Patent Application
20080013586
There is an ongoing need for laser systems that produce high power while maintaining good output beam quality, and that can be directly diode pumped. Typically, for a diode pumped laser system, pump light from one or more diode pump sources is directed to a solid state laser gain material or crystal that then emits or amplifies laser radiation at a desired wavelength. For such a system, not all of the pump light is converted to desirable laser radiation; some of the pump light is converted to heat in the laser gain material. Temperature increases and gradients that result from this heat in the laser gain material can lead to several detrimental effects on the performance of the laser system. Examples of such detrimental effects can include bulk thermal lensing, surface bulging and changes to fundamental properties of the laser gain material such as the upper-state lifetime, thermal conductivity and thermal expansion of the material. Although some or most of this heat may be removed using some type of cooling apparatus, the heat generation within the lasing material and resulting temperature variations may still give rise to some or all of these detrimental effects.
Each of these detrimental effects can have a negative effect on the laser system output characteristics. For example, a thermal lens created within the laser gain material as a result of heat generated by the pumping process may limit the maximum achievable output power for the laser system, and may also lead to degradation in the output beam quality. In addition, thermally induced mechanical stresses that result within the laser gain material can also affect the output power and beam quality, and can ultimately lead to fracture or cracking of the laser gain material. Other detrimental effects may also be manifested as a result of the anisotropic properties, such as anisotropic thermo-mechanical and optical properties, of many desirable laser gain materials. This anisotropy may lead to a time varying absorption of pump light by the laser gain material if the polarization of pump light incident on the laser gain material does not remain constant over time as well. In addition, other detrimental effects on the lasing efficiency of a laser system may result from the anisotropic properties. What has been needed are methods and devices for pumping laser gain materials that reduce the detrimental effects of heating of the laser gain material. What has also been needed are robust laser systems, such as solid state laser systems, that produce high quality beams which are stable over time. What has also been needed are devices and methods to produce more uniform absorption of pump light in a laser gain material. What has also been needed are methods to produce narrow band pump light suitable for laser gain materials.
In one embodiment, a laser system includes a laser gain material and a narrow band pump source configured to produce pump light at a wavelength at or near an intersection of absorption coefficients for pump light having different polarizations in the laser gain material. In another embodiment, a laser system includes a solid state laser gain material, at least one diode pump source and at least one Volume Bragg Grating (VBG) optically coupled to the diode pump source.
In another embodiment, a method of pumping a solid state laser system includes pumping a laser gain material with a narrow band pump light having a wavelength centered substantially at an intersection of absorption coefficients for different polarizations of pump light in the laser gain material. In yet another embodiment a method of pumping a laser gain material includes emitting pump light from a diode pump source, narrowing the bandwidth of the pump light and directing the pump light into the laser gain material.
In one embodiment, a laser system includes a laser gain material and a narrow band pump source configured to produce pump light at or near a wavelength where the absorption coefficients for pump light having different polarizations are substantially equal in the laser gain material. In another embodiment, a laser system includes a solid state laser gain material, at least one diode pump source and at least one VBG optically coupled to the diode pump source.
In another embodiment, a method of pumping a solid state laser system includes pumping a laser gain material with a narrow band pump light having a wavelength centered substantially at a point where the absorption coefficients for different polarizations of pump light are substantially equal in the laser gain material. In yet another embodiment a method of pumping a laser gain material includes emitting pump light from a diode pump source, narrowing the bandwidth of the pump light and directing the pump light into the laser gain material.
These features of embodiments will become more apparent from the following detailed description when taken in conjunction with the accompanying exemplary drawings.
