JP5848211B2 - Self-referencing interference apparatus and method - Google Patents

Description

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

In recent years, a new technique for directly measuring the frequency of light has been developed based on a microwave reference frequency that is equal to or lower than a microwave that can be measured by an electrical method (see Non-Patent Documents 1 to 7). As a laser light source used in these techniques, there is a passive mode-locked laser including a resonator device having a length L. This laser light source generates pulsed laser light having a repetition frequency f _rep [= c / (2L)] (c is the speed of light) (see Non-Patent Documents 1 to 3 and 5 to 7).

In addition, a pulse laser light source of a phase modulation system that uses a CW light source that generates continuous (CW) light as a light source and converts the phase of the CW light source into an optical pulse train having a constant repetition frequency (f _rep ) is also used. (See Non-Patent Document 8). In the pulsed laser light oscillated by these lasers, pulses as indicated by “optical carrier envelope” in FIG. 4 are arranged at equal time intervals T (= 1 / f _rep ) on the time axis. In FIG. 4 , a pulse due to a change in the “optical carrier electric field” is shown as an envelope of the optical carrier electric field, and one pulse is shown.

On the other hand, on the frequency axis, as shown in FIG. 5 , an aggregate of a large number of modes (line spectra) arranged in a comb shape at equal frequency intervals f _rep . An assembly of laser beams (line spectra) arranged in a comb shape on the frequency axis is an “optical frequency comb”.

When attention is paid to one pulse of the pulsed laser beam, as shown in FIG. 4 , the peak of the optical carrier envelope and the peak of the optical carrier electric field are shifted due to the difference between the group velocity of light and the phase velocity. This difference between the peak of the optical carrier envelope and the peak of the optical carrier electric field phase is called the carrier envelope phase. Further, as shown in FIG. 5 , the optical frequency comb is also offset by the offset optical frequency f ₀ in response to a shift in the carrier envelope phase between repeated pulses.

By the way, the offset frequency f ₀ described above is expressed as “f ₀ = when the shift between adjacent pulses (envelope) of the phase of the optical carrier electric field (carrier envelope phase) measured from the peak of the optical carrier envelope is Δφ. (Δφ / 2π) × f _rep ”. Since the interval on the frequency axis of the optical frequency comb is given by f _rep , the spectral frequency of each mode can be expressed as f _n = n × f _rep + f ₀ (where n is an integer).

If the laser repetition frequency (f _rep ) and the carrier envelope phase difference can be fixed, the frequency of each mode of the optical frequency comb can be stabilized, and the frequency reference that connects the microwave frequency to the optical frequency region Become. The optical frequency comb in which the frequency of each mode is accurately determined will be applied not only to high-precision frequency standards and related basic physics, but also to fields such as communication, precision measurement, and quantum information communication.

As a conventional method for detecting the offset frequency (f ₀ ) described above, there is an f ₁ -2f ₂ self-referencing interferometry. This method uses the apparatus shown in FIG . This apparatus includes a nonlinear optical medium 702 that uses supercontinuum (SC) light in which a spectrum band of an optical frequency comb composed of an optical pulse train having a repetition frequency f _rep obtained from a frequency comb generator 701 is expanded by one octave or more, and a nonlinear A dichroic mirror 703 is provided for separating the optical frequency comb that has passed through the optical medium 702 into a long wavelength component and a short wavelength component.

  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 device is generated by the nonlinear optical crystal 704 that generates the second harmonic (second harmonic) of the separated long wavelength component, and the short wavelength component and the nonlinear optical crystal 704 that are separated by the dichroic mirror 703. A polarization beam splitter 705 that interferes with the second harmonic of the long wavelength component, a detector 706 that detects a beat signal (light beat) of light generated by the polarization beam splitter 705, and a beat detected by the detector 706 A feedback control unit 707 that controls the frequency comb generation unit 701 based on the signal is provided.

Using the above-described apparatus, first, the spectrum band of the optical frequency comb of the repetition frequency f _rep generated by the frequency comb generator 701 is expanded by the nonlinear optical medium 702 to obtain SC light. Next, the obtained SC light is spatially separated into a short wavelength component (f ₁ = m × f _rep + f ₀ ) and a long wavelength component (f ₂ = n × f _rep + f ₀ ) by the dichroic mirror 703. To do. Next, from the long wavelength component (f ₂ = n × f _rep + f ₀ ) separated by the dichroic mirror 703, the second harmonic [2f ₂ = 2 × (n × f _rep + f ₀ )] is _generated by the nonlinear optical crystal 704. generate.

Next, the second harmonic (2f ₂ ) of the long wavelength component and the short wavelength component (f ₁ ) obtained as described above are caused to interfere by the polarization beam splitter 705, and a beat signal generated by this interference is detected. This is detected by the unit 706. Accordingly, it is possible to measure the carrier envelope offset frequency of the frequency difference f ₀ (= f ₁ −f ₂ ) between f ₁ and f ₂ that appears when m = 2n is satisfied as the smallest term of the frequency.

