Laser-diode-excited laser apparatus, fiber laser apparatus, and fiber laser amplifier in which laser medium doped with one of Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup. Number:7,154,930 from the United States Patent and Trademark Office (PTO) owispatent A solid-state laser crystal constituting a laser-diode-excited solid-state laser apparatus or an optical fiber constituting a fiber laser apparatus or fiber laser amplifier is doped with one of Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup.3+ so that a laser beam is emitted from the solid-state laser crystal or the optical fiber, or incident light of the fiber laser amplifier is amplified, by one of the transitions from .sup.5S.sub.2 to .sup.5I.sub.7, from .sup.5S.sub.2 to .sup.5I.sub.8, from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, from .sup.4F.sub.3/2 to .sup.6H.sub.11/2, from .sup.5D.sub.0 to .sup.7F.sub.2, from .sup.4F.sub.9/2 to .sup.6H.sub.13/2, from .sup.4F.sub.9/2 to .sup.6H.sub.11/2, from .sup.4S.sub.3/2 to .sup.4I.sub.15/2, from .sup.2H.sub.9/2 to .sup.4I.sub.13/2, and from .sup.5D.sub.4 to .sup.7F.sub.5. The above solid-state laser crystal or optical fiber is excited with a GaN-based compound laser diode.<H amplifier, apparatus, Laser, diode, exc, 7154930.html, Patent, pto, 7154930

Title: Laser-diode-excited laser apparatus, fiber laser apparatus, and fiber laser amplifier in which laser medium doped with one of Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup.

Abstract: A solid-state laser crystal constituting a laser-diode-excited solid-state laser apparatus or an optical fiber constituting a fiber laser apparatus or fiber laser amplifier is doped with one of Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup.3+ so that a laser beam is emitted from the solid-state laser crystal or the optical fiber, or incident light of the fiber laser amplifier is amplified, by one of the transitions from .sup.5S.sub.2 to .sup.5I.sub.7, from .sup.5S.sub.2 to .sup.5I.sub.8, from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, from .sup.4F.sub.3/2 to .sup.6H.sub.11/2, from .sup.5D.sub.0 to .sup.7F.sub.2, from .sup.4F.sub.9/2 to .sup.6H.sub.13/2, from .sup.4F.sub.9/2 to .sup.6H.sub.11/2, from .sup.4S.sub.3/2 to .sup.4I.sub.15/2, from .sup.2H.sub.9/2 to .sup.4I.sub.13/2, and from .sup.5D.sub.4 to .sup.7F.sub.5. The above solid-state laser crystal or optical fiber is excited with a GaN-based compound laser diode.

Patent Number: 7,154,930 Issued on 12/26/2006 to Ohtsuka, et al.

Inventors: Ohtsuka; Hisashi (Kaisei-machi, JP), Okazaki; Yoji (Kaisei-machi, JP), Katoh; Takayuki (Kaisei-machi, JP)
Assignee: Fuji Photo Film Co., Ltd. (Kanagawa-ken, JP)

Appl. No.: 10/952,768

Filed: September 30, 2004

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
10143806	May., 2002	6816532

Foreign Application Priority Data


May 15, 2001 [JP]			144466/2001
May 23, 2001 [JP]			153578/2001
May 25, 2001 [JP]			157139/2001

Current U.S. Class: 372/75 ; 372/6

Current International Class: H01S 3/30 (20060101); H01S 3/091 (20060101); H01S 3/094 (20060101)

Field of Search: 372/75,6,39,40

References Cited [Referenced By]

U.S. Patent Documents


5084881	January 1992	Farries et al.
5084890	January 1992	Brierley
5388110	February 1995	Snitzer
5525932	June 1996	Kelkar et al.
5727007	March 1998	Smart et al.
6125132	September 2000	Okazaki
6154589	November 2000	Gavrilovic et al.
6411432	June 2002	Kasamatsu
6490309	December 2002	Okazaki et al.
6510276	January 2003	Jain et al.
6650677	November 2003	Itoh et al.

Foreign Patent Documents


3-86405	Apr., 1991	JP
4-318988	Nov., 1992	JP
2001-36168	Feb., 2001	JP
2001-36175	Feb., 2001	JP

Other References

L Esterowitz et al, Blue light emission by a Pr:LiYF4-laser operated at room temperature, Journal of Applied Physics, vol. 48.No. 2, Feb. 1977, Cambridge, Massachusetts. cited by other .
D. Sandrock et al, Efficient Continuous Wave-Laser Emission of Pr3+-Doped Fluorides at Room Temperature, Applied Physics B 58, 149-151 (1994), Hamburg, Germany. cited by other .
R.B. Smart et al, CW room temperature operation of praseodymium-doped fluorozirconate glass fibre lasers in the blue-green, green and red spectral regions, Optics Communications 86 (1991) 337-340, Noth Holland. cited by other .
Etsuko Ishikawa et al, Rare earth doped fiber for optical amplifiers operating around 1.3.mu.m, Technical Report of IEICE, LQE95-30 (Jun. 1995) p. 83-89, Tokyo, Japan. cited by other.

Primary Examiner: Rodriguez; Armando
Assistant Examiner: Rogers; Kelly
Attorney, Agent or Firm: Sughrue Mion, PLLC

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 10/143,806 filed May 14, 2002 now U.S. Pat. No. 6,816,532.

