Nonuniform birefringence in highly reflective substrate-transferred <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>GaAs</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Al</mml:mi> </mml:mrow> <mml:mrow> <mml:mn

Using a custom-built scanning system, we generated maps of birefringence on reflection at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>λ</a:mi> <a:mo>=</a:mo> <a:mn>1064</a:mn> </a:mrow> </a:math> nm from single-crystal GaAs/ <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>Al</b:mi> <b:mrow> <b:mn>0.92</b:mn> </b:mrow> </b:msub> <b:msub> <b:mi>Ga</b:mi> <b:mrow> <b:mn>0.08</b:mn> </b:mrow> </b:msub> <b:mi>As</b:mi> </b:mrow> </b:math> Bragg reflectors (henceforth “AlGaAs coatings”). Ten coatings were bonded to fused silica substrates and one remained on the epitaxial growth wafer. The average phase difference on reflection between beams polarized along the fast and slow axes of the coating was found to be <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>ψ</c:mi> <c:mo>=</c:mo> <c:mn>1.09</c:mn> <c:mo>±</c:mo> <c:mn>0.18</c:mn> </c:mrow> </c:math> mrad, consistent with values observed in high-finesse optical reference cavities using similar AlGaAs coatings. Scans of substrate-transferred coatings with diameters between 18 and 194 mm showed birefringence nonuniformity at a median level of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mn>0.1</d:mn> </d:mrow> </d:math> mrad. A similar epitaxial multilayer that was not substrate transferred, but remained on the growth wafer, had by far the least birefringence nonuniformity of all mirrors tested at <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mn>0.02</e:mn> </e:mrow> </e:math> mrad. On the other hand, the average birefringence of the epi-on-wafer coating and substrate-transferred coatings was indistinguishable. Excluding nonuniformity found at the location of crystal and bonding defects, we conclude that the observed nonuniformity was imparted during the substrate transfer process, likely during bonding. Quantifying the impact on the scatter loss in a LIGO (Laser Interferometer Gravitational wave Observatory) interferometer, we find that birefringence nonuniformity at the levels seen here is unlikely to have a significant impact on performance. Nonetheless, future efforts will focus on improved process control to minimize and ultimately eliminate the observed nonuniformity.