The carrier envelope offset frequency obtained by this measurement is compared with the microwave reference frequency obtained from the outside. By applying feedback control to the magnitude of nonlinear dispersion in the resonator in the frequency comb generator 701 so that the result of this comparison is constant, the offset frequency of the optical frequency comb generated by the frequency comb generator 701 (f ₀ ) will be able to stabilize. The repetition frequency (f _rep ) of the optical frequency comb can be fixed by feeding back to the laser resonator length in the frequency comb generation unit 701 based on the repetition frequency signal detected by the detection unit 706. It is.

An optical frequency comb stabilized light source that detects and makes constant a carrier envelope offset frequency (f ₀ ) and a mode frequency interval (f _rep ) using the above-described f ₁ -2f ₂ self-referencing interferometry includes a resonator device. Have been developed based on a mode-locked laser (see Non-Patent Documents 1 to 6).

However, in the f ₁ -2f ₂ self-referencing interferometry, the optical frequency comb needs to be SC light with a bandwidth of 1 octave or more. For this purpose, a high-intensity ultrashort pulse laser light source and a photo with a large nonlinear constant are required. A nick crystal fiber or the like is required. There is a method of using a 2f ₁ -3f ₂ self-referencing interferometer instead of such a f ₁ -2f ₂ self-referencing meter (see Non-Patent Documents 10 and 11).

This method uses the apparatus shown in FIG . This apparatus uses a non-linear optical medium 802 having a spectral band of an optical frequency comb composed of an optical pulse train having a repetition frequency f _rep generated by a frequency comb generator 801 as SC light, and an optical frequency comb that has passed through the non-linear optical medium 802 as a long length. A dichroic mirror 803 that separates into a wavelength component and a short wavelength component is provided. In this apparatus, the nonlinear optical medium 802 may generate SC light having a ratio of the frequency band of the long wavelength component to the short wavelength component of 2: 3 or more due to the self-phase modulation effect.

  The apparatus also includes a nonlinear optical crystal 804 that generates a second harmonic of a separated short wavelength component, a nonlinear optical crystal 814 that generates a third harmonic of a separated long wavelength component, and a nonlinear optical crystal 814. A polarization beam splitter 805 that interferes with the third harmonic of the long-wavelength component generated in step 1, a detector 806 that detects a beat signal (light beat) of the light generated by the polarization beam splitter 805, and a detector 806 that detects the beat signal. A feedback control unit 807 that controls the frequency comb generation unit 801 from the beat signal that is generated.

Using the above-described apparatus, first, the spectral band of the optical frequency comb of the repetition frequency f _rep generated by the frequency comb generator 801 is in a state where the ratio of the long wavelength component to the short wavelength component is 2: 3 by the nonlinear optical medium 802. To expand to SC light. Next, the obtained SC light is spatially separated into a short wavelength component (f ₁ = m × f _rep + f ₀ ) and a long wavelength component (f ₂ = n × f _rep + f ₀ ) by the dichroic mirror 803. To do.

Next, from the long wavelength component (f ₂ = n × f _rep + f ₀ ) separated by the dichroic mirror 803, the third harmonic [3f ₂ = 3 × (n × f _rep + f ₀ )] is _generated by the nonlinear optical crystal 814. generate. Further, the second harmonic [2f ₁ = 2 × (m × f _rep + f ₀ )] is generated by the nonlinear optical crystal 804 from the short wavelength component (f ₁ = m × f _rep + f ₀ ) separated by the dichroic mirror 803. Let

Next, the third harmonic wave (3f ₂ ) of the long wavelength component and the second harmonic wave (2f ₁ ) of the short wavelength component obtained as described above are caused to interfere by the polarization beam splitter 805 and generated by this interference. The detecting unit 806 detects the beat signal to be used. In order to satisfy 3n = 2m, in other words, when wavelength conversion is performed so that the wavelengths of the generated second harmonic and third harmonic are approximately equal to each other and 2f ₁ and 3f ₂ are caused to interfere with each other, the carrier envelope offset frequency ( f ₀ ) can be detected.

  For example, from the center frequency of the communication wavelength band of the center frequency f = 193.1 THz (λ = 1552 nm) generated by the frequency comb generator 801, the frequency band is 2: 3 or more in the nonlinear optical medium 802 such as a photonic crystal fiber. Broadband SC light (1200 to 1800 nm). Next, the SC light is separated into a short wavelength component (1200 to 1500 nm) and a long wavelength component (1500 to 1800 nm) by a dichroic mirror 803 having a cutoff frequency of 1500 nm.

Next, the nonlinear optical crystal 814 generates a third harmonic (λ _L / 3 = 600 nm) of light (f ₂ ) of λ _L = 1800 nm, for example, among the long wavelength components of the SC light, and the nonlinear optical crystal 804 generates the SC. For example, a double wave (λ _H / 2 = 600 nm) of light (f ₁ ) having a wavelength of λ _H = 1200 nm, for example, is generated. Thereafter, when these are caused to interfere with each other, the carrier envelope offset frequency (f ₀ ) is detected.

The greatest advantage of the method using the 2f ₁ -3f ₂ self-referencing interferometry is that the frequency band of light only needs to be expanded to 3: 3 or more, so that the carrier envelope offset frequency is detected even with a pulse laser light source with low pulse energy. It is a point that becomes possible.

