Coherent light generators – Optical fiber laser
Reexamination Certificate
1999-02-17
2001-05-29
Kim, Robert H. (Department: 2877)
Coherent light generators
Optical fiber laser
C372S102000, C385S037000, C385S123000, C385S124000, C385S126000, C385S127000
Reexamination Certificate
active
06240108
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical fiber lasers, and more particularly, to an optical fiber laser having reduce parasitic lasing.
BACKGROUND ART
A cladding-pumped fiber laser typically comprises a single mode fiber core disposed within a relatively large multi-mode cladding. This inner cladding is surrounded by a second, outer cladding to prevent radiation from propagating out of the inner cladding. The fiber is positioned between two mirrors or gratings to define a laser cavity. Light from a pump laser, such as a laser diode, is injected into the end of the cladding. Typically, the length of the fiber is sufficient for absorbing most of the pure radiation, which propagates in the core, by the active laser species located in the core. This is advantageous since radiation can be coupled into the multi-mode cladding without the high tolerances typically required for coupling light directly into a single mode core. The core is typically doped with rare-earth ions, which are the active lasing species. The ions absorb photons delivered by the pump laser. Photons are then emitted by the ions at a wavelength characteristic of the particular dopant species.
Of particular importance are high power, multi-watt fiber lasers that operate at shorter wavelength, i.e. at less than 1065 nm. The radiation is used to pump Erbium/Ytterbium (Er/Yb) fiber amplifiers operating in the 1.55 &mgr;m. Cladding-pumped Yb fiber lasers that produce multi-watt outputs at 1065 nm have been demonstrated with special high-brightness diode laser sources. However, efficient operation of such lasers at less than 1065 nm is difficult to achieve with commercially available low brightness pump sources, such as diode-coupled fiber bundles.
One problem with the previously known cladding-pumped Yb fiber lasers is that parasitic lasing and amplified spontaneous emission (ASE) occur at a longer wavelength than the design wavelength. When parasitic operations occur, lasing at the design wavelength cannot be achieved, unless the device efficiency is sacrificed by reducing the length of the fiber laser. These limitations become more significant in high power multi-watt applications.
Remedies against parasitic lasing include incorporating highly-selective Bragg gratings as resonator reflectors and angle-cleaved fiber ends for reducing the feedback at the parasitic wavelength. In addition, the fiber laser length may be reduced until parasitic lasing gives way to operation at the desired wavelength. Shortening the length of the fiber, however, is highly undesirable since it reduces the amount of absorbed pump power. This reduces the overall efficiency of the system.
Thus, there is a need for an improved cladding-pumped laser structure that operates at lower wavelengths and reduces parasitic lasing.
SUMMARY OF THE INVENTION
One object of the invention is to prevent parasitic lasing in an optical fiber laser so that it operates at a shorter wavelength without sacrificing its efficiency by reducing the fiber length.
In one aspect of the invention, an optical fiber laser has an optical fiber having a first end and a second end. The fiber has a core with an undoped central region and a collar surrounding the core. The collar is doped with rare-earth ions to form the active laser medium. An inner cladding surrounds the core. A first grating is disposed at the first end of the optical fiber, and a second grating is disposed at the second end. The laser is activated by coupling an optical pump to the first cladding.
One advantage of the invention is that a shorter design wavelength than that normally formed in an optical fiber laser of the same length.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.
REFERENCES:
patent: 5881197 (1999-03-01), Dong et al.
patent: 5991314 (1999-11-01), Ionov et al.
patent: 6137813 (2000-10-01), Ionov et al.
Flores Ruiz Delma R.
Grunebach Georgann S.
Hughes Electronics Corporation
Kim Robert H.
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