*Advanced Functional Materials*

widely applied for fabricating tellurite glass microsphere cavities and other compound glass microsphere cavities resulting in a good shape and high Q factor. The fabrication method using a CO2 laser is described here.

on the molten glass. A microscope image of a tellurite microspheres fabricated in

Although the resulting microsphere cavity made of molten glass fiber includes a glass fiber attached at a pole of the sphere, the light field is mainly concentrated around the equatorial plane of the microsphere cavity, and hence the loss induced by the fiber to the whispering gallery mode (WGM) resonance is negligible [89, 90]. In general, the fiber rods are only used to hold the microspheres in place and to facilitate light coupling. However, in recent years, some other uses of fiber rods have attracted increasing attention. In 2017, Murphy et al. [91] reported an alternative method for precise coupling control using the fiber rod. In the experiment, 980 nm laser light was input into the fiber rod, and the coupling distance between the microsphere cavity and the tapered fiber was precisely controlled by heating the connection between the microsphere and the fiber rod using the 980 nm laser. The adjustment range of microsphere cavity position was from 0.61 0.13 μm

The previously mentioned method for fabricating tellurite glass microspheres was only capable of producing one microsphere at a time, and the powder floating represents an alternative method for preparing glass microspheres in large quantities. In this method, the tellurite glass was ground into powder and poured into a high temperature furnace, which was placed vertically with a proper protective gas (nitrogen or inert gas) from the bottom to top. The tellurite glass powder melts and forms into microspheres due to surface tension at high temperature. In addition, the protective gas reduced the falling speed of the glass powder and increased the exposure time of the powder to the high temperature in the furnace. Additionally, the method isolates the glass powder from the atmosphere [93]. For some glass materials with less stringent experimental requirements, the microspheres can be

Tellurite microspheres prepared using this method have no attached fiber rods, which is different from melting glass fiber or sol-gel methods. Using the powdered method, a large number of microspheres with different diameters can be prepared simultaneously, which is beneficial to the selection of experimental size and enables the integration and commercialization of the microsphere laser on a mass produced

basis. **Figure 25** is a schematic diagram of fabrication of microspheres, and **Figure 26** is a microscope image of the microspheres fabricated using the powder

Tellurite glass has emerged as a promising material for use in microsphere resonators in the near infrared wavelength region and tellurite glass microsphere lasers have been widely reported. In 2002, Sasagawa et al. reported continuouswave oscillation in an Nd3+-doped tellurite glass microsphere laser at 1.06 μm for the first time [96]. Tellurite glass microspheres with diameters in the range of 50 μm to a few hundred micrometers were fabricated by melting using a Kanthal wire heater. Resonances were excited in the microsphere pumped using a 800 nm laser, the threshold of the output laser was measured as 81 mW, and the emission

Later in 2005, an Er3+-doped tellurite glass microsphere laser was reported by Peng et al. [97]. The threshold of 1561 nm microsphere laser with 0.5 wt% Er2O3 doped was measured to be as low as 1.4 mW, and the maximum output power

this manner is shown in **Figure 24**.

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

to 3.49 0.13 μm.

floating method.

**279**

*3.1.2 Powder floating method*

formed without the use of protective gas [94].

**3.2 Tellurite glass microsphere lasers**

spectrum is shown in **Figure 27**.

A schematic diagram of the experimental setup for manufacturing a microsphere resonator is shown in **Figure 23**. The main positioning and alignment instrument used in the experiment was a precision three-dimensional (3D) translation stage, a continuous CO2 laser with an output wavelength of 10.6 μm, and a ZnSe lens for focusing. The experimental step for fabricating the tellurite microsphere resonator was divided into three stages. In the first step, the tellurite fiber was mounted vertically on the 3D translation stage and a weight hung at the end of the fiber. The ZnSe lens was used to focus the laser beam on the tip of the tellurite fiber, causing it to absorb the incident light which resulted in a temperature rise. The glass softened and gradually stretched into a tapered fiber under the influence of the weight. The heating was terminated when the waist diameter of the tapered fiber reached around 30 μm. In the second step, the tapered fiber was accurately cleaved at the waist region to obtain a half-tapered fiber. In the third step, using a ZnSe lens once more to focus the laser beam on the end of the half-tapered fiber, the tellurite microsphere was formed at the fiber end due to the surface tension acting

