WO2001029941A1

WO2001029941A1 - Beam correcting laser amplifier

Info

Publication number: WO2001029941A1
Application number: PCT/US2000/028252
Authority: WO
Inventors: Harry Rieger; Serge Cambeau
Original assignee: Jmar Research, Inc.
Priority date: 1999-10-15
Filing date: 2000-10-12
Publication date: 2001-04-26
Also published as: AU1082001A; EP1226636A1; EP1226636A4; JP2003524889A

Abstract

A four-pass conjugate optical amplifier (5) with a linearly polarized optical beam (20) using two PBS units (30 and 40), two mirros (70 and 80) a DPR (50) and NPR (6) unit.

Description

BEAM CORRECTING LASER AMPLIFIER

Field Of The Invention

The present invention relates to an apparatus for the amplification of a light energy such as a laser beam, and more particularly to amplification in which the light energy is processed in four passes through an amplification device.

Background Of The Invention Laser discharges having high power and brightness are very desirable. In one application, such laser discharges are used in the production of ultraviolet and X-ray radiation. For example, a high brightness laser beam can generate over IO¹⁴ W/cm² on targets such as copper tape. The resulting hot plasma then emits x-rays. These x-rays can be used for advanced

lithography in achieving 0.1 μm feature size or less. Similarly, the Extreme UV (EUV)

discharge from laser plasma can also be used for lithography, tomography, microscopy, spectroscopy, and more. Other applications for the high powered lasers include micro- machining, target ranging and industrial applications.

Laser amplifiers take an input light beam from a light source and intensify the input beam to produce the laser discharge. Increasingly brighter, more powerful laser discharges are achieved by increasing amplification power. However, known laser amplifiers have design limitations that limit achievable power and brightness gains.

One limit on high-powered laser amplification is the _δ-integral effect. This effect describes the positive relationship between a material's index of refraction and the intensity of illumination. As a result, a light beam with a non-uniform intensity distribution, such as the Gaussian intensity profile, has higher indexes of refraction in areas of higher intensity. Varying illumination also occurs with non-uniform pumping input energy, again resulting in varying indexes of refraction. The index of refraction determines the phase velocity of light and the effective optical path length. As a result, phase and beam delays occur in the areas of higher intensity, distorting the focus of the light beam and limiting brightness and power gains. A varying index of refraction also alters the optical path of the beam, causing the whole beam or portions of the beam to collapse into focus points. The 5-integral effects become more pronounced under high power amplification because of the greater variances in illumination levels.

As a result of the 5-integral effects and other sources of distortion to the light beam (such as optical imperfections in the laser path), high-powered amplification in known laser amplifiers creates areas of heat accumulation known as "hot spots." The hot spots occur in areas of imperfections that disrupt the laser, causing it to release energy into surrounding areas. Hot spots may also form as a result of non-uniform pumping that causes varying levels of heat within the amplifier. As an area heats, it further distorts the laser, leading to still greater heat accumulation. This cycle of increasing heat and distortion continues until either the laser amplifier breaks down or interference prevents further gains in laser brightness and power. For the above reasons, known laser amplifier designs are prone to hot spot formation and, as a result, have limited laser brightness and power gains. Hot spot formation also causes the known laser amplifier to be inefficient since much of the amplifying energy is lost as waste heat. Furthermore, known laser amplifiers are not well designed to withstand hot spots and rapidly break down under high-powered amplification, requiring expensive repair and replacement of parts.

To manage these high temperatures, a means of active heat removal is required. Conventionally, the non-optical surfaces of the laser crystal are cooled by the forced convection of a fluid, which is usually water. Alternatively, the surface can be thermally contacted to a heat sink of sufficient mass to absorb the waste power. These methods have been employed. But due to the geometry of the active laser volume and the relatively low thermal conductivity of the laser crystal, high temperatures and high temperature gradients persist.

Accordingly, there is a need for a laser amplifier that produces high power and brightness laser discharges but minimizes the formation of harmful hot spots. Furthermore, there is a need for a laser amplifier that produces high power and brightness, but is better able to withstand the formation of hot spots.

Summary Of The Invention The present invention provides a system to produce an output laser beam with high power and high brightness. In particular, the present invention provides a four pass laser amplifier that takes a polarized input laser beam from an external source, directs the input beam through four amplifying passes through the amplifier, and then allows the amplified beam to exit as the desired output. The apparatus includes a first and a second polarizing beam splitter

["PBS"]; a directional polarization rotator ["DPR"], a non-directional polarization rotator

["NPR"]; a first and a second reflector; and a pumping module. The first and second PBSs are positioned in the laser path to transmit the input beam but deflect light having a polarization normal to the original input beam. The first PBS allows the original input beam to enter the laser amplifier but directs away fully amplified beam as output.

