JPH05173019A - Polarization canceler - Google Patents

Abstract

PURPOSE:To narrow a full-width at half maximum in an infrared area by engraving specific grating grooves on the surface of an anisotropic crystal so that the depth sum of grooves on a crystal with 2d thickness is twice the depth sum of grooves on a crystal with (d) thickness. CONSTITUTION:Two anisotropic crystals A, B having respectively different thickness d, 2d are arranged so that an 45 deg. angle is made by respective optical axis directions. Since the thickness of the crystal B is twice that of the crystal A, the B always applies twice the phase difference of the A independently of incident wavelength. Grating grooves with a sufficiently short period (L1+L2) as compared with working wavelength are engraved on the surface of the crystal A with the thickness (d) in the direction parallel with the lagging axis of the crystal A so as to obtain total depth (t) on all the sides of the crystal A. On the other hand, grooves with total depth 2t are engraved on the surface of the crystal B with the thickness 2d. Since the groove direction is set up in parallel with the lagging axis, the grooves are parallel with the optical axis when the crystal is an optically uniaxial positive crystal or vertical to the optical axis in the case of a negative crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Rio-type depolarizer used in a measurement device using a photoelectric detector, an optical experiment, or the like, in a measurement in which the polarization characteristic of the detector is a problem.

[0002]

Photoelectric detectors generally have polarization characteristics. There is no problem if the light incident on the detector is isotropic light such as unpolarized light or circularly polarized light, but when anisotropic light such as linearly polarized light or elliptically polarized light is incident, the polarization characteristics of the detector Therefore, even if the light has the same energy, the output varies depending on the vibration direction. Such a polarization characteristic causes a significant inconvenience to the measurement, but in order to avoid this influence, it is necessary to convert the light into an isotropic form and make it incident.

For depolarization, a Rio-type depolarizer, a scattering plate, an integrating sphere, etc. are usually used. However, the scattering plate has a fatal defect that the amount of incident light is reduced, and the integrating sphere has a defect that the device becomes bulky. In that respect, in principle, there is no loss of light quantity, and a compact Rio-type depolarizer is the most excellent, but if this incident light does not have a sufficient wavelength width, a sufficient elimination effect cannot be obtained. There is a drawback.

As shown in FIG. 3, in the Rio depolarizer, two anisotropic crystals A and B having thicknesses d and 2d are arranged such that the directions of their optical axes are 45 ° to each other. Since the crystal B has a thickness twice that of the crystal A, the crystal B always gives a phase difference of A to A for any incident wavelength.

The central wavelength of the incident light is λ ₀ , and the half-value width of the wavelength is Δ.
If λ ₀ , in order for the Rio-type depolarizer to exert a sufficient elimination effect, λ _{0 is} required during transmission through the thickness d of the crystal A.
_{-Δλ 0/2 (= λ 1} ) of the wavelength of light and λ ₀ + Δλ _0/2
The phase difference difference δ _A (λ ₁ ) −δ _A (λ ₂ ) given between the ordinary ray and the extraordinary ray of the light of wavelength (= λ ₂ ) must be 2π or more (BH Billing S, J.Opt. .Soc.Am. 41 (1
951) 966).

That is, the principal refractive indices of the ordinary ray and the extraordinary ray of the anisotropic crystal are n _w (λ) and n _e (λ), respectively, and Δ
If n (λ) = | n _e (λ) −n _w (λ) | is set, then δ _cr (λ) = 2π / λΔn (λ) d (1), so δ _cr (λ ₁ ) −δ _cr ( λ ₂ ) = 2πd {Δn (λ ₁ ) / λ ₁ −Δn (λ ₂ ) / λ ₂ } ≧ 2π (2) must be satisfied.

Wavelength half-width Δλ ₀ required for incident wavelength
It can be said that the smaller is the better depolarizer.
As is clear from equation (2), the larger d is, the more Δn (λ)
Is larger or λ ₀ is smaller, Δλ ₀ can be smaller. However, the required half-width becomes wider as the wavelength becomes longer.

For example, in the case of an artificial quartz Rio depolarizer with d = 1 mm and 2d = 2 mm, the relationship between the incident wavelength λ ₀ and the minimum half-value width Δλ ₀ can be calculated from the equation (2) as shown in FIG. 4a. Become.

[0009]

In the short wavelength region such as ultraviolet light, the required half width may be narrow,
When the wavelength used is in the long wavelength region such as infrared light, a wide half width is required.

However, there is a limit to widen the half-value width of the incident light, and in the case of spectroscopic measurement, the measurement itself may become unreliable. That is, the conventional Rio-type depolarizer has a drawback that it cannot obtain a sufficient effect in the infrared region. In order to overcome the drawbacks of the conventional Rio-type depolarizer, the present invention has an object to narrow the half width required especially in the infrared region.

[0011]

The structure of the Rio-type depolarizer of the present invention will be described with reference to FIGS. As shown in FIG. 1, the basic structure of the resolver composed of two crystal plates A and B is the same as that of the conventional Rio type, but in the present invention, a crystal having a large number of grooves formed on the surface is used. FIG. 2 shows a cross section of a grating groove carved on the surface.

Lattice grooves having a period (L ₁ + L ₂ ) sufficiently shorter than the used wavelength λ ₀ are formed on the surface of the crystal having a thickness d in a direction parallel to the slow axis of the crystal regardless of whether the crystal is on one side or both sides. The total depth is t, and the surface of the crystal having a thickness of 2d is totaled with a depth of 2t. Since the direction of the groove is parallel to the slow axis, when the crystal is a uniaxial positive crystal, it is parallel to the optical axis and when it is a negative crystal, it is perpendicular to the optical axis.

