JP2017207687A - Broadband stabilization light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance a light damage resistance of a WG type EOM.SOLUTION: An electrooptical phase modulator 102 and an electrooptic intensity modulator 103 have an optical waveguide type electrooptic modulator structure including: a substrate 111; a core 112 containing lithium niobate having a higher refraction index than the substrate 111 and formed separately from the substrate 111; and an electrode structure 114 for adding a signal of a frequency to the core 112.SELECTED DRAWING: Figure 1B

Description

  The present invention relates to a broadband stabilized light source that generates broadband light.

  In recent years, a broadband light source has been demanded because of its high potential. Broadband light sources have been researched and developed in various organizations, and further commercialization has been actively conducted. Recently, studies on application of a broadband light source to the fields of optical communication and optical measurement have been activated. In the fields of optical communication and optical measurement, expectations for light sources with uniform optical frequency intervals are increasing in addition to broadband characteristics (see Non-Patent Document 1).

  Many broadband light source technologies have been proposed to meet such expectations. Until now, a mode-locking method has been developed as a method for generating broadband light. The mode-locking method is a technique for obtaining a broadband light source by generating extremely short light pulses by fixing the phase relationship between oscillation modes in a laser oscillator. Has been used. Furthermore, in recent years, a method for generating broadband light using a mode-locked Ti sapphire solid-state laser and a photonic crystal fiber (PCF) has become common in order to achieve a wider band.

  In recent years, optical pulses from a short pulse laser are incident on a nonlinear medium such as a highly nonlinear fiber (HNLF), and the spectrum is expanded using the nonlinear effect generated thereby. Studies on technologies for obtaining light of a further ultra-wideband extending to the wavelength have been activated. This broadband light source is called supercontinuum light because of its wide spectral continuity, and since the oscillation spectrum is regularly arranged in a comb shape on the optical frequency axis, it is also called an optical frequency comb due to its optical spectrum shape. It is. This broadband light source is being actively developed for application to light sources for optical communication and optical frequency measurement.

  For the generation of ultra-wideband light called supercontinuum light or optical frequency comb described above, a plurality of methods and configurations have been proposed depending on the difference in short pulse lasers that generate optical pulses.

  For example, the electro-optic modulation method does not use a general mode-locking technique, and generates a light pulse by phase-modulating CW light by the electro-optic effect and intensity-modulating the phase-modulated light. Broadband light is obtained by expanding the spectrum by inputting into the fiber. This electro-optic modulation method can be configured using commercially available modulators and optical components, and is therefore being actively studied (see Non-Patent Document 2).

  As described above, in a recent broadband light source, an optical pulse generated by a short pulse laser is incident on a nonlinear medium such as a highly nonlinear fiber, and the spectrum is expanded by using the nonlinear optical effect generated thereby to obtain an ultra-wideband light source. The method has become commonplace.

  The optical output of the optical pulse output from the short-pulse laser is the same as the optical pulse output from the short-pulse laser in any method of generating a broadband light by entering an optical pulse generated by an electro-optic modulation type short-pulse laser into a nonlinear medium such as a highly nonlinear fiber. Currently, it is limited to about 10 mW. Because of this background, it is difficult to increase the bandwidth by nonlinear effects at present. In these studies, an optical pulse is amplified using an optical amplifier, such as an Er-doped fiber amplifier (EDFA), to the power level necessary to generate four-wave mixing in a highly nonlinear fiber for spectral broadening. As a result, the bandwidth is increased.

However, the configuration using this optical amplifier has the following problems.
As described above, in order to generate four-wave mixing (FWM) in a highly nonlinear fiber, it is necessary to increase the output of an optical pulse with an EDFA, but a spontaneous emission light (ASE) noise added by the EDFA is generated in a wide band. This is a cause of deteriorating the signal / noise ratio (SNR) of light.

  It is known that ASE noise added excessively by EDFA deteriorates the SNR of light generated by four-wave mixing for spectrum expansion (see Non-Patent Document 3 and Non-Patent Document 4). The influence of this ASE noise appears more strongly as the input power to the EDFA is lower. For this reason, in order to ensure a high SNR of broadband light, it is extremely important to keep the input power to the EDFA high, that is, to increase the output of the optical pulse from the short pulse laser. In addition to having a wide wavelength range, a broadband light source has a high SNR, which is extremely important for light source applications.

For example, when a broadband light source is used for optical frequency measurement, first, a beat signal generated from the broadband light by the self-reference method is photoelectrically converted, and a carrier envelope offset frequency (f _ceo ) and a frequency interval f _rep are set from the RF spectrum. . Further, the beat signal f _beat is determined by causing interference between the broadband light and the measured laser, and the optical frequency of the measured laser is determined.

In this case, the degradation of the SNR of the broadband light caused by the ASE noise from the spectrum expansion EDFA increases the noise floor on the RF spectrum after photoelectric conversion. As a result, it becomes difficult to identify and discriminate frequencies such as _fceo , causing an increase in miscounts and degrading frequency measurement accuracy. Therefore, when a broadband light source is used for optical frequency measurement, it is indispensable to secure a high input power to the EDFA for optical pulse amplification and generate broadband light with a high SNR.

