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 Characteristics of L-band Erbium-doped Fiber Amplifier

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

Background technology In recent years, with the development of wavelength division multiplexing optical communication technology, the bandwidth required by the optical fiber communication system is getting wider and wider. ) extended to the long-wave band (L-band). After several years of hard work, the L-band EDFA has gradually entered into practical use, but there are still some technical problems to be solved. One of the problems that needs to be solved urgently is the temperature dependence of the gain spectrum, that is, the change of the gain spectrum with temperature.

In the wavelength division multiplexing system, the optical amplifier needs to amplify signals of several or even dozens of different wavelengths at the same time. In order to ensure the overall performance of the system, it is required that the gain of the amplifier for each channel is the same, that is, the gain spectrum is flat. For this reason, people use optical filters to level the originally uneven gain spectrum. When the number of input channels or the power of each channel changes and the gain spectrum changes, some methods such as adjusting the pump power or adjusting the auxiliary injection optical power are used to lock the gain spectrum. Gain spectrum, these techniques are collectively referred to as gain equalization techniques. In addition, people also found that the gain spectrum of EDFA will change with the change of ambient temperature, especially the L-band amplifier, due to the temperature sensitivity of the erbium-doped fiber itself and the relatively long erbium-doped fiber used, the change of the ambient temperature is more than C Waveguides are more sensitive. Figure 1 shows the measured results of an L-band EDFA: by using optical filters and adjusting the pump power, an ideal flat filter spectrum is obtained at room temperature (26°C), but when the temperature is at 26°C To 70 ℃, the gain spectrum also changes. The higher the temperature, the higher the gain of the short-band and the lower the gain of the long-band, that is, the gain spectrum is tilted, and the difference between gain increase/decrease can reach as much as 3dB in severe cases. The tilt of the gain spectrum will destroy the balanced transmission of the system, which cannot be tolerated by the WDM system.

In order to avoid or compensate the EDFA's gain spectrum tilt due to temperature changes, some methods have been proposed. One of the most direct methods is to put the erbium-doped fiber into the insulation box to keep the temperature of the erbium-doped fiber constant, but this not only increases the design requirements for packaging and circuits, but also brings about volume and power consumption. cost. Another way is to research and make erbium-doped fibers with opposite temperature characteristics, and then mix the two kinds of fibers with different tendencies. However, this method is not practical until the research on erbium-doped fibers with "reverse" temperature characteristics is mature. . Japan's J.Nakagawa et al. also proposed a way to use the automatic gain control (AGC) and automatic temperature control (ATC) functions at the same time, that is, to simultaneously adjust the attenuation of the interpolated variable attenuator and the injected auxiliary optical power to achieve Temperature compensation. Although this method is feasible, there are two parameters that need to be adjusted at the same time when the temperature changes, and the adjusted parameters are also related to the gain balance of the amplifier at a constant temperature. Such a complex control relationship is very important for the intelligent microcomputer control commonly required in modern engineering It is very disadvantageous.

SUMMARY OF THE INVENTION The purpose of the present invention is to overcome the deficiencies of the prior art and propose a new compensation method for the temperature-dependent gain spectrum slope characteristic of L-band EDFA. In this method, only one parameter needs to be adjusted, and the relationship between the adjusted parameter and the temperature variation is simple, which is beneficial to the use of a single-chip microcomputer to realize intelligent control.

The present invention proposes a compensation method suitable for the temperature-dependent gain spectrum inclination characteristic of the L-band erbium-doped optical fiber amplifier. When the temperature changes, only adjusting the attenuation of the variable optical attenuator can keep the gain spectrum flat and the gain value unchanged;

There is a linear relationship between the attenuation adjustment amount ΔA (unit: dB) and the temperature change amount ΔT (unit: ° C) of the adjusted interpolation optical attenuator:

ΔA=CLΔT

In the formula, C is a constant determined by the parameters of the erbium-doped fiber used and the optical path structure, which can be calculated by fitting method according to the measured ΔT and ΔA data; L is the total length of the erbium-doped fiber in the amplifier.

