CA1249650A - Frequency doubling a laser beam by using intracavity type ii phase matching - Google Patents
Frequency doubling a laser beam by using intracavity type ii phase matchingInfo
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- CA1249650A CA1249650A CA000520349A CA520349A CA1249650A CA 1249650 A CA1249650 A CA 1249650A CA 000520349 A CA000520349 A CA 000520349A CA 520349 A CA520349 A CA 520349A CA 1249650 A CA1249650 A CA 1249650A
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
- H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
- H01S3/109—Frequency multiplication, e.g. harmonic generation
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/35—Non-linear optics
- G02F1/37—Non-linear optics for second-harmonic generation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08054—Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
- H01S3/0813—Configuration of resonator
- H01S3/0815—Configuration of resonator having 3 reflectors, e.g. V-shaped resonators
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
- H01S3/1123—Q-switching
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Nonlinear Science (AREA)
- Optics & Photonics (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- General Physics & Mathematics (AREA)
- Lasers (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A frequency-doubler for a laser is disclosed in which a Type II SHG crystal is oriented to generate a second harmonic frequency beam in response to the orthogonal components of a fundamental beam. After the fundamental beam makes a round trip through the SHG crystal, any differential phase delays between the orthogonal components or between the E and O rays of the fundamental beam due to birefringence are eliminated to improve the efficiency and stability of the cavity.
A frequency-doubler for a laser is disclosed in which a Type II SHG crystal is oriented to generate a second harmonic frequency beam in response to the orthogonal components of a fundamental beam. After the fundamental beam makes a round trip through the SHG crystal, any differential phase delays between the orthogonal components or between the E and O rays of the fundamental beam due to birefringence are eliminated to improve the efficiency and stability of the cavity.
Description
124g650 BACKGROUND OF T~E INVENTION
The field of the invention concerns a frequency-doubled laser, and in particular a method and apparatus for generating a frequency-doubled beam using Type I I phase-matching in an intracavity second harmonic generation crystal.
DESCRIPTION OF THE PRIOR ART
Second Harmonic Generation (SHG) provides a means ofdoubling the frequency of a laser source. In this process, a fundamental electromagnetic wave in a non-linear medium induces a polarization wave with a frequency that is doubled 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 ~econd harmonic polarizations yield the SHG intensity, the intensity is limited by the phase retardation. A technique, known as phase-matching, is designed to overcome this difficulty by utilizing in uniaxial and biaxial crystals the natural biref~ingence, i.e. the difference in the phase velocity as a function of polarization, to offset the dispersion effect so that the fundamental and second harmonic wave can propagate in phase.
,,, lZ4~650 There a~e two well known types of phase-matching, which employ the polarization vectors of the incident fundamental wave in different ways.
In Type I 2hase-matching, the fundamental wave i~
polarized perpendicular to the crystal's optic axis (an O or ordinary ray) and the induced second harmonic wave is polarized parallel to the optical axis (an E or extraordinary ray). (A
method utilizing Type I phase-matching i8 described in U.S.
Patent No. 4,413,342.) Since the fundamental wave i6 polarized along the optic axes of the crystal, there is no change in its linear polarization when it exits from the crystal. ~n intracavity Type 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 O and E rays by requiring its polacization to be 45 with respect to the optic axis of the crystal; the output second harmonic wave which results is linearly polarized parallel to the optic axis (an E
ray). Here, the phase velocities of the O and E rays of the incident fundamental wave are different due to the natural birefringence of the crystal. In general, the linear polarization of this input fundamental wave is turned into an elliptical polarization as it propagates through the crystal.
The magnitude of the phase retardation between O and E rays is '~ ` `~
lZ4~i50 the product of the index difference in the material and the effective optical path.
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 polariza~ion will not in 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 c~ystal provides a phase retardation between the polarization components resolved along its O and E axes. This retardation, which doubles on the return trip of the fundamental 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 usually dependent 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 difficult to maintain during standard laser operation. Due to these problems, Type II SHG has typically been employed in an extracavity arrangement in which the polarization of the 124'J650 exiting fundamental wave from the SHG crystal is unimportant.
Of course the advantage that the higher power density intracavity fundamental wave within the laser cavity has in generating second hacmonic, is lost.
Many lasers can have the temporal form of their output power altered by a process known as Q-switching. Here, a special device which alters the optical quality or 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 inver~ion. When the Q-switch is turned off (returning the resonator quickly to its high Q state) this excess population is utilized ~o produce a high-intensity, Q-switched pulse.
Since most Q-switches are electronically controlled, the process is repeatable at high repetition rates making a Q-switched laser a useful source of high intensity pulses.
Peak pulse intensities many thousands of times the la~er's continuous wave output power level can be generated. Because of the superior focusability and enhanced material interaction cf shorter wavelengths, it is often of interest to frequency-double the output of Q-switched lasers.
~.,.
124~50 SUMMARY OF TH~ INVENTION
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 be compensated for upon return passage of the fundamental wave through the SHG crystal.
It is another object of the invention to provide laser fcequency-doubling apparatus with a laser medium in which the fundamental beam incident on the laser medium maintains its original linear or random polarization.
A further object is to provide a system in which the output, frequency-doubled beam, has a known polarization.
The system includes a laser harmonic generating means for generating the second harmonic frequency of the fundamental frequency emitted by the laser which may be a Q-switched laser, means for dynamically compensating for any phase lags generated in the fundamental beam passing through said harmonic generating means, a first highly reflecting mirror at the fundamental frequency, and a second mirror. The first and second mirrors are positioned to form a cavity for the laser, the harmonic generator and the compensating means.
12~650 BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawings in 5 which:
Figures la, lb and lc illustrate second harmonic laser generators according to this invention; and Figures 2a, 2b and 2c show alternate embodiments of the invention.
While the invention will be described in conjunction with an example embodiment, it will be understood that it is not intended to limit the invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the aepended claims.
DET~ILED DESCRIPTION OF THE INVENTION
In the following description, similar features in the drawings have been given similar reference numerals.
