JP2628171B2 - Optical spectrum analyzer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical spectrum analyzer for obtaining a power spectrum curve relating to the wavelength of light.

2. Description of the Related Art An optical spectrum analyzer is a device that causes light coming from a sample to interfere with an optical interferometer and observes the interference light to obtain a power spectrum related to the wavelength of the light from the sample.

FIG. 5 is a block diagram showing a configuration example of a conventional optical spectrum analyzer. The light emitted from the light source 11 is the sample 12
And the light transmitted through the sample 12 is observed by the optical interferometer 13. The light incident on the optical interferometer 13 is condensed by a condenser lens 14, becomes almost parallel light, enters the beam splitter 15, and is directly reflected by the beam splitter 15 and two lights reflected at right angles. And divided into These two light beams are reflected by the first and second reflecting mirrors 16A and 16B and re-enter the beam splitter 15, where they are reflected in a right angle direction, or travel straight as they are so that they are combined into one light beam and interfered with. Become. The interference light is focused by the condenser lens 17 and enters the detector 18.

In this optical interferometer 13, the first reflecting mirror 16A is a movable mirror,
The second reflecting mirror 16B is a fixed mirror. That is, the first reflecting mirror 16A
In the direction of incidence of light, a periodic minute vibration centering on the reference position is given by the driving device 19 in the direction in which the light is incident. It changes according to the vibration of the mirror 16A, that is, over time. For this reason, there is a difference in the optical path length traced by the two light beams, and the optical path difference traced by the two light beams changes with time. Therefore, the interference state of the interference light obtained by combining these spectral components into one changes with time. The detector 18 converts the interference light into an electric signal corresponding to the intensity and outputs the electric signal. The electric signal output from the detector 18 is converted into a digital signal by an AD converter 21, and this digital signal is supplied to a zoom circuit 22. The zoom circuit 22 includes frequency converters 23A and 23B,
Digital filters 24A, 24B and resampling means 25A, 2
5B. The local oscillator signals cos (2πf _c nΔt) and sin (2πf _c nΔt) are supplied to the frequency converters 23A and 23B, respectively.
There is provided a digital signal by the local oscillation signal, that it is possible to shift the center frequency of the frequency range of the digital converted interference signal to an arbitrary frequency f _c. The digital filters 24A and 24B remove unnecessary harmonic components attached when the frequency converters 23A and 23B are operated so that the center frequency of the interference signal is shifted to a desired frequency from the interference signal.

The digitized complex interference signal shifted to the desired center frequency is supplied to resampling means 25A and 25B, respectively.
The data is sampled at each resampling period t, and the output data of the first frequency converter 23A is stored in the complex memory 26 as real part data, and the output data of the second frequency converter 23B is stored as imaginary part data. When one frame of data, for example, 1024 pairs of data, one pair each of real part data and imaginary part data, is read into the complex memory 26, these sets of data are read by an FFT converter or a DFT converter.
The Fourier transform is supplied to 27 and the result of the Fourier transform is stored in the memory 28.

One frame of 1024 pairs of complex data sampled from the repeatedly supplied interference signal is Fourier-transformed for each frame, and a power spectrum curve having a frequency on the horizontal axis is obtained. When a large number of spectral curves are obtained for each frame, the accuracy of the optical spectral curve can be improved by averaging the spectral curves.

"Problems to be Solved by the Invention" Simply averaging a large number of spectral curves obtained by measuring an interference signal many times cannot obtain a spectral curve having a sufficiently satisfactory resolution. Further, since the Fourier transform is performed for each frame to be observed, the time required for the arithmetic processing is long. Furthermore, there are many points that are not practically satisfactory, for example, a desired part of the spectrum curve cannot be extracted and observed.

"Means for Solving the Problems" The present invention solves the above problems. That is, in the present invention, the interference signal of the optical interferometer is converted into an electric signal by the detector, and the electric signal is further converted into a digital signal. This digital signal is frequency-converted by a first digital frequency converter with a sine wave local signal, and frequency-converted by a second digital frequency converter with a cosine wave local signal.

The outputs orthogonally transformed by these frequency converters, that is, complex data, are converted into signals of a predetermined band via first and second digital filters, respectively, and supplied to first and second resampling means. The first and second resampling means repeatedly sample the supplied digital complex data at a predetermined cycle, and divide a predetermined number of complex data in one frame around the complex data when the interference signal has zero optical path difference. The minute signal is stored in the complex memory.

In the present invention, the instantaneous phase between the interference signal and the local oscillation signal at a predetermined optical path difference is obtained, and the complex data for each measurement frame is rotated by the difference between the predetermined phase and each instantaneous phase. , The average of each complex data is taken.

