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Timestamp: 2014-12-23 01:51:19
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Patent US6570892 - Passively mode-locked fiber lasers - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA passive mode-locked linear-resonator fiber laser using polarization-maintaining fibers and a saturable absorber to produce ultra short pulses and a long-term reliable operation with reduced maintenance. Such a fiber laser can be configured to produce tunable pulse repetition rate and tunable laser...http://www.google.com/patents/US6570892?utm_source=gb-gplus-sharePatent US6570892 - Passively mode-locked fiber lasersAdvanced Patent SearchPublication numberUS6570892 B1Publication typeGrantApplication numberUS 10/123,942Publication dateMay 27, 2003Filing dateApr 16, 2002Priority dateJul 11, 1997Fee statusPaidAlso published asUS6097741, US6373867Publication number10123942, 123942, US 6570892 B1, US 6570892B1, US-B1-6570892, US6570892 B1, US6570892B1InventorsHong Lin, Katherine Y. LinOriginal AssigneeCalmar Optcom, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (11), Non-Patent Citations (9), Referenced by (28), Classifications (11), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetPassively mode-locked fiber lasersUS 6570892 B1Abstract A passive mode-locked linear-resonator fiber laser using polarization-maintaining fibers and a saturable absorber to produce ultra short pulses and a long-term reliable operation with reduced maintenance. Such a fiber laser can be configured to produce tunable pulse repetition rate and tunable laser wavelength.
FIELD OF THE INVENTION The present invention relates to fiber optical devices and lasers, and more specifically, to mode-locked fiber lasers.
Fiber lasers have been developed as a new generation of compact, inexpensive and robust light sources. In essence, a fiber laser is an optically-pumped resonator with a doped-fiber as the gain medium. As the gain exceeds the total optical loss in the resonator, a laser oscillation can be generated. Many different dopants can be used to achieve laser oscillations at different wavelengths. Atomic transitions in rare-earth ions can be used to produce lasers from visible wavelengths to far infrared wavelengths (e.g., 0.45 μm�3.5 μm). Er-doped fiber lasers for producing optical pulses at 1.55 μm are particularly useful for optical fiber communication since the optical loss in the commonly used silica fibers is minimum at about 1.55 μm.
SUMMARY OF THE INVENTION The present disclosure describes a passive mode-locked fiber laser with a simple linear cavity and a saturable absorber to generate femtosecond pulses with a peak power up to and greater than tens of watts.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing one embodiment of a polarization-maintaining linear-cavity fiber laser.
DETAILED DESCRIPTION OF THE INVENTION Various embodiments of the invention are now described in detail with specific reference to �fiber�. Although the term �fiber� is in general understood as an optical fiber comprising a center core and an outer portion that contains an optical beam, �fiber� is used throughout this disclosure to include any optical waveguiding conduit such as optical fibers and waveguides.
Polarization-maintaining single-mode (�PM�) fibers or polarizing single-mode fibers are preferable for any fibers in the optical path of the laser pulses, i.e., the doped fiber 130 and other undoped fiber segments linking various optical elements in the resonator. Fiber segments 140 a, 140 b, 160 a, 160 b, and 121 a for linking optical couplers 140, 160 and optical collimator 121 are such examples. Preferably, fiber segments 144 and 164 in the couplers 140 and 160 are also PM or polarizing fibers although regular fibers may be used. A polarization-maintaining fiber is configured to have well-defined principal axes for two mutually orthogonal polarizations. A polarizing fiber has a single principal polarization axis. These two types of fibers can be configured so that a principal axis is essentially not influenced by environmental conditions, such as fiber position, temperature, and stress. Therefore, the polarization of a beam propagating in such a fiber can be maintained. In the following description, �polarization-maintaining fiber� fiber will be used to include any fiber or optical waveguide that can preserve an optical polarization of a beam in a resonator.
