Optical interferometer, optical interferometer system, and optical path difference measurement method

Abstract

The optical interferometer comprises: a broadband light source that outputs a first light in a given frequency domain with suppressed relative intensity noise (RIN); a wavelength divider that divides the first light into a first component and a second component with a center wavelength different from the first component and outputs the second component; a first optical path that propagates the first component output from the wavelength divider; a second optical path that propagates the second component output from the wavelength divider; and a wavelength synthesizer that combines the first component propagating in the first optical path and the second component propagating in the second optical path as a second light output.

Description

Technical Field

[0001] This invention relates to optical interferometers, optical interferometer systems, and methods for measuring optical path difference. Background Technology

[0002] Optical interferometers are a long-established technology. Since the invention of the laser in the 1960s, optical interferometers (laser interferometers) using lasers as light sources have made significant progress. The invention of the laser enabled the use of coherent light; the development of computers facilitated the mathematical processing of interferometer data; and the development of single-mode optical fibers enabled the realization of more complex and compact optical paths. These factors have driven the development of optical interferometers, especially laser interferometers.

[0003] Laser interferometers have a wide range of applications, but if we narrow it down to distance measurement, their applications include high-resolution precision measurement of surface shapes and distance measurement of several kilometers with long coherence lengths. In particular, the ultimate application of the latter is LIGO for gravity wave detection.

[0004] By leveraging the high coherence of lasers, distance measurements can be performed at ranges from several kilometers to tens of kilometers. However, the structure of devices for such long-distance measurements becomes complex, making it difficult to develop small and compact devices that are easily portable. Therefore, time-of-flight (TOF) measurements, which measure the time it takes for a light pulse emitted from a light source to hit an object and reflect back, are widely used for distance measurements exceeding tens of meters. This is primarily due to advancements in semiconductor laser technology, which easily enables the production of high-speed, high-output light pulses. For example, distance measurement is a crucial technology in recently popular autonomous driving and driver assistance systems, and LiDAR using laser-based TOF methods is becoming increasingly prevalent.

[0005] Prior art literature

[0006] Patent documents

[0007] Patent Document 1: International Publication No. 2022 / 054860

[0008] Patent Document 2: International Publication No. 2023 / 106348

[0009] Patent Document 3: International Publication No. 2023 / 190885

[0010] Non-patent literature

[0011] Non-patent literature 1: FERCHER, Adolf F., et al. Optical coherence tomography-principles and applications. Reports on progress in physics, 2003, 66.2: 239.P.260 OCT light source.

[0012] Non-patent literature 2: DERICKSON, Dennis. Fiber optic test and measurement.Fiber optic test and measurement / edited by Dennis Derickson. Upper SaddleRiver, 1998., pp.177-194.

[0013] Non-patent document 3: NAZARATHY, Moshe, et al. Spectral analysis of optical mixing measurements. Journal of Lightwave technology, 1989, 7.7: 1083-1096.

[0014] Non-patent literature 4: GALLION, Philippe; DEBARGE, Guy. Quantum phase noise and field correlation in single frequency semiconductor laser systems. IEEEJournal of Quantum Electronics, 1984, 20.4: 343-349.

[0015] Non-patent literature 5: TKACH, R.; CHRAPLYVY, A. Phase noise and linewidth in an InGaAsP DFB laser. Journal of Lightwave Technology, 1986, 4.11: 1711-1716. Summary of the Invention

[0016] -The problem the invention aims to solve-

[0017] However, when using Time-of-Flight (TOF), the power of the reflected light cannot be significantly attenuated due to its inherent principle. Therefore, there are technical difficulties in achieving the required high-output, short-pulse light when using TOF. On the other hand, if distance measurement is performed using light interference, similar to optical interferometers that utilize the coherence of lasers, it is relatively easy to maintain a high power for the reference light and ensure a high signal-to-noise ratio (SNR).

[0018] When using an optical interferometer for distance measurement, the propagation delay time difference caused by the difference in the length of the two optical paths in the optical interferometer is represented as FSR (Free Spectral Range) in the interference light.

[0019] However, for distance measurements exceeding tens of kilometers, the linewidth of the laser needs to be around several kHz. Furthermore, to achieve a small, economical, and portable distance measurement device, a semiconductor laser must be used, but the linewidth of semiconductor lasers is typically around several MHz. With the development and widespread adoption of digital coherent modulation and demodulation technology in fiber optic communication in recent years, semiconductor lasers with linewidths below 100 kHz are being developed, but it is not easy to stably achieve a linewidth of around several kHz.

[0020] The present invention was made in view of the above circumstances, and its object is to provide a small and simple optical interferometer that can be applied to various purposes, as well as an optical interferometer system and a method for measuring optical path difference.

[0021] -Methods for solving problems-

[0022] To address the aforementioned issues and achieve the objective, an optical interferometer according to one aspect of the present invention comprises: a broadband light source that outputs first light in a given frequency domain with suppressed relative intensity noise (RIN); a wavelength divider that divides the wavelength of the first light into a first component and a second component with a center wavelength different from the first component and outputs the second component; a first optical path that propagates the first component output from the wavelength divider; a second optical path that propagates the second component output from the wavelength divider; and a wavelength synthesizer that combines the first component propagating in the first optical path and the second component propagating in the second optical path as a second light output.

[0023] Alternatively, the full width at half maximum (FWHM) of the power spectrum for the wavelength of the first component can be narrower than the half width at half maximum (FWHM) of the power spectrum for the wavelength of the second component.

[0024] Alternatively, the full width at half maximum (FWHM) of the power spectrum for the wavelength of the first light can be greater than 5 nm and less than 30 nm.

[0025] Alternatively, a delay control device may be provided in the first optical path or the second optical path.

[0026] Alternatively, neither the first optical path nor the second optical path may have a polarization control mechanism.

[0027] Alternatively, the optical path difference between the first optical path and the second optical path can be sub-cm to tens of km.

[0028] An optical interferometer according to one aspect of the present invention comprises: a broadband light source that outputs first light in a given frequency domain with suppressed relative intensity noise (RIN); a power divider that divides the first light power into a third component and a fourth component and outputs them; a first optical path that propagates the third component output from the power divider; a second optical path that propagates the fourth component output from the power divider; and a power combiner that combines the third component propagating in the first optical path and the fourth component propagating in the second optical path as a fifth light output.

[0029] Alternatively, the optical path difference between the first optical path and the second optical path can be sub-cm to sub-m.

[0030] An optical interferometer according to the present invention comprises: a broadband light source that outputs a first light having periodic fluctuations in the power spectrum of the light; a power divider that divides the first light power into a third component and a fourth component and outputs them; a first optical path that propagates the third component output from the power divider; a second optical path that propagates the fourth component output from the power divider; and a power combiner that combines the third component propagating in the first optical path and the fourth component propagating in the second optical path and outputs them as a fifth light.

[0031] Alternatively, the optical path difference between the first optical path and the second optical path can be sub-cm to 100m.

[0032] Alternatively, the optical interferometer may include a polarizer disposed between the broadband light source and the power divider.

[0033] Alternatively, the broadband light source may include: two primary light sources that output primary light with RIN suppressed in the given frequency domain; and a polarization wave synthesizer that synthesizes the two polarization waves of the primary light from the two primary light sources and outputs them as the first light.

[0034] Alternatively, the amount of RIN suppression in the fifth light can be mitigated compared to the amount of RIN suppression in the first light.

[0035] Alternatively, the first optical path may include a spatial optical path in which either the first component or the third component propagates toward the object to be measured and returns after reaching the object to be measured.

[0036] Alternatively, the first optical path may include an optical fiber for measuring the length of the object.

[0037] Alternatively, the RIN suppression at the given frequency in the first light can be 10 dB or more.

[0038] Alternatively, the corner frequency at which the suppression of RIN begins in the first light can be above 1 GHz.

[0039] An optical interferometer system according to one aspect of the present invention comprises: the optical interferometer; a light receiver for receiving the second light or the fifth light and outputting a current signal corresponding to the received light; and an electric spectrum analyzer for displaying the spectrum of the input current signal in the frequency domain.

[0040] An optical interferometer system according to one aspect of the present invention comprises: the optical interferometer; a light receiver for receiving the second light or the fifth light and outputting a current signal corresponding to the received light; and a processing device for processing information contained in the input current signal in the frequency domain.

[0041] One aspect of the optical path difference measurement method of the present invention is an optical path difference measurement method performed by the optical interferometer system, which obtains the RIN spectrum of the second light or the fifth light, obtains the delay time τ0 from the FSR (free Spectral Range) of the obtained RIN spectrum, and estimates the optical path difference L between the first optical path and the second optical path by the following formula: L=τ0·c / n, where c is the speed of light in vacuum and n is the effective refractive index of the optical path constituting the optical path difference.

[0042] -The Effects of the Invention-

[0043] According to the present invention, an optical interferometer with a small and simple structure is provided, which can be applied to a wide variety of applications. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the optical interferometer involved in Implementation Method 1.

[0045] Figure 2 yes Figure 1 A schematic diagram of the light source shown.

[0046] Figure 3 It means Figure 2 A diagram showing a portion of the light source.

[0047] Figure 4 This is a diagram showing an example of the power spectrum of the first light.

[0048] Figure 5This is a diagram showing an example of the power (Pf) of the output light relative to Ib.

[0049] Figure 6 This is a schematic diagram representing the RIN spectrum when RIN is suppressed.

[0050] Figure 7A This is a diagram showing an example of the power spectrum for the wavelength of the first light.

[0051] Figure 7B This is a diagram showing an example of the power spectrum for the wavelength of the first component.

[0052] Figure 7C This is a diagram showing an example of the power spectrum for the wavelength of the second component.

