WO1991018317A1 - Generation of harmonics from laser output - Google Patents

Generation of harmonics from laser output Download PDF

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Publication number
WO1991018317A1
WO1991018317A1 PCT/GB1991/000800 GB9100800W WO9118317A1 WO 1991018317 A1 WO1991018317 A1 WO 1991018317A1 GB 9100800 W GB9100800 W GB 9100800W WO 9118317 A1 WO9118317 A1 WO 9118317A1
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Prior art keywords
laser
angle
lens
linear device
laser beam
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Application number
PCT/GB1991/000800
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French (fr)
Inventor
Graham Alexander Naylor
Timothy George Michael Freegarde
Original Assignee
Oxford Lasers Limited
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Publication date
Application filed by Oxford Lasers Limited filed Critical Oxford Lasers Limited
Publication of WO1991018317A1 publication Critical patent/WO1991018317A1/en

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    • 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
    • G02F1/37—Non-linear optics for second-harmonic generation
    • 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

Definitions

  • Frequency doubling generates a beam of coherent radiation at twice the frequency (half the wavelength) of the original, fundamental laser wavelength.
  • Frequency doubling can be performed by passing the radiation through a non-linear medium, such as a non-isotropic, uniaxial or bi-axial crystal.
  • the radiation emanating from the non-linear medium can be made to be in phase with the original, incident laser beam and will then reinforce the harmonic radiation constructively to give an optimum conversion efficiency, if the speeds of propagation of the incident (fundamental) and output (harmonic) wavelengths are the same. This is called the condition of phase matching.
  • the present invention provides a system for use with a laser, which will substantially achieve the condition of phase matching for harmonic conversion of the fundamental laser beam, despite variability in the pointing direction of the output beam.
  • an optical system for producing harmonics from the output of a laser comprising a non-linear device, characterised in that the system includes optical means (22, L ⁇ , L 9 , L for receiving a laser beam (25, 31) and for presenting said beam (25, 31) to the non-linear device (21, 33) at an angle of incidence which substantially does not vary with variation of the receiving angle (A, B) of the laser beam (25, 31).
  • the present invention provides a laser system which includes such an optical system arranged to produce harmonics from the output of the laser.
  • Figure 1 shows a beam from a laser incident on a non-linear medium
  • Figures 2 and 3 are vector diagrams.
  • Figure 4 shows laser beams passing through a non-linear medium.
  • Figure 5 is a diagram of a laser system using the invention, and,
  • Figure 6 is a diagram of a laser system using an alternative version of the present invention.
  • a coherent light beam 10 from a laser has a polarisation which can be resolved into polarisation components referred to as ordinary and extraordinary.
  • the ordinary polarisation component lies perpendicular to the plane defined by the incident beam and the optic axis and the extraordinary polarisation component lies in the plane defined by the incident beam and the optic axis.
  • the beam 10 is the fundamental beam which is incident on a non ⁇ linear device 13 to produce an output harmonic beam 15.
  • Figure 2 shows a vector plot, n Q , of the amplitude of the refractive index seen by the ordinary ray, and the direction of the ray, in relation to the optic axis 16 of the non-linear medium 13, exiting from a non-linear device 13, for the full 360° of incidence of a ray incident on device 13.
  • Figure 2 also shows a vector plot, n , of the amplitude of the refractive index of the extraordinary ray and the direction of the ray, exiting from device 13, for the full 360° of incidence.
  • the amplitude, n Q , of the refractive index of the ordinary ray is unaffected by its disposition relative to the optic axis 16 and its vector plot is a circle.
  • the extraordinary ray encounters
  • SUBSTITUTE SHEET an index of refraction, n e , which varies according to its disposition relative to the optic axis 16 and, as a result, its vector plot is an ellipse.