#### **Figure 23.**

*Schematic diagram of the experimental setup for making a tellurite glass microsphere. (a) A ZnSe lens is used to focus a CO2 laser beam on the tellurite fiber. (b) The waist region of tapered fiber is cleaved. (c) A tellurite microsphere is obtained by focusing a CO2 laser beam on the end of the cleaved tapered [92].*

**Figure 24.** *Microscope image of tellurite microsphere made with CO2 laser.*

#### *Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

widely applied for fabricating tellurite glass microsphere cavities and other compound glass microsphere cavities resulting in a good shape and high Q factor. The

A schematic diagram of the experimental setup for manufacturing a microsphere resonator is shown in **Figure 23**. The main positioning and alignment instrument used in the experiment was a precision three-dimensional (3D) translation stage, a continuous CO2 laser with an output wavelength of 10.6 μm, and a ZnSe lens for focusing. The experimental step for fabricating the tellurite microsphere resonator was divided into three stages. In the first step, the tellurite fiber was mounted vertically on the 3D translation stage and a weight hung at the end of the fiber. The ZnSe lens was used to focus the laser beam on the tip of the tellurite fiber, causing it to absorb the incident light which resulted in a temperature rise. The glass softened and gradually stretched into a tapered fiber under the influence of the weight. The heating was terminated when the waist diameter of the tapered fiber reached around 30 μm. In the second step, the tapered fiber was accurately cleaved at the waist region to obtain a half-tapered fiber. In the third step, using a ZnSe lens once more to focus the laser beam on the end of the half-tapered fiber, the tellurite microsphere was formed at the fiber end due to the surface tension acting

*Schematic diagram of the experimental setup for making a tellurite glass microsphere. (a) A ZnSe lens is used to focus a CO2 laser beam on the tellurite fiber. (b) The waist region of tapered fiber is cleaved. (c) A tellurite*

*microsphere is obtained by focusing a CO2 laser beam on the end of the cleaved tapered [92].*

fabrication method using a CO2 laser is described here.

*Advanced Functional Materials*

**Figure 23.**

**Figure 24.**

**278**

*Microscope image of tellurite microsphere made with CO2 laser.*

on the molten glass. A microscope image of a tellurite microspheres fabricated in this manner is shown in **Figure 24**.

Although the resulting microsphere cavity made of molten glass fiber includes a glass fiber attached at a pole of the sphere, the light field is mainly concentrated around the equatorial plane of the microsphere cavity, and hence the loss induced by the fiber to the whispering gallery mode (WGM) resonance is negligible [89, 90]. In general, the fiber rods are only used to hold the microspheres in place and to facilitate light coupling. However, in recent years, some other uses of fiber rods have attracted increasing attention. In 2017, Murphy et al. [91] reported an alternative method for precise coupling control using the fiber rod. In the experiment, 980 nm laser light was input into the fiber rod, and the coupling distance between the microsphere cavity and the tapered fiber was precisely controlled by heating the connection between the microsphere and the fiber rod using the 980 nm laser. The adjustment range of microsphere cavity position was from 0.61 0.13 μm to 3.49 0.13 μm.

#### *3.1.2 Powder floating method*

The previously mentioned method for fabricating tellurite glass microspheres was only capable of producing one microsphere at a time, and the powder floating represents an alternative method for preparing glass microspheres in large quantities. In this method, the tellurite glass was ground into powder and poured into a high temperature furnace, which was placed vertically with a proper protective gas (nitrogen or inert gas) from the bottom to top. The tellurite glass powder melts and forms into microspheres due to surface tension at high temperature. In addition, the protective gas reduced the falling speed of the glass powder and increased the exposure time of the powder to the high temperature in the furnace. Additionally, the method isolates the glass powder from the atmosphere [93]. For some glass materials with less stringent experimental requirements, the microspheres can be formed without the use of protective gas [94].

