JPH085459A - Michelson interferometer - Google Patents

Abstract

PURPOSE:To realize an optical measuring device, which can always accurately measure the optical power spectrum regardless of the condition that the incident light is polar ized. CONSTITUTION:A device is provided with a first light receiving unit 15a, which receives one of the outgoing beams of two directing after the interference for synthesis of the reflected light from a movable reflecting mirror 13 and a fixed reflecting mirror 14 by a beam splitter 12 and which converts it into the electrical signal 11 and supplies it to a polarization correcting circuit 20, and a second light receiving unit 15b, which receives the other outgoing beams of the two directions after the interference for synthesis of the reflected beam from the movable reflecting mirror 13 and the fixed reflecting mirror 13 by the beam splitter 12 and which converts it into the electrical signal 12 and supplies it to the polarization correcting circuit 20. The device is also provided with a polarization correcting circuit 20, which receives the signal 11 from the first light receiving unit 15a and the signal 12 from the second light receiving unit 15b and generates the signal 11LP obtained by filtering the high area of the signal 11 in a LPF 16a and the signal 11HP obtained by filtering the low area of the signal 11 in a LPF 16b and which performs the computing {11+12X11HP/11LP}.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device for obtaining an optical interference waveform and an optical power spectrum which are not affected by an error in measurement data due to the polarization state of incident light in the field of optical measurement.

[0002]

2. Description of the Related Art Measurement of an optical power spectrum by a Michelson interferometer according to the prior art will be described with reference to an example of an optical measurement block diagram shown in FIG. The configuration is a beam splitter 12, a movable reflecting mirror 13, a fixed reflecting mirror 14,
Light receiver 15, HPF 16, AD converter 17, and FF
The T processing unit 18 and the display unit 19 form a measurement system.

The beam splitter 12 receives the measurement incident light E0.
In response to the incident light, the light is bifurcated into reflection / transmission on the incident surface of 45 degrees and emitted toward the movable reflecting mirror 13 and the fixed reflecting mirror 14. Then, both the light rays reflected by the movable reflecting mirror 13 and the fixed reflecting mirror 14 are made incident again on the incident surface of 45 degrees to cause a synthetic interference of both the light rays. The light beams after the interference become the emitted lights E1 and E2. The fixed reflecting mirror 14 receives the reflected light beam from the beam splitter 12, reflects it with a fixed optical path length L1, and returns it. On the other hand, the movable reflecting mirror 13 has a structure in which an arbitrary optical path length L2 can be set by an external driving means and an arbitrary moving speed can be given. The optical path difference between the two is continuously changed so that the beam splitter 12 can generate an interference fringe having a desired wavelength. The light receiver 15 receives the light beam E1 after the above interference and converts it into an electric signal. In this electric signal, the AC amplitude component which is the interference fringe is filtered by the high-pass filter HPF16 and taken out, and this is converted into a digital signal by the AD converter 17, and then the FFT processing unit 1
Fourier transform is performed at 8, and the power spectrum of the measurement incident light corresponding to the wavelength is displayed at the display unit 19.

By the way, the beam splitter 12 has a polarization dependency in which the reflectivity / transmittance changes depending on the polarization state. Due to this dependency, even if the incident power is the same, if the polarization state is different, the measured output level will change. This will be described below by showing a calculation formula. With the symbol of the formula, the amplitude of the input light is E0
The amplitude reflectivity of the beam splitter 12 for P-polarized light is √ (Rp), the amplitude transmissivity is √ (Tp), the amplitude reflectivity for S-polarized light is √ (Rs), and the amplitude transmissivity is √.
(Ts). The wave number K of the incident light is K = 2π / λ, where λ is the wavelength. Calculation formulas based on these are the following formulas 51 to 56.