Japanese translation of PCT publication No. 2002-539627 JP 2006-209067 A

D. J. Jones et al., "Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis", SCIENCE, vol.288, pp.635-639, 2000. R. Holzwarth et al., "Optical Frequency Synthesizer for Precision Spectroscopy", PHYSICAL REVIEW LETTERS, vol.85, no.11, pp.2264-2267, 2000. K. Sugiyama et al., "Frequency Control of a Chirped-Mirror-Dispersion-Controlled Mode-Locked Ti: Al203 Laser for Comparison between Microwave and Optical Frequencies", Proceedings of SPIE, vol.4269, pp.95-104. 2001 . T. R. Schibli et al., "Frequency metrology with a turnkey all-fiber system", OPTICS LETTERS, vol.29, no. 21, pp.2467-2469, 2004. T. M. Fortier et al., "Octave-spanning Ti: sapphire laser with a repetition rate 1 GHz for optical frequency measurements and comparisons", OPTICS LETTERS, vol.31, no.7, pp.1011-1013, 2006. I. Hartl et al., "Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology", OPTICS EXPRESS, vol.13, no.17, pp.6490-6496, 2005. M. Kourogi et al., "Limit of Optical-Frequency Comb Generation Due to Material Dispersion", IEEE JOURNAL OF QUANTUM ELECTRONICS, vol.31.no.12, pp.2120-2126, 1995. T. Kobayashi et al., "Optical Pulse Compression Using High-Frequency Electrooptic Phase Modulation", IEEE JOURNAL OF QUANTUM ELECTRONICS, vol.24.no.2, pp.382-387, 1988. http://optipedia.info/laser/ultrashort-pulse/supercontinuum/sc-light/ 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. TM Ramond et al., "Phase-coherent link from optical to microwave frequencies by means of the broadband continuum from a 1-GHz Ti: sapphire femtosecond oscillator", OPTICS LETTERS, vol.27, no.20, pp.1842-1844 , 2002.

However, in the second harmonic generation used in the 2f ₁ -3f ₂ interferometry, as shown in FIG. 8 , the second harmonic (λ _L / 2) of light (λ _L ) of a certain wavelength and the second harmonic are generated . The third harmonic (λ _L / 3) is generated by taking the sum frequency with the light of the wavelength λ _L used to generate the harmonic. Here, 1 / (λ _L / 3) = 1 / λ _L + 1 / (λ _L / 2). For this reason, there exists a problem that conversion efficiency falls.

As an example of wavelength conversion, in order to obtain a third harmonic (λ _L / 3 = 600 nm) of a long wavelength component (λ _L = 1800 nm) of SC light, the second harmonic of light of a certain wavelength (λ _L = 1800 nm) is used. The third harmonic (600 nm) is generated by taking the sum frequency of the wave (λ _L / 2 = 900 nm) and the second harmonic (λ _L = 1800 nm).

For example, let η (0 <η <1) be the wavelength conversion efficiency from λ _L to the second harmonic λ _L / 2. Further, in order to generate the third harmonic, the conversion efficiency of λ _L / 2 and λ _{L into} the sum frequency is η ′ (0 <η ′ <1). Here, the light of lambda _L for generating lambda _L of light, and third harmonics to produce a lambda _L / 2 are the same to obtain the SC light, for generating lambda _L / 2 lambda _{Since the} remaining λ _L light intensity due to the use of _L light is (1−η), the conversion efficiency up to the third harmonic is η (1−η) η ′. η (1-η) has a maximum value of 0.25 when η = 0.5. Therefore, even if there is an element that generates (converts) the second harmonic at a rate of η = 0.5 as an ideal element and generates (converts) the third harmonic at a rate of 100%, In principle, the conversion efficiency of the third harmonic generated cannot exceed 25%. In such a state where the conversion efficiency is low, the strength of the beat signal due to interference cannot be obtained, and the offset frequency cannot be detected with high accuracy.

  The present invention has been made to solve the above-described problems, and it is an object of the present invention to be able to detect an offset frequency with high accuracy without significantly expanding the spectrum band of an optical frequency comb. .

The self-referencing interference apparatus according to the present invention has an optical frequency input within a range (n is an integer greater than or equal to 2) in which light having a long wavelength component and a short wavelength component with a frequency band ratio of n: (n + 1) is obtained. a band expansion means for generating an optical spectrum bandwidth expanded so by band expansion optical pulse comb, a separating means for separating from the band expanding optical pulse long wavelength components and short wavelength components, the first converted beam Ri by short wavelength component a first converting light generating means for generating, interference to the interference and the second converted light generating means for generating a second converted beam Ri by long wavelength components, the first converted light and the second converted light Means, and at least detection means for detecting an optical signal having a frequency difference between the first converted light and the second converted light generated by the interference between the first converted light and the second converted light . converted light generating means, when n is 2, the first group among the short-wavelength component First converted light that is the second harmonic from the light is generated. When n is 3 or more, the first basic light obtained from the second harmonic generated from the first basic light and the short wavelength component Generates a first converted light by generating a sum frequency with n-2 first group converted light generating lights each having a different wavelength, and the second converted light generating means has a long wavelength component. Of the second harmonics generated from the second fundamental light and n-1 second group- converted light generating lights having different wavelengths from the second fundamental light obtained from the long wavelength component. The second converted light is generated by generating the sum frequency.