Tellurite microspheres prepared using this method have no attached fiber rods, which is different from melting glass fiber or sol-gel methods. Using the powdered method, a large number of microspheres with different diameters can be prepared simultaneously, which is beneficial to the selection of experimental size and enables the integration and commercialization of the microsphere laser on a mass produced basis. **Figure 25** is a schematic diagram of fabrication of microspheres, and **Figure 26** is a microscope image of the microspheres fabricated using the powder floating method.

#### **3.2 Tellurite glass microsphere lasers**

Tellurite glass has emerged as a promising material for use in microsphere resonators in the near infrared wavelength region and tellurite glass microsphere lasers have been widely reported. In 2002, Sasagawa et al. reported continuouswave oscillation in an Nd3+-doped tellurite glass microsphere laser at 1.06 μm for the first time [96]. Tellurite glass microspheres with diameters in the range of 50 μm to a few hundred micrometers were fabricated by melting using a Kanthal wire heater. Resonances were excited in the microsphere pumped using a 800 nm laser, the threshold of the output laser was measured as 81 mW, and the emission spectrum is shown in **Figure 27**.

Later in 2005, an Er3+-doped tellurite glass microsphere laser was reported by Peng et al. [97]. The threshold of 1561 nm microsphere laser with 0.5 wt% Er2O3 doped was measured to be as low as 1.4 mW, and the maximum output power

**Figure 25.** *Schematic diagram of fabrication of microsphere by powder floating method [95].*

**Figure 26.** *Microspheres fabricated by powder floating method.*

achieves 124.5 μW. **Figure 28** shows the relationship between the output laser power and the 1480 nm pump power.

1.47 μm and 1.9 μm bands using a tellurite glass microsphere [101]. The output spectrum of the Tm3+-doped tellurite glass laser is shown in **Figure 29**, which shows the laser emission in the S band and at 1.9 μm. The average output power is plotted as a function of the average pump power in **Figure 30**. The threshold of the laser in the S band is 4.6 mW, while the thresholds measured for 1.9 μm are 3.0 mW and 4.8 mW, respectively. The differential quantum efficiency in the S band and at

*The microsphere laser pumped by a 1480 nm laser. The Er2O3 doping concentration is 0.5 wt%, and the diameter of the microsphere is 32 μm. The maximum output power is 124.5 μW. The inset shows the single-*

*I11/2 transition of Nd3+ ions in tellurite glass microsphere at various pumping*

In 2019 [3], Li et al. fabricated Tm3+-Ho3+ co-doped tellurite glass samples to solve the problem of the population inversion and obtained a 1.47 μm output using a tellurite glass microsphere laser. **Figure 31(a)** shows the output spectrum at 1.47 μm of Tm3+-Ho3+ co-doped tellurite glass microspheres when pumped using a 802 nm

attenuated through the energy transfer process in Tm3+-Ho3+ co-doped tellurite

F4 energy level is

1.9 μm were calculated as 1.4% and 1.1% for bidirectional lasing.

**Figure 27.** *Emission spectra for <sup>4</sup>*

*powers [96].*

**Figure 28.**

**281**

*F3/2* ! *<sup>4</sup>*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

*mode profile of this L-band microsphere laser [97].*

laser source. It is clear from **Figure 31(b)** that the lifetime of <sup>3</sup>

The output wavelength of the laser around 1.9 μm, and 1.47 μm band is generated from the transition of Tm3+ ions: <sup>3</sup> F4 ! <sup>3</sup> H6 and <sup>3</sup> H4 ! <sup>3</sup> F4 [98]. Wu et al. proposed a microcavity laser based on a Tm3+-doped tellurite glass microsphere at 1.9 μm [99]. However, there are two problems in realizing a laser at the wavelength 1.47 μm. Firstly, the lifetime of the <sup>3</sup> H4 level is shorter than that of the <sup>3</sup> F4 level in Tm3+ ions, so the transition is sometimes described as self-terminating [100]. Secondly, the glass host material should have very low phonon energy, as in the case of silica and phosphate glass lasers, and amplification is essential. Tellurite and other heavy metal fluoride glasses have been considered as key materials for thuliumdoped fiber amplifier operation in the S band, mainly due to their lower phonon energies (�580 cm�<sup>1</sup> ) [12]. In 2004, Sasagawa et al. solved the population inversion problem in Tm3+ ions and realized a cascade laser with output wavelengths in the