Solid state lasers have a wide range of applications in industry and elsewhere due, in large part, to their compact size, reliability and cost effectiveness. However, present solid state lasers do have limitations which may limit their usefulness in some applications. Many of these limitations may result from detrimental effects due to heating, and, particularly, non-uniform heating of the laser gain material by inefficiencies in the pumping of the laser gain material. Other detrimental effects may result from the anisotropic properties of some desirable laser gain materials alone, or in combination with heat effects. For example, desirable laser gain materials such as neodymium doped vanadate (Nd:YVO4) or ytterbium doped tungstates (Yb:KGW, Yb:KYW or KYbW, collectively referred to as Yb:tungstate) have polarization and wavelength dependent absorption properties as well as thermomechanical properties that are orientation dependant. The absorption coefficients in some embodiments of Nd:YVO4 have a ratio of about 3.5:1 at 808 nm and about 20:1 at 880 nm. Alternatively, for example, the absorption coefficients in Yb:tungstate have a ratio of about 3:1 at 930 nm and about 6:1 at 980 nm. This anisotropy can lead to a time varying absorption if the polarization of the pump light does not remain constant.
Diode pump sources that include optical fibers or conduits to couple the light to the laser gain material typically provide unpolarized light so that the influence of environmental factors may be reduced or minimized. However, this has the disadvantage that the pump light generally experiences at least two different absorption coefficients in the laser gain material or crystal. Since it is desirable to have most or substantially all of the pump light absorbed by the laser gain material or crystal, it may be desirable that the doping and length of the laser gain crystal be chosen to absorb the pump light that has a polarization direction or orientation that is most weakly absorbed by the laser gain material. Typically, a laser gain material of Nd:YVO4 having a neodymium concentration of about 1% requires crystals of about 4 mm in length to absorb most of the light polarized along the more weakly absorbed of the two polarization directions. As a result, the light that is polarized along the strongest absorption is often completely absorbed in the first few millimeters of the laser gain material, which may be a crystal, or over less than about-one half the length of the laser gain material required to completely absorb light energy having the more weakly absorbed polarization orientation. This results in more heat being deposited near the pumped input surface of the laser gain crystal, which may result in increased thermal lensing, surface bulging and stresses near the pumped surface of the laser gain material, as well as, possibly, changes to at least some of the fundamental laser gain crystal or material properties.
One way to mitigate the effect of unequal absorption is to reduce the doping concentration of the laser gain material and lengthen the laser crystal so that the fraction of the pump light is absorbed remains essentially the same, but the heat is deposited over a larger volume in the laser gain crystal. This has been described in U.S. Pat. No. 6,185,235 to Chen, et al., filed Nov. 24, 1998, which is incorporated by reference herein in its entirety. However, this technique merely reduces the consequences of the anisotropic absorption rather than addressing the cause. Also, this method is limited to the extent that it is not possible to lower the doping and increase the crystal length ad infinitum.
In many laser gain materials, the absorption coefficients for pump light also vary differently with wavelength as well as crystal orientation. It is thus often possible to find a crystal orientation and pump wavelength where the absorptions are equal or nearly equal, for the different pump light polarization directions or orientations. Using this technique, the differential absorptions in the laser gain material between two polarizations can be greatly reduced. In addition, if the data curves on a graph representing the coefficient of absorption of pump light versus wavelength of pump light for each polarization are not only equal in magnitude, but also cross one another, the variation of pump light absorption with wavelength can be reduced. This technique has been discussed in U.S. Pat. No. 6,891,876 to Sutter et al., filed Aug. 30, 2002, which is incorporated by reference herein in its entirety.
By way of illustration, and without limitation, Nd:YVO4 may be used as a laser gain material with pump light having wavelengths near 808 nm. The absorption coefficients (a) for Nd:YVO4 are shown in
However, the absorption of pump light having a wavelength near 815 nm is relatively small compared to the absorption near the peak absorption wavelength of pump light. For example, in Nd:YVO4, the value of a near a pump light wavelength of 815 nm is less that about 15% of the value near the peak absorption wavelength. As a result, the gain crystal would have to be about seven times longer or about seven times more highly doped in order to achieve the same overall absorption of pump light, but longer crystals that maintain good optical properties are not readily available and/or have significantly higher costs. Higher doping is also not a desirable solution because as the doping concentration in a laser gain material is increased, other effects that adversely affect the laser performance, such as quenching of the upper laser level, can become significant.
