KR19980028131A - Erbium-doped optical amplifiers for maintaining gain equalization - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an erbium-added optical amplifier for maintaining a state in which a gain difference with a wavelength is minimized in a wavelength division multiplex optical transmission system. In order to solve the problem of degrading the performance of the system by being affected by the external environment when used in the split multiplex optical transmission system, the present invention provides three optical isolators (21a, 21b, 21c), and a wavelength division optical fiber combiner (22). , An excitation light source 23, an erbium-doped optical fiber (A) for amplifying signal light and natural emission light, an optical detector 26 connected to an optical fiber directional coupler 25, a photonic directional coupler 25, and amplified nature in the 1530 nm region. It consists of an optical filter β for extracting only the emitted light and an excitation light intensity adjusting unit 28 for equalizing the gain, so that the gain is amplified natural emission even if the external environment changes. By the by the principle that in proportion to the intensity of the spontaneous emission light component of the 1530nm region extracted by the optical filter 27 as a reference light is adjustable to keep the painter constant gain of the state optimum etc. gain the gain of the optical amplifier.

Description

Erbium-doped optical amplifiers for maintaining gain equalization

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an erbium-added optical amplifier for maintaining gain equalization, and more particularly, to an erbium-added optical amplifier for maintaining gain equalization according to wavelengths in order to compensate for loss of optical fiber in a wavelength division multiplex optical transmission system. will be.

In general, erbium-added optical amplifiers compensate for optical path losses in optical transmission systems, increasing transmission distances.

For this purpose, the erbium-added optical amplifier must maintain an appropriate gain and noise figure for a given environment, that is, input signal light intensity and excitation light intensity.

Another important fact in the case of erbium-doped optical amplifiers used in multichannel optical transmission systems is the transmission equalization characteristics, i.e. the gain equalization between transmission wavelengths.

Erbium-doped optical amplifiers have amplification factor and noise figure depending on the wavelength due to the fundamental characteristics of gain erbium ions. In particular, the non-uniformity of these wavelengths leads to a change in the final light intensity and signal-to-noise ratio between the transmitted and transmitted signal channels through several erbium-added optical amplifiers.

Therefore, in order for the entire optical transmission system to operate without error, the standard of the optical transmission system should be determined based on the channel with the worst quality. In this case, the high quality channel has more margin than necessary, so that the entire optical transmission system is inefficient. do.

In order to eliminate this disadvantage, in general, in a multi-channel optical transmission system, the optical intensity and signal-to-noise ratio between channels in the receiver are adjusted to be uniform.

For this purpose, a method using an optical filter exhibiting spectral characteristics opposite to the gain spectrum of an erbium-doped optical amplifier (EDFA), and controlling the light intensity between transmission optical channels before transmitting the optical signal, There is a method to make the intensity constant, and to control the length and excitation light intensity of the erbium-added optical amplifier in the EDFA to maintain the gain equalization in the EDFA itself.

However, even if the light intensity and the signal-to-noise ratio between the channels are uniformly adjusted, if the input signal light intensity or the excitation light intensity of the EDFA is changed, the equalization characteristics at the receiving end may change, which may change the system performance.

Therefore, in order to construct a robust optical transmission system, gain equalization and maintenance of EDFA are essential. In the present invention, a gain equalization maintaining method of gain equalized EDFA is described using the various gain equalization methods listed above.

When the length of the erbium-doped optical fiber is determined, the gain representing the state in which the gain of EDFA including this is equalized is determined as one value. That is, the minimum gain of the gain difference (ΔG) of the wavelength of the erbium doped fiber length L is a single value G _0.

For the same length of erbium-doped optical fiber, if the gain is greater than G3, the gain on the short wavelength side is greater than the gain on the long wavelength side, and when the gain is less than G ₀ , the gain on the long wavelength side is greater than the gain on the short wavelength side.

Since the gain representing the equalization state of the gain spectrum is determined singularly for a given erbium-doped fiber length, the gain equalization and automatic maintenance of this EDFA can be achieved by using a fixed gain holding method representing the equalized gain for a given erbium-doped fiber. do.

The equalization method between channels does not optimize the erbium-added optical amplifier itself, but maintains the equalized state even if an optical filter having a spectrum opposite to the gain spectrum of the transmission optical signal or an erbium-added optical amplifier is used. Maintain a fixed gain in the situation.

In other words, the erbium-added optical amplifier has a fundamental characteristic that the gain is different depending on the wavelength, and when used as it is in the wavelength division multiplex optical transmission system, there occurs a problem of degrading the performance of the system.

Accordingly, in general, in the wavelength division multiplex optical transmission system, the gain characteristics of an erbium-added optical amplifier are evenly used.

However, even if these characteristics are optimized, the optimized characteristics change when the external environment changes. Therefore, there is a need to maintain the gain equalization characteristics according to the wavelength even if the external environment changes.

