Palsdottir, Bera

Poulsen, Ove

Shi, Yuan

Thirstrup, Carsten

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Direct measurement of pump-induced phase modulation in erbium-doped fibresThirstrup, Carsten; Shi, Yuan; Poulsen, Ove; Palsdottir, BeraPublished in:Proceedings of 5th European Quantum Electronics ConferencePublication date:1994Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Thirstrup, C., Shi, Y., Poulsen, O., & Palsdottir, B. (1994). Direct measurement of pump-induced phasemodulation in erbium-doped fibres. In Proceedings of 5th European Quantum Electronics Conference (pp. 198-199). IEEE.198 f EQEC'94 f THURSDAY MORNING ing spectrum with increasing of the pumping current. Within the limits of the free-running cw generation regime (I, = 120 mA), no evidence of chirping in the spectrum is observed. The modal chirp appears when passing to the mode-lock- ing regime (I, = 130 mA), and its value increases with pumping current. As mode-locking develops, the laser spec- trum becomes markedly asymmetric (I, = 150 mA). This effect can be explained by the fact that the dispersion of the laser active medium decreases with wave- length. At I, = 150 mA, the minimal value of time-bandwidth product ATAV = 0.52 was registrated. It should be noted that the optical pulses obtained are not trans- form-limited ones due to the chirp effect and it is possible to compress them in a medium with a proper dispersion. With further increase in pumping current (I, = 170 mA), the lasing spectrum broadens (with the corresponding increase in the time-bandwidth product) and becomes symmetric again, which is supposedly due to gain saturation for longer wave- length modes. An important advantage of the lasers investigated is that their entire spectrum lies in the amplification band of erbium- doped fibers. Being combined with fiber amplifiers, these lasers could serve as a source of soliton pulses with a very high repetition rate. It should be also noted that the measurement of laser character- istics was being carried out during ap- proximately 50 hours of operation under cw pumping and no degradation of spec- tra or pulse parameters was observed. CW passive mode-locking was ob- served for the first time in InGaAsP laser diodes with saturable absorber regions created by the ion implantation. Optical pulses of 650 fs width at 110-GHz repe- tition rate were obtained in the spectral range of 1.53-1.54 pm. No changes in spectra and autocorrelation traces were observed after 50 hours of cw operation. *Institute of Physics and Technology, Russian Academy of Sciences, 26 Politekhnicheskaya str., St. Petersburx 194021 1. Y. K. Chen, M. C. Wu, T. Tanbun-Ek, R. A. Logan, M. A. Chin, Appl. Phvs. Lett. 58(12), 1253-1255 (1991). 2. K. k. Lau, IEEE j: of Quant. El. 26(2), 250-261 (1990). J.  H. Zarrabi, E. L. Portnoi, A. V. Chelnokov, Appl. Phys. Lett., 1526- 1528 (1991). 3. QThF 1100 Rooms T & U Special Nonlinear Materials Jo Zyss. CNET, France, Presider QThFl 1100 Dynamics and origin of the optical Kerr effect in CdSe nanoparticles in the resonant regime M. Ghanassi, M. C. Schanne-Klein, D. Ricard, C. Flytzanis, Laboratoire d'Optique Quantique du C.N.R.S., Ecole Polytechnique, 91128 Palaiseau Cedex, France In the resonant regime, the optical Kerr effect of CdSe-doped glasses is due to photoexcitation of carriers whose pres- ence modifies the optical properties of the CdSe quantum dots. The recovery time, which govems the repetition rate at which such devices could be operated, is equal to the recombination time of these carriers. At low laser intensity, the recom- bination time may be shortened by the photodarkening effect.' At higher inten- sity, nonlinear processes such as Auger recombination may play a significant role as aleady suggested* although the first data could not clearly make the differ- ence between a cubic (Auger) process or a quadratic one. Using three different time-resolved techniques, optical phase conjugation, nonlinear absorption, and time-resolved luminescence, we clearly observed that, for a fresh sample, at high laser intensity and for not too long delay times, carrier recombination is dominated by Auger processes. All these data lead to an Auger constant on the order of a few cm6 s-'. When the sample has been photo- darkened, both the linear nonradiative decay and the Auger process are important. Understanding the origin of the non- linearity in these media is also of impor- tance since such artificial materials can, in principle, be tailored to meet specific requirements. The optical Kerr effect may be due to free carriers in which case it corresponds to saturation of the first elec- tronic transition and to induced absorp- tion between the corresponding one-pair state and two-pair states. Altematively, the photoexcited carriers may be trapped at the surface of the nanoparticles and, through the static electric field they create for example, they