JP6093246B2 - Mid-infrared wavelength conversion light source - Google Patents

Description

  The present invention relates to a wavelength conversion light source, and more specifically to a wavelength conversion light source that generates light in the mid-infrared wavelength region suitable for gas sensing and 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.

  Although various types of wavelength conversion elements can be used, a waveguide type wavelength conversion element using a quasi-phase matching in which a nonlinear optical constant is periodically modulated is most promising from a practical viewpoint. In order to form a periodic modulation structure with a nonlinear constant, a method of alternately inverting the sign of the nonlinear constant, or substantially alternately arranging a portion with a large nonlinear constant and a portion with a small nonlinear constant can be considered.

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

FIG. 1 shows a configuration of a conventional wavelength conversion light source. A light source 10 shown in FIG. 1 includes a LiNbO ₃ substrate 11 on which an optical waveguide 12 is formed, a multiplexer 15, two semiconductor lasers (not shown) that output signal light 13 and excitation light 14, respectively. Consists of The signal light 13 and the excitation light 14 are multiplexed by a multiplexer 15 and are incident on an optical waveguide 12 formed on a LiNbO ₃ substrate 11 whose polarization is periodically inverted. In the optical waveguide 12, converted light 16 that is a difference frequency light between the signal light 13 and the excitation light 14 is generated. Assuming that the wavelength of the signal light is λ ₁ , the wavelength of the converted light (idler light) is λ ₂ , and the wavelength of the excitation light is λ ₃ , these three wavelengths satisfy the following (Equation 1).
1 / λ ₃ = 1 / λ ₂ + 1 / λ ₁ (Formula 1)

For example, if the signal light wavelength λ ₁ is 1.56 μm and the excitation light wavelength λ ₃ is 1.06 μm, converted light with λ ₂ = 3.31 μm can be generated. When the refractive index at the signal light wavelength λ ₁ is n ₁ , the refractive index at the converted light wavelength λ ₂ is n ₂ , the refractive index at the pumping light wavelength λ ₃ is n ₃ , and the modulation period of the nonlinear constant is Λ, the phase mismatch amount Δβ is given by the following (formula 2).
Δβ = 2π (n ₃ / λ ₃ −n ₂ / λ ₂ −n ₁ / λ ₁ ) (Formula 2)

  The conversion efficiency η is expressed by the following (Equation 3) with respect to the phase mismatch amount Δβ shown in the above (Equation 2). Here, L represents the length of the light traveling direction of the nonlinear medium of the wavelength conversion light source.

From (Equation 3), the conversion efficiency η becomes maximum when Δβ = 2π / Λ. For example, when the pumping light wavelength λ ₃ is fixed, the signal light wavelength λ ₁ and the pumping light wavelength λ ₃ satisfying a so-called quasi-phase matching condition where the phase mismatch amount Δβ = 2π / Λ given by (Equation 2) is satisfied. Depends on the chromatic dispersion of the refractive index of the nonlinear material and is determined substantially uniquely when the modulation period Λ is determined. If the signal light wavelength λ ₁ or the pumping light wavelength λ ₃ is changed from a so-called quasi phase matching wavelength that satisfies the quasi phase matching condition, the conversion efficiency decreases according to (Equation 2) and (Equation 3).

FIG. 2 shows a change in conversion efficiency with respect to the phase mismatch amount in the wavelength conversion light source. In FIG. 2, the horizontal axis represents (Δβ-2π / Λ) L / π, and the vertical axis represents the conversion efficiency normalized with the maximum value of the conversion efficiency in the wavelength conversion light source as 1. For example, when the pumping light wavelength λ ₃ is fixed and the signal light wavelength λ ₁ is converted, the wavelength band corresponding to the phase mismatch amount at which the conversion efficiency shown in FIG. When a LiNbO ₃ waveguide having a length L of 50 mm is used, it is about 6.8 nm when converted to a converted light wavelength in the 3.31 μm band, which is narrow.

Therefore, as shown in Patent Document 1, the group speeds of the excitation light of the wavelength 1 μm band and the converted light of the wavelength 3 μm band are matched, the signal light of the wavelength 1.55 μm band is fixed, and the excitation light of the wavelength 1 μm band When the LiNbO ₃ waveguide having a length L of 50 mm is used, the wavelength band corresponding to the phase mismatch amount at which the conversion efficiency shown in FIG. 2 is half of the maximum value is converted in the 3.31 μm band. When converted to the light wavelength, it spreads to about 123 nm. Further, the patent document discloses a wavelength conversion light source having a wide spectrum in a wavelength of 3 μm using short pulse light using a Yb-doped fiber for light of a wavelength of 1 μm.

Here, it will be described from another point of view that the excitation light wavelength tolerance is wide in the domain-inverted LiNbO ₃ in which the group velocity matching is achieved. FIG. 3 shows the phase matching relationship between the excitation wavelength and the signal light wavelength in the case of the polarization inversion period Λ = 30 μm of bulk LiNbO ₃ . In FIG. 3, a combination of phase matching in bulk LiNbO ₃ when the polarization inversion period Λ = 30 μm when the pumping light wavelength and the signal light wavelength are changed is shown by a curve of Λ = 30 μm. That is, since the phase matching is achieved at each pumping light wavelength and signal light wavelength on the curve showing Λ = 30 μm in FIG. 3, (Equation 2) is satisfied, and (Equation 3) becomes η = η _max .

