Refractive index measuring device
A photodiode includes semiconductor layers and a gate insulating layer provided on a buried insulating layer formed on a substrate and has a diffraction grating portion in which a plurality of groove portions are formed in a two-dimensional lattice shape, on the gate insulating layer. Measurement light is guided by an optical system including a photoelastic modulator and is incident on the photodiode. The measurement light is emitted from the light source device in a state of being linearly polarized light having a predetermined wavelength and is converted at a predetermined frequency by the optical system such that states in which the measurement light becomes linearly polarized light beams of two orthogonal directions are repeated. In addition, electric signals from the photodiode in the state in which the measurement light becomes the linearly polarized light beams of the two orthogonal directions are lock-in detected.
1. A refractive index measuring device, comprising:
a photodiode configured to have a substrate made of a semiconductor or a metal, a buried insulating layer formed on the substrate, a semiconductor layer including a p-type semiconductor layer and an n-type semiconductor layer formed to be arranged along a predetermined region on the buried insulating layer, a gate insulating layer formed on the semiconductor layer, and a diffraction grating portion disposed on the gate insulating layer and having groove portions formed two-dimensionally in a planar conductive member;
a light source configured to emit linearly polarized light having a predetermined wavelength;
an optical system configured to include a photoelastic modulator, convert the light such that two states in which the light becomes linearly polarized light beams of two orthogonal directions are alternately repeated at a predetermined frequency, and guide the converted light to the photodiode; and
an amplifier configured to perform lock-in detection on electric signals output from the photodiode when the light is incident on the photodiode through the optical system, wherein the lock-in detection comprises detecting a differential voltage of the electrical signals at two timings, at the predetermined frequency, when the light becomes the linearly polarized light beams of the two orthogonal directions,
wherein the groove portions are arranged at a predetermined first grating pitch in a first direction and are arranged at a predetermined second grating pitch in a second direction crossing the first direction.
2. The refractive index measuring device according to claim 1 , further comprising a refractive index measuring device which is configured to calculate a refractive index based on the electric signals that are lock-in detected by the amplifier and based on a difference in quantum efficiency associated with a reference refractive index.
3. The refractive index measuring device according to claim 2 , wherein
two arrangement directions of the groove portions formed two-dimensionally are orthogonal to each other in the diffraction grating portion,
the refractive index measuring device further comprising a sample/hold circuit configured to be provided at a front step of the amplifier and sample/hold the electric signals at each time when the light becomes linearly polarized light beams of directions along the arrangement directions.
4. The refractive index measuring device according to claim 1 , wherein
in the diffraction grating portion, the groove portions are formed in a two-dimensional lattice shape and a period of the groove portions and an incidence angle of the light with respect to the diffraction grating portion are set such that the electric signals with respect to the linearly polarized light beams of the two orthogonal directions are equalized with respect to a wavelength of a predetermined light source and a predetermined reference refractive index, and
the amplifier performs the lock-in detection on the electric signals at time when the light becomes linearly polarized light beams of directions along two orthogonal arrangement directions of the groove portions formed in the two-dimensional lattice shape.
5. The refractive index measuring device according to claim 1 , wherein the optical system includes a ¼ wavelength plate at least between the light source and the photoelastic modulator or between the photoelastic modulator and the photodiode.
6. The refractive index measuring device according to claim 1 , further comprising a logarithmic conversion circuit configured to be provided at a rear step of the photodiode and logarithmically convert the electric signals.
7. The refractive index measuring device according to claim 1 , wherein the first direction and the second direction are orthogonal to each other.
8. The refractive index measuring device according to claim 1 , wherein the first grating pitch and the second grating pitch are equal to each other.
9. The refractive index measuring device according to claim 1 , wherein each of the groove portions has an opening of a square shape.
10. The refractive index measuring device according to claim 1 , wherein
two arrangement directions of the groove portions formed two-dimensionally are orthogonal to each other in the diffraction grating portion, and
further comprising a refractive index measuring device which is configured to calculate a refractive index based on the electric signals that are lock-in detected by the amplifier and based on a difference in quantum efficiency associated with a reference refractive index,
wherein the refractive index measuring device comprises a sample/hold circuit configured to be provided at a front step of the amplifier and sample/hold the electric signals at each time when the light becomes linearly polarized light beams of directions along the arrangement directions.