Coupling of Multiple Laser Beams to a Single Optical Fiber

The field of the invention is that of an apparatus and method for coupling a parallel array of laser beams into a single optical fiber for the transmission of higher optical power. If multiple helium-neon lasers, each with a high output power of around 50 mW or more are used, the focussed laser light transmitted in the optical fiber can be applied effectively for photodynamic therapy for cancer treatment.

It is currently known that photodynamic therapy (PDT) using hematoporphyrin and its derivatives (HpD) requires red light at the wavelength range of around 630 nm. This corresponds to one of the broad absorption maxima of HpD. For FDT applications, red laser outputs from argon ion pumped dye lasers (Forbes et. al., 1980), gold vapour lasers (Forbes et. al., 1984) and helium-neon lasers (Perria et. al., 1980) have been reported. Dye lasers can be tuned to output in the red from 620 to 630 nm whereas the gold vapour laser has an output at 628 nm and the helium-neon laser at 632.8 nm. All three lasers can deliver red laser light equally effectively to an optical fiber for photodynamic therapy. FIG. 1 shows how a simple lens is used to focus a laser beam into an optical glass fiber (typically from 400 to 1000 um diameter) for delivery of red laser light for PDT. Laser output power of a few hundred milliwatts (at least 600 mW as quoted J. A. S. Carruth and A. L. McKenzie, 1985 ) are necessary. Both the argon ion pumped dye laser and the gold vapour laser can deliver these powers. However, both types of lasers are technically difficult to operate. High electrical power consumption (usually from a 3-phase supply), water cooling, long start up time and elaborate maintenance procedures are needed.

On the other hand, helium-neon lasers are relatively simple lasers to use and can be operated from the usual domestic single phase supply. Helium-neon lasers are also known to be reliable with long operating life. However, presently available commercial helium-neon lasers can give at most 100 mW of stable output power. A single helium-neon laser has therefore too low optical power for practical photodynamic therapy. The possibility of folding a series of helium-neon lasers, either end-on or in folded geometry, has not been reported to be successful due to the difficulties in the alignment of such a long cavity laser due to the small bore size normally required of helium-neon laser tubes. The possibility of using a parallel array of helium-neon lasers, effectively coupled together, has not been conceived as a possible alternative to the dye laser or gold vapour laser systems for photodynamic therapy.

This invention relates to a parallel array of lasers with a beam delivery system for directing multiple laser beams for focussing into a single optical fiber efficiently. Such an array of lasers, with an arbitrary number of units, can be beam steered and simultaneously focussed into a single optical fiber. If high power helium-neon lasers are used, this arrangement can result in sufficient red laser light power to be focussed into a single fiber and transmitted effectively for photodynamic therapy. Such a helium-neon laser array is easy to operate requiring very little maintenance.

FIG. 1 illustrates a known concept of focussing a single laser beam into an optical fiber.

FIG. 2 shows the method of steering a linear array of two laser beams forming two near parallel beams that can be independently steered for focussing completely into a single optical fiber.

FIG. 3 shows the method of steering a square array of 2.times.2 laser beams for focussing using a single converging device into a single optical fiber.

FIG. 4 shows the method of steering a linear array of n.times.1 array of laser beams for focussing into a single optical fiber.

While it might be known that more than one laser beam can be simultaneously focussed into a single optical fiber, no practical application has been published. Due to the constraints in the physical dimensions of each laser, no scheme has been published for the incoherent addition of multiple laser beams into a single optical fiber.

FIG. 2 shows how a linear array of 2 laser beams can be beam steered for focussing into a single optical fiber. The arrangement of the 2 beam steering mirrors is paramount to this method. Mirrors M1 and M2 are positioned to enable the two beams to become near parallel and at most two or three millimeters apart, limited by the physical spacing and positioning of the mirrors and their mounts. Mirrors M3 and M4 will enable the directions of the two laser beams to be independently steered through another right angle before incident on the focussing device. While mirrors 3 and 4 can be replaced by a single mirror, the use of two mirrors instead of 1 constitutes a better method due to ease of alignment of the two beams for optimum focussing into the converging device. Since both laser beams can be independently steered, they can be both focussed simultaneously into a single fiber placed at or near the focal point of a light converging device. A converging lens is illustrated although a reflecting concave mirror can similarly be used. The f-number of the focussing device is chosen sufficiently large so that all the converged beams will enter the fiber at an angle smaller than the acceptance angle of the optical fiber used. Further, the fiber diameter has to be chosen sufficiently large so that optimum light acceptance is achieved, taking into considerations the different beam diameters at the focal point due to spherical aberration. The 2 laser beams need not be parallel to the optic axis of the converging device for optimum focussing into the fiber. An aspheric lens or a parabolic mirror can also be used. For maximum coupling of laser light into the fiber, the reflecting mirrors should have the highest reflectivities possible to the laser beams and the converging lens should be anti-reflection coated to the laser wavelength.

FIG. 3 shows how four laser beams, in a square 2.times.2 array, can be beam steered to be focussed completely into a single optical fiber. Mirrors 1 to 4 enable the four beams, near parallel and physically separated some distance apart, to be brought closer together in one direction as discussed in the case of FIG. 3. Mirrors 5 to 8 enable the beams to be brought closer together in the other direction to form a neatly packed 2.times.2 laser beams for focussing into the optical fiber. Each of the 4 beams can be independently aligned easily and quickly for maximum coupling into the optical fiber.

FIG. 4 shows the generalised method of steering a linear array of n.times.1 laser beams to become near parallel and very closely spaced laser beams for focussing into a single optical fiber. Miniature mirror mounts are used to ensure that small spacings are obtained between each laser beams.

Based on the methods outlined embodied in the discussions for FIG. 2 and FIG. 3 for the 2.times.2 laser array, and FIG. 4 for the n.times.1 linear laser array, any square n.times.n (n>1) array of laser beams can be generalised and focussed simulataneously into a single optical fiber.

For easier maintenance of the laser array, some of the inner laser positions need not be filled. Thus, for the 3.times.3 array, the innermost laser should best be left vacant for easier maintenance of the 8 lasers placed on the outside.

While the above describe the best mode of steering a n.times.n array of laser beams for focussing into a single optical fiber, there are obviously many possible modifications and variations to the arrangement of the beam steering optics with the same end results in light of the above teachings. It is therefore to be understood that within the scope of the appended claims that the invention may be practiced otherwise than as specifically described.

1. I. J. Forbes et. al., `Phototherapy of . . . haematoporphyrin derivatives`, Med. J. Aust., 1980, 2, 489-493.

2. I. J. Forbes et. al., `Multidisciplinary approach . . . human cancers`, Prog. Clin. Biol. Res., 1984, 170, 693-708.

3. C. Perria et. al., `Fast attempts . . . human gliomas`, J. Neurosurg. Sci. 1980, 24, 119-129.

4. J. A. S. Carruth and A. L. McKenzie, `Preliminary report . . . head and neck`, Eur. J. Surg. Oncol., 1985, 11, 47-50.