[go: up one dir, main page]

WO1998031081A1 - Laser a largeur de bande basse frequence - Google Patents

Laser a largeur de bande basse frequence Download PDF

Info

Publication number
WO1998031081A1
WO1998031081A1 PCT/GB1998/000094 GB9800094W WO9831081A1 WO 1998031081 A1 WO1998031081 A1 WO 1998031081A1 GB 9800094 W GB9800094 W GB 9800094W WO 9831081 A1 WO9831081 A1 WO 9831081A1
Authority
WO
WIPO (PCT)
Prior art keywords
grating
fibre
light
laser
pitch
Prior art date
Legal status (The legal status 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 status listed.)
Ceased
Application number
PCT/GB1998/000094
Other languages
English (en)
Inventor
Raymond John Chaney
Nigel Joseph Copner
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Renishaw PLC
Original Assignee
Renishaw PLC
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 Renishaw PLC filed Critical Renishaw PLC
Priority to EP98900588A priority Critical patent/EP0894350A1/fr
Publication of WO1998031081A1 publication Critical patent/WO1998031081A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • 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
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14—External cavity lasers
    • H01S5/146—External cavity lasers using a fiber as external cavity
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02171—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes
    • G02B6/02176—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes due to temperature fluctuations

Definitions

  • the present invention relates to a laser, such as a semiconductor laser diode (“laser”) having a narrow frequency bandwidth, and which may be used, for example, in interferometry to enable measurement of a displacement.
  • a laser such as a semiconductor laser diode (“laser”) having a narrow frequency bandwidth, and which may be used, for example, in interferometry to enable measurement of a displacement.
  • Laser diodes are known to emit laser light over a relatively wide frequency bandwidth. A consequence of this is that light emitted from the diode will not have the same phase, at least to the accuracy, and over distances which are required in interferometry.
  • the diode is thus said to have a short "coherence length".
  • the present invention relates to a technique whereby the coherence length of e.g. a laser diode may be increased to the extent that the diode may provide light which is useful for interferometric purposes.
  • a first aspect of the present invention provides for an increase in the coherence length of a laser by coupling to the laser a substantially monomode optical fibre which contains a grating in the form of a plurality of longitudinally spaced fibre regions having a refractive index which differs to that of the fibre core, the fibre regions being spaced apart by a distance corresponding to the desired wavelength of emission from the diode, and wherein the fibre is retained within a temperature controlled cavity.
  • the grating selectively reflects a relatively narrow frequency/ wavelength of the emitted light back into the laser, which will in turn result in the laser lasing about a frequency bandwidth centralised upon the frequency corresponding to the wavelength of the reflected light, thus achieving a reduction in the frequency bandwidth of the diode.
  • a fibre having regions of higher refractive index is manufactured by exposing the relevant regions of the fibre to incident ultraviolet light.
  • Fig 1 is a schematic view of a linear displacement laser interferometer employing a laser diode stabilised in accordance with the present invention
  • Fig 2 is an oscilloscope display
  • Fig 3 is a schematic representation of a method of creating a grating in a fibre
  • Figs 4A-D illustrate the manufacturing process shown in Fig 3 ;
  • Fig 5 is a further oscilloscope display.
  • a linear displacement laser interferometer includes a laser diode 10, which generates an output beam of linear polarised laser light.
  • the output beam is launched, via suitable optical coupling 11, into a substantially monomode optical fibre 12, having a plurality of microscopic regions 14 whose refractive indices differ to that of the remainder of the fibre (typically up to a difference in refractive indices of about 1 part in 10000) ; in the present example they have a higher refractive index.
  • the regions 14 are spaced apart by a distance corresponding to the desired output wavelength of the laser diode and are referred to collectively as a "grating" 16.
  • the fibre 12 is retained in a temperature controlled cavity 20, to prevent variations in the distance between the regions 14 of the grating.
  • the fibre 12 and grating 16 operate to stabilise the frequency of light emitted from the diode (which is itself temperature controlled) , and to reduce the frequency bandwidth over which light is predominantly emitted.
  • the diode emits a wavefront of light of the desired output wavelength
  • a proportion which depends upon the difference in refractive index between the main body of the fibre 12 and regions 14
  • the pitch of the grating is the same as the wavelength of the light of the wavefront.
  • Light from the emission end 18 of the fibre is collimated by means of a lens 22, and is subsequently incident upon a polarising beamsplitter cube 30, oriented at 45° to the direction of polarisation of the laser light.
  • a fraction of the incident beam is reflected at the beamsplitter 30 into a retroreflector 32, mounted in a stationary position with respect to the beamsplitter 30, and which in conjunction with the beamsplitter 30 forms a reference arm of an interferometer.
  • the fraction of the light beam which passes undiverted through beamsplitter 30 is incident upon a further retroreflector 34, mounted to an object whose linear displacement with respect to the beamsplitter it is desired to determine and which forms the measurement arm of the interferometer.
  • Light reflected at retroreflectors 32,34 is recombined at the beamsplitter 30, and, after passage through a k waveplate 50, interferes to generate an interference beam 40.
  • the interference beam 40 is incident upon a non-polarising beamsplitter cube 60, which splits interference beam 40 into two fractions, each of which are incident upon a photodetector 80 via polaroids 90, which select different phases (e.g. 0° and 90°), of interference beam 40.
  • Movement of the retroreflector 34 in the direction of the undiverted light beam will result in a change in the relative optical path lengths of the beams in the reference and measurement arms, which will result in a shift in the interference pattern and a consequent change in the intensity of light incident upon the photodetectors 80.
  • the resulting intensity modulation is transduced into a pair of phase-shifted correspondingly modulating signals at photodetectors 80, from which the displacement of the retroreflector 34 relative to a reference position may be determined.
