Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
2001-08-03
2004-11-02
Wong, Don (Department: 2828)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S021000
Reexamination Certificate
active
06813302
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lasers and, more particularly, to a method and apparatus for producing eyesafe high-peak-power laser radiation from a diode-pumped solid-state laser.
BACKGROUND OF THE INVENTION
Diode-pumped solid-state lasers are generally considered the most practical source of laser radiation for applications requiring high efficiency and compact, low-weight, and rugged packaging. Laser diode pump sources have high electrical-to-optical conversion efficiency, and the narrow-band spectral output of laser diodes can be chosen to closely match the absorption bands of solid-state laser materials. As a result, heat loads in diode-pumped solid-state lasers are significantly lower than for the flashlamp-pumped solid-state lasers that have been largely supplanted by diode-pumped lasers for applications requiring compactness and high efficiency.
For many applications the laser radiation must be in the “eyesafe” band, that is having a wavelength longer than 1.4 microns. Laser radiation in this range is strongly absorbed by liquid water. As the fluid in the human eye consists primarily of water, laser radiation in this range does not damage the retina because it is absorbed before reaching the retina.
Several solid-state lasers operate in the eyesafe band, have absorption features suitable for diode pumping, and have millisecond duration fluorescence lifetimes. These lasers are doped with trivalent rare-earth ions and are well-known. An advantage of these lasers is that their long fluorescence lifetimes (i.e. energy storage times) readily allow the production of high-energy Q-switched pulses, and their relatively low emission cross-sections result in pulsewidths that are hundreds of nanoseconds in duration. These pulsewidths are ideal for active remote sensing instrumentation such as coherent detection laser radar systems. Examples of these lasers include:
1) the 1.53-micron ytterbium/erbium co-doped glass (Yb,Er:glass) laser
2) the 2.05-micron thulium/holmium co-doped yttrium lithium fluoride (Tm,Ho:YLF) laser
3) the 2.01-micron thulium-doped yttrium aluminum garnet (Tm:YAG) laser
4) the upper-state-pumped 2.10-micron Ho:YAG laser
5) the upper-state-pumped 1.6-micron erbium-doped bulk crystal (for example, Er:YAG) laser.
Existing eyesafe diode-pumped solid-state lasers have deficiencies that make them non-ideal for critical applications such as remote sensing.
The 1.53-micron Yb,Er:glass laser does not perform well at high average power owing to the poor thermo-mechanical properties of the glass host material. This limits operation of the laser to low pulse repetition frequencies (PRF), thereby severely limiting the sensitivity of remote sensing systems based on this laser.
In the 2.05-micron Tm,Ho:YLF laser, the Tm ion absorbs the pump light and transfers the excitation energy to the lasant Ho ion. This laser suffers from upconversion loss from the upper laser state between pairs of Tm and Ho ions, which reduces the efficiency and energy storage capacity of the laser medium. Energy storage is an important consideration for applications such as remote sensing (where high energy laser pulses enable long-range sensing), because a long energy storage lifetime is required to convert the intrinsically continuous wave (cw) or quasi-cw laser diode pump power into high peak power. In typical Tm,Ho:YLF lasers the energy storage time is reduced from an intrinsic lifetime of 10 ms by upconversion to a lifetime of approximately 1 ms.
The 2.01-micron Tm:YAG laser similarly suffers from upconversion loss, in this case between pairs of Tm ions in the upper laser state. As a result the energy storage lifetime in a typical Tm:YAG laser is reduced from an intrinsic lifetime of 8 ms by upconversion to approximately 3 ms.
The detrimental effects of upconversion loss can be eliminated by pumping the Ho:YAG laser directly into its upper laser state. In this case a low concentration of Ho can be utilized, which suppresses the concentration-dependent upconversion process. The Ho:YAG laser can not be operated efficiently when pumped directly into the upper state by laser diodes, however, because presently there are no laser diodes with sufficiently high power and brightness in the wavelength range (1.85 to 1.95 microns) of strong absorption for efficient upper-state pumping of Ho:YAG. Instead, the Ho:YAG laser must be pumped by an intermediate solid-state laser, for example the 1.94-micron Tm:YALO laser or the 1.91-micron Tm:YLF laser. These lasers have sufficient brightness for pumping the Ho:YAG laser, but have the disadvantage of requiring liquid coolants.
The upper-state-pumped 1.6-micron Er-doped bulk crystal laser operates in a similar fashion to that of the upper-state-pumped Ho:YAG laser. In the case of the Er-doped laser strong absorption for upper-state pumping occurs at 1.533 microns. As in the case of the Ho:YAG laser, laser diodes do not exist presently with sufficiently high power and brightness for efficient upper-state pumping of the Er-doped laser.
A need has existed for many years for efficient eyesafe lasers suitable for remote sensing applications. Such devices are required to operate in the Q-switched mode with high pulse energy and long pulsewidth (hundreds of nanoseconds). In recent years development efforts for such devices have concentrated on solid-state lasers operating at wavelengths near 2 microns. The 2.01-micron Tm:YAG laser and the 2.05-micron Tm,Ho:YLF laser (and variations on these such as the 2.02-micron Tm:LuAG laser) are generally considered to be the most practical sources of efficient, eyesafe, high-energy, long-pulsewidth, Q-switched laser radiation.
In particular, the upper-state pumped 1.6-micron Er-doped bulk crystal laser has been considered to be impractical owing to the lack of a suitable pump source. The 1.6-micron Er-doped laser is known to operate with two pump sources: 1) flashlamp-pumped Yb,Er:glass lasers, and 2) 1.5-micron InGaAs laser diodes. Neither of these pump devices provides output radiation that is suitable for efficient operation of the 1.6-micron Er-doped laser. High-power Yb,Er-doped, Raman-shifted Yb-doped and Raman-shifted Nd-doped fiber lasers have been commercially available for several years; however, these devices have not been previously proposed as pump sources for the upper-state pumped 1.6-micron Er-doped laser. Use of guided-wave lasers, such as fiber lasers (typically having a doped core of circular cross section) and bulk waveguide lasers (typically having a doped region of rectangular cross section), as the pump source is an aspect of the subject invention. In a guided-wave laser the laser radiation is confined to the laser medium (which occupies the same space as the laser resonator) by total internal reflection from the transverse surfaces of the guided-wave structure. The guided-wave geometry is in contrast to conventional lasers in which the laser resonator is defined by discrete mirrors at the ends of a free-space resonator. Guided-wave lasers have the advantage of a small resonator cross section that results in a low laser threshold and efficient removal of heat from the laser medium. An aspect of the present invention is that the pump source of the Er-doped bulk crystal laser is either a Yb,Er-doped guided-wave laser, a Raman-shifted Yb-doped guided-wave laser, or a Raman-shifted Nd-doped guided-wave laser.
A suitable pump laser for the Er-doped laser must meet the following requirements: 1) the pump laser must have sufficiently high electrical-to-optical conversion efficiency, 2) the pump laser must be of small size and low weight, 3) the pump wavelength must be strongly absorbed by the Er-doped laser material, 4) the pump brightness must be sufficiently high for efficient pumping of the Er-doped laser medium, and 5) the pump linewidth must be sufficiently narrow for efficient pumping of the Er-doped laser medium. The flashlamp-pumped Yb,Er:glass pump laser does not meet requirements 1 and 2, while the InGaAs pump laser does not meet requirements 4 and 5.
The flashlamp-pumped Yb,Er:glass las
Henderson Sammy W.
Stoneman Robert C.
Coherent Technologies, Inc.
Vy Hung Tran
Wong Don
Young James R.
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