# Numerical Simulation of 980 nm-LD-PumpedYb3+-Er3+-Tm3+-Codoped Fiber Amplifier for  1500 nm and 1600 nm Bands

Authors: Chun Jiang
Journal: Advances in OptoElectronics
 (2009)

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## Abstract

The theoretical model ofYb3+-Er3+-Tm3+-codoped fiber amplifier pumped by 980 nm laser
is proposed,  and the rate and power propagation equations are
numerically solved to analyze the dependences of the gains at
1500 nm and 1600 nm bands on the activator
concentrations, fiber length, pump power, and signal wavelength.
The numerical results show that our model is in good agreement
with experimental result, and with pump power of 200 mW and
fiber length varying from 0.15 to 1.5 m, the gains at the
two bands may reach 10.0–20.0 dB when the codoping
concentrations of  Yb3+, Er3+, and Tm3+ are in the ranges 1.0–3.0×1025, 1.0–3.0×1024, and 1.0–3.0×1024 ions/m3, respectively. The fiber parameters  may be optimized to flatten the gain spectra.


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## Body

## 1. Introduction

The All-wave fiber without residual OH group is being considered for Coarse Wavelength Division Multiplexing (CWDM) transmission systems and metro area optical networks because it has low-loss bandwidth of about 400 nm (1250–650 nm). WDM technology has been the most important technology of large-capacity optical transmission system, and optical amplifiers are key devices of WDM system. Although Fiber Raman Amplifier (FRA) using split-band and multipump schemes might simultaneously amplify multichannel signal within 130nm bandwidth, it would increase the power penalty of the system and degrade the performance of the system since the introduction of the multiplexing/de-multiplexing caused additional insertion loss [1]. Another kind of FRA could provide distributed amplification within 140 nm bandwidth using pump-signal interleaving method, but its gain spectra had relatively large ripple [1]. Later, a novel multiband FRA was reported theoretically providing flat bandwidth of 200 nm with ripple of 1 dB [2]. However, the FRA requires higher pump power due to its lower pump efficiency. Rare-earth doped fiber amplifiers have higher gain and pump efficiency, and in the past decades the researches on  rare-earth-doped fiber amplifiers have been focusing on the singly-doped fiber amplifiers, and all of these amplifiers have their own bandwidths, Er3+-doped fiber amplifier (EDFA) with new parallel configuration [3–5] was reported providing gain bandwidth of more than 100 nm, and Tm3+-doped- and Pr3+-doped fiber amplifiers [6–9]  could provide the gains in the ranges 1400–1500 nm, 1280–1340 nm, respectively. Recent reports on emission properties of Er3+-Tm3+-codoped silicate and telluride fibers showed that the combination of the emission at 1500 nm windows with that at 1400 nm windows in a single fiber may generate a large seamless emission spectra with emission width up to 200 nm in the codoped system [10–14]. The  upconversion and energy transfer in Yb3+-Er3+-Tm3+- and Er3+-Tm3+-codoped glasses were reported [15]. Energy transfer and upconversion luminescence of Bismuth, Tm3+/Ho3+, Tb3+/Yb3+, Er3+/Tm3+/Yb3+-codoped inorganic materials for display systems were reported [16–30]. In our previous works [31, 32], we presented the numerical model of Er3+-Tm3+ codoped telluride fiber amplifier pumped at 800 nm and calculated the dependence of the gains at 1470 nm and 1530 nm bands on the fiber parameters, and analyzed the transmission performance of the WDM system based on the codoped amplifier. Although the rate equations of Er3+-Tm3+, Er3+-Yb3+-Tm3+-codoped fluoride glasses was reported in [15], the equations just considered the upconversion of infrared to visible light for display system, did not consider the spontaneous and stimulated emissions of 1470 nm, 1530 nm, and  1630 nm bands. Other references on this codoped system just concentrated on the spectral properties. Owing to importance of the double and multidoped systems for broadband amplification of telecommunication wavelength, a theoretical model will be desirable for designing and optimizing the codoped broadband amplifiers. In present paper, we propose a new numerical model of Yb3+  and Er3+  and Tm3+ codoped fiber system pumped by 980 nm laser for amplification of the signal at 1470, 1530, and 1630 nm bands. This model considers the excited state absorption and upconversion of Er3+  and Tm3+  ions and cross-relaxation of Tm3+-Er3+. The dependence of the gain spectra covering 1500 and 1600 nm bands on codoping concentrations, fiber length, and pump power is calculated and analyzed.


