JP6504658B2 - Broadband optical frequency comb source and method of generating broadband optical frequency comb - Google Patents

Description

  The present invention relates to an optical frequency comb light source, and more specifically, to a wide band optical frequency comb light source generating light over a wide band from the visible region to the mid infrared region suitable for measurement and spectroscopy, and a method of generating the wide band optical frequency comb It is about

  Light having a discrete spectrum of equal frequency intervals is called an optical frequency comb, and a light source emitting an optical frequency comb is a very useful light source in measurement, spectroscopy, and other fields. If the optical frequency comb can be obtained in several wavelength ranges such as the visible range and the mid-infrared range, frequency comparison with the reference frequency becomes possible in various wavelength ranges. Furthermore, the optical frequency comb light source is also useful as a light source such as dual comb spectroscopy and coherent tomography.

In the optical frequency comb, a plurality of line spectra appearing at a constant frequency interval are regarded as combs, and the interval of the combs is the repetition frequency of the light pulse, and the optical frequency of each comb is an integral multiple of the frequency of the interval between adjacent combs The offset optical frequency is added to Therefore, in the optical frequency comb, the optical frequency of each line spectrum can be determined by the frequency of the interval of the comb and the integer value. That is, each optical frequency which comprises an optical frequency comb is given by following Formula.
f _n = f ₀ + nf _rep (1)

Here, f _rep is an optical frequency interval of adjacent combs, n is an integer, f ₀ is the remainder of the optical frequency, and the relationship of 0 ≦ f ₀ <f _rep is satisfied. The optical frequency comb can be regarded as a collection of light of various optical frequencies f _n . In particular, f ₀ is called a carrier envelope offset frequency and may be denoted as f _CEO .

  As a method of generating such an optical frequency comb, a Ti-sapphire-comb using Ti-sapphire shown in Non-patent Document 1 and an Er-comb in which Er-doped fiber is inserted into a fiber loop shown in Non-patent Document 2 are known. It is done.

Most important in the optical frequency comb is a spectral band in which the optical frequency comb can be obtained. Various techniques have been devised to widen this band, that is, to realize a wider band.
For example, in a broadband optical frequency comb close to one octave (a wavelength relationship between a short wavelength side and a long wavelength side is twice) of a visible region having a wavelength of 500 to 1000 nm, the broadened Ti sapphire comb shown in Non-Patent Document 3 or Visible light is obtained by second harmonic generation of Er comb using a photonic crystal fiber (PCF: Photonic crystal fiber) shown in Non-Patent Document 4. However, these optical frequency combs in the visible region require the use of a spatial optical system when using Ti sapphire or PCF, and the instability of the spatial optical system causes the problem that the robustness required for long operation is not obtained. there were.

  On the other hand, the broadband optical frequency comb near 1 octave in the near infrared region with a wavelength of 1000-2400 nm is highly nonlinear fiber (or Er comb (or Yb comb) using Er-doped fiber (or Yb-doped fiber) as gain medium. HNLF: obtained by spectrally broadening a highly nonlinear fiber). This configuration is all based on optical fiber, is robust against environmental disturbances, and can operate for a long time.

  And, the broadband optical frequency comb in the mid-infrared region with a wavelength of 2400 nm or more is the optical frequency comb in the near-infrared region, and periodically poled lithium niobate (PPLN: Periodically poled lithium niobate) which is a second-order nonlinear optical crystal. Obtained by wavelength conversion using. Since PPLN is fabricated as an optical module and stably maintained coupling with the optical fiber, the optical frequency comb in the mid-infrared region is also robust against environmental disturbances and can operate for a long time.

  As an optical frequency comb in the mid-infrared region, for example, one having a wavelength range of 3.2 to 4.8 μm is obtained in Non-Patent Document 5, and one having a wavelength range of 2.8 to 5.4 μm in Non-Patent Literature Is obtained. However, in the example of the optical frequency comb disclosed in Non-Patent Documents 5 and 6, since the tilt of the crystal is changed to obtain a wide-band spectrum using PPLN having a chirped inversion period, a part of the above wavelength range It is only possible to generate light of a wide band and not to generate a wide band optical frequency comb at a time.

