Laser diodes are mass produced through manufacturing processes similar to those used for building silicon wafer based integrated circuits. These laser diodes have high electrical to optical conversion efficiency and high brightness (referring to the combined angular and spatial concentration of light emitted from the source). Though the light emitted from multiple laser diodes is not coherent (in phase with each other), large numbers of laser diodes can be grouped together to ‘pump’ or illuminate a ‘gain’ medium (a glass slab or optical fiber that has been impregnated with a rare-earth element) such that a coherent ‘seed’ laser source passing through the gain medium will be amplified and still remain coherent. In this way, the incoherent optical energy of laser diodes can be converted into a coherent and highly energetic laser source that is then useful for any manner of medical, scientific, and industrial purpose.
The light emitting area or stripe of a typical high power multimode laser diode is very small, on the order of 1 um in height by 95-190 μm in stripe width. The 1 μm height is set by the guiding layers that form the semiconductor junction of the laser diode, and the width is set somewhat by the desired output power. For example, laser diodes in the 980 nm class of emitting wavelength are produced with approximately 85 mW of output power per μm of emitter width, meaning that a typical 95 μm wide emitter would have a rated output power of 8 W. With reference to FIG. 1, the emitter divergence of a laser diode of this type would be on the order of 10 degrees (full width angle) in the parallel axis direction (θ∥, in FIG. 1) and 40 degrees (full width angle) in the perpendicular axis (θ⊥, in FIG. 1). Emitter size and angular divergence can be multiplied together to form a specification called ‘etendue’, whereby the brightness of a laser diode is then its etendue divided by its output power. Brightness for a laser diode can be improved by increasing the output power for a given emitter size and divergence angle, or by reducing the divergence angles or emitter size (reducing etendue) for a given output power.
For the laser diode shown in FIG. 1 with emitting area A, the etendue is given by:etendueLD=A·θ⊥·θν
and the brightness, B, of the laser diode is simply the output power P (in Watts) divided by the etendue (in units of cm2 sr, where sr is for steradian, the solid angle):
      B    LD    =      P          etendue      LD      
It is generally advantageous for high power laser amplifiers to have their incoherent pumping sources (groups of laser diodes) illuminate their gain medium with the highest possible brightness. The high brightness of the pump source allows a coherent seed laser to efficiently extract the available pump energy from the gain medium for maximum amplification while retaining good coherent beam quality (the ability to focus to a small spot).
Typically, pump energy from laser diodes is delivered to the gain medium via a multimode (large etendue) optical fiber. The highest brightness of a pump source is generally achieved when the pump output fiber has the smallest etendue possible for the greatest amount of contained optical power. Contemporary pump sources are generally built in one of two types. In a first type, commonly referred to as a Fiber Coupled Single Emitter Device or FCSED, individual laser diodes are coupled one-to-one into small etendue multimode optical fibers within hermetic packaging. Those fibers are then bundled together and spliced to a single, larger etendue multimode fiber. In a second type, commonly referred to as a Fiber Coupled Multi Emitter Device or FCMED, the output of multiple laser diodes is combined through complex bulk lensing systems onto the input face of a multimode fiber.
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