  • Figure 3 shows the same vector plots for the fundamental beam (lw) but additionally shows a similar vector plot for the harmonic (2w), which comprises an ordinary ray 17 and extraordinary ray 18.
  • the loci for the harmonic are a circle for the ordinary ray 17 and an ellipse for the extraordinary ray 18.
  • the efficient generation of the harmonic is critically dependent on the quality of the primary beam.
  • the beam quality can be improved so as to be close to the diffraction limit using either an unstable cavity or an injection controlled oscillator, the quality of the output beam from the laser is ultimately limited by instability in the pointing direction of the output beam.
  • Such instability can arise due to air of varying temperature, and thus varying optical density, passing through the laser beam in a random, varying manner. This problem is particularly acute in the vicinity of hot objects, such as laser windows. This effect can produce a random wandering of the angle
  • FIG. 4 there is shown a non-linear crystal 20 with a beam which is incident on the crystal at an angle of incidence X at which constructive reinforcement occurs.
  • the angle of incidence can, however, vary randomly so that the beam can quite suddenly become incident on device 21 at any angle such as an angle X*.
  • the arrangement shown in Figure 5 is used to focus the output beam 25 from a laser 23 to a focal plane in non-linear device 21.
  • a convex lens 22 is positioned at a distance D from the laser 23, the lens having a focal length F and being positioned such that substantial focussing of the laser output will occur in the non-linear device 21.
  • the distance I) is arranged to be equal to focal length F and the beam emanating from laser 23 is caused to wander (as shown in broken lines, to be received at lens 22 at angle A) because of heat coming from laser window 24, the effect will be for the beam passing through device 21 to be displaced sideways within device 21 but the positioning of the lens will keep a constant angle of incidence of the beam to the device 21.
  • the focal length of the focussing lens and thus the cone angle of the beam incident on the device 21, are chosen so that the focus is not so tight as to cause damage to the crystal.
  • the cone angle can be made smaller so that a lens having a longer focal length is required.
  • An optical path length of over twice the focal length is required for frequency conversion. To double the frequency of high power lasers, such as copper-vapour
  • the focal length of the ' lens required becomes too long to fit into a practical system.
  • the beam may be telescoped down to reduce the cone angle when focussed into the non-linear device, but this generally increases the variation in beam angle.
  • the arrangement shown in Figure 6 can then be used as this allows a beam waist to be formed with a narrow cone angle whilst requiring a relatively modest amount of space.
  • beam waist angle variations are only cancelled for perturbations to the laser beam direction arising from a single plane, the solution is sufficiently insensitive to the source of the angle variation, that perturbations arising in the vicinity of several laser windows at different optical positions may, to all intents and purposes, be cancelled.
  • a laser 30 outputs a fundamental beam 31 which is focussed by a convex lens L-*_, positioned distance ⁇ from the laser, to a focus at a point distance £ from lens L ⁇ .
  • a concave or diverging lens _2 of focal length t ⁇ - i- 3 positioned a distance (c - a) from lens L-*_.
  • the beam will move sideways within the non-linear device but will maintain its angle of incidence and so the phase matching condition will be sustained.
  • the slight wandering of the beam is likely to be beneficial as it will reduce the local thermal loading on the non- linear device.
  • Lasers which have successfully been frequency doubled or tripled are those operating in the infrared or visible range so as to generate light in the visible or ultra-violet ranges respectively.
  • An example is the efficient conversion (about 50%) of light at 1.06 um from a
  • frequency doubling copper vapour lasers which have fundamental wavelengths of 510.6nm and 578.2nm to generate ultra-violet wavelengths of 255nm and 289nm.
  • the technique of the present invention can be used for frequency conversion in all these frequencies as they all require the phase- matching condition to be met.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Optics & Photonics (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
  • Lasers (AREA)

