JP2018205546A - Widebnad light generator - Google Patents

Abstract

To provide a wideband light generator.SOLUTION: A wideband light generator of the present invention comprises a mode synchronous fiber laser, and a dispersion adding unit composed of a single mode fiber laser for compressing the pulse width of each pulse in an optical pulse train generated by the mode synchronous fiber laser and outputting wideband light. The wideband light generator is further characterized in that the mode synchronous fiber laser consists of a phase synchronous loop, including an electrooptical phase modulator within the phase synchronous loop, the electrooptical phase modulator including an electrooptical phase modulator structure that includes a substrate, a core containing lithium niobate of higher refractive index than the substrate and joined to the substrate, and an electrode structure for applying a modulation signal to the core.SELECTED DRAWING: Figure 9

Description

  The present invention relates to a broadband light generator.

In recent years, a broadband light source has been demanded due to its high potential, and research and development of a broadband light source and further commercialization are actively conducted in various organizations. Recently, studies on application of broadband light sources to the fields of optical communication and optical measurement have been activated. In the fields of optical communication and optical measurement, in addition to broadband characteristics, expectations for light sources with uniform optical frequency intervals are expanding (Non-Patent Document 1).
Many broadband light source technologies have been proposed to meet these 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 the oscillation modes in a laser oscillator. Has been used. Furthermore, in recent years, a generation method using a mode-locked Ti sapphire solid-state laser and a photonic crystal fiber (PCF) has become common for higher bandwidth.

  In recent years, optical pulses from short pulse lasers are incident on nonlinear media such as highly nonlinear fibers (HNLF), and the spectrum is expanded by nonlinear effects that occur in these technologies. Studies on obtaining ultra-wideband light that spans a wide range are intensifying. This broadband light source is called supercontinuum light because of its broad spectrum continuity, or because the oscillation spectrum is regularly arranged in a comb shape on the optical frequency axis, it is also called an optical frequency comb from the shape of the optical spectrum. It has been 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, a plurality of methods and configurations have been proposed depending on the difference between short pulse lasers that generate optical pulses.

  The method using a nonlinear fiber loop type mode-locked laser is a method of generating an optical pulse by using an erbium-doped optical fiber amplifier (EDFA) and a fiber loop and utilizing nonlinear polarization rotation generated in the loop. The broadband light source using this fiber loop has a good affinity with fiber parts and has the advantage of high stability.

  As described above, in recent broadband light sources, an optical pulse generated by a short pulse laser is incident on a nonlinear medium such as a highly nonlinear fiber, and a method of obtaining an ultra-wideband light source by expanding the spectrum by the nonlinear optical effect generated there It is the target.

Nikkei Science July 2008 Issue, P86-94 Translated by Inaba Satoshi “The Most Accurate Measure Hikari Com” W. Imajuku and A.I. Takada, IEEE J. et al. Quantum Electron. 39, 799 (2003). E. Yamazaki, A .; Takada, and T.K. Morioka, IEEE J.M. SeI. Top. Quantum Electron. 12, 536 (2006). Yoshiaki Nakajima, 18 January 2010 / Vol. 18, no. 2 / OPTICS EXPRESS 1667 Kana Iwakuni, 18 June 2012 / Vol. 20, no. 13 / OPTICS EXPRESS 13769 M.M. Asobe, T .; Umeki, and O.I. Tadanaga, “Phase sensitive amplification with noise figure bellow the 3 dB quantum limit using CW pumped PPLN waveguide,” Optics 4- 13 Exp 20 Enbutsu, Koji, et al. "Integratedquasi-phase-matched second-harmonic generator and electro-optic phase modulator for low-quantitative 33-40.

