JP2018084482A - Light spectrum measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light spectrum device having an optical filter with characteristics which do not affect the measurement accuracy.SOLUTION: The light spectrum measurement device using a grating spectrometer includes an unnecessary order cut operation unit, the operation unit being for estimating an unnecessary order light spectrum in which a wavelength λa appears in a measured value of a wavelength λb of an unnecessary order diffraction light on the basis of the rate of the diffraction efficiency between a measurement target order diffraction light for the wavelength λa and an unnecessary order diffracted light, and for correcting the measured value of the wavelength λb, using the estimated unnecessary order light spectrum.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical spectrum measuring apparatus using a diffraction grating spectrometer.

  An optical spectrum measurement device is a measuring instrument that analyzes and analyzes the optical power corresponding to each wavelength with respect to input light, and is intended for evaluation of optical fiber transmission systems and characteristics of optical communication devices. It is widely used for measurement.

  FIG. 10 is a diagram showing a measurement principle of a conventional optical spectrum measurement apparatus 500. As shown in FIG. Input light to be measured is divided into narrow wavelength slots by an optical bandpass filter 521 and converted into an electrical signal by a photodiode 540. Then, it is amplified by the amplifier 550 and converted into a digital signal by the A / D converter 560.

  An optical spectrum is obtained by plotting a signal obtained by sweeping the center wavelength of the optical bandpass filter 521, and is displayed on the display device 570 as a measurement result. The optical bandpass filter 521 is a mechanical device using a diffraction grating as a wavelength dispersion element and is called a monochromator. In the optical band-pass filter 521, the center wavelength is swept by changing the angle of the diffraction grating arranged on the rotary stage by the position control unit 526 provided with a motor.

The output wavelength of the diffraction grating spectrometer 520 is given by [Equation 1].
Here, m is the diffraction order, d is the groove interval of the diffraction grating 522, θa is the angle between the incident light and the outgoing light of the diffraction grating 522, θ is the normal of the diffraction grating 522, and the center of the incident light and the outgoing light. The angle between (See FIG. 11).

  As can be seen from [Equation 1], when light of wavelength λ enters the diffraction grating 522, it is diffracted at various angles depending on the value of the diffraction order m. Here, since m is an integer, the diffraction angle is a discrete value.

Therefore, when a wide wavelength range (λa to λb) is incident on the diffraction grating 522, as shown in FIG. For example, since the diffraction angle θ _{800 at which 800} nm of the first-order diffracted light can be extracted is the same as the diffraction angle θ ₄₀₀ at which the second-order diffracted light of 400 nm is extracted, the spectra overlap at that angle.

  Therefore, when it is desired to extract 800 nm of the first-order diffracted light, it is necessary to shield 400 nm light. For this purpose, the conventional optical spectrum measuring apparatus 500 includes an optical filter 530 that cuts unnecessary orders. A plurality of types of optical filters 530 are prepared for each wavelength band, and are used by switching appropriately.

Japanese Patent Laid-Open No. 02-085729 JP 2005-249777 A

  Since the conventional optical spectrum measuring apparatus 500 uses the optical filter 530, the measurement accuracy depends on the characteristics of the optical filter 530. For example, the shielding ability of the optical filter 530 is limited, and complete shielding is not possible in the wavelength band to be shielded.

  For example, when an optical filter 530 that shields 400 nm to 900 nm is used to extract a wavelength of 1000 nm or longer, if the shielding ability of the optical filter 530 is 99.9%, an unnecessary diffraction order of 0.1% Light leaks, which adversely affects the accuracy of the measurement results.

  Therefore, an object of the present invention is to provide an optical spectrum device in which the characteristics of an optical filter do not affect the measurement accuracy.

