JP2016133438A - Spectral instrument and spectral detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spectral instrument and a spectral detection method that cancel effect of a fringe by reflection on an end face of a waveguide type wavelength conversion element without branching an optical path into two.SOLUTION: A spectral instrument comprises: a waveguide type wavelength conversion element 10 which generates new conversion light through secondary nonlinear optical effect as signal light and excitation light are made incident; and an optical receiver 24 which receives the conversion light transmitted through a multi-path cell 31, and spectrally diffracts the light received by the optical receiver 24 by sweeping a wavelength of the signal light and thereby sweeping a wavelength of the conversion light. The spectral instrument is provided with a temperature control part 32 which periodically modulates a temperature of the waveguide type wavelength conversion element 10 to periodically modulate a refractive index of the waveguide type wavelength conversion element 10, and obtains a spectrum by averaging the output of the optical receiver 24 with time over one period of modulation or longer, or statistically averaging the output a plurality of times.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a spectroscopic device and a spectroscopic detection method using a wavelength conversion light source, and more specifically to a spectroscopic device and a spectroscopic detection method using a wavelength conversion light source suitable for gas sensing and substance optical spectroscopy.

  In recent years, the problem of global warming has been highlighted, and in order to detect methane, carbon dioxide and the like with high sensitivity, a mid-infrared light source that outputs a wavelength of 2 to 5 μm is required. In such a wavelength range, research and development of semiconductor lasers have been conventionally performed. However, the present situation is that a light source that can be easily used at room temperature has not been realized. Therefore, a technique for generating light in a wavelength region that is difficult to directly generate from such a light source by using wavelength conversion using a nonlinear optical effect is known.

Further, since the generation of dioxins can be suppressed by controlling the combustion of the incinerator by observing the concentrations of CO ₂ , CO, O _2, etc., the observation of the absorption line of O ₂ existing at 0.76 μm is attracting attention. A surface emitting laser is used as a laser that generates light in the 0.76 μm band suitable for a gas absorption line. However, since the output cannot be increased, a wavelength conversion using a non-linear optical effect is used. A technique for generating light in the 76 μm band has attracted attention.

  Various types of wavelength conversion elements can be used, but from a practical point of view, a waveguide type wavelength conversion element using quasi-phase matching obtained by periodically modulating a nonlinear optical constant is most promising. . In order to form a periodic modulation structure having a nonlinear optical constant, a method of alternately inverting the sign of the nonlinear optical constant or arranging substantially large and small portions of the nonlinear optical constant is conceivable.

In a ferroelectric crystal such as LiNbO ₃ (lithium niobate), since the sign of the nonlinear optical constant corresponds to the polarity of the spontaneous polarization, the sign of the nonlinear optical constant can be reversed by inverting the spontaneous polarization. . As a method for generating the mid-infrared wavelength region, as shown in Non-Patent Document 1, there is known a method using difference frequency generation by a waveguide type wavelength conversion element using two semiconductor lasers and pseudo phase matching. Yes. Further, as a method for emitting light in the 0.76 μm band, a method based on second harmonic generation by a waveguide type wavelength conversion element as shown in Non-Patent Document 2 is known.

FIG. 7 shows the configuration of a wavelength conversion light source based on the difference frequency generation. The light source 50 shown in FIG. 7 includes a LiNbO ₃ substrate 51 on which an optical waveguide 52 is formed, a multiplexer 55, two semiconductor lasers (not shown) that output signal light 53 and excitation light 54, respectively. Consists of The signal light 53 and the excitation light 54 are combined by a multiplexer 55 and are incident on an optical waveguide 52 formed on a LiNbO ₃ substrate 51 that is periodically poled. In the optical waveguide 52, converted light 56 that is a difference frequency light between the signal light 53 and the excitation light 54 is generated. Assuming that the signal light wavelength of the signal light 53 is λ _a , the converted light wavelength of the converted light 56 is λ _b , and the excitation light wavelength of the excitation light 54 is λ _c , these three wavelengths satisfy the following (formula A).
1 / λ _c = 1 / λ _b + 1 / λ _a (Formula A)

For example, if the signal light wavelength λ _a is 1.56 μm and the excitation light wavelength λ _c is 1.06 μm, the converted light 56 with the converted light wavelength λ _b = 3.31 μm can be generated. When the refractive index at the signal light wavelength λ _a is n _a , the refractive index at the converted light wavelength λ _b is n _b , and the refractive index at the pumping light wavelength λ _c is n _c , nonlinear optics given by the following (formula B) When a constant modulation period Λ ₀ is set, the converted light 56 is efficiently generated.
n _c / λ _c −n _b / λ _b −n _a / λ _a −1 / Λ ₀ = 0 (formula B)

Incidentally, a case where sum frequency generation and second harmonic generation, which are second-order nonlinear optical effects, are used in the same manner will be described. Explaining in accordance with (Formula A), a phenomenon in which light of wavelengths λ _a and λ _c is input and new light λ _b is generated is called sum frequency generation. Furthermore, wavelength λ _a and wavelength λ _c are the same. The case of wavelength is called second harmonic generation. The modulation period Λ ₀ of the nonlinear optical constant that should be set in order to efficiently generate converted light is the same as in (Formula B).