  Below, the problem of a broadband light source is described. The optical output of the optical pulse output from the short pulse laser is the current status in any method that generates a broadband light by injecting an optical pulse generated by a fiber loop type short pulse laser into a nonlinear medium such as a highly nonlinear fiber. Because it is limited to about 10 mW and it is difficult to increase the bandwidth by using the nonlinear effect as it is, in these studies, EDFA and the like are used up to the power level necessary for generating four-wave mixing in the highly nonlinear fiber for spectrum expansion. Broadening the bandwidth by amplifying an optical pulse using an optical amplifier. 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 the EDFA deteriorates the SNR of light generated by four-wave mixing for spectrum expansion. (Non-Patent Document 3). Since the influence of the ASE noise becomes stronger as the input power to the EDFA is lower, in order to secure a high SNR of broadband light, the input power to the EDFA is kept high, that is, the output of the optical pulse from the short pulse laser is increased. Is extremely important. 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, beat signals generated from the broadband light by the self-reference method are OE-converted, and the carrier envelope offset frequency (fceo) and pulse repetition frequency (frep) are further converted from the RF spectrum to the broadband light. The beat signal fbeat is determined and the optical frequency of the laser to be measured is determined. However, the SNR degradation of the broadband light caused by the ASE noise from the spectrum expansion EDFA is caused by the RF spectrum after OE conversion. Raise the upper noise floor. As a result, it becomes difficult to identify and discriminate frequencies such as fceo, which causes an increase in miscounts and deteriorates frequency measurement accuracy. Therefore, high input power to the EDFA for optical pulse amplification is ensured and a high SNR wideband It is essential to generate light.

  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, this problem can be avoided by securing as much as possible the input power to the EDFA, that is, the output of the short pulse laser. For this purpose, the pulse output from the short pulse laser may be increased by using a high output LD for fiber loop excitation. However, in the current light source configuration, it is difficult to solve the problem of SNR degradation of broadband light only by applying such a high-power excitation LD.

  The biggest reason is the low optical input power tolerance of the modulator which is an optical component constituting the short pulse laser. Hereinafter, the optical input power tolerance of this modulator will be described.

  FIG. 1 shows a basic configuration of a broadband light generating apparatus which is a conventional broadband light source. FIG. 1 shows a configuration in which the short pulse laser 100 is connected to an optical amplifier 110, the optical amplifier 110 is connected to a dispersion compensating optical fiber 120, and the dispersion compensating optical fiber 120 is connected to a highly nonlinear fiber 130.

  FIG. 2 shows a mode-locked (fiber) laser 100a using a nonlinear fiber loop laser as one form of the short pulse laser 100 constituting the broadband light generator. FIG. 2 shows an optical isolator 201, an erbium-doped fiber (EDF) 203, a phase modulator 204, a polarization controller 205, an optical coupler 206, an optical coupler 207, and a non-linearity via a single mode (optical) fiber (SMF) 202. A configuration in which a fiber loop laser excitation LD light source 208 is connected is shown. The mode-locked laser using the nonlinear fiber loop laser shown in FIG. 2 generates an optical pulse by using the nonlinear polarization rotation generated in the loop by the EDF 203 and the fiber loop. Here, this nonlinear fiber loop laser loses the stability of the phase in the loop due to the change of the loop length due to the change of the environment and the influence of acoustic noise, and it is difficult to oscillate stably for a long time as it is.

  Therefore, it is common to form a phase-locked loop (PLL) for a long-term stable operation in a short pulse laser with a fiber loop. The PLL operates as follows. A phase modulator indicated by 204 is inserted into the loop, and the compensation signal obtained from the feedback circuit is applied to the phase modulator 204 so that the output pulse power becomes maximum, thereby canceling the phase change in the loop. Here, in order to absorb the influence of various disturbances in the operating environment of the light source, it is necessary to use a phase modulator having a high response speed. Of course, the phase modulator is also required to have low loss.

  Phase modulators are indispensable for nonlinear fiber loop type short pulse lasers as shown in FIG. 2. However, these modulators require a high drive voltage and the presence of the modulator is a pulse from a short pulse laser. Decreasing output power has been a problem.

  In the nonlinear fiber loop type, an EO phase modulator using a bulk crystal, which is a phase modulator having a high response speed, has been used in order to realize a stable operation by a PLL in the studies so far. FIG. 3 shows a configuration example of an EO phase modulator 400 using a bulk crystal. The phase modulator 400 includes a lens 402, an electrode 404, and an EO crystal 403 in contact with the electrode 404. The electrode 404 is connected to the AC power source 405. The EO effect is capable of very fast refractive index change / phase modulation obtained by the Pockels effect due to the electron response in the oxide crystal material, which is a much wider band than that of a phase modulator using a piezo element. Indicates. Therefore, in recent years, a broadband light source using an EO phase modulator in a PLL has been studied, and an extremely long-term stable operation has been confirmed in optical frequency measurement using the broadband light source (Non-Patent Document 4). However, because of its high speed, it can follow the phase fluctuation accompanying the fluctuation of the fiber loop length due to environmental changes at a high speed. However, since it is a bulk crystal type, it is difficult to reduce the interval between the electrodes 404, and thus the drive voltage is high. there were. A further problem is the light coupling efficiency. There is a problem in that the optical coupling efficiency between the optical fiber 401 and the EO crystal 403 is low, leading to light loss in the loop and a decrease in the output power of the optical pulse.

