AU705771B2 - Eyesafe optical parametric system pumped by solid state lasers - Google Patents

Description

WO 97/05677 I PCT/AU96/00477 EYESAFE OPTICAL PARAMETRIC

SYSTEM

PUMPED BY SOLID STATE

LASERS

Technical Field This invention relates to the generation of an eyesafe laser beam with wavelength conversion in an optical parametric system.

Background Art A system of this kind is disclosed in United States Patent 5181211 to Burnham et al, which works from the reported understanding that laser beams having a wavelength in the range of 1.5um to 2.2um are completely absorbed by the vitreous humor of the eye cornea, thereby avoiding any damage to the retina. The patent proposes the combination of a solid state laser, such as a diode-pumped neodymium laser, with a non-linear crystal parametric converter in an associated resonant optical cavity. In order to ensure the highest efficiency of operation, the patent requires non-critical phase matching in that the incident beam from the laser is transmitted parallel to one of the principal axes of the converter crystal. The output beam is said to be polarised in a direction parallel to a second of the principal axes and the idler beam is polarised in a direction parallel to the third principal axis. A preferred non-linear crystal is potassium titanyl phosphate

(KTP).

The present invention stems from an appreciation that, contrary to United States Patent 5181211, beam propagation parallel to a principal axis is not necessary and that beam propagation at an angle with t10' with respect to a principal axis can achieve parametric oscillation with no significant degradation of efficiency. Beam propagation parallel to a principal axis is also very difficult to align. Moreover, if the beam propagation is specifically off-axis, the output wavelength of the parametric converter can be tuneable according to the offset angle. This offers much greater flexibility than the type proposed in United States Patent 5181211.

Disclosure of the Invention The invention accordingly provides a solid state laser beam generating system comprising, in combination: WO 97/05677 PCT/AU96/00477 -2solid state laser means for producing a first laser beam having a wavelength range outside the eyesafe range 1.5,um to 1.

8 ,um; and converter means arranged to respond to the first beam by outputting a second beam having a wavelength within said eyesafe range; wherein said converter means includes an element of non-linear structure having three defined principal axes and said system is configured such that, in operation, said first beam travels in a direction offset from one of said principal axes by an angle greater than 2°.

Preferably, the solid state laser means is a diode-pumped neodymium laser and said offset angle is in the range 2° to 100, said offset angle is in the range of 20 to 100, depending on the output wavelength required. Preferred neodymium lasers are Nd:YAG and Nd:YVO 4 lasers operating respectively at 1.0642um and 1.0643um. Other lasers which may be utilised include an Nd:YAP laser operating at 1 0 7 2 6 /m or 1.0795/m or an Nd:YLF laser operating at 1.047/um or 1.053um.

The converter means is preferably an optical parametric system such as an optical parametric oscillator (OPO), optical parametric generator (OPG) or an optical parametric amplifier (OPA). The preferred non-linear element is a non-linear crystal and the preferred such crystal is potassium titanyl phosphate (KTP). The converter means can be either within the pump laser cavity (intracavity) or external to the pump laser cavity (extracavity) Brief Description of the Drawings The drawing is a schematic diagram of an eyesafe solid state laser beam generating system according to the preferred embodiment of the invention.

Best Modes for Carrying Out the Invention As shown in the drawing, the eyesafe laser beam generating system comprises a pump laser 1 such as a Nd:YAG laser pumped by diode array, which produces an output laser beam 2 having a wavelength of 1.0642,um. The beam 2 passes through energy condensing optics 3 of known type before passing to an optical parametric system 4.

The optical parametric system comprises mirrors 5,6 and a non-linear crystal 7 such as a KTP crystal.

SUBSTITUTE SHEET (RULE 26) WO 97/05677 PCT/AU96/00477 -3- The input beam 2 travels through the crystal at an offset angle of between 20 and 100 to a principal axis to provide an output in the eyesafe wavelength range of 1.5um to 1.8man. The output wavelength can be tuned by adjusting the offset angle.