[0005]

[Equation 1]

In this calculation formula, the total incident light intensity incident on the beam splitter 12 is formula 51. Of the light rays received by the light receiver 15, the amplitude output E1p of the P-polarized component is
And the amplitude output E1S of the S-polarized component is expressed by equation 53. From these formulas, the intensity I1P of the optical power P component becomes the formula 54, and the intensity I1S of the optical power S component becomes the formula 55. The total light intensity I1 received by the photodetector 15 is calculated as the sum of both P and S components and is represented by Expression 56.

From this equation, the strength of both I1P and I1S is
It can be seen that it changes depending on the polarization state. That is, even with the same incident light intensity, RPTP in the expression of Equation 54 and RSTS in the expression 55 are parameters that change depending on the polarization state, and the relationship between them is RPTP ≠ RSTS. As a result, it means that the electric signal output level in the light receiver 15 varies depending on the polarization state. This change in output level is
Depending on the polarization characteristics of the beam splitter 12,
For example, it may change by about 3 dB. For this reason, in an application in which measurement is performed while changing the polarization state, an error factor on the measuring instrument side is large, which is inconvenient in use.

Here, in order to express the ratio of the polarization state, if the ratio on the S-polarized side is M and the ratio on the P-polarized side is (1-M), the light intensity on the S-polarized side is (E0S) 2 = M (E0) 2,
The light intensity on the P-polarized side is (E0P) 2 = (1-M) (E0) 2. Substituting these into the above equations 54, 55 and 56,
The intensity I1P of the P component is given by equation 61, and the intensity I1S of the S component is
Is given by equation 62 and the total light intensity I1 which is the sum of both P and S components
Becomes the equation 63, and these equations are shown below.

[0009]

[Equation 2]

In the calculation formula 63, the right side is substituted as S (M) as in the polarization-dependent portion of the formula 64 in order to simplify the expression. Due to this polarization dependent element S (M), the output level at the light receiver 15 changes.

[0011]

As described above, when the polarization state of incident light changes, the optical power spectrum may not be measured correctly due to the polarization dependent element S (M), which may cause a measurement error. . This imposes a practical limitation on the measurement of a light source whose polarization state changes, which is inconvenient in practice and is not preferable. Therefore, an object of the present invention is to realize an optical measuring device that can always measure a correct optical power spectrum regardless of the polarization state of incident light.

[0012]

FIG. 1 shows a first solution according to the present invention. In order to solve the above problems, in the configuration of the present invention, the movable reflecting mirror 13 and the fixed reflecting mirror 1
The first light receiver 15a which receives one of two outgoing light rays after the interference of the reflected light from the beam splitter 4 by the beam splitter 12 and converts it into an electric signal I1 and supplies it to the polarization correction circuit 20.
Is provided to receive the other of the outgoing light rays in two directions after the reflected light from the movable reflecting mirror 13 and the fixed reflecting mirror 14 are interference-combined by the beam splitter 12, and convert them into an electric signal I2 and supply them to the polarization correction circuit 20. The second photoreceiver 15b is provided to receive the signal I1 from the first photoreceiver 15a and the signal I2 from the second photoreceiver 15b, and the LPF 16a generates a high-pass filtered signal I1LP from the signal I1. At signal I1
Generate a low-pass filtered signal I1HP from {I1 + I
2 × I1HP / I1LP} polarization correction circuit 20 for performing arithmetic processing
Is provided.