may also modify the optical properties. Time resolving the nonlinear response at low laser intensity and in the degen- erate case for samples having experi- enced various degrees of photodarken- ing, we have been able to access the relevant mechanisms. Usually, the re- sponse shows both a fast component due to free carriers and a slow one due to trapped carriers. The larger the darken- ing dose is, the smaller the time constant of the fast component and the smaller the magnitude of the slow component. The dominant mechanism is thus observed to depend on the origin of the sample and on its past history. Time-resolving the nonlinear response is the most reliable technique of determining it. 1. P. Roussignol, D. Ricard, J. Lukasik, C. Flytzanis, J. Opt. Soc. Am. B 4, 5 (1987). P. Roussignol, M. Kull, D. Ricard, F. de Rougemont, R. Frey, C. Flytzanis, Appl. Phys. Lett. 51, 1882 (1987) and 54, 1705 (1989). 2. OThFZ 1115 Direct measurement of pump-induced phase modulation in erbium-doped fibres Carsten Thirstrup, Yuan Shi, Ove Poulsen, Bera l'dlsdbttir,' Mikroelektronik Centre!, Technical University of Denmark, Building 345E, 2800 Lyngby, Denmark A system, which is able to determine pump induced phase modulation in Er- doped fibres for phase shifts bp ranging from - n / 2 0  to several TI, has been de- veloped. The system (see Fig. l) consists of a Mach-Zehnder interferometer using a polarization maintaining 3-dB fibre coupler. One arm of the interferometer includes an Er-doped fibre pumped by a 980-nm Tisapphire laser. Light from a tunable semiconductor signal laser (A, = 1.5 pm-1.6 pm) is coupled to both inter- ferometric arms. A differential detection scheme improves the sensitivity and eliminates influence of amplified spon- taneous emission. The phase in the ref- erence arm is linearly modulated in time, 9, = wt, by means of a piezo-electric crystal. This results in a response of the detected signal, which in the case of no pump power can be written; A L t  cos(wt + 44, and in presence of pump power; ALP x cos(wt + 60 + 44, where $o is an arbitrary phase and 4$ is the pump-induced phase shift of the sig- nal, &, is determined by measuring the phase difference of the cosine waveforms before and after the pump is tumed on by an optical chopper. QThFZ Fig. 1. Schematic of the setup for directly measuring pump-induced phase modulation in Er-doped fibres. T.S.: tunable semiconductor laser; SE: single mode fibre; M.O.: microscope objective; POL.: polarizer; B.S.: beam splitter; M.: mirror; WDM 980 nm/1550 nm wavelength demultiplexer; S.L.: spherical lens; E.D.F.: Er-doped fibre; P.E.: piezo-electric crystal; S.F.C.: (polarization maintaining) 3-dB single mode fibre coupler; F.: pump block filter; P.D.: photodiode. Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 08,2010 at 09:01:36 UTC from IEEE Xplore.  Restrictions apply. THURSDAY MORNING 1 EQEC‘94 f 199 ErJt-concentration: a_ 0 . 7 ~ 1 0 ~ ~  cm?-8.0~1 0lp cm? a, Vc c  .- .- t o  I 0 0 -5 - al LT-10 0 5 10 15 20 25 Absorbed optical power (mW) QThFZ Fig. 2. Refractive index change as function of wavelength for Er-doped fibres with a do in concentration of 0.7 X l O ’ ’ ~ m 8 - ~  (square symbols), 2 X cm-3 (cross symbols), and 8 X 1019 (triangular symbols). The pump power at 980 nm is 4 mW. - 161 , , , , , , , , , , ,. /!4 Er’+-concentration: A as%%fl 0.7~10‘9 cm-3 x 4  m ._ al U - 4  .- I ,e 1500 1520 1540 1560 1580 1600 Wavelength ( n m )  QThF2 Fig. 3. Refractive index change as function of absorbed pump power for Er-doped fibers with doping concentrations of 0.7 X 1019 cm-3 (square symbols) and 8 X l O I 9  cm-3 (triangular symbols) at different wavelengths as indicated. In Fig. 2, the change in refractive in- dex defined as, An = (X,/(~T~L)+,, where L is the length of the fibre, is plotted for three fibres with E?*-doping concentra- tions of 0.7 X 1019 cm-3 (square symbols), 2 X 1019 cm-3 (cross symbol) and 8 X 10’’ ~111.~ (triangular symbols) as function of wavelength at an absorbed pump power of 4 mW. The An increases with increas- ing doping concentration and changes sign at A, = 1535 run, which is situated just below a resonance wavelength of 1530 nm. The slope of the curves and therefore the dispersion are maximum near resonance. The dispersion for the fi- bre with the highest doping concentra- tion is calculated to be -200 ps/nm/km at 1530 nm. Figure 3 shows An as function of ab- sorbed pump power for the fibres with Er3’<oncentrations of 0.7 x 10l9 cm-’ (square symbols) and 8 X l O I 9  cm-3 (tri- angular symbols). The wavelengths de- picted in Fig. 3, A, = 1565 nm and 1568 nm are situated below resonance and the wavelengths A, = 1522 nm and 1520 nm are situated above resonance, where a larger An can be obtained. The An satu- rates at higher pump powers for the fibre with the higher doping concentration. Thermal effects are believed to be negli- gible, since a thermal contribution to An