[0053] Figure 7D This is a diagram showing an example of the power spectrum for the wavelength of the second light.

[0054] Figure 8 It is an overlapping representation Figure 7A The power spectrum of the first light and Figure 7D The power spectrum of the second light.

[0055] Figure 9 This is a diagram showing an example of the RIN spectrum of the first light and the RIN spectrum of the second light.

[0056] Figure 10 This is a diagram showing the fluctuation of the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 1m.

[0057] Figure 11 This is a diagram showing the fluctuation of the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 100m.

[0058] Figure 12 This is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 1 km.

[0059] Figure 13 This is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 10 km.

[0060] Figure 14A This is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 40 km.

[0061] Figure 14BThis is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 40 km.

[0062] Figure 15A This is a diagram illustrating the FSR in a well-known optical interferometer.

[0063] Figure 15B This is a diagram illustrating the FSR in a well-known optical interferometer.

[0064] Figure 15C This is a diagram illustrating the FSR in a well-known optical interferometer.

[0065] Figure 16 This is a diagram showing an example of the RIN spectrum of the second light when the length of the optical fiber containing the object of length measurement in the first optical path is 100m, without swapping the first component and the second component, and with swapping them.

[0066] Figure 17 This is a diagram showing an example of the RIN spectrum of the second light when the length of the optical fiber containing the object of length measurement in the first optical path is 1 km, without swapping the first component and the second component, and with swapping them.

[0067] Figure 18A This is a graph showing the RIN spectrum when Is is set to 0 mA and Ib is set to 1000 mA.

[0068] Figure 18B This is a graph showing the RIN spectrum when Is is set to 20mA and Ib is set to 1000mA.

[0069] Figure 18C This is a graph showing the RIN spectrum when Is is set to 25 mA and Ib is set to 1000 mA.

[0070] Figure 18D This is a graph showing the RIN spectrum when Is is set to 30mA and Ib is set to 1000mA.

[0071] Figure 18E This is a graph showing the RIN spectrum when Is is set to 50 mA and Ib is set to 1000 mA.

[0072] Figure 18F This is a graph showing the RIN spectrum when Is is set to 100mA and Ib is set to 1000mA.

[0073] Figure 18G This is a graph showing the RIN spectrum when Is is set to 100mA, and Ib is set to 200mA, 300mA, or 1000mA.

[0074] Figure 19 It is represented by magnifying the area around the concave region with a wavelength of 1445nm. Figure 7D A graph of the wavelength spectrum.

[0075] Figure 20 This is a diagram showing the conditions under which interference occurs in a wavelength-splitting interferometer.

[0076] Figure 21A It is a magnified diagram of the power spectrum of the first light when Is is set to 0mA.

[0077] Figure 21B It is a magnified diagram of the power spectrum of the first light when Is is set to 20mA.

[0078] Figure 21C It is a magnified diagram of the power spectrum of the first light when Is is set to 30mA.

[0079] Figure 21D It is a magnified diagram of the power spectrum of the first light when Is is set to 50mA.

[0080] Figure 21E This is a magnified diagram of the power spectrum of the first light when Is is set to 75mA.

[0081] Figure 21F It is a magnified diagram of the power spectrum of the first light when Is is set to 100mA.

[0082] Figure 22 This is a diagram showing an example of the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 1m, and the relationship between the polarization waves of the first component and the second component is changed.

[0083] Figure 23 This is a schematic diagram of the optical interferometer involved in Embodiment 2.

[0084] Figure 24 This is a schematic diagram of the optical interferometer involved in Embodiment 3.

[0085] Figure 25 This is a schematic diagram of the optical interferometer involved in Embodiment 4.

[0086] Figure 26A This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers involved in embodiments 2 and 3.

[0087] Figure 26B This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers involved in embodiments 2 and 3.

[0088] Figure 27A This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers involved in embodiments 3 and 4.

[0089] Figure 27B This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers involved in embodiments 3 and 4.

[0090] Figure 28 This is a diagram of an example of the RIN spectra of the first, second, and fifth beams in the optical interferometers described in comparative embodiments 1 and 2.

[0091] Figure 29 This is a schematic diagram of the optical interferometer involved in Embodiment 5.

[0092] Figure 30 This is a schematic diagram of the optical interferometer involved in Implementation Method 6.

[0093] Figure 31 This is a schematic diagram of the optical interferometer involved in Embodiment 7.

[0094] Figure 32 This is a diagram showing an example of the RIN spectrum of the fifth light in a seedless power-splitting interferometer with a structure corresponding to Embodiment 2.

[0095] Figure 33 This is a diagram showing an example of the power spectrum of the first light in the power-divided interferometer or the optical interferometer according to Embodiment 2.

[0096] Figure 34 This is a diagram showing an example of the RIN spectrum of the first light in the seedless power splitting interferometer or the optical interferometer according to Embodiment 2.

[0097] Figure 35 This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometer according to Embodiment 2, where the light source is replaced by an EDFA.

[0098] Figure 36 This is a diagram showing an example of the power spectrum of the first light output from the EDFA.

[0099] Figure 37 This is a diagram showing an example of the RIN spectrum of the first light output from the EDFA.

[0100] Figure 38 This is a schematic diagram of the optical interferometer system involved in Implementation Method 8. Detailed Implementation

[0101] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below. Furthermore, in the accompanying drawings, the same reference numerals will be appropriately used to label the same parts, and repeated descriptions will be omitted where appropriate.

[0102] The ASE (Amplified Spontaneous Emission) light from EDFA (Erbium Doped Fiber Amplifier), SOA (Semiconductor Optical Amplifier), and SLD (Super Luminescent Diode) is known as representative of incoherent light (Non-Patent Document 1). The coherence length lc, which represents the coherence of light, is expressed as shown in Equation (1) below.

[0103] [Mathematical Expression 1]

[0104]

[0105] Here, λ0 is the center wavelength of the light, and Δλ is the full width at half maximum (FWHM) of the power spectrum. For example, with λ0 of 1425 nm and Δλ of 30 nm, the coherence length lc is only 70 μm. Therefore, the wider the FWHM, the shorter the coherence length. Furthermore, to obtain a longer coherence length, the FWHM needs to be narrowed. In the case of lasers, the power spectrum is often linear, so the FWHM is sometimes referred to as the linewidth.

[0106] The inventors have disclosed techniques for suppressing relative intensity noise (RIN) light (RIN-suppressed light) (Patent Documents 1-3). Through further in-depth research on RIN-suppressed light, the inventors surprisingly discovered that, despite its wide field of view (FWHM), RIN-suppressed light exhibits coherence comparable to that of lasers with narrow linewidths. Furthermore, the inventors conceived that by using such RIN-suppressed light as a light source to construct an optical interferometer, an optical interferometer with novel and useful characteristics could be realized, thus completing this invention.

[0107] (Implementation Method 1)

[0108] Figure 1This is a schematic diagram of the optical interferometer according to Embodiment 1. The optical interferometer 1000 according to Embodiment 1 includes a light source 100, a wavelength divider 200, a first optical path 300, a second optical path 400, and a wavelength synthesizer 500. The optical interferometer 1000 is connected to a photodetector 2000 for measuring the RIN spectrum and an electrospectral analyzer 3000. The optical interferometer 1000, the photodetector 2000, and the electrospectral analyzer 3000 constitute an optical interferometer system.

[0109] Light source 100 is an example of a broadband light source, and its output is a first light L1, which is a RIN-suppressed light that suppresses relative intensity noise (RIN) in a given frequency domain. The first light L1 is a light with a relatively wide FWHM.

[0110] A wavelength splitter 200 receives a first optical beam L1, splits the wavelength of the first optical beam L1 into a first component L11 and a second component L12, and outputs the splitter. Specifically, the wavelength splitter 200 outputs the first component L11 to a first optical path 300 and the second component L12 to a second optical path 400. The center wavelength of the second component L12 is different from that of the first component L11. The wavelength splitter 200 may be configured, for example, to include a WDM (Wavelength Division Multiplexing) coupler.

[0111] In this embodiment, the first optical path 300 is composed of optical fibers. The first optical path 300 includes, for example, an optical fiber 301 for which the length is to be measured (the optical fiber 301 for which the length is to be measured). The first component L11 propagates in the first optical path 300.

[0112] In this embodiment, the second optical path 400 is composed of optical fiber. The second component L12 propagates in the second optical path 400. The optical path length of the second optical path 400 is shorter than that of the first optical path 300 by the amount of optical fiber 301.

[0113] Wavelength synthesizer 500 combines the first component L11 propagating in the first optical path 300 and the second component L12 propagating in the second optical path 400, and outputs it as the second light L2. Wavelength synthesizer 500 can be configured, for example, to include a WDM coupler, or it can be configured as an optical coupler that includes a 3dB coupler to combine two lights at a given power ratio.

[0114] The photodetector 2000 receives the second light L2 and outputs a current signal corresponding to the intensity of the received light to the electrospectral analyzer 3000. The electrospectral analyzer 3000 measures the RIN spectrum of the second light L2 based on the input current signal. The electrospectral analyzer 3000 is an example of an electrospectral analyzer that displays the spectrum of the input electrical signal in the frequency domain.

[0115] The optical interferometer 1000 has a structure similar to that of a Mach-Zehnder type optical interferometer, but it differs from at least the Mach-Zehnder type optical interferometer in that it has a wavelength divider 200 that divides the input light into wavelengths and outputs the wavelength. Hereinafter, the structure having a wavelength divider 200 like that of the optical interferometer 1000 will sometimes be referred to as a wavelength-divided interferometer.

[0116] Next, the structure and characteristics of the light source 100 will be explained, followed by an explanation of the characteristics of the first light L1, the first component L11, the second component L12, and the second light L2.