  The problem of degradation of the SNR of such a broadband light source is the same in application fields such as optical communication and gas sensing, and must be avoided because it adversely affects transmission capacity and sensing sensitivity.

  In other words, the above-described problem can be avoided by ensuring as much as possible the output of the short pulse laser input to the EDFA. For this purpose, the pulse output from the short pulse laser may be increased by applying a high-power CW light source.

  However, in the current light source configuration, it is difficult to solve the problem of SNR degradation of broadband light only by applying such high-power CW light. The biggest reason for this is the low optical input power tolerance of the modulator, which is an optical component constituting the short pulse laser. The optical input power tolerance of this modulator will be described below.

  First, a basic configuration of a current general broadband light source will be described with reference to FIG. FIG. 7 is a configuration diagram showing a configuration of a conventional general broadband light source. This broadband light source includes a short pulse laser 501, an optical amplifier 502, a dispersion compensating optical fiber 503, and a highly nonlinear optical fiber 504.

  Further, the short pulse laser 501 is obtained by electro-optic modulation with the configuration shown in FIG. The light output from the CW light source 511 is phase-modulated by the electro-optic phase modulator 512, and the positive chirp portion is generated by pulsing the electro-optic intensity modulator 513. Next, the generated positive chirped pulse is propagated through a 1.55 μm band single mode optical fiber (SMF) 514 which is a negative dispersion medium to compress the pulse to obtain an optical pulse (see Non-Patent Document 5). In this configuration, a low-loss optical waveguide (WG) modulator having high speed and good coupling efficiency with an optical fiber is required.

  Among the WG type modulators, the WG type electro-optic modulator (EOM) using a Ti diffusion LN crystal has high compatibility with optical connection in addition to high speed, and it has a low driving voltage, so that it can be used for commercial light. It is a widely used modulator with a track record of being used to generate modulated signals in fiber communication systems. Hereinafter, a conventional EO phase modulator (EOPM) using a Ti diffusion LN crystal waveguide (Ti diffusion WG) will be described with reference to FIG. FIG. 9 is a configuration diagram illustrating a configuration example of the EOPM.

  This EOPM includes an input side optical fiber 601, a lens 602a, a lens 602b, a Ti diffusion WG 603, an output side lens 604a, a lens 604b, and an optical fiber 605. Further, the Ti diffusion WG 603 is provided with an electrode 606a and an electrode 606b with a waveguide portion interposed therebetween, and a voltage can be applied by a power source 607.

  This EOPM confines light by an optical waveguide structure formed in the Ti diffusion WG 603. In this EOPM, the distance between the electrodes 606a and 606b can be narrowed to the order of μm by a microfabrication technique used in a semiconductor device.

  As a result, the driving voltage can be greatly reduced as compared with the bulk type EOPM shown in FIG. Further, since it is of the waveguide type, the optical coupling efficiency between the input optical fiber 601 and the output optical fiber 605 is high, so that it has an advantage of low loss. 10 includes an input-side optical fiber 701, a lens 602, a Ti diffusion crystal portion 703, an output-side lens 704, and an optical fiber 705. The bulk-type EOPM shown in FIG. Further, the Ti diffusion crystal part 703 is provided with electrodes 706a and 706b, and a voltage can be applied by a power source 707.

  In a broadband light source using an electro-optic modulation type short pulse laser described with reference to FIG. 8, in many cases, a Ti diffused WG type EOM is used, and broadband light is obtained by applying phase modulation and intensity modulation to CW light. Is generated.

S. Candiff, J. E, J. Hall, “The Most Accurate Measure Hikari Com”, Nikkei Science, July, 86-94, 2008. A. Ishizawa et al., "Octave-spanning frequency comb generated by 250 fs pulse train emitted from 25 GHz externally phase-modulated laser diode for carrier-envelope-offset-locking", Electronics Letters, vol.45, no.19, pp.1343-1344, 2010. W. Imajuku and A. Takada, "Noise Figure of Phase-Sensitive Parametric Amplifier Using a Mach-Zehnder Interferometer With Lossy Kerr Media and Noisy Pump", IEEE Journal of Quantum Electronics, vol.39, no.6, pp.799- 812, 2003. E. Yamazaki et al., "Noise Figure of Parametric Wavelength Conversion With Quasi-Phase-Matched LiNbO3 Waveguide", IEEE Journal of Selected Topics in Quantum Electronics, vol.12, no.4, pp.536-543, 2006. 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. M. Asobe et al., "Phase sensitive amplification with noise figure below the 3 dB quantum limit using CW pumped PPLN waveguide", Optics Express, vol.20, no.12, pp.13164-13172, 2012. K. Enbutsu et al., "Integrated quasi-phase-matched second-harmonic generator and electro-optic phase modulator for low-noise phase-sensitive amplification", Optics Letters, vol.40, no.14, pp.3336-3339 , 2015.

