Application of OFDR Optical Frequency Domain Reflectometry in Optical Fiber Communication

The development of optical fiber communication plays an important role in my country's economic construction. Optical fiber communication has incomparable advantages: broadband transmission frequency, less loss and consumption. The construction of optical fiber communication started in the 1990s and has been developed on a large scale.

As a transmission network carrying a large amount of information, optical fiber communication has certain risks and instability. In order to ensure the smooth operation and safety of optical fiber communication, it is necessary to develop a tool or instrument that can accurately measure the characteristics of optical fiber communication. Optical frequency domain reflection can accurately detect the characteristics of optical fiber communication. Optical frequency domain reflection mainly analyzes the time difference and optical path difference of the scattered light of the optical fiber to detect the optical fiber communication.

Optical Fiber Communication

Introduction to the principle of OFDR optical frequency domain reflectometry

1. Scattering in the fiber

When light travels through an inhomogeneous medium, it travels in all directions. This is the scattering of light. For example, a clear sky appears blue, and sea water is blue. This is the result of the scattering of sunlight (shorter wavelengths of blue light are scattered by atmospheric particles) ). Similarly, when light is transmitted in an optical fiber, due to the uneven distribution of refractive index in the optical fiber, scattering will also occur, mainly in three forms: Rayleigh scattering, Brillouin scattering and Raman scattering.

Scattering is the result of light waves interacting with particles in the fiber medium. In Rayleigh scattering, after the incident light is scattered, the wavelength and frequency do not change, which is a kind of elastic scattering; in Brillouin scattering, the incident light interacts with the acoustic wave field in the fiber, and light with a frequency higher than the original incident light will appear. and light with frequencies lower than the original incident light. Raman scattering produces similar results, both being inelastic scattering.

At present, OTDR technology is mature, and it is mostly used for the diagnosis of integrated optical circuits and the detection of optical communication network faults. However, due to the limitation of the contradiction between the pulse width of detection light, spatial resolution and dynamic range, it is difficult to meet the requirements of large dynamic range and high space at the same time. resolution, not suitable for high-precision measurement. In the field of temperature and strain sensing, BOTDR, BOTDA and BOFDA technologies based on Brillouin scattering are mostly used. Among them, BOFDA technology can achieve a spatial resolution of up to 2 cm, but the entire test system is very complicated and the measurement time is long.

OFDR technology is a technology that uses swept-frequency light source coherent detection technology to detect optical signals in optical fibers. Because it is not limited by the contradiction between spatial resolution and dynamic range, it has high spatial resolution (optical measurement up to 10μm), large dynamic range, high test sensitivity, suitable for short-distance high-precision monitoring fields such as internal analysis of optical devices, civil engineering simulation tests, vehicle structure research, etc.

2. Optical coherence detection

The basic principle of optical coherent detection is basically the same as that of radio wave heterodyne detection, so it is also called optical heterodyne detection. It is a detection technology that uses the coherence of light to mix the probe light containing the measured signal and the reference light as a reference under certain conditions, and output the difference frequency signal of the two light waves.

Coherent detection is an indirect detection technology that converts high-frequency optical signals to intermediate-frequency signals that are easy to detect. It has the advantages of high conversion gain, strong detection capability, and high signal-to-noise ratio. It is widely used in optical communication and measurement fields.

3. OFDR (Optical Frequency Domain Reflectometry) Principle

OFDR (Optical Frequency Domain Reflectometry) is a back-reflection technology based on Rayleigh scattering in the optical fiber. The linear sweep light emitted by the light source is divided into two paths by the coupler, and one path enters the fiber to be tested, and the light is transmitted at each position of the fiber. The Rayleigh scattering signal is continuously generated, and the signal light is backward, and is coupled with another reference light to the detector for coherent mixing. Different positions of the fiber to be tested have different optical frequencies, and the frequency difference between the signal light and the reference light is also different.

The light intensity at each position in the fiber to be measured can be obtained by frequency measurement. The frequency corresponds to the position of the fiber, and the light intensity corresponds to the reflectivity and return loss at this position.

When light travels forward in an optical fiber, when defects occur in the optical fiber and losses occur, the Rayleigh scattering signals generated at different positions carry the loss information. The frequency detection of the Rayleigh scattered signal light can accurately locate the splices, bends, breakpoints, etc. along the optical fiber. OFDR technology realizes the diagnosis of optical fiber links through the above principles.

