Wavelength variable optical band-pass filter, spectrometer using the same, and method for controlling wavelength variable optical band-pass filter

Abstract

To increase variable speeds in a wavelength variable bass-pass filter of SBS type.SOLUTION: Provided is a wavelength variable band-pass filter using Simulated Brillouin Scattering (SBS), comprising: a light source for outputting SBS pump light based on a set wavelength; an SBS medium which the SBS pump light and the signal light to be filtered are inputted to, and which outputs part of SBS-amplified signal light; and a path control unit located between the light source and the SBS medium, for forwarding input from the SBS medium to an output terminal while outputting input from the light source to the SBS medium. The light source is a Dispersion-Tuned Mode-Locked Laser (DTMLL).SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a tunable optical bandpass filter capable of increasing the tuning speed, a spectroscopic measurement device using the same, and a method for controlling the tunable optical bandpass filter. [Background technology]

[0002] Conventionally, optical tunable bandpass filters (OTBPFs) have been considered essential components in optical communications and sensing. A comparison of various optical filter methods is shown in Table 1. This Table 1 was created by the inventors of the present invention based on Non-Patent Documents 4 and 5.

[0003] [Table 1]

[0004] In Table 1, the grating type is a method that uses a diffraction grating. The Fabry-Perot (FP) type uses the transmission / reflection of a resonator using a pair of opposing mirrors, and includes the fiber Fabry-Perot (FFP), liquid crystal FP, and micromachine FP types. The ring resonator type uses a ring resonator instead of an FP resonator. The cascade MZI type uses the interference pattern of a Mach-Zehnder interferometer. The fiber Bragg grating (FBG) type is a type in which a periodic diffraction grating is written into a fiber.

[0005] The "Acousto-optic tunable filter" uses the acousto-optic effect, and the "Electro-optic tunable filter" uses the electro-optic effect. Both are methods of achieving filtering by spatially modulating the refractive index of a medium using acoustic waves and an electric field to create a diffraction grating. The "Arrayed waveguide grating" utilizes the interference of output light from an arrayed waveguide. The "Active filter" is a method in which signal light is input into a laser resonator and filtering is performed using the resonator's filtering characteristics, while simultaneously amplifying the light using the laser active layer.

[0006] The stimulated Brillouin scattering (SBS) type is a method that utilizes signal filtering by Brillouin amplification using SBS (Non-Patent Document 5). This method utilizes the narrow amplification bandwidth of Brillouin amplification, which is several tens of MHz, to selectively amplify a portion of the spectral components of the light under measurement, thereby achieving narrow-band filtering. Spectroscopic analysis devices based on this method have been proposed (Patent Document 1, Non-Patent Document 1) and are commercially available (Non-Patent Document 2).

[0007] The following three specifications are required for the OTBPF described above. (1) Variable filter width and narrow band (2) Wavelength tunable range (3) Variable speed

[0008] Table 1 shows typical specifications for various methods. In conventional technology, there were methods that satisfied one or two of the required specifications (1) to (3), but it was difficult to satisfy all of (1) to (3) simultaneously. In particular, with regard to (1) filter width variability and narrow band, it was difficult to achieve a narrow band of 10 GHz or less.

[0009] Among the methods listed in Table 1, the SBS method is a particularly promising method that can achieve a narrow bandwidth of several tens of MHz to several tens of GHz and a wavelength tuning range of 40 nm or more. However, since its wavelength tuning mechanism depends on the wavelength tuning performance of the SBS pump light source, the tuning speed of the filter is limited by the tuning speed of the wavelength tunable light source. The wavelength tuning mechanism of a wavelength tunable laser used as a pump light source generally relies on inserting an OTBPF into the laser resonator and changing its center wavelength. In this case, the OTBPF inserted into the resonator is also one of the various methods listed in Table 1, so ultimately the tuning speed is limited by the tuning speed of the other filters.

[0010] As a wavelength tunable laser with a wavelength tuning mechanism that does not rely on OTBPF, there is a method that utilizes a phenomenon called dispersion tuning in an actively mode-locked laser. Hereinafter, this will be referred to as a dispersion-tuned mode-locked laser (DTMLL) (Non-Patent Documents 3, 6-11). The principle behind this will be explained below.

[0011] Figure 1 shows the basic configuration of a DTMLL. As shown in Figure 1, a DTMLL 100 comprises a gain medium 1, a modulator 2, a signal source 3 that drives the modulator 2, a power splitter 4, a dispersion medium 5, and an isolator 6, all connected in a ring configuration. An actively mode-locked laser is a laser in which a modulator is inserted into the laser resonator and modulated to forcibly generate mode-locked oscillation. Mode-locking is achieved when the modulation frequency is an integer multiple of the free spectral range (FSR) of the resonator. The FSR of the resonator, or "f FSR " is the speed of light c, the group refractive index n g , where L is the cavity length, FSR = c / n g It is given by "L".

[0012] If the dispersion in the resonator is strong, as shown in Fig. 2, gSince f has wavelength dependence due to group delay dispersion (GVD), FSR In this situation, a certain modulation frequency f m When "f m =Nf FSR Mode locking occurs at wavelengths that satisfy the relationship "(N = 1, 2, ...)". Therefore, f m By making the OTBPF variable, the oscillation wavelength can be tuned. This wavelength tuning principle does not depend on the tuning of the OTBPF, so it is possible to increase the speed, and since the wavelength tuning range is determined by the gain bandwidth of the gain medium, it is possible to achieve a wide bandwidth. Non-Patent Document 3 shows that by using a semiconductor optical amplifier (SOA) with a wide gain bandwidth, it is possible to achieve a wavelength tuning range of 83 nm and a high-speed wavelength tuning of 1 MHz. This laser is being applied to high-speed strain measurement using FBG and optical coherence tomography. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Patent No. 4414341 [Non-patent literature]

