JP6305975B2 - Integrated light reflectometer - Google Patents

Description

Related applications

  This application is a provisional patent application No. 1 filed on Mar. 14, 2012. Claim priority based on 61/610533. US provisional patent application no. The entire content of 61/610533 is incorporated herein by reference in its entirety.

Statement about federal-sponsored research or development

  At least a portion of the present disclosure was conceived or first practically implemented under a contract with the US Department of Energy.

  The present disclosure relates to an apparatus and method for inspecting the optical integrity of an optical fiber (FO) facility. More particularly, the present disclosure tests the optical integrity of FO equipment in high resolution (eg, less than 1 cm, eg, less than 2 mm), inexpensively and efficiently with a single sweep over 1 kilometer. It relates to an integrated light reflectometer and related methods.

  Optical communication technologies including optical fibers and lasers are the mainstay of the Internet and high capacity computing. Meeting the computing and communication needs of the next decade requires progress across a broad front of research and development, including intelligent and seamless networks, as well as optical signal generation, transmission, switching and routing. To do. Institutions and companies have access to such fast-growing, high-speed global communications networks, but the infrastructure still gives individuals access to make full use of the optical power at the FO facility. Not ready to serve.

Known FO diagnostic equipment has two expensive categories:
1) Relatively low resolution long range units for telecommunications and large data networks (such units typically use optical time domain reflection (OTDR) 2) consisting of ultra-high resolution laboratory equipment using optical frequency domain reflection (OFDR) that provides resolutions of less than 1 mm.

  The term “coherent” or “coherence” as used herein means a uniform wavelength (or frequency). Therefore, the term “coherence length” means the distance of air such that the wavelength of the laser light is uniform in the air.

  OFDR uses a tunable, high coherence laser source. OFDR lasers can provide a wide optical frequency sweep that can be converted to very high spatial resolution in a reflectometer, but the cost of the laser source is very high and the suppliers are limited. Standard distributed feedback (DFB) lasers are much cheaper and can be tuned over a narrower wavelength range, but the wavelength variability of DFB lasers is sufficient for resolution in the sub 1 cm region. However, DFB sources typically do not provide sufficient coherence to be used for measurements beyond a fiber length of about 1 m.

  To support continuous large-scale FO deployments such as home-to-data center communications networks, these are much cheaper and with high resolution to locate obstacles in these more compact environments. There are identified needs for testing FO equipment.

  In view of the above, the present disclosure provides a new, integrated light reflectometer that can further develop optical communications at a high reliability level in a cost effective manner.

  To support continuous large-scale FO deployment for communications, sensing, advanced lighting systems, and other FO platforms, the integrated light reflectometer of the present disclosure provides optical integrity of the FO equipment. Allows to be tested at a much lower cost with higher resolution. The integrated light reflectometer of the present disclosure provides inexpensive optical measurements with a resolution of less than 1 cm over a kilometer range. The integrated light reflectometer provides a complete and / or customizable solution.

  Therefore, to help current and future FO equipment operate at peak efficiency, an integrated light reflectometer makes it easier for network operators to identify where fiber connection problems end. Like that. Similarly, one key area of improvement addressed by the present disclosure is a reduction in the cost of high resolution communication test and measurement equipment. The integrated optical reflectometer of the present disclosure is a reflectometer with high resolution and reasonably short range to be used for FO network maintenance, supporting large scale FO network deployments. This technology creates a new FO network in the neighborhood, office buildings, fiber-to-the-home, local area network, wide area network, and self-contained FO platform including mobiles such as aircraft and ships. to support. Some of these FO platforms extend at most several hundred meters (<1 kilometer). And it is desirable for the optical interrogation system to find optical disturbances within a range of less than one centimeter to make the necessary repairs to maintain the integrity of the entire optical network.

The present disclosure provides that the laser output is observed using an interferometer (eg, a measurement unit) and feedback correction is applied to the detected signal before the light is used in the reflectometer. The interferometer is used to measure the instantaneous wavelength / frequency of the laser light source. This is to determine whether there is a difference between the actual frequency of the laser light output from the laser light source at that moment and the target frequency of the laser light source. The interferometer includes first and second laser paths that divide laser light output by the laser light source, and the first and second laser paths have different lengths. An interference pattern is generated when the laser light traveling along the first laser path is recombined with the laser light traveling along the second laser path. The interference pattern is observed by a detector. Based on the change in laser phase represented by the observed interference pattern, a measured difference between the actual frequency and the target frequency is derived. The output of the interferometer can be considered an “error signal” because it represents the difference between the actual frequency of the laser source and the target frequency . Based on the measured difference between the actual frequency and the target frequency in the interferometer, a feedback correction is performed in real time for the laser light source to adjust the laser frequency to the target frequency. Applied. Thereby, a highly coherent laser oscillation frequency output can be obtained from the laser light source.

  Thus, the present disclosure uses a phase / frequency controlled feedback mechanism based on the lasing frequency observed with an interferometer for all points in a long fiber (eg, 1 kilometer or more) in a single measurement. Generate a corrected signal that works on A high feedback correction gain is made possible by using an optimized feedback phase response to correct for laser variations. The same feedback mechanism also provides a point at which the wavelength control signal should be injected to ensure control linearity and provide accurate measurement results that exceed the natural coherence length of the DFB laser source.

  Further improvements, advantages and features of the present disclosure are described in more detail below with reference to exemplary embodiments shown in the drawings. In the drawing

FIG. 1A is a diagram illustrating a configuration of a laser light source and a measurement unit (for example, an interferometer).

FIG. 1B is a diagram illustrating an arrangement of a laser light source and a measurement unit (for example, an interferometer).

FIG. 2 is a block diagram of an integrated light reflectometer according to an exemplary embodiment of the present disclosure.

FIG. 3 shows a MATLAB controlled electro-optic model of a wavelength-managed DFB laser with a test fiber interferometer for laser source interrogation.