Another embodiment of the present invention may be as illustrated in Figure 3 of United States Patent 5181211 (the disclosure of which is incorporated herein by reference and a copy of which is annexure 1 in this specification), except that the non-linear KTP crystal in the optical parametric oscillator is oriented to achieve the offset relationship with respect to the first laser beam from the solid state laser means in accordance with the present invention.

Depending on the application and the available input wavelength and required output wavelength, it may be desirable to use either X-cut or Y-cut crystals for the non-linear conversion. The tuneable output wavelength of the parametric converter with off-axis beam propagation can be achieved for both X and Y crystals, allowing for both types of crystals to be applied to produce tuneable eyesafe output wavelengths, with conversion efficiencies very close to the conversion efficiencies obtained for parallel alignment.

The tuneable output wavelength of the parametric converter with off axis beam propagation can be achieved for X and Y cut crystals, although the X-cut crystal (beam propagation along X-axis) results in high non-linear coefficient than the Y-cut crystal (beam propagation along Y-axis).

Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The described arrangement has been advanced merely by way of explanation and many modifications may be made thereto without departing from the spirit and scope of the invention, which includes every novel feature and combination of novel features herein disclosed.

SUBSTITUTE SHEET (RULE 26) WO 97/05677 WO 9705677PCT/AU96/00477 Annexure 1 United States Patent Application No. 5,181,211 WO 97/05677 PCT/AU96/00477 Ill~li1111 I 111USO05IS121 IA United States Patent (1 Burnhamn, et al.

[11) Patent Number: (45] Date of Patent: 5,181,211 Jan. 19, 1993 [54] EYE-SAFE LASER SYSTEM [751 Inventors: Ralph L Burnham, Reston; Jeffrey J.

Kasiaskif, Fairfax; Larry R. Marshall, Reston, all of Va.' (73] Assignee: Fibertck, Inc., Herndon. Va.

[21] AppI. No.: 702,531 [22] Filed: May 20, 1991 [51] lInt. C1.5 HOIS 3/10 (52] U.S. 372/21; 372/106; 372/20; 372/93: 372/23 [58] Field of Search 372/3, 98, 99, 20.,21, 372/23 [56] References Cited U.S. PATENT DOCUMENTS 5,065.046 1!/1991 37221 OTHER PUBLICATIONS Burnham, Stolzenbergcr, Pinto; "Infrared Optical Parametric Oscillator in Potassium Titanyl Phosphate"; Jan.

1989; IEEE Photonics Technology Letters vol. 1, No.

1.

Primary Examiner-U on Scott, Jr.

(573

ABSTRACT

An eye-safe laser operating at high efficiency, pulse repeftiion rate and power output is described. The laser comprises a diode-array pumped laser having a pumping Wavelength range which produces a polarized output beam. The output beam passes through. a nonlinear tunable parametric converter crystal having X, Y, and Z principal axes. Noncritical phase matching is produced in said laser by phase matching for a beam propagation parallel to a principal axis which results in a high efficiency of laser operation. The nonlinear tunable parametric converter crystal converts the wavelength of an otherwise unsafe laser beam output to one that is harmlessly absorbed by the human eye.

4 Claims, I Drawing Sheet 3,668,420 6/1972 3,949,323 4/1976 4,231.838 11/1980 4.272,733 6/1981 4.761,202 8/1998 4,93.166 8/1990 5.025.446 6/1991 5,053.641 10/1991 372/3 Bierlein et 332/7.51 Gier 156/600 Walling et, al. 372/41 Bordui et .1 156/621 372/92 372/21 Cheng et al 372/21

SIGNAL

IDLER

PUMP LASER SUBSTITUTE SHEET (RULE 26) WO 97/05677 PCT/AU96/00477 -6- U.S. Patent Jani. 19, 1993 5,181,211 -C

DIODE

LASER

RD POLARIZERa aROD FIGURE 1

SIGNAL

IDLER

FIGURE 2 r -LASER CAVITY 20- ]D'IODES OPO CAVITY1 FIGURE 3 SUBSTITUTE SHEET (RULE 26) WO 97/05677 PCT/AU96/00477 -7- 5,181,211 1I EYE-SAFE LASER SYSTEM BACKGROUND OF THE INVENTION The present invention generally relates to laser sys- 5 tems and more particularly, is concerned with a laser system that generates a beam that is safe to the human eye.