[0117] <Structure of the Light Source>

[0118] Figure 2 yes Figure 1 The diagram shows a light source 100. The light source 100 includes a light source module 10 and driving devices 101 and 102.

[0119] The light source module 10 includes a seed light source 11 as an SOA, an optical isolator 12, an enhancement amplifier 13 as an SOA, an optical isolator 14, and an output optical fiber 15. The seed light source 11, optical isolator 12, enhancement amplifier 13, and optical isolator 14 are optically cascaded in this order via optical fibers, optical components, etc. That is, the light source module 10 has a multi-level connected SOA.

[0120] Seed light source 11 outputs seed light LS with a given frequency band. Here, seed light LS is the ASE light of SOA. The given frequency band is, for example, a broadband band of 25 nm or more. Optical isolator 12 transmits seed light LS and inputs it to enhancement amplifier 13, and prevents return light traveling from the enhancement amplifier 13 side from inputting to seed light source 11. Optical isolator 12 prevents or reduces the instability of seed light source 11 due to the input of return light.

[0121] The boost amplifier 13 optically amplifies the input seed light LS and outputs it as the first light L1. The optical isolator 14 allows the first light L1 to pass through and enter the output fiber 15, and prevents light traveling from the output fiber 15 from entering the boost amplifier 13. The optical isolator 14 prevents or reduces the instability of the boost amplifier 13 due to the input of return light.

[0122] The output optical fiber 15 is an optical fiber that guides the first light L1 to the outside of the light source module 10. That is, the light source 100 outputs the first light L1 to the outside.

[0123] The drive devices 101 and 102 are known drive devices for SOA. Drive device 101 supplies drive current C1 to seed light source 11. Drive device 102 supplies drive current C2 to enhancement amplifier 13.

[0124] Figure 3 It means Figure 2 A diagram showing a portion of the light source 100. The enhancement amplifier 13 has a first end face 13a and a second end face 13b opposite to each other. The enhancement amplifier 13 receives seed light LS from the first end face 13a and outputs first light L1 to the outside from the second end face 13b.

[0125] The first end facet 13a and the second end facet 13b undergo reflection reduction treatments such as AR (Ant-Reflection) coating. Furthermore, the first end facet 13a and the second end facet 13b can also achieve reflection reduction by tilting them relative to the optical axis of the optical amplification waveguide of the enhancement amplifier 13. This configuration is also known as a tilted waveguide configuration.

[0126] <Characteristics of Light Sources>

[0127] The characteristics of light source 100 are explained. Figure 4 This is a diagram showing an example of the power spectrum of the output light (first light) of the boost amplifier. Figure 5 This means that when the driving current C1 supplied to the seed light source 11 is set to Is and the driving current C2 supplied to the enhancement amplifier 13 is set to Ib, Is is set to 50mA and Ib is set to 800mA. Figure 4 The power spectrum of the output light is roughly Gaussian in shape, with a center wavelength (measured using RMS) of 1445 nm and an FWHM of approximately 28 nm. Additionally, the whisker-like fluctuations visible in the spectrum are due to the absorption of moisture within the measuring instrument.

[0128] Figure 5 This is a diagram showing an example of the power (Pf) of the first light relative to Ib. Figure 5 In the configuration, Is is set to 50mA. Figure 5 In this case, it can be known that Pf is approximately 200mW when Ib is 1000mA. Additionally, Figure 4 , Figure 5 These are examples of light source modules that use semiconductor optical amplifiers with substantially the same characteristics, serving as both seed light sources and enhancement amplifiers.

[0129] Here, the seed light LS is an ASE light, and the enhancement amplifier 13 operates in a gain saturation state, so the first light L1 becomes a light that suppresses RIN. Furthermore, the first light L1 has a smooth wavelength spectrum (power spectrum) that also suppresses fluctuations in FP (Fabry-Perot) oscillations (Patent Documents 1-3).

[0130] Figure 6 This is a schematic diagram representing the RIN spectrum when RIN is suppressed. In Figure 6 In the middle, the level of line 210 is the level of ASE-ASE inter-ASE beat frequency noise in the ASE light.

[0131] The power spectral width of the ASE light in a typical SOA is tens of nm, which is several THz when expressed in terms of frequency. The measurement bandwidth of RIN is small enough to be tens of GHz, so RIN is calculated by the following equation (2) (Non-Patent Document 2).

[0132] RIN=0.66 / Δν ASE [Hz -1 ]···(2)

[0133] Here, 0.66 is the coefficient when the power spectrum of the ASE light is Gaussian, Δν ASE It is the FWHM of the power spectrum.

[0134] For example, Figure 4 The power spectrum of the output light shown is Gaussian, with an FWHM of approximately 30 nm. Therefore, if the above equation (1) is used, the RIN is calculated to be approximately -127 dB / Hz.

[0135] In contrast, the RIN of the first light L1 output from the light source 100 is suppressed in a low-frequency region 211, which is lower than the corner frequency 213. The low-frequency region 211 is an example of a given frequency domain. The low-frequency region 211 in which the RIN is suppressed is also referred to as the RIN suppression region.

[0136] In this specification, RIN suppression means that RIN is suppressed compared to the RIN calculated by equation (2) above in the low-frequency region below the corner frequency. The amount of RIN suppression is defined by the amount of reduction in RIN from the level of ASE-ASE inter-beat frequency noise in the RIN suppression region, but this RIN suppression amount can be 10 dB or more, and further can be 20 dB or more, and further can be 30 dB or less, and further can be 40 dB or more.

[0137] Furthermore, if the corner frequency 213 is set to fc [Hz], then fc is represented by the following equation (3). (Patent Document 3)

[0138] fc=1 / (D·Δλ·L)···(3)

[0139] Here, equation (3) is the corner frequency of a light beam after propagation in a certain optical fiber, Δλ[nm] is the full width at half maximum (FWHM) of the wavelength spectrum of the light beam, L[km] is the length of the optical fiber, and D[ps / nm / km] is the absolute value of the wavelength dispersion of the optical fiber at the center wavelength of the light beam. The corner frequency fc is, for example, above 1 GHz, but it can also be above 10 GHz, above 20 GHz, above 30 GHz, or above 40 GHz.

[0140] <Characteristics of the first ray, the first component, the second component, and the second ray>

[0141] Figures 7A to 7D This is a diagram showing an example of the power spectrum for the wavelengths of the first light, the first component, the second component, and the second light. Figure 7A This is the power spectrum of the first light L1. Figure 7B This is the power spectrum of the first component L11. Figure 7C This is the power spectrum of the second component L12. Figure 7D This is the power spectrum of the second light L2.

[0142] It is important to note that, regarding the second light, the first optical path does not include the optical fiber of the object being measured. The power spectrum is measured under the condition that the optical path length of the first optical path is approximately the same as that of the second optical path.

[0143] exist Figure 7A In the example shown, the center wavelength of the power spectrum of the first light L1 is 1425 nm. Furthermore, the first component L11 and the second component L12 are components of the first light L1 separated by the wavelength divider 200 with a boundary of approximately 1443 nm. As a result, in Figure 7D In the image, a recess appears at a wavelength of approximately 1443 nm. This recess is sometimes referred to as the wavelength cutoff point.

[0144] In addition, by Figure 7B , Figure 7C It can be seen that the full width at half maximum (FWHM) of the power spectrum of the first component L11 is wider than the full width at half maximum (FWHM) of the power spectrum of the second component L12.

[0145] Figure 8 It is an overlapping representation Figure 7A The power spectrum of the first light and Figure 7D The power spectrum of the second light. From Figure 8 It can be seen that the power spectrum PS1 of the first light L1 and the power spectrum PS2 of the second light L2 have almost the same shape.

[0146] Figure 9 This is a diagram showing an example of the RIN spectrum of the first light and the RIN spectrum of the second light. From Figure 9It can be seen that when the optical path length of the first optical path is approximately the same as that of the second optical path, the RIN spectrum RS1 of the first light L1 and the RIN spectrum RS2 of the second light L2 have almost the same shape.

[0147] <Characteristics of the second light when the first optical path includes an optical fiber for measuring the length>

[0148] Next, the characteristics of the second light L2 when the first optical path 300 includes an optical fiber 301 for measuring the length will be explained.

[0149] Figure 10 This is a diagram showing the fluctuation of the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 1m. Figure 11 This is a diagram showing the fluctuation of the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 100m. Figure 12 This is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 1 km. Figure 13 This is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 10 km. Figure 14A , Figure 14B This is a diagram showing the fluctuations in the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 40 km. Figure 14B It is Figure 14A A magnified portion of the image.

[0150] exist Figures 10-14A , Figure 14B The RIN spectrum shown exhibits periodic fluctuations in frequency, but the period varies depending on the length of the fiber 301 being measured.

[0151] about Figures 10-14A , Figure 14B The fluctuations in the RIN spectrum shown were analyzed by the inventors based on the FSR (Free Spectral Range) of the RIN spectrum measured in a known optical interferometer.

[0152] Figures 15A-15C The diagram illustrates the FSR in a known optical interferometer (see also Non-Patent Document 2). Figure 15AA measurement system 5000 incorporating a known Mach-Zehnder type optical interferometer is shown. The measurement system 5000 includes an optical coupler 5001 as a 3dB coupler, a first optical path 5002, a second optical path 5003, an optical coupler 5004 having the same structure and function as the optical coupler 5001, and a light receiver 5005. Here, the optical path length of the first optical path 5002 is longer than the optical path length of the second optical path 5003. Such a measurement system 5000 is also used when measuring the linewidth of a test light. For example, such linewidth measurement methods are widely used in fiber optic communications, especially digital coherent communication systems, for performance evaluation of signal lasers.