  Further, in Non-Patent Document 7, Er comb is bifurcated, and for one Er comb, high power of 1.55 μm band light is achieved by the Er-doped fiber amplifier, and for the other Er comb, high power is achieved by the Er-doped fiber amplifier. Then, an optical frequency comb whose spectrum is broadened by HNLF is input to the Yb-doped fiber amplifier to increase the power of the 1.04 μm band light, and the high-output 1.55 μm band light and the high output 1.04 μm band light It is disclosed that an optical frequency comb is generated in a 2.8-3.6 μm wavelength band in the mid-infrared region by inputting the signal to the PPLN by combining the However, the configuration disclosed in Non-Patent Document 7 can not obtain a broadband optical frequency comb exceeding one octave in the mid-infrared region.

  In addition, in Patent Document 1, a broadband optical frequency comb light source having a configuration including an Er-doped fiber amplifier having an Er-comb input and a highly nonlinear fiber connected to the Er-doped fiber amplifier is close to one octave in the near infrared region. The optical frequency comb output from the broadband optical frequency comb light source is input to PPLN, and PPLN has light having wavelength λp and wavelength λs selected from among the multiple lights of the input optical frequency comb Wide band frequency exceeding 1 octave of 2-4.5 μm mid-infrared region by outputting converted light of wavelength λi satisfying the equation of 1 / λi = 1 / λp-1 / λs by difference frequency generation with light It is disclosed to generate a comb. However, even in the example of Patent Document 1, it is not possible to obtain an optical frequency comb in the visible wavelength band.

JP, 2014-235174, A

DJJones, SADiddams, JKranka, A. Stentz, RS Windeler, JLHall, and STCundiff, “Carrier envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis”, Science, vol. 288, pp. 635 -639, 2000 H. Inaba, Y. Daimon, F.-L. Hong, A. Onae, Ka. Minoshima, TR Schibli, H. Matsumoto, M. Hirano, T. Okuno, M. Onishi, and M. Nakazawa, "Long- term measurement of optical frequencies using a simple, robust and low-noise fiber based frequency comb, Optics Express, vol. 14, pp. 5223-5231, 2006 J. C. Knight, "Photonic crystal fibers", Nature, vol. 424, pp. 847-851, 2003 A. Onae, T. Ikegami, K. Sugiyama, F.-L. Hong, K. Minoshima, H. Matsumoto, K. Nakagawa, M. Yoshida, and S. Harada, "Optical frequency link between an acetylene stabilized laser at 1542 nm and an Rb stabilized laser at 778 nm using a two-colour mode-locked fiber laser ", Optics Communications, Vol. 183, pp. 181-187, 2000 C. Erny, K. Moutzouris, and J. Biegert, "Mid-indifference difference-frequency generation of ultrashort pulses tunable between 3.2 and 4.8 μm from a compact fiber source", Optics Letters, Vol. 32, pp. 1138-1140, 2007 A. Sell, R. Scheu, A. Leitentstorfer, and R. Huber, "Field-resolved detection of phase-locked infrared transients from a compact Er: fiber system tunable between 55 and 107 THz", Applied Physics Letters, vol. 93 , 251 107 2008 S. A. Meek, A. Poisson, G. Guelachvili, T. W. Hansch, and N. Picque, "Fourier transform spectroscopy around 3 m with a broad difference frequency comb", Applied Physics B, vol. 114, pp. 573-578, 2014

  As described above, conventionally, there has been a problem that coherent optical frequency combs can not be obtained over a very wide wavelength range close to three octaves ranging from the visible region to the mid-infrared region.

  The present invention has been made to solve the above problems, and is a wide band optical frequency comb light source capable of emitting a coherent optical frequency comb over a very wide wavelength range extending from the visible region to the mid-infrared region. An object of the present invention is to provide a method of generating an optical frequency comb.

