1. Introduction

Increasing capacity of fiber communication systems necessitates the development of fiber amplifiers and lasers operating in the spectral region 1300–1500 nm. Substantial progress has been achieved in the construction of Bi-doped fiber lasers and amplifiers (see review [1] and references therein). Bi-doped fibers exhibit IR luminescence and optical gain in the wavelength range 1140–1550 nm. However, the efficiency of Bi fiber lasers is comparatively low. Increasing the efficiency of fiber lasers seems difficult without an adequate model for optical active Bi centers responsible for luminescence and amplification. While several models based essentially on the presence of Bi atoms in a glass have been proposed in the literature (e.g., Bi⁵⁺ [2, 3], Bi⁺ [4–7], Bi₂-, Bi₂ ²-, Bi₂/Bi₂ -[8, 9]), neither of them has been experimentally confirmed.

Another approach to the origin of IR active centers in glasses was offered in [10]. It was shown that glasses doped with Pb, Sn, Sb [10], and Te [11] reveal within certain experimental conditions approximately the same luminescence spectra in IR region as Bi-doped glass. Relying on this data, point defects in glass were proposed as possible candidates for optical active centers regardless of the sort of doping element.

It should be noted that IR and visible luminescence in a wavelength region λ<1 µm was observed in a number of Pb-doped crystals (see, e.g. [12–14]). Color centers in these crystals were formed using preliminary x-irradiation. But the active centers in Pb- and Bi-doped glasses do not need x-ray radiation for their formation. For this reason models of color centers in Pb-doped crystals can hardly be used for active centers in Pb-doped glasses.

This paper reports the first observation of the luminescence and optical gain in Pb-doped germanosilicate fibers and studies the optical characteristics of Pb-doped and Bi-doped fibers of the same core composition.

2. Characteristics of fabricated fiber

We fabricated the preforms for Pb-doped germanosilicate (GSPb) fibers by MCVD-technique. Germanosilicate glass of the fiber core was deposited from the vapor phase. Pb was incorporated by a solution technique: a porous germanosilicate glass layer on the inner wall of the substrate tube was infiltrated with a water solution of Pb(NO₃)₂. The preform core composition was measured using X-ray spectral microanalysis. The preform contained 5 at% of germanium, 0.03 at% of Pb, the rest was oxygen and silicon. The maximal value of refraction index difference between the core and the fiber cladding was Δn≈0.03. The mode field diameter in the single mode GSPb fiber was equal to 3.1 µm.

3. Results

The optical loss spectrum for the GSPb fiber is shown in Fig. 1(line 1). Since optical properties of similar objects – Bi-doped optical fibers – depend tangibly on temperature (see, e.g. [1]), it is essentially to note that all experimental results in this paper were obtained at room temperature. Fig.1 shows also the loss spectrum of the gemanosilicate fiber with the same concentration of Ge as GSPb but doped with Bi (GSB) instead of Pb. The Bi concentration in GSB fiber was less than 0.02 at% (this value corresponds to the sensitivity threshold of our method of measurement). Optical properties of GSB fiber, optical ampification and laser generation in it were described in more details earlier [15]. The optical loss spectrum of GSPb fiber includes an absoption band (or bands) with maximum at the wavelength λ<400 nm. The long wavelength edge of this band is distinctly observed at λ=600 nm. The loss spectrum contains also complicated (plateau-like) band between 600 and 1150 nm. In the long wavelength region of this spectrum we observe reduction of loss level from 1 dB/m to 0.1 dB/m in the range 1150–1600 nm. Absorption bands at ≈940, 1240 and 1400 nm belong to OH groups superimposed on the GSPb spectrum. This is confirmed by a coincidence of the position and shape of these bands with similar bands observed in absorption spectrum of the germanosilicate fiber of the same composition but without Pb doping. And finally an increase of loss level in the region 1620–1700nm is observed. Comparison of GSPb and GSB fiber loss spectra in Fig. 1 indicates an essential change in spectra after substitution of Pb for Bi in germanosilicate glass. Energy level structures of optical active centers associated with Pb (PbAC) and with Bi (BiAC) are thus substantially different.

 

Fig. 1. Optical loss spectra of GSPb (1) and GSB (2) fibers.

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GSPb fiber, being pumped by radiation at various wavelengths (Fig. 2a), reveals IR and visible luminescence. GSPb fiber under pump radiation at λ_p=1058 nm (single mode Yb fiber laser) luminesces in wavelength band with maximum at λ_max=1140 nm and full width at half maximum (FWHM) ≈90 nm (Fig. 2a, line 1). Being excited at λ_p=975 nm (single mode laser diode) GSPb fiber shows luminescence in the same band (Fig. 2a, line 2). The additional peak at 1018 nm corresponds to the Raman scattering in the fiber with a frequency shift of ≈440 cm^-1.