FIG. 4 is a diagram showing fiber Bragg grating (FBG) reflection intensity with respect to wavelength at three different temperatures.

FIG. 5 is a diagram showing return signals from three FBGs and a reference signal from a wideband reflector.

FIG. 6 is a diagram showing a graph in which the laser light frequency is swept with respect to time.

7 shows a simulation plot of the FO test network device in FIG. 3 (from top to bottom) cavity electron count, cavity intensity, cavity optical phase, control interferometer output field, and measured interferometer output field. Several important parameters are shown, including

FIG. 8 is a diagram illustrating an example of post-processing data from the test apparatus for numerically quantifying and comparing the apparatus performance.

FIG. 9 is a diagram showing a self-heterodyne configuration using an asymmetric Mach-Zehnder interferometer.

  Exemplary embodiments of the present disclosure will be described with reference to the drawings. The following description sets forth specific details, such as particular embodiments, procedures, techniques, for purposes of explanation and not limitation. It is to be understood that the embodiments described below are exemplary and other embodiments may be used apart from these specific details. In some instances, detailed descriptions of well-known methods, interfaces, circuits, and devices have been omitted so as not to obscure the description of the present disclosure. In some of the drawings, individual blocks are shown. Those skilled in the art will record the functions of those blocks in software programs and non-transitory computer readable media in conjunction with a suitably programmed digital microprocessor or general purpose computer using separate hardware circuitry. It will be appreciated that the data may be implemented using an application specific integrated circuit (ASIC) and / or using one or more digital signal processors (DSPs). .

  In the description of exemplary embodiments of the present disclosure, examples of types of laser light sources are provided as lasers including DFB laser diodes. It should be understood that these are given as examples of laser light sources, and the present disclosure is not limited thereto.

  At the beginning of the detailed description of the present disclosure, a discussion of the principles of OFDR is provided with reference to various examples to better explain the unique solutions of the present disclosure.

  A reflectometer is used to examine the properties of a waveguide along the length of the waveguide by analyzing the reflection of the signal injected at one end. In OFDR, the signal is coherent light having an optical wavelength (or optical frequency) that changes steadily.

  In one example of OFDR, light steadily increases in optical frequency from a nominal frequency of 200 THz (wavelength 1500 nm) at a rate of 10 MHz per microsecond, and this frequency sweep can be maintained for a period of 1000 microseconds. Light reflected from discontinuities in the fiber (eg, defects, connectors, end faces, etc.) at a distance of 1 km will generate a round trip delay of about 10 μs (typical values for the speed of light in the fiber). Use). Thus, during the optical frequency sweep, the reflected light will be from the laser source when the generated optical frequency is 100 MHz slow. This particular frequency difference between the reflected light and the current laser output frequency will be a property of all light from this distance as long as the sweep is maintained. Reflected light from all other distances will have different characteristic frequency differences.

  The optical frequency / wavelength may be difficult to measure directly with high resolution. However, very small frequency differences can be easily detected by interference between the two light sources. In this case, the reflected light energy can interfere with a portion of the light extracted directly from the laser. This configuration is sometimes called self-heterodyne. The difference in optical frequency between these two interfering optical signals produces an intensity modulation frequency equal to the interference beat, or the difference in optical frequency between the two optical input signals. The intensity modulation of the light after interference can be converted into an electrical signal using a photodetector. Since we have good tools for further processing in the electrical domain, it is useful to move the signal to the electrical domain. In this configuration, reflection from a point 1 km away now produces an electrical signal with a characteristic property of 100 MHz. The frequency identifies where the reflection originated, and the amplitude of the beat frequency indicates the intensity of the reflection at that point. It is clear that the frequency spectrum of the signal from the photodetector contains analogs of the reflection profile along its length of the fiber. Here, the zero frequency represents the launch end and the fiber length is represented on a scale where the scale is 100 MHz = 1 km (ie, 100 kHz / m). This frequency spectrum is often generated by a fast Fourier transform (FFT) of the beat signal from the detector.

  In the above, the laser light was assumed to be coherent. Therefore, the laser is an optical frequency oscillator. Here, the phase can be predicted for any length of time into the future. However, this is not the case in practice. All oscillators are subject to some disturbances, in particular because the oscillation phase of a semiconductor laser is destroyed by spontaneous emission into the cavity of the semiconductor laser. Stimulated photon emission enhances the lasing wave energy in a constructive phase. On the other hand, each spontaneously emitted photon has a random phase, and therefore adds a random code and a small disturbance in amplitude to the laser oscillation phase. Over time, the effect of these random disturbances is to wander the laser phase away from the ideal target phase (ie, it was completely coherent) in a “random walk” style. If the random contribution is very small, it may be possible to predict the laser phase in some useful way for the future. This effective prediction time is the laser coherence time. Further, the coherence time can be expressed as the laser oscillation coherence length by dividing the light speed. Thus, for example, a laser with a coherence time of 3 μs has a coherence length of 1 km in air. Taking the above example, assuming a laser with a coherence time of 3 μs, the coherence length in the fiber is about 600 m (the speed of light is slower in the fiber), so we reflected to a distance of 300 m in the fiber. Rate measurements can be taken (ie, 600 m round trip). Thus, lasers with longer coherence lengths can be used for distant reflectance measurements.

  Also, the above discussion of coherence shows that the laser phase is well defined for a certain time and beyond that it is unknown, or the coherence function has a high value for the coherence length and exceeds that distance. It should be noted that it can mean falling to nothing. However, the typical coherence function is Lorenzian, which is a statistical distribution with a slow tail and a long tail. A coherence “cut-off” point can be defined as a point where the laser light is 50% coherent, or a point where the resulting interference fringes have a contrast of 50%, but the degree of coherence changes only slowly over a wide range. Although a single diagram for coherence length may be somewhat misleading, it should be understood that such a diagram is provided to quantify coherence length / time. Thus, in describing OFDR operation, the numerical values used can be approximated to a degree. However, as noted herein, such an approximation does not affect the use or understanding of such coherence length or time.