Typically, diode pump sources have a full-width half maximum bandwidth equal to or greater than about two nanometers. As a result, the effective absorption coefficient experienced by the pump source is an average over about a few nanometers of the narrow band values. As an example, the effective value of absorption coefficient for Nd:YVO4 with an input bandwidth of about 2.5 nm is shown in
As such, many of these problems may be addressed by use of a relatively narrow bandwidth wavelength stabilized diode pump source for pumping laser gain materials. Various embodiments that can be used to achieve substantially fixed wavelength operation from a diode include a Volume Bragg Grating (VBG) such as those manufactured by PD-LD, Inc. in Pennington, N.J., or manufactured by Ondax, Inc. in Monrovia, Calif. These devices can be used to provide a narrow spectral bandwidth from the pump diode, as well as a stabilized central wavelength. Such embodiments have been described in U.S. Provisional Patent Application titled “Line Narrowed Laser Diode System and Method of Use”, Ser. No. 60/623,376, filed by Treusch et al. on Oct. 28, 2004, which is incorporated by reference herein in its entirety. Alternatively, a wavelength selective thin-film coating can be utilized. However, any embodiment that provides an appropriately narrow band and/or wavelength stable pump light source may be used in the embodiments discussed herein.
The use of narrow band, wavelength stabilized pump light sources provides a central wavelength that is very well stabilized. As such, it is relatively straightforward to operate at any point on an absorption curve of a laser gain material, even points where the absorption changes relatively quickly with respect to pump light wavelength, because the pump light wavelength will remain approximately fixed. In addition, because the pump light bandwidth is relatively narrow, the effective values of the absorption coefficients are much closer to the narrow band values, which often result in a higher absorption. This enables the pump light wavelength to be chosen based solely on other properties of the laser gain material, without having to consider how those properties change with pump light wavelength which provides flexibility when designing a laser system. For example, the pump light wavelength may be chosen based on how it will effect the thermo-optical and/or thermo-mechanical properties of the laser gain material or crystal, how it will affect other important properties of the laser such as the intensity required in the laser material to reach transparency or both. Also, the absorption is higher than for the case of broadband pumping so that more conventional shorter, lower doped crystals may still be used.
The laser gain material 14 may include a variety of materials including gases, liquids and solids including semiconductor materials. Specific materials may include neodymium doped vanadate (Nd:YVO4), ytterbium doped tungstates (Yb:KGW, Yb:KYW or KYbW, collectively known as Yb:tungstate), Yb:SYS, Yb:BOYS, Yb:GdCOB, Yb:CaF, Nd:KGW, Cr:LiSAF, Cr:LiCAF, Ti:sapphire, Nd:YLF, Nd:GdVO4 or any other suitable material. Some or all of these gain materials 14 may be incorporated and used in a variety of configurations such as bars, optical fibers, sections of optical fibers and the like. Some embodiments of the laser gain material 14 may have a length of about 8 mm to about 16 mm and a transverse dimension of about 2 mm to about 6 mm, however, other suitable lengths and transverse dimensions may also be used. The transverse cross section of the laser gain material 14 may be square, rectangular, round or any other suitable shape.
The diode pump source 12 is coupled to a bandwidth narrowing element in the form of a VBG 22 which may be enclosed within a housing of the diode pump source 12 or disposed externally to the diode pump source 12 and optically coupled to the diode pump source 12 with an optical coupler such as an optical conduit in the form of an optical fiber. An output of pump light from the VBG 22 is optically coupled to a pump source output element 26 by an optical coupler in the form of an optical fiber or optical fiber bundle 28 but any other suitable optical coupler may be used. The VBG 22 may serve to provide optical feedback to the diode pump source 12 which causes the diode pump source 12 to emit a narrow band light energy output. As an alternative, a narrow band spectal filter could be used in order to produce a narrow band output from the diode pump source 12. The pump source output element 26 includes a polarization scrambling element 30 and collimating optics 32 that are configured to collimate the pump light emitted from the optical fiber 28 and the polarization scrambling element 30. The polarization scrambling element 30 generally may be used to produce pump light output that is substantially un-polarized in some embodiments. Embodiments of the VBG 22 may be configured to emit embodiments of narrow band pump light having a bandwidth of less than about 0.7 nm, specifically, a bandwidth of about 0.2 nm to about 0.7 nm, and more specifically, a bandwidth of about 0.3 nm to about 0.5 nm.
