WO1987004016A1

WO1987004016A1 - Laser resonator

Info

Publication number: WO1987004016A1
Application number: PCT/US1986/002512
Authority: WO
Inventors: Robin A. Reeder
Original assignee: Hughes Aircraft Company
Priority date: 1985-12-20
Filing date: 1986-11-24
Publication date: 1987-07-02
Also published as: JPS63501994A; EP0252116A1; IL80664A; US4740986A; ES2005085A6; KR880701028A; IL80664A0

Abstract

A reflectance-outcoupled laser resonator (10) with an improved electro-optical Q-switch (13, 18, 19, 21) which is alignment stable. The laser performance is insensitive to angular movements of the resonator elements. This resonator requires fewer optical components and lower total cost than other resonators which are designed to perform similar functions. The invention includes a laser medium (12), two prisms (13, 21), a polarizer (18), an electro-optic crystal (19) and an alignment wedge (20). A polarization preserving folding prism (13) allows great flexibility in resonator design by passing any arbitrary polarization state without change. A double end-reflector prism (21) serves as both cavity end reflectors of the resonator and also serves as a component of the Q-switch (13, 18, 19, 21).

Description

LASER RESONATOR

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lasers and to apparatus for .alignment-stable, reflectance-outcoupled electro-optically Q-switched laser resonators.

2 . Background Information

The utility of lasers derives from the unique properties of laser light, including high brightness, monochromaticity, low beam divergence, and coherence. These attributes make the radiation from lasers completely different than the radiation from previously known sources, and open up a wide area of application that includes rangefinding, tracking, motion sensing, communi¬ cations, seis ography, holography, and various defense applications.

Many communications and measurement applications of laser technology require series of pulses of laser radiation which have prescribed energy levels, periods of pulse duration, and intervals between pulses. A series of laser pulses may be produced by a laser which operates in conjunction with a Q-switch. A Q-switch is an apparatus, which is used to obtain short, intense bursts of oscillation from lasers. These devices are 1 well known in the electro-optics arts and are described in the text Optoelectronics: An Introduction, by J. Wilson and J. F. B. Hawkes, published by Prentice-Hall .in 1983.

.5 Q-switching techniques may utilize active or passive components to introduce time or intensity dependent losses into the laser resonator. A Q-switch constrains lasing action within a laser medium by preventing the buildup of oscillations within the

10 resonator. The switching which occurs is a change in the Q or quality factor of the resonator. The quality factor is a quotient which is proportional to the energy stored in a resonator divided by the energy dissipated per. cycle. A Q-switch functions by altering

15 the optical pathway within a resonator. When a high loss condition is imposed on a laser cavity, energy which enters the laser medium from an excitation source is dissipated before atoms or molecules in the medium can be stimulated to laser oscillation. If a large

20. population inversion is built up within the laser medium and the Q of the resonator is suddenly increased by eliminating the cavity loss conditions, laser action will suddenly begin. Once the quality factor of the resonator is sharply increased, laser oscillations

25 build--up rapidly within the laser cavity and all available energy is emitted in one/ large pulse which substantially depopulates the upper lasing level and shuts down the laser oscillation.

Q-switching may be accomplished using an active

30 component such as a rotating mirror or prism. This technique requires one of the resonator reflectors to be rotated at a high angular velocity so that optical losses within the resonator prevent lasing except for the brief interval in each rotation cycle when the

35 1 reflectors are substantially parallel. Such systems utilize mirrors or prisms that rotate up to 60,000 revolutions per minute and require costly motor and control apparatus which are susceptible to mechanical 5 failure and misalignment.

Electro-optic, magneto-optic, and acousto-optic modulators may also be used as Q-switches. An electro- optic crystal, for example, may be placed within a laser resonator and alter the quality factor of the

10 cavity by changing the polarization of the radiation in the cavity, in response to an electrical signal applied to the crystal. Acoustical devices are employed to deflect some of the laser beam out of the cavity by diffracting light off sound waves and thereby

15 diminish_rrt§* the quality factor of the resonator.

Saturable absorbers can be employed as passive Q-switches. A transparent cell containing a normally opaque, bleachable dye in a suitable solvent is placed within a laser cavity in order to absorb some portion

20 of incident excitation energy. When the dye saturates and can no longer absorb excitation radiation, the dye loses its original opacity and functions as a Q-switch by increasing the optical efficiency of the laser.

The optical feedback loop of a- laser resonator

25 fundamentally comprises an active gain medium bounded by reflectors at each end of the resonator cavity. These reflectors must be maintained in precise alignment so that laser radiation emanating from the gain medium can make a myriad of transits through the medium between

30 the reflectors. Resonators are susceptible to high energy losses if the reflectors are even slightly

**. misaligned. Losses large enough to completely disable a resonator can be generated if one of the resonator reflectors is^" t ilted off its alignment axi s by only a few mil li radi ans . • The use of a Q-swi tch wi thin a resonator cavity exacerbates typical , systemic misalignment" losses. ^— Another source of energy losses which plaguέ' the operation of- a laser resonator is dimensional instability within the resonator cavity. Dimensional instability results from the unwanted relative movement of components of the laser resonator. Any change in the optical alignment of. the resonator components produces a loss which adversely affects the laser output power and beam quality.

