JP6964464B2 - Wavelength calibration method for tunable laser - Google Patents

Description

The present invention relates to a method of calibrating the absolute wavelength of a tunable laser.

When calibrating the absolute wavelength of a tunable laser that irradiates a tunable laser light, an optical frequency standard spectrum is used. This is an absolute frequency assigned to the light absorption intensity spectrum for a given substance, which is a strict ruler for calibration. Such an optical frequency standard spectrum is defined by an institution such as NIST (National Institute of Standards and Technology). The frequency of the light to be measured can be specified by superimposing the absorption spectrum of the laser light to be calibrated on the optical frequency standard spectrum.

Here, when trying to calibrate the laser beam in the tunable laser apparatus using the above-mentioned optical frequency standard spectrum, a measuring device such as a tunable wavemeter must be used at the same time. This is because the frequency continuous tuning range of the tunable laser device is extremely narrow, and is ^{only about 30 GHz (1 [cm -1} ]).

In other words, when trying to search the optical standard spectrum for overlapping and matching frequency regions, the absorption spectrum of the laser beam to be calibrated is ^{only about 1 [cm -1} ], so a wide range of optical frequency standard spectra. Since it is not possible to estimate the frequency domain to be collated, a huge amount of work is required for the search, and the calibration cannot be actually completed. Therefore, first, the frequency of the laser beam to be calibrated is measured with a wavelength meter, and the calibration is performed by strictly superimposing the optical frequency standard spectrum of the measured frequency region and the absorption intensity spectrum of the laser beam to be calibrated. The reality is that.

Japanese Unexamined Patent Publication No. 2000-101177

A wavelength meter is required for wavelength calibration of a tunable laser as described above. However, there is a problem that the wavemeter is expensive. Therefore, there is a demand for a technique capable of easily performing wavelength calibration of a tunable laser without using a wavelength meter.

In order to solve the above problems, the present invention provides the following wavelength calibration method for a tunable laser. That is, as the first invention, among the absorption intensity spectra of laser light to a predetermined substance known in advance as standard data for calibration, the frequency distribution of absorption peaks having an absorption intensity equal to or higher than the threshold value with the predetermined absorption intensity as a threshold. , The frequency distribution of the absorption peaks selected in descending order of the absorption intensity of the absorption intensity spectrum of the laser light to the same substance of the wavelength variable laser to be calibrated is compared with each absorption intensity to be calibrated. A method for calibrating a wavelength of a variable wavelength laser that determines a correct frequency , wherein the threshold value is for a peak having the maximum absorption intensity in the absorption spectrum of the laser light to the predetermined substance known in advance as standard data for calibration. A method for calibrating a wavelength of a variable wavelength laser determined by a ratio is provided.

Further, as a second invention, the selection of the absorption peak of the tunable laser to the same substance into the laser beam is set regardless of the threshold applied to the standard data for calibration. provides a wavelength calibration method of the wavelength tunable laser according to the first inventions of selecting those with absorption strength of at least.

Further, as a third invention, the comparison searches for a frequency interval in the absorption peak frequency distribution of the calibration standard data that matches the absorption peak frequency interval having the selected absorption intensity to be calibrated. provides a wavelength calibration method of the tunable laser according to the first invention or second inventions is the processing.

Further, as a fourth invention, in the absorption intensity spectrum of laser light to a predetermined substance known in advance as standard data for calibration, the frequency distribution of the absorption peak and the laser of the wavelength variable laser to be calibrated to the same substance. This is a wavelength calibration method for a variable wavelength laser that determines the correct frequency corresponding to each absorption intensity to be calibrated by comparing with the frequency distribution of an absorption peak having an absorption intensity equal to or higher than a predetermined threshold of the light absorption intensity spectrum. The threshold provides a wavelength calibration method for a wavelength-variable laser, which is determined by a ratio of the absorption intensity of the laser light to the predetermined substance, which is known as standard data for calibration, to the peak having the maximum absorption intensity.