The optical attenuator can be inserted between two original stages of the optical fiber amplifier, or inserted in the middle of a section of optical fiber of a certain stage, that is, the original stage is re-divided into two stages. The specific insertion position can be designed through the conventional optimization method of the erbium-doped fiber amplifier according to the principle of having the least impact on the performance of the amplifier.

The optical attenuator and its control method of the present invention have become a common technical means in optical fiber communication equipment, and there are many commercially available optical attenuator products;

The monitoring of the temperature change is a commonly used technical means in modern general instruments and meters. At present, the commercially available erbium-doped fiber amplifier modules are generally equipped with temperature sensing elements and the temperature change is used as one of the monitoring parameters of the working state of the module.

Working principle of the present invention:

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

${\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; )</span>\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; l</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, α _si , g ^* _si and l _si are the absorption coefficient, emission coefficient and background loss coefficient of erbium-doped fiber to the i-th wavelength light respectively, and they all vary with different wavelengths, and i includes the signal light of each channel and pump light. L is the total length of the erbium-doped fiber in the amplifier. $\stackrel{\&OverBar;}{N_2}\&equiv;\frac{1}{L}{\&Integral;}_0^L{N}_2\left(z\right)\mathrm{dz},$ Be in the average value of the erbium ion concentration of upper energy level in the erbium-doped fiber along the length L of the erbium-doped fiber, _NT is the doping concentration of erbium ion in the fiber, then, It is called the average inversion degree of erbium ions along the length of the fiber. Theoretical results also show that the average degree of inversion It is determined by the number of pump photons consumed in the erbium-doped fiber and the number of signal photons increased:

$<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; L\&zeta;</span>}\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; hv</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}^{\mathrm{<span\; class="notranslate">\; out</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; si</span>}}^{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Here, P _si ^out and P _si ⁱⁿ are respectively the output optical power of each channel, the input optical power, or the remaining output pump power and the input pump power, hv _si is the energy of each channel photon or pump photon, ζ is the saturation parameter of the erbium-doped fiber. It can be seen from (1) that as long as the average inversion degree is maintained The gain of EDFA to each channel can be kept constant; from (2), it can be seen that when the input signal changes, as long as the input pump power is properly adjusted, the average inversion degree can be kept unchanged, which is the EDFA gain Fundamentals of Equilibrium.

When the ambient temperature changes, the distribution of erbium ions on each sub-energy level in the same energy level changes, that is, the absorption coefficient α _si and the emission coefficient g ^* _si of each wavelength change, according to the formula (1) It can be seen that this results in the slope of the temperature-dependent gain spectrum of the erbium-doped fiber.

Usually, the input signal optical power of an amplifier is much smaller than the input pump optical power, and the increased signal photon number (that is, the total output power of each channel) is completely determined by the consumed pump photon number; that is Say, if the remaining pump power is negligibly small, then It is completely determined by the input pump light power. The research also shows that the absorption coefficient and emission coefficient of light at 980nm wavelength do not change much with temperature. Therefore, the applicant concluded that if a 980nm laser is used as the pump, it can be approximately considered that the average inversion degree does not change with temperature, and the temperature-dependent gain spectrum tilt is completely determined by the emission coefficient g ^* _si and absorption of each signal wavelength The coefficient α _si changes with temperature. Further theoretical research shows that when the ambient temperature changes ΔT, the gain change ΔG _K is:

${\mathrm{<span\; class="notranslate">\; \&Delta;G</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 4.343</span>}{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="L\; kT"\; filenames="C0314089400111.png"\; state="\{\{state\}\}">L</figure-callout></span><figure-callout\; id="0"\; label="L\; kT"\; filenames="C0314089400111.png"\; state="\{\{state\}\}">\; </figure-callout>}}{{{\mathrm{<span\; class="notranslate">\; </span>}}_{}}^{}}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}<span\; class="notranslate">\; -</span>\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; h\&upsi;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; kT</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span>\frac{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&upsi;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; h\&upsi;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Among them, T ₀ is a certain median value in the temperature range in question, such _as room temperature; ε(υ _K ) is the energy required to excite erbium ions from the lower energy level to the upper energy level; According to the functions obtained through experimental measurements, the above quantities can be approximately considered to be independent of temperature. The formula (3) shows that the signal gain of each wavelength is directly proportional to the temperature variation ΔG _K and the temperature variation ΔT.