  In addition to the curve of Λ = 30 μm, FIG. 3 shows a curve in which the converted light wavelength uniquely determined by (Equation 1) is displayed in contour lines. That is, for example, when the excitation light wavelength is 1 μm and the signal light wavelength is 1.5 μm, the converted light wavelength is 3 μm because it is on the curve of the converted light 3 μm. At this time, when the signal light wavelength is fixed to 1.54 μm and the excitation light wavelength is changed from 1.05 μm to 1.08 μm, the line segment indicated by the excitation light wavelength and the signal light wavelength is a curve of approximately Λ = 30 μm. Therefore, even if the excitation light wavelength is changed, phase matching is performed with a polarization inversion period that is close to a polarization inversion period of approximately Λ = 30 μm. In other words, the phase mismatch amount shown in FIG. 2 does not change greatly even when the pumping light wavelength is changed, and only a slight deviation from 0 does not significantly reduce the normalized conversion efficiency. Therefore, a wide phase matching wavelength width can be obtained even if the excitation light wavelength is changed.

JP 2011-203376 A

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”, 2006, APPLIED PHYSICS LETTERS, Vol.88, No.6, 061101-1-061101-3. Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki, “Direct-bonded QPM-LN ridge waveguide with high damage resistance at room temperature”, 2003, Electronics Letters, Vol.39, No .7, p.609-611. K. Iwakuni, H. Inaba, Y. Nakajima, T. Kobayashi, K. Hosaka, A. Onae, FL. Hong, “Narrow linewidth comb realized with a mode-locked fiber laser using an intra-cavity waveguide electro-optic modulator for high-speed control ”, 2012, OPTICS EXPRESS, Vol.20, No.13, p.13769-13776. A. Ishizawa, T. Nishikawa, A. Mizutori, H. Takara, S. Aozasa, A. Mori, H. Nakano, A. Takada, M. Koga, “Octave-spanning frequency comb generated by 250 fs pulse train emitted from 25 GHz externally phase-modulated laser diode for carrier-envelope-offset-locking ”, 2010, Electronics Letters, Vol. 46, No. 19, p.1343-1344.

  In the mid-infrared region, hydrocarbon gases such as methane and ethane are present in the wavelength range of 3.0 to 3.4 μm, carbon monoxide is present in the 2.3 μm band and 4.6 μm band, 2.7 μm band and 4.3 μm. Various gases, such as carbon dioxide in the band and nitrous oxide in the 4.5 μm band, are scattered at 2 to 4.5 μm. In the mid-infrared light source that generates the difference frequency in the nonlinear optical medium by combining the excitation light of the 1 μm wavelength light and the signal light of the 1.55 μm wavelength as described above, the 1 μm wavelength light is swept. Thus, a wide tuning range of 123 nm can be provided in the wavelength 3.3 μm band.

  However, such a conventional wavelength conversion light source cannot efficiently generate converted light over a wide wavelength range, and can measure only one kind of gas such as methane gas existing in a wavelength range of 3.0 to 3.4 μm. There is a problem that it cannot be applied when it is necessary to sense various gases scattered in the wavelength range of 2 to 4.5 μm.

  The present invention has been made in view of such problems, and its object is to generate light in the mid-infrared wavelength region over a wide wavelength range in the 2 to 4.5 μm band. Provided is a mid-infrared wavelength conversion light source having a generation wavelength range of 1 μm or more which is at least 10 times the wavelength range of the order of 0.1 μm shown in Patent Document 1 There is to do.

A mid-infrared wavelength conversion light source according to claim 1 of the present invention includes an optical frequency comb-type light generation unit, an Er-doped fiber amplifier connected to the optical frequency comb-type light generation unit, and the Er-doped fiber amplifier. An optical frequency comb light source that outputs at least an optical frequency comb; and a nonlinear optical medium that receives the optical frequency comb output from the optical frequency comb light source and generates a second-order nonlinear optical effect. wherein with the door, said nonlinear optical medium comprises a LiNbO ₃ waveguide having a polarization reversal structure polarization direction is periodically inverted nonlinear optical material, in the LiNbO ₃ optical waveguide of the nonlinear optical medium, input The difference 1 / λi = 1 / λp−1 / λs is satisfied by the difference frequency generation between the light having the wavelength λp and the light having the wavelength λs selected from a large number of lights of the optical frequency comb. And outputs the converted light to wavelength .lambda.i, by the difference frequency generation is performed in a plurality of combinations among the light having respective wavelengths of the optical frequency comb, a plurality of converted light .lambda.i is 2~4.5μm The polarization inversion period is set so that the polarization inversion structure satisfies a phase matching condition in which λi is 2 to 4.5 μm with respect to the wavelength λp and the wavelength λs. .

  The wavelength-converted light source according to claim 2 of the present invention is the wavelength-converted light source according to claim 1 of the present invention, and a filter that attenuates light of 2 μm or less is disposed after the nonlinear optical medium. It is characterized by.

