Method and apparatus for measurement of reflectivity for high quality mirrors

A method for evaluation of reflectivity for high quality mirrors comprising the steps of:

• defining a first optical cavity by commonly disposing first and second mutually spaced mirrors placed along a longitudinal axis with reflecting surfaces perpendicular to perpendicular said longitudinal axis and facing each other, said first mirror having predetermined reflectivity;
• directing a radiation beam into said first optical cavity along said longitudinal axis, said radiation beam produced by a radiation source and transmitted through a beamsplitter which is interposed between said first optical cavity and said radiation source on said longitudinal axis;
• measuring a first optical transmission signal of said first optical cavity by a first detector, said first detector positioned on said longitudinal axis behind said second mirror;
• measuring a first relative reflection signal of said first optical cavity by a second detector, said second detector located in a deflection plane of said beamsplitter;
• forming a second optical cavity by reversing the position of said first and said second mirrors respectively;
• aligning said second optical cavity to obtain a second optical transmission signal from said first detector;
• measuring by said second detector a second relative reflection signal of said second optical cavity;
• obtaining intrinsic cavity losses of said first optical cavity from the relationship of said predetermined reflectivity and said first and second relative reflection signals;
• determining a reflection coefficient of said second mirror as a function of said predetermined reflectivity and said first and second relative reflection signals;
• and
• evaluating reflectivity of said second mirror by comparing said reflection coefficient with said intrinsic cavity losses.

A method of verification of reflectivity measurement for high quality mirrors comprising the steps of:

• defining a first optical cavity by commonly disposing first and second mutually spaced mirrors placed along a longitudinal axis with reflecting surfaces perpendicular to said longitudinal axis and facing each other, said first mirror having predetermined reflectivity;
• directing a radiation beam into said first optical cavity along said longitudinal axis, said radiation beam produced by a radiation source and transmitted through a beamsplitter which is interposed between said first optical cavity and said radiation source on said longitudinal axis;
• measuring a first optical transmission signal of said first optical cavity by a first detector, said first detector positioned on said longitudinal axis behind said second mirror;
• measuring a first relative refection signal of said first optical cavity by said second detector, said second detector located in a deflection plane of said beamsplitter;
• forming a second optical cavity by reversing the position of said first and said second mirrors respectively;
• aligning said second optical cavity to obtain a second optical transmission signal from said first detector;
• measuring by a second detector a second relative reflection signal of said second optical cavity;
• determining a reflection coefficient of said second mirror as a function of said predetermined reflectivity and said first and second relative reflection signals;
• forming a third optical cavity from said second optical cavity by substituting a third mirror in place of said first mirror, said third mirror having a reflection coefficient substantially equal to the reflection coefficient of said second mirror;
• aligning said third optical cavity by adjusting said third mirror to maximize the signal of said first detector;
• measuring a third relative reflection signal of said third optical cavity by a second detector;
• and
• comparing said reflection coefficient of said second mirror by matching it with a reflection coefficient of said third mirror.

Apparatus for measurement of reflectivity for high quality mirrors comprising:

• a radiation source for producing a radiation beam of coherent light;
• an optical cavity bounded by a pair of mirrors on a longitudinal axis, said pair of mirrors spaced apart and displaceable with respect to one another;
• means for alignment of said optical cavity;
• means for mode matching of said radiation source and said optical cavity, said mode matching means placed between said radiation source and said optical cavity on said longitudinal axis;
• a beamsplitter for permeating a radiation beam from said radiation source to permeate said optical cavity and deflecting a reflected beam, said beamsplitter interposed between said radiation source and said optical cavity on said longitudinal axis;
• a first detector for measuring optical transmission signals from said optical cavity, said optical cavity disposed between said beamsplitter and said first detector on said longitudinal axis;
• a second detector for measuring optical reflection signals from said optical cavity deflected by said beamsplitter, said second detector disposed in the deflection plane of said beamsplitter;
• and
• means for registration of said optical transmission and reflection signals from said first and second detectors, said first and second detectors electrically connected to said means for registration.