  As described above, application of the Ti diffusion WG type EOM is being studied in a broadband light source configured to broaden the optical pulse light from a short pulse laser by using a highly nonlinear fiber. However, the Ti diffusion WG type EOM has a problem of characteristic deterioration depending on input power called “optical damage” due to the photorefractive effect.

  Here, the problem of optical damage in this Ti diffusion WG type EOM will be described using a two-dimensional optical waveguide structure. FIG. 11 is a cross-sectional view showing a configuration of a conventional EOPM (also referred to as an EO phase shifter) using a Ti diffusion WG. FIG. 11 shows a cross section perpendicular to the waveguide direction. This EOPM includes a core 802 in which Ti is diffused to increase the refractive index on a substrate 801 made of lithium niobate (LN) crystal, and the core 802 forms an optical waveguide (WG).

  It can be considered that the refractive index distribution in the core 802 is approximately Gaussian and continuously spread. This refractive index distribution state is shown in FIGS. 12A and 12B. 13A and 13B show the mode field shape of light propagating through the Ti diffusion WG in the x and y axis directions. (A) of FIG. 12A and FIG. 12B are refractive index distributions when no optical damage occurs. Moreover, (a) of FIG. 13A and FIG. 13B is a mode field shape when optical damage does not occur.

  In the substrate 801 on which the core 802 is formed, carriers are excited and diffused by crystal defects or impurities generated when Ti is diffused at a high temperature, and a photorefractive effect in which the refractive index changes is likely to occur. When the input light power to the Ti diffusion WG in this state is gradually increased, a refractive index change approximately proportional to the light intensity distribution occurs in the region of the core 802 due to the photorefractive effect. As a result, as shown in FIG. 12A and FIG. 12B (b), the refractive index of the central portion of the Ti diffusion WG increases compared to the surroundings.

  With this increase in refractive index, the relative refractive index difference Δn of the Ti diffusion WG increases, and as shown in FIGS. 13A and 13B (b), the light confinement effect is increased, the mode field diameter is reduced, and the light at the center is reduced. The intensity is higher than when low input power is incident. As a result, the refractive index at the center further increases.

  Thus, in the Ti diffusion WG described above, when the input power is increased, a self-convergence phenomenon in which narrowing of the mode field and the refractive index distribution continuously occurs due to the interaction. In addition, since the optical waveguide structure is asymmetric with respect to the y-axis, the mode field center position also changes, causing an axial deviation from the mode field of the optical fiber. Furthermore, when the input power is increased, the waveguide mode can no longer exist, and the input light spreads over the entire substrate 801. Since the coupling efficiency between the Ti diffusion WG and the input / output optical fiber depends on the overlap integral of the optical electric field distribution, the coupling efficiency decreases due to the mode field change and the axis deviation on the Ti diffusion WG side, and the insertion loss of the modulator module There is an important problem that will increase.

  In order to evaluate the influence of optical damage in the above-described conventional Ti diffusion WG, the time-dependent change of insertion loss when light of high input power was incident on a commercially available fiber input / output type Ti diffusion LNEOPM was measured. The result is shown in FIG. In FIG. 14, the x-axis is the incident time of light, and the y-axis is the amount of change in insertion loss. In the evaluation, the experiment was performed under two conditions of 100 mW and 200 mW as the average input power of the pulsed light. As shown in FIG. 14, it was found that the loss increases as the incident time (operation time) becomes longer, and the loss increases as the input power increases.

  As described above, even when a Ti diffusion WG type EOPM, which is a general waveguide type modulator, is used, since it does not have high optical damage resistance, the input power to the EOPM is limited due to optical damage. For this reason, it is extremely difficult to increase the output of the light pulse from the short pulse laser. Similar to the problem of the decrease in optical power in the configuration using the bulk type EOPM, as described above, the input power to the EDFA for spectrum expansion decreases, so that the ASE from the optical amplifier increases in the input light to the HNLF. In other words, the signal-to-noise ratio SNR of broadband light whose spectrum has been expanded by four-wave mixing has been degraded.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to further improve the optical damage resistance in the WG type EOM.

  A broadband stabilized light source according to the present invention includes a light source that outputs continuous laser light, an electro-optic phase modulator that generates a pulse train having a pulse repetition frequency by modulating the phase of the laser light at a desired frequency. An optical optical intensity modulator that modulates the intensity of the optical pulse train generated by the electro-optical phase modulator with frequency, and a broadband light by compressing the pulse width of each optical pulse of the optical pulse train generated by the electro-optical intensity modulator. The electro-optic phase modulator and electro-optic intensity modulator are made of lithium niobate having a refractive index higher than that of the substrate. The optical waveguide type electro-optic modulator structure includes a core formed on the body and an electrode structure for applying a frequency signal to the core.

  In the broadband stabilized light source, the electro-optic intensity modulator has a Mach-Zehnder interference structure having two electro-optic modulator structures.