  The mid-infrared wavelength conversion light source according to claim 3 of the present invention is the wavelength conversion light source according to claim 1 or 2 of the present invention, and a branching unit that branches the output light of the optical frequency comb light source, The Yb-doped fiber amplifier that inputs one output light branched by the branching means, the other output light branched by the branching means, and the output light of the Yb-doped fiber amplifier are combined, and the combined light Is further provided with a multiplexing means for outputting to the nonlinear optical medium.

Infrared wavelength conversion light source in the claim 4 of the present invention is a wavelength conversion light source according to claim 1 of the present invention, the nonlinear optical material, Mg in LiNbO ₃ or LiNbO _3, Zn, Sc And at least one selected from the group consisting of In is made of a material containing as an additive.

Infrared wavelength conversion light source in the claim 5 of the present invention is a wavelength conversion light source according to claim 1 of the present invention, the polarization inversion period of the poled structure lambda ([mu] m) is, the LiNbO ₃ against the optical waveguide path of the core area a (μm ^2), and satisfies the -280 / a + 29 ≦ Λ ≦ -280 / a + 32.

A mid-infrared wavelength conversion light source according to a sixth aspect of the present invention is the wavelength conversion light source according to any one of the first to fifth aspects of the present invention, wherein the optical frequency comb seed light generating means is an Er addition. A fiber loop including at least a fiber and an isolator; a pumping light source that outputs pumping light for pumping the Er-doped fiber; and a multiplexing unit that combines the pumping light output from the pumping light source into the fiber loop; And a demultiplexing unit that extracts light from the fiber loop and outputs the light to the Er-doped fiber amplifier.

A mid-infrared wavelength conversion light source according to a seventh aspect of the present invention is the wavelength conversion light source according to any one of the first to fifth aspects of the present invention, wherein the optical frequency comb light generating means is an output light. At least, and a modulation means for modulating the output light of the semiconductor laser.

  According to the present invention, a mid-infrared optical frequency comb light source having a very wide converted light by difference frequency generation is realized by inputting light having a wide band generated from an optical frequency comb light source to a nonlinear optical medium. If the output light is observed with a spectroscope after absorption of the gas, it is possible to perform spectroscopy in a wide wavelength band. Also, another so-called mid-infrared optical frequency comb light source having a different repetition frequency is prepared, and the light output from the two mid-infrared optical frequency comb light sources is received by one light receiver and observed by an electric spectrum analyzer. Using dual comb spectroscopy, an absorption spectrum can be observed instantaneously.

It is a figure which shows the structure of the conventional wavelength conversion light source. It is a figure which shows the change of the conversion efficiency with respect to the phase mismatching amount in a wavelength conversion light source. Poling period of the bulk LiNbO ₃ is a diagram showing the phase-matching relationship between the excitation wavelength and the signal light wavelength in the case of lambda = 30 [mu] m. It is a figure which shows the phase matching relationship between an excitation wavelength and a signal light wavelength in the case where the polarization inversion period of bulk LiNbO ₃ is Λ = 20, 25, 30, 35 μm. It is a figure which shows the phase matching relationship between the excitation wavelength and signal light wavelength in the case where the polarization inversion period of bulk LiNbO ₃ is Λ = 20, 25, 30, 35 μm in the case of using a Yb-doped fiber amplifier. It is a figure which shows the phase matching relationship between an excitation wavelength and a signal light wavelength in the case where the polarization inversion period of bulk LiNbO ₃ is Λ = 20, 25, 30, 35 μm when an Er-doped fiber amplifier is used. It is a figure which shows the phase matching relationship of an excitation wavelength and a signal light wavelength in case the polarization inversion period of LiNbO3 which has a waveguide structure is (LAMBDA) = 28, 29, 30, ₃₁ micrometers. It is a figure which shows the structure of the wavelength conversion light source which concerns on Example 1 of this invention. Is a diagram showing an input spectrum and the transmitted light spectra of LiNbO ₃ waveguide in the wavelength conversion light source according to Embodiment 1 of the present invention. Is a diagram showing an input spectrum and the transmitted light spectra of LiNbO ₃ waveguide in the wavelength conversion light source according to a second embodiment of the present invention. It is a figure which shows the structure of the wavelength conversion light source which concerns on Example 3 of this invention. Is a diagram showing an input spectrum and the transmitted light spectra of LiNbO ₃ waveguide in the wavelength conversion light source according to a third embodiment of the present invention. It is a figure which shows the structure of the wavelength conversion light source which concerns on Example 4 of this invention. Is a diagram showing an input spectrum and the transmitted light spectra of LiNbO ₃ waveguide in the wavelength conversion light source according to a fourth embodiment of the present invention. It is the schematic of the structure of the wavelength conversion light source which concerns on this invention.

  FIG. 15 shows a schematic diagram of a configuration of a wavelength conversion light source according to the present invention. As shown in FIG. 15, the wavelength conversion light source 100 according to the present invention includes an optical frequency comb light source 110 and a nonlinear optical medium 120. The optical frequency comb light source 110 includes an optical frequency comb seed light generation unit 101, an Er-doped fiber amplifier 102 connected to the subsequent stage of the optical frequency comb seed light generation means 101, and a high frequency connected to the subsequent stage of the Er-doped fiber amplifier 102. A nonlinear fiber 103.