FIG. 2 shows a second solution according to the present invention. In order to solve the above problems, in the configuration of the present invention, one of two outgoing light rays after the reflected light from the movable reflecting mirror 13 and the fixed reflecting mirror 14 is interference-combined by the beam splitter 12 and an electric signal is received. First AD after conversion to I1
A first light receiver 15a for supplying to the converter 17a is provided, and the reflected light from the movable reflecting mirror 13 and the fixed reflecting mirror 14 is subjected to interference synthesis by the beam splitter 12, and the other of the two outgoing light beams is received to receive an electric signal. The second AD converter 17 by converting to I2
b is provided with a second light receiver 15b, and the first light receiver 15b is provided.
The first AD converter 17a that receives the signal a and quantizes and converts the signal I1D to the polarization correction calculator 25 is provided, and the signal I2D that is quantized and converted by the signal from the second light receiver 15b is provided.
Second AD converter 17 that supplies 2D to the polarization correction calculation unit 25
b is provided. Then, the signal I1D of the first AD converter 17a
And the signal I2D from the second AD converter 17b,
The F means generates a high-pass filtered signal I1LPD from the signal I1D, and the HPF means generates a low-pass filtered signal I1HPD from the signal I1D. From these, {I1D + I2D × I1HPD
/ I1LPD} is used as a constituent means for providing the polarization correction calculation unit 25 for performing the calculation process of [I1LPD].

FIG. 3 shows a third solution according to the present invention. In order to solve the above problems, in the configuration of the present invention, one of two outgoing light rays after the reflected light from the movable reflecting mirror 13 and the fixed reflecting mirror 14 is interference-combined by the beam splitter 12 and an electric signal is received. First AD after conversion to I1
A first light receiver 15a for supplying to the converter 17a is provided, and the reflected light from the movable reflecting mirror 13 and the fixed reflecting mirror 14 is subjected to interference synthesis by the beam splitter 12, and the other of the two outgoing light beams is received to receive an electric signal. The second AD converter 17 by converting to I2
b is provided with a second light receiver 15b, and the first light receiver 15b is provided.
An analog switch 24 for switching the signal of “a” or the signal of the second light receiver 15b and supplying it to the AD converter 17 is provided. Then, the signal from the analog switch 24 is received, quantized and converted, and the converted signal is subjected to the polarization correction operation unit 2
5 is provided with an AD converter 17, which receives signals I1D and I2D from the AD converter 17 to generate a high-pass filtered signal I1LPD from the signal I1D by the LPF means and a low-pass signal from the signal I1D by the HPF means. Is generated to generate a filtered signal I1HPD, and a polarization correction calculation unit 25 for calculating {I1D + I2D × I1HPD / I1LPD} from these signals is provided.

[0015]

The function is to obtain the total incident power I0 of the measurement light branched by the beam splitter 12 by adding the signals of the first light receiver 15a and the second light receiver 15b. By the polarization correction circuit 20 or the polarization correction calculation unit 25,
By performing the calculation of Expression 104, the polarization dependent element S (M)
Has the effect of removing. As a result, a measurement function that does not depend on the polarization state of the measurement light can be realized.

[0016]

【Example】

(Embodiment 1) An embodiment of the present invention will be described with reference to FIG. 1 in the case of an optical measurement block that eliminates polarization dependence by two light receivers and a polarization correction circuit. The configuration is such that the beam splitter 12, the movable reflecting mirror 13, the fixed reflecting mirror 14, the first light receiver 15a, the second light receiver 15b, the polarization correction circuit 20, the AD converter 17, and the FFT processing unit 1 are provided.
8 and the display unit 19 form a measurement system. The polarization correction circuit 20 includes an adder 21, a divider 22, a multiplier 23, an LPF 16a, and an HPF 16b. The beam splitter 12, the movable reflecting mirror 13, and the fixed reflecting mirror 14, which form the optical system, are the same as in the conventional case. In addition, the FFT processing unit 18 after the AD converter 17,
The display unit 19 is also similar to the conventional one.

The outgoing light E1 of the two outgoing lights E1 and E2 after interference by the beam splitter 12 is the first photodetector 15a.
It is received by and converted into a light intensity signal I1. The other emitted light E2
Is received by the second light receiver 15b and converted into a light intensity signal I2. Both receivers use receivers with uniform sensitivity characteristics. In the polarization correction circuit 20, an arithmetic processing means for receiving the electric signals of both light receivers and eliminating the polarization dependent element S (M) is implemented, and then supplied to the AD converter 17.