## 2. Theoretical Model

Figure1 shows the schematic of the energy levels and electron transitions and energy transfer process of an Er3+-Tm3+-Yb3+ codoped telluride glass system pumped by 980 nm. With the excitation of 980 nm pump, the electrons of Yb3+  ions are excited from the ground state (2F7/2) to the excited state (2F5/2) and the energy at the 2F5/2  level transfers to the 4I11/2 level of Er3+  due to matching energy gap between the two levels, meanwhile, the energy at the level also can transfer to the 3F4  level of Tm3+  ion through multiphonon process. With the excitation, moreover, the electrons of Er3+  ions are excited from the ground state  4I15/2 to the excited state  4I11/2, then nonradiately relax to the 4I13/2  level through multiphonon process, and transit from the level to the ground state (4I15/2 level), emitting photons at 1500 nm band. In addition, the electrons at 4I11/2 level of Er3+ ions can be excited to the 4F7/2 level due to excited state absorption (ESA), and the electrons at the 4F7/2 level transit to the ground state with emission of red photon. On the other hand, energy transfer also can take place between Er3+-Tm3+ and Yb3+-Tm3+ via cross-relaxation process.  The energy at the 4I13/2 level of Er3+ ions and the 2F5/2 level of Yb3+  ions  can transfer to the  3F4, 3F2  levels of Tm3+ ions via the electron transition from the  3F4  level to 3F2  level of Tm3+ at which the electron nonradiately relax to 3H4 level from which is excited to the 1G4 level via another cross-relaxation process. Electron at the 1G4 level  transit to the 3F2,  3F4, and  3H6 levels with emission of lights at 1630, 650, and  476 nm bands, respectively. The electron at 3F2  level nonradiately relaxes to 3H4  level and then transites to the 3F4 level with emission of light at 1470 nm band. The electrons at 3F4 level transit to ground state (3H6), emitting the light at 1680 nm band.Figure 1
The schematic of energy level and transition configuration and energy transfer process of Yb3+,  Er3+  and  Tm3+  codoped telluride fiber system pumped with 980 nm  LD.According to Figure1, a rate equation group can be written as follows:
(1)∂N1∂t=(W12-W13)N1+(W21+A21)N2+WET28N2N7+W41N4-WET63N6N1,∂N2∂t=W12N1-(W21+A21)N2+A32N3-WET28N2N7,∂N3∂t=-A32N3+WET63N6N1-W34N3+W13N1-WET64N6N3,∂N4∂t=W34N3+WET64N6N3-W41N4,∂N6∂t=W56N5-WET63N6N1-WET64N6N3-WET68N6N7-WET6-10N6N8-WET6-11N6N9-A65N6,∂N7∂t=A87N8+W97N9-WET68N6N7-WET28N2N7+A11-7N11,∂N8∂t=-A87N8+WET68N6N7+WET28N2N7-(W8-10+WET6-10N6)N8+A11-8N11,∂N9∂t=A10-9N10-WET6-11N6N9-W97N9,∂N10∂t=-A10-9N10+(W8-10+WET6-10N6)N8-W10-11N10+W11-10N11+A11-10N11,∂N11∂t=W10-11N10-A11-10N11-W11-10N11+WET6-11N6N9-(A11-8+A11-7)N11,