In order to solve the above-mentioned problems, an optical spectrum measuring apparatus according to the present invention is an optical spectrum measuring apparatus using a diffraction grating spectrometer, which has a diffraction efficiency of a measurement target order diffracted light and an unnecessary order diffracted light with respect to a wavelength λa. Based on the ratio, the unnecessary order light spectrum that appears in the measured value of the wavelength λb where the wavelength λa is the unnecessary order diffracted light is estimated, and the measured value of the wavelength λb is corrected using the estimated unnecessary order light spectrum. A cut operation unit is provided.
Here, the unnecessary order cut calculation unit can further estimate the unnecessary order light spectrum based on a resolution bandwidth ratio between the wavelength λb and the wavelength λa.
In addition, the optical filter may further include an optical filter that blocks the wavelength λb of the unwanted order diffracted light, and the unnecessary order cut calculation unit may further estimate the unwanted order light spectrum based on a cutoff characteristic of the optical filter.
Moreover, it is preferable that the said unnecessary order cut calculating part has recorded the ratio of the said diffraction efficiency previously.

  According to the present invention, an optical spectrum device is provided in which the characteristics of the optical filter do not affect the measurement accuracy.

It is a block diagram which shows 1st Example of the optical spectrum measuring apparatus of this invention. It is a figure for demonstrating the principle of an unnecessary order cut. It is a figure which shows the example of the diffraction efficiency of 1st order diffracted light and 2nd order diffracted light. It is a flowchart explaining operation | movement of the optical spectrum measuring apparatus of 1st Example. It is a figure explaining calculation of a function K (λ). It is a figure explaining the parameter of resolution bandwidth. It is a block diagram which shows 3rd Example of the optical spectrum measuring apparatus of this invention. It is a figure which shows the example of the shielding characteristic of the optical filter for unnecessary order cut. It is a figure explaining the optical spectrum measuring device which changes a diffraction order according to a measurement wavelength. It is a figure which shows the measurement principle of the conventional optical spectrum measuring apparatus. It is a figure explaining the output wavelength of a diffraction grating spectrometer. It is a figure explaining the phenomenon in which the spectrum of an adjacent order partially overlaps.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of an optical spectrum measuring apparatus according to the present invention. As shown in the figure, the optical spectrum measurement apparatus 100 includes a diffraction grating spectrometer 120, a photodiode 140, an amplifier 150, an A / D converter 160, a display device 170, and an unnecessary order cut calculation unit 180.

  The diffraction spectroscope 120 changes the angle of the diffraction band disposed on the rotary stage and the optical bandpass filter 121 composed of a monochromator using the diffraction grating as a wavelength dispersion element by using a motor. A position control unit 126 that sweeps the center wavelength is provided.

  The optical spectrum measuring apparatus 100 of the first embodiment does not include an unnecessary-order cut optical filter, but includes an unnecessary-order cut calculation unit 180 instead. The unnecessary order cut calculation unit 180 estimates the optical spectrum that appears according to the unnecessary order, and subtracts it from the measurement result to cut the unnecessary order component. For this reason, the characteristics of the optical filter do not affect the measurement accuracy. The unnecessary order cut calculation unit 180 includes an estimation table 181 and is referred to when unnecessary order cut calculation is performed.

  Here, the principle of unnecessary order cut according to the present invention will be described. Here, an optical spectrum measurement apparatus that measures a measurement wavelength band from 800 nm to 1800 nm using first-order diffracted light is taken as an example. It is assumed that an optical filter that cuts unnecessary order diffracted light is not used. When monochromatic light of 850 nm and monochromatic light of 1300 nm are simultaneously input to this apparatus, in the measurement result, a peak appears at 1700 nm, which is twice 850 nm, in addition to the peaks at 850 nm and 1300 nm, as shown in FIG. .

Since the peak at 1700 nm is the second-order diffracted light at 850 nm, it is a spectrum measured by unnecessary order. Here, as shown in [Equation 2], the peak height P ₁₇₀₀ of ₁₇₀₀ nm can be expressed as a value obtained by multiplying the peak height P _{850 of 850} nm by a coefficient K.
However, eta _850Nm1st is the diffraction efficiency of first order diffracted light in the _850nm, η 850nm2nd is a diffraction efficiency second-order diffracted light in the 850 nm. The diffraction efficiency is a value specific to a component such as a diffraction grating to be used, and can be acquired in advance. That is, if K is known, the peak height of the second-order diffracted light can be estimated based on the peak height P ₈₅₀ at ₈₅₀ nm, so that the spectrum with an unnecessary order of 1700 nm can be cut.