  In order to avoid the problem of high driving voltage and optical loss due to connection of this bulk crystal type EO phase modulator, in recent years it has been studied to apply a waveguide type electro-optic phase modulator (EOPM) which is rich in connectivity with fibers. Has been made. In Non-Patent Document 5, a long-time optical frequency measurement experiment has been successful with broadband light generated using a waveguide type EOPM.

  Among the waveguide type EOPMs, the WG type electro-optic modulator (EOM) using Ti diffused LN crystal is a commercial optical fiber communication system because of its high compatibility with optical connection and low driving voltage. It is a widely used modulator with a track record of being used for generating modulation signals. FIG. 4 shows a configuration example of a conventional EO phase modulator (EOPM) 500 using a Ti diffusion LN crystal waveguide (Ti diffusion WG503). Light can be confined by the waveguide structure, and the distance between the electrodes 404 can be narrowed down to the μm order by microfabrication technology. Therefore, the driving voltage is greatly increased compared to the bulk type EO phase modulator EOPM shown in FIG. It can be reduced. Furthermore, since it is of the waveguide type, the optical coupling efficiency with the input / output optical fiber 401 is high, so that it has the advantage of low loss.

  As described above, the application of the Ti diffusion WG type EOM is being studied in a broadband light source having a configuration in which optical pulse light from a short pulse laser is widened with 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. 5 shows a cross-sectional view of a conventional EOPM (also referred to as an EO phase shifter) 500 using a Ti diffusion WG. An optical buffer layer 603 is formed on the LN substrate 602, and an electrode 404 is formed on the optical buffer layer 603. The electrode 404 is connected to the AC power source 405. In the Ti diffusion WG 503, the core 601 is formed by increasing the refractive index by diffusing Ti on the surface of the LN crystal substrate 602. It can be considered that the refractive index distribution in the core 601 is generally Gaussian and continuously spread. FIG. 6 schematically shows an overview of the refractive index distribution in the x- and y-axis directions of the Ti diffusion WG. FIG. 7 schematically shows an overview of the mode field shape in the x- and y-axis directions of light propagating through the waveguide having the Gaussian refractive index profile shown in FIG. Curves (a) in FIGS. 6 and 7 are the refractive index distribution and the mode field shape when no optical damage occurs, respectively. Here, in the LN substrate 602, 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 waveguide in this state is gradually increased, the refractive index change in the core region is approximately proportional to the light intensity distribution due to the photorefractive effect, and the refractive index at the center of the guided light increases compared to the surroundings. (Curve (b) in FIG. 6). Accordingly, the relative refractive index difference Δn of the waveguide is increased, the light confinement effect is increased, the mode field diameter is reduced (curve (b) in FIG. 7), and the light intensity at the center is higher than that at the time of low input power incidence. . As a result, the refractive index of the central portion further increases. When the input power is increased in this way, a self-convergence phenomenon in which the mode field and the refractive index profile are continuously narrowed by the interaction appears. In addition, since the waveguide structure is asymmetric with respect to the y-axis, the center position of the mode field also changes, causing an axial deviation from the mode field of the optical fiber. When the input power is further increased, the waveguide mode can no longer exist and the input light spreads over the entire substrate. Since the coupling efficiency between the waveguide 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 axial deviation on the waveguide side, and the insertion loss of the modulator module increases. There is an important problem that will occur.

  In order to evaluate the influence of this optical damage, the time-dependent change in insertion loss was measured when light of high input power was incident on a commercially available fiber input / output Ti diffused LNEOPM. FIG. 8 shows a graph of measurement results. In FIG. 8, the x-axis is the incident time of light, and the y-axis is the amount of change in insertion loss. In the evaluation, an experiment was performed under two conditions of 100 mW and 200 mW as an average input power of pulsed light. As can be seen from FIG. 8, as a result of the evaluation, it was found that the loss increases as the incident time becomes longer, and the loss increases as the input power increases.

