CN1233127C - Compensation method for temperature correlated gain spectrum characteristic of L-band Er-doped fiber amplifier - Google Patents

Abstract

The present invention belongs to the technical field of optical fiber amplifier design in fiber communication, which belongs to a compensation method for the temperature correlated gain spectrum characteristic of an L-band Er-doped fiber amplifier. A variable optical attenuator is inserted between Er-doped fiber sections of the fiber amplifier. When the environment temperature which the Er-doped fibers are at is changed, the attenuation value of the variable optical attenuator is adjusted in order to keep the gain spectrum flat and the gain value invariant. The linear relation between the attenuation regulation amount delta A (dB) of the interpolation optical attenuator and the temperature variation delta T (DEG C) is regulated to satisfy delta A=CL delta T. The method of the present invention only needs to regulate one parameter. The regulated parameter has simple relation with the temperature variation, which is favorable to using a single chip computer to realize intelligent control.

Description

Compensation method for temperature-dependent gain spectrum characteristic of L-band erbium-doped fiber amplifier

The invention belongs to the technical field of optical fiber amplifier design in optical fiber communication, and particularly relates to a compensation method for temperature-dependent gain spectrum tilt characteristics of an L-band erbium-doped optical fiber amplifier.

Background artin recent years, with the development of wavelength division multiplexing optical communication technology, the bandwidth required by an optical fiber communication system is becoming wider and wider, and accordingly, the erbium-doped fiber amplifier (EDFA) technology is also expanding from the original conventional band (C band) to the long band (L band). Over the course of several years of effort, L-band EDFAs have been gradually put into practical use, but some technical problems remain to be solved, among which one of the issues that is urgently to be solved is the temperature dependence of the gain spectrum, i.e., the variation of the gain spectrum with temperature.

In a wavelength division multiplexing system, an optical amplifier needs to amplify several to several tens of signals with different wavelengths at the same time, and in order to ensure the overall performance of the system, the gain of the amplifier for each channel is required to be the same, i.e., the gain spectrum is flat. Therefore, an optical filter is used to level out an originally uneven gain spectrum, and when the gain spectrum changes due to changes in the number of input channels or power of each channel, some methods such as adjusting pump power or adjusting auxiliary injection optical power are used to lock the gain spectrum, which are collectively called gain equalization techniques. It has also been found that the gain spectrum of an EDFA changes with changes in ambient temperature, and in particular L-band amplifiers are more sensitive to changes in ambient temperature than C-waveguides due to the temperature sensitivity of the erbium doped fibre itself and the longer length of erbium doped fibre used. Fig. 1 shows the measured results for an L-band EDFA: by using optical filters and adjusting the pump power, a relatively ideal flat filtered spectrum is obtained at room temperature (26 ℃), but the gain spectrum changes when the temperature varies between 26 ℃ and 70 ℃. The higher the temperature, the higher the gain of the short wave band and the lower the gain of the long wave band, i.e. the gain spectrum is tilted, and the difference of the gain increase/decrease can reach as much as 3dB when the temperature is serious. This gain spectrum tilt condition will destroy the equalized transmission of the system, which cannot be tolerated by the wavelength division multiplexing system.

To avoid or compensate for this gain spectrum tilt of the EDFA due to temperature variations, several approaches have been proposed. One of the most direct methods is to put the erbium-doped fiber into a thermal insulation box to keep the temperature of the erbium-doped fiber constant, but this not only increases the design requirements in the aspects of packaging and circuit, but also brings the cost in the aspects of volume and power consumption. Another approach is to manufacture erbium-doped fibers with opposite temperature trends and then mix two fibers with different trends, but this approach has not been practical before the studies on erbium-doped fibers with "reverse" temperature characteristics have been mature. Nakagawa et al in japan also propose a method of simultaneously using an Automatic Gain Control (AGC) and an Automatic Temperature Control (ATC) function, i.e., simultaneously adjusting the attenuation amount of an interpolated variable attenuator and the injected auxiliary optical power to achieve temperature compensation. Although this method is feasible, two parameters need to be adjusted simultaneously when the temperature changes, and the adjusted parameters are also related to the gain balance of the amplifier under the constant temperature, so that the complex control relationship is very disadvantageous to the intelligent microcomputer control generally required in modern engineering.

