TUESDAY AFTERNOON / CLEO'99 / 155

I"' """""""" " " " ' "" T=80K ' " ' I

Photon wavelength (A) CTuK65 Fig. 2. Edge-emission spectra at 80 K for different optical pump powers. At 0.3 kW/cm2, below threshold, we observe a large spontaneous emission spectrum from the AQW. At 6 kW/cmz we are above threshold and lasing is observed. In- creasing further the optical power at 30 kW/cm2 induces a 3 nm redshift of the lasing wavelength.

In Fig. 2, we show the edge-emission spectra obtained in optical pumping experiments from the sample of Fig. 1 inserted in aseparate- confinement-heterostructure waveguide. The sample is made of a CdTe-based heterostruc- ture emitting at 800 nm and is grown by mo- lecular beam epitaxy. The lasing and the charg- ing of the structure is provided by a pulsed NdYAG 532 nm laser. Above threshold, we observe a 3 nm redshift at T = 80 K as the optical power increases from 6 to 30 kW/cmz, which demonstrates the feasibility of the de- vice. Furthermore, tuning is observed up to room temperature but the tuning range is re- duced because of an increase of the lasing threshold. Ongoing work is focusing on opti- mising the structure parameters in order to obtain larger tuning range at room tempera- ture. *Also with INFM and Dipartimento di Fisica, Cittadella Universitaria, 09042 Sestu, Italy *+Laboratoire de Spectromttrie Physique, CNRS, Univ. J. Fourier, 38402 Saint Martin d'Hhres, France 1. S. Murata, I. Mito, Optical and Quantum

Electronics 22, 1 (1990). 2. M. Yamanishi, I. Suemune, Jpn. J. Appl.

Phys. 22, L22 (1983). 3. L.Y. Liu, E.E. Mendez, H. Meier, Appl.

Phys. Lett. 60,2971 (1992). 4. V. Ortiz, N.T. Pelekanos, G. Mula, Appl.

Phys. Lett. 72,963 (1998).

CTuK66

Ultralow-blrefrlngence measurement In optical fibers by use of a flber optical loop mirror

E.A. Kuzin, B. Ibarra Escamilla, J.M. Estudillo Ayala, INAOE, A.P.51 y216, Puebla, 72000, Mexico; E-mail: ekuz@inaoep.mx The most of the experimental techniques pro- posed to measure fiber birefringence use a sec- tion of a fiber with a length of several beat

CTuK66 Fig. 1. Experimental setup.

"'..I I

Fiber twist, degree.

CTuK66 Fig. 2. Typical dependence of the transmission coefficient on the fiber twist.

1engths.lJ These techniques are convenient for high-birefringent fibers, but it is difficult to use them for low-birefringent fibers. Recently the proposal to use a twist method for low- birefringent fibers has been published.' High accuracy of this technique was demonstrated but it requires a quite difficult procedure ofthe adjustment of polarizers and principal axes of the fiber.

It is well known that a Fiber Optical Loop Mirror (FOLM) does not require polarized lights at an input. This feature has been used for polarization independent sensors and WDMs.*p5 Polarization independence may simplify drastically the adjustment when the FOLM is used for polarization measurements. In this report we show for the first time that the FOLM consisted from a coupler with output ports connected by the fiber to be measured provides a very simple and accurate measure- ment of the birefringence of low-birefringent fibers.

Figure 1 presents the experimental configu- ration. A laser team from a cw Nd:YAG laser is splitted by a 0.45/0.45 cube beamsplitter and introduced to a span of a low birefringent fiber. The coupling of the beam to the fiber was adjusted to provide equal powers of the coun- terpropagating beams in the fiber. We twisted the fiber and measured the dependence of the transmission coefficient on the fiber twist.

Figure 2 presents an example of the experi- mental results. The numerical simulations show that the maximum value of the transmis- sion coefficient depends on the ratio between a beat length and fiber loop length. Fig. 3 pre- sents the calculated dependence (solid line). We have made measurements for several spans of the fiber with length in the range from 0.5 m to 1.5 m. We have fitted the experimental val- ues of the transmission coefficient with calcu-

0.0 \I 0.0 0.2 0.4 0.6 0.8 1.0

Fiber length per beat length ratio

CTuK66 Fig. 3. Dependence of the maxi- mum transmission on the ratio of the fiber length per beat length. Solid line presents theoretical result, square point present experimental values.

lated ones and found the beat length to be equal to 2.1 m. The experimental values of the transmission coefficient is presented in Fig. 3 by square points. To calculate the fiber length per beat length ratio we used the value of beat ratio equal to 2.1 m. As we can see the agree- ment between experimental and calculated re- sults is within the accuracy better then 10%.

It follows from Fig. 3 that the most accurate measurements may be made if the fiber length per beat length ratio lies in the range from 0.1 to 0.4. This corresponds the near-linear depen- dence of the transmission on the twist. For the implementation of the experiment the most appropriate fiber length lies in the range from 0.5m to 5 m. So this technique may be used efficiently for fibers with the beat length in the range from 1 m to 50 m. 1. R. Calvani, R. Caponi, and F. Cisternino,

J. Lightwave Technology, v. 7 (1989), p. 1187. 2. T. Okoshi, S . Ryu, and K. Emura, Opt.

Communs. v. 2 (1981), p. 134. 3. A. Rodriguez, A.V. Khomenko, R. Cortes,

and Garcia-Weidner, Optics Letters, v. 22 (1997), p. 877. D.B. Mortimore, Journal of Light wave Technology, v. 7, (1988) p. 1217. E.A. Kuzin, J. Sanchez-Mondragon, M.A. Basurto-Pensado, B. Ibarra-Escamilla, and M. Ramirez-Renero, Optik, v. 106 (1996) p. 172. X. Fang and R.O. Claus, Optics Letters, v. 20 (1995), p. 2146.

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CTuK67

Effects of temperature on radiative transitions of erbium In lead-tellurium- germanate glass

S.U. Egarievwe, I.D. Diaz-Rodriguez,, K. Chattopadhay, S.H. Morgan, A. Burger, NASA Center for Photonic Materials and Device, Department of Physics, Fisk University, Nashville, Tennessee 37208-3051 USA; E-mail: segariev@du bois.fisk.edu Er3+ has been shown to be a good rare-earth dopant of crystals and glasses for practical ap- plications as laser-active media and infrared-to- visible signal converters.'-a The effects of tem- perature on E?' transitions in the host materials has been a subject of many investigations aimed at obtaining efficient systems for fiber-laser and fiber-communications application^.'-^ Several