FIG. 8 shows a conventional spectroscopic device using a light source utilizing difference frequency generation. It has a mid-infrared light source 60 composed of a waveguide-type wavelength conversion element 61, a multiplexer 62, a signal light source 63, and an excitation light source 64. The converted light emitted from the waveguide-type wavelength conversion element 61 is air. It propagates in the space and its intensity is measured by the light receiver 65. At this time, if the pumping light wavelength λ _c is fixed and the signal light wavelength λ _a is changed, the converted light wavelength λ _b changes according to (Equation A). Alternatively, when the signal light wavelength λ _a is fixed and the excitation light wavelength λ _c is changed, the converted light wavelength λ _b is changed according to (Equation A).

Utilizing this fact, the shape of the absorption line of the gas in the mid-infrared region is observed by changing the converted light wavelength λ _b . Here, the excitation light wavelength λ _c was fixed at 0.98 μm, the signal light wavelength λ _a was changed from 1.5626 μm to 1.5638 μm, and the converted light wavelength λ _b was changed from 2.625 μm to 2.6285 μm. The distance between the mid-infrared light source 60 and the light receiver 65 is 1 m. FIG. 9 shows a graph of the converted light wavelength on the horizontal axis and the received light intensity on the vertical axis. A dent in the output is visible in the center. This was due to a decrease in strength due to the influence of moisture (water vapor) contained in the air in a 1 m space. The decrease was large when the humidity was high, and the decrease was small when the humidity was low. That is, the degree of decrease varies according to the moisture concentration, and the moisture concentration can be determined from the magnitude.

  Thus, the absorption of water in the 2.6 μm band is very strong, so that the shape of the absorption line can be easily observed. However, when the gas concentration is low, the absorption intensity is not large even in the mid-infrared region, and the absorption intensity is usually 1% or less. When the absorption intensity falls below 1%, it becomes difficult to accurately grasp the absorption line intensity due to fluctuations caused by baseline noise when there is no absorption.

  Therefore, wavelength modulation spectroscopy for observing the second derivative component of the absorption line described in Non-Patent Document 3 and Non-Patent Document 4 is used. In this method, the wavelength of the laser light used for observation is modulated by a sine wave (sine wave) having a frequency f by a minute amount, and a component whose intensity changes at the frequency 2f of the second harmonic is detected by a lock-in amplifier. . In many cases, this is performed by changing the injection current of the distributed feedback laser diode (DFB-LD). In order to observe the absorption line shape, it is also necessary to observe the wavelength dependence of the absorption line. Therefore, in addition to the minute sine wave of frequency f, a sawtooth wave or triangular wave having a large amount of change is superimposed to superimpose the DFB-LD. Modulate the current injection amount. FIG. 10 shows a schematic diagram of the actual absorption amount and the shape of the observed 2f component. The 2f component takes the maximum value when the absorption amount is the largest.

O. Tadanaga, T. Yanagawa, Y. Nishida, H. Miyazawa, K. Magari, M. Asobe, H. Suzuki, "Efficient 3-μm difference frequency generation using direct-bonded quasi-phase-matched LiNbO3 ridge waveguides", APPLIED PHYSICS LETTERS, Vol.88, No.6, 2006, pp.061101-1-061101-3. O. Tadanaga, M. Asobe, Y. Nishida, H. Miyazawa, K. Yoshino, H. Suzuki, "763-nm Laser Light Source for Oxygen Monitoring Using Second Harmonic Generation in Direct-Bonded Quasi-Phase-Matched LiNbO3 Ridge Waveguide , "IEICE Trans. Electron. Vol. E89-C, No.7, 2006, pp.1115-1117. I. Linnerud, P. Kaspersen, T. Jager, "Gas monitoring in the process industry using diode laser spectroscopy", Appl. Phys. B, Vol. 67, 1998, pp.297-305. A. Tokura, M. Asobe, K. Enbutsu, T. Yoshihara, S. Hashida, and H. Takenouchi, "Real-Time N2O Gas Detection System for Agricultural Production Using a 4.6-μm-Band Laser Source Based on a Periodically Poled LiNbO3 Ridge Waveguide ", Sensors, Vol. 13, 2013, pp.9999-10013 Kikuo Takemoto and 3 others, “Near-infrared sensing technology for biological and environmental measurement”, Science Forum, Inc., 1999, pp.257-261, pp.269-276

