ISSN 1063-7850, Technical Physics Letters, 2006, Vol. 32, No. 5, pp. 442–444. © Pleiades Publishing, Inc., 2006. Original Russian Text © I.V. Aleksandrov, G. Melo Melchor, D.N. Vinogradov, M.P. Petrov, 2006, published in Pis’ma v Zhurnal Tekhnichesko œ Fiziki, 2006, Vol. 32, No. 10, pp. 56–60. 442 † The modern technology of optical fibers (OFs) fre- quently employs the doping of silica with germanium oxide, phosphorus oxide, and other impurities in order to obtain fibers with preset properties. Germanium is the main impurity that is capable of providing the required refractive index profile in the core of a silica fiber. It is also known that doping with germanium ensures an increase in the gain of fiber-optic amplifiers due to the phenomenon of stimulated Raman scattering (SRS) [1]. This gain is relatively small (on the order of 5 dB/(km W)) in standard OFs (containing about 4 mol % GeO 2 ) but increases with the dopant concen- tration. For example, a fiber containing 75 mol % GeO 2 can provide a gain of up to 300 dB/(km W) for the radi- ation component with a Stokes shift of 440 cm –1 [1]. The SRS effect in doped OFs can also be used for the creation of dynamic optical memory, all-optical logics, and optical pulse shape correctors [2]. Bragg diffrac- tion gratings recorded in germanium-doped OFs are the key components in fiber-optic communication systems and transducers. On the other hand, the presence of var- ious impurities in silica fibers can also lead to negative consequences—in particular, to structural defects that lead to increased optical losses [3]. Despite the consid- erable interest in doped OFs and extensive research in this field, many important aspects of the structural and optical properties of such fibers, in particular, the parameters of vibrations of the silicon- and germa- nium-containing groups, are still insufficiently studied. This Letter presents the results of investigations of the SRS phenomenon in germanium-doped silica OFs with the GeO 2 concentration varied between 10 and † Deceased. 45 mol %. We observed a variable spectral shift of the overtones of the SRS bands, and it was suggested that the appearance of this shift is related to a change in the force constants and the dissociation energies of the vibrating complexes. The investigation was performed using an experi- mental setup described in detail elsewhere [4]. We have studied step-index-profile silica (quartz glass) OFs in plastic sheaths. The OFs with low concentrations of germanium (obtained from the Fiber-Optic Research Center at the Prokhorov Institute of General Physics, Moscow) had a core diameter of 50 µ m, which is typi- cal of multimode fibers. The fiber length was varied from 5 to 100 m. The samples of OFs with a germanium oxide content of 45 mol % (Cornig Inc., Germany) had a core diameter of 6 µ m and a length of 30 m. Pulsed laser radiation was focused on the fiber input face using a microlens with a number aperture below that of the fiber to provide effective beam feeding into the fiber core. In all cases, lower modes were excited in the paraxial region in order to avoid the propagation of light via the sheath. The SRS was mostly studied using single pulses of second-harmonic radiation ( λ = 532 nm) of a YAG:Nd laser with a pulse duration of 35 ps and a peak power of 10 4 –10 5 W. In some cases, the fiber was excited by pulse trains. The pumping radi- ation and the Stokes components of the SRS signal from the fiber output face were measured by an STE1 spectrograph (LOMO Company, Russia) equipped with a compact diode array photodetector. When necessary, the pumping radiation and the Stokes components could be projected from the spectrograph output onto a screen placed in the focal plane for visual observation. Correlation between the Dopant Concentration and the Anharmonicity of Bending Vibrations in Silica Optical Fibers I. V. Aleksandrov † , G. Melo Melchor, D. N. Vinogradov, and M. P. Petrov* Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia * e-mail:
[email protected] Received January 17, 2006 Abstract —The generation of high-order Stokes components in a silica optical fiber doped with germanium oxide to 10 and 45 mol % has been studied using a picosecond pulsed stimulated Raman scattering (SRS) tech- nique. An increase in the dopant concentration leads to a decrease in the anharmonicity of bending vibrations in the fiber. It is suggested that the observed effect is related to an increase in the dissociation energy and force constants of the vibrating complexes. PACS numbers: 78.20.