GB2182197A

GB2182197A - Laser frequency doubler

Info

Publication number: GB2182197A
Application number: GB08624607A
Authority: GB
Inventors: Kuo-Ching Liu
Original assignee: Quantronix Inc
Current assignee: Quantronix Inc
Priority date: 1985-10-29
Filing date: 1986-10-14
Publication date: 1987-05-07
Also published as: DE3631909C2; FR2589290B1; GB8624607D0; DE3631909A1; FR2589290A1; GB2182197B; CA1249650A

Abstract

A frequency doubler for a laser in which a Type II SHG crystal 14 is oriented to generate a second harmonic frequency beam 24 in response to the orthogonal components E and O of a fundamental beam 22. After the fundamental beam makes a round trip through the SHG crystal 14, any differential phase delays between E and O rays of the fundamental beam due to birefringence are eliminated to improve the efficiency and stability of the cavity. The beam from the laser rod 18 may be polarized by polarizer 16 or may be incident on the SHG crystal 14 with a random polarization direction. <IMAGE>

Description

SPECIFICATION Frequency doubling a laser beam by using intracavity type II phase matching The invention relates to a frequency-doubled laser and to a method and apparatus for generating a frequency-doubled beam using Type II phase-matching in an intracavity second harmonic generation crystal.

Second Harmonic Generation (SHG) provides a means of doubling the frequency of a laser source. In this process, a fundamental electromagnetic wave in a nonlinear medium induces a polarization wave with a frequency that is double that of the fundamental wave. Because of dispersion in the refractive index of the medium, the phase velocity of such a wave is a function of its frequency, so the phase of the induced second harmonic polarization wave is retarded from that of the fundamental wave. Since the vector sum of all the generated second harmonic polarizations yields the SHG intensity, the intensity is limited by the phase retardation.Atechnique, known as phase matching, is designed to overcome this difficulty by utilizing the natural birefringence in uniaxial crystals, that is to say, the difference in the phase velocity as a function of polarization, to offset the dispersion effect so that the fundamental and second-harmonic waves can propagate in phase.

There are two well known types of phase matching, which employ the polarization vectors ofthe in cidentfundamental wave in different ways.

In Type I phase matching, the fundamental wave is polarized perpendicular to the crystal's optic axis (and O or ordinary ray) andthe induced Second Harmonic wave is polarized parallel to the optical axis (and E or extraordinary ray). (A method utilizing Type I phase matching is described in U.S. Patent No.

4,413,342.) Since the fundamental wave is polarized along the optical axes ofthe crystal, there is no change in its linear polarization when it exits from the crystal. An intracavityType I SHG arrangement can easily be adopted to take advantage of the higher power density available within the laser cavity because the introduction of the SHG crystal will not produce a significant polarization loss.

In Type II phase matching, the linearly polarized fundamental wave is equally divided into 0 and E rays by requiring its polarization to be 45" with respectto the optical axis of the crystal; the output second harmonic wave which results in linearly polarized parallel to the optical axis (and E ray). Here, the phase velocities of the 0 and E rays of the incident fundamental wave are different because of the natural birefringenceofthecrystal. In general,thelinear polarization ofthis input fundamental wave is turned into an elliptical polarization as it propagates through the crystal. The magnitude ofthe phase retardation between 0 and E rays is the product ofthe index difference in the material and the effective opti caipath.

When such a Type II crystal is placed inside a laser resonator, this phase retardation can cause serious power loss because the laser's original linear polarizationwill notin general be properly maintained.

When the laser is randomly polarized, as is the case in multimode lasers when the laser active medium is not naturally birefringent and no polarizing elements are employed intracavity, the Type II SHG crystal provides a phase retardation between the polarization components resolved along its 0 and E axes. This retardation, which doubles on the return trip ofthefun- damental beam through the Type II SHG crystal can affect the stability and output power of the laser by affecting the laser's ability to optimize its polarization relative to thermal or other induced birefringent effects in the laser active medium.

One can attempt to compensate this phase retardation using a passive device such as a Babinet-Soleil compensator. However, the retardation is usuallydependent upon temperature and variations in temperature can be induced either by the ambient environment or by self-absorption of the laser radiation (fundamental and/or second harmonic) in the crystal itself. Such passive compensation thus becomes difficultto maintain during standard laser operation.

