KR0162758B1 - Optical fiber amplifier - Google Patents

Abstract

In the present invention, an erbium-doped fiber is inserted as a gain medium between the optical isolator and the optical filter in a two-stage amplifier having a conventional optical isolator and an optical filter that best block the forward and reverse ASE. We propose a three-stage EDFA structure that can simultaneously obtain a wide dynamic range with low noise figure by efficiently distributing excitation light sources.

Since the optical fiber amplifier has a wide dynamic range and a high small signal gain under a low noise figure, the optical fiber amplifier can be used as a preamplifier or a relay amplifier in an optical communication system, and can be used in a variety of light intensity equalizers in an optical switching network.

Description

Fiber optic amplifier

Figure 1 (a) is a conventional two stage optical fiber amplifier having an optical isolator between each stage.

(b) is a conventional two-stage optical fiber amplifier having a narrow band optical filter between each stage.

(c) is a conventional two-stage optical fiber amplifier having an optical isolator and a narrow-band filter simultaneously between each stage.

2 is a three-stage optical fiber amplifier according to the present invention.

3 is a comparison between the output signal light intensity of the conventional optical fiber amplifier and the output signal light intensity of the present invention.

The present invention relates to an optical fiber amplifier having a low noise figure and a wide operating range.

Erbium-doped fiber amplifiers (EDFAs) have the advantages of low insertion loss, high gain, low noise figure, and a wide gain band of more than 30nm.

However, there is a trade-off between EDFA's gain and noise figure since there is an inverse relationship between density inversion and signals, depending on the length of the erbiun-doped fiber (EDF) for a constant excitation light source intensity. ) Exists.

Thus, by inserting an optical isolator or narrow band optical filter in the middle of the EDF and constructing a two-stage amplifier that excites it as a limiting excitation light source, the effect of amplified spontaneous emission (ASE) proceeding in the forward and reverse directions is achieved. Reduce both noise figure and high gain.

In Figures 1 (a) to (c), conventional optical fiber amplifiers are shown.

In (a) to (c) of FIG. 1, reference numerals 1 and 1a denote wavelength division multiplex (WDM), 2 and 2a denote an erbium-doped optical fiber (EDF), and 3 denote an optical isolator. And 4 represents an optical filter, respectively.

(A) of FIG. 1 divides the optical fibers 2 and 2a into two stages, and inserts an optical isolator 3 in the middle to block the reversed amplified natural emission and excites in the forward direction to remove the residual excitation light source which is not absorbed at the first stage. The structure of the two stage optical fiber amplifier used for the 2nd stage using the wavelength division optical fiber coupler (WDM) 1a is shown.

However, this structure is not effective for small signals below -30 dBm because the optical isolator 3 does not block the forward ASE.

FIG. 1 (b) shows a structure in which the narrow band optical filter 4 is inserted in the middle of the optical fiber.

In this structure, the forward and reverse ASEs are blocked, but due to the bidirectional propagation characteristics of the optical filter 4, the forward and reverse ASEs corresponding to the bandwidth 4 of the filter are amplified while going backward and forward to consume the pump light source, and the EDF Due to its very high gain, oscillations may be caused by unwanted reflections that can occur across the fiber amplifier for low input light intensities below -35dBm.

In addition, since the loss of the optical filter is about 3dB, the output light intensity decreases with the increase in the signal light intensity for the large signal due to the attenuation action of the optical filter and the strong excitation light source consumption at the first stage.

FIG. 1 (b) shows a structure in which a narrow band optical isolator 3 and an optical filter 4 are simultaneously inserted in the middle of an optical fiber.

This structure provides the highest gain for small signals because it effectively blocks the forward and reverse ASEs, but for large signals the output light intensity increases with increasing signal intensity due to the loss of the optical filter and the large consumption of the excitation light source at the first stage. Have a decreasing bond.

Conventional two-stage amplifier can get low noise figure and high gain at the same time, but it is not enough gain for small signal input because the length of first stage erbium-doped fiber (EDF) must be shortened to reduce noise figure.

In this case, since the output strength of the first stage, which is the input of the second stage after passing through the isolator or filter, is low, the second stage does not have a saturation output. Therefore, if the length of the first stage is shortened to obtain a low noise figure, the input signal light intensity region (dynamic region) with a constant output light intensity is narrowed. If the length of the first stage is increased to increase the gain of the first stage, the dynamic range becomes wider but the noise index is It gets worse, so there's a trade-off between the two.