On the other hand, from formula (1), it can also be obtained that the relationship between the gain of each wavelength and the change of the average inversion number under the condition of constant temperature is also a linear relationship:

$\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 4.343</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L\&Delta;</span>}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Therefore, as long as the population inversion level is changed by some method, the gain spectrum tilt caused by temperature change can be corrected.

Fig. 2 is the experimental result of the applicant trying to correct the gain spectrum by changing the input pump power to change the particle population inversion level. It can be seen from the figure that when the temperature rises, the method of appropriately reducing the pump power can indeed restore the flatness of the gain spectrum, but because the average particle number inversion level decreases, the gain level also decreases. In order to compensate for the decrease of the gain level, other measures must be taken, such as inserting a variable optical attenuator in the EDFA to adjust the amplifier gain: when the temperature is low, the inserted attenuation is larger, and the pump power is reduced after the temperature rises. At the same time, the attenuation is also adjusted accordingly. But this method has two parameters that need to be adjusted, and the control relationship is relatively complicated, which is not ideal.

For this reason, the present invention proposes a method that only needs to adjust one parameter of the interpolation variable optical attenuator to realize the compensation of the slope characteristic of the temperature-dependent gain spectrum. The optical path structure of the amplifier principle using this method is shown in Figure 3. The output light of the pump laser L31 is combined with the input signal light through the wavelength division multiplexer W31 and sent to the erbium-doped optical fiber E31 for variable temperature compensation. Optical attenuator V31 is inserted between two sections of erbium-doped optical fiber 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 degree of the fiber E32 part, and then the average inversion degree of the entire amplifier is also changed; on the other hand, the actual net gain of the amplifier is equal to The sum of the gains of the two erbium-doped fibers subtracts the losses of the interpolated optical attenuator and other components. Therefore, adjusting the variable optical attenuator also changes the actual net gain of the amplifier. It is the double function of the interpolation optical attenuator to adjust the average inversion degree and the gain level of the amplifier, so only one parameter needs to be adjusted to achieve the purpose of maintaining a constant gain while correcting the tilt of the temperature-related gain spectrum.

间有线性关系：In the principle optical path shown in Figure 3, all the 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, that is, all the input light entering the second section of erbium-doped fiber All pass through the attenuation control of the attenuator V31. Under such conditions, the attenuation ΔA of the attenuator and the change of the average inversion degree of the amplifier can be obtained through theoretical derivation

There is a linear relationship between:

$\mathrm{<span\; class="notranslate">\; \&Delta;A</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 4.343</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; L</span>}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; ]</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

是当衰减器的衰减为零时光纤E32部分的平均反转度，对于实际的放大器，它们都是常量。对比(5)、(4)和(3)可以看到，只要调整内插光衰减器的衰减量以达到ΔG_K+ΔG_K′-ΔA≈0的要求，便可以补偿由温度引起的各波长增益变化。当满足这一要求时，进一步的理论推导可以得出，衰减器的衰减变化量和温度变化量之间满足线性关系：Here, _L2 is the length of fiber E32,

is the average inversion degree of the E32 part of the fiber when the attenuation of the attenuator is zero. For the actual amplifier, they are all constant. Comparing (5), (4) and (3), it can be seen that as long as the attenuation of the interpolation optical attenuator is adjusted to meet the requirement of ΔG _K +ΔG _K ′-ΔA≈0, each wavelength caused by temperature can be compensated Gain changes. When this requirement is met, further theoretical derivation can conclude that the attenuation change of the attenuator and the temperature change satisfy a linear relationship:

ΔA＝CLΔT (6)

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

Here, C is a constant related to the parameters of the erbium-doped fiber used and the optical path structure, which can be calculated by (7), or calculated by fitting method based on the measured ΔT and ΔA data, and L is the erbium-doped optical fiber in the amplifier. The total length of the fiber.

