JP6016132B2 - Self-referencing interferometer - Google Patents

Description

  The present invention relates to a self-referencing interference apparatus that detects a carrier envelope offset frequency of an optical frequency comb used for measuring the frequency of light.

As a high-precision frequency standard, research on optical frequency combs has been actively conducted. As shown in FIG. 6, the optical frequency comb is an aggregate of a number of emission line spectra arranged in a comb shape at equal frequency intervals f _rep on the frequency axis. The spectral frequency f _n of the optical frequency comb can be expressed as “f _n = n × f _rep + f _CEO (where n is an integer)” using the frequency interval f _rep and the carrier envelope offset frequency f _CEO .

Further, when the optical frequency comb is viewed on the time axis, the pulses as shown by “optical carrier envelope” in FIG. 7 are pulsed laser beams arranged at equal intervals T (= 1 / f _rep ) on the time axis. . There is an “optical carrier electric field” in the optical carrier envelope. The difference between the optical carrier envelope and the peak of the optical carrier electric field is called “carrier envelope phase (CEP)”, and the difference in CEP between adjacent pulses on the time axis is called “carrier envelope phase difference (Δφ)”. The carrier envelope phase difference (Δφ) is caused by the difference between the group velocity and the phase velocity of light passing through the medium in the laser resonator.

There is a relationship “f _CEO = (Δφ / 2π) × f _rep ” between the carrier envelope phase difference (Δφ) and the carrier envelope offset frequency (f _CEO ) of the optical frequency comb.

Since f _rep is generally several MHz to several GHz and f _CEO is 0 <f _CE <f _rep , these exist in the microwave region. By stabilizing f _rep and f _CEO existing in the microwave region, it becomes possible to stabilize an optical frequency comb of about several hundred THz.

For the stabilization of the optical frequency comb, a phase locked circuit (phase-locked loop circuit, PLL circuit) or the like is used. f _rep and f _CEO are detected by a photodiode and compared with microwave reference frequencies f _ref1 and f _ref2 obtained from the outside. The optical frequency comb is stabilized by feedback control to the laser resonator length so that the difference between f _rep and f _ref1 becomes zero, and the nonlinearity of the medium in the laser resonator so that the difference between f _CEO and f _ref2 becomes zero. Feedback control to dispersion. f _rep can be detected as a repetitive signal of pulsed laser light when light of an optical frequency comb is directly applied to a photodiode, and f _CEO can be detected by generating a beat signal by a self-reference interferometer described later.

As a most typical detection method of the carrier envelope offset frequency f _CEO using a beat signal, there is an f-to-2f self-referencing interference method (see Non-Patent Document 1). In this method, first, an optical frequency comb is incident on a highly nonlinear fiber such as a photonic crystal fiber to generate supercontinuum light (SC light).

Next, SC light low frequency components (offset frequency: f _CEO) is incident to the nonlinear medium second harmonic (offset frequency: 2f _CEO) generates, SC Mitsutaka frequency component (offset frequency: f _CEO) and the interference Let In a region where these two lights overlap on the frequency axis, a beat signal of frequency f _CEO appears as shown in FIG.

Here, in order for the second harmonic of the SC light low frequency component and the SC light high frequency component to overlap, assuming that the SC light spreads from f _min to f _{max in the} frequency domain, f _max / f _min > 2. That is, it must be spread over one octave. This time, when this is seen in the wavelength region, the wavelength and the frequency have an inverse relationship, so it is necessary that the second harmonic on the long wavelength side is included in the SC light. For example, the fact that 1050 nm of the second harmonic of 2100 nm is included in the SC light component is a condition for detecting the beat signal by the f-to-2f self-referencing interferometry [FIG. 8B].

In addition to the above-described method, there is a 2f-to-3f self-referencing interference method in which f _CEO is detected by interfering with the third harmonic of the SC light low frequency component and the second harmonic of the high frequency component (non-patent document). 2). For example, this is a method in which a third harmonic of 1800 nm (600 nm) interferes with a second harmonic of 1200 nm (600 nm) [FIG. 8C].

The advantage of the above-described f-to-2f self-referencing interferometry is that f _CEO can be detected if a nonlinear process is used only once to generate a second harmonic. However, in the f-to-2f self-referencing interferometry, the SC light must be spread over one octave.

  On the other hand, the 2f-to-3f self-referencing interferometry can be stabilized by generating SC light of 2/3 octave or more. On the other hand, the 2f-to-3f self-referencing interferometry uses the second-order nonlinear process in the nonlinear optical crystal twice for the generation of the third harmonic wave, or uses the third-order nonlinear process to produce a beat signal that is weak. The problem is that the S / N ratio is small.

Here, an interferometer used for f _CEO detection will be described. For example, as shown in FIG. 9, there is a Mach-Zehnder type f-to-2f self-referencing interferometer that divides an optical path. This apparatus uses an optical pulse train having a repetition frequency f _rep obtained from a frequency comb generator 301, a nonlinear optical medium 302 for generating SC light in a band of 1 octave or more, and SC light that has passed through the nonlinear optical medium 302 as a long wavelength component. And a dichroic mirror 303 that separates into short wavelength components.

  SC light is broadband laser pulse light, and is generally generated by nonlinear optical effects such as self-phase modulation, cross-phase modulation, four-wave mixing, and Raman scattering when ultrashort pulse light is incident on a nonlinear optical material. The At present, it is possible to generate SC light having a broadband property of one octave or more by using a highly nonlinear fiber which is a nonlinear material.

