Frequency Doubling a Q-Switched Laser Beam by Using Intracavity Type II Phase Matching

1. Field of the 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 II phase-matching in an intracavity second harmonic generation crystal.

2. Description of the Prior Art

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. A technique, known as phase matching, is designed to overcome this difficulty by utilizing in uniaxial and biaxial crystals the natural birefringence, 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.

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

In Type I phase matching, the fundamental wave is 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 is described in U.S. Pat. No. 4,413,342.) Since the fundamental wave is polarized along the optic axes of the crystal, there is no change in its linear polarization when it exits from the crystal. An 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 polarization to be at 45.degree. 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 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, if the laser's original polarization is linear it will not in general be properly maintained. 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 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 harmonic, 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 exciting 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 (returning the resonator quickly to its high Q state) this excess population is utilized to 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 higher than the laser's continuous wave output power level can be generated. Because of the superior focusability and enhanced material interaction of shorter wavelengths, it is often of interest to frequency-double the output of Q-switched lasers.

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 frequency doubling apparatus with a laser medium in which the fundamental beam incident on the laser medium maintains its original 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 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.

FIG. 1 illustrates a second harmonic laser generator according to this invention; and

FIG. 2 shows an alternate embodiment of the invention.

Referring now to FIG. 1, 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 polarizer 16, an active laser medium 18, a Q-switch 19 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 and the beam generated by the laser rod is Q-switched by switch 19 in the conventional manner. The pumping reflector and pump lamp are well known in the art and therefore have not been shown in FIG. 1 for the sake of clarity.

Crystal 14 is a known second harmonic generator crystal such as a KTP (potassium titanyl phosphate) crystal. Importantly, crystal 14 is oriented with its optic axis, shown by arrow Z in FIG. 1, at an angle of 45.degree. 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 FIG. 1, the O and E axes of crystal 14 are oriented at an angle of 45.degree. 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 FIG. 1 is oriented parallel to the polarization of the laser 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.

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.degree. 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 is rotated by 90.degree. and is then transmitted through mirror 10. As shown by the arrow it is linearly polarized at 45.degree. to the vertical direction.

As the fundamental beam 22 with its linear polarization oriented at 45.degree. to the Z axis propagates through the SHG crystal, the birefringence causes a phase retardation to occur between fundamental O and E components.

In FIG. 1 it is assumed that after passing through crystal 14, the O component of the fundamental beam 22 lags behind the E component.

In fact obviously, 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 polarizer 16 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 FIG. 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 10, 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 90.degree. so that, as shown in FIG. 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 10, 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 crystal 14 have identical linear polarizations resulting in no loss in the laser resonator.

Under the conditions described above, the harmonic beam generated within crystal 14 comprises a component E.sub.D at 45.degree. to the vertical as shown. Since in many applications it is desirable to obtain a frequency-doubled laser beam of a known 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.degree.. If plate 12 were unspecified at the second harmonic frequency, the frequency doubled beam would have an arbitrary elliptical polarization. As a result of utilizing a half-wave plate, beam 24 exiting from the optical cavity is linearly polarized 45.degree. to the vertical axis as shown.

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.

Another embodiment of the invention is shown in FIG. 2. In this embodiment, the frequency doubled laser 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, 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 FIG. 1. 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 necessary on 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.

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, the O and E components of fundamental beam 122 are phase shifted with respect to each other as described in the previous embodiment. Also, as in the previous embodiment, the fundamental frequency quarter-wave plate 116 and mirror 118 are used to rotate the O and E components by 90.degree. 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 110. 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 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. Pat. 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 polarization rotated by 90.degree. 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 medium 110. Mirror 120 completes the optical cavity while switch 119 Q-switches the laser output in the conventional manner. 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.

Obviously numerous other modifications may be made to the invention without departing from its scope as defined in the appended claims.