As a result, the first PBS allows the first amplifying pass and ends the fourth and final path. The second PBS transmits either the original input beam or the fully amplified beam but deflects a partially amplified beam so that it returns for more amplification. Thus, the second PBS deflects the second pass to initiate the third pass, while not affecting the first or fourth passes.

The first and the second PBSs generally have polarizing coatings on an outside surface.

However, the preferred embodiment of the present invention uses PBSs having the polarizing coatings on an interior surface. In particular, these preferred PBSs are cubed-shaped, having the polarizing coating on an interior diagonal plane. This positioning of the polarizing coating avoids the hot spots that commonly form at the boundary of the polarizing coating and air because the polarizing coating no longer contacts air.

Located in the beam path between the first and second PBSs is the DPR. The DPR is a component or set of components that has no polarization effect on the beam travelling from the first to the second PBS while rotating the polarization of the beam travelling from the second to the first PBS. As a result, the second PBS transmits light from the first PBS while the first PBS deflects light from the second PBS. The DPR allows the original input beam to enter amplification while rotating the fully amplified beam so that the first PBS deflects it away as output. In general, the DPR is combination of a Faraday rotator and a λ/2 wave plate. Amplification of the input beam occurs in the pumping module that is positioned in the beam path, downstream from the two PBSs and the DPR. The pumping module contains one or more light sources that add additional optical energy to the input beam to increase its brightness and power. The light source(s) may be a flash pump or other type of lamp, but it is preferably one or more sets of one or more laser diodes. The use of laser diodes is advantageous because the laser diodes produce laser energy very efficiently, allowing greater higher amplification with the same input energy.

In general, the pumping module contains an optical pathway composed of a laser crystal composed of a solid state material, such as yttrium-aluminum-garnet ["YAG"], doped with active materials such as Nd, Yb, Ho, or Er. The laser crystal increases pumping efficiency because its atoms become energized and emit more light in the presence of the optical energy of the input beam and the pumping energy.

One embodiment of the present invention uses cladding around laser crystal to improve amplification performance. The cladding is a substantially clear solid-state material of similar composition to the laser crystal. Adding cladding improves amplification performance by decreasing the variances in the illumination levels of the input beam. For the greatest pumping efficiency, the laser diodes emit light transverse to the laser crystal and to the path of the input beam. In the preferred embodiment, pumping efficiency is further improved by using an odd number of odd numbered laser diodes around the laser crystal because the laser diodes then receive and return much of the pumping energy that bypasses the input beam.

As discussed above, it is very desirable to have substantially uniform pumping energy in the laser amplifier. Pumping uniformity is improved by increasing the size of the sets of transverse laser diodes because increasing the number decreases the angle between the laser diodes. Furthermore, increasing the size of the sets of laser diodes increases the total amount of pumping energy (by increasing the number of input sources) and allows for more powerful amplification. Accordingly, the preferred embodiment uses sets of five or more laser diodes, instead of the sets of three found in known laser amplifiers.

To achieve even greater uniformity in pumping energy, one embodiment of the present invention rotates each set of laser diodes so that the pumping energy reaches the laser crystal from different directions. For example, with laser amplifiers using sets of five laser diodes for transverse pumping, each set of laser diodes rotates 36°.

One problem with commercially available laser diodes is that they vary significantly in output power and wavelength. To overcome this problem, a embodiment of the present invention uses laser diodes that are hand-selected to have substantially similar output wavelengths. These laser diodes are then wired in parallel so that each laser diode receives substantially identical electrical input. If the electrical input is limited to the level required for the least powerful laser diode, all the laser diodes produce substantially similar output power.

Changing the orientation of the laser diodes also increases the uniformity and efficiency of the pumping. Laser diodes are produced in long, narrow arrays that are generally orientated parallel to the input beam path. The preferred embodiment of the present invention, however, uses closely spaced laser diodes arrays oriented perpendicular to the input beam path. This positioning produces greater pumping uniformity because of the decreased spacing between the laser diode arrays. Furthermore, amplification power increases because this configuration uses a greater number of laser diodes. These two features of increasing the number laser diodes and decreasing spacing between the laser diodes also increase the durability of the laser amplifier. In particular, the amplifier continues to operate well even if any single laser diode fails because the adjacent laser diodes sufficiently compensate for the lost pumping energy.

One embodiment of the present invention also uses micro-lenses and micro-mirrors that direct the emissions from the laser diodes. As a result, amplification efficiency improves since more pumping energy reaches the input beam. Furthermore, pumping uniformity improves by positioning the micro-lenses and micro-mirrors so that the focal plane of the pumping energy is in front of the laser crystal. Another embodiment of the present invention uses aspherical lenses to further improve amplifier performance. However, with the perpendicular laser diode orientation described above, the laser amplifier has sufficiently uniform pumping energy to perform very well even without employing the micro-lenses and the micro-mirrors.