Referring now to Figures la, lb, and lc, a frequency-doubling laser system comprises the following elements aligned along a common optical axis 8 as shown: a mirror 10, a quarter-wave plate 12, an SHG crystal 14, a ~olarizer 16, and .~
. ~
~, 12~36SV
active laser medium 18 and a second mirror 20. Laser 18 is adapted to generate a laser beam at a predetermined fundamental frequency along 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 reflector with a pumping lamp. These latter laser elements are well known in the art and therefore have not been shown in Figure 1 for the sake of clarity.
Crystal 14 is a known second harmonic qenerator crystal such as a KTP (potassium titanyl phosphate) crystal.
Crystal 14 is oriented with its optic axis, shown by arrow Z in Figures la and lb, at an angle of 45 with respect to the angle of polarization of the beam from laser 18. Thus, for example, if the fundamental beam F from laser 18 is polarized vertically, then as shown in these two figures, the 0 and E
axes of crystal 14 are oriented at an angle of 45 from the vertical.
Plate 12 is selected to operate as a quarter-wave plate at the fundamental frequency and simultaneously as a half wave plate at the second harmonic frequency. The optical axis of the plate (or its perpendicular indicated by arrow Q in Figures la and lb) i8 oriented parallel to the polarization of the laser beam.
Mirror 10 is highly reflective at the fundamental frequency and highly transmissive at ~he second harmonic frequency. Mirror 20 is highly reflective at the fundamental 1~9~i50 frequency. Mirrors 10 and 20 are positioned and arranged to form a resonating optical cavity for the fundamental beam generated by active laser medium 18, with the SHG crystal 14 and plate 12 disposed within the cavity.
Polarizer 16 is used to polarize the laser beam in the vertical direction (V).
As this initial beam 22 propagates through the crystal 14, the crystal, in response to both the O and E components of the beam 22 generates a beam 24 having double the frequency of the fundamental beam oriented at 45 to the vertical (an E ray) as shown. Beam 24 is transmitted through plate 12, and mirror 10 out of the cavity. Because plate 12 acts as a half-wave plate, the beam at the doubled-frequency transmitted is rotated by 90 and then through mirror 10. As shown by the arrow it i6 linearly polarized at 45 to the vertical direction.
As the fundamental beam 22 with its linear polarization oriented at 45 to the Z axis propagates through the SHG crystal, the birefringence causes a phase retardation to occur between fundamental 0 and E components.
In Figures la and lb it is assumed that after passing through crystal 14, The 0 component of the fundamental beam 22 lags behind the E component.
Without any phase lag compensatory means, the fundamental beam reflected from mirror 10 and back through the SHG crystal will exhibit twice the phase retardation shown after one pass and the polarization of the beam reentering the 124~?~i5() polarizer 16 (Figures la, lb) will not in general be linear and vertical, resulting in significant and undesirable polarization loss.
Therefore, in the present invention, beam 22 is passed from SHG crystal 14 through 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 of the fundamental beam incident on crystal 14. After reflection by mirror lO, the beam 22' passes again through quarter-wave plate 12. As a result of the two passes through plate 12, the polarization components of beam 22 have been rotated by 9O so that, as shown in Figure 1, the orientation of the E and O components of beam 22' are reversed with respect to the orientation of the components of beam 22. However component O still lags behind E. The reflected beam 22' then passes through crystal 14 but this second time, vertical component E is differentially phase shifted by an amount identical to the first differential phase shift with respect to O so that the components E and O of the beam 22' as it leaves the crystal 14 are now in phase and combine to yield linear polarization F'. Therefore by interposing plate 12 between crystal 14 and mirror lO, the birefringent effects of the SHG crystal are successfully self-compensated and thereby eliminated.
As a result, the fundamental beam 22 incident on crystal 14 and the fundamental beam 22' exiting from the ~24~9650 crystal 14 have identical linear polarizations resulting in no loss in the laser resonator.
Under the conditions described above, the harmonic beam ge~erated within crystal 14 comprises a component ED at 45 to the vertical as shown. S~nce in many applications it is desirable to obtain a freguency-doubled laser beam of a ~nown linear polarization, plate 12 is constructed to act simultaneously as a half-wave plate at the second harmonic frequency thereby rotating beam 24 by 90. If plate 12 were unspecified at the second harmonic frequency, the frequency-doubled beam would have an arbitrary elloptical polarization.
As a result, beam 24 exiting from the optical cavity is linearly polarized along the ordinary axis as shown.
In Figure lc, the beam emitted by laser active medium 18 has a random polarization and is shown in Figure 1 as being resolved into two orthogonal components V and H.
In Figure lc, crystal 14 is oriented with its optic axis, shown by arrow Z in Figure 1, parallel to one of the components of the beam from laser active medium 18, for example, component V. Thus, for example, component V from laser 18 is oriented vertically along the Y axis and component H horizontally along X. Then as shown in Figure lc, the E and O axes of crystal 14 are oriented parallel and perpendicular to the vertical.
Plate 12 is selected to operate as a quarter-wave plate at the fundamental frequency. The optical axis of the 12~3~;50 plate indicated by arrow Q in Figure lc i8 oriented at 45 to the V component of the fundamental beam.
Mirror 10 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. Mirror 20 is highly reflective at the fundamental frequency. Mirrors 10 and 20 are positioned and arranged to form a resonating optical cavity for the fundamental beam generated by active laser medium 18, with the SHG crystal 14 and plate 12 disposed within the cavity.
As the initial beam 22 propagates through the crystal 14, the crystal, in response to both the V and H components (the Q and E rays) of the linearly polarized beam 22, generates a beam 24 having double the frequency of the fundamental beam oriented along the vertical (an E ray) as shown in Figure lc.
15 Beam 24 is transmitted through plate 12, and mirror 10 out of the cavity.
As the fundamental beam 22 with its vertical and horizontal polarizations oriented parallel and perpendicular to the Z axis, propagates through the SHG crystal, the bire-fringence causes a phase retardation to occur betweenfundamental V and H components (E and 0 rays respectively) of fundmental beam 22.
In Figure lc it is assumed that after passing through crystal 14, the 0 ray of the fundamental beam 22 lags behind the E ray.