  As described above, even if the 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 by optical damage. Therefore, it is extremely difficult to increase the output of the light pulse from the short pulse laser. Therefore, the input light to the highly nonlinear fiber (HNL) is reduced because the input power to the EDFA for spectrum expansion decreases as described above, as well as the problem of the decrease in optical power in the configuration using the bulk type EOPM. As a result, the ASE from the optical amplifier is increased, and the signal-to-noise ratio SNR of the broadband light whose spectrum is expanded by four-wave mixing is deteriorated. Therefore, such a broadband light source with little SNR degradation has been desired.

  In order to solve such problems, an object of the present invention is to provide a broadband light generation device that applies a WG type EOPM having high optical damage resistance and has a high SNR and can operate stably for a long period of time.

  As a result of diligent research on the technology related to the broadband light generating device, the inventors can solve the problem by applying EOPM using a direct junction type LN waveguide to the WG type EOPM constituting the short pulse laser in the broadband light generating device. I found out. In recent years, a low-noise optical amplification technique using the nonlinear optical effect of an LN waveguide rich in optical damage resistance manufactured by a method called a direct bonding method, compared with the Ti diffusion LN waveguide poor in optical damage resistance described so far, EOPM has been reported (Non-Patent Documents 6 and 7), and it has been confirmed that the waveguide structure functions stably without destroying the watt-class input optical power. In the direct bonding method, a light-damage resistant LN crystal (ZnLN, MgLN respectively) substrate doped with ZnO or MgO is directly bonded and annealed with another substrate having a low refractive index to form a waveguide structure. This direct bonding method is a technique that improves the optical damage resistance by using ZnLN or MgLN for the core layer of the waveguide and allows high-power light to enter. As a result of the study, the inventors have been able to increase the output of the optical pulse by using this direct junction LN waveguide for the WG for the EOPM for the PLL, and ensure a high input power to the optical amplifier for the spectrum expansion. As a result, it was concluded that a broadband light generating device having a higher SNR than conventional broadband light generating devices and capable of stable operation for a long period of time can be realized.

In order to achieve such an object, one aspect of the broadband light generator of the present invention is:
A mode-locked fiber laser;
In a broadband light generator having a dispersion imparting unit composed of a single mode optical fiber that outputs a broadband light by compressing a pulse width of each optical pulse of an optical pulse train generated by the mode-locked fiber laser,
The mode-locked fiber laser comprises a phase-locked loop, and has an electro-optic phase modulator in the phase-locked loop;
The electro-optic phase modulator is an optical waveguide type including a substrate, a core bonded to the substrate including lithium niobate having a higher refractive index than the substrate, and an electrode structure for applying a modulation signal to the core. A broadband light generator comprising an electro-optic modulator structure.

  The electro-optic phase modulator may include a lumped constant electrode structure.

  The electro-optic phase modulator may have a coplanar electrode structure.

  The substrate includes a crystal of lithium tantalate, and the core includes a crystal of lithium niobate doped with at least one of Zn or Mg.

  According to the present invention, it is possible to realize a broadband light generator that has a higher SNR than conventional broadband light generators and can operate stably for a long period of time.

It is a figure which shows the structural example of the conventional broadband light generator. It is a figure which shows the structural example of the short pulse laser using a fiber loop mode-locked laser. It is a figure which shows the example of the conventional phase modulator for fiber loop optical path length adjustment using a bulk crystal. It is a figure which shows the example of the conventional type EO phase modulator using Ti diffused LN crystal waveguide. It is a figure which shows the structural example of the conventional type | mold phase modulator using Ti diffused LN crystal waveguide. (a) It is a conceptual diagram of the refractive index change of the Ti diffusion LN crystal waveguide core area | region by optical damage, Comprising: It is a figure which shows the refractive index distribution of an x-axis direction. (b) It is a conceptual diagram of the refractive index change of the Ti diffusion LN crystal waveguide core area | region by optical damage, Comprising: It is a figure which shows the refractive index distribution of a y-axis direction. (a) It is a conceptual diagram of the mode field change of the fundamental propagation mode of Ti diffusion LN crystal waveguide by optical damage, Comprising: It is a figure which shows the mode field distribution of an x-axis direction. (b) It is a conceptual diagram of the mode field change of the fundamental propagation mode of Ti diffusion LN crystal waveguide by optical damage, Comprising: It is a figure which shows the mode field distribution of a y-axis direction. It is a figure which shows the insertion loss change of the conventional EO modulator using Ti diffused LN crystal waveguide. 1 is a diagram illustrating a configuration of a broadband light generation apparatus according to Embodiment 1. FIG. 1 is a diagram showing a configuration of an EO phase modulator using a direct junction type LN crystal waveguide according to Embodiment 1. FIG. It is a figure which shows the comparison of the insertion loss change of the direct junction type | mold LN crystal waveguide based on Embodiment 1, and the conventional EO modulator. It is a figure which shows the broadband light generation device and optical frequency measurement system which concern on Embodiment 1. FIG. 6 is a diagram illustrating a measurement result of a carrier envelope offset frequency spectrum in the optical frequency measurement system according to Embodiment 1. FIG. 6 is a diagram showing a configuration of a coplanar EO phase modulator using a direct junction LN crystal waveguide according to Embodiment 2. FIG.