The use of lasers in recent years has been continually 0 increasing. The greater the power of lasers, the more risk there is to the people who may come into contact with the system.

Specifically, when a collimated beam of visible light enters the eye cornea, it passes through or is otherwise 1 absorbed by the vitreous humor. The portion of the beam that is not absorbed is focused by the eye lens onto the retina. Under normal conditions, the light energy is converted by the retina into chemical energy, stimulating optical sensations. Eye injury results because the 20 focused high energy laser beam cannot be absorbed and causes damage to the retina. This damage does not occur when conventional sources of illumination are exposed to the eye because the light is emitted in all directions and produce a sizeable (rather than focused) 25 image on the retina that can be safely absorbed. It has been determined in the industry that laser beams having a wavelength in the range of 1.5 um to 2.2 umrn is completely absorbed by the vitreous humor thereby alleviating any damage to the retina. 30 Laser system used as optical radar and communicalion transmitters in populated locations need to be operated so as to avoid eye damage.

Up to the present, eye-safe lasers generally had low efficiency. Two of the predominant eye-safe lasers are 35 based on laser emission in erbium-doped solid-state host materials pumped by pulsed gas discharge lamps or laser diodes, or on frequency conversion of a ncodymium laser using stimulated Raman scattering in a molecular gas such as methane. These devices have many 40 shortcomings. The erbium lasers have low efficiency (typically less than owing to the low stimulated emission coefficient of the laser transition in erb um 3 ions at a 1.54 um output and to the low efficiency for 4 optical pumping with the visible flash lamp. Further, the erbium laser can only be operated in a pulsed mode.

Stimulated Raman conversion requires a cell containing a high pressure flammable gas. This gas is excited by a neodymium pump laser to emit stimulated radiation in s the eye-safe region. The Raman conversion therefore is 0 not amenable to continuous wave operation. In addi- a tion, since the Raman process deposits energy in the a conversion medium, causing thermal distortions, the iw eye-safe Raman laser cannot be conveniently operated 55 at high average power or repetition rate.

Consequently, a need -exists for the availability of a Z laser system operating in an eye-safe frequency with f acceptable efficiency, a high pulse repetition rate and T high average output power. 60 t* SUMMARY OF THE INVENTION 01 The present invention provides an eye-safe laser op- p crating at high efficiency, pulse repetition rate and ra power output without having the deficiencies of the 65 w aforementioned prior art systems. The inventive system ca employes, as a key element, a tunable parametric con- di verier to convert the wavelength of an otherwise unsafe an laser beam output to one that is harmlessly absorbed by the human eye.

The preferred embodiment of the laser system utiliz.

ing the present invention includes a diode-array pumped Nd:YAG, or Nd:YLF solid state laser to provide a polarized output beam. The tunable parametric convertor is a nonlinear crystal having defined principal X, Y and Z axes. The preferred crstal must be capable of being incorporated both internal and external to the laser cavities to provide a means to convert the wavelength of the laser sources. Such non-linear crystal can take the form of either an optical parametric oscillator (OPO) or parametric amplifier (OPA). OPOs and OPAs generate two waves of longer wavelength than the input wave. Their output can be tuned over a wide range of wavelengths from a single input source at a fixed wavelength.

For efficient energy transfer between the input and output waves, it is necessary that the waves remain in phase. In order for this to occur, the refraction index of the crystal for the input wave must be the same as for the output wave. However, almost all materials have normal dispersion in the optical region. Therefore, the output waves will generally lag behind the input wave providing for less efficient energy transfer. This means that the output intensity will be much less than the input intensity.

Crystals that can offset the undesired dispersion have the property known as birefringence. Such property means that the refractive index of a non-linear crystal for a given input wave depends on: a. The frequency (or reciprocally, wavelength) of the wave; b. The direction of propagation of the wave, relative to the axes of the crystal; c. The polarization of the wave; d. Properties of the crystal itself, including temperature.