The subject matters disclosed in this specification are related to the subject matters disclosed in the following copending, commonly-assigned U.S. patent applications:

(1) U.S. Pat. No. 6,125,132 filed by Yoji Okazaki (one of the applicants of the present patent application) on Apr. 28, 1998 and entitled "LASER DIODE PUMPED SOLID STATE LASER, FIBER LASER AND FIBER AMPLIFIER," corresponding to Japanese patent application Nos. 10(1998)-6369 and 10(1998)-6370, which are laid open in Japanese Unexamined Patent Publication Nos. 11(1999)-17266 and 11(1999)-204862; and

(2) U.S. Ser. No. 09/621,241 filed by Yoji Okazaki (one of the applicants of the present patent application) and Takayuki Katoh (another of the applicants of the present patent application) on Jul. 21, 2000 and entitled "LASER-DIODE-PUMPED LASER APPARATUS IN WHICH Pr-DOPED LASER MEDIUM IS PUMPED WITH GaN-BASED COMPOUND LASER DIODE," corresponding to Japanese patent application Nos. 11(1999)-206817 and 11(1999)-206573, which are laid open in Japanese Unexamined Patent Publication Nos. 2001-36168 and 2001-36175.

The contents of the above copending, commonly-assigned U.S. patent applications (1) and (2) and the corresponding Japanese patent applications are incorporated in this specification by reference.

Claims

What is claimed is:

1. A fiber laser apparatus comprising: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Sm.sup.3+, and emits a second laser beam generated by one of a first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, a second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, and a third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 when the optical fiber is excited with said first laser beam.

2. A fiber laser apparatus according to claim 1, wherein said second laser beam is generated by said first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2 and is in a wavelength range of 556 to 576 nm.

3. A fiber laser apparatus according to claim 1, wherein said second laser beam is generated by said second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2 and is in a wavelength range of 605 to 625 nm.

4. A fiber laser apparatus according to claim 1, wherein said second laser beam is generated by said third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 and is in a wavelength range of 640 to 660 nm.

5. A fiber laser apparatus according to claim 1, wherein said core of said optical fiber is doped with no rare-earth ion other than Sm.sup.3+.

6. A fiber laser apparatus according to claim 1, wherein said GaN-based compound laser diode has an active layer made of one of InGaN, InGaNAs, and GaNAs materials.

7. A fiber laser amplifier comprising: a GaN-based compound laser diode which emits an excitation laser beam; and an optical fiber which has a core doped with Sm.sup.3+, and amplifies incident light which has a wavelength within a wavelength range of fluorescence generated by one of a first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, a second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, and a third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 when the optical fiber is excited with said excitation laser beam.

8. A fiber laser amplifier according to claim 7, wherein said fluorescence is generated by said first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2 and is in a wavelength range of 556 to 576 nm.

9. A fiber laser amplifier according to claim 7, wherein said fluorescence is generated by said second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2 and is in a wavelength range of 605 to 625 nm.

10. A fiber laser amplifier according to claim 7, wherein said fluorescence is generated by said third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 and is in a wavelength range of 640 to 660 nm.

11. A fiber laser amplifier according to claim 7, wherein said core of said optical fiber is doped with no rare-earth ion other than Sm.sup.3+.

12. A fiber laser amplifier according to claim 7, wherein said GaN-based compound laser diode has an active layer made of one of InGaN, InGaNAs, and GaNAs materials.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser-diode-excited solid-state laser apparatus in which a solid-state laser crystal doped with a rare-earth ion is excited with a laser diode (semiconductor laser) so as to emit a laser beam.

The present invention also relates to a laser-diode-excited solid-state laser apparatus in which a solid-state laser crystal doped with a rare-earth ion is excited with a laser diode (semiconductor laser), and which is arranged to emit ultraviolet light.

The present invention further relates to a laser-diode-excited fiber laser apparatus in which a core of an optical fiber doped with a rare-earth ion is excited with a laser diode (semiconductor laser) so as to emit a laser beam.

The present invention furthermore relates to a laser-diode-excited fiber laser amplifier in which a core of an optical fiber doped with a rare-earth ion is excited with a laser diode (semiconductor laser) so as to amplify incident light by utilizing fluorescence generated by the excitation of the core.

2. Description of the Related Art

(1) Solid-State Laser

Gas-laser-excited solid-state laser apparatuses in which a Pr.sup.3+-doped solid-state laser crystal is excited with a gas laser such as an Ar laser are known as disclosed in Journal of Applied Physics, vol. 48, No. 2, pp. 650 653 (1977) and Applied Physics, B58, pp. 149 151 (1994). These solid-state laser apparatuses can generate a laser beam in a blue wavelength range of 470 to 490 nm by a transition from .sup.3P.sub.0 to .sup.3H.sub.4 and a laser beam in a green wavelength range of 520 to 550 nm by a transition from .sup.3P.sub.1 to .sup.3H.sub.5. Therefore, the above solid-state laser apparatuses can be used as light sources for recording a color image in a color sensitive material.

In addition, another solid-state laser apparatus which emits a laser beam having a wavelength in the blue or green wavelength range is known. For example, the Japanese Unexamined Patent Publication No. 4(1992)-318988 corresponding to the Japanese Patent Application No. 3(1991)-086405, which is assigned to the present assignee, discloses a laser-diode-excited solid-state laser apparatus in which a solid-state laser beam is converted into a second harmonic, i.e., the wavelength of the solid-state laser beam is reduced, by arranging a nonlinear optical crystal in a resonator.

Further, InGaN-based compound laser diodes and ZnMgSSe-based compound laser diodes which emit laser beams in the blue and green wavelength ranges have recently been developed.