According to the configuration of the present invention, the interference signal output from the optical interferometer is averaged by adjusting the phase between the measurement data of each time, and the averaged data is adjusted by adjusting the phase. Fourier transformed.

Embodiment FIG. 1 is a block diagram showing an embodiment of an optical spectrum analyzer according to the present invention. Parts corresponding to those in FIG. 5 are denoted by the same reference numerals, and redundant description will be simplified.

The Michelson interferometer 13 is supplied with, for example, light from the white light source 11 transmitted through the sample 12. The transmitted light is split into two by the beam splitter 15, reflected by the first and second reflecting mirrors 16A and 16B, and made into one light again by the beam splitter 15 and interfered. When the first reflecting mirror 16A is driven back and forth in the light traveling direction by the driving device 19, the interference state of light changes according to the movement of the reflecting mirror 16A. The interference light is provided to the detector 18, and an electrical signal corresponding to the change in the interference state, that is, an interference signal is output from the detector 18.

Now, when the electric field traveling in the x-axis direction is expressed in a complex form, E = Re [a exp (j2πν (t−x / v))] = Re [aexp (−j2πνx / v) · exp (2πν)
t)] = Re [Aexp (j2πνt)] (1) where A is a complex amplitude, and A = aexp (−jΦ) = aexp (−j2πνx / v) where Where, a: amplitude, ν: frequency, and v: velocity.

If the movable mirror 16A of the Michelson interferometer 13 is moved by the distance d from the reference position where the optical path difference is zero, the optical path difference between the two light beams is 2d, and interference corresponding to the optical path difference 2d occurs. This interference, the intensity of light incident on the detector ^{18, <E 2> = <(} E 1 + E 2) (E 1 + E 2) *> = <A 1 2> + <A 2 2> + (< ^{_{a 1 a 2 *> + <A}} 1 * a 2>) from the _{I d = I 1 + I 2} +2 (I 1 I 2) 1/2 cos (Φ 1 -Φ 2) be expressed as ... (2) it can.

In this optical interferometer 13, the phase difference ΔΦ between the two branched lights
= Φ ₁ -Φ ₂ corresponds to the optical path difference 2d. The wavelength of monochromatic light is λ
Then, Φ ₁ −Φ ₂ = 2π (2d) / λ = 2π ν2d / C = 2π ν τ = 2π ν (nΔt) where nΔt: sampling point and zero optical path difference correspond to n = 0, and n = −m,..., 0,.

Using this relationship, the AC component I _{dAC of the} interference light intensity I _d
Is represented by the following equation.

I _dAC = 2I ₀ cos (2πν nΔt) = I ₀ (exp (j2πν · nΔt) + exp (−j2πν · nΔf)) (3) The interference signal output from the detector 18 is shown in the figure. However, if necessary, it is supplied to an AD converter 21 via an amplifier and converted into a digital signal, and this digital signal is further converted into a first digital frequency converter 23A and a second digital frequency converter 23B.
And are quadrature-modulated by these frequency converters 23A and 23B.

That is, the first digital frequency converter 23A is given cosine local signal cos (2πf _c nΔt) is a digital signal is frequency-converted by the cosine wave local signal. Further, the second digital frequency converter 23B is given a sinusoidal local signal -sin (2πf _c nΔt), the digital signal is frequency-converted by the sine wave local signal, therefore,
Complex data in which the output of the first frequency converter 23A is real part data and the output of the second frequency converter 23B is imaginary part data is output. The output of the frequency-converted first and second frequency converters 23A and 23B is output to the first and second digital filters 24A and 24A.
4B respectively. These first and second digital filters 24A and 24B have a function of a low-pass filter, and cut a signal component in a high-frequency region generated when the frequency of the digital interference signal is converted. That is, _{[I 0 {exp (j2π ν nΔt} ) + exp (-j2π ν nΔt)} × exp (-j2πf c nΔt + ψ) ] _{LPF = I 0 exp (j2π (} ν-f c) nΔt) · exp (-jψ) ... A complex interference signal represented by (4) is obtained. However, f _c is the oscillation frequency of the local oscillation signal.

The complex interference signals output from the first and second digital filters 24A and 24B are supplied to first and second resampling means 25A and 25B, respectively, and the digital values at that time are sequentially sampled and output in synchronization with the sampling signals. Is done. The sampling outputs of the first and second resampling means 25A and 25B are respectively supplied to the complex memory 26 and are sequentially stored in the complex memory 26 in a cyclic manner. The complex memory 26 is a real part storage area 26
A and the imaginary part storage area 26B are separated, and the sampling data supplied from the first and second sampling means 25A and 25B are stored in the real part storage area 26A and the imaginary part storage area 26B, respectively. For example, the sampling data is sequentially stored from address 0 to address 1, address 2,... Of the real part storage area 26A and the imaginary part storage area 26B of the complex memory 26, and the last address of the complex memory 26, for example, 1023 When the data is stored in the address, the data supplied from the resampling means 25A and 25B is stored by updating the data previously stored in the address 0, and then the addresses 1 and 2 are stored. Are sequentially updated and stored. In other words, the complex memory 26 always has the latest
1024 pairs of data are stored.