[0153] When test light is input to optical coupler 5001, optical coupler 5001 splits the test light into a first component and a second component with a given power ratio. The first component propagates in the first optical path 5002, and the second component propagates in the second optical path 5003. Optical coupler 5004 combines the first component propagating in the first optical path 5002 and the second component propagating in the second optical path 5003, and outputs the combined light to photodetector 5005. Photodetector 5005 receives the combined light and outputs a current signal i(t) corresponding to the intensity of the received light. Here, t is time.

[0154] i(t) is a function of the phase difference φ between the first and second components in the synthesized light. Specifically, as... Figure 15B That is, it changes periodically relative to the phase difference. Therefore, i(t) can also be written as i(Φ). FSR is defined by the interval between the peaks of i(Φ) as shown in equation (4) below. In addition, τ0 is the delay time of the first component propagating in the first optical path 5002 relative to the second component propagating in the second optical path 5003.

[0155] FSR=1 / τ0···(4)

[0156] According to non-patent document 2, if the linewidth (FWHM) of the frequency of the test light is set as Δν, then Δν can be expressed by RIN through mathematical processing of i(t). First, if the electric field input to the light receiver 5005 is set as E... T Then E T As shown in equation (5) below.

[0157] [Mathematical Expression 2]

[0158]

[0159] Here, P1 and P2 are the powers of the first and second components, respectively, and v0 is the center frequency of the test light.

[0160] i(t) is expressed as shown in equation (6) below.

[0161] [Mathematical Expression 3]

[0162]

[0163] Here, R[A / W] is the sensitivity of the photodetector 5005.

[0164] Next, the autocorrelation function of i(t) is obtained, and the Fourier transform of this autocorrelation function is performed according to the Wiener-Khintchine theorem to obtain the power spectral density S. i S i (f) is expressed as in equation (7) below.

[0165] [Mathematical Expression 4]

[0166]

[0167] The first term on the right side of equation (6) is the DC component of the received light power, the second term is the shot noise component, and the third term is the interference component. In addition, f[Hz] is the frequency of the horizontal axis of the RIN spectrum obtained by measuring i(t) with an electrospectral analyzer.

[0168] S mix (f) is expressed as in equation (8) below.

[0169] [Mathematical Expression 5]

[0170]

[0171] RIN[Hz -1 The power spectral density is obtained by normalizing the power spectral density by the square of the average power.

[0172] Equation (8) allows for the division of cases by using Δν·τ0 as a parameter. That is, Figure 15C This is a graph representing the RIN spectrum when Δν is 30 MHz. However, if Δν·τ0 ≥ 1, then... Figure 15C As shown, no fluctuations appear in the RIN spectrum. In this case, the linewidth of the experimental light can be determined from the RIN spectrum (for further detailed academic papers, see non-patent literature 3, 4, and 5). To satisfy the condition of this inequality, it is obvious that if the linewidth Δν of the experimental light is narrow, the delay time τ0 must be increased. On the other hand, when Δν·τ0 < 1, due to interference, such as Figure 15C As shown, fluctuations appear in the RIN spectrum. The frequency interval of these fluctuations is 1 / τ0. In this specification, this frequency interval of fluctuations is also referred to as FSR.

[0173] Apply the above methods Figures 10-14A , Figure 14BThe result. First, the delay time generated by the optical fiber of the object whose length is measured is expressed as shown in the following equation (9).

[0174] τ0=L / (c / n)···(9)

[0175] Here, L is the length of the optical fiber being measured, c is the speed of light in a vacuum, and n is the effective refractive index of the optical fiber being measured. L represents an example of optical path difference, and n represents an example of the effective refractive index of the optical path that constitutes the optical path difference.

[0176] Next, the FSR is calculated using equations (4) and (9), and this calculated value is compared with that based on... Figures 10-14A , Figure 14B The measured FSR values ​​were compared with those obtained by calculating the intervals of fluctuations in the data. The results are shown in Table 1. Additionally, n was set to 1.45 in the calculated values. As shown in Table 1, the measured values ​​and calculated values ​​are in good agreement.

[0177] [Table 1]

[0178]

[0179] From these results, we can say that, Figure 4 Figure 7 Figure 8 As shown, RIN-suppressed light exhibits a very special property of high coherence even at FWHM values ​​of approximately 30 nm. Furthermore, according to... Figure 14A , Figure 14B Even with a stripe length difference of 40 km between the first optical path 300 and the second optical path 400, fluctuations are still observed in the RIN spectrum. As shown in equation (8) above, interference even with a stripe length difference of 40 km implies that Δν·τ0<1 holds true and Δν<5kHz holds true. In the case of a laser light source, this is equivalent to a linewidth of less than 5kHz. Of course, the FWHM of general ASE light is relatively wide, so the concept of linewidth does not exist. However, from the perspective of interference, it can be considered that the RIN suppression light equivalently exhibits the same characteristics as a laser with a linewidth of less than 5kHz. These results are quite different from the previously widely accepted characteristics of ASE light, but they are experimental facts.

[0180] Additionally, in this specification, when referring to... Figure 1 When the optical interferometer 1000 of that configuration receives the first light L1, such as Figures 10-14A , Figure 14B If a fluctuation is confirmed in the RIN spectrum, it is recorded that the first light L1 has interferometry (coherence) or has produced interference.

[0181] Based on the above results, the optical interferometer 1000 according to Embodiment 1 utilizes the unique property of the RIN-suppressed light output by the light source 100, which has high coherence comparable to that of a laser with an extremely narrow linewidth, to measure the length of the optical fiber 301 over a wide measurement range (e.g., from a few centimeters to tens of kilometers or more), making it applicable to various applications. Furthermore, the light source 100, which uses ASE light as a seed source, is an extremely small and simple structure compared to a laser device that outputs a laser with the same linewidth. Therefore, the optical interferometer 1000 is a small and simple structure, and is an optical interferometer applicable to various applications.

[0182] Furthermore, in the optical interferometer 1000, the light source 100 can be a continuous wave (CW) light source, and the first light L1 can be a CW light. In this case, compared with the case where the first light L1 is a pulsed light, the average power of the second light L2 will not decrease, which is advantageous in terms of the light-receiving sensitivity in the photodetector 2000.

[0183] Furthermore, in the above embodiment, the full width at half maximum (FWHM) of the power spectrum of the first component L11 is wider than the half-value linewidth of the power spectrum of the second component L12. However, the present invention is not limited to this, and the full WHM of the power spectrum of the second component L12 may also be wider than the half-value linewidth of the power spectrum of the first component L11.

[0184] Figure 16 This is a diagram showing an example of the RIN spectrum of the second light in the optical interferometer according to Embodiment 1, when the length of the optical fiber for length measurement included in the first optical path is 100m, without swapping the first component and the second component, and with swapping them.

[0185] The case where the first component L11 and the second component L12 are not interchanged refers to the case where the first component L11 propagates in the first optical path 300 and the second component L12 propagates in the second optical path 400. The case where the first component L11 and the second component L12 are interchanged refers to the case where the first component L11 propagates in the second optical path 400 and the second component L12 propagates in the first optical path 300.

[0186] RIN spectrum RS3 is the RIN spectrum without swapping the first and second components, while RIN spectrum RS4 is the RIN spectrum with swapped first and second components. Furthermore, line 214 indicates the location of the corner frequency in RIN spectrum RS3. Additionally, the corner frequency in RIN spectrum RS4 was not observed and is considered to be above 40 GHz. Figure 16The results show that when the first and second components are swapped, i.e., when the full width at half maximum (FWHM) of the power spectrum of the first component L11 is narrower than the full width at half maximum (FWHM) of the power spectrum of the second component L12, the RIN suppression region is wider. This corresponds to the fact that the corner frequency fc of the component with smaller Δλ in equation (3) is larger (higher).

[0187] Figure 17 This is a diagram showing an example of the RIN spectrum of the second light in the optical interferometer according to Embodiment 1, when the length of the optical fiber for length measurement included in the first optical path is 1 km, with and without swapping the first and second components.

[0188] RIN spectrum RS5 is the RIN spectrum without swapping the first and second components, while RIN spectrum RS6 is the RIN spectrum with swapped first and second components. Furthermore, line 215 indicates the location of the corner frequency in RIN spectrum RS5. Additionally, the corner frequency of RIN spectrum RS6 was not observed and is considered to be above 40 GHz. Figure 17 The results also indicate that when the full width at half maximum (FWHM) of the power spectrum of the first component L11 is narrower than the FWHM of the power spectrum of the second component L12, the RIN suppression region is wider. This corresponds to the larger (higher) corner frequency fc of the component with smaller Δλ in equation (3). Furthermore, comparing... Figure 16 and Figure 17 This means that if the difference in optical path length between the first optical path 300 and the second optical path 400 increases, the RIN suppression region becomes narrower. This corresponds to the fact that the corner frequency fc of the side with larger L in equation (3) is smaller (lower).

[0189] Therefore, if it is desirable to ensure a wide interference band in the frequency range, it is preferable to configure the optical interferometer 1000 such that the first component and the narrower FWHM component of the second component are input into the optical path containing the fiber containing the length measurement object.

[0190] Furthermore, the inventors discovered that, according to Figure 16 , Figure 17 In the RIN spectrum, fluctuations occur only in the RIN suppression region below the corner frequency, and no fluctuations occur at frequencies higher than the corner frequency. This can be described as light with characteristics significantly different from ordinary lasers, exhibiting coherent light properties that cause interference in the region below the corner frequency, and incoherent light properties in the region above the corner frequency.

[0191] <Changes in the RIN spectrum of the second light relative to Is or Ib>

[0192] exist Figure 10The following diagram illustrates the fluctuation of the RIN spectrum under the following conditions: in the optical interferometer 1000, the length of the optical fiber to be measured is 1 m, the driving current Is supplied to the seed light source 11 is 100 mA, and the driving current Ib supplied to the enhancement amplifier 13 is 1000 mA. The following section shows the changes in the RIN spectrum caused by varying Is.

