JPH0345330B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polarization measuring device, and in particular, the present invention relates to a polarization measurement device, and in particular, to a polarization measurement device that measures a minute rotation angle of a polarization plane based on an AC differential amplification method.
This invention relates to a device that can perform measurements easily and efficiently.

As shown in Figure 1, the conventional method for measuring the rotation angle of the polarization plane using a polarization azimuth oscillator is as follows:
When the light from the sample C is polarized by a polarizer P, and the polarization plane of the light emitted from the sample C is oscillated at the frequency ω of the oscillator G using a polarization azimuth oscillator H, the analyzer is perpendicular to the polarizer. There is a method that takes advantage of the fact that the ω component of the frequency of light emitted from A is proportional to the angle of rotation of the plane of polarization by sample C. In this method, in order to increase the ω component of the signal, that is, the frequency, by increasing the width of the polarization azimuth oscillator H, when the rotation angle of the polarization plane is small, the ω component is independent of the rotation angle of the polarization plane. Since the second harmonic component of ω becomes much larger, it becomes noise and reduces the S/N of the output signal of photodetector D.
There were flaws that made it worse. In other words, the output E of the detector D is given by the angle of rotation of the plane of polarization by the sample C as θ, the amplitude A of the polarization azimuth oscillator H, and the frequency ω. Under crossed Nicols between the polarizer P and analyzer A, then E=Ksin ²
(θ+Asinωt), but when θ, A《1, E
KA ² /2 + 2KA θ sin ωt - (KA ² /2) cos 2 ωt, and when the amplitude A is increased in order to increase the ω component, the 2ω component, which is unrelated to θ, increases in A ² and deteriorates the S/N.

In addition, the signal from the conventional photodetector D is passed through a lock-in amplifier (Lock in Amp) LA to reduce the frequency to ω.
A method of extracting only the ω component is also known, but this is affected by the performance of the lock-in amplifier LA (extracting only the ω component, excluding the 2ω component), and increasing the amplitude of the polarization azimuth oscillator H reduces the dynamic The drawback was that it exceeded the range.

This method also had the disadvantage that a time constant was applied at the signal processing stage in the lock-in amplifier, making high-speed readout extremely difficult.

The purpose of the present invention is to use a polarization azimuth oscillator to measure minute rotation angles of the plane of polarization, divide the light into light that passes through the sample, and light that does not pass through the sample, and measure the difference in the final electrical signal output of each. It is an object of the present invention to provide a polarization measurement method that improves the S/N ratio by removing harmonic components unrelated to the rotation angle of the polarization plane by a sample of interest.

Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 2 shows a configuration diagram of an embodiment of the present invention, and parts equivalent to those in FIG. 1 are given the same reference numerals. In Figure 2, B receives the output light of the polarization orientation oscillator H, and generates two light beams.
A beam splitter that separates into L ₁ and L ₂ , A ₁ and A ₂ are analyzers arranged so that the polarization direction is orthogonal to the polarizer P, and F receives the output of photodetectors D ₁ and D ₂ . It is a differential amplifier.

Light from a light source S is polarized by a polarizer P, its polarization plane is vibrated at a frequency ω using a polarization azimuth oscillator H, and the beam splitter B separates it into two light beams L ₁ and L ₂ . As described in connection with FIG. 1, the beam L ₁ passes through the sample C and the analyzer A ₁ and then enters the photodetector D ₁ . The output signal I ₁ of photodetector D ₁ is
It is as shown by equation (1).

I ₁ = K ₁ sin ² (θ + Asinωt) ...(1) Also, after the light beam L ₂ is polarized by the analyzer A ₂ ,
Input to photodetector _D2 . The output signal from this photodetector D ₂ is I ₂ =K ₂ sin ² (Asinωt) (2).

The gain of the differential amplifier is A′=K/K ₁ , A″=K/K ₂
Then, the output E of the differential amplifier is E=K
{sin ² (θ+Asinωt)−sin ² (Asinωt)}=Ksinθsi
n
(θ+2Asinωt), where θ, A《1, EKθ ² −
2KAθsinωt. This result does not include the 2ω component, indicating that even when θ is small, it is possible to detect θ with a good S/N ratio by increasing the amplitude A of the polarization azimuth oscillator within the range where the approximation holds. There is.