  • the outputs from the photodetectors may be fed to X and Y inputs of an oscilloscope.
  • the trace on the oscilloscope is a circular Lissajous, with a well defined line (as shown in Fig 2) which may be used to generate a value representing the displacement of the movable retroreflector.
  • Coherent component beams of the interference beam 40 are generated, as described above, by means of the reflection at grating 16 of a selected frequency/wavelength of the laser light emitted from the diode.
  • the degree of frequency/wavelength selectivity, and the extent to which the grating 16 reflects the selected frequency/wavelength light determine the magnitude of the frequency bandwidth (i.e. coherence length) of light emitted.
  • the frequency/wavelength selectivity may be determined by the geometry of the grating 16, or alternatively by some external control of the operating characteristics of the laser, such as (in the case of a laser diode) the drive current and/or operating temperature.
  • the reflectivity is determined by the number of regions 14 of higher refractive index and/or the extent to which their refractive index differs with respect to the refractive index of the fibre core (" ⁇ n") .
  • the frequency/wavelength of the laser light and the geometry (e.g. pitch of the refractive regions) of the grating must be matched. This can be achieved either by adjusting the frequency/wavelength of the output beam, or the pitch of the grating 16 relative to the laser (as will be described later) . If a particular frequency/wavelength of light is required, then the grating pitch will need to be matched to the desired frequency/wavelength; if the precision of the emitted frequency/wavelength is not an issue, the laser output may be adjusted so that it matches the grating characteristics.
  • the following conditions must be satisfied: (i) the distance between the grating 16 and the rear reflective face of the laser must be a half integer multiple of the desired wavelength; and (ii) the distance between reflective faces of the laser must be a half integer multiple of the desired wavelength.
  • these conditions can be met by either adjusting the output frequency/wavelength of the laser, or the position of the grating 16 relative to the laser.
  • Fig 3 Techniques for exposing fibres to create a grating are known per se, and one such technique, illustrated in Fig 3 involves directing a beam of U.V. light through a phase grating to create an interference pattern in which the fibre 12 is positioned.
  • Exposure of a plurality of fibre regions 14 to a periodic intensity distribution of U.V. light will result in the light conductive core 100 of the fibre having a variation in refractive index substantially as illustrated in Fig 4B. Because the difference in refractive index between exposed and unexposed parts of the core 100 (i.e.
  • ⁇ n must be smaller than the difference in refractive indices of the core 100 and cladding 110, the value of ⁇ n is such that very little reflection will take place at an individual region 14. It is for this reason (among others) that a plurality of such regions are required in the form of a grating 16 i.e. to produce the requisite total reflectivity. It should also be noted that the susceptibility of the fibre core 100 to a change in refractive index as a result of the incidence of U.V. light is non-linear. That is to say that a completely unexposed fibre will initially undergo a relatively rapid rate of change of refractive index upon exposure to U.V. , but that the rate of change of refractive index with incident U.V. intensity will subsequently be lower.
  • a grating which has been created within a fibre may be erased, substantially or in part, by exposing the entire grating region to U.V. light.
  • the previously unexposed regions of the fibre will undergo a more rapid increase in refractive index than those which have previously been exposed, thereby enabling the entire region of the grating to be "washed out” to a constant refractive index after a given exposure time.
  • This phenomenon is illustrated in Fig 4C, whereby a short exposure of U.V.
  • a further grating may be created in the region of the previously washed out grating. This is the case even though the new grating lies within a region of the fibre core 100 which has a higher fundamental refractive index than that of the rest of the fibre core 100.
  • a newly created grating, located on the site of a washed out grating is illustrated in Fig 4D.
  • a grating 16 within an optical fibre to provide a stable coherent laser source: a matching of the output wavelength with the grating position and pitch; and the degree of reflectivity of a appropriately matched grating.
  • the characteristics of the laser and the grating geometry/position will first be matched. This is done by creating a very weak grating within the fibre at what is calculated to be an appropriate location, and investigating the coherence of the output laser source as a result.
  • this may be performed by washing out the existing grating, and creating a further grating with a different pitch (the pitch of the gratings being changed as a result of a corresponding alteration in the mutual angles of incidence of the interfering U.V. beams) .
  • the pitch of the gratings being changed as a result of a corresponding alteration in the mutual angles of incidence of the interfering U.V. beams
  • Matching of laser wavelength and grating pitch is performed by adjusting either the grating pitch (as explained in relation to Figs 4C and 4D above) , and/or the drive current and/or operating temperature of the laser diode.
  • adjustments may then be made (either to increase or decrease) to the reflective property of the grating 16 by changing the extent of exposure of the regions 14 of the grating 16 to U.V. light as described above.
  • the process of adjusting the reflectivity of the grating is repeated until the requisite coherence length has been attained.
  • the coherence length may be determined by means of an oscilloscope.
  • the trace on an oscilloscope due to microscopic movement of the retroreflector 34 can be seen in Fig 4.
  • This trace represents a fraction of a circular arc. It can be seen that the line defining the segment of the arc is relatively thick and ill-defined, representing a short coherence length for the laser diode 10 (by contrast to the "ideal" trace shown in Fig 2) .
  • the fibre 12 is thus repeatedly exposed to the U.V. interference pattern until the oscilloscope trace becomes sufficiently well-defined; representing an acceptable signal as a result of the incidence of an interference beam formed from coherent component beams .
  • the present has been exemplified by using regions of differing refractive index to cause reflection in the fibre. Any one or more fibre regions which have a reflective effect on light transmitted along the fibre should be regarded as lying within the scope of the invention, irrespective of their refractive index or indices.