The connection receives the light intensity signal I1 from the first photodetector 15a, connects it to one input end of the LPF 16a and HPF 16b and the adder 21, and connects the output end of the LPF 16a to one input end of the divider 22. Connect to, HPF16b
Is connected to one input end of the multiplier 23, receives the light intensity signal I2 from the second photodetector 15b, and receives the adder 21
Connected to the other input terminal of the adder 21, the output terminal of the adder 21 is connected to the other input terminal of the divider 22, and the output terminal of the divider 22 is connected to the other input terminal of the multiplier 23. The output terminal of the multiplier 23 is supplied to the AD converter 17.

The LPF 16a receives the light intensity signal I1 after the first light receiver 15a, and outputs a frequency component I1LP, which is a so-called DC component, lower than the interference fringe frequency signal. On the other hand, the HPF 16b receives the light intensity signal I1 and outputs the interference fringe frequency component I1HP, so-called AC component. The adder 21 receives the signals I1 and I2 from the two photodetectors, adds I0 = I1 + I2, and then supplies them to the divider 22. The divider 22 receives the signal I0 from the adder 21 and L
It receives I1LP from the PF 16a, divides it by (I0 / I1LP), and then supplies it to the multiplier 23. The multiplier 23 receives the signal (I0 / I1LP) from the divider 22 and I1HP from the HPF 16b, multiplies by (I0 / I1LP) × I1HP, and then supplies it to the AD converter 17.

Next, regarding the calculation basis for eliminating the polarization dependence in the polarization correction circuit 20, the following calculation formulas 71 to 71 are given.
Reference numeral 78 will be described.

[0021]

(Equation 3)

In the above formula, the total incident light intensity incident on the beam splitter 12 is formula 71. First light receiver 15
The calculation formula of the emitted light E1 on the a side is the calculation formula of the conventional description, and is shown in Formulas 61, 62, and 63. On the other hand, in the calculation formula of the emitted light E2 on the side of the second light receiver 15b, the amplitude output E2p of the P-polarized component is expressed by the formula 72, and the amplitude output E2S of the S-polarized component is expressed by the formula 73. From these, the intensity I2P of the optical power P component becomes formula 74, and the intensity I2S of the optical power S component becomes
Becomes Equation 75. As a result, the total light intensity I2 received by the second photodetector 15b is obtained as the sum of both P and S components, and is represented by Expression 76. Here, the polarization ratio received on the second light receiver 15b side is calculated using (1-M). This total light intensity I
Assuming that there is no absorption in the beam splitter 12 in Equation 76 of 2, RP + TP = 1, RS + TS = 1, RP2 + T
Since P2 = 1−2RPTP and RS2 + TS2 = 1−2RSTS, these are substituted to obtain the expression 77. In this equation 77, there is the same part as the equation 63, and when this is substituted as I1, the equation 78 is obtained. Assuming that the beam splitter 12 has no absorption, the incident power (E0) 2 = I0 = I1 + I2
The relationship is established.

Electric signal I1 received by the first light receiver 15a side
The relation between the output signal and the output signal after being filtered by the LPF 16a and the HPF 16b is I1 = I1LP + I1HP.
Here, in order to simplify the expression, an element when the optical path length is changed, that is, an AC component element due to interference is defined as A = cos {kn
(L1-L2)}. From this A, equation 63 becomes I1 = I0
It can be expressed as (1 + A) S (M). In this formula, LPF1
The output signal after 6a is: I1LP = I0 × S (M). ． ． The output signal after the HPF 16b corresponds to the expression 101 and has the following formula: I1HP = I0 * S (M) * A. ． ． Corresponds to the expression 102. In order to obtain the power spectrum of the light, the output signal I1HP after the HPF 16b may be Fourier-transformed. However, since this expression includes the polarization dependent element S (M), it cannot be correctly measured as it is. .
Then, by substituting S (M) of the equation 101 into the equation 102, I0 * A = I0 * I1HP / I1LP is obtained. This formula 104 indicates that the measurement does not depend on the polarization state because the polarization dependent element S (M) is not included. It is the polarization correction circuit 20 that executes the calculation of the equation 104.

