JPS63182531A - Optical spectrum analyzer - Google Patents

Abstract

PURPOSE:To obtain high resolution by finding the spectrum of measurement light roughly by using a light interfering means or photocell, and processing this spectrum data and finding a detailed spectrum. CONSTITUTION:The measurement light is incident on a Farbry-Peot etalon 1, and only light with specific wavelength determined by the distance between mirrors 2 and 3 is transmitted, projected, and incident on a photodetector 5. The output of the detector 5 is passed through an A/D conversion part 10 and stored in a storage means 11. The output of a mirror moving means 8 is inputted to and stored in the means 11. The output of this means 11 is supplied to a Fourier transformation part 12, a division part 13, and a reverse Fourier transformation part 14 in order and processed, and the resulting spectrum is displayed on a display part 15. In this constitution, the resolution of the spectrum of the device is not limited to the resolution of the light interfering means and photocell and the high resolution is easily obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an improvement in an optical spectrum analyzer that measures the spectrum of measurement light or output light of a light source.

<Prior Art> An optical spectrum analyzer is a device that measures the spectrum of measurement light, and is widely used to measure the wavelength characteristics of output light from a light source and the characteristics of optical components such as optical filters. The configuration of such an optical spectrum analyzer is
As shown in the figure. In FIG. 5, 1 is a Fabry-Perot etalon, which includes a mirror 2.3 therein. Measurement light enters the Fabry-Perot etalon 1 from the mirror 2 side, and only light of a specific wavelength determined by the distance between the mirrors 2 and 3 is transmitted and exits from the mirror 3 side. This light is focused by a lens 4 and is incident on a photodetector 5. Photodetector 5 converts the intensity of the incident light into an electrical signal. The output of this photodetector 5 is amplified by an amplifier 6 and input to a display section 7. A mirror moving means 8 moves the mirror 3 in the direction of the mirror 2 or in the opposite direction. The output of the mirror moving means 8, ie, the position signal of the mirror 3, is input to the display section 7.

In such a configuration, the distance between mirrors 2 and 3! Then, light interferes in the Fabrylot etalon 1, and the output light has a frequency ν■, νm = Cm/ (2n
n cos (θ) ) ・(1) C: Speed of light m: Integer n: Refractive index in Fabryault etalon 1 1: Mirror 2
Distance θ between and 3: Only light that satisfies the incident angle of the measurement light is transmitted. When the position of the mirror 3 is changed by the mirror moving means 8 and the distance 1iltl between the mirrors 2 and 3 is changed, the interfering frequency ν changes, and the intensity of the output light of the Fabry-Perot Eta [1-1 is measured. It changes according to the spectrum of light. This intensity change is plotted on the vertical axis as described above (
1) By displaying the wavelength determined by the equation on the horizontal axis on the display section 7, the spectrum of the measurement light can be obtained.

<Problems to be Solved by the Invention> However, the wavelength resolution of such an optical spectrum analyzer is determined by the width of the peak of the transmission characteristic of the Fabry-Perot etalon. FIG. 6 shows the transmission characteristics of the Fabry-Perot etalon. In this figure, the horizontal axis is the frequency of incident light, the vertical axis t(ν) is the transmittance, and R is the reflectance of the mirror. In order to increase the resolution, the width of the peak of the transmission characteristic must be narrowed, but as is clear from Figure 6, in order to narrow the width of the peak, the reflectance of the mirror must be made high (see Figure 6). However, it is difficult to reduce absorption and scattering and to increase the fouling rate.Also, the characteristics shown in Figure 6 are theoretical values, and in reality the width of the peak of the transmission characteristics may vary due to various reasons. tends to widen. Therefore, there is a limit to narrowing the width of the peak, which has the disadvantage of not being able to increase the resolution.

<Objective of the Invention> An object of the present invention is to provide an optical spectrum analyzer that can improve wavelength resolution without narrowing the width of the peak of transmission characteristics.