In the above self-referencing interference apparatus, when n is 2, the first converted light generating means includes a plurality of regions in which the polarization of the crystal is inverted in adjacent regions connected in series and arranged in each region. The length in the arrangement direction of the first polarization inversion array crystal portion is composed of one group corresponding to the wavelength of the first basic light, and when n is 3 or more, In addition, a plurality of regions in which the polarization of the crystal is inverted in adjacent regions are composed of a plurality of groups connected in series, and the length in the array direction of each region of each group is the first group conversion A first group polarization inversion array crystal unit corresponding to each wavelength of the light for generating light; and the second converted light generation means includes a plurality of regions in which the polarization of the crystal is inverted in adjacent regions. A group in which the length of each region in the arrangement direction corresponds to the wavelength of the second basic light. A plurality of groups in which a second domain-inverted array crystal portion and a plurality of regions where the polarization of the crystal is inverted in adjacent regions are connected in series and arranged in each region of each group The length of the direction is comprised from the 2nd group polarization inversion array crystal | crystallization part corresponding to each wavelength of each light of the 2nd group conversion light production | generation light.

In addition, the self-reference interference method according to the present invention provides light that is input in a range in which light having a long wavelength component and a short wavelength component having a frequency band ratio of n: (n + 1) is obtained (n is an integer of 2 or more). first step by expanding the light spectrum band frequency comb to generate a band expansion light pulse, a second step of separating from the band expanding optical pulse long wavelength components and short wavelength components, the first conversion Ri by short wavelength component a third step of generating a light, and a fourth step of generating a second converted beam Ri by long wavelength components, a fifth step of interfering with the first converted light and the second converted light, a first At least a sixth step of detecting an optical signal having a frequency difference between the first converted light and the second converted light generated by the interference between the converted light and the second converted light. In the case of the second, the second basic light from the first basic light in the short wavelength component is When the first converted light to be harmonic is generated and n is 3 or more, the second harmonic generated from the first basic light and the first basic light obtained from the short wavelength component are respectively wavelengths. The first converted light is generated by generating a sum frequency with the n-2 first group converted light generating lights different from each other, and the fourth step includes the second basic light in the long wavelength component. By generating a sum frequency of the second harmonics generated from the second basic light obtained from the long wavelength component and the n-1 second group converted light generating lights each having a different wavelength. 2 converted lights are generated.

  As described above, according to the present invention, it is possible to obtain an excellent effect that the offset frequency can be detected with high accuracy without significantly expanding the spectrum band of the optical frequency comb.

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 illustrating a configuration example of a quasi phase matching type wavelength conversion element. FIG. 3 is a flowchart for explaining the self-referencing interference method according to the embodiment of the present invention. FIG. 4 is an explanatory diagram for explaining the phase of an optical carrier wave in an optical pulse. FIG. 5 is an explanatory diagram showing the state of a line spectrum on the frequency axis of an optical pulse. FIG. 6 is a configuration diagram showing the configuration of the self-referencing interference apparatus. FIG. 7 is a configuration diagram showing the configuration of the self-referencing interference apparatus. FIG. 8 is an explanatory diagram for explaining the generation of the third harmonic.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration of a self-referencing interference apparatus according to an embodiment of the present invention. This self-referencing interference device includes a frequency comb generator 101, a nonlinear optical medium (band expanding means) 102, a dichroic mirror (separating means) 103, an optical crystal ( first converted light generating means) 104, an optical crystal ( second A conversion light generation unit) 105, a beam splitter (interference unit) 106, a detection unit 107, and a feedback control unit 108.

The frequency comb generator 101 outputs a frequency comb having a repetition frequency 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. Further, the frequency comb generator 101 modulates the phase of the CW light source by applying a microwave with a repetition frequency f _rep from the outside to the CW light source that generates continuous (CW) light by a phase modulator or the like. A pulse laser light source by a phase modulation method that converts the repetition frequency (f _rep ) into a constant optical pulse train and outputs it may be used.

  The nonlinear optical medium 102 receives the optical frequency comb generated from the frequency comb generator 101, and the optical spectrum band of this optical frequency comb is divided into a long wavelength component and a short wavelength component with a frequency band ratio of n: (n + 1). Is expanded in a range where n light is obtained (n is an integer of 2 or more) to generate a band-enlarged light pulse (SC light). The nonlinear optical medium 102 may be composed of, for example, a photonic crystal fiber. Due to the self-phase modulation effect generated in a photonic crystal fiber or the like, SC light in a frequency band in a range in which each component in which the ratio of frequency bands is longer wavelength component: short wavelength component = n: (n + 1) or more is obtained is generated. The dichroic mirror 103 has a desired cutoff frequency, and spatially separates the long wavelength component and the short wavelength component from the SC light obtained from the nonlinear optical medium 102.