It is worth pointing out that due to the consideration of gain, noise figure, output power, and gain spectrum flatness, the actual amplifier structure is more complex than that shown in Figure 3. Generally, multiple erbium-doped fibers, multiple pump lasers, and wavelength division multiplexing are used. The optical attenuator V used for temperature characteristic compensation can be inserted between the original two stages, or in the middle of a certain section of optical fiber. The original level was re-divided into two levels. The specific insertion position can be designed through the conventional optimization method of the erbium-doped fiber amplifier according to the principle of having the least impact on the performance of the amplifier.

A practical multi-stage erbium-doped fiber amplifier generally has a corresponding pump source for each section of erbium-doped fiber. For example, for the structure shown in Figure 3, we can use another wavelength division multiplexer to connect the pump laser of another The light is sent into the optical fiber E32. At this time, since the input light input to the optical fiber E32 does not pass through the attenuation control of the attenuator V, the attenuation change amount ΔA of the attenuator and the change amount of the average inversion degree represented by the formula (5) The linear relationship between them cannot be fully established. However, our further theoretical and numerical simulation research results show that in the usual operating temperature range, this linear relationship can still be considered to be true within the error range allowed by engineering applications. Relevant details will be demonstrated through numerical simulation methods and experimental data in conjunction with examples.

As mentioned earlier, if a 980nm laser is used as the pump, it can be approximated that the average inversion degree does not change with temperature, and thus the gain change ΔG _K with the ambient temperature change ΔT satisfies the linear relationship (3) and the attenuation The linear control relationship (6) of the controller. If a 1480nm laser is used as the pump, the average inversion degree will vary slightly with temperature; this change is ignored temporarily, the above theoretical analysis and the proposed temperature compensation method including the linear control relationship shown in (6) are completely applicable , while the change in the average inversion degree caused by the temperature change will only slightly change the gain level. That is to say, if the requirement of compensation accuracy is slightly relaxed, the method proposed by the present invention is also fully applicable to the case of using pump light with a wavelength of 1480nm.

Features of the present invention:

The interpolation optical attenuator adopting the method of the present invention can simultaneously play the dual functions of adjusting the average inversion degree and adjusting the gain level, so when the ambient temperature changes, only one parameter of the attenuator needs to be adjusted to recover the flat gain spectrum. At the same time, the gain level is kept constant; moreover, in the usual operating temperature range of the erbium-doped fiber amplifier, in order to keep the gain spectrum flat and the gain constant, the attenuation variation of the interpolation attenuator and the variation of the ambient temperature are approximately linear. Such a simple control operation relationship is very suitable for the use of single-chip microcomputer control, which is conducive to the realization of fast intelligent control of small-volume modules.

Description of drawings

Figure 1 shows the gain spectrum of a typical L-band EDFA without temperature compensation under different temperature conditions - the gain spectrum is inclined when the temperature changes;

Figure 2 shows the effect of adjusting the pump power for temperature compensation—the gain level decreases while the gain spectrum is restored to flatness;

Fig. 3 adopts the interpolation variable attenuator of the inventive method to carry out the principle structural diagram of the L-band EDFA of temperature characteristic compensation;

Fig. 4 is the optical path structure diagram of an L-band EDFA embodiment using the interpolation variable attenuator of the present invention to compensate for temperature characteristics;

Fig. 5 is the average inversion degree variation obtained by simulating and calculating the optical path structure of the embodiment of the present invention shown in Fig. 4 when the third section of erbium-doped optical fiber is injected with different pump powers The relationship with the attenuation change ΔA of the attenuator—approximate linear relationship;

Figure 6. is the gain spectrum actually measured at different temperatures after temperature compensation using the method of the present invention. When the temperature is 26, 40, 55, and 70°C, the attenuation of the attenuator is respectively 4.82dB, 3.72dB, and 3.02dB and 2.12dB, the gain change after compensation does not exceed 0.3dB;

Fig. 7 is the relationship between the attenuation and temperature of the variable optical attenuator in the process of temperature compensation using the method of the present invention, wherein the curve is the calculation result according to (6), and the triangle point is the experimental value.