In one embodiment, the present invention uses a fluid, such as water, to cool the laser crystal. The fluid is contained within a casing composed of a substantially clear material such as glass or plastic. The casing and fluid are located between the optical pathway and the light source of pumping energy. In one embodiment of the present invention, the fluid layer is circulated to help deter localized heat accumulation. In another embodiment, the casing is adapted to form a lens that focuses the pumping energy from the laser diodes and directs it toward the optical pathway. This change allows the increased efficiency that comes from optical manipulation of the pumping energy without the relatively expensive micro-lenses and micro- mirrors. In another embodiment, the laser amplifier uses the cooling fluid to adjust the temperature of the laser crystal to correspond with the pumping energy. In one embodiment, the present invention uses a pair of closely spaced laser crystals positioned in series along the path of the input beam. This configuration allows for increased amplification of the input beam by increasing the number of pumping energy sources.

However, the preferred embodiment of the present invention uses a lens and a 90° rotator between the two laser crystals to help compensate for phase, focus, and polarity changes in the input beam caused by pumping. In particular, one laser crystal counteracts the changes that occur in the other laser crystal. For optimal compensation, the two laser crystals have substantially similar physical characteristics. This similarity is accomplished by manufacturing the laser crystal from the same boule. Coring the rods from the same location in the same crystal boule achieves even greater uniformity in the two laser crystal.

The NPR is in the beam path, downstream from the pumping module. The NPR rotates the polarization of the input beam 45° degrees on each pass. As a result, the beam is orthogonally polarized in the second pass (after 90° degrees of rotation) so that the second PBS deflects it away to begin the third pass. The NPR also returns the beam to initial polarization in the fourth pass (after 445° or 180° degrees of rotation), allowing second PBS to transmit the beam and from the first PBS to deflect the beam after rotations by the DPR, as described above. The NPR may be λ/4 wave plate. The preferred embodiment of the present invention, however, uses a Faraday rotator to achieve higher amplification and better performance.

As described above, the present invention also contains two reflectors. The first reflector is in the beam path downstream from the NPR and returns the beam after the first and third passes to begin second and the fourth passes. The second reflector is in the deflection path of the second PBS to receive and return the beam between the second third passes. The reflectors are generally mirror-like structures that receive and return the input beam.

In a preferred embodiment, the first reflector is a Porro prism. Porro prisms are 45°-90°- 45° solid structures constructed of substantially clear materials. This structure allows the Porro prism to efficiently reflect the entering light energy without the reflexive coating commonly used to create mirrors. Furthermore, the use of the Porro prism increases uniformity of the laser beam by inverting the beam after the first and third passes to homogenizing beam imperfections.

In another embodiment, the substrate of the first and the second reflexive devices are composed of sapphire. In contrast to materials such as glass or plastics, sapphire has the physical property of rapidly diffusing localized heat. Thus, sapphire helps prevent the accumulation of heat that causes hot spots.

These and other features and advantages of the present invention will be appreciated from review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout.

Brief Description Of The Drawings And Addenda

FIG. 1 is a schematic diagram of an apparatus in accordance with the present invention; FIG. 2 is a schematic diagram of the preferred embodiment of an apparatus in accordance with the present invention;

FIG. 3 is a perspective view of a known polarizing beam splitter used in an embodiment of an apparatus in accordance with the present invention;

FIG. 4 is a perspective view of a cube-shaped polarizing beam splitter used in an embodiment of an apparatus in accordance with the present invention;

FIG. 5 is a perspective view of a Porro prism reflector used in an embodiment of an apparatus in accordance with the present invention;

FIG. 6 is a schematic illustration of orientation of laser diode arrays in prior art laser amplifiers; FIG. 7 is a schematic illustration of the orientation of laser diode arrays in the preferred embodiment of an apparatus in accordance with the present invention;

FIG. 8 is a cross-sectional view along the 8-8 plane FIG. 7 of a pumping module used in an embodiment of an apparatus in accordance with the present invention; FIG. 9 is a cross-sectional view along the 8-8 plane FIG. 7 of a pumping module used in another embodiment of an apparatus in accordance with the present invention;

FIG. 10 is a cross-sectional view along the 8-8 plane FIG. 7 of a pumping module used in another embodiment of an apparatus in accordance with the present invention;

Addendum A is a two-page further description of the present invention, entitled, "Master Oscillator";

Addendum B is a ten-page further description of the present invention, entitled, "High Brightness And Power Solid State Laser Amplifiers"; and

Addendum C is a five-page further description of the present invention, entitled, "High Brightness and Power Nd.YAG Laser".

Detailed Description Of The Invention The invention is described in the following Detailed Description and in the Figures 1-10 and Addendum A, Addendum B and Addendum C, which are referred to and incorporated herein by this reference. In accordance with the present invention, a device is provided for producing high power and brightness laser discharges. As illustrated in FIGS. 1 and 2, the present invention provides a laser amplifier 5 that receives and amplifies an input beam 20 from an external laser source 10.

