WO1993023900A1 - Fast optics for low optical damage - Google Patents

Fast optics for low optical damage Download PDF

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Publication number
WO1993023900A1
WO1993023900A1 PCT/US1993/004412 US9304412W WO9323900A1 WO 1993023900 A1 WO1993023900 A1 WO 1993023900A1 US 9304412 W US9304412 W US 9304412W WO 9323900 A1 WO9323900 A1 WO 9323900A1
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WO
WIPO (PCT)
Prior art keywords
lasing medium
pumping radiation
radiation
entrance surface
pumping
Prior art date
Application number
PCT/US1993/004412
Other languages
French (fr)
Inventor
Michael O. Hale
Original Assignee
Amoco Corporation
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 Amoco Corporation filed Critical Amoco Corporation
Publication of WO1993023900A1 publication Critical patent/WO1993023900A1/en

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Classifications

    • 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
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • 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
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1601—Solid materials characterised by an active (lasing) ion
    • H01S3/162—Solid materials characterised by an active (lasing) ion transition metal
    • H01S3/1625—Solid materials characterised by an active (lasing) ion transition metal titanium
    • 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
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/163—Solid materials characterised by a crystal matrix
    • H01S3/1631—Solid materials characterised by a crystal matrix aluminate
    • H01S3/1636—Al2O3 (Sapphire)

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

Methods and apparatus for directing and converging pumping radiation through a lasing medium as a beam of pumping radiation to shape the beam as a means of distributing the concentration of energy of the pumping radiation transmitted through the lasing medium in order to provide minimal optical damage to the pumping radiation input surface for the lasing medium while maintaining maximum laser gain.