  In addition, this apparatus combines the wavelength converter 304 that generates the second harmonic of the SC light long wavelength component, and the SC light short wavelength component that has been separated by the dichroic mirror 303 and passed through the delay control unit 310. A polarization beam splitter 305 to be waved and a detection unit 306 that reads a beat signal of an optical frequency comb generated by the polarization beam splitter 305 are provided. The wavelength conversion unit 304 is composed of a nonlinear optical crystal.

Using the apparatus described above, first, the frequency comb generator 301 generates an optical frequency comb having a repetition frequency f _rep . Next, the optical frequency comb is incident on the nonlinear optical medium 302 to obtain broadband SC light. Next, the obtained SC light is spatially separated into a short wavelength component (f ₁ ) and a long wavelength component (f ₂ ) by the dichroic mirror 303. Next, the wavelength converter 304 generates a second harmonic wave (2f ₂ ) of the SC light long wavelength component (f ₂ ).

Next, the double wave (2f ₂ ) and the short wavelength component (f ₁ ) of the long wavelength component obtained above are combined by the polarization beam splitter 305 to detect the beat signal generated by the interference of the two lights. Read in part 306. As described above, the carrier envelope offset frequency (f _CEO ) can be measured as the frequency difference between f ₁ and 2f ₂ .

The carrier envelope offset frequency (f _CEO ) obtained by this measurement is compared with the microwave reference frequency (f _rep2 ) obtained from the outside. The offset frequency (f _CEO ) of the optical frequency comb is stabilized by applying feedback control to the magnitude of the nonlinear dispersion in the resonator in the frequency comb generator 301 so that the difference between f _CEO and f _rep2 becomes zero. can do. The repetition frequency (f _rep ) stabilization of the optical frequency comb is performed by comparing the repetition frequency (f _rep ) read by the detection unit 306 with the microwave reference frequency (f _ref1 ), and the difference between f _rep and f _rep1 is obtained. This can be realized by applying feedback to the resonator length of the laser in the frequency comb generator 301 so as to be zero.

  In addition, a collinear type collinear f-to-2f self-reference interference that causes the SC light long wavelength component to interfere with the SC light short wavelength component with a single wavelength converter without dividing the optical path. There is also a law.

  The advantage of the Mach-Zehnder type described above is that the delay control unit 310 and the like can be adjusted so that the two divided optical path lengths coincide with each other, and the temporal overlap of light can be maximized by this configuration. On the other hand, a disadvantage of the Mach-Zehnder type is that it is greatly affected by fluctuations of the interferometer itself when the frequency is stabilized.

  On the other hand, the collinear type is advantageous in that there is no fluctuation of the interferometer itself (see Non-Patent Document 3). Further, since two beams come out on the same straight line in the collinear type, there is an advantage that the difficult alignment work required in the Mach-Zehnder type becomes unnecessary in the collinear type. On the other hand, in the collinear type, in order to obtain temporal overlap, it is necessary to adjust the phase velocity delay in one wavelength conversion unit, but there is a problem that this is not easy.

  In the past research, the above-described f-to-2f type self-referencing interferometer is the mainstream, and both the Mach-Zehnder type and the collinear type have been commercialized as laser stabilization techniques.

  On the other hand, although the 2f-to-3f self-referencing interferometer is realized at the laboratory level, since the conversion efficiency of the third harmonic is generally small, the beat obtained by the 2f-to-3f self-referencing interferometer is low. The signal-to-noise ratio of the signal is reduced. Therefore, it has not been commercialized.

On the other hand, by the inventors' research, by using a ridge waveguide by a quasi-phase-matched lithium niobate (PPLN) having two types of pitch lengths (Λ ₁ , Λ ₂ ), it is tripled. It was experimentally shown that waves can be generated with high efficiency (see Non-Patent Document 4).

B. R. Washburn et al., "Phase-locked, erbium-fiber-laser-based frequency comb in the near infrared", OPTICS LETTERS, vol.29, no.3, pp.250-252, 2004. U. Morgner et al., "Nonlinear Optics with Phase-Controlled Pulses in the Sub-Two-Cycle Regime", PHYSICAL REVIEW LETTERS, vol.86, no.24, pp.5462-5465, 2001. M. Pawlowska et al., "Collinear interferometer with variable delay for carrier-envelope offset frequency measurement", REVIEW OF SCIENTIFIC INSTRUMENTS, vol.80, 083101, 2009. K. Hitachi et al., "Periodically poled lithium niobate ridge waveguides with high conversion efficiency for 2f-to-3f self-referencing interferometer", ELECTRONICS LETTERS, vol.49, no.2, 2013. C. Langrock et al., "Generation of octave-spanning spectra inside reverse-proton-exchanged periodically poled lithium niobate waveguides", OPTICS LETTERS, vol.32, no.17, pp.2478-2480, 2007.

  As described above, the 2f-to-3f self-referencing interferometry is advantageous over the f-to-2f self-referencing interferometry in that it can be stabilized by generating SC light of 2/3 octave or more. On the other hand, there is a problem that a collinear type that is not affected by the fluctuation of the interferometer itself is not realized in frequency stabilization.

  The present invention has been made to solve the above problems, and stabilizes the optical frequency comb without being affected by the frequency fluctuation of the interferometer even with respect to SC light having a spectral band of less than 1 octave. An object of the present invention is to realize a collinear type 2f-to-3f self-referencing interference apparatus.