  In the broadband stabilized light source, the electro-optic phase modulator and the electro-optic intensity modulator may be formed on the same substrate.

  In the broadband stabilized light source, the substrate may be made of a lithium tantalate crystal, and the core may be made of a lithium niobate crystal doped with at least one of Zn or Mg.

  As described above, according to the present invention, it is possible to obtain an excellent effect that the optical damage resistance in the WG type EOM becomes higher.

FIG. 1A is a configuration diagram showing a configuration of a broadband stabilized light source according to Embodiment 1 of the present invention. FIG. 1B is a cross-sectional view showing a partial configuration of the broadband stabilized light source according to Embodiment 1 of the present invention. FIG. 2A is a perspective view illustrating a configuration example of the electro-optic intensity modulator 103. FIG. FIG. 2B is a perspective view illustrating a configuration example of the electro-optic phase modulator 102. FIG. 3 shows the insertion loss of the optical waveguide type electro-optic modulator constituting the electro-optic phase modulator 102 and electro-optic intensity modulator 103 of the broadband stabilized light source in Embodiment 1 of the present invention with respect to the input optical power. It is a characteristic view which shows the result of having measured change with time. FIG. 4 is a configuration diagram showing the configuration of the frequency measurement system for evaluating the SNR improvement effect of the broadband stabilized light source in the first embodiment of the present invention. FIG. 5 is a characteristic diagram showing frequency measurement results of the optical waveguide type electro-optic modulator used in the broadband stabilized light source according to the first embodiment implemented using FIG. FIG. 6 is a perspective view showing a partial configuration of the broadband stabilized light source according to Embodiment 2 of the present invention. FIG. 7 is a configuration diagram showing a configuration of a conventional general broadband light source. FIG. 8 is a configuration diagram showing the configuration of a conventional short pulse laser 501 of a general broadband light source. FIG. 9 is a configuration diagram showing a configuration of a conventional EO phase modulator using a Ti-diffused LN crystal waveguide. FIG. 10 is a configuration diagram showing the configuration of a conventional EO phase modulator using a bulk LN crystal. FIG. 11 is a sectional view showing a configuration of a conventional EO phase modulator using a Ti-diffused LN crystal waveguide. FIG. 12A is a characteristic diagram showing a refractive index distribution state in the core 802 in a conventional EO phase modulator using a Ti-diffused LN crystal waveguide. FIG. 12B is a characteristic diagram showing a refractive index distribution state in the core 802 in a conventional EO phase modulator using a Ti-diffused LN crystal waveguide. FIG. 13A is a characteristic diagram showing a mode field shape in the core 802 in a conventional EO phase modulator using a Ti-diffused LN crystal waveguide. FIG. 13B is a characteristic diagram showing a mode field shape in the core 802 in a conventional EO phase modulator using a Ti-diffused LN crystal waveguide. FIG. 14 is a characteristic diagram showing the results of measuring the time-dependent change in insertion loss when light is incident on a commercially available fiber input / output Ti diffused LNEOPM.

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

[Embodiment 1]
First, Embodiment 1 of the present invention will be described. FIG. 1A is a configuration diagram showing a configuration of a broadband stabilized light source according to Embodiment 1 of the present invention. FIG. 1B is a cross-sectional view showing a partial configuration of the broadband stabilized light source according to Embodiment 1 of the present invention.

  The broadband stabilized light source includes a light source 101, an electro-optic phase modulator 102, an electro-optic intensity modulator 103, and a dispersion imparting unit 104.

  The light source 101 outputs a continuous laser beam. The electro-optic phase modulator 102 phase-modulates the laser beam at a desired frequency to generate an optical pulse train having a pulse repetition frequency. The electro-optic intensity modulator 103 modulates the intensity of the optical pulse train generated by the electro-optic phase modulator 102 with the above frequency. For example, a signal having a desired frequency may be generated and used using a signal generator (not shown).

  The dispersion imparting unit 104 is composed of a single mode optical fiber, and compresses the pulse width of each optical pulse of the optical pulse train generated by the electro-optic intensity modulator 103 to output broadband light. Note that the electro-optical intensity modulator 103 may be disposed before or after the electro-optical phase modulator 102. The electro-optic intensity modulator 103 selects one of the down or up-chirped ones by the electro-optic phase modulator 102 to transmit light, and removes the DC base portion generated by the electro-optic phase modulator 102. Thereby, it is possible to contribute to suppression of the DC component when the dispersion imparting unit 104 compresses the pulse width of each pulse.

  The broadband stabilized light source according to the first embodiment includes an optical amplifier 105, a dispersion compensating optical fiber 106, and a highly nonlinear optical fiber 107. The optical amplifier 105 is, for example, an Er-doped optical fiber amplifier, and amplifies the light intensity of the light (optical pulse train) output from the dispersion providing unit 104. For example, optical amplification is performed in stages using nonlinear effects such as a progressive self-phase modulation effect in the optical amplifier 105, and the spectral bandwidth is expanded without causing optical splitting.