The inventor of the present invention diligently studied the configuration of a light source capable of obtaining output light over a wide wavelength band in a wavelength range of 2 to 4.5 μm where various gases exhibit large absorption. As a result, by inputting the optical frequency comb output from the optical frequency comb light source 110 as shown in FIG. 15 to the nonlinear optical medium 120, various pumping light wavelengths and signal light wavelengths at each wavelength of the optical frequency comb are obtained. Can be realized, and by generating a difference frequency of each combination using a nonlinear optical medium 120 having a LiNbO ₃ optical waveguide having periodic polarization inversion, various wavelengths that maintain phase matching conditions in a wide wavelength range It was discovered that the light can be effectively converted into a broadband mid-infrared wavelength region. The operation principle will be described below.

  As the optical frequency comb light source 110 shown in FIG. 1, for example, there is an optical frequency comb light source as shown in Non-Patent Document 3 or Non-Patent Document 4, but the output of the highly nonlinear fiber is 1 for both optical frequency comb light sources. It spreads over a wavelength range of 0.0 μm to 2.1 μm. Such a light source amplifies a group of laser beams having a frequency difference of equal intervals including laser beams of at least two wavelengths by an Er-doped fiber amplifier, and has the frequency difference by four-wave mixing in a highly nonlinear fiber. Are successively generated on the long wavelength side and the short wavelength side, and the wavelength width of the generated light is expanded.

Each wavelength of the optical frequency comb output from the optical frequency comb light source 110 satisfies the following (Equation 4) at various integers n using the repetition frequency f _rep , f _CEO indicating the remainder of the frequency, and an integer n. It can be regarded as an aggregate of f _n wavelengths.
f _n = f _CEO + n · f _rep (Formula 4)

The light having each frequency f _n is laser light, that is, the optical frequency comb can be said to be a lump of laser light. When a laser beam group having this frequency group is input to the nonlinear optical medium 120, if n = a and n = b (a> b), the difference between the two frequencies is obtained by the difference frequency generation. Light having a frequency f ′ _ab without f _CEO, which is a surplus term shown in the following (Formula 5), is generated.
f ′ _ab = ( _ab ) f _rep (Formula 5)

FIG. 4 shows the phase matching relationship between the excitation wavelength and the signal light wavelength when the polarization inversion period of bulk LiNbO ₃ is Λ = 20, 25, 30, 35 μm. In FIG. 4, in addition to the curve of Λ = 30 μm shown in FIG. 3, curves of Λ = 20 μm, Λ = 25 μm, and Λ = 35 μm are added, and the excitation light wavelength and the signal light wavelength are compared with those shown in FIG. The graph which expanded the range of is shown. Also in FIG. 4, bulk LiNbO ₃ is used as the nonlinear optical medium.

The light output from the optical frequency comb light source 110 to the nonlinear optical medium 120 is an optical frequency comb having a wavelength range extending to a band of 1 to 2.1 μm. The light of the optical frequency comb becomes both excitation light and signal light. The light in the optical frequency comb can be obtained and exists in the wavelength region 401 where the hatching regions 402 and 403 shown in FIG. 4 overlap. The light existing in the optical frequency comb follows (Equation 4), that is, the light exists at every optical frequency interval f _rep of the optical frequency comb. This frequency interval f _rep can be determined by the optical path length of an optical loop including an Er-doped fiber when the optical frequency comb light source 110 takes a form as shown in Non-Patent Document 3, and the optical frequency comb light source 110 Can be defined by the modulation frequency of the modulation means. This frequency interval f _rep can be set to about 50 MHz.

  On the other hand, in the graph of the relationship satisfying the phase matching condition in the case of various polarization inversion periods shown in FIG. 4, the relationship satisfying the phase matching condition is satisfied only on a limited curve, but also shown in FIG. Thus, even if it deviates from the phase matching condition, the light is not converted at all, and the conversion efficiency slowly decreases with a certain width. As described above, this phase matching width is obtained by converting the pumping light wavelength in the 1 μm band to the converted light wavelength in the 3.31 μm band when the signal light wavelength in the 1.55 μm band is changed. Although it is about 6.8 nm and narrow, it is about 100 GHz when converted to the optical frequency, and is sufficiently wide as compared with the optical frequency comb frequency width of 50 MHz. That is, in the pumping light and signal light of the optical frequency comb having a mesh interval of about 50 MHz, the combination of the pumping light and signal light existing within the phase matching allowable width of 100 GHz efficiently generates converted light by the difference frequency generation. To do.

  In the phase matching condition of Λ = 30 μm in FIG. 4, the excitation light and the signal light having the excitation light wavelength in which the curve of Λ = 30 μm exists in the wavelength region 401 where the hatching regions 402 and 403 shown in FIG. 4 overlap. Conversion light is generated by the signal light of the wavelength. As for the converted light wavelength generated at this time, it is understood that converted light having a wavelength of 2 to 4.5 μm can be generated by reading the relationship between the converted light wavelengths shown in FIG. Furthermore, since the generated converted light having a wavelength of 2 to 4.5 μm is also present in the nonlinear optical medium 120, this becomes a new signal light, and the range of the mid-infrared light wavelength generated from 4.5 to 4.7 μm is expanded.