[0014] [Non-Patent Document 1] JMS Domingo, J. Pelayo, F. Villuendas, CD Heras and E. Pellejer, "Very high resolution optical spectrometry by stimulated Brillouin scattering," in IEEE Photonics Technology Letters, vol. 17, no. 4, pp. 855-857, April 2005, doi: 10.1109 / LPT.2005.843946. [Non-patent document 2] ZEPREN Solutions, PHOTONICS TEST & MEASUREMENT SOLUTIONS (brochure) “https: / / zepren.com / wp-content / uploads / 2024 / 01 / 2024-01-Photonics-TM-Catalog-v1.1.pdf.” [Non-patent document 3] Y. Takubo and S. Yamashita, "High-speed dispersion-tuned wavelength-swept fiber laser using a reflective SOA and a chirped FBG," Opt. Express 21, 5130-5139 (2013). [Non-patent document 4] D. Sadot and E. Boimovich, "Tunable optical filters for dense WDM networks," in IEEE Communications Magazine, vol. 36, no. 12, pp. 50-55, (1998). [Non-patent document 5] T. Tanemura, Y. Takushima, and K. Kikuchi, "Narrowband optical filter, with a variable transmission spectrum, using stimulated Brillouin scattering in optical fiber," Opt. Lett. 27, 1552-1554 (2002). [Non-patent document 6] S. Li and KT Chan, “Electrical wavelength tunable and multiwavelength actively mode-locked fiber ring laser,” Appl. Phys. Lett. 72, 1954-1956 (1998). [Non-Patent Document 7] S. Yamashita and M. Asano, "Wide and fast wavelength-tunable mode-locked fiber laser based on dispersion tuning," Opt. Express 14, 9299-9306 (2006) [Non-patent document 8] Y. Nakazaki and S. Yamashita, "Fast and wide tuning range wavelength-swept fiber laser based on dispersion tuning and its application to dynamic FBG sensing," Opt. Express 17, 8310-8318 (2009) [Non-Patent Document 9] Yuya Takubo, Study on performance improvement of dispersion-tuned fiber lasers for OCT applications, PhD thesis, 2015. [Non-Patent Document 10] K. Tamura and M. Nakazawa, “Dispersion-tuned harmonically mode-locked fiber ring laser for self-synchronization to an external clock,” Opt. Lett. 21, 1984-1986 (1996). [Non-Patent Document 11] Y. Hasegawa, T. Shirahata and S. Yamashita, "Analysis of Dynamic Properties of Dispersion-Tuned Swept Lasers," in Journal of Lightwave Technology, vol. 33, no. 1, pp. 219-226, (2015). Summary of the Invention [Problem to be solved by the invention]

[0015] To achieve high-speed tunable bandpass filters, it is considered effective to use a DTMLL as a pump light source for an SBS-type optical filter. However, the spectral properties of conventional DTMLLs, specifically the longitudinal mode spacing and spectral width, are not suitable for use as a pump light source for an SBS-type optical filter, and there is a problem that they cannot be used as is.

[0016] As a premise, the Brillouin shift f B and Brillouin gain bandwidth δf B are typically expressed as f B ~11GHz,δf B The DTMLL is a mode-locked laser, and according to Non-Patent Document 10, the time waveform of its output is Gaussian. The repetition frequency of the pulse train is f rep Then, the spectrum shape is the Fourier transform of the time waveform, and has a Gaussian envelope with a longitudinal mode spacing of f l =f rep In the following, the "spectral envelope width" is simply defined as the spectral width δ fe The spectral width (spectral linewidth) of each longitudinal mode is defined as δf lw "

[0017] DTMLL is usually a fiber laser with a cavity length of several tens of meters, so δf lw It is believed that the linewidth is narrow on the order of 100 kHz. Conventional knowledge suggests that under certain conditions, the longitudinal mode spacing of the DTMLL is l and spectral width δf e are f l ~770MHz, δf e When this DTMLL output light is used as SBS pump light, Δf lw <<δf B Therefore, the width of the SBS amplification band created by one longitudinal mode is approximately δf B As shown in Figure 3, the spectral shape of the entire SBS amplification band is a superposition of contributions from each longitudinal mode, and as a result, the overall spectral shape is determined by fl This results in a comb shape with spacing of about 770 MHz, which is not suitable for an optical filter shape.

[0018] Regarding the spectral width, the amplification bandwidth in Brillouin amplification is at most f B This is because in the SBS process, the Stokes light on the low-frequency pump side generates gain, and at the same time, the anti-Stokes light on the high-frequency pump side is absorbed. Figure 4 shows this diagrammatically. As shown in Figure 5, f B 2F from B When multiple pump lights are used with frequency differences in the range, the gain, pump, and loss frequencies may overlap and cancel each other out. Therefore, the bandwidth of the pump light = δf e f B Even if the amplification band is expanded beyond this, the maximum B Furthermore, δf e >f B In this situation, the overlap between gain and absorption causes a deterioration in the extinction ratio and an increase in the Brillouin threshold power, which may adversely affect the characteristics of the optical filter. e ~40GHz and δf e >f B Therefore, in order to avoid the above situation, it is necessary to narrow the spectrum.

[0019] The present invention has been made in view of the above circumstances, and aims to provide a tunable optical bandpass filter that can increase the tuning speed in an SBS-type tunable optical filter, a spectrometer using the same, and a method for controlling the tunable optical bandpass filter.

[0020] (1) In order to achieve the above object, the present invention provides the following means: That is, the tunable optical bandpass filter of the present invention is a tunable optical bandpass filter using stimulated Brillouin scattering (SBS), and includes: a light source that outputs SBS pump light based on a set wavelength; an SBS medium that receives the SBS pump light and signal light to be filtered and outputs a portion of the signal light that has been SBS-amplified; and a path controller that is provided between the light source and the SBS medium and outputs the input from the light source to the SBS medium and the input from the SBS medium to an output terminal, wherein the light source is a dispersion-tuned mode-locked laser (DTMLL).