Abstract

A system for producing harmonics from a laser beam in which the beam is passed through a non-linear device such as a non-linear crystal. The harmonic ouptut from the crystal must be in phase with the original laser beam on exiting the crystal for the two constructively to reinforce one another. The angle at which the laser beam leaves the laser can change randomly due to such effects as local heating around the laser window and the present invention provides an optical system which ensures that the angle of incidence of the laser beam on the crystal does not alter even when the laser beam wanders due to this effect so that constructive reinforcement of the two beams is unaffected by the random movement of the laser beam.

Description

Generation of Harmonics from Laser Output
Many commercially available lasers produce light at one or more fixed wavelengths. The usefulness of such lasers can be increased by the conversion of these wavelengths to other wavelengths. One wavelength conversion technique, referred to as frequency doubling, generates a beam of coherent radiation at twice the frequency (half the wavelength) of the original, fundamental laser wavelength. Frequency doubling can be performed by passing the radiation through a non-linear medium, such as a non-isotropic, uniaxial or bi-axial crystal. The radiation emanating from the non-linear medium can be made to be in phase with the original, incident laser beam and will then reinforce the harmonic radiation constructively to give an optimum conversion efficiency, if the speeds of propagation of the incident (fundamental) and output (harmonic) wavelengths are the same. This is called the condition of phase matching.
However, non-linear materials inherently produce dispersion and birefringence in the light beam and the condition of equal fundamental and harmonic speeds can only be met under certain conditions. The present invention provides a system for use with a laser, which will substantially achieve the condition of phase matching for harmonic conversion of the fundamental laser beam, despite variability in the pointing direction of the output beam.
According to the present invention there is provided an optical system for producing harmonics from the output of a laser, the optical system comprising a non-linear device, characterised in that the system includes optical means (22, L^, L9, L for receiving a laser beam (25, 31) and for presenting said beam (25, 31) to the non-linear device (21, 33) at an angle of incidence which substantially does not vary with variation of the receiving angle (A, B) of the laser beam (25, 31).
SUBSTITUTE SHEET Preferably, the present invention provides a laser system which includes such an optical system arranged to produce harmonics from the output of the laser.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which.
Figure 1 shows a beam from a laser incident on a non-linear medium, Figures 2 and 3 are vector diagrams.
Figure 4 shows laser beams passing through a non-linear medium. Figure 5 is a diagram of a laser system using the invention, and, Figure 6 is a diagram of a laser system using an alternative version of the present invention.
Referring now to Figure 1, a coherent light beam 10 from a laser (not shown) has a polarisation which can be resolved into polarisation components referred to as ordinary and extraordinary. The ordinary polarisation component lies perpendicular to the plane defined by the incident beam and the optic axis and the extraordinary polarisation component lies in the plane defined by the incident beam and the optic axis. The beam 10 is the fundamental beam which is incident on a non¬ linear device 13 to produce an output harmonic beam 15. Figure 2 shows a vector plot, nQ, of the amplitude of the refractive index seen by the ordinary ray, and the direction of the ray, in relation to the optic axis 16 of the non-linear medium 13, exiting from a non-linear device 13, for the full 360° of incidence of a ray incident on device 13. Figure 2 also shows a vector plot, n , of the amplitude of the refractive index of the extraordinary ray and the direction of the ray, exiting from device 13, for the full 360° of incidence. The amplitude, nQ, of the refractive index of the ordinary ray is unaffected by its disposition relative to the optic axis 16 and its vector plot is a circle. However, the extraordinary ray encounters
SUBSTITUTE SHEET an index of refraction, ne, which varies according to its disposition relative to the optic axis 16 and, as a result, its vector plot is an ellipse.
Figure 3 shows the same vector plots for the fundamental beam (lw) but additionally shows a similar vector plot for the harmonic (2w), which comprises an ordinary ray 17 and extraordinary ray 18. The loci for the harmonic are a circle for the ordinary ray 17 and an ellipse for the extraordinary ray 18.
As seen in Figure 3 there are four positions in which the vector n of the extraordinary ray 12 of the fundamental beam has the same amplitude as the vector of the ordinary ray 17 of the second harmonic beam (where the two intersect) and this is the condition for constructive reinforcement between the beams. In fact, these four vectors define four points on a cone (shown in broken lines and out of the plane of the Figure) because the vectors can be rotated at the same angle to the optic axis Z to define cones. When the incident beam is oriented to one of these angles within the non-linear medium, the frequency conversion will be enhanced.
When using a non-linear medium to produce harmonics of light beams, the efficient generation of the harmonic is critically dependent on the quality of the primary beam. Although the beam quality can be improved so as to be close to the diffraction limit using either an unstable cavity or an injection controlled oscillator, the quality of the output beam from the laser is ultimately limited by instability in the pointing direction of the output beam. Such instability can arise due to air of varying temperature, and thus varying optical density, passing through the laser beam in a random, varying manner. This problem is particularly acute in the vicinity of hot objects, such as laser windows. This effect can produce a random wandering of the angle