  When an optical component is inserted in the optical path, in general, periodic modulation is added to the wavelength dependence of transmittance due to multiple reflection of light. This periodic modulation of transmission intensity by multiple reflection is called fringe (interference fringes). The waveguide type wavelength conversion element is also a kind of optical component, and is affected by reflection from the end face of the waveguide. In order to minimize the influence of reflection on the end face of the waveguide, the end face of the waveguide is cut obliquely to increase the coupling to the reflection mode, or a non-reflective coating (AR coating) is applied to the end face of the waveguide. The fringe of about 0.1% may not be able to be removed. If such a fringe is present, the wavelength dependence of the output fluctuates, and when attempting to observe the absorption intensity below the fluctuation, the fringe dominates, so the 2f component is buried and the discrimination is made. Becomes difficult.

  For example, originally, as shown in FIG. 11 (b), the 2f component indicating the absorption line is present at the position of the one-dot chain line at the center, but when the surrounding fringe is large, FIG. As shown in a), the 2f component is buried in the fringe, and it is difficult to specify the position of the absorption line.

  Therefore, as described in Non-Patent Document 5, in general, the output light is branched into two optical paths, one is received without receiving gas absorption, and the other is received after receiving gas absorption. A method of observing minute absorption by subtracting or dividing the received light intensity to offset the influence of fringe is known. However, when such an optical path is branched into two, there is a problem that the output of the light source is reduced by half, or two optical receivers are required and the optical system becomes complicated.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a spectroscopic device and a spectroscopic detection method that reduce the effect of fringes due to reflection of the end face of a waveguide type wavelength conversion element without branching the optical path into two. And

A spectroscopic device according to a first invention for solving the above-mentioned problems is as follows.
It consists of a second-order nonlinear optical medium having a periodically poled structure, and generates light of a new third wavelength by the second-order nonlinear optical effect by entering light of the first wavelength and light of the second wavelength. And at least one of the light of the first wavelength and the light of the second wavelength, and a waveguide type wavelength conversion element to be received and a light receiver that receives the light of the third wavelength transmitted through the measurement object In the spectroscopic device that performs spectroscopy by sweeping the wavelength of the third light by sweeping the wavelength of the light,
Providing temperature modulation means for periodically modulating the temperature of the waveguide type wavelength conversion element, and periodically modulating the refractive index of the waveguide type wavelength conversion element;
Spectral spectra are obtained by averaging the output of the photoreceiver over time over one period of the modulation or by statistically averaging the output multiple times.

A spectroscopic device according to a second invention for solving the above-mentioned problems is as follows.
In the spectroscopic device according to the first invention,
The temperature modulation means uses, as the temperature modulation intensity of the waveguide type wavelength conversion element, a temperature modulation intensity at which a phase difference of the waveguide type wavelength conversion element due to a temperature change during temperature modulation is 2π or more. And

A spectroscopic device according to a third invention for solving the above-described problems is as follows.
In the spectroscopic device according to the first or second invention,
The waveguide type wavelength conversion element is made of a material in which at least one selected from the group consisting of Mg, Zn, Sc, and In is added to LiNbO ₃ or LiNbO ₃ as an additive.

A spectroscopic detection method according to a fourth invention for solving the above-mentioned problem is as follows.
The light of the first wavelength and the light of the second wavelength are incident on a waveguide type wavelength conversion element made of a second order nonlinear optical medium having a periodically poled structure, and the second order nonlinearity of the waveguide type wavelength conversion element. By generating a light of a new third wavelength by the optical effect, and sweeping the wavelength of at least one of the light of the first wavelength and the light of the second wavelength, Sweep wavelength,
Spectroscopy in which light of the third wavelength generated by the waveguide type wavelength conversion element is incident on a measurement object, and the light of the third wavelength transmitted through the measurement object is received by a light receiver to perform spectroscopic measurement In the detection method,
While periodically modulating the temperature of the waveguide type wavelength conversion element, and periodically modulating the refractive index of the waveguide type wavelength conversion element,
Spectral spectra are obtained by averaging the output of the photoreceiver over time over one period of the modulation or by statistically averaging the output multiple times.

A spectroscopic detection method according to a fifth invention for solving the above-mentioned problem is as follows.
In the spectroscopic detection method according to the fourth invention,
As the temperature modulation intensity of the waveguide type wavelength conversion element, a temperature modulation intensity at which a phase difference of the waveguide type wavelength conversion element due to a temperature change during temperature modulation is 2π or more is used.