[0021] (2) The wavelength tunable optical bandpass filter of the present invention is characterized by further comprising a control unit that controls the dispersion-tuned mode-locked laser to set the longitudinal mode spacing of the SBS pump light to a frequency smaller than the Brillouin bandwidth and to set the spectral width of the SBS pump light to the Brillouin shift frequency or less.

[0022] (3) Furthermore, when the gain medium of the dispersion tuning mode-locked laser is a semiconductor optical amplifier (SOA), the tunable optical bandpass filter of the present invention further comprises a modulation unit that inputs SBS pump light output by the dispersion tuning mode-locked laser and modulates the longitudinal mode spacing of the SBS pump light to a frequency smaller than the Brillouin bandwidth.

[0023] (4) In the tunable optical bandpass filter of the present invention, the modulation section outputs a plurality of periodic passbands by controlling the modulation frequency.

[0024] (5) The tunable optical bandpass filter of the present invention further comprises a power adjustment mechanism that adjusts the power of the SBS pump light output by the dispersion tuning mode-locked laser, and the control unit controls the amount of power adjustment of the power adjustment mechanism so that the optical power of the SBS pump light has an intensity that allows a desired SBS amplification gain to be obtained in the SBS medium, and if the power of the input SBS pump light has wavelength dependency, so as to equalize it, and also controls the Brillouin shift frequency and Brillouin gain bandwidth of the SBS medium based on the wavelength, material, temperature, and strain of the medium.

[0025] (6) In the wavelength tunable optical bandpass filter of the present invention, the dispersion tuning mode-locked laser includes at least a gain medium, a modulator, and a dispersion medium, and is configured as at least one of a ring resonator and a Fabry-Perot resonator.

[0026] (7) In the wavelength tunable optical bandpass filter of the present invention, the dispersion tuning mode-locked laser maintains a constant modulation frequency and dynamically modulates dispersion.

[0027] (8) Furthermore, in the wavelength tunable optical bandpass filter of the present invention, when the SBS pump light output from the dispersion tuning mode-locked laser is single-polarized and the SBS medium is polarization-maintaining, the wavelength tunable optical bandpass filter may be configured to input the SBS pump light output via a half-wave plate at an angle of 45° to the polarization axis of the SBS medium; to split the SBS pump light into two and polarization-combine them using a polarization beam splitter; to fusion-splice the SBS pump light and a polarization-maintaining circulator at an angle of 45° to their respective polarization axes; or to provide a polarization scrambler between the dispersion tuning mode-locked laser and the SBS medium to average the polarization dependence of the SBS pump light.

[0028] (9) In the tunable optical bandpass filter of the present invention, a polarization beam splitter is further provided at the output end to perform polarization splitting of the output of the path control unit.

[0029] (10) A spectrometer according to the present invention is characterized by comprising the tunable optical bandpass filter according to (1) or (3) above.

[0030] (11) Furthermore, the method for controlling a wavelength tunable optical bandpass filter of the present invention is a method for controlling a wavelength tunable optical bandpass filter using stimulated Brillouin scattering (SBS), which comprises adjusting the longitudinal mode spacing of the SBS pump light by Δf l and the Brillouin bandwidth is δf B and the spectral width of the SBS pump light is δf e and the Brillouin shift frequency is f B When the SBS pump light source, the dispersion tuning mode-locked laser (DTMLL), is used, Δf l <δf B and δf e <f B The control is characterized by performing control that satisfies the following. [Effects of the Invention]

[0031] According to the present invention, it is possible to increase the tuning speed in an SBS type tunable optical filter. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 is a diagram illustrating the basic configuration of a DTMLL. [Figure 2] FIG. 1 is a diagram illustrating the principle of wavelength tuning. [Figure 3] FIG. 10 shows longitudinal mode spacing and filter results. [Figure 4] FIG. 1 is a diagram showing a schematic diagram of SBS amplification and absorption. [Figure 5]FIG. 2 is a diagram schematically illustrating the arrangement of pump light and the frequency relationship of Brillouin gain. [Figure 6] 1 is a block diagram showing a schematic configuration of a tunable optical bandpass filter according to the present invention; [Figure 7] FIG. 10 is a diagram showing a configuration in which a modulator is inserted into a DTMLL output. [Figure 8] FIG. 10 is a diagram showing a comparison between the resonator length and 50 / FSR, which is a comparison result of Non-Patent Document 3. [Figure 9] FIG. 10 is a diagram showing an example of calculation of the relationship between the amount of dispersion and Δfe. [Figure 10] FIG. 10 is a diagram showing a modified example of the first embodiment. [Figure 11A] FIG. 10 is a diagram showing a reflective ring type. [Figure 11B] FIG. 1 is a diagram showing a reflective SOA ring type. [Figure 11C] FIG. 10 is a diagram showing a configuration using a reflective dispersion medium. [Figure 11D] This is a diagram showing an arrangement in which a polarizing beam splitter 20 and a Faraday rotator mirror 21 are used instead of a circulator. [Figure 11E] FIG. 10 is a diagram illustrating a gain modulation type. [Figure 11F] FIG. 1 is a diagram illustrating a distributed modulation type. [Figure 11G] FIG. 10 is a diagram showing a playback mode synchronization type. [Figure 12A] FIG. 1 is a diagram illustrating an example of the configuration of a DTMLL having a Fabry-Perot resonator structure. [Figure 12B] FIG. 1 shows a diagram in which the mirror and gain medium are replaced with RSOAs. [Figure 12C] This figure shows a configuration in which the mirror and gain medium are replaced with RSOAs, and the dispersion medium is further replaced with a reflective dispersion medium. [Figure 13] FIG. 10 is a diagram illustrating an example of a variable dispersion medium. [Figure 14A] FIG. 10 is a diagram showing a configuration in which the DTMLL output light is input at an angle of 45° with respect to the polarization axis of the SBS medium. [Figure 14B]This is a diagram showing a method in which the DTMLL output light is split into two and the polarizations are combined using a polarization beam splitter. [Figure 14C] FIG. 14B shows a configuration in which the same state as in FIG. 14A is achieved by fusion splicing the DTMLL output and the polarization-maintaining circulator at an angle of 45° with respect to their respective polarization axes. [Figure 14D] FIG. 10 is a diagram showing a configuration in which a polarization scrambler is inserted into the DTMLL output to average the polarization dependency. [Figure 15] FIG. 10 is a diagram showing a configuration in which a polarization beam splitter is connected to the output of a circulator. [Figure 16] FIG. 10 is a diagram showing an example of a spectrometer configured using the DTMLL having the configuration described in the first and second embodiments. [Figure 17] 8 is a configuration example of a spectrometer based on the configuration of FIG. 7. DETAILED DESCRIPTION OF THE INVENTION