Because of these problems, Type II SHG has typically been employed in an extracavityarrangmentin which the polarization of the existing fundamental wave from the SHG crystal is unimportant. Of course the advantage of being able to use the higher power densityintracavityfundamental wave within the laser cavity in generating the second harmonic wave is lost.

Many lasers can have the temporal form oftheir output power altered by a process known as Qswitching. Here, a special device which alters the optical quality of Q of the resonator is inserted into the beam within the resonator cavity. This "Q-switch" can be activated to produce enough optical loss to overcome the optical gain or amplification supplied by the laser active medium, thereby inhibiting oscillation. If the source exiting the laser active medium is maintained on during the low-Q period, energy is stored in the laser active medium in the form of an excess population inversion. When the Q-switch is turned off(returingthe resonatorquicklyto its high Q state) this excess population is utilized to produce a high-intensity, Q-switched pulse.Since most Qswitches are electronically controlled, the process is repeatable at high repetition rates making a Qswitched laser a useful source of high intensity pulses. Peak pulse intensities many thousands of times higher than the laser's continuous wave output power level can be generated. Because of the sup eriorfocusability and enhanced material interaction of shorter wavelengths, it is often of interesttofrequ- ency-doublethe output of Q-switched lasers.

The invention provides a frequency-doubling laser comprising: first and second mirrors constructed and arranged to form an optical cavity therebetween; a laser active medium disposed within said cavity for generating a beam at a preselected fundamental frequency and having a linear polarization which can be resolved into two orthogonal components; a crystal for generating via Type II phase-matching a linearly polarized second harmonic beam in response to said fundamental beam, said crystal having an ordinary and an extraordinary axis, said orthogonal components of said fundamental beam being oriented along said axes, said crystal further generating a dif ferential phase delay between said orthogonal components; and means for eliminating on the return trip through said crystal said differential delay.

The invention also provides a frequency-doubled laser comprising: a first mirror; a laser active medium for generating a fundamental beam at a preselected frequency and linear polarization resolved into two orthogonal components; a crystal with its optic axis at an angle of 45" with respect to said preselected linear polarization having an ordinary and an extra-ordinary axis oriented along said orthogonal components to generate a second harmonic beam in response to both orthogonal components, said crystal further generating a differential phase delay between said orthogonal components; means for interchanging upon first passage through said means, reflection from the second mirror and return passage through said means the respective orientations of said orthogonal components to eliminate by return passage through said SHG crystal said differential delay; and a second mirror, said first mirror and second mirror being arranged to form an optical cavityforsaid laser active medium, crystal and interchanging means.

The laser may include a polarizer, preferably located between the laser-active medium and the said crystal, arranged so to select the linear polarization of the fundamental beam that the said orthogonal components are of equal amplitude.

The invention further provides a frequencydoubled laser comprising: first and second mirrors constructed and arranged to form an optical cavity therebetween; a laser active medium disposed within said cavity for generating a beam at a preselected fundamental frequencythe beam having a random polarization which can be resolved into two coplanar components; a crystal for generating via Type II phase-matching a linearly polarized second harmonic beam in response to said components, said crystal having an ordinary and an extraordinary axis, said orthogonal rays of said fundamental beam components being oriented along said axes, said crystal further generating a differential phase delay between said orthogonal rays; and means forelim- inating on the return trip through said crystal said differential delay.

The invention also provides a frequency-doubled laser comprising: a first mirror; a laser active medium for generating a fundamental beam at a preselected frequency with random polarization, which polarization can be resolved into two coplanarorth- ogonal components; a crystal with its optic axis par allel to one of said components having an ordinary and an extraordinary axis oriented along said orthogonal rays to generate a second harmonic beam in response to said orthogonal components, said crystal further generating a differential phase delay between said orthogonal rays; means for interchanging upon first passage through said means, re flection from the second mirror and return passage through said means the respective orientations of said orthogonal rays to eliminate by return passage through said SHG crystal said differential delay; a second mirror, said first mirror and second mirror being arranged to form an optical cavityforsaid laser active medium, crystal and interchanging means.

