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The work in the thesis entitled “ERBIUM AND YTTERBIUM DOPPED PHOSPHOTELLURITE GLASSES FOR LASERS AND OPTICAL AMPLIFIERS” was carried out by me under the supervision of Dr. The increase in phonon energy and Tg with the addition of phosphate in tellurite glass would facilitate the fabrication of waveguide laser and amplifier.

Table 1.1: Solid State Lasers, DP=Diode pumped.
Table 1.1: Solid State Lasers, DP=Diode pumped.

Rare earth doped photonic devices

Erbium doped fibre amplifier(EDFA)

Chemical stability is required to cut and polish to good optical quality in the required size and geometry. The measured photoluminescence spectrum of commercial Yb-Er doped phosphate glass and the different spectral bands are shown in Fig.

Figure 1.1: Loss of a full spectrum fibre of silica glass, where S Band is 1460-1530 nm, C Band is 1530-1550 nm, L Band is 1550 to 1610 nm [12].
Figure 1.1: Loss of a full spectrum fibre of silica glass, where S Band is 1460-1530 nm, C Band is 1530-1550 nm, L Band is 1550 to 1610 nm [12].

Erbium doped waveguide amplifier (EDWA)

Fibre lasers

Laser glasses

The high non-linear refractive index and the low phonon energy make the tellurite glass fibers uniquely suitable for non-linear and laser applications. Spectroscopic properties of Er doped boron tellurite [50] and germano-tellurite [41] glasses reported for 1.5 µm amplification.

Erbium doped glasses

Energy levels

All these LS-coupled levels are again Stark split in the central field approximation, giving rise to the broadening of the absorption and emission spectra. The ions at level E2 can then return to the ground state by releasing their energy in the form of photons.

Figure 1.3: Different energy levels of Er doped silicate glass with the respective lifetimes.
Figure 1.3: Different energy levels of Er doped silicate glass with the respective lifetimes.

Ion-Ion Interactions

The lifetime of the excited ions in the state where the ESA occurs is also a factor influencing this effect. Another reason for the quenching is the clustering of the rare earth ions in the host matrix.

Figure 1.4: Different energy transfer mechanisms of Er and Er-Yb doped glass. (a) Yb-Er energy transfer, (b) Er-Er energy transfer and co-operative upconversion, (c) Pump ESA, (d) Fluorescence quenching
Figure 1.4: Different energy transfer mechanisms of Er and Er-Yb doped glass. (a) Yb-Er energy transfer, (b) Er-Er energy transfer and co-operative upconversion, (c) Pump ESA, (d) Fluorescence quenching

Yb doped glasses

Energy levels

Stimulated emission of Yb3+ involves transitions between Stark levels of the 2F7/2 and 2F5/2 electronic states at wavelengths in the range 1.01–1.06µm [72]. These are the only levels of the 4f13 ground configuration; levels of excited configurations and charge transfer states are in the ultraviolet.

Research scheme for rare earth doped laser glasses

Absorption cross-section

Judd-Ofelt theory

The Judd-Ofelt intensity parameters Ωt can be derived from the electric-dipole contributions of the experimental oscillator strengths using a least-squares fit. The Ωt parameters are important for investigating the local structure and bonding in the vicinity of rare earth ions in the glass.

Emission cross-section

JO, where τm is the measured lifetime and τJO O is the radiation lifetime calculated from the JO analysis. It is a parameter of the efficiency of a rare earth host, more is η, better is the host.

Bulk modification in transparent materials by an ultrafast laser

The refractive index changes observed at low energy are attributed to localized melting and rapid solidification of the glass [91]. As a result, energy is transferred to the substrate during the excitation of the electrons by the laser pulse, which causes the area around the focal point to heat up.

Figure 1.7: (a) Longitudinal (b) Transverse writing geometry of channel waveguides using a femtosecond laser.
Figure 1.7: (a) Longitudinal (b) Transverse writing geometry of channel waveguides using a femtosecond laser.

Superfluorescent fibre sources

The presence of spontaneous emission degrades the performance of the fiber amplifier by adding unwanted power near the signal wavelength in the form of noise. Many of the complications known to SFS and Er-doped fiber amplifiers are avoided such as excited state absorption (ESA) and concentration quenching by inter-ion energy transfer.