[0033] The inventors focused on the fact that the tuning speed cannot be improved in an SBS-type tunable optical filter, and discovered that when a DTMLL is used as a pump light source, the longitudinal mode spacing of the DTMLL output light can be controlled to a frequency smaller than the Brillouin bandwidth to make the transmission characteristics of the SBS-type optical filter a single passband, and that the extinction ratio can be maximized and the Brillouin threshold minimized by controlling the spectral width to be equal to or less than the Brillouin shift frequency, thereby improving the tuning speed of the SBS-type tunable optical filter, leading to the present invention.

[0034] That is, the tunable optical bandpass filter of the present invention is a tunable optical bandpass filter using stimulated Brillouin scattering (SBS), and comprises: a light source that outputs SBS pump light based on a set wavelength; an SBS medium that receives the SBS pump light and signal light to be filtered and outputs a portion of the signal light that has been SBS-amplified; and a path controller that is provided between the light source and the SBS medium and outputs the input from the light source to the SBS medium and outputs the input from the SBS medium to an output terminal, wherein the light source is a dispersion-tuned mode-locked laser (DTMLL).

[0035] As a result, the inventors have made it possible to increase the tuning speed in a wavelength tunable optical bandpass filter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0036] Fig. 6 is a block diagram showing a schematic configuration of a wavelength tunable optical bandpass filter according to the present invention. As shown in Fig. 6, the wavelength tunable optical bandpass filter 101 is composed of a setting input unit 7, a control unit 8, a DTMLL 9, a power adjustment mechanism 10, an optical circulator 11, an SBS medium 12, an optical isolator 13, a signal light input unit 14, and a signal light output unit 15. The setting input unit 7 accepts settings for the OTBPF, such as the center wavelength of the bandpass and the passband width, and provides the setting information to the control unit 8.

[0037] The DTMLL 9 ​​varies the oscillation wavelength depending on the frequency applied to the modulator in the resonator. The control unit 8 maintains an internal frequency-wavelength correspondence table, determines the frequency based on the set wavelength input from the setting input unit 7, and sets this in the DTMLL 9. As mentioned above, the DTMLL 9 ​​has the configuration shown in Figure 1, and is composed of a gain medium 1, a modulator 2 and a signal source 3 that drives the modulator 2, a power splitter 4, a dispersive medium 5, and an isolator 6. These components form the resonator of the DTMLL 9.

[0038] Each component of the DTMLL 9 ​​is designed to satisfy the aforementioned "Condition 1 regarding longitudinal mode spacing" and "Condition 2 regarding spectral width." Design guidelines will be described later. In a properly designed DTMLL 9, the frequency of the internal modulation signal source is controlled by a control unit 8, and the DTMLL 9 ​​oscillates at the desired wavelength. The output light of the DTMLL 9 ​​is input to a power adjustment mechanism 10, where it is amplified by an optical amplifier provided in the power adjustment mechanism 10 or attenuated by an optical attenuator provided in the power adjustment mechanism 10, so that the optical power is such that the desired SBS amplification gain is obtained. The output light of the power adjustment mechanism 10 is input to an SBS medium 12 from left to right on the paper via an optical circulator 11, which serves as a path control unit.

[0039] The "signal light to be filtered" is input to the SBS medium 12 from the signal light input unit 14 in the opposite direction to the SBS pump light, from right to left on the paper. An optical isolator 13 is provided between the SBS medium 12 and the signal light input unit 14, and is configured to prevent the pump light from being input to the signal light source. A portion of the signal light is amplified by SBS amplification and output to the signal light output unit 15 via the optical circulator 11.

[0040] Next, we will explain the design guidelines for the DTMLL 9. The above-mentioned "Condition 1 regarding the longitudinal mode spacing" and "Condition 2 regarding the spectral width" can be considered using the following relational expressions based on the analysis methods described in Non-Patent Documents 7 to 9.

[0041] The longitudinal mode spacing of the DTMLL output light is δf l Then, δf l is the modulation frequency f of the DTMLL m It is determined by δf l =f m In an actively mode-locked laser, f m is the FSR of the resonator, i.e., "f FSR " and the minimum f m is f m =f FSR f FSRis given by the following equation:

[0042]

number

[0043] Also, the modulation frequency f m wavelength λ m is given by the following equation:

[0044]

number

[0045] The maximum wavelength tunable range Δλmax determined from the FSR is given by the following equation:

[0046]

number

[0047] Spectral width δf of the DTMLL output light e is given by the following equation:

[0048]

number

[0049] According to Non-Patent Document 9, when an SOA is used as a gain medium, the nonlinearity inherent in the SOA causes a large chirp in the low modulation frequency range, broadening the spectral width. Under the conditions in Non-Patent Document 9, the modulation frequency at which the spectral width is minimized is estimated to be around 2 GHz, which may make it difficult to simultaneously satisfy Conditions 1 and 2 above. To resolve this problem, as shown in Figure 7, a modulator 16 and a modulation signal generator 16-2 are provided as a modulation unit outside the DTMLL 9 ​​(resonator), and sidebands are generated so that the longitudinal mode spacing is smaller than the Brillouin bandwidth, thereby satisfying Condition 1.