Here, for the purpose of explanation, monochromatic light of 850 nm and monochromatic light of 1300 nm are taken as an example, but the diffraction efficiency of the first-order diffracted light and the diffraction efficiency of the second-order diffracted light are obtained at each wavelength, and the coefficient K is determined. That is, as shown in [Equation 3], the coefficient K can be considered as a function K (λ) of the wavelength. However, η _λ1st is the diffraction efficiency of the first-order diffracted light at the wavelength λ, and η _λ2nd is the diffraction efficiency of the second-order diffracted light at the wavelength λ.
From this, when the height of the first-order diffracted light spectrum of wavelength λ is P (λ), the second-order diffracted light spectrum Pcal (2λ) of wavelength λ appearing at wavelength 2λ as an unnecessary order can be expressed as [Equation 4]. .
Therefore, if the measurement spectrum of the wavelength 2λ is P (2λ), the spectrum P0 (2λ) of the first-order diffracted light from which unnecessary order components are cut can be expressed as [Equation 5].
Incidentally, the diffraction efficiency of the first-order diffracted light and the second-order diffracted light generally has characteristics as shown in FIG. That is, the diffraction efficiency of the second-order diffracted light is high in the range of 400 nm to 900 nm, and appears as an unnecessary order spectrum in the range of 800 nm to 1800 nm.

  Therefore, λ of the function K (λ) may be obtained in a range of approximately 400 nm to 900 nm where the diffraction efficiency of the second-order diffracted light is substantially high. Then, using the obtained function K (λ) and the measured spectrum of 400 nm to 900 nm, the spectrum of the second-order diffracted light appearing at 800 nm to 1800 nm may be estimated and subtracted from the measured value as an unnecessary order component.

  Next, the operation of the optical spectrum measuring apparatus 100 of the first embodiment will be described with reference to the flowchart of FIG. The operation of the optical spectrum measuring apparatus 100 can be divided into a prior phase and a measurement phase. The prior phase is a phase in which a function K (λ) used for estimating the spectrum of the second-order diffracted light is created and recorded, and may be performed once at the time of manufacture or the like. However, it may be updated again during maintenance. The measurement phase is a phase in which actual optical spectrum measurement is performed, and is repeatedly performed by the user.

  First, as an advance phase, an estimation table 181 is created (S101). In this process, a function K (λ) is obtained based on actual measurement or the like and recorded as the estimation table 181. Since the function K (λ) is different for each individual, it is preferable to perform the prior phase for each individual.

  The function K (λ) can be obtained by [Equation 3]. Therefore, for the wavelength band with high diffraction efficiency of the second-order diffracted light, the optical power of the first-order diffracted light and the optical power of the second-order diffracted light are measured while sequentially changing the single light color of the wavelength λ, and the ratio May be K (λ).

  For example, as shown in FIG. 5A, when a single light color of 400 nm is incident as a wavelength with high diffraction efficiency of the second-order diffracted light, the optical power of the first-order diffracted light of 400 nm and the optical power of the second-order diffracted light of 800 nm. K (400 nm) can be obtained by calculating the ratio of. The finer the wavelength interval of the single light color, the more accurate the function K (λ) can be obtained. Data between measurement points can be obtained by interpolation calculation.

  In the measurement phase, the incident light to be measured is measured by sweeping the center wavelength of the optical bandpass filter 121 (S102). The measurement result at this point includes an unnecessary order component.