The wide band optical frequency comb light source of the present invention comprises an optical frequency comb type light generating means for generating an optical frequency comb having a comb-like spectrum having peaks arranged at equal intervals on a frequency axis, and an output from the optical frequency comb type light generating means An Er-doped fiber amplifier for amplifying the optical frequency comb, a highly nonlinear fiber for expanding the band of the optical frequency comb amplified by the Er-doped fiber amplifier, and the optical frequency comb output from the highly nonlinear fiber , and a nonlinear optical medium to emit secondary nonlinear optical effect, the nonlinear optical medium, a light having a light wavelength lambda ₂ having a wavelength lambda ₁ of a number of light included in the optical frequency comb input by the second-order nonlinear difference frequency generation of the effect of the former light, 1 / lambda ₃ = a ₁ / λ 1 -1 / λ ₂ of the light of wavelength lambda ₃ satisfies the formula, the lambda _1, long-wave than lambda ₂ The outputs as a light by sum frequency generation or second harmonic generation of second-order nonlinear optical effect of the light and former light having the light wavelength lambda ₂ having the wavelength λ _{1, 1} / λ ₄ = The light of wavelength λ ₄ satisfying the equation of 1 / λ ₁ + 1 / λ ₂ is output as the light having a wavelength shorter than that of λ ₁ and λ ₂ , and of the large number of lights included in the input optical frequency comb The light of wavelength λ ₆ satisfying the equation of 1 / λ ₆ = 1 / λ ₅ + 1 / λ ₄ by sum frequency generation using light of wavelength λ ₅ and light of wavelength λ ₄ as the original light It is characterized in that it is output as light having a wavelength shorter than ₁ and λ ₂ .

Also, in one example of the configuration of broadband optical frequency comb light source of the present invention, the optical frequency comb seed light generating means includes a fiber loop, the Er-doped fiber disposed in the fiber loop, disposed in said fiber loop , A polarization controller disposed in the fiber loop, an isolator disposed in the fiber loop, an excitation light source for outputting excitation light for exciting the Er-doped fiber, and the excitation It comprises: multiplexing means for multiplexing the pump light output from the light source with the light propagating in the fiber loop, and demultiplexing means for extracting the light propagating in the fiber loop and outputting the light to the Er-doped fiber amplifier It is characterized by

Further, one configuration example of the wide band optical frequency comb light source according to the present invention further includes a polarization controller disposed in a light propagation path between the optical frequency comb type light generation means and the Er-doped fiber amplifier; The polarization controller controls the polarization state of the optical frequency comb input to the nonlinear optical medium .

Further, according to the broadband optical frequency comb generation method of the present invention, an optical frequency comb type light generation step of generating an optical frequency comb having a comb shaped spectrum having peaks arranged at equal intervals on a frequency axis; The amplification step of amplifying with a doped fiber amplifier, the first band expansion step of expanding the band of the amplified optical frequency comb with a highly nonlinear fiber, and the band of the optical frequency comb obtained in the first band expansion step And a second band expansion step of further expanding the nonlinear optical medium that emits the second nonlinear optical effect, wherein the second band expansion step includes a plurality of optical frequency combs included in the first band expansion step. Of the light of wavelength λ ₁ and the light of wavelength λ ₂ among the light of the second-order nonlinear optical effect, the equation of 1 / λ ₃ = 1 / λ ₁ −1 / λ ₂ The light of the wavelength λ ₃ satisfying the above is output as light of a longer wavelength than the λ ₁ and λ ₂ , and second-order nonlinearity using the light having the wavelength λ ₁ and the light having the wavelength λ ₂ as the original light by sum frequency generation or second harmonic generation of optical _{effects, 1 / λ 4 = 1 /} λ 1 + 1 / λ light of wavelength lambda ₄ which satisfies the _second expression, the lambda _1, the light having a shorter wavelength than lambda ₂ The sum frequency generation using the light of wavelength λ _{5 and} the light of wavelength λ ₄ as the original light among the many lights included in the optical frequency comb obtained in the first band expansion step. It is characterized by including a step of outputting light of wavelength λ ₆ satisfying the equation 1 / λ ₆ = 1 / λ ₅ + 1 / λ ₄ as light of a wavelength shorter than that of λ ₁ and λ _2. .

