A four-pass laser amplifier module comprising: an enclosure having a first end and a second end; an input window; a mirror optically coupled to the input window and disposed in a first plane; a first amplifier head disposed along an optical amplification path in a second plane substantially parallel to the first plane, wherein the first amplifier head is adjacent the first end of the enclosure; a second amplifier head disposed along the optical amplification path in the second plane and adjacent the second end of the enclosure; and a first cavity mirror disposed along the optical amplification path and operable to reflect light after a first amplification pass through the first amplifier head and the second amplifier head in an opposite direction, whereby the light reflected by the first cavity mirror passes through the second amplifier head and the first amplifier head in this order as a second amplification pass and wherein the mirror, the amplifier heads (129, 136) and the first cavity mirror are located inside the enclosure, further comprising: a reflective polarizer optically coupled to the mirror and disposed in the second plane; and a quarter wave plate that receives the light from the reflective polarizer, the quarter wave plate being disposed along the optical amplification path, wherein the laser amplifier module further comprises a Pockels cell disposed along the optical amplification path and between the first amplifier head and a second cavity mirror disposed in the second plane. an enclosure having a first end and a second end; an input window; a mirror optically coupled to the input window and disposed in a first plane; a first amplifier head disposed along an optical amplification path in a second plane substantially parallel to the first plane, wherein the first amplifier head is adjacent the first end of the enclosure; a second amplifier head disposed along the optical amplification path in the second plane and adjacent the second end of the enclosure; and a first cavity mirror disposed along the optical amplification path and operable to reflect light after a first amplification pass through the first amplifier head and the second amplifier head in an opposite direction, whereby the light reflected by the first cavity mirror passes through the second amplifier head and the first amplifier head in this order as a second amplification pass and wherein the mirror, the amplifier heads (129, 136) and the first cavity mirror are located inside the enclosure, further comprising: a reflective polarizer optically coupled to the mirror and disposed in the second plane; and a quarter wave plate that receives the light from the reflective polarizer, the quarter wave plate being disposed along the optical amplification path, wherein the laser amplifier module further comprises a Pockels cell disposed along the optical amplification path and between the first amplifier head and a second cavity mirror disposed in the second plane. a reflective polarizer optically coupled to the mirror and disposed in the second plane; and a quarter wave plate that receives the light from the reflective polarizer, the quarter wave plate being disposed along the optical amplification path, wherein the laser amplifier module further comprises a Pockels cell disposed along the optical amplification path and between the first amplifier head and a second cavity mirror disposed in the second plane.