FIG. 1 schematically illustrates a laser amplifier 5. A beam 20 travels through the laser amplifier 5, and the beam is "pumped" on each pass to become brighter and more powerful. As illustrated in FIG. 1, the beam 20 passes through the amplifier 5 four times before exiting as output 15. Accordingly, this type of laser amplifier configuration is known as a four-pass amplifier. Although the FIG. 1 shows the four passes not overlapping, the beam 20 is actually collinear in all the passes. It should be appreciated however, that the teachings of the present invention may also be applied to other types of laser amplifier configurations as well. The basic operation of the laser amplifier 5 is now discussed. A laser source 10, such as a pulsed laser oscillator or a laser diode, creates the beam 20. The beam 20 is initially horizontally polarized (p-polarized) and oscillates parallel to the paper in FIG. 1. This orientation allows the input beam 20 to pass through a first PBS 30. A DPR 50 polarization does not effect the polarization of the beam as it travels from the first PBS 30 to a second PBS 40. The beam 20 remains in p-polarization and passes through the second PBS 40. The beam 10 then enters a pumping module 90 where pumping increases the power and the brightness of the beam 20. Upon exiting the pumping module 90, the beam is rotated 45° by a NPR 60. A first reflector 70 then receives and reflects the beam 20 to end the first pass.

The light beam 20 begins a second pass upon reflection from the first mirror 110. After reflection, the NPR 60 rotates the light beam 20 an additional 45° for a total of 90° from p- polarization. As a result, the beam 20 is vertically polarized, also known as s-polarization, during the second pass. Since the beam 20 is no longer p-polarized, the light beam 10 will not pass though the second PBS 40 after pumping in pumping module 90. Instead, the second PBS 40 diverts the beam 20 to a second reflector mirror 80. Upon reflection from the second reflector 80, the beam 20 begins a third amplifying pass.

Similar to the first and second passes, the pumping module 90 again amplifies the beam 20, and the NPR 60 again rotates beam 20 is another 45°.

After reflection from the first reflector 70, the beam 20 begins a fourth and final amplification pass. The NPR 60 rotates the beam 20 another 45° to return it to p-polarization. The pumping module 90 then amplifies the beam 20 for the fourth time. Because the beam 20 is again in p-orientation, it passes through the second PBS 50. Then, the beam 20 returns to the DPR 50, which now rotates the orientation of the beam 20 90° to s-polarization. As a result, the first PBS 30 deflects the fully amplified beam 20 to exit the laser amplifier 5 as output 15.

The individual components of the laser amplifier 5 are now discussed in more detail. As discussed above, the beam 20 is directed though the laser amplifier 5 by the first and second PBSs, respectively 30 and 40. Generally, both first PBS 30 and second PBS 40 are substantially similar. It should be appreciated, however, that these two elements may differ without significant effect to the performance of the laser amplifier 5.

The first and second PBSs, 30 and 40, are a well-known technology. In particular, first and second PBSs, 30 and 40, are devices that allows properly polarized light to pass substantially unhindered while deflecting other light energy. These devices are commonly available through commercial channels. For example, CVI Technology Inc of Albuquerque, New Mexico sells numerous models of polarized beam splitters.

FIG. 3 illustrates the first and second PBSs, 30 and 40, used in one embodiment of the present invention. These first and second PBSs, 30 and 40, have a polarizing coating 35 applied on an outside surface of a substantially clear material 45. The PBS, 30 or 40, illustrated in FIG. 3 often suffers from heat accumulation at the air boundary. This heat accumulation is sufficient to steer the laser beam and misalign the laser system. To address this problem, the preferred embodiment of the present invention, as illustrated in FIGS 2 and 4, uses first and second PBSs, 30 and 40, having a substantially cube shape. In particular, FIG. 4 illustrates the polarizing coating 45 located along an internal diagonal plane within the substantially clear material 45. This structure for the first and second PBSs, 30 and 40, is desirable because it helps avoid the formation of air pockets in the polarizing coating 35. As a result of fewer air pockets, the cube- shaped PBSs, 30 and 40, illustrated in FIG 4 produce very good beam alignment stability. Another component of laser amplifier 5 illustrated in FIG. 1 is the DPR. As described above, the DPR has no polarization effect on the beam 20 travelling from the first PBS 30 to the second PBS 40 while rotating the polarization of the beam travelling from the second PBS 40 to the first PBS 30. As a result, the second PBS 40 transmits light from the first PBS 30 while the first PBS 30 deflects light from the second PBS 40. As illustrated in FIG. 2, the DPR generally contains a λ/2 wave plate 100 (also known as a retardation plate) and a Faraday rotator 110.