However, the light sources for use in recording a color image in a color image recording apparatus are required to be small in size, light in weight, and inexpensive. Nevertheless, the above gas-laser-excited solid-state laser apparatus using the Pr.sup.3+-doped solid-state laser crystal is not suitable for use in recording a color image in a color image recording apparatus since the gas laser in the gas-laser-excited solid-state laser apparatus are large, heavy, and expensive.

On the other hand, since the efficiency of wavelength conversion in the conventional laser-diode-excited solid-state laser apparatuses in which a wavelength of a solid-state laser beam is reduced by using a nonlinear optical crystal is not sufficiently high, it is difficult to obtain high output power. In addition, in such laser-diode-excited solid-state laser apparatuses, an etalon or the like is inserted for limiting the oscillation mode to a single mode. Therefore, loss in the resonator is great, and thus achievement of high output power becomes more difficult.

Further, in order to match phases in the wavelength conversion in the above laser-diode-excited solid-state laser apparatuses, highly accurate temperature control is required, and therefore the outputs of the laser-diode-excited solid-state laser apparatuses are not stable. Furthermore, the numbers of parts are increased by the provision of the nonlinear optical crystal and the etalon. Therefore, the laser-diode-excited solid-state laser apparatuses become expensive.

When InGaN-based compound laser diodes are used, the oscillation wavelengths of the InGaN-based compound laser diodes increase with increase in the indium contents, and theoretically it is possible to obtain laser beams in the blue wavelength range of 470 to 490 nm or in the green wavelength range of 520 to 550 nm. However, since the quality of the crystal deteriorates with the increase in the indium content, it is practically impossible to sufficiently increase the indium content, and the upper limit of the lengthened wavelength is about 450 nm.

In addition, blue light can be obtained by other laser diodes having an active layer made of an InGaNAs or GaNAs material. The oscillation wavelengths in these laser diodes can also be increased by doping the active layer with arsenic. However, since the quality of the crystal also deteriorates with the increase in the arsenic content, the upper limit of the wavelength realizing high output power is about 450 to 460 nm.

Further, the conventional ZnMgSSe-based compound laser diodes cannot continuously oscillate at wavelengths below 500 nm at room temperature, and the lifetimes of the conventional ZnMgSSe-based compound laser diodes are at most about a hundred hours.

In order to solve the above problems, the copending, commonly-assigned U.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent Publication No. 11(1999)-17266 disclose a laser-diode-excited solid-state laser apparatus which is inexpensive, and can emit a laser beam in the blue or green wavelength range with high efficiency, high output power, and high output stability. In this laser-diode-excited solid-state laser apparatus, a Pr.sup.3+-doped solid-state laser crystal is excited with a GaN-based compound laser diode.

(2) Ultraviolet Laser

Highly efficient, high output power ultraviolet lasers which continuously oscillate in the ultraviolet wavelength range are required, for example, for applications in ultraviolet lithography, fluorometric analysis of organic cells using laser excitation, and the like.

GaN-based compound semiconductor lasers having an active layer made of an InGaN, InGaNAs, or GaNAs material are known as lasers which oscillate in the ultraviolet wavelength range. Recently, GaN-based compound semiconductor lasers which can continuously oscillate for a thousand hours at the wavelength of 400 nm with output power of several milliwatts have been provided.

On the other hand, wavelength-conversion solid-state lasers which output ultraviolet laser beams having wavelengths of 400 nm or below are known. In these wavelength-conversion solid-state lasers, wavelengths of laser oscillation light are shortened to the ultraviolet wavelengths by second harmonic generation (SHG) or third harmonic generation (THG) using nonlinear optical crystals.

However, the conventional GaN-based compound semiconductor lasers cannot emit laser light with output power of 100 mW or more in a single transverse mode, although such laser light is required in many applications. In addition, the oscillation efficiency in the conventional GaN-based compound semiconductor lasers which emit laser light having wavelengths of 380 nm or below is low, and the lifetimes of such GaN-based compound semiconductor lasers are very short.

On the other hand, wavelength-conversion solid-state lasers which output ultraviolet laser beams having wavelengths of 400 nm or below are known. In these wavelength-conversion solid-state lasers, wavelengths of laser oscillation light are shortened to the ultraviolet wavelengths by second harmonic generation (SHG) or third harmonic generation (THG) using nonlinear optical crystals.

However, solid-state laser mediums which realize efficient oscillation in the wavelength range of 700 to 800 nm have not yet been found. Therefore, it is difficult to obtain ultraviolet laser beams with high output power from the wavelength-conversion solid-state lasers in which the wavelengths of the laser light are shortened by second harmonic generation (SHG).

In addition, the efficiency of the wavelength-conversion solid-state lasers in which the wavelengths of the laser light are shortened by third harmonic generation (THG) is essentially low, and the conventional THG wavelength-conversion solid-state lasers can oscillate in only a pulse mode. In order to realize continuous oscillation, i.e., in order to maintain resonance of THG light of the fundamental wave, highly accurate temperature adjustment of a resonator with a precision of 0.01.degree. C. is required. However, such accurate temperature adjustment is practically difficult in terms of cost.

In order to solve the above problems, the copending, commonly-assigned U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36175 disclose a laser-diode-excited solid-state laser apparatus in which a solid-state laser beam is converted into a second harmonic by using an optical wavelength conversion element so that ultraviolet light is obtained.

The above laser-diode-excited solid-state laser apparatus comprises: a solid-state laser crystal which is doped with at least one rare-earth ion including at least Pr.sup.3+; a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam for exciting the solid-state laser crystal; and an optical wavelength conversion element which performs wavelength conversion on a solid-state laser beam generated by excitation of the solid-state laser crystal so as to generate ultraviolet laser light.

