Dispersive device having beam expanding optical system and dispersive element and wavelength selective switch having dispersive device
A dispersive device has a beam expanding optical system which includes first and second prisms each having a pair of faces inclined relative to each other, and expands light containing a plurality of wavelength components by passing the light through each of the faces of the first and second prisms; and a dispersive element which emits the light expanded by the beam expanding optical system, at different diffraction angles by the respective wavelength components. A direction of variation of an output angle of the light emitted from the beam expanding optical system due to temperature change is configured to be a direction to suppress variation of the diffraction angles of the respective wavelength components emitted from the dispersive element due to the temperature change.
1. A dispersive device comprising:
a beam expanding optical system which includes a first prism and a second prism each having a pair of faces inclined relative to each other, and expands light containing a plurality of wavelength components by passing the light through each of the faces of the first prism and the second prism; and
a dispersive element which emits the light expanded by the beam expanding optical system, at different diffraction angles by the respective wavelength components,
wherein a direction of variation of an output angle of the light emitted from the beam expanding optical system due to temperature change is configured to be a direction to suppress variation of the diffraction angles of the respective wavelength components emitted from the dispersive element due to the temperature change.
2. The dispersive device according to claim 1 , wherein a direction of variation of an output angle of the light emitted from the first prism due to the temperature change is identical with a direction of variation of an output angle of the light emitted from the second prism due to the temperature change.
3. The dispersive device according to claim 2 , wherein a plus or minus sign of a refraction angle of the light in the first prism is different from a plus or minus sign of a refraction angle of the light in the second prism, and
wherein plus or minus signs of refractive index changes of the first prism and the second prism to the temperature change are different from each other.
4. The dispersive device according to claim 3 , wherein the beam expanding optical system includes three or more prisms each having a pair of faces inclined relative to each other, and
wherein the first prism and the second prism are two prisms located nearer to the dispersive element out of the three or more prisms.
5. The dispersive device according to claim 4 , wherein the beam expanding optical system includes N prisms where N is an even number of 4 or more, and
wherein the prisms other than the first prism and the second prism are comprised of a same material as one prism of the first prism and the second prism and have the same apex angle as the one prism.
6. The dispersive device according to claim 2 , wherein a plus or minus sign of a refraction angle of the light in the first prism is identical with a plus or minus sign of a refraction angle of the light in the second prism, and
wherein plus or minus signs of refractive index changes of the first prism and the second prism to the temperature change are identical with each other.
7. The dispersive device according to claim 6 , wherein the beam expanding optical system includes three or more prisms each having a pair of faces inclined relative to each other,
wherein the three or more prisms includes the first prism and the second prism, and
wherein an optical axis of the light entering the beam expanding optical system is parallel to an optical axis of the light emitted from the beam expanding optical system.
8. The dispersive device according to claim 6 , wherein plus or minus signs of the refractive index changes of the first prism and the second prism to the temperature change are minus.
9. The dispersive device according to claim 1 , wherein the dispersive element has a diffraction grating, and
wherein when an incident angle of the light to the diffraction grating is θ in , a diffraction angle of each of the wavelength component is θ out , a period of the diffraction grating is Λ g , a beam magnification of the diffraction grating is M g , an angular dispersion of the diffraction grating is D g , a wavelength in vacuum of each of the wavelength component is λ, a temperature of the diffraction grating is T, and a refractive index of a medium around the first prism and the second prism is no, a temperature dependence dθ out /dT of the diffraction angle θ out expressed by the formula below is zero
ⅆ
θ
out
ⅆ
T
=
1
M
g
ⅆ
θ
in
ⅆ
T
-
D
g
λ
{
1
n
0
Λ
g
ⅆ
ⅆ
T
(
n
0
Λ
g
)
}
.
10. The dispersive device according to claim 9 , wherein a change of the refractive index no of the medium around the first prism and the second prism is taken into account in the formula.
11. The dispersive device according to claim 1 , wherein a variation of the output angle of the light from the beam expanding optical system due to the temperature change is set so as to substantially zero variations of the diffraction angles of the respective wavelength components emitted from the dispersive element due to the temperature change.
12. A wavelength selective switch comprising:
the dispersive device according to claim 1 ;
an input section which inputs the light containing the plurality of wavelength components;
a condensing element which condenses the plurality of wavelength components emitted from the dispersive device;
a light deflecting element which independently deflects the plurality of wavelength components condensed by the light condensing element by the respective wavelength components; and
an output section which outputs the plurality of wavelength components deflected by the light deflecting element by the respective wavelength components.