Further, as a fifth invention, the comparison searches for whether or not a frequency interval matching the absorption peak frequency interval having an absorption intensity equal to or higher than a predetermined threshold of the calibration target exists in the frequency distribution of the absorption peak of the calibration standard data. provides a wavelength calibration method of the wavelength tunable laser according to the fourth invention is a process.

According to the present invention having the above configuration, wavelength calibration can be performed without using a wavelength meter or the like.

The figure which shows a part of the absorption intensity spectrum in the near-infrared light region of iodine monobromide molecule. The figure which shows an example of the absorption intensity spectrum of the laser light with respect to the iodine monobromide molecule of the tunable laser to be calibrated. The figure which shows an example of the peak interval in the absorption intensity spectrum to be calibrated. The figure about the interval measurement between peak A and peak B in the standard data for calibration. Diagram of the number of spectra searched for that match the calibration target using multiple peak intervals. The figure which shows the correct answer rate of the search result of the spectrum which matches the calibration target using a plurality of peak intervals.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to these embodiments, and can be implemented in various embodiments without departing from the gist thereof.
<Embodiment>

The wavelength calibration method of the wavelength variable laser according to the present invention includes the frequency distribution of the absorption peak and the wavelength variable laser to be calibrated in the absorption intensity spectrum of the laser light to a predetermined substance known in advance as standard data for calibration. The wavelength of the wavelength-variable laser that determines the correct frequency corresponding to each absorption intensity to be calibrated by comparing with the frequency distribution of the absorption peak having the absorption intensity equal to or higher than a predetermined threshold of the absorption intensity spectrum of the laser light to the same substance. This is a calibration method.

The wavelength calibration method of the tunable laser of the present embodiment described below can be realized as a combination of hardware and software. Specifically, if a computer is used, the CPU, main memory, bus, or secondary storage device (hardware, non-volatile memory, storage media such as CD and DVD, and reading drive for those media), information, etc. Hardware components such as input devices, display devices, variable wavelength lasers, fabric perot resonators, electro-optical modulators, gas cells, A / D converters, and other external peripheral devices used for input, and their external peripheral devices. Interfaces, communication interfaces, driver programs and other application programs for controlling those hardware, user interface applications, and the like. Then, by the arithmetic processing of the CPU according to the program expanded on the main memory, the data input from the input device or other interface and stored on the memory or hard disk is processed and accumulated, or the above hardware or the above-mentioned hardware or the like. Instructions to control the software are generated. Alternatively, the functional block of this system may be realized by dedicated hardware.

Moreover, each embodiment described in this specification can be realized not only as an apparatus but also as a method. Further, a part of such a device can be configured as software. Further, a software product used to execute such software on a computer, and a recording medium on which the product is fixed are naturally included in the technical scope of each embodiment described in the present specification (the present specification). The same is true throughout.

Widely used tunable lasers include, for example, titanium sapphire laser (Ti: Al ₂ O ₃ Laser) and gallium arsenide semiconductor laser (GaAs) that oscillate in the near-infrared light region. Laser). As a predetermined substance for measuring the absorption intensity spectrum of these variable wavelength lasers, a substance having many absorption lines in the absorption intensity spectrum of light in the near infrared light region is preferable. This is because the absorption intensity at the absorption line is an index for comparison and collation during calibration. Such substances include interhalogen compounds such as iodine monobromide (IBr) and iodine monochloride (ICl).

FIG. 1 is a diagram showing a part of an absorption intensity spectrum of an iodine monobromide molecule, which is one of the interhalogen compounds, in the near infrared light region. The wave number ([cm ^-1 ]) is shown on the horizontal axis to indicate the continuously variable range of the frequency of the tunable laser. In this figure, the value of the wave number swayed on the horizontal axis is the absolute frequency, and the vertical axis is the value obtained by dividing the difference between the maximum intensity and the minimum intensity of the absorption intensity. As shown in the figure, there are a plurality of remarkable peaks of absorption intensity in the continuously variable frequency range. Since these peaks serve as effective indexes when collating in wavelength calibration, they are suitable as standard data for calibration.

