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

Abstract

This optical interferometer comprises: a broadband light source that outputs first light for which relative intensity noise (RIN) in a prescribed frequency domain has been suppressed; a wavelength divider that wavelength-divides the first light into a first component and a second component having a different center wavelength from the first component and outputs the components; a first optical path through which the first component output from the wavelength divider propagates; a second optical path through which the second component output from the wavelength divider propagates; and a wavelength combiner that combines the first component that has propagated through the first optical path with the second component that has propagated through the second optical path, and outputs the combined components as second light.

Description

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

[0001] The present invention relates to an optical interferometer, an optical interferometer system, and an optical path difference measurement method.

[0002] Optical interferometers are a technology with a long history. Since the invention of the laser in the 1960s, optical interferometers using lasers as light sources (laser interferometers) have made great strides. The invention of the laser made coherent light available, advances in computers made it easier to mathematically process interferometer data, and the development of single-mode optical fibers made it possible to realize more complex and compact optical paths. These are some of the reasons for the development of optical interferometers, especially laser interferometers.

[0003] Laser interferometers have a wide range of applications, but if we focus on distance measurement, we can see that their application areas include precise measurement of surface shapes by taking advantage of their high resolution, and distance measurement of several kilometers by taking advantage of their long coherence length. The ultimate application of the latter is LIGO, which detects gravitational waves.

[0004] Taking advantage of the high coherency of laser light, it is possible to measure distances of several kilometers to several tens of kilometers. However, the configuration of devices for such long-distance distance measurements is complex, and it is not easy to realize a small, compact device that is easily transportable. 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, reflect, and return, are generally used for distance measurements of several tens of meters or more. This is largely due to advances in semiconductor laser technology, which have made it easy to produce high-speed, high-power light pulses. For example, distance measurement is an essential technology for autonomous driving and driver assistance, which have recently attracted a lot of attention, and TOF-based LiDAR using laser light is becoming increasingly popular.

[0005] International Publication No. WO 2022 / 054860 International Publication No. WO 2023 / 106348 International Publication No. WO 2023 / 190885

[0006] 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.DERICKSON, Dennis. Fiber optic test and measurement. Fiber optic test and measurement / edited by Dennis Derickson. Upper Saddle River, 1998., pp.177-194.NAZARATHY, Moshe, et al. Spectral analysis of optical mixing measurements. Journal of Lightwave technology, 1989, 7.7: 1083-1096.GALLION, Philippe; DEBARGE, Guy. Quantum phase noise and field correlation in single frequency semiconductor laser systems. IEEE Journal of Quantum Electronics, 1984, 20.4: 343-349.TKACH, R.; CHRAPLYVY, A. Phase noise and linewidth in an InGaAsP DFB laser. Journal of Lightwave Technology, 1986, 4.11: 1711-1716.

[0007] However, when using TOF, due to its principle, it is inevitable that the power of the reflected light will be significantly attenuated. Therefore, when using TOF, there is a technical difficulty in that a high-power short pulse light is required. On the other hand, if distance measurement is performed using the interference of light, such as in an optical interferometer that utilizes the coherency of laser light, it is relatively easy to maintain a high power of the reference light and ensure a high signal-to-noise ratio (SNR).

[0008] When distance measurement is performed using an optical interferometer, a propagation delay time difference caused by a difference in two optical path lengths in the optical interferometer appears as a free spectral range (FSR) in the interference light.

[0009] However, to measure distances of tens of kilometers or more, the linewidth of the laser light needs to be on the order of several kHz. Furthermore, to realize a small, economical, and portable distance measurement device, the use of a semiconductor laser is essential, but the linewidth of semiconductor lasers is generally as wide as on the order of several MHz. With the recent advancement and spread of digital coherent modulation and demodulation technology in optical fiber communications, semiconductor lasers with linewidths below 100 kHz are being developed, but it is not easy to stably achieve a linewidth of on the order of several kHz.

[0010] The present invention has been made in view of the above, and an object of the present invention is to provide an optical interferometer that is small and has a simple configuration and that can be used for a variety of purposes, an optical interferometer system including the same, and an optical path difference measurement method.

[0011] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is an optical interferometer including: a light source that outputs first light in which relative intensity noise (RIN) in a predetermined frequency range is suppressed; a wavelength splitter that wavelength-divides the first light into a first component and a second component having a center wavelength different from that of the first component and outputs the resultant light; a first optical path along which the first component output from the wavelength splitter propagates; a second optical path along which the second component output from the wavelength splitter propagates; and a wavelength combiner that combines the first component that has propagated through the first optical path and the second component that has propagated through the second optical path and outputs the resultant light as second light.

[0012] The full width at half maximum of the power spectrum for the wavelength of the first component may be narrower than the line width at half maximum of the power spectrum for the wavelength of the second component.

[0013] The full width at half maximum of the power spectrum for the wavelength of the first light may be 5 nm or more and 30 nm or less.

[0014] The optical interferometer may include a delay control device provided in the first optical path or the second optical path.

[0015] The optical interferometer may not include a polarization control mechanism in the first optical path and the second optical path.

[0016] The optical path difference between the first optical path and the second optical path may be from sub-centimeter to several tens of kilometers.

[0017] One aspect of the present invention is an optical interferometer including: a light source that outputs first light having suppressed relative intensity noise (RIN) in a predetermined frequency range; a power divider that power-divides the first light into a third component and a fourth component and outputs the components; a first optical path along which the third component output from the power divider propagates; a second optical path along which the fourth component output from the power divider propagates; and a power combiner that combines the third component propagated through the first optical path and the fourth component propagated through the second optical path and outputs the combined light as fifth light.

[0018] The optical path difference between the first optical path and the second optical path may be sub-cm to sub-m.

[0019] One aspect of the present invention is an optical interferometer including: a broadband light source that outputs a first light having periodic ripples on the power spectrum of the light; a power divider that power-divides the first light into a third component and a fourth component and outputs the components; a first optical path along which the third component output from the power divider propagates; a second optical path along which the fourth component output from the power divider propagates; and a power combiner that combines the third component propagated through the first optical path and the fourth component propagated through the second optical path and outputs the combined light as a fifth light.

[0020] The optical path difference between the first optical path and the second optical path may be sub-centimeter to 100 m.

[0021] The optical interferometer may include a polarizer disposed between the broadband light source and the power splitter.

[0022] The broadband light source may include two original light sources that output original light with suppressed RIN in the specified frequency region, and a polarization combiner that polarization combines the two original lights from the two original light sources and outputs the first light.

[0023] The amount of RIN suppression in the fifth light may be increased relative to the amount of RIN suppression in the first light.

[0024] The first optical path may include a spatial optical path along which the first component or the third component propagates toward a measurement object, reaches the measurement object, and then returns.

[0025] The first optical path may include an optical fiber whose length is to be measured.

[0026] The first light may have a suppression amount of RIN at frequencies in the predetermined frequency range of 10 dB or more.

[0027] The corner frequency at which the suppression of RIN in the first light begins may be 1 GHz or higher.

[0028] One aspect of the present invention is an optical interferometer system including the optical interferometer, a photoreceiver that receives the second light or the fifth light and outputs a current signal corresponding to the received light, and an electrical spectrum analyzer that displays the spectrum of the input current signal in the frequency domain.

[0029] One aspect of the present invention is an optical interferometer system including the optical interferometer, a photoreceiver that receives the second light or the fifth light and outputs a current signal corresponding to the received light, and a processing device that processes information contained in the input current signal in the frequency domain.

[0030] One aspect of the present invention is an optical path difference measurement method executed by the optical interferometer system, comprising: acquiring an RIN spectrum of the second light or the fifth light; and calculating a delay time τ from an FSR (free spectral range) of the acquired RIN spectrum. ０ and estimates the optical path difference L between the first optical path and the second optical path using the following formula: L=τ ０ ·c / n where 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.

[0031] According to the present invention, an optical interferometer is provided which is small and has a simple configuration and is applicable to a variety of uses.