[0193] Figure 18A , Figure 18B , Figure 18C , Figure 18D , Figure 18E , Figure 18F These are the RIN spectra for setting Is to 0mA, 20mA, 25mA, 30mA, 50mA, 100mA, and Ib to 1000mA, respectively. Furthermore, Figure 18G This is a graph showing the RIN spectrum when Is is set to 100mA, and Ib is set to 200mA, 300mA, or 1000mA.

[0194] like Figures 18A to 18F As shown, when Is increases, RIN is suppressed, and the amplitude of the fluctuation increases. In the figure, the solid line represents the level of beat noise between ASEs (-127 dB / Hz) and the level of suppressed RIN (RIN suppression level). The difference between these two levels is the amount of RIN suppression. For example, in Figure 18A When Is = 0 mA, the suppressed RIN level is -137 dB / Hz, therefore the RIN suppression is 10 dB, and the fluctuation amplitude is also small, but... Figure 18F When Is = 100mA, the level of suppressed RIN is -150dB / Hz, so the RIN suppression reaches 23dB, and the amplitude of the fluctuation also increases significantly.

[0195] In addition, Figures 18A to 18F In the case where the FWHM of the first light is approximately 28 nm, the ASE-ASE beat noise level, which serves as the upper limit of RIN, is -127 dB / Hz. Here, the upper limit of RIN depends on the FWHM of the first light; if the FWHM of the first light is 5 nm, the upper limit of RIN is approximately -120 dB / Hz. In this case, the RIN suppression reaches (150-120) = 30 dB. Furthermore, if the suppressed RIN level is -160 dB / Hz, the RIN suppression reaches (140-120) = 40 dB.

[0196] In addition, such as Figure 18GAs shown, when Ib is small, the corner frequency is low, but if Ib is increased, the corner frequency becomes high. In the RIN spectrum, fluctuations only occur in the RIN suppression region below the corner frequency, making measurement difficult when the corner frequency is low and the fiber 301 is short. Therefore, to ensure a wider measurement range, it is preferable to operate the enhancement amplifier 13 with a sufficiently increased Ib and high gain saturation.

[0197] <On the First Light of FWHM>

[0198] Figure 19 It is represented by magnifying the area around the concave region (wavelength cutoff point) with a wavelength of approximately 1443nm. Figure 7D The wavelength spectrum diagram. For interference to occur in a wavelength-splitting interferometer such as the optical interferometer 1000, at least one set of modes needs to exist around the wavelength split point, and the frequency difference of these modes needs to be within the bandwidth of the photodetector 2000. Furthermore, the first beam L1 needs to have a wavelength-splitting frequency (FWHM) that includes more than two modes and allows for wavelength splitting within the bandwidth of the photodetector 2000. Figure 20 This is a diagram illustrating the conditions under which interference occurs in a wavelength-splitting interferometer. Figure 20 In the diagram, the wavelength side shorter than the wavelength division point is designated as the first component L11, and the wavelength side longer is designated as the second component L12. Furthermore, the thick arrows represent at least one set of modes intersecting the wavelength division point, while the dashed arrows represent other modes. Additionally, Be, Δν, Δλ, and FWHMminimum represent the frequency band of the photodetector 2000, the mode spacing, the wavelength cutoff width of the wavelength divider 200, and the minimum FWHM of the light source, respectively.

[0199] Figures 21A to 21F This is a magnified graph of the power spectrum of the first beam L1 in the optical interferometer 1000, with Is set to 0mA, 20mA, 30mA, 50mA, 75mA, and 100mA respectively. Additionally, Ib is set to 1000mA.

[0200] exist Figures 21A to 21F In the light, periodic fluctuations are observed in the power spectrum. The wavelength interval of these fluctuations is approximately 0.2 nm. Such fluctuations are believed to be caused by FP modes resulting from reflections at the two end faces of the SOA (as an enhancement amplifier 13) or reflections in optical components adjacent to these end faces. Even if reflections at the two end faces of the SOA or reflections in optical components adjacent to these end faces are suppressed as much as possible, such FP modes cannot realistically be zero.

[0201] The inventors have disclosed a situation in which such fluctuations and RIN suppression occur simultaneously (Patent Document 1).

[0202] Depend on Figures 21A to 21F It can be seen that when Is increases, the amplitude of the fluctuation decreases when it increases from 20mA to 30mA, and then tends to increase again when it increases from 50mA to 75mA.

[0203] Therefore, if Is is set to an appropriate value (e.g., 30mA to 50mA in this example), fluctuations are suppressed and the spectrum becomes smooth (see reference). Figure 4 However, even with this fluctuation suppressed, it is presumed that FP modes are buried by ASE light. As a result, ASE-ASE beat frequency noise is suppressed in the RIN suppression region, thus enabling the extraction of beat signals between FP modes.

[0204] Furthermore, when the wavelength interval of the fluctuation is approximately 0.2 nm as described above, the beat frequency (i.e., the mode interval of the FP mode) is approximately 25 GHz. Therefore, when the photodetector 2000 detects this beat frequency, the photodetector 2000 needs to have a bandwidth of 25 GHz or higher. Additionally, in the experiments for which results were disclosed in this specification, according to... Figure 9 It can also be seen that a photodetector capable of detecting 40GHz signals is used.

[0205] Considering the above points, an example of the preferred FWHM for the first light will be explained. First, consider... Figures 7B to 7D , Figure 19 If we consider the wavelength division characteristics of the achievable wavelength divider 200, then the FWHM is preferably 5nm or higher. However, in Figure 19 In this recess, the width is also greater than 0.2 nm, so multiple modes exist in the recess. Considering the power attenuation of the modes contributing to the interference due to the recess, and the resulting reduction in the amplitude of the beat signal, the wavelength divider 200 preferably has sufficient wavelength cutoff characteristics, with a narrower width and a deeper depth of the recess.

[0206] On the other hand, if the FWHM of the first light L1 is relatively wide, the power per mode becomes relatively small. Furthermore, starting from the frequency band of the photodetector 2000, at most two modes, separated by wavelength division points, contribute to the interference, while other modes contained in the first or second component do not contribute to the interference, or even if they do, cannot be detected. Regarding this, to sufficiently ensure the power per mode, the FWHM is preferably 30 nm or less. Furthermore, the FWHM can be 25 nm or less, 20 nm or less, 15 nm or less, or 10 nm or less. If the FWHM is narrow and the power per mode is high, it is advantageous, for example, for a long measurement distance (such as the length of the fiber in the object being measured), even with large optical losses.

[0207] Furthermore, if the spectral shape of the first light L1 is, for example, Gaussian, then the power distribution to each mode can be easily controlled according to the wavelength division point.

[0208] <Polarization Wave Dependence>

[0209] In the optical interferometer 1000 according to Embodiment 1, the inventors constructed an experimental system in which the length of the optical fiber 301 was set to 1m, and a polarization wave controller was inserted into the first optical path 300 and the second optical path 400. Then, in this experimental system, they measured how the RIN spectrum changed in a state where the polarization wave state of the first component L11 in front of the wavelength synthesizer 500 was consistent with the polarization wave state of the second component L12, and in a state where they were orthogonal.

[0210] Figure 22 This is a diagram illustrating an example of the RIN spectrum of the second light when the length of the optical fiber being measured in the first optical path is 1 m, and the relationship between the polarization waves of the first and second components is altered. Figure 22 In the diagram, RIN spectrum RS7 represents the case where the polarization states of the waves are orthogonal, while RIN spectrum RS8 represents the case where the polarization states of the waves are uniform. For example... Figure 22 As shown, experiments revealed that in RIN-suppressed light, there is no significant difference in interference characteristics between the cases where the polarization states are orthogonal and those where they are uniform.

[0211] Interference generally occurs when light waves are of uniform polarization. That is, it is widely understood that no interference occurs between mutually orthogonal polarized waves. In contrast, Figure 22 The results shown are presumed to be because, due to the broad FWHM of the power spectrum of the first light L1, which serves as the RIN suppression light, the polarization wave is scrambled only through a very short section of fiber that is not a fixed-polarization wave fiber, which connects the light source 100 and the wavelength splitter 200. This is a property of great significance in applications. It is particularly significant in optical fibers, but the polarization wave state of light often changes during its propagation. Therefore, in order to achieve a good interference state with good sensitivity, it is necessary to control the polarization light to make it uniform. However, this is not necessary in the optical interferometer 1000, which uses the RIN suppression light in Embodiment 1. That is, the first optical path 300 and the second optical path 400 can achieve an interference state even without a polarization wave control mechanism such as a polarization wave controller.

[0212] (Implementation methods 2, 3 and 4)

[0213] Next, the optical interferometers according to Embodiments 2, 3, and 4 will be described. Hereinafter, the optical interferometers according to Embodiments 2, 3, and 4 will be described first, and then the characteristics of the optical interferometers of Embodiments 2, 3, and 4 will be described.

[0214] <Structure of Implementation Method 2>

[0215] Figure 23 This is a schematic diagram of the optical interferometer according to Embodiment 2. The optical interferometer 1000A according to Embodiment 2 has a structure in which the wavelength divider 200 in the optical interferometer 1000 according to Embodiment 1 is replaced with a power divider 200A, and the wavelength combiner 500 is replaced with a power combiner 500A. The optical interferometer 1000A, the light receiver 2000, and the electron spectrum analyzer 3000 constitute an optical interferometer system.

