KR20030075301A - Optical tunable microwave filter using higher mode - Google Patents

Abstract

PURPOSE: An optical synchronous microwave filter using a higher order mode is provided to form various delay lines from one short distributive compensating optical fiber by using two mode converters. CONSTITUTION: An optical synchronous microwave filter using a higher order mode includes a laser diode(10), a modulator(20), the first mode converter(30), the second mode converter(30'), and a receiver(40). The laser diode(10) is used for generating a carrier signal. The modulator(20) is used for modulating an output signal of the laser diode(10). The first mode converter(30) is used for distributing the power of the modulated signal of the modulator(20) into two modes. The second mode converter(30') is used for converting the modes to one mode. The receiver(40) is used for receiving the converted signal of the second mode converter(30').

Description

Optical tunable microwave filter using higher mode

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optically tuned microwave filter using a higher order mode, and more particularly, to a filter constituting a plurality of delay lines using one short dispersion compensation fiber through two mode converters. .

In general, an essential element for fabricating a filter is a delay line that will change the phase of the signal. In order to make this delay line, most of the filters proposed so far have used high-dispersion fibers (5) having a large dispersion value, as can be seen in FIG. That is, the signal from the laser 1 is modulated and amplified through the RF modulator 2 and the amplifier 3, and the modulated signal is distributed through the divider 4 (in the ratio of 1: 8 in FIG. 1). being). In addition, a plurality of lines 8 and paths through which such signals can pass are made, and a plurality of delay lines are formed by connecting optical fibers 5 having a large dispersion of length different for each line 8. As such, the signal passing through each delay line is detected by a photodiode, which is a detector 6, and the characteristics of the filter can be obtained by viewing the detected frequency characteristics. The signal sensed by the detector 6 is then output through an 8: 1 RF Power combiner 7.

However, in the conventional method as described above, one optical fiber having a large dispersion value constitutes one delay line. In addition, since there must be a detector in each path, the required number of delay lines is needed. In addition, to construct a desired delay line, an optical fiber with a large dispersion value of several tens of meters or more is required. This is a disadvantage in terms of filter size and price.

Therefore, there is a need for a method capable of constructing multiple delay lines with one short optical fiber and one detector.

An object of the present invention is to solve the problems of the prior art as described above, by using a high order mode that can configure a plurality of delay lines in one sensor using one short dispersion compensation optical fiber through two mode converters. An optical tuning microwave filter is provided.

1 is a conventional filter configuration diagram.

2 is a block diagram of a filter according to the present invention.

3 is a schematic diagram of a variant embodiment according to the present invention;

<Description of Drawing>

Laser (1), modulator (2), amplifier (3), splitter (4), optical fiber (5), detector (6), synthesizer (7), line (8), laser diode (10), single mode optical fiber ( 12, hollow fiber 14, 14 ', distributed compensation fiber 16, modulator 20, first mode converter 30, second mode converter 30', receiver 40

Term description

The terms used in the description of the present invention will be briefly described as follows.

Higher mode: A mode other than the main mode with the lowest cutoff frequency.

Dispersion Compensation Fiber (DCF): One of the wavelength division multiplexed transmission methods that are attempted for transmission of 10 Gbps or more. How to compensate for variance.

Single Mode Fiber: In the wavelength of use, it is an optical fiber with only one transmitable propagation mode. It has a very small core diameter of less than 10um and has only one form of light propagation. Since there is almost no distortion (distortion) of the signal, long-distance transmission of the signal is possible.

Hollow Optical Fiber: Hollow optical fiber.

Group Velocity: The speed at which a signal waveform propagates, the reciprocal of the rate of change of the phase constant for each frequency in a particular mode.

Group Index: The speed of light in a vacuum divided by the group speed of a mode for a mode in which the medium has a refractive index of n.

Delay Line: A means of creating a time delay.

Configuration

The overall configuration of the present invention will be described with reference to FIG.

The present invention is a filter composed of the first and second mode converters 30 and 30 ', the single mode optical fiber 12 and the dispersion compensation optical fiber 16, the laser diode 10, the modulator 20 and the receiver 40. to be.