Description

Fast Qptic-ς for Low Optical Damage
Field of the Invention
The present invention relates to optically-pumped laser sources and amplifiers, and more particularly to methods and apparatus for distributing pump radiation in a lasing medium to minimize the density of me pumping radiation needed to secure a predetermined gain from the lasing medium.
Background of the Invention
Many laser systems comprise a solid-state lasing medium that is pumped by a high energy pulsed optical pumping source.
Laser systems such as Nd.YAG-pumped Ti:sapphire and Nd.YAG- pumped Forsterite are two of many high energy pulsed pump source/solid-state lasing medium combinations. The current state of the art for coupling pump radiation into the solid-state lasing medium involves shaping the input beam with lenses to achieve the appropriate flux or fluence at the entrance surface of the lasing medium, which is assumed to be constant area through the lasing medium. Once the lasing medium has been pumped, the stored energy is extracted from the lasing medium as laser radiation. of predetermined wavelength or wavelengths.
Optical damage at the faces of the lasing medium may be caused by any of many disturbances that frequently occur during operation of a laser system. The most common of these disturbances are due to airborne particulates deposited on the pump radiation input surface of the lasing medium, excess energy in the pump radiation beam and plasma breakdown in the air above the pump radiation input surface of the lasing medium. In most cases the probability, of optical damage is approximately an exponential function of the difference in intensity of the pump radiation flux from the radiation flux damage threshold of the lasing medium. Therefore, it is desirable for the pump radiation flux to be as removed from the damage threshold flux level as possible. The only methods used in the current state of the art to control the problems of optical damage at the pump radiation input surface of the lasing medium have involved making the pump beam more spatially uniform at the pump radiation input surface of the lasing medium or by increasing the spot size of the pump radiation on the pump radiation input surface of the lasing medium. Creating a uniform beam requires a specially designed and fabricated pump source and is a difficult task to achieve in commercial systems, increasing the spot size of the pump radiation reduces the concentration of the distributed pump energy and thus reduces the gain of the lasing medium.
Summary of the Invention
Th* invention comprises methods and apparatus for directing and converging pumping radiation through a lasing medium as a beam of pumping radiation to shape the beam as a means of distributing the concentration of energy of the pumping radiation transmitted through the lasing medium in order to provide minimal optical damage to the pumping radiation input surface for the lasing medium while maintaining maximum laser gain.
In the preferred embodiment, the invention comprises a method of pumping a laser system that comprises an a lasing medium that is pumped by pumping radiation to increase the gain of said lasing medium with a reduced concentration of pumping radiation energy distributed within said lasing medium, comprising the stθDs of: propagating said pumping radiation toward an entrance surface of said lasing medium; and converging said propagated radiation to form a converging beam that distributes said directed pumping radiation over a predetermined area of said entrance surface as it passes through said entrance surface of said lasing medium to converge at a focal point of predetermined distance beyond said entrance surface to control the distribution of said pumpinα radiation energy within said lasing medium and to secure substantially uniform gain from said lasing medium as a function of penetration depth of said pump radiation in said lasing medium from said entrance surface.
In the preferred embodiment, the invention comprises apparatus for pumping a laser system that comprises a lasing medium that is pumped with pumping radiation to increase the gain of said lasing medium with a reduced concentration of pumping radiation energy distributed within said lasing medium, comprising: means for propagating said pumping radiation toward an entrance surface of said lasing medium; and means for converging said propagated radiation to form a converging beam that distributes said directed pumping radiation over a predetermined area of said entrance surface as it passes through said entrance surface of said lasing medium to converge at a focal point of predetermined distance beyond said entrance surface to contol the distribution of said pumping radiation energy within said lasing medium and to secure substantially uniform gain from said lasing medium as a function of penetration depth of said pump radiation in said lasing medium from said entrance surface.
Description of the Drawings
Figure 1 is a schematic diagram of a preferred embodiment of the invention.
Figure 2 is a graphical representation of the integrated gain along the axis of the lasing medium and as a function of radius out from the Gaussian peak.
Figure 3 is a graphical representation of the on axis integrated gain calculated as a function of focal length.
Figure 4 is a graphical representation of the cross sectional area of the pumping radiation as a function of focal length of the lens element.
Figure 5 is a graphical representation of the percentage area increase of the pumping radiation as a function of focal length of the lens element.
Figure 6 is a graphical representation of damage probability as a function of focal length of the lens element. Description of the Invention
Figure 1 is a schematic diagram of a preferred embodiment of the invention, wherein a laser system 2 comprises a lasing medium 4, typically a solid-state crystal, such as T sapphire or Forsterite. The laser 2 also comprises means for propagating pumping radiation, typically a source of pumping radiation 6, such as a Nd.ΥAG laser. At least a portion of the pumping radiation 6 is directed substantially along an optical path 8 that passes through the lasing medium 4.
The laser medium 4 has an entrance surface 10 that typically supports a dichroic optical coating 12 applied to it. The dichroic coating 12 is relatively transmissive for the pumping radiation and relatively reflective for wavelengths at which the laser system 2 is expected to lase. The laser system 2 also comprises an output coupler 14 positioned along the optical path 8 proximate an exit surface 16 of the lasing medium 4. The output coupler 14 has a surface 18 that supports an optical coating 20 that is at least partially reflective for wavelengths at which the laser system 2 is expected to lase.
An optical cavity 22 is thus formed between the dichroic coating 12 that is supported by the entrance surface 10 and the optical coating 20 on one of the surfaces of the output coupler 18. The spacing between the dichroic optical coating 16 and the reflective coating 22 may be adjusted to resonate radiation for any of the wavelengths at which the laser system is expected to lase.
Of course, the optical cavity 22 of the laser system 2 may comprise any other well-known optical cavity configuration to provide sufficient positive feedback to sustain oscillation. Also, the laser system 2 may alternatively comprise a laser amplifier, wherein the optical cavity 22 is not needed, the input radiation is coupled to the laser system 2 via an input wavelength division multiplexer (WDM) and the amplified output radiation is extracted with an output WDM, as well known in the art. A means for converging the pumping radiation that is propagated toward the entrance surface 10, typically a lens element 26 that has a predetermined focal length f, is positioned a predetermined distance L, from the entrance face of the lasing medium 4. The predetermined distance L is a fraction of the predetermined focal length f. The lasing medium 4 is a predetermined thickness dL. The pumping radiation that passes through the lens element 26 along the optical path 8 is converged onto the entrance face 10 of the lasing medium 4 at the predetermined distance L. As the converging pcmping radiation beam propagates through the crystal, the cross sectional area of the pumping radiation beam decreases, thereby increasing the flux and fiuence.