The invention aims to overcome the defects of the prior art and provides a novel compensation method for the temperature-dependent gain spectrum tilt characteristic of an L-band EDFA. The method only needs to adjust one parameter, the relation between the adjusted parameter and the temperature variation is simple, and the method is beneficial to realizing intelligent control by adopting a single chip microcomputer.

The invention provides a compensation method for the temperature-dependent gain spectrum tilt characteristic of an L-band erbium-doped fiber amplifier, which is characterized in that a variable optical attenuator is inserted between erbium-doped fiber sections of the fiber amplifier, and when the ambient temperature of the erbium-doped fiber is changed, the gain spectrum can be kept flat and the gain value is unchanged by only adjusting the attenuation of the variable optical attenuator;

the attenuation adjustment quantity delta A (unit: dB) and the temperature variation quantity delta T (unit: DEG C) of the interpolation optical attenuator are adjusted to be in a linear relation:

ΔA＝CLΔT

in the formula, C is a constant determined by parameters of the erbium-doped optical fiber and an optical path structure, and can be deduced by a fitting method according to actually measured data of delta T and delta A; l is the total length of the erbium doped fiber in the amplifier.

The optical attenuator can be inserted between the two original stages of the optical fiber amplifier, or can be inserted between a section of the optical fiber of a stage, namely, the original stage is divided into two stages again. The specific insertion positions can be designed by conventional optimization methods of erbium-doped fiber amplifiers according to the principle of minimal impact on amplifier performance.

The optical attenuator and the control method thereof become the common technical means in the optical fiber communication equipment at present, and various commercially available optical attenuator products exist;

the monitoring of the temperature variation is a common technical means in modern instruments and meters, and a temperature sensing element is generally installed in an erbium-doped fiber amplifier module sold commercially at present and the temperature variation is used as one of monitoring parameters of the working state of the module.

The working principle of the invention is as follows:

according to the theoretical research results, the gain of an erbium-doped fiber amplifier can be expressed as:

$G_{\mathrm{si}}=\exp \left[(({\mathrm{\α}}_{\mathrm{si}}+g_{\mathrm{si}}^{*})\frac{\stackrel{\‾}{N_2}}{N_T}-({\mathrm{\α}}_{\mathrm{si}}+l_{\mathrm{si}}))L\right]---\left(1\right)$

wherein alpha is_si，g^* _siAnd l_siThe absorption coefficient, emission coefficient and background loss coefficient of the i-th wavelength of the erbium-doped fiber are different with different wavelengths, and i comprises signal light and pump light of each channel. L is the total length of the erbium doped fiber in the amplifier. $\stackrel{\‾}{N_2}\≡\frac{1}{L}{\∫}_0^L{N}_2\left(z\right)\mathrm{dz},$ Is the average concentration of erbium ions at the upper level in the erbium-doped fiber along the length L of the erbium-doped fiber, N_TThe doping concentration of erbium ions in the fiber, then,referred to as the average degree of inversion of erbium ions along the length of the fiber. The theoretical research results also show that the average degree of inversionThe number of consumed pump photons and the number of growing signal photons in the erbium-doped fiber are jointly determined:

$-\mathrm{L\ζ}\frac{\stackrel{\‾}{N_2}}{N_t}=\underset{i}{\mathrm{\Σ}}\frac{1}{{\mathrm{hv}}_{\mathrm{si}}}[P_{\mathrm{si}}^{\mathrm{out}}-P_{\mathrm{si}}^{\mathrm{in}}]---\left(2\right)$

here, P_si ^outAnd P_si ⁱⁿRespectively, the output optical power, the input optical power, or the output residual pump power and the input pump power, hv_siIs the energy of each channel photon or pump photon, and ζ is the saturation parameter of the erbium doped fiber. As can be seen from the formula (1), as long as the average degree of inversion is maintainedThe gain of the EDFA to each channel can be kept constant without changing; from (2), it can be seen that when the input signal is changed, the average inversion can be kept unchanged as long as the input pump power is properly adjusted, which is the basic principle of EDFA gain equalization.