Optical crystal 104 generates a first converted beam Ri by short wavelength component separated by the dichroic mirror 103, the optical crystal 105 generates a second converted beam Ri by long wavelength component separated by the dichroic mirror 103. First, when n is 2, the optical crystal 104 generates first converted light that becomes the second higher harmonic than the first basic light in the short wavelength component. In addition, when n is 3 or more, the optical crystal 104 has n-2 pieces of wavelengths different from each other in the second harmonic generated from the first basic light and the first basic light obtained from the short wavelength component. The first converted light is generated by generating a sum frequency with the first group converted light generation light.

On the other hand, the optical crystal 105 includes (n−1) first harmonics having different wavelengths from the second harmonic generated from the second fundamental light in the long wavelength component and the second fundamental light obtained from the long wavelength component. The second converted light is generated by generating a sum frequency with the two group- converted light generating light .

  The first converted light and the second converted light thus obtained are caused to interfere with each other by the beam splitter 106, and the first converted light generated by the interference between the first converted light and the second converted light. The detection unit 107 detects an optical signal (beat signal) having a frequency difference between the first converted light and the second converted light. Further, the feedback control unit 108 controls the output of the optical frequency comb in the frequency comb generation unit 101 based on the beat signal detected by the detector 107.

In the optical crystal 104, the first converted light is generated. On the other hand, in the optical crystal 105, the second converted light is generated . By interfering a first converted beam and the second converted light that occurred, the carrier envelope offset frequency (f ₀₎ by measuring the obtained beat signal (light beat) can be detected.

In addition, the measurement value by the detection unit 107 is compared with an external microwave reference frequency, and the feedback control unit 108 controls the frequency comb generation unit 101 to perform feedback control based on the magnitude of the deviation from the microwave reference frequency. As a result, the offset frequency (f ₀ ) of the output optical frequency comb can be stabilized.

For example, feedback may be performed on the magnitude of the nonlinear dispersion in the resonator constituting the frequency comb generator 101 based on the magnitude of the deviation from the microwave reference frequency. Further, the laser repetition frequency (f _rep ) can be fixed 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 107. It is.

As described above, according to the above-described embodiment, first, the necessary SC light has a frequency band ratio within a range in which each component having a long wavelength component: short wavelength component = n: (n + 1) or more is obtained. The frequency band may be sufficient, and it is not necessary to expand it by one octave or more, and lower pulse energy may be used. In addition, when n is 3 or more, the second harmonic generated from the fundamental light in the short wavelength component of the first converted light and the fundamental light obtained from the short wavelength component have different wavelengths. Since it is generated by generating a sum frequency with -1 first group conversion light generation light, high conversion efficiency can be obtained, and the intensity of the beat signal due to interference is increased. This makes it possible to detect the offset frequency with high accuracy.

Hereinafter, the optical crystal ( first converted light generation means) 104 and the optical crystal ( second converted light generation means) 105 will be described in more detail. These may be composed of a quasi-phase matching type wavelength conversion element having a domain-inverted structure with a periodic pitch length. As shown in FIG. 2A , the quasi-phase matching type wavelength conversion element 301 is composed of a plurality of regions 302 made of a crystal having nonlinear optical characteristics and connected in series. The crystal polarization is reversed. Light enters and transmits in the direction of this arrangement.

  In general, there is a difference in phase velocity between the fundamental light and the wavelength converted light generated by the crystal having nonlinear optical characteristics because the refractive index is different. For this reason, the phase of the wavelength-converted light generated one after another as the basic light propagates through the crystal gradually shifts in phase. Among these, when the phase difference between the wavelength-converted lights generated at two points separated by a certain distance Λ becomes π, they cancel each other and conversely attenuate the intensity. As a method of suppressing this state and increasing the intensity stably, there is a quasi phase matching type wavelength conversion element.

  In the quasi phase matching type wavelength conversion element, the wavelength converted light generated here by reversing the polarization (direction of polarization) of the crystal where the phase difference between the fundamental light and the wavelength converted light is π (distance Λ). Invert the phase of. According to this structure, if the polarization is not reversed, the states that cancel each other are intensified, and the intensity of wavelength-converted light that is generated one after another can be constantly increased as it propagates through the crystal. As described above, in the quasi-phase matching method, the wavelength is converted by quasi-phase-matching by periodic polarization reversal in which the polarization of the crystal is reversed at a distance Λ corresponding to the wavelength λ of the basic light.

One of the major features of the quasi phase matching type wavelength conversion element 301 is that the corresponding wavelength and conversion method (harmonic generation, optical parametric oscillation, etc.) can be appropriately set by adjusting the polarization inversion period. For example, in the wavelength conversion element 301 for generating the second harmonic, the length Λ in the arrangement direction of each region 302 that becomes the polarization inversion period is “Λ = m × λ / {2 × (n _2ω −n _ω )”. } (1) ". Here, λ is the wavelength of the fundamental light, n _ω is the refractive index of the wavelength of the fundamental light in the region 302, n _2ω is the refractive index of the second harmonic wavelength in the region 302, and m is a constant. is there. As shown in the equation (1), the polarization inversion period Λ is determined by the wavelength λ of the incident basic light. Therefore, the wavelength conversion element 301 corresponding to various wavelengths λ is manufactured by appropriately designing the period Λ. Can do.

