Coherent light generators – Particular resonant cavity – Mirror support or alignment structure
Reexamination Certificate
2001-03-23
2003-08-12
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Mirror support or alignment structure
Reexamination Certificate
active
06606340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to grating-tuned external cavity lasers and more particularly to a method and apparatus for generating a continuously-tunable, low-noise laser beam in a grating-tuned external cavity laser.
2. Description of Related Art
Grating-tuned external cavity lasers produce continuously-tunable laser beams consisting of light with high coherence and very narrow linewidth. To obtain high coherence and narrow linewidth, a grating is generally employed to disperse the emission from a light source or gain element, and feed it back to the gain medium at a wavelength selected by a tuning device. Tunable laser beams can be produced either by rotating a grating in a Littrow-type arrangement, or a reflector in a Littman-type configuration. Littman-type tunable laser systems are described in the publications, “Spectrally Narrow Pulse Dye Laser Without Beam Expander,” by Michael G. Littman and Harold J. Metcalf,
Applied Optics
, Vol. 17, No. 14, pages 2224-2227, Jul. 15, 1978, and “Narrowband Operation Of A Pulsed Dye Laser Without Intracavity Beam Expansion” by l. Shoshan, N. N. Dannon, and U. P. Oppenheim,
Journal of Applied Physics
, Vol. 48, pages 4495-4497, 1977. A single-longitudinal-mode (very narrow linewidth) frequency tunable pulsed dye laser was described in the publication, “Single-Mode Pulsed Tunable Dye Laser,” by M. G. Littman, Optics Letters, Vol. 23, pages 138-140, 1978. This single-longitudinal mode laser provides a foundation for producing tunable narrow-bandwidth lasers.
FIG. 1
shows a prior art grating-tuned external cavity laser capable of producing a laser beam which is tunable over a broad range of wavelengths by rotation of a tuning reflector. Laser system
100
comprises pivot
102
, base
104
, plane reflector
106
, gain medium
108
, diffraction grating
110
, tuning reflector
112
, rotatable unit
114
, output laser beam
116
and first-order diffracted radiation
118
.
In the prior art system of
FIG. 1
, a proximal end of rotatable unit
114
is pivotably connected to base
104
by pivot
102
. Tuning reflector
112
is mounted on rotatable unit
114
forming an acute angle with respect to diffraction grating
110
, which is mounted on an upper surface of base
104
. Plane reflector
106
and gain medium
108
are mounted on base
104
and are disposed to produce a laser beam which is incident on diffraction grating
110
at a grazing angle, thereby generating output laser beam
116
and first-order diffracted radiation
118
.
In operation, rotating arm
114
pivots around pivot
102
such that tuning reflector
112
moves relative to diffraction grating
110
. Plane reflector
106
and gain element
108
generate a laser beam which is incident on diffraction grating
110
at a grazing angle. Part of this laser beam is reflected as output laser beam
116
and exits laser system
100
. The rest of the laser beam incident on diffraction grating
110
is diffracted and reflected to generate a light radiation pattern which includes first-order diffracted radiation
118
. First-order diffracted radiation
118
retro-reflects off tuning reflector
112
and is again incident on diffraction grating
110
.
Upon further diffraction and reflection by diffraction grating
110
, a portion of first-order diffracted radiation
118
enters gain element
108
and plane reflector
106
, thereby forming an external feedback laser cavity for laser system
100
. The wavelength of output laser beam
116
depends on the angle formed by grating surface
110
and the reflective surface of tuning reflector
112
, which may be adjusted by pivoting rotatable unit
114
around pivot
102
. Consequently, the wavelength of output laser beam
116
may be tuned by pivoting rotatable unit
114
around pivot
102
. Accurate positioning of pivot
102
enables mode-hop-free, continuous tuning of output laser beam
116
over the entire emission band of gain element
108
.