  Next, an unnecessary order light spectrum is estimated for a band (900 to 1800 nm in the example of FIG. 5) affected by the second-order diffracted light of other wavelengths (S103). The unnecessary order light spectrum is estimated according to [Equation 4] in the measurement light spectrum in the band (400 to 900 nm in the example of FIG. 5) that is not affected by the second-order diffracted light of other wavelengths. Multiply

  For the band affected by the second-order diffracted light of other wavelengths, the measurement light spectrum is corrected by subtracting the unnecessary order light spectrum estimated from the measurement light spectrum for each wavelength λ according to [Equation 5] ( S104).

  Finally, the corrected measurement light spectrum is output as a measurement result to the display device 170 or the like (S105). The output measurement result is a highly accurate value obtained by cutting unnecessary order components without using an optical filter.

  Next, a second embodiment of the present invention will be described. With the optical spectrum measuring apparatus 100 of the first embodiment, it is possible to obtain a highly accurate measurement result obtained by cutting unnecessary order components. This accuracy is further increased by increasing the estimation accuracy of the optical spectrum that appears according to the unnecessary order.

Therefore, in the second embodiment, considering that the value of the optical power measured by the optical spectrum measuring apparatus 100 is the optical power per resolution bandwidth called optical power density, the unnecessary shown in [Expression 4] The second-order diffracted light spectrum Pcal (2λ) of the wavelength λ appearing at the wavelength 2λ as the order component is obtained by the equation shown in [Expression 6].
Here, PB is a resolution bandwidth and is given by [Equation 7]. Where ε is the output slit width in the diffraction grating spectrometer 120, f is the focal length of the focusing mirror in the diffraction grating spectrometer 120, and β is the angle formed between the normal line of the diffraction grating and the diffraction angle (see FIG. 6).
The resolution bandwidth PB is a value unique to the diffraction grating spectrometer 120 and can be acquired in advance. Therefore, it is obtained together with the function K (λ) and recorded as the estimation table 181 in the prior phase.

  Next, a third embodiment of the present invention will be described. FIG. 7 is a block diagram showing a third embodiment of the optical spectrum measuring apparatus of the present invention. The same blocks as those in the first embodiment are denoted by the same reference numerals. As shown in this figure, the optical spectrum measuring apparatus 101 includes a diffraction grating spectrometer 120, an unnecessary order cutting optical filter 130, a photodiode 140, an amplifier 150, an A / D converter 160, a display device 170, and an unnecessary order cutting calculation. Part 180 is provided. That is, in the third embodiment, the unnecessary order cut optical filter 130 is used together with the unnecessary order cut calculation unit 180.

In the third embodiment, the second-order diffracted light spectrum Pcal (2λ) of the wavelength λ appearing at the wavelength 2λ as the unnecessary order shown in [Equation 4] is obtained by the equation shown in [Equation 8].
Here, OD is a shielding characteristic of the unnecessary-order-cutting optical filter 130 as illustrated in FIG. Unnecessary order cut calculation unit 180 performs unnecessary order cut calculation on the unnecessary order diffracted light which is shielded and sufficiently reduced by unnecessary order cut optical filter 130, so that a more accurate optical spectrum can be obtained as a measurement result. it can. At this time, since the unnecessary order cut calculation is performed in consideration of the shielding characteristic of the unnecessary order cutting optical filter 130, the influence of the characteristics of the optical filter on the measurement accuracy can be removed.

  The shielding characteristic is a value unique to the unnecessary-order cutting optical filter 130 and can be acquired in advance for each wavelength. Therefore, it is obtained together with the function K (λ) and recorded as the estimation table 181 in the prior phase. In [Equation 8], the resolution bandwidth PB may be omitted.

  By the way, as the principle of unnecessary order cut according to the present invention, the measurement wavelength band from 800 nm to 1800 nm has been described using an example of an optical spectrum measurement apparatus that uses first-order diffracted light. Some of them enable high-efficiency measurement in a wide wavelength band by changing the diffraction order according to the measurement wavelength.