In the present invention, in the absorption spectrum of laser light to a predetermined substance as shown in FIG. 1, a frequency distribution having an absorption intensity equal to or higher than the threshold value with the predetermined absorption intensity as a threshold value is used as standard data for calibration. According to the example of FIG. 1, the center wave number at the three peaks of _{“A std} ”, “B _std ”, and “C _{std”, which have relatively high absorption intensities, is the standard data for calibration.} As shown in the figure, the center wave number of each peak is "A _std = 14047.36549", "B _std = 14047.50706", and "C _std = 14047.80195".

Although FIG. 1 ^{shows the absorption intensity spectrum in the range from 14047.0 [cm -1} ] to 14048.0 [cm ^-1 ], the absorption intensity spectrum in a wider range exists as the standard data for calibration. _{In addition, although only the three peaks “A std} ”, “B _std ”, and “C _std ” are illustrated in FIG. 1, the standard data for calibration includes the center wave number and absorption of all peaks within the measured range. The data is associated with intensity.

Such standard data for calibration is stored in advance in a storage device such as a non-volatile memory provided in the computer that executes the wavelength calibration method of the wavelength variable laser according to the present invention, and is appropriately read out by the CPU and used for processing by the CPU. Will be done.

FIG. 2 is a diagram showing an example of measurement data of the absorption intensity spectrum of the laser light with respect to the iodine bromide molecule of the tunable laser to be calibrated. ^{The wave number ([cm -1} ]) is taken on the horizontal axis as in the absorption intensity spectrum as the standard data for calibration shown in FIG. 1, but the given value is the relative wavelength. The relative wavelength can be imparted to the absorption intensity spectrum of the tunable laser to be calibrated by using the interference fringes generated by the Fabry-Perot resonator or the like. In this way, the absorption intensity spectrum of the tunable laser to be calibrated within the wavelength variable range is measured, and a relative wavelength is also given.

In the absorption intensity spectrum in the measurement data to which the relative wavelength is assigned in this way, the CPU included in the computer that executes the wavelength calibration method of the wavelength variable laser according to the present invention executes a predetermined program and encloses the iodine monoxide molecule. The signal generated by detecting the laser beam passing through the gas cell and the interference fringes generated by the Fabry-Perot resonator is subjected to a predetermined process, and the generated absorption intensity spectrum to be calibrated is stored in a non-volatile memory or the like. Stored in the device.

Then, by selecting a plurality of absorption intensity peaks existing in the tunable range and searching for a peak interval that matches the interval between those peaks from the calibration standard data, the absorption intensity spectrum to be calibrated is absolute. Give a frequency. The peak selected from the absorption intensity spectrum to be calibrated can be arbitrarily selected, and for example, it can be selected in descending order of absorption intensity. Further, a peak having an absorption intensity equal to or higher than the threshold value may be selected with an arbitrary absorption intensity as a threshold value so that a plurality of peaks having a strong absorption intensity are selected. It should be noted that this threshold value is set regardless of the threshold value applied to the calibration standard data described later.

FIG. 2 is a conceptual diagram showing an embodiment in which four peaks are selected in descending order of absorption intensity from the absorption intensity spectrum to be calibrated. Here, four peaks (a, b, c, d) are selected. Then, the interval between those peaks is used for comparison with the standard data for calibration. As shown in the figure, the interval α between the peak a and the peak b, the interval β between the peak b and the peak c, the interval γ between the peak c and the peak d, and the interval δ between the peak a and the peak d are used.

The peak interval is obtained by taking the difference in the center wave number (center frequency) of each peak. The wave number given to the absorption spectrum to be calibrated is the relative wave number, but by taking a difference, it is possible to compare with the peak interval in the calibration standard data to which the absolute frequency is given.

On the other hand, the absorption intensity spectrum of the standard data for calibration to be compared contains a huge number of peaks, and it is not preferable to search for matching peak intervals for all peaks because it is a large amount of calculation. Therefore, in response to the selection of a peak having a strong absorption intensity in the calibration target, a threshold value based on the absorption intensity is also set in the calibration standard data to narrow down the peak to be compared.

