1. Field of the Invention
The present invention relates to fiber laser oscillators that have an optical fiber including a core portion. The core portion contains a laser activation material so that a laser beam is generated and amplified within the core portion when a pumping light enters the optical fiber.
2. Description of the Related Art
Conventionally, a variety of fiber laser oscillators have been proposed in order to obtain a laser beam that is extremely high in quality while using a pumping light that has a relatively low beam quality.
A conventional fiber laser generator of an end pumping type generally uses an optical fiber 10 as shown in FIG. 9(C). The optical fiber 10 has a core portion 12 that is positioned centrally with respect to a cross section of the optical fiber 10. The core portion 12 may permit the transmission of a single mode laser beam and may be doped with a rare earth element (such as Nd and Er) or the like. The core portion 12 has a fiber-like configuration and may have a diameter of about 2 to 12 μm. The optical fiber 10 has a first cladding portion 14 (permitting transmission of a pumping light Lin) surrounding the core portion 12. The refractive index of the first cladding portion 14 is lower than the refractive index of the core portion 12 in order to confine an output laser beam Lout within the core portion 12. In addition, the optical fiber 10 has a second cladding member 16 surrounding the first cladding portion 14. The refractive index of the second cladding member 16 is lower than the refractive index of the first cladding portion 14, so that the pumping light Lin is confined within the first cladding portion 14.
When the pumping light Lin enters the optical fiber 10 to transmit through the core portion 12 (or collides with the core portion 12), the rare earth element contained in the core portion 12 is excited to generate the output laser beam Lout, and the single mode output laser beam Lout remains within the core portion 12. The output laser beam Lout has a relatively small diameter (which may depend on the diameter of the core portion 12) and has a relatively small divergence angle (which may depend on the wavelength of the output laser beam Lout and the refractive indexes of the core portion 12 and the first cladding portion 14). Therefore, the quality of the output laser beam Lout is extremely high. The quality of the output laser beam Lout may be represented by a product of the diameter of the outgoing light and half the divergence angle of the outgoing light. The beam quality may become higher as the product becomes smaller. However, because the area at the end surface of the optical fiber is relatively small, the energy of the pumping light Lin is relatively low. As a result, there has been a desire for an increased amount of output energy.
In this specification, the term “optical fiber” is used to mean an optical fiber having a core member and a cladding member covering the core member, for use with a fiber laser oscillator, unless otherwise indicated.
U.S. Pat. No. 5,999,673 proposes methods shown in FIGS. 8(A) and 8(B). In the method shown in FIG. 8(A), an optical fiber 10z is wound around the optical fiber 10 for use with a fiber laser oscillator and provides a guide for a pumping light to be directed to the optical fiber 10. In the method shown in FIG. 8(B), the optical fiber 10z, for providing a guide for the pumping light, extends along the optical fiber 10 and is gradually joined to the optical fiber 10 at the circumferential surface. Therefore, when the pumping light Lin enters the optical fiber 10z, the incident pumping light Lin is guided to the optical fiber 10 via the joint portion.
Japanese Laid-Open Patent Publication No. 2001-015835 proposes a laser beam generator shown in FIGS. 8(C) and 8(D), in which a core portion 12 is arranged to extend within a flat plane. In addition, prisms 4a and 4b are disposed on an upper surface of the generator so that the pumping light Lin enters via an increased area provided by the prisms 4a and 4b (in this case, the pumping light Lin enters from a circumferential surface of the optical fiber and not from an end surface). As a result, an outgoing laser beam Lout may have a relatively larger energy.
Japanese Laid-Open Patent Publication No. 10-190097 proposes a laser device shown in FIG. 9(A), in which an optical fiber 10 is coiled and then bound by a UV curable resin so as to have a configuration similar to a cylindrical block. A pumping light may be emitted from the outer peripheral side of the laser device.
Japanese Laid-Open Patent Publication No. 11-284255 proposes a fiber laser device shown in FIG. 9(B), in which an optical fiber 10 is wound around a circumferential surface of a cylindrical rod 50z made of glass. A pumping light emitted from a laser diode 30z may enter the glass cylindrical rod 50z and may be transmitted to a position adjacent to the circumferential surface of the glass cylindrical rod 50z via a collimating lens 56 and a prism 54. The pumping light transmitted to the position adjacent to the circumferential surface of the glass cylindrical rod 50z may be reflected within the glass cylindrical rod 50z, thereby pumping the wound optical fiber 10 so that a laser beam is outputted.