  The above-described quasi-phase matching type wavelength conversion element is not limited to the second harmonic, but includes sum frequency generation (SFG), difference frequency generation (DFG), and optical parametric oscillation (Optical Parametric Oscillation: It is also widely used for various wavelength conversions utilizing second-order nonlinearity such as OPO). Further, the quasi-phase matching type wavelength conversion element is also referred to as PP-XX (XX is a material name) because it has the characteristic periodic polarization inverted structure described above (XX is a material name), and PPLN , PPMgLN, and PPMgSLT.

The optical crystal 104 and the optical crystal 105 can be configured by the above-described quasi phase matching type wavelength conversion element. For example, the optical crystal 105 can be composed of a quasi-phase matching type wavelength conversion element having n types of Λ polarization inversion structures. In this case, in the optical crystal 105, first, a plurality of regions whose crystal polarization is inverted in adjacent regions are connected in series and arranged, and the length of each region in the arrangement direction is the second basic light. A portion corresponding to the wavelength (second domain-inverted array crystal portion) is provided. In addition, the optical crystal 105 is composed of a plurality of groups in which a plurality of regions in which the polarization of the crystal is inverted in adjacent regions are connected in series, and the length in the arrangement direction of each region of each group Are provided with portions (second group polarization inversion array crystal portions) respectively corresponding to the wavelengths of the n second group conversion light generation lights each having a wavelength different from that of the second basic light.

In the optical crystal 105, the second basic light having a long wavelength component becomes incident light. The second converted light can be generated by using _n wavelengths (λ ₁ , λ ₂ ,..., Λ _n ) among them. For this purpose, a first portion (second polarization-inverted array crystal portion) composed of a plurality of length Λ ₁ regions corresponding to the wavelength λ ₁ and a plurality of length Λ ₂ regions corresponding to the wavelength λ ₂ are used. comprising a second portion, the optical crystal from the third portion, the n moiety consisting regions of the plurality of length lambda _n corresponding to ... wavelength lambda _n comprising a plurality of length lambda ₃ of the region corresponding to the wavelength lambda ₃ 105 may be configured.

In the first portion, the second harmonic of the wavelength lambda ₁ of the light _{(λ 1 '= λ 1/} 2) to generate. The pitch length Λ _{1 at} this time is given by “Λ ₁ = λ ₁ / [2 × {n (λ ₁ ′) −n (λ ₁ )}]... (2)” from the above-described equation (1). Good. Here, n (λ) represents the refractive index of a crystal having nonlinear optical characteristics with respect to the wavelength λ. At this time, light of wavelengths λ ₂ , λ ₃ ,..., Λ _n is transmitted without being wavelength-converted in the first portion because the corresponding pitch length is not Λ ₁ .

In the second part, the sum frequency of λ ₂ , which is a wavelength different from λ _1, and the second harmonic (λ ₁ ′) generated in the first part is taken, and wavelength converted light of wavelength λ ₂ ′ is converted. Generated. The pitch length Λ _{2 at} this time is “1 / Λ ₂ = n (λ ₂ ′) / λ ₂ ′ −n (λ ₁ ′) / (λ ₁ ′) −n (λ ₂ ) / λ _2. (3) ”. By changing the pitch length Λ ₂ obtained by the equation (3), it is possible to select which wavelength λ ₂ ′ of the SC light is used.

Hereinafter, in the k-th (2 ≦ k ≦ n) portion, the sum frequency of λ _k−1 ′ and λ _k is taken to obtain wavelength converted light of wavelength λ _k ′. For this purpose, the pitch length Λ _k of each region in the k-th portion is set to “1 / Λ _k = n (λ _k ′) / λ _k ′ −n (λ _k−1 ′) / (λ _k−1 ′) −n (λ _k ) / λ _k (3) ”.

The greatest feature of the optical crystal 105 configured as described above is that the second converted light is not generated only from the wavelength λ ₁ , but the light of wavelengths (λ ₁ , λ ₂ ,..., Λ _n ) is generated. Since it is used, highly efficient converted light can be generated.

For example, consider the case for simplicity n = 2, the this case, as shown in FIG. 2 (b), may be the wavelength converting element 311 of the quasi phase matching. The wavelength conversion element 311 includes a plurality of regions 312 that are formed of crystals having nonlinear optical characteristics and connected in series, and a plurality of regions 313 that are formed of crystals having nonlinear optical characteristics and connected in series. ing. Each region 312 has a length Λ ₁ in the arrangement direction corresponding to the wavelength λ _{1 of the} basic light, and each region 313 has a length Λ ₂ in the arrangement direction corresponding to the wavelength λ ₂ having a wavelength different from that of the basic light. The second harmonic of the fundamental light having the wavelength λ ₁ is generated by the first portion including the region 312.

The conversion efficiency at which the second harmonic is generated by the first portion composed of the region 312 is η (0 <η <1). In the second portion composed of the region 313, the light of wavelength λ _{2 and} the light of the second harmonic If the conversion efficiency for generating the sum frequency is η ′, the conversion efficiency for obtaining the second converted light is given by η × η ′. Since there is no term limiting the maximum value such as “1-η” here, it is possible in principle that the conversion efficiency exceeds 25%.

