Passively modelocked fiber laser

Details Passively modelocked fiber laser Passively modelocked fiber laser

2003-03-25

2004-09-28

Wong, Don (Department: 2828)

Coherent light generators

Optical fiber laser

C372S018000

Reexamination Certificate

active

06798793

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
The application is based on and claims priority under 35 U.S.C. §119 with respect to a Japanese Patent Application 2002-082518, filed on Mar. 25, 2002, the entire content of which is incorporated herein by reference.
FIELD OF THE INVENTION
This invention generally relates to a passively modelocked fiber laser capable of providing an extra-short pulse, and more particularly, this invention pertains to a fiber isolator which removes a return laser beam returning from a cavity part to a laser diode.
BACKGROUND OF THE INVENTION
A typical passively modelocked fiber laser is shown in FIG.
3
. The passively modelocked fiber laser is mainly composed of a pump part
20
and a cavity part
30
. The pump part
20
includes a laser diode
21
which generates a laser beam (laser energy) with a wavelength of 980 nm as an excitation light, a fiber isolator
22
, and a single mode fiber
23
which propagates the laser beam generated at the laser diode
21
to the fiber isolator
22
. The cavity part
30
includes a wavelength splitter
31
which guides the laser beam with the wavelength of 980 nm from the pump part
20
, an erbium doped fiber
32
(a gaining medium) which amplifies the laser beam with a wavelength of 1560 nm by the excitation light having the wavelength of 980 nm, and a pair of metallic mirrors
33
and
34
(a reflecting means) which reflects a laser beam in the cavity part
30
.
The cavity part
30
further includes collimator optical systems
35
and
36
which form an optical path between the metallic mirrors
33
and
34
, a beam splitter
37
(an output means) which outputs the laser beam with the wavelength of 1560 nm, and a single mode fiber
38
which propagates the laser beam with the wavelength of 1560 nm between the wavelength splitter
31
and the collimator optical system
36
. Propagation between the pump part
20
and the cavity part
30
is performed by a single mode fiber
24
.
The laser beam with a wavelength of 980 nm generated at the laser diode
21
is propagated as single mode propagation because of the single mode fibers
23
,
24
, and
38
propagating a laser beam with a cutoff wavelength or larger than the cutoff wavelength. The laser beam guided by the wavelength splitter
31
is amplified to the laser beam with the wavelength of 1560 nm at the erbium doped fiber
32
.
Part of a return laser beam (1560 nm) which returns to the fiber isolator
22
via the single mode fiber
24
from the cavity part
30
is discharged out of an optical fiber (not shown) which is part of the fiber isolator
22
, or total internal reflection of the return laser beam (1560 nm) within the optical fiber is not generated and the return laser beam leaks outside. Consequently, the return laser beam incident on the laser diode
21
and with a wavelength of 1560 nm can be prevented by an inexpensive device as well as a downsized device.
However, in the conventional fiber isolator
22
, all of the return laser beam is not discharged from the optical fiber, and part of the return laser beam returns to a core of the fiber within isolator
22
by being reflected on a boundary part between the optical fiber and the outside. Additionally, part of a laser beam once discharged outside is affected by a wavelength absorption characteristic of a coating, or some other apparatus, located on or around the optical fiber. For example, protected by a white coated tube, the laser beam leaking from the optical fiber is reflected by the white coated tube, returns to the inside of the optical fiber again, and is incident on the laser diode
21
. When a return laser beam returning to a fiber isolator is incident on a laser diode, the following problems may arise; unsteady oscillation of a laser beam at a laser diode, dropping of signal-to-noise (S/N) ratio of a laser beam which has to be guided, or deterioration of the M
2
factor. The M
2
factor is an international standard for evaluating quality of a laser beam quantitatively.
The present invention, therefore, seeks to provide an improved passively mode locked fiber laser capable of solving the foregoing problems, for example, unsteady oscillation of a laser beam, the reduction of S/N ratio, and the deterioration of the M
2
factor, when a return laser beam is incident on a laser diode.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, a passively modelocked fiber laser includes a laser diode generating laser energy, a wavelength splitter guiding the laser energy generated at the laser diode to a cavity part, and a gaining medium for amplifying the laser energy in the cavity part. The passively modelocked fiber laser further includes a reflecting means, such as the mirrors shown in
FIG. 3
, for reflecting the laser energy along an optical axis passing through the gaining medium, an output means, such as the beam splitter
37
shown in
FIG. 3
, for outputting the laser energy generated in the cavity part, and a fiber isolator provided between the laser diode and the wavelength splitter. In the foregoing passively modelocked fiber laser according to the invention, the fiber isolator has an optical fiber having a curved shape. The optical fiber includes a core for propagating the laser energy, a clad for coating the core, and an optical absorption film for coating the clad.
According to another aspect of the present invention, the optical fiber has a coiled shape, and a value of a refractive index of the optical absorption film is equal to or greater than a value of a refractive index of the clad.


REFERENCES:
patent: 5689519 (1997-11-01), Fermann et al.
patent: 5701319 (1997-12-01), Fermann
patent: 5923686 (1999-07-01), Fermann et al.
patent: 6275512 (2001-08-01), Fermann
patent: 2002/0172486 (2002-11-01), Fermann
patent: 2003/0147579 (2003-08-01), Hammer

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