Although the laser-diode-excited solid-state laser apparatus disclosed in the U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36175 can solve the aforementioned problems, the wavelength of the ultraviolet light which can be generated by the disclosed laser-diode-excited solid-state laser apparatus is limited to about 360 nm.

(3) Fiber Laser

As disclosed in the Technical Report of the Institute of Electronics, information and Communication Engineers in Japan, LQE95-30 (1995) p.30 and Optics Communications 86 (1991) p.337, laser-diode-excited fiber laser apparatuses which comprise a laser diode and an optical fiber having a core made of a Pr.sup.3+-doped fluoride are known. In the laser-diode-excited fiber laser apparatuses, the optical fiber is excited with the laser diode so as to generate a laser beam.

In addition, as disclosed in the above references, optical fiber amplifiers which comprise a laser diode and an optical fiber having a Pr.sup.3+-doped core are also known. In these optical fiber amplifiers, the optical fiber is excited with the laser diode so that fluorescent light is generated by the excitation of the optical fiber, and incident light of the optical fiber is amplified by the energy of the fluorescent light when the wavelength of the incident light is included in the wavelength range of the fluorescent light.

Further, Optics Communications 86 (1991) p.337 discloses an Ar-laser-excited, Pr.sup.3+-doped fiber laser apparatuses, and laser oscillations at the wavelengths of 491, 520, 605, and 635 nm using excitation light having a wavelength of 476.5 nm have been reported.

The above laser-diode-excited fiber laser apparatuses and Ar-laser-excited, Pr.sup.3+-doped fiber laser apparatuses can emit blue or green laser beams, and the above optical fiber amplifiers can amplify blue or green laser beams. In this respect, it is considered that these apparatuses and amplifiers can be used as constituents of light sources for recording a color image in a color sensitive material.

However, in order to operate the above Ar-laser-excited, Pr.sup.3+-doped fiber laser apparatuses or Pr.sup.3+-doped fiber laser amplifiers with high power of a few watts to several tens of watts, for example, for recording a color image, a water cooling system is necessary. Therefore, the size is increased, and the lifetime and the efficiency are reduced.

In order to solve the above problems, the copending, commonly-assigned U.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent Publication No. 11(1999)-204862 disclose a fiber laser apparatus which can efficiently emit a laser beam in a blue or green wavelength range with high output power and high stability in the output and beam quality, and can be formed in a small size. In this fiber laser apparatus, an optical fiber having a core doped with Pr.sup.3+ is excited with a GaN-based compound laser diode.

In addition, the U.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent Publication No. 11(1999)-204862 also disclose a fiber laser amplifier which can efficiently amplify a laser beam in a blue or green wavelength range with high output power and high stability in the output and beam quality, and can be formed in a small size. In this fiber laser amplifier, an optical fiber having a core doped with Pr.sup.3+ is excited with a GaN-based compound laser diode so as to amplify incident light of the optical fiber when the wavelength of the incident light is in the wavelength range of fluorescent light generated by the excitation of the optical fiber.

Further, the U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36168 disclose a fiber laser apparatus in which an optical fiber having a core codoped with Pr.sup.3+ and at least one of Er.sup.3+, Ho.sup.3+, Dy.sup.3+, Eu.sup.3+, Sm.sup.3+, Pm.sup.3+, and Nd.sup.3+ is excited with a GaN-based compound laser diode. The U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36168 also disclose a fiber laser amplifier in which an optical fiber having a core codoped with Pr.sup.3+ and at least one of Er.sup.3+, Ho.sup.3+, Dy.sup.3+, Eu.sup.3+, Sm.sup.3+, Pm.sup.3+, and Nd.sup.3+ is excited with a GaN-based compound laser diode so as to amplify incident light of the optical fiber when the wavelength of the incident light is in the wavelength range of fluorescent light generated by the excitation of the optical fiber.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a laser-diode-excited solid-state laser apparatus which uses a GaN-based compound laser diode as an excitation light source, and can emit laser light in a wide wavelength range which is not covered by the conventional laser-diode-excited solid-state laser apparatuses which uses a Pr.sup.3+-doped solid-state laser crystal.

A second object of the present invention is to provide a laser-diode-excited solid-state laser apparatus which is inexpensive, and can continuously emit ultraviolet light having a wavelength longer or shorter than 360 nm with high output power and high efficiency.

A third object of the present invention is to provide a fiber laser apparatus which uses a GaN-based compound laser diode as an excitation light source, and can emit laser light in a wide wavelength range which is not covered by the conventional fiber laser apparatuses which use a GaN-based compound laser diode as an excitation light source.

A fourth object of the present invention is to provide a fiber laser amplifier which uses a GaN-based compound laser diode as an excitation light source, and can amplify laser light in a wide wavelength range which is not covered by the conventional fiber laser amplifiers which use a GaN-based compound laser diode as an excitation light source.

(I) According to the first aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a GaN-based compound laser diode which emits an excitation laser beam; and a solid-state laser crystal which is doped with Ho.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and a second transition from .sup.5S.sub.2 to .sup.5I.sub.8 when the solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatus according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The solid-state laser beam is generated by the first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and is in the wavelength range of 740 to 760 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.5S.sub.2 to .sup.5I.sub.8 and is in the wavelength range of 540 to 560 nm. (iii) The solid-state laser crystal is doped with no rare-earth ion other than Ho.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Ho.sup.3+ is 420 nm.

