IL29404A - Laser including non-linear absorption means for controlling peak intensity - Google Patents

Description

Κ·»Β»Π flpsig ηΐϊ>ϊ? Q-H W-IR na^o »g3D« ynan t ^ LASER INCLUDING NON-LINEAR ABSORPTION MEANS FOR CONTROLLING PEAK INTENSITY The present invention relates to lasers. More particularly, the invention relates to solid state lasers.

In the prior art solid state lasers of the conventional type utilize typically a xepon flashlamp as an optical pump and a ruby or neodymium glass rod as an emissive element oriented along an optical path between reflecting surfaces of a Fabry-Perot cavity.

For Q-switching, one of the reflecting surfaces may be rotated to inhibit lasing until a desired threshold is obtained. Another device used for Q-switching is a dye solution which bleaches out above a selected radiation intensity and becomes transparent to permit lasing at that level.

The peak power at which such devices can operate is limited in the first instance to a level below which the emissive 17483/68 at selected intervals, i.e. the laser is pulsed.

In prior art Q-switched lasers, the pulse of radiation is limited in length to from 10 to 200 nanoseconds. The flash which precipitates the pulse extends in duration for approximately 100-4000 microseconds. The pulse length of a given prior art Q-switching type laser is not variable.

It is therefore an object of the invention to provide an improved solid state laser exhibiting a higher power output.

A further object of the invention is to provide an improved solid state laser which is more efficient.

Still another object of the invention is to provide an improved solid state laser exhibiting variable pulse length.

A still further object of the invention is to provide an improved solid st¾te laser exhibiting longer pulSes.

Still another ob ect of the invention is to provide an improved solid state laser having uniform power output.

A still further object of the invention is to provide an impfS wd solid state laser which generates multiple pulses upon injection of a single pulse of enabling energy.

Yet another object of the invention is to provide an improved solid state laser having bnger life and requiring less frequent replacement of emissive mater In accordance with the invention there is File provided an improved laser characterized as including a source of enabling energy, a resonant cavity including a Q-switching means and an active laser medium disposed within said resonant cavity and responsive to said enabling energy for producing pulses of plane polarized radiatioA along a path including said laser medium and wherein said improvement comprises a non-linear abesrption means intercepting said path for controlling the peak intensity of said radiation to a selected maximum level.

In one form of the invention the source of enabling energy is an optical pump. The stimulated emission means is a transparent medium formed of a material susceptible of stimulated emission and the cavity means includes a pari of optical reflecting surfaces for reflecting light energy successively through the medium.

In another form of the invention the control means include a non-linear absorption element traversing the optical path between the medium and the reflecting surfaces.

In still another form of the invention the element is formed of a material which generates a harmonic frequency of the radiation and absorbs the harmonic frequency radiation.

In still another form of the invention the element is adjustable to vary the crystallographic orientation with respect to the light energy from the laser to vary the degree and. character of absorption. Here the element is formed of a single crystal material having a selected crystallographic orientation.

In another form of the invention ·οοη χ lifier transparent stimulated emission medium is oriented along the optical path and the optical pumping means provide enabling energy in common for the first and second stimulated emission means.

The term "control" as used herein includes but is not limited to: - 3a - 1) control dynamically achieved by introducing nonlinear absorption wherein the absorption rate in- creases with increasing intensity of incident radiation until the system losses just balance the system ■ gain; and 2) control by saturating a medium through which radiation is transmitted.

Other and further objects of the invention will be apparent from the following description taken in connection with the accompanying drawings, and its scope-' will be pointed out in the appended claims.

IN THE DRAWINGS: Fig. 1 is a schematic diagram of a laser embodying the invention; Fig. 2 is a schematic diagram of a modification_ of the laser in Fig. 1; , Fig. 3 is a graph of a series of curves illustrating the operation of the invention; and Fig. 4 is a schematic diagram of still another modification of the laser in Fig. 1.

DESCRIPTION AND EXPLANATION OF THE APPARATUS IN FIG. 1 Referring now to the drawings and with particular refer ence to Fig. 1, there is here illustrated a schematic diagrapm of a laser apparatus embodying the present invention. While the following description is taken with respect to a laser, it will be apparent that the principles of the invention are applic factor on the frequency of operation is the relaxation time associated with the excited states of the emissive material.