  The self-referencing interference apparatus according to the present invention includes SC light generating means for generating SC light having a plurality of emission line spectra having equal frequency intervals and having an optical spectrum band extending over 2/3 octave or more, and a short wavelength of the SC light. The first converted light having the second harmonic wave from the component is generated, and the second converted light having the third harmonic wave from the long wavelength component of the SC light is generated, and the generated first converted light and second converted light are generated. Wavelength conversion interference means for causing interference with light, and detection means for detecting a difference frequency component generated by interference between the first converted light and the second converted light. The wavelength conversion interference means is located in the preceding stage. A first periodically poled array crystal part and a second periodically poled array crystal part located in the subsequent stage, wherein the first periodically poled array crystal part inverts the polarization of the crystal in periodically adjacent regions. The length of each region in the array direction is The second periodic polarization inversion array crystal part is composed of a group of nonlinear optical crystals corresponding to the generation of the second harmonic wave of the long wavelength component of the C light. The length of the arrangement direction of the region is composed of a group of nonlinear optical crystals corresponding to the generation of the second harmonic wave of the short wavelength component of the SC light, and the second converted light is generated in the first periodically poled array crystal portion. The second harmonic of the first long wavelength component of the SC light and the second long wavelength component of the SC light are generated by taking the sum frequency in the second periodically poled array crystal portion, The converted light is generated by generating a second harmonic from the short wavelength component of the SC light in the second periodically poled array crystal part.

  In the self-referencing interference apparatus, the first long wavelength component of the SC light and the second long wavelength component of the SC light for generating the second converted light may have different wavelengths.

  In the self-referencing interference device, the second periodic polarization inversion array crystal portion has a length of each of a set of array directions composed of two adjacent regions where the polarization of the crystal is inverted gradually increases toward the rear end. Or you may make it shorten. In addition, the first periodically poled array crystal portion is configured so that the length of each of the set of two adjacent regions in which the polarization of the crystal is inverted gradually increases or decreases toward the rear end. May be.

  In the self-referencing interference apparatus, the wavelength conversion interference unit may have an optical waveguide structure in which target light is propagated in a multimode.

  As described above, according to the present invention, the collinear type 2f− that can stabilize the optical frequency comb without being affected by the frequency fluctuation of the interferometer even with respect to the SC light having a spectrum band of less than 1 octave. An excellent effect that a to-3f self-referencing interference apparatus can be realized is obtained.

FIG. 1 is a configuration diagram showing a configuration of a self-referencing interference apparatus according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing the configuration of the wavelength conversion interference unit 103 that constitutes the self-referencing interference apparatus according to the embodiment of the present invention. FIG. 3 is an explanatory diagram schematically showing the SC light fundamental wave, the second harmonic wave, and the third harmonic wave in the wavelength conversion interference unit 103. FIG. 4 shows the wavelength of the second harmonic and the third harmonic when Λ ₁ = 25.46 μm (λ _2R · λ _3L ) and the fundamental wave used to generate them from the Selmeier equation in PPLN. FIG. 6 is a characteristic diagram in which a center wavelength (λ _{1L ′} · λ _1R ) is calculated as a function of Λ ₂ . FIG. 5 is a characteristic diagram showing experimental results of wavelength conversion using the wavelength conversion interference unit 103 composed of a PPLN ridge optical waveguide. FIG. 6 is an explanatory diagram showing the state of a line spectrum on the frequency axis of an optical pulse. FIG. 7 is an explanatory diagram showing the carrier envelope phase of the optical carrier wave in the optical pulse. FIG. 8 is an explanatory diagram of the concept of detecting the carrier envelope offset frequency (f _CEO ) of the optical frequency comb. FIG. 9 is a block diagram showing the configuration of an interferometer used in carrier envelope offset frequency (f _CEO ) detection.

  Hereinafter, embodiments of the present invention will be described with reference to FIG. 1 and FIG. FIG. 1 is a configuration diagram showing a configuration of a self-referencing interference apparatus according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing the configuration of the wavelength conversion interference unit 103 that constitutes the self-referencing interference apparatus according to the embodiment of the present invention.

  This self-referencing interference apparatus includes a frequency comb generation unit 101, a nonlinear optical medium (band expanding means) 102, a wavelength conversion hall game 103, a detection unit 104, and a feedback control unit 105.

The frequency comb generator 101 generates an optical pulse train having a repetition frequency f _rep . The spectrum of such an optical pulse train becomes a plurality of bright line spectra having the same frequency interval f _rep . That is, the frequency comb generator 101 outputs an optical frequency comb with a frequency interval f _rep . The frequency comb generator 101 may be, for example, a passive mode-locked laser whose repetition frequency f _rep is determined by the resonator length L, and may be a titanium sapphire laser / fiber laser or the like.

Alternatively, the frequency comb generator 101 applies the microwave of the repetition frequency f _rep to the intensity modulator / phase modulator from the outside to the CW light source that generates continuous (CW) light, and thereby the intensity of the CW light source. In addition, a pulse laser light source of an intensity / phase modulation method that modulates the phase, converts the optical pulse train into a constant repetition frequency (f _rep ), and outputs it may be used.

The nonlinear optical medium 102 receives the optical frequency comb generated from the frequency comb generator 101 and expands the optical spectrum band of this optical frequency comb to 2/3 octave or more to generate SC light. The nonlinear optical medium 102 may be composed of, for example, a photonic crystal fiber. Alternatively, a nonlinear optical crystal having a nonlinear optical effect may be used. The frequency comb generator 101 and the nonlinear optical medium 102 constitute SC light generating means for generating SC light having a plurality of emission line spectra having the same frequency interval f _rep and having an optical spectrum band extending over 2/3 octave or more. To do.