According to the present invention, the optical frequency comb having a wide band generated by the optical frequency comb type light generation means, the Er-doped fiber amplifier and the highly nonlinear fiber is included in the optical frequency comb by inputting to the nonlinear optical medium. The light of longer wavelength than λ ₁ and λ ₂ is output due to the second-order nonlinear optical effect using the light having the wavelength λ ₁ and the light having wavelength λ ₂ as the original light, and the light is shorter than λ ₁ and λ ₂ Since light of wavelengths can be output, a broadband optical frequency comb light source can be realized that emits coherent optical frequency combs over a very wide wavelength range ranging from the visible region to the mid-infrared region. Further, according to the present invention, it is possible to realize a broadband optical frequency comb light source which is robust against environmental disturbance and can operate for a long time. By absorbing the optical frequency comb emitted from the wide band optical frequency comb light source of the present invention into a gas and observing the output light after absorption of the gas with a spectroscope, it becomes possible to disperse in a very wide wavelength band . Besides such absorption spectroscopy, it can be used, for example, for coherent tomography and calibration of spectrometers, and is particularly useful in the visible region.

It is a figure which shows the structure of the broadband optical frequency comb light source which concerns on embodiment of this invention. It is a figure which shows the output light spectrum of the broadband optical frequency comb light source which concerns on embodiment of this invention. It is a diagram illustrating the principle which can be wavelength-converted into a wide band mid-infrared optical frequency comb output from the highly nonlinear fiber in the embodiment of the present invention in periodically poled LiNbO ₃ waveguide. It is a figure which shows the coherency of the output light of the mid-infrared area | region of the broadband optical frequency comb light source which concerns on embodiment of this invention. FIG. 7 is a diagram showing the coherency of the output light of the visible region of the wide band optical frequency comb light source according to the embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment
FIG. 1 is a view showing the configuration of a wide band optical frequency comb light source according to an embodiment of the present invention. The wide band optical frequency comb light source includes an optical frequency comb type light generating means 1 for generating an optical frequency comb, an Er-doped fiber amplifier 2 for amplifying an optical frequency comb output from the optical frequency comb type light generating means 1, and an optical frequency A polarization that is disposed in an optical fiber between the comb seed light generating means 1 and the Er-doped fiber amplifier 2 and controls the polarization input from the optical frequency comb seed light generating means 1 to the Er-doped fiber amplifier 2 to an arbitrary polarization state Wave controller 3, high nonlinear fiber 4 for expanding the frequency band of the optical frequency comb amplified by Er-doped fiber amplifier 2, and optical frequency comb output from high nonlinear fiber 4 as input to emit second-order nonlinear optical effect a periodically-poled LiNbO ₃ (lithium niobate) waveguide 5 is a nonlinear optical medium, the output light from the periodically poled LiNbO ₃ waveguide 5 And a fluoride lens 6 for converting the line light.

  In the present embodiment, a fiber-comb type having an Er-doped fiber as a gain medium is used as the optical frequency comb type light generation means 1. That is, the optical frequency comb seed light generating means 1 is disposed in the fiber loop 10, the Er-doped fiber 11 disposed in the fiber loop 10, and the fiber loop 10, and any polarized light propagating in the fiber loop 10 is It outputs a polarization controller 12 for controlling the polarization state, a polarizer 13 disposed in the fiber loop 10, an isolator 14 disposed in the fiber loop 10, and excitation light for exciting the Er-doped fiber 11. An excitation LD (Laser Diode) 15 that is an excitation light source, a coupler 16 that is a multiplexing unit that inserts the excitation light output from the excitation LD 15 into the fiber loop 10, and light that propagates through the fiber loop 10 is extracted and Er-doped It comprises the splitter 17 which outputs to the fiber amplifier 2.

  The polarizer 13 is inserted in order to cut out the light propagating in the fiber loop 10 only in a polarization state in which polarization rotation due to light intensity occurs. The isolator 14 is inserted to determine the traveling direction of the oscillating laser light. In order to excite the Er-doped fiber 11 in the fiber loop 10, the light from the pump LD 15 disposed outside is inserted into the fiber loop 10 via the coupler 16. In the present embodiment, a 1.48 μm band laser is used as the excitation LD 15 that oscillates continuously. The light (optical frequency comb) propagating in the fiber loop 10 is extracted through the splitter 17 and input to the Er-doped fiber amplifier 2 through the polarization controller 3.