  As described above, any physical point along the fiber length is identified at a particular frequency after self-heterodyne detection. This characteristic frequency is proportional to the distance, but it is also proportional to the rate of change of the lasing light frequency. In order to maintain high resolution, exemplary embodiments of the present disclosure provide that the frequency detected from that point is constant. In the above example, it is desirable to resolve the reflection at a point about 1 km away to 1 cm (1 / 100,000), and linearity of the optical frequency sweep speed of 1 / 100,000 is required. However, a wavelength tunable laser cannot be easily tuned with such tuning linearity.

  As stated, the measurement distance can be limited by laser coherence. There are three main technologies that overcome this limitation. That is, 1) a technique using a high coherence wavelength tunable laser light source as described above, 2) U.S. Pat. 7,515,276 (hereinafter referred to as “Frogat”), or a technique by compensating the raw measurement data of laser non-coherence before interpreting the result, or 3) In accordance with the solution of the present disclosure described in detail, a technique by correcting for laser incoherence in a laser source.

  Technology 1) is a viable solution. There are several high quality tunable laser sources that can produce similar high quality measurement results over a multi-kilometer range. However, these lasers are often micromechanical structures that include laser devices combined with MEMS components and mirrors, and are generally expensive to manufacture. In contrast, medium quality common DFB semiconductor lasers are manufactured at very low cost as light sources for telecommunications systems. DFB lasers have a coherence length that allows OFDR measurements in a very limited span, more typically about 1 m. This is often not a useful range.

  The Frogat technique 2) has been used as a means to enable inexpensive DFB lasers to be used when making long length measurements. In principle, the Frogat technique involves measuring the laser phase variation over time and electronically recording the measurements, thereby compensating for the measurements at the detector at some point in the past past. There is a history record of the actual laser phase (or approximation) that can be used. Thus, Frogat observes the phase of the DFB laser with an interferometer, and then uses that information (digitally delayed) to generate a historical record of phase / frequency gradual changes, then Based on past history records, it has been proposed to empirically compensate for OFDR measurements from the fiber region beyond the laser coherence length. The delay used for the laser observation data is equal to the delay to the compensated region as a correction term in the actual reflectometer measurements at distances beyond the laser coherence length. However, the compensation is only applied to a small window size around the compensated delay. To see the entire fiber length, one needs to make many partial readings.

  The present disclosure provides a new technique for observing gradual changes in laser phase and immediately using this information in a feedback loop to correct for laser phase variations, thereby increasing laser coherence length / time. . By correcting the laser operation with the laser light source, the collected measurement data can be applied over the entire measurement length, as in a system using a highly coherent tunable laser light source according to the technique of 1) above. The present disclosure provides that the laser power is observed using an interferometer and feedback correction is applied to the laser light source to correct the optical frequency of the laser light source. Thus, the present disclosure uses a phase / frequency controlled feedback mechanism based on the lasing frequency observed with an interferometer to produce a corrected signal that works for all points in a long fiber in a single measurement. To do. High feedback correction gains are possible by using an optimized feedback phase response to correct for laser variations. The same feedback mechanism also provides the point at which the wavelength control signal that is used to provide the optical frequency sweep necessary to use the DFB laser as the OFDR light source is injected.

FIG. 1A is a block diagram illustrating a configuration of a laser light source 110 and a measurement unit 120 (for example, an interferometer). Although the laser light source 110 has a target frequency, as described above, the laser light source does not always output laser light at this target frequency. The measurement unit 120 is configured to measure the difference between the actual frequency output by the laser light source 110 at the current moment and the target frequency of the laser light source 110.

  An example of laser power interferometry for observing (eg, detecting) the optical phase / frequency of the laser light source 110 will now show how this observation provides real-time feedback control for the laser light source 110. This will be described with reference to FIG. 1A.

  The instantaneous laser phase and / or optical frequency can be observed using an interferometer on some optical output from the laser source 110. For example, in the Mach-Zehnder interferometer of the measurement unit 120, the laser light is divided into two paths (short path and long path) and then recombined. When the light is recombined, interference occurs, which can be constructive or destructive. According to an exemplary embodiment, the interferometer can be constructed from a fiber and a 2 × 2 fiber coupler. The first 2 × 2 coupler splits the laser light into two fiber paths. One input of the second 2 × 2 coupler is fused directly to one output of the first coupler. Along with that, the other input / output pair is connected through an additional length of fiber. One or both of the outputs of the second 2 × 2 coupler may be connected to a photodetector for observing the interference result. If the path length difference from one input to the output of this interferometer is the exact number of wavelengths, the interference at that output will be constructive and the optical output will be the maximum value (approximately equal to the total input light level). Become. On the other hand, if the path length difference from one input to the output of the interferometer is a half wavelength difference in wavelength (if these are 2 × 2 couplers with an equal division ratio, this is the case. It may not be obvious, but it occurs with those couplers.) The second output level is close to zero. If the wavelength is changed and the length of the first path is changed to be half a wavelength longer than the above, the first output becomes zero (cancellation interference) and the second output becomes the maximum value (construction). Interference). In the point between the above two conditions, the intensity at either output is a function of the oscillation wavelength, and if the fiber length difference is multiple wavelengths of light, the output intensity is a very sensitive function of optical frequency. Can be. It is also taken into account that on a shorter time scale (shorter than the propagation delay through the path difference), the output intensity is a direct function of the instantaneous change in laser phase. This is because light from longer paths has not yet experienced that phase change.

FIG. 1B shows a block diagram of an alternative embodiment. There, the measurement unit 130 includes a recirculating interferometer instead of the Mach-Zehnder interferometer shown in FIG. 1A. Nevertheless, the measurement units 120 and 130 in FIGS. 1A and 1B each measure the difference between the frequency output by the laser source 110 at the current moment and the target frequency of the laser source 110. It is configured.