One of the major concerns in designing and fabricating a^' laser resonator is to substantially eliminate dimensional instability by increasing the alignment insensitivity of the resonator.

For laser systems that are manufactured in large ty designs that employ a

ive part are highly desirable. Moreover, the design should ideally result in a laser cavity and means of radiation outcoupling that result in a performance which is not degraded by temperature change, physical shock, and other environmental hazards; particularly if the laser system is used in the field. The reflectors placed at either end of the cavity need not be plane mirrors, but must be aligned so that multiple reflections occur with little loss. The resonator cavity is usually very sensitive to changes in alignment of the mirrors. A small tilt of one mirror, for example, subjects the resonator cavity to a large loss of energy. Typically, a relatively small misalignment in the resonator cavity can prevent the operation of the laser transmitter. In addition, the loss of energy through misalignment is made more likely by the use of Q-switching. Q-switches retard stimulated emission by preventing the buildup of oscillations in the resonant cavity. When a high level of population inversion is reached in the laser medium, a Q-switch is triggered to suddenly restore the resonant cavity.

Previous efforts to achieve alignment insensitivity have included the use of concave mirrors, opposite corner-cube reflectors tilted against correlative

Brewster angles, and internal reflection prisms arranged with either parallel or obliquely crossed rooflines. In U.S. Patent No. 4,420,836, Harper discloses an optical resonator cavity configuration using a unitary mirror with oppositely directed convex and concave reflective surfaces that reverse both ends of a laser beam propagating from a laser rod disposed between two total internal reflection prisms rigidly positioned with perpendicularly crossed virtual rooflines. The rooflines of the internal reflection prisms are perpendicular to the axis of the laser beam and to the optical axes of the optical resonator components. The unitary mirror with oppositely directed reflective surfaces of opposite sign positioned between opposite ends of the beam enhances the insensitivity of the resonator cavity to misalignment.

In U.S. Patent No. 3,896,397, de Wit et al . describe an improved acousto-optically Q-switched laser. Corner-cube and Porro prism retro-reflectors are combined and provide improved tolerances for three of the four mounting adjustments. Since the reflecting element near the laser is a corner cube, and since the feedback reflector is a Porro prism, only a single close tolerance adjustment of the reflectors is necessary. 1 U.S. Patent No..3,959,740—Dewhirst discloses a configuration of crystalline quartz wedges utilized as polarizers in combination with an electro-optic switch which constitutes a laser Q-switch. The two wedges are •5^' identical and- are positioned on either side of the switch. These wedges are oriented such that each compensates for the angular deviation and dispersion of the other. The principal advantage of the disclosed wedge polarizer configuration is that Dewhirst¹s con- 0 figuration does not require calcite for the polarizing material, since a large angular separation of the polarizations is not required. The electro-optic switch is oriented with a Porro prism such that in the off condition, both polarizations are misaligned angularly 5 with the Porro^' prism. The quartz wedges are considerably less expensive-, and are more easily mounted and aligned in the laser. Quartz is also more durable than calcite and is not as easily damaged by the laser beam.

In U.S. Patent No. 3,982,203, de Wit discloses an 0 improved, optically pumped, acousto-optically Q-switched laser which produces significantly increased¹ output energy. Optical coatings, with their inherent limitations on the maximum energy of the output pulse, are eliminated. At the reflecting surfaces, Porro prisms replace 5 conventional mirrors. At the nonreflecting or trans¬ mitting surfaces, conventional antireflecting coatings are eliminated by placing the respective elements at the Brewster angle for the dominant polarization of the Q-switching material. 0

5 In U.S. Patent No. RE 29,421, Scott discloses a laser system having an electronically selectable gain which employs an acousto-optical beam deflector to variably control the Q of a laser cavity in response to an electronic signal.

None of the preceding inventions completely solves the concomitant problems of polarization conservation, alignment stability, insensitivity to mechanical and thermal changes, and the difficulties of mass producing reflectance outcoupled laser resonators which include a minimum number of reliable components. Such a solution would satisfy a long felt need manifested by the current efforts of the laser and optoelectronics industries, which continue to attempt to develop laser resonators which can cope with the constantly increasing demands for improved performance as described above.

SUMMARY OF THE INVENTION The present invention is a low-cost, rugged laser resonator with improved Q-switch using fewer components than previous conventional resonators. It enploys two prisms of novel design, one of which serves as both cavity end reflectors and which is used as part of the Q-switch. The second of this pair serves as a folding prism that preserves the polarization state of incident radiation.

The present invention uses fewer, less expensive components than previous designs, provides a laser resonator for which there is less distortion and loss, and is stable against misalignment from motion of the end-reflector elements. The present invention is less susceptible to- environmental hazards such as temperature extremes or physical shock, and can be fabricated with mechanical tolerances on its optical mounting surfaces that are not as critical as in conventional designs. The improved Q-switch contained in the present invention does not permit prelasing (lasing before the Q-switch is opened) to take place, even for extremely high input energies. The present invention provides higher efficiency than^" for conventional designs, and it avoids the temperature- alignment problem association with the use of calcite wedge Q-switches.