According to the present invention, a predetermined amount of data is stored in the complex memory 26 centering on the interference data when the optical path difference of the optical interferometer 13 is zero. For this reason, the point in time when the intensity of the interference light becomes the strongest is output from the optical interferometer 13 and supplied to the means 31 for controlling the amount of data.
, The writing of data therefrom is prohibited. That is, when the optical path difference 2d by the first and second reflecting mirrors 16A and 16B becomes zero due to the driving of the first reflecting mirror 16A, the intensity of the interference light becomes the largest, and this optical path difference becomes zero. The data is output from the optical interferometer 13, and is configured so that approximately half the amount of the complex memory 26, in this case, for example, 512 data is updated and stored from that time. Therefore, complex memory
26, 1024 pairs of complex data for one frame are stored so as to be 511 pairs or 512 pairs before and after the time when the intensity of the interference light becomes maximum.

By the way, as can be seen from equation (4), the phase of the complex interference signal is rotated by the phase difference ψ between the phase of the interference signal from the detector 18 and the phase of the local oscillation signal. However, for example, there is no special correlation between the interference signal obtained when the optical path difference is zero in the Michelson interferometer 13 and the local oscillation signal. The phase difference ψ from the local oscillation signal takes a random value. Therefore, simply adding and averaging the complex data representing the complex interference signals of the respective frames having the phases shifted from each other as described above does not result in obtaining accurate data.

In view of such a point, the present invention is configured so that the phases of the complex interference signals obtained for each measurement frame are matched and the addition / average is obtained. For this purpose, the phase included in the complex interference signal of each frame is examined.
That is, predetermined data, for example, complex data (R + jY) obtained when the optical path difference is zero is read from 1024 pairs of data stored in the complex memory 26 for one frame,
The instantaneous phase defined by the complex data (R + jY) is given to the means 32 for calculating the instantaneous phase.
When the optical path difference is zero, there must always be the largest data as an interference signal. Therefore, complex data having the least noise component is stored in the complex memory 26. The instantaneous phase ψ is obtained by examining the phase defined by this high-quality complex data, that is, the complex number of R + jY.

The obtained instantaneous phase represents a phase difference Δi between the interference signal and the quadrature modulation signal of the current observation frame.
The phase difference Δi has a different value for each observation frame. However, if the interference signal of each frame is rotated and corrected by the phase difference に 対 す る with respect to the quadrature modulation signal obtained for each frame in this manner, the signal phase of each frame can be matched. .

That is, in the present invention, the phase θ appropriately determined according to the obtained instantaneous phase ψi for each frame.
The phase of each frame is rotated by the phase rotation means 33 by the phase angle (θ−ψi).

That is multiplied by exp j (ψi-θ) with respect to the phase rotation unit 33 complex complex interference signal represented by _{(R + jY) I 0 exp} (j2π (ν-f c) nΔt) · exp (-jψi) An operation is performed. _{: I 0 exp (j2π (ν} -f c) nΔt) · exp (-jψi) × exp j (ψi-θ) = I 0 exp (j2π (ν-f c) nΔt) · exp (-jθ) given In each frame adjusted to the phase θ (usually, θ = 0), an average of complex data for each corresponding sampling point is obtained. That is, not only the sampling points when the optical path difference of the complex interference signal is zero, but also the complex interference signals at the other sampling points are all in the same phase θ, and thus the complex interference signal is represented as a vector on the complex plane. When expressed, the vectors point in the same direction θ, and the averaging means 34 adds and averages the vectors for each corresponding sampling point of each frame. The obtained 1024 pairs of complex data are supplied to the FFT converter or DFT converter 27, and are Fourier-transformed by an ordinary method to obtain a power spectrum curve expressed in a frequency axis.

FIG. 2 is a diagram for explaining that when the supplied interference signal contains noise, the effect of the noise is reduced by the present invention. That is, the complex interference signal supplied from the optical interferometer 13 and subjected to quadrature modulation has a vector on a complex plane. It is represented by This vector Is the signal vector And noise vector Are superimposed. In the present invention, the vector , That is, in the complex plane, each vector Are rotated to a predetermined angle θ and then added to obtain an average. Noise vector Has no regularity, and the variance indicating the average size is represented by σ ² / N. This is, for example, the signal vector Noise vector within an arc of radius σ / N ^1/2 from the tip of You can draw as if there is. In this way, the higher the average number is, the more the samples appearing in a conjugate relation to each other increase, and therefore the noise vector Are offset and become smaller.