Laser active media such as a Nd-YAG laser rod pro ducing a fundamental beam at 1064 nm are not natu rally birefringent, but in use in a laser some degree of birefringence may be produced by thermal stress. As a result, such a medium tends to produce a laser beam that is randomly polarized in the sense that, although at any instant the beam may be plane polarized, the plane of polarization fluctuates rapidly at random. The time-scale of the changes in polarization is typically small compared with the duration of a pulse from a Q-switched laser. If an intracavity polarizer is provided to impose on the laser a particular plane of polarization, then some loss of power output will tend to result because not all parts of the laser rod will always be able to lase efficiently in that plane.If, on the other hand, no polarizer is provided, then the components of the fundamental beam parallel to the ordinary ane extraordinary axes of the SHG crystal will not always be of equal amplitudes and some loss of efficiency in the generation ofthe second harmonic beam may result. In either case, however, the fluctuations in efficiency will average outovertheduration of a Q-switched pulse and the plane of polarization of the second-harmonic beam will be determined bythe orientation of the SHG crystal.

It is a principal object of the invention to overcome the disadvantages of a system using intracavity Type II phase matching for SHG by having the effect of birefringence of the SHG crystal on the polarization of the fundamental wave compensated for upon return passage ofthe fundamental wave through the SHG crystal.

It is another object ofthe invention to provide laser frequency-doubling apparatus with a laser medium in which the fundamental beam incident on the laser medium maintains its original polarization.

Afurther object is to provide a system in which the output, frequency-doubled beam, has a known polarization.

One form of the system includes a laser harmonic generating means for generating the second harmonicfrequencyofthefundamental frequency emitted by the laser, means for dynamically compensat- ing for any phase lags generated in the fundamental beam passing through the said harmonic generating means, a first highly reflecting mirror atthefundamental frequency, and a second mirror. The first and second mirrors are positioned to form a cavityforthe laser, the harmonic generator and the compensating means.

Various forms of frequency-doubling laser constructed in accordance with the invention will now be described by way of example with referencetothe accompanying drawings, in which: Figure 7 shows somewhat schematicallyafirst form of laser; Figure2showssomewhatschematicallya second form of laser; Figure 3 shows somewhat schematically a third form of laser; Figure4shows somewhat schematicalíy a fourth form of laser; Figure 5shows somewhat schematically a fifth form of laser; and Figure 6shows somewhat schematically a sixth form of laser.

Referring to the drawings, and initially to Figure 1, afirstform offrequencydoubling laser system com prisesthefollowing elements aligned along a common optical axis 8 as shown: a mirror 10, a quarterwave plate 12, an SHG crystal 14, a polarizer 16, an active laser medium 18 and a second mirror 20. The laser 18 is arranged in use to generate a laser beam at a predetermined fundamental frequency along the common axis 8. For example, the laser may be a YAG laser which emits a beam at a wavelength of 1064 nm. The active laser medium, a laser rod, may be included within a pumping medium, a laser rod, may be included with a pumping reflectorwith a pumping lamp. These latter laser elements are well known in the art and therefore have not been shown in Figure 1 forthe sake of clarity.

The crystal 14 is a known second harmonic generator crystal such as a KTP (potassium titanyl phosphate) crystal. It is important that the crystal 14 is oriented with its optic axis, shown by an arrow Z in Figure 1, atan angle of 454to the plane of polarization ofthe beam from the laser 1 8. Thus, for example, if the fundamental beam Ffrom the laser 18 is pol arized vertically then, as shown in Figure 1,the 0 and E axes of the crystal 14 are oriented at an angle of45" from the vertical.

The plate 12 is selected to operate as a quarter wave plate atthefundamental frequency and simultaneously as a half wave plate at the second har monicfrequency. The optical axis of the plate (or its perpendicular indicated by arrow Q in figure 1) is or ented parallel to the polarization ofthe laser beam.

The first mirror 10 is highly reflective at the funda mentalfrequencyand highlytransmissiveatthe second harmonic frequency. The second mirror 20 is highly reflective at the fundamental frequency. The mirrors 10 and 20 are positioned and arranged to form a resonating optical cavity for the fundamental beam generated by the active laser medium 18, with the SHG crystal 14 and the plate 12 disposed within the cavity.

The polarizer16 is usedto polarizethe laserbeam in the vertical direction (V).