According to the present invention, an object of the present invention is to maintain gain equalization when an external environment such as an input signal light intensity or an excitation light intensity changes in an erbium-doped optical amplifier in which a gain according to a wavelength is equalized.

1 is a light spectrum of a typical erbium-added optical amplifier.

2 is a block diagram of an erbium-added optical amplifier for gain equalization of the present invention and a realization device for measuring the characteristics thereof.

3 is a measurement result of changing the signal light intensity incident to the erbium-containing optical amplifier as the gain and gain equalization characteristics of the amplifier with respect to the intensity of the optical signal reflected from the optical filter.

4 is a gain equalization characteristic diagram for an optical signal intensity reflected by an optical filter within an arbitrary force signal light intensity and an excitation light intensity.

Fig. 5 is a gain equalization characteristic diagram of change in signal light intensity when the excitation light intensity is adjusted to maintain the intensity of the optical signal reflected by the optical filter at an appropriate value.

* Explanation of symbols for main parts of the drawings

10 signal source 15 optical attenuator

20: Erbium-added optical amplifier

21a, 21b, 21c: first to third optical isolators

22: wavelength division optical fiber coupler 23: excitation light source

24: Erbium-doped optical fiber 25: Optical fiber directional coupler

26: light detector 27: optical filter

28: excitation light intensity control unit 30: optical spectrum analyzer

Ah, attached to the present invention. When described in detail based on the drawings as follows.

1 shows the gain spectrum of EDFA.

In the case of EDFA operating under normal conditions, the gain is very large in the 1530 nm region, and the gain spectrum is relatively uniform in the 1545 nm to 1560 nm region.

Since the gain in the 1530 nm region is so large that it passes through several EDFAs, the gain in this region is much larger than the gain in other wavelengths, saturating the gain in other bands, so in this case only a narrow band in the 1530 nm region is available.

So, in general, the 1530nm region is completely removed to eliminate saturation due to this component, and instead, the wider band 1545nm to 1560nm region is used as the signal band.

For this purpose, the amplified spontaneous emission components of the 1530 nm band are removed by using an optical filter such as an optical fiber diffraction grating after passing through each amplifier.

The amplified spontaneous emission intensity of this removed 1530 nm region is proportional to the gain.

In this invention, the amplified natural emission component of the 1530 nm region extracted with the optical filter is used as the reference light, and the gain of the optical amplifier is adjusted to keep the gain of the optimum gain synchronous at a constant state.

FIG. 2 shows a structure of an EDFA capable of performing amplified natural emission removal in a 1530 nm region and gain equalization maintaining function using the same, and an experimental apparatus for testing its performance.

In the present invention, four light sources located between 1545 nm and 1557 nm were multiplexed using a 4X4 optical fiber directional coupler to measure gain gain characteristics of the EDFA.

Accordingly, the erbium-added optical amplifier 20 for maintaining the state of gain assimilation of the present invention will be described in connection with an experimental apparatus.

In order to measure the degree of gain equalization of the erbium-containing optical amplifier 20, a signal light source 10 having four light sources (eg, 1545 nm, 1548.2 nm, 1553 nm, and 1557 nm) in the range of 1545 nm to 1557 nm, and the signal light source 10 Optical spectrum attenuator 15 for adjusting the intensity of the signal light output from the optical spectrum, and optical spectrum analyzer 30 for measuring the optical spectrum of the output of the erbium-containing optical amplifier 20 and the erbium-containing optical amplifier 20 of the present invention The gain equalization characteristics of EDEA are measured. Measurement results thereof will be described with reference to FIGS. 3 to 5 to be described later.

The erbium-containing optical amplifier 20 includes a first light isolator 21a that blocks reflected light and passes transmitted light to the input signal light to measure the degree of gain equalization, and the transmitted signal light. A wavelength division optical fiber coupler 22 for coupling the excitation light to excite (optical pump) the laser medium, and an erbium-doped optical fiber that amplifies the signal light passed through the wavelength division optical fiber coupler 22 and amplifies the natural emission light. A second optical isolator 21b for blocking the light reflected by the output light amplified through the erbium-doped optical fiber 24 and allowing the transmitted light to pass therethrough, and the second optical isolator 21b. The optical fiber directional coupler 25 for extracting the components of the 1530 nm region reflected and transmitted through the amplified signal light and the natural emission light passing through the natural light of the 15530 nm region extracted from the optical fiber directional coupler 25. An optical detector 26 for measuring the light intensity of the outgoing light, an optical filter 27 for passing the amplified spontaneous emission group in the 1530 nm region through the optical fiber directional coupler 25 and passing the wavelength component of the signal light; The excitation light intensity change of the excitation light source 23 based on the natural emission light intensity of the extracted 1530 nm region is connected to one side of the optical fiber anti-corrosive coupler 25 by the feedback method. The excitation light intensity adjusting unit 28 to adjust and the wavelength component of the remaining signal light passing through the optical filter 27 are composed of a third optical isolator 21c for transmitting. Referring to the operation of the present invention by such a configuration as follows.