where N1, N2, N3,N4 are the population densities of Er3+ at energy levels 1, 2, 3, and 4,  and  N5  and  N6  are the population densities of Yb3+  at energy levels 1 and  2 and N7, N8,N9, N10,N11 are the population densities of Tm3+ at energy levels 1, 2, 3, 4, and 5, W13, W56,  and W79   are the pump transition rates, Wij(i,j=1~11) is the transition rate  between energy levels i and j of the activating ions (Er3+, Yb3+, Tm3+), and WETij(i,j=1~11) is the transfer rate between energy level i and j. Aij(i,j=1~11) is the spontaneous transition rate  between energy level i and j. Ner, Nyb, Ntm  are total concentrations of Er3+, Yb3+, and Tm3+ ions, respectively. The transition  rates are as follows:
(2)W13=σ13PPhνPAeff,W56=σ56PPhνPAeff,W8-10=σ8-10PPhνPAeff,Wij=σijPshνsAeff,
where σ13,σ56,σ8–10   are the pump absorption cross-section, σij is the absorption  and emission cross-sections of the transitions between levels i and j. Aeff  is the effective cross-section area of the fiber.The power propagations of the pump and signal and amplified spontaneous emission (ASE) along the fiber are described by differential equation group (3), where PP is the pump power at 980 nm, PS1, PS2,  and PS3 are the power of the signals at 1470 nm, 1530 nm, and 1630 nm, respectively. PASE1, PASE2,PASE3  are the ASE powers in the 1470, 1530, and 1630 bands, and Γ_1470,  Γ_1530,  Γ_1630, and Γ_λASE  are overlap factors at 1470 nm, 1530 nm, and 1630 nm, and ASE wavelength, respectively, and υs,υp are signal and pump frequencies, respectively. h and α(v) are Plank constant, the frequency-independent back-ground loss of the active fiber, respectively,(3)dPS1dz=Γ1470(N9σ98-N8σ89)PS1-α1470PS1,dPS2dz=Γ1530(N2σ21-N1σ12)PS2-α1530PS2,dPS3dz=Γ1630(N11σ11-10-N10σ10-11+N8σ87)PS3-N7σ78PS3-α1630PS3,dPPdz=-Γ980[(N1σ13-N3σ31)+(N5σ56-N6σ65)+(N10σ10-8-N8σ8-10)]PP-α980PP,∂PASE1∂z=Γ(λASE1)(σ98N9-σ89N8)PASE1+Γ(λASE1)2hΔASE1υσ98N9-αPASE1,∂PASE2∂z=Γ(λASE2)(σ21N2-σ12N1)PASE2+Γ(λASE2)2hΔASE2υσ21N2-αPASE2,∂PASE3∂z=Γ(λASE3)(σ11-10N11-σ10-11N10+σ8-7N8)PASE3-σ7-8N7PASE3+Γ(λASE3)2hΔυASE3(N8σ8-7+σ11-10N11)-αPASE3.The above power propagation equation group forms a system of coupled differential equations, which will be solved by numerical integration using Newton and Lung-Kutta methods along the active fiber.  It was assumed that the energy transfer ratesWET_YE,WET_YT,WET_ET   were increasing functions of NYb,NEr,   and NTm  [33–35], and expressed in the following equation:
(4)WET_YE=1.0×10-22+4.0×10-49[(NYbNEr)1/2-1.0×1025],WET_ET=1.0×10-22+4.0×10-49[(NErNTm)1/2-1.0×1025],WET_YT=1.0×10-22+4.0×10-49[(NYbNTm)1/2-1.0×1025].