The Er-doped fiber amplifier 2 amplifies the optical frequency comb output from the optical frequency comb type light generation means 1. When the optical frequency comb amplified by the Er-doped fiber amplifier 2 is input to the highly nonlinear fiber 4, the frequency difference between the respective light components constituting the optical frequency comb in the highly nonlinear fiber 4 by four wave mixing (the above-mentioned optical frequency interval f _The light having _rep ) is successively generated on the long wavelength side and the short wavelength side, and the wavelength width of the generated light is expanded. In the present embodiment, the light spectrum can be extended to a wavelength range of 1 μm to 2.4 μm. The optical frequency comb output from the highly nonlinear fiber 4 is input to the periodically poled LiNbO ₃ waveguide 5 in accordance with the TM polarization of the waveguide.

Periodically poled LiNbO ₃ waveguide 5 has a periodically inverted structure polarization of LiNbO ₃ is a second-order nonlinear optical material. In the present embodiment, the periodically poled LiNbO ₃ waveguide 5 is manufactured by the direct bonding method. Direct bonding method, a substrate of LiNbO ₃ having a Zn added 7Mol% as a core layer, previously subjected to polarization reversal in the LiNbO ₃ substrate, and bonding and this LiNbO ₃ substrate, a LiTaO ₃ substrate made of a cladding layer directly, Thereafter, the LiNbO ₃ substrate is thinned and confined in the lateral direction by dicing to form a ridge type optical waveguide structure. The thickness of the core layer was 12 μm, and the width of the core layer was 16 μm. The polarization inversion period 反 転 was 28.55 μm, and the waveguide length was 24 mm.

FIG. 2 shows an output spectrum when the output light from the periodically poled LiNbO ₃ waveguide 5 is converted into parallel light by the fluoride lens 6 and dispersed by a spectroscope. The output values on the vertical axis in FIG. 2 are indicated by relative intensities. Here, the output light spectrum (200 of FIG. 2) in the short wavelength region 350 to 1700 nm including the visible region, the output light spectrum (201 of FIG. 2) of the near infrared region 1200 to 2400 nm, and the mid infrared region 2500 to The output light spectrum (at 202 in FIG. 2) of 4500 nm was observed by different light spectrum analyzers. The noise floor is shown in FIG. 2 because three different optical spectrum analyzers are used.

  The visible comb (optical frequency comb in the visible region) extends to 500 nm or less, and the mid-infrared comb (optical frequency comb in the mid-infrared region) extends to 4400 nm. In the spectrum of the mid-infrared comb, the absorption of atmospheric water is seen at 2.6 μm, and the absorption line of atmospheric carbon dioxide is seen at 4.3 μm. As described above, according to the wide band optical frequency comb light source of the present embodiment, it has been found that wide band light is output in the visible region to the mid-infrared region at least from the wavelength 500 nm to 4400 nm.

Next, the principle of wavelength conversion of the optical frequency comb in the near infrared region output from the highly non-linear fiber 4 to the mid infrared region by the periodically poled LiNbO ₃ waveguide 5 will be described using FIG. In the periodically poled LiNbO ₃ waveguide 5, idler light, which is the difference frequency light between the signal light and the excitation light, is generated. Assuming that the wavelength of the excitation light is λ ₁ , the wavelength of the signal light is λ ₂ , and the wavelength of the idler light is λ ₃ , these three wavelengths λ ₁ , λ ₂ , λ ₃ are generated in the difference frequency generation by the second-order nonlinear optical effect. The following formula is satisfied.
1 / λ ₃ = 1 / λ ₁ -1 / λ ₂ (2)

Each of the light included in the optical frequency comb output from the highly nonlinear fiber 4 can be either signal light or excitation light. Also, when the waveguide mode refractive index at wavelength λ ₃ is n _idler , the waveguide mode refractive index at wavelength λ ₁ is n _pump , and the waveguide mode refractive index at wavelength λ ₂ is n _signal By setting the polarization inversion period を 満 た す satisfying the equation, wavelength conversion can be performed with high efficiency.
n _pump / λ ₁ -n _signal / λ ₂ -n _idler / λ ₃ -1 / Λ = 0 (3)