Although not shown in FIG. 4, when Λ = 29 μm or the like, converted light having a wavelength of 3 to 5 μm can be obtained. Furthermore, when a nonlinear optical element with Λ = 25 μm is used, converted light having a wavelength of 5 to 6 μm can be obtained, and a converted light width of 1 μm can be provided. The widest wavelength width can be obtained when the signal light wavelength abruptly phase-matched near the excitation light wavelength of 1 μm becomes longer like the above-described nonlinear optical element with Λ = 30 μm. On the other hand, if the polarization inversion period is long, such as Λ = 33 μm, a curve indicating phase matching protrudes from the wavelength region 401 where hatching overlaps, and converted light is not generated. Therefore, if possible, in the case of bulk LiNbO ₃ , it is desirable to select an element with Λ = 29 to 32 μm.

  In this way, by inputting the optical frequency comb whose wavelength is spread to the band of 1 to 2.1 μm into the nonlinear optical medium using the optical frequency comb light source, the optical frequency comb input to the nonlinear optical medium is 1 to 2. Since it has various wavelengths between 1 μm, it is possible to construct a mid-infrared light source that can cover a wide wavelength range by generating converted light by combining pump light of various pump light wavelengths and signal light of signal light wavelengths. it can.

  Here, consider a case where a fiber amplifier is used to enhance the converted light output in the wavelength conversion light source 100 shown in FIG. In general, a Yb-doped fiber amplifier can be used when amplifying light of a wavelength of 1 μm band, and an Er-doped fiber amplifier can be used when amplifying light of a wavelength of 1.55 μm. Here, in the wavelength conversion light source 100 shown in FIG. 15, the output from the highly nonlinear fiber 103 is branched by, for example, a branching unit that branches light having a wavelength of 1.2 μm or less and light having a wavelength of 1.2 μm or more. A Yb-doped fiber amplifier is connected to the output side of the branching means that transmits light of 1.2 μm or less, and the output of the Yb-added fiber amplifier and the output side of the branching means that transmits wavelengths of 1.2 μm or more are combined to produce nonlinear optics. Consider the case of input to the medium 104.

In this case, FIG. 5 shows the phase matching relationship between the excitation wavelength and the signal light wavelength in the case of using periodically poled bulk LiNbO ₃ as the nonlinear optical medium. As shown in FIG. 5, the pumping light wavelength range is limited by the Yb-doped fiber amplifier as compared to the case shown in FIG. 4, but when Λ = 30 μm, the conversion of the wavelength range of 2 to 4 μm is performed. Light is obtained.

  On the other hand, instead of the Yb-doped fiber amplifier, consider using an Er-doped fiber amplifier on the branch port side of the branching means that transmits 1.2 μm or more. FIG. 6 shows the phase matching relationship between the excitation wavelength and the signal light wavelength at that time. As shown in FIG. 6, considering the curve Λ = 30 μm and the hatched region 601, converted light in the wavelength range of 3 to 4 μm can be obtained, but the wavelength range is limited to 1 μm. . When the Er-doped amplifier is arranged in the subsequent stage of the highly nonlinear fiber, the light spread in the 1 to 2 μm band is filtered within the band of the Er-doped fiber amplifier. It is preferable not to install an Er-doped fiber amplifier between the fiber and the nonlinear optical medium.

Next, consider that the nonlinear optical medium has a waveguide structure. As a waveguide fabrication method, for example, there is a method of fabricating a direct junction ridge waveguide as shown in Non-Patent Document 1. In this direct junction ridge waveguide, the LiNbO ₃ substrate serving as the core layer is first subjected to polarization inversion, the LiNbO ₃ substrate and the LiTaO ₃ substrate serving as the cladding layer are directly joined, and then the LiNbO ₃ substrate is reduced to a thickness of about 10 μm. The film is thinned and confined in the lateral direction by dicing to form a waveguide structure.

FIG. 7 shows the phase matching relationship between the pumping light wavelength and the signal light wavelength in the case of a LiNbO ₃ waveguide having a core thickness of 10 μm and a core width of 14 μm manufactured by the direct bonding method. As shown in FIG. 7, it can be seen that when Λ = 28 μm, converted light of 2 to 4.2 μm can be obtained. Further, it can be seen that the generation wavelength range becomes narrower as Λ = 29 μm and 30 μm. On the other hand, although not shown, when Λ is shortened to 27 μm, converted light in a wide wavelength range of 3 to 5 μm can be obtained. In view of the above, in the case of a LiNbO ₃ waveguide having a core thickness of 10 μm and a core width of 14 μm, a polarization inversion period of 27 μm to ₃₀ μm is considered optimal.