When the environmental temperature changes, the distribution of erbium ions on each energy level within the same energy level changes, that is, the absorption coefficient alpha of each wavelength changes_siAnd an emission coefficient g^* _siThis causes a temperature-dependent gain spectrum tilt characteristic of the erbium-doped fiber, as can be seen from equation (1).

In general, the input signal optical power of an amplifier is much smaller than the input pump optical power, and the number of increased signal photons (i.e. the total output power of each channel) is completely determined by the number of consumed pump photons; that is, if the remaining pump power is small and negligible, thenIs determined entirely by the input pump light power. Studies have also shown that the absorption coefficient and emission coefficient of light at 980nm wavelength do not change much with temperature. The applicant has then concluded that if a 980nm laser is used as a pump, the average degree of inversion can be approximated as not varying with temperature, and that the temperature dependent gain spectrum tilt is entirely due to the emission coefficient g at each signal wavelength^* _siAnd absorption coefficient alpha_siCaused by temperature changes. Further theoretical studies have shown that the amount of change Δ G in the gain when the ambient temperature changes Δ T_KComprises the following steps:

${\mathrm{\&Delta;G}}_K=\frac{4.343{\mathrm{\&alpha;}}_{K_0}\left({\mathrm{\&upsi;}}_K\right)\mathrm{<figure-callout\; id="0"\; label="L\; kT"\; filenames="C0314089400111.png"\; state="\{\{state\}\}">L</figure-callout>}}{{{\mathrm{kT}}_{}}^{}}\left\{B\left({\mathrm{\&upsi;}}_K\right)\right[1-\frac{\stackrel{\&OverBar;}{N_2}}{N_T}-\frac{\stackrel{\&OverBar;}{N_2}}{N_T}\exp \left(\frac{\mathrm{\&epsiv;}({\mathrm{\&upsi;}}_K-{\mathrm{h\&upsi;}}_K)}{{\mathrm{kT}}_0}\right)]-\frac{\stackrel{\&OverBar;}{N_2}}{N_T}[\mathrm{\&epsiv;}\left({\mathrm{\&upsi;}}_K\right)-{\mathrm{h\&upsi;}}_K\left]\right\}\mathrm{\&Delta;T}---\left(3\right)$

wherein, T₀Is some median value within the temperature range in question, such as room temperature; ε (upsilon)_K) Is the energy required to excite the erbium ion from a lower energy level to an upper energy level; b (upsilon)_K) Are functions that can be measured experimentally, and all of the above quantities can be considered approximately independent of temperature. (3) The formula shows the variation quantity delta G of the gain of each wavelength signal along with the temperature_KAnd is proportional to the temperature change amount deltat.

On the other hand, the relationship of the gain of each wavelength varying with the average inversion number under the constant temperature condition is also linear relationship obtained from the equation (1):

$\mathrm{\&Delta;}{G_K}^{\&prime;}=4.343({\mathrm{\&alpha;}}_K+g_K^{*})\mathrm{L\&Delta;}(\stackrel{\&OverBar;}{N_2}/N_T)---\left(4\right)$

therefore, the gain spectrum tilt due to temperature change can be corrected by changing the population inversion level by some method.

Fig. 2 is an experimental result of the applicant's trial of varying the input pump power to change the population inversion level to correct the gain spectrum. It can be seen from the figure that the gain spectrum can be restored to be flat by properly reducing the pump power when the temperature is increased, but the gain level is also reduced because the average particle number inversion level is reduced. To compensate for the decrease in gain level, other measures must be taken, such as inserting a variable optical attenuator into the EDFA to adjust the amplifier gain: the inserted attenuation is large when the temperature is low, and the attenuation is correspondingly reduced while the pumping power is reduced after the temperature is increased. However, such a method has two parameters to be adjusted, and the control relationship is relatively complicated and not ideal.