FIG. 2 illustrates an exemplary embodiment of an apparatus 200 that includes a laser. The optical frequency of the laser is stabilized, and thus the coherence length of the laser is increased according to the present disclosure. The apparatus 200 uses the interferometer 120 of FIG. 1A to observe the oscillation frequency of the laser light source 110. The interference measurement is fed to a phase / frequency control feedback mechanism for controlling the frequency / wavelength of the laser light output by the laser light source 110 in real time. Reference numeral 210 in FIG. 2 indicates a feedback control unit. The feedback control unit maintains a constant frequency of the laser output from the laser light source 110 based on the difference measured by the interferometer 120 between the actual frequency of the laser light source 100 and the target frequency. The laser light source 110 is controlled in real time. Thereby, the frequency of the laser light emitted from the laser light source 110 is adjusted to the target frequency. The output of the interferometer 120 can be considered an “error signal”. This is because the output represents the difference between the actual frequency of the laser light source 110 and the target frequency. Based on the difference between the actual frequency measured at the interferometer and the target frequency, the feedback control unit 210 applies feedback correction in real time to the laser source 110 to adjust the laser frequency to the target frequency and Correction, thereby obtaining a highly coherent lasing frequency output of the laser light source 110.

As described above, the time variation of the laser phase / frequency measured by the interferometer is based on a reflected (delayed) signal that is self-heterodyne with laser light generated at different instants. According to an exemplary embodiment of the present disclosure, real-time measurement of the laser frequency is used immediately in the feedback loop to instantly estimate the frequency error between the actual frequency output by the laser source 110 and the target frequency. correcting (e.g., by an infinite impulse response (IIR) filter shown in Figure 2).

  This is achieved by providing two design features. First, the optical frequency detection and feedback loop must cause a minimum delay that is small compared to the required correction bandwidth. This means that the short fiber length used in the interferometer and feedback detector (s) (if fiber is used, other integrated optical implementations to keep the waveguide length small) And high speed, low delay electronic components used in the feedback loop. Second, means are provided for applying correction tuning to a laser light source 110 having sufficient bandwidth to correct for frequency errors as they occur. DFB lasers are typically tuned by temperature changes using, for example, an attached Peltier cooler, which has a time response on the order of seconds (s) and is used in our feedback design Too late for. Also, a DFB laser can be tuned slightly by the current flowing through the laser (current changes also modulate the light intensity from the laser). This is the mechanism applied in this disclosure. Here, the main effect is a temperature change from laser current heating, but a more rapid change is in the number of electrons in the laser cavity, which affects the refractive index and hence the oscillation wavelength. The present disclosure takes advantage of available mechanisms to close the feedback loop and minimize disruption to the laser phase (otherwise resulting in a low coherence length). Although some short-term (nanosecond scale) laser phase noise is acceptable, the feedback loop quickly restores the laser frequency to maintain coherence on a time scale of a few microseconds to tens of microseconds. A laser for OFDR equipment is suitable for this coherence characteristic.

  Tuning linearity has been found to be important for high resolution instruments. Most tunable lasers (including the “high quality” devices described above) have moderate tuning linearity but are nevertheless insufficient for many OFDR measurements. The laser is tuned by adjusting the temperature, adjusting the angles of the MEMS mirror and the diffraction grating, and so on. These “open loop” mechanisms have some non-linearity. The solution of the present disclosure is to apply lasing frequency control within the feedback loop described above.

  As described above, the feedback stabilization technique of the present disclosure is effective in stabilizing the phase at only one fixed frequency. Frequency tuning can be added to this by changing the optical phase (the feedback loop locks to that optical phase). For example, a quadrature feedback loop with digital phase rotation can be provided for fine tuning.

  One means of realizing this frequency control using a feedback loop involves tuning by drawing a long fiber of the interferometer. It works as follows. The feedback loop is satisfied by a certain voltage at the output of the feedback photodetector. This voltage is generated by a constant phase relationship between the light in the short and long arms of the interferometer. When the fiber is drawn, to maintain this phase relationship, the wavelength is increased so that the same number of wavelengths fits in the new optical fiber length and the same phase relationship at the output coupler. If this is not achieved, the output voltage of the detector changes and the laser current changes to pull the oscillation frequency, so that the state is restored. Thereby, the laser wavelength is adjusted to satisfy the new long fiber length. The fiber can be stretched by the small amount necessary for tuning by winding the fiber tightly onto, for example, a Piezo cylinder and changing the dimensions of the cylinder with applied voltage. The laser tuning range is limited by the current tuning range that can be achieved with the laser light source 110 used. The degree of stretching required to reach this limit is quite small compared to the tension that can be tolerated by the fiber. This wavelength tuning mechanism was demonstrated in the model shown in FIG. FIG. 3 shows a MATLAB-simulated (Simulink) electro-optic model of a wavelength-managed DFB laser with a test fiber interferometer for laser source interrogation.

  According to an exemplary embodiment, another technique for tuning includes rotating the phase of the feedback lock state. The quadrature interferometer output can be achieved by various means, for example by using equally divided 4 × 4 couplers for the output coupler of the interferometer. According to an exemplary embodiment, the coupler can have four outputs representing a combination of two optical paths in four quadrature relationships. Orthogonal pairs of these can have a 90 degree relationship in their output intensity. By detecting both, an orthogonal coordinate representation of the optical phase relationship in the interferometer is obtained. The outputs of these detectors can be summed at an appropriate ratio (eg, rectangular to polar mapping) to satisfy any phase relationship when the feedback loop is locked. By rotating the mapping from the Cartesian coordinates to the polar coordinates, the locked optical phase can be rotated and the laser wavelength is changed to satisfy the lock condition. The rotation can be made through 360 degrees or more. Each full rotation adds another wavelength to the asymmetric length of the interferometer while ramping the lasing wavelength as the rotation proceeds. This phase rotation and laser tuning can continue until the limit of the laser current tuning range is reached.

