Abstract:
A lens duct is used for pump delivery and the laser beam is accessed through an additional component called the intermediate beam extractor which can be implemented as part of the gain element, part of the lens duct or a separate component entirely.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the use of lens ducts to deliver pump light to laser gain materials, and more specifically, it relates to means for delivering pump light to a laser gain element while maintaining access for a laser or probe beam. 
     2. Description of Related Art 
     The design of solid-state lasers is greatly determined by the architecture for pump light delivery and similarly, the brightness (W/cm2-steradian) of pump light. For instance, some devices utilize pump light impinging on a face of a slab or along the barrel of a rod in a geometry commonly referred to as side-pumping. However, in some cases, it is desirable to utilize an end-pumping geometry in order to achieve high delivery efficiency, or due to limitations in the absorption length due to dopant concentration. 
     Because of limitations in diode array brightness, it is often desirable to confine the pump light in the gain element via total internal reflection (TIR), since the Rayleigh range may be much less than the absorption length. The limited diode brightness also places limitations on access to the laser beam if the pump light fills the entire cross-sectional area of the gain element. One geometry for diode-array end-pumping uses a dichroic coating on one endface of the laser gain element to achieve high reflectivity at the laser wavelength and high transmission for the pump wavelength over its angular distribution. This allows the pump light to be delivered through this endface while the laser beam is reflected off of the same face and extracted from the laser medium through another face, usually the other endface. However, the use of the dichroic coating places restrictions on how close the pump and laser wavelengths can be, as well as the angular distribution of the pump light. In addition, there are situations where pump light enters the gain element through a surface and it would be desirable to access the laser beam through the same surface, for instance to pump both ends of a slab or rod. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an intermediate beam extractor (IBE). 
     It is another object of the present invention to provide means to access a laser beam through the pumped face of a laser gain element by appropriate design of the gain element and the pump delivery optical train. 
     Still another object of the invention is to provide designs for laser amplifiers and laser oscillators that utilize one or more lensing ducts and one or more intermediate beam extractors such that pump light may be delivered to a gain medium while maintaining access to the laser beam. 
     Another object of the invention is to provide an intermediate beam extractor with turning mirrors made therein to redirect light back into the laser gain medium. 
     It is another object of the invention to provide laser oscillators and amplifiers that utilize on each end of a laser gain medium a laser diode array optical pump source directed through a lensing duct and an intermediate beam extractor while maintaining access the the laser beam. 
     Another object of the invention is to provide a dot mirror placed on an intermediate beam extractor such that pump light is funneled around the dot mirror. 
     The invention is an intermediate beam extractor (IBE). A lens duct is used to deliver pump light through the IBE into a laser gain medium. The laser beam is accessed through the intermediate beam extractor which can be implemented as part of the gain element, part of the lens duct or a separate component entirely. The IBE comprises an optically transparent material and a plurality of continuously extending flat sides interconnecting the first and second end faces of the IBE. The first end face has a greater cross-section than the second end face and the first end face has a greater cross-section than the smaller cross-section area end face of a lensing duct. The first end of the IBE is positioned to receive a pump beam after it exits the smaller cross-section area end of the lensing duct. U.S. Pat. No. 5,307,430, titled “Lensing Duct”, describes the lensing duct in detail. 
     The intermediate beam extractor is included in several embodiments of lasers and laser amplifiers. One embodiment is an apparatus, comprising a lensing duct having an optically transparent material, first and second ends and a plurality of continuously extending flat sides interconnecting the ends, where said one end of the lensing duct has a greater cross-section than the other end. A laser diode array is used for production of a pump laser beam, where the laser diode array is configured to direct the pump laser beam onto the larger cross-section end of the lensing duct and the pump laser beam exits from the smaller cross-section end of the lensing duct. The IBE is positioned to receive the pump beam after it exits from the smaller cross-section end of the lensing duct. A laser gain medium is positioned to receive the pump laser beam after it exits the IBE. 
     A dual-end-pumped slab laser gain module has been designed where pump light is delivered to both ends of a slab gain element with the laser beam entering and exiting at an angle greater than that of the pump light. An important aspect of the design is the means to extract the laser beam with minor impact on pump delivery efficiency. The additional optical component can be fabricated as part of the gain element or as a separate component. The optical components may also have beneficial properties in suppressing parasitic modes, in addition to their purpose of pump light delivery and laser beam extraction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B show a sketch of a laser gain element of arbitrary shape. 
     FIG. 2 shows a schematic of a diode-array end-pumped zigzag slab laser gain element utilizing an intermediate beam extractor to enable the pump light to be delivered to the slab while allowing access for the laser beam. 
     FIG. 3 shows an embodiment where a laser slab is pumped through an IBE located at both ends of the laser slab. 
     FIG. 4A illustrates a laser design utilizing the IBE of the present invention. 
     FIG. 4B shows an embodiment where the laser light enters and exits on the side of the IBE. 
     FIG. 5 shows a design the laser beam is confined entirely to the interior of the IBE using turning mirrors. 