[0216] A power divider 200A receives a first light L1 from a light source 100, divides the power of the first light L1 into a third component L13 and a fourth component L14, and outputs them. The power divider 200A outputs the third component L13 to a first optical path 300 and the fourth component L14 to a second optical path 400. The power divider 200A is configured as an optical coupler that, for example, divides two lights into two components at a given power ratio. This optical coupler could also be, for example, a 3dB coupler.

[0217] Power combiner 500A combines the third component L13 propagating in the first optical path 300 and the fourth component L14 propagating in the second optical path 400, and outputs them as the fifth light L5. Power combiner 500A is configured to include an optical coupler that combines two lights, for example, at a given power ratio. This optical coupler can also be, for example, a 3dB coupler.

[0218] <Structure of Implementation Method 3>

[0219] Figure 24 This is a schematic diagram of the optical interferometer according to Embodiment 3. The optical interferometer 1000B according to Embodiment 3 has a structure in which a polarizer 600 is disposed between the light source 100 and the power divider 200A of the optical interferometer 1000A according to Embodiment 2. The polarizer 600 is, for example, a polarization synthesizer (PBC). The polarizer 600 is configured such that its polarization wave direction is consistent with the polarization wave direction of the first light L1 from the light source 100. As a result, the polarizer 600 outputs a first light L1B with a higher degree of polarization than the first light L1. Furthermore, the pigtail fiber connecting the light source 100 and the polarizer 600 is preferably a fixed-polarization fiber, but it can also be a conventional non-fixed-polarization fiber.

[0220] The power divider 200A is input to the first optical beam L1B, which divides the power of the first optical beam L1B into a third component L13B and a fourth component L14B and outputs them. The power combiner 500A combines the third component L13B propagating in the first optical path 300 and the fourth component L14B propagating in the second optical path 400 and outputs them as the fifth optical beam L5B.

[0221] <Structure of Implementation Method 4>

[0222] Figure 25 This is a schematic diagram of the optical interferometer according to Embodiment 4. The optical interferometer 1000C according to Embodiment 4 has a structure in which the light source 100 of the optical interferometer 1000 according to Embodiment 2 is replaced by the light source 100C.

[0223] Light source 100C comprises two primary light sources 100C1 and 100C2 and a polarization synthesizer 100C3. The two primary light sources 100C1 and 100C2 output primary beams L01 and L02, respectively, which suppress the RIN, in the same manner as the first beam L1. Both primary light sources 100C1 and 100C2 can have the same structure as light source 100. The polarization synthesizer 100C3 combines the two primary beams L01 and L02, which are in mutually orthogonal polarization states, to output the first beam L1C. In this case, the pigtail fiber connecting the primary light sources 100C1 and 100C2 and the polarization synthesizer 100C3 is preferably a fixed-polarization fiber.

[0224] The power divider 200A is input to the first optical beam L1C, which splits the power of the first optical beam L1C into a third component L13C and a fourth component L14C and outputs them. The power combiner 500A combines the third component L13C propagating in the first optical path 300 and the fourth component L14C propagating in the second optical path 400 and outputs them as the fifth optical beam L5C.

[0225] The aforementioned optical interferometers 1000A, 1000B, and 1000C all have the structure of a Mach-Zehnder type optical interferometer. Hereinafter, in contrast to the wavelength-division interferometer 1000 described in Embodiment 1, the structure of optical interferometers 1000A, 1000B, and 1000C will sometimes be described as a power-division interferometer.

[0226] Furthermore, in embodiments 2 to 4, the photodetector 2000 receives the fifth light L5, L5B, or L5C and outputs a current signal corresponding to the intensity of the received light to the electrospectral analyzer 3000. The electrospectral analyzer 3000 measures the RIN spectrum of the fifth light L5, L5B, or L5C based on the input current signal. The electrospectral analyzer 3000 is an example of an electrospectral analyzer that displays the spectrum of the input electrical signal in the frequency domain. Furthermore, in embodiments 2 to 4, the optical interferometers 1000A, 1000B, or 1000C, the photodetector 2000, and the electrospectral analyzer 3000 constitute an optical interferometer system.

[0227] <The Characteristics of the Fifth Light>

[0228] Next, the characteristics of the fifth light will be explained. Figure 26A , Figure 26B This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometer described in embodiments 2 and 3 when the optical fiber 301 is not included. Additionally, in Figure 26A In the diagram, RIN spectrum RS9 represents the RIN spectrum of the first beam L1 or L1B in optical interferometer 1000A or 1000B. RIN spectrum RS10 represents the RIN spectrum of the fifth beam L5 in optical interferometer 1000A. RIN spectrum RS11 represents the RIN spectrum of the fifth beam L5B in optical interferometer 1000B. Furthermore, Figure 26B This represents a portion of the RIN spectrum RS10 and RS11.

[0229] The RIN spectra of the first light L1 in optical interferometer 1000A and the first light L1B after passing through polarizer 60° in optical interferometer 1000B are approximately the same. Furthermore, this RIN spectrum is consistent with... Figure 9 The RIN spectrum RS1 is also roughly the same.

[0230] On the other hand, such as Figure 26A , Figure 26B As shown, the RIN spectra RS10 and RS11 in optical interferometers 1000A and 1000B, which are power-divided interferometers, are compared with, for example... Figure 16 The characteristics of the RIN spectra RS3 and RS4 in the optical interferometer 1000, which is a wavelength-splitting interferometer, are quite different. Specifically, the RIN suppression is significantly reduced in RIN spectra RS10 and RS11. However, in RIN spectrum RS11, the RIN level is as low as -135 dB / Hz. Furthermore, in RIN spectra RS10 and RS11, fluctuations are almost invisible in the RIN suppression region, but they are more pronounced in the high-frequency region above 10 GHz, such as the corner frequency. The FSR of these fluctuations is about 5 GHz. This characteristic is similar to that of so-called white light, but the interference length is longer than that of white light. In addition, an FSR of 5 GHz means that the difference in optical path length between the first optical path 300 and the second optical path 400, which does not include fiber 301, is about 4 cm. Thus, the optical interferometers 1000A and 1000B, which are power-splitting interferometers, are also sensitive to a small difference in optical path length such as 4 cm. Furthermore, it is believed that the interference amplitude of RIN spectrum RS11 is large, and the interference is high compared with that of RIN spectrum RS10.

[0231] then, Figure 27A , Figure 27B This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers involved in embodiments 3 and 4. Figure 27AThis is the RIN spectrum of the fifth beam in the 1000B optical interferometer, assuming the fiber optic cable 301 has a length of 1m. Furthermore, Figure 27B This is the RIN spectrum of the fifth beam in the 1000C optical interferometer, assuming the fiber 301 has a length of 1m. Figure 27A , Figure 27B The results show that, compared with RIN-suppressed light passing through a polarizer, the RIN-suppressed light obtained by polarizing two mutually orthogonal polarized wave states can reduce the RIN suppression amount and decrease the interference amplitude. This means that it is possible to generate high-output, wide-bandwidth (large FWHM) quasi-white light with suppressed interference, and that the RIN suppression amount and interference can be controlled by appropriately setting the RIN-suppressed light.

[0232] Based on the above results, the optical interferometers 1000A, 1000B, and 1000C described in Embodiments 2-4 are power-splitting interferometers, but they have a simpler structure compared to the wavelength-splitting interferometer 1000 described in Embodiment 1, which includes a wavelength divider 200. Furthermore, the optical interferometers 1000A, 1000B, and 1000C can generate simulated white light (quasi-white light) with suppressed interference, and therefore are optical interferometers applicable to various purposes. For example, the optical interferometers 1000A, 1000B, and 1000C are suitable for obtaining light with interference characteristics that have a delay comparable to the length of an optical fiber of approximately several meters.

[0233] Furthermore, the coherence length of broadband light, such as the ASE light in SOA, is typically around tens of μm, as mentioned above. Interference using such low-coherence light is called white interference, and it is widely used, for example, as OCT (Optical Coherence Tomography). Utilizing the short coherence length, high-resolution distance measurement can be achieved.

[0234] then, Figure 28This is a diagram illustrating an example of the RIN spectra of the first, second, and fifth beams in the optical interferometers described in comparative embodiments 1 and 2. Specifically, RIN spectrum RS20 is the RIN spectrum of the first beam L1, RIN spectrum RS21 is the RIN spectrum of the second beam L2 in optical interferometer 1000, and RIN spectrum RS22 is the RIN spectrum of the fifth beam L5 in optical interferometer 1000A. Furthermore, the fiber optic cable 301 has a length of 1 km. Corner frequency 216 is the corner frequency in RIN spectrum RS21. Additionally, the corner frequency of RIN spectrum RS20 is 40 GHz or higher. The level of line 210 represents the level of beat frequency noise between ASEs. Line L220 represents the level of suppressed RIN in RIN spectrum RS20. Line L230 represents the level of suppressed RIN in RIN spectrum RS21. Line L240 represents the level of suppressed RIN in RIN spectrum RS22. Arrow Ar1 indicates that the RIN suppression in the RIN spectrum RS22 of the fifth light L5 is approximately 7 dB. Arrow Ar2 indicates that the RIN suppression in the RIN spectrum RS22 of the fifth light L5 is approximately 20 dB less severe than the RIN suppression in the RIN spectrum RS20 of the first light L1. Arrow Ar3 indicates that the RIN suppression in the RIN spectrum RS21 of the second light L2 is approximately 7 dB less severe than the RIN suppression in the RIN spectrum RS20 of the first light L1.

[0235] (Implementation Method 5)

[0236] Figure 29 This is a schematic diagram of the optical interferometer according to Embodiment 5. The optical interferometer 1000D has a structure in which the first optical path 300 in the optical interferometer 1000 according to Embodiment 1 is replaced by the first optical path 300D.

