High stability interferometer for measuring small changes in refractive index and measuring method using the interferometer

A high stability interferometer for measuring small changes in refractive index, comprising:

• a light source for producing spatially coherent light;
• a diffraction grating provided with grating lines and placed in a path of the light from said light source for diffracting the light passing through it into beams which include +1, -1 and zero order beams;
• means for moving said diffraction grating in a direction for causing said grating lines to intersect a path of the light from said light source;
• a first Fourier Transform lens having a first input plane and a first output plane, and arranged so that said diffraction grating is in the first input plane through which at least said +1, -1 and zero order beams pass and are thereby brought to a focus in the first output plane;
• a sample cell provided with means for continuously supplying a sample to said sample cell and placed in a path of the +1 or -1 order beam that has passed through said lens;
• a mirror placed in the first output plane of said first lens for reflecting back toward said diffraction grating the beams that have passed through said first lens;
• a second Fourier Transform lens having a second input plane and a second output plane, and arranged so that said diffraction grating is in the second input plane so as to bring the +1 and -1 order beams recombined with the common zero order beam at said diffraction grating to a focus in the second output plane;
• photodetection means placed in the second output plane of said second lens for detecting two focused spots of the +1 and -1 order beams that have passed through said second lens;
• and
• data processing means for determining the refractive index of the sample from the phase difference between the two focused spots detected by said photodetection means;
• whereby the refractive index of the sample continuously supplied to said sample cell is continuously measured from a difference in phase between the two focused spots whose intensities fluctuate with the movement of said diffraction grating.

A method for measuring small changes in refractive index of an object with high stability, comprising the steps of:

• diffracting spatially coherent light into beams inclusive of +1, -1 and zero order beams with a diffraction grating;
• causing the diffracted -1, -1 and zero order beams to be incident on a first Fourier Transform lens arranged so that the diffraction grating is in an input plane of the first lens;
• causing the +1 or -1 order beam output from the first lens to pass through a sample and then bringing the beams output from the first lens to a focus in an output plane of the first lens;
• causing a mirror placed in the output plane of the first lens to reflect the focused beams back toward the diffraction grating via the first lens, thereby recombining the +1 and -1 order beams with the common zero order beams at the diffraction grating;
• bringing the recombined +1 and -1 order beams to a focus in an output plane of a second Fourier Transform lens arranged so that the diffraction grating is in an input plane of the second lens;
• detecting two focused spots of the +1 and -1 order beams;
• and
• determining the refractive index of the sample from the phase difference between the two focused spots.