The pump optical coupler 140 is disposed relative to the doped fiber segment 130 to couple the pump beam in the fiber 142 into the doped fiber segment 130. Two undoped fiber segments 140 a and 140 b may be used to optically coupled the coupler 140 into the resonator. The fiber 140 a is connected to one end of the doped fiber 130 by, for example, fusion splicing. The pump beam is directed into the doped fiber 130 to produce population inversion at a desired optical transition. This initiates spontaneous emission at the desired laser wavelength defined by the optical transition and builds up the laser oscillation due to the optical feedback as the output power of the pump light source 150 exceeds a threshold level. Since the laser wavelength is different from the pump wavelength, the pump optical coupler 140 should be a wavelength-division multiplexer (�WDM�). For Er-doped fiber laser, the WDM 140 is configured for coupling energy between two different channels near a 1.55-μm lasing region (e.g., from 1520 nm to 1580 nm) and near a 0.98-μm pump region (e.g., from 965 nm to 995 nm).
FIGS. 3A-3F show alternative embodiments of the first reflective element 110. FIG. 3A uses a lens 114 a in place of the collimator 114 in FIG. 1. The lens 114 a is placed away from the grating 112 with a spacing larger than the lens focal length f to image the free end facet of the fiber 111 to the grating 112. Since the beam is a Gaussian beam, the beam on the grating surface is essentially collimated within the Rayleigh length range. The output facet of the fiber 111 as shown is perpendicular to the fiber core. This facet is preferably coated with an anti-reflection coating at the pump wavelength to reduce the reflection of the pump beam coupled into the resonator from the coupler 140. This coating may also be anti-reflective at the laser wavelength to reduce a sub-cavity effect due to intra-cavity reflection at the laser wavelength. FIG. 3B is similar to FIG. 3A except that the output facet 111 a of the fiber 111 is polished at an angle of several degrees (typically, about 6��8�) to reduce optical reflection at both laser wavelength and pump wavelength. The angle-polished fiber 111 is tilted at an angle with respect to the optic axis of the lens 114 a in order to achieve proper optical coupling. FIG. 3C shows an embodiment that uses a reflector 113 with a high reflectivity at the laser wavelength and a low reflectivity at the pump wavelength. FIG. 3D places a lens 114 a away from the output facet of the fiber 111 by a focal length to collimate the beam incident to the high reflector 113. This optical configuration can also replace the focusing configuration shown in FIGS. 3A-3C.
A fiber laser in accordance with the invention may be operated either in a soliton mode or in a non-soliton mode. Soliton is a special nonlinear phenomenon in which an optical pulse maintains its shape and spectral profile essentially unchanged during propagation in fibers. An optical pulse traveling in a fiber is subject to the fiber dispersion so that different frequency components in the pulse travel at different group velocities. This dispersion causes pulse broadening in the time domain. In addition, a pulse also experiences a nonlinear effect �self phase modulation� (�SPM�) caused by the intensity dependence of the refractive index of the fiber. SPM can lead to new frequency components in high intensity pulses, thus effectively broadening the pulse in the frequency domain. Soliton pulses are generated from a fiber laser when the fiber dispersion is negative, i.e., the group velocity of a high frequency component is higher than that of a low frequency component, and the group velocity dispersion compensates for SPM.
FIG. 8A shows a side view of an adjustable mount 800 for holding the optical collimator 114 and 121 in the fiber laser 100 in FIG. 1. The mount 800 has a base 810 with a first base part 810 a and a second base part 810 b in a shape of the letter �L�. The base 810 is made of a rigid material such as aluminum or steel. A slot 812 is formed in the first base part 810 to change the position of the second base part 810 b upon being compressed by a set of screws 814 a and 814 b. Screws 814 a adjust the position of the second base part 810 b in a direction opposite to that controlled by screws 814 b. A slot 812 b is formed in the second base part 810 b to split a portion thereof to form an adjustable part 810 c. The second base part 810 b has a through hole 820 to hold a collimator 802. Screws 816 on the adjustable part 810 c are used to adjust the relative tilt of the part 810 c in order to control the orientation of the collimator 802. FIGS. 8B and 8C show different side views of the mount 800 along the lines 8B�8B and 8C�8C, respectively. This design also eliminates the conventional ball bearing.
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