As the initial laser beam 22 propagates through the crystal 14, the crystal, in response to both the O and E components ofthe beam 22, generates a beam 24 having double the frequency of the fundamental beam and oriented at 450to the vertical (an E ray) as shown. The second-harmonic beam 24 is transmitted through the plate 12 and the first mirror 10 outofthe cavity. Because the plate 12 acts as a half-wave plate at the second harmonic, the beam 24 atthe doubled frequencytransmitted is rotated by 90" and then passes through the first mirror 10. As shown bythe arrow it is linearly polarized at 450 to the vertical dir ection.

As the fundamental beam 22 with its linearpolariz ation oriented at 45" to the Z axis propagates through the SHG crystal 14, the birefringence causes a phase retardation to occur between fundamental 0 and E components.

As shown in Figure 1, it is assumed that, after passing through crystal 14,theOcomponentofthefundamental beam 22 lags behind the E component.

In fact obviously, without any phase lag compensatory means, the fundamental beam reflected from the mirror 10 and backthrough the SHG crystal 14 will exhibit twice the phase retardation shown after one pass and the polarization ofthe beam re-entering the polarizer 16will not in general be linearandvertical, resulting in significant and undesirable polarization loss.

Therefore, in the present apparatus, the beam 22 is passed from the SHG crystal 14through the plate 12 which is a quarter-wave plate of the fundamental frequency. In Figure 1, as previously mentioned, the plate 12 is shown with its optic axis parallel (or perpendicular) to the polarization ofthe fundamental beam incident on the crystal 14. After reflection by the mirror 10, the beam 22' passes again through the quarter-wave plate 12. As a result of the two passes through the plate 12, the polarization components of beam 22 have been rotated by 90" so that, as shown in Figure 1 ,the orientation ofthe E and 0 components of the reflected beam 22' are reversed with respect to the orientation of the components of the beam 22, but component 0 still lags behind component E.The reflected beam 22' then passes through the SHG crystal 14 but this second time, component E is the ordinary component 0' and is retarded relative to component 0, which is now the extraordinary component E8, by an amount identical to the amount by which component 0 was previously retarded relativeto component E. The components E and 0 of the beam 22' as it leaves the SHG crystal 14 are now in phase and combine to yield a linear polarization F' parallel to the original polarization F. Therefore, by interposing the plate 12 between the SHG crystal 14 and the first mirror 10, the birefringent effects ofthe SHG crystal are successfully self-compensated and thereby eliminated.

As a result, the fundamental beam 22 incident on the crystal 14 and the fundamental beam 22' exiting from the crystal 14 have identical linear polarizations resulting in no polarization loss in the laser resonator.

Underthe conditions described above, the second harmonic beam 24 generated with the SHG crystal 14 comprises a component ED polarized at 45" to the ver- tical as shown. Since in many applications it is desirable to obtain a frequency-doubled laser beam of a known linear polarization, the plate 12 is constructed to act simultaneously as a half-wave plate at the second harmonicfequencythereby rotating the second-harmonic beam 24 by 900. (If the properties of the plate 12were unspecified atthesecond harmonic frequency,thefrequencydoubled beam would in general have an arbitrary elliptical polarization.) As a result, the beam 24 exiting from the optical cavity is linearly polarized along the ordinary axis as shown.

It should be appreciated that the plate 12 and the SHG crystal 14 accomplish their intended purposes dynamically. In the present invention,the phase lag is automatically and accurately corrected regardless ofthetemperature of the crystal.

Referring now to Figure 2, a second form offrequ- ency-doubling laser comprises a three-mirror cavity with a first mirror 112, an SHG crystal 114, a quarterwave plate 116, a second mirror 118, a third mirror 120, a laser active medium 110 and a polarizer 128.

The laser 110, the crystal 114, the quarter-wave plate 116 and the polarizer 128 function in a manner identicaltotheircounterparts inthefirstform of laser, shown in figure 1. Thethird mirror 120 is highly reflective atthe fundamental frequency and highly transmissive at the second harmonic frequency. The first mirror 112 is positioned and arranged to focus the output ofthe laser lOon the SHG crystal 1 14for effective second harmonic generation. The second mirror 118 is highly reflective both atthe fundamental frequency and at the second harmonicfrequency.