.;~
124~9650 Without any phase lag compensatory means, the fundamental beam reflected from mirror 10 and back through the SHG crystal will exhibit twice the phase retardation shown after one pass and the polarization of the beam reentering the laser active medium 18 will not in general be the same as that initially leaving 18 possibly resulting in significant and undesirable losses or instability in laser 18.
Therefore, in the present invention, beam 22 is passed from SHG crystal 14 through plate 12 which is a quarter-wave plate of the fundamental frequency. In Figure 1, as previously mentioned, the plate 12 îs shown with its optic axis at 45O to the component V of the fundamental beam incident on crystal 14. After reflection by mirror 10, the beam 22' passes again through quarter-wave plate 12. As a result of the two passes through quarter-wave plate 12, the V and H rays of beam 22 have been rotated by 90 so that, as shown in Figure 1, the orientation of the E and O rays of beam 22' are reversed with respect to the orienta~ion of the components of beam 22.
However ray O still lags behind E. The reflected beam 22' then passes through crystal 14 but this second time, ray E is differentially phase shifted by amount identical to the first differential phase shift with respect to O so that the rays E
and O of the beam 22' as it leaves the crystal 14 are now in phase and combined to yield fundamental beam components V and H
in the same phase as originally left crystal 12. Therefore by interposing plate 12 between crystal 14 and mirror 10, the 1249~5(:~
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 crystal 14 and the fundamental beam components V' and H~ exiting from the crystal 14 have identical phase relationships resulting in no loss or instability in the laser resonator.
It should be appreciated that plate 12 and crystal 14 accomplish their intended purposes dynamically. In the present invention, the phase lag is automatically and accurately corrected regardless of the temperature of the crystal.
If necessary, a Q-switch 16 may be added between laser 18 and mirror 20 to Q-switch the laser beam in the normal manner.
Another embodiment of the invention is shown in Figures 2a, 2b, and 2c. In the embodiment of Figure 2a, the frequency-doubler comprises a three-mirror cavity with a mirror 112, an SHG crystal 114, a quarter-wave 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 and quarter-wave plate 116 and polarizer 128 function in a manner identical to their counterparts in the embodiment of Figure la. Mirror 120 is highly reflective at the fundamental frequency, mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition mirror 112 is also positioned and arranged to focus the output ,, . ~ . ~
~Z~3~iSO
of laser 110 on crystal 114 for effective second harmonic generation. Mirror 118 is highly reflective at the fundamental frequency and at the second harmonic frequency.
In ope~ation, a fundamental beam 122 produced by laser active medium llO is reflected and focused by mircor 112 on crystal 114. The crystal generates a linearly second harmonic beam 124. After propagation through crystal 114, the O and E
components of fundamental beam 122 are phase shifted with respect to each other as described in the embodiment of Figure la. Also, as in this previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O and E component by 9O after reflection so that passage of beam 122' back through crystal 114 puts all components back in phase and restores the polarization to that linear polarization which initially left laser active medium llO. On the return trip through crystal 114, beam 122' generates second harmonic beam 126, which is colinear with reflected second harmonic beam 124'.
Thus, in this embodiment, the second harmonic genecated on the return trip of the fundamental is not lost so the potential exists for a second harmonic power gain of a factor of two. Interference may occur between these beams which will affect the stability of the SHG output intensity.
In order to overcome this undesirable effect, the polarizations of the beams 124' and 126 are made orthogonal using a technique similar to that described in U.S. Patent No. 4,413,342. Plate lZ4~i50 116, is simultaneously made a quarter-wave plate at the second harmonic frequency. Beam 124 will, upon passage through 116.
reflection from 118 and return through 116, have its polarization rotated by 90 and thereby be orthogonal and non-interfering with beam 126. Beams 124~ and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 is reflected by mirror 112 toward laser 110. Mirror 120 completes the optical cavity. Plate 116 compensates for the phase shift in the O and E components of the fundamental beam as previously described thereby insuring that beams 122 and 122' have the same linear polarization.
Another embodiment of the invention is shown in Figure lb. In this embodiment, the frequency-doubled laser comprises 15 a three-mirror cavity with a 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 Q-switch 119 and a polarizer 128. The laser 110, the crystal 114, quarter-wave plate 116, Q-switch 119 and polarizer 128 function in a manner identical to their counterparts in the embodiment of Figure lb. Mirror 120 is highly reflective at the fundamental frequency, mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition mirror 112 is also positioned and arranged to focus if 25 necessary on the output of laser 110 on crystal 114 for effective second harmonic generation. Mirror 118 is highly "~.
J 2~50 reflective at the fundamental frequency and at the second harmonic frequency.
In operation, a fundamental beam 122 produced by laser active medium 110 is reflected and focused by mirror 112 on crystal 114. The crystal generates a linearly polarized second harmonic beam 124. After propagation through crystal 114, O
and E components of fundamental beam 122 are phase shifted with respect to each other as described in the embodiment of Figure lb. Also, as in this previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O and E components by 90 after reflection so that pa6sage of beam 122' back through crystal 114 puts all components back in phase and restores the polarization to that linear polarization which initially left laser active medium llO. On the return trip through crystal 114, beam 122' generates second harmonic beam 126, which is colinear with reflected second harmonic beam 124~.
Thus, in this embodiment, the second harmonic generated on the return trip of the fundamental through the SHG
crystal is not lost so the potential exi6ts for a second harmonic power gain of a factor of two. Interference may occur between these beams which will affect the stability of the SHG
output intensity. In order to overcome this undesirable effect, the polarizations of the beams 124' and 126 are made orthogonal using a technique similar to that described in U.S.
Patent No. 4,413,342. Plate 116, is simultaneously made a lZ~96SO
quarter-wave plate at the second harmonic frequency. Beam 124 will, upon passage through 116, reflection from 118 and return through 116, have its polarization rotated by 90 and thereby be orthogonal and non-interfering with beam 126. Beams 124~
and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 i~
reflected by mirror 112 toward laser medium 110. Mirror 120 completes the optical cavity while switch 119 Q-switches the laser output in the conventional manner. Plate 116 compensate6 for the phase shift in the 0 and E components of the fundamental beam as previously described thereby insuring that beams 122 and 122' have the same linear polarization.
