Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
1999-05-03
2001-06-19
Lee, John D. (Department: 2874)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S019000, C372S020000, C372S099000
Reexamination Certificate
active
06249537
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to laser oscillators, and in particular to single longitudinal mode operation of a coupled cavity, self seeding laser pumped by a pulsed source.
BACKGROUND OF THE INVENTION
Lasers that are tunable over a wide range of wavelengths and have a narrow linewidth are desirable for a number of applications, including, for example, laser radar, isotope separation, remote sensing, medicine, and lithography. One such tunable laser uses a diffraction grating inside the laser cavity and a gain medium that, when pumped, fluoresces over a broad range of wavelengths. The angle of the diffraction grating inside this so-called Littman cavity is adjusted so that only the desired wavelength of light is amplified by the cavity.
The main difficulty with the Littman cavity is that its grating has a low diffraction efficiency. Most of the cavity light is lost due to ordinary mirror, or specular, reflection from the grating. Therefore the Littman cavity gives only a low powered laser.
This problem was partially solved by Lee, Cha, Kim, and Ko in Optics Letters 20 (1995) pp. 710-712 and U.S. Pat. No. 5,633,884. Lee et al. use a Littman cavity coupled to a second, slave oscillator. The light in the Littman cavity is amplified by the slave oscillator to overcome the power restrictions of the Littman cavity operating alone.
In this coupled oscillator approach, the gain medium is activated using an external laser pulse. Light first builds up in the Littman cavity. The power in the Littman cavity then “seeds” the gain medium, whose induced emission circulates in and is amplified by the slave oscillator. The light circulating in the slave oscillator is specularly reflected from the grating. Therefore, light that was previously lost to specular reflection in the Littman cavity alone is recaptured and amplified in the method of Lee et al. The self-seeding coupled oscillators yield a laser with increased power.
However, the laser of Lee et al. still has some drawbacks. For the narrowest linewidth possible, the laser should operate in a single longitudinal mode. However, the laser of Lee et al. typically operates in several longitudinal modes at once, thereby giving a broader than optimum linewidth.
OBJECTS AND ADVANTAGES
It is therefore a primary object of the present invention to provide a Littman oscillator coupled to a slave oscillator, the components of the coupled cavity oscillators being arranged so that the Littman oscillator operates in a single longitudinal mode. It is also an object of the present invention to provide a method for efficient self-seeding of the coupled oscillators. These objects yield a tunable laser having a minimal linewidth and a high power.
SUMMARY OF THE INVENTION
A self-seeding laser comprises a slave oscillator cavity having the following elements positioned sequentially to provide a first beam path of length L
1
: an end mirror, a gain medium, a grating having a groove spacing d, and an output mirror. The first beam path has an angle of incidence &thgr; upon the grating, and the beam path is defined by zeroth order diffraction of the grating. The laser also comprises a tuning mirror facing the grating, for reflecting light that is first-order diffracted away from the first beam path by the grating. The tuning mirror provides a Littman cavity of length L
2
comprising the end mirror, the gain medium, the grating, and the tuning mirror. An angle &phgr; between a normal to the tuning mirror and a normal to the grating is adjusted to specify a wavelength &lgr; of light that circulates in the Littman cavity.
The gain medium is pumped by a pulsed pump beam. The pump beam activates a pump region within the gain medium, the pump region having a radius w. After the gain medium is pumped, a mode build-up time is required for a longitudinal mode in the Littman cavity to build up energy. Only the mode having wavelength &lgr; builds up appreciable energy in the Littman cavity, because other modes walk off of the pump region due to the dispersion of the grating. An adjacent mode walk-off time t
W
is defined as the time it takes for the modes adjacent to the mode of wavelength &lgr; to walk off of the pump region. Adjacent mode walk-off time t
w
is equal to 2 (2 d w cos &thgr;)
½
L
2
/(&lgr;c), where c is the speed of light. Single longitudinal mode operation of the laser is obtained by selecting the angle of incidence &thgr;, the groove spacing d, and the radius w to make t
w
greater than the mode build-up time.
Immediately after the gain medium is pumped by the pump beam, the slave oscillator cavity has a first gain, and the Littman cavity has a second gain. The gains depend on the lengths L
1
and L
2
of the cavities. The lengths are adjusted so that the first gain is less than the second gain, thereby enabling the Littman cavity to “seed” the slave oscillator cavity.
REFERENCES:
patent: 5081630 (1992-01-01), Lowenthal et al.
patent: 5577058 (1996-11-01), Kafka et al.
patent: 5633884 (1997-05-01), Lee et al.
patent: 5889800 (1999-03-01), Kafka et al.
patent: 6016323 (2000-01-01), Kafka et al.
Ko, Do-Kyong et al., “Self-seeding in a Dual-cavity-type Pulsed Ti: Sapphire Laser Oscillator” Optics Letters, vol. 20 No. 7, Apr. 1, 1995, pp. 710-712.
Merriam Andrew
Yin Guang-Yu
Lee John D.
Lumen Intellectual Property Services
The Board of Trustees of the Leland Stanford Junior University
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