## 3. Results and Discussion

### 3.1. Comparison with Experimental Results

The spontaneous  spectra  of Yb3+-Er3+-Tm3+-telluride glasses excited near 980 nm were measured with two emission peaks [36]. The ratio of Yb3+, Er3+, Tm3+  ion concentrations in the glass samples with thickness of 1.5 mm was 10:1:1 and   10:1:2, two stronger emission peaks were observed  in the samples.  To verify our theoretical model, same parameters as the sample are used to calculated the luminescence intensity, and calculated emission spectra and measured emission spectra from the reference are normalized and plotted in Figure 2 where solid line represent calculated spectra, and the measured spectra from [36] are denoted using circle, it is shown that calculated spectra are in good agreement with the measured, verifying feasibility of our model.Figure 2
Comparison of calculated normalized emission intensity of  Yb3+-Er3+-Tm3+  codoped telluride glass excited at 980 nm  with measured normalized emission intensity, the measured results from [36].


### 3.2. Activator Concentration Dependence

The gain spectra is calculated by solving numerically the equation groups (1)–(3). Table 1 is the spectroscopic parameters for the Er3+-doped and Tm3+-doped telluride glass fiber for this calculation, the parameters of Yb3+ is from [36], the core diameter of fiber is 5 μm and numerical aperture is 0.21, and the input power of signal is -30 dBm.Table 1
Spectroscopic parameters of Er3+-doped and Tm3+  doped telluride fiber for numerical calculation.
ParameterSymbolValueUnitRemarksCore diameter2r5.0μmBackground lossα0.1dB/mSpontaneous emission rate (Er3+)A10172.41/s[37]A401804.31/sA2110001/sA32510001/sA54510001/sA65510001/sSpontaneous emission rate (Tm3+)a213111/s[37]a321311/sEnergy transfer rateWET11401/s[38]WET39851/sTransition rate of ASEA874001/s[37]A411804.31/sPump absorption cross sectionσp11.0 × 10-25m2[38]σp21.0 × 10-26m2σp31.0 × 10-25m2Stimulated emission cross section (Er3+)σ218.6 × 10-25m2[38]σ128.5 × 10-25m2Pump emission cross section (Er3+)σ311.0 × 10-26m2[38]Stimulated emission cross section (Tm3+)σ974.5 × 10-25m2[38]σ794.5 × 10-25m2σ878.6 × 10-25m2σ788.6 × 10-25m2Dependence of the gain spectra in the range 1450–1700 nm on Er3+ concentration is shown Figure 3(a).  With Yb3+ concentration of 20.0×1024 ions/m3 and Tm3+ concentration of 1.0×1024 ions/m and fixed fiber length at 1.5 m and when the Er3+ concentration increases from 1.0×1024  to 3.0×1024  ions/m3, the gains at the range 1510–1630-nm increase, where the gain at the 1530 nm  increases from 10.0 to 21.0 dB,  the gains beyond 1630 nm keep constant. Figure 3(b) demonstrates variation of the gain spectra with Tm3+ concentration. With Yb3+ concentration of 20.0×1024 ions/m3 and Er3+ concentration of 1.0×1024 ions/m and fixed fiber length at 1.5 m and when the Tm3+  concentration increases from 1.0×1024 to 3.0×1024   ions/m3, the gains at the range 1510–1650 nm increase, where the gain at the 1530 nm  increases from 10.5 to 17.8 dB,  and the gain at 1630 nm  increases from 7.0 to 9.8-dB . Effect of Yb3+ concentration on the gain spectra is shown Figure 3(c).  With  both Er3+ and Tm3+ concentrations of 1.0×1024 ions/m3 and fixed fiber length at 1.5 m and when the Yb3+ concentration increases from 10.0×1024  to 30.0×1024  ions/m3, the gains at the range 1510–1610 nm decrease, and the gain at the 1610–1650 nm  increase.Variation of the gain spectra with Er3+  ion concentration  (a), Tm3+  ion concentration (b), and Yb3+ ion concentration (c). Fiber length,  pump power, and  input signal power are 1.5 m, 980 nm, 200 mW, -30 dBm, respectively.
(a)(b)(c)


### 3.3. Fiber Length Effect

Effect of fiber length on the gain spectra is shown Figure4.  With Yb3+ concentration of 20.0×1024  ions/m3, Er3+ concentration of 1.0×1024 ions/m and Tm3+ concentration of 3.0×1024  ions/m,  when fiber length increases from 0.15 to 0.25 m, the gains at the range 1500–1630 nm increase, where the gain at the 1530 nm increases from 12.5 to 24.5 dB, and the gains at 1630 nm decreases from 10.0 to 5.0 dB .Figure 4
Dependence of the gain spectra with fiber length, the concentrations of Yb3+,  Er3+, Yb3+, ion are 20.0 × 1024, 1.0 × 1024, 3.0 × 1024 ions/m3,  respectively, and  pump power, and  input signal power are 200 mW, -30 dBm, respectively.


### 3.4. Variation with Pump Power

Variation of the gain spectra with pump power is shown in Figure5.  With Yb3+  concentration of 20.0 × 1024  ions/m3,  Er3+  concentration of 1.0 × 1024 ions/m and Tm3+ concentration of 3.0 × 1024 ions/m and fiber length of 1.5 m,  when pump power increases from 100 to 300 mW, the gains at the range 1500–1610 nm increase, where the gain at the 1530 nm  increases from -14.0 to 27.0 dB, and the gain at 1630 nm decreases from 10.0 to -1.0 dB.Figure 5
Effect of pump power on gain spectra. Concentrations of Yb3+, Er3+,  Yb3+ ion are 20.0 × 1024, 1.0 × 1024,  3.0 × 1024 ions/m3,  respectively. Fiber length, input signal power are 1.5 m,  -30 dBm, respectively.


## 3.1. Comparison with Experimental Results

The spontaneous  spectra  of Yb3+-Er3+-Tm3+-telluride glasses excited near 980 nm were measured with two emission peaks [36]. The ratio of Yb3+, Er3+, Tm3+  ion concentrations in the glass samples with thickness of 1.5 mm was 10:1:1 and   10:1:2, two stronger emission peaks were observed  in the samples.  To verify our theoretical model, same parameters as the sample are used to calculated the luminescence intensity, and calculated emission spectra and measured emission spectra from the reference are normalized and plotted in Figure 2 where solid line represent calculated spectra, and the measured spectra from [36] are denoted using circle, it is shown that calculated spectra are in good agreement with the measured, verifying feasibility of our model.Figure 2
Comparison of calculated normalized emission intensity of  Yb3+-Er3+-Tm3+  codoped telluride glass excited at 980 nm  with measured normalized emission intensity, the measured results from [36].