  According to the present invention, due to the refractive index modulation by the temperature modulation of the waveguide type wavelength conversion element, the periodic wavelength dependence of the optical receiver output caused by the fringe of the waveguide type wavelength conversion element changes with time. Become. Then, the average wavelength dependence of the receiver output is averaged (cancelled) by averaging the receiver output over time over one modulation period or statistically averaging multiple outputs. Wavelength dependent intensity oscillation component (constant fringe) caused by reflection at the end face of the waveguide type wavelength conversion element is canceled to improve the signal-to-noise ratio of the spectrum and observe minute absorption lines be able to.

It is a figure which shows the simulation result of the temperature modulation amplitude dependence of a fringe intensity | strength component, (a) shows the change of fringe intensity | strength components in the case of the temperature modulation amplitude for phase difference 1 / 4pi, (b) is a figure. The change of the fringe intensity component when the temperature modulation amplitude is equal to the phase difference ½π is shown, (c) is the change of the fringe intensity component when the temperature modulation amplitude is the phase difference π, and (d) is the phase difference. The change of the fringe intensity component when the temperature modulation amplitude is a phase difference of 3 / 2π is shown, (e) shows the change of the fringe intensity component when the temperature modulation amplitude is a phase difference of 2π, and (f) is The change of the fringe intensity component when the temperature modulation amplitude is 3π of the phase difference is shown. In the waveguide type wavelength conversion element of the wavelength conversion light source used for the spectroscopic device concerning the present invention, it is a lineblock diagram showing an example of the composition of the element for temperature control, (a) is the side view, (b) is FIG. FIG. 7 is a configuration diagram showing another example of the configuration of the temperature control element in the waveguide type wavelength conversion element of the wavelength conversion light source used in the spectroscopic device according to the present invention, (a) is a side view thereof, (b) ) Is a top view, and (c) and (d) are cross-sectional views of the waveguide type wavelength conversion element. It is a block diagram which shows an example of the spectroscopic device which concerns on this invention. It is a wave form diagram which shows the observation result of the absorption line observed with the spectrometer of Example 1, (a) is a case where there is no temperature modulation, (b) is a case where there is temperature modulation. It is a wave form diagram which shows the observation result of the absorption line observed with the spectrometer of Example 3, (a) is a case where there is no temperature modulation, (b) is a case where there is temperature modulation. It is a block diagram which shows the conventional wavelength conversion light source. It is a block diagram which shows the spectrometer which used the conventional wavelength conversion light source. It is a graph which shows the example at the time of observing the absorption line of gas with a large absorption factor. It is a schematic diagram which shows the example at the time of observing the absorption line of gas with a small absorption factor. (A) is a waveform diagram showing the observation result of the 2f component when there is a fringe, and (b) is a waveform diagram showing the observation result of the 2f component when there is no fringe.

When light having a wavelength λ is passed through an optical component having a refractive index n and a length L, the optical path difference δ between the transmitted light and the light reflected and transmitted from both end faces can be expressed by the following equation (1). .
δ = 4πnL cos θ / λ (1)

Here, θ is the refraction angle of the optical component. In the case of the waveguide type wavelength conversion element, since light propagates in a specific direction, θ = 0 can be set.
δ = 4πnL / λ = 2mπ (m: integer) (2)

Under the condition of the above formula (2), when the phase difference is generated for one wavelength, superposition occurs at the same phase, so that the transmitted light is maximized. Therefore, the relationship that becomes the position of the fringe peak can be expressed as the following equation (3), where m is an integer.
λ = 2nL / m (3)

Now, it is known that when the temperature of the optical element changes, the refractive index changes accordingly, which is called the thermo-optic effect. If the temperature is changed to the first order term, the above equation (2) can be written as the following equation (4).
δ = 4πnL / λ × (1 + Δn / n) (4)

When Δt is a temperature difference and dn / dt is a temperature derivative of the refractive index, Δn is expressed by the following equation (5).
Δn = dn / dt × Δt (5)

Therefore, the phase difference Δδ due to temperature change is expressed by the following equation (6).
Δδ = 4πL / λ × dn / dt × Δt (6)

If Δδ changes by 2π, that is, if a temperature change occurs by a value represented by the following expression (7), the fringe moves by one period, and therefore the temperature is changed with a temperature modulation width of Δt ₂ π. If it can, the interference intensity can be controlled. Δt ₂ π is a temperature modulation width when Δδ changes by 2π.
Δt ₂ π = λ / (2Ldn / dt) (7)

For example, when λ = 1.39 μm, the element temperature is 40 ° C., and the element length is 48 mm, Δt ₂ π = 0.36 ° C. Here, the temperature differential dn / dt of the refractive index is a numerical value depending on the wavelength and temperature, and dn / dt of lithium niobate (bulk) is already known. Here, Δt ₂ π was calculated using dn / dt of lithium niobate at a wavelength of 1.39 μm and a temperature of 40 ° C. Although dn / dt in the waveguide is different from the bulk value, it can be calculated using the bulk value if the waveguide structure is determined.

