Many therapeutic uses of lasers are based on photo-reactions of tissues, or dyes which are absorbed preferentially by only diseased tissue, that are wavelength specific. For instance, certain types of dyes absorb a specific wavelength of light. Photo-reaction by-products in tissues contaminated with specific dyes, as well as the absorbed heat itself, are used for therapeutic purposes. For instance, cancerous tissue treated with Purpurin is irradiated with 659 nanometer red laser light, to cause singlet oxygen byproducts, which attack the cancerous tissue.
Reliable and practical lasers which generate light at a specific wavelength with sufficient power to achieve the therapeutic objectives present complicated design problems. Gain media in practical laser systems are limited to relatively few lasing wavelengths. Therefore, a non-linear optic element is needed to generate wavelengths outside the gain profile of the laser media. Specifically, non-linear optical crystals, such as KDP, KD*P, KTP, LBO, BBO, and others, are used for harmonic generation in resonant cavities, as is well known in the art. For a discussion of harmonic generation in solid state laser systems, see Koechner, "SOLID-STATE LASER ENGINEERING", 2d Edition, Springer-Verlag, 1988, pp. 477-518, and particularly pp. 514-518. See, also, Kleinman, et al.. "SECOND-HARMONIC GENERATION OF LIGHT BY FOCUSED LASER BEAMS", PHYSICAL REVIEW, Vol. 145, No. 1, May 1966, pp. 338-346; and Boyd, et al., "SECOND-HARMONIC GENERATION OF LIGHT WITH DOUBLE REFRACTION", PHYSICAL REVIEW, Vol. 137, No. 4a, Feb. 1965, pp. A1305-A1320.
The amount of power available in these laser systems using harmonic generation has been limited by a variety of parameters. For instance, the anisotropic crystals used for harmonic generation are birefringent. It may happen that this birefringence causes the extraordinary and ordinary ("e" and "o" beams in a uniaxial crystal) or simply orthogonally polarized beams (since both are considered to be extraordinary in the case of a biaxial crystal) to diverge within the crystal. Because overlap of the orthogonally polarized beams at the lasing wavelength in the crystal is necessary for non-linear interaction between the two orthogonal electric fields which will result in power produced at the desired sum frequency output wavelength, wavelengths which have a high divergence angle, or walkoff, within the crystals have not been efficiently generated in the past.
Therefore, for practical laser systems in the prior art, it has been difficult to generate laser beams at certain important wavelengths with sufficient power to achieve therapeutic purposes. For instance, red lasers having an output power greater than about 2 watts have not been practical. The only system known by the Applicants which produces a power in the red greater than 2 watts are large krypton gas ion lasers. These laser systems are impractical for therapeutic purposes because of their immense size and the high power requirements. Further, these systems only generate red outputs at specific wavelengths with very small powers. Only when all the red lines of a prior art krypton gas ion laser are combined, is the maximum power known by Applicant to have been produced repeatedly about 41/2 watts. However, such high power gas lasers are not practical for medical applications.
In practical laser systems, such as frequency doubled Nd:YAG lasers, the maximum output power available has been about 2 watts. This limited amount of output power, which is further attenuated by systems for delivering the beam to therapeutic sites, limits the speed with which a desired dosage of radiation can be administered. Therefore, certain therapies take an unnecessarily long period of time to accomplish.
For instance, certain photodynamic therapy is based on depositing certain dyes in the tissue to be treated. As mentioned above, a dye known to have useful photoreactions at 659 mm is known as Purpurin. When Purpurin is preferentially taken up at cancer cell sites, and irradiated with red light at essentially 659 nanometers in wavelength, therapeutic effects such as singlet oxygen generation and heating on the cancerous tissue are achieved. However, the prior art systems have been unable to deliver light at 659 wavelengths at greater than about 2 watts. Therefore, the speed with which this Purpurin therapy or other therapy requiring red laser light can be accomplished has been significantly limited. It will be appreciated that dyes other than Purpurin can be employed.
Accordingly, it is desirable to provide a practical laser system which provides significant output power at practical pumping powers using non-linear optics, such as used in second harmonic generation. More particularly, it is desirable to provide an output beam in the red having a power of greater than 4 watts for many therapeutic purposes.