SUBSTITUTE SHEET of incidence of the beam at tfie non-linear device and a consequent random loss of the preferred condition for constructive reinforcement. Referring to Figure 4, there is shown a non-linear crystal 20 with a beam which is incident on the crystal at an angle of incidence X at which constructive reinforcement occurs. The angle of incidence can, however, vary randomly so that the beam can quite suddenly become incident on device 21 at any angle such as an angle X*. The arrangement shown in Figure 5 is used to focus the output beam 25 from a laser 23 to a focal plane in non-linear device 21. A convex lens 22 is positioned at a distance D from the laser 23, the lens having a focal length F and being positioned such that substantial focussing of the laser output will occur in the non-linear device 21. When the distance I) is arranged to be equal to focal length F and the beam emanating from laser 23 is caused to wander (as shown in broken lines, to be received at lens 22 at angle A) because of heat coming from laser window 24, the effect will be for the beam passing through device 21 to be displaced sideways within device 21 but the positioning of the lens will keep a constant angle of incidence of the beam to the device 21. This will mean that, provided the laser has been positioned to project its beam towards the non-linear device 21 with the correct orientation, such sideways displacement of the beam will not affect the constructive reinforcement of the fundamental beam by the harmonic. The focal length of the focussing lens and thus the cone angle of the beam incident on the device 21, are chosen so that the focus is not so tight as to cause damage to the crystal. For high power lasers the cone angle can be made smaller so that a lens having a longer focal length is required. An optical path length of over twice the focal length is required for frequency conversion. To double the frequency of high power lasers, such as copper-vapour
SUBSTITUTE SHEET lasers, the focal length of the' lens required becomes too long to fit into a practical system. The beam may be telescoped down to reduce the cone angle when focussed into the non-linear device, but this generally increases the variation in beam angle. The arrangement shown in Figure 6 can then be used as this allows a beam waist to be formed with a narrow cone angle whilst requiring a relatively modest amount of space. Although beam waist angle variations are only cancelled for perturbations to the laser beam direction arising from a single plane, the solution is sufficiently insensitive to the source of the angle variation, that perturbations arising in the vicinity of several laser windows at different optical positions may, to all intents and purposes, be cancelled. In Figure 6, a laser 30 outputs a fundamental beam 31 which is focussed by a convex lens L-*_, positioned distance ά from the laser, to a focus at a point distance £ from lens L^. A concave or diverging lens _2 of focal length t^- i-3 positioned a distance (c - a) from lens L-*_. This produces an almost parallel beam 32 which is finally focussed by another convex lens L^, positioned a distance (b + a) from lens I*2> to form an incident beam at a non-linear device 33 positioned a distance e_ from lens ^.
If the beam emanating from laser 30 receives a small transient deflection through a small angle B to be received at lens L^ at angle B relative to the original beam, the sideways displacement of the beam at lens L^ will be D-^, at lens L it will be T>2» at distance &_ from lens L the virtual focus will be displaced by distance Do, at lens LA by distance DA and at the non-linear device 33 by distance Dc. The following then obtains, B1 = (d * B) = (c * B)
SUBSTITUTE SHEET
Figure imgf000008_0001
= ( (c - a) * B) + ( (a * d * B) / c)
D4 = D2 + ((b * D3) / a) + ((D2 * b) / a) * ((a - f2) / f2) = ((c - a) * B) + ((a * d * B) / c) + ((b * c * B) / a) + [((c*b) / a) - b + ((b*d) / c) * ((a-f2) / f2)] * B
D5 = ((e * D3) / a) = ((e * c * B) / a) The condition for no variation in the angle of the beam waist is:- D4 = D5 For the case when a = f2, this simplifies to, b = (e + (a2 / c) - a - (a2 * d) / c) An example solution for a practical system would be:- e = lm a = 0.18m d = 2m c = lm => b = 0.79m If there were a second source of perturbation of the beam, for example a second window, at a distance d = 4m, then the change in angle in the waist would be 0.4A, which is still quite small. An optimal solution would then be chosen for about d = 3m.
The beam will move sideways within the non-linear device but will maintain its angle of incidence and so the phase matching condition will be sustained. The slight wandering of the beam is likely to be beneficial as it will reduce the local thermal loading on the non- linear device.
Lasers which have successfully been frequency doubled or tripled are those operating in the infrared or visible range so as to generate light in the visible or ultra-violet ranges respectively. An example is the efficient conversion (about 50%) of light at 1.06 um from a
SUBSTITUTE SHEET neodyniumtyttrium /aluminium /garnet laser, to 532nm (green) and by tripling to 355nm. There is also interest in frequency doubling copper vapour lasers which have fundamental wavelengths of 510.6nm and 578.2nm to generate ultra-violet wavelengths of 255nm and 289nm. The technique of the present invention can be used for frequency conversion in all these frequencies as they all require the phase- matching condition to be met.
SUBSTITUTE SHEET