(II) According to the second aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a GaN-based compound laser diode which emits an excitation laser beam; and a solid-state laser crystal which is doped with Sm.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, a second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, and a third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 when the solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatus according to the second aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv). (i) The solid-state laser beam is generated by the first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2 and is in the wavelength range of 556 to 576 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2 and is in the wavelength range of 605 to 625 nm. (iii) The solid-state laser beam is generated by the third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 and is in the wavelength range of 640 to 660 nm. (iv) The solid-state laser crystal is doped with no rare-earth ion other than Sm.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Sm.sup.3+ is 404 nm.

(III) According to the third aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a GaN-based compound laser diode which emits an excitation laser beam; and a solid-state laser crystal which is doped with Eu.sup.3+, and emits a solid-state laser beam by a transition from .sup.5D.sub.0 to .sup.7F.sub.2 when the solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatus according to the third aspect of the present invention may also have one or any possible combination of the following additional features (i) and (ii). (i) The solid-state laser beam is in the wavelength range of 579 to 599 nm. (ii) The solid-state laser crystal is doped with no rare-earth ion other than Eu.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Eu.sup.3+ is 394 nm.

(IV) According to the fourth aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a GaN-based compound laser diode which emits an excitation laser beam; and a solid-state laser crystal which is doped with Dy.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and a second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 when the solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatus according to the fourth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The solid-state laser beam is generated by the first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and is in the wavelength range of 562 to 582 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 and is in the wavelength range of 654 to 674 nm. (iii) The solid-state laser crystal is doped with no rare-earth ion other than Dy.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Dy.sup.3+ is 390 nm.

(V) According to the fifth aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a GaN-based compound laser diode which emits an excitation laser beam; and a solid-state laser crystal which is doped with Er.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.4S.sub.3/2 to .sup.4I.sub.15/2 and a second transition from .sup.2H.sub.9/2 to .sup.4I.sub.13/2 when the solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatus according to the fifth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The solid-state laser beam is generated by the first transition from .sup.4S.sub.3/2 to .sup.4I.sub.15/2 and is in the wavelength range of 530 to 550 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.2H.sub.9/2 to .sup.4I.sub.13/2 and is in the wavelength range of 544 to 564 nm. (iii) The solid-state laser crystal is doped with no rare-earth ion other than Er.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Er.sup.3+ is 406 or 380 nm.

(VI) According to the sixth aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a GaN-based compound laser diode which emits an excitation laser beam; and a solid-state laser crystal which is doped with Tb.sup.3+, and emits a solid-state laser beam generated by a, transition from .sup.5D.sub.4 to .sup.7F.sub.5 when the solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatus according to the sixth aspect of the present invention may also have one or any possible combination of the following additional features (i) and (ii). (i) The solid-state laser beam is in the wavelength range of 530 to 550 nm. (ii) The solid-state laser crystal is doped with no rare-earth ion other than Tb.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Tb.sup.3+ is 380 nm.

In addition, in the first to sixth aspects of the present invention, the GaN-based compound laser diode may have an active layer made of one of InGaN, InGaNAs, and GaNAs materials.

(VII) The advantages of the first to sixth aspects of the present invention are as follows. (i) Since the rare-earth ions, Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup.3+ have their absorption bands in the wavelength range of 380 to 420 nm, it is relatively easy to excite the rare-earth ions with a GaN-based compound laser diode. The wavelength range of 380 to 430 nm is a wavelength range in which the GaN-based compound laser diodes can oscillate with relative ease. In particular, the currently available GaN-based compound laser diodes can achieve their maximum output power in the wavelength range of 400 to 410 nm. Therefore, when a solid-state laser crystal doped with at least one of the rare-earth ions, Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup.3+ is excited with a GaN-based compound laser diode, it is possible to make a great portion of the excitation light absorbed by the solid-state laser crystal, and achieve high efficiency and high output power. (ii) In addition, as individually exemplified before; the wavelength bands of the fluorescence generated by the excitation of the solid-state laser crystals doped with the rare-earth ions, Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, Er.sup.3+, and Tb.sup.3+ are distributed in a wide wavelength range. Therefore, it is possible to realize a laser-diode-excited solid-state laser apparatus which can emit laser light having a wavelength which no laser light capable of being generated by the conventional laser-diode-excited solid-state laser apparatuses has. (iii) On the other hand, the thermal conductivity coefficients of the GaN-based compound laser diodes are about 130 W/m.degree. C., and much greater than the thermal conductivity coefficients of the ZnMgSSe-based compound laser diodes, which are about 4 W/m.degree. C. In addition, since the dislocation mobility in the GaN-based compound laser diodes is very low, compared with that in the ZnMgSSe-based compound laser diodes, the COD (catastrophic optical damage) thresholds of the GaN-based compound laser diodes are very high. Therefore, it is easy to obtain GaN-based compound laser diodes having a long lifetime and high output power. Since the laser-diode-excited solid-state laser apparatuses according to the first to sixth aspects of the present invention use a GaN-based compound laser diode as an excitation light source, the laser-diode-excited solid-state laser apparatuses according to the first to sixth aspects of the present invention can have a long lifetime, and emit a laser beam with high output power.