[0032] FIG. 1 is a schematic diagram of an optical interferometer according to a first embodiment. FIG. 2 is a schematic diagram of a light source shown in FIG. 1. FIG. 3 is a diagram illustrating a portion of the light source shown in FIG. 2. FIG. 4 is a diagram illustrating an example of a power spectrum of a first light. FIG. 5 is a diagram illustrating an example of the power (Pf) of output light relative to Ib. FIG. 6 is a diagram illustrating an RIN spectrum in which RIN is suppressed. FIG. 7A is a diagram illustrating an example of a power spectrum versus wavelength of the first light. FIG. 7B is a diagram illustrating an example of a power spectrum versus wavelength of the first component. FIG. 7C is a diagram illustrating an example of a power spectrum versus wavelength of the second component. FIG. 7D is a diagram illustrating an example of a power spectrum versus wavelength of the second light. FIG. 8 is a diagram illustrating an overlap of the power spectrum of the first light in FIG. 7A and the power spectrum of the second light in FIG. 7D. FIG. 9 is a diagram illustrating an example of the RIN spectrum of the first light and the RIN spectrum of the second light. FIG. 10 is a diagram illustrating ripples appearing in an example of the RIN spectrum of the second light when the length of an optical fiber included in the first optical path and whose length is to be measured is 1 m. FIG. 11 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 100 m. FIG. 12 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 1 km. FIG. 13 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 10 km. FIG. 14A is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 40 km. FIG. 14B is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 40 km. FIG. 15A is a diagram explaining the FSR in a known optical interferometer. FIG. 15B is a diagram explaining the FSR in a known optical interferometer. FIG. 15C is a diagram explaining the FSR in a known optical interferometer.FIG. 16 is a diagram showing an example of the RIN spectrum of the second light when the first component and the second component are not swapped and when they are swapped, when the length of the optical fiber included in the first optical path and the length of which is to be measured is 100 m. FIG. 17 is a diagram showing an example of the RIN spectrum of the second light when the first component and the second component are not swapped and when they are swapped, when the length of the optical fiber included in the first optical path and the length of which is to be measured is 1 km. FIG. 18A is a diagram showing the RIN spectrum when Is is set to 0 mA and Ib is set to 1000 mA. FIG. 18B is a diagram showing the RIN spectrum when Is is set to 20 mA and Ib is set to 1000 mA. FIG. 18C is a diagram showing the RIN spectrum when Is is set to 25 mA and Ib is set to 1000 mA. FIG. 18D is a diagram showing the RIN spectrum when Is is set to 30 mA and Ib is set to 1000 mA. FIG. 18E shows the RIN spectrum when Is is set to 50 mA and Ib is set to 1000 mA. FIG. 18F shows the RIN spectrum when Is is set to 100 mA and Ib is set to 1000 mA. FIG. 18G shows the RIN spectrum when Is is set to 100 mA and Ib is set to 200 mA, 300 mA, or 1000 mA. FIG. 19 shows the wavelength spectrum of FIG. 7D enlarged around the depression with a wavelength of 1445 nm. FIG. 20 shows the conditions under which interference is established in a wavelength division interferometer. FIG. 21A shows an enlarged view of the power spectrum of the first light when Is is set to 0 mA. FIG. 21B shows an enlarged view of the power spectrum of the first light when Is is set to 20 mA. FIG. 21C shows an enlarged view of the power spectrum of the first light when Is is set to 30 mA. Fig. 21D is an enlarged view of the power spectrum of the first light when Is is set to 50 mA. Fig. 21E is an enlarged view of the power spectrum of the first light when Is is set to 75 mA. Fig. 21F is an enlarged view of the power spectrum of the first light when Is is set to 100 mA. Fig. 22 is a view showing an example of the RIN spectrum of the second light when the relationship between the polarization of the first component and the polarization of the second component is changed when the length of the optical fiber included in the first optical path and whose length is to be measured is 1 m.FIG. 23 is a schematic diagram of an optical interferometer according to the second embodiment. FIG. 24 is a schematic diagram of an optical interferometer according to the third embodiment. FIG. 25 is a schematic diagram of an optical interferometer according to the fourth embodiment. FIG. 26A is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers according to the second and third embodiments. FIG. 26B is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers according to the second and third embodiments. FIG. 27A is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers according to the third and fourth embodiments. FIG. 27B is a diagram showing an example of the RIN spectrum of the fifth light in the optical interferometers according to the third and fourth embodiments. FIG. 28 is a diagram comparing examples of the RIN spectra of the first, second, and fifth lights in the optical interferometers according to the first and second embodiments. FIG. 29 is a schematic diagram of an optical interferometer according to the fifth embodiment. FIG. 30 is a schematic diagram of an optical interferometer according to the sixth embodiment. FIG. 31 is a schematic diagram of an optical interferometer according to the seventh embodiment. 32 is a diagram showing an example of the RIN spectrum of the fifth light in a seedless power division interferometer having a configuration corresponding to the second embodiment. FIG. 33 is a diagram showing an example of the power spectrum of the first light in a seedless power division interferometer or an optical interferometer according to the second embodiment. FIG. 34 is a diagram showing an example of the RIN spectrum of the first light in a seedless power division interferometer or an optical interferometer according to the second embodiment. FIG. 35 is a diagram showing an example of the RIN spectrum of the fifth light when the light source in the optical interferometer according to the second embodiment is replaced with an EDFA. FIG. 36 is a diagram showing an example of the power spectrum of the first light output from the EDFA. FIG. 37 is a diagram showing an example of the RIN spectrum of the first light output from the EDFA. FIG. 38 is a schematic diagram of an optical interferometer system according to the eighth embodiment.

[0033] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiment described below. Furthermore, in the description of the drawings, the same parts are appropriately designated by the same reference numerals, and duplicated explanations are appropriately omitted.

[0034] Amplified spontaneous emission (ASE) light from an erbium-doped fiber amplifier (EDFA), a semiconductor optical amplifier (SOA), and a super luminescent diode (SLD) is known as a representative example of incoherent light (see Non-Patent Document 1). The coherence length lc, which indicates the coherency of light, is expressed by the following equation (1): Here, λ0 is the central wavelength of the light, and Δλ is the full width at half maximum (FWHM) of the light's power spectrum. For example, if λ0 is 1425 nm and Δλ is 30 nm, the coherence length lc is only 70 μm. This means that if the FWHM is wide, the coherence length is short. Also, to obtain a long coherence length, it is necessary to narrow the FWHM. In the case of laser light, the power spectrum is often linear, so the FWHM is sometimes called the linewidth.

[0035] The present inventors have disclosed techniques related to light with suppressed relative intensity noise (RIN) (Patent Documents 1 to 3). After further intensive research into RIN-suppressed light, the inventors surprisingly found that RIN-suppressed light has high coherency equivalent to that of laser light with a relatively narrow linewidth, despite its wide FWHM. The inventors then conceived the idea that an optical interferometer with novel and useful properties could be realized by using such RIN-suppressed light as a light source, and thus completed the present invention.

[0036] 1 is a schematic diagram of an optical interferometer according to the first embodiment. The optical interferometer 1000 according to the first embodiment includes a light source 100, a wavelength splitter 200, a first optical path 300, a second optical path 400, and a wavelength combiner 500. The optical interferometer 1000 is connected to an optical receiver 2000 for measuring the RIN spectrum and an electrical spectrum analyzer 3000. The optical interferometer 1000, the optical receiver 2000, and the electrical spectrum analyzer 3000 constitute an optical interferometer system.

[0037] The light source 100 is an example of a broadband light source, and outputs a first light L1 that is RIN-suppressed light in which the relative intensity noise (RIN) in a predetermined frequency range is suppressed. The first light L1 is light with a relatively wide FWHM.

[0038] The wavelength splitter 200 receives a first light L1, splits the first light L1 into a first component L11 and a second component L12, and outputs the split light. Specifically, the wavelength splitter 200 outputs the first component L11 to a first optical path 300 and outputs the second component L12 to a second optical path 400. The second component L12 has a different center wavelength from the first component L11. The wavelength splitter 200 includes, for example, a WDM (Wavelength Division Multiplex) coupler.

[0039] In this embodiment, the first optical path 300 is made of an optical fiber. The first optical path 300 includes, for example, an optical fiber 301 whose length is to be measured (an optical fiber 301 whose length is to be measured). The first component L11 propagates through the first optical path 300.

[0040] In this embodiment, the second optical path 400 is made of an optical fiber. The second component L12 propagates through 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 length of the optical fiber 301.

[0041] The wavelength combiner 500 combines the first component L11 propagated through the first optical path 300 and the second component L12 propagated through the second optical path 400, and outputs the combined light as a second light L2. The wavelength combiner 500 may be configured to include, for example, a WDM coupler, or may be configured to include an optical coupler that combines two lights at a predetermined power ratio, such as a 3 dB coupler.

[0042] The optical receiver 2000 receives the second light L2 and outputs a current signal corresponding to the intensity of the received light to the electrical spectrum analyzer 3000. The electrical spectrum analyzer 3000 measures the RIN spectrum of the second light L2 based on the input current signal. The electrical spectrum analyzer 3000 is an example of an electrical spectrum analyzer that displays the spectrum of the input electrical signal in the frequency domain.

[0043] The optical interferometer 1000 has a configuration similar to that of a Mach-Zehnder optical interferometer, but differs from the Mach-Zehnder optical interferometer in that it includes a wavelength splitter 200 that splits input light into wavelengths and outputs the split light. Hereinafter, a configuration including a wavelength splitter 200 such as the optical interferometer 1000 may be referred to as a wavelength division interferometer.

[0044] Next, the configuration and characteristics of the light source 100 will be described, followed by the characteristics of the first light L1, the first component L11, the second component L12, and the second light L2.