Similarly, when various core sizes were intensively studied and the optimum polarization inversion period was obtained, the following relationship was found with respect to the range between the core size and the polarization inversion period. For the core area A (μm ² ), it is desirable that the polarization inversion period Λ (μm) satisfy the following (formula 6).
−280 / A + 29 ≦ Λ ≦ −280 / A + 32 (Formula 6)

[Example 1]
FIG. 8 shows a wavelength conversion light source according to the first embodiment of the present invention. FIG. 8 shows a pump LD 810, a fiber loop 820, an Er-doped fiber amplifier 830, a polarization controller 840, a highly nonlinear fiber 850, and a periodically poled LiNbO ₃ waveguide (hereinafter referred to as LiNbO ₃ waveguide). A wavelength conversion light source 800 including 860 and an optical filter 870 made of Ge that cuts light of 2 μm or less is shown.

  In the fiber loop 820, an Er-doped fiber 821, a polarization controller 822, an isolator 823 for determining the traveling direction of the oscillating laser light, and an outside of the fiber loop 820 for exciting the Er-doped fiber 821. A coupler 824 that inserts the pumping light output from the pumping LD 810 into the fiber loop 820 and a branching unit 825 for outputting the light in the fiber loop 820 to the outside are arranged. In the wavelength conversion light source 800 according to the first embodiment, a 1.48 μm band laser is used as a pumping LD, and a fiber laser type fiber loop 820 using an Er-doped fiber 821 as a gain medium as an optical frequency comb type light generating means. Using.

The LiNbO ₃ waveguide 860 was produced by a direct bonding method. As the core of the LiNbO ₃ waveguide 860, LiNbO ₃ added with 7 mol% of Zn and LiTaO ₃ as the cladding were used, and a ridge type optical waveguide was formed by dicing. The core thickness of the LiNbO ₃ waveguide 860 was 10 μm, and the core width was 14 μm. The polarization inversion period Λ of the LiNbO ₃ waveguide 860 was 29.0 μm, and the element length was 50 mm.

The pump light output from the pump LD 810 is input to the fiber loop 820 via the coupler 824, and the output light of the fiber loop 820 is output to the Er-doped fiber amplifier 830 via the branching means 825, and then to the Er-doped fiber amplifier 830. And output to the polarization controller 840 and the highly nonlinear fiber 850, and the output is injected into the LiNbO ₃ waveguide 860.

Figure 9 shows the spectrum after passing through the transmitted light spectrum and filter output from an input light spectrum and LiNbO ₃ waveguide 860 into LiNbO ₃ waveguide 860 in the first embodiment. In FIG. 9, the output value is shown as a relative intensity. As shown in FIG. 9, the optical frequency comb whose wavelength spreads in the 1 to 2 μm band is transmitted through the LiNbO ₃ waveguide 860 to be converted into transmitted light having a wavelength in the range of 2 to 4.5 μm. It can be seen that light in the mid-infrared region is selectively output by passing through. That is, with the wavelength conversion light source 800 according to Example 1, output light could be obtained in a wide range of 2.5 μm.

In Example 1, using a LiNbO ₃ with the addition of Zn as the core of the LiNbO ₃ waveguide 860. By using LiNbO ₃ to which Zn is added, optical damage can be prevented particularly when the intensity of the excitation light with a short wavelength is large. LiNbO ₃ to which Mg, Sc, In or the like is added in addition to Zn can be used in the same manner for the purpose of preventing optical damage.

  In the configuration of the first embodiment, the polarization controllers are inserted in two places, but may be added where necessary. Further, the system may be configured by a polarization preserving system and the polarization controller may be eliminated. In the first embodiment, the mechanism for adjusting the length of the fiber loop 820 is not used. However, since the repetition frequency of the optical frequency comb is determined by the fiber loop length, the fiber frequency is stabilized in order to stabilize the repetition frequency. A long control mechanism may be inserted. In addition, the output of the highly nonlinear fiber 850 may be branched and monitored in order to control the pump LD 810 to stabilize the wavelength of the optical frequency comb or to output a control signal.

[Example 2]
As the wavelength conversion light source according to the second embodiment, a light source that is different from the first embodiment only in the domain-inverted LiNbO ₃ waveguide is used. In Example 2, a LiNbO ₃ waveguide having a polarization reversal period Λ of 27.5 μm of polarization reversal LiNbO ₃ was used. Figure 10 shows the transmitted light spectrum outputted from the optical spectrum and LiNbO ₃ waveguide to LiNbO ₃ waveguide in the second embodiment. In FIG. 10, the output value is shown by relative intensity. As shown in FIG. 10, the optical frequency comb whose wavelength spreads in the range of 1 to 2 μm transmits through the LiNbO ₃ waveguide, thereby allowing transmission in the wavelength range of 3 to 5 μm on the long wavelength side compared to Example 1. It can be seen that it has been converted to light. That is, also in Example 2, output light could be obtained in a wide range of 2 μm.

[Example 3]
In FIG. 11, the wavelength conversion light source which concerns on Example 3 of this invention is shown. FIG. 11 shows, for example, an LD 1110 that outputs light in a 1.55 μm band, a modulation means 1120 including a phase modulator 1121 and an intensity modulator 1122, a polarization controller 1130, an Er-doped fiber amplifier 1140, and a highly nonlinear fiber. A wavelength converted light source 1100 with 1150 and a LiNbO ₃ waveguide 1160 is shown. The wavelength conversion light source 1100 according to the third embodiment is obtained by changing the portion of the optical frequency comb type light generation unit including the fiber loop 820 of the first embodiment into an LD 1110 and a modulation unit 1120.