The λ/2 waveplate 100 is an optical element having two principal axes, slow and fast,

that transform beam 20 into two mutually perpendicular polarized light streams. Upon exiting

the λ/2 waveplate 100, two emerging light streams recombine to form a beam 20 with

polarization rotated by 90°. Because the λ/2 waveplate 100 is an optical element such as a

combination of lenses, the resulting polarization rotation is relative to the direction of light beam

20. The λ/2 waveplate 100 is a well-known technology and is commonly commercially

available. For example, CVI Laser Corp. of Albuquerque New Mexico, manufactures numerous

models of λ/2 waveplates. The selection of the particular model depends on the desired size of

λ/2 waveplate 100 and wavelength of beam 20.

In the DPR 50, the λ/2 waveplate 100 is paired with the Faraday Rotator 110. In contrast

to the λ/2 waveplate 100, the Faraday rotator 110 does not contain optically active materials.

Instead, the Faraday rotator 110 operates by imposing a strong magnetic field on the input beam 20. This strong magnetic field causes the light beam 20 to rotate its polarization in the direction of the field, regardless of the direction traveled by the input beam 20. For example, the Faraday rotator 110 rotates the polarization of input beam 20 clockwise on the first pass but counterclockwise on the fourth pass. Faraday rotators are a well-known technology and are commonly commercially available. For example, Electro-Optics Technology, Inc. of Traverse City, Michigan sells manufactures numerous models of Faraday rotators. As illustrated in FIGS. 1 and 2, the laser amplifier 5 also contains NPR 60. As previously described, the NPR 60 rotates the polarization of the input beam 20 45° on each pass.

In one embodiment, the NPR 60 is a λ/4 waveplate. However, a λ/4 waveplate, accomplishes

proper polarization rotation only if the incoming polarization of the input beam 20 is linear and known. Unfortunately, the polarization of input beam 20 is generally neither linear nor known because of birefringence caused during pumping. Although birefringence can be minimized through careful design of the laser amplifier 5, it almost inevitably occurs under high-powered

operation. Tests confirm that high average power operation with a λ/4 waveplate as the NPR 60

is generally unsuccessful because the amplifier then behaves as a free running laser without proper rotation of input beam 20.

To address this problem, a preferred embodiment of the present invention uses a Faraday rotator for NPR 60 to turn the polarization by 45° per pass. The residual polarization rotation not compensated in the first pass can be compensated on the second pass. Tests show that the laser amplifier 5 with the NPR 60 comprising of a Faraday rotator can operate beyond the 100 watts level.

FIGS. 1 and 2 illustrate another two elements of the laser amplifier 5, first reflector 70 and second reflector 80. The first and second reflectors, 70 and 80, are generally mirror-like components able to withstand and redirect the energy and brightness of input beam 20.

In one embodiment, the first second reflectors, 70 and 80, have a sapphire (Al₂O₃) substrate. Using sapphire to form the first and second reflectors 70 and 80 is advantageous because sapphire has high heat conductivity and is very effective in preventing localized heat accumulation. Without the use of sapphire substrates, the first and second reflectors 70 and 80 may heat and distort the input beam 20 under high-powered amplification,

In another embodiment, the first reflector 70 is a Porro prism. As illustrated in FIG. 5, a Porro prism is a 45°-90^o-45° prism that reflects beam 20 180°. The Porro prism is composed on substantially clear material such as glass or plastic. The input beam 20 enters the Porro prism though the long diagonal plane 73, and then reflects off of the two short planes 76 and 79 that define a right angle. Using a Porro prism for second reflector 70 is highly desirable because it enhances the uniformity of the laser beam shape. In particular, the Porro prism inverts the cross section of the input beam 20 (either vertically or horizontally) to helps homogenize much of the non-uniformity in the input beam 20.

As illustrated in FIGS. 1 and 2, the laser amplifier 5 also contains pumping module 90. As previously discussed, the pumping module 90 amplifies input beam 20 on each of the four passes through the amplifier. The pumping module 90 generally includes an optical path and a light source. As the input beam 20 passes along the optical path, the energy source provides optical energy to pump the input beam 20.

As illustrated in FIGS. 6-10, the optical path is generally a laser rod 160. FIGS 6 and 7 show the directions, 162 and 164, of input beam as it travels through the laser rod 160. In a preferred embodiment, laser rod 160 is composed of a solid state media, such as YAG, doped with active elements such as Nd, Yb, Ho, Er, or the like. This laser rod 160 of solid state material is desirable because these materials allow high power, yet efficient pumping of input beam 20. Furthermore, laser rods 160 composed of Nd.YAG and like materials are widely commercially available. Change the characteristics of the laser rod 160, such as its composition or physical dimensions, alters its performance characteristics. Accordingly, it should be appreciated that laser rod 160 is selected to provide desired performance.