  FIGS. 1A to 1F show simulation results of measuring the fringe intensity by sweeping the wavelength when periodic modulation of the temperature modulation frequency F [Hz] is performed with various temperature modulation amplitudes Δt. Show. The horizontal axis represents the wavelength sweep time and corresponds to the wavelength. The intensity amplitude of the fringe is normalized so that the value on the vertical axis is 1. When the amplitude is Δt, the modulation intensity is 2Δt because ± Δt modulation is applied. As can be seen from these results, when temperature modulation is performed with a modulation intensity (modulation width) at which the phase difference is less than 2π, it appears that periodicity remains, but with a modulation intensity at which the phase difference is 2π or more. That is, when temperature modulation is performed with a temperature modulation amplitude equal to or greater than that obtained by Equation (7), a result that can be regarded as a nearly random signal is obtained. In such a case, the fringe component can be removed by a time average of one or more periods of modulation or a statistical average of a plurality of spectra.

In the case of wavelength conversion using the second-order nonlinear optical effect, the wavelength at which wavelength conversion occurs most efficiently is called the phase matching wavelength λ _PM . When the temperature of the element changes and the refractive index changes, this value also changes and the wavelength conversion efficiency changes. That is, the output fluctuates. For this reason, it is desirable to modulate the temperature within a range in which the phase matching wavelength λ _PM is not significantly changed. It is known that the temperature dependence of λ _PM is generally expressed by the following formula (8) even when the pumping light wavelength or the signal light wavelength is changed when the element length is around 50 mm in the mid-infrared region. Yes. Note that λ _PM0 is a phase matching wavelength when Δt = 0, that is, at the reference temperature in the temperature modulation.
λ _PM [nm] _{≈λ PM0} + 0.1 × Δt (8)

If the variation width of λ _PM is within about 0.1 nm, the output is less likely to be adversely affected. Therefore, when Δt = 0 is the center of modulation, the upper limit is about ± 0.5 ° C. for Δt. Is desirable.

  Next, the configuration of the wavelength conversion light source used in the spectroscopic device according to the present invention will be described with reference to FIGS. As the wavelength conversion light source, a waveguide type wavelength conversion element 10 is used. The waveguide type wavelength conversion element 10 uses a direct bonding method in which the core layer is made of lithium niobate and the cladding layer is made of lithium tantalate. A ridge-type waveguide having a periodically poled structure is produced in the ridge-type waveguide.

  Then, a Peltier element 12, which is a temperature control element that can be temperature-controlled by an electric current, is attached to a metal plate carrier 11 with an adhesive or the like. A metal plate base 13 is bonded onto the Peltier element 12, and the waveguide type wavelength conversion element 10 is bonded onto the base 13. A thermistor 14 that can refer to the temperature from the resistance value is installed on the base 13, and the temperature is monitored, temperature control and temperature modulation are performed using this element. In the figure, reference numerals 12a and 12b denote Peltier element wirings, and reference numerals 14a and 14b denote thermistor wirings.

  3A to 3D show a configuration in which a Peltier element 16 and a thermistor 17 are further provided on the waveguide type wavelength conversion element 10 via a base 15 made of a metal plate. In this case, minute temperature modulation is performed by the upper Peltier element 16 near the waveguide on the upper surface of the waveguide type wavelength conversion element 10, and overall average temperature control is performed by the lower Peltier element 12. Since the monitoring elements are independent, the frequency for feedback can be assigned to an optimum value, which is suitable for more precise control. In the figure, reference numerals 16a and 16b indicate Peltier element wirings, and reference numerals 17a and 17b indicate thermistor wirings.

  When the waveguide type wavelength conversion element 10 is manufactured, as shown in FIG. 3C, when a waveguide is manufactured by forming a groove 10b in the substrate 10a by dicing, a Peltier element is formed above the waveguide. However, as shown in FIG. 3D, the groove 10b is filled with a resin 10c or the like to reinforce the strength, so that the Peltier is formed on the upper portion of the waveguide. The element 16 or the like can be attached.

[Example 1]
This embodiment will be described with reference to FIG. 2, FIG. 4 and FIG.

In this embodiment, a ridge-type waveguide using a direct bonding method in which the core layer is made of lithium niobate and the clad layer is made of lithium tantalate is manufactured for the waveguide type wavelength conversion element 10. In this ridge-type waveguide, a periodic polarization reversal structure with a reversal period Λ ₀ = 26.1 μm was fabricated to 48 mm. The thickness and width of the core layer of the ridge-type waveguide are 13 μm and 20 μm, respectively. The end face of the waveguide type wavelength conversion element 10 is cut at an angle of 6 degrees and AR coated.