[0045] <Configuration of Light Source> Fig. 2 is a schematic diagram of the light source shown in Fig. 1. This light source 100 includes a light source module 10 and driving devices 101 and 102.

[0046] The light source module 10 includes a seed light source 11 which is an SOA, an optical isolator 12, a booster amplifier 13 which is also an SOA, an optical isolator 14, and an output optical fiber 15. The seed light source 11, the optical isolator 12, the booster amplifier 13, and the optical isolator 14 are optically cascaded in this order by optical fibers, optical elements, etc. In other words, the light source module 10 includes SOAs connected in multiple stages.

[0047] The seed light source 11 outputs seed light LS having a predetermined band. Here, the seed light LS is ASE light from an SOA. The predetermined band is, for example, a wide band, such as a wavelength band of 25 nm or more. The optical isolator 12 transmits the seed light LS and inputs it to the booster amplifier 13, while blocking returning light traveling from the booster amplifier 13 side from inputting to the seed light source 11. The optical isolator 12 prevents or reduces instability in the operation of the seed light source 11 due to the input of returning light.

[0048] The booster amplifier 13 optically amplifies the input seed light LS and outputs it as first light L1. The optical isolator 14 transmits the first light L1 and inputs it to the output optical fiber 15, and prevents light traveling from the output optical fiber 15 from inputting to the booster amplifier 13. The optical isolator 14 prevents or reduces instability in the operation of the booster amplifier 13 due to the input of returned light.

[0049] 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.

[0050] The driver 101 supplies a drive current C1 to the seed light source 11. The driver 102 supplies a drive current C2 to the booster amplifier 13.

[0051] Fig. 3 is a diagram showing a part of the light source 100 shown in Fig. 2. The booster amplifier 13 has a first end face 13a and a second end face 13b facing each other. The booster amplifier 13 receives the seed light LS from the first end face 13a and outputs the first light L1 to the outside from the second end face 13b.

[0052] The first end face 13a and the second end face 13b are subjected to a reflection reduction treatment such as AR (Ant-Reflection) coating. Furthermore, the first end face 13a and the second end face 13b may be subjected to a reflection reduction treatment by being inclined with respect to the optical axis of the optical amplification waveguide included in the booster amplifier 13. Such a structure is also called an oblique waveguide structure.

[0053] <Characteristics of Light Source> The characteristics of the light source 100 will now be described. FIG. 4 is a diagram showing an example of the power spectrum of the output light (first light) of the booster amplifier. FIG. 5 shows a case where the drive current C1 supplied to the seed light source 11 is Is and the drive current C2 supplied to the booster amplifier 13 is Ib, where Is is set to 50 mA and Ib is set to 800 mA. The power spectrum of the output light in FIG. 4 has a roughly Gaussian spectral shape, with a center wavelength (measured using the RMS method) of 1445 nm and a FWHM of approximately 28 nm. Note that the whisker-like fluctuations visible on the spectrum are due to moisture absorption in the measuring device.

[0054] Fig. 5 is a diagram showing an example of the power (Pf) of the first light relative to Ib. In Fig. 5, Is is set to 50 mA. In the case of Fig. 5, it can be seen that when Ib is 1000 mA, Pf is approximately 200 mW. Note that Figs. 4 and 5 show examples of light source modules fabricated using semiconductor optical amplifiers having approximately similar characteristics as the seed light source and the booster amplifier, respectively.

[0055] Here, since the seed light LS is ASE light and the booster amplifier 13 is operating in a gain saturated state, the first light L1 is light with suppressed RIN. Furthermore, the first light L1 has a smooth wavelength spectrum (power spectrum) with suppressed ripples of FP (Fabry-Perot) oscillation (Patent Documents 1 to 3).

[0056] 6 is a diagram showing a typical RIN spectrum in which RIN is suppressed, in which the level of line 210 is the level of ASE-to-ASE beat noise in the ASE light.

[0057] The power spectrum width of ASE light from a normal SOA is several tens of nanometers, which corresponds to several THz in frequency. Since the RIN measurement band is sufficiently small, at several tens of GHz, the RIN is calculated by the following formula (2) (Non-Patent Document 2): RIN = 0.66 / Δν ＡＳＥ [Hz －１ ] ... (2) where 0.66 is a coefficient when the power spectrum of the ASE light is Gaussian, and Δν ＡＳＥis the FWHM of the power spectrum.

[0058] For example, the power spectrum of the output light shown in FIG. 4 is Gaussian and has an FWHM of about 30 nm, so that using the above formula (1), the RIN is calculated to be about -127 dB / Hz.

[0059] In contrast, the RIN of the first light L1 output from the light source 100 is suppressed in a frequency region 211 lower than the corner frequency 213. The low frequency region 211 is an example of a predetermined frequency region. The low frequency region 211 in which the RIN is suppressed is also called an RIN suppressed region.

[0060] In this specification, "RIN suppression" means that the RIN is suppressed below the RIN calculated by the above formula (2) in a frequency region lower than the corner frequency. The amount of RIN suppression is defined as the amount of RIN reduction from the level of ASE-to-ASE beat noise in the RIN suppressed region, and this RIN suppression amount may be 10 dB or more, or may be 20 dB or more, or may be 30 dB or less, or may be 40 dB or more.

[0061] Furthermore, if the value of the corner frequency 213 is fc [Hz], fc is expressed by the following formula (3): (Patent Document 3) fc=1 / (D·Δλ·L) (3) Here, if formula (3) is the corner frequency after a certain light has propagated through a certain optical fiber, Δλ [nm] is the full width at half maximum of the wavelength spectrum of the certain light, L [km] is the length of the certain optical fiber, and D [ps / nm / km] is the absolute value of the chromatic dispersion of the certain optical fiber at the center wavelength of the certain light. The corner frequency fc is, for example, 1 GHz or higher, but may also be 10 GHz or higher, 20 GHz or higher, 30 GHz or higher, or 40 GHz or higher.

[0062] 7A to 7D are diagrams showing examples of power spectra for the wavelengths of the first light, the first component, the second component, and the second light. Fig. 7A shows the power spectrum of the first light L1, Fig. 7B shows the power spectrum of the first component L11, Fig. 7C shows the power spectrum of the second component L12, and Fig. 7D shows the power spectrum of the second light L2.

[0063] However, it should be noted that with regard to the second light, the power spectrum is measured when the first optical path does not include the optical fiber whose length is to be measured, and the optical path lengths of the first optical path and the second optical path are approximately the same.

[0064] In the example shown in Fig. 7A, the center wavelength of the power spectrum of the first light L1 is 1425 nm. The first component L11 and the second component L12 are components obtained by splitting the first light L1 by the wavelength splitter 200 at a wavelength of approximately 1443 nm. As a result, a dip appears at the wavelength of approximately 1443 nm in Fig. 7D. This dip is sometimes called a wavelength division point.

[0065] As can be seen from FIGS. 7B and 7C, the full width at half maximum of the power spectrum of the first component L11 is wider than the full width at half maximum of the power spectrum of the second component L12.

[0066] Fig. 8 is a diagram showing the power spectrum of the first light in Fig. 7A superimposed on the power spectrum of the second light in Fig. 7D. As can be seen from Fig. 8, the power spectrum PS1 of the first light L1 and the power spectrum PS2 of the second light L2 have almost the same shape.

[0067] 9 is a diagram illustrating an example of the RIN spectrum of the first light L1 and the RIN spectrum of the second light L2. As can be seen from Fig. 9, when the optical path length of the first optical path is approximately the same as the optical path length 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.

[0068] <Characteristics of the Second Light When the First Optical Path Includes an Optical Fiber of Which the Length is to be Measured> Next, the characteristics of the second light L2 when the first optical path 300 includes an optical fiber 301 of which the length is to be measured will be described.

[0069] Fig. 10 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 1 m. Fig. 11 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 100 m. Fig. 12 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 1 km. Fig. 13 is a diagram showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 10 km. Figs. 14A and 14B are diagrams showing ripples appearing in an example of the RIN spectrum of the second light when the length of the optical fiber included in the first optical path and whose length is to be measured is 40 km. Fig. 14B is an enlarged view of a portion of Fig. 14A.

[0070] The RIN spectra shown in FIGS. 10 to 14A and 14B show ripples that are periodic in frequency, but the period varies depending on the length of the optical fiber 301 whose length is to be measured.

[0071] The inventors performed the following analysis of the ripples in the RIN spectrum shown in FIGS. 10 to 14A and 14B based on the FSR (Free Spectral Range) of the RIN spectrum measured by a known optical interferometer.