[0050] Furthermore, in the configuration shown in Figure 7, f l >>f B By doing so, it is possible to create transmission characteristics with multiple periodic passbands, similar to a WDM interleaver. The configuration shown in Figure 7 is expected to be useful for demultiplexing WDM signals and extracting each comb component of a frequency comb.

[0051] Next, we will explain the wavelength tuning speed of the DTMLL9. m When the switching frequency is changed, the oscillation wavelength of the DTMLL9 does not change instantly, but rather passes through a transient state before reaching a steady state. To quantitatively analyze the time required for the oscillation wavelength to reach a steady state, numerical analysis based on the Ginzburg-Landau equation is required (Non-Patent Documents 9 and 11).

[0052] An example of a numerical analysis of an actively mode-locked laser with a configuration similar to that of the DTMLL of this embodiment is disclosed in the reference document "Y. Hasegawa, T. Shirahata, and S. Yamashita, "Analysis of Dynamic Properties of Dispersion-Tuned Swept Lasers," in Journal of Lightwave Technology, vol. 33, no. 1, pp. 219-226, (2015)." As shown in "Figs. 9 to 11" of this reference, a steady state is reached after approximately 50 revolutions. Since the time required for a pulse to make one revolution around the resonator is 1 / FSR, the time required to reach steady state is estimated to be 50 / FSR. Furthermore, Non-Patent Document 3 compares the wavelength tuning speed of the DTMLL by changing the resonator length. Figure 8 shows the comparison results of Non-Patent Document 3, comparing the resonator length and 50 / FSR. As shown in Figure 8, the two results are consistent on the same order of magnitude, which is considered to support the validity of the above discussion.

[0053] [Example 1] A more specific example of the basic configuration shown in Figure 6 will be described. Here, the desired operating wavelength is assumed to be the C-band of 1530 nm to 1565 nm. The components of the DTMLL 9 ​​shown in Figure 1, including the gain medium 1, modulator 2, signal source 3 that drives modulator 2, power splitter 4, dispersion medium 5, and isolator 6, are as follows:

[0054] The gain medium 1 is an erbium-doped fiber amplifier (EDFA) with a gain bandwidth covering the entire C-band. The modulator 2 is a Mach-Zehnder modulator using lithium niobate (LiNbO3: LN). The signal source 3 that drives the modulator 2 is a signal generator that outputs a radio frequency sine wave. The power splitter 4 taps 10% of the power out of the resonator. The dispersion medium 5 is 40m of dispersion compensating fiber with a dispersion parameter of -100ps / nm / km. The isolator 6 is an optical isolator with an isolation of 20dB or more. The power adjustment mechanism for the output of the DTMLL 9 ​​is the EDFA mentioned above.

[0055] A standard single-mode fiber is used as the SBS medium 12 shown in Figure 6. The Brillouin shift frequency f B and Brillouin bandwidth δf B are respectively f B ~11GHz,δf B The longitudinal mode spacing of the DTMLL output light is Δf l Then, from condition 1, δf l Set to 5MHz.

[0056] In equation (1), c = 3 × 10 8 m / s,n g = 1.462, then f m =f FSR =δf l = 5MHz. L is 40m.

[0057] The proportionality coefficient of the wavelength obtained from equation (2) is 9.75 nm / kHz when λ0 = 1530 nm and D = -100 ps / nm / km.

[0058] The wavelength tunable range calculated from equation (3) is Δλ max = 50 μm, which is sufficiently wider than the desired wavelength band. The actual wavelength tunable range used as a filter is 35 nm, so the corresponding frequency change is 3.591 kHz.

[0059] Furthermore, from equation (4), when M = 0.5, D = -100 ps / nm / km, and L = 40 m are used, δf e =3.1GHz, which is the Brillouin shift f B It is smaller than 11GHz and satisfies condition 2. The variable speed is 50 / f FSR Therefore, it is estimated to be 10 μs.

[0060] [Example 2] In the second embodiment, a dispersion compensation module that can achieve a large dispersion amount with a short fiber length is used instead of a dispersion compensation fiber. By using the dispersion compensation module, the spectral width can be narrowed further. Figure 9 shows the relationship between the dispersion amount and Δf e For example, if a chirped FBG (CFBG) with a dispersion of -1000 ps / nm is used with L=40 m, Δf e = 800MHz, which makes it possible to narrow the passband width.

[0061] [Example 3] FIG. 10 is a diagram showing a modification of the first embodiment. As a difference from the basic configuration of FIG. 6, the control unit 8 also controls the power adjustment mechanism 10 and the SBS medium 12. For example, when the output optical power of the DTMLL 9 ​​has wavelength dependency, the power adjustment mechanism 10 controls the amount of power adjustment to equalize it. In addition, in the SBS medium 12, generally, f B and δf B Since depends on the wavelength, the material of the medium, the temperature, and the strain, f B and δf B Control.

[0062] The SBS amplification characteristics in single mode fibers are disclosed in the following references:

[0063] Reference: "M. Nikles, L. Thevenaz and PA Robert, "Brillouin gain spectrum characterization in single-mode optical fibers," in Journal of Lightwave Technology, vol. 15, no. 10, pp. 1842-1851, Oct. 1997."