In the wavelength conversion light source 1100 according to the third embodiment, the excitation light output from the continuously oscillating 1.55 μm LD 1110 is modulated by the modulation means 1120 including the phase modulator 1121 and the intensity modulator 1122, and the output is added by Er. Input to the fiber amplifier 1140. The output of the Er-doped fiber amplifier 1140 is input to the highly nonlinear fiber 1150 and input to the LiNbO ₃ waveguide 1160. In Example 3, the same LiNbO ₃ waveguide 1160 as the LiNbO ₃ waveguide 860 according to Example 1 was used.

Figure 12 shows the transmitted light spectrum outputted from the input light spectrum and LiNbO ₃ waveguide 1160 to the LiNbO ₃ waveguide 1160 in the third embodiment. In FIG. 12, the output value is shown by relative intensity. As shown in FIG. 12, the optical frequency comb whose wavelength spreads in the 1 to 2 μm band is transmitted through the LiNbO ₃ waveguide 1160, thereby being converted into transmitted light having a wavelength range of 2 to 4.5 μm. Recognize. That is, also in Example 3, output light could be obtained in a wide range of 2.5 μm.

  In the third embodiment, the modulation unit 1120 including the phase modulator 1121 and the intensity modulator 1122 is used. The modulation means 1120 determines the repetition frequency of the optical frequency comb. The modulation frequency of the modulation means 1120 may be stabilized so that a part of the optical system of this configuration is separately observed and the repetition frequency is stabilized.

  In the third embodiment, the polarization controller 1130 is inserted between the intensity modulator 1122 and the Er-doped fiber amplifier 1140, but the position may be changed as appropriate. Further, the polarization controller may be omitted by assembling an optical system in the polarization preserving system. In the third embodiment, the continuously oscillating LD 1110 is driven by constant current control. Since fluctuation of the oscillation wavelength of the LD 1110 affects the overall wavelength of the optical frequency comb, a part of the optical system of this configuration may be observed separately and fed back to the LD to stabilize the oscillation wavelength.

[Example 4]
In FIG. 13, the wavelength conversion light source which concerns on Example 4 of this invention is shown. FIG. 13 shows an LD 1310, a modulation means 1320 comprising a phase modulator 1321 and an intensity modulator 1322, a polarization controller 1330, an Er-doped fiber amplifier 1340, a highly nonlinear fiber 1350, an amplification means 1360, and LiNbO _3. A wavelength converted light source 1300 with a waveguide 1370 is shown. The amplifying unit 1360 includes a duplexer 1361, a Yb-doped fiber amplifier 1362, a multiplexer 1363, and a polarization controller 1364. The wavelength conversion light source 1300 according to the fourth embodiment uses almost the same configuration as the wavelength conversion light source 1100 according to the third embodiment, but the amplification unit 1360 is inserted between the highly nonlinear fiber 1350 and the LiNbO ₃ waveguide 1370. This is different from the wavelength conversion light source 1100 according to the third embodiment.

In the wavelength conversion light source 1300 according to the fourth embodiment, a demultiplexer 1361 that demultiplexes input light into light having a wavelength of 1.2 μm or less and light having a wavelength of 1.2 μm or more is disposed after the highly nonlinear fiber 1350. Then, a Yb-doped fiber amplifier 1362 is arranged at a port that transmits a wavelength of 1.2 μm or less. The output of the Yb-doped fiber amplifier 1362 and the output of the port that transmits light having a wavelength of 1.2 μm or more of the demultiplexer 1361 are combined by the combiner 1363 and passed to the LiNbO ₃ waveguide 1370 via the polarization controller 1364. I input it. With this configuration, the LiNbO ₃ waveguide 1370 receives input light whose light intensity near the wavelength of 1 to 1.1 μm is enhanced by the Yb-doped fiber amplifier 1362 of the amplifying unit 1360.

14 shows a transmitted light spectrum outputted from the input light spectrum and LiNbO ₃ waveguide 1370 to the LiNbO ₃ waveguide 1370 in the fourth embodiment. In FIG. 14, the output value is shown by relative intensity. As shown in FIG. 14, the optical frequency comb whose wavelength spreads in the 1 to 2 μm band is transmitted through the LiNbO ₃ waveguide 1370, thereby being converted into transmitted light in the wavelength range of 2 to 4.5 μm. Recognize. That is, also in Example 4, output light could be obtained in a wide range of 2.5 μm.

  Since the relative light intensity is shown in FIG. 14, the entire light level seems to be lowered. However, when light of 2 μm or less is blocked by Ge and the output light intensity of 2 μm or more is measured without passing through the spectroscope, Example The wavelength conversion light source 1300 according to No. 4 was able to obtain an output 10 times that of the second embodiment.

  In the fourth embodiment, the polarization controller 1330 is inserted between the intensity modulator 1322 and the Er-doped fiber amplifier 1340, but the position may be changed as appropriate. Further, the polarization controller may be omitted by assembling an optical system in the polarization preserving system. In the fourth embodiment, similarly to the second embodiment, a part of the optical system having this configuration may be separated and observed, and the modulation frequency of the modulation unit may be stabilized so as to stabilize the repetition frequency. Further, a part of the optical system of this configuration may be observed separately and fed back to the LD 1310 to stabilize the oscillation wavelength.