–e, 42.79.Jn DOI: 10.1134/S106378500605021X TECHNICAL PHYSICS LETTERS Vol. 32 No. 5 2006 CORRELATION BETWEEN THE DOPANT CONCENTRATION 443 Typical SRS spectra of silica fibers containing 10 and 45 mol % of germanium oxide are presented in the figure. A distinctive feature of the SRS spectrum of low-loss fibers is the appearance of a large number of overtones (high-order Stokes components). As can be seen, a Stokes component representing the fundamental mode (with a frequency shift of about 440 cm –1 ) in the SRS spectrum is accompanied by overtones up to the seventh Stokes order. The frequency shift observed for the fiber with a dopant content of 45 mol % exhibited a clear increase as compared to the Stokes shift in a fiber containing 10 mol % GeO 2 . Using the SRS data, we calculated the anharmonicity constants of the samples under study. As can be seen from the figure, the band- width of the fundamental mode (440 cm –1 ) in slightly and highly doped samples is the same, which implies that the efficiency of the interaction of this mode with the other oscillations is independent of the dopant con- tent. At the same time, the bandwidths of high-order components exhibit broadening that is also independent of the doping level. The fundamental oscillations responsible for the SRS in germanium-doped silica fibers are well known and reliably interpreted [5]. The most intense Raman band, with a Stokes shift of 440 cm –1 , is related to the bending vibrations of tetrahedral silicon groups, while the anomalously narrow band at 490 cm –1 is assigned to the vibrations of defect groups, in which silicon is miss- ing in one of the bonds. The introduction of germanium leads to the weakening (or, at a sufficiently large dopant content, even to the disappearance) of the latter band. This behavior indicates that germanium atoms are trapped at the defects. In other words, these atoms only occupy the vacant (missing silicon) lattice sites, rather than destroying the Si–O bonds. Strict theoretical calculation of the SRS spectrum in a complex polyatomic glassy matrix structure with allowance for the anharmonicity of oscillations is still impossible. However, qualitative analysis can be per- formed using a theory of anharmonic oscillations for diatomic molecules. According to the data of Sverdlov et al. [6], the measured vibrational frequency ν and the overtone frequency ν ov are related to the fundamental harmonic frequency ω and the anharmonicity constant χ as (1) and so on for the higher overtones. In the case of χ � 1, this constant and the harmonic frequency are given by the following formulas: (2) (3) ν ω 1 2χ–( ), νov 2ω 1 3χ–( ), …= = χ ha/ 4π 2Dµ( )1/2[ ],= ω K /µ( )1/2/2π,= where h is the Planck constant, a is a numerical coeffi- cient (dependent on the particular form of the potential of the anharmonic oscillator), D is the energy of disso- ciation of the diatomic molecule, µ is the reduced mass of the molecule, and K is the force constant. For a doped fiber containing 10 mol % GeO 2 , the χ values were calculated previously [4, 7]. For the bands with frequencies near 440 cm –1 , we obtained χ ≈ 4 × 10 –3 (the particular calculation was performed for ω = 446 cm –1 ). The analogous calculation (using data pre- sented in the figure) for a fiber containing 45 mol % of GeO 2 yields χ ≈ 3 × 10 –3 (with an error of about ± 10%). As can be seen from relations (2) and (3), the substi- tution of germanium for silicon in a SiO 2 molecule can lead to changes in the D , K , and µ values. A change in the molecular mass must not play a significant role, since the SRS in the case of bending vibrations of tetra- hedral groups is predominantly related to the vibrations of oxygen, rather than to the vibrations of silicon or ger- manium [5]. Therefore, the anharmonicity constant most probably decreases (and the frequency of funda- mental molecular oscillations increases) due to an increase in the D and K values, since the ionization potential is higher for germanium than for silicon (7.85 and 7.39 eV, respectively). The observed significant broadening of the high- order Stokes components as compared to the first com- ponent is readily explained by the inhomogeneous character of broadening (i.e., by the spread of χ and ω values for various tetrahedra), since this factor leads to an increase in the bandwidth with increasing number of the Stokes component. However, it should be borne in mind that these are only purely qualitative conclusions based on an oversimplified model used for the descrip- tion of the anharmonicity of oscillations in doped OFs. At the same time, the experimental investigations of high-order Stokes components provide for a unique 2500 30002000150010005000 ∆ν , cm –1 I , a.u. 440 2 × 440 3 × 440 4 × 440 5 × 440 6 × 440 7 × 440 SRS spectra of germanium-doped silica fibers containing (solid curve) 10 mol % and (dashed curve) 45 mol % GeO 2 ( ∆ν is the Stokes frequency shift, I is the normalized SRS intensity in arbitrary units). 444 TECHNICAL PHYSICS LETTERS Vol. 32 No. 5 2006 ALEKSANDROV et al. possibility of detecting very slight changes in the struc- ture of silica (quartz glass) fibers. Such sensitivity usu- ally cannot be achieved in the measurements of Raman and IR spectra and low-order Stokes components in the SRS spectra. REFERENCES 1. A. S. Kurkov and E. M. Dianov, Kvantovaya Élektron. (Moscow) 34 , 881 (2004). 2. V. I. Belotitskii, E. A. Kuzin, M. P. Petrov, and V. V. Spi- rin, Electron. Lett. 29 , 49 (1993). 3. D. L. Grisom, Proc. SPIE 541 , 38 (1985). 4. I. V. Aleksandrov, Z. V. Nesterova, and G. T. Petrovskii, J. Non-Cryst. Solids 123 , 223 (1990). 5. G. E. Walfaren and J. Stone, Appl. Spectr. 29 , 337 (1975). 6. L. M. Sverdlov, M. A. Kovner, and E. P. Krainov, Vibra- tional Spectra of Polyatomic Molecules (Nauka, Mos- cow, 1970; Wiley, New York, 1974). 7. Z. V. Nesterova and I. V. Aleksandrov, Zh. Prikl. Spek- trosk. 45 , 670 (1986). Translated by P. Pozdeev