[0064] Fig. 7, Fig. 10, and Fig. 12 in this reference show the dependence of the Brillouin shift on the core material (amount of Ge doping), temperature, and strain. For example, the shift is larger for pure silica core fiber than for Ge-doped fiber. By controlling the temperature and strain constantly or dynamically, the performance as an optical filter can be tuned.

[0065] [Example 4] Although an example of the configuration of the DTMLL 9 ​​is shown in FIG. 1 and Example 1, the present invention is not limited to this, and various other configurations can be adopted. FIGS. 11A to 11G are diagrams showing a DTMLL with a ring resonator structure. Also, FIGS. 12A to 12C are diagrams showing example configurations of a DTMLL with a Fabry-Perot resonator structure. All configurations can be all-fiber types, all-space optical types, or combinations of these. Each component can be a bulk optical element or an integrated type, or combinations of these.

[0066] Figure 11A shows a reflective ring type. Part of the ring is a reflective type configuration using a circulator and a mirror. In this configuration, the optical pulse passes through the dispersive medium 5 twice, on the outbound and inbound paths, which increases the amount of dispersion.

[0067] Figure 11B shows a reflective SOA ring configuration. This is a reflective ring configuration using a reflective SOA (RSOA) 18. By using the RSOA 18, mirrors can be omitted, simplifying the configuration. In Figure 11B, the dispersion medium 5 is placed inside the ring, but it can also be placed in the path between the RSOA 18 and the optical circulator 11.

[0068] Fig. 11C shows a configuration using a reflective dispersion medium. A specific example is a chirped FBG (CFBG), but this is not limiting. By adopting this configuration, the mirror can also be omitted.

[0069] 11D is a diagram showing the use of a polarization beam splitter 20 and a Faraday rotator mirror 21 as a path control section instead of a circulator. This is the configuration shown in Non-Patent Document 9. Even if the dispersion medium 5 is a non-polarization-maintaining type, this configuration makes it possible to keep the polarization state in the resonator constant, which contributes to stabilizing the oscillation state.

[0070] FIG. 11E shows a gain modulation type. This is a configuration in which the gain of the gain medium 1 is modulated. As a specific example, this can be achieved by modulating the injection current of the SOA, but this method is not limited to this. In this configuration, the modulator can be omitted, which simplifies the configuration.

[0071] FIG. 11F shows a dispersion modulation type. This is a configuration that utilizes the fact that the wavelength can be changed by dynamically modulating the dispersion using a variable dispersion medium 22 while keeping the modulation frequency constant. A specific example of the variable dispersion medium 22 is a grating pair, but is not limited to this. Both the modulation frequency and the dispersion may be modulated. Also, by dynamically varying the dispersion, Δf e is variable, and the passband width of the optical bandpass filter can be varied.

[0072] Figure 11G shows a regeneratively mode-locked configuration. The configurations described so far are based on an actively mode-locked laser, where the input to the modulator is generated from a radio frequency signal source. However, the input signal to the modulator can also be a regeneratively mode-locked laser configuration, where the clock signal is extracted from the laser output itself. In this case, the frequency is tunable by adjusting the electrical bandpass filter (BPF) in the clock extraction circuit.

[0073] Figure 12A shows a Fabry-Perot resonator. It consists of a gain medium 1, a modulator 2, and a dispersion medium 5 sandwiched between a pair of mirrors. At least one of the mirrors is a partial reflector to extract the output. This configuration is particularly suitable for spatial optical systems, and allows for a simple and compact resonator structure.

[0074] Figure 12B shows a configuration in which the mirror and gain medium are replaced with RSOAs, which simplifies the resonator structure.

[0075] 12C is a diagram showing a case where the dispersion medium is replaced with a reflective dispersion medium, which further simplifies the resonator structure.

[0076] Any or all of the polarization rotation reflection, gain modulation, dispersion modulation, and regenerative mode-locking configurations described in FIGS. 11A to 11G can also be combined with the Fabry-Perot configuration shown in FIGS. 12A to 12C.

[0077] The structure of the DTMLL 9 ​​has been explained so far, but various combinations of the components, namely the gain medium 1, modulator 2, signal source 3, and dispersive medium 5, are possible. That is, the gain medium 1 may be any medium that has gain in the desired wavelength range. For example, solid-state media include "Ti:sapphire," "Nd:YAG," various SOAs, and various fiber amplifiers. Furthermore, fiber amplifiers include, but are not limited to, rare-earth doped fiber amplifiers such as erbium, Raman amplifiers, Brillouin amplifiers, and optical parametric amplifiers.

[0078] Liquid media include, but are not limited to, various dye types, and gaseous media include "He-Ne", "Ar", "Kr", and "CO2".

[0079] Modulators are classified into three modulation methods: phase modulation, frequency modulation, and intensity modulation. Modulation principles include the thermo-optic effect, electro-optic effect, acousto-optic effect, magneto-optic effect, and electro-absorption effect.

[0080] A sine wave is usually used as the signal source, but other waveforms may also be used. For example, Non-Patent Document 9 shows that the variable speed can be increased by using a pulse waveform instead of a sine wave waveform.

[0081] The dispersion medium may be either an optical fiber type or a bulk type. Optical fibers include, but are not limited to, common single-mode fibers, dispersion-compensating fibers (including single-mode and multimode fibers), and photonic crystal fibers. Bulk types include, but are not limited to, CFBG, "VIPA (virtually imaged phased array)," grating pairs, all-pass filters, lattice types, and "LCoS (liquid crystal on silicon)." Of these, VIPA, grating pairs, all-pass filters, lattice types, and LCoS types can also be used as tunable dispersion media.