The present invention comprises a laser resonator which is stable against misalignment because the laser performance is . insensitive to motions of the resonator elements. Fewer elements are used than in conventional designs, including two specially designed prisms. A double end-reflector prism serves the function of both end reflectors of the resonator cavity, and in addition, can serve as a^'- component of the resonator Q-switch. A polarization conserving folding prism preserves the polarization state of the incident radiation in addition to folding the. beam. The Q-switch design makes use of a mulitlayer thin-film polarizer and a quarter-wave electro-optic crystal .

The laser resonator comprises substantially six optical elements: the four elements mentioned above plus a laser rod and an alignment wedge. The 100% reflector for the resonator is a roof on the double end- reflector prism that makes use of total internal reflection to ^'reflect the incident radiation. The reflective out coupling surface is a dielectric-coated face of the double- end— reflector prism. The polarization conserving folding prism folds the resonator beam path without affecting the polarization state of the radiation. The laser rod and alignment wedge are standard components which are readily available commer c l items and which are well known to persons ordinarily skilled in the laser and optoelectronics arts. The electro-optical Q-switch includes the quarter- wave electro-optic crystal, the multilayer thin-film polarizer, and the double end-reflector prism. Radiation passing through the polarizer is linearly polarized at a 45° angle with respect to the longitudinal axis of the double end-reflector prism. With no voltage applied to the electro-optic crystal, the polarization state of the radiation is unaffected as it passes through the crystal. The radiation returning from the double end-reflector prism is not passed by the polarizer because its polarization is shifted 90° from its initial state. When a quarter-wave voltage is applied to the electro- optic crystal, a double pass through the crystal will change the polarization state in such a way that the returning radiation has its polarization aligned with the axis of the polarizer. Thus, the radiation will pass through the polarizer on the return path only when the quarter-wave voltage is applied. The polarization conserving folding prism is necessary to preserve the polarization state of the radiation returning from the dielectric reflective surface of the double end-reflector prism. The radiation is then aligned with the axis of the polarizer.

It is, therefore, an object of the present invention to provide a laser resonator with fewer parts than many conventional resonators.

It is also an object of the present invention to provide a laser resonator with less expensive optical components than many conventional designs. An additional object of this important invention is to provide a laser resonator which suffers from less distortion and energy losses because there are fewer optical components in the resonating cavity than in conventional designs. Another object of the invention is to provide a folded laser resonator which is stable against misalign¬ ment caused by motion of the end reflector element or of the folding. element . Yet another object of this invention to provide a laser resonator which is less susceptible to environmental hazards such as temperature extremes or mechanical shock than prior designs.

A further object of the present invention to provide a laser resonator for which mechanical tolerances on optical mounting surfaces are not as critical as in conventional designs.

Another object of this innovative design is to provide a laser resonator which is more easily manu- factured than conventional resonators and which is easily maintained in the field.

An additional object of the apparatus described in this patent application is to provide an electro- optical Q-switch that results in a higher efficiency for the laser resonator than in conventional designs.

Another object of the present inventioή.to provide an electro-optical Q-switch that substantially eliminates the phenomenon of prelasing, even at extremely high input energies. A further object of the invention is to provide an electro-optical Q-switch that avoids the changes in alignment with temperature 'and prelasing problems which plague calcite and quartz wedge Q-switches.

Yet another object of this invention is to provide a single optical element which serves as both end reflectors of a laser resonator, and which also serves as part of an electro-optical Q-switch in the same resonator. Still another object of the present invention is to provide an improved optical feedback configuration that is compatible with a wide variety of laser resonator devices. This resonator also provides a folding prism of novel design which conserves the polarization state of the radiation incident on it, regardless of the initial polarization state of the incident radiation.

A greater appreciation of other aims and objects, along with a more complete and comprehensive understanding of the present invention, may be achieved through the study of the following detailed description and through reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective drawing of the laser resonator with improved Q-switch, which includes a polarization conserving folding prism and a double end- reflector prism. FIG . 2 is a perspect ive drawi ng of the polarization prese rvi ng foldi ng prism , showi ng its act ion on an i nput beam of l i nearly polarized light .

FIG . 3 is a drawi ng of the double end-reflector prism, showing an isometric view and three orthogonal views .

FIG. 4 shows the state of polarization of the radiation at various points of the beam path within the laser resonator, for the two cases of zero voltage applied to the electro-optic crystal and a quarter- wavelength volt age appli ed to the electro-optic crys tal .

FIG . 5 is a graph of the sensit ivi ty of laser output energy to x- or y-t ilt of the double end-reflector prism, compared to the alignment sens it ivi ty of a conve nt ional resonator . FIG . 6 i,s a comparison of laser beam dive rge nce

• caused by misalignment of the wedge in a conventional resonator to the change in beam divergence as a funct ion

• of y-t ilt in the double end-re lector prism in the present invention .

FIG . 7 is a comparison of the sens itivi ty of beam steeri ng to wedge misalignment in the conventional resonator and the sens itivi ty of beam s teering to tilt of the double ^' end-reflector prism in the present invention .