  Therefore, the feedback control unit 210 is configured to control the laser light source 110 so as to maintain a linear change speed of the frequency of the laser light output from the laser light source 110. The mapping of the at least two quadrature interferometer output signals is added to the interferometer phase so that the feedback loop can be locked in any optical phase relationship within the interferometer 120. As a result, the mapping can be rotated to “wind” the light cycle / wavelength into or out of the interferometer 120. The feedback loop of feedback control unit 210 adjusts the laser frequency to accommodate these changes to maintain feedback lock. As a result, the laser frequency is accurately swept.

  Each wavelength added to the asymmetric length of the interferometer represents an exact change in lasing wavelength and frequency. Thus, the tuning is very accurate, as represented by a count of wavelengths “wound” into or out of the interferometer. This can be realized as a digital and accurate tuning mechanism. Interpolation between these integer wavelength counts, controlled by Cartesian to polar mapping, is accomplished using a multiplying DAC (Digital to Analog Converter) and is digitally controlled. While this is naturally not as digitally accurate as counting wavelengths, it can be designed to be accurate and the digital coefficients can be easily compensated to increase accuracy. The overall result of this technique is a laser that can be tuned with very high digital accuracy sufficient for many OFDR applications without requiring additional signal processing or compensation methods (within that laser's current tuning range). so). By achieving this, the combination of laser and feedback mechanism can be used as a high coherence laser source (eg, as a black box). The laser source can be used to directly make longer range OFDR measurements without further compensation, yielding results that are valid over the entire length of the measurement in each measurement. Thus, the rest of the design to complete the OFDR can be very simple, simple and low cost.

  As described above, the orthogonal information can be generated from an interferometer using a 4 × 4 coupler. Similar results can be obtained by other means. For example, a 3x3 coupler provides an approximation to a quadrature output, but for a 4x4 coupler it would include an altered mapping between its output and the phase to be locked. In the integrated light reflectometer, the 4 × 4 coupler may be composed of other elements. For example, it can be constructed from 2 × 2 elements.

  It may be desirable to achieve a wider tuning range than can be achieved by current tuning alone. In an exemplary embodiment, temperature tuning may be added within the feedback loop. Temperature tuning is not sufficiently linear in nature to tune the OFDR laser wavelength. However, if that temperature tuning is added to the laser in the wavelength control feedback loop as described above, the feedback loop control (current tuning) can generate the correction terms needed to linearize the laser tuning. In the wider tuning range with temperature tuning, the same high degree of linearity can be achieved (as in the pure current tuning mechanism above).

  One limitation of temperature tuning is the relatively slow response of temperature changes in response to heating and cooling inputs. According to an exemplary embodiment, magnetic refrigeration can be used to change the temperature of the laser chip more quickly. Magnetic refrigeration is a phenomenon in which the temperature of a material can be changed by the application of a magnetic field. When the magnetic field is removed, the temperature returns to its previous value. There is no heat generated or removed and the process is adiabatic. This means an “instantaneous” temperature change due to an instantaneous change in magnetic field strength. By attaching a feedback stabilized laser to a piece of magnetic refrigeration material (eg, gadolinium (Gd) or an alloy thereof), the laser temperature is not instantaneous (to change the temperature of the laser chip, the magnetic refrigeration material Because the thermal energy still needs to be exchanged between the laser chip and the laser chip).

  An exemplary embodiment of the present disclosure includes a temperature adjustment component that includes a magnetic refrigeration material for adjusting the temperature of the laser light source 110 in a mechanism for temperature tuning the laser light source 110. The temperature adjusting component can rapidly tune the temperature of the laser light source 110 independently or in conjunction with the feedback control unit 210. Accordingly, an exemplary embodiment of the present disclosure comprises a laser light source (eg, laser light source 110 of FIGS. 1A and 1B) and a temperature adjustment component that includes a magnetic refrigeration material that adjusts the temperature of the laser light source. Thus, an apparatus for controlling the frequency of laser light output from the laser light source is provided.

  Exemplary embodiments of the present disclosure have been described as being configured to output laser light into an optical component such as an FO facility. However, the present disclosure is not limited thereto. The embodiments of the present disclosure can also be applied to free-standing or stand-off implementations in which laser light is transmitted to a region of interest through any medium such as air, for example. It is. As used herein, an optical component can be any one or more of optical fiber, optical coupler, optical connector, optical switch, optical integrated optical waveguide, liquid, air, and free space.

  An exemplary embodiment of a fiber structure has been described as including, for example, a 2 × 2 coupler, as described above. The present disclosure is not limited to this. For example, an integrated optical solution can be provided in which the laser, coupler and detector are integrated on a small substrate.

  Accordingly, exemplary embodiments of the present disclosure combine an electro-optic controller with a standard DFB laser source. The electro-optic controller has a defined coherence that can be used to perform reflectance measurements over long fiber lengths (up to several kilometers, eg up to 5 kilometers), with a single measurement, with a resolution of a few millimeters. In order to manage the wavelength so as to facilitate the optical frequency sweep. The laser control configuration of the present disclosure is modeled using a modeling system. The modeling system showed proven accuracy in other optics with the expected results in FIG.

  The reflectometer technology of the present disclosure that combines an electro-optic controller with a laser light source such as DFB manages the output wavelength, thereby providing reflectivity over a long fiber length, eg, 1 kilometer or more, with a resolution of a few millimeters Facilitates optical frequency sweeping with defined coherence that can be used to make meter measurements. Accordingly, the present disclosure uses wavelength modeling with a very long coherence time and wavelength tuning range from conventional telecommunications grade laser sources using optical modeling techniques to evaluate and design DFB linewidth stabilization. Enable design. Accordingly, exemplary embodiments of the present disclosure provide a laser light source configured to output laser light having a coherence length of at least about a hundred meters.

  This new reflectometer technology is a mobile, self-contained FO platform for neighborhood / office / government FO networks (eg, WAN, LAN, etc.) and also includes vehicles such as airplanes, ships and land , And as a hand-held / integrated measurement / demodulation tool for FO sensing. Many of these FO platforms extend at most several hundred meters (<1 km). The optical interrogation system should position the optical interference accurately (within <1 cm) to maintain the overall FO system integrity and perform the necessary repairs (not possible with current technology).