In operation, a fundamental beam 122 produced by the laser active medium 110 is reflected and focused bythe first mirror 112 onto the SHG crystal 114. The crystal generates a linearly-polarised second harmonic beam 124. After propagation through the crystal 114, the 0 and E components ofthefunda- mental beam 122 are phase shifted with respect to each other as described above with reference to Figure 1.Also, as in the first form of laser, thefunda- mental frequencyquarter-wave plate ll6andthe second mirror 118 are used to rotate the 0 and E components by 90" after reflection so that the passage of the reflected beam 122' backthrough crystal 114 puts all components backin phaseandrestoresthepol- arization to that linear polarization which initially left laser active medium 110. On the return trip through the SHG crystal 114, the reflected fundamental beam 122' generates a second harmonic beam 126, which is colinearwith the reflected second harmonic beam 124'.

Thus, in this laser, the second harmonic beam 126 generated on the return trip of the fundamental beam 122' is not lost so the potential exists for a second harmonic power gain of a factor of two compared with the firstform of laser. If interference were to occur between the second harmonic beams 124' and 126, it would affect the stability ofthe SHG output intensity. In orderto overcome this undesirable effect, the polarizations ofthe beams 124' and 126 are made orthogonal using a technique similar to that described in United States Patent Specification No.

4,413,342. The quarter-wave plate 116 is simultaneously made a quarter-wave plate at the second harmonicfrequency. The second-harmonic beam 124will upon passagethrough the plate 116, reflec- tion from the second mirror 118, and return through the plate 116 as the reflected beam 124' have its polarization rotated by 90 and thereby be orthogonal and non-interfering with the beam 126. The secondharmonic beams 124' and 126 arethen coupled out through the highlytransmissive first mirror 112.

The reflected fundamental beam 122' after passing through the SHG crystal 114, is reflected bythefirst mirror 112towardsthe laser 110. The third mirror 120 completes the optical cavity. The quarter-wave plate 1 compensatesforthe phase shift in the 0 and E components ofthefundamental beam as previously described thereby ensuring that the beams 122 and 122' have the same linear polarization.

Referring now to Figure 3, a third form offrequ ency-doubling laser system comprises the following elements aligned along a common optical axis 8 as shown: afirst mirror 10, a quarter-wave plate 12,an SHG crystal 14, a polarizer 16, an active laser medium 18, a Q-switch 19 and a second mirror 20. The laser 18 is arranged in operation to generate a laser beam at a predetermined fundamental frequency along the common axis 8. For example, the laser may be a YAG laserwhich emits a beam at a wavelength of 1064 mm. The active laser medium, a laser rod, may be included within a pumping reflectorwith a pumping lamp and the beam generated by the laser rod is Q switched bythe switch 19 in the conventionai manner.The other components ofthe third form of laser are the same as those of the first form of laser.

They have been accorded the same reference num erals and the description of their structure and function will not be repeated.

Referring now to Figure 4 of the drawings, a fourth form of frequency-doubled laser comprises a three mirror cavity with a first mirror 112, an SHG crystal 114, a quarter-wave plate 116, a second mirror 118, a third mirror 120, a laser active medium 110, a Qswitch 119 and a polarizer 128. The laser 110, the SHG crystal 114, the quarter-wave plate 116, the Q-switch 119, and the polarizer 128 function in a manneridentical to theircounterparts in the third form of laser, shown in Figure 3. The components apart from the Q-switch 119 are identical to, and function in the same manner as, the corresponding components in the second form of laser, shown in Figure 2. They have been given the same reference numerals and their description will not be repeated. The switch 119 Q-switches the laser output in the conventional manner.

Referring now to Figure 5, a fifth form offrequencydoubling laser comprises the following elements aligned along a common optical axis 8 as shown: an active laser medium 18, an optional Q-switch 19, and a second mirror 20. The laser 18 is arranged in use to generate a laser beam at a predetermined fundamental frequency along the common axis 8. For example, the laser may be a YAG laser which emits a beam at a wavelength of 1064 nm. The active laser medium, a laser rod, may be included within a pumping reflectorwith a pumping lamp. These latter laser elements are well known in the art and therefore have not been shown in Figure forthe sake of clarity.

The beam 222 emitted by laser active medium 18 has a random polarization and is shown in Figure 5 as being resolved into two orthogonal components V and H.