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 depicts a laser resonator 10 which includes a solid state laser rod 12 having a longitudinal axis parallel to beam path 11, a polarization preserving folding prism 13, a thin film polarizer 18, an electro- optic crystal- 19, an alignment wedge 20, and a double end-reflector prism 21. The lase,r rod is typically made of neodymium-doped yttrium aluminum garnet (Nd:YAG) or a chromium-aluminum-oxide composition. The rod

.absorbs excitation energy from an incandescent lamp or . flashtube (not. shown) in order to create a population inversion of excited electrons or atoms within the gain media. Thin-film polarizer 18 is a conventional optical device which comprises a plurality of dielectric films having alternating indices of refraction. These films polarize incident radiation in the same way a series of glass plates separated by air spaces polarize a beam of light. Radiation which is not passed or absorbed by polarizer 18 is. reflected away from beam path 11. A large portion of this reflected radiation is absorbed by beam stops (not shown) which are deployed on either side of polarizer 18. Electro-optic crystal 19 may be a Kerr or Pockels Cell. In the preferred embodiment of the present invention, crystal 19 is fabricated from lithium niobate. 1 Alignment wedge 20 is also a conventional transparent optical element which is essentially a wedged cylindrical or disc-shaped piece of optically-isotropic material. One of the circular end surfaces of the wedge is aligned 5 ^' approximately perpendicular to beam path 11, while the circular end surface which faces the double end-reflector prism 21 is inclined from the direction normal to beam path 11. Wedge 20 may be made of glass. It corrects optical deviation produced by any of the resonator

10 elements.

The radiation beam path 11 is folded by polarization conserving folding prism 13. Four total internal reflections of the beam occur within prism 14, which is shown best in FIG. 2. The resonant cavity is

15 defined by partially reflecting side surface 22a and totally reflecting chisel roof surfaces 24a, 24b of the double end-reflector prism 21. Partially reflecting side surface 22a constitutes the outcoupling means for the laser radiation. A dielectric coating is applied

20^' to side surface 22a in order to provide a partially •reflecting surface. Beam path 11 is perpendicular to this surface 22a. With the exception of the total internal reflection surfaces, the other surfaces which

- lie in beam path 11 are coated with an antiτreflectlve 25 material . -

The resonant cavity is defined by partially reflecting side surface 22a and totally reflecting roof 25 of double end-reflector prism 21. The partially reflecting side surface 22a constitutes the outcoupling 3.0. means for the laser radiation. The partially reflecting side surface 22a is fabricated by applying a dielectric coating to either side face of the double end-reflector prism that is normal to the beam axis 11. The other surfaces in the beam path are antijtreflection coated 35 except for the total internal reflection surfaces. 1 The polarization preserving folding prism 13, which is best shown in FIG. 2(a), is a radiation folding waveguide that maintains the relative phase relationships of the components of the polarization vectors of the 5 beam which enters the prism 13. FIG. 2(a) depicts the polarization preserving folding prism 13 in a perspective view and for the special case of linearly polarized radiation. The longitudinal axis of prism 13 is parallel to its longest edge which runs from top to bottom in

10 the drawing. .Radiation from laser rod 12 enters prism 13 at point A through a hexagonal, anti-ref lecting front surface 14 which is coated with an anti-reflective material in order to permit the laser radiation to enter and exit prism 13 with minimal losses due to

15 surface interactions... This entrance beam, which strikes prism 13 in a; direction normal to the plane of hexagonal side 14, is diverted ninety degrees at point B when it . • strikes the interior of the inclined, rectangular, total internal. eflection side 15 of prism 13. Two

20 ^•' more total internal reflections at points C and D

'._ ." redirect the _.beam through another 180 degre.es. Points C and D are 'located on the interior of upper and lower total internal-reflection surfaces 17a and 17b of prism 13. • The two planes, containing this pair of folding

25. surfaces 17a_r-b. are oriented, forty-five degrees from ^" the longitudinal axis of prism 13 which is parallel to the long edge of the inclined, rectangular, total internal reflection side 15. Once the beam is reflected off lower total internal reflection surface 17b, it

30 impinges upon the interior of side 15 at point E and is then reflecte .through another ninety degrees before it passes out of .the polarization conserving folding prism 13 at point F. The exit beam, like the entrance beam, travels in a direction perpendicular to the plane

35 containing hexagonal side 14. FIG. 2(a) shows the preferred embodiment of prism 13. This version of the present invention may be viewed as an evolution of- a conventional folding prism, which is trapezoidal in cross-section, having a base and a top surface* parallel to the base which are joined by a pair of side surfaces which are inclined toward the center of the prism. The polarization preserving folding prism 13 differs from a conventional folding prism in that prism 13 has been coupled at its base to s a simple right-triangular iso eles prism. FIG. 2(a) reveals that the end surfaces 16a,.b of prism 13 are both adjacent surfaces 14 and 15, and are separately adjacent to surfaces 17a, b. Surfaces 16a,. b are isαceles triangles having hypotenuse legs that form the transverse edges of rectangular side surface 15. This design provides the desired reflections at points B and E.