should be either positive or negative and independent of wavelength, in contrary to the results shown in Fig. 3. The proposed system has proven to be useful to measure pump-induced phase shifts directly and refractive index changes in Er-doped fibres. The results show that a highly doped fibre should be used if refractive index modulation is de- sirable (e.g., in optical switches). In ap- plications where the dispersion is un- wanted (e.g., in soliton transmission systems), a tradeoff between gain and re- fractive index modulation should be made for the operating wavelength. “Lycom Al.,  NKT All6 75, 2605 Brmdby, Denmark Input ’ energy (PJ) 0 QThF3 Fig. 1. Output energy versus input energy and best fit (solid curves). Femtosecond data at 605 nm. QThF3 (Invited) 1130 Two-photon absorption of semiconductor crystallites in doped glasses G. P. Banfi, V. Degiorgio, M. Ghigliazza, H. M. Tan; A. Tomaselli, Dipartimiento di Elettronica, Uniuersitri di Pauia, Via Abbiategrasso 209, Pavia, Italy One of the questions conceming semi- conductor doped glasses (SDG) is whether the nonlinearities of the crystal- lites at frequencies well below bandgap are affected by the electronic confine- ment. The problem has both conceptual and practical implications. Large effects even for modest confinement have been suggested by theoretical models, but the data so far reported are not conclusive to this regard.’-2 We here report the first quantitative determination of Imxf’, the imaginary part of the third order suscep- tibility of the crystallites in SDG. The ten samples investigated are commercial SDG manufactured by Schott, and con- tain CdTe (RG850-RG830) or CdSSe (CG-495 to RG 715) crystallites. In order to obtain Zmxf’, one first de- rives, through a nonlinear transmission measurement, psE, the two-photon ab- sorption (TPA) coefficient of the SDG (psE is related to Imx& through p = (w(q,c2n’)-’lm 13)) and then makes use of the relation x i  = x& P f,, with f the local field correction factor, and f, the volume fraction occupied by the crystallites.” The crucial quantities in the procedure are Since the volume fraction cannot be obtained neither from a wet analysis, nor from the initial composition of the SDG melt, this problem was solved by using small angle neuron scattering measure- ments (performed at the Cold Neutron facility at NIST, Gaithersburg, Maryland) which provided accurate values off, and of the average radius R of the crystallites for each sample! For our samples, fv = 1.5 - 5%, R = 2-11 nm. Some transmission data are shown in Fig. 1. In order to obtain a reliable psm from an otherwise simple transmission measurement one should take into ac- count the effect of collisional absorption. In fact, the optical pulse is attenuated not only by TPA, but also by the carriers ex- cited by TPA. In the measurements at 1.06 pm (were we employed 30 ps PSm and f, 10-20110 ’ 112 ’ lj4 ’ 116 ’ 118 ’ 1L, Eglhw QThF3 Fig. 2. Plot of the scaled quantity (w/wo)’Im~(3J versus E,(hw). Optical measurements at: 0.6 pm (squares), 1.06 Fm (circles). The uncertainty for glasses with E,/(fiw) > stems from the indeterminacy in collision absorption: the two open circles encompass the possible extreme values. The numbers associated to the experimental points are the radii of the nanocrystals in angstroms. The solid line gives (w/w;’iO)‘lm~‘~’ for bulk semiconductors. pulses), we could account for collisional absorption, at least for three glasses, by comparing results obtained with samples of different thickness. Another possibility is to employ much shorter pulses, which generate a small number of free carriers. Measurement were done at LENS, Flor- ence, with 200 fs pulses at A = 0.6 pm. The available wavelength allowed only the study of OG 495, 570, 590. The results can be seen in Fig. 2 where we plotted the scaled quantity (w/ w,)41mx‘3’, with wo the frequency of 1.06 pm radiation, as a function of Eg(fiw), with E, the energy gap and fiw the pho- ton energy. The solid line gives the the- oretical value of (w/w,)‘Im~~~’ for a bulk semiconductor, and, as indicated by Van Stryland? is in agreement with bulk data. Imx:’ must be quite small, if we note that deviations from the bulk values are not evident even for the small crystallites of the GG 495 glass (R = 2 nm). We can con- clude that semiconductor nanocrystals with a size ranging between 2 and 11 nm, the TPA coefficient is comparable to that the bulk semiconductor in the whole ac- cessible range of E,/(hw) values. Such a result calls for a reconsideration of the Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 08,2010 at 09:01:36 UTC from IEEE Xplore.  Restrictions apply. 

Direct measurement of pump-induced phase modulation in erbium-doped fibres

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Direct measurement of pump-induced phase modulation in erbium-doped fibres

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