On the other hand, when the fiber to be tested is placed in the external temperature field or strain field, the fiber is affected by temperature or strain, and the refractive index distribution inside the fiber will change, and the frequency of the corresponding Rayleigh scattering signal light will also change. The frequency measurement of the Rayleigh scattering signal light can correspond to the change of the external temperature field or strain field. Thus, distributed optical fiber sensing is realized.

Development Status of OFDR

OFDR has three main applications: optical communication network diagnosis, integrated optical path diagnosis and tomography technology. The difference between these applications is that they have different requirements on the OFDR system. The technical difference is mainly in the modulation mode of the light source part.

When applied in chromatography technology, the measurement range is required to be several millimeters, and the measurement accuracy is several tens of microns.

In order to seek the commercialization of OFDR systems, many foreign research institutes have studied and discussed the OFDR systems using semiconductor lasers as light sources. They tried to use various methods to modulate the semiconductor laser light source in the frequency domain to meet the requirements of OFDR systems, such as current injection method, temperature modulation method, extra-cavity grating modulation method or extra-cavity electro-optic phase modulation method.

Integrated optical diagnostics require larger measurement ranges than chromatography techniques. Experts obtained an OFDR system with a resolution of 50 μm and a measurement range of 25 mm using the indium phosphide optical waveguide structure.

When modulating the light source, the change of injection current, residual amplitude modulation and nonlinear frequency chirp will degrade the resolution of the system. Using the frequency equalizer can linearize the frequency chirp, optimize the resolution of the system, make the resolution of the system reach 1mm, and make the measurement range reach 1m.

Diagnosis of an optical communication network requires the use of a light source with a wavelength of 1.3 μm or 1.55 μm, and the measurement range of the OFDR system must be much larger. Using the ND: YAG laser with a wavelength of 1.32 μm as the light source, a longer coherence length was obtained, which enabled the measurement range to reach 50 km, and the resolution in the experiment reached 380 m. The Er-Yb laser with a wavelength of 1.55cm was used as the light source, and an Er-doped fiber amplifier was used to obtain a resolution of 50m and a measurement range of 30km. With the increasing maturity of light source frequency modulation technology, the resolution of OFDR has been greatly improved. Using the SSB modulation technique, the resolution of cm order was successfully obtained when the range was greater than 5km.

Advantages of Optical Frequency Domain Reflectometry

The detection of optical communication network includes the diagnosis of integrated optical circuits and the detection of optical communication network failures. The former is generally only of the order of centimeters or even millimeters. The diagnosis of the latter generally uses a light source with a wavelength of 1.3 μm or 1.55 μm, and the range reaches the kilometer level. A large range requires a large dynamic range and high light source optical power. Obviously. The contradiction between OTDR resolution and dynamic range can not solve this problem well, while OFDR can meet it. It has the advantages of high sensitivity and high spatial resolution.

1. High sensitivity

Because the optical power of the reference light is relatively large, it can generally reach several tens of milliwatts. On the other hand, the power of the back-scattered light signal of the fiber is very small. It's only about -45dB of the incident light, which leads to the conclusion. The sensitivity of OFDR detection method is much higher than that of OTDR detection method. That is to say, under the condition of the same dynamic range, the optical power of the light source required by OFDR is much smaller.

2. High spatial resolution

Spatial resolution refers to the ability of a measurement system to distinguish between two adjacent measurement points on the fiber under test. High spatial resolution means that the distance between the measurement points that can be distinguished is short, that is, there are more information points that can be measured on the fiber, which can better reflect the characteristics of the entire fiber to be measured. In the OTDR system, the resolution is limited by the width of the probe light pulse. The narrower the probe light pulse width, the higher the resolution, the smaller the light pulse energy, and the smaller the signal-to-noise ratio.

The spatial resolution in an OFDR system can correspond to the ability to distinguish intermediate frequency signals corresponding to two adjacent measurement points of the fiber under test, and the ability to distinguish intermediate frequency signals is closely related to the receiver bandwidth of the spectrum analyzer used in the system. Obviously, the smaller the receiver bandwidth, the stronger the ability to distinguish two different frequency signals, the smaller the noise level introduced at the same time, and the improved signal-to-noise ratio. Therefore, the OFDR system can obtain high spatial resolution at the same time. large dynamic range.