  The dispersion compensating optical fiber 106 generates a shorter pulse than the optical pulse amplified by the optical amplifier 105. The optical pulse train output from the dispersion compensating optical fiber 106 is input to the highly nonlinear optical fiber 107 and output as wide spectrum band light (SC light). If a material having a high nonlinear susceptibility is used in the highly nonlinear optical fiber 107, desired wide spectrum band light can be generated with high efficiency, and the minimum threshold value of the energy supplied from the light source 101 can be suppressed low.

  Here, as shown in FIG. 1B, the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 are composed of a substrate 111 and lithium niobate having a higher refractive index than the substrate 111, and are formed separately from the substrate 111. And an optical waveguide type electro-optic modulator structure provided with an electrode structure 114 for applying a frequency signal to the core 112. In the example shown in FIG. 1B, an insulating layer 113 formed so as to cover the core 112 is provided, and an electrode structure 114 is formed with the insulating layer 113 interposed therebetween.

  For example, as shown in the perspective view of FIG. 2A, the electro-optic intensity modulator 103 has a Mach-Zehnder interference structure including two electro-optic modulator structures formed on the substrate 111a. On the board | substrate 111a, the input part core 112a comprised from the lithium niobate of refractive index higher than the board | substrate 111a, the 1st arm core 112b, the 2nd arm core 112c, and the output part core 112d are provided. The substrate 111a is made of, for example, a lithium tantalate crystal. Each core is made of, for example, a crystal of lithium niobate doped with at least one of Zn or Mg.

  The input core 112a branches into two, a first arm core 112b and a second arm core 112c, and is coupled to the output core 112d. A first electrode 114a is formed on the second arm core 112c via an insulating layer 113a. A second electrode 114b is formed on the first arm core 112b via an insulating layer 113a. A third electrode 114c is also formed on the insulating layer 113a.

  On the plane of the substrate 111a, each electrode is disposed so that the third electrode 114c and the second electrode 114b sandwich the first electrode 114a. By adopting a coplanar electrode structure in this way, an electric field perpendicular to the substrate is applied to the first arm core 112b and the second arm core 112c with high efficiency. An electric field is applied to the first arm core 112b and the second arm core 112c at the above frequency according to a signal having a desired frequency that is input. The second arm core 112c has a refractive index at the above frequency due to an electro-optic effect (Pockels effect). Changes.

  For this reason, the phase of the light input from the input core 112a and branched and passed to the first arm core 112b and the second arm core 112c is modulated at the above frequency. As a result, the light output from the output core 112d to which the first arm core 112b and the second arm core 112c are coupled is intensity-modulated at the frequency of the input signal.

  Next, as shown in the perspective view of FIG. 2B, the electro-optic phase modulator 102 is composed of one electro-optic modulator structure formed on the substrate 111b. On the board | substrate 111b, the input part core 112d comprised from the lithium niobate of refractive index higher than the board | substrate 111b, the modulation | alteration part core 112e, and the output part core 112f are provided. The substrate 111b is made of, for example, a lithium tantalate crystal. Each core is made of, for example, a crystal of lithium niobate doped with at least one of Zn or Mg.

  The input unit core 112e, the modulation unit core 112e, and the output unit core 112f are continuously formed in a straight line. A first electrode 114d is formed on the modulator core 112e via an insulating layer 113b. A second electrode 114e and a third electrode 114f are also formed on the insulating layer 113b so as to sandwich the first electrode 114d on the plane of the substrate 111a. By adopting the coplanar electrode structure in this way, an electric field perpendicular to the substrate is applied to the modulation unit core 112e with high efficiency.

  Therefore, an electric field is applied to the modulator core 112e at the above frequency by the input signal having a desired frequency. For this reason, the phase of the light input from the input unit core 112e and passing through the modulation unit core 112e is modulated at the above frequency. As a result, the phase of the light that passes through the modulation unit core 112e and is output from the output unit core 112f is modulated at the frequency of the input signal.

  As described above, the optical waveguide type electro-optic modulator structure composed of the substrate 111 and the core 112 made of lithium niobate having a higher refractive index than the substrate 111 and formed separately from the substrate 111, Since the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 are configured, the optical damage resistance in the WG type EOM can be further increased.

  Unlike the case where the core is configured by diffusion of Ti, in the core 112 in the first embodiment, the presence of crystal defects and impurities is suppressed, and the photorefractive effect is suppressed. Further, the structure of the optical waveguide can be made symmetric in a cross section perpendicular to the waveguide direction. As a result, according to the first embodiment, it is possible to further increase the optical damage resistance.

  Further, since the optical waveguide type electro-optic modulator structure described above has a ridge type core, the relative refractive index difference of the optical waveguide can be made larger than that of the Ti diffusion type WG, and the core portion is reached. Light can be confined strongly, and the guided light can be effectively modulated.