  Hereinafter, embodiments of the broadband light generating apparatus of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the description of the embodiments described below, and it is obvious to those skilled in the art that various changes in form and details can be made without departing from the spirit of the invention disclosed in this specification and the like. is there. In addition, structures according to different embodiments can be implemented in appropriate combination. Note that in the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals, and repetitive description thereof is omitted.

[First Embodiment]
FIG. 9 shows a configuration example of an embodiment of a broadband light generation apparatus 1000 using a short pulse laser and a highly nonlinear fiber by a nonlinear fiber loop. FIG. 9 shows a broadband light generator 1000 including a mode-locked laser 100a using a fiber loop, an Er-doped fiber (amplifier), and a highly nonlinear fiber. In the fiber loop 102, an Er-doped fiber 103, a polarization controller 105 that controls the propagating polarized light to an arbitrary polarization state, an isolator 101 for determining the traveling direction of the oscillating laser light, and an Er-doped fiber 103 are provided. A coupler 106 for inserting the pumping light output from the pumping LD 108 arranged outside the fiber loop 102 for pumping into the fiber loop 102 and a branching unit 107 for outputting the light in the fiber loop 102 to the outside. Has been placed. Further, a PLL phase modulator 1001 is inserted in the fiber loop 102. The fiber loop is composed of a single mode (optical) fiber. The optical intensity of the optical pulse train output by the branching means 107 is amplified by the optical amplifier 110, the pulse width is compressed by the dispersion compensating optical fiber 120 functioning as a dispersion providing unit, and the nonlinear optical effect is obtained by the highly nonlinear fiber 130. The light is output to the outside by extending the pulse width of the optical pulse to a wide band. In the present specification, a portion of the dispersion compensating optical fiber 120 configured from a single mode optical fiber is referred to as a dispersion providing unit.

In the present embodiment, a phase modulator 1001 using a direct junction LN waveguide for compensating for an optical path length change for PLL is manufactured by the following method. FIG. 10 shows a cross-sectional structure of the phase modulator according to the present embodiment. First, a 3-inch z-cut ZNLN substrate having a high refractive index and a 3-inch z-cut MgLN substrate 1102 having a lower refractive index are directly utilized using the intermolecular force of the hydroxyl group adsorbed on the wafer surface. After bonding, an annealing process is performed to form an oxygen bond at the bonding interface, and the two sheets are bonded. Thereafter, a ZnLN substrate used for the waveguide core thickness is ground and polished to a desired core thickness to produce a slab type waveguide. Then, an etching mask having a waveguide pattern is formed on the core layer by photolithography. Further, a ridge-shaped core 1101 was formed on the core surface on which the etching mask was formed by dry etching using CF and Ar gas, and a channel optical waveguide was manufactured. The core contains crystals of lithium niobate doped with Mg. Thereafter, for example, an SiO ₂ film to be the optical buffer layer 1103 is formed on the surface on which the ridge-shaped core is formed by an ECR plasma CVD method. Next, in order to form a modulation electrode, a gold thin film was formed by EB vapor deposition, and then a photoresist having an electrode pattern was formed on the optical buffer layer by photolithography. Thereafter, the gold was removed by wet etching to form a lumped constant electrode. As shown in FIG. 10, a voltage is applied between the electrode 1105 on the ridge-shaped core and the electrode 1104 on the etched surface beside the ridge-shaped core, and a large electro-optic constant r ₃₃ of the LN crystal is applied to the incident light of TM polarized light. The electric field perpendicular to the substrate is designed to be applied with high efficiency in the ridge-shaped core so that it can be used. The film thickness of the electrode 1104 and the film thickness of the electrode 1105 are substantially equal. Since the waveguide structure in the present embodiment is a ridge type, the relative refractive index difference of the waveguide can be made larger than that of the Ti diffusion type WG, and thus light can be strongly confined in the ridge-shaped core. Since this is possible, it is possible to effectively modulate the guided light. In this embodiment, an SiO ₂ film is used as the optical buffer layer 1103, but an SiN _x film may be used.