Thus, we have here illustrated a laser generally indicated at 10; having an emissive rod 11 which is inherently capable of stimulated emission. The rod 11 may be formed, for example, o: ruby or neodymium glass. An optical pump 12 such as a zenon flashlamp provides the enabling energy for stimulated emission in the rod 11. A cylindrical reflector, not shown, surrounds the lamp 12 and rod 11. The rod 11 is disposed along an optical path indicated at 13 in a Fabry-Perot cavity between the reflecting surfaces provided by a prism 14 and a semi-transparent reflecting prism 15. A control element 16 is disposed between the rod 11 and the reflecting surface of the prism 14. The control element is typically a non linear absorption device which increases its absorption rate as the intensity increases tending to hold, down the intensity of any given ray. Normally, lasing actions tend to take place in the rod in a filamentary fashion so that the result ant beam at the surfaces of the rod consists of a plurality of point sources of light or infrared energy. The term "optical", or "light" as used herein includes, but is not limited to, all wavelengths of light including infrared and ultraviolet. The prism 14 is shown rotatable for Q-switching. purposes to develop a higher threshold of operation for the beam.

The control element 16 may be formed, for example, of. cadmium sulfide which exhibits the property of nonlinear absorption, i.e., absorption increases as the intensity increases. The photons cannot be distinguished from the second harmonic. A discussion of second harmonic generation and two photon absorption may be found in an article entitled "Non Linear Optics" by R.¥. Minck, R.W. Terhune and C.C. Wang in APPLIED OPTICS, October, 1966 pages 1595-1612. Note particularly Figs. 7-10 on pages 1603 and 1604. Thus, any tendency for the laser to produce sharp intense peaks along a filament is eliminated, thereby protecting the rod from hot spots and overheating and consequently, shattering. It is deemed feasible to form the ,rod 11 of a material which exhibits stimulated emission and which also absorb's energy in a manner required to maintain the intensity of the energy at a reduced level. For cadmium sulfide, the element 16 is oriented at a Brewster angle' of approximately 70° to avoid introducing additional Fabry-Perot reflection surfaces to reduce losses in the optical cavity. The Brewster angle is chosen because the light coming from the rod 11.., is plane polarized.

Note that the prism 15 may itself be a Fabry-Perot cavity. The reflecting surfaces 15a and 15b provide the cavity and the energy is internally reflected therebetween. Energy from within the prism 15 combines with energy reflectivity of the prism The theory of the operation of the laser is well-known in the art and well-described in the literature particularly in the context of a Fabry-Perot cavity. More particularly, the theory of such a laser is outlined in Patent No. 2,929,922 issued to Townes and Schawlow. The prism 14 is shown rotatable to enable Q-switching and thus to increase the peak power output applied to the lamp. By varying the voltage to the lamp, the intensity may be varied. In the present laser this results in varying the length of the output pulses from 10 nanoseconds to over 2000 nanoseconds. A variable voltage source 17 is shown coupled to the lamp':.l2.

In an apparatus built and tested a model No.1009 laser as manufactured by Applied Lasers, Inc. of Stoneham, Massachusetts was modified to include a control element in the position indicated in Fig. 1. A ruby rod 3" long by ¾" diameter was used. The Brewster angle chosen was approximately 70°·. The control element was formed of cadmium sulfide in an elliptical disc of approximately 18mm X 50mm shaped as shown in Fig.- 3. The thickness of the control element determines the degree of absorption which takes place. In the sample tested, the thickness was approximate-ly 5mm. The cadmium sulfide was a single crystal of excellent optical quality. The faces were fabricated parallel to within . two (2) seconds of arc. The crystallographic C axis was oriented along the major axis of the ellipse.

A typical!' frequency of operation is 6943A°. In the apparatus tested the input energy was 800-1600 joules and the peak output power was approximately 10^ watts.

DESCRIPTION AND EXPLANATION OF THE APPARATUS IN FIG. 2 Referring now to Fig. 2 there is here illustrated an oscillator amplifier laser capable of producing high ovitput powers for extremely long pulse duration, e.g., over 2 microseconds.