The SHG crystal 214 is a known second harmonic generator crystal such as a KTP (potassiumtitanyl phosphate) crystal. The SHG crystal 214 is oriented with its optic axis, shown by an arrow Z in Figure 5, parallel to one ofthe components ofthe beam from the laser active medium 18, for example, component V. Thus, for example, componentVfrom laser 18 is oriented vertically, along the axis, and component H horizontally, along the X axis. Then, as shown in Figure 5,the and 0 axis oftheSHG crystal 214are oriented parallel and perpendicular to the vertical.

The plate 212 is selected to operate as a quarter wave plate at the fundamental frequency. The optical axis ofthe plate 212, indicated by an arrow Q in Figure 1 is oriented at 45" to the V component ofthe fundamental beam.

The first mirror 10 is highly reflective atthefunda- mental frequency and highlytransmissiveatthe second harmonic frequency. The second mirror 20 is highly reflective at the fundamental frequency. The mirrors 10 and 20 are positioned and arranged to form a resonating optical cavity for the fundamental beam generated by the active laser medium 18, with the SHG crystal 214 and the plate 212 disposed within the cavity.

As the initial beam 222 propagates through the SHG crystal 214, the crystal, in response to both the V and H components (the 0 and E rays) of the linearly polarized beam 222, generates a beam 224 having double the frequency of the fundamental beam or ented along the vertical (and E ray) as shown. The second-harmonic beam is transmitted through the plate 212 and through thefirst mirror 10 outofthe cavity.

As the fundamental beam 222 with its vertical and horizontal polarization, oriented parallel and per pendicularto the optic axis Z, propagates through the SHG crystal, the birefringence causes a phase retardation to occur between the V and H components (E and 0 rays, respectively) ofthefundamental beam.

In Figure 5 it is assumed that, after passing through theSHG crystal 214,the0 rayofthefundamental beam 222 lags behind the E ray.

Without any means for compensating forthe phase lag, the fundamental beam 222' reflected from the first mirror 10 and backthrough the SHG crystal 214would exhibit twice the phase retardation shown after one pass, and the polarization of the beam reentering the laser active medium 18 would not in general be the same as that ofthe beam 222 initially leaving the laser active medium, which could result in significant and undesirable losses or instability in laser 18.

Therefore, in the present laser, the fundamental beam 222 is passed from the SHG crystal 214through the plate 212 which is a quarter-wave plate ofthefun damental frequency. In Figure 5, as previously mentioned, the plate 212 is shown with its optic axis at 45" tothecomponentVofthefundamental beam 222 in cident on the SHG crystal 214. After reflection by the firstmirrorl0,thereflectedfundamental beam 222' passes again through the quarter-wave plate 212.As a result ofthe two passes through the quarter-wave plate 212, components V and H ofthe beam 222 have been rotated by 9050 that, as shown in Figure 5, the orientation of theE and 0 rays of the reflected beam 222' are reversed with respect to the orientation of the components of the fundamental beam 222. That is to say, the extraordinary ray E which is the vertical component V of the fu ndamenta I beam 222 becomes the ray E' which is the horizontal component H' ofthe reflected beam 222' which is the ordinary ray, and similarlythe ordinary ray 0, which is the horizontal component H of the fundamental beam becomes the ray 0' which is the vertical component of V' ofthe reflected beam and is an extraordinary ray. Ray O lagged behind ray E, and ray 0' still lags behind ray E'.The reflected beam 222, then passesthroughthe SHG crystal 214 and, this second time, ray E' is retarded relativeto ray 0' by an amount identical tothe amount by which ray 0 was previously retarded relativeto ray E sothatthe rays E' and O' ofthe reflected beam 222' as it leaves the SHG crystal 214 are now in phase and combinetoyieldfundamental beam components V and H in the same phase as originally left crystal 12. Therefore by interposing the quarter-wave plate 212 between the SHG crystal 214 and the first mirror 10, the birefringent effects of the SHG crystal are successfully self-compensated and thereby eliminated.

As a result, the fundamental beam components V and H incident on the SHG crystal 214 and thefunda- mental beam components V' and H ' exiting from the SHG crystal have identical phase relationships resulting in no loss or instability in the laser resonator.

The components V and H are in effect interchanged to form the components V' and H'. If the components V and H were not of equal amplitude, then the reflected fundamental beam 222' will not be polarized in thesameplaneasthefundamental beam 222 but when the fundamental beam is randomly polarized that has been found to be acceptable.