     FIG. 6 shows a schematic of a laser rod with a flanged end cap and a dot mirror. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B show a sketch of a laser gain element of arbitrary shape, for which optical surfaces can be identified as surface  10  and surface  20  (FIG. 1B only). The invention involves the partial occlusion of said optical surfaces with a diode array or pump light delivery system such as a lens duct or other system. The remaining non-occluded portion of the optical surface serves to access the coherent laser beam. FIGS. 1A and 1B also encompass combinations of laser gain elements and pump delivery optics with dashed lines showing possible breaks between the laser gain element and the pump delivery optics. Several practical devices are based on this concept. 
     FIG. 2 shows a sketch of a laser slab  22  using a lens duct  24  to deliver pump light from diode array  26 . Laser slab  22  includes a highly reflective coating  28  on one end. An intermediate beam extractor  30  is located between lens duct  24  and an end of laser slab  22 . The intermediate beam extractor (IBE)  30 , is the subject of this disclosure. This component acts as a continuation of the lens duct which serves to deliver the pump light from a large diode laser array to a smaller cross-sectioned laser slab. 
     Various embodiments of the lens duct usable in the present invention are disclosed in U.S. Pat. No. 5,307,430, titled “Lensing Duct”, incorporated herein by reference. Other related disclosure of hollow lensing ducts may be found in U.S. patent application Ser. No. 09/182155, filed Oct. 21, 1998 titled “Hollow Lensing Duct”, incorporated herein by reference. 
     The feature which distinguishes the IBE from the lens duct is that the IBE not only serves to in-couple pump radiation to the slab but also serves to in-couple or out-couple the laser radiation that is to be amplified or (that has been amplified) by the slab. The in-coupling and out-coupling of laser radiation is accomplished by having the IBE flange out at a larger opening angle than does the lens duct. This allows the IBE to have two faces, one on either side of the center that serve to let laser beams exit and enter the device. 
     The configuration of FIG. 2 reveals a top view of a zig-zag laser amplifier in which the beam to be amplified is passed into the slab through one face of the IBE and then double passes the slab prior to exiting the other face of the IBE. This particular configuration is pumped at one end and may have a HR coating applied to the other end, or alternatively, the laser beam could exit the slab and be redirected back into the slab with external optics. Specifically, light from laser diode array  26  propagates through lens duct  24  and IBE  30  to optically pump laser slab  22 . A laser beam  32  is directed through IBE  30  into laser slab  22 . After undergoing total internal reflection within laser slab  22 , laser beam  32  reflects from the highly reflective coating  28 , to exit from the laser slab  22  and the IBE  30  as amplified laser beam  34 . 
     FIG. 3 shows an embodiment where the slab is pumped at both ends. Pump radiation from diode array  40  propagates through lens duct  42  and IBE  44  into laser slab  46 . Pump rediation from diode array  48  propagates through lens duct  50  and IBE  52  into laser slab  54 . In this configuration the laser beams  56  and  58  which are to be amplified in the zig-zag slab enter and exit the slab after a single pass through the laser slab  46 . This embodiment in particular has applicability to many quasi-three level laser schemes in which intense bleach wave pumping is a prerequisite to efficient laser operation 
     FIG. 4A illustrates a laser design utilizing an embodiment of the IBE of the present invention. Pump light from a laser diode array generates laser light along an optical axis defined by a path between high reflector  60  and output coupler  62 . This embodiment may be pumped from a single end or from both ends. One embodiment of the single ended pump configuration includes a highly reflective coating  64  on one end of laser slab  66  and an IBE  74  on the other laser slab end. The IBE  74  shown in FIG. 4A can be replaced with IBE  75  shown in FIG. 4B, which includes laser beam access portions  78  and  79 , shown to be integral to IBE  75 . Another embodiment can be used for either single or double ended pumping by using an IBE  76  on the end of the laser slab  66  instead of the highly reflecting coating  64 . Note the formation of mirror surfaces  70  and  72  on the end of IBE  68 . These mirror surfaces function to re-direct the output radiation from the slab back into the slab through two reflections, one at each turning mirror. 
     FIG. 5 shows a design of IBE  84  where the laser beam is confined entirely to the interior of IBE  84  using turning mirrors  86  and  88 . In this design, the intermediate beam extractor includes integral turning mirrors for manipulating the laser beam without actually extracting the laser beam from the device. In this particular device, turning mirrors  86  and  88  are fabricated on the IBE  84  itself by cutting appropriately oriented surfaces directly into the body of the IBE  84 . The function of these turning mirrors is simply to re-direct the output radiation from the slab  89  back into the slab through two reflections, one at each turning mirror. The advantage of this approach over one in which the pump beam is delivered into the end of a slab through a dichroic coating that serves to pass the pump but reflect the laser light is that the need for the dichroic coating is eliminated. In many laser systems this approach would be attractive as it may not be possible to fabricate a dichroic coating with the required specifications. 
     FIG. 6 is an implementation of the present invention which is applicable to both rod and slab geometries. FIG. 6 shows a schematic of a laser rod  90  with a flanged end cap  92  and a dot mirror  94 . In this design, the flanged end cap  92  is larger than the laser rod  90  itself. The pump light is delivered to this flange through lens duct  96  while being directed around dot mirror  94  used to reflect the laser beam. In this way the laser beam is reflected with minimal impact on the pump light delivery. This design would be of value, e.g., in a situation where a dichroic coating could not be used on the end face (e.g. because the pump and laser wavelengths were very close). 
     The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.