PI ^■4CT c) are vectorial depictions of

Λ. 2(b) and 2( the states of polarization of the entrance and exit beams. Since the incident radiation is plane polarized by thin-film polarizer 18, the vectorial representation illustrated by FIG. 2(b) shows the electric intensity vector V, having some arbitary polarization angle theta. After impinging upon prism 13 and undergoing four total internal reflections, the beam issuing from prism 13 retains the same phase relationship between the vectorial components V_x and Vy of the beam's polarization vector V. The polarization vector V retains the same angle relative to the x and y axes, theta degrees from the y axis. FIGS. 2(b) and 2(c) are not identical because the two beaπrs are travelling in opposite directions. The apparent inclination of the polarization vector V in the two drawings is relative to the direction of travel of the beam. If the plane containing the x and y axes , -* >_*__aΛ<-*--3__r in FIG. 2(c) is rotated 180 degrees around the y-axis, then both ββ-fcs— of vector-T -eomponent-s appear parallel, indicating the polarization conserving feature of prism 13. The polarization conserving folding prism 13 is designed to ensure that equal numbers of P- and S-type reflections of mutually perpendicular components of a polarized light beam occur as the light beam traverses the prism. A -P-type reflection is one in which the electric intensity vector of the incident light wave is parallel to the plane of incidence, which is defined as the plane containing both the electric intensity vector and a vector describing the direction of the light wave (such as the. Poynting vector). An S-type reflection (from the German "senkrecht") refers to a reflection in which the electric intensity vector is perpendicular to .the plane of incidence.

In FIG..2, a linearly polarized beam with the electric intensity vector oriented at an angle θ with respect to the y-axis is shown impinging at point A of the surface 14, which is polished flat and has an . antireflection coating on it. The electric intensity - vector can be .resolved^' into two mutually perpendicular components along the x and y axes of the reference ^' coordinate system -.shown. The y component undergoes total internal .reflections at points B, C, D, and E from surfaces 15, 17(a), 17(b), and 15 (again), respectively^',^' which can be characterized sequentially as S-type, P-*>t.ype, P-type , and S-type. The reflections ' of the x component of polarization, on the other hand, can be characterized as P-type at point B (from surface ^• 15), S-type at point C (from surface 17(a)), S-type at point D (from surface 17(a)), and P-type at point E 1

( from surface 15 again) . When polarized light is totally internally reflected at the plane boundary between two dif ferent dielectric media , the P- and S-type components of polarization , in general , undergo different phase shifts . In order to preserve the i nitial s tate of polarization of the incident rad i ation , the phase dif ference between the x and y components of polarization must not be allowed to change . This is accomplished in the polar ization prese rvi ng foldi ng prism 13 because; the x and y components of polarization both undergo two P-type and two S-type reflections .

The polarization conserving folding prism 13 was tested in a laser resonator of such a design that the use of a conventional folding prism would have produced large losses due to polarization effects. No polari¬ zation induced losses were observed when the polarization conserving folding prism was used. Since the vertical and horizontal, components of incident radiation undergo equal numbers of P (parallel to the plane of incidence) and S (perpendicular to the plane of incidence) reflections from the total internal reflection surfaces of the prism, any phase shifts that occur on reflection are exactly the same for both the horizontal and vertical components of the incident ^'radiation, and thus there is no net relative phase- shift. This same property of preserving the' polarization state of incident radiation is also possessed by the double end-reflector prism. Double end-reflector prism 21 is shown in one isometric and three orthogonal views in FIGS. 3(a), (b), (c), and (d). Prism 21 is essentially a four- sided structure having a slanted roof surface 23 at its top end and terminating in an identical pair of roof surfaces 24a b divided by a roof crest line 25 at 1 its other e d. In the preferred embodiment, the slanted roof surface 23_. is inclined forty-five degrees from the prism's transverse axis which runs along its upper end as depicted in FIG. 3(a). The angle between lower

5 surfaces 24a arid 24b, which give the bottom portion of prism 21 the appearance of a wedge or chisel, is also forty-five degrees. The front of prism 21, shown as surface 22a in FIG. 3(a), is the outcoupling surface of the resonator 10. The back surface, which is identified 10 in FIG. 3(c) as 22b, is coated with an anti^r-ef lect ive material and is polished flat. The slanted roof surface ' 23 and chisel roof surfaces 24a ^{^}b are also polished flat in order to preserve the optical integrity of the beams which are reflected from both sides of these 15 surfaces.

Prism 21 serves as both end reflectors for the laser resonator cavity. The roof surfaces 24a ^""b prism 21 serve as the 100% reflector by means -of total internal reflection, and outcoupling of the laser 20 ^' radiation is effected through a dielectric coating on either one of the prism faces 22a, b perpendicular to the roof edge .

After leaving the laser rod 12, radiation traverses the polarization preserving folding prism 13, passes ^'25*. through thin-film polarizer 18, electro-optic crystal 19, ^■ and the alignment wedge 70, and impinges upon double-end reflector prism .21. The radiation undergoes a first total internal reflection off the totally reflecting slanted, upper roof surface 23 of prism 21. After the 30 beam travels along, the Longitudinal axis of the double .end-reflector ^'.prism 21, it undergoes two total internal reflections at roof faces 24a,^{^}b, travels back along . the longitudinal axis, undergoes a second total internal reflection at slanted surface 23, and exits back toward •35 the- electro-optic crystal 19. The double end-reflector prism i s also used as an element of the Q-switch in addition to its role as both end reflectors of the resonating cavity. The polarization

5 statβiof the. radiation at various places in the cavity are shown in FIGS. 4(a, b, c). The radiation emitted by laser rod 12 is assumed to be randomly polarized.