In general, the diameter of a core portion 12 of an optical fiber 10 is set to be about 2 to 12 μm in order to improve the quality of a generated laser beam. As noted previously, the quality of an output laser beam Lout may be represented by the product of the diameter of the outgoing light and half the divergence angle of the outgoing light. The beam quality may become higher as the product becomes smaller. The diameter of a first cladding portion 14 is set to be about several hundred to several thousand μm.
In case of the known end pumping type fiber laser oscillator shown in FIG. 9(C), the area of the end surface of the cladding portion 14, to which the pumping light Lin enters, is small. In addition, the cross sectional area of the core portion 12 is very small in comparison with the area of the end surface of the cladding portion 14. Therefore, the probability of the transmission of the pumping light Lin through the core portion 12 is low, resulting in a low oscillating efficiency. Further, it is very difficult to target the incident pumping light Lin so as to collide with the core portion 12, because the pumping light Lin is relatively low in quality. Therefore, it is very difficult to generate an output laser beam Lout with a large amount of energy.
In case of the known art disclosed in U.S. Pat. No. 5,999,673 (see FIGS. 8(A) and 8(B)), there is a limitation to the diameter of the optical fiber 10z used for providing a guide for the pumping light. Therefore, the amount of the pumping light Lin that can be entered through the optical fiber 10z is limited. In order to produce an output laser beam Lout with a large amount of energy, a large number of optical fibers 10z (i.e., such as several hundred to several thousand) must be used for connections. This may cause difficulties in manufacturing the fiber laser oscillator. Further, the fiber laser oscillator must have a relatively large size.
In addition, because of the increases in the number of parts (optical fibers 10z in this case), the probability of errors occurring may be relatively high. As a result, in some cases the pumping light Lin may not be highly effectively in entering the optical fiber 10. It is difficult to connect the optical fibers 10z so as to have the same connecting conditions for each optical fiber 10z. In addition, it is also difficult to position the optical fibers 10z so as to align with the path of the pumping light Lin. In usual cases having these types of difficulties, some degree of error is inevitable.
In case of the known art disclosed in Japanese Laid-Open Patent Publication No. 2001-015835 (see FIGS. 8(C) and 8(D)), the circumferential surface of the optical fiber, which has the core portion 12 arranged within a flat plane and covered by the cladding portion 14, is machined so as to allow placement of the prisms 4a and 4b upon the side surface. However, in order to substantially entirely absorb the pumping light Lin that enters via the prisms 4a and 4b, the device must have a considerable length in the longitudinal direction. Therefore, the device is constrained to have a relatively large size.
In addition, because the pumping light Lin must enter the prisms 4a and 4b in the form of parallel light beams, a collimating lens is required for converting the pumping light Lin to parallel beams. Consequently, additional elements, i.e., collimating lenses and prisms are required for transmitting the pumping light Lin to the optical fiber. Therefore, there exists a possibility that the oscillating efficiency may be lowered due to errors in configurations or positions of the additional elements.
In case of the known art disclosed in Japanese Laid-Open Patent Publication No. 10-190097 (see FIG. 9(A)), the pumping light Lin is emitted from the outer peripheral side of the block-like optical fiber 10. Therefore, it is not possible to completely confine the pumping light Lin. Some portion of the pumping light L may not be used for pumping. As a result, the oscillating efficiency of the device may be lowered.
In case of the known art disclosed in Japanese Laid-Open Patent Publication No. 11-284255 (see FIG. 9(B)), the pumping light Lin may enter the glass cylindrical rod 50z and may be transmitted to a position adjacent to the circumferential surface of the glass cylindrical rod via the prism 54. However, the pumping light Lin should enter the prism 54 in the form of parallel light beams. If a semiconductor laser is used for generating the pumping light Lin, the device requires a collimating lens to convert the pumping light Lin into parallel light beams. Therefore, for the same reason as discussed in connection with Japanese Laid-Open Publication No. 2001-015835, the oscillating efficiency of the device may be lowered. In addition, the volume of the core portion 12 is very small relative to the total volume of the portions through which the pumping light Lin is transmitted. The probability that the pumping light Lin collides with the core portion 12 is low. Therefore, the efficiency is low for this reason as well.