The pumping radiation flux, at any point between the lens and f is
f'z-f» - « r- (z 2. f)2 < >
where z is the distance from the lens along the optical path 8, E is energy of the pumping radiation, f is the focal length of the lens element 26 and r is the radius of the pumping radiation at the 26. This relationship indicates that for points along the optical path 8 within the lasing medium 4 that approach the focal point, the flux goes up rapidly due to the conical shape of the laser beam.
As the pumping radiation propagates through the crystal, the pump radiation is absorbed and produces an excited-state distribution of
N(z,f) - φ(z,f) exp(-α (z - L)) (2)
where α is the absorption coefficient of the laser crystal at the pump laser wavelength. As the flux increases along z so does the excited-state density.
In Figure 1 , the distribution of the pumping radiation energy that passes through the lasing medium 4 is represented by the region of the lasing medium 4 within dashed lines 28. For purposes of comparison, the distribution of collimated or nearly-coliimated pumping radiation that would be passed through the lasing medium 4 is represented by the region within the lasing medium 4 within dashed lines 30.
For the case of the the standard collimated or nearly- coliimated pumping radiation generally applied according to the prior art, the entrance surface 6 has a given flux and the excited- state density is distributed throughout the lasing medium 4 to provide a certain on-axis gain. According to the present invention, the flux of the pumping radiation at the entrance surface 10 is less than the '.oihrnated beam and the excited-state density near the entrance surface 10 is also less. However, as the pumping radiation propagates through the lasing medium 4 it also converges, so the excited-state density is greater than in the collimated pump beam case. When the gain on axis is integrated along the length of the lasing medium 4, the two pumping geometries can have the same gain while the geometry according to the invention has a lower pump flux. Thus, optical damage with the geometry according to the present invention is less than for the collimated case, while the gain on axis and the laser performance is the same.
The gain in the lasing medium 4 determines the performance of the laser system 2. The gain is the integrated population inversion times the simulated emission cross section of the lasing medium 4, or
L + dL g(r,f) - J σ N(z.f) dz (3)
L
where σ is the stimulated emission cross section of the lasing medium 4. For pumping radiation of substantially Gaussian cross sectional distribution, the gain is easily calculated and is shown in Figure 2.
Figure 2 is a graphical representation of the integrated gain along the axis of the lasing medium and as a function of radius out from the Gaussian peak for different focal lengths of the lens element 26. Lines 32, 34, 36 and 38 represent focal lengths of 100, 150, 300 and 600 mm, respectively. The lasing medium 4 is positioned at 2/3 of the focal length for each of the focal lengths represented in Figure 2 so that the spot size of the pumping radiation on the entrance surface 10 is a constant for each of the focal lengths.
in this example, the diameter of the pumping radiation is taken to be 1 mm, and the energy is assumed to be 1 in arbitrary units. An absorption coefficient of 0.24/mm and a length of 18 mm is used for the lasing medium 4, matching typical parameters for Tksapphire lasers. It is easily seen in Figure 2 that the integrated gain at a radius of zero is enhanced significantly by using a short focal length for the lens element 26. However, the gain at a large radius, such as 0.5 mm, is reduced. Normal operation of pulsed sources of pumping radiation will not be effected by a reduction in the gain of the outer areas of the lasing medium 4, since the laser system 2 operates primarily on the center portion of the gain region of the lasing medium 4 and the outer region has little or no effect on the operation of the lasing system 2.
Figure 3 is a graphical representation of the on axis gain, g(0,f), wherein line 40 represents integrated gain calculated as a function of focal length under the same conditions described above.
Figure 3 clearly illustrates the dramatic effect of using very short focal length lenses as the gain increases exponentially near zero.
To determine the improvement or reduction in optical damage problems, the relative cross sectional areas of the pumping radiation that propagates through the lasing medium 4 for constant gain should be determined. This can be solved setting the following constraints
g(0,f) - g(0,f) (4)
Figure 4 is a graphical representation of the cross sectional area of the pumping radiation as a function of focal length of the lens element 26 for three different values of g π r2/E, wherein lines 42, 44 and 46 represent values of 6, 15 and 50, respectively. These results show a clear increase in the cross sectional area of the pumping radiation at the shorter focal lengths.
This effect is made clearer when we plot the percentage area increase for the same data, as shown in Figure 5. Lines 48, 50 and 52 represent the values of 6, 15 and 50, respectively. The increase in area is approximately exponential as the focal length reduces. Focal lengths of 100 mm can easily produce area increases of 30 to 40 percent. Shorter focal lengths can produce even greater area increases.
The lasing medium 4 is damaged when a disturbance occurs that permanently changes the optical absorption characteristics of the lasing medium 4. An example of such a disturbance can be the settling of a dust spec on the entrance surface 10 followed by a pulse of pumping radiation that is large enough to vaporize or melt the dust spec. The effect is that the entrance surface 10 is permanently damaged and its absorption characteristics, and subsequently its lasing characteristics, are both altered permanently. Such disturbances that cause optical damage are one of the most serious problems with solid-state laser systems. The theoretical description of optical damage is very complicated. However, a simplified model may be considered for determination of the improvement in the optical damage probabilities. It may be assumed that the damage probability is some exponential function of the difference between the flux or fluence of the pumping radiation and the optical damage threshold flux or fluence.
Figure 6 is a graphical representation of damage probability as a function of focal length of the lens element 26 for a value of g π r2/E equal to 50. This data had a thirty two percent increase in spot siz? ' • a 100 mm focal length as compared to the 1000 mm focal iength. The relative damage probability for the short focal length is almost a factor of four times less than for the collimated or long focal length case. The collimated pumping radiation flux at the entrance surface 10 is 3 J/cm2 for the 1000 cm focal length and the optical damage threshold is assumed to be 10 J/cm2. These values correspond roughly to the values for Ti.sapphirβ crystals.
Thus there has been described herein methods and apparatus for directing and converging pumping radiation through a lasing medium as a beam of pumping radiation to shape the beam as a means of distributing the concentration of energy of the pumping radiation transmitted through the lasing medium in order to provide minimal optical damage to the pumping radiation input surface for the lasing medium while maintaining maximum laser gain, it will be understood that various changes in the details, materials, steps and arrangements of parts that have been described and illustrated above in order to explain the nature of the invention may be made by those of ordinary skill in the art within the principle and scope of the present invention as expressed in the appended claims.