It is FIG. 3 which showed the relationship of each wavelength about the case of polarization inversion period (psi) = 26.0 micrometers, 28.55 micrometers, and 30.0 micrometers using the above relationship. Here, the wavelength lambda ₂ of the signal light horizontal axis, taking the wavelength lambda ₁ of the excitation light, the wavelength lambda ₃ of the idler on the vertical axis. In FIG. 3, 300 indicates the wavelength λ ₁ of the excitation light when the polarization inversion period Λ = 28.55 μm, 301 indicates the wavelength λ ₃ of the idler light when the polarization inversion period Λ = 28.55 μm, and 302 indicates the polarization represents the wavelength lambda ₁ of the excitation light when the inversion period Λ = 26.0μm, 303 represents the wavelength lambda ₃ of the idler light when the polarization inversion period Λ = 26.0μm, 304 are polarization inversion period lambda = 30. represents the wavelength lambda ₁ of the excitation light when the 0 .mu.m, 305 represents the wavelength lambda ₃ of the idler light when the polarization inversion period lambda = 30.0.

Figure 3 shows the efficiency relationship between an excitation wavelength lambda ₁ and the idler wavelength lambda ₃ to occur when the polarization inversion period Λ and is the value of the wavelength lambda ₂ and the periodically poled LiNbO ₃ waveguide 5 of the signal light There is. That is, for example, when the wavelength of signal light λ ₂ = 1.8 μm and the polarization inversion period Λ = 28.55 μm, the wavelength λ ₁ of excitation light of about 1 μm and the wavelength λ ₃ of idler light of about 2.2 μm In the middle infrared region of 2.2 μm.

Possible values of the wavelength lambda ₁ of the signal light of the wavelength lambda ₂ and the excitation light is the value of the wavelength range 1~2.4μm optical frequency comb output from the highly nonlinear fiber 4 (306 range in FIG. 3) . It can be understood from FIG. 3 that a wide wavelength of 2 to 4.4 μm can be generated in the polarization inversion period around Λ = 28.55 μm.

Next, the principle of wavelength conversion of the optical frequency comb output from the highly nonlinear fiber 4 to a wide band in the visible region by the periodically poled LiNbO ₃ waveguide 5 will be described. The visible wavelength conversion is generated based on the sum frequency generation of the second-order nonlinear optical effect with the optical frequency comb in the wavelength region 1 to 2.4 μm output from the highly nonlinear fiber 4 as the original light. Assuming that the wavelength of the excitation light is λ ₁ , the wavelength of the signal light is λ ₂ , and the wavelength of the idler light is λ ₄ , these three wavelengths λ ₁ , λ ₂ , λ ₄ can be used in sum frequency generation with the second-order nonlinear optical effect. The following formula is satisfied.
1 / λ ₄ = 1 / λ ₁ + 1 / λ ₂ (4)
At this time, in the case where the relationship of λ ₁ = λ ₂ is satisfied, it is particularly referred to as second harmonic generation, but it can be considered as a kind of sum frequency generation.

As in the case of difference frequency generation, each of the light contained in the optical frequency comb output from the highly nonlinear fiber 4 can be either signal light or excitation light. The inventors have found that, even if phase matching is not performed, broad band light expanded to 1000 to 2400 nm is input to the nonlinear optical medium to generate light in a wide visible range due to sum frequency generation and second harmonic generation ( Figure 2). Furthermore, in FIG. 2, light of 500 nm or less is also generated, and sum frequency generation of light contained in the optical frequency comb output from the highly nonlinear fiber 4 and light of λ ₄ generated by sum frequency generation also occurs simultaneously. That is, a wavelength λ satisfying the following equation by sum frequency generation in which the light of wavelength λ _{5 and} the light of wavelength λ ₄ among many lights included in the optical frequency comb output from the highly nonlinear fiber 4 are original light A light of ₆ is generated.
1 / λ ₆ = 1 / λ ₅ + 1 / λ ₄ (5)
By the way, sum frequency generation should not be performed efficiently considering the fundamental mode of the waveguide and the polarization inversion period Λ = 28.55 μm, but the waveguide index of the higher order waveguide mode and the higher order polarization Actually, light of various wavelengths is efficiently generated in the visible region due to the influence of the inversion period.

