Phase modulation device and laser microscope
A phase modulation device corrects wave front aberrations generated by an optical system including an objective lens disposed on an optical path of a light flux. The phase modulation device includes a phase modulation element which includes a plurality of electrodes, and modulates the phase of the light flux in accordance with a voltage applied to each electrode, and a control circuit which controls the voltage to be applied to each electrode. The control circuit controls the voltage to be applied to each electrode in such a manner that the light flux is imparted with a phase modulation amount in accordance with a phase modulation profile having a polarity opposite to the polarity of a phase distribution to be determined according to a relational equation representing a relationship between a numerical aperture of the objective lens and a ratio between third-order spherical aberration and fifth-order spherical aberration.
1. A phase modulation device for correcting wave front aberrations generated by an optical system including an objective lens disposed on an optical path of a light flux of coherent light to be emitted from a coherent light source, comprising:
a phase modulation element which comprises a plurality of annular electrodes in a concentric form, the center of which is an optical axis of the optical system, and modulates a phase of the light flux transmitting through the objective lens in accordance with a voltage applied to each of the annular electrodes; and
a control circuit which controls the voltage to be applied to each of the plurality of annular electrodes, wherein
the control circuit controls the voltage to be applied to each of the plurality of annular electrodes in such a manner that the light flux is imparted with a phase modulation amount in accordance with a phase modulation profile which has a polarity opposite to a polarity of a phase distribution of the wave front aberrations and is determined according to a relational equation representing a relationship between a numerical aperture of the objective lens and a ratio between third-order spherical aberration and fifth-order spherical aberration when the phase distribution of the wave front aberrations is resolved using Zernike polynomials; and
the control circuit applies a voltage to one of the annular electrodes corresponding to a first position where the phase modulation amount of the phase modulation profile is maximum in such a manner that a maximum value of the phase modification amount is generated on the first position and applies a voltage to another one of the annular electrodes corresponding to a second position where the phase modulation amount of the phase modulation profile is minimum in such a manner that a minimum value of the phase modulation amount is generated on the second position.
2. The phase modulation device according to claim 1 , wherein the objective lens is an immersion objective lens, and the numerical aperture of the objective lens satisfies the following condition:
1.15≦NA≦1.27
where NA represents the numerical aperture of the objective lens, and the relational equation is represented by the following equation:
1.91
NA
-
1.87
-
1.4
A
2
≤
B
A
≤
1.91
NA
-
1.87
+
1.4
A
2
where A represents a third-order spherical aberration component, B represents a fifth-order spherical aberration component, and NA represents the numerical aperture of the objective lens.
3. The phase modulation device according to claim 1 , wherein the objective lens is an immersion objective lens, and the numerical aperture of the objective lens satisfies the following condition:
1.05≦NA≦1.27
where NA represents the numerical aperture of the objective lens, and the relational equation is represented by the following equation:
1.37
NA
-
1.21
-
1.4
A
2
≤
B
A
≤
1.37
NA
-
1.21
+
1.4
A
2
where A represents a third-order spherical aberration component, B represents a fifth-order spherical aberration component, and NA represents the numerical aperture of the objective lens.
4. The phase modulation device according to claim 1 , wherein the objective lens is a dry objective lens, and the numerical aperture of the objective lens satisfies the following condition:
0.75≦NA≦0.95
where NA represents the numerical aperture of the objective lens, and the relational equation is represented by the following equation:
1.06
NA
-
0.64
-
1.4
A
2
≤
B
A
≤
1.06
NA
-
0.64
+
1.4
A
2
where A represents a third-order spherical aberration component, B represents a fifth-order spherical aberration component, and NA represents the numerical aperture of the objective lens.
5. The phase modulation device according to claim 1 , wherein the phase modulation profile is determined in such a manner that a phase modulation amount on an optical axis of the optical system is equal to a phase modulation amount at an end of an active region, the active region being a region capable of phase-modulating a light flux on the phase modulation element.
6. The phase modulation device according to claim 1 , wherein the phase modulation profile is determined in such a manner that a root mean square value of the phase modulation profile is minimized.
7. The phase modulation device according to claim 1 , wherein the phase modulation element is a liquid crystal element.
8. The phase modulation device according to claim 7 , wherein the control circuit adjusts the phase modulation profile in accordance with a wavelength of the coherent light.
9. The phase modulation device according to claim 1 , wherein
the annular electrodes are connected to each other by one or more resistors, and
a voltage applied to each of the annular electrodes corresponding to a position other than the first and second positions being determined by dividing a difference between the voltage to be applied to the annular electrode whose phase modulation amount is maximum, and the voltage to be applied to the annular electrode whose phase modulation amount is minimum, by a resistance value of corresponding resistor connected between the annular electrodes.
10. The phase modulation device according to claim 1 , wherein
the control circuit applies a voltage to an outermost peripheral annular electrode, as well as to the annular electrodes corresponding to the first position where the phase modulation amount is maximum and corresponding to the second position where the phase modulation amount is minimum.
11. A laser microscope, comprising:
a coherent light source which irradiates coherent light;
a first optical system disposed on an optical path of a light flux of the coherent light, and including an objective lens to focus the light flux on a specimen;
a second optical system which transmits a light flux including specimen information derived from the specimen to a detector; and
the phase modulation device of claim 1 , wherein
the phase modulation element of the phase modulation device is disposed between the coherent light source and the objective lens.