Further, in the quasi phase matching type wavelength conversion element having the above-described configuration, the second converted light can be generated by one element. Compared with an optical system in which n quasi-phase matching elements are arranged in series, the optical parts can be saved, so that the Fresnel reflection loss from the optical parts and the end faces of the quasi-phase matching elements is eliminated. Another feature of the element is that the optical coupling loss between the laser beam and the quasi phase matching element can be reduced.

The same applies to the optical crystal 104 that generates the first converted light . In the optical crystal 104, when n is 2, a plurality of regions in which the polarization of the crystal is inverted in adjacent regions are connected in series and the length in the array direction of each region is the first basic What is necessary is just to be comprised from the 1st polarization inversion array crystal part which consists of 1 group corresponding to the wavelength of light. The first basic light is obtained from a short wavelength component. When n is 3 or more, the optical crystal 104 is arranged by connecting a plurality of regions in which the polarization of the crystal is inverted in adjacent regions in addition to the first domain-inverted array crystal portion described above. a first group polarization inversion array crystal portion composed of n-1 groups, the length of each region of each group in the array direction corresponding to each wavelength of the first group converted light generation light; It only has to have. The first group converted light generation light is n−1 pieces of light each having a different wavelength from the basic light obtained from the short wavelength component.

Next, the self-referencing interference method according to the embodiment of the present invention will be described with reference to the flowchart of FIG. 3 by taking n = 2 as an example.

  First, in step S401, an optical frequency comb that has been input within a range in which light having a long wavelength component and a short wavelength component having a frequency band ratio of 2: 3 can be obtained by a nonlinear optical medium such as a photonic crystal fiber. The optical spectrum band is expanded to generate band-expanded light pulses (SC light) (first step). Next, in step S402, for example, a long wavelength component and a short wavelength component are separated from the SC light using a dichroic mirror having a desired cutoff frequency (second step).

Next, in step S403, first converted light of the second harmonic is generated from the first basic light in the short wavelength component (third step). For example, the short wavelength component of the SC light including the first basic light having the frequency f ₁ is incident on the wavelength conversion element 301 described above. If the frequency f _{1 of} incident light used for wavelength conversion is “f ₁ = m × f _rep + f ₀ ”, the frequency of the first converted light that is the second harmonic is 2f ₁ = 2 × (mx f _rep + f ₀ ). Incidentally, n is 3 or more cases, the first and the second harmonic wave generated from the fundamental light of each wavelength from the first basic light obtained from the short-wavelength component in a short wavelength component are different n- The first converted light is generated by generating a sum frequency with the two first group converted light generation lights.

Next, in step S404, it generates a second converted beam Ri by long wavelength components. At this time, the second harmonic generated from the second fundamental light in the long wavelength component and other light having different wavelengths from the second fundamental light obtained from the long wavelength component (second group converted light generation) The second converted light is generated by generating a sum frequency with the light .

For example, the wavelength converting element 311, a second fundamental beam of frequency f ₂ and from this to the incident long wavelength component of the SC light including light of a frequency f ₂ 'of different frequencies. If the frequencies of the two lights used for wavelength conversion are f ₂ (= n × f _rep + f ₀ ) and f ₂ ′ (= n ′ × f _rep + f ₀ ) (where f _{2 to} f ₂ ′), The frequency of the second harmonic generated by the group of the plurality of regions 312 of the wavelength conversion element 311 is 2f ₂ = 2 (n × f _rep + f ₀ ). The frequency of KazuAmane wave generated by the group consisting of a plurality of regions _{313, 2f 2 + f 2 '=} (2n + n') is a × f _rep + 3f _0. Accordingly, the second converted beam that occur is (integer multiple of the repetition frequency f _rep) (offset frequency f ₀₎ × 3 +.

  Next, in step S405, the first converted light and the second converted light are caused to interfere (fifth step), and in step S406, the first converted light generated by the interference between the first converted light and the second converted light. An optical signal (optical beat) having a frequency difference between the first converted light and the second converted light is detected (sixth step).

In order to satisfy 2n + n ′ = 2m, in other words, wavelength conversion is performed so that the wavelengths of the generated first converted light and second converted light are substantially equal, and the first converted light and the second converted light are converted. It is possible to detect the carrier envelope offset frequency (f ₀ ) by causing the light of the light to interfere with each other.

  For example, from the center frequency of the communication wavelength band of the center frequency f = 193.1 THz (λ = 1552 nm), first, SC light having a frequency band of long wavelength component: short wavelength component = 2: 3 or more with a photonic crystal fiber or the like (1200 to 1810 nm) is generated. Next, the SC light is separated into a short wavelength component (1200 to 1500 nm) and a long wavelength component (1500 to 1810 nm) with a dichroic mirror having a cutoff frequency of 1500 nm.