Landscapes

  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Optical Transform (AREA)

Abstract

L'invention concerne une source de lumière laser produisant une lumière hautement cohérente en bande étroite, et qui injecte une lumière de diode laser (10) dans une fibre optique (12) monomode comprenant un réseau de Bragg (16), constitué de zones de fibres (14) à indice de réfraction supérieur. On enferme toute la longueur de la fibre, à partir du laser jusqu'à et y compris le réseau de Bragg (16), à l'intérieur d'une cavité (20) à température contrôlée, de façon à prévenir toute altération du pas du réseau ou l'écart réseau/laser imputable à la dilatation thermique. On créé le réseau par exposition de la fibre à une configuration périodique d'ultraviolets, laquelle est, à son tour, créé par passage d'un rayonnement ultraviolet dans un réseau de phase. Au cours de la fabrication, on adapte la longueur d'onde de sortie laser, ainsi que le pas du réseau et l'écart réseau/laser, puis on accorde l'étendue de la réflectivité du réseau (16) en fonction de la cohérence de la lumière émise dans le réseau (16).
PCT/GB1998/000094 1997-01-10 1998-01-12 Laser a largeur de bande basse frequence Ceased WO1998031081A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
EP98900588A EP0894350A1 (fr) 1997-01-10 1998-01-12 Laser a largeur de bande basse frequence

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9700417.0 1997-01-10
GB9700417A GB9700417D0 (en) 1997-01-10 1997-01-10 Low frequency bandwidth laser diode

Publications (1)

Publication Number Publication Date
WO1998031081A1 true WO1998031081A1 (fr) 1998-07-16

Family

ID=10805772

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1998/000094 Ceased WO1998031081A1 (fr) 1997-01-10 1998-01-12 Laser a largeur de bande basse frequence

Country Status (3)