The filter according to the present invention comprises a laser diode 10 for generating a carrier signal; A modulator (20) for modulating the signal generated by the laser diode (10); Power of the modulated signal is input through the modulator 20, and the power of the input signal, that is, the power of the signal transmitted in one mode LP01 in the single mode optical fiber 12 is distributed and compensated optical fiber. A first mode converter 30 for distributing into two modes LP01 and LP02 within 16; When the signals of the two modes LP01 and LP02 having a time delay are input through the first mode converter 30, the signals of the two modes LP01 and LP02 are input again to the single mode optical fiber 12. A second mode converter 30 'that converts to one mode LP01 present in the apparatus; The receiver 40 is configured to receive the signal converted by the second mode converter 30 '.

Here, the first mode converter 30 and the second mode converter 30 ′ connect the single mode optical fiber 12 and the dispersion compensation optical fiber 16, and have a radius of 3.4 μm and a center radius of 0.6 μm. Hollow optical fibers 14, 14 '.

A description of the modes LP01 and LP02 according to the present invention is as follows.

In general, there are TE mode, TM mode, and Hybrid mode in the optical fiber. Here, the TE (Transverse Electric) mode is a case where there is an H-field (magnetic field component) but no E-field (electric field component) in the direction of the electromagnetic wave, and the TM (Transverse Magnetic) mode is the direction of the electromagnetic wave. There is an E-field but no H-field. In addition, unlike the TE and TM modes, the hybrid mode is a mode in which the E-field or the H-field exists in the traveling direction of the electromagnetic wave. In addition, the refractive index of each mode may be obtained through a characteristic equation obtained by applying a boundary condition from a wave equation, and the characteristic equation is different for each mode (TM, TE, Hybrid).

However, when the refractive indices of the optical fiber core and the cladding are almost the same, there are modes in which the values of the refractive indices of the modes are substantially the same. It is called. In LPmn of LP mode, m is an index included in a characteristic equation obtained by applying boundary conditions to a waveform equation, and n is a root order obtained by substituting a given index m value into the characteristic equation. In other words, LP01 is a mode having a refractive index corresponding to the first value obtained by inserting m = 0 into the characteristic equation of LPmn, and LP02 corresponds to a second value obtained by inserting m = 0 into the characteristic equation of LPmn. It may be referred to as a mode having a refractive index.

action

The operation principle according to the configuration of the present invention described above will be described with reference to FIG.

First, when the laser diode 10 generates a carrier signal, the generated signal passes through the modulator 20 and is subjected to intensity modulation. Here, the power of the modulated signal is transmitted only to one mode existing in the single mode optical fiber 12, that is, the LP01 mode, and the power of the transmitted signal is input to the first mode converter 30.

The first mode converter 30 is a single mode optical fiber 12 and the dispersion compensation optical fiber 16 is connected to the hollow optical fiber 14, the role is as follows.

The first mode converter 30 serves to distribute the power of the signal transmitted only in the LP01 mode, and the power of the modulated signal transmitted only in the LP01 mode in the single mode optical fiber 12 may cause the first mode converter 30 to lose weight. Through this mode, the shape of the mode changes to a ring shape, and the ring-shaped LP01 mode is dispersed in two modes existing in the dispersion compensation optical fiber 16, that is, LP01 mode and LP02 mode at a power ratio of 50:50, respectively. do. In this way, the first mode converter 30 serves to make the signal proceeding in one mode LP01 distributed to two modes LP01 and LP02. In addition, as described above, the mode changes to a ring shape while passing through the hollow optical fiber 14, so that the coupling is performed to the LP02 mode existing in the dispersion compensation optical fiber 16 more efficiently. to be.

The two divided modes LP01 and LP02 pass through the dispersion compensation optical fiber 16 with different group indexes and different group velocities within the dispersion compensation optical fiber 16. This means that even if the two modes LP01 and LP02 have the same length, a time delay occurs due to the speed difference. In this manner, the two modes LP01 and LP02 having a time delay pass through the second mode converter 30 'again. In the second mode converter 30', the signals of the two modes LP01 and LP02 are again returned to the single mode optical fiber. It converts to one mode LP01 existing in (12). As such, the signal converted into the LP01 mode existing in the single mode optical fiber 12 passed through the second mode converter 30 ′ is received by the receiver 40. Here, like the first mode converter 30, the second mode converter 30 ′ is a hollow optical fiber 14 ′ which connects the single mode optical fiber 12 and the dispersion compensation optical fiber 16.