[0237] The first optical path 300D includes spatial optical systems 300D1 and 300D2, a measurement object 300D3, and a spatial optical path 300D4. Spatial optical system 300D1 is a spatial optical system that collimates the first component L11 output from wavelength divider 200 and outputs it into space towards the measurement object 300D3; it includes, for example, a lens. The measurement object 300D3 is the object for distance or velocity measurement. Spatial optical path 300D4 is an optical path in which the first component L11 propagates towards the measurement object 300D3 and is reflected by the measurement object 300D3, then propagates towards spatial optical system 300D2. Spatial optical system 300D2 is a spatial optical system that receives the first component L11 reflected by the measurement object 300D3 and combines it with wavelength synthesizer 500; it also includes, for example, a lens. The wavelength synthesizer 500 combines the first component L11 propagating in the spatial optical path 300D4 and the second component L12 propagating in the second optical path 400 as the second light L2D output. That is, the first optical path 300D includes the spatial optical path 300D4 from the first component L11 propagating toward the object to be measured 300D3 and returning after reaching the object to be measured 300D3.

[0238] By analyzing the fluctuations appearing in the RIN spectrum of the second light L2D using the optical interferometer 1000D, the optical path length of the spatial optical path 300D4 can be determined, similar to that of the optical interferometer 1000. The distance to the object being measured 300D3 can then be calculated based on this optical path length. Furthermore, the moving speed of the object being measured 300D3 can be calculated by analyzing the time variation of the fluctuations. Here, the moving speed is the relative moving speed of the optical interferometer 1000D with respect to the wavelength divider 200 and the wavelength synthesizer 500.

[0239] (Implementation Method 6)

[0240] Figure 30 This is a schematic diagram of the optical interferometer according to Embodiment 6. The optical interferometer 1000E has a structure in which the first optical path 300 in the optical interferometer 1000A according to Embodiment 2 is replaced with the first optical path 300D in Embodiment 5.

[0241] Space optical system 300D1 collimates the third component L13 output from power splitter 200A and outputs it into space towards the object 300D3. Space optical path 300D4 is the optical path through which the third component L13 propagates towards the object 300D3 and is reflected by the object 300D3, then propagates towards space optical system 300D2. Space optical system 300D2 receives the third component L13 reflected by the object 300D3 and combines it with power combiner 500A. Power combiner 500A combines the third component L13 propagating in space optical path 300D4 and the fourth component L14 propagating in the second optical path 400 as the fifth light L5E output. That is, the first optical path 300D includes space optical path 300D4 until the third component L13 propagates towards the object 300D3 and returns after reaching the object 300D3.

[0242] According to the optical interferometer 1000E, by analyzing the fluctuations appearing in the RIN spectrum of the fifth ray L5E, the optical path length of the spatial optical path 300D4 can be determined in the same way as in the case of the optical interferometer 1000A, and the distance to the object to be measured 300D3 can be calculated based on this optical path length. Furthermore, the moving speed of the object to be measured 300D3 can be calculated by analyzing the time variation of the fluctuations. In particular, the optical interferometer 1000E is suitable for distance measurement at the centimeter level or below.

[0243] (Implementation Method 7)

[0244] Figure 31 This is a schematic diagram of the optical interferometer according to Embodiment 7. The optical interferometer 1000F has a structure in which the first optical path 300 in the optical interferometer 1000 according to Embodiment 1 is replaced by the first optical path 300F.

[0245] A delay control device 301F is provided in the first optical path 300F. The delay control device 301F is a device that can control the delay time of light in the first optical path 300F according to the modulation signal from the signal source 700, and for example, it has electrodes for changing the effective refractive index of the first optical path 300F.

[0246] In this configuration, the first component L11 propagating in the first optical path 300F and the second component L12 propagating in the second optical path 400 are combined to form the second light L2F output from the wavelength synthesizer 500, which becomes the modulation signal. In this configuration, the optical interferometer 1000F functions as a modulator.

[0247] Furthermore, in the optical interferometer 1000F, the delay control device is located in the first optical path, but it can also be located in the second optical path. Further, the delay control device can also be located in both the first and second optical paths.

[0248] Furthermore, in the above embodiments, the seed light source 11 is an SOA, and the seed light LS is the ASE light of the SOA. However, the seed light source 11 can also be a rare earth type such as EDFA with added fiber amplifier, Raman fiber amplifier or SLD, and the seed light LS can also be the ASE light of these.

[0249] However, the inventors discovered that, in Figure 23 In the optical interferometer 1000A of Embodiment 2 shown, with the length of the optical fiber 301 set to 1m, Is is set to 0mA, that is, the SOA, which serves as the seed light source 11, is set to not output the seed light LS, and Ib is set to 1000mA. The output fifth light L5 is measured, and the result has the same characteristics as... Figure 27A , Figure 27B The characteristics of different situations. Hereinafter, the situation in which no seed light LS is output in a power splitting interferometer such as the optical interferometer 1000A, that is, the first light is simply the ASE light of the enhancement amplifier 13, is sometimes referred to as a seedless power splitting interferometer, and the light source in this case is sometimes referred to as a seedless light source.

[0250] Figure 32 This is a diagram showing an example of the RIN spectrum of the fifth light in a seedless power-splitting interferometer with the structure corresponding to Embodiment 2. Here, the seedless power-splitting interferometer with the structure corresponding to Embodiment 2 is the device in Embodiment 2 where Is is set to 0 mA. (Comparison) Figure 32 and Figure 27A , Figure 27B It can be seen that, in Figure 32 Fluctuations in the RIN suppression region present on the low-frequency side Figure 27A , Figure 27B It disappeared. Additionally... Figure 33 This is a diagram showing an example of the power spectrum of the first light in the power-divided interferometer or the optical interferometer 1000A according to Embodiment 2. Figure 34 This is a diagram showing an example of the RIN spectrum of the first light in the seedless power splitting interferometer or the optical interferometer 1000A according to Embodiment 2.

[0251] Therefore, in the optical interferometer 1000A, the inventors replaced the light source 100, which is a broadband light source, with an EDFA, based on setting the length of the optical fiber 301 to 1m, and used the first light as the ASE light of the EDFA to measure the RIN spectrum of the fifth light.

[0252] Figure 35 This is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometer 1000A according to Embodiment 2, where the light source 100 is replaced by an EDFA. Figure 36This is a diagram showing an example of the power spectrum of the first light output from the EDFA. Figure 37 This is a diagram showing an example of the RIN spectrum of the first light output from the EDFA. Figure 35 In, also with Figure 32 Similarly, fluctuations can be observed in this situation. However, in Figure 32 As can be seen, above 10 GHz, compared to frequencies below 10 GHz, the amplitude of the fluctuation becomes larger; conversely, at... Figure 35 In the middle, the amplitude of the fluctuation actually becomes smaller.

[0253] like Figure 32 , Figure 35 As shown, the study investigates the presence of significant interferometric fluctuations in such high-frequency regions within a power-division interferometer. Based on the research of the inventors, in... Figure 21A In the case of Is = 0 mA, i.e., without a specific light source, the power spectrum of the first light exhibits fluctuations. Therefore, in a power-splitting interferometer, the FP mode, which grows to the degree of fluctuation in the power spectrum, is presumed to be the main cause of the interferometry. Furthermore, as in... Figures 21A to 21F As described in the explanation, it is presumed that the FP mode can exist in a state of being buried by the ASE light. Therefore, even in cases where the first light is an ASE light with EDFA, for example, there is a certain oscillation mode that is presumed to exist but is difficult to observe. The existence of this oscillation mode can be confirmed by whether there are fluctuations in the RIN spectrum of the fifth light.

[0254] Therefore, the optical interferometer according to another embodiment of the present invention can be configured as follows: a broadband light source that outputs a first light having periodic fluctuations in the power spectrum; a power divider that divides the first light power into a third component and a fourth component and outputs them; a first optical path that propagates the third component output from the power divider; a second optical path that propagates the fourth component output from the power divider; and a power combiner that combines the third component propagating in the first optical path and the fourth component propagating in the second optical path and outputs it as a fifth light. Specifically, such an optical interferometer can be configured by setting the SOA, which serves as the seed light source 11, to a state where it does not output the seed light LS, or by replacing the light source 100 with an EDFA. In addition, the amplitude of the fluctuations in the power spectrum of the light is, for example, 0.5 dB or more, and for example, 10 dB or less.

[0255] (Implementation Method 8)

[0256] Figure 38 This is a schematic diagram of the optical interferometer system according to Embodiment 8. The optical interferometer system 10000 has a component that... Figure 1 The optical interferometer system shown, consisting of optical interferometer 1000, light receiver 2000, and electrospectral analyzer 3000, has its electrospectral analyzer 3000 replaced by a processing device 4000.

[0257] The processing device 4000 is configured, for example, to include a known electrical signal processing device or a computer. The processing device 4000 performs electrical processing on the current signal input from the photodetector 2000. Through such electrical processing, information contained in the current signal is mathematically processed (calculated, etc.) in the frequency domain.

[0258] Alternatively, the optical interferometer 1000 in the optical interferometer system 10000 according to Embodiment 8 can be replaced with any of the optical interferometers according to other embodiments 1 to 6.

[0259] In any of the above-described embodiments 8 or other optical interferometer systems, the following optical path difference measurement method can be implemented: obtaining the RIN spectrum of the second or fifth light, obtaining the delay time τ0 from the FSR of the obtained RIN spectrum, and estimating the optical path difference L between the first and second optical paths using the following formula.

[0260] L=τ0·c / n

[0261] Here, c is the speed of light in a vacuum, and n is the effective refractive index of the optical path that constitutes the optical path difference. Furthermore, the relationship between FSR and the delay time τ0 is expressed by the following equation.