This polarization state is depicted in Segment I of the beam path diagram shown in FIG. 4(a). The path of the beam within the resonator has been divided into four beam path Segments labelled I, II, III, and IV in FIG.

4(a). FIGS. 4(b) and 4(c) are vectorial representations of the polarization states of the laser beam at Segments I, II, III, and IV for both the outward and return ^• paths of the beam through the entire resonator 10. FIG. 4(c) differs from FIG. 4(b) in that a quarter-wave voltage has now been imposed across the electro-optic crystal 19. When such a voltage is applied to this crystal 19, the crystal acts like a quarter-wave plate which converts the incident linearly polarized light into circularly polarized light.

The orthogonal arrows in the upper right portion of FIG. 4(b) indicate that the laser radiation emerging from rod 12 on its way to double end-reflector prism 21 is unpolarized. This polarization state is unchanged by polarization conserving folding prism 13, so the next representation under "II" is the same. Thin-film polarizer 18 changes the polarization states of the radiation in Segments I and II, so that Segment III is represented by a vector inclined forty-five degrees from the vertical. When the electro-optic crystal 19 is not energized, the polarization angle is unchanged when the radiation passes through it. Therefore, the representations in FIG. 4(b) for Segments II and IV in the outbound pass to the totally reflective mirror surfaces 24a, b are identical. 1 The polarization states are also preserved as the radiation passes through double end-reflector prism 21. The vector illustrations are rotated through ninety degrees in the bottom row only because the direction of 5 the return beam is now anti-∑parallel to the outbound beam. Since crystal .19 is not operating, the polarization state is not altered as the radiation transits from Segment IV back to Segment III. Now, however, the beam is blocked by thin-film polarizer 18 since the polari- Q zation state of the beam does not match that which is passed by the polarizer. Accordingly, no beam passes back to Segments II and I in FIG. 4(b).

FIG. 4(c) reveals what happens to the beam when crystal 19 is_:.activated. When the radiation passes

15 through the cell 19, the incident linearly polarized light is changed to right-circularly polarized radiation. When the beam travels back from the double end-reflector prism 21 in the opposite direction, it appears as left- circularly polarized radiation. As the beam passes

2.0 . through the cell 19 on its way back to laser rod 12, the cell changes the beam back to its former, linearly polarized condition. This polarization state is the state which thin-f ilm.^'polarizer 18 will pass, which enables . the lasing process to occur.

•25 Q-switched laser breadboard was extensively

^• tested in which the double end-reflector prism was used as an element of the Q-switch and as the cavity end reflectors. The performance of the laser did not deteriorate when the double end-reflector prism was

30 tilted by lOO.microradians. The performance of a similar conventional laser resonator would greatly deteriorate if .^'either end reflector were tilted by

^• ." this same amount. For a conventional resonator, the

^"35- output, E, decreased 20% and the beam divergence increased 35%, as can be seen from FIGS. 5 and 6. In FIG. 5, ' the output energy, E, measured in millijoules for a conventional resonator is shown by curve 30. Even for a tiny amount _.of misalignment, the conventional resonator fails to operate. Only 0.3 illiradians of deflection causes the conventional resonator to lose one-half of its output power. . Curve 32 depicts experimental data for the present invention. Curve 32 reveals that the present invention is almost totally insensitive to misalignment from the x axis in either the end-reflector prism 21 or the folding prism 13. Curves 34 and 36 illustrate that the present invention is also sub¬ stantially insensitive to misalignment from the y axis. FIG. 6. eveals that for misalignment of the resonator from- he y axis by an angle of about 200 microradians,-.a* conventional resonator suffers an increase in beam divergence (curve ^'38) which is nearly twice as bad as the beam divergence- experienced by the present invention as- llustrated by curve 40.

FIG. 7 shows that conventional resonators are plagued by enormous beam steering error caused by reflector misalignment. Even when the double end- reflector prism 21 is out of alignment by as much as 200 microradians, beam steering error remains relatively low (curve 44) compared to the large error of 700 microradians for.the conventional resonator (curve 42). . As FIG. 7 shows, the beam steering changes by an amount four times the "tilt of either end reflector (for the particular resonator length tested), whereas the change in line of sight for the resonator using the double end- reflector pri m is equal to the tilt of double end- reflector prism 21.

22

Some deterioration of the performance of the laser breadboard was observed for larger tilts of double end-reflector prism- 21. A tilt of 500 microradians caused a 5% decrease in output energy, but actual ly reduced beam divergence by 9% . The magnitude of this tilt applied to either end mirror of a conventional resonator is -enough to cause the laser to stop lasing .

Laser resonator conf igurations using the double end-reflector' prism such as the ones depicted in FIGS .

10 1 and 4 are stable^' against misalignment due to moveme nts of either the double end-reflector prism or the folding pr ism ( e ither polarization-preserving or conventional ) . Motion of the double end-reflector prism about the x and • y axes shifts the beam line of sight by an amount equal to the angular motion , but the output beam remains i normal to the- outcoupling surf ace of the double end- re-rle . ^c.-^P** .- — re-fie-e-t©*-! prism. The beam line of s ight is unaf fected by angular motion of the folding pr ism.