## 3.2. Activator Concentration Dependence

The gain spectra is calculated by solving numerically the equation groups (1)–(3). Table 1 is the spectroscopic parameters for the Er3+-doped and Tm3+-doped telluride glass fiber for this calculation, the parameters of Yb3+ is from [36], the core diameter of fiber is 5 μm and numerical aperture is 0.21, and the input power of signal is -30 dBm.Table 1
Spectroscopic parameters of Er3+-doped and Tm3+  doped telluride fiber for numerical calculation.
ParameterSymbolValueUnitRemarksCore diameter2r5.0μmBackground lossα0.1dB/mSpontaneous emission rate (Er3+)A10172.41/s[37]A401804.31/sA2110001/sA32510001/sA54510001/sA65510001/sSpontaneous emission rate (Tm3+)a213111/s[37]a321311/sEnergy transfer rateWET11401/s[38]WET39851/sTransition rate of ASEA874001/s[37]A411804.31/sPump absorption cross sectionσp11.0 × 10-25m2[38]σp21.0 × 10-26m2σp31.0 × 10-25m2Stimulated emission cross section (Er3+)σ218.6 × 10-25m2[38]σ128.5 × 10-25m2Pump emission cross section (Er3+)σ311.0 × 10-26m2[38]Stimulated emission cross section (Tm3+)σ974.5 × 10-25m2[38]σ794.5 × 10-25m2σ878.6 × 10-25m2σ788.6 × 10-25m2Dependence of the gain spectra in the range 1450–1700 nm on Er3+ concentration is shown Figure 3(a).  With Yb3+ concentration of 20.0×1024 ions/m3 and Tm3+ concentration of 1.0×1024 ions/m and fixed fiber length at 1.5 m and when the Er3+ concentration increases from 1.0×1024  to 3.0×1024  ions/m3, the gains at the range 1510–1630-nm increase, where the gain at the 1530 nm  increases from 10.0 to 21.0 dB,  the gains beyond 1630 nm keep constant. Figure 3(b) demonstrates variation of the gain spectra with Tm3+ concentration. With Yb3+ concentration of 20.0×1024 ions/m3 and Er3+ concentration of 1.0×1024 ions/m and fixed fiber length at 1.5 m and when the Tm3+  concentration increases from 1.0×1024 to 3.0×1024   ions/m3, the gains at the range 1510–1650 nm increase, where the gain at the 1530 nm  increases from 10.5 to 17.8 dB,  and the gain at 1630 nm  increases from 7.0 to 9.8-dB . Effect of Yb3+ concentration on the gain spectra is shown Figure 3(c).  With  both Er3+ and Tm3+ concentrations of 1.0×1024 ions/m3 and fixed fiber length at 1.5 m and when the Yb3+ concentration increases from 10.0×1024  to 30.0×1024  ions/m3, the gains at the range 1510–1610 nm decrease, and the gain at the 1610–1650 nm  increase.Variation of the gain spectra with Er3+  ion concentration  (a), Tm3+  ion concentration (b), and Yb3+ ion concentration (c). Fiber length,  pump power, and  input signal power are 1.5 m, 980 nm, 200 mW, -30 dBm, respectively.
(a)(b)(c)


## 3.3. Fiber Length Effect

Effect of fiber length on the gain spectra is shown Figure4.  With Yb3+ concentration of 20.0×1024  ions/m3, Er3+ concentration of 1.0×1024 ions/m and Tm3+ concentration of 3.0×1024  ions/m,  when fiber length increases from 0.15 to 0.25 m, the gains at the range 1500–1630 nm increase, where the gain at the 1530 nm increases from 12.5 to 24.5 dB, and the gains at 1630 nm decreases from 10.0 to 5.0 dB .Figure 4
Dependence of the gain spectra with fiber length, the concentrations of Yb3+,  Er3+, Yb3+, ion are 20.0 × 1024, 1.0 × 1024, 3.0 × 1024 ions/m3,  respectively, and  pump power, and  input signal power are 200 mW, -30 dBm, respectively.


