KR20070080048A - Differential mode delay measurement system for a multimode fiber by use of an intermodal interferometer and optical frequency chirping - Google Patents

Abstract

A differential mode delay measurement system for an MMF(Multimode Optical Fiber) using an intermodal interference with varied optical frequency is provided to easily increase resolving power due to the characteristic of an OFDR(Optical Frequency Domain Reflectometer) using a wavelength variable laser, and to overcome the limit of measurement length by measuring the light although the intensity of the light is low. A differential mode delay measurement system for an MMF(30) using an intermodal interference with varied optical frequency is composed of a wavelength variable laser(10) oscillating the laser light having the wavelength repeatedly varied in a linear form within a specific frequency band; a mode scrambler(20) installed at the front end of the MMF, to generate all available modes by transmitting the laser light oscillated from the wavelength variable laser to the MMF; an optical detector(40) detecting the light after converting the light passing through the MMF into the parallel light; and a delayed time calculating unit computing the delayed time between the modes by using an interference signal generated from the optical detector, and calculating a DMD(Differential Mode Delay) value from the delayed time between the modes.

Description

Differential mode delay measurement system for a multimode fiber by use of an intermodal interferometer and optical frequency chirping

1 is a block diagram of a device for measuring the inter-mode delay time of the multimode optical fiber of the present invention.

Figure 2 is a view showing the optical frequency with time of each mode after the inter-mode delay occurs as the light generated from the wavelength variable laser of the present invention passes through the multi-mode optical fiber.

3 is a view showing the relationship between the interference between the three modes generated in the multimode optical fiber of the present invention.

4 is a diagram showing a relationship between a frequency of a signal generated by interference between two modes and a delay time between modes in the present invention.

5 is a view showing the inter-mode delay time obtained by using the inter-mode interference phenomenon generated in the multi-mode optical fiber in the present invention.

FIG. 6 is a waveform diagram of inter-mode interference signals generated in an optical fiber having three modes according to the present invention. FIG.

FIG. 7 is a diagram showing the inter-mode delay time of this optical fiber obtained from the interference signals between the three modes obtained in FIG. 6 according to the present invention. FIG.

8 is a diagram showing the inter-mode delay time caused by the inter-mode interference of a commercial optical fiber having many modes in the present invention.

FIG. 9 is a diagram illustrating a mode-to-mode delay time measured by a conventional method using a short pulse for the multimode optical fiber used in FIG. 8. FIG.

The present invention relates to the analysis of the modal delay characteristics (modal delay) of the multimode optical fiber (MMF), in particular, in analyzing the delay characteristics between the optical fiber mode, there is no limitation of the measurement length of the optical fiber, the implementation of the device is simple The present invention relates to a method for measuring inter-mode delay time of a multimode optical fiber using inter-mode interference with a variable optical frequency.

Recently, attention has been focused on Gigabit Ethernet using 850 nm VCSEL.

Multimode optical fiber (MMF) research is also actively underway to enable such Gigabit Ethernet.

This is an analysis of the modal delay between modes, which is one of the important characteristic evaluation items in the study of MMF.

In order to evaluate the performance of the multimode optical fiber, it is necessary to analyze the delay characteristics between the modes. Recently, many studies have been conducted on this, and various measurement methods have been studied.

In addition, as optical fibers using plastics are developed in addition to optical fibers made of amorphous glass, characteristics evaluation of multimode optical fibers other than the 850 nm wavelength band is required.

The conventional method for analyzing the intermodal delay characteristics of a multimode fiber uses a sampling oscilloscope to sample the time delay between separated pulses while passing the multimode fiber using a short pulse. Measured in the time domain.

When using this time-domain measurement method, a short pulse laser and a high speed oscilloscope are essential components. Since such a short pulse laser and a high speed oscilloscope are expensive, optical fibers are required. This increases the cost of inspection and thus increases the price of the fiber.

In addition, it is difficult to equip the production line with expensive equipment for the evaluation of the optical fiber, which takes a long time for evaluation, which may hinder mass production and sales.