An optical filter 27 is inserted at the output end of the EDFA to remove (extract) the amplified natural emission light in the 1530 nm region, and the amplified natural emission light component in the 1530 nm region extracted by the optical filter 27 is optically directed. Pulled out by the combiner 25.

The light intensity of the natural emission light component thus extracted is detected by the photodetector 26.

The natural emission light amplified and amplified by the erbium-doped optical fiber 24 and the amplified signal light are reflected by the optical filter 27 and the components of the 1530 nm region are reflected and the wavelength components (components of the 1550 nm wavelength band) of the remaining signal light are transmitted. Go to the output of the amplifier.

At this time, the component of the 1530 nm region reflected by the optical filter 27 is extracted by the optical fiber directional coupler 25 between the second optical isolator 21b and the optical filter 27 and enters the photo detector 26.

By adjusting the excitation light intensity excited by the excitation light source 23 so that the light intensity of the component detected by the photo detector 26 is the light intensity at a gain that keeps the gain of the given EDFA uniform with respect to the wavelength, Even if the input signal light intensity or the excitation light intensity of the EDFA changes, the excitation light intensity is readjusted by a feedback method to maintain the state of gain equalization.

As a result of such control, the state of the output from the erbium-containing optical amplifier is measured by the optical spectrum analyzer 30.

3 shows the gain, degree of gain assimilation, and light intensity in the extracted 1530 nm region for a particular erbium-doped fiber length.

The degree of gain mobilization according to the wavelength is inversely proportional to the gain and is proportional to the light intensity of the extracted 1530 nm region. For the amplifier used for the measurement, the gain representing the state of gain equalization is about 20 dB and the light intensity in the 1530 nm region extracted in this state is about -23 dBm. 4 shows the light intensity of the extracted 1530 nm region with respect to the degree of gain equalization measured while varying the input signal light and excitation light intensity of EDFA.

At a certain degree of gain equalization, the light intensity in the extracted 1530 nm region is always constant. Thus, using this method, EDFA can be adjusted to any gain equalization state required by the system, as well as maintaining gain equalization.

5 is based on the light intensity of the extracted 1530nm region when the optimal gain equalization state is used for the EDFA used for measurement, so that the light intensity of the extracted 1530nm region is kept constant when the input signal light intensity incident to the EDFA is changed. The change in the degree of gain equalization was measured while adjusting the excitation light intensity.

Even if the input signal light intensity per channel varies from -30dBm to -15dBm, the gain equalization is maintained within about 0.4dB.

The input signal light intensity region is a value limited by the excitation light intensity, for example, and the pressure signal light intensity region can be continuously increased when the excitation light intensity is sufficient.

As described above, the present invention can automatically adjust and maintain the gain equalization state of the equalized EDFA.

The present invention as described above has the advantage of improving the system performance in the wavelength division multiplexing system because the gain equalization characteristic according to the wavelength is maintained even if the external environment changes.

Claims (3)

The first optical isolator 21a which blocks the reflected light with respect to the input signal light and passes the transmitted light to measure the gain equalization degree, and combines the transmitted signal light with the excitation light excited by the excitation light source 23. Through the erbium-doped optical fiber 24 and the erbium-doped optical fiber 24 which amplify the signal light passing through the wavelength-divided optical fiber coupler 22 and amplify the natural emission light. A second optical isolator 21b that blocks reflected light with respect to the amplified output light and passes the transmitted light, and a 1530 nm region in which signal light and natural emission light passed through the second optical isolator 21b are transmitted and reflected. An optical fiber directional coupler 25 for extracting a component of the light, a photodetector 26 and an optical fiber for measuring the light intensity of natural emission light in the 1530 nm region extracted by the optical fiber directional coupler 25 An optical filter 27 which extracts only the amplified natural emission light of the 1530 nm region passing through the directional coupler 25 and passes the wavelength component of the remaining signal light, and is connected to one side of the optical fiber directional coupler 25 with equal gain. The excitation light intensity adjusting unit 29 and the optical filter 27 for adjusting the excitation light intensity change of the excitation light source 23 by the feedback method on the basis of the extracted natural emission light intensity roll in the extracted 1530 nm region to maintain the The erbium-added optical amplifier for maintaining the state of gain equalization, characterized by comprising a third optical isolator (21c) for transmitting the wavelength component of the remaining signal light passing through. The erbium-added optical amplifier according to claim 1, wherein the optical filter (27) reflects the components of the 1530 nm wavelength band and transmits the components of the 1530 nm wavelength band. 2. The light source according to claim 1, wherein the excitation light intensity control unit 29 has light at a gain such that the light intensity of the component detected by the photo detector 26 maintains a gain of a predetermined erbium-added optical amplifier with respect to a wavelength. An erbium-added optical amplifier for maintaining the state of gain equalization characterized by adjusting the excitation light intensity to be the intensity.