It should be appreciated that the quarter-wave plate 212 and the SHG crystal 214accomplish their intended purposes dynamically, and the phase lag is automatically and accurately corrected regardless of the temperature of the crystal.

If necessary a Q-switch 19 may be added between the laser 18 and the second mirror 20 to Q-switch the laser beam in the normal manner.

Referring now to Figure 6, a sixth form offrequency doubling laser comprises a three-mirror cavity with afirst mirror 112, an SHG crystal 314, a quarterwave plate 316, a second mirror 118, athird mirror 120, and a laser active medium 110. The laser 1 10,the SHG crystal 314, and the quarter-wave plate 316 func- tion in manner identical to their counterparts in the fifth form of laser, shown in Figure 5.

Thethird mirror 120 is highly reflectiveatthefun- damental frequency, the first mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition,thefirstmirror 112can be positioned and arranged to focus the output of the laser 110 on the SHG crystal 31 4for effective second-harmonic generation. The second mirror 118 is highly reflective at the fundamental frequency and at the second harmonicfrequency.

In operation, afundamental beam 322 having random polarization produced bythe laser active medium 110 is reflected and focused bythefirst mirror 112 on to the SHG crystal 314. The crystal generates a second harmonic beam 324. After propagation through the SHG crystal 314, the O and E rays of the fundamental beam 322 are phase-shifted with respectto each other as has been described above with reference to the fifth form of laser according to the invention.Also, as described above,thefunda mental-frequency quarter-wave plate 316 and the second mirror 118 are used to rotate the 0 and E rays by 90" after reflection so that passage of the reflected fundamental beam 322' backthrough the SHG crystal 314 puts all components back in phase and restores the polarization to that polarization which initially left the laser active medium 110. On there turntripthroughthe SHG crystal 314,thefunda- mental beam 322' generates a second harmonic beam 326, which is colinearwiththe beam 324, from thesecond mirror 118 as a second harmonic beam 324'.

Thus, in this form of laser the second-harmonic beam 326 generated on the return trip ofthefunda- mental beam 322' is not lost, so the potential exists for a second harmonic power gain of a factor of two.

Interference may occur between the secondharmonic beams 324' and 326 which would affect the stability ofthe SHG output intensity. in orderto overcomethis undesirable effect, the polarizations ofthe beams 124' and 126 are made orthogonal using a technique similarto that described in United States Patent Specification No.4,413,342. The quarter-wave plate 316 is simultaneously made a quarter-wave plate atthe second harmonic frequency. The secondharmonic beam 324 will, upon passage through the quarter-wave plate 316, reflection from the second mirror 118, and return through thequarter-wave plate, have its polarization rotated by 90" and thereby be orthogonal and non-interfering with the beam 126. The second-harmonic beams 124' and 126 are then coupled outthrough the highly-transmissive first mirror 112.

The reflected fundamental beam 322', after passing through the SHG crystal 314, is reflected bythe first mirror 11 towards the laser 110. The third mirror 120 completes the optical cavity. The quarterwave plate 116 compensates for the phase shift in the O and E rays ofthefundamental beam as previously described, thereby insuring that the fundamental beams 322 and 322' have the same randon polarization.

A Switch 119 may be added to Q-switch the fundamental beam as described above.

Claims

1. Afrequency-doubled lasercomprising: first and second mirrors forming an optical cavity therebetween; a laser-active medium disposed within the optical cavity for generating a fundamental beam at a pre-selected fundamental frequency and having a linear polarization which can be resolved into two orthogonal components; a crystal for generating via Type II phase-matching a linearly-polarized second harmonic beam in response to said fundamental beam,the said crystal having an ordinary and an extraordinary axis, the said orthogonal components of the said fundamental beam being oriented along the said axes, the said crystal further generating a differential phase delay between the said orthogonal components; and means for eliminating on the return trip through said crystal said differential delay; to produce a frequency-doubled output beam.

2. Afrequency-doubled laser comprising: a first mirror; a laser-active medium for generating a fundamental beam at a preselected frequency and having a linear polarization resolved into two othogonal components; a crystal with its optic axis at an angle of 45" with respect to the said preselected linear polarization having an ordinary and an extraordinary axis oriented along the said orthogonal components to generate a second harmonic beam in response to both orthogonal components, the said crystal causing a differential phase delay between the said orthogonal components; means for interchanging the respective orientations of the said orthogonal components to eliminate the said differential phase delay by return passage through the said crystal; and a second mirror; the first and second mirrors being arranged to form an optical cavity for the laser active medium,the said crystal, andthesaid interchanging means.