  In the third and fourth embodiments, the optical frequency comb type light generating means is composed of a 1.55 μm-band LD and a modulation means. However, as in the first and second embodiments, a 1.48 μm-band pumping LD and a fiber loop are used. And at least an Er-doped fiber, an isolator for determining the traveling direction of the oscillating laser light, and pumping light output from a pumping LD disposed outside the fiber loop for pumping the Er-doped fiber. Obviously, an optical frequency comb type light generating means having a configuration in which a coupler inserted into the fiber loop and a branching means for outputting the light in the fiber loop 820 to the outside may be used. Further, in the configuration shown in FIGS. 11, 13, and 15, the optical filter that cuts light of 2 μm or less made of Ge is not provided in the subsequent stage of the nonlinear optical medium as in the configuration shown in FIG. 11, 13, and 15, it is obvious that the optical filter may be disposed after the nonlinear optical medium.

10, 100, 800, 1100, 1300 Wavelength conversion light source 11 LiNbO ₃ substrate 12 Optical waveguide 13 Signal light 14 Excitation light 15, 1363 Multiplexer 16 Conversion light 101 Optical frequency comb type light generation means 102, 830, 1140, 1340 Er Doped fiber amplifier 103, 850, 1150, 1350 Highly nonlinear fiber 110 Optical frequency comb light source 120 Nonlinear optical medium 810 Excitation LD
820 Fiber loop 821 Er doped fiber 822, 840, 1130, 1330, 1364 Polarization controller 823 Isolator 824 Coupler 825 Branch means 860, 1160 Periodically poled LiNbO ₃ waveguide 870 Filter 1110, 1310 LD
1120, 1320 Modulating means 1121, 1321 Phase modulator 1122, 1322 Intensity modulator 1360 Amplifying means 1361 Demultiplexer 1362 Yb-doped fiber amplifier

Claims (7)

An optical frequency comb seed light generating means, an Er-doped fiber amplifier connected to the optical frequency comb seed light generating means, and a highly nonlinear fiber connected to the Er-doped fiber amplifier are provided, and outputs an optical frequency comb. An optical frequency comb light source;
An optical frequency comb output from the optical frequency comb light source, and a nonlinear optical medium that emits a second-order nonlinear optical effect; and
The nonlinear optical medium includes a LiNbO ₃ optical waveguide having a polarization inversion structure in which the polarization direction of the nonlinear optical material is periodically inverted ,
In the LiNbO ₃ optical waveguide of the nonlinear optical medium, 1 / λi is generated by the difference frequency generation between the light having the wavelength λp and the light having the wavelength λs selected from the multiple lights of the input optical frequency comb. = 1 / λp-1 / outputs converted light of a wavelength .lambda.i satisfying the formula of [lambda] s, by the difference frequency generation is performed in a plurality of combinations among the light having respective wavelengths of the optical frequency comb, .lambda.i Generate a plurality of converted light of 2 to 4.5 μm ,
The mid-infrared wavelength, wherein the domain-inverted structure has a domain-inverted period set so as to satisfy a phase matching condition in which λi is 2 to 4.5 μm with respect to the wavelength λp and the wavelength λs Conversion light source.   2. The mid-infrared wavelength conversion light source according to claim 1, wherein a filter for attenuating light of 2 [mu] m or less is disposed after the nonlinear optical medium. Branching means for branching the output light of the optical frequency comb light source;
A Yb-doped fiber amplifier for inputting one output light branched by the branching means;
And further comprising: multiplexing means for combining the other output light branched by the branching means and the output light of the Yb-doped fiber amplifier, and outputting the combined light to the nonlinear optical medium. The mid-infrared wavelength conversion light source according to claim 1 or 2. The nonlinear optical material, a LiNbO ₃ or LiNbO ₃ Mg, Zn, Sc, and according to claim 1, wherein at least one selected from the group consisting of In, characterized in that the made of a material which is contained as an additive Mid-infrared wavelength conversion light source. The poling period of the poled structure lambda ([mu] m) is, with respect to the LiNbO ₃ optical waveguide path of the core area A (μm ^2),
−280 / A + 29 ≦ Λ ≦ −280 / A + 32
The mid-infrared wavelength conversion light source according to claim 1 , wherein: The optical frequency comb seed light generating means,
A fiber loop including at least an Er-doped fiber and an isolator;
An excitation light source that outputs excitation light for exciting the Er-doped fiber;
Multiplexing means for multiplexing the excitation light output from the excitation light source to the fiber loop;
Infrared wavelength conversion light source in according to any one of claims 1 to 5, characterized in that taking out the light in the fiber loop and a demultiplexing means for outputting to the Er-doped fiber amplifier. The optical frequency comb seed light generating means includes a semiconductor laser for continuously oscillating an output light, in any one of claims 1 to 5, characterized in that at least and a modulation means for modulating the output light of said semiconductor laser The mid-infrared wavelength conversion light source described.

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