It is therefore possible, given an input wave of a given wavelength and polarization, to change the direction of propagation through the crystal by rotating the crystal so that the input wave is in phase with an output wave of particular wavelength and lolarization. This technique is called "phase matching". Phase matching helps ensure high conversion efficiency.

Each crystal has defined X. Y atd Z axes, which arc known as the principal axes. Phase matching is achieved >y selecting the direction of beam propagation with espect to the principal axes such that all three waves pump, signal and idler) remain in phase as they travel hrough the crystal. The phase matching angle is generlly unique in that phase matching can only be achieved n that direction for particular input and generated iavelengths and polarizations.

If phase matching is achieved in a direction that is not arallel to one of the principal crystalline axes Y, or then the input and generated waves will "walk off' rom one another as they propagate through the crystal.

his is caused by "double refraction" in the crystal, and he angle at which the input and generated wave walk ff is known as the "walkoff angle". Also, if the beams o not propagate parallel to one of the principal axes, hase matching only occurs over an extremely narrow nge of angles such that small divergence of the input ave will prevent phase matching. This leads to practi- I difficulties. Phase-matching along a propagation rection not parallel to a principal crystal axis Y, id Z) is termed critical phase matching.

SUBSTITUTE SHEET (RULE 26) WO 97/05677 -8- PCT/AU96/00477 5,181,211 Noncritical phase matching occurs when one may phase match for beam propagation parallel to a principal crystal axis. In this case, small deviations from the phase matching angle do not have such large effects as in critical phase matching, nor is there any walkoff 5 angle between the beams. Non critical phase matching results in the highest efficiency of operation.

An OPO can be tuned, given an input wave of particular wavelength, by rotating the crystal and phase matching with a output wave of a different wavelength. 10 An OPO will be very efficient if the crystal is aligned such that noncritical phase matching occurs.

The preferred crystal for use in the present invention is Potassium Titanyl Phosphate (KTP). The KTP crystal has a high linear coefficient, a high linear coefficient, 15 a high damage threshold and a large angular and temperature acceptance range.

BRIEF DESCRIPTION OF DRAWINGS In the accompanying drawings: 2 0 FIG. 1 illustrates an embodiment of the invention utilizing an optical parametric oscillator, pumped by a solid-state laser.

FIG. 2 is a schematic illustrating the conversion of beam wavelength utilizing the embodiment shown in 25 FIG. 1.

FIG. 3 illustrates an alternative embodiment of the invention in which the crystal is located within the optical cavity of the pump laser.

DETAILED EXPLANATION OF THE

INVENTION

Referring now to the drawings, and more particularly FIG. 1, there is shown an eye safe laser system, 35 generally designated 10, which incorporates the preferred embodiment of the present invention. The laser system 10 includes a pump laser, generally designated and an optical parametric oscillator, generally designated 30. Pump laser 20 consists of an Nd Yag laser 40 crystal being pumped by a solid-state diode array. Such pump lasers are well known and readily available.

Other laser crystals, such as Nd YLF may be utilized. It should be understood that the pump laser 20 consists of known elements familiar to those versed in the use of 45 solid state lasers. The pump laser could be any other source generating single transverse mode (TEMoo) output in the wavelength range 1.0 um to 1.1 umrn.

Still referring to FIG. 1, the optical parametric oscillator or OPO 30 is shown to include mirrors 32 and 36 50 and wavelength converter 34. Mirror 32 is coated for high transmission of beam wavelengths between 1.0 um and 1.1 um and high reflection of wavelengths between um and 2.2 um.

Mirror 36 is coated to have a high reflectance for 55 wavelengths between 1.0 um and 1.1 um and partially reflective to wavelengths between 1.5 um and 2.2 um. It is preferable to coat mirror 36 so that between 50% aind of signals having wavelengths between 1.5 um and 2.2 um are reflected. It is not essential that mirror 36 60 reflect wavelengths between 1.0 um and 1.1 um, but such reflection does lower the input energy required to operate the device.