In addition, in the time-domain measurement method as described above, the minimum length of the optical fiber capable of measuring the delay time between modes can be confirmed that the mode is separated when the length of the optical fiber is longer than 500 m.

* However, using a laser having a general pulse width of 100 ps,

* The mode-to-mode delay time (DMD) of multimode fiber is within 0.2ps / m

Therefore, in order to grasp the delay characteristics between modes of the multimode optical fiber within 1km, a measuring method capable of measuring even a short length is required.

In addition, since the interferometer is used in the inter-mode delay measurement using the optical frequency domain reflectometer (OFDR) in the prior art has a somewhat complicated device configuration.

The present invention overcomes the limitations of the measurement length, which is a problem of the conventional method, and simplifies the configuration apparatus as compared to the existing method, and also has a distinction from the existing method, and makes it possible to measure the delay time between modes of the multimode optical fiber, which is easy and simple at low cost. To propose a method for measuring the inter-mode delay time of a multi-mode optical fiber using the inter-mode interference phenomenon having a variable optical frequency.

A typical optical frequency domain reflectometer (OFDR) system consists of a Michelson interferometer and a tunable laser.

The light reflected from the sample fiber of one arm and the light reflected from the other arm interfere with each other, and the system is used to analyze the cutting position or various physical characteristics of the sample fiber by analyzing the frequency of the obtained interference signal. .

The present invention is a method for evaluating the characteristics of a multimode optical fiber having one cutting surface but several modes, instead of a single mode optical fiber having several cutting surfaces. This reduces bandwidth in short-range optical communications using multimode fiber.

Therefore, many fiber manufacturers want to measure the differential mode delay (DMD or modal dispersion) incurred in multimode fiber and are provided as basic information in actual fiber products.

The present invention is a method of measuring the DMD, but using the OFDR technology is a method that can be measured without the existing interferometer.

The present invention,

Wavelength-variable lasers that oscillate laser light with repeatable linearly variable wavelengths within a specific frequency range, and all modes that can be generated by delivering laser light oscillated from wavelength-variable lasers installed in front of the multimode optical fiber to the multimode optical fiber. A mode decomposer, a photodetector for converting and detecting light passing through the multimode optical fiber into parallel light, and a delay time calculating means for calculating a delay time between modes using an interference signal generated by the photodetector. Characterized in that it comprises a.

With reference to the embodiment shown in the accompanying drawings of the present invention having such a feature will be described in detail the configuration and its operation.

1 shows a configuration apparatus for measuring the inter-mode delay time of a multimode optical fiber.

A wavelength-variable laser 10 for oscillating laser light having a wavelength that is repeatedly linearly changed in a specific frequency region, and a multi-mode laser light oscillated from a wavelength-variable laser is installed in front of the multimode optical fiber (MMF) 30. And a mode decomposer 20 for generating all modes that can be generated by transmitting to the optical fiber, and a photodetector 40 for converting and detecting light passing through the multimode optical fiber 30 into parallel light.

It includes a delay time calculation means for calculating the delay time between the modes by using the interference signal generated by the photodetector 40.

The photodetector 40 includes a collimator 41 for converting light passing through the multimode optical fiber 30 into parallel light, and a photodetector 42 for detecting parallel light converted through the collimator 41. It includes.

The present invention having such a configuration,

The laser beam oscillates linearly from the wavelength tunable laser 10.

The waveform shown in the dotted line on the wavelength tunable laser 10 of FIG. 1 represents the optical frequency with time of the oscillated light.

The tunable laser 10 oscillates the laser while repeatedly changing in a specific frequency region.

The laser light thus oscillated is introduced into the multimode optical fiber (MMF) 20 which is a sample optical fiber.

A mode scrambler 30 is mounted to make all the modes possible in front of the multimode fiber 20. This mode decomposer 30 has been studied a lot, and recently it is commercialized and used.

The variable wavelength light is divided into many modes while passing through the mode resolver, and the divided modes are delayed with each other as they pass through the multi-mode optical fiber 20.

Figure 2 shows the optical frequency with time in each mode after the inter-mode delay occurs as the light generated from the wavelength variable laser passes through the multi-mode optical fiber.