  In addition, by disposing the insulating layer 113 serving as an optical buffer layer, the effective dielectric constant of the high-frequency signal propagating through the electrode structure 114 is lowered to approach the refractive index of light so that speed matching can be achieved. Become.

  In addition, since the electro-optic intensity modulator 103 has a Mach-Zehnder interference structure, the magnitude of the phase shift amount in the two arms is the same, and the sign of the change amount can always be reversed, and push-pull operation is possible. Therefore, the half-wave drive voltage Vπ can be greatly reduced.

  In addition, the high light confinement effect due to the core made of lithium niobate makes it possible to reduce the dimensions such as the width of the core, reduce the spacing between the electrodes, and increase the density. It becomes possible.

  As a result of intensive investigations on the conventional broadband light source technology by the inventors, the problem of optical damage resistance is achieved by applying EOM using a direct junction type LN waveguide to the WG type EOM constituting the short pulse laser in the broadband light source. Was found to be solvable.

  In recent years, a low-noise optical amplification technique or an EOPM using the nonlinear optical effect of an optical damage resistant LN waveguide manufactured by a method called a direct bonding method, compared to the Ti diffusion LN waveguide having a low optical damage resistance described so far. Has been reported (see Non-Patent Document 6 and Non-Patent Document 7). With this technology, it has been confirmed that the waveguide structure functions stably without being destroyed even with a watt-class input optical power.

  In the direct bonding method, an optical waveguide structure is formed by bonding a substrate made of LN crystal (ZnLN or MgLN) with high optical damage resistance doped with ZnO or MgO by directly bonding and heating to another substrate having a low refractive index. To do. This is a technique that improves the optical damage resistance by allowing the substrate portion made of ZnLN or MgLN produced by this direct bonding method to be the core of the optical waveguide and allows high-power light to enter.

  As a result of investigations, the inventors have been able to increase the output of an optical pulse by using a direct junction LN waveguide for an EOM WG, and to ensure a high input power to an optical amplifier for spectrum expansion. As a result, it was concluded that a broadband light source that has a higher SNR than conventional broadband light sources and can operate stably for a long period of time can be realized.

  Next, a method for manufacturing the optical waveguide type electro-optic modulator structure in the first embodiment will be described.

First, a ZnLN substrate (diameter 3 inches) made of z-cut lithium niobate doped with Zn is prepared. An LT substrate (3 inches in diameter) made of LiTaO ₃ having a lower refractive index than this substrate is prepared. Next, using the intermolecular force of the hydrogen group adsorbed on the surface of each substrate, the ZnLN substrate and the LT substrate are directly bonded (contacted), and then heat treatment is performed to form an oxygen bond at the bonding interface. Let them join. Thereby, a state in which a ZnLN layer made of z-cut lithium niobate is formed on the LT substrate is obtained.

  Next, the ZnLN layer on the LT substrate is thinned to a desired core thickness. For example, the ZnLN layer may be thinned by well-known grinding / polishing. Next, the thinned ZnLN layer is patterned by a known photolithography technique and etching technique to form a core. First, a photosensitive resist is applied on the thinned ZnLN layer to form a resist layer. Next, the formed resist layer is patterned by a photolithography technique to form a core-shaped mask pattern.

Next, for example, by etching the thin ZnLN layer by reactive ion etching using a carbon fluoride gas and Ar gas, the mask pattern shape is transferred to the thinned ZnLN layer. Thereafter, if the mask pattern is removed, a ridge-shaped core made of z-cut lithium niobate (crystal) is obtained on the LT substrate, and a channel optical waveguide using this core is manufactured. Next, an insulating layer can be formed by depositing SiO ₂ on the LT substrate on which the core is formed, for example, by the well-known ECR plasma CVD method.

  Next, a lift-off mask having an opening at an electrode formation position is formed on the insulating layer. As described above, a lift-off mask may be formed using a photosensitive resist by using a photolithography technique. Next, an electrode material is deposited on the lift-off mask. For example, Au is deposited by a well-known plating method to form an Au film. The Au film may be formed to a thickness of about 30 μm. Thereafter, the lift-off mask is removed. By removing the lift-off mask, the Au film in the region other than the opening is also removed at the same time as the lift-off mask, and the Au film remains at the electrode forming position, and an electrode is formed (lift-off method).

  Next, an optical waveguide type electro-optic modulator constituting the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 of the broadband stabilized light source in the first embodiment is manufactured, and the time-dependent change of the insertion loss with respect to the input optical power Was measured. The measurement results are shown in FIG. In FIG. 3, the x-axis is the light incident time, and the y-axis is the amount of change in insertion loss. Measurement was performed under two conditions of 100 mW and 200 mW as the average input power of the pulsed light.

  As shown in FIG. 3, the change in insertion loss is extremely small even when light is incident for a long time of 100 hours or more. As described above, it can be seen that the optical waveguide type electro-optic modulator constituting the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 of the broadband stabilized light source in the first embodiment has high optical damage resistance.