When the long wavelength component (including λ ₂ = 1800 nm and λ ₂ ′ = 1810 nm) of the SC light is used as the incident light, the wavelength λ ₃ of the wavelength converted light (second converted light) in the optical crystal 105 is 1 / _{λ 3 = 1 / (λ 2} /2) + 1 / λ 2 ' from lambda ₃ = a 601.1Nm. Further, when the short wavelength component (including λ ₁ = 1202.2 nm) of the SC light is used as the incident light, the double wave (first converted light) of λ ₁ = 1202.2 nm obtained from the optical crystal 104 is obtained. wavelength λ _1/2 is a 601.1nm. When these are caused to interfere, a beat signal (f ₀ ) is detected.

This measured value is compared with an external microwave reference frequency, and based on the magnitude of the deviation from the microwave reference frequency, the feedback control unit 108 uses the nonlinearity in the resonator constituting the frequency comb generation unit 101. Perform feedback control on the size of the variance. By this control, the offset frequency (f ₀ ) of the optical frequency comb obtained from the frequency comb generator 101 can be stabilized. The repetition frequency (f _rep ) of the frequency comb output from the frequency comb generation unit 101 is the length of the resonator of the laser constituting the frequency comb generation unit 101 based on the repetition frequency signal detected by the detection unit 107. It is possible to fix by feeding back.

  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 ... Dichroic mirror (separation means), 104 ... Optical crystal ( 1st conversion light production | generation means), 105 ... Optical crystal ( 2nd conversion) (Light generation means), 106 ... beam splitter (interference means), 107 ... detection section, 108 ... feedback control section.

Claims (3)

A band obtained by expanding the optical spectrum band of the input optical frequency comb in a range in which light of a long wavelength component and a short wavelength component in which the ratio of frequency bands is n: (n + 1) is obtained (n is an integer of 2 or more). A band expanding means for generating an expanded light pulse;
Separating means for separating a long wavelength component and a short wavelength component from the band-expanded optical pulse;
A first converted light generating means for generating a first converted beam Ri by the short wavelength components,
A second converted light generating means for generating a second converted beam Ri by the long wavelength components,
Interference means for causing the first converted light and the second converted light to interfere with each other;
And at least detecting means for detecting an optical signal having a frequency difference between the first converted light and the second converted light generated by interference between the first converted light and the second converted light,
The first converted light generation means, when n is 2, generates the first converted light that is a second higher harmonic than the first basic light in the short wavelength component, and n is 3 or more. In the case of (2), n-2 first group converted light generations having different wavelengths from the second harmonic generated from the first basic light and the first basic light obtained from the short wavelength component, respectively. Generating the first converted light by generating a sum frequency with the light for use,
The second converted light generation means has a wavelength different from that of the second harmonic generated from the second basic light in the long wavelength component and the second basic light obtained from the long wavelength component. The self-referencing interference apparatus, wherein the second converted light is generated by generating a sum frequency with n-1 second group converted light generation lights. The self-referencing interferometer according to claim 1,
The first converted light generation means includes:
When n is 2, a plurality of regions in which the polarization of the crystal is inverted in adjacent regions are connected in series and arranged so that the length in the arrangement direction of each region is equal to the wavelength of the first basic light. It is composed of a first domain-inverted array crystal part consisting of a corresponding group,
When n is 3 or more, in addition to the first domain-inverted array crystal part, a plurality of regions in which the polarization of the crystal is inverted in adjacent regions are connected in series to form an n−1 group. A first group polarization inversion array crystal portion configured so that the length in the arrangement direction of each region of each group corresponds to each wavelength of the first group converted light generation light,
The second converted light generation means includes:
A plurality of regions in which the polarization of the crystal is inverted in adjacent regions are connected in series and arranged, and the length of each region in the arrangement direction is made up of one group corresponding to the wavelength of the second basic light. 2 domain-inverted array crystal parts;
The second group conversion is made up of n groups in which a plurality of regions whose crystal polarizations are inverted in adjacent regions are connected in series, and the length in the array direction of each region of each group is the second group conversion And a second group polarization inversion array crystal part corresponding to each wavelength of the light generating light. The optical spectrum band of the optical frequency comb that has been input (n is an integer of 2 or more) is expanded within the range in which long-wavelength component and short-wavelength component light with a frequency band ratio of n: (n + 1) is obtained. A first step of generating a light pulse;
A second step of separating a long wavelength component and a short wavelength component from the band expanding optical pulse;
A third step of generating the first converted light Ri by the short wavelength components,
A fourth step of generating a second converted beam Ri by the long wavelength components,
A fifth step of causing the first converted light and the second converted light to interfere with each other;
And at least a sixth step of detecting an optical signal having a frequency difference between the first converted light and the second converted light generated by interference between the first converted light and the second converted light,
In the third step, when n is 2, the first converted light that is the second higher harmonic than the first basic light in the short wavelength component is generated, and when n is 3 or more, The second harmonic wave generated from the first basic light and the first basic light obtained from the short wavelength component are n-2 first group converted light generating lights each having a different wavelength. The first converted light is generated by generating a sum frequency,
In the fourth step, the second harmonic generated from the second fundamental light in the long wavelength component and the second fundamental light obtained from the long wavelength component have n-1 different wavelengths. A self-reference interference method, wherein the second converted light is generated by generating a sum frequency with the second group converted light generation light.

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