~ nother embodiment of the invention is shown in Figure 2c. In this embodiment, the frequency-doubled laser comprises 15 a three-mirror cavity with a mirror 112, an SHG crystal 114, a quarter-wave plate 116, a second mirror 118, a third mirror 120, and a laser active medium 110. The laser 110, the crystal 114, and quarter-wave plate 116 function in a manner identical to their counterparts in the embodiment of Figure lc. Mirror 120 is highly reflective at the fundamental frequency, mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition mirror 112 can also be positioned and arranged to focus the output of laser 110 on crystal 114 for effective second harmonic generation. Mirror 118 is highly reflective at the fundamental frequency and at the second harmonic frequency.
iz~g~50 In operation, a fundamental beam having random polarization 122 produced by laser active medium llO is reflected and focused by mirror 112 on crystal 114. The crystal generates a second harmonic beam 124. After propagation through crystal 114, the O and E rays of fundamental beam 122 are phase shifted with respect to each other as described in the embodiment of Figure lc. ~lso, as in this previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O
and E rays by 90 after reflection so that pasfiage of beam 122' back through crystal 114 puts all components back in phase and restores the polarization to that polarization which initially left laser active medium llO. On the return trip through crystal 114, beam 122' generates second harmonic beam 126, which is colinear with reflected second harmonic beam 124'.
Thus, in this embodiment, the second harmonic generated on the return trip of the fundamental is not lost so the potential exists for a second harmonic power gain of a factor of two. Interference may occur between these beams which will affect the stability of the SHG output intensity.
In order to overcome this undesirable effect, the polarizations of the beams 124' and 126 are made orthogonal using a technique similar to that described in U.S. Patent No. 4,413,342. Plate 116, is simultaneously made a quarter-wave plate at the second harmonic frequency. Beam 124 will, upon passage through 116, reflection from 118 and return through 116, have its ~ '`
12'~ 5~
polarization rotated by 90 and thereby be orthogonal and non-interfering with beam 126. Beams 124' and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 is reflected by mirror 112 toward laser 110. Mirror 120 completes the optical cavity. Plate 116 compensates for the phase shift in the O and E rays of the fundamental beam as previously described thereby insuring that beams 122 and 122' have the same random polarization.
~ Q-switch 128 may be added to Q-switch the fundamental beam as described above.
Obviously numerous other modifications may be made to the invention without departing from its scope as defined in the appended claims.
The field of the invention concerns a frequency-doubled laser, and in particular a method and apparatus for generating a frequency-doubled beam using Type I I phase-matching in an intracavity second harmonic generation crystal.
DESCRIPTION OF THE PRIOR ART
Second Harmonic Generation (SHG) provides a means ofdoubling the frequency of a laser source. In this process, a fundamental electromagnetic wave in a non-linear medium induces a polarization wave with a frequency that is doubled 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 ~econd harmonic polarizations yield the SHG intensity, the intensity is limited by the phase retardation. A technique, known as phase-matching, is designed to overcome this difficulty by utilizing in uniaxial and biaxial crystals the natural biref~ingence, i.e. the difference in the phase velocity as a function of polarization, to offset the dispersion effect so that the fundamental and second harmonic wave can propagate in phase.
,,, lZ4~650 There a~e two well known types of phase-matching, which employ the polarization vectors of the incident fundamental wave in different ways.
In Type I 2hase-matching, the fundamental wave i~
polarized perpendicular to the crystal's optic axis (an O or ordinary ray) and the induced second harmonic wave is polarized parallel to the optical axis (an E or extraordinary ray). (A
method utilizing Type I phase-matching i8 described in U.S.
Patent No. 4,413,342.) Since the fundamental wave i6 polarized along the optic axes of the crystal, there is no change in its linear polarization when it exits from the crystal. ~n intracavity Type 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 O and E rays by requiring its polacization to be 45 with respect to the optic axis of the crystal; the output second harmonic wave which results is linearly polarized parallel to the optic axis (an E
ray). Here, the phase velocities of the O and E rays of the incident fundamental wave are different due to the natural birefringence of the crystal. In general, the linear polarization of this input fundamental wave is turned into an elliptical polarization as it propagates through the crystal.
The magnitude of the phase retardation between O and E rays is '~ ` `~
lZ4~i50 the product of the index difference in the material and the effective optical path.
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 polariza~ion will not in 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 c~ystal provides a phase retardation between the polarization components resolved along its O and E axes. This retardation, which doubles on the return trip of the fundamental 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 usually dependent 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 difficult to maintain during standard laser operation. Due to these problems, Type II SHG has typically been employed in an extracavity arrangement in which the polarization of the 124'J650 exiting fundamental wave from the SHG crystal is unimportant.
Of course the advantage that the higher power density intracavity fundamental wave within the laser cavity has in generating second hacmonic, is lost.
Many lasers can have the temporal form of their output power altered by a process known as Q-switching. Here, a special device which alters the optical quality or 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 inver~ion. When the Q-switch is turned off (returning the resonator quickly to its high Q state) this excess population is utilized ~o produce a high-intensity, Q-switched pulse.
Since most Q-switches are electronically controlled, the process is repeatable at high repetition rates making a Q-switched laser a useful source of high intensity pulses.
Peak pulse intensities many thousands of times the la~er's continuous wave output power level can be generated. Because of the superior focusability and enhanced material interaction cf shorter wavelengths, it is often of interest to frequency-double the output of Q-switched lasers.
~.,.
124~50 SUMMARY OF TH~ INVENTION
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 be compensated for upon return passage of the fundamental wave through the SHG crystal.
It is another object of the invention to provide laser fcequency-doubling apparatus with a laser medium in which the fundamental beam incident on the laser medium maintains its original linear or random polarization.
A further object is to provide a system in which the output, frequency-doubled beam, has a known polarization.