Means for Solving the Problems - In order to solve the above problems, the present invention provides an optical interference means capable of changing the interference wavelength of the incident measurement light, and an optical interference means for changing the interference wavelength of the incident light. a photodetector for converting the light intensity into an electrical signal, a control means for changing the interference wavelength of the optical interference means, an output of the control means and an output of the photodetector are inputted, and these outputs are stored. and a calculation means for calculating the spectrum of the measurement light by Fourier calculation or conpoligion calculation from the data stored in the storage means and the characteristics specific to the optical interference means.

Furthermore, a light source whose output light wavelength can be varied is used as a light source, and a light cell that transmits or does not transmit only light of a specific wavelength is used instead of the optical interference means.

Measurement using optical interference means or a light source whose output light wavelength can be varied and a light cell that transmits or does not transmit only light of a specific wavelength = 6- Light at that wavelength to obtain an outline of the spectrum,
A roughly detailed spectrum is obtained from this rough data by Fourier calculation or convolution calculation.

(Embodiment 2) FIG. 1 shows an embodiment of the optical spectrum analyzer according to the present invention. The same elements as in FIG. 11 is an AD converter, which inputs the output of the photodetector 5 and converts it into a digital signal. 11 is a storage means;
The output of is input and stored. The output of the mirror moving means 8 is also input to the storage means 11 and stored therein. 12
is a Fourier transform unit to which the output of the storage means 12 is input, and performs a discrete Fourier transform based on the input data. Reference numeral 13 denotes a dividing unit, into which the output j of the Fourier transform unit 12 and a function T(ω) representing the characteristics of the Fabry-Perot etalon 1 are input, and the ratio thereof is calculated. 14
is an inverse Fourier transform unit to which the output of the division unit 13 is input and performs discrete inverse Fourier transform. A display section 15 displays the spectrum calculated by the inverse Fourier transform section 14. Note that the Fabry-Perot etalon 1 functions as an optical interference means, and the mirror moving means 8 functions as a control means for changing the interference wavelength of this optical interference means.

Next, the operation of this embodiment will be explained. The transmission characteristic S(ν) of Fabry-Perot etalon 1 is S(ν) = (1-R)
2/((1-R)2+4Rsin2 (πmshi/shiTl
t))...(2) R: Reflectance of mirror C: Frequency of light rn: Integer C2 Expressed by the peak frequency of the second m-th transmitted light. Here, ν bamboo is Fabry-Perot etalon 1
It depends on the characteristics of , and is expressed by the above equation (1). Further, the transmission characteristics are as shown in FIG.

In Figure 6, δ on the horizontal axis is the normalized frequency, and δ = 2π
mshi/sil. As is clear from the above equation (1), the peak frequency ν of the transmitted light is the mirror 2 of the Fabry-Perot etalon 1.
．． Therefore, by changing the distance l using the mirror moving means 8, the transmitted light peak frequency ν
It is possible to change the morbidity. Let the light intensity spectrum obtained in this manner be p(ν).

In the above equation (2), it is possible to set shishisasa+shi and replace ν with the difference shi between the peak frequency νsasa of the transmitted light and the frequency ν to be measured. In addition, by changing the distance Ml between the mirrors, the peak frequency ν of the transmitted light can be changed to △ν positive. /
Tn2) Δl, '1B (3). In order to perform measurements accurately, it is necessary to design the measurement light so that the spectral spread of the measurement light is less than half of the peak frequency ν positive interval νF of the transmitted light of the Fabry-Perot etalon 1. If the width of the spectrum to be measured is νF/4, the peak frequency ν of the transmitted light must be changed within this range, so Δν, -sit-/4. From the above equation (1), since F - SITL / m, the change in the coefficient of ν' shown in the above equation (3) △(πm/ν
lice) is Δ(πm/ν bamboo)=(πm/ν bamboo 2)·(ν bamboo/(4m))−π/(4ν bamboo). Normally, in the case of light, νWI~10I4 is very large, so the change in the coefficient of ν' shown in the -F equation is very small. Therefore, by changing the distance tlJ, lfr, the peak frequency νsasa is changed. The form of equation (2) remains unchanged. Since the band of measurement light can be considered to be limited to +/-F/4, the function obtained by multiplying S(ν') by the window function W(ν') is expressed as t (ν -) - S (ν −) ・IJノ
(ν ′)・・・・・・・・・〈4), and the Fourier transformed function of this function is T(ω)−J
(t(c)1). J is an operator representing Fourier transform.