  As a result of converting the phase modulation of the present embodiment into intensity modulation by the retardation method and evaluating the response speed of the modulation, it was found that the intensity-modulated light sufficiently responded to the application of a sinusoidal voltage in the order of MHz. . Next, a fiber input / output EOM module was fabricated using this directly bonded LN waveguide, and the change over time in the insertion loss with respect to the input optical power was measured. FIG. 11 shows a graph of measurement results. As in FIG. 8, in FIG. 11, the x-axis is the light incident time, and the y-axis is the amount of change in insertion loss. Also, the average input power condition of the pulsed light was tested under two conditions of 100 mW and 200 mW as in the experiment in FIG. As can be seen from FIG. 11, as a result of the evaluation, it was revealed that the change in insertion loss was very small even when light was incident for a long time of 100 hours or more, and the high optical damage resistance of the direct junction LN waveguide was confirmed.

  Next, in order to evaluate the characteristics of the broadband light generator 1000 in which this EOM module is applied to a phase modulator for PLL, an optical frequency measurement experiment of broadband light was performed. Since the generated broadband light has an optical frequency band of one octave or more, it is possible to use interference between the broadband light and its second harmonic. Therefore, the SNR improvement effect of broadband light generated by the f-2f self-referencing method was evaluated.

  FIG. 12 shows a broadband light generation apparatus 1000 and an optical frequency measurement system 1200 according to this embodiment. In FIG. 12, 1301 is a polarization controller, 1302 is a periodically poled lithium niobate crystal (PPLN) waveguide, 1303 is an optical bandpass filter, and 1304 is a photodetector. In the optical frequency measurement system according to the present embodiment, the second harmonic (SHG) of the broadband light is generated in the PPLN waveguide, and at the same time, the broadband light and the SHG light are interfered to obtain a beat signal. By detecting this beat signal with an RF spectrum analyzer, the carrier envelope offset frequency fceo of broadband light was measured. FIG. 13 shows a graph of the measured beat signal spectrum. When the beat signal spectrum was evaluated, the limit of fseo SNR when Ti diffusion EOPM was applied to PLL was about 40 dB, whereas the fseo SNR in the present embodiment was 51 dB or more. I understood. In the configuration in which the Ti diffusion EOPM is applied to the PLL modulator, the optical pulse output is limited to an average power of 10 mW in order to avoid optical damage, whereas an EOM made of a direct junction LN waveguide having high optical damage resistance. Since the output of the fiber laser can be increased to 100 mW by applying the EDFA, sufficiently large input power to the EDFA used for pulse amplification can be secured, and the influence of excess noise in the EDFA can be effectively suppressed. It was possible to greatly improve the SNR of the fceo signal by 10 dB or more.

[Second Embodiment]
Next, Embodiment 2 of the present invention will be described with reference to FIG. FIG. 14 is a diagram showing a cross section of the phase modulator of the broadband light generation apparatus according to Embodiment 2 of the present invention. In the second embodiment, a traveling wave type electrode that responds more rapidly to the fact that the electric field application electrode of the phase modulator used in the first embodiment is a lumped constant type electrode is loaded on the waveguide core. Is different. This traveling wave type electrode was produced by the following method.