The system includes a laser harmonic generating means for generating the second harmonic frequency of the fundamental frequency emitted by the laser which may be a Q-switched laser, means for dynamically compensating for any phase lags generated in the fundamental beam passing through said harmonic generating means, a first highly reflecting mirror at the fundamental frequency, and a second mirror. The first and second mirrors are positioned to form a cavity for the laser, the harmonic generator and the compensating means.
12~650 BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawings in 5 which:
Figures la, lb and lc illustrate second harmonic laser generators according to this invention; and Figures 2a, 2b and 2c show alternate embodiments of the invention.
While the invention will be described in conjunction with an example embodiment, it will be understood that it is not intended to limit the invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the aepended claims.
DET~ILED DESCRIPTION OF THE INVENTION
In the following description, similar features in the drawings have been given similar reference numerals.
Referring now to Figures la, lb, and lc, a frequency-doubling laser system comprises the following elements aligned along a common optical axis 8 as shown: a mirror 10, a quarter-wave plate 12, an SHG crystal 14, a ~olarizer 16, and .~
. ~
~, 12~36SV
active laser medium 18 and a second mirror 20. Laser 18 is adapted to generate a laser beam at a predetermined fundamental frequency along 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 reflector with a pumping lamp. These latter laser elements are well known in the art and therefore have not been shown in Figure 1 for the sake of clarity.
Crystal 14 is a known second harmonic qenerator crystal such as a KTP (potassium titanyl phosphate) crystal.
Crystal 14 is oriented with its optic axis, shown by arrow Z in Figures la and lb, at an angle of 45 with respect to the angle of polarization of the beam from laser 18. Thus, for example, if the fundamental beam F from laser 18 is polarized vertically, then as shown in these two figures, the 0 and E
axes of crystal 14 are oriented at an angle of 45 from the vertical.
Plate 12 is selected to operate as a quarter-wave plate at the fundamental frequency and simultaneously as a half wave plate at the second harmonic frequency. The optical axis of the plate (or its perpendicular indicated by arrow Q in Figures la and lb) i8 oriented parallel to the polarization of the laser beam.
Mirror 10 is highly reflective at the fundamental frequency and highly transmissive at ~he second harmonic frequency. Mirror 20 is highly reflective at the fundamental 1~9~i50 frequency. Mirrors 10 and 20 are positioned and arranged to form a resonating optical cavity for the fundamental beam generated by active laser medium 18, with the SHG crystal 14 and plate 12 disposed within the cavity.
Polarizer 16 is used to polarize the laser beam in the vertical direction (V).
As this initial beam 22 propagates through the crystal 14, the crystal, in response to both the O and E components of the beam 22 generates a beam 24 having double the frequency of the fundamental beam oriented at 45 to the vertical (an E ray) as shown. Beam 24 is transmitted through plate 12, and mirror 10 out of the cavity. Because plate 12 acts as a half-wave plate, the beam at the doubled-frequency transmitted is rotated by 90 and then through mirror 10. As shown by the arrow it i6 linearly polarized at 45 to the vertical direction.
As the fundamental beam 22 with its linear polarization oriented at 45 to the Z axis propagates through the SHG crystal, the birefringence causes a phase retardation to occur between fundamental 0 and E components.
In Figures la and lb it is assumed that after passing through crystal 14, The 0 component of the fundamental beam 22 lags behind the E component.
Without any phase lag compensatory means, the fundamental beam reflected from mirror 10 and back through the SHG crystal will exhibit twice the phase retardation shown after one pass and the polarization of the beam reentering the 124~?~i5() polarizer 16 (Figures la, lb) will not in general be linear and vertical, resulting in significant and undesirable polarization loss.
Therefore, in the present invention, beam 22 is passed from SHG crystal 14 through 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 of the fundamental beam incident on crystal 14. After reflection by mirror lO, the beam 22' passes again through quarter-wave plate 12. As a result of the two passes through plate 12, the polarization components of beam 22 have been rotated by 9O so that, as shown in Figure 1, the orientation of the E and O components of beam 22' are reversed with respect to the orientation of the components of beam 22. However component O still lags behind E. The reflected beam 22' then passes through crystal 14 but this second time, vertical component E is differentially phase shifted by an amount identical to the first differential phase shift with respect to O so that the components E and O of the beam 22' as it leaves the crystal 14 are now in phase and combine to yield linear polarization F'. Therefore by interposing plate 12 between crystal 14 and mirror lO, the birefringent effects of the SHG crystal are successfully self-compensated and thereby eliminated.
As a result, the fundamental beam 22 incident on crystal 14 and the fundamental beam 22' exiting from the ~24~9650 crystal 14 have identical linear polarizations resulting in no loss in the laser resonator.
Under the conditions described above, the harmonic beam ge~erated within crystal 14 comprises a component ED at 45 to the vertical as shown. S~nce in many applications it is desirable to obtain a freguency-doubled laser beam of a ~nown linear polarization, plate 12 is constructed to act simultaneously as a half-wave plate at the second harmonic frequency thereby rotating beam 24 by 90. If plate 12 were unspecified at the second harmonic frequency, the frequency-doubled beam would have an arbitrary elloptical polarization.
As a result, beam 24 exiting from the optical cavity is linearly polarized along the ordinary axis as shown.
In Figure lc, the beam emitted by laser active medium 18 has a random polarization and is shown in Figure 1 as being resolved into two orthogonal components V and H.
In Figure lc, crystal 14 is oriented with its optic axis, shown by arrow Z in Figure 1, parallel to one of the components of the beam from laser active medium 18, for example, component V. Thus, for example, component V from laser 18 is oriented vertically along the Y axis and component H horizontally along X. Then as shown in Figure lc, the E and O axes of crystal 14 are oriented parallel and perpendicular to the vertical.
Plate 12 is selected to operate as a quarter-wave plate at the fundamental frequency. The optical axis of the 12~3~;50 plate indicated by arrow Q in Figure lc i8 oriented at 45 to the V component of the fundamental beam.
Mirror 10 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. Mirror 20 is highly reflective at the fundamental frequency. Mirrors 10 and 20 are positioned and arranged to form a resonating optical cavity for the fundamental beam generated by active laser medium 18, with the SHG crystal 14 and plate 12 disposed within the cavity.
