-13 1998-07-08 CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. Procédé et dispositif de mesure du coéfficient Kerr de non-linéarité dans un fibre optique monomode

Also Published As

Publication number Publication date
AU8527191A (en) 1992-03-30
JPH06504381A (ja) 1994-05-19
GB9019011D0 (en) 1990-10-17

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