## 3.4. Variation with Pump Power

Variation of the gain spectra with pump power is shown in Figure5.  With Yb3+  concentration of 20.0 × 1024  ions/m3,  Er3+  concentration of 1.0 × 1024 ions/m and Tm3+ concentration of 3.0 × 1024 ions/m and fiber length of 1.5 m,  when pump power increases from 100 to 300 mW, the gains at the range 1500–1610 nm increase, where the gain at the 1530 nm  increases from -14.0 to 27.0 dB, and the gain at 1630 nm decreases from 10.0 to -1.0 dB.Figure 5
Effect of pump power on gain spectra. Concentrations of Yb3+, Er3+,  Yb3+ ion are 20.0 × 1024, 1.0 × 1024,  3.0 × 1024 ions/m3,  respectively. Fiber length, input signal power are 1.5 m,  -30 dBm, respectively.


## 4. Discussion

In above results, one notes that with increasing Yb3+ concentration from 1.0 × 1025 to 3.0 × 1025  ions/m3, the gains at the range 1510–1610 nm decrease, and the gains at the 1610–1650 nm increase, showing different Yb3+ ion concentration dependences of gain spectra in different wave band. These different dependences, we think, may be explained using energy transfer from Yb3+  to Er3+  and from Yb3+ to Tm3+ ions. When Yb3+  ion concentration increases, the pump absorption induced by Yb3+ increases and the pump absorption by Er3+  decreases, this leads to (1) increased energy transfer from Yb3+  to Tm3+  ions and increased population inversion between 3H4  and 3F4  of  Tm3+  ions and thus increased gain at 1630 nm band, and (2) increased energy transfer from Yb3+ to Er3+ ions, but reduced pump absorption of Er3+  probably dominates over the increased energy transfer, thus results in reduced population inversion between the 4I13/2  and  4I15/2  levels of Er3+  and decreased gain at 1530 nm band.Meanwhile, one also notes that with increasing pump power, the increasing of the gain spectra in 1500–1610 nm and the decreasing of the gain spectra in 1610–1650 nm show different pump-power effect on the gain spectra in different wave band. These different effects  may be explained using energy transfer from Yb3+  to Er3+  and from Yb3+ to Tm3+  ions. With the power increasing, the pump absorptions caused by Yb3+  and Er3+  and Tm3+  increase simultaneously, this leads to  increased population inversion between the 4I13/2 and 4I15/2  levels of Er3+  and thus increased gain at 1500–1610 nm band. The efficiency of  the increasing of pump absorption caused by Tm3+  is less than that caused by Er3+  because the former depends highly on the population number of the 3F4  level of Tm3+  ions coming from energy transfer from Yb3+ and Er3+ to Tm3+. Moreover, increasing pump absorption can increase the population of the 3F2  level of Tm3+ ions, hence the population inversion between  1G4  and  3F2 of  Tm3+  ions and the gain at 1610–1650 nm band is reduced if the lifetime at  1G4  level is shorter than that at  3F2  level.It is shown from Figures3, 4, and 5 that two weak peaks in the gain spectra appear at 1470 nm and 1680 nm, resulting separately from the transitions from the  3H4  to  3F4  levels and  from  3H4  to  3H6 levels of  Tm3+  ions. If  both pumps at 800 nm and 980 nm are used, the gain spectra covering 1470 nm, 1530 nm, 1630 nm, 1680 nm bands may appear, further study is needed to  optimize the fiber parameter and pump configuration to reduce the ripple of  the gain spectra.


## 5. Conclusions

In conclusion, we present the numerical model of  Yb3+-Er3+-Tm3+-codoped fiber amplifier pumped by 980 nm laser and analyzed the dependences of the gains at 1530 nm and 1630 nm on the activator codoping concentrations, fiber length, signal wavelength. The numerical results showed that our model was in good agreement with experimental result, and with pump power of 200 mW and fiber length varying from 0.15 to 1.5-m, the gains at the two bands could reach 10.0–20.0 dB when the codoping concentrations of  Yb3+,  Er3+, and  Tm3+ were in the ranges 1.0–3.0 × 1025, 1.0–3.0 × 1024, 1.0–3.0 × 1024  ions/m3, respectively.  Two weak peaks in the gain spectra appeared at 1470 nm and 1680 nm, resulting separately from the transitions from the  3H4  to  3F4  levels and  from  3H4  to  3H6 levels of  Tm3+  ions. If  both pumps at 800 nm and 980 nm will be used, the gain spectra covering 1470 nm, 1530 nm, 1630 nm, 1680 nm bands may appear. The optimization of the fiber parameter and two-pump configuration will be desirable for reducing  the ripple of  the gain spectra.



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*Source: 278105-2009-06-28.xml*