Narrow bandwidth pump light may be emitted from the diode pump source 12 and directed from the pump source output element 26 into the laser gain material 14 in order to pump the laser gain material 14 and produce a laser beam that may be emitted from the third reflective optic 20. In this way, the detrimental effects that degrade the laser system performance which are functions of wavelength variation or bandwidth may be reduced or eliminated. The laser system 10 may also include the use of pump diode source 12 that emits pump light having a wavelength that encompasses or is otherwise at or near an intersection of data curves representing different pump light polarizations on a graph of pump light absorption coefficient versus wavelength of pump light. The pump diode source 12 may also emit pump light having a wavelength that is centered or substantially centered at a wavelength where absorption coefficients of pump light of different polarizations are substantially the same. The characteristics discussed above may be applied to an optical system where the laser gain material 14 is configured for use as either a laser oscillator or laser amplifier.
By way of example and without limitation,
In
Using a broadband pump source near 806.5 nm would give some improvement over pumping near the absorption peak of the gain material because the relative difference between the effective absorption coefficients is reduced. However, because the total absorption is reduced for the case of broadband pumping near 806 nm, it may be necessary to increase the doping and/or the length of the laser gain material to keep the total absorbed pump light power constant, and this increases the heat load near the input face of the laser gain material. In this example, the doping is increased by about 30% to keep the total absorption substantially constant. Narrow band pumping, on the other hand, allows the laser gain material parameters to remain unchanged, and can further reduce the relative difference between the effective absorption coefficients.
It is important to point out that the embodiments discussed herein do not require that the curves of the pump light absorption versus pump light wavelength for both polarization orientations cross at or near the operating point, or that the values be equal. As demonstrated by the above example, the absorption values may be close enough over some (possibly narrow) bandwidth for some embodiments so that the absorptions for each polarization will be more or substantially equal throughout the laser gain material.
Embodiments may also be applied to any laser gain material that has polarization or direction dependant absorption coefficients, and is not limited to the specific embodiments discussed herein. Examples of other embodiments may include Nd:YVO4 when pumped near 880 nm and Yb:tungstate when pumped near 940 nm and near 981 nm. It will also be appreciated that embodiments can generally be applied to materials other than laser gain materials that absorb laser radiation and produce thermo-optical, thermo-mechanical or other detrimental effects. They can further be applied to enable the efficient and stable pumping of relatively narrow absorption features such as those present in Yb:tungstate near 980 nm.
Again, by way of example and without limitation, consider the absorption characteristics of Nd:YVO4 near 880 nm. Data illustrating the absorption coefficients for pump light having polarizations both parallel and perpendicular to the c-axis are shown together with data illustrating the effective coefficient for unpolarized light in
In
This is further illustrated in
Note that because the peak absorptions for the two polarizations are offset with respect to each other in wavelength, even if a broadband pump source were used, the best absorption would be obtained at a wavelength near 879 nm, which is on the short wavelength side of the main absorption peak. This would also provide better heat distribution properties because the differential absorption between the two polarizations is lower than at the peak absorption near 880 nm.
Embodiments discussed herein may refer to the same laser gain material or families of materials. The method and device embodiments discussed herein may be used with existing laser gain materials, and do not require new dopings or laser gain material dimensions, such as length. This allows the possibility to improve the performance of existing lasers by simply replacing the existing broadband pump modules or sources with narrow band pump modules or sources at an optimized or otherwise desirable wavelength.
With regard to the above detailed description, like reference numerals used therein refer to like elements that may have the same or similar dimensions, materials and configurations. While particular forms of embodiments have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the embodiments of the invention. Accordingly, it is not intended that the invention be limited by the forgoing detailed description.
This application claims priority under 35 U.S.C. section 119(e) from U.S. Provisional Patent application Ser. No. 60/714,576 titled “Narrow Band Diode Pumping of Laser Gain Materials”, filed Sep. 6, 2005, by Spence, D. et al. which is also incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60714576 | Sep 2005 | US |