FIG. 3 shows an example of application to a magneto-optical memory reading device. In Figure 3, S _L is a laser light source,
H _F is a farad cell, J is an optical lens, K is a magnetic disk, and T is a low-pass filter. Laser light from a laser light source S passes through a polarizer P and becomes linearly polarized light.Furthermore, the plane of polarization passes through a Faraday cell H _F , where the plane of polarization is modulated at a frequency ω that is sufficiently larger than the bit frequency of the memory. It is separated into two beams. One beam passes through the analyzer A ₁ orthogonal to the polarizer P and enters the photodetector D ₁ as a reference beam. The other one is reflected by the magnetic disk K, and depending on the magnetization state ±M, ±θ
The plane of polarization is rotated by the amount of light that passes through detector A ₂ and enters photodetector D ₂ . In this case, analyzer A ₂ has magnetization −
If it is placed perpendicular to the polarization plane of the reflected light from the M state, the polarization plane of the reflected light from the +M state will be relative to A ₂ .
It will rotate by 2θ. Therefore, the photodetector
By passing the outputs of D ₁ and D ₂ through differential amplifier F,
The high frequency component of ω is canceled out from the output, and the state ±M of the magneto-optical memory K is such that the ω component is detected as 0 (-M) or 4KAθ (+M), that is, a binary code of 0 or 1, The S/N ratio of the signal becomes good. In this case, even if θ is small, by increasing the modulation amplitude A of the Faraday cell, the S/N can be increased without applying a time constant in the signal processing stage. Reading is possible.

When using a photodetector as a photodetector, the root mean square value of the signal current in the polarization plane modulation method L ^-2 _S =
1/2 (P _O eηhν4θA) ² (where P ₀ : laser light power of laser light source S ₀ , η : quantum efficiency, θ : polarization plane rotation angle by magnetic disk K, A : modulation amplitude), shot noise Root mean square value = 2eP _O eη/hν×A ² /2×Δν(Δ
ν: band width of the entire detector), S/N=P _O 8θ ² /hνΔν
/η, and the minimum detected rotation angle θmin at which S/N=1
is A=5°, Δν=10MHz, P _O =10MW (λ=0.63μ
m), η = 0.1, it becomes 4″.

FIG. 4 shows another embodiment of the present invention,
The difference from Fig. 1 is that two polarization azimuth oscillators are used, one by H 1 after being reflected by beam splitter B, and the other by H ₁ after passing through sample C.
The point is that the plane of polarization is modulated by _H2 . In Fig. 4, A ₃ and A ₄ are analyzers, D ₃ and D ₄ are photodetectors, and F' is a differential amplifier. As shown by the thick line in Figure 4, the output of the differential amplifier F' is connected to the lock-in amplifier.
The phase is detected by LA, which is then negatively fed back to the polarization orientation oscillator _H2 through the DC amplifier DA. At this time,
If the loop gain A from the photodetector D ₃ to the DC amplifier DA is sufficiently large (A≫1), the output signal e ₀ of the DC amplifier DA is e ₀ = A/1 + CAθiθi/C (C is the unit output signal θi is the rotation angle of the polarization plane of the polarization _azimuth oscillator for
It becomes independent of the system's lube gain and light source intensity.
Therefore, the effects of fluctuations due to light source fluctuations, amplifier drift, etc. are removed. In addition, by increasing the time constant (τ) of the lock-in amplifier, the band width (1/τ) of the detection system can be made very narrow, which reduces the shot noise of the photodetector and increases the S/N ratio.
It becomes possible to measure the polarization angle of

According to the present invention, when measuring the rotation angle of the polarization plane, in order to differentially remove the second harmonic component of the modulation frequency that is unrelated to the rotation angle, the modulation amplitude is increased and the S/N Since it is possible to take a large value, there is an advantage that the rotation angle of the plane of polarization can be easily and accurately measured.

[Brief explanation of drawings]

Fig. 2 is a principle diagram of the measurement system using this method, Fig. 1 is a measurement system diagram of the conventional method, and Fig. 3 is an embodiment of the present invention.
FIG. 4 shows a modification of the present invention. S: light source, P: polarizer, H: polarization azimuth oscillator,
B: Beam splitter, C: Sample, A ₁ , A ₂ : Analyzer, D ₁ , D ₂ : Photodetector, F: Differential amplifier,
J: Lens, K magnetic disk, H _F : Faraday cell, T: Low pass filter, LA: Lock in
AmP, DA: DC amplifier.

Claims (1)

[Scope of Claims] 1. a linearly polarized light generator; a first polarization azimuth oscillator that can receive the linearly polarized light from the linearly polarized light generator and rotate the plane of polarization at a predetermined frequency by applying an alternating current signal; A first measurement comprising: a first analyzer that receives the output light of the first polarization orientation oscillator; and a first photodetector that receives the output light of the first analyzer and converts it into an electrical signal. a second analyzer that receives the light that has passed through the sample by irradiating the branched light of the light that has passed through the first polarization orientation oscillator, or the light that has been reflected from the sample; and the second analyzer. a second photodetector that receives the output light of the linearly polarized light and converts it into an electrical signal; a second polarization azimuth oscillator that receives the reflected light from the sample; a third analyzer that receives the output light of the polarization azimuth oscillator; and a third photodetector that receives the output light of the third analyzer. one of the first measurement system, the second measurement system, and the third measurement system of the third measurement system, the first photodetector, and the second light A polarization measuring device comprising: a differential amplifier that receives the output of the detector or the third photodetector and cancels out the predetermined frequency component by taking the difference between the two.