Conclusion

From the infrared transmission spectrum we found that the maximum phonon energy of phosphotellurite glasses is also high (~1290 cm−1). Addition of phosphate, due to its intermediate phonon energy (~ 1200 cm−1) in tellurite glass is expected to reduce the lifetime of the 4I11/2 level from multiphonon relaxation.

Experiments

The absorption of the glass samples in the NIR around 1.5 µm was taken using the Perkin Elmer Spectrum BX FTIR spectrometer with 1 cm−1 resolution. The fluorescence lifetime of the erbium Er3+: 4I13/2 level of the glass samples is measured with an InGaAs detector and a 200MHz digital storage oscilloscope (Tektronics TDS 2024).

Physical and thermal properties

The glass transition temperature Tg and refractive index of the glasses were measured using a differential scanning calorimeter (Perkin Elmer DSC-7) and a prism coupler (Metricon 2010), respectively. The results obtained with DSC show that the Tg values ​​of phosphotellurite glass are much higher than those of sodium tellurite glass (NT) (Table 2.1), making them suitable for fabrication of planar waveguides by ion exchange.

Bulk phonon energy

Therefore, the maximum phonon energy of the phosphorite tellurite glasses is obtained as ~1290 cm−1, which is higher than that of the sodium tellurite (NT) glass and many other tellurite hosts, and less than only for boron tellurite glass reported in literature [50] . From the figure it is clear that there is a high probability of depopulation of the excited atoms in the laser plane E2 via multiphonon relaxation in borate glasses due to there much higher phonon energy.

Figure 2.2: DSC scan of NT(solid), NPT1(dash), NPT2(dot) and LPT(dash-dot)
Figure 2.2: DSC scan of NT(solid), NPT1(dash), NPT2(dot) and LPT(dash-dot)

Multiphonon relaxation

Absorption and emission cross-sections

The peak emission diameter and the bandwidths for all the sample are reported in Table 2.6. The absorption and emission cross-sections of NT, NPT1 and LPT differ very little, while those of NPT2 are slightly high.

Figure 2.5: Absorption cross-section spectra around 980 nm for samples NT(solid), NPT1(dash), NPT2(dot) and LPT(dash-dot).
Figure 2.5: Absorption cross-section spectra around 980 nm for samples NT(solid), NPT1(dash), NPT2(dot) and LPT(dash-dot).

Judd-Ofelt analysis and quantum efficiency

However, the replacement of the alkali ion sodium with lithium in phosphotellurite glasses reduced the cross-sections. The values ​​of the Judd-Ofelt intensity parameters Ωt, rms deviation δrms are listed in Table 2.9.

Figure 2.9: The room temperature absorption spectrum of the samples NT(solid), NPT1(dash), NPT2(dot) and LPT(dash-dot) in the wavelength range λ= 350 nm to 1100 nm
Figure 2.9: The room temperature absorption spectrum of the samples NT(solid), NPT1(dash), NPT2(dot) and LPT(dash-dot) in the wavelength range λ= 350 nm to 1100 nm

Conclusion

The absorption spectra show that there is a blue shift of the absorption edge of phosphotellurite glass compared to NT. The calculated radiation lifetime of the phosphotellurite glasses is high compared to sodium tellurite glasses.

Experiment

The effect of Yb-Er coding on the fluorescence properties under 980 nm excitation is also studied in phosphotellurite glass. Fluorescence spectra of the samples at room temperature are recorded in an optical spectrum analyzer (Agilent 86142B) using a cw 980 nm laser diode (JDS uniphase) as the excitation source.

Figure 3.1: XRD pattern of a sample before and after the annealing.
Figure 3.1: XRD pattern of a sample before and after the annealing.

Glass transition temperature

This is due to the hijacking of the glass network by the PO4 tetrahedra incorporated into the chains. The decrease in the A/B ratio is indicative of inhibition of the chains of -Te-O- Te- and -Te-O-P-.