Mode 2 arrives τ1 than Mode 1 and Mode 3 arrives τ2 later than Mode 1.

When light having different optical frequencies arrives at the same time in the photodetector 40, they interfere with each other to generate an interference signal. The frequency of the signal is the difference between the optical frequencies of the two interfering lights.

Therefore, in FIG. 2, interference 1 between Mode 1 and Mode 2, interference 2 between Mode 2 and Mode 3, and finally interference 3 between Mode 1 and Mode 3 appear.

3 shows a relationship diagram in which an interference occurs in the form of an actual mode.

Without splitting light, Mode 1, intact shape, Mode 2 in two, and Mode 3 in four cause interference after each other.

4 is a diagram illustrating a relationship between a frequency of a signal generated by interference between two modes and a delay time between modes.

In general, when light having different frequencies meets interference, the interference signal generated as described above is simply expressed as follows.

here,

만큼 시간 지연을 가진 두 모드의 광주파수. fi and fj are respectively

Optical frequency of both modes with time delay.

φ is an arbitrary phase constant.

는 레이저에서 생성한 광주파수의 변화율로서 도 4에서 있는 삼각형의 기울기를 나타낸다. In FIG. 4, the two modes interfere with each other to generate a frequency of Δfij.

Denotes the gradient of the triangle in FIG. 4 as the rate of change of the optical frequency generated by the laser.

을 측정할 수 있다. Since this value is a characteristic of the laser, it is the known value before the measurement and is the time interval between the two modes we want from this value and the frequency of the measured signal.

Can be measured.

The relationship between these values can be expressed as Equation 2 below.

Figure 5 shows the inter-mode delay time obtained by using the inter-mode interference phenomenon generated in the multi-mode optical fiber.

In FIG. 2, Fourier transforms the interference signals generated from the three interferences ①, ②, and ③ that are possible between the three modes to obtain a frequency for each interference, and the delay time between the modes is obtained using Equation 2.

이며 도표상의 위치는 2이다. 5, the delay time of the first interference ①

And position on the chart is 2.

이며 도표상의 위치는 10이다. The delay time of the last interference ③ is

And the position on the chart is 10.

The interference ② is represented by the difference between the two delay times, and the position on the chart is 8.

Therefore, by analyzing the signals generated by the interference between the modes generated in the multi-mode optical fiber 30 to obtain a graph as shown in FIG.

And the last delay time here is the differential mode delay of the mode of the multi-mode optical fiber 30 we want to find.

가 이 광섬유의 DMD 값이 된다. That is, in Figure 5 corresponding to ③

Becomes the DMD value of this optical fiber.

6 shows an inter mode interference signal generated in an optical fiber having three modes as an example.

The device is configured as shown in FIG. 1, and the light generated while varying a specific optical frequency region by the wavelength tunable laser 10 in the 1550 nm band is inserted into the multimode optical fiber 30.

In the present invention, the wavelength is measured at a wavelength of 1550 nm, but in the case of a multimode optical fiber, a 1310 nm or 850 nm laser is used, and in the case of a polymer multimode optical fiber, a laser of 650 nm is used. 10) can be extended to all wavelengths.

In order to facilitate the analysis, an optical fiber 30 having a mode having three different light shapes as in FIG. 3 was manufactured and used.

These three modes interfere with each other after about 50 m of the optical fiber 30 to generate an interference signal as shown in FIG.

The frequency component of the generated interference signal is obtained by using a Fourier transform, and the delay time is obtained by using Equation 2.

7 shows the inter-mode delay time obtained from the optical fibers having these three modes,

It is an actual example of the graph shown in FIG.

The interference ① of FIG. 7 is an interference between Mode 1 and Mode 2 of FIG. 3, the interference ② is an interference between Mode 2 and Mode 3, and finally the interference ③ represents an interference between Mode 3 and Mode 1.

The delay times of interferences ①, ②, ③ are 5.19, 6.02, and 10.38 ps / m, respectively.

In general, when expressing the inter-mode delay time (DMD) of the multi-mode optical fiber 30, the delay time is expressed by dividing the length of the measured optical fiber, the unit is ps / m.