Limiting factors and development status of OFDR

1. Limitations of light source phase noise and coherence

The above analysis assumes that the light source is monochromatic, but the actual signal source will produce a large phase noise and show it through a limited spectral width. The phase noise will reduce the spatial resolution and shorten the length of the optical fiber that can be reliably measured. That is, the data measured by the optical fiber after a certain length cannot accurately reflect the size of the scattered signal, so that the transmission characteristics of the optical fiber cannot be accurately analyzed.

2. The limitation of the nonlinearity of the light source sweep frequency

Due to the influence of temperature changes, device vibration, grid voltage fluctuations and other conditions, the actual lasers will cause changes in the position of the light source resonator, which will affect the changes of the output light spectrum, causing the nonlinearity of the sweep frequency, which will broaden the OFDR measurement. The range of the difference frequency signal in the system, which limits the size of the spatial resolution of OFDR.

3. Polarization limitation of light waves

Since the OFDR method adopts a coherent detection scheme, it is obvious that if the polarization directions of the signal light and the reference light on the photosensitive surface of the photodetector are orthogonal, the information of the optical fiber measurement point corresponding to the signal light will be lost. Therefore, the stability of the polarization of the light wave must be guaranteed

Application of Optical Frequency Domain Reflectometer (OFDR) in Military Equipment

In the above-mentioned shipboard high-speed optical fiber network, weapons and ammunition using optical fiber guidance, or local devices using optical fiber to transmit information, there are a large number of optical fiber connectors or optical fiber bending, etc., the network link structure is complex, and the number of optical devices is large; network work The environment is harsh, the temperature changes greatly, and the vibration shock is severe; the reliability detection of such networks is related to national security, and it is necessary to have a high fault resolution during maintenance and repair and be able to locate the inside of the device. OTDR technology obviously can not meet the above requirements, and OFDR has the ability to meet this application requirement. OFDR can effectively detect the reflection and loss characteristics of each optical device in the link, while OTDR is difficult to effectively detect the condition of optical devices in the link due to low distance resolution. It is shown that OFDR can effectively and accurately detect the faults of optical fibers and components in short- and medium-distance dedicated optical fiber networks.

Application of aerospace equipment

Manned spaceflight and large aircraft, as symbols of national scientific and technological strength, have developed rapidly, and my country has also included them in major special projects and major scientific projects in the medium and long-term scientific and technological development plan. The development of large aircraft and manned spaceflight will inevitably put forward new requirements for the transmission capacity, anti-interference ability and volume weight of its internal communication network. Optical fiber is known for its transmission bandwidth, anti-electromagnetic Unique advantages such as corrosion and no fire hazard make it the best technology choice to support this development need.

The link distance of the network is short (tens of meters to several kilometers), the structure is complex, and the number of optical devices is large, which requires the fault to be precisely located inside the device. Therefore, optical fiber link detection equipment with a positioning accuracy of the order of millimeters and a distance range of several kilometers is required. Optical time domain reflectometry (OTDR) obviously cannot meet the above measurement requirements, while OFDR has the ability to meet this application requirement. .

At present, the communication system of domestic military aircraft generally adopts the "1+N+1" mode, "1" represents the length of the multimode fiber in the switch chassis, and "N" represents the length of the optical cable between the two chassis.

Application of land military equipment

The long-range system of strategic and tactical communication in the military communication application on land, the local area network of communication between bases, etc. do not need to use high-resolution OFDR because of the long communication distance of optical cable.

Due to the inherent advantages of low transmission loss and frequency bandwidth of optical fiber, the application of optical fiber in radar system is first used to connect the radar antenna and the radar control center, so that the distance between the two can be expanded from within 300m of the original coaxial cable to 2 ~5km. Using optical fiber as the transmission medium, its frequency band can cover X-band (8-12.4GHz) or Ku-band (12.4-18GHZ). The application of optical fiber in microwave signal processing is mainly optical fiber delay line signal processing. Advanced high-resolution radars require low-loss, high-time-bandwidth product delay devices for signal processing. The traditional coaxial delay line, surface acoustic wave (SAW) delay line, charge coupled device (CCD), etc. can no longer meet the requirements. Optical fiber delay lines can not only meet the above requirements, but also can be packaged into a small package. Most of the delay lines used in phased array radar signal processing are multimode fibers.

In the above-mentioned short- and medium-distance applications, especially fiber-optic delay lines packaged in small boxes, only high-resolution OFDR can detect potential failures during maintenance.