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

**Figure 27.**

*Emission spectra for <sup>4</sup> F3/2* ! *<sup>4</sup> I11/2 transition of Nd3+ ions in tellurite glass microsphere at various pumping powers [96].*

#### **Figure 28.**

achieves 124.5 μW. **Figure 28** shows the relationship between the output laser

*Schematic diagram of fabrication of microsphere by powder floating method [95].*

The output wavelength of the laser around 1.9 μm, and 1.47 μm band is gener-

F4 ! <sup>3</sup>

proposed a microcavity laser based on a Tm3+-doped tellurite glass microsphere at 1.9 μm [99]. However, there are two problems in realizing a laser at the wavelength

Tm3+ ions, so the transition is sometimes described as self-terminating [100]. Secondly, the glass host material should have very low phonon energy, as in the case of silica and phosphate glass lasers, and amplification is essential. Tellurite and other heavy metal fluoride glasses have been considered as key materials for thuliumdoped fiber amplifier operation in the S band, mainly due to their lower phonon

problem in Tm3+ ions and realized a cascade laser with output wavelengths in the

H6 and <sup>3</sup>

H4 ! <sup>3</sup>

H4 level is shorter than that of the <sup>3</sup>

) [12]. In 2004, Sasagawa et al. solved the population inversion

F4 [98]. Wu et al.

F4 level in

power and the 1480 nm pump power.

*Microspheres fabricated by powder floating method.*

ated from the transition of Tm3+ ions: <sup>3</sup>

1.47 μm. Firstly, the lifetime of the <sup>3</sup>

energies (�580 cm�<sup>1</sup>

**280**

**Figure 25.**

*Advanced Functional Materials*

**Figure 26.**

*The microsphere laser pumped by a 1480 nm laser. The Er2O3 doping concentration is 0.5 wt%, and the diameter of the microsphere is 32 μm. The maximum output power is 124.5 μW. The inset shows the singlemode profile of this L-band microsphere laser [97].*

1.47 μm and 1.9 μm bands using a tellurite glass microsphere [101]. The output spectrum of the Tm3+-doped tellurite glass laser is shown in **Figure 29**, which shows the laser emission in the S band and at 1.9 μm. The average output power is plotted as a function of the average pump power in **Figure 30**. The threshold of the laser in the S band is 4.6 mW, while the thresholds measured for 1.9 μm are 3.0 mW and 4.8 mW, respectively. The differential quantum efficiency in the S band and at 1.9 μm were calculated as 1.4% and 1.1% for bidirectional lasing.

In 2019 [3], Li et al. fabricated Tm3+-Ho3+ co-doped tellurite glass samples to solve the problem of the population inversion and obtained a 1.47 μm output using a tellurite glass microsphere laser. **Figure 31(a)** shows the output spectrum at 1.47 μm of Tm3+-Ho3+ co-doped tellurite glass microspheres when pumped using a 802 nm laser source. It is clear from **Figure 31(b)** that the lifetime of <sup>3</sup> F4 energy level is attenuated through the energy transfer process in Tm3+-Ho3+ co-doped tellurite

#### **Figure 29.**

*Emission spectrum of a Tm3+-doped tellurite microsphere laser with diameter of 104 μm. (Inset) OSA emission spectrum [101].*

#### **Figure 30.**

*Average laser output power against average pump power for a Tm3+-doped tellurite microsphere laser. (Inset) Laser emission spectrum at average pump power of 4.0 mW [101].*

all-solid version, which provided greater options for using different pump sources, producing higher coherence. In the case of high-power output and stable MIR SC

*Fluorescence decay curves of Tm3+-Ho3+ co-doped and Tm3+-doped tellurite glass samples at 1.9 μm. The inset figure shows that the lifetime of Tm3+ at 1.9 μm is 2.32 ms in Tm3+-doped tellurite glass samples [3].*

*(a) Laser emission spectrum from the Tm3+-Ho3+ co-doped microsphere when the pump power was set to*

*2.5 mW. (b) Energy level diagram and energy transfer model in tellurite glass [3].*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

microstructured one, and hence the fluorotellurite fiber is a promising candidate for high-power Mid-IR laser emission. Potentially this technology could be expected to reach the hundred-watt output level even after losses with careful design for heat

Tellurite glass microsphere resonators have overcome the limitations associated with traditional resonators in terms of glass materials. In the future, it is envisaged that tellurite glass microsphere resonators will have wide-ranging applications in photonics, having a high Q value and fast response. Meanwhile, doping rare earth ions in different host materials is expected to achieve higher-power output and more efficient lasers accessing different wavelength ranges, most notably in the

generation, the all-solid tellurite fiber performed much better than the

management, fiber structure, and pump parameter optimization.

infrared band.