A laser system similar to the prior art system shown in
FIG. 1
is described in the publication, “Novel Geometry for Single-Mode Scanning of Tunable Lasers,” by Michael G. Littman and Karen Liu, Optics Letters, Vol. 6, No.3, pages 117, 118, March, 1981. A mode-hop-free, Littman cavity laser system with broad-range tuning capabilities is set forth in the publication, “Synchronous Cavity Mode and Feedback Wavelength Scanning in Dye Laser Oscillators with Gratings,” by Harold J. Metcalf and Patrick McNicholl,
Applied Optics
, Vol. 24, No. 17, pages 2757-2761, Sep. 1, 1985. The publication “Scanning Geometry for Broadly Tunable Single-Mode Pulsed Dye Lasers,” by Guangzhi Z. Zhang and Kohzo Hakuta,
Optics Letters
, Vol. 17, No. 14, pages 997-999, Jul. 15, 1992, describes a dye laser system capable of continuously tuning a single-longitudinal-mode laser beam over a range of more than 190 cm
−1
by employing a predefined rotation pivot for the tuning reflector and grating.
Various configurations of grating-tuned, Littman-type, external laser cavity systems capable of providing continuous, broadband, mode-hop-free laser beams have been disclosed in U.S. Pat. No. 5,319,668 to Luecke, U.S. Pat. No. 5,867,512 to Sacher, U.S. Pat. No. 5,771,252 to Lang, U.S. Pat. No. 5,802,085 to Lefevre, et al and the publication “Continuously Tunable Diode Lasers,” by Timothy Day, Frank Luecke, and Michel Brownell,
Lasers
&
Optronics
, No. 6, June, 1993, pp. 15-17. According to these publications, accurate positioning of the pivot is paramount to obtain continuous, broadband tuning capability over the entire emission bandwidth of the gain medium.
Lowering the lasing threshold for grating-tuned external cavity lasers increases the laser power output in the presence of optical power loss occurring inside the laser cavity due to grating diffraction. A method for reducing power loss was described in the publication, “Lasing Threshold Reduction for Grating-Tuned Laser Cavities,” by Guangzhi Z. Zhang and Dennis Tokaryk,
Applied Optics
, vol. 36, No. 24, pages 5855-5858, Aug. 20, 1997. This publication introduced a laser system that utilized potentially wasted optical power in an effective feedback configuration.
Mode-hop-free, broadband tunable lasers have been extensively used in a wide range of applications, including laser spectroscopy, optical metrology, in-situ process monitoring and test and measurement of optical passive components in Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber systems.
The output of grating-tuned, external cavity laser systems in the prior art generally consists of two spectral components: (1) a laser beam; and (2) background light radiation comprising Source Spontaneous Emission (“SSE”) and Amplified Spontaneous Emission (“ASE”) light radiation. The laser beam is the desired output component and consists of substantially coherent, narrow-linewidth laser light. The SSE and ASE radiation, however, constitutes an undesirable incoherent noise background which is emitted directly by the gain element.
The laser beam component of the laser output couples with the SSE and ASE background radiation component in space and time. Although the SSE and ASE background radiation is usually weak in power as compared to the laser output, it has a significant effect in many sensitive applications including test and evaluation of optical passive components and fibers and Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber data-transmission systems. Consequently, there is a need to filter out SSE and ASE background radiation from the output of grating-tuned, external cavity laser systems to obtain a coherent, narrow-linewidth, noise-free output laser beam.
A few types of grating-tuned external cavity laser systems that could suppress SSE and ASE background noise have been described in the publications, “Using Diode Lasers for Atomic Physics”, by Carl E. Wieman and Leo Hollberg, Review of Scientific Instruments, vol. 62, pages 1-19, January, 1991 and “Impact of Source Spontaneous
Wu I-Fan
Zhang Guangzhi Z.
Al-Nazer Leith A
Ip Paul
Lumen Intellectual Property Services Inc.
New Focus Inc.
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