Therefore, the invention provides a method for realizing the temperature-dependent gain spectrum tilt characteristic compensation only by adjusting one parameter of the interpolated variable optical attenuator. The amplifier principle optical path structure using this method is shown in fig. 3, the output light of the pump laser L31 is merged with the input signal light by the wavelength division multiplexer W31 and sent into the erbium-doped fiber E31, and the variable optical attenuator V31 for temperature compensation is inserted between two sections of erbium-doped fibers E31 and E32. Adjusting the optical attenuator can change the optical power input from the optical fiber E31 to the optical fiber E32, thereby changing the average inversion of the optical fiber E32 portion, and the average inversion of the whole amplifier is changed; on the other hand, the actual net gain of the amplifier is equal to the sum of the two erbium-doped fiber gains minus the losses of the interpolated optical attenuator and other components, so adjusting the variable optical attenuator also changes the actual net gain of the amplifier. The interpolation optical attenuator plays the dual roles of adjusting the average inversion and the gain level of the amplifier, so that the aim of correcting the temperature-dependent gain spectrum inclination and keeping the gain constant can be fulfilled only by adjusting one parameter.

In the principle optical path shown in fig. 3, all signal light and pump light of the second section of erbium-doped fiber E32 are provided by the output of the first section of erbium-doped fiber E31, i.e. all input light entering the second section of erbium-doped fiber is controlled by the attenuation of attenuator V31. Under such conditions, the attenuator attenuation amount Δ A and the change amount of the average degree of inversion of the amplifier can be obtained by theoretical derivation

There is a linear relationship between:

$\mathrm{\&Delta;A}=4.343[({\mathrm{\&alpha;}}_p+g_p^{*})L-\frac{L}{L_2{(\stackrel{\&OverBar;}{N_2}/N_T)}_2}]\mathrm{\&Delta;}(\stackrel{\&OverBar;}{N_2}/N_T)---\left(5\right)$

here, L₂Is the length of the optical fiber E32,

is the average inversion of the section of fiber E32 when the attenuation of the attenuator is zero, and is constant for practical amplifiers. As can be seen by comparing (5), (4) and (3), the attenuation of the interpolated optical attenuator is adjusted to Δ G_K+ΔG_KThe requirement of' - Δ A ≈ 0 makes it possible to compensate for gain variations of the respective wavelengths due to temperature. When this requirement is satisfied, further theoretical derivation can be made that the linear relationship between the amount of change in attenuation and the amount of change in temperature of the attenuator is satisfied:

ΔA＝CLΔT (6)

wherein, $C=\frac{\frac{4.343{\mathrm{\&alpha;}}_K}{\mathrm{k\&Delta;\&lambda;}{{\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="C0314089400111.png"\; state="\{\{state\}\}">T</figure-callout>}}_0}^2}\left\{B\left({\mathrm{\&lambda;}}_K\right)\right[1-\frac{\stackrel{\&OverBar;}{N_2}}{N_T}-\frac{\stackrel{\&OverBar;}{N_2}}{N_T}\exp \left(\frac{\mathrm{\&epsiv;}\left({\mathrm{\&lambda;}}_K\right)-\mathrm{hc}/{\mathrm{\&lambda;}}_K}{{\mathrm{kT}}_0}\right)]-\frac{\stackrel{\&OverBar;}{N_2}}{N_T}[\mathrm{\&epsiv;}\left({\mathrm{\&lambda;}}_K\right)-\mathrm{hc}/{\mathrm{\&lambda;}}_K\left]\right\}}{1-\frac{({\mathrm{\&alpha;}}_K+g_K^{*})}{({\mathrm{\&alpha;}}_K+g_K^{*})-\frac{1}{L_2{(\stackrel{\&OverBar;}{N_2}/N_T)}_2}}}---\left(7\right)$

where C is a constant related to the parameters of the erbium-doped fiber used and the optical path structure, and can be calculated by (7) or can be estimated by fitting from the measured Δ T and Δ a data, and L is the total length of the erbium-doped fiber in the amplifier.

