ZBLAN

Relatively smaller amounts of very high purity A1F. are used ia ultra low loss optical fiber—duotide glass compositions, the most common of which is ZBLAN containing tirconium, barium, lanthanum, aluminum, and sodium (see Fiber optics). High purity A1F. is also used ia the manufacture of aluminum siUcate fiber and ia ceramics for electrical resistors (see Ceramics AS electrical materials Refractory fibers). 

For ZBLAN, the IR edge is in the 5-8 m range with a 50% transmission located at 7.1 /on (1400 cm-1) for a 2 mm-thick sample. This is to be compared with silica glass whose transparency starts decreasing at 3 pm. The IR edge is due to multiphonon absorption processes related to the fundamental vibration frequencies... 

Day and France found that the addition of lead fluoride to ZBLAN fibers caused, however, a marked increase of the Rayleigh coefficient (A = 1.12 dB km-1 nm4) [20], [21], Very likely, this is due to an increase of the concentration of submicron crystallites related to the lower stability of ZBLAN-Pb glass, as compared to pure ZBLAN. 

Typical of sodium-containing glasses such as ZBLAN or BIG-Na, another way to increase the refractive index is to substitute Li+ for Na+ ions, despite the lower polarizability of lithium ions because of their small size. Actually, Li+ ions are so small that they induce a local collapse of the glassy network resulting in an increase of the densification and of the refractive index [10,24,25]. This property is utilized for the elaboration of fluoride glass planar waveguides by ionic exchange, as described in Sec. 5.3. 

On the other hand, a significant decrease of the refractive index can be obtained by substitution of alkali ions or aluminum ions for Ba2+ in both ZBLAN and BIG glasses [22,25], or by substitution of hafnium for zirconium in ZBLA [2],... 

Extensive research including the study of radiative and non-radiative properties of rare-earth ions has been carried out. Especially, the Judd-Ofelt theory has been applied to most rare-earth — fluoride-glass combinations. Typical Judd-Ofelt parameters are reported in Table 3 for ZBLAN glass [31-34], An exhaustive list of such parameters for glasses and crystals can be found in Ref. [35]. 

In most cases, the Judd-Ofelt parameters are calculated with good confidence considering that results from various laboratories are convergent. They are found to adequately predict the radiative properties — lifetimes and branching ratios — of several transitions especially for Ho3+, Er3+, and Tm3+ ions in ZBLAN as well as in fluoroindate glasses. 

Judd-Ofelt parameters Q, (r = 2, 4, 6) of rare-earth ions in ZBLAN fluoride glasses... 

One of the main spectroscopic properties that differentiate fluoride glasses from silica-based glasses is the low multiphonon emission rate. These non-radiative relaxations that may strongly compete with radiative processes in rare-earth ions are nearly three orders of magnitude lower in ZBLAN glass than in silicate, as shown in Fig. 2. This property is directly related to the fundamental vibration modes of the host and, therefore, varies basically in the same manner as the infrared absorption edge. 

Raman spectroscopy or far-IR spectroscopy can determine the fundamental vibration frequencies of the host. However, these methods give information about the whole glass matrix and do not account for the local nature of electron-phonon interactions. So, the fundamental frequencies are preferably determined by recording the phonon-side bands (PSB) of rare-earth transitions or by studying the temperature-dependence of multiphonon relaxations [42,43]. The phonon energies determined by PSB spectroscopy, which is the most direct method, are usually lower (400 cm-1 in ZBLAN) than those measured by other methods ( 500 cm-1) suggesting that weak M—F bonds are coupled to the rare-earth [43]. 

Lasing has been demonstrated at 1.06 /tm in Nd3+-doped ZBLAN and BIG fluoride glass rods pumped by an alexandrite laser and xenon flashlamps, respectively [71,72], Fig. 6 shows the 1.06 //m laser output energy out of Nd3+-doped and Cr3+ Nd3+-codoped fluoroindate glass rods of 40 mm length. In presence of Cr3+ ions, which are efficient absorbers of excitation light from flashlamps,... 

With an emission at 1.3 /tm, Nd3+ is a potential candidate for optical amplification in the second telecommunication window. However, optical gain remains limited because of amplified spontaneous emission at 1.05 fim and also because of strong ESA at 1.3 /on. This ESA process, which takes place from 4F3/2 to (4G7/2, 2K13/2) and to 4G9/2 as shown in Fig. 5, is so probable that no gain is achieved around 1.3 /tm in silica fibers. In ZBLAN fluoride fibers, gain is achieved at 1.34... 

Fig. 7. Excited state absorption in the 1.3 /tm region in ZBLAN Nd3+ glass.

Room-temperature lasing has been demonstrated in the ultraviolet at 381 nm and in the violet at 412 nm with Nd3+-doped ZBLAN fibers of about 40 cm in length [164,165]. The transitions involved are4D3/2 4L i/2 and 2P3/2 —> 4In/2, asdepicted... 

Green coherent light has been obtained as well. Up-conversion of 801 nm light from a single laser diode leads to 3 mW of 544 nm light at the output of an Er3+-doped ZBLAN fiber [175]. Higher output powers are attained with large pump... 

Characteristics of rare-earth-doped ZBLAN fluoride glass fiber lasers, (i) incident, (1) launched, and (a) absorbed pump powers, (uc) up-conversion pumping... 

Contrary to all the laser lines reported in Table 4, lasing at 3.9 /tm with ZBLAN Ho3+ fiber is achieved at liquid-nitrogen temperature. CW output power of 11 mW is obtained with 900 mW launched pump power at 885 nm. It must be noted that such lasers are of prime importance for military and space applications because they lie within an atmospheric window transparent from 3 /tm to 5 /tm. Very few lasers exist in this spectral region. 

Characteristics of fluoride glass optical amplifiers. All fibers are ZBLAN-based glass except when otherwise indicated. (LD) laser diode ( ) gain obtained with two amplification units. 

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