Figure 3.3: (a) IR absorption spectra with peak fit of Sample NT, (b) IR absorption spectra with peak fit of Sample NPT1, where dotted lines are the gaussian fits with the original data plotted in solid lines and dot-dashed lines are the fit to the experimenta
Figure 3.3: (a) IR absorption spectra with peak fit of Sample NT, (b) IR absorption spectra with peak fit of Sample NPT1, where dotted lines are the gaussian fits with the original data plotted in solid lines and dot-dashed lines are the fit to the experimenta

Fluorescence bandwidth

The reason for this is the breaking of the glass network and the formation of more TeO3+δ and TeO3 polyhedra. The comparison of the two structures shows that the addition of phosphate in the sodium tellurite glass changed the local environment of Er3+ ions.

Figure 3.6: Comparison of emission cross-section spectra of two sample 0.25Er and 1.75Yb.
Figure 3.6: Comparison of emission cross-section spectra of two sample 0.25Er and 1.75Yb.

Radiative properties

Other significant trend observed is the decrease in peak absorption cross section with the increasing Er concentration. Addition of Yb reduced the emission cross-section of the Er-doped glass at 1535 nm as shown in the table and in Fig.

Figure 3.12: Absorption spectra of the 1.75Yb glass sample.
Figure 3.12: Absorption spectra of the 1.75Yb glass sample.

Conclusion

The glasses show high absorption and emission cross sections and a longer lifetime of the Yb3+ transition: 2F5/2→2F7/2. The dependence of the absorption and emission cross sections and the lifetime of the Yb3+ laser level on many glass compositions, such as borate, phosphate, silicate, have been extensively studied in the past.

Experimental procedure

The emission cross-section spectra are calculated from the measured absorption cross-section spectra using the reciprocity method (RM) [133] and the integral method of reciprocity (IMR) [134]. The room temperature fluorescence spectra are recorded on an optical spectrum analyzer that uses the same laser diode as the excitation source.

Physical and thermal properties

The VHN is 299.3 for 30PT5Yb, which is the highest among all the samples used in this study. From a comparison of the VHN of 30PT3Yb (284.5) and that of 35PT3Yb (273.8), it is further deduced that the microhardness value decreased with the increase in P2O5 concentration.

Figure 4.1: Absorption cross-section spectra (solid line ) and stimulated emission cross- cross-section spectra calculated by reciprocity method(RM)(dash line ) and integral method of reciprocity(IMR)(dotted line) are shown for the samples 30PT1Yb, 30PT3Yb
Figure 4.1: Absorption cross-section spectra (solid line ) and stimulated emission cross- cross-section spectra calculated by reciprocity method(RM)(dash line ) and integral method of reciprocity(IMR)(dotted line) are shown for the samples 30PT1Yb, 30PT3Yb

Spectroscopic properties

It is also possible to determine the stimulated emission cross section from the absorption cross section and radiative lifetime spectra of Yb3+ ions using a slightly modified procedure called the integral reciprocity method (IMR)[134]. The absorption and stimulated emission cross-section spectra (σe(RM) and σe(IMR)) of the 35PT3Y sample are shown in Figures 4.1 and 4.2.

Laser performance parameters

The measured fluorescence spectrum of two samples with the same Yb concentration and different phosphate concentrations is given in Fig 4.3. The fluorescence decay curves for all the samples are shown in Fig 4.4 and the measured fluorescence decay times are given in Table 4.3.

Figure 4.4: Measured fluorescence decay data for 30PT1Y (circle), 30PT3Y (square), 30PT5Y (diamond) in (a) and 35PT1Y (circle), 35PT3Y (square) in (b)
Figure 4.4: Measured fluorescence decay data for 30PT1Y (circle), 30PT3Y (square), 30PT5Y (diamond) in (a) and 35PT1Y (circle), 35PT3Y (square) in (b)

Discussion

The emission cross sections calculated by both methods have large errors in the wavelength region above ~1040 nm where the absorption cross section is very close to zero. The change in the shape of the emission cross-section in this region is related to the dopant concentration or glass composition.

Figure 4.5: Comparison of different Yb:hosts reported and glasses in the present study with respect to emission cross-section and I min
Figure 4.5: Comparison of different Yb:hosts reported and glasses in the present study with respect to emission cross-section and I min

Conclusion

Channel waveguides written with ultrafast lasers in erbium-doped phosphate glasses for integrated amplifiers and lasers operating in the C-band have been demonstrated [88, 16]. Moreover, surface damage during photolithographic processing hindered the fabrication of ion-exchanged channel waveguides in Er-doped tellurite glass [109].