(VIII) According to the seventh aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam; a solid-state laser crystal which is doped with at least one rare-earth ion including Ho.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and a second transition from .sup.5S.sub.2 to .sup.5I.sub.8 when the solid-state laser crystal is excited with the excitation laser beam; and an optical wavelength conversion element which converts the solid-state laser beam into ultraviolet laser light by wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatus according to the seventh aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The solid-state laser beam is generated by the first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and has a wavelength of about 750 nm, and the ultraviolet laser light has a wavelength of about 375 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.5S.sub.2 to .sup.5I.sub.8 and has a wavelength of about 550 nm, and the ultraviolet laser light has a wavelength of about 275 nm. (iii) The solid-state laser crystal is doped with no rare-earth ion other than Ho.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Ho.sup.3+ is 420 nm.

(IX) According to the eighth aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam; a solid-state laser crystal which is doped with at least one rare-earth ion including Sm.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, a second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, and a third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 when the solid-state laser crystal is excited with the excitation laser beam; and an optical wavelength conversion element which converts the solid-state laser beam into ultraviolet laser light by wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatus according to the eighth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv). (i) The solid-state laser beam is generated by the first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2 and has a wavelength of about 566 nm, and the ultraviolet laser light has a wavelength of about 283 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2 and has a wavelength of about 615 nm, and the ultraviolet laser light has a wavelength of about 308 nm. (iii) The solid-state laser beam is generated by the third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 and has a wavelength of about 650 nm, and the ultraviolet laser light has a wavelength of about 325 nm. (iv) The solid-state laser crystal is doped with no rare-earth ion other than Sm.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Sm.sup.3+ is 404 nm.

(X) According to the ninth aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam; a solid-state laser crystal which is doped with at least one rare-earth ion including Eu.sup.3+ and emits a solid-state laser beam generated by a transition from .sup.5D.sub.0 to .sup.7F.sub.2 when the solid-state laser crystal is excited with the excitation laser beam; and an optical wavelength conversion element which converts the solid-state laser beam into ultraviolet laser light by wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatus according to the ninth aspect of the present invention may also have one or any possible combination of the following additional features (i) and (ii). (i) The solid-state laser beam has a wavelength of about 589 nm, and the ultraviolet laser light has a wavelength of about 295 nm. (ii) The solid-state laser crystal is doped with no rare-earth ion other than Eu.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Eu.sup.3+ is 394 nm.

(XI) According to the tenth aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam; a solid-state laser crystal which is doped with at least one rare-earth ion including Dy.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and a second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 when the solid-state laser crystal is excited with the excitation laser beam; and an optical wavelength conversion element which converts the solid-state laser beam into ultraviolet laser light by wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatus according to the tenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The solid-state laser beam is generated by the first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and has a wavelength of about 572 nm, and the ultraviolet laser light has a wavelength of about 286 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 and has a wavelength of about 664 nm, and the ultraviolet laser light has a wavelength of about 332 nm. (iii) The solid-state laser crystal is doped with no rare-earth ion other than Dy.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Dy.sup.3+ is 390 nm.

(XII) According to the eleventh aspect of the present invention, there is provided a laser-diode-excited solid-state laser apparatus including: a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam; a solid-state laser crystal which is doped with at least one rare-earth ion including Er.sup.3+, and emits a solid-state laser beam generated by one of a first transition from .sup.4S.sub.3/2 to .sup.4I.sub.15/2 and a second transition from .sup.2H.sub.9/2 to .sup.4I.sub.13/2 when the solid-state laser crystal is excited with the excitation laser beam; and an optical wavelength conversion element which converts the solid-state laser beam into ultraviolet laser light by wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatus according to the eleventh aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The solid-state laser beam is generated by the first transition from .sup.4S.sub.3/2 to .sup.4I.sub.15/2 and has a wavelength of about 540 nm, and the ultraviolet laser light has a wavelength of about 270 nm. (ii) The solid-state laser beam is generated by the second transition from .sup.2H.sub.9/2 to .sup.4I.sub.13/2 and has a wavelength of about 554 nm, and the ultraviolet laser light has a wavelength of about 277 nm. (iii) The solid-state laser crystal is doped with no rare-earth ion other than Er.sup.3+.

The excitation wavelength of the solid-state laser crystal which is doped with Er.sup.3+ is 406 or 380 nm.

In addition, in the seventh to eleventh aspects of the present invention, the optical wavelength conversion element may be realized by a nonlinear optical crystal having a periodic domain-inverted structure.