In the present invention, in the two-stage amplifier having both the optical isolator and the optical filter at the same time, which is the structure that best blocks the forward and reverse ASE, the EDF is inserted as the gain medium between the optical isolator and the optical filter, and the pump light source is efficiently distributed to lower noise. We present a three-stage EDFA structure that can obtain an exponent and a wide dynamic range simultaneously.

The optical fiber amplifier according to the present invention includes a first wavelength division optical fiber coupler for receiving an input optical signal, a first erbium-doped optical fiber 2 connected to the first wavelength division optical fiber coupler, and a second erbium-added optical fiber in series. An optical isolator, a second erbium-added optical fiber, an optical filter, a third erbium-added optical fiber, a second wavelength division optical fiber coupler connected in parallel with both ends of the optical isolator, and the first wavelength division optical fiber combiner And a third wavelength split optical fiber coupler having one end connected to the other end between the optical filter and the second erbium-doped optical fiber.

In the optical fiber amplifier of the present invention, the length of the first erbium-doped optical fiber is relatively short enough to have a gain of about 30 dB for the small signal input, and the third erbium-added optical fiber has a relative output such that a high output strength can be obtained. Make it long enough.

2 shows the structure of a three stage optical fiber amplifier according to the present invention.

In Fig. 2, reference numerals 1, 1a and 1b denote wavelength-dividing optical fiber couplers, 2, 2a and 2b denote erbium-doped optical fibers, and 3 and 4 denote optical isolators and optical filters, respectively.

Referring to FIG. 2, the length of the first erbium-doped optical fiber 2 of the first stage is selected short enough to have only a gain of about 20 dB for the small signal input, and between the optical isolator 3 and the optical filter 4 The second erbium-added optical fiber 2a of appropriate length is inserted, and the third erbium-added optical fiber 2b of the third stage is held long enough to obtain a high output intensity.

At this time, the second erbium-doped optical fiber 2a serves to amplify the intensity of the input light again so that the output of the third erbium-doped optical fiber 2b can be saturated for a small signal of -30 dBm.

In addition, the erbium-doped fiber optic amplifier (EDFA) increases the noise figure as the intensity of the pump light source increases, but maintains a constant noise figure over a certain intensity, thereby separating the excitation light source according to the value.

The first erbium-added optical fiber 2 and the second erbium-added optical fiber 2a are directly connected to each other by the intensity of the excitation light having a constant noise index, and the third erbium-added optical fiber 2b is excited as the excitation light source.

If the excitation light source is connected in series to excite the first and second erbium-doped optical fibers (2, 2a) and the third erbium-added optical fiber (2b), the excitation light is greatly consumed at the first and second ends, thereby increasing the intensity of the output light. Since the third erbium-doped optical fiber 2b to be determined is not sufficiently excited, the intensity of the output light is lowered.

3 is a comparison diagram of output light intensity when the same input signal light source is applied to the conventional optical fiber amplifier shown in FIG. 1 and the optical fiber amplifier of the present invention shown in FIG.

Referring to FIG. 3, the output light intensity (*) of the present invention is higher than the output light intensity (x, ◇, Δ) by any conventional configuration.

That is, the three-stage optical fiber amplifier of the present invention as described above has a wider dynamic range and higher small signal gain than the conventional two-stage amplifier under low noise figure.

Since the optical fiber amplifier has a wide dynamic range and a high small signal gain under a low noise figure, the optical fiber amplifier can be used as a preamplifier or a relay amplifier in an optical communication system, and can be used in a variety of light intensity equalizers in an optical switching network.

Claims (1)

A first wavelength division optical fiber coupler 1 for receiving an input optical signal, a first erbium-doped optical fiber 2 connected to the first wavelength division optical fiber coupler 1, and the first erbium-added optical fiber 2; The optical isolator 3, the second erbium-added optical fiber 2a, the optical filter 4, the third erbium-added optical fiber 2b, which are sequentially connected in series, and both ends of the optical isolator 3 in parallel therewith. One end is connected to the second wavelength division optical fiber coupler (1a) and the first wavelength division optical fiber coupler (1) to be connected, and the other end is connected between the optical filter (4) and the third erbium-added optical fiber (2b). A third wavelength division optical fiber coupler 1b; The length of the first erbium-doped optical fiber 2 is relatively short enough compared to the second and third erbium-doped optical fibers 2a and 2b to have a gain of about 20 dB for the small signal input, and the third The optical fiber amplifier characterized in that the erbium-added optical fiber (2b) is relatively long enough compared with the first and second erbium-doped optical fibers (2, 2a) to obtain a high output strength.