As the initial beam 22 propagates through the crystal 14, the crystal, in response to both the V and H components (the Q and E rays) of the linearly polarized beam 22, generates a beam 24 having double the frequency of the fundamental beam oriented along the vertical (an E ray) as shown in Figure lc.
15 Beam 24 is transmitted through plate 12, and mirror 10 out of the cavity.
As the fundamental beam 22 with its vertical and horizontal polarizations oriented parallel and perpendicular to the Z axis, propagates through the SHG crystal, the bire-fringence causes a phase retardation to occur betweenfundamental V and H components (E and 0 rays respectively) of fundmental beam 22.
In Figure lc it is assumed that after passing through crystal 14, the 0 ray of the fundamental beam 22 lags behind the E ray.
.;~
124~9650 Without any phase lag compensatory means, the fundamental beam reflected from mirror 10 and back through the SHG crystal will exhibit twice the phase retardation shown after one pass and the polarization of the beam reentering the laser active medium 18 will not in general be the same as that initially leaving 18 possibly resulting in significant and undesirable losses or instability in laser 18.
Therefore, in the present invention, beam 22 is passed from SHG crystal 14 through plate 12 which is a quarter-wave plate of the fundamental frequency. In Figure 1, as previously mentioned, the plate 12 îs shown with its optic axis at 45O to the component V of the fundamental beam incident on crystal 14. After reflection by mirror 10, the beam 22' passes again through quarter-wave plate 12. As a result of the two passes through quarter-wave plate 12, the V and H rays of beam 22 have been rotated by 90 so that, as shown in Figure 1, the orientation of the E and O rays of beam 22' are reversed with respect to the orienta~ion of the components of beam 22.
However ray O still lags behind E. The reflected beam 22' then passes through crystal 14 but this second time, ray E is differentially phase shifted by amount identical to the first differential phase shift with respect to O so that the rays E
and O of the beam 22' as it leaves the crystal 14 are now in phase and combined to yield fundamental beam components V and H
in the same phase as originally left crystal 12. Therefore by interposing plate 12 between crystal 14 and mirror 10, the 1249~5(:~
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 crystal 14 and the fundamental beam components V' and H~ exiting from the crystal 14 have identical phase relationships resulting in no loss or instability in the laser resonator.
It should be appreciated that plate 12 and crystal 14 accomplish their intended purposes dynamically. In the present invention, the phase lag is automatically and accurately corrected regardless of the temperature of the crystal.
If necessary, a Q-switch 16 may be added between laser 18 and mirror 20 to Q-switch the laser beam in the normal manner.
Another embodiment of the invention is shown in Figures 2a, 2b, and 2c. In the embodiment of Figure 2a, the frequency-doubler comprises a three-mirror cavity with a mirror 112, an SHG crystal 114, a quarter-wave 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 and quarter-wave plate 116 and polarizer 128 function in a manner identical to their counterparts in the embodiment of Figure la. Mirror 120 is highly reflective at the fundamental frequency, mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition mirror 112 is also positioned and arranged to focus the output ,, . ~ . ~
~Z~3~iSO
of laser 110 on crystal 114 for effective second harmonic generation. Mirror 118 is highly reflective at the fundamental frequency and at the second harmonic frequency.
In ope~ation, a fundamental beam 122 produced by laser active medium llO is reflected and focused by mircor 112 on crystal 114. The crystal generates a linearly second harmonic beam 124. After propagation through crystal 114, the O and E
components of fundamental beam 122 are phase shifted with respect to each other as described in the embodiment of Figure la. Also, as in this previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O and E component by 9O after reflection so that passage of beam 122' back through crystal 114 puts all components back in phase and restores the polarization to that linear polarization which initially left laser active medium llO. On the return trip through crystal 114, beam 122' generates second harmonic beam 126, which is colinear with reflected second harmonic beam 124'.
Thus, in this embodiment, the second harmonic genecated on the return trip of the fundamental is not lost so the potential exists for a second harmonic power gain of a factor of two. Interference may occur between these beams which will affect the stability of the SHG output intensity.
In order to overcome this undesirable effect, the polarizations of the beams 124' and 126 are made orthogonal using a technique similar to that described in U.S. Patent No. 4,413,342. Plate lZ4~i50 116, is simultaneously made a quarter-wave plate at the second harmonic frequency. Beam 124 will, upon passage through 116.
reflection from 118 and return through 116, have its polarization rotated by 90 and thereby be orthogonal and non-interfering with beam 126. Beams 124~ and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 is reflected by mirror 112 toward laser 110. Mirror 120 completes the optical cavity. Plate 116 compensates for the phase shift in the O and E components of the fundamental beam as previously described thereby insuring that beams 122 and 122' have the same linear polarization.
Another embodiment of the invention is shown in Figure lb. In this embodiment, the frequency-doubled laser comprises 15 a three-mirror cavity with a 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 Q-switch 119 and a polarizer 128. The laser 110, the crystal 114, quarter-wave plate 116, Q-switch 119 and polarizer 128 function in a manner identical to their counterparts in the embodiment of Figure lb. Mirror 120 is highly reflective at the fundamental frequency, mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition mirror 112 is also positioned and arranged to focus if 25 necessary on the output of laser 110 on crystal 114 for effective second harmonic generation. Mirror 118 is highly "~.
J 2~50 reflective at the fundamental frequency and at the second harmonic frequency.
In operation, a fundamental beam 122 produced by laser active medium 110 is reflected and focused by mirror 112 on crystal 114. The crystal generates a linearly polarized second harmonic beam 124. After propagation through crystal 114, O
and E components of fundamental beam 122 are phase shifted with respect to each other as described in the embodiment of Figure lb. Also, as in this previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O and E components by 90 after reflection so that pa6sage of beam 122' back through crystal 114 puts all components back in phase and restores the polarization to that linear polarization which initially left laser active medium llO. On the return trip through crystal 114, beam 122' generates second harmonic beam 126, which is colinear with reflected second harmonic beam 124~.