First, a ridge-shaped core 1402 was joined on an LT substrate (lithium tantalate substrate) 1401. This ridge-shaped core contains a crystal of lithium niobate doped with Zn, and can also be called a ZnLN layer. Next, an optical buffer layer 1403 was formed so as to cover the ridge-shaped core 1402. The optical buffer layer is made of, for example, a SiO ₂ film. Photoresist was formed on the optical buffer layer at locations other than the electrodes by photolithography. Thereafter, gold, which is an electrode material, is formed by plating on a portion of the optical buffer layer where the photoresist is not formed to a thickness of 30 μm, and unnecessary resist is removed to thereby remove the central conductor 1404 and the ground conductor 1405. A coplanar electrode (CPW electrode) 1406 made of the above was formed. The center conductor 1404 overlaps with the ridge-shaped core 1402, and the ground conductor 1405 does not overlap with the ridge-shaped core 1402. Further, the film thickness of the center conductor 1404 and the film thickness of the ground conductor 1405 are different. Similarly to the first embodiment, as shown in FIG. 14, a voltage is applied between the electrode (center conductor) 1404 on the ridge-shaped core and the electrode (ground conductor) 1405 on the side of the ridge-shaped core, and the incident light of TM polarized light is applied. On the other hand, an electric field perpendicular to the substrate is designed to be applied with high efficiency in the ridge-shaped core so that the large electro-optic constant r ₃₃ of the LN crystal can be used. Further, as shown in FIG. 14, an optical buffer layer 1403, for example, a SiO ₂ film is disposed around the electrode, and the effective dielectric constant of the high-frequency signal propagating through the electrode is lowered, and the speed matching is made close to the refractive index of light. Thus, a coplanar electrode structure that efficiently converts a high-frequency signal voltage into a phase change is obtained. In this embodiment, an SiO ₂ film is used as the optical buffer layer 1403, but an SiN _x film may be used.

  As a result of using the phase modulator to which the coplanar electrode structure manufactured as described above is applied, the response speed of the optical path length change compensation for PLL is the phase modulator to which the lumped constant electrode used in the first embodiment is applied. Compared with the conventional broadband light generator, the stability of the phase synchronization in the fiber loop was improved, and the continuous measurement time of the broadband light fceo could be measured for 10 times or more compared with the first embodiment.

  The present invention can be applied to a broadband light generator.

100 short pulse laser 100a mode-locked (fiber) laser 101 isolator
102 Fiber loop 103 Er-doped fiber 105 Polarization controller 106 Coupler 107 Branch means 108 Pumping LD
DESCRIPTION OF SYMBOLS 110 Optical amplifier 120 Dispersion compensation optical fiber 130 Highly nonlinear fiber 201 Optical isolator 202 Single mode (optical) fiber (SMF)
203 Erbium-doped fiber (EDF)
204 Phase modulator 205 Polarization controller 206, 207 Optical coupler 208 Nonlinear fiber loop laser excitation LD light source 400 Phase modulator 401 Optical fiber 402 Lens 403 EO crystal 404 Electrode
405 AC power supply 500 EOPM
503 Ti diffusion WG
601 Core 602 LN (crystal) substrate 603 Optical buffer layer 1000 Broadband light generator 1001 PLL phase modulator 1101 Core 1102 MgLN substrate 1103 Optical buffer layer 1105 Electrode 1104 Electrode 1200 Optical frequency measurement system 1301 Polarization controller 1302 Periodic polarization inversion niobium Lithium acid crystal (PPLN) waveguide 1303 Optical band pass filter 1304 Photo detector 1401 LT substrate 1402 Core 1403 Optical buffer layer 1404 Electrode (center conductor)
1405 Electrode (Grounding conductor)
1406 Coplanar electrode (CPW electrode)

Claims (4)

A mode-locked fiber laser;
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 mode-locked fiber laser and outputs broadband light;
In a broadband light generator having
The mode-locked fiber laser comprises a phase-locked loop, and has an electro-optic phase modulator in the phase-locked loop;
The electro-optic phase modulator is
An optical waveguide type electro-optic modulator structure comprising a substrate, a core bonded to the substrate including lithium niobate having a higher refractive index than the substrate, and an electrode structure for applying a modulation signal to the core A broadband light generator characterized by that. The broadband light generator according to claim 1,
The electro-optic phase modulator has a lumped-constant electrode structure. The broadband light generator according to claim 1,
The electro-optic phase modulator includes a coplanar electrode structure, and a broadband light generator. In the broadband light generating device according to any one of claims 1 to 3,
The substrate comprises a crystal of lithium tantalate;
The said core contains the crystal | crystallization of lithium niobate doped with at least one of Zn or Mg. The broadband light generator characterized by the above-mentioned.