The resonator' s angular alignment is unaf fected ^'20 ^• by the motion of either the double end-reflector pr ism 21 or the folding prism 13. Initial alignment is made with an int a- resonator alignment wedge ( not shown) . When the resonator is properly aligned , the output radiation exits normal to the outcoupling surf ace 22 2*5 ^' of the double nd-rref lector prism 21. The alignment ^' insens itivity of^' the resonator is best unders tood by - tracing the path of a ray which is normal to the ^"'■ ^{: ■}• outcoupling surface. 22. After travers ing the length of the resonator, through folding prism 13 and the double 30 : . end-reflector prism^' 21, the ray is retroref lected and arrives in proper- alignment normal to the outcoupling surface 22. . If the double end-reflector prism 21 is tilted about the x axis , then the ray str ikes the roof

35 1 ' off center, but the ray is still in the retroref lect ive plane. Tilting the double end-reflector prism 21 about ^'. the y axis only, shifts the ray along the crest 26 without decentering its point of impingement. Rotation 5 about, the y axis changes the orientation of both the

■ ray and the retroref lect ive plane by the same amount so that the retroref lection is undisturbed. Tilting the double end-re lector prism 21 about the z axis decenters the ray at the^' roof crest 25, but does not move it out 10_. of the retrore lect ive plane.

The folding prism 13 is also insensitive to motion about all three- axes. Displacement from the x axis only decenters: the ray at roof crest 25 but does not misalign the resonator. The same is true for motion 15*- about the z axis. Displacement from the y axis only moves the ray -along roof crest 25, and thus no decentering or misalignment- occurs.

-The present invention represents a marked ^{■ •} improvement over conventional laser resonator designs 20. ^'. which include^'an output reflector, a folding prism, an electro-opt ic.aϊ Q-switch comprising a half-wave lithium ^■ niobate crystal., two crystal wedges, an alignment wedge - and a 100% reflector or Porro prism. The crystal wedges are usually made of either quartz or calcite. The use 25 of either the calcite or quartz wedges has certain disadvantages which are avoided by the present invention. Two problems which..^'are inherent in the use of the wedges are- prelasing (lasing before the Q-switch is opened) and shifts in alignment due to temperature changes. 30 The optical properties of calcite and quartz are such that thicker wedges^' of quartz must be used to produce . the same resistance to prelasing as that of calcite.

35- 1 However, the effect of temperature change is less for quartz than for calcite. The tendency for prelasing to occur is inversely related to the angular displacement between the ordinary and extraordinary rays, so that

^•5. calcite is superior to quartz in this respect. On the other hand, Q-switches using calcite wedges are more susceptible to misalignment due to changes in temperature.

In addition, such conventional Q-switches are very costly.

The Q-switch of the present invention is lower in cost

of the present invention has lower distortion and losses than for a conventional resonator. It will not prelase even at extremely high input energies and is stable

.15 against misalignment. It is much less sensitive to slight movements of the resonator end reflectors than conventional resonators, in the sense that the performance of the laser^' will not deteriorate.

Because -of all these advantages over previous

20. designs, the present invention should find wide application^' in; the manufacture of lasers and laser

35 1^' The terra "laser radiation", which appears many . times in this' specification, should be understood to connote a broad range of radiant energy which is not ^• limited to energy within the narrow confines of the 5 visible spectrum. The advantages gained by using the present invention in a laser resonator with a solid state, rod may also be exploited in lasers having gaseous or liquid phase- gain media. ^• ' Although the' present invention has been described

10 ^'•^' in detail with reference to a particular preferred embodiment, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the 15 • invention.

20*

25

30

TNG: lm [ 338 -1 ]

35

Claims

CLAIMSWhat is Claimed, is:1 - 1. A radiation folding waveguide comprising: reflector means for transmitting incident• radiation; said reflector means includi ng a first surf ace 5 which: has a- longitudinal axis along its longest •dimension ,.i.s substantially anti-reflect ive , is. disposed substantially perpendicular 0 - ' to said incident radiation , and which receives and passes sa id incident .radiation; said- reflector means further including a segond surface adapted to reflect radiation by total 5- internal reflection; and said reflector means further includi ng a• plurality of folding surfaces for conducting said• incident radiation by total internal reflection .

1. .

2. A polarization conserving , radiation folding element, comprising:^' reflector, means for receiving and redirect ing incident radiation^' away, from said reflector means in a •5 , • direction substantially ^'anti-parallel to a direct ion of incidence ; said., reflector means including a first surf ace .-" which:

has a longitudinal axi s along i ts longest 10 dimension , is substantially anti-reflective, .- is disposed substantially perpendicular to said incident radiation, .and which receives and passes said 15. .incident .radiation; said ^'reflector means further including a second surface adapted to reflect radiation by total internal • reflection; said reflector means further including a pair of •20. surfaces perpendicular to said first surface and disposed at equal angles of inclination relative to said •^• . ' longitudinal axis in order to redirect said incident radiation by total internal reflection; said reflector means also having a plurality of 25 ' . ^'.- total internal,reflection points including equal numbers of P-type and S-type*reflections so that said reflector means preserves an incident polarization state of said incident radiation;- a d ^' said reflector means having said internal reflection ^■30. ' points located so that said reflector means is substanti lly*alignment stable for rotations about its axes.