[0013]

The larger one of the two principal refractive indices n _w and n _e of the anisotropic crystal is n _s , and the smaller one is n _f . n _s represents the refractive index of light whose electric vector oscillates in the direction parallel to the slow axis, and n _f represents the refractive index of light oscillating in the direction parallel to the fast axis. In the positive crystal, n _s = n _e and n _f = n _w , and in the negative crystal, n _s = n _w and n _f = n _e . However, the above-mentioned refractive indexes all have dispersion, and are a function depending on the incident wavelength.

A crystal having a structure whose cross section is as shown in FIG. 2 has optical anisotropy caused by the physical properties of the crystal and anisotropy caused by the structure (M. Born and E. Wolf). .Principle
s of Optics (Pergamon, New York) P.707).

That is, the refractive index of light oscillating in the direction parallel to the slow axis is n ₂ , and the refractive index of light oscillating in the vertical direction is n.
_{If it is 1} , we can write as follows.

N ₂ (λ) = ({n ² _s (λ) -1} q + 1) ^1/2 (3) n ₁ (λ) = 1 / ({1 / n ² _f (λ) -1} q + 1 ) ^1/2 (4) However, q = L ₁ / (L ₁ + L ₂ ) (5) A groove of total depth t is carved in the crystal of thickness d, but the phase difference δ _st (λ ) Becomes δ _st (λ) = 2π / λ {n ₂ (λ) -n ₁ (λ)} t (6). Further, since the phase difference due to the anisotropy of the crystal itself is δ _cr (λ) = 2π / λ | n _e (λ) −n _w (λ) | d (7), the crystal A having the thickness d is transmitted. The sum of the phase differences δ _A (λ) that light receives is δ _A (λ) = δ _st (λ) + δ _cr (λ) = 2π / λ [{n ₂ (λ) -n ₁ (λ)} t + | n _e (λ) −n _w (λ) | d] (8).

On the other hand, in the crystal B, t → 2t, d in (8)
→ 2d can be replaced, so the phase difference is δ _B (λ) = 4π / λ [{n ₂ (λ) −n ₁ (λ)} t + | n _e (λ) −n _w (λ) | d] = 2δ _A (λ) (9) Although n ₂ , n ₁ , n _e , and n _w are all wavelength-dependent quantities, the relationship δ _B = 2δ _A always holds regardless of the wavelength.

The minimum full width at half maximum required for this cancellation to function sufficiently is δ _A (λ ₁ ) −δ _A (λ ₂ ) = δ _cr (λ ₁ ) −δ _cr (λ ₂ ) + δ _st (λ ₁ ) − δ _st (λ ₂ ) ≧ 2π (10), but as is clear from comparison with Eq. (2), the difference in retardation due to structural anisotropy δ _st (λ ₁ ) − δ _st (Λ
_{With the addition of 2} ), the full width at half maximum can be narrower than the conventional resolver.

[0019]

[Example] Two artificial crystal plates A having a thickness of 1 mm and 2 mm,
Each of the crystal plates of the Rio-type depolarizer made of B has a depth of 1 μm on each side and 2 μm on each side, and a total of 2 μm and 4 μm.
Consider the case where the grid grooves of (1) are engraved at the same density of 1200 lines / mm. Since the artificial quartz is a positive crystal, the groove direction is parallel to the optical axis direction. If the ratio of L ₁ : L ₂ is 1: 1, then L ₁ = L ₂ = 417 nm, and if the wavelength used is infrared light with λ = 1200 nm or more, λ >>
It becomes L ₁ and L ₂ , and structural anisotropy can occur. The lattice grooves of 1200 lines / mm can be manufactured by using the holographic exposure method and the ion beam etching technique.

The relationship between the incident wavelength λ ₀ and the required full width at half maximum Δλ ₀ for this canceller can be calculated from the equation (10) and becomes as shown in FIG. 4b.

In the embodiment, the case was considered in which grooves having a depth of 1 μm and 2 μm were engraved on both surfaces of two crystal plates A and B, respectively. It may be 2 μm or 4 μm each, or may be a marking line having a depth of 2 μm on one side of A or both sides of B.
Anyway, the total depth of the grooves engraved on the A and B crystal plates should be in a ratio of 1: 2.

[0022]

The Rio-type depolarizer is required to have a full width at half maximum in accordance with the incident wavelength in order to obtain a sufficient elimination effect.
Since the full width at half maximum becomes wider as the wavelength becomes longer, the conventional canceller was difficult to use in the infrared region, but the present invention can narrow the full width at half maximum required particularly in the infrared region. It became possible, and the usable wavelength range of the Rio depolarizer was expanded.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a Rio-type depolarizer of the present invention.

FIG. 2 is a cross-sectional view of lattice grooves engraved on the crystal surface of the Rio-type depolarizer of the present invention.

FIG. 3 is a block diagram of a conventional Rio-type depolarizer.

FIG. 4 is a graph showing the relationship between the incident center wavelength and the required minimum half value width for the conventional type and the Rio type depolarizers of the examples of the present invention.

[Explanation of symbols]

 A, B ... Anisotropic crystal of thickness d, 2d

Claims (1)

[Claims] 1. A Rio-type depolarizer composed of two anisotropic crystals of thickness d and 2d, each of which has a surface parallel to the slow axis and is sufficiently smaller than the wavelength used. A lattice pitch of a short pitch is engraved on one side or both sides, and the total depth of the grooves engraved on the crystal with a thickness of 2d is the total depth of the grooves engraved on the crystal with a thickness of d. A depolarizer characterized by being twice as large as