  Here, as shown in FIG. 9, according to the diffraction efficiency of the first-order diffracted light and the second-order diffracted light, the band from 400 nm to 550 nm uses the second-order diffracted light, and the band from 550 mm to 1200 nm uses the first-order diffracted light. A fourth embodiment of the present invention will be described using an optical spectrum measuring apparatus as an example.

  In the fourth embodiment, for example, the second-order diffracted light of 400 nm and the first-order diffracted light of 800 nm are obtained at the same rotation angle of the diffraction grating, so that an optical filter for cutting unnecessary orders is essential. That is, when measuring the second-order diffracted light of 400 nm, an optical filter that shields 800 nm is required, and when measuring the first-order diffracted light of 800 nm, an optical filter that shields 400 nm is required. For this reason, it can be set as the structure similar to 3rd Example shown in FIG.

  If there is no optical filter, for example, when monochromatic light of 800 nm is input, the first-order diffracted light of 800 nm and the second-order diffracted light of 400 nm are measured, and the respective peak heights are equal.

  Similarly, when 400-nm monochromatic light is input, the first-order diffracted light of 800 nm and the second-order diffracted light of 400 nm are measured, and the respective peak heights are equal.

  Therefore, when there is no optical filter, the wavelength λ measured with the second-order diffracted light and the wavelength 2λ measured with the first-order diffracted light appear as unnecessary order light spectra in the mutual measurement with the coefficient K = 1. Become.

In the fourth embodiment, the unnecessary-order cutting optical filter 130 is indispensable for mounting. Therefore, when the shielding characteristic of the unnecessary-order cutting optical filter 130 is OD, it appears at the wavelength λ measured by the second-order diffracted light. Unnecessary diffracted light of wavelength 2λ measured with the next diffracted light can be expressed by [Equation 9], and the measurement value of wavelength λ can be corrected by [Equation 10]. Here, since K = 1, K becomes unnecessary.
Further, the unnecessary diffracted light having the wavelength λ measured by the second-order diffracted light appearing at the wavelength 2λ measured by the first-order diffracted light can be expressed by [Equation 11], and the correction of the measurement result of the wavelength 2λ can be expressed by [Equation 12]. ] Can be performed.
Here, the wavelength range in which the unnecessary order component is cut can be the range of the wavelength λ measured by the second-order diffracted light and the corresponding 2λ range. For example, in the example of FIG. 9, λ may be in the range of 400 nm to 550 nm, and 2λ may be in the range of 800 nm to 1100 nm.

DESCRIPTION OF SYMBOLS 100 ... Optical spectrum measuring device 101 ... Optical spectrum measuring device 120 ... Diffraction grating spectrometer 121 ... Optical band pass filter 126 ... Position control part 130 ... Optical filter 140 for unnecessary order cut ... Photodiode 150 ... Amplifier 160 ... A / D conversion 170 ... Display device 180 ... Unnecessary order cut calculation unit 181 ... Estimation table

Claims (4)

An optical spectrum measurement apparatus using a diffraction grating spectrometer,
Based on the ratio of the diffraction efficiency of the measurement target order diffracted light and the unnecessary order diffracted light with respect to the wavelength λa, the unnecessary order light spectrum that appears in the measured value of the wavelength λb where the wavelength λa is the unnecessary order diffracted light is estimated,
An optical spectrum measuring apparatus, comprising: an unnecessary order cut calculation unit that corrects a measured value of the wavelength λb using the estimated unnecessary order optical spectrum. The unnecessary order cut calculation unit is:
2. The optical spectrum measuring apparatus according to claim 1, wherein the unnecessary-order optical spectrum is estimated based on a resolution bandwidth ratio between the wavelength λb and the wavelength λa. An optical filter for blocking the wavelength λb of the unwanted order diffracted light;
The unnecessary order cut calculation unit is:
The optical spectrum measurement apparatus according to claim 1, wherein the unnecessary order light spectrum is estimated based on a cutoff characteristic of the optical filter. The unnecessary order cut calculation unit is:
The optical spectrum measuring apparatus according to claim 1, wherein the ratio of the diffraction efficiency is recorded in advance.