[0072] 15A to 15C are diagrams illustrating the FSR of a known optical interferometer (see also Non-Patent Document 2). FIG. 15A shows a measurement system 5000 including a known Mach-Zehnder optical interferometer. The measurement system 5000 includes an optical coupler 5001, which is a 3 dB coupler, a first optical path 5002, a second optical path 5003, an optical coupler 5004 having the same configuration and function as the optical coupler 5001, and an optical receiver 5005. Here, the optical path length of the first optical path 5002 is assumed to be longer than the optical path length of the second optical path 5003. Such a measurement system 5000 is also used to measure the linewidth of test light. For example, such a linewidth measurement method is widely adopted for evaluating the performance of signal lasers in optical fiber communications, particularly digital coherent communication systems.

[0073] When test light is input to optical coupler 5001, optical coupler 5001 splits the test light into a first component and a second component at a predetermined power ratio. The first component propagates through first optical path 5002, and the second component propagates through second optical path 5003. Optical coupler 5004 combines the first component propagated through first optical path 5002 and the second component propagated through second optical path 5003, and outputs the combined light to optical receiver 5005. Optical receiver 5005 receives the combined light and outputs a current signal i(t) corresponding to the intensity of the received light, where t is time.

[0074] i(t) is a function of the phase difference φ between the first and second components in the combined light, and specifically, it changes periodically with respect to the phase difference φ as shown in FIG. 15B. Therefore, i(t) can also be written as i(Φ). FSR is defined as the interval between peaks of i(Φ) as shown in the following equation (4). Note that τ ０ is the delay time of the first component propagating through the first optical path 5002 relative to the second component propagating through the second optical path 5003. FSR=1 / τ ０ ... (4)

[0075] According to Non-Patent Document 2, if the frequency linewidth (FWHM) of the test light is Δν, Δν can be expressed in terms of RIN by mathematically processing i(t). First, the electric field input to the photodetector 5005 is E Ｔ Then, E Ｔ is expressed as the following equation (5). where P1 and P2 are the powers of the first and second components, respectively, and ν ０ is the center frequency of the test light.

[0076] i(t) is expressed as in the following equation (6). Here, R[A / W] is the sensitivity of the photodetector 5005.

[0077] Next, the autocorrelation function of i(t) is calculated, and the autocorrelation function is Fourier transformed according to the Wiener-Khintchine theorem to obtain the power spectral density S ｉ We ask for S. ｉ (f) is expressed as the following formula (7). The first term on the right side of equation (6) is the DC component of the received optical power, the second term is the shot noise component, and the third term is the interference component. Note that f [Hz] is the frequency on the horizontal axis of the RIN spectrum obtained when i(t) is measured with an electrical spectrum analyzer.

[0078] S ｍｉｘ (f) is expressed as the following formula (8).

[0079] RIN [Hz －１ ] is a quantity obtained by normalizing the power spectral density by the square of the average power.

[0080] Equation (8) is Δν τ ０ That is, FIG. 15C shows the RIN spectrum when Δν is 30 MHz, but Δν·τ ０ ≥ 1, no ripples appear in the RIN spectrum, as shown in Figure 15C. In this case, the linewidth of the test light can be determined from the RIN spectrum (see Non-Patent Documents 3, 4, and 5 for detailed academic papers). In order to satisfy the condition of this inequality, if the linewidth Δν of the test light is narrow, the delay time τ ０ It is clear that Δν・τ must be increased. ０ <1, ripples appear in the RIN spectrum due to interference, as shown in Fig. 15C. The frequency interval of the ripples is 1 / τ ０ In this specification, the frequency interval of this ripple is also referred to as FSR.

[0081] The above-described method is applied to the results of Figures 10 to 14A and 14B. First, the delay time caused by the optical fiber whose length is to be measured is expressed as in the following equation (9). τ ０ = L / (c / n) (9) where L is the length of the optical fiber whose length is to be measured, c is the speed of light in a vacuum, and n is the effective refractive index of the optical fiber whose length is to be measured. L is an example of an optical path difference, and n is an example of the effective refractive index of the optical path that constitutes the optical path difference.

[0082] Next, the FSR was calculated using equations (4) and (9), and this calculated value was compared with the measured FSR obtained from the ripple spacing in Figures 10 to 14A and 14B. The results are shown in Table 1. Note that n was set to 1.45 in the calculated values. As shown in Table 1, the measured and calculated values ​​were in good agreement.

[0083]

[0084] From these results, it can be said that RIN-suppressed light has a very unique property of exhibiting high coherency despite a large FWHM of approximately 30 nm, as shown in Figures 4, 7, and 8. Furthermore, as shown in Figures 14A and 14B, even when the difference in length between the first optical path 300 and the second optical path 400 is 40 km, ripples are still observed in the RIN spectrum. As shown in the above-mentioned formula (8), the fact that interference occurs even with a difference in length of 40 km means that Δν τ ０ < 1 and Δν < 5 kHz. In the case where the light source is a laser, this corresponds to a linewidth of less than 5 kHz. Of course, since ASE light generally has a wide FWHM, the concept of linewidth does not exist, but from the viewpoint of interference, RIN-suppressed light can be considered to exhibit characteristics equivalent to laser light with a linewidth of less than 5 kHz. These results are far removed from the characteristics of ASE light that have been widely believed in the past, but they are experimental facts.

[0085] In this specification, when a first light L1 is input into an optical interferometer 1000 configured as shown in FIG. 1 and ripples are observed in the RIN spectrum as shown in FIGS. 10 to 14A and 14B, it is said that the first light L1 has coherency or that interference is occurring.

[0086] From the above results, the optical interferometer 1000 according to the first embodiment can measure the length of the optical fiber 301 over a wide measurement range (for example, from several centimeters to several tens of kilometers or more) by utilizing the extremely unique property of the RIN-suppressed light output by the light source 100, which has high coherency equivalent to that of laser light with an extremely narrow linewidth, and is therefore applicable to a variety of applications. Furthermore, the light source 100, which uses ASE light as a seed light source, has an extremely small and simple configuration compared to laser devices that output laser light with a similar linewidth. Therefore, the optical interferometer 1000 is a small and simple optical interferometer that can be applied to a variety of applications.

[0087] Furthermore, in the optical interferometer 1000, the light source 100 may be a continuous wave (CW) light source, and the first light L1 may be CW light. In this case, the average power of the second light L2 does not decrease compared to when the first light L1 is pulsed light, and this is advantageous in terms of the light receiving sensitivity of the optical receiver 2000.

[0088] In the above embodiment, the full width at half maximum of the power spectrum of the first component L11 is wider than the half width at half maximum of the power spectrum of the second component L12. However, the present invention is not limited to this, and the full width at half maximum of the power spectrum of the second component L12 may be wider than the half width at half maximum of the power spectrum of the first component L11.

[0089] Figure 16 shows an example of the RIN spectrum of the second light in the optical interferometer of embodiment 1, when the length of the optical fiber included in the first optical path and whose length is to be measured is 100 m, and when the first component and the second component are not swapped and swapped.

[0090] The case where the first component L11 and the second component L12 are not interchanged means that the first component L11 propagates through the first optical path 300, and the second component L12 propagates through the second optical path 400. The case where the first component L11 and the second component L12 are interchanged means that the first component L11 propagates through the second optical path 400, and the second component L12 propagates through the first optical path 300.

[0091] RIN spectrum RS3 is the RIN spectrum when the first and second components are not swapped, while RIN spectrum RS4 is the RIN spectrum when the first and second components are swapped. Line 214 indicates the position of the corner frequency of RIN spectrum RS3. The corner frequency of RIN spectrum RS4 was not observed and is believed to be 40 GHz or higher. The results in Figure 16 show that when the first and second components are swapped, i.e., when the full width at half maximum of the power spectrum of the first component L11 is narrower than the full width at half maximum of the power spectrum of the second component L12, the RIN-suppressed region is wider. This corresponds to the fact that, in equation (3), a smaller Δλ corresponds to a larger (higher) corner frequency fc.

[0092] Figure 17 shows an example of the RIN spectrum of the second light in the optical interferometer of embodiment 1, when the length of the optical fiber included in the first optical path and whose length is to be measured is 1 km, and when the first component and the second component are not swapped and swapped.

[0093] RIN spectrum RS5 is the RIN spectrum when the first and second components are not swapped, while RIN spectrum RS6 is the RIN spectrum when the first and second components are swapped. Line 215 indicates the position of the corner frequency of RIN spectrum RS5. The corner frequency of RIN spectrum RS6 was not observed and is believed to be 40 GHz or higher. The results in Figure 17 also indicate that the RIN-suppressed region is wider when the full width at half maximum of the power spectrum of the first component L11 is narrower than the full width at half maximum of the power spectrum of the second component L12. This corresponds to a larger (higher) corner frequency fc in equation (3). Furthermore, comparing Figures 16 and 17 indicates that the RIN-suppressed region narrows as the optical path length difference between the first optical path 300 and the second optical path 400 increases. This corresponds to a smaller (lower) corner frequency fc in equation (3).

[0094] Therefore, if it is desired to ensure a wide frequency interference band, it is preferable to configure the optical interferometer 1000 so that the component with the narrower FWHM of the first and second components is input to the optical path including the optical fiber whose length is to be measured.

