1. What is optical frequency domain reflectometry?

Optical frequency domain reflectometry (OFDR) has attracted the interest of researchers because of its high-precision measurement in various ranges and its large dynamic range. The light source required by the OFDR system should be a linear swept narrow linewidth single longitudinal mode laser, so the requirements for the light source are very high, which also leads to the lack of domestic research on OFDR. Because OFDR can be applied to high-precision measurements in various ranges and has a large dynamic range, it has attracted the interest of many researchers.

The optical frequency domain reflectometer structure includes a linear swept frequency light source, a Michelson interferometer, a photodetector, and a spectrometer (or a signal processing unit), etc., based on optical heterodyne detection.

The continuous light, which is linearly swept with the frequency as the center, enters the Michelson interferometer structure through the coupler and is divided into two beams. One beam returns through the mirror, and its optical path is fixed, which is called the reference beam, and the other beam enters the fiber to be measured. Rayleigh scattering occurs due to the microscopic inhomogeneity of the refractive index of the fiber. Part of the backscattered light satisfies the numerical aperture of the fiber and returns to the injection end, which is called signal light. If the propagation length satisfies the light coherence condition, the signal light and the reference light will be mixed on the photosensitive surface of the photodetector. When the frequency of the photocurrent corresponding to the Rayleigh backscattered signal at any point X on the fiber to be measured is set to 0, the frequency is proportional to the scattering point position x. As long as the frequency is less than the cut-off response frequency of the photodetector. The photodetector will output the photocurrent of the corresponding frequency, the amplitude of which is proportional to the backscattering coefficient and the optical power at the fiber x, so as to obtain the scattering attenuation characteristics along the fiber to be tested, and the maximum value of the test frequency can be obtained. to derive the length of the fiber to be measured.

2. 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, while the diagnosis of the latter generally uses a light source with a wavelength of 1310 or 1550, 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 cannot solve this problem well, while OFDR can satisfy it, which has the advantages of high sensitivity and high spatial resolution.

High sensitivity

The OTDR directly detects the back Rayleigh scattered light signal of the optical fiber, and the output optical power 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. Just about the -45dB position of the incident light, so that the conclusion can be drawn. 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.

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 the 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.

3. Test methods in different optical devices

Optical devices are divided into two parts according to the source, optical passive devices and optical active devices.

Optical passive components: optical jumpers, pigtails;

AWG,WDM,PLC;

…

Optical active devices: mainly optical transceiver modules

In fact, whether it is an optical active device or an optical passive device, when testing with an OFDR tester, it is to test the optical link of the product in a passive state. Analyze and diagnose the optical link to determine whether there is an abnormal state; the obtained test data includes = distance position, insertion loss, and return loss.

four.LUNA OFDR tester

Luna Innovations Incorporated (LUNA), the world's leading advanced optical fiber technology company, was founded in 1990. With its technological innovation and practical experience in many fields, it provides customers with industry-leading sensing, test and measurement, monitoring and control products and solutions. Committed to applying new technologies to measure, monitor, improve and protect critical processes in industries such as telecommunications, medical, compliance materials and defense.

Among the many products of LUNA, OFDR is very representative. It is divided into three series: 4600, 6400, 6200. The 4600 series is laboratory level, with high precision and long test length; the 6400 series is the production line level, with low price and complete functions; the 6200 series is a portable instrument, which is convenient for outdoor operation and testing.

Performance parameters

Test Challenges

The complete characterization of optical waveguide structures requires both optical frequency domain response and optical time domain response. In fact, whether it is a frequency domain test or a time domain test, our first and most important step is to use an end face detector to check whether the end face of the connector is clean.

5. Relevant test plan

Six. Specific case 1

Customer Name: XX Company

Test requirements: test abnormal conditions at the coupling of the coherent optical module chip

Test process: Since the distance between the customer chip and the FA is very close, and there is a medium in the middle, interference fringes are found during the test (the distance between the interference fringes in the picture above is 8mm), but the end of the FA can still be well judged The distance from the front-end position of the chip, so that subsequent product development analysis can be carried out.

Six. Specific case 2

Customer Name: XX Company

Test requirements: test abnormal conditions at the ferrule tail handle of AWG products

Test process: Because the abnormal conditions of some products are very slight, resulting in exceeding the equipment range, there is no way to accurately test, but in the vast majority of cases, correct judgment can be made, even if the two abnormal positions are one millimeter away, there is nothing. The problem. At the same time, it can be determined how much the insertion loss and return loss are at these two locations.

Seven. Conclusion

Linpu Instruments was founded in 1992, and it has passed its 30th anniversary. Our company focuses on the optical communication test business, and has a number of international famous brands of test instruments. At the same time, our company also has a lot of practical experience to help more customers who want to join the development of optical communication.

With the development of optical communication and integrated optics, measurement techniques with high spatial resolution are increasingly required. As a high-spatial-resolution measurement technique with broad application prospects, OFDR is being paid more and more attention. With the development of domestic science and technology, the research on OFDR will surely attract people's attention and be carried out.