Thus, in this embodiment, the second harmonic generated on the return trip of the fundamental through the SHG
crystal is not lost so the potential exi6ts for a second harmonic power gain of a factor of two. Interference may occur between these beams which will affect the stability of the SHG
output intensity. In order to overcome this undesirable effect, the polarizations of the beams 124' and 126 are made orthogonal using a technique similar to that described in U.S.
Patent No. 4,413,342. Plate 116, is simultaneously made a lZ~96SO
quarter-wave plate at the second harmonic frequency. Beam 124 will, upon passage through 116, reflection from 118 and return through 116, have its polarization rotated by 90 and thereby be orthogonal and non-interfering with beam 126. Beams 124~
and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 i~
reflected by mirror 112 toward laser medium 110. Mirror 120 completes the optical cavity while switch 119 Q-switches the laser output in the conventional manner. Plate 116 compensate6 for the phase shift in the 0 and E components of the fundamental beam as previously described thereby insuring that beams 122 and 122' have the same linear polarization.
~ nother embodiment of the invention is shown in Figure 2c. In this embodiment, the frequency-doubled laser comprises 15 a three-mirror cavity with a mirror 112, an SHG crystal 114, a quarter-wave plate 116, a second mirror 118, a third mirror 120, and a laser active medium 110. The laser 110, the crystal 114, and quarter-wave plate 116 function in a manner identical to their counterparts in the embodiment of Figure lc. Mirror 120 is highly reflective at the fundamental frequency, mirror 112 is highly reflective at the fundamental frequency and highly transmissive at the second harmonic frequency. In addition mirror 112 can also be positioned and arranged to focus the output of laser 110 on crystal 114 for effective second harmonic generation. Mirror 118 is highly reflective at the fundamental frequency and at the second harmonic frequency.
iz~g~50 In operation, a fundamental beam having random polarization 122 produced by laser active medium llO is reflected and focused by mirror 112 on crystal 114. The crystal generates a second harmonic beam 124. After propagation through crystal 114, the O and E rays of fundamental beam 122 are phase shifted with respect to each other as described in the embodiment of Figure lc. ~lso, as in this previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O
and E rays by 90 after reflection so that pasfiage of beam 122' back through crystal 114 puts all components back in phase and restores the polarization to that polarization which initially left laser active medium llO. On the return trip through crystal 114, beam 122' generates second harmonic beam 126, which is colinear with reflected second harmonic beam 124'.
Thus, in this embodiment, the second harmonic generated on the return trip of the fundamental is not lost so the potential exists for a second harmonic power gain of a factor of two. Interference may occur between these beams which will affect the stability of the SHG output intensity.
In order to overcome this undesirable effect, the polarizations of the beams 124' and 126 are made orthogonal using a technique similar to that described in U.S. Patent No. 4,413,342. Plate 116, is simultaneously made a quarter-wave plate at the second harmonic frequency. Beam 124 will, upon passage through 116, reflection from 118 and return through 116, have its ~ '`
12'~ 5~
polarization rotated by 90 and thereby be orthogonal and non-interfering with beam 126. Beams 124' and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 is reflected by mirror 112 toward laser 110. Mirror 120 completes the optical cavity. Plate 116 compensates for the phase shift in the O and E rays of the fundamental beam as previously described thereby insuring that beams 122 and 122' have the same random polarization.
~ Q-switch 128 may be added to Q-switch the fundamental beam as described above.
Obviously numerous other modifications may be made to the invention without departing from its scope as defined in the appended claims.
Claims (34)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A frequency-doubled 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 futher generating a differential phase delay between said orthogonal components; and means for eliminating on the return trip through said crystal said differential delay.
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 futher generating a differential phase delay between said orthogonal components; and means for eliminating on the return trip through said crystal said differential delay.
2. 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 extraordinary 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 cavity for said laser active medium, crystal and interchanging means.
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 extraordinary 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 cavity for said laser active medium, crystal and interchanging means.
3. The frequency-doubled laser of claim 2 wherein said interchanging means is further adapted to maintain the linear polarization of the second harmonic beam.
4. The frequency-doubled laser of claim 2 wherein said interchanging means is a plate positioned for dual passes of the fundamental beam after a pass through the crystal to exchange the ordinary and extraordinary components of the fundamental beam before said fundamental beam reenters the crystal.
5. The frequency-doubled laser of claim 4 wherein said plate is a quarter-wave plate at the fundamental frequency.
6. The frequency-doubled laser of claim 5 wherein said plate is a half-wave plate at the second harmonic frequency.
7. The frequency-doubled laser of claim 2 wherein said second mirror is highly reflective at the fundamental frequency.
8. The frequency-doubled laser of claim 7 wherein said second mirror is highly transmissive at the doubled-frequency.
9. The frequency-doubled laser of claim 2 further comprising a third mirror for reflecting and focusing the fundamental beam from the laser on the crystal.
10. The frequency-doubled laser of claim 9 wherein said third mirror is further provided for coupling said second harmonic beam out of said cavity.
11. The frequency-doubled laser of claim 9 wherein said interchanging means for the fundamental orthogonal components also interchanges the second harmonic frequency components to preclude interference.
12. The frequency-doubled laser of claim 9 wherein said interchanging means is a quarter-wave plate at the fundamental frequency and a quarter-wave plate of the second harmonic frequency.
13. A Q-switched frequency-doubled 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 differential phase delay betwen said orthogonal components;
a Q-switch for Q-switching the output of the laser active medium; and means for eliminating on the return trip through said crystal said differential delay.
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 differential phase delay betwen said orthogonal components;
a Q-switch for Q-switching the output of the laser active medium; and means for eliminating on the return trip through said crystal said differential delay.
14. A Q-switched 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 extraordinary 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:
a Q-switch for Q-switching the output of the laser active medium; and a second mirror, said first mirror and second mirror being arranged to form an optical cavity for said laser active medium, crystal and interchanging means.
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 extraordinary 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:
a Q-switch for Q-switching the output of the laser active medium; and a second mirror, said first mirror and second mirror being arranged to form an optical cavity for said laser active medium, crystal and interchanging means.