[0095] 16 and 17, the inventors have found that ripples in the RIN spectrum occur only in the RIN-suppressed region below the corner frequency, and no ripples occur at frequencies higher than the corner frequency. This means that RIN-suppressed light has the properties of coherent light, where interference occurs, below the corner frequency, and incoherent light above the corner frequency, making it light with characteristics significantly different from ordinary laser light.

[0096] <Change in RIN spectrum of second light with change in Is or Ib> Figure 10 shows the state of ripple in the RIN spectrum when the length of the optical fiber to be measured in optical interferometer 1000 is 1 m, Is, which is the drive current supplied to seed light source 11, is 100 mA, and Ib, which is the drive current supplied to booster amplifier 13, is 1000 mA. The following shows the change in the RIN spectrum when Is is changed.

[0097] 18A, 18B, 18C, 18D, 18E, and 18F show RIN spectra when Is is set to 0 mA, 20 mA, 25 mA, 30 mA, 50 mA, and 100 mA, and Ib is set to 1000 mA, respectively, and FIG. 18G shows RIN spectra when Is is set to 100 mA, and Ib is set to 200 mA, 300 mA, and 1000 mA.

[0098] As shown in Figures 18A to 18F, increasing Is suppresses RIN and increases the amplitude of the ripple. In the figures, the solid lines indicate the ASE-ASE beat noise level (-127 dB / Hz) and the suppressed RIN level (RIN suppression level). The difference between these two levels is the RIN suppression amount. For example, when Is = 0 mA in Figure 18A, the suppressed RIN level is -137 dB / Hz, so the RIN suppression amount is 10 dB and the ripple amplitude is small. However, when Is = 100 mA in Figure 18F, the suppressed RIN level is -150 dB / Hz, so the RIN suppression amount reaches 23 dB and the ripple amplitude also increases significantly.

[0099] 18A to 18F, the FWHM of the first light is approximately 28 nm, so the level of the ASE-ASE beat noise as the upper limit of RIN is -127 dB / Hz. Here, the upper limit of RIN depends on the FWHM of the first light, and 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 amount reaches (150 - 120) = 30 dB. Furthermore, if the suppressed RIN level is -160 dB / Hz, the RIN suppression amount reaches (140 - 120) = 40 dB.

[0100] 18G, when Ib is small, the corner frequency is low, but when Ib is increased, the corner frequency becomes higher. In the RIN spectrum, ripple occurs only in the RIN-suppressed region below the corner frequency, so when the corner frequency is low, measurement becomes difficult when the length of the optical fiber 301 is short. Therefore, to ensure a wider measurement range, it is preferable to operate the booster amplifier 13 in a state where the gain saturation is high by setting Ib sufficiently large.

[0101] <Regarding the FWHM of the First Light> FIG. 19 is an enlarged view of the wavelength spectrum of FIG. 7D , showing the area surrounding the depression (wavelength division point) at a wavelength of approximately 1443 nm. For interference to occur in a wavelength division interferometer such as the optical interferometer 1000, at least one pair of modes must exist on either side of the wavelength division point, and the frequency difference between the modes must be within the bandwidth of the optical receiver 2000. Furthermore, the first light L1 must include two or more modes and have an FWHM sufficient to allow wavelength division within the bandwidth of the optical receiver 2000. FIG. 20 is a diagram showing the conditions for interference to occur in a wavelength division interferometer. In FIG. 20 , the first component L11 is on the shorter wavelength side of the wavelength division point, and the second component L12 is on the longer wavelength side. Furthermore, the bold arrows indicate at least one pair of modes on either side of the wavelength division point, and the dashed arrows indicate other modes. Furthermore, Be, Δν, Δλ, and FWHM minimum respectively indicate the bandwidth of the optical receiver 2000, the mode spacing, the wavelength cutoff width of the wavelength splitter 200, and the minimum FWHM of the light source.

[0102] 21A to 21F are enlarged views of the power spectrum of the first light L1 when Is is set to 0 mA, 20 mA, 30 mA, 50 mA, 75 mA, and 100 mA, respectively, in the optical interferometer 1000. Note that Ib is set to 1000 mA.

[0103] 21A to 21F, periodic ripples appear in the optical power spectrum. The wavelength interval of the ripples is approximately 0.2 nm. These ripples are thought to be ripples due to Fabry-Perot (FP) modes caused by FP resonance formed by reflections at both end facets of the SOA, which is the booster amplifier 13, or by optical components adjacent to those end facets. Even if reflections at both end facets of the SOA or by optical components adjacent to those end facets are suppressed as much as possible, it is practically impossible to reduce such FP modes to zero.

[0104] The simultaneous occurrence of such ripples and RIN suppression has already been disclosed by the present inventors (Patent Document 1).

[0105] As can be seen from Figures 21A to 21F, when Is was increased, the amplitude of the ripple became smaller when it increased from 20 mA to 30 mA, but then tended to become larger again when it increased from 50 mA to 75 mA.

[0106] Therefore, if Is is set to an appropriate value (for example, 30 mA to 50 mA in this example), the ripples are suppressed and the spectrum becomes smooth (see Figure 4). However, even when the ripples are suppressed, they are presumed to exist as FP modes buried in the ASE light. As a result, in the RIN suppression region, ASE-to-ASE beat noise is suppressed, making it possible to extract beat signals between FP modes.

[0107] When the wavelength spacing of the ripples is approximately 0.2 nm as described above, the beat frequency (i.e., the mode spacing of the FP mode) is approximately 25 GHz. Therefore, in order for the photoreceiver 2000 to detect this beat frequency, the photoreceiver 2000 needs to have a bandwidth of 25 GHz or more. Note that, in the experiments whose results are disclosed in this specification, a photoreceiver capable of detecting a 40 GHz signal was used, as can be seen from FIG. 9 and other figures.

[0108] Taking the above points into consideration, an example of a preferable FWHM of the first light will be described. First, considering Figures 7B to 7D, 19, etc., and considering the wavelength division characteristics of the wavelength splitter 200 that can be realized, it is preferable that the FWHM be 5 nm or more. However, even in Figure 19, the width of the depression is 0.2 nm or more, so multiple modes exist within this depression. Considering that the depression attenuates the power of the mode that contributes to interference, thereby reducing the amplitude of the beat signal, it is preferable that the wavelength splitter 200 have sufficient wavelength blocking characteristics, and that the width and depth of the depression are narrower and deeper.

[0109] On the other hand, if the FWHM of the first light L1 is relatively wide, the power per mode will be relatively small. Furthermore, given the bandwidth of the optical receiver 2000, at most two modes on either side of the wavelength division point will contribute to interference, and other modes included in the first or second component will not contribute to interference, or even if they do contribute, they will be undetectable. In this regard, to ensure sufficient power per mode, the FWHM is preferably 30 nm or less. Alternatively, the FWHM may be 25 nm or less, 20 nm or less, 15 nm or less, or 10 nm or less. A narrow FWHM and high power per mode are advantageous, for example, even when the measurement distance (e.g., the length of the optical fiber to be measured) is long and large optical loss occurs.

[0110] Furthermore, if the spectral shape of the first light L1 is, for example, Gaussian, it is easy to control the power distribution to each mode according to the wavelength division points.

[0111] <Polarization Dependence> The inventors configured an experimental system in the optical interferometer 1000 according to the first embodiment, in which the length of the optical fiber 301 was set to 1 m and polarization controllers were further inserted in the first optical path 300 and the second optical path 400. Then, in this experimental system, they measured how the RIN spectrum changed when the polarization states of the first component L11 and the second component L12 before the wavelength multiplexer 500 were made to coincide with each other and orthogonal to each other.

[0112] 22 shows an example of the RIN spectrum of the second light when the relationship between the polarization of the first component and the polarization of the second component is changed when the length of the optical fiber included in the first optical path and the length of which is to be measured is 1 m. In FIG. 22, RIN spectrum RS7 is obtained when the polarization states are orthogonal, and RIN spectrum RS8 is obtained when the polarization states are identical. As shown in FIG. 22, it was experimentally found that there is no significant difference in the interference characteristics of RIN-suppressed light when the polarization states are orthogonal and identical.

[0113] Generally, interference occurs when the polarizations of light coincide. It is widely understood that interference does not occur between orthogonal polarizations. In contrast, the results shown in Figure 22 are presumably due to the broad FWHM of the power spectrum of the first light L1, which is RIN-suppressed light, resulting in polarization scrambling even when propagating through a very short optical fiber, such as the non-polarization-maintaining portion of the optical fiber connecting the light source 100 and the wavelength splitter 200. This is a very significant property in practical applications. Light often changes its polarization state during propagation, particularly in optical fibers. Therefore, to achieve an interference state with high sensitivity, it is necessary to control and synchronize the polarization. However, this is not necessary in the optical interferometer 1000 using RIN-suppressed light, such as in the first embodiment. In other words, the first optical path 300 and the second optical path 400 can achieve an interference state without requiring a polarization control mechanism such as a polarization controller.