  FIG. 4 shows an experimental system of the spectroscopic device of this example. This experimental system includes a waveguide type wavelength conversion element 10, a multiplexer 21, a signal light source 22 that outputs signal light (first wavelength light), and an excitation that outputs excitation light (second wavelength light). A mid-infrared light source 20 including a light source 23 is included. A 1.39 μm DFB-LD is used as the signal light source 22 and a 1.064 μm DFB-LD is used as the pumping light source 23, and each output light is combined by a multiplexer 21 which is a fiber-type coupler. And is input to the waveguide type wavelength conversion element 10. In the waveguide type wavelength conversion element 10, 4.54 μm mid-infrared light, which is converted light (light of the third wavelength), is generated by the difference frequency generation. The output light from the waveguide type wavelength converting element 10 is extracted only by mid-infrared light by a Ge filter, filled with air as outside air, and passed through a 10 m multi-pass cell 31 (measurement object) that is decompressed to 10 mTorr. The light receiver 24 receives the light.

  As described above with reference to FIG. 2, the thermistor 14 is provided together with the Peltier element 12 on the base 13 to which the waveguide type wavelength conversion element 10 is bonded, and the thermistor 14 is used to monitor the temperature. The temperature control unit 32 and the power source 33 are used to perform temperature control and temperature modulation of the waveguide type wavelength conversion element 10 by the Peltier element 12 (temperature modulation means).

  The signal light source 22 controls the injection current of the LD inside the light source by voltage input from the signal generator 25. In order to measure the 2f intensity, the signal generator 25 uses 1 Hz as the injection current to the signal light source 22. While modulating with a sawtooth wave, a sine wave with a frequency f of 15 kHz is superimposed so that the signal light source 22 can modulate and sweep the signal light as the input light. On the other hand, in the excitation light source 23, the wavelength of the excitation light that is input light is fixed. With such signal light and excitation light, the wavelength of the converted light emitted from the waveguide type wavelength conversion element 10 can be modulated and swept.

  The optical signal received by the light receiver 24 is input to the lock-in amplifier 26. The lock-in amplifier 26 detects the 2f intensity, and the time dependency of the detected 2f intensity can be observed with the oscilloscope 27. Yes. Note that a synchronization signal having a frequency f is input from the signal generator 25 to the lock-in amplifier 26, and a sawtooth wave synchronization signal is input from the signal generator 25 to the oscilloscope 27.

  In the experimental system described above, the signal light and the excitation light are combined by the multiplexer 21 and input to the waveguide type wavelength conversion element 10. Then, the optical signal received by the light receiver 24 is input to the lock-in amplifier 26, and the 2f intensity is detected by the lock-in amplifier 26. When the time dependence of 2f intensity is observed with the oscilloscope 27, the fluctuation of the fringe intensity change due to temperature modulation can be smoothed by the time constant of the lock-in amplifier 26 (because the time constant is long, the time average over one period of the modulation) Output), the intensity change due to fringing is reduced. Alternatively, the intensity change due to fringe is reduced by taking a statistical average of a plurality of spectra. In this way, the absorption line signal can be clearly observed.

  In this example, the wavelength dependence of the intensity change due to the fringe of the signal light mainly affects the output of the mid-infrared light that is the difference frequency light, and it is necessary to remove the fringe component of the signal light. is there. FIG. 5A shows an oscilloscope waveform when the waveguide-type wavelength conversion element 10 is not temperature-modulated. Although there was a disturbance in the center, an almost sine wave type output waveform caused by the fringe of the output light was obtained. In FIG. 5A and later-described FIG. 5B, the horizontal axis indicates the time corresponding to the wavelength, and the vertical axis indicates the signal intensity of the 2f component.

  In this experimental system, temperature modulation of the waveguide type wavelength conversion element 10 is performed. The temperature control unit 32 uses a power source 33 to feedback-control the amount of current flowing through the Peltier element 12 based on the temperature display of the thermistor 14 on the base 13, and in addition to the overall temperature control, a small Δt Temperature modulation was performed in a sinusoidal shape indicated by Δtsin (2πFT) (T is time). The minute temperature modulation may be 0.2 ° C. (modulation intensity is 0.4 ° C.) as the temperature modulation amplitude by the calculation of the above-described equation (7), and the temperature modulation is applied with this amplitude. Modulation was performed at a temperature modulation frequency F of 103.7 Hz. In this embodiment, as shown in FIG. 2, feedback control is performed by using one Peltier element 12 and the thermistor 14, respectively. However, as shown in FIG. 3, two Peltier elements 12, 16 and the thermistor are used. 14 and 17 can be used for more precise control.