Here the difference in arrival time between the fastest and slowest modes, or DMD, is 10.38 ps / m.

8 is a DMD measured in the same manner as above, using a commercially available multimode optical fiber generally used for short-range optical communication of 850 nm.

Therefore, unlike FIG. 7, it can be seen that many interferences between modes occur.

In FIG. 8, the dotted line is measured without the mode decomposer 20 in front of the optical fiber 30, and the solid line is measured by generating all the modes that can be generated in the optical fiber 30 with the mode decomposer 20.

When not passing through the mode decomposer 20, there is less interference between modes, and when the mode decomposer 20 passes, a lot of modes are generated, and as shown in FIG. 8, much interference occurs.

The last location of interference here is 1.36 ps / m, which is the DMD of this fiber.

9 is a DMD result of the 450 m optical fiber used in FIG. 8 measured with a short pulse laser and a high speed oscilloscope which are generally used for DMD measurement.

The fastest mode is 0.72 ps / m and the slowest mode is 2.10 ps / m with a difference of about 1.38 ps / m.

It can be seen that this value is almost the same as 1.36 ps / m obtained using the apparatus presented in the present invention.

Comparing FIG. 8 measured with the apparatus of the present invention and FIG. 9 measured with the conventional method, the measured optical fiber lengths are 50 m and 450 m, respectively. As shown in Figures 8 and 9, the present invention can be easily measured because the length is short, and the delay time increases even when the length is longer.

The method of measuring the differential mode delay of the optical fiber using the self-mode interference generated in the wavelength tunable laser and the multimode optical fiber of the present invention is a revolutionary innovation in the complexity and economics of the conventional method. It is expected to bring.

Due to the characteristics of the optical frequency domain reflectometer (OFDR) using a wavelength tunable laser, it is easy to increase the resolution, and it is effective as a method of overcoming the measurement length limitation of the conventional method because the measurement can be performed even if the intensity of light to be measured is small.

Claims (5)

Wavelength-variable lasers that oscillate laser light with repeatable linearly variable wavelengths within a specific frequency range, and all modes that can be generated by delivering laser light oscillated from wavelength-variable lasers installed in front of the multimode optical fiber to the multimode optical fiber. Calculate the delay time between modes by using the mode resolver to make it, the photodetector converting the light passing through the multimode optical fiber into parallel light, and the interference signal generated by the photodetector, And a delay time calculating means for calculating a delay time (DMD) value of a mode optical fiber. The method of claim 1, The delay time calculation means Calculating the delay time between modes by performing Fourier transform on the generated interference signal and converting the frequency into a frequency, and calculating the delay time between modes according to the ratio of the optical frequency conversion ratio of the converted frequency band wavelength tunable laser. Inter-mode delay time measurement system of multimode fiber using inter-mode interference with varying optical frequencies. Transmitting laser light having a wavelength that is repeatedly linearly changed in a specific frequency region to a multimode optical fiber through a mode decomposer, A process of Fourier transforming the interference signal generated by the photodetector passing through the multi-mode optical fiber into frequency components; Calculating a delay time between modes by using the converted interference signal frequency between each mode and a predetermined laser optical frequency conversion rate; Method for measuring the inter-mode delay time of the multi-mode optical fiber using the inter-mode interference phenomenon having a variable optical frequency, characterized in that it comprises the step of obtaining the delay time (DMD) value of the multi-mode optical fiber from the calculated inter-mode delay time . The method of claim 3, wherein In the process of calculating the delay time between modes, A method for measuring inter-mode delay time of a multimode optical fiber using inter-mode interference with a variable optical frequency, characterized in that the inter-mode delay time is calculated according to the ratio of the optical frequency conversion ratio of the converted frequency band wavelength tunable laser. The method according to claim 3 or 4, In the process of obtaining the delay time (DMD) value of the multimode optical fiber from the calculated inter-mode delay time, The delay time (DMD) value of the multimode optical fiber is obtained by the difference in arrival time (mode delay time) between the fastest mode and the slowest mode. How to measure the delay time between modes.

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