(Embodiment 2) An embodiment of the present invention will be described with reference to FIG. 2 in the case of an optical measurement block that eliminates polarization dependence by two light receivers, two AD converters, and a polarization correction operation unit. To do. The configuration is the beam splitter 12
A movable reflecting mirror 13, a fixed reflecting mirror 14, a first light receiver 15a, a second light receiver 15b, and a first AD converter 17a.
A second AD converter 17b, a polarization correction calculator 25,
The FFT processing unit 18 and the display unit 19 form a measurement system. A beam splitter 12 forming an optical system,
The movable reflecting mirror 13 and the fixed reflecting mirror 14 are the same as conventional ones. Further, the FFT processing unit 18 and the display unit 19 are also similar to the conventional one.

According to the present invention, instead of the polarization correction circuit 20 implemented by the circuit in the first embodiment, the two AD converters directly quantize the digital signal, and then digitally the above equation 10 is used.
4 is a means for executing polarization correction by a software operation means or a hardware operation means. That is, in the polarization correction calculation unit 25, addition, division, and
The same operation is performed by multiplication, digital LPF, and digital HPF means.

(Application Example) In the above description of the first embodiment, the connection of the polarization correction circuit 20 has been described in the example shown in FIG.
There are other connection configurations for executing the arithmetic expression shown in Formula 104, and if necessary, the addition / multiplication / division operation order may be changed, and the same operation can be performed.

In the above description of the second embodiment, the two AD converters 17a and 17b are used to quantize a digital signal and supply it to the polarization correction operation section 25. However, instead of this, FIG. As shown in, using the 2-input / 1-output analog switch 24, the signals from the light receivers 15a and 15b are alternately switched and input to one AD converter 17 to be quantized into digital signals I1D and I2D. It may be configured as described above, and can be implemented in the same manner.

[0028]

Since the present invention is configured as described above, it has the following effects. By adding the signals of the first light receiver 15a and the second light receiver 15b, it is possible to obtain the total incident power I0 of the measurement light split by the beam splitter 12. The polarization-dependent element S (M) can be deleted by the polarization correction circuit 20 or the polarization correction calculation unit 25, and the measurement independent of the polarization state of the measurement light becomes possible. As a result, the correct optical power level can always be measured, reliable measurement data can be provided, and an optical measurement device having good measurement accuracy can be realized.

[0029]

[Brief description of drawings]

FIG. 1 is an example of an optical measurement block diagram of an optical power spectrum by a Michelson interferometer in which polarization dependence is eliminated by two light receivers and a polarization correction circuit of the present invention.

FIG. 2 is an example of a block diagram of optical measurement of an optical power spectrum by a Michelson interferometer in which polarization dependence is eliminated by two light receivers, two AD converters, and a polarization correction calculation unit according to the present invention.

FIG. 3 is an example of an optical measurement block diagram of an optical power spectrum by a Michelson interferometer in which polarization dependence is eliminated by two light receivers, an analog switching device, one AD converter, and a polarization correction operation unit of the present invention. is there.

FIG. 4 is an example of a conventional optical measurement block diagram of an optical power spectrum by a Michelson interferometer.

[Explanation of symbols]

 12 splitter 13 movable reflecting mirror 14 fixed reflecting mirror 15, 15a, 15b light receiver 16a LPF (low-pass filter) 16, 16b HPF (high-pass filter) 17, 17a, 17b AD converter 18 FFT processing unit 19 display unit 20 polarization correction circuit 21 Adder 22 Divider 23 Multiplier 24 Analog Switch 25 Polarization Correction Calculation Unit

Claims (3)