DAR dmo 1 20 is disposed between a semi-transparent reflecting prism 21 and 2 a Q-switching reflecting prism 22. A pair of flashlamps 23 and 2 3 illuminate both the oscillator rod and an amplifier rod formed of 4 material exhibiting stimulated emission 25. A reflector 26 surrounds both rods 20 and 25 and is used to optically pump both 6 rods simultaneously. The rod 25 is preferably fashioned of a 7 longer piece of material and larger in diameter to provide a 8 higher power output. Again, a control element 27 is shown dis¬ 9 posed between the rod 20 and, the reflecting surface of the pris 22. The prism 22 is rotatable as described- above for Q-switching 11 purposes . 12 In the prior art oscillator amplifier, if the same 13 optical pump wer.e used for both rods, the oscillator rod would 14 shatter. Here, however, the pulse length is increased and higher 15 oscillator energies are obtained. The maximum power within the 16 oscillator rod is substantially controlled by the non linear ele17 ment rather than by the flashlamp intensity. 18 The laser of the present invention provides a laser 19 having many advantages. More particularly, the average power in 20 time is increased. The average power possible across the cross- 21 section of the rod is increased. The pumping power possible is 22 increased and the overall' efficiency of the laser is increased. 23 By reducing the peak pulses along a filament by using 241 the control element in the manner of the present invention, the average power of a Q-switched laser may be substantially increas26 ed. A further advantage is that the laser will more readily alternately broken line in the curve (a). The curve (e) of Fig.3 corresponds with the intensity illustrated by the dashed line in the curve (a) „ The curve (e) further illustrates the generation of multiple pulses with a single flash of enabling energy.

DESCRIPTION AND EXPLANATION OF THE LASER IN FIG. 4 Referring now to Fig. 4, there is here illustrated a modification of the laser in Fig. 1. Here like reference numerals correspond with like parts ίη Fig. 1.

Here an optical transformer has been added to increase the intensity at the control element 16. •'The transformer includes a condensing lens 30 and collimating lens 31 to concentrate the energy on the control element 16. In this manner an element with a given threshold controlling action may be used with a wide range of energies. Threshold control may be provided from below 9 18 " 2 one (1) watt per cm to over 10 watts per cm . The transformer may be reversed to use a divergent lens system which reduces the intensity of radiation at the control element.

From the foregoing discussion it will be apparent that the present invention has broad application to the field of laser technology.

While there has hereinbefore been presented what are at present considered to be .the preferred embodiments of the invention, it will be apparent to those of ordinary skill in the art that many modifications and changes may be made thereto with

Claims (3)

1. WHAT IS CLAIMED ISj 1. An improved laser characterized as including a source of enabling energy, a resonant cavity including a Q-switching means and an active laser medium disposed within said resonant cavity and responsive to said enabling energy for producing pulses of plane polarized radiation along a path including said laser medium and wherein said improvement comprises a non-linear absorption means intercepting said path for controlling the peak intensity of said radiation to a selected, maximum level.

2. Apparatus as recited in Claim 1 wherein said non-linear absorption means is disposed in said path within said resonant cavity adjacent said laser mediu

3. Apparatus as recited in Claim 2 wherein said non-linear abeorption means is a single crystal having a selected crystallographic orientation, having plane parallel faces and disposed with said faces oriented at Brewster's angle relative to said radiation '" path. h. Apparatus as recited in Claim .1 wherein said non-linear abeerption means is formed of a material which generates a harmonic frequency of said radiation and absorbs said harmonic frequency radiation. - 11 5* Apparatus as recited in Claim 1 wherein said non-linear absorption is formed of a photo-conductive material. 6. Apparatus as recited in Claim 1 wherein said active laser medium is ruby, and said non-linear absorption means is a single crystal of cadmium sulfide. 7. Apparatus as recited in Claim 1 further including a second active laser medium disposed in said path and receiving enabling energy from said source of enabling energy. 8. Apparatus as recited in Claim 1 further including means for varying the intensity of said enabling energy to thereby vary the length of said radiation pulses . 9· . Apparatus as recited in Claim 1 further including an optical transformer disposed in said path between said non-linear absorption means and said active laser medium. - 12 -