[0114] (Embodiments 2, 3, and 4) Next, we will explain optical interferometers according to embodiments 2, 3, and 4. Below, we will first explain the optical interferometers according to embodiments 2, 3, and 4, and then explain the characteristics of the optical interferometers according to embodiments 2, 3, and 4.

[0115] 23 is a schematic diagram of an optical interferometer according to embodiment 2. The optical interferometer 1000A according to embodiment 2 has a configuration in which the wavelength splitter 200 in the optical interferometer 1000 according to embodiment 1 is replaced with a power splitter 200A, and the wavelength combiner 500 is replaced with a power combiner 500A. The optical interferometer 1000A, the photodetector 2000, and the electrical spectrum analyzer 3000 constitute an optical interferometer system.

[0116] The power divider 200A receives the first light L1 from the light source 100, divides the power of the first light L1 into a third component L13 and a fourth component L14, and outputs the divided components. The power divider 200A outputs the third component L13 to the first optical path 300 and the fourth component L14 to the second optical path 400. The power divider 200A includes, for example, an optical coupler that divides two lights at a predetermined power ratio. The optical coupler may be, for example, a 3 dB coupler.

[0117] The power combiner 500A combines the third component L13 propagated through the first optical path 300 and the fourth component L14 propagated through the second optical path 400, and outputs the combined light as a fifth light L5. The power combiner 500A includes, for example, an optical coupler that combines two lights at a predetermined power ratio. The optical coupler may be, for example, a 3 dB coupler. <Configuration of Embodiment 3> FIG. 24 is a schematic diagram of an optical interferometer according to Embodiment 3. The optical interferometer 1000B according to Embodiment 3 has a configuration in which a polarizer 600 is provided 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 combiner (PBC). The polarization direction of the polarizer 600 is set to coincide with the polarization direction of the first light L1 from the light source 100. As a result, the polarizer 600 outputs the first light L1B having a higher degree of polarization than the first light L1. Note that the pigtail fiber connecting the light source 100 and the polarizer 600 is preferably a polarization-maintaining fiber, but may be a normal optical fiber that is not polarization-maintaining.

[0118] The power divider 200A receives the first light L1B, divides the power of the first light L1B into a third component L13B and a fourth component L14B, and outputs the divided light. The power combiner 500A combines the third component L13B that has propagated through the first optical path 300 and the fourth component L14B that has propagated through the second optical path 400, and outputs the combined light as a fifth light L5B.

[0119] 25 is a schematic diagram of an optical interferometer according to embodiment 4. An optical interferometer 1000C according to embodiment 4 has a configuration in which the light source 100 of the optical interferometer 1000 according to embodiment 2 is replaced with a light source 100C.

[0120] The light source 100C includes two original light sources 100C1 and 100C2 and a polarization combiner 100C3. The two original light sources 100C1 and 100C2 output original light sources L01 and L02, respectively, with RIN suppressed similarly to the first light L1. The two original light sources 100C1 and 100C2 may both have the same configuration as the light source 100. The polarization combiner 100C3 polarization-combines the two original light sources L01 and L02, which are orthogonal in polarization state to each other, and outputs the resulting first light L1C. In this case, the pigtail fiber connecting the original light sources 100C1 and 100C2 and the polarization combiner 100C3 is preferably a polarization-maintaining fiber.

[0121] The power divider 200A receives the first light L1C, divides the power of the first light L1C into a third component L13C and a fourth component L14C, and outputs the divided light. The power combiner 500A combines the third component L13C that has propagated through the first optical path 300 and the fourth component L14C that has propagated through the second optical path 400, and outputs the combined light as a fifth light L5C.

[0122] The above-described optical interferometers 1000A, 1000B, and 1000C all have the configuration of a Mach-Zehnder optical interferometer. Hereinafter, the optical interferometer 1000 according to the first embodiment will be referred to as a wavelength division interferometer, whereas the configurations of the optical interferometers 1000A, 1000B, and 1000C will be referred to as power division interferometers.

[0123] In addition, in embodiments 2 to 4, the optical receiver 2000 receives the fifth light L5, L5B, or L5C and outputs a current signal corresponding to the intensity of the received light to the electrical spectrum analyzer 3000. The electrical spectrum analyzer 3000 measures the RIN spectrum of the fifth light L5, L5B, or L5C based on the input current signal. The electrical spectrum analyzer 3000 is an example of an electrical spectrum analyzer that displays the spectrum of an input electrical signal in the frequency domain. In addition, in embodiments 2 to 4, the optical interferometer 1000A, 1000B, or 1000C, the optical receiver 2000, and the electrical spectrum analyzer 3000 constitute an optical interferometer system.

[0124] <Characteristics of Fifth Light> Next, the characteristics of the fifth light will be described. Figures 26A and 26B are diagrams showing examples of the RIN spectrum of the fifth light when the optical fiber 301 is not included in the optical interferometers according to the second and third embodiments. In Figure 26A, RIN spectrum RS9 represents the RIN spectrum of the first light L1 or L1B in the optical interferometer 1000A or 1000B. RIN spectrum RS10 represents the RIN spectrum of the fifth light L5 in the optical interferometer 1000A. RIN spectrum RS11 represents the RIN spectrum of the fifth light L5B in the optical interferometer 1000B. Figure 26B also shows portions of the RIN spectra RS10 and RS11.

[0125] The RIN spectra of the first light L1 in the optical interferometer 1000A and the first light L1B in the optical interferometer 1000B, which is the first light L1 after passing through the polarizer 600, are substantially the same. This RIN spectrum also substantially matches the RIN spectrum RS1 in FIG.

[0126] On the other hand, as shown in Figures 26A and 26B, the RIN spectra RS10 and RS11 of the optical interferometers 1000A and 1000B, which are power division interferometers, have significantly different characteristics from the RIN spectra RS3 and RS4 of the optical interferometer 1000, which is a wavelength division interferometer, as shown in Figure 16. Specifically, the RIN suppression amount was significantly reduced in the RIN spectra RS10 and RS11. However, the RIN level in the RIN spectrum RS11 was low, at approximately -135 dB / Hz. Furthermore, in the RIN spectra RS10 and RS11, ripples were almost absent in the RIN-suppressed region, but were more pronounced in the high-frequency region above 10 GHz, which is higher than the corner frequency. This ripple had an FSR of approximately 5 GHz. These characteristics are similar to those of so-called white light, but the interference length is longer than that of white light. Note that an FSR of 5 GHz is considered to mean that the optical path length difference between the first optical path 300, which does not include the optical fiber 301, and the second optical path 400 is approximately 4 cm. Thus, the optical interferometers 1000A and 1000B, which are power division interferometers, are sensitive to even a slight optical path length difference such as 4 cm. Furthermore, the RIN spectrum RS11 has a larger interference amplitude and is considered to be more coherent than the RIN spectrum RS10.

[0127] Next, Figures 27A and 27B are diagrams showing examples of the RIN spectrum of the fifth light in the optical interferometers according to the third and fourth embodiments. Figure 27A shows the RIN spectrum of the fifth light in the optical interferometer 1000B, where the optical fiber 301 has a length of 1 m. Figure 27B shows the RIN spectrum of the fifth light in the optical interferometer 1000C, where the optical fiber 301 has a length of 1 m. The results in Figures 27A and 27B indicate that RIN-suppressed light obtained by polarization combining two RIN-suppressed lights with orthogonal polarization states can reduce the amount of RIN suppression and the amplitude of interference compared to RIN-suppressed light that has passed through a polarizer. This indicates that it is possible to generate high-power, broadband (large FWHM) quasi-white light with reduced coherence and that the amount of RIN suppression and coherence can be controlled by appropriately setting the RIN-suppressed light.

[0128] From the above results, the optical interferometers 1000A, 1000B, and 1000C according to the second to fourth embodiments are power division interferometers, but have a simpler configuration than a wavelength division interferometer including a wavelength splitter 200 such as the optical interferometer 1000 according to the first embodiment. Furthermore, the optical interferometers 1000A, 1000B, and 1000C are optical interferometers applicable to a variety of applications because they can generate pseudo-white light (quasi-white light) with reduced coherence. For example, the optical interferometers 1000A, 1000B, and 1000C are suitable for obtaining light with interference characteristics of a delay corresponding to the length of an optical fiber of about several meters.

[0129] As mentioned above, the coherence length of broadband light such as the ASE light from an SOA is typically on the order of several tens of micrometers. Interference using such low-coherence light is called white light interference, and is widely used, for example, in optical coherence tomography (OCT). Taking advantage of the short coherence length, high-resolution distance measurement is achieved.