[0262] FSR=1 / τ0

[0263] For example, the processing device 4000 stores a program that enables a computer to perform such an optical path difference measurement method. The processor of the computer included in the processing device 4000 executes the program, thereby enabling the optical path difference measurement method to be performed.

[0264] <Example of measurement range>

[0265] In the optical interferometers (i.e., wavelength-splitting interferometers, power-splitting interferometers, or seedless power-splitting interferometers) and optical interferometer systems described in the above embodiments, the optical path difference (measurement range) suitable for measurement will be explained. Furthermore, the object of measurement below is an optical fiber used for length measurement, and the effective refractive index of the optical fiber is 1.45.

[0266] First, the lower limit of the measurement is determined by the frequency band of the optical receiver 2000. Here, the FSR is approximately 20 GHz when the fiber length is 1 cm, so the lower limit of the measurement is approximately sub-cm (i.e., less than 1 cm). For example, if the frequency band of the optical receiver 2000 is 100 GHz, measurements of approximately 2 mm can be performed.

[0267] Furthermore, the upper limit of the measurement is determined, for example, by the degree of interferometry in the optical interferometer. In the case of a wavelength-splitting interferometer, as mentioned above, the upper limit is, for example, tens of kilometers such as 40 km.

[0268] Furthermore, in the case of a power-splitting interferometer, fluctuations begin to occur around 1 GHz above where RIN suppression begins to ease, so the upper limit is, for example, sub-m such as 0.2 m (e.g., including tens of cm).

[0269] Furthermore, in the case of a power-division interferometer without a power division interferometer, the upper limit is longer compared to the case of a power-division interferometer, but the upper limit is, for example, 100m.

[0270] In addition, in the aforementioned optical interference system, the interference light is analyzed by converting it into photocurrent through a photodetector, but the analysis method is not limited to this.

[0271] Furthermore, the present invention is not limited to the embodiments described above. Structures formed by appropriately combining the above structural elements are also included in the present invention.

[0272] For example, the structure of the optical interferometer 1000F, which is a wavelength-splitting interferometer according to Embodiment 7, can also be applied to the optical interferometers 1000A to 1000C, which are power-splitting interferometers according to Embodiments 2 to 4, to function as a modulator.

[0273] When the bandwidth of the modulation signal is lower than the corner frequency of the first light, a wavelength-splitting interferometer as described in Embodiment 7 is preferred. In this case, the noise characteristics of the light source itself are close to the shot noise limit, which is advantageous in terms of noise characteristics. On the other hand, when the bandwidth of the modulation signal is higher than the corner frequency of the first light, a power-splitting interferometer is preferred. Furthermore, since the first light is a RIN-suppressed light with a wide FWHM, it is more suitable for spatial transmission that can achieve higher output compared to transmission using optical fibers with wavelength dispersion.

[0274] Furthermore, those skilled in the art can readily derive further effects and variations. Therefore, the invention is not limited to the embodiments described above, and various modifications are possible.

[0275] Industrial availability

[0276] This invention can be used in optical interferometers.

[0277] -Explanation of Figure Markers-

[0278] 10: Light source module

[0279] 11: Seed Light Source

[0280] 12, 14: Optical isolators

[0281] 13: Enhancement Amplifier

[0282] 13a: First end face

[0283] 13b: Second end face

[0284] 15: Output fiber

[0285] 100, 100C: Light source

[0286] 100C1, 100C2: Original light source

[0287] 100C3: Polarization Wave Synthesizer

[0288] 101, 102: Drive unit

[0289] 200: Wavelength splitter

[0290] 200A: Power Divider

[0291] 211: Low-frequency region

[0292] 213, 216: Corner frequencies

[0293] 214, 215: Lines

[0294] 300, 300D, 300F: First optical path

[0295] 300D1, 300D2: Space Optical System

[0296] 300D3: Measured object

[0297] 300D4: Spatial Optical Path

[0298] 301: Fiber Optic

[0299] 301F: Delay control equipment

[0300] 400: Second optical path

[0301] 500: Wavelength synthesizer

[0302] 500A: Power Combiner

[0303] 600: Polarizing filter

[0304] 700: Signal Source

[0305] 1000, 1000A, 1000B, 1000C, 1000D, 1000E, 1000F: Optical Interferometers

[0306] 2000, 5005: Photodetector

[0307] 3000: Electron Spectrometer

[0308] 4000: Processing unit

[0309] 5000: Measurement System

[0310] 5001, 5004: Optical couplers

[0311] 5002: The First Optical Path

[0312] 5003: Second Optical Path

[0313] 10000: Optical Interferometer System

[0314] Ar1, Ar2, Ar3: Arrows

[0315] C1, C2: Drive current

[0316] L01, L02: Original Light

[0317] L1, L1B, L1C: First Light

[0318] L11: First component

[0319] L12: Second component

[0320] L13, L13B, L13C: Third component

[0321] L14, L14B, L14C: Fourth Component

[0322] L2, L2D, L2F: Second light

[0323] L5, L5B, L5C, L5E: Fifth Light

[0324] LS: Seed Light

[0325] PS1, PS2: Power Spectrum

[0326] RS1, RS2, RS3, RS4, RS5, RS6, RS7, RS8, RS9, RS10, RS11, RS20, RS21, RS22: RIN spectrum.

Claims

1. An optical interferometer, characterized in that, have: A broadband light source outputs the first light whose relative intensity noise in a given frequency domain is suppressed, i.e., the RIN is suppressed. A wavelength splitter divides the first optical wavelength into a first component and a second component whose center wavelength is different from the first component, and outputs the result. The first optical path propagates the first component output from the wavelength divider; The second optical path propagates the second component output from the wavelength divider; as well as A wavelength synthesizer combines the first component propagating in the first optical path and the second component propagating in the second optical path, and outputs them as a second light output.

2. The optical interferometer according to claim 1, wherein, The full width at half maximum (FWHM) of the power spectrum for the wavelength of the first component is narrower than the full width at half maximum (FWHM) of the power spectrum for the wavelength of the second component.

3. The optical interferometer according to claim 1, wherein, The full width at half maximum (FWHM) of the power spectrum for the wavelength of the first light is greater than 5 nm and less than 30 nm.

4. The optical interferometer according to claim 1, wherein, The optical interferometer includes a delay control device, which is disposed in the first optical path or the second optical path.

5. The optical interferometer according to claim 1, wherein, Neither the first optical path nor the second optical path has a polarization control mechanism.

6. The optical interferometer according to claim 1, wherein, The optical path difference between the first optical path and the second optical path is sub-cm to tens of km.

7. An optical interferometer, characterized in that, have: A broadband light source outputs the first light whose relative intensity noise in a given frequency domain is suppressed, i.e., the RIN is suppressed. A power divider divides the first optical power into a third component and a fourth component and outputs them; The first optical path propagates the third component output from the power divider; The second optical path propagates the fourth component output from the power divider; as well as The power combiner combines the third component propagating in the first optical path and the fourth component propagating in the second optical path, and outputs them as the fifth optical output.

8. The optical interferometer according to claim 1, wherein, The optical path difference between the first optical path and the second optical path is sub-cm to sub-m.

9. An optical interferometer, characterized in that, have: A broadband light source that outputs first light with periodic fluctuations in the power spectrum of light; A power divider divides the first optical power into a third component and a fourth component and outputs them; The first optical path propagates the third component output from the power divider; The second optical path propagates the fourth component output from the power divider; as well as The power combiner combines the third component propagating in the first optical path and the fourth component propagating in the second optical path, and outputs them as the fifth optical output.

10. The optical interferometer according to claim 1, wherein, The optical path difference between the first optical path and the second optical path is sub-cm to 100m.

11. The optical interferometer according to claim 7 or 9, wherein, The optical interferometer includes a polarizer disposed between the broadband light source and the power divider.

12. The optical interferometer according to claim 7, wherein, The broadband light source has the following features: Two primary light sources output the original light with RIN suppressed in the given frequency domain; and A polarization wave synthesizer combines the two original light polarization waves from the two original light sources and uses them as the first light output.

13. The optical interferometer according to claim 7 or 9, wherein, The suppression of RIN in the fifth light is mitigated compared to the suppression of RIN in the first light.

14. The optical interferometer according to any one of claims 1, 7, and 9, wherein, The first optical path includes a spatial optical path in which either the first component or the third component propagates toward the object to be measured and returns after reaching the object to be measured.

15. The optical interferometer according to any one of claims 1, 7, and 9, wherein, The first optical path includes an optical fiber for measuring the length of the object.

16. The optical interferometer according to claim 1 or 7, wherein, The RIN suppression at the given frequency in the first light is greater than 10 dB.

17. The optical interferometer according to any one of claims 1, 7, and 9, wherein, In the first light, the corner frequency at which RIN suppression begins is above 1 GHz.

18. An optical interferometer system, characterized in that, have: The optical interferometer according to any one of claims 1, 7 and 9; The light receiver receives the second light or the fifth light and outputs a current signal corresponding to the received light. as well as The electric spectrum analyzer displays the spectrum of the input current signal in the frequency domain.

19. An optical interferometer system, characterized in that, have: The optical interferometer according to any one of claims 1, 7 and 9; The light receiver receives the second light or the fifth light and outputs a current signal corresponding to the received light. as well as The processing device processes the information contained in the input current signal in the frequency domain.

20. A method for measuring optical path difference, which is the method for measuring optical path difference performed by the optical interferometer system of claim 19, characterized in that, Obtain the RIN spectrum of the second or fifth light. The delay time τ0 is obtained from the free spectral region of the obtained RIN spectrum, i.e., FSR. The optical path difference L between the first optical path and the second optical path can be estimated using the following formula. L=τ0·c / n Here, c is the speed of light in a vacuum, and n is the effective refractive index of the optical path that constitutes the optical path difference.

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