When the temperature of the waveguide type wavelength conversion element 10 is modulated and the obtained spectrum is statistically averaged, as shown in FIG. 5B, a peak is observed in the vicinity of the center, and the theoretically predicted N ₂ O concentration It coincided with the position of the absorption line. Although the fluctuation intensity of the fringe decreased due to the rounding due to the time constant of the lock-in amplifier 26, the fringe could be effectively removed by performing statistical averaging of the spectrum. As described above, the effect of fringe has been successfully suppressed by performing periodic refractive index modulation by temperature modulation.

Here, the N ₂ O absorption line in the vicinity of 4.54 μm was observed, but the same effect can be obtained with other gases having different wavelength bands.

[Example 2]
In this example, the same wavelength type wavelength conversion element as that used in Example 1 is used, and the system of the spectroscopic apparatus is basically the same as that shown in FIG. Only different parts will be described with reference to FIG.

  In the first embodiment, the wavelength of the excitation light source 23 is fixed, and the wavelength of the signal light source 22 is modulated by a signal in which a sawtooth wave having a frequency of 1 Hz and a sine wave having a frequency of 15 kHz emitted from the signal generator 25 are superimposed. However, in this embodiment, the excitation light source 23 is modulated with a sine wave having a frequency of 15 kHz, and the signal light source 22 is modulated with a sawtooth wave having a frequency of 1 Hz. As described above, in the first embodiment, modulation is performed using a signal in which a sine wave and a sawtooth wave are superimposed on signal light. However, in this embodiment, modulation and sweep are performed by sharing roles between excitation light and signal light. Yes. Even in this case, as shown in (Formula A), the wavelength of the converted light itself is subjected to the modulation that is superimposed in both the first embodiment and the present embodiment.

  In the case of the present example, since the wavelength sweep is performed by the excitation light, it is sufficient that the temperature modulation amplitude is 0.15 ° C. (modulation intensity is 0.3 ° C.) according to the calculation of Expression (7) described above. The temperature modulation frequency was 205.3 Hz.

When the waveform was observed with the oscilloscope 27 in the same manner as in Example 1, when the temperature modulation of the waveguide type wavelength conversion element 10 was not performed, there was a disturbance in the center, but it was almost caused by the fringe of the output light. When a sine wave type output waveform was obtained and temperature modulation was performed, a peak almost identical to the theoretically predicted position of the N ₂ O absorption line was observed near the center.

[Example 3]
This embodiment will also be described with reference to FIGS.

In this embodiment, a ridge-type waveguide using a direct bonding method in which the core layer is made of lithium niobate and the clad layer is made of lithium tantalate is manufactured for the waveguide type wavelength conversion element 10. In this ridge-type waveguide, a periodic polarization reversal structure having a reversal period Λ ₀ = 17.6 μm was fabricated to 48 mm. The thickness and width of the core layer of the ridge-type waveguide are 8 μm and 14 μm, respectively. The end face of the waveguide type wavelength conversion element 10 is cut at an angle of 6 degrees and AR coated.

  Here, the signal light source 22 and the excitation light source 23 are the same input light source, and a 1.526 μm DFB-LD is used, and this output light is input to the waveguide type wavelength conversion element 10. In the waveguide type wavelength conversion element 10, red light of 0.763 μm, which is converted light, is generated by the second harmonic generation. The output light from the waveguide type wavelength conversion element 10 propagates 1 m of air (measuring object), which is outside air, and is received by the Si light receiver (light receiver 24).

  The wavelength of the input light source is modulated and swept by a signal generated by superimposing a sawtooth wave with a frequency of 1 Hz and a sine wave with a frequency of 15 kHz emitted from the signal generator 25. The output from the light receiver 24 is input to the lock-in amplifier 26, a 30 kHz component that is a double component of the 15 kHz sine wave is detected, and the output is input to the oscilloscope 27. The oscilloscope 27 is synchronized with the 1 Hz sawtooth wave, and the generated wavelength is modulated by the sawtooth wave. Therefore, the time axis corresponds to the change of the wavelength, and the spectrum of the 2f component from the lock-in amplifier 26 is observed. ing.

  FIG. 6A shows an oscilloscope waveform when temperature modulation is not performed. Although there was a disturbance in the center, an almost sine wave type output waveform caused by the fringe of the output light was obtained. In FIG. 6A and FIG. 6B described later, the horizontal axis indicates the time corresponding to the wavelength, and the vertical axis indicates the signal intensity of the 2f component.