(XIII) The advantages of the seventh to eleventh aspects of the present invention are as follows. (i) When a Ho.sup.3+-doped solid-state laser crystal, e.g., a Ho.sup.3+-doped YAG crystal, is excited with a GaN-based compound laser diode (at an excitation wavelength of 420 nm), a solid-state laser beam in the near infrared range is generated by a transition from .sup.5S.sub.2 to .sup.5I.sub.7. Therefore, when this solid-state laser beam is converted into a second harmonic by wavelength conversion using an optical wavelength conversion element, ultraviolet light having a high intensity and a wavelength longer than 360 nm can be obtained. That is, a solid-state laser beam having a wavelength of, for example, about 750 nm can be obtained by the above transition. Thus, when this solid-state laser beam is converted into a second harmonic, ultraviolet light having a high intensity and a wavelength of about 375 nm is obtained. (ii) In addition, when the Ho.sup.3+-doped solid-state laser crystal is excited with the GaN-based compound laser diode, a solid-state laser beam having a wavelength of about 550 nm is generated by a transition from .sup.5S.sub.2 to .sup.5I.sub.8. Therefore, when this solid-state laser beam is converted into a second harmonic by wavelength conversion using an optical wavelength conversion element, ultraviolet light having a high intensity and a wavelength of about 275 nm, which is shorter than 360 nm, is obtained. (iii) Although the construction for realizing the wavelength conversion of the solid-state laser beam into a third harmonic is complex, the construction for realizing the wavelength conversion of the solid-state laser beam into a second harmonic is simple. Therefore, the laser-diode-excited solid-state laser apparatus using the wavelength conversion into a second harmonic is inexpensive. (iv) In the laser-diode-excited solid-state laser apparatus according to the eighth aspect of the present invention, in which a Sm.sup.3+-doped solid-state laser crystal is used, solid-state laser beams having wavelengths of about 566, 615, and 650 nm can be generated by excitation, for example, at the excitation wavelength of 404 nm, as explained before. Therefore, when these solid-state laser beams are converted into second harmonics by wavelength conversion, ultraviolet light beams having wavelengths of about 283, 308, and 325 nm can be obtained, respectively. (v) In the laser-diode-excited solid-state laser apparatus according to the ninth aspect of the present invention, in which a Eu.sup.3+-doped solid-state laser crystal is used, a solid-state laser beam having a wavelength of about 589 nm can be generated by excitation, for example, at the excitation wavelength of 394 nm, as explained before. Therefore, when this solid-state laser beam is converted into a second harmonic by wavelength conversion, an ultraviolet light beam having a wavelength of about 295 nm can be obtained. (vi) In the laser-diode-excited solid-state laser apparatus according to the tenth aspect of the present invention, in which a Dy.sup.3+-doped solid-state laser crystal is used, solid-state laser beams having wavelengths of about 572 and 664 nm can be generated by excitation, for example, at the excitation wavelength of 390 nm, as explained before. Therefore, when these solid-state laser beams are converted into second harmonics by wavelength conversion, ultraviolet light beams having wavelengths of about 286 and 332 nm can be obtained, respectively. (vii) In the laser-diode-excited solid-state laser apparatus according to the eleventh aspect of the present invention, in which an Er.sup.3+-doped solid-state laser crystal is used, solid-state laser beams having wavelengths of about 540 and 554 nm can be generated by excitation, for example, at the excitation wavelength of 406 or 380 nm, as explained before. Therefore, when these solid-state laser beams are converted into second harmonics by wavelength conversion, ultraviolet light beams having wavelengths of about 270 and 277 nm can be obtained, respectively. (viii) As explained before, the rare-earth ions, Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, and Er.sup.3+ have their absorption bands in the wavelength range of 380 to 420 nm, in which the currently available GaN-based compound laser diodes can easily oscillate. In particular, the currently available GaN-based compound laser diodes can achieve their maximum output power in the wavelength range of 400 to 410 nm. Since the solid-state laser crystals respectively doped with the rare-earth ions, Ho.sup.3+, Sm.sup.3+, Eu.sup.3+, Dy.sup.3+, and Er.sup.3+ are excited with a GaN-based compound laser diode in the laser-diode-excited solid-state laser apparatuses according to the seventh to eleventh aspects of the present invention, a great portion of the excitation light is absorbed by the solid-state laser crystal, and high efficiency and high output power can be achieved. (ix) In addition, as explained before, the GaN-based compound laser diodes have a great thermal conductivity coefficient and a high COD (catastrophic optical damage) threshold. Therefore, it is easy to obtain GaN-based compound laser diodes having a long lifetime and high output power. The laser-diode-excited solid-state laser apparatuses according to the seventh to eleventh aspects of the present invention, in which a GaN-based compound laser diode is used as an excitation light source, can have a long lifetime, and emit a laser beam with high output power.

(XIV) According to the twelfth aspect of the present invention, there is provided a fiber laser apparatus including: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Ho.sup.3+, and emits a second laser beam generated by one of a first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and a second transition from .sup.5S.sub.2 to .sup.5I.sub.8 when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the twelfth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The second laser beam is generated by the first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and is in the wavelength range of 740 to 760 nm. (ii) The second laser beam is generated by the second transition from .sup.5S.sub.2 to .sup.5I.sub.8 and is in the wavelength range of 540 to 560 nm. (iii) The core of the optical fiber is doped with no rare-earth ion other than Ho.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Ho.sup.3+ is 420 nm.

(XV) According to the thirteenth aspect of the present invention, there is provided a fiber laser apparatus including: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Sm.sup.3+, and emits a second laser beam generated by one of a first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, a second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2, and a third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the thirteenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv). (i) The second laser beam is generated by the first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2 and is in the wavelength range of 556 to 576 nm. (ii) The second laser beam is generated by the second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2 and is in the wavelength range of 605 to 625 nm. (iii) The second laser beam is generated by the third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 and is in the wavelength range of 640 to 660 nm. (iv) The core of the optical fiber is doped with no rare-earth ion other than Sm.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Sm.sup.3+ is 404 nm.

(XVI) According to the fourteenth aspect of the present invention, there is provided a fiber laser apparatus including: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Eu.sup.3+, and emits a second laser beam generated by a transition from .sup.5D.sub.0 to .sup.7F.sub.2 when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the fourteenth aspect of the present invention may also have one or any possible combination of the following additional features (i) and (ii). (i) The second laser beam is in the wavelength range of 579 to 599 nm. (ii) The core of the optical fiber is doped with no rare-earth ion other than Eu.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Eu.sup.3+ is 394 nm.