1 ^•'

3. A beam folding, polarization preserving, alignment stable, intermediate reflector prism for use in a laser resonator that emits a plurality of beams comprising:

5 '^{• ' '} ■ a first- hexagonal exterior surface whi ch : has *a longitudinal axi s parallel to a ^• pair - of longitudinal edges , 'includes a pa ir of transverse edges orthogonal to said longitudinal edges , 10 is coated wi th an anti-reflect ive material , is aligned substantially perpendicular • .to said beams, and which receives and passes said beams; 15 a second rectangular surface which: shares a common longitudinal edge with said first hexagonal exterior surface, has an identical pair of transverse edges perpendicular to said longitudinal edge 20 . that each form forty-five degree angles

^'with said first hexagonal exterior surface, •and which is adapted to reflect said beams at two reflection points by total internal reflection; 25 an identical pair of rectangular surfaces which: each- have a pair of transverse edges •aligned perpendicular to said longitudinal axis, are, arranged such that their interior 30 ^• surfaces face toward said second rectangular surface, . _r ' are each contiguous and perpendicular to said- first hexagonal exterior surface, and are each inclined forty-five degrees from 35..- . ' said longitudinal axis in order to redirect

,*,- ^•;^' -ss^'aid beams at central reflection points

_. ^".by total internal reflection; an identical pair of right, isoceles, triangular end surfaces which:. each separately share one of said transverse

.edges of said second rectangular surface, each separately share one of said transverse edges of said first hexagonal surface, •and. which each separately share one of said transverse edges of said identical

^' pair of rectangular surfaces; said .reflector prism also having four total internal reflection points including two P-type and two S-type reflections so that said reflector prism preserves an incident polarization state of each of said beams; and said reflector prism having said four internal reflection points located so that said reflector prism is • substantially alignment stable for rotations about ^'■^'• its three axes".

^•'

4. A -refl.ectance-outcoupled radiation waveguide comprising: ..- a substantially anti-reflective surface ^■ aligned perpendicular. to. an incident beam of radiation adapted to receive and pass said incident radiation; a first substantially total reflector means, a second. substantially total reflector means, and, a partial reflector means; . said, first substantially total reflector means: being disposed to receive said incident beam from said substantially anti-reflective surface and being^' disposed to transmit said incident ■ beam by total internal reflection to said second substantially total reflector means; said second substantially total reflector means further including a plurality of roof reflector surfaces for -returning said incident beam back to said first substantially total reflector means and then out 20_. ^' of said ref lectanc.e-outcoupled radiation waveguide through said substantially anti-reflective surface; and said -partial reflector means including an outcoupling surface aligned substantially perpendicular to an incident beam.

1 5. A ref lectance-outcoupled , polarization preserving, alignment stable, double end-reflector prism for a laser ^' esonator comprising: a pentagonal, substantially anti-reflective 5_. surface having:- a first, transverse edge end; a_. second, chisel edge end; two longitudinal edges running in the direction of said prism's longest dimension; 10 said pentagonal surface being aligned perpendicular to an incident beam of _.laser radiation adapted to receive and ^•pass said incident laser radiation; a slanted, substantially totally reflective 15 roof surface having: an upper transverse edge orthogonal to •said longitudinal edges of said pentagonal surface and ^• said slanted roof surface forming an 20^' • ^• -angle of forty-five degrees with said longitudinal edges of said pentagonal ..surface; said^" slanted roof surface and said pentagonal surface further having a common transverse edge at said first,^' transverse edge end of said pentagonal surface; an identical pair of substantially totally reflective, rectangular, chisel roof surfaces coupled to said pentagonal' s.urf ace at said second, chisel edge end of said pentagonal surface; a reflectance-outcoupling surface aligned

^' normal to an^' output .beam from said laser; said double end-reflector prism further having a plurality of total internal reflection points including equal numbers. of P-type and S-type reflections so that said double end-reflector prism preserves an incident polarization state of said incident laser radiation; and said; doub e end-reflector prism also having said .internal ^' ref lection points located so that said double end-reflector prism is substantially alignment stable for rotations about its three axes.

^'

6. A Q-switch comprising: a radiation folding waveguide as claimed in ' Claim 1; a reflectance-outcoupled radiation waveguide as claimed in ^'Claim 4; a polarizer; and an electro-optic crystal.

7. A Q-switch comprising: a be.am folding, polarization preserving, align¬ ment stable ,-• intermediate reflector prism as claimed in Claim 3; ^• a. ref lectahce-outcoupled , polarization preserving, alignment stable, double end-reflector prism as claimed in. Claim 5; a polarizer; and an electro-optic crystal. ^' 8. An improved laser resonator combination comprising: a Q-switch as claimed in Claim 6; a laser gain medium; and an alignment wedge.

^' 9. An impro.ved laser resonator combination comprising: a Q-switch- as claimed in Claim 7; a laser gain medium; and an alignment wedge.