Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-09-28
2003-08-19
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S098000, C372S092000, C372S099000, C372S009000
Reexamination Certificate
active
06608847
ABSTRACT:
BACKGROUND OF THE INVENTION
Frequency tunable semiconductor diode lasers provide versatile optical tools for telecommunications, metrology, spectroscopy and other uses. Many such tunable lasers use a diffraction grating with a movable reflector to select a desired wavelength from the beam diffracted by the grating. A diode gain medium is employed that has an antireflection (AR) coating on one facet thereof. Light emitted from the AR coated facet is diffracted by a grating and directed to a movable reflector, which feeds light back to the grating and gain medium. Rotation of the reflector selects the wavelength diffracted by the grating and allows the laser to be tuned to a desired output wavelength. Translational motion of the reflector is frequently employed in conjunction with the rotational motion to couple the cavity optical path length to the selected wavelength and provide mode-hop free tuning. Grating-tuned external cavity lasers are typically arranged in the Littman-Metcalf configuration with a “folded cavity”, which permits compact-sized external cavity laser devices suitable for many commercial uses.
The optical output of grating-tuned external cavity lasers of this sort may be collected as the light emitted from a rear, partially reflective facet of the gain medium, or as the grating reflection of light directly from the gain medium. This provides a relatively high output power, but includes “noise” in the form of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) from the gain medium. One approach to providing a spectrally “clean” output from grating-tuned external cavity lasers has been to simply insert a beam coupler directly into the laser cavity between the grating and gain medium. A partially reflective surface on the beam coupler directs a portion of the light returning from the grating outside the cavity. This partially reflected light is at the selected wavelength and has been spatially separated from the propagation direction of the spontaneous emission light by the grating. This spectrally clean output may then be coupled into a fiber for use in applications requiring high spectral purity.
This relatively simple approach to providing a spectrally pure output beam has some important drawbacks. One of the attractive features of folded cavity lasers is the small or compact size that is possible for commercial lasers. Directing optical output outside of the folded cavity results in a substantial increase in the overall size and complexity of the external cavity laser device. Further, the introduction of a beam coupler into the laser cavity results in a significant intracavity optical loss. The insertion of a beam coupler into the laser cavity always results in the extra optical loss from the opposite reflection off the partially reflective surface of the beam coupler from the spectrally cleaned light that is collected and use. The spectral cleansing provided by beam couplers thus is obtained with a corresponding sacrifice in laser output power.
There is a need for an external cavity laser apparatus that provides suppression of spontaneous emission light from laser output, that is simple and compact in design, which provides high laser output power, and which collects loss components associated with spectral cleaning as usable laser outputs. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.
SUMMARY
The invention provides a laser apparatus and method with compact cavity design that provides suppression of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) light with minimal intracavity loss. The apparatus comprises a gain medium emitting a light beam along an optical path, a tuning element positioned in the optical path and configured feed back light of a selected wavelength to the gain medium and configured to define a first output beam directed along a first output path, a partial reflector located in the optical path and positioned to create a second output beam directed along a second output path substantially parallel to the first output path; and having a spontaneous emission component that is spatially separated from the selected wavelength.
By way of example, and not of limitation, the apparatus may further comprise an optical fiber positioned with respect to the second output path such that light at the selected wavelength is selectively received by or coupled into the optical fiber. A reflector may be positioned in the optical path after the tuning element to define an external laser cavity with a facet of the gain medium. The reflector may be movable with respect to the tuning element to define the selected wavelength. The tuning element may comprise a grating, an etalon, an interference filter, or other optical element or capable of providing wavelength selection.
In certain embodiments, the apparatus may comprise a beam coupler positioned in the optical path, with the partial reflector located on a facet of the beam coupler. The beam coupler may further comprise an antireflection coating on one or more facets that are opposite from the partial reflector on the beam coupler. In certain embodiments, the beam coupler may be configured to define a third output beam traveling a third output path that is substantially parallel to the first and second output paths.
By way of further example, the external cavity of the apparatus, in some embodiments, has a folded external cavity design and comprises a reflective rear facet on the gain medium and a reflector positioned in the optical path after the tuning element, wherein the reflector and a rear facet of the gain medium define the external laser cavity. The tuning element comprises a tuning grating capable of selecting a specific wavelength for output from the external cavity. The external cavity is folded with respect to the tuning grating, and the reflector is movable with respect to the grating to provide wavelength selection. A first output beam is reflected from the tuning grating along a first output path, and contains noise associated with source spontaneous emission (SSE) and/or amplified spontaneous emission (ASE) associated with current pumping of the gain medium.
A beam coupler, which may comprise a simple optical flat with a partially reflective surface and an anti-reflection-coated surface, is positioned in the optical path between the gain medium and the tuning grating. The beam coupler is positioned in the optical path to receive light diffracted from the tuning grating and to reflect a portion of this light out of the external cavity as a second output beam along a second output path that is substantially or approximately parallel to the first output path. The second output beam comprises a portion of the light diffracted from the tuning-grating toward the gain medium, and which is intercepted or picked off by the beam coupler before the light can be fed back to or otherwise return to the gain medium. The beam coupler may, in certain embodiments, have a wedge or prism configuration.
In the second output beam, the spontaneous emission light generated in the gain medium has been spatially separated from light at the selected wavelength by operation of the tuning grating, and has dispersion characteristics for generating a low-noise output. The second output beam is coupled to an optical fiber positioned and configured to selectively receive the light at the tuned or selected wavelength, and to selectively exclude light associated with spontaneous emission. Since the second output path is substantially or approximately parallel to the first output path, coupling of the both the first and second output beams into fibers can be achieved without sacrificing the overall compact size of the external cavity laser apparatus. This arrangement of the output paths allows for ease of alignment and detection of either the first or second output beam from the laser apparatus.
In certain embodiments, the beam coupler may be configured to provide a third output beam along a third output
Davidson Andrew
Hand Carter
Reed Murray
Robinson David
Wang Weizhi
Cannon Alan W.
Jr. Leon Scott
New Focus Inc.
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