  Next, based on the temperature display of the thermistor 14 on the base 13, the amount of current flowing through the Peltier device 12 is feedback-controlled, and in addition to the overall temperature control, a minute temperature modulation of Δt is Δtsin (2πFT). The sine wave shape shown was applied (T is time). The minute temperature modulation may be 0.225 ° C. (modulation intensity is 0.45 ° C.) as the temperature modulation amplitude by the calculation of the above-described equation (7), and the temperature modulation is applied with this amplitude. Modulation was performed at a temperature modulation frequency F of 92.3 Hz. Also in this embodiment, feedback control is performed by one Peltier element 12 and the thermistor 14 as shown in FIG. 2, but two Peltier elements 12, 16 and the thermistor 14 as shown in FIG. , 17 can also be controlled more precisely.

When the temperature was modulated and the obtained spectrum was statistically averaged, as shown in FIG. 6B, a peak was observed in the vicinity of the center, which coincided with the theoretically predicted position of the O ₂ absorption line. Although the fluctuation intensity of the fringe decreased due to the rounding due to the time constant of the lock-in amplifier 26, the fringe could be effectively removed by performing statistical averaging of the spectrum. As described above, the effect of fringe has been successfully suppressed by performing periodic refractive index modulation by temperature modulation.

[Other variations]
In Examples 1 to 3, lithium niobate which is a second-order nonlinear optical crystal is used for the core layer. However, a material in which protons are diffused in lithium niobate or another second-order nonlinear optical medium may be used. In particular, using lithium niobate or lithium tantalate, or using a material containing at least one selected from the group consisting of Mg, Zn, Sc and In as an additive in lithium niobate or lithium tantalate Is desirable. In the first to third embodiments, the difference frequency generation and the second harmonic generation are shown. However, even if wavelength conversion is performed using other second-order nonlinear optical effects such as sum frequency generation, the same effect can be obtained. can get.

  The present invention is suitable for gas sensing and substance optical spectroscopy.

DESCRIPTION OF SYMBOLS 10 Waveguide type | mold wavelength conversion element 12, 16 Peltier element 14, 17 Thermistor 20 Mid-infrared light source 21 Multiplexer 22 Signal light source 23 Excitation light source 24 Light receiver 25 Signal generator 26 Lock-in amplifier 27 Oscilloscope 32 Temperature control part 33 Power supply

Claims (5)

It consists of a second-order nonlinear optical medium having a periodically poled structure, and generates light of a new third wavelength by the second-order nonlinear optical effect by entering light of the first wavelength and light of the second wavelength. And at least one of the light of the first wavelength and the light of the second wavelength, and a waveguide type wavelength conversion element to be received and a light receiver that receives the light of the third wavelength transmitted through the measurement object In the spectroscopic device that performs spectroscopy by sweeping the wavelength of the third light by sweeping the wavelength of the light,
Providing temperature modulation means for periodically modulating the temperature of the waveguide type wavelength conversion element, and periodically modulating the refractive index of the waveguide type wavelength conversion element;
A spectroscopic device that obtains a spectrum by averaging the output of the light receiver over time over one period of the modulation or by statistically averaging the output multiple times. The spectroscopic device according to claim 1,
The temperature modulation means uses, as the temperature modulation intensity of the waveguide type wavelength conversion element, a temperature modulation intensity at which a phase difference of the waveguide type wavelength conversion element due to a temperature change during temperature modulation is 2π or more. A spectroscopic device. The spectroscopic device according to claim 1 or 2,
The spectroscopic device, wherein the waveguide type wavelength conversion element is made of a material containing at least one selected from the group consisting of Mg, Zn, Sc and In as an additive in LiNbO ₃ or LiNbO ₃ . The light of the first wavelength and the light of the second wavelength are incident on a waveguide type wavelength conversion element made of a second order nonlinear optical medium having a periodically poled structure, and the second order nonlinearity of the waveguide type wavelength conversion element. By generating a light of a new third wavelength by the optical effect, and sweeping the wavelength of at least one of the light of the first wavelength and the light of the second wavelength, Sweep wavelength,
Spectroscopy in which light of the third wavelength generated by the waveguide type wavelength conversion element is incident on a measurement object, and the light of the third wavelength transmitted through the measurement object is received by a light receiver to perform spectroscopic measurement In the detection method,
While periodically modulating the temperature of the waveguide type wavelength conversion element, and periodically modulating the refractive index of the waveguide type wavelength conversion element,
A spectral detection method characterized in that a spectral spectrum is obtained by time averaging the output of the light receiver over one period or more of the modulation, or by statistically averaging the output a plurality of times. The spectroscopic detection method according to claim 4.
A spectral detection method using a temperature modulation intensity at which a phase difference of the waveguide type wavelength conversion element is 2π or more due to a temperature change during temperature modulation, as the temperature modulation intensity of the waveguide type wavelength conversion element.

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