Specific embodiments The compensation method for the temperature-dependent gain spectrum characteristic of the L-band erbium-doped fiber amplifier proposed by the present invention is described in detail in conjunction with the embodiments and accompanying drawings as follows:

An embodiment of the optical path structure adopted by the method of the present invention is shown in FIG. 4 . This is a cascade connection consisting of three pump sources L41, L42, L43, three wavelength division multiplexers W41, W42, W43, three isolators I41, I42, I43 and three sections of optical fiber E41, E42, E43 Optical path structure, the variable optical attenuator V41 used for temperature compensation is placed between the second section of erbium-doped optical fiber E42 and the third section of erbium-doped optical fiber E43, and F41 is a flat filter. The erbium-doped fiber used is Lucent MP1480 L092202 fiber, and the pump source is a 980nm semiconductor laser. The lengths of the erbium-doped fibers E41 and E42 are 3.0 m and 40.0 m, respectively, and the corresponding pump powers are 60 mW and 120 mW, respectively. Here, the first section of the short fiber and low-power pump is used to generate the C-waveguide ASE injected into the second section, and its function is to suppress the amplified spontaneous emission power leaked backward in the second section of erbium-doped fiber, which can greatly improve the pumping efficiency. Pu efficiency. The length of the third section of erbium-doped fiber is 7.3m, and the corresponding maximum pump power can reach 280mW.

随光衰减器衰减变化量ΔA的关系，如图5所示。这里，计算了第三段掺铒光纤的泵浦功率PLD3分别为270mw、180mw、73mw以及0mW的几种情况，其中PLD3＝0mW时，输入到第三段掺铒光纤的所有光功率都经过V31的衰减控制，计算结果完全满足线性关系，与本发明的理论推导结果一致。其它当第三段掺铒光纤的泵浦功率不为零的时候，ΔA与

的关系可以近似满足线性关系，特别当ΔA不太大的时候，线性吻合相当好。针对图4所示实施例结构和所用掺铒光纤的具体参数，根据(7)计算得到(6)中的比例系数C＝0.00127dB/m/k。In order to study the situation of applying the present invention to practical amplifiers, namely whether (5) formula is established under the situation that the pumping power directly injected into the third section of erbium-doped optical fiber after the interpolation attenuator is not zero, the present invention aims at Fig. The optical path structure of the embodiment shown in 4 obtains the average inversion degree by numerical simulation method

The relationship with the attenuation variation ΔA of the optical attenuator is shown in Figure 5. Here, several cases where the pump power PLD3 of the third erbium-doped fiber is calculated are 270mw, 180mw, 73mw and 0mW respectively. When PLD3=0mW, all the optical power input to the third erbium-doped fiber passes through V31 The attenuation control of the calculation result fully satisfies the linear relationship, which is consistent with the theoretical derivation result of the present invention. Others When the pump power of the third erbium-doped fiber is not zero, ΔA and

The relationship of can approximately satisfy the linear relationship, especially when ΔA is not too large, the linear fit is quite good. For the structure of the embodiment shown in FIG. 4 and the specific parameters of the erbium-doped fiber used, the proportional coefficient C in (6) is calculated according to (7) = 0.00127dB/m/k.

Figure 6 shows the actual measured gain spectrum of the experimental device shown in Figure 4 at different temperatures. When the temperature is 26, 40, 55, and 70°C, the attenuation of the attenuator is 4.82dB, 3.72dB, and 3.02dB. and 2.12dB, from the curve in the figure, it can be seen that the gain change after compensation does not exceed 0.3dB, and the effect is quite good. Figure 7 shows the relationship between the attenuation and temperature of the variable optical attenuator in the above temperature compensation process, wherein the curve is the calculation result according to (6), and the triangle point is the actual value measured. It can be seen that the theoretical calculation value and the experimental value The values are in good agreement, indicating that the linear attenuation variation and temperature relationship can be used for control, and a good compensation effect can be achieved.

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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