(XVII) According to the fifteenth aspect of the present invention, there is provided a fiber laser apparatus including: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Dy.sup.3+, and emits a second laser beam generated by one of a first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and a second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the fifteenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The second laser beam is generated by the first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and is in the wavelength range of 562 to 582 nm. (ii) The second laser beam is generated by the second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 and is in the wavelength range of 654 to 674 nm. (iii) The core of the optical fiber is doped with no rare-earth ion other than Dy.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Dy.sup.3+ is 390 nm.

(XVIII) According to the sixteenth aspect of the present invention, there is provided a fiber laser apparatus including: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Er.sup.3+, and emits a second laser beam generated by one of a first transition from .sup.4S.sub.3/2 to .sup.4I.sub.15/2 and a second transition from .sup.2H.sub.9/2 to .sup.4I.sub.13/2 when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the sixteenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The second laser beam is generated by the first transition from .sup.4S.sub.3/2 to .sup.4I.sub.15/2 and is in the wavelength range of 530 to 550 nm. (ii) The second laser beam is generated by the second transition from .sup.2H.sub.9/2 to .sup.4I.sub.13/2 and is in the wavelength range of 544 to 564 nm. (iii) The core of the optical fiber is doped with no rare-earth ion other than Er.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Er.sup.3+ is 406 or 380 nm.

(XIX) According to the seventeenth aspect of the present invention, there is provided a fiber laser apparatus including: a GaN-based compound laser diode which emits a first laser beam; and an optical fiber which has a core doped with Tb.sup.3+, and emits a second laser beam generated by a transition from .sup.5D.sub.4 to .sup.7F.sub.5 when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the seventeenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The second laser beam is in the wavelength range of 530 to 550 nm. (ii) The core of the optical fiber is doped with no-rare-earth ion other than Tb.sup.3+. The excitation wavelength of the core of the optical fiber doped with Tb.sup.3+ is 380 nm.

(XX) According to the eighteenth aspect of the present invention, there is provided a fiber laser amplifier including: a GaN-based compound laser diode which emits an excitation laser beam; and an optical fiber which has a core doped with Ho.sup.3+, and amplifies incident light which has a wavelength within a wavelength range of fluorescence generated by one of a first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and a second transition from .sup.5S.sub.2 to .sup.5I.sub.8 when the optical fiber is excited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the eighteenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The fluorescence is generated by the first transition from .sup.5S.sub.2 to .sup.5I.sub.7 and is in the wavelength range of 740 to 760 nm. (ii) The fluorescence is generated by the second transition from .sup.5S.sub.2 to .sup.5I.sub.8 and is in the wavelength range of 540 to 560 nm. (iii) The core of the optical fiber is doped with no rare-earth ion other than Ho.sup.3+.

The excitation wavelength of the optical fiber which is doped with Ho.sup.3+ is 420 nm.

(XXI) According to the nineteenth aspect of the present invention, there is provided a fiber laser amplifier including: a GaN-based compound laser diode which emits an excitation laser beam; and an optical fiber which has a core doped with Sm.sup.3+, and amplifies incident light which has a wavelength within a wavelength range of fluorescence generated by one of a first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2, a second transition from .sup.4G.sub.9/2 to .sup.6H.sub.7/2, and a third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 when the optical fiber is excited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the nineteenth aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv). (i) The fluorescence is generated by the first transition from .sup.4G.sub.5/2 to .sup.6H.sub.5/2 and is in the wavelength range of 556 to 576 nm. (ii) The fluorescence is generated by the second transition from .sup.4G.sub.5/2 to .sup.6H.sub.7/2 and is in the wavelength range of 605 to 625 nm. (iii) The fluorescence is generated by the third transition from .sup.4F.sub.3/2 to .sup.6H.sub.11/2 and is in the wavelength range of 640 to 660 nm. (iv) The core of the optical fiber is doped with no rare-earth ion other than Sm.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Sm.sup.3+ is 404 nm.

(XXII) According to the twentieth aspect of the present invention, there is provided a fiber laser amplifier including: a GaN-based compound laser diode which emits an excitation laser beam; and an optical fiber which has a core doped with Eu.sup.3+, and amplifies incident light which has a wavelength within a wavelength range of fluorescence generated by a transition from .sup.5D.sub.0 to .sup.7F.sub.2 when the optical fiber is excited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the twentieth aspect of the present invention may also have one or any possible combination of the following additional features (i) and (ii). (i) The fluorescence is in the wavelength range of 579 to 599 nm. (ii) The core of the optical fiber is doped with no rare-earth ion other than Eu.sup.3+.

The excitation wavelength of the core of the optical fiber doped with Eu.sup.3+ is 394 nm.

(XXIII) According to the twenty-first aspect of the present invention, there is provided a fiber laser amplifier including: a GaN-based compound laser diode which emits an excitation laser beam; and an optical fiber which has a core doped with Dy.sup.3+, and amplifies incident light which has a wavelength within a wavelength range of fluorescence generated by one of a first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and a second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 when the optical fiber is excited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the twenty-first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii). (i) The fluorescence is generated by the first transition from .sup.4F.sub.9/2 to .sup.6H.sub.13/2 and is in the wavelength range of 562 to 582 nm. (ii) The fluorescence is generated by the second transition from .sup.4F.sub.9/2 to .sup.6H.sub.11/2 and is in the wavelength range of 654 to 674 nm. (iii) The core of the optical fiber is doped with no rare-earth ion other than Dy.sup.3+.

The excitation wavelength of the optical fiber which is doped with Dy.sup.3+ is 390 nm.

(XXIV) According to the twenty-second aspect of the present invention, there is provided a fiber laser amplifier including: a GaN-based compound laser diode which emits an excitation laser beam; and an optical fiber which has