  Next, the result of evaluating the characteristics of the broadband stabilized light source in the first embodiment will be described. In this evaluation, an optical frequency measurement experiment of broadband light obtained from a broadband stabilized light source was performed. Since the broadband light to be generated has an optical frequency band of one octave or more, it is possible to use interference between the broadband light and the second harmonic. Therefore, the SNR improvement effect of the broadband light generated from the broadband stabilized light source was evaluated by the well-known f-2f self-referencing method.

  First, the configuration of the frequency measurement system will be described with reference to FIG. This measurement system includes a polarization controller 201, a wavelength conversion interference unit 202, a filter 203, and a light detection unit 204.

  The wavelength conversion interference unit 202 generates first converted light that is a second harmonic wave from the short wavelength component of the broadband light generated from the broadband stabilized light source 200 in Embodiment 1 and passed through the polarization controller 201, and Second converted light that is a third harmonic wave is generated from the long wavelength component of the broadband light, and the generated first converted light and second converted light are caused to interfere with each other. Due to this interference, a beat signal including a difference frequency component between the first converted light and the second converted light is generated.

The filter 203 removes noise components such as ASE (Amplified Spontaneous Emission) noise from the beat signal output from the wavelength conversion interference unit 202. The light detection unit 204 detects the beat signal from which the noise component has been removed by the filter 203. The light detection unit 204 is, for example, an RF spectrum analyzer. The difference frequency component detected by the light detection unit 204 includes the carrier envelope offset frequency f _CEO .

The wavelength conversion interference unit 202 first generates a double wave whose frequency is “(carrier envelope offset frequency f _CEO ) × 2 + (integer multiple of repetition frequency f _rep )”. Further, the wavelength conversion interference unit 202 generates a third harmonic whose frequency is “(carrier envelope offset frequency f _CEO ) × 3 + (integer multiple of repetition frequency f _rep )”. The second harmonic wave and the third harmonic wave interfere with each other, and the beat signal obtained as a result is measured by the light detection unit 204, whereby the carrier envelope offset frequency (f _CEO ) can be detected.

  The wavelength conversion interference unit 202 includes, for example, a first periodic polarization inversion array crystal part located in the previous stage and a second periodic polarization inversion array crystal part located in the subsequent stage. These may be constituted by, for example, an optical waveguide (ridge waveguide) having a ridge structure composed of periodically poled lithium niobate (PPLN) using lithium niobate as a crystal material. What is necessary is just to set it as dual pitch PPLN provided with the period reversal structure of a (two) pitch which differs in a front | former stage and a back | latter stage.

Each periodic polarization inversion array crystal part has a plurality of crystal parts arranged in series in a state where the polarization of adjacent crystal parts is inverted. In the first periodically poled array crystal part, the period is Λ _1, and in the second periodically poled array crystal part, the period is Λ ₂ . The period Λ ₁ corresponds to the generation of the second harmonic of the long wavelength component, and the period Λ ₂ corresponds to the generation of the second harmonic of the short wavelength component.

  In the wavelength conversion interference unit 202 having the above-described configuration, the second long-wave component and the second long-wave component generated in the first periodic polarization-reversed array crystal unit are the second periodic polarization. The second converted light is generated by taking the sum frequency (third harmonic wave) in the inverted array crystal part. In the second periodically poled array crystal portion, the first converted light is generated by taking the second harmonic wave of the SC light short wavelength component.

As described above, in the first periodic polarization inversion array crystal portion of the wavelength conversion interference unit 202, the second harmonic wave (λ _2L = λ _1L ) of the fundamental light (wavelength λ _1L ; first long wavelength component) with SC light. / 2), and in the second periodically poled array crystal part, the sum frequency of the second harmonic (λ _2L ) and the fundamental light with the SC light (wavelength λ _1L ′; second long wavelength component) (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 arrayed crystal portion uses the second harmonic wave (λ _2R = λ _1R / 2; first converted light) of the fundamental light (wavelength λ _1R ; short wavelength component) with SC light. ) Is generated. When the generated third harmonic (λ _3L ; second converted light) and second harmonic (λ _2R ; first converted light) overlap in the wavelength region, they interfere with each other and can detect a beat signal. Thus, the carrier envelope offset frequency f _{ceo of the} broadband light generated from the broadband stabilized light source 200 can be measured.

The measurement results are shown in FIG. When the beat signal spectrum was evaluated, the SNR of _fceo was limited to about 40 dB when the conventional optical waveguide type electro-optic modulator structure in which the core was formed by diffusion of Ti was used. It was found that the SNR of f _ceo was 49 dB or more in the broadband stabilized light source 200 of the first embodiment.

In the conventional broadband optical light source, the optical pulse output is limited to an average power of 10 mW in order to avoid optical damage. However, by applying an optical waveguide type electro-optic modulator having high optical damage resistance, the output power can be reduced. The output can be increased to 100 mW. As a result, a sufficiently large input power to the optical amplifier used for pulse amplification can be secured, the influence of excess noise in the optical amplifier can be effectively suppressed, and the SNR of the _fceo signal is greatly improved to about 10 dB. It is possible.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described with reference to FIG. FIG. 6 is a perspective view showing a partial configuration of the broadband stabilized light source according to Embodiment 2 of the present invention. In the second embodiment, the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 are formed on the same substrate 311. Other configurations are the same as those of the first embodiment described above, and the description thereof is omitted below.