[Claims] 1. A movable reflecting mirror (13) having a beam splitter (12), a movable reflecting mirror (13) and a fixed reflecting mirror (14) for measuring an optical spectrum of incident light of unknown polarization state. ) And the reflected light from the fixed reflecting mirror (14) are interference-combined by the beam splitter (12) and receive one of the two outgoing light beams and convert them into an electric signal (I1) for polarization correction circuit (20). The first light receiver (15
a) is provided, and the reflected light from the movable reflecting mirror (13) and the fixed reflecting mirror (14) are subjected to interference synthesis by the beam splitter (12), and the other of the outgoing light beams in two directions is received and the electric signal (I2) is received. The second photodetector (15)
b) is provided and receives the signal (I1) from the first light receiver (15a) and the signal (I2) from the second light receiver (15b),
The PF (16a) generates a high-frequency filtered signal (I1LP) from the signal (I1), and the HPF (16b) generates the signal (I1).
Generate a low-pass filtered signal (I1HP) from 1),
A Michelson interferometer characterized by comprising a polarization correction circuit (20) for performing {(I1 + I2) × I1HP / I1LP} arithmetic processing and having the above. 2. A movable reflector (13) having a beam splitter (12), a movable reflector (13) and a fixed reflector (14) for measuring an optical spectrum of incident light of unknown polarization state. ) And the reflected light from the fixed reflecting mirror (14) are interference-combined by the beam splitter (12) and receive one of two outgoing light beams and convert into an electric signal (I1) to convert it into a first AD converter (17a). ) To the first light receiver (1
5a) is provided, and the reflected light from the movable reflecting mirror (13) and the fixed reflecting mirror (14) is subjected to interference synthesis by the beam splitter (12), and the other of the two outgoing light beams is received to receive an electric signal (I2). To the second AD converter (17b) after being converted into
5b) is provided, the first AD converter (17a) that receives the signal from the first light receiver (15a), quantizes and converts the signal, and supplies the signal (I1D) to the polarization correction calculation unit (25) is provided, A second AD converter (17b) is provided, which receives the signal from the second photodetector (15b), quantizes and converts the signal, and supplies the signal (I2D) to the polarization correction calculation unit (25). By receiving the signal (I1D) of (17a) and the signal (I2D) from the second AD converter (17b),
The LPF means generates a high-frequency filtered signal (I1LPD) from the signal (I1D), the HPF means generates a low-pass filtered signal (I1HPD) from the signal (I1D + I2D) × I1HPD / I1LPD} A Michelson interferometer, which is provided with a polarization correction operation unit (25) for performing operation processing of [I1LPD] and is equipped with the above. 3. A movable reflecting mirror (13) having a beam splitter (12), a movable reflecting mirror (13) and a fixed reflecting mirror (14) for measuring an optical spectrum of incident light of unknown polarization state. ) And the reflected light from the fixed reflecting mirror (14) are interference-combined by the beam splitter (12) and receive one of two outgoing light beams and convert into an electric signal (I1) to convert it into a first AD converter (17a). ) To the first light receiver (1
5a) is provided, and the reflected light from the movable reflecting mirror (13) and the fixed reflecting mirror (14) is subjected to interference synthesis by the beam splitter (12), and the other of the two outgoing light beams is received to receive an electric signal (I2). To the second AD converter (17b) after being converted into
5b) is provided, and an analog switch (24) for switching the signal of the first photoreceiver (15a) or the signal of the second photoreceiver (15b) and supplying it to the AD converter (17) is provided. Receiving the signal from the vessel (24),
Quantization conversion is performed, and the converted signal is subjected to polarization correction calculation unit (25)
Is provided with an AD converter (17), which receives signals (I1D, I2D) from the AD converter (17) and generates a high-pass filtered signal (I1LPD) from the signal (I1D) by the LPF means. Then, the signal (I1D) is sent by HPF means.
A polarization correction calculation unit (25) for generating a low-pass filtered signal (I1HPD) from the generated signal and performing {(I1D + I2D) × I1HPD / I1LPD} calculation processing from these signals is provided. Michelson interferometer.