Claims

What is claimed is:
1. A method of pumping a laser system that comprises an a lasing medium that is pumped by pumping radiation to increase the gain of said lasing medium with a reduced concentration of pumping radiation energy distributed within said lasing medium, comprising the steps of:
propagating said pumping radiation toward an entrance surface of said lasing medium; and
converging said propagated radiation to form a converging beam that distributes said directed pumping radiation over a predetermined area of said entrance surface as it passes through said entrance surface of said lasing medium to converge at a focal point of predetermined distance beyond said entrance surface to control the distribution of said pumping radiation energy within said lasing medium and to secure substantially uniform gain from said lasing medium as a function of penetration depth of said pump radiation in said lasing medium from said entrance surface.
2. Apparatus for pumping a laser system that comprises a lasing medium that is pumped with pumping radiation to increase the gain of said lasing medium with a reduced concentration of pumping radiation energy distributed within said lasing medium, comprising:
means for propagating said pumping radiation toward an entrance surface of said lasing medium; and
means for converging said propagated radiation to form a converging beam that distributes said directed pumping radiation over a predetermined area of said entrance surface as it passes through -aid entrance surface of said lasing medium to converge at a focal point of predetermined distance beyond said entrance surface to control the distribution of said pumping radiation energy within said lasing medium and to secure substantially uniform gain from said lasing medium as a function of penetration depth of said pump radiation in said lasing medium from said entrance surface.
PCT/US1993/004412 1992-05-08 1993-05-10 Fast optics for low optical damage WO1993023900A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US88049392A 1992-05-08 1992-05-08
US07/880,493 1992-05-08

Publications (1)

Publication Number Publication Date
WO1993023900A1 true WO1993023900A1 (en) 1993-11-25

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Country Status (1)

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Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
OPTICS LETTERS. vol. 13, no. 5, May 1988, NEW YORK US pages 380 - 382 C.H. MULLER III ET AL. '2.0 J Ti:sapphire laser oscillator' *

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