It is worth proposing that, in consideration of gain, noise factor, output power, gain spectrum flatness and the like, the actual amplifier is more complex than the structure shown in fig. 3, and generally adopts a multi-stage erbium-doped fiber, a plurality of pump lasers, a wavelength division multiplexer, a plurality of isolators, filters and the like to form a multi-stage complex structure, and at this time, an optical attenuator V for temperature characteristic compensation can be inserted between the original two stages, or can be inserted between a certain section of fiber, that is, the original one stage is subdivided into two stages. The specific insertion positions can be designed by conventional optimization methods of erbium-doped fiber amplifiers according to the principle of minimal impact on amplifier performance.

For example, in the structure shown in fig. 3, the light from another pump laser can be sent to the fiber E32 by another wavelength division multiplexer, and since the input light to the fiber E32 does not pass through the attenuation control of the attenuator V, the attenuation change Δ a and the change of the average inversion degree of the attenuator represented by the formula (5) are all controlledThe linear relationship between them does not hold fully. However, our further theoretical and numerical simulation results show that in the normal working temperature range, the linear relationship can still be considered to be true within the error range allowed by engineering application. Details will be given by the method of numerical simulation and experimental data in connection with the examples.

As mentioned above, if a 980nm laser is used as a pump, it can be approximated that the average inversion does not change with temperature, and thus the gain change Δ G is obtained_KThe variation Delta T with the environmental temperature satisfies the linear relation (3) and the linear control relation (6) of the attenuator. If 1480nm laser is adopted as a pump, the average inversion degree can be slightly changed along with the temperature; the change is temporarily ignored, the above theoretical analysis and the proposed temperature compensation method including the linear control relationship shown in equation (6) are fully applicable,whereas changes in average inversion due to temperature changes will only cause slight changes in gain level. That is, if the requirement of compensation accuracy is relaxed slightly, the method proposed by the present invention is also fully applicable to the case of using 1480nm pump light.

The invention has the characteristics that:

the interpolated optical attenuator adopting the method can simultaneously play the dual roles of adjusting the average inversion and adjusting the gain level, so that when the ambient temperature changes, the gain level can be kept unchanged while the gain spectrum is restored to be flat by only adjusting one parameter of the attenuator; in addition, in the normal working temperature range of the erbium-doped fiber amplifier, in order to keep the gain spectrum flat and the gain unchanged, the attenuation variation of the interpolation attenuator and the variation of the environment temperature are approximately in a linear relationship, so that the simple control operation relationship is very suitable for using a single chip microcomputer to control, and the small-volume module which is in favor of realizing the rapid intelligent control is provided.

Drawings

FIG. 1 is a gain spectrum of a typical L-band EDFA without temperature compensation under different temperature conditions, where the gain spectrum is tilted with temperature changes;

FIG. 2 shows the effect of temperature compensation by adjusting the pump power, i.e., the gain level decreases while the gain spectrum is restored to be flat;

FIG. 3 is a schematic diagram of an L-band EDFA structure for temperature characteristic compensation by using the interpolation variable attenuator of the present invention;

FIG. 4 is a diagram of an optical path structure of an embodiment of an L-band EDFA for temperature characteristic compensation using an interpolating variable attenuator according to the method of the present invention;

FIG. 5 is a graph showing the average inversion variation when a third erbium-doped fiber segment is injected with different pump powers, which is obtained by simulation calculation of the optical path structure shown in FIG. 4 according to the embodiment of the present inventionA relationship with attenuator attenuation change Δ A, an approximately linear relationship;

FIG. 6 shows the gain spectra actually measured at different temperatures after temperature compensation by the method of the present invention, the attenuators have attenuation amounts of 4.82dB, 3.72dB, 3.02dB and 2.12dB at temperatures of 26, 40, 55 and 70 ℃ respectively, and the variation of the compensated gain is not more than 0.3 dB;

FIG. 7 is a graph showing the relationship between the attenuation amount of the variable optical attenuator and the temperature during the temperature compensation process using the method of the present invention, in which the curve is a calculation result according to the equation (6) and the triangular points are experimental values.