In addition, as the window function W (she), for example, W (she) −1 (−νF/2 × νF/2)−〇
<Sees other than the above) are used.

In the configuration shown in FIG. 1, the light intensity spectrum p<ν') of the measurement light stored in the storage means 11 is
It is transformed into a discrete frame. It becomes P(ω)-7(p(ν')). This P(ω) is input to the divider 13, and is divided into J:EP(ω)/T
(ω) is taken. This value is subjected to discrete inverse Fourier transform in the inverse Fourier transform unit 14, and the spectrum f(ν) of the measurement light is
is required. That is, f(ν)=,7-1 tp(ω)/T(z)))...
・・・・・・(5) It becomes. Chi1 is an operator representing inverse Fourier transform. That is, if the light intensity detected by the photodetector 5 when the peak frequency of the transmitted light of the Fabry-Perot etalon 1 changes by Δν() is p(Δνsasa), then p(Δνsasa>=ff(ν) )1(C△νt)dC flash...(6) The functions f and t are shown in equations (4) and (5>, respectively.These If the functions obtained by Fourier transforming the functions p, f, and t of are P<ω), F(ω), and T(ω), respectively, then the above equation (6) becomes P(ω)−F(ω)T(ω) From this relationship, F(ω) is obtained and inverse Fourier transform is performed to obtain the spectrum f(ν) of the measurement light.This spectrum is displayed on the display unit 15.

A second embodiment of the present invention is shown in FIG. 2, and the same reference numerals as in FIG. 1 are used for elements without d5, and their explanations are omitted.

In this embodiment, a spectrum is obtained by performing a convolution operation without performing a Fourier transform. That is, from the above formula (5), f(ν)=”'(P(ω)/King(ω))-J-1('J
f tp <ν))/T(ω))−f“p(c)ij
(C) dC = p * t・・・・・・・・・・・・
(7) 1, (ν)-7-' (1/T (ω))
becomes. However, 4 is an operator indicating convolution. In FIG. 2, 16 is a convolution calculation section. This convolution calculation unit 16 executes the calculation of equation (7).

FIG. 3 shows a third embodiment. This embodiment is used when measuring the spectrum of output light from a light source in which only the wavelength can be shifted without changing the shape of the optical spectrum. An example of such a light source is a semiconductor radar. When a semiconductor laser changes its injection current, the wavelength of the output light shifts, but its spectrum does not change as much as /υ within a range where the injection current is small. Note that the same elements as in FIG. 1 are given the same reference numerals and their explanations will be omitted. In Figure 3,
It is assumed that 17 is a semiconductor laser, and the relationship between its injection current and the center wavelength of output light is known.

The light emitted from the semiconductor laser 17 is made into parallel light by the lens 18, and is incident on the Fabry-Perot etalon 19, where it becomes -1,:3.
- The Fabry-Perot etalon 19 has a fixed interference frequency and functions as a photocell. Reference numeral 20 denotes injection current control means, which controls the injection current of the semiconductor laser 17. In this configuration, the central frequency of the output light of the semiconductor laser 17 is ν■, the difference between this central frequency ν and the frequency ν of the output light of the semiconductor sheath "17" is ν', and the spectrum of the semiconductor laser 17 is If f(ν'), the output light intensity p(Δνt) of the 7-application Perot etalon 1 when the central frequency ν changes by Δν■ is p(Δν electric)=f″lf(Sh′+ΔSi bamboo) )・A t (ν ′ ) dν − −f″′f′(−ξ+Δν□)1(ξ)dξ=f:6t
<Δshi1−ξ)1(ξ)dξ (8). Note that t (c) is a function representing the characteristics of the Fabry-Perot etalon 1 shown in equation (4) above, and the modification of the above equation is 1(
It takes advantage of the fact that ν′) is an even function. When both sides of this equation are Fourier-transformed, it becomes P<<ω)-(ω)・T(ω), which is the same equation as the embodiment in FIG. The spectrum can be obtained by performing discrete Fourier transform, dividing by T(ω) in the division section 13, and performing discrete inverse Fourier transform in the inverse Fourier transform section 14.Equation (8) can be calculated using the above (8). Since it is the same as equation 7), a conpolicing operation may be performed instead of the Fourier transform.