The wavelength conversion interference unit 103 generates a first converted light that is a second harmonic wave from the SC light short wavelength component, and generates a second converted light that is a third harmonic wave from the SC light long wavelength component The first converted light and the second converted light are caused to interfere with each other. In addition, the detection unit 104 detects a beat signal including a difference frequency component between both converted lights, which is generated due to interference between the first converted light and the second converted light. This difference frequency component includes a carrier envelope offset frequency f _CEO . Further, the feedback control unit 105 compares the beat signal (f _CEO ) detected by the detection unit 104 with the microwave reference frequency f _ref2 obtained from the outside so that the difference between f _CEO and f _ref2 becomes zero. The frequency comb generator 101 performs feedback control on the nonlinear dispersion of the medium in the laser resonator. Further, the repetition frequency (f _rep ) of the optical frequency comb is also compared with the microwave reference frequency (f _ref1 ), and feedback control is performed on the laser resonator length in the frequency comb generator 101 so that the difference becomes zero.

The wavelength conversion interference unit 103 first generates a double wave having a frequency of (carrier envelope offset frequency f _CEO ) × 2 + (integer multiple of repetition frequency f _rep ). Further, the wavelength conversion interference unit 103 generates a third harmonic whose frequency is (carrier envelope offset frequency f _CEO ) × 3 + (integer multiple of repetition frequency f _rep ). The second harmonic and the third harmonic interfere with each other, and by measuring the beat signal obtained as a result, the carrier envelope offset frequency (f _CEO ) can be detected. Therefore, this self-referencing interference device does not divide the optical path, and the wavelength conversion interference unit 103 causes the second harmonic of the SC light short wavelength component and the third harmonic of the SC light long wavelength component to interfere with each other. Shape) type 2f-to-3f self-referencing interferometer.

Further, the measurement value obtained by the detection unit 104 described above is compared with an external microwave reference frequency, and the feedback control unit 105 causes the frequency comb generation unit 101 to make a difference from the microwave reference frequency f _ref2 zero. By performing feedback control, the offset frequency (f _CEO ) of the output optical frequency comb can be stabilized.

For example, the feedback control unit 105 may perform feedback on the magnitude of nonlinear dispersion in the resonator that constitutes the frequency comb generation unit 101. The laser repetition frequency (f _rep ) can be stabilized by feeding back to the resonator length of the laser constituting the frequency comb generation unit 101 based on the repetition frequency signal from the detection unit 104. Is possible.

  According to the embodiment described above, since the optical path is not divided, there is an advantage that fluctuation of the interferometer itself can be suppressed when the frequency is stabilized. In this respect, it is superior to a Mach-Zehnder type system that divides the light path. Further, since two beams come out on the same straight line in the collinear type, there is an advantage that the difficult alignment work required in the Mach-Zehnder type becomes unnecessary in the collinear type. Further, according to the embodiment, compared with an f-to-2f type collinear self-referencing interferometer, it is excellent in that it is sufficient to use SC light that is spread by 2/3 octave or more.

  Next, the wavelength conversion interference unit 103 will be described in more detail. As shown in FIG. 2, the wavelength conversion interference unit 103 includes a first periodic domain-inverted array crystal unit 103a at the front stage and a second period domain-inverted array crystal unit 103b at the rear stage. The SC light generated from the nonlinear optical medium 102 is incident from the tip (incident end) of the first periodically poled array crystal portion 103a and is emitted from the rear end (emitter end) of the second periodically poled array crystal portion 103b. To do. In this example, the first periodically poled array crystal part 103a and the second periodically poled array crystal part 103b are integrated.

The first periodically poled array crystal portion 103a is composed of a group of nonlinear optical crystals in which a plurality of regions 131 are arranged in series, and the polarization of the crystals is inverted in adjacent regions 131. The length of each region 131 in the arrangement direction corresponds to the second harmonic generation of the SC light long wavelength component. In FIG. 2, the length of two adjacent regions 131 whose polarizations are reversed is the pitch length Λ ₁ .

The second periodic domain-inverted array crystal part 103 b is composed of a group of nonlinear optical crystals in which a plurality of regions 132 are arranged in series, and the polarization of the crystals is inverted in adjacent regions 132. The length of each region 132 in the arrangement direction corresponds to the second harmonic generation of the SC light short wavelength component. In FIG. 2, the length of two adjacent regions 132 whose polarizations are reversed is the pitch length Λ ₂ .

  In the wavelength conversion interference unit 103 configured as described above, the second harmonic of the first long wavelength component of the SC light generated in the first periodically poled array crystal unit 103a, and the second long wavelength component of the SC light, However, the second converted light is generated by taking the sum frequency (third harmonic wave) in the second periodically poled array crystal part. Further, in the second periodically poled array crystal unit 103b, the first converted light is generated by taking the second harmonic wave of the SC light short wavelength component.

As described above, the second periodic wave (λ _2L = λ) of the fundamental light with the SC light (wavelength λ _1L ; first long wavelength component) is generated in the first periodically poled array crystal unit 103 a of the wavelength conversion interference unit 103. _1L / 2), and the second periodic polarization inversion array crystal part 103b generates the second harmonic wave (λ _2L ) and the fundamental light with the SC light (wavelength λ _1L ′; second long wavelength component). A sum frequency (wavelength λ _3L ; second converted light) is generated (where 1 / λ _3L = 1 / λ _2L + 1 / λ _1L ′).

At the same time, the second periodic polarization-reversed array crystal unit 103b uses the second harmonic wave (λ _2R = λ _1R / 2) of the fundamental light with the SC light (wavelength λ _1R ; short wavelength component); first conversion Light). When the generated third harmonic (λ3L; second converted light) and second harmonic (λ2R; first converted light) overlap in the wavelength region, they interfere with each other, making it possible to detect a beat signal. .