Pumping at λ_p=800 nm results in weak luminescence with λ_max=1050 nm. Finally, luminescence spectrum of GSPb fiber pumped by the second harmonic radiation of the solid state Nd laser at λ_p=457 nm reveals a broad asymmetric band in the range 600–1000 nm with λ_max=700 nm. The observed luminescence spectra of GSPb fiber differ essentially from luminescence spectra of GSB fiber shown in Fig. 2b. This fiber being pumped at λ_p=1058 nm (Fig. 2b, line 1) and λ_p=975 nm shows very weak luminescence below threshold sensitivity of our recording scheme. Pumping at λ_p=800 nm results in bright luminescence band with maximum at 1400 nm [15]. Luminescence spectrum of GSB fiber pumped at λ_p=457 nm (Fig. 2b, line 4) reveals a broad complicated band between 600 and 1000 nm with two superimposed narrow bands with λ_max=825 and 930 nm. What luminescent spectra of GSPb and GSB fibers have in common is the broad band luminescence in the wavelength range 600–1000 nm under λ_p=457 nm radiation. Because of this, the above-mentioned luminescence band can be presumably attributed to the glass matrix defects. All other luminescence bands in GSPb and GSB fiber spectra are different and, apparently, are associated with doping atoms. So, distinctions between luminescence spectra of GSPb and GSB fibers is further evidence of the difference in structure of PbAC and BiAC in germanosilicate glass.

 

Fig. 2. Luminescence spectra of GSPb (a) and GSB (b) fibers excited at λ_p=1058 nm (1), λ_p=975 nm (2), λ_p=800 nm (3), and λ_p=457 nm (4). Marks ✱ label Raman scattering peaks at the line 2(a) and line 1(b).

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It is interesting to note the similarity of luminescence spectra of GSPb and Bi-doped aluminosilicate fibers [15] (fibers have considerably different core compositions) pumped at the same wavelength λ_p=1058 nm. These spectra (Fig. 3) have close values of λ_max, but the FWHM of the luminescence band in Bi-doped aluminosilicate fiber is substantially higher: ≈150 nm.

 

Fig. 3. Luminescence spectra of GSPb fiber (1) and Bi-doped aluminosilicate fiber (2) excited at λ_p=1058 nm.

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We have also observed and measured the on/off optical gain in GSPb fiber pumped at λ_p=1058 nm. A simple scheme of linear optical amplifier, similar to the one used in [16], was used for measuring optical gain.

A light emitting diode was used as a signal source in the wavelength band 1100–1300 nm. The results obtained are shown in Fig. 4. The maximal value of the on/off gain g(λ) was observed at the wavelength 1140 nm.

 

Fig. 4. On/off gain spectra for 40 m long PbGS fiber pumped at λ_p=1058 _HM. The pump power is indicated for each spectrum. The inset shows gain saturation at the wavelengths 1140 and 1180 nm.

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The value of g(λ) increased with pump power reaching saturation at ≈200 mW. This corresponds to the saturation pump intensity of ≈2 MW/cm². The maximal value of g(λ) reached ≈0.25 dB/m. That is substantially (4 times) lower than low-level signal absorption in GSPb fiber at the same wavelength. For this reason real amplification was not attained in our GSPb fiber.

The luminescence decay time was also measured. The GSPb fiber was pumped by pulse radiation (duration 1 ms) at the wavelength λ_p=975 nm. Luminescence intensity was measured in the wavelength band 1000–1300 nm. The time dependence of the luminescence intensity I (t) after the pump turn-off is shown in Fig. 5.

 

Fig. 5. Luminescence decay in GSPb fiber pumped by pulse radiation at 975 nm.

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The observed luminescence decay can be described with sufficient accuracy as a sum of two exponents I(t)=A ₁exp·(t/τ₁)+A ₂·exp(t/τ₂), where τ₁=4.5µs, τ₂=40µ and A ₂/A ₁≈2. For comparison, luminescence relaxation in volume specimens of Pb-doped aluminogermanate glass also includes short- and long-lived exponents [10]. The time constant of the latter is ~400 µs. For the GSPb fiber, the corresponding time constant of the long-lived component is an order of magnitude lower. There is substantial difference between luminescence spectra (e.g. at pump wavelength 800 nm) of these two glasses. These facts suggest a different structure of active centers in GSPb fibers and aluminogermanate glass [10] due to differences both in composition and in the technology of preparation.

The data obtained enable us to make some estimations. On the assumption that the gain saturation intensity of the GSPb fiber can be estimated as I_sat=hω/στ, where hω is the pump quantum energy, σ is absorption cross section of pump radiation, and τ~10µs is the effective time of luminescence decay. The measured value of I_sat~2MW/cm ² sat enables us to estimate σ-10^-20 cm ². But in GSPb fiber at λ_p=1058 nm the low signal absorption coefficient n dB m AC α=σ·n_AC≈1dB/m (here n_AC is the PbAC density). From the last equation n_AC can be estimated as n_AC~2·10¹⁷ cm ^-3, whereas the direct measurement of Pb atoms concentration gives the value n_Pb~6·10¹⁸ cm ^-3. The discrepancy between n_AC and n_Pb values indicates that the active center concentration in the core of the GSPb fiber is ~30 times lower than concentration of Pb. Therefore, only one in thirty Pb atoms takes part in the formation of the active centers in the core glass.

4. Conclusion

In summary, we have constructed a Pb-doped germanosilicate optical fiber which exhibits luminescence and optical gain in the wavelength range 1100–1200 nm. The concentration of the PbAC in core glass is shown to be approximately 1.5 orders lower than the concentration of Pb atoms. The experimental data obtained (the absorption, luminescent and optical gain spectra) show that PbAC and BiAC in germanosilicate optical fibers are essentially different. Further researches are necessary for understanding the physical nature of NIR active centers in Pb- and Bi-doped glasses and fibers.