In the preferred embodiment illustrated in FIG. 2, the laser amplifier 5 contains a pumping module 90 comprising a first and a second amplification sub-modules, 120 and 130. Both the first and the second sub-modules, 120 and 130, contain a laser rod 160 and an input light source. Using the first and second amplification sub-module 120 and 130 to pump input beam 20 is advantageous because amplifier performance is improved through the use of an inter- rod lens 140 and a 90° rotator 150.

For example, under high average pumping, a laser rod 160 composed of Nd.YAG or like materials behaves as a positive lens, altering the path of light beam 5. However, the input beam 20 must remain parallel throughout the four passes in order to achieve desirable power and brightness gains. As illustrated in FIG. 2, the inter-rod lens 140 is added between the first and second sub-modules 120 and 130. The inter-rod lens 140 is adapted to counteract the thermal lensing of laser rods 160 in sub-modules 120 and 130, returning the light beam 20 to the desired path. The inter-rod lens 140 is selected to correspond to the thermal lensing and to withstand the rigors of laser amplification.

The heating of the laser rods 160 during pumping also effects the input beam 20. In particular, thermal stress in the laser rod 160 causes birefringence in the input beam 20 that rotates the polarization unevenly within the beam cross-section (like a four leaf clover). To compensate for this uneven rotation in the polarization of input beam 20, the 90° rotator 150 is placed between the first and second amplification sub-modules 120 and 130. By rotating the input beam 20 by 90° between the first and the second amplification sub-modules 120 and 130, birefringence that occurs in one amplification sub-module is countered in the second amplification sub-module. The direction of the rotation caused by 90° rotator 150 is unimportant. Accordingly, 90° rotator 150 may be any device that rotates the polarization of input beam 20. For example, 90° rotator 150 may be either a waveplate or a Faraday rotator, as described above.

For proper correction of the birefringence, the laser rods 160 in first and second amplification sub-modules 120 and 130 must have substantially similar thermal stress characteristics. Accordingly, a preferred embodiment of laser amplifier 5 has laser rods 160 in first and second amplification sub-module 120 and 130 that are cored from the same crystal boule. Furthermore, even greater uniformity in the two laser rods 160 is achieved by coring the rods from the same location in the crystal boule.

In other embodiments of laser amplifier 5, design changes are made to laser rod 160 in order to improve high-powered operation. For example, in the embodiments illustrated in FIGS. 6-10, the laser rod 160 is cooled by a fluid 190 contained in a tube 170. The fluid 190 is a substantially clear fluid such as water that does not substantially disrupt pumping. Similarly, tube 170 is a substantially clear material such as glass or plastic that does not significantly effect pumping. In one embodiment, the fluid 230 that surrounds the laser rod 160 is stagnant.

However, to provide more stable temperatures, another one embodiment of the laser amplifier 5 has fluid 190 that flows around laser rod 1600 to prevent heat accumulation. For example, laser amplifier 5 may have a pump that moves the fluid 190. However, it should be appreciated that other means may be employed to cause the movement of fluid 190.

In another embodiment, a layer of substantially clear cladding, such as a clear YAG material, is added around laser rod 160. For efficient pumping, the input beam 20 should overfill the aperture of laser rod 160. However, overfilling the aperture causes input beam 20, which enters with a Gaussian profile, to exit with a "top hat" profile with a concentric, ringed pattern.

The loss of the Gaussian profile limits power and brightness gains in input beam 20. A Gaussian profile in input beam 20 is better preserved by softening the aperture laser rod 160 with clear cladding that reduce diffraction rings. For example, optical diffusion bonding allows the creation of the laser rod 160 with cladding. Adding cladding to laser rod 160 also improve the cooling uniformity, since fluctuation in the temperature of fluid 190 are buffered by the cladding. As illustrated in FIGS. 6-10, the light source in pumping module 90 is generally one or more laser diodes 180. The use of the laser diodes 180 as a source for pumping energy is desirable because the laser diodes 180 produce high quality pumping energy having a specific wavelength and a coherent structure. The laser diodes 180 are semiconductor devices that receive electrical energy and emit electromagnetic energy. The laser diodes 180 are widely available in commercial channels.

In one embodiment, the laser diode 180 is any array of elements measuring

approximately 1 cm long by 300 μm thick (thickness of a wafer) by 1 mm deep. The light

emission comes from multiple apertures along the length (about 100 μm/aperture long by 1 μm

wide). The wavelength of the preferred embodiment of the laser diode si 80 is about 805.5 nm with a bandwidth of less than 5 nm. The emission divergence from the laser diodes 180 is about 50° in the horizontal direction (known as the fast axis), and about 5° in the vertical direction (known as the slow axis). FIGS. 8-10 illustrates the first and second amplification sub-modules, 120 and 130 with an odd number of laser-diodes 180. This configuration is desirable because it positions the laser diode 180 to receive and reflect the pump radiation from other laser diodes 180 that pass through the laser rod 160. Thus, the pumping action is more efficient with an odd number of laser diodes 180, since more of the pumping energy is retained in the laser rod 160. However, it should be appreciated that amplification sub-module 120 and 130 may contain an even number of laser diode 180.