Here, in principle, the first long wavelength component (λ _1L ) and the second long wavelength component (λ _1L ′) of the SC light may be the same wavelength or different wavelengths. It is possible to configure a self-referencing interference apparatus according to the above. However, for the reason described later, when these wavelengths are different, the first converted light or the second converted light can be generated more efficiently. Therefore, the detection unit 104 detects a beat signal having a larger SN ratio. Will be able to.

Next, there are two types (first periodically poled array crystal parts 103a, 103a, 103a, 103a, 103a, 103b, which make the center wavelengths of the second harmonic wave (first converted light) and the third harmonic wave (second converted light) closer to each other. A method for determining the quasi phase matching lengths (Λ ₁ , Λ ₂ ) of the second periodically poled array crystal portion 103b) will be described.

The third harmonic generation process in the two types of quasi-phase matching devices takes the second harmonic of the SC fundamental wave (wavelength λ _1L ) at the first periodic domain-inverted array crystal part 103a, which is the first stage, and the second stage. 2 periodic polarization-reversed array crystal part 103b is composed of a two-stage second-order nonlinear process that takes the sum frequency of the second harmonic and the SC fundamental wave (wavelength λ _1L ′), and λ _1L and λ _1L ′ are equal Is common.

In order to increase the overlap in the wavelength region, it is advantageous to intentionally make λ _1L 'shorter than λ _1L compared to the case where λ _1L ' is equal (λ _1L = λ _1L '). It is important to determine the length (Λ ₁ , Λ ₂ ). In practice, if two types of pitch lengths are designed so that λ _1L is as large as possible and λ _1L ′ is small within the range in which the SC light spreads, these overlaps will increase.

In order to explain how to determine the pitch length (Λ ₁ , Λ ₂ ) of the wavelength conversion interference unit 103 which is a quasi-phase matching device, _first , how many pitch lengths are determined and which wavelength of light is converted to which wavelength I will state.

  In a normal nonlinear optical crystal medium, the phase speed differs between the fundamental wave light and the wavelength-converted light depending on which position in the crystal the wavelength conversion is performed. For this reason, the phases of the wavelength-converted light are scattered and cancel each other, and as a result, the conversion efficiency is greatly reduced. This can be solved by the quasi phase matching device. In the quasi-phase matching device, when the phase of the fundamental wave light and the wavelength converted light is shifted by π, the polarization of the nonlinear crystal is inverted, and the wavelength of the wavelength converted light is shifted by π to perform wavelength conversion with high efficiency. In other words, when the combination of the fundamental wave light and the wavelength converted light is such that the distance when the phase of the fundamental wave light and the wavelength converted light is shifted by π coincides with half of the quasi phase matching pitch length (Λ / 2). Furthermore, wavelength conversion is performed with high efficiency. This condition is called a quasi phase matching condition.

Assuming that the refractive index when the wavelength of light incident on the nonlinear optical crystal is λ is n (λ), the sum frequency (λ _f ) is generated with high efficiency from the fundamental wave (λ _s , λ _i ). In order to determine the pitch length Λ of quasi phase matching, “1 / Λ = {n (λ _f )} / λ _f − {n (λ _s )} / λ _s − {n (λ _i )} / λ _i ... (1) ”may be satisfied. Here, the relational expression “1 / λ _f = 1 / λ _s + 1 / λ _i (2)” holds between the wavelength λ _f of the sum frequency and the fundamental waves λ _s and λ _i .

In particular, in the case of the second harmonic, λ _s and λ _i are equal to each other, so that λ _f = λ _s / 2 from Equation (2), and the quasi phase matching condition of Equation (1) is “1 / Λ = {n (Λ _f ) −n (2λ _f )} / λ _f (3) ”.

Next, the operation of the wavelength conversion interference unit 103 when there are _two pitch lengths (Λ ₁ , Λ ₂ ) using the above three relational expressions will be described with reference to FIG. Which SC light component (λ _1L , λ _1L ′, λ _1R ) is used for wavelength conversion in the first periodic polarization inversion array crystal portion 103 a and the second periodic polarization inversion array crystal portion 103 b of the wavelength conversion interference unit 103. Which harmonic (λ _2L , λ _2R ) is generated, and the wavelength of the sum frequency of the second harmonic (λ _2L ) and the SC light component (λ _1L ′) generated by the first periodically poled crystal unit 103a The number of (λ _3L ) will be described.

The relational expression between the second harmonic wave (λ _2L ) generated in the first periodically poled array crystal part 103a (pitch length Λ ₁ ) and the fundamental wave (λ _1L ) used for wavelength conversion is expressed by “ 1 / Λ ₁ = {n (λ _2L ) −n (2λ _2L )} / λ _2L (4) ”, and“ λ _2L = λ _1L / 2 (5) ”. From the equation (4), the second harmonic wave (wavelength: λ _1L / 2) of the SC light shown in FIG. 3A is generated [FIG. 3B].

Next, in the second periodically poled array crystal part 103b (pitch length Λ ₂ ), the second harmonic wave (λ _2L ) generated in the first periodically poled array crystal part 103a and a certain component (λ The sum frequency with _1L ') is generated. When the pitch length Λ ₂ is fixed, the SC light component (λ _1L ′) to be used and the wavelength of the sum frequency to be generated (λ _3L ) are determined. These relational expressions are “1 / Λ ₂ = {n (λ _3L )} / Λ _3L − {n (λ _2L )} / λ _2L − {n (λ _1L ′)} / λ _1L ′ (6) ”,“ 1 / λ _3L = 1 / λ _2L + 1 / λ _1L '(7) ".