15. The frequency-doubled laser of claim 14 wherein said interchanging means is further adapted to maintain the linear polarization of the second harmonic beam.
16. The frequency-doubled laser of claim 14 wherein said interchanging means is a plate positioned for dual passes of the fundamental beam after a pass through the crystal to exchange the ordinary and extraordinary components of the fundamental beam before said fundamental beam reenters the crystal.
17. The frequency-doubled laser of claim 16 wherein said plate is a quarter-wave plate at the fundamental frequency.
18. The frequency-doubled laser of claim 17 wherein said plate is a half-wave plate at the second harmonic frequency.
19. The frequency-doubled laser of claim 14 wherein said second mirror is highly reflected at the fundamental frequency.
20. The frequency-doubled laser of claim 19 wherein said second mirror is highly transmissive at the doubled frequency.
21. The frequency-doubled laser of claim 14 further comprising a third mirror for reflecting and focusing the fundamental beam from the laser on the crystal.
22. The frequency-doubled laser of claim 21 wherein said third mirror is further provided for coupling said second harmonic beam out of said cavity.
23. The frequency-doubled laser of claim 21 wherein said interchanging means for the fundamental orthogonal components also interchanges the second harmonic frequency components to preclude interference.
24. The frequency-doubled laser of claim 21 wherein said interchanging means is a quarter-wave plate at the fundamental frequency and a quarter-wave plate of the second harmonic frequency.
25. A frequency-doubled 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 the 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 axis, said crystal further generating a differential phase delay between said orthogonal rays; and means for eliminating on the return trip through said crystal said differential delay.
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 the 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 axis, said crystal further generating a differential phase delay between said orthogonal rays; and means for eliminating on the return trip through said crystal said differential delay.
26. 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 coplanar orthogonal components:
a crystal with its optic axis parallel 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, reflection 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 and optical cavity for said laser active medium, crystal and interchanging means.
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 coplanar orthogonal components:
a crystal with its optic axis parallel 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, reflection 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 and optical cavity for said laser active medium, crystal and interchanging means.
27. The frequency-doubled laser of claim 26 further comprising a Q-switch for Q-switching the fundamental beam.
28. The frequency-doubled laser of claim 26 wherein said interchanging means is a plate positioned for dual passes of the fundamental beam after a pass through the crystal to exchange the ordinary and extraordinary of the fundamental beam before said fundamental beam reenters the crystal.
29. The frequency-doubled laser of claim 28 wherein said plate is a quarter-wave plate at the fundamental frequency.
30. The frequency-doubled laser of claim 26 wherein said second mirror is highly reflective at the fundamental frequency.
31. The frequency-doubled laser of claim 30 wherein said second mirror is highly transmissive at the doubled-frequency.
32. The frequency-doubled laser of claim 26 further comprising a third mirror for reflecting and focusing the fundamental beam from the laser on the crystal.
33. The frequency-doubled laser of claim 32 wherein said third mirror is further provided for coupling said second harmonic beam out of said cavity.
34. The frequency-doubled laser of claim 33 wherein said interchanging means for the fundamental orthogonal components also interchanges the second harmonic frequency components to preclude interference.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/792,474 US4637026A (en) | 1985-04-17 | 1985-10-29 | Frequency doubling a Q-switched laser beam by using intracavity Type II phase matching |
| US06/792,477 US4617666A (en) | 1985-04-17 | 1985-10-29 | Frequency doubling a laser beam by using intracavity type II phase matching |
| US792,477 | 1985-10-29 | ||
| US792,474 | 1985-10-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1249650A true CA1249650A (en) | 1989-01-31 |
Family
ID=27121284
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000520349A Expired CA1249650A (en) | 1985-10-29 | 1986-10-10 | Frequency doubling a laser beam by using intracavity type ii phase matching |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1249650A (en) |
| DE (1) | DE3631909C2 (en) |
| FR (1) | FR2589290B1 (en) |
| GB (1) | GB2182197B (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4748631A (en) * | 1987-05-06 | 1988-05-31 | International Business Machines | Modulated laser source for optical storage |
| JP3066966B2 (en) * | 1988-02-29 | 2000-07-17 | ソニー株式会社 | Laser light source |
| DE4032323A1 (en) * | 1990-10-11 | 1992-04-16 | Adlas Gmbh & Co Kg | SINGLE FASHION LASER |
| JP4891526B2 (en) * | 2004-01-23 | 2012-03-07 | ミヤチテクノス株式会社 | Laser welding equipment |
| LT5968B (en) * | 2012-01-27 | 2013-10-25 | Valstybinis mokslinių tyrimų institutas Fizinių ir technologijos mokslų centras | Method and device for sum-frequency generation of light pulses |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4408329A (en) * | 1978-12-04 | 1983-10-04 | The United States Of America As Represented By The Secretary Of The Navy | Laser device with intraresonator harmonic generator |
| US4346314A (en) * | 1980-05-01 | 1982-08-24 | The University Of Rochester | High power efficient frequency conversion of coherent radiation with nonlinear optical elements |
| US4331891A (en) * | 1980-05-01 | 1982-05-25 | The University Of Rochester | High power efficient frequency conversion of coherent radiation with nonlinear optical elements |
| US4413342A (en) * | 1980-11-20 | 1983-11-01 | Quantronix Corporation | Method and apparatus for frequency doubling a laser beam |
-
1986
- 1986-07-22 FR FR868610632A patent/FR2589290B1/en not_active Expired - Fee Related
- 1986-09-19 DE DE3631909A patent/DE3631909C2/en not_active Expired - Fee Related
- 1986-10-10 CA CA000520349A patent/CA1249650A/en not_active Expired
- 1986-10-14 GB GB8624607A patent/GB2182197B/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| FR2589290A1 (en) | 1987-04-30 |
| DE3631909C2 (en) | 1996-01-04 |
| FR2589290B1 (en) | 1990-08-17 |
| DE3631909A1 (en) | 1987-04-30 |
| GB2182197A (en) | 1987-05-07 |
| GB2182197B (en) | 1989-09-20 |
| GB8624607D0 (en) | 1986-11-19 |
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