3. Afrequency-doubled laser comprising: first and second mirrors constructed and arranged to form an optical cavitytherebetween; a laser active medium disposed within the optical cavityforgenerating a beam at a preselected fundamental frequency, the beam having a random or arbitrary linear polarization that can be resolved into two components polarized in orthogonal planes; a crystal for generating via Type II phase-matching a linearlypolarized second-harmonic beam in response to the said components of the fundamental beam, the said crystal having an ordinary and an extraordinary axis, each said axis lying in the plane of polarization of a respective one of the said orthogonal components of said fundamental beam, the crystal tending to cause a relative phase shift between the said orthogonal components of the fundamental beam; and means for causing the said phase shift to be eliminated on the return trip ofthe fundamental beam through the said crystal.

4. Afrequency-doubled laser comprising: a first mirror; a multimode laser-active medium that is not naturally birefringentfor generating a fundamental beam art a preselected frequency, with the funda- mental beam having a random or arbitrary polarization, which polarization can be resolved into two coplanar orthogonal components; a crystal with its optic axis parallel to one of said components having an ordinary and an extraordinary axis oriented along the said orthogonal rays to generate a second harmonic beam in response to the said orthogonal components, the said crystal further generating a differential phase delay between the said orthogonal rays; means for interchanging upon first passage through said means, reflection from the second mirror and return passage through said means the respective orientations ofthe said orthogonal rays to eliminate by return passage through said SHG crystal and the said differential delay; and a second mirror; the first mirror and the second mirror being arranged to form an optical cavity for the laser-active medium, the said crystal, and the said interchanging means.

5. Afrequency-doubled laser as claimed in any one of claims 1 to 4, comprising a Q-switch forQ- switching the fundamental beam.

6. Afrequency-doubled laser as claimed in any one of claims 1 to 5, wherein the said interchanging means maintains the linear polarization ofthe second harmonic beam.

7. A frequency-doubled laser as claimed in any one of claims 1 to 6, wherein the said interchanging means is a plate positioned for dual passes ofthe fundamental beam to exchange the ordinary and ex traordinarycomponentsofthefundamental beam before the fundamental beam re-enters the crystal.

8. A frequency-doubled laser as claimed in claim 7, wherein the said plate is a quarter-wave plate at the fundamental frequency.

9. Afrequency-doubled laser as claimed in claim 8, wherein the said plate is a half-wave plate atthe second harmonic frequency.

10. Afrequency-doubled laser as claimed in any one of claims 1 to 9, wherein the second mirror is highly reflective at the fundamental frequency.

11. Afrequency-doubled laser as claimed in claim 10, wherein the second mirror is highly trans- missive at the doubled frequency.

12. Afrequency-doubled laser as claimed in any one of claims 1 to 11, comprising a third mirrorfor reflecting and focusing the fundamental beam from the laser onto the crystal.

13. Afrequency-doubled laser as claimed in claim 12, wherein the third mirror is further provided for coupling the second harmonic beam out ofthe said cavity.

14. Afrequency-doubled laser as claimed in claim 12 or claim 13, wherein the said interchanging means forthe fundamental orthogonal components also interchanges the second-harmonic-frequency components to preclude interference.

15. Afrequency-doubled laser as claimed in any one of claims 12 to 14, wherein the said interchanging means is a quarter-wave plate at the fundamental frequency and a quarter-wave plate at the second harmonicfrequency.

16. Afrequency-doubled laser as claimed in claim 7 or claim 8, wherein the said plate is between the said crystal and the point at which thefrequency- doubled beam leaves the optical cavity, and the said plate is a half-wave plate at the doubled frequency.

17. Afrequency-doubled laser as claimed in any one of claims 1 to 8, wherein the said crystal is between the interchanging means and the point at which the frequency-doubled beam leaves the optical cavity, and the interchanging means is arranged to rotate through 900 the plane of polarization of the frequency-doubled beam that passes through it twice.

18. Afrequency-doubled laser substantially as hereinbefore described with reference to, and as shown in, any of the acco m panying drawings.