[0130] Next, FIG. 28 is a diagram comparing examples of the RIN spectra of the first, second, and fifth light in the optical interferometers according to the first and second embodiments. Specifically, RIN spectrum RS20 is the RIN spectrum of the first light L1, RIN spectrum RS21 is the RIN spectrum of the second light L2 in the optical interferometer 1000, and RIN spectrum RS22 is the RIN spectrum of the fifth light L5 in the optical interferometer 1000A. The length of the optical fiber 301 is 1 km. Corner frequency 216 is the corner frequency of the RIN spectrum RS21. The corner frequency of the RIN spectrum RS20 is 40 GHz or higher. The level of line 210 indicates the level of ASE-ASE beat noise. Line L220 indicates the level of suppressed RIN in the RIN spectrum RS20. Line L230 indicates the level of suppressed RIN in the RIN spectrum RS21. A line L240 indicates the suppressed RIN level in the RIN spectrum RS22. An arrow Ar1 indicates that the RIN suppression amount in the RIN spectrum RS22 of the fifth light L5 is approximately 7 dB. An arrow Ar2 indicates that the RIN suppression amount in the RIN spectrum RS22 of the fifth light L5 is alleviated by approximately 20 dB compared to the RIN suppression amount in the RIN spectrum RS20 of the first light L1. An arrow Ar3 indicates that the RIN suppression amount in the RIN spectrum RS21 of the second light L2 is alleviated by approximately 7 dB compared to the RIN suppression amount in the RIN spectrum RS20 of the first light L1.

[0131] 29 is a schematic diagram of an optical interferometer according to embodiment 5. The optical interferometer 1000D has a configuration in which the first optical path 300 in the optical interferometer 1000 according to embodiment 1 is replaced with a first optical path 300D.

[0132] The first optical path 300D includes spatial optical systems 300D1 and 300D2, a measurement object 300D3, and a spatial optical path 300D4. The spatial optical system 300D1 is a spatial optical system that collimates the first component L11 output from the wavelength splitter 200 and outputs it into space toward the measurement object 300D3, and includes, for example, a lens. The measurement object 300D3 is an object for which distance or movement speed is measured. The spatial optical path 300D4 is an optical path along which the first component L11 propagates toward the measurement object 300D3 and the first component L11 reflected by the measurement object 300D3 propagates toward the spatial optical system 300D2. The spatial optical system 300D2 is a spatial optical system that receives the first component L11 reflected by the measurement object 300D3 and couples it to the wavelength combiner 500, and includes, for example, a lens. The wavelength combiner 500 combines the first component L11 propagated through the spatial optical path 300D4 and the second component L12 propagated through the second optical path 400, and outputs the combined light as second light L2D. That is, the first optical path 300D includes the spatial optical path 300D4 along which the first component L11 propagates toward the measurement object 300D3, reaches the measurement object 300D3, and then returns.

[0133] According to the optical interferometer 1000D, by analyzing the ripples appearing in the RIN spectrum of the second light L2D, the optical path length of the spatial light path 300D4 can be measured in the same manner as in the optical interferometer 1000, and the distance to the measurement object 300D3 can be calculated from the optical path length. Furthermore, by analyzing the change in the ripples over time, the movement speed of the measurement object 300D3 can be calculated. Here, the movement speed is the movement speed of the optical interferometer 1000D relative to the wavelength splitter 200 and the wavelength combiner 500.

[0134] 30 is a schematic diagram of an optical interferometer according to embodiment 6. The optical interferometer 1000E has a configuration 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.

[0135] The spatial optical system 300D1 collimates the third component L13 output by the power divider 200A and outputs it into space toward the measurement object 300D3. The spatial optical path 300D4 is an optical path along which the third component L13 propagates toward the measurement object 300D3 and the third component L13 reflected by the measurement object 300D3 propagates toward the spatial optical system 300D2. The spatial optical system 300D2 receives the third component L13 reflected by the measurement object 300D3 and couples it to the power combiner 500A. The power combiner 500A combines the third component L13 propagated through the spatial optical path 300D4 and the fourth component L14 propagated through the second optical path 400, and outputs the combined light as fifth light L5E. That is, the first optical path 300D includes a spatial optical path 300D4 along which the third component L13 propagates toward the measurement object 300D3, reaches the measurement object 300D3, and then returns.

[0136] The optical interferometer 1000E can measure the optical path length of the spatial light path 300D4 by analyzing the ripples that appear in the RIN spectrum of the fifth light L5E, as with the optical interferometer 1000A, and can calculate the distance to the measurement object 300D3 from the optical path length. Furthermore, by analyzing the temporal change in the ripples, the movement speed of the measurement object 300D3 can be calculated. The optical interferometer 1000E is particularly suitable for distance measurements on the order of centimeters or less.

[0137] 31 is a schematic diagram of an optical interferometer according to embodiment 7. The optical interferometer 1000F has a configuration in which the first optical path 300 in the optical interferometer 1000 according to embodiment 1 is replaced with a first optical path 300F.

[0138] The first optical path 300F is provided with a delay control device 301F. The delay control device 301F is a device that can control the delay time of light in the first optical path 300F by a modulation signal from the signal source 700, and includes, for example, an electrode for changing the effective refractive index of the first optical path 300F.

[0139] In this case, the first component L11 propagating through the first optical path 300F and the second component L12 propagating through the second optical path 400 are combined to produce a modulated signal as second light L2F output from the wavelength combiner 500. In this case, the optical interferometer 1000F functions as a modulator.

[0140] Although the delay control device is provided in the first optical path in the optical interferometer 1000F, it may be provided in the second optical path, or may be provided in both the first optical path and the second optical path.

[0141] In addition, in the above embodiment, the seed light source 11 is an SOA and the seed light LS is the ASE light of the SOA, but the seed light source 11 may also be a rare-earth doped optical fiber amplifier such as an EDFA, a Raman optical fiber amplifier, or an SLD, and the seed light LS may be the ASE light of these.

[0142] Incidentally, the present inventors measured the output fifth light L5 of the optical interferometer 1000A according to the second embodiment shown in Fig. 23 by setting the length of the optical fiber 301 to 1 m, Is to 0 mA, i.e., a state in which the SOA serving as the seed light source 11 does not output the seed light LS, and Ib to 1000 mA, and found that the result has characteristics different from those in Fig. 27A and 27B. Hereinafter, a state in which the seed light LS is not output in a power division interferometer such as the optical interferometer 1000A, i.e., the first light is simply the ASE light of the booster amplifier 13, may be referred to as a seedless power division interferometer, and the light source in this case may be referred to as a seedless light source.

[0143] FIG. 32 is a diagram showing an example of the RIN spectrum of the fifth light in a seedless power division interferometer having a configuration corresponding to embodiment 2. Here, the seedless power division interferometer having a configuration corresponding to embodiment 2 is one in which Is is set to 0 mA in embodiment 2. Comparing FIG. 32 with FIGS. 27A and 27B , it can be seen that the ripples present in the RIN-suppressed region on the low frequency side in FIG. 32 have disappeared in FIGS. 27A and 27B . FIG. 33 is a diagram showing an example of the power spectrum of the first light in a seedless power division interferometer or the optical interferometer 1000A according to embodiment 2. FIG. 34 is a diagram showing an example of the RIN spectrum of the first light in a seedless power division interferometer or the optical interferometer 1000A according to embodiment 2.

[0144] Therefore, the inventors of the present invention set the length of the optical fiber 301 to 1 m in the optical interferometer 1000A, replaced the light source 100 as a broadband light source with an EDFA, and measured the RIN spectrum of the fifth light using the ASE light of the EDFA as the first light.

[0145] Fig. 35 is a diagram showing an example of the RIN spectrum of the fifth light when the light source 100 is replaced with an EDFA in the optical interferometer 1000A according to the second embodiment. Fig. 36 is a diagram showing an example of the power spectrum of the first light output from the EDFA. Fig. 37 is a diagram showing an example of the RIN spectrum of the first light output from the EDFA. It can be seen that ripples exist in Fig. 35 as in Fig. 32 . However, it can be seen that the amplitude of the ripples is larger at frequencies of 10 GHz or higher than at frequencies below 10 GHz in Fig. 32 , whereas the amplitude of the ripples is smaller in Fig. 35 .

[0146] We will now consider what causes the presence of ripples that exhibit significant coherence in the high-frequency range in a seedless power division interferometer, as shown in Figures 32 and 35. According to the inventors' investigations, when Is = 0 mA, as shown in Figure 21A, i.e., in the seedless light source state, ripples appear in the power spectrum of the first light. Therefore, in a seedless power division interferometer, it is presumed that the FP mode, which has grown to the point where it appears as a ripple in the power spectrum, is the main cause of coherence. As mentioned in the explanation of Figures 21A to 21F, it is presumed that the FP mode can exist while being buried in the ASE light. Therefore, even if the first light is, for example, ASE light from an EDFA, it is presumed that some kind of oscillation mode exists, and although the oscillation mode is difficult to see, it is presumed that it exists. The presence or absence of such an oscillation mode can be confirmed by the presence or absence of ripples in the RIN spectrum of the fifth light.