By the operation as described above, by measuring the frequency response while increasing the modulation frequency of the signal obtained through the receiver 40 can obtain the characteristics of the filter.

On the other hand, the tuning method of the filter proposed in the present invention is as follows.

When the laser wavelength of the laser diode 10 is changed, the group velocity of each of the modes LP01 and LP02 has a value different from that before the wavelength is changed by the mode characteristic existing in the dispersion compensation optical fiber 16. This changes the time delay of the two modes LP01 and LP02 at the ends of the dispersion compensation fiber 16. That is, since the time delay is the inverse of the filter period, the filter characteristics can be easily adjusted.

Modifiable Embodiment

A deformable embodiment of the filter proposed in the present invention is as follows.

First, other mode converters can be used to produce filters suitable for the application. That is, the degree of power coupling (efficiency) between the modes LP01 and LP02 is controlled by adjusting the diameter of the hollow optical fiber and the diameter of the empty space existing therein. Therefore, since the diameter of the hollow optical fiber (including the diameter of the empty space existing therein) is changed, the shape of the manufactured filter is also changed (not shown).

Second, another filter characteristic can be obtained by using a plurality of laser diodes. That is, as shown in FIG. 3, when several laser diodes 10 having different wavelengths are connected, the LP01 mode in which the wavelength of the laser diode 10 differs from each other in the single optical fiber 12 is present. In addition, each LP01 mode having a different wavelength enters into the distributed compensation optical fiber 16 through the first mode converter 30, and each LP01 mode having a different wavelength within the distributed compensation optical fiber 16 combines the modes into the LP01 and LP02 modes. coupling takes place. Each mode LP01 and LP02 in the dispersion compensation optical fiber 16 have group refractive index values corresponding to their wavelengths, and thus have different group speeds. Due to this speed difference, a time delay occurs, and a signal converted into one mode LP01 existing in the single optical fiber 12 through the second mode converter 30 'causes the receiver 40 to fail. Is received via. As such, using different laser diodes with different wavelengths, different filter characteristics may be obtained.

As described above, unlike the conventional filter, the filter proposed in the present invention can configure several delay lines using one short dispersion compensation fiber through two mode converters. Tuning can achieve an easy effect. In addition, by controlling the diameter size of the hollow fiber as a mode converter or by connecting a plurality of laser diodes having different wavelengths, there is an effect that can obtain filter characteristics different from the existing ones.

Claims (5)

A filter consisting of two mode converters, single mode fiber and distributed compensation fiber, laser diode, modulator and receiver, Laser diodes for generating carrier signals, A modulator for modulating the signal generated by the laser diode, Power of the modulated signal is input through the modulator, and the power of the input signal, that is, the power of the signal transmitted in one mode LP01 in the single mode optical fiber, is divided into two modes (in the distributed compensation optical fiber). A first mode converter for distributing to LP01, LP02, When the signals of the two modes LP01 and LP02 having a time delay are input through the first mode converter, the signals of the two modes LP01 and LP02 are again present in a single mode optical fiber. A second mode converter to convert to LP01, And a receiver for receiving the signal converted by the second mode converter. The method of claim 1, wherein the first mode converter and the second mode converter is a single mode optical fiber and the dispersion compensation optical fiber, a radius of 3.4um and the center radius of the hollow optical fiber (hollow optical fiber) having a hollow space of 0.6um A wide tuning microwave filter using a higher order mode, characterized in that. 3. The optically tuned microwave filter of claim 1, wherein the power distribution from the first mode converter to each mode (LP01, LP02) is distributed at a ratio of 50:50. 4. The high-order mode of claim 1 or 2, wherein the degree of power coupling (efficiency) between the modes LP01 and LP02 is adjustable by adjusting the diameter of the hollow optical fiber and the diameter of the empty space existing therein. Tunable microwave filter using a. The optically tuned microwave filter using the higher-order mode of claim 1, wherein a plurality of laser diodes generating the carrier signal are connected to each other to manufacture a filter having different characteristics.