Also, as illustrated in FIGS. 8-10, the laser diodes 180 are generally spaced evenly around the laser rod 160. This design is common for transverse pumping and helps prevent hot- spot formation by having greater uniformity in the pumping energy applied to the laser rod 160. However, it should be appreciated that laser diodes 180 can be positioned in any configuration around laser rod 160.

FIG. 8-10 illustrates a preferred embodiment of the first and second amplification sub- modules, 120 and 130, with the laser rod 160 pumped by sets of five laser-diode 180. Configurations of the first and second amplification sub-modules, 120 and 130, with sets of five laser-diodes 180 deliver higher average power than configurations with three laser diodes 180. 18

Furthermore, pump uniformity is improved with five laser diode 180 because the angle separating each laser diode 180 is decreased. It should be appreciated however that even larger numbers of laser diode 180 could be implemented.

In an embodiment illustrated in FIG. 9, pumping uniformity is further improved by rotating the sets of laser diodes 180. For example, in configurations having sets of five laser diodes 180, each set of the laser diodes 180 is rotated 36° along the long axis of the laser rod 160 with respect to adjacent sets of laser diodes 180.

Proper positioning of the laser diodes 180 around laser rod 160 does not ensure even deposition of pumping energy. Another cause for uneven pumping is variances in the laser diodes 180. For example, the laser diodes 180 often vary in power, wavelength, and bandwidth.

In today's state of the art, it is impossible to request substantially similar specifications for the laser diodes 180 at a reasonable cost.

The non-uniformity in the output energy of the laser diodes 180 degrades performance of the laser amplifier 5. For example, the variance in output wavelengths is undesirable because a laser diode 180 with a center wavelength that is closer to the peak absorption in the laser rod 160 would exhibit more absorption at the edge of the rod than the center of the rod. Similarly, variances in the output energy of the laser diodes 180 are undesirable because laser diode 180 with higher power causes stronger energy deposition in the adjacent portion of the laser rod 160. One method to improve uniformity and performance is to select laser diodes 180 that closely correspond in wavelength, and then compensate for variations in output power.

Compensation for variations in output power is achieved by adding a parallel load to the laser diodes 180 that exhibit higher power. By draining the proper amount of input from the laser diodes 180 of higher power, output power is reduced to correspond to the least powerful laser diode 180. Pumping performance can be further improved by adjusting the temperature of the laser diode 180. This adjustment is accomplished, for example, by adjusting a chiller that cools the fluid 190. The wavelength of the pumping energy changes with adjustments to the temperature of the laser diode 180. For example, experiments show that the peak wavelength for the pumping output of one type of laser diode 180 shifts about 1 nm per 3.5 °C change in temperature. Thus, adjustments to the temperature allow tuning of the peak wavelength of the output of the laser diodes 180 to correspond to the laser rod 160. When the peak wavelength of the pumping energy corresponds to the desired absorption wavelength of the laser rod 160, pumping efficiency and uniformity improve. Orientation of the laser diodes is also important. As previous described, the laser diodes

180 are generally, long, thin arrays. FIG. 6 illustrates a known configuration for laser diode 180 along laser rod 160 where the laser diodes 180 are oriented parallel to the long axis of the laser rod 160. This configuration causes problems that limit the performance and the reliability of the laser amplifier 5. For example, laser diodes 180 of producing varying levels of pumping energy tend to form localized hot spots in the laser rod 160 that degrade the performance of the laser amplifier 5.

Another problem with the known configuration of laser diode 180 in FIG. 6 is that it has no redundancy in the pumping output. As a result, the failure of any single laser component of laser diode 180 will require that the entire array be replaced. Furthermore, this configuration for the laser diodes 180 requires the use of micro-lenses 200 (illustrated in FIG. 8-10) to achieve uniform pumping of laser rod 160. It is desirable to design amplifier modules 120 and 130 to operate without micro-lenses 200 because resulting embodiments of laser amplifier 5 have reduced the costs and complexity.

FIG. 7 illustrates the orientation of the laser diodes 180 in the preferred embodiment of the present invention. In the preferred embodiment, the laser diodes 180 are oriented perpendicular to the long axis of the laser rod 160. One advantage of this configuration is that laser diodes 180 are closely spaced, causing the pumping radiation to be substantially homogenized due to the wide divergence of pumping radiation along the fast axis. As previously discussed, illuminating the laser rod 160 with more uniform pumping radiation minimizes hot spots, therefore reducing thermal stress and non-uniform gain. Furthermore, there is no need for micro-lenses 200 in the configuration of laser diodes 180 presented in FIG. 7 because of the substantial homogeneity of the pump radiation. This configuration for the laser diodes 180 is also more durable than the known configuration of FIG. 6 because the failure of any laser diode 180 can be offset by power adjustments to the remaining laser diodes 180.