  US government agencies use FO technology for a wide range of applications, for example, air, sea, ground and space. FO applications can often be highly specialized with very specific projects and range conditions, requiring rigorous testing and harsh environment certification to ensure reliability and performance in the field Sometimes. For example, aircraft is a rapidly growing FO application. Used as a communication link between ground control and antenna controlling UAV, FO provides a very fast and efficient means for transmitting a wide range of data over long distances. Typically, an aircraft FO links aircraft positioning / control and information / data transmission over a long distance with high bandwidth and high speed. For ground vehicles and automotive applications, FO is of course the ideal choice for lighting, communication and sensing requirements. Finally, for ship systems, FO systems are for mission-critical on-board communication systems and ship-to-land links to provide data, telephone and other services to docked ships. Has been deployed. These connections allow high speed, high bandwidth communication to and from the ship without using an onboard radio transmission / reception system.

  The present disclosure also provides a call for coherent or interference measurements, such as a distributed sensor system using a fiber Bragg grating (FBG), an external Fabry-Perot interferometer (EFPI), or other transducer. Works as a hanging system. In a typical distributed fiber sensor system, it is first necessary to provide means for measuring temperature, pressure, stress / strain and vibration in the optical fiber, and secondly, several measurements on a single fiber. It is also necessary to provide a means for multiplexing. For example, a wavelength-managed DFB laser used with an FBG sensor can meet both requirements.

  First, the FBG can be designed to reflect light at a wavelength within the laser sweep range. The reflection wavelength is determined by the grating period of the FBG. Due to strain or temperature changes, the fiber dimensions may change and / or the core refractive index may change. It changes the reflection wavelength, as shown in FIG. 4 (showing the reflection intensity of the FBG for wavelengths at three different temperatures). By measuring the reflected peak wavelength from the fiber, the strain or temperature in the FBG can be determined. This process requires a tunable laser source, but does not necessarily require high source coherence.

  If the light source wavelength is swept rapidly, the wavelength between the current laser and any FBG reflected signal having a wavelength seen by the light source before the round trip delay time to the FBG as shown in FIG. It may be possible to determine the difference (FIG. 5 shows the signals returned from the three FBGs and the reference signal from the wideband reflector). This wavelength difference may be a fraction of the laser wavelength, but can still reach kilohertz or megahertz with respect to frequency. This frequency range can be easily captured and processed electronically. The frequency difference can be detected using coherent light detection (eg, by mixing two optical signals on a photodetector) to produce a beat frequency. Each of the FBGs located at different points in the fiber has a unique round trip distance and therefore has the unique beat frequency shown in FIG. 6 (FIG. 6 shows that the laser light frequency is swept over time). It is shown that.) Delayed reflections from distant FBGs indicate earlier optical frequencies that are offset from the optical frequencies of nearby reference reflectors. This system requires a coherent light source to facilitate coherent detection. Here, the coherence may be at least as long as the round trip distance in the fiber for the farthest FBG (although not necessarily).

  While FBG is used above as an example of a transducer in a fiber, there are other possible transducers including EFPI, and even the inherent backscattering characteristics of the fiber itself. The present disclosure obtains the best performance considering the expected laser tuning range and speed, the sensitivity factor of the particular transducer, the measurement range and sensitivity of the desired system, and the desired measurement spatial resolution and total length To support a number of optimizations to be made.

  In sensing applications for the energy market, there is a desire to provide high resolution strain, temperature, vibration and pressure for the energy generation and distribution industry. Fiber optic sensing systems have been developed to address these markets because of the high level of accuracy and the ability to operate in harsh environments at high temperatures and pressures. In addition, transformers (which are the largest capital investment of power companies) have traditionally been managed manually, retroactively and inefficiently. Next-generation sensors deployed in substations prevent utilities from causing unplanned failures, reduce maintenance costs, and extend useful transformer life. Wind energy sensing products have been developed to enable observation and status tracking of wind power generation systems. These new sensing applications are addressed by the FO sensing system. The FO sensing system observes the pitch to maximize generator output and blade, drivetrain and tower load reduction while providing cost saving benefits to the turbine manufacturer. For health management of mobile platforms, aircraft, ships, spacecraft, etc., temperature and other physical parameters, as well as strain, are measured and observed. It will provide a safer and more efficient transportation system for the future.

  According to the present disclosure, an integrated light reflectometer manages the output wavelength of a laser light source, thereby enabling long fiber lengths of up to about 5 kilometers in a single measurement with a resolution of a few millimeters. Thus, it will facilitate optical frequency sweeps with a defined coherence that can be used to make reflectometer measurements. The laser control configuration is modeled using a highly specialized modeling system. The modeling system showed proven accuracy in other optical systems. Using these unique tools, the present disclosure improves upon past DFB linewidth stabilization and wavelength tuning designs to achieve very long coherence times and small (but sufficient) from normal communication grade laser sources. ) Get the wavelength tuning range.

  FIG. 7 shows a simulation plot of the FO test network device in FIG. 3, including (from top to bottom) cavity electron number, cavity intensity, cavity optical phase, control interferometer output field, and measured interferometer output field. Some important parameters are shown. Although not shown here, electrical circuit waveforms (eg, laser and detector currents and voltages) will also be observed.

  For the integrated light reflectometer, an efficient light source is used that obtains the best performance from a communication grade DFB laser. Therefore, a figure of merit for comparing the light sources is performed to fine tune the device. One such quantitative figure of merit is the laser linewidth. We can measure this with post-processing data taken from the above model. In that model, we can read the optical phase directly and thus derive a Fast Fourier Transform (FFT) from this information.