The positioning of mirror 32, KTP crystal 34 and mirror 36 as explained above results in a resonant opti- 65 cal cavity wherein the unconverted pump laser beam is reflected back into converter 34 from output mirror 36 for further processing.

In the preferred embodiment converter 34 is a crystal having a high non-linear coefficient, a high damage threshold and a large angular and temperature acceptance range. Specifically, crystal 34 is Potassium Titanyl Phosphate (KTiOPO4 or "KTP" for short) and has defined principal axes and defined by convention familiar to those versed in the physics of crystals. Hereinafter convener 34 will be referred to as crystal 34 for case of explanation.

Crystal 34 is cut into a convenient shape for mounting, and in the preferred embodiment shown in FIG. I is a rectangular prism. The cut is made so that the principal axes of the crystal are perpendicular to the faces of the rectangular prism, so that the incident beam propagates along a principal axis when it strikes perpendicular to the face of the rectangular crystal 34. The input wave is aligned with the appropriate face. Phase matching is thus achieved so as to convert the 1.06 um pump beam propagating parallel to the X-axis to 1.6 um wavelength or 1.54 um wavelength for the same pump beam propagating parallel to the Y-axis.

The operation of laser system 10 can be explained by referring to FIG. 2. Pump laser 20 generates pump beam 101 having a wavelength between 1.0 um and 1.1 um. Pump beam 101, polarized parallel to the axis of crystal 34 propagates through lens 40 and mirror 32 into crystal 34. The orientation of crystal 34 is such that pump beam 101 travels parallel to the axis. The effect of crystal 34 on pump beam 101 results in two output beams, one having a wavelength between um-2.2 um referred to in FIG. 2 as signal 102 and the other having a wavelength between 3.0 um-3.2 um referred to as idler 103. Any pump beam that is not converted during the initial pass through crystal 34 is reflected back into the OPO for further processing.

Signal 102 continues in the direction polarized parallel to the axis of crystal 34 while idler 103 is polarized in a direction parallel to the axis of crystal 34. With crystal 34 oriented as described above and shown in FIG. 1 and 2, non-critical phase matching results. In this orientation, the highest conversion efficiency in an eyesafe wavelength is obtained. The crystal may be made longer to further increase efficiency, especially when lower power input beams are utilized.

Resulting idler wave having a 3.2 um wavelength is allowed to pass out of the oscillator without reflection.

An alternate embodiment interchanges the roles of the signal and idler beams; the idler can be resonated in the oscillator cavity, allowing the signal to pass out of oscillator without reflection.

It is to be understood that the surfaces of mirrors 32 and 36 may be flat, concave or convex.

Mode matching is accomplished by providing lens between pump laser 20 and OPO 30. Lens 40 focuses the pump laser beam into OPO 30 so that mode matching is accomplished. Mode matching is a known procedure wherein the size of the pumped laser beam spot in KTP crystal 34 is equal to the signal spot in the resonator.

The resonator in which the KTP resides has a given mode shape defined by the mirrors employed to form that resonator. The mode of a resonator describes the path taken by rays that repetitively reflect off each mirror such that they re-trace the same path upon each reflection. The lens is used to alter the shape of the input beam so that it matches the shape of the OPO resonator mode, If the OPO employs flat mirrors then mode matching can occur without the need of a lens, since in this case the resonator mode is a beam of parallel light; SUBSTITUTE SHEET (RULE 26) -9- WO 97/05677 PCT/AU96/00477 5,181,211 o 6 then mode matching is achieved simply by using a low from the spirit and scope of the invention or sacrificing divergence laser beam to provide the input wave to the all of its material advantages, the form hereinbefore OPO. If the OPO uses identical concave mirrors, a lens described being merely a preferred or exemplary emmust he provided. In that instance the mode will have a bodiment thereof.

