Coherent light generators – Particular beam control device – Producing plural wavelength output
Reexamination Certificate
2001-02-28
2004-02-24
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Producing plural wavelength output
C372S024000
Reexamination Certificate
active
06697393
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser amplifier and oscillator using a medium doped with a rare earth element as a gain medium and using a semiconductor laser as an excitation light source, and a method and apparatus for laser amplification using the same.
2. Description of the Related Art
A laser apparatus using a crystal or glass medium doped with a rare earth element as an active medium, that is, a laser oscillator and amplifier or the like is widely applied to information communication industries and mechanical engineering fields. An optical fiber amplifier in a large yield solid laser for metal processing and an optical fiber communication system is a typical example of its application.
The efficiency, size, service life, and mechanical stability of the laser oscillator and amplifier are mainly determined depending on an excitation light source. A semiconductor laser used as an excitation light source is superior to a solid laser and a fiber laser in these respects. Therefore, considering equipment applicability, it is preferred to employ a system (LD excitation system) using a semiconductor laser (laser diode: LD) as an excitation light source. A solid laser and a fiber laser used as an excitation light source have many more disadvantages than the semiconductor laser in various respects. In particularly, apart from the fact that the solid and fiber lasers are disadvantageous as compared with the LD in view of efficiency, size, mechanical stability, and service life, these lasers are disadvantageous in that a small number of passive optical parts in a bandwidth of 1.05 &mgr;m in wavelength is provided. For example, an optical isolator with its low loss and high isolation is not practically used.
In such a laser apparatus, energy is supplied to rare earth ion by means of light excitation. Thus, in order to ensure operation with high efficiency, it is particularly important to select a wavelength of an excitation light source. However, some kinds of rare earth ions do not match well a wavelength of a semiconductor laser in optimal excitation wavelength, i.e., ion absorption wavelength bandwidth, making it difficult to ensure LD excitation caused by a semiconductor laser with its single wavelength. Thus, it may be required to use a light source other than LD. In particular, in the case where an ion forming a self-termination system whose laser low level service life is longer than laser high level service life is used as a gain medium, as shown in the following example, the wavelength of excitation light is limited more significantly, thus making it more difficult to ensure LD excitation.
As an example, a case of a Tm (thulium) ion in a fluorine glass will be described here. When a rare earth element such as Tm is doped in a medium such as fluorine glass, the element is ionized in the medium to form a Tm ion.
FIG. 1
is an energy level chart showing a conventional method of exciting a thulium fiber amplifier. In addition, the wavelength of the conventional excitation light is explicitly shown in the figure.
FIG. 2
is a graph illustrating an ASE (amplified spontaneous emission) spectrum when a transition shown in
FIG. 1
is generated. As shown in
FIG. 1
, in a fiber amplifier in which a Tm ion is doped in a core, 1.04 to 1.07 &mgr;m (hereinafter, referred to a bandwidth of 1.05 &mgr;m) is used as an excitation wavelength, whereby light amplification of 1.47 &mgr;m bandwidth in wavelength (transition from
3
F
4
to
3
H
4
) can be achieved. In the figure, the light amplification is explicitly shown as a transition “a”. In addition, at this time, an ASE spectrum as shown in
FIG. 2
can be obtained. In more detail, this fact is disclosed in IEEE Journal of Quantum Electronics, Vol. 31, page 1880, 1995; Japanese Patent Application No. 11-156745; and Optics Letters, Vol. 24, page 1684, 1999.
In such a fiber amplifier, as shown in
FIG. 1
, an excitation photon with its 1.05 &mgr;m bandwidth causes Tm ion base level absorption (transition from
3
H
6
to
3
H
5
), and further causes non-radiation transition (not shown) and excitation state absorption (transition from
3
H
4
to
3
F
2
or transition from
3
F
4
to
1
G
4
). Then, an inversion distribution is formed between
3
F
4
-
3
H
4
levels due to two-stage transition. The reason why this technique is efficient is that Tm ion base level absorption spectrum and excitation level absorption spectrum are superimposed in wavelength of 1.05 &mgr;m, thus making it possible to ensure excitation with single excitation light of 1.05 &mgr;m in wavelength.
However, in the above-mentioned Tm doped fiber amplifier, it is difficult to ensure excitation of 1.05 &mgr;m bandwidth with a semiconductor laser. This is because, although laser light oscillation of 1.05 &mgr;m bandwidth in a semiconductor laser is reported in some research papers, an apparatus capable of achieving a practical yield power level, for example, a transverse single mode yield of about 500 mW, does not exist in research level and commercially available level. For example, as disclosed in Applied Physics Letters, Vol. 69, page 248, 1996, the current semiconductor laser yield of 1.06 &mgr;m in wavelength is about 200 mW. The current semiconductor laser yield disclosed in the paper is too small to ensure excitation in the above-mentioned Tm doped fiber amplifier. And the yield of commercially available semiconductor laser is smaller than that.
For such reasons, in a conventional Tm-doped fiber amplifier, as an excitation light source of 1.05 &mgr;m bandwidth, for example, there is used LD excited solid lasers such as Nd: YAG, Nd: YLF, Yb: YAG; or LD excited fiber laser such as Yb doped fiber lasers, for example.
On the other hand, although excitation light source other than 1.05 &mgr;m bandwidth, for example, excitation of 0.79 &mgr;m bandwidth that directly excites
3
F
4
level, for example, (wavelength of 0.77 to 0.80 &mgr;m and transition “b” in
FIG. 1
) and 0.67 &mgr;m bandwidth that excites
3
F
2
level (a wavelength of 0.64 to 0.68 &mgr;m and transition “c” in
FIG. 1
) can perform LD excitation, for example, as disclosed in Electronics Letters, Vol. 25, page 1660, 1989, the ion number density of laser low level (
3
H
4
) increases, and an inversion distribution cannot be maintained in a constant state, making it impossible to ensure operation with high efficiency. This is because the laser low level service life is about 10 msec in Tm ion, which is longer than the laser high level service life (
3
F
4
service life is 1.3 msec). Such a system is called self-termination system, which is observed in rare earth elements Er (Erbium) and Ho (Holmium) or the like other than Tm.
In a laser amplifier and oscillator using a rare earth element forming its self-termination system, in order to ensure operation with high efficiency, it is essentially required to provide excitation light serving to excite ion from a base level to laser low level or an energy level above the laser low level; and excitation light serving to excite ion from laser low level to laser high level, and to form an inversion distribution. As described above, the excitation light of 1.05 &mgr;m bandwidth in Tm can play these two roles at the same time, but LD excitation is impossible.
As shown in the above example, in the case where a medium in which an ion forming its self-termination system is doped is employed as a gain medium, an excitation wavelength is limited, thus making it difficult to ensure LD excitation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a laser amplifier, a method and an apparatus for laser amplification, and a laser oscillator, which uses a medium doped with a rare earth element forming its self-termination system transition, in which semiconductor laser excitation is enabled and high efficiency, miniaturization, extended service life, and highly stable operation can be ensured at the same time.
A laser amplifier according to the present invention is directed to a laser amplifier using
Kasamatsu Tadashi
Yano Yutaka
Ip Paul
NEC Corporation
Whitham Curtis & Christofferson, P.C.
Zahn Jeffrey N
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