Experiments show the configuration illustrated in FIG. 7 allows for higher powered pumping and greater gains from the same levels of pump energy. For example, a free running laser operation in one test yields over 35% conversion from laser diode output to laser output, indicating very efficient pumping. Furthermore, high average power tests show that a water- cooled package with the configuration illustrated in FIG. 7 can handle over a twenty-percent

duty cycle. In other tests, the laser diodes 180 were operated for about 200 μs and at repetition

rates up to 1000 Hz.

As previously described and as illustrated in FIGS. 8-10, the performance of the laser amplifier 5 can be further improved by using the first and second amplification sub-module, 120 and 130, having micro-lenses 200 positioned output bath of the laser diodes 180. The purpose of the micro-lens 200 is to collimate the pump radiation along the slow axis and therefore guide the light from the laser diode bar 180 toward the laser rod 160. Thus pumping efficiency is increased, allowing greater brightness and power with the same pump energy input. In general, the micro-lenses 200 are spherical because aspherical lenses are very expensive. However, use of micro-lenses 200 with aspherical shapes may allow even greater performance of laser amplifier 5. Proper positioning of the micro-lens 200 is important to achieve desired pumping performance gains. For example, when the laser rod 160 is at the focal plane of the micro-lens

200, there is strong deposition of the pumping energy in the center of the laser rod 160. As a result of this strong beam intensity at the center of the laser rod 160, pumping is very efficient, but damage to the laser rod 160 and overall poor quality of input beam 20 occur above 20 watts. By setting the laser diodes 180 and micro-lenses 200 so that the focal plane is in front of the laser rod 160, the pumping energy is diverging as it enters the laser rod 160. As a result of this divergence, the deposition of pumping energy into laser rod 160 is significantly more uniform. For example, tests show that laser amplifiers 5 with this configuration can achieve output 15 with power greater than 100 watts without damaging the laser rod 160. Further increases in the divergence of the pumping energy results in increasingly uniform deposition at the expense of losing pumping power.

FIG. 10 illustrates an embodiment of the present invention where the micro-lens 200 is built in to the tube 170 that holds the fluid 190. To perform this task, energy deposition measurements are taken from one laser diode 180 and then overlapped it 5 times to simulate 5 diode array pumping.

In another embodiment, reflecting walls (not illustrated) are also positioned along the sides of the laser diodes 180 to reflect stray energy along the fast axis. The combination of micro-lens 200 and the reflecting walls provides that substantially all the radiation from the laser diode 180 gets to the laser rod 160.

Thus, it is seen that an apparatus and method for creating a high power, high brightness laser amplifier are provided. One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments which are presented in this description for the purpose of illustration and not limitation, and the present invention is limited only by the claims that follow. It is noted that equivalents for the particular embodiment discussed in this description may practice the invention as well.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A laser amplifier for amplifying an input beam comprising: a first polarizing beam splitter in the path of the input beam; a second polarizing beam splitter in the path of the input beam, downstream from said first polarizing beam splitter; a first polarization rotator positioned in the path of the input beam between said first and said second polarizing beam splitters, wherein said first polarization rotator transmits the input beam received from said first polarizing beam splitter with a polarization suitable pass through said second polarizing beam source, and wherein said first polarization rotator transmits the input beam received from said second polarizing beam splitter with a polarization suitable for deflection by said first polarizing beam splitter; a first reflector positioned in the deflection path of said second polarizing beam splitter; at least one pumping module positioned in input beam path, downstream from said second polarizing beam splitter, said pumping module comprises: a laser crystal; and at least one light source; a 45° Faraday Rotator positioned in the input beam path, downstream from said pumping module; and a second reflector positioned in the input beam path, downstream from said 45° Faraday

Rotator.

2. A laser amplifier for amplifying an input beam comprising: a first polarizing beam splitter in the path of the input beam; a second polarizing beam splitter in the path of the input beam, downstream from said first polarizing beam splitter; a first polarization rotator positioned in the path of the input beam between said first and said second polarizing beam splitters, wherein said first polarization rotator transmits the input beam received from said first polarizing beam splitter with a polarization suitable pass through said second polarizing beam source, and wherein said first polarization rotator transmits the input beam received from said second polarizing beam splitter with a polarization suitable for deflection by said first polarizing beam splitter; a first reflector positioned in the deflection path of said second polarizing beam splitter; at least one pumping module positioned in input beam path, downstream from said second polarizing beam splitter, said pumping module comprises: a laser crystal; and at least one laser diode array positioned for transverse pumping of the laser crystal and orientated so that the long axis of the laser diode is perpendicular to the long axis said laser crystal; a 45° Rotator positioned in the input beam path, downstream from said pumping module; and a second reflector positioned in the input beam path, downstream from said 45° Rotator.