waist (smallest size) at the center of the resonator, and 5 What is claimed is: the pump will need to be focused to match the resonator 1. A solid state laser beam producing system comprismode. ing: It is noted that the wavelength of the output signal a first means for producing a first beam having a can be changed by rotating the crystal with respect to pumping wavelength range of from 1.0 um to 1.1 any cf its principal X. Y and Z axes. 10 um; a second means, nonlinear and rotatable with Referring now to FIG. 3, an alternative embodiment respect to three defined X, Y, and Z principal axes is illustrated in which OPO 30' is placed in what is for converting said first beam into a second signal called an "intracavity configuration", because the OPO beam, and a third idler beam; said second beam cavity is now inside the laser cavity. In this configura- having a signal wavelength of from 1.5 um to 2.2 tion. pump laser 20' and OPO 30' both utilize the same 15 urn and said third beam having an idler wavelength mirror 36'. The OPO resonator is formed by mirrors 32' given by the frequency difference between the said and 36'. As indicated above, the crystal 34' is oriented first beam and said second beam and wherein said for non-critical phase matching, but is now located first beam travels parallel to one of the said princiwithin the optical cavity of pump laser 20'. The mirrors pal axes of the said second means; and said second perform the same functions as in the embodiment illus- 20 beam is polarized by polarization means in a directrated in FIG. 1. Mirror 36' reflects the pump and par- tion parallel to a second of said three principal axes tially transmits the signal at 1.6 um; and mirror 32' trans- and said third beam is polarized in a direction paralmits the pump and reflects the signal. lel to the third of said principal axes such that non- In operation, the embodiment shown in FIG. 3 pro- critical phase matching is achieved and wherein vides a lower power threshold for wavelength conver- 25 said second converted signal beam has an eyesafe sion than that of the embodiment of FIG. 1, resulting in wavelength of no more than 2.2 um.

the ability for a successful parametric conversion utiliz- 2. The system of claim I wherein said second means ing continuous wave operation. Further, the efficiency is a nonlinear KTiOPO 4 crystal.

of the intracavity configuration can be higher because 3. The system of claim 1 wherein a laser rod, said the pump power that is not converted to the 1.5 umr-2.2 30 polarizer and a Q-switch, and reflection means, make up um wavelength in crystal 34' is reflected back into crys- a laser cavity inside of which said laser beam can resotal 34' for subsequent conversion. nate and wherein a tunable parametric convener is It is thought that the eyesafe laser system apparatus located outside of the laser cavity.

and method of the present invention and many of its 4. The system of claim 3 wherein a lens is disposed attendant advantages will be understood from the fore- 35 between said first means and a tunable parametric oscilgoing description and it will be apparent that various lator, housing said second means to produce mode changes may be made in the form, construction and matching.

arrangement of the parts thereof without departing SUBSTITUTE SHEET (Rule 26)

Claims (8)

1. A solid state laser beam generating system comprising, in combination: solid state laser means for producing a first laser beam having a wavelength range outside the eyesafe range of 1.5 1 um to 1.8,um; and converter means arranged to respond to the first beam by outputting a second beam having a wavelength within said eyesafe range; wherein said converter means includes an element of non-linear structure having three defined principal axes and said system is configured such that, in operation, said first beam travels in a direction offset from one of said principal axes by an angle greater than 2'.

2. A solid state laser beam generating system as claimed in claim 1, wherein said direction is offset by an angle of from 20 to 100.

3. A solid state laser beam generating system as claimed in any one of claim 1 or claim 2, wherein said solid state laser means is a neodymium laser.

4. A solid state laser beam generating system as claimed in claim 3, wherein said neodymium laser is diode-pumped.

A solid state laser beam generating system as claimed in claim 3, wherein said neodymium laser is one of the group comprising Nd:YAG lasers, Nd:YVO 4 lasers, Nd:YAP lasers and Nd:YLF lasers.

6. A solid state laser beam generating system as claimed in any one of claims 1 to wherein said converter means is one of an optical parametric oscillator, an optical WO 97/05677 PCT/AU96/00477 11 parametric generator or an optical parametric amplifier.

7. A solid state laser beam generating system as claimed in any one of claims 1 to 6, wherein said non-linear element is a non-linear crystal.

8. A solid state laser beam generating system as claimed in claim 7, wherein said non- linear crystal is potassium titanyl phosphate.