Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2002-02-26
2004-09-07
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S107000
Reexamination Certificate
active
06788726
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. Rotational movement of the reflector with respect to a pivot point 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 an important drawback: the partially reflective surface of the beam coupler has the disadvantage of reciprocity. As a simple mirror, the beam coupler simultaneously reflects an equal portion of the beam traveling from the gain medium towards the grating. The insertion of a conventional beam coupler into the laser cavity thus always results in an 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 are many uses for external cavity lasers having output with high spectral purity, including medical, metrological and optical communications areas. The low power of currently available spectrally pure laser output has, however, limited the commercial use of external cavity lasers in these areas. There is accordingly a need for an external cavity laser apparatus that provides suppression of spontaneous emission light from laser output without significant optical loss, and which is simple, compact and inexpensive in design. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.
SUMMARY
The invention is a laser apparatus and method that provides for suppression of source spontaneous emission (SSE) and amplified spontaneous emission (ASE) light in laser output with minimal intracavity loss. The apparatus comprises, in general terms, a gain medium emitting a light beam, a wavelength selection element positioned in the light beam, and a non-reciprocal pickoff positioned in the light beam to receive light returning from the wavelength selection element to the gain medium. The wavelength selection element may be tunable.
The non-reciprocal pickoff may comprise a linear polarizer positioned in the light beam, together with a non-reciprocal polarization rotator positioned in the light beam after the polarization-dependent beam splitter. The non-reciprocal pickoff may further comprise a reciprocal polarization rotator positioned in the light beam after the polarization-dependent beam splitter. The non-reciprocal polarization rotator and the reciprocal polarization rotator may be balanced with respect to each other, such that the non-reciprocal polarization rotator and the reciprocal polarization rotator each define substantially equal angles of polarization rotation. The rotators are configured to cancel out each other's rotational effect on the polarization orientation of outward-bound light from the gain medium towards the wavelength selection element, and to produce an additive rotational effect on light returning towards the gain medium from the wavelength selection element. In certain embodiments, the gain medium and the polarization-dependent beam splitter may be angularly positioned with respect to each other at an angle that is equal or substantially equal to the angle of rotation defined by the non-reciprocal rotator, such that the gain medium and polarization-dependent beam splitter effectively provide the effect of a reciprocal polarization rotator.
The invention also provides methods of laser operation that comprise, in general terms, emitting a light beam from a gain medium along an optical path, positioning a wavelength selection element in the optical path, positioning a non-reciprocal pickoff in the optical path, feeding spectrally clean light back to the gain medium by the wavelength selection element, and picking off, by the non-reciprocal pickoff, a portion of spectrally clean light traveling the optical path towards the gain medium. The non-reciprocal pickoff may be positioned between the gain medium and the wavelength selection element. The positioning of the non-reciprocal pickoff may comprise positioning a polarization-dependent beam splitter in the optical path between the gain medium and the wavelength selection element, and positioning a non-reciprocal polarization rotator in the optical path between the polarization-dependent beam splitter and the wavelength selection element.
In certain embodiments the methods may comprise angularly positioning the polarization-dependent beam splitter and the gain medium with respect to the non-reciprocal polarization rotator at an angle that is substantially equal to the angle of polarization rotation defined by the non-reciprocal polarizer. The positioning of the non-reciprocal pickoff may, in other embodiments, comprise positioning a reciprocal polarization rotator in the optical path between the polarization-dependent beam splitter and the wavelength selection element. The methods may additionally comprise positioning a reflector in the optical path after the tuning element. In certain embodiments, the methods may further comprise defining an external laser cavity between the reflector and a reflective facet of the gain medium.
The invention also provides methods for generating spectrally clean laser output which, in general terms, comprise emitting a light beam from a gain medium outward along an optical path, allowing the outward traveling light beam to interact with a wavelength selection element, returning a spectrally cleaned light beam along the optical path to the gain medium from the wavelength selection element, and non-reciprocally picking off a portion of the returning, spectrally cleaned light beam from the optical path.
The non-reciprocally picking off may comprise passing the outward light beam through a linear polarizer such as a polarization-dependent beam splitter to linearly polarize the light beam, rotating the polarization orientation of spectrally clean light that is returned to the polarization-dependent beam splitter, and reflecting along an output path, by
Farinas Alejandro D.
Hand Carter F.
Reed Murray
Zhang Guangzhi Z.
Law Office of Alan W. Cannon
New Focus Inc.
Nguyen Tuan N.
Wong Don
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