FIG. 3 is a conceptual diagram showing peaks narrowed down for comparison by a threshold value in the standard calibration data. As shown, there are many peaks in the range ^{from 14047.0 [cm -1} ] to 14048.0 [cm ^-1]. Therefore, the difference between the maximum absorption intensity and the minimum absorption intensity of the standard data for calibration is used as a percentage, and the relative value of the maximum absorption intensity is used as a threshold value. The six peaks (A, B, C, D, E, F) in the figure are peaks when the threshold value is set to "70%". As shown, when the threshold is set to "60%" of the maximum absorption intensity, more peaks are extracted.

Next, a search for matching peak intervals when using four peak intervals (α, β, γ, δ) generated by the four peaks (a, b, c, d) shown in FIG. 2 will be described. .. First, it is searched whether the same frequency interval as the interval α exists in the frequency distribution of the absorption intensity peak of the calibration standard data. That is, for one peak existing in the standard data for calibration, it is searched for whether there is another peak whose interval α from the peak is the interval α. This process is performed for all peaks present in the calibration standard data.

Then, when the interval ε between the peak A and the peak B shown in FIG. 3 is found as an interval equal to the interval α, further search is performed in the same direction as the search for the other peak from one peak, and then the search is performed. A comparison is made as to whether or not the distance ζ between the existing peak C and the peak B is equal to the distance β between the peak b and the peak c shown in FIG. If they are equal, it is compared whether or not the interval η between the peak D and the peak C existing next to each other is equal to the interval γ between the peak c and the peak d. If they are equal, a comparison is made as to whether or not the interval θ between the peak A and the peak D is equal to the interval δ between the peak a and the peak d.

By comparing the peak intervals while sequentially shifting the peak intervals composed of a plurality of peaks in this way, a predetermined value existing in the absorption intensity spectrum to be calibrated from among a large number of peak intervals in the calibration standard data. It is possible to narrow down the series of peaks at the same interval as the series of peaks of, and finally derive the series of matching peaks. Then, the absolute frequency, which is the center frequency of the matched peak in the absorption intensity spectrum of the calibration standard data, is determined as the center frequency of the matched peak in the calibration target.

Such a comparison of peak intervals is performed by a CPU included in a computer that executes the wavelength calibration method of the tunable laser according to the present invention, which executes a predetermined program and is stored in a storage device such as a non-volatile memory. A plurality of peaks are obtained by reading the absorption intensity spectrum and acquiring a plurality of absorption intensities selected as peaks and a center frequency corresponding to the absorption intensity, and then performing a process of calculating the difference between the acquired plurality of center frequencies. The interval is acquired and stored in the storage device.

Subsequently, the CPU executes a predetermined program, and the interval between one peak having an absorption intensity equal to or higher than the threshold value of the calibration standard data stored in a storage device such as a non-volatile memory and another peak adjacent thereto. Is calculated from the difference in the center frequencies of the respective peaks, and the CPU executes a comparison process of whether or not the calculated interval and the peak interval in the calibration target match. In this way, through the process of searching the calibration standard data for an interval equal to the peak interval in the calibration target, the absolute frequency of each peak forming the equal peak interval in the calibration standard data is given to the peak to be calibrated. The absolute frequency is determined by this.

Further, a predetermined error may be allowed when determining the agreement between the peak interval in the calibration standard data and the peak interval in the calibration target. For example, as shown in FIG. 4, when comparing with the peak interval in the calibration target, when measuring the interval between the peak A and the peak B in the calibration standard data, for example, ± 0.003 of the center wave number of the peak B [ Allow an error of about ^{cm -1].} This is because an error may occur in a measuring device or the like for measuring the absorption intensity spectrum to be calibrated.

In addition, in order to suppress an enormous increase in the amount of calculation and perform an efficient search, a case where a threshold value is set for extracting a peak to be compared in the standard data for calibration has been described, but the present invention has set it. As a means of solving the problem, it is natural that all peaks are used as comparison targets without setting a threshold value.