  The electro-optic phase modulator 102 is composed of one electro-optic modulator structure formed on the substrate 311. On the substrate 311, an input core 312a, a modulator core 312b, and a connecting core 312c made of lithium niobate having a higher refractive index than that of the substrate 311 are provided. The substrate 311 is made of, for example, a lithium tantalate crystal. Each core is made of, for example, a crystal of lithium niobate doped with at least one of Zn or Mg.

  The input unit core 312a, the modulation unit core 312b, and the connection unit core 312c are continuously formed in a straight line. A first electrode 314a is formed on the modulator core 312b with an insulating layer 313 interposed therebetween. In addition, a second electrode 314 b and a third electrode 314 c are also formed on the insulating layer 313 so as to sandwich the first electrode 314 a on the plane of the substrate 311. By adopting the coplanar electrode structure in this way, an electric field perpendicular to the substrate is applied to the modulation unit core 312b with high efficiency.

  Next, the electro-optic intensity modulator 103 has a Mach-Zehnder interference structure with two electro-optic modulator structures formed on the substrate 311. On the substrate 311, a first arm core 312 d, a second arm core 312 e, and an output core 312 f made of lithium niobate having a higher refractive index than the substrate 311 are provided. Each core is made of, for example, a crystal of lithium niobate doped with at least one of Zn or Mg.

  A connecting portion core 312c continuing from the electro-optic phase modulator 102 branches into two, a first arm core 312d and a second arm core 312e, and is coupled to the output portion core 312f. A first electrode 314a is formed on the second arm core 312e via an insulating layer 313. A second electrode 314b is formed on the first arm core 312d via an insulating layer 313. A third electrode 314 c is also formed on the insulating layer 313.

  On the plane of the substrate 311, each electrode is disposed so that the third electrode 314 c and the second electrode 314 b sandwich the first electrode 314 a. By adopting a coplanar electrode structure in this way, an electric field perpendicular to the substrate is applied to the second arm core 312e with high efficiency.

By configuring as described above, compared to a configuration in which the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 are connected by an optical fiber, the optical waveguide-fiber coupling loss and the optical connector connection loss are essentially reduced. Can be eliminated. As a result, the output of the light pulse (broadband light) can be further increased. As described above, according to the second embodiment in which the electro-optic phase modulator 102 and the electro-optic intensity modulator 103 are formed on the same substrate, when the characteristics are evaluated in the same manner as described above, the SNR of f _ceo is 52 dB or more. I found out.

  As described above, according to the present invention, an electro-optic modulator structure of an optical waveguide type using a core made of lithium niobate having a refractive index higher than that of the substrate and formed separately from the substrate, and electro-optics Since the phase modulator and the electro-optical intensity modulator are configured, the optical damage resistance in the WG type EOM can be further increased.

  DESCRIPTION OF SYMBOLS 101 ... Light source, 102 ... Electro optical phase modulator, 103 ... Electro optical intensity modulator, 104 ... Dispersion imparting part, 105 ... Optical amplifier, 106 ... Dispersion compensation optical fiber, 107 ... High nonlinear optical fiber, 111 ... Substrate, 112 ... core, 113 ... insulating layer, 114 ... electrode structure.

Claims (4)

A light source that outputs continuous laser light;
An electro-optic phase modulator that phase-modulates the laser light at a desired frequency to generate an optical pulse train having a pulse repetition frequency of the frequency;
An electro-optic intensity modulator that modulates the intensity of the optical pulse train generated by the electro-optic phase modulator at the frequency;
A dispersion imparting unit composed of a single mode optical fiber that compresses the pulse width of each optical pulse of the optical pulse train generated by the electro-optic intensity modulator and outputs broadband light;
The electro-optic phase modulator and the electro-optic intensity modulator are
An optical waveguide-type electricity comprising a substrate, a core made of lithium niobate having a higher refractive index than the substrate and formed separately from the substrate, and an electrode structure for applying a signal of the frequency to the core A broadband stabilized light source characterized by comprising an optical modulator structure. The broadband stabilized light source of claim 1,
2. The broadband stabilized light source, wherein the electro-optic intensity modulator has a Mach-Zehnder interference structure by two electro-optic modulator structures. The broadband stabilized light source according to claim 1 or 2,
The electro-optic phase modulator and the electro-optic intensity modulator are formed on the same substrate. In the broadband stabilized light source according to any one of claims 1 to 3,
The substrate is composed of a crystal of lithium tantalate,
The said core is comprised from the crystal | crystallization of lithium niobate doped with at least one of Zn or Mg. The broadband stabilized light source characterized by the above-mentioned.