Finally, the relational expression between the pitch length Λ _{2 of} the second periodically poled array crystal part 103b, the generated second harmonic (λ _2R ), and the SC light component (λ _1R ) used for wavelength conversion is “1”. / Λ ₂ = {n (λ _2R ) −n (2λ _2R )} / λ _2R (8) ”, and“ λ _2R = λ _1R / 2 (9) ”. As a result, in FIG. 3C, a second harmonic (wavelength: λ _2R ) and a third harmonic (wavelength: λ _3L ) for taking a beat signal are determined.

The wavelength conversion interference unit 103 may be composed of, for example, periodically poled lithium niobate (PPLN) using lithium niobate (LiNbO ₃ ; LN) as a crystal material. However, the crystal material is not limited to LN, and may be any nonlinear optical crystal that can generate a sum frequency, a second harmonic, or the like. For example, MgLN, MgSLT, etc. may be used.

Hereinafter, the wavelength of the third harmonic (λ _3L ) and the second harmonic (λ _2R ) when the pitch lengths are Λ ₁ and Λ ₂ are calculated for the wavelength conversion interference unit 103 composed of PPLN. The refractive index n (λ) of PPLN is given by Selmeier's equation “n (λ) = [5.35583 + 4.629 × 10 ⁻⁷ f + (0.100473 + 3.862 × 10 ⁻⁸ f) / {λ ² − (0 20692-0.89 × 10 ⁻⁸ f) ² } + (100 + 2.657 × 10 ⁻⁵ f) / (λ ² -11.34927 ² ) −1.5334 × 10 ⁻² λ ² ] ^1/2 .. (10) ". Here, f is a function of the temperature T (° C.) and is “f = (T−24.5) × (T + 570.82) (11)”. Here, T = 25 ° C.

  Hereinafter, a case will be considered in which an erbium-doped fiber laser (center wavelength: 1556 nm) is used as the laser light source, and the SC light is expanded to about 1100 to 1800 nm by a highly nonlinear fiber.

When the pitch length Λ ₁ is determined so that λ _1L = 1800 nm is used as the fundamental wave, Λ ₁ = 25.46 μm from Equation (4).

Next, λ _3L and λ _1L ′ when Λ ₂ is specified are obtained using equations (6) and (7), and λ _2R is obtained using equation (8).

FIG. 4 shows the wavelength of the second harmonic (λ _2R ) and third harmonic (λ _3L ) when Λ ₁ = 25.46 μm [FIG. 4 (a)] and the SC light used to generate them. The center wavelength (λ _1L ′, λ _1R ) [FIG. 4B] is plotted as a function of Λ ₂ .

As shown in FIG. 4B, λ _1L = λ _1L ′, that is, the sum formed by the first periodically poled array crystal part 103 a and the second periodically poled array crystal part 103 b of the wavelength conversion interference unit 103. When the frequency (λ _3L ) is 1/3 of λ _1L (point A), the wavelength of the corresponding second harmonic / third harmonic is λ _2R from points B and C in FIG. = 618, λ _3L = 600 nm, and the difference between these is 18 nm.

Whether the harmonics and third harmonics with central wavelengths λ _2R and λ _3L overlap each other in the wavelength region depends on the line width of the generated second and third harmonics and the second and third harmonics. It is determined by the magnitude relationship with the difference in the center wavelength. If the line width of the second harmonic wave is Δ _2R and the line width of the third harmonic wave is Δ _3L , the condition that the two spectra overlap is “| λ _2R −λ _3L | <(Δ _2R + Δ _3L ) / 2 ... (12) ".

From the experimental results to be described later, in the wavelength conversion interference unit 103 in the ridge waveguide by PPLN, even when λ _1L and λ _1L ′ are substantially equal, the second and third harmonics overlap and a beat signal can be obtained. Recognize.

In order to increase the SN ratio of the beat signal, it is necessary to bring the two wavelengths λ _2R and λ _3L closer to each other and increase the overlap between the second harmonic and the third harmonic. In order to realize this from the calculation in FIG. 4B, the pitch length Λ ₂ may be determined so that λ _1L ′ is shorter than λ _1L . For example, if λ _1L ′ = 1417 nm (point D), the difference between the two wavelengths can be reduced, and the pitch length at this time is Λ ₂ = 7.851 μm, and the two wavelengths are λ _2R = 557.8 and λ _3L , respectively. = 550.4 nm (E point, F point), and the difference between them is 7.4 nm. At this time, since λ _1R = 1115.6 nm (point G) is included in the SC light components 1100 to 1800 nm, a second harmonic can also be generated. If the SC light component spreads to the shorter wavelength side, the pitch length Λ ₂ can be further shortened to make the wavelengths of the second and third harmonics closer.

Another reason that λ _1L ′ and λ _1L should take different values is that the SC light wavelength component λ _1L ′ is not used for the second harmonic generation of the first periodically poled array crystal part 103a. As a result, a high power triple wave can be generated. On the other hand, when λ _1L ′ and λ _1L are completely equal, the intensity of the SC light wavelength component λ _1L ′ is reduced during the generation of the sum frequency, and as a result, the intensity of the third harmonic is reduced.

The above process can be applied to SC light spreading over 2/3 octave. For example, when the SC light extends from 1100 to 1900 nm, the overlap of two wavelengths can be maximized by determining the quasi-phase matching length so that, for example, λ _1L = 1900 and λ _1L ′ = 1307 nm. At this time, Λ ₁ = 27.70, Λ ₂ = 7.68 μm, λ _2R = 553.8, λ _3L = 550.1, and the difference between them is 3.7 nm. Note that λ _2R and λ _3L coincide with each other when λ _1L = λ _1L '/ 2. In order to satisfy this condition, SC light of one octave or more must be expanded, and one octave is not expanded. However, it does not take advantage of f _CEO detection.