[0147] Therefore, an optical interferometer according to another embodiment of the present invention can be configured as an optical interferometer including: a broadband light source that outputs first light having periodic ripples in the power spectrum of the light; a power divider that power-divides the first light into third and fourth components and outputs the resulting components; a first optical path along which the third component output from the power divider propagates; a second optical path along which the fourth component output from the power divider propagates; and a power combiner that combines the third component propagated through the first optical path and the fourth component propagated through the second optical path and outputs the combined light as fifth light. Specifically, such an optical interferometer can be configured by setting the SOA serving as the seed light source 11 in the optical interferometer 1000A according to the second embodiment to a state in which it does not output seed light LS, or by replacing the light source 100 with an EDFA. The amplitude of the ripples in the power spectrum of the light is, for example, 0.5 dB or more and, for example, 10 dB or less.

[0148] 38 is a schematic diagram of an optical interferometer system according to embodiment 8. The optical interferometer system 10000 has a configuration in which the electric spectrum analyzer 3000 of the optical interferometer system including the optical interferometer 1000, the photodetector 2000, and the electric spectrum analyzer 3000 shown in FIG.

[0149] The processing device 4000 is configured to include, for example, a known electrical signal processing device or a computer. The processing device 4000 electrically processes the current signal input from the optical receiver 2000. By such electrical processing, it is possible to mathematically process (calculate, etc.) the information contained in the current signal in the frequency domain.

[0150] The optical interferometer 1000 in the optical interferometer system 10000 according to the eighth embodiment may be replaced with any of the optical interferometers according to the first to sixth embodiments.

[0151] In any of the above-described eighth embodiment and the other optical interferometer systems described above, the RIN spectrum of the second light or the fifth light is acquired, and the delay time τ ０ and estimates the optical path difference L between the first optical path and the second optical path using the following formula: L=τ ０ where 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. ０ The relationship between FSR and τ is expressed by the following formula: FSR=1 / τ ０ For example, the processing device 4000 stores a program that causes a computer to execute such an optical path difference measurement method, and the optical path difference measurement method can be performed by the processor of the computer provided in the processing device 4000 executing the program.

[0152] <Example of Measurement Range> The optical path difference (measurement range) suitable for measurement in the optical interferometer (i.e., wavelength division interferometer, power division interferometer, or seedless power division interferometer) and optical interferometer system according to the above-described embodiments will be described. Note that in the following, the measurement object is an optical fiber whose length is to be measured, and the effective refractive index of the optical fiber is 1.45.

[0153] First, the lower limit of measurement is determined by the bandwidth of the optical receiver 2000. Here, since the FSR is approximately 20 GHz when the length of the optical fiber is 1 cm, the lower limit of measurement is on the order of sub-centimeter (i.e., approximately 1 cm or less). For example, if the bandwidth of the optical receiver 2000 is 100 GHz, measurement of approximately 2 mm is possible.

[0154] The upper measurement limit is determined by the degree of coherence in the optical interferometer, for example: in the case of a wavelength division interferometer, the upper limit is, as mentioned above, a few tens of km, for example 40 km.

[0155] In the case of a power division interferometer, ripples start to occur at frequencies above 1 GHz where RIN suppression begins to relax, so the upper limit is sub-millimeter (including, for example, several tens of cm), such as 0.2 m.

[0156] Also, for a seedless power division interferometer the upper limit is longer than for a power division interferometer, the upper limit being, for example, 100 m.

[0157] In the above optical interference system, the interference light is analyzed by converting it into a photocurrent by a photoreceiver, but the analysis method is not limited to this.

[0158] Furthermore, the present invention is not limited to the above-described embodiment, and configurations in which the above-described components are appropriately combined are also included in the present invention.

[0159] For example, the configuration of the optical interferometer 1000F according to the seventh embodiment, which is a wavelength division interferometer, may be applied to the optical interferometers 1000A to 1000C according to the second to fourth embodiments, which are power division interferometers, and configured to function as a modulator.

[0160] When the bandwidth of the modulated signal is lower than the corner frequency of the first light, it is preferable to apply a wavelength division interferometer as in embodiment 7. 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 modulated signal is higher than the corner frequency of the first light, it is preferable to apply a power division interferometer. Furthermore, since the first light is RIN-suppressed light and has a wide FWHM, it is more suitable for spatial transmission that takes advantage of its high output than for transmission through an optical fiber with chromatic dispersion.

[0161] Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

[0162] The present invention can be used in optical interferometers.

[0163] 10: Light source module 11: Seed light source 12, 14: Optical isolator 13: Booster amplifier 13a: First end face 13b: Second end face 15: Output optical fiber 100, 100C: Light source 100C1, 100C2: Original light source 100C3: Polarization combiner 101, 102: Driving device 200: Wavelength splitter 200A: Power splitter 211: Low frequency region 213, 216: Corner frequency 214, 215: Line 300, 300D, 300F: First optical path 300D1, 300D2: Spatial optical system 300D3: Measurement object 300D4: Spatial optical path 301: Optical fiber 301F: Delay control device 400 : Second optical path 500 : Wavelength combiner 500A : Power combiner 600 : Polarizer 700 : Signal source 1000, 1000A, 1000B, 1000C, 1000D, 1000E, 1000F : Optical interferometer 2000, 5005 : Photoreceiver 3000 : Electrical spectrum analyzer 4000 : Processing device 5000 : Measurement system 5001, 5004 : Optical coupler 5002 : First optical path 5003 : Second optical path 10000 : Optical interferometer system Ar1, Ar2, Ar3 : Arrows C1, C2 : Drive current L01, L02 : Original light L1, L1B, L1C : First light L11 : First component L12 : Second component L13, L13B, L13C : Third component L14, L14B, L14C : Fourth component L2, L2D, L2F : Second light L5, L5B, L5C, L5E : Fifth light LS : Seed light PS1, PS2 : Power spectrum RS1, RS2, RS3, RS4, RS5, RS6, RS7, RS8, RS9, RS10, RS11, RS20, RS21, RS22 : RIN spectrum

Claims

1. An optical interferometer comprising: a broadband light source that outputs first light with relative intensity noise (RIN) suppressed in a predetermined frequency range; a wavelength divider that wavelength-divides the first light into a first component and a second component having a different center wavelength from the first component and outputs them; a first optical path through which the first component output from the wavelength divider propagates; a second optical path through which the second component output from the wavelength divider propagates; and a wavelength combiner that combines the first component that has propagated through the first optical path and the second component that has propagated through the second optical path and outputs them as second light.

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

3. The optical interferometer according to claim 1, wherein the full width at half maximum of the power spectrum with respect to the wavelength of the first light is 5 nm or more and 30 nm or less.

4. The optical interferometer according to claim 1, further comprising a delay control device provided in the first optical path or the second optical path.

5. The optical interferometer according to claim 1, not having a polarization control mechanism in the first optical path and the second optical path.

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

7. An optical interferometer comprising: a broadband light source that outputs first light with relative intensity noise (RIN) suppressed in a predetermined frequency range; a power divider that power-divides the first light into a third component and a fourth component and outputs them; a first optical path through which the third component output from the power divider propagates; a second optical path through which the fourth component output from the power divider propagates; and a power combiner that combines the third component that has propagated through the first optical path and the fourth component that has propagated through the second optical path and outputs them as fifth light.

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

9. A broadband light source that outputs first light having periodic ripples on the optical power spectrum, a power splitter that splits and outputs the first light into a third component and a fourth component, a first optical path through which the third component output from the power splitter propagates, a second optical path through which the fourth component output from the power splitter propagates, and a power combiner that combines the third component that has propagated through the first optical path and the fourth component that has propagated through the second optical path and outputs them as fifth light. An optical interferometer comprising:

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

11. The optical interferometer according to claim 7 or 9, further comprising a polarizer provided between the broadband light source and the power splitter.

12. The broadband light source according to claim 7, comprising two original light sources that output original light with RIN suppressed in the predetermined frequency region, and a polarization combiner that polarization-combines the two original lights from the two original light sources and outputs them as the first light.

13. The optical interferometer according to claim 7 or 9, wherein the suppression amount of RIN in the fifth light is relaxed with respect to the suppression amount 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 through which the first component or the third component propagates toward the measurement object and returns after reaching the measurement object.

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

16. The optical interferometer according to claim 1 or 7, wherein the suppression amount of RIN at the frequency in the predetermined frequency region in the first light is 10 dB or more.

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

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

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

20. A method for measuring an optical path difference executed by the optical interferometer system according to claim 19, comprising: obtaining an RIN spectrum of the second light or the fifth light; obtaining a delay time τ from an FSR (free Spectral Range) of the obtained RIN spectrum ０ ; and estimating an optical path difference L between the first optical path and the second optical path by the following formula. Optical path difference measurement method. L = τ ０ · 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.

Images