FIG. 4 shows an example of an absorption cell used in place of the Fabry-Perot etalon, which is the optical cell of the embodiment shown in FIG. As shown in FIG. 4, the absorption cell is a transparent container made of glass or the like in which a standard substance that absorbs only light of a specific wavelength, such as 06, is sealed. Light enters the absorption cell from one side and exits from the other, as shown by the arrow. By using this absorption cell, the structure of the device can be simplified.

In the embodiment shown in FIG. 1, the mirror 3 in the Fabry-Perot etalon 1 is moved to change the central frequency of its peak, but the incident angle or the refractive index n may also be changed.

Further, in these embodiments, a Fabry-Perot etalon was used as the optical interference means, but other means may be used.

Furthermore, although a dedicated arithmetic unit is used for the Fourier operation and the convolution operation, a general-purpose processor may also be used.

Effects of the Invention> As described above in detail based on the embodiments, in the present invention, a rough spectrum of measurement light is obtained using an optical interference means or an optical cell, and data of this spectrum is subjected to Fourier calculation or component computation. A detailed spectrum was obtained by performing a recall operation. Therefore, the spectral resolution of the device is not limited by the resolution of the optical interference means or optical cell, and high resolution can be easily achieved. Furthermore, by adding a computing unit to a conventional optical spectrum analyzer, it becomes possible to increase the resolution.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an optical spectrum analyzer according to the present invention, FIGS. 2 and 3 are block diagrams showing an example of an optical spectrum analyzer according to the present invention, and FIG. 4 is a block diagram showing an example of an optical cell. 5 is a configuration block diagram showing a conventional optical spectrum analyzer, and FIG. 6 is a characteristic curve diagram showing the characteristics of a Fabry-Perot etalon. 1.19...Fabry-Perot etalon, 2,3...
Mirror, 5... Photodetector, 8... Mirror moving means,
10... AD conversion section, 11... storage means, 12...
-Fourier transform unit, 13...Divide unit, 14...Inverse Fourier transform unit, 15...Display unit, 16...Convolution calculation unit, 17...Semiconductor laser, 20...
- Injection current control means. 171

Claims (5)

[Claims] (1) An optical interference means into which measurement light is incident and whose interference wavelength can be varied; a photodetector into which the output light of the optical interference means is input and which converts the light intensity into an electrical signal; and an interference wavelength of the optical interference means. a storage means to which the outputs of the control means and the photodetector are input; 1. An optical spectrum analyzer comprising: calculation means for determining the spectrum of measurement light. (2) A light source whose output light wavelength can be varied; a light cell into which the output light of this light source enters and transmits or does not transmit only light of a specific wavelength; and a light cell into which the output light of this light cell is input and whose light intensity is A photodetector for converting into an electrical signal, a control means for changing the wavelength of the output light of the light source, a storage means into which the outputs of the control means and the photodetector are input, and data stored in the storage means. and a calculation means for calculating the spectrum of the output light of the light source by Fourier calculation or convolution calculation from the characteristics specific to the optical cell. (3) The optical spectrum analyzer according to claim 1, wherein a Fab-Perot etalon is used as the optical interference means. (4) The optical spectrum analyzer according to claim 2, wherein a Fabry-Perot etalon is used as the optical cell. (5) The optical spectrum analyzer according to claim 2, wherein an absorption cell encapsulating a standard substance that absorbs light of a specific wavelength is used as the optical cell.