1. Field of the Art
The present invention relates to a hollow waveguide for transmitting high output ultraviolet laser beams at a high efficiency.
2. Prior Art
Lasers having wavelengths is the ultraviolet region (e.g. an excimer laser) generally have a high energy levels. Those lasers are utilized in a very wide range of applications, such as in laser CVD, surface reforming, photochemical synthesizing and the like. Besides, in the medical field, lasers are employed for in vivo operation in combination with the endoscope. In an industrial field, lasers are used for hyperfine machining.
A requirement essential to utilization of the lasers in the industrial and medical fields is to establish a wave guide technology for guiding laser beams to a desired spot at high efficiency. For example, excimer laser beams having a short wavelength and large peak power are harmful to a living body. However, there exists a difficult problem in terms of transmitting the laser beams safely to a remote part. Thus, it is desired, particularly from the medical sector, that, a waveguide be capable of easily and safely transmitting such laser beams to a position which needs the laser beams.
The following are conventional methods, itemized by (1) through (3), of transmitting high energy laser beams of a laser having a wavelength in the ultraviolet region (e.g., an eximer laser).
(1) A method based on a quartz system optical fiber:
As illustrated in FIG. 6, a core 51 is formed of quartz having a high transmissivity to light in the ultraviolet region. A cladding 52 covering the core 51 is composed of quartz to which an impurity such as fluorine is doped to give thereto a refractive index smaller than that of the core 51. Laser beams l are transmitted while being reflected by an interface between the core 51 and the cladding 52. A hydroxyl group may be doped in the core to reduce the absorption of a specific wavelength. Note that FIG. 6(a) is a cross-sectional view depicting the glass fiber, and FIG. 6(b) is a longitudinal sectional view thereof.
(2) A method based on an aluminum tube waveguide:
An aluminum tube 53, as illustrated in FIG. 7, is obtained by forming aluminum into a tubular shape, exhibiting a high reflectivity to the light of the ultraviolet region. The inner surface thereof is polished to increase the reflectivity. This method uses a so-called hollow waveguide for transmitting the beams l while being reflected by the inner surface of the aluminum tube 53. Note that FIG. 7(a) is a cross-sectional view showing the aluminum tube, and FIG. 7(b) is a longitudinal sectional view thereof.
(3) A method based on an aluminum plate rectangular hollow path:
As illustrated in FIG. 8, the reflection plates are two metal flat plates 54, 54 which are elongate in the light traveling direction. Dielectric spacers 55 are sandwiched therebetween at bilateral ends. A spacing of the hollow path is kept constant. The beams l are transmitted while being reflected by the two metal flat plates 54, 54. These metal flat plates 54, 54 are formed of aluminum having a high reflectivity to light in the ultraviolet region. The inner surface of the hollow path is polished. The dielectric substance serving as a spacer may be formed of Teflon. Note that FIG. 8(a) is a cross-sectional view showing the aluminum plate rectangular hollow path, and FIG. 8(b) is a longitudinal sectional view thereof.
The above-mentioned conventional method (1) based on the quartz system optical fiber, however, presents the following problems. The damage threshold value of an incident end surface against the laser beam is low, although the transmitting efficiency is good. Thus, if an energy density of the incident light is large, the incident end surface is destroyed. Besides, light absorption appears in a wavelength corresponding to an inter-atom bond energy of the material of the core 51. Therefore, in specific wavelengths, the transmissivity is extremely low. In addition, time-variations in the transmissivity become large.
In the method (2) based on the aluminum tube waveguide, the incident power can be increased. However, in this method, the inner surface polishing process becomes harder with a narrower diameter of the waveguide. Besides, the reflectivity of the inner surface decreases with the passage of time due to oxidization, resulting in a drop in the transmitting efficiency.
In the method (3) based on the aluminum plate rectangular hollow path, the incident power can be increased as in the case of method (2). In addition, the inner surface polishing process of the reflection plate can be facilitated. It is, however, difficult to manufacture the path with a smaller sectional area (e.g., 1 mm.sup.2 or thereabouts). Also, as in the case of method (2), the reflectivity of the inner surface drops due to the oxidization, and the transmitting efficiency is thus decreased.