Claims

1 An optical system for producing harmonics from the output of a laser, the optical system comprising a non-linear device, characterised in that the system includes optical means (22, L^, L2, L4) for receiving a laser beam (25, 31) and for presenting said beam (25, 31) to the non-linear device (21, 33) at an angle of incidence which substantially does not vary with variation of the receiving angle (A, B) of the laser beam (25, 31).
2 An optical system as claimed in claim 1, characterised in that the optical means (22, L-^, L2, L^) are arranged to produce a focus of the beam (25, 31) in said non-linear device (21, 33).
3 An optical system as claimed in claim 2, characterised in that the optical means (22, L-*_, L , L4) are arranged such that variation of the receiving angle (A, B) of the laser beam causes transverse movement of the beam incident on said non-linear device (21, 33) but does not vary the angle of incidence.
4 An optical system as claimed in claim 2, characterised in that said optical means comprise a first lens (L-,) and a second lens (L2) arranged to produce a substantially parallel beam directed toward said non-linear device (21, 33).
5 An optical system as claimed in claim 4, characterised in that said first lens (L-^) is a convex lens and said second lens (L2) is a concave lens.
6 An optical system as claimed in claim 5, characterised in that said optical means includes a third lens (L^) arranged to focus said beam in the non-linear device (21, 33).
7 An optical system as claimed in claim 6, characterised in that said third lens (L4) is a convex lens.
SUBSTITUTE SHEET A laser system comprising a laser and an optical system arranged for producing harmonics from the output of a laser, the optical system comprising a non-linear device, characterised in that the system includes optical means (22, L^, L2, L^) for receiving a beam (25, 31) from said laser (23, 30) and for presenting said beam (25, 31) to the non-linear device (21, 33) at an angle of incidence which substantially does not vary with variation of the receiving angle (A, B) of the laser beam (25, 31). A laser system as claimed in claim 8, characterised in that the optical means (22, L-^, L2, L^) are arranged to produce a focus of the beam (25, 31) in said non-linear device (21, 33). A laser system as claimed in claim 9, characterised in that the optical means (22, L^, L2, L^) are arranged such that variation of the receiving angle (A, B) of the laser beam causes transverse movement of the beam incident on said non-linear device (21, 33) but does not vary the angle of incidence.
SUBSTITUTE SHEET
PCT/GB1991/000800 1990-05-23 1991-05-21 Generation of harmonics from laser output WO1991018317A1 (en)

Applications Claiming Priority (2)

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GB9011534.6 1990-05-23
GB909011534A GB9011534D0 (en) 1990-05-23 1990-05-23 Generation of harmonics from laser output

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9841655B2 (en) 2015-07-01 2017-12-12 Kla-Tencor Corporation Power scalable nonlinear optical wavelength converter

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4312588A (en) * 1979-01-20 1982-01-26 Canon Kabushiki Kaisha High-speed recording apparatus
JPS6356976A (en) * 1986-08-27 1988-03-11 Nec Corp Light converging equipment of wavelength conversion type pulse laser

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4312588A (en) * 1979-01-20 1982-01-26 Canon Kabushiki Kaisha High-speed recording apparatus
JPS6356976A (en) * 1986-08-27 1988-03-11 Nec Corp Light converging equipment of wavelength conversion type pulse laser

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
APPLIED PHYSICS LETTERS. vol. 55, no. 19, November 6, 1989, NEW YORK US pages 1943 - 1945; T. NISHIKAWA ET AL.: 'Angle tuning characteristics of second harmonic generation in KTiOPO4 ' *
PATENT ABSTRACTS OF JAPAN vol. 12, no. 277 (E-640)(3124) July 30, 1988 & JP-A-63 56 976 (NEC CORP. ) March 11, 1988 see the whole document *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9841655B2 (en) 2015-07-01 2017-12-12 Kla-Tencor Corporation Power scalable nonlinear optical wavelength converter

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