  In order to generate such a broadband optical frequency comb, this can be achieved by optimizing the polarization state inputted to the highly nonlinear fiber 4 by the polarization controller 3 provided in the front stage of the highly nonlinear fiber 4.

Next, the coherence of the output light of the broadband optical frequency comb light source of the present embodiment was examined. First, the output light of the broadband optical frequency comb light source was observed with an InSb light receiver through a filter that transmits light of a wavelength longer than 2400 nm, which is a mid-infrared region. The result of observing the output signal of the InSb light receiver with an electric spectrum analyzer is shown in FIG. According to FIG. 4, it is possible to observe up to the eighth harmonic peak of the signal corresponding to the optical frequency interval f _rep of about 50 MHz, and it is confirmed that the coherency of the output light is very good.

Furthermore, in order to confirm the coherency in the visible region, the output light from the periodically poled LiNbO ₃ waveguide 5 is caused to interfere with the He—Ne laser light of wavelength 633 nm, and the spectrum of the interference light is observed with a light spectrum analyzer. It is shown in FIG. Here, the resolution bandwidth RBW of the optical spectrum analyzer is 300 kHz, and the image signal bandwidth VBW is 3 kHz. According to FIG. 5, in addition to the signal corresponding to the optical frequency interval f _rep of about 50 MHz, the signal corresponding to the carrier envelope offset frequency f _CEO indicated by the arrow in the figure can also be observed, and the optical frequency comb with good coherency even in the visible region It can be seen that is output.

In the near-infrared region, the near-infrared light frequency comb obtained by broadening the optical frequency comb generated by the light frequency comb type light generation means 1 with the highly nonlinear fiber 4 is periodically poled LiNbO. It goes without saying that the coherency of the output light in the near infrared region is good because the light is transmitted through the _three waveguides 5 and output.

Although the present embodiment has been described using the optical frequency comb type light generation means 1 (Er comb) using the Er-doped fiber 11 as a gain medium, another optical frequency comb type light generation means 1 may be used. An optical frequency comb light source may be used.
In the present embodiment, a mechanism for adjusting the length of the fiber loop 10 is not used, but the optical frequency interval f _rep of the optical frequency comb is determined by the length of the fiber loop 10, so _A fiber length control mechanism may be inserted to stabilize _rep .

Further, in the present embodiment, as the second-order nonlinear optical material constituting the core layer of the periodically poled LiNbO ₃ waveguide 5, the one obtained by adding a very small amount of Zn to LiNbO ₃ is used, but no, LiNbO ₃ itself or it may be used in which other elements LiNbO ₃ was added a small amount. For example Zn in LiNbO _3, Mg, Sc, those at least one of In, but that is added can be used as a second-order nonlinear optical material. Other second order nonlinear optical materials may also be used.

In the present embodiment, the length of the periodically poled LiNbO ₃ waveguide 5 is 24 mm. However, when waveguides of various lengths are tested, the length of the periodically poled LiNbO ₃ waveguide 5 is 20 The length of the spectral band of the outgoing light does not increase even if it is longer than 30 mm, and the light output decreases if it is short.

  The present invention can be applied to a technique for generating a wide band optical frequency comb.

DESCRIPTION OF SYMBOLS 1: Optical frequency comb type light generation means 2. Er: doped fiber amplifier 3, 12 Polarization controller 4. Highly nonlinear fiber 5. Periodically poled LiNbO ₃ waveguide 6. Fluoride lens 10 Fiber Loop 11 11 Er doped fiber 13 Polarizer 14 Isolator 15 Pumping LD 16 Coupler 17 Demultiplexer.