Country Link
EP (1) EP0894350A1 (fr)
GB (1) GB9700417D0 (fr)
WO (1) WO1998031081A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001024326A1 (fr) * 1999-09-29 2001-04-05 Optical Technologies Italia S.P.A. Procede de fabrication de laser a fibre
WO2005085748A1 (fr) * 2004-03-04 2005-09-15 Renishaw Plc Tete de lecture optique

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1988005614A1 (fr) * 1987-01-20 1988-07-28 Hewlett-Packard Company Laser a cavite fibreuse externe syntonisable par bande large
WO1994017448A2 (fr) * 1993-01-29 1994-08-04 British Telecommunications Public Limited Company Emballage pour dispositif optique
WO1996000997A1 (fr) * 1994-06-28 1996-01-11 Sdl, Inc. Diode laser stabilisee par un reseau sur une fibre

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1988005614A1 (fr) * 1987-01-20 1988-07-28 Hewlett-Packard Company Laser a cavite fibreuse externe syntonisable par bande large
WO1994017448A2 (fr) * 1993-01-29 1994-08-04 British Telecommunications Public Limited Company Emballage pour dispositif optique
WO1996000997A1 (fr) * 1994-06-28 1996-01-11 Sdl, Inc. Diode laser stabilisee par un reseau sur une fibre

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
CHRISTENSEN D A ET AL: "LASER DIODE COHERENCE LENGTH VARIATION FOR BALANCING FIBER OPTIC INTERFEROMETERS", OPTICAL ENGINEERING, vol. 33, no. 6, 1 June 1994 (1994-06-01), pages 2034 - 2038, XP000454692 *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001024326A1 (fr) * 1999-09-29 2001-04-05 Optical Technologies Italia S.P.A. Procede de fabrication de laser a fibre
WO2005085748A1 (fr) * 2004-03-04 2005-09-15 Renishaw Plc Tete de lecture optique

Also Published As

Publication number Publication date
GB9700417D0 (en) 1997-02-26
EP0894350A1 (fr) 1999-02-03

Similar Documents

Publication Publication Date Title
US8711367B2 (en) Position-measuring device
US6724485B1 (en) Interferometric measuring device for determining the profile or the pitch of especially rough surfaces
KR100951618B1 (ko) 광주파수 발생기를 이용한 절대거리 측정방법 및 시스템
JP5759692B2 (ja) 温度測定装置及び温度測定方法
CN100439859C (zh) 利用光纤光栅的光纤干涉型在线微位移测量系统
EP1906137A1 (fr) Procédé et dispositif pour générer une longueur d'onde synthétique
US20160223317A1 (en) Displacement detection apparatus
US7046368B2 (en) Position measuring arrangement
JP4871791B2 (ja) 光コヒーレンストモグラフィ装置
JP3513432B2 (ja) 光周波数領域反射測定装置および光周波数領域反射測定方法
US20130088722A1 (en) Measurement apparatus
CN110864622A (zh) 一种偏分式双波长调频连续波激光干涉仪
JP7578769B2 (ja) 変位検出装置
WO1998031081A1 (fr) Laser a largeur de bande basse frequence
JP7547539B2 (ja) 変位検出装置
JP4027352B2 (ja) 光ファイバプローブ装置
KR101792632B1 (ko) 측정 불가 구간과 방향 모호성이 없는 절대거리 측정을 위한 분광형 간섭계 시스템
RU2141621C1 (ru) Интерферометрическое устройство для измерения физических параметров прозрачных слоев (варианты)
CN111896123A (zh) 波分式多波长调频连续波激光干涉仪及其测量方法
JP2002131139A (ja) 光ファイバセンサ並びに光ファイバの調節方法及び装置
US20240183650A1 (en) Optical measurement device
Heininger Longitudinal cavity mode referenced spline tuning for widely tunable MG-Y branch semiconductor laser
Seta et al. Interferometric absolute distance measurement utilizing a mode-jump region of a laser diode
JP4604879B2 (ja) 波長モニタ
Boukar et al. Investigation of Laser Diode Coherence and Fringes Visibility

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): JP US

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): AT BE CH DE DK ES FI FR GB GR IE IT LU MC NL PT SE

WWE Wipo information: entry into national phase

Ref document number: 1998900588

Country of ref document: EP

WWE Wipo information: entry into national phase

Ref document number: 09142466

Country of ref document: US

121 Ep: the epo has been informed by wipo that ep was designated in this application
WWP Wipo information: published in national office

Ref document number: 1998900588

Country of ref document: EP

WWW Wipo information: withdrawn in national office

Ref document number: 1998900588

Country of ref document: EP