[0082] [Example 5] Figure 13 is a diagram showing an example of a tunable dispersion medium. Even for a medium such as fiber, in which it is normally difficult to tune dispersion, as shown in Figure 13, multiple dispersion media with different dispersion values ​​can be prepared and switched between using an optical switch, making it possible to use it as a tunable dispersion medium. Naturally, VIPA, grating pairs, all-pass filters, lattice filters, and LCoS can also be used. This configuration is useful as a mechanism for varying the passband width of an optical bandpass filter. When the amount of dispersion changes, the proportionality coefficient between frequency and wavelength changes according to equation (2). Therefore, when dynamically changing the amount of dispersion, a mechanism is required to switch the frequency-wavelength correspondence table held by the control unit according to the amount of dispersion.

[0083] [Example 6] Figure 14 shows the schematic configuration of a wavelength tunable optical bandpass filter to which polarization diversity is applied. SBS efficiency generally depends on polarization. Therefore, if you want to support both polarizations, it is effective to use a polarization diversity configuration. An example of this configuration is shown in Figure 14. Here, we assume that the SBS medium is polarization-maintaining and the DTMLL output is single-polarized.

[0084] Figure 14A shows a configuration in which the DTMLL output light is input at a 45° angle with respect to the polarization axis of the SBS medium. As shown in Figure 14A, by inserting a half-wave plate into the DTMLL output and setting it at an appropriate angle, the DTMLL output light is input at a 45° angle with respect to the polarization axis of the SBS medium. Figure 14B shows a method in which the DTMLL output light is split into two and polarization-combined using a polarization beam splitter. Figure 14C shows a configuration similar to Figure 14A, in which the DTMLL output and a polarization-maintaining circulator are fusion-spliced ​​at a 45° angle with respect to their respective polarization axes. Figure 14D shows a configuration in which a polarization scrambler is inserted into the DTMLL output to average the polarization dependence. The above configuration makes it possible to apply polarization diversity to a wavelength-tunable optical bandpass filter.

[0085] [Example 7] Fig. 15 is a diagram showing a configuration in which a polarization beam splitter is connected to the output of a circulator. In the seventh embodiment, by adopting a configuration in which a polarization beam splitter is connected to the output of a circulator in addition to the configurations described in Figs. 14A to 14D, it becomes possible to split the input light into polarizations and output the split light.

[0086] [Example 8] Example 8 shows an example of a spectroscopic analysis device (hereinafter referred to as "spectroscope") configured using the above-described wavelength-tunable optical bandpass filter. Fig. 16 is a diagram showing an example of a spectrometer 120 configured using the DTMLL 9 ​​configured as described in Examples 1 and 2. In Fig. 16, various settings of the spectrometer 120, specifically, the center wavelength, span, resolution, etc., are set from a setting input unit 38. A control unit 39 sets the modulation frequency of the DTMLL 9 ​​and the dispersion amount of the variable dispersion medium according to the set measurement conditions.

[0087] 16, the DTMLL output is input to a PM EDFA 31 used as a power adjustment mechanism, where it is adjusted to an appropriate optical intensity. The polarization-maintaining fiber output from the PM EDFA 31 is fusion-spliced ​​so that its polarization axis is at 45° with respect to the polarization axis of the PM circulator 110, and the SBS pump light is input via the PM circulator 110 to a polarization-maintaining single-mode fiber 35 used as an SBS medium. The signal light to be measured, filtered by Brillouin amplification, is input via the PM circulator 110 to a polarization beam splitter 20, where it is polarization-separated, input to a photodetector (PD) 41, and A / D converted by an A / D converter 40.

[0088] With the above configuration, the control unit 39 can obtain power information for each wavelength of the signal light to be measured, and after appropriate processing, displays the results on the data display unit. As a result, it is expected to function as a high-speed tunable spectrometer with a tuning speed of up to 10 μs and a resolution of up to 3 GHz (all-fiber configuration) or up to 800 MHz (using chirped FBG).

[0089] Figure 17 shows an example of a spectrometer configuration based on the configuration in Figure 7. This configuration uses an SOA as the gain medium, and the modulation frequency is, for example, around 2 GHz. In this case, an LN phase modulator is inserted into the DTMLL output, and it is modulated with an electrical signal generated by a signal generator. The signal generator that supplies the modulation signal to the modulator generates, for example, a "2 Gbaud NRZ signal" by Nyquist filtering a fixed pattern with α = 0. By setting the length of the fixed pattern to be long enough so that the longitudinal mode spacing of the electrical signal spectrum is 5 MHz or less, a single continuous passband can be generated. The rest of the configuration is similar to that in Figure 16.

[0090] Comparing the configurations of Figures 16 and 17, in Figure 16, the phase modulator and signal generator in the DTMLL output section can be omitted, but an intensity modulator is required inside the DTMLL. The analog bandwidth of the phase modulator and signal generator must support modulation frequencies from DC to the DTMLL, for example, up to high frequencies of 2 GHz, while the modulator inside the DTMLL only needs to be up to about 5 MHz, so it is thought that the configuration of Figure 16 can achieve lower costs.

[0091] On the other hand, in Fig. 16, a long resonator length of several tens of meters is required to achieve a small FSR of 5 MHz, and the oscillation state of the DTMLL may be susceptible to disturbances such as vibration and temperature fluctuations. In contrast, the configuration in Fig. 17 requires a short resonator length, so the influence of disturbances is thought to be reduced.