The compensation method for the temperature-dependent gain spectrum characteristic of the L-band erbium-doped fiber amplifier according to the present invention is described in detail below with reference to the following embodiments and accompanying drawings:

an embodiment of the optical path structure used in the method of the present invention is shown in fig. 4. The optical fiber is a cascade optical path structure which is composed of three pump sources L41, L42 and L43, three wavelength division multiplexers W41, W42 and W43, three isolators I41, I42 and I43, three optical fibers E41, E42 and E43, a variable optical attenuator V41 for temperature compensation is arranged between a second section of erbium-doped optical fiber E42 and a third section of erbium-doped optical fiber E43, and F41 is a flat filter. The erbium-doped fiber is Lucent MP 1480L 092202 fiber, and the pumping source is 980nm semiconductor laser. The lengths of the erbium doped fibres E41 and E42 were 3.0m and 40.0m respectively, with corresponding pump powers of 60mW and 120mW respectively. The short fiber, low power pump first section is used to generate C waveguide ASE injected to the second section, which acts to suppress the amplified spontaneous emission power of the reverse leakage in the second section erbium doped fiber, thus greatly improving the pump efficiency. The length of the third section of erbium-doped fiber is 7.3m, and the maximum value of corresponding pumping power can reach 280 mW.

For the purpose of studying the application of the present invention to practical amplifiers, it was found that the pump power injected directly into the third erbium-doped fiber section after the add attenuator is notIf the equation (5) is satisfied in the case of zero, the present invention obtains the average degree of inversion by a numerical simulation method for the optical path structure of the embodiment shown in fig. 4

The relationship with the attenuation change amount Δ a of the optical attenuator is shown in fig. 5. Here, several cases were calculated in which the pump power PLD3 of the third erbium-doped fiber segment was 270mW, 180mW, 73mW, and 0mW, respectively, where, when PLD3 was 0mW, all the optical power input to the third erbium-doped fiber segment was subjected to attenuation control by V31, and the calculation results completely satisfied the linear relationship, which is consistent with the theoretical derivation results of the present invention. Other times when the pump power of the third erbium-doped fiber section is not zero, the sum of Delta A and Delta B

The relationship of (a) can be approximated to satisfy a linear relationship, and particularly when Δ a is not too large, the linear agreement is quite good. For the structure of the embodiment shown in fig. 4 and the specific parameters of the erbium-doped fiber used, the proportionality coefficient C in (6) is 0.00127dB/m/k calculated according to (7).

FIG. 6 shows the gain spectra actually measured at different temperatures for the experimental apparatus shown in FIG. 4, wherein the attenuations of the attenuators are respectively 4.82dB, 3.72dB, 3.02dB and 2.12dB at the temperatures of 26, 40, 55 and 70 ℃, and the variation of the gain after compensation is not more than 0.3dB, which is seen from the curve in the figure, and the effect is quite good. Fig. 7 shows the relationship between the attenuation and the temperature of the variable optical attenuator during the temperature compensation process, wherein the curve is the calculation result according to the equation (6), and the triangle points are the measured actual values, so that the theoretical calculation value and the experimental value are well matched, which indicates that the linear attenuation variation and the temperature relationship are adopted for control, thereby achieving a good compensation effect.

Claims (1)

1. A compensation method for the temperature-dependent gain spectrum tilt characteristic of an L-band erbium-doped fiber amplifier is characterized in that a variable optical attenuator is inserted between erbium-doped fiber sections of the fiber amplifier, and when the ambient temperature of the erbium-doped fiber is changed, the gain spectrum can be kept flat and the gain value is kept unchanged by only adjusting the attenuation of the variable optical attenuator;

the attenuation adjustment quantity delta A (dB) of the interpolation optical attenuator and the temperature variation quantity delta T (DEG C) are in a linear relation:

ΔA＝CLΔT

in the formula, C is a constant determined by parameters of the erbium-doped optical fiber and an optical path structure, and can be deduced by a fitting method according to actually measured data of delta T and delta A;

l is the total length of the erbium doped fiber in the amplifier.

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