Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1998-07-07
2001-11-06
Davie, James W. (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S023000
Reexamination Certificate
active
06314116
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to lasers, and more particularly to improved multiple-color lasers.
2. Description of Related Art
Multiple-wavelength laser systems are used for a variety of applications. In the context of this invention, multiple wavelengths may be taken to mean two or more wavelengths that can be distinguished from each other and be used to convey independent information to the observer or detection apparatus.
One multi-wavelength application is full-color holograms. These holograms present holographic, full-color images. For obvious reasons, such holograms are much preferred to the older generation of monochromatic holograms. Color holograms may be recorded on holographic panchromatic materials. Ultra-high resolution, single-layer, silver-halide emulsions and new photo polymer materials also may be used for this purpose.
A variety of recording setups might be used for full-color holography. However, it appears that the single beam Denisyuk recording scheme has produced the best results with the simplest apparatus. Three laser wavelengths, such as the colors red, green, and blue, are needed for the recording. Suitable colors can be selected from different lasers conventionally in use in holographic recordings: argon, krypton, diode-pumped frequency-doubled Nd:YAG, helium-neon, and helium-cadmium lasers.
Multiple lasers are needed in this arrangement. A prior art color hologram recording set up is illustrated in FIG.
1
.
FIG. 1
shows color holography laser system
100
, including HeNe laser
102
; krypton laser
104
; argon laser
106
; first beam mirrors
108
,
109
, and
110
; second beam mirror
112
, and dichroic beam mirrors
114
, and
116
; optional coherence monitoring system
118
; shutter
120
, spatial filter
122
, hologram recording film
124
, and object
130
.
HeNe laser
102
, krypton laser
104
, and argon laser
106
are installed on an independent vibration-isolation system isolated from an optical table surface (not shown). The beams emitted by the lasers are redirected to shutter
120
using first beam mirrors
108
,
109
, and
110
; second beam mirror
112
, and dichroic beam mirrors
114
, and
116
. An optional coherence monitoring system may be placed on the beam path between dichroic beam mirror
116
and the shutter. When the shutter is in the open position, a beam passes through spatial filter
122
and illuminates hologram recording film
124
and object
130
. The object is positioned on a side of the hologram recording film opposite from the spatial filter.
In operation, the three colors of light emitted by HeNe laser
102
, krypton laser
104
, and argon laser
106
are combined into a single “white light” beam using first beam mirrors
108
,
109
, and
110
; second beam mirror
112
, and dichroic beam mirrors
114
, and
116
. Optional coherence monitoring system
118
may be used to monitor beam wavelength purity. The beam is then directed onto shutter
120
, which serves to control illumination. When the shutter is open, the beam passes through spatial filter
122
, and illuminates hologram recording film
124
and object
130
. The light rays reflected from object
130
interfere with the beam incident on the hologram recording film to form a hologram, which is recorded by the hologram recording film.
Such an arrangement represented a significant improvement over earlier full-color hologram methods. Using the dichroic filters in combining laser beams permitted a shortened and simplified exposure procedure without changing mirror positions between exposures as was necessary before using dichroic mirrors. Furthermore, the light intensity and red-green-blue ratio on the recording plane were much less likely to remain undisturbed after initial set-up. This reduced the need for check-up and calibration between hologram recordings.
However, problems still remain with this arrangement. The use of multiple lasers and multiple optical elements significantly increases the cost of the recording system, making it commercially less feasible to produce inexpensive, custom holograms. Additionally, the relatively large number of laser systems and optical elements increases the possibility of failures and beam misalignments.
These problems result primarily from the fact that while conventional laser systems may emit multiple wavelengths, the desired colors are not available from any single laser gain medium. Further, the ratios of the available power at the various desired colors do not necessarily match those needed for conventional films. Additionally, the separation between the wavelengths is such that the emitted colors are not suitable for use in full-color holography. Therefore, the conventional solution, as illustrated in
FIG. 1
, has been to use multiple lasers with the attendant problems noted above.
Likewise, in other multiple-wavelength applications, such as three-wavelength laser Doppler velocimetry, conventional systems suffer from a number of shortcomings. Three-wavelength laser Doppler velocimetry systems using conventional lasers would require multiple lasers, with attendant cost and reliability issues.
FIGS. 2A
,
2
B, and
2
C show a prior art three-dimensional laser Doppler velocimetry system. Shown in
FIG. 2A
are lasers
202
,
204
,
206
, and optical fiber network
208
. Shown in
FIG. 2B
are first focusing optic
210
, X-direction beam
212
, Y-direction beam
214
, second focusing optic
216
, Z-direction beam
218
, measurement volume
220
, and fiber optic network
208
. Shown in
FIG. 2C
are second focusing optic
216
, Z-direction beam
218
, measurement volume
220
, back scattered beam
222
, detector
224
, and signal path
226
to a signal analyzer (not shown).
Lasers
202
,
204
, and
206
are optically coupled to optical fiber network
208
. Optical fiber network
208
is optically coupled to first focusing optic
210
, and to second focusing optic
216
. Detector
224
is positioned in the optical path behind focusing optic
216
in such a way as to capture back scattered light from measurement volume
220
. Detector
224
is coupled by signal path
226
to a signal analyzer (not shown).
In operation, lasers
202
,
204
, and
206
emit light into fiber optic network
208
. Each of lasers
202
,
204
, and
206
emit on a single wavelength or a single color. Light transmitted by the fiber optic network is delivered to first focusing optic
210
, and second focusing optic
216
. First focusing optic
210
serves to focus X-direction beam
212
and Y-direction beam
214
that are used to measure velocities in measurement volume
220
in both the X and Y directions. The Z direction measurement is made by light transmitted by fiber optic network
208
focused through second focusing optic
216
and directed as Z-direction beam
218
to measurement volume
220
. Results from the measurement volume are captured via back scattered beams, for example, back scattered beam
222
, shown in FIG.
2
C. The back scattered beam is collected through second focusing optic
216
onto detector
224
. The signal from the detector is then transmitted to an analyzer via signal path
226
. Although
FIG. 2C
shows an arrangement for the detection for measurements in the Z direction, similar detector arrangements may be used to monitor the X and Y directions as well.
A problem with current laser Doppler velocimetry systems is that they use laser sources with relatively broad linewidths. These linewidths are typically on the order of 6-8 Giga-hertz measured full width at half maximum. Such relatively broad linewidths translate to relatively short spatial coherence lengths which limit the size of the measurement volume as defined by the interference fringes at the intersection of the two beams. In addition, the relatively broad linewidths also limit the contrast of the interference fringes and thus the signal-to-noise ratio of the laser Doppler velocimetry measurement.
There is therefore a need for a single laser system that addresses these deficiencies in the prior art.
SUMM
Feitisch Alfred
Klavuhn Kurt G.
von Gunten Marc K.
Wright David L.
Davie James W.
Spectra Physics Lasers, Inc.
Wilson Sonsini Goodrich & Rosati
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