[0092] As described above, according to this embodiment, by using a DTMLL as an SBS pump light source in an SBS-type tunable optical bandpass filter, it is possible to improve the wavelength tuning speed. In this case, by using a DTMLL that satisfies the following conditions 1 and 2, it is possible to achieve high-speed wavelength tuning, a single passband, a high extinction ratio, and minimized Brillouin threshold. Condition 1 is achieved by controlling the longitudinal mode spacing of the DTMLL output light to a frequency smaller than the Brillouin bandwidth. This results in a single passband transmission characteristic of the SBS-type optical filter. Condition 2 is achieved by controlling the spectral width to be equal to or less than the Brillouin shift frequency. This maximizes the extinction ratio and minimizes the Brillouin threshold. Furthermore, the tunable optical bandpass filter according to this embodiment can also be applied to a spectrometer. As a result, it is possible to achieve a tunable optical bandpass filter that simultaneously satisfies a wide wavelength tuning range, a narrow bandwidth, and high-speed wavelength tuning. According to the inventors' calculations, it is expected that a high-speed tunable optical bandpass filter and spectrometer with a wavelength tunable range of 1530 to 1565 nm, a tuning speed of about 10 μs, and a resolution of about 800 MHz can be realized. [Explanation of symbols]

[0093] 1 Gain medium 2. Modulator 3 Signal source 4 Power Splitter 5 Dispersion medium 6 Isolator 7 Setting input section 8 Control Unit 9 DTMLL 10 Power adjustment mechanism 11 Optical Circulator 12 SBS medium 13 Optical isolator 14. Signal light input section 15 Signal light output section 16 Modulator 16-2 Modulation signal generator 17. Mirror 18 Reflective SOA 19 Reflective Dispersive Media 20 Polarization beam splitter 21 Faraday Rotating Mirror 22 Variable Dispersion Medium 23 Clock extraction circuit 24 Electrically Variable Bandpass Filter 25 Partial Mirror 26 resonator 27 Dispersion Medium 1 28 Dispersion Medium 2 29 Dispersion Medium 3 30 Optical Switch 31 PM EDFA 32 Half-wave plate 33 Polarization-maintaining SBS medium 34 Polarization Scrambler 35 Polarization-maintaining single-mode fiber 37 Data display section 38 Setting input section 39 Control Unit 40 A / D converter 41 Photodetector 42 LN intensity modulator 43 Signal Generator 44 LN phase modulator 101 Tunable Optical Bandpass Filter 110 PM Circulator 120 spectrometer

Claims

1. A tunable optical bandpass filter using stimulated Brillouin scattering (SBS), a light source that outputs SBS pump light based on a set wavelength; an SBS medium to which the SBS pump light and the signal light to be filtered are input and which outputs a portion of the signal light amplified by SBS; a path control unit provided between the light source and the SBS medium, for outputting an input from the light source to the SBS medium and outputting an input from the SBS medium to an output terminal; A wavelength tunable optical bandpass filter, wherein the light source is a dispersion-tuned mode-locked laser (DTMLL).

2. 2. The tunable optical bandpass filter according to claim 1, further comprising a control unit that controls the dispersion-tuned mode-locked laser so that the longitudinal mode spacing of the SBS pump light is set to a frequency smaller than the Brillouin bandwidth and the spectral width of the SBS pump light is set to a Brillouin shift frequency or less.

3. 3. The tunable optical bandpass filter according to claim 2, further comprising a modulation unit that receives SBS pump light output from the dispersion tuning mode-locked laser and modulates the longitudinal mode spacing of the SBS pump light to a frequency smaller than the Brillouin bandwidth when the gain medium of the dispersion tuning mode-locked laser is a semiconductor optical amplifier (SOA).

4. 4. The tunable optical bandpass filter according to claim 3, wherein said modulation section outputs a plurality of periodic passbands by controlling a modulation frequency.

5. a power adjustment mechanism for adjusting the power of the SBS pump light output from the dispersion tuning mode-locked laser; 3. The tunable optical bandpass filter according to claim 2, wherein the control unit controls the power adjustment amount of the power adjustment mechanism so that the optical power of the SBS pump light becomes an intensity at which a desired SBS amplification gain is obtained in the SBS medium, and if the power of the input SBS pump light has wavelength dependency, so that it is equalized, and also controls the Brillouin shift frequency and the Brillouin gain bandwidth of the SBS medium based on the wavelength, material, temperature, and strain of the medium.

6. the dispersion-tuned mode-locked laser comprises at least a gain medium, a modulator, and a dispersion medium; 2. The tunable optical bandpass filter according to claim 1, wherein the filter has at least one of a ring resonator and a Fabry-Perot resonator.

7. 7. The tunable optical bandpass filter according to claim 6, wherein said dispersion-tuned mode-locked laser maintains a constant modulation frequency and dynamically modulates dispersion.

8. When the SBS pump light output from the dispersion-tuned mode-locked laser is a single-polarized wave and the SBS medium is a polarization-maintaining type, a configuration in which the SBS pump light is inputted via a half-wave plate at a polarization angle of 45° with respect to the polarization axis of the SBS medium; a configuration in which the SBS pump light is split into two beams and polarization-combined using a polarization beam splitter; a configuration in which the SBS pump light and the polarization-maintaining circulator are fusion-spliced ​​at an angle of 45° to the respective polarization axes of the SBS pump light and the polarization-maintaining circulator; or 2. The tunable optical bandpass filter according to claim 1, wherein a polarization scrambler is provided between the dispersion-tuned mode-locked laser and the SBS medium, and the polarization dependence of the SBS pump light is averaged.

9. 9. The tunable optical bandpass filter according to claim 8, further comprising a polarization beam splitter at the output end for polarization splitting the output of the path control section.

10. 4. A spectrometer comprising the tunable optical bandpass filter according to claim 1.

11. A method for controlling a wavelength tunable optical bandpass filter using stimulated Brillouin scattering (SBS), comprising: The longitudinal mode spacing of the SBS pump light is δf ｌ year, The Brillouin bandwidth is δf Ｂ year, The spectral width of the SBS pump light is δf ｅ year, The Brillouin shift frequency is f Ｂ When For the dispersion tuning mode-locked laser (DTMLL), which is the source of the SBS pump light, δf ｌ <δf Ｂ and δf ｅ <f Ｂ A method for controlling a wavelength tunable optical bandpass filter, characterized by performing control that satisfies the following:

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