Experimental procedures

Centimeter-long channels have been written by us inside the glass using ultrashort (45 fs) laser pulses. The spectral broadening due to self-phase modulation of the laser is observed by transmitting unfocused laser light with an energy of 343 µJ through the glass sample.

Table 5.1: Physical properties, Judd-Ofelt (JO) parameters, radiative lifetime and fluores- fluores-cence properties of the tellurite glass.
Table 5.1: Physical properties, Judd-Ofelt (JO) parameters, radiative lifetime and fluores- fluores-cence properties of the tellurite glass.

Results and discussion

Bulk glass properties

The emission cross section of the glass was obtained from the emission spectrum according to standard procedures [56]. The radiation lifetime obtained from the JO analysis was used in the calculation of the spectrum.

Figure 5.2: Absorption spectrum of the erbium-doped glass.
Figure 5.2: Absorption spectrum of the erbium-doped glass.

Channel waveguides

These facets are indicated in the side views of the waveguides that we depict in Fig. The average Te/P ratio of the bulk glass outside the channel was found to be 1.9.

Figure 5.5: . (a) Laser-written channels observed through an optical microscope at writing speeds of 0.01 cm/s (top channel), 0.02 cm/s (middle channel), and 0.03 cm/s (bottom channel) with 3 µJ energy
Figure 5.5: . (a) Laser-written channels observed through an optical microscope at writing speeds of 0.01 cm/s (top channel), 0.02 cm/s (middle channel), and 0.03 cm/s (bottom channel) with 3 µJ energy

Conclusion

The Te/P ratio near the edge of the channel (position 1 in the figure) is 1.4, and the ratio in the middle of the channel (position 2) is 3.0. The spectral characteristics of the ASE doped phosphotellurite glass fiber were investigated using fibers of different lengths and pump powers.

Experimental procedures

The objective of this chapter is to fabricate and study the spectroscopic properties of Yb-doped and Yb-Er co-doped phosphotellurite (PT) glass suitable for SFS. The output from the doped fiber was fed into the OSA through a standard single-mode fiber.

Results and discussion

Bulk glass properties

The emission cross-section spectrum and measured fluorescence decay used to calculate the lifetime are shown in Figure. The emission cross-section spectrum was obtained from the measured fluorescence spectrum using equation (1.12) and using the measured lifetime.

Yb doped fiber ASE

For the 10 cm fiber, the residual pump power is visible in the given ASE spectrum. In the case of the 10 cm fiber, the pump power is available along its entire length and hence the forward ASE.

Table 6.1: Physical, spectroscopic properties, and Laser parameters of the bulk glass having the composition 25P 2 O 5 -74.6TeO 2 -0.4Yb 2 O 3 (FY)
Table 6.1: Physical, spectroscopic properties, and Laser parameters of the bulk glass having the composition 25P 2 O 5 -74.6TeO 2 -0.4Yb 2 O 3 (FY)

Yb-Er doped fiber ASE

The absorption cross section of Er in the glass is very low compared to the emission cross section in the wavelength range 1550 nm-1650 nm. When the pump is not sufficient enough to amplify the Er-ASE signal, it undergoes absorption in the shorter wavelength end of the spectrum, where the difference in emission cross section and absorption cross section is very small.

Figure 6.4: Normalised ASE of the Yb doped fibre of different lengths 10 cm(solid line), 26 cm(dashed), 36 cm(dotted).
Figure 6.4: Normalised ASE of the Yb doped fibre of different lengths 10 cm(solid line), 26 cm(dashed), 36 cm(dotted).

Conclusion

Jorgensen, Excited State Phenomena in Glassy Materials, Handbook on the Physics and Chemistry of Rare Earth Metals, North Holland, Amsterdam, 1987.

Figure

Figure 1.1: Loss of a full spectrum fibre of silica glass, where S Band is 1460-1530 nm, C Band is 1530-1550 nm, L Band is 1550 to 1610 nm [12].
Figure 1.2: Photoluminescence (PL) spectrum of a erbium doped phosphate glass (Schott IOG), shows the emission at around 1535 nm.
Figure 1.3: Different energy levels of Er doped silicate glass with the respective lifetimes.
Figure 1.6: Research scheme for the spectroscopic study of rare-earth doped glasses.
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References

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