That is, if noise variance sigma ² is _{applied, I 0 exp (j2π (ν} -f c) nΔt) · exp (-jψ) + u ...
... (5) This vector The ([psi-theta) by phase rotation, on average N _{times, I 0 exp (j2π (ν} -f c) nΔt) · exp (-jθ) + <u> ......
(6) is obtained. The variance of the noise is σ ² / N by the average. That is,
When N measurements are repeated and averaged, the noise energy decreases as −5 log ₁₀ Ndbm. That is, each time the number of times of measurement is doubled, it changes by -1.505 dbm, and it can be seen that the number decreases.

FIG. 3 is a diagram showing the relationship between the number of averaging in the optical spectrum analyzer according to the present invention and the magnitude of noise appearing in the averaging result. The solid line indicates the theoretical value, and the dotted line indicates the experimental value. According to the theoretical value, the influence of noise is reduced at a rate of -1.5 db every time the average number is doubled. However, in this experiment 12
Up to the eighth, it is almost the same as the theoretical value, but after that, it is shown that it did not decrease to the theoretical value for some reason.

FIG. 4 is a diagram showing a measurement example. FIG. 4A is a measurement example using a conventional optical spectrum analyzer that simply averages a large number of measurement data, and FIGS. 4B and 4C are measurement examples using an optical spectrum analyzer according to the present invention. It is an example of the obtained spectrum curve. As shown in FIGS. 4B and 4C, according to the optical spectrum analyzer of the present invention, even fine structures that cannot be obtained in FIG. 4A appear. Note that B and C are the cases where the number of measurements is 32 and 8192, respectively, and show that the noise decreases as the number of measurements increases.

[Effects of the Invention] As described above, according to the present invention, the interference signal is averaged for each sampling point with the same phase, so that an accurate spectrum curve can be obtained. Further, since the conversion to the power spectrum curve in the frequency axis expression is performed only once, the calculation time required for the DFT conversion or the FFT conversion is short. Furthermore, since DFT or FFT conversion is performed only once, DF
Noise generated at the stage of executing T or FFT conversion can be minimized.

Further, if the resampling period of the resampling unit and the output of the unit for storing a predetermined amount of data are controlled, an arbitrary portion can be enlarged to an arbitrary magnification and sampled as necessary. Accordingly, a high-resolution spectrum curve with a short processing time can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical spectrum analyzer according to the present invention, and FIG. 2 is a diagram in which the effect of noise contained in the measured data is reduced by averaging the measured data of each set in phase. FIG. 3 is a diagram for explaining the fact that the noise contained in the data decreases as the number of measurements increases, and FIG. 4A shows an example of measurement data obtained by a conventional spectrum analyzer. FIGS. 4B and 4C are diagrams showing examples of measurement data obtained by the spectrum analyzer according to the present invention, and FIG. 5 is a diagram showing a configuration example of a conventional optical spectrum analyzer.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takahiro Yamaguchi 1-32-1 Asahicho, Nerima-ku, Tokyo Inside the Advantest Co., Ltd. (72) Inventor Toru Nakanishi 1-32-1 Asahicho, Nerima-ku, Tokyo Stock Advantest Co., Ltd. (72) Inventor Norio Arakawa 1-32-1 Asahicho, Nerima-ku, Tokyo Stock Company Advantest Co., Ltd. (56) References JP-A-62-180228 (JP, A)

Claims (1)

(57) [Claims] An optical interferometer, a detector for converting output light of the optical interferometer into an electric signal, an AD converter for converting an output of the detector into a digital signal, and a sine wave localizer for converting the digital signal A first digital frequency converter that converts the frequency of the digital signal with a signal, a second digital frequency converter that converts the frequency of the digital signal with a cosine wave local signal, and outputs of the first and second frequency converters are supplied to a predetermined band. , A first and a second digital filter having a pass band, a first and a second resampling means for resampling the output of the first and the second digital filter, and an output of the first and the second resampling means. A complex memory to be stored; means for storing a predetermined amount of data centered on zero optical path difference of the optical interferometer in the complex memory; and means for determining an instantaneous phase of a local signal at a predetermined optical path difference An optical spectrum analyzer comprising: a stage; phase rotation means for rotating the phase of each measurement data by an amount of a difference between a predetermined phase and an instantaneous phase; and averaging means for complex-averaging each of the phase-rotated data.