sorption cross section was calculated to be 4.4 X 10- l9 cm’. This value gives only a lower limit of the cross section because the actual concentration of Cr2+ ions in the sample is unknown. The luminescence spectrum excited at 1900 nm shows a strong band centered at 2250 nm with a FWHM of 450 nm and a life- time of 1.4 ks. When cooling the sample to 15 K the lifetime increased up to 3.7 ps. Under the assumption that the radiative decay rate is con- stant with temperature, any change in the mea- sured lifetime is interpreted as being due to the onset of nonradiative decay. The room- temperature quantum efficiency is then esti- mated to be as high as 38% (ratio between 300 and 15 K lifetimes). Using the estimated quan- tum efficiency and room temperature lifetime, the emission cross section was calculated ac- cording to McCumber’s theory to be 2.7 X lo-’’ cm’. The emission cross section of Cr2+:CdMnTe is significantly larger than that of the commercial laser material Tisapphire.

Room-temperature laser operation of Cr2+:Cdo~8,Mno~15Te was demonstrated using a hand-polished, uncoated, 3-4-mm-thick sample that was placed in a cavity consisting of a flat high reflector (R > 99% @ 2350 nm) and a curved output coupler (R = 95% @ 2350 nm). For the excitation of Cr2+ ions the pulsed 1907 nm output from a H, Raman-Cell pumped by a Nd:YAG laser was employed. Laser activity centered at 2525 nm was achieved with a slope efficiency of -4.5%. De- tailed studies on the laser behavior are cur- rently in progress and will be published in a forthcoming paper.’ A higher efficiency of the Cr2+:Cd,,,,Mn,,,,Te laser can be expected af- ter optimizing the crystal preparation. *Brimrose Corporation ofAmerica, 5020 Camp- bell Blvd., Baltimore, Maryland 21236 1. L. D. DeLoach, R. H. Page, G. D. Wilke,

S. A. Payne, W. F. Krupke, IEEE J. Quan- tum Electron. 32, No. 6,885 (1996). U. Hommerich et al., to be published. 2.

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CThT8 6:15 pm

Characteristics of diode-pumped lasers using CO’+ doped zinc chalcogenides

Yang Zhao, Cunkai Wu, Department of Electrical and Computer Engineering, Wayne State University, Detroit, Michigan 48202

Transition metal-doped 11-VI semiconductors as lasing media have attracted attention, since they exhibit interesting absorption and emis- sion spectra for IR laser applications.’-6 Re- cently, a new diode-pumped Cr:ZnSe laser has been demonstratedwith an output wavelength of 2.35 micrometers.’ This laser showed tun- able emission from 2 to 3 micrometers without adverse effects of nonradiative decay. This work has opened a new field of transition metal-doped 11-VI semiconductors as lasing media. In this paper, we present a design of a tunable Co:ZnTe laser with an output wave- length from 3 to 4 micrometers, pumped by powerful AlGaAs semiconductor lasers. Com- pared with the Cr:ZnSe laser, the CO laser has several new features including longer output wavelength, and the pumping source, which is low cost and reliable. In addition, since CO has

THUR5

‘*‘ pumping

CThT8 Fig. 1 Energy levels of CO’+ in 11-VI semiconductors. The pumping laser excites the population from the ground level to 4T,(P) level. These populations then experience nonradiative relaxation to 4T,(F) level. Lasing occurs from 4T,(F) to 4A,(F).

higher solubility in 11-VI semiconductors, doping control is much easier and quality of these crystals can be better.’

Our design is based on the spectroscopic characteristics of CO’+ in 11-VI semiconduc- tors.’-6 Figure 1 shows relevant energylevels as well as pumping and lasing transitions. The crystal has 4A,(F) to 4T2(F), 4T1(F), and 4T,(P) absorption bands. The absorption is very strong between 4A2(F) to 4T1(P) (wave- length near 810 nm and cross section - lo-’’ cm’) and relatively weak between 4A,(F) and 4T,(F) (cross section - ~m’).’,~ The emission spectrum shows that a wide tuning range in the mid-IR emission is possible from 4T,(F) to 4A,(F) transition.’ The emission cross section of this transition is on the order of

cm2.1 The emission lifetime is around 290 PS.’-~

We select the absorption from 4A,(F) to 4T,(P) as the pumping mechanism, since the wavelength of this transition matches that of AlGaAs lasers. The population transition from the 4T1(P) to the 4T,(F) level is fast nonradia- tive relaxation. With enough pumping, popu- lation inversion is achieved for lasing from 4T,(F) to 4A,(F).

Using a three-level model, we can estimate the slope efficiency of this laser. The slope effi- ciency can be calculated from’

s = L u,u,u,u,u, where L is the loss factor related to cavity loss and absorption of the laser medium, and Ui ( i = 1-5) are pump source efficiency, radia- tion transfer efficiency, absorption efficiency, efficiency of transfer energy from the ground state to the upper laser level, and beam overlap efficiency, respectively. Using the material pa- rameters for Co:ZnