Laser welding of crystalline sapphire for use in future generation gravitational wave detector suspensions

Since the first direct detection of a gravitational wave signal in 2015, advancements in technology have resulted in incremental improvements to the current detector network. Future detectors will exploit new technologies to deliver an order of magnitude improvement in performance by increasing their scale and directly targeting thermal and other noise sources. With many next-generation gravitational wave detectors planning to implement crystalline materials as replacements for fused silica, there is an increasing need for novel techniques in suspension jointing. This work will report on the development and characterisation of a repeatable and repairable laser welding technique for crystalline sapphire. Utilizing a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>CO</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> </a:math> laser within a crystal growth machine, specifically designed for laser-heated pedestal growth, we established a reliable method for laser welding sapphire stock pieces of various millimetre scale diameters. Experimental characterisation revealed an upper limit of the weld region to be <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mo>≤</c:mo> <c:mn>200</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi mathvariant="normal">μ</c:mi> <c:mi mathvariant="normal">m</c:mi> </c:mrow> </c:math> , and no measurable detriment to the thermal conductivity or tensile strength due to a welded joint. A thermal conductivity of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mn>4587</g:mn> <g:mtext> </g:mtext> <g:mtext> </g:mtext> <g:msup> <g:mtext>Wm</g:mtext> <g:mrow> <g:mo>−</g:mo> <g:mn>1</g:mn> </g:mrow> </g:msup> <g:mtext> </g:mtext> <g:msup> <g:mi mathvariant="normal">K</g:mi> <g:mrow> <g:mo>−</g:mo> <g:mn>1</g:mn> </g:mrow> </g:msup> </g:math> at 20 K and a minimum unbroken tensile strength of 1.1 GPa were measured on a welded sample. In addition, cryogenic mechanical loss measurements produced a lowest loss of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mn>3.2</j:mn> <j:mo>×</j:mo> <j:msup> <j:mn>10</j:mn> <j:mrow> <j:mo>−</j:mo> <j:mn>7</j:mn> </j:mrow> </j:msup> </j:math> at 14 K, allowing a weld loss estimate of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mn>5.1</l:mn> <l:mo>×</l:mo> <l:msup> <l:mn>10</l:mn> <l:mrow> <l:mo>−</l:mo> <l:mn>6</l:mn> </l:mrow> </l:msup> </l:math> . These initial findings indicate significant potential for sapphire welding as a viable concept in next-generation cryogenic detector suspensions and provide a strong justification for further development of this jointing method.