**283**

**Figure 32.**

**Figure 31.**

glass. The Tm3+ ions are excited from <sup>3</sup> F4 to the <sup>3</sup> H4 energy level by the 802 nm pump laser, and the lifetime of Tm3+-doped and Tm3+-Ho3+ co-doped material are shown in **Figure 32**. The emission process originates from the Tm3+: <sup>3</sup> H4 ! <sup>3</sup> F4 transition, and the energy transfer efficiency of the Tm3+: <sup>3</sup> F4 level to Ho3+: <sup>5</sup> I7 level is 34.9% in Tm3+-Ho3+ co-doped tellurite glass sample.

#### **3.3 Summary**

The last two decades have witnessed significant progress of tellurite fiber-based SC light sources, whose original progress was primarily implemented through the development of microstructured and all-solid fiber devices. The microstructured fiber demonstrated greater flexibility in tailoring the dispersion profile than the

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

#### **Figure 31.**

*(a) Laser emission spectrum from the Tm3+-Ho3+ co-doped microsphere when the pump power was set to 2.5 mW. (b) Energy level diagram and energy transfer model in tellurite glass [3].*

#### **Figure 32.**

glass. The Tm3+ ions are excited from <sup>3</sup>

**3.3 Summary**

**282**

**Figure 30.**

**Figure 29.**

*spectrum [101].*

*Advanced Functional Materials*

F4 to the <sup>3</sup>

The last two decades have witnessed significant progress of tellurite fiber-based SC light sources, whose original progress was primarily implemented through the development of microstructured and all-solid fiber devices. The microstructured fiber demonstrated greater flexibility in tailoring the dispersion profile than the

pump laser, and the lifetime of Tm3+-doped and Tm3+-Ho3+ co-doped material are

*Average laser output power against average pump power for a Tm3+-doped tellurite microsphere laser. (Inset)*

*Emission spectrum of a Tm3+-doped tellurite microsphere laser with diameter of 104 μm. (Inset) OSA emission*

shown in **Figure 32**. The emission process originates from the Tm3+: <sup>3</sup>

transition, and the energy transfer efficiency of the Tm3+: <sup>3</sup>

is 34.9% in Tm3+-Ho3+ co-doped tellurite glass sample.

*Laser emission spectrum at average pump power of 4.0 mW [101].*

H4 energy level by the 802 nm

F4 level to Ho3+: <sup>5</sup>

H4 ! <sup>3</sup>

F4

I7 level

*Fluorescence decay curves of Tm3+-Ho3+ co-doped and Tm3+-doped tellurite glass samples at 1.9 μm. The inset figure shows that the lifetime of Tm3+ at 1.9 μm is 2.32 ms in Tm3+-doped tellurite glass samples [3].*

all-solid version, which provided greater options for using different pump sources, producing higher coherence. In the case of high-power output and stable MIR SC generation, the all-solid tellurite fiber performed much better than the microstructured one, and hence the fluorotellurite fiber is a promising candidate for high-power Mid-IR laser emission. Potentially this technology could be expected to reach the hundred-watt output level even after losses with careful design for heat management, fiber structure, and pump parameter optimization.

Tellurite glass microsphere resonators have overcome the limitations associated with traditional resonators in terms of glass materials. In the future, it is envisaged that tellurite glass microsphere resonators will have wide-ranging applications in photonics, having a high Q value and fast response. Meanwhile, doping rare earth ions in different host materials is expected to achieve higher-power output and more efficient lasers accessing different wavelength ranges, most notably in the infrared band.

*Advanced Functional Materials*