  In the above description, it has been described that the wavelength conversion interference unit 103 is configured with two different pitch lengths. However, the pitch length of the first periodic polarization inversion array crystal unit 103a or the second periodic polarization inversion array crystal unit 103b is set as follows. Even if the configuration is gradually changed (lengthened or shortened) toward the rear end, the two spectra can be overlapped. As described above, the pitch length is a length in the arrangement direction of a pair of two adjacent regions where the polarization of the crystal is inverted.

Hereinafter, a description will be given of an example of a design in which the pseudo phase matching length of the second periodically poled array crystal portion 103b is linearly changed from Λ ₂₀ to Λ ₂₁ . In the following, a case where the wavelength conversion interference unit 103 is composed of PPLN will be described. The pitch length of the first periodically poled array crystal part 103a is fixed to 25.46 μm (a second harmonic of 900 nm is generated), and the pitch length Λ ₂ of the second periodically poled array crystal part 103b is set to 7.851 8. Gradually change to 227 μm.

In the second periodically poled array crystal portion 103b, a third harmonic or a second harmonic is generated at any pitch of 7.851 to 8.227 μm. At this time, the wavelengths that match the quasi-phase matching condition are λ _3L = 550.4 to 557.8 nm and λ _2R = 557.8 to 566.4 nm. The quasi phase matching condition can be satisfied in the wavelength range determined by the pitch length. In addition, since the spectral line width is wide by Δ _{2R for} the second harmonic and Δ _{3L for} the third harmonic, the overlap between the second harmonic and the third harmonic is increased.

  In addition, when it is set as such a structure, the peak power with respect to the conversion light of a certain 1 wavelength becomes weak. Thus, the length of the variable pitch length and the peak power for one wavelength are in a trade-off relationship. In actual design, it is necessary to set the pitch length in consideration of this trade-off. By performing the same as described above, the first periodically poled array crystal portion 103a is formed so that the length of each of the set of two adjacent regions in which the polarization of the crystal is inverted gradually increases toward the rear end. It is also possible to make the state longer (or shorter).

  Another method for increasing the overlap in the wavelength region of the second harmonic and the third harmonic is to make the wavelength conversion interference unit 103 into an optical waveguide structure. In the case of an optical waveguide structure, it is desirable to have an optical waveguide structure capable of multimode propagation in the wavelength band of light handled by the wavelength conversion interference unit 103. By adopting a multi-mode optical waveguide structure, various light propagation modes exist in the optical waveguide. Since the effective refractive index differs depending on the propagation mode, the wavelength converted as a result is different, and as a result, the line widths of the second harmonic and the third harmonic can be increased.

  As the number of modes of the optical waveguide increases, the angle at which light is totally reflected in the optical waveguide increases, and the light travels obliquely with respect to the wavelength conversion interference unit 103, so that the effective pseudo phase matching length increases. As a result, the conversion wavelength shifts to the long wavelength side. Therefore, when the propagation mode of the optical waveguide is set to the multimode, the line width of the converted light is increased, and the line widths of the second harmonic and the third harmonic can be increased. The number of modes present in the optical waveguide can be controlled by changing the width of the optical waveguide. Further, the light intensity in each mode of the optical waveguide can be changed by changing the incident angle of the light and the NA of the lens.

Compared with a bulk wavelength conversion interference unit 103 in which only a single mode exists, it is more advantageous to use an optical waveguide structure. The conversion wavelength width of the bulk wavelength conversion interference unit 103 is expressed as follows: “Δβ = {n (λ _f ) / λ _f −n (λ _i ) / λ _i −n (λ _s ) / λ _s − 1 / Λ} (13) ”. Here, the conversion efficiency η is given by the following equation using the maximum conversion efficiency η _max .

  In Expression (14), L is the length of the wavelength conversion interference unit 103.

In the formula (14), eta is eta fundamental give _max / 2 or more to become such _{Δβ (λ i, λ s)} , and there the combination of the wavelength converted light (lambda _f) is, satisfy this lambda of _f (Maximum value) − (Minimum value) is given as the spectral line width of the converted wavelength.

  In general, when the single wavelength light source is the signal light and idler light, Δβ = 0 can be obtained by gradually changing the pitch length (Λ), and the conversion efficiency can be maximized. Further, the line width greatly depends on the length L of the optical waveguide. The longer the length, the narrower the line width.

On the other hand, when the light source is SC light, λ _i and λ _s can be freely set within a range where the SC light is spread, and as a result, the line width is widened. As a rough estimate, if L = 20 mm and the SC light spreads to about 1100 to 1900 nm, the double wave having a fundamental wave of 1200 nm has a line width (spectrum width) of about 10 nm. Therefore, when the shift of the center wavelength of the second harmonic and the third harmonic is larger than this, it is difficult to detect the beat signal. In this regard, when the wavelength conversion interference unit 103 has an optical waveguide structure, it has a role of widening the line width due to the multimode propagation effect, so that it is possible to ensure an overlap between the two conversion wavelengths.

Next, experimental results will be described. The result of the overlap of the second harmonic and the third harmonic in the PPLN ridge optical waveguide actually having two different quasi-phase matching conditions will be described with reference to FIG. An erbium-doped fiber laser (average power 300 mW, repetition frequency f _rep = 250 MHz, pulse width 100 fs) is incident on a highly nonlinear fiber to generate SC light (1100-1900 nm), followed by two different pseudo-phase matchings The light was incident on a PPLN ridge optical waveguide having conditions.

