Multifunction CO 2 Waveguide Laser For Application In Systems

Abstract

Multifunction CO2 waveguide laser for application in systemsIan Park and Chris WilliamsFerranti Industrial Electronics Ltd.Dunsinane Avenue, Dundee DD2 3PN, ScotlandABSTRACTDesign criteria for a compact, sealed, high power CO2 waveguide laser are discussed. The laser is capable of being operatedcontinuous wave (cw) and with amplitude or frequency modulation (AM or FM) according to the type of intra -cavity crys-tal used. It is anticipated that the device will find application in a variety of coherent detection based systems.1. INTRODUCTIONIn order to provide a versatile and cost effective CO9 laser source for use in a variety of system configurations, the basicgain cell has to be configured to support a number of different output formats. RF- excited waveguide based technology isappropriate for this application because of its inherent high volumetric efficiency and ruggedness. In our design, it is a rela-tively simple matter to change between frequency modulated, Q- switched or cavity dumped modes of operation, in additionto the standard cw output.Within such a multifunction device it is desirable to embody the following basic design features:(1) High output power /compact size(2) Ruggedness and good frequency stability(3) Operation in a wide range of ambient temperatures(4) Long shelf and active lives(5) A high quality output mode (suitable for coherent detection).Each of these topics will be discussed in more detail before moving on to deal with the various infra- cavity modulationformats.2. BASIC CAVITY DESIGN2.1 High output power /compact sizeA schematic of the waveguide laser cavity is shown in figure 1. The waveguide is 2.5mm square. This choice is the result ofoptimising the constraints of waveguide loss, fold coupling loss and gain bandwidth requirement. In order to obtain highoutput power in a sealed device a long active length is required. Since the device must also be compact, it is necessary tofold the optical cavity. This must be accomplished with minimal loss. The two basic fold techniques involve either a ora geometry. The losses associated with a waveguide fold are quantified in the expression for the fold's effective singlepass reflectivity. The major contributions to this come from the mode coupling loss and the reflectivities of the surfaces ofthe fold reflectors. In our design of U fold, inter -guide coupling is achieved by means of a ZnSe prism. This rotates thedirection of beam propagation through 180 degrees via two Brewster angle interfaces and a single total internal reflection.This does not require any optical coatings since the transmission of the Brewster faces is at least 99.7% and the total inter-nal reflectivity approaches 100 %. In a V fold, a single reflector provides inter -guide coupling and while mirrors are avail-able with quoted reflectivities of greater than 99 %, it is our experience that the dielectric coating used to enhance the reflec-tivity of the metal substrate is prone to rapid damage when exposed to the laser discharge. A realistic long term reflectivityfor nominal total reflectors is 98.5% and this value has been used to generate figure 2 which shows the effective reflectivi-ties cf comparable V and U folds (the graph also makes use of standard expressions for the plane mirror EH 11 couplingloss' and the bulk absorption of ZnSe). The U fold method offers significantly better performance because of its lower lossin addition to its inherently more compact geometry. This value of net fold loss together with previously determined valuesfor laser gain, saturation intensity and waveguide loss' enable a Rigrod based numerical calculation of the cw output power