Claims (4)

An optical frequency comb type light generating means for generating an optical frequency comb having a comb-shaped spectrum in which peaks are equally spaced on a frequency axis;
An Er-doped fiber amplifier for amplifying the optical frequency comb output from the optical frequency comb type light generation means;
Highly nonlinear fiber which expands the band of the optical frequency comb amplified by this Er-doped fiber amplifier,
An optical frequency comb output from this highly nonlinear fiber as an input, and a nonlinear optical medium that emits a second-order nonlinear optical effect,
The non-linear optical medium generates a difference frequency of a second-order non-linear optical effect that uses light having wavelength λ ₁ and light having wavelength λ ₂ among a large number of lights contained in the input optical frequency comb as original light the light of the wavelength lambda ₃ that satisfies _{1 / λ 3 = 1 / λ} 1 -1 / λ 2 of the formula, the lambda _1, and outputs a light having a longer wavelength than lambda _2, the light having the wavelength lambda ₁ Wavelength λ satisfying the equation of 1 / λ ₄ = 1 / λ ₁ + 1 / λ ₂ by sum frequency generation or second harmonic generation of a second-order nonlinear optical effect based on the light having the wavelength λ ₂ and the light having the wavelength λ _{2 4} is output as light having a wavelength shorter than the wavelengths λ ₁ and λ ₂ , and light of wavelength λ ₅ and light of the wavelength λ ₄ among a large number of lights included in the input optical frequency comb the light of the wavelength lambda ₆ satisfying the equation of sum frequency generation by _{1 / λ 6 = 1 / λ} 5 + 1 / λ 4 to former light the door, the lambda _1, lambda ₂ light having a shorter wavelength than Broadband light frequency comb source characterized by and output. In the broadband light frequency comb light source according to claim 1 Symbol placement,
The optical frequency comb type light generation means is
Fiber loop,
An Er-doped fiber disposed in the fiber loop,
A polarizer disposed in the fiber loop;
A polarization controller disposed in the fiber loop;
An isolator disposed in the fiber loop;
An excitation light source for outputting excitation light for exciting the Er-doped fiber;
Combining means for combining the pump light output from the pump light source with the light propagating in the fiber loop;
A wide-band optical frequency comb light source comprising: branching means for extracting light propagating in the fiber loop and outputting the light to the Er-doped fiber amplifier. The broadband optical frequency comb light source according to claim 1 or 2
And a polarization controller disposed in a propagation path of light between the optical frequency comb type light generation means and the Er-doped fiber amplifier.
The wide band optical frequency comb light source characterized in that the polarization controller controls the polarization state of the optical frequency comb input to the nonlinear optical medium. An optical frequency comb type light generation step of generating an optical frequency comb having a comb-shaped spectrum in which peaks are equally spaced on a frequency axis;
Amplifying the optical frequency comb with an Er-doped fiber amplifier;
A first band expansion step of expanding the band of the amplified optical frequency comb with a highly nonlinear fiber;
And a second band expansion step of further expanding the band of the optical frequency comb obtained in the first band expansion step with a nonlinear optical medium that emits a second-order nonlinear optical effect,
Said second band expansion step, the former light and light having a light wavelength lambda ₂ having a wavelength lambda ₁ among a plurality of light contained in the obtained optical frequency comb in the first band extending step By the difference frequency generation of the second-order nonlinear optical effect, the light of wavelength λ ₃ satisfying the equation of 1 / λ ₃ = 1 / λ ₁ −1 / λ ₂ is output as light of longer wavelength than the λ ₁ and λ _2. 1 / λ ₄ = 1 / λ ₁ by sum frequency generation or second harmonic generation of second-order nonlinear optical effects using light having the wavelength λ ₁ and light having the wavelength λ ₂ as the original light. Light of wavelength λ ₄ satisfying the formula of + 1 / λ ₂ is output as light of a shorter wavelength than λ ₁ and λ ₂ , and a large number of the optical frequency combs obtained in the first band expansion step are output. by sum frequency generation to former light and light of a wavelength of light and the wavelength lambda ₄ wavelength lambda ₅ out of the light _{1 / λ 6 = 1 / λ} 5 + 1 / λ 4 Broadband light frequency comb generation method characterized by the light of the wavelength lambda _6, comprising outputting the lambda _1, the light having a shorter wavelength than lambda ₂ satisfying.