8, the device performance numerically quantified, for comparison, shows an example of the post-processing data from the above-described test device. In this case, the laser cavity phase FFT provides a measure of the laser linewidth, which is reduced to an acceptable value. Data manipulation is well-satisfied in Simulink and MATLAB environments. There, the key to efficient development is an accurate model that has been validated for practical and measurable cases.

  As described above, it is not easy to directly measure the optical phase with a physical light source, and therefore, the laser line width cannot be directly measured. Instead, heterodyne with its own delayed copy of the laser light, ie self-heterodyne, is used to measure the phase noise. If this is done well with a very long delay, well beyond the coherence length of the light source, the measured line width appears to be the product of two different light sources with the same line width; One of the line widths can be easily evaluated. However, if the coherence is quite high, this also requires a very long fiber delay, since the typical Lorentz-shaped coherence distribution has a very long tail. Instead, we can use a shorter interferometer that causes the light to interfere with a slightly delayed version of the light itself, thereby obtaining a measurement of phase noise. Fortunately, this is a principle that can be accurately modeled and implemented in practice.

  FIG. 9 shows a self-heterodyne configuration using an asymmetric Mach-Zehnder interferometer. When there is a possibility of polarization rotation in the fiber, the polarization matching of the second optical coupler needs to be ensured in practice, so that it is shown somewhat simplified. In that model, it is easy to ensure this polarization matching sufficiently by design.

  The integrated optical reflectometer of the present disclosure provides a low cost instrument platform that includes a reflectometer core module for FO equipment. The platform can be incorporated into a handheld device with packaging and display for installation and maintenance personnel. The platform can also be used as an integrated part of the FO network continuous observation system. This disclosure provides network operators with the data they need in summary form to facilitate problem reporting, and more details to provide maintenance personnel with specific information needed to solve the problem. A simple report. With the high resolution of the present disclosure, they can identify problems within individual connectors and optics. A very low cost, integrated light reflectometer can be used in research and education networks (REN), small / home offices (SOHO), LANs and multiple locations to ensure network observation and reliability. It can be integrated across certain buildings and other markets. The present disclosure provides for the first time equipment required in terms of cost. There, these devices are newly designed and can be retrofitted to existing networks.

  Many of these new FO network platforms are developed and serviced by small and medium sized technology companies with limited resources and capabilities. In addition to larger customers, SMEs (SMEs) face unique challenges when coming to FO network testing. Existing test equipment in the optical communications market focuses on serving large enterprises, while these small and medium businesses are not served. The present disclosure, as well as large customers, provides FO test solution options at affordable prices that are available to this unserviced customer.

  The present disclosure meets several market needs. For example, the present disclosure provides for the development of inexpensive measuring equipment with low manufacturing cost per equipment, high resolution (km resolution, <1 cm resolution). The present disclosure also provides a very flexible and configurable optical network interrogation platform. The platform eliminates the overwhelming myriad of test equipment options and is compatible with all currently available FO sensing elements (EFPI, FBG, LPG, etalon, dispersion, etc.). Furthermore, the present disclosure provides either a handheld configuration or a simple optoelectronic card that can be incorporated into a network to observe faults and performance as well as a sensing system. Internet downloadable software upgrades can be used to keep the device at the forefront of performance.

  The present disclosure also provides a method for performing the operational function of the integrated light reflectometer described herein, integrated light reflection in an OFDR device for measuring fiber properties. Provided with the use of a rate meter.