It is also preferable that the calibration method is performed together with the use of the tunable laser. A variable wavelength laser is used for gas analysis, etc., but a gas cell in which laser light that guides the gas cell to be analyzed is separated by a beam splitter or the like, is guided to the gas cell to be separated, and an iodine monobromide molecule is sealed. By also guiding the light, the calibration of the variable wavelength laser and the gas analysis can be performed at the same time. As a result, gas analysis based on a calibrated tunable laser is performed, and accurate gas analysis such as imparting an absolute frequency to the absorption intensity spectrum in the gas cell becomes possible, for example. By configuring the tunable laser so that the calibration method can be performed at the same time as the original use of the tunable laser, it can be realized as a tunable laser to which the calibration function by the calibration method is added.
<Verification of this calibration method>

FIG. 5 shows the number of spectra searched for that match the calibration target using a plurality of peak intervals. The vertical axis is the average number of hits. The horizontal axis is the number of peaks used in this calibration method. The number to be searched is 1 using the peak data with high intensity among the peak data (intensity and wave number) acquired in the ^{1380-14000 [cm -1} ] region as the measurement data of the absorption intensity spectrum to be calibrated. It is executed for each peak data, and this is performed 329 times, and the total number of searches output from the execution result is divided by the total number of peak files 329. The maximum intensity threshold was set to 45 [%].

This time, we confirmed the number of searches when multiple peaks were used by changing the peak error threshold. When three peaks are used and the peak error threshold is 0.003 [cm ^-1 ] or more, the number of hits is 170 or more. It was also confirmed that the number of hits increased proportionally by lowering the peak error threshold even when a plurality of peak intervals were used. When the number of peaks used was 4, the number of hits was narrowed down compared to the number of 3 peaks, and the number of hits was 31 at the ^{peak error threshold of 0.003 [cm -1].} The number of hits also decreased for peaks of 5 or more, and the average number of hits converged to a value close to 1.

FIG. 6 shows the correct answer rate of the search result of the spectrum matching the calibration target using a plurality of peak intervals. The vertical axis is the correct answer rate, and the horizontal axis is the number of peaks used for spectrum search. It was confirmed whether or not there was data having the same wave number as that described in the peak file among the hits in FIG.

This time, it is assumed that the value with the lowest squared error using the peak error threshold is the correct answer. The correct answer was given when the difference between the peak with the lowest squared error and the peak described in the peak data was 0.1 [cm ^{-1] or less.} FIG. 6 shows the data with the highest correct answer rate in this system. When the maximum intensity threshold was 45 [%], the peak error threshold was 0.006 [cm ^-1 ], and the number of peaks used was 8, a correct answer rate of 90.3 [%] could be obtained.
<Effect>

According to the wavelength calibration method of the tunable laser of the present embodiment, wavelength calibration can be performed without using a wavelength meter or the like.

Claims (3)

Of the absorption intensity spectra of laser light to a predetermined substance known in advance as standard data for calibration, the frequency distribution of absorption peaks having an absorption intensity equal to or higher than the threshold value with the predetermined absorption intensity as a threshold, and the wavelength-variable laser to be calibrated. A variable wavelength laser that determines the correct frequency corresponding to each absorption intensity to be calibrated by comparing the frequency distribution of multiple absorption peaks selected in descending order of absorption intensity of the laser beam absorption intensity spectrum to the same substance. It is a wavelength calibration method of
The threshold is a wavelength calibration method for a tunable laser, which is determined by the ratio of the absorption intensity of the laser light to the predetermined substance, which is known as standard data for calibration, to the peak having the maximum absorption intensity. The selection of the absorption peak of the wavelength tunable laser to be calibrated into the laser beam to the same substance shall have an absorption intensity equal to or higher than the threshold set regardless of the threshold applied to the calibration standard data. The wavelength calibration method for a tunable laser according to claim 1 to be selected. The comparison is a process of searching whether a frequency interval matching the absorption peak frequency interval having the selected absorption intensity to be calibrated exists in the frequency distribution of the absorption peak of the calibration standard data. Item 2. The wavelength calibration method for a tunable laser.

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