The pitch lengths of the PPLN ridge optical waveguide used in the experiment are Λ ₁ = 23.75 and Λ ₂ = 10.16 μm, and the sizes of λ _1L and λ _1L ′ are designed to be almost equal in the ridge optical waveguide. doing. In order to examine how much the second and third harmonic components of the generated visible light spectrum [FIG. 5A] are present, a 1600 nm short-pass filter [FIG. 5C is placed in front of the PPLN ridge optical waveguide. ) Gray dotted line: corresponding to 2nd harmonic] The spectrum of visible light when a long-pass filter of 1300 nm [FIG. 5 (b) gray solid line: corresponding to 3rd harmonic] is limited to limit the SC light is also shown. The solid black line in FIG. 5A is a visible light spectrum when no filter is inserted.

In this data, the center wavelength of the third harmonic is λ _3L = 599.52, the center wavelength of the second harmonic is λ _2R = 611.93 nm, and the spectrum center is separated by 12.41 nm. Since the spectral wavelength width of the solid line is large, they overlap in the wavelength region. When this light was actually incident on a photodetector serving as a detector, a beat signal could be observed with an SN ratio of 35 dB or more on average. This indicates that it is possible to simultaneously generate a second harmonic and a third harmonic with a single wavelength conversion interference unit, and to overlap two lights in the wavelength region.

  As described above, according to the present invention, the collinear 2f-to-3f self-referencing interferometry can be realized. In addition, a self-referencing interferometer can be assembled with one chip, and the number of optical components can be suppressed as much as possible.

  In addition, since it is a collinear system in which the second harmonic and the third harmonic are emitted simultaneously, the fluctuation of the interferometer itself can be suppressed as compared with the Mach-Zehnder type that divides the light path. Further, since the two beams come out on the same straight line in the collinear type, there is an advantage that the alignment work required in the Mach-Zehnder type is unnecessary in the collinear type. In addition, since it is a 2f-to-3f interferometer, the frequency can be stabilized even for SC light that does not spread by one octave.

  By the way, an example in which the second harmonic and the third harmonic are overlapped by one type of quasi phase matching device and a beat signal is output has been reported (see Non-Patent Document 5). Compared with this example, our system uses a two-stage second-order nonlinear effect using both the first periodically poled array crystal part and the second periodically poled array crystal part of the wavelength conversion interference part. Since a highly efficient third harmonic is generated, a beat signal with a high S / N ratio can be obtained.

Furthermore, according to the present invention, it is possible to increase the overlap of the second harmonic and the third harmonic in the wavelength region. By increasing the overlap, it is possible to increase the intensity of the beat signal even if the output of the second harmonic and the third harmonic is the same. Furthermore, by using two fundamental wavelengths of the SC light (λ _1L , λ _1L ′) as fundamental waves for generating the third harmonic wave, the fundamental wave at the time of the third harmonic generation is compared with a simple third harmonic generation process. Since (λ _1L ′) is not reduced by the second harmonic generation process, it becomes possible to output light with higher efficiency, and as a result, the SN ratio of the beat signal can be increased.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 101 ... Frequency comb generation part, 102 ... Nonlinear optical medium (band expansion means), 103 ... Wavelength conversion interference part, 103a ... 1st periodic polarization inversion array crystal part, 103b ... 2nd periodic polarization inversion array crystal part, 104 ... detection unit, 105 ... feedback control unit, 131,132 ... area.

Claims (5)

SC light generating means for generating SC light having a plurality of emission line spectra having equal frequency intervals and having an optical spectrum band extending over 2/3 octave or more;
The first converted light that is the second harmonic wave from the short wavelength component of the SC light is generated, and the second converted light that is the third harmonic wave is generated from the long wavelength component of the SC light, and the generated first light is generated. Wavelength conversion interference means for causing the converted light and the second converted light to interfere with each other,
And at least detecting means for detecting a difference frequency component generated by interference between the first converted light and the second converted light,
The wavelength conversion interference means includes a first periodic polarization reversal array crystal part located in the front stage and a second periodic polarization reversal array crystal part located in the rear stage,
In the first periodically poled array crystal part, the polarization of the crystal is periodically reversed in adjacent regions, and the length in the array direction of each region corresponds to the generation of the second harmonic wave of the long wavelength component of the SC light. It consists of a group of nonlinear optical crystals,
The second periodically poled array crystal part has a periodically inverted crystal polarization in a region adjacent to each other, and the length in the array direction of each region corresponds to the generation of the second harmonic wave of the short wavelength component of the SC light. It consists of a group of nonlinear optical crystals,
The second converted light has a second harmonic wave of the first long wavelength component of the SC light generated in the first periodically poled array crystal portion and a second long wavelength component of the SC light. , By taking a sum frequency in the second periodically poled array crystal part,
The first converted light is generated by generating a second harmonic wave from a short wavelength component of the SC light in the second periodically poled array crystal part. The self-referencing interferometer according to claim 1,
The self-referencing interference apparatus, wherein the first long wavelength component of the SC light and the second long wavelength component of the SC light for generating the second converted light have different wavelengths. The self-referencing interference apparatus according to claim 1 or 2,
In the second periodic domain-inverted array crystal portion, the length of each of a pair of array directions composed of two adjacent regions in which the polarization of the crystal is inverted gradually increases or decreases toward the rear end. A self-referencing interference device. The self-referencing interference apparatus according to any one of claims 1 to 3,
The first periodically poled array crystal portion is characterized in that the length of each of a pair of array directions composed of two adjacent regions in which the polarization of the crystal is inverted becomes gradually longer or shorter toward the rear end. A self-referencing interference device. The self-referencing interference apparatus according to any one of claims 1 to 4,
The self-referencing interference apparatus, wherein the wavelength conversion interference means has an optical waveguide structure in which target light is propagated in a multimode.