It will be appreciated by persons skilled in the art that the present invention may be implemented in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed herein are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes, ranges, and equivalent ranges that fall within the meaning are intended to be embraced therein.
The matters described in the following items 1 to 23 are within the disclosure scope at the beginning of the application of the present application.
[Item 1]
A laser light source configured to output laser light at a target frequency;
A measuring unit for measuring a deviation between an actual frequency output by the laser light source in a current period and a target frequency of the laser light source;
A feedback control unit configured to control the laser light source to maintain a constant frequency of laser output from the laser light source based on the measured deviation between the actual frequency and a target frequency; By which the frequency of the laser light transmitted from the laser light source is adjusted to the target frequency.
[Item 2]
In the apparatus according to item 1,
The feedback control unit is configured to maintain a greater linear change rate of the frequency of the laser light output from the laser light source than when the laser light source does not have a controlled target frequency. A device that is configured to control.
[Item 3]
In the apparatus according to item 1,
The feedback control unit includes an infinite impulse response filter configured to compensate for the characteristics of the measurement unit used for frequency measurement.
[Item 4]
In the apparatus according to item 3,
The feedback control unit adds a mapping of at least two quadrature output signals of the measurement unit to the phase of the measurement unit, so that feedback control of the laser light source by the feedback control unit is optional in the measurement unit. A device that is configured to be locked in an optical phase relationship.
[Item 5]
An OFDR device,
Comprising the apparatus according to item 2,
The OFDR apparatus outputs laser light from the laser light source to a point of the optical component to be tested that is spaced from the laser light source, and the instantaneous frequency of the output laser light and the laser light reflected from the point Configured to measure the frequency difference between
The feedback control unit maintains a constant difference frequency between the frequency of the laser light output from the laser light source and the instantaneous frequency reflected from the point, whereby the difference frequency is An OFDR device configured to characterize the reflectance profile of the optical component in direct proportion to the distance to a point, and wherein the amplitude of the reflected laser light is proportional to the reflectance at the point.
[Item 6]
In the OFDR device according to item 5,
The optical component is an OFDR device that is at least one of an optical fiber, an optical coupler, an optical connector, an optical switch, an optical integrated optical waveguide, liquid, air, and free space.
[Item 7]
In the OFDR device according to item 6,
The OFDR apparatus, wherein the reflectance profile of the optical fiber is measurable with a length exceeding the reflectance profile measurable by the laser light source not controlled by the feedback control unit.
[Item 8]
In the OFDR device according to item 6,
The OFDR apparatus, wherein the reflectance profile of the optical fiber can be measured in one measurement at a distance greater than 1 meter.
[Item 9]
In the apparatus according to item 1,
The feedback control unit is configured to stabilize the actual frequency range of the laser light source to the target frequency.
[Item 10]
In the apparatus according to item 1,
A temperature control component including a magnetic refrigeration material attached to the laser light source;
The temperature adjusting component is an apparatus configured to adjust the temperature of the laser light source controlled by the feedback control unit.
[Item 11]
Measuring the deviation between the actual frequency output by the laser source in the current period and the target frequency of the laser source,
Based on the measured deviation between the actual frequency and the target frequency, the laser light source is controlled to maintain a constant frequency of the laser output from the laser light source, and thereby from the laser light source. A method in which the frequency of the transmitted laser beam is adjusted to the target frequency.
[Item 12]
In the method according to item 11,
A method of controlling the laser light source so as to maintain a linear change speed of the frequency of the laser light output from the laser light source.
[Item 13]
In the method according to item 11,
A method of compensating control of the laser light source through an infinite impulse response filter so as to compensate for the characteristics of the measurement used for frequency measurement of the laser light source.
[Item 14]
In the method according to item 13,
A mapping of at least two quadrature output signals of the measurement unit for measuring the deviation is added to the phase of the measurement unit, so that the feedback control of the laser light source is in any optical phase relationship within the measurement unit. How to be locked.
[Item 15]
In the method according to item 12,
In the OFDR device, output laser light from the laser light source to the point of the optical component to be tested that is spaced from the laser light source,
Measure the frequency difference between the output laser light and the instantaneous frequency of the laser light reflected from the point,
Maintaining a constant difference frequency between the frequency of the laser light output from the laser light source and the instantaneous frequency reflected from the point, so that the difference frequency is directly at the distance from the laser to the point. A method of characterizing the reflectance profile of the optical component in proportion and with the amplitude of the reflected laser light being proportional to the reflectance at the point.
[Item 16]
In the method according to item 15,
The method in which the optical component is at least one of an optical fiber, an optical coupler, an optical connector, an optical switch, an optical integrated optical waveguide, liquid, air, and free space.
[Item 17]
In the method according to item 16,
The method wherein the reflectance profile of the optical fiber is measurable at a length that exceeds the reflectance profile measurable by the laser light source where the target frequency is not controlled.
[Item 18]
In the method according to item 16,
The method wherein the reflectance profile of the optical fiber is measurable in a single measurement at a distance greater than 1 meter.
[Item 19]
In the method according to item 11,
A method of stabilizing an actual frequency range of the laser light source at the target frequency.
[Item 20]
In the method according to item 11,
Controlling the laser light source
Adjusting the temperature of the laser light source with a magnetic refrigeration material.
[Item 21]
A laser light source configured to output laser light;
A temperature control component including a magnetic refrigeration material attached to the laser light source;
The temperature adjusting component is an apparatus configured to control the frequency of the laser light output from the laser light source by adjusting the temperature of the laser light source.
[Item 22]
Adjusting the temperature of a laser light source configured to output laser light by attaching a magnetic refrigeration material to the laser;
A method for controlling the frequency of the laser light output from the laser light source based on the adjusted temperature of the laser light source.
[Item 23]
A laser light source configured to output laser light having a coherence length of at least about 100 meters.

Claims (10)

A laser light source configured to output laser light at a target frequency;
A measurement unit for deriving a measured difference between an actual frequency of the laser light source at a certain moment and a target frequency of the laser light source;
The measurement unit further comprises:
Comprising first and second laser paths for splitting the laser light output by the laser light source, the first and second laser paths having different lengths;
Equipped with a detector to observe the interference pattern,
When the laser light traveling along the first laser path is recombined with the laser light traveling along the second laser path, the interference pattern is generated,
The measuring unit, based on the change of the laser phase represented by the interference pattern that is the observed derives the measured difference between the actual frequency and the target frequency,
A feedback control unit configured to control the laser light source to maintain a constant frequency of laser output from the laser light source based on the measured difference between the actual frequency and the target frequency. the provided, whereby equipment the frequency of the laser beam transmitted from the laser light source is Ru is adjusted to the target frequency of the laser light source. The apparatus of claim 1.
The feedback control unit includes an infinite impulse response filter configured to compensate for the characteristics of the measurement unit used for frequency measurement. An OFDR device,
Comprising the apparatus of claim 1,
The OFDR apparatus outputs laser light from the laser light source to a point of the optical component to be tested that is spaced from the laser light source, and the instantaneous frequency of the output laser light and the laser light reflected from the point Configured to measure the frequency difference between
The feedback control unit maintains a constant frequency difference between the frequency of the laser light output from the laser light source and the instantaneous frequency reflected from the point, whereby the frequency difference is An OFDR device configured to characterize the reflectance profile of the optical component in direct proportion to the distance to a point, and wherein the amplitude of the reflected laser light is proportional to the reflectance at the point. The OFDR device according to claim 3,
The optical component is an OFDR device that is at least one of an optical fiber, an optical coupler, an optical connector, an optical switch, an optical integrated optical waveguide, liquid, air, and free space. The OFDR device according to claim 4,
The OFDR apparatus, wherein the reflectance profile of the optical fiber is measurable with a length exceeding the reflectance profile measurable by the laser light source not controlled by the feedback control unit. The OFDR device according to claim 4,
The OFDR apparatus, wherein the reflectance profile of the optical fiber can be measured in one measurement at a distance greater than 1 meter. The apparatus of claim 1.
The feedback control unit is configured to stabilize the actual frequency range of the laser light source to the target frequency. The apparatus of claim 1.
The control of the laser light source includes adjusting the temperature of the laser light source with a magnetic refrigeration material. The apparatus of claim 1.
The control of the laser light source includes adjusting the temperature of the laser light source by a current tuning mechanism. The apparatus of claim 2.
The feedback control unit adds a mapping of at least two quadrature output signals of the measurement unit to the phase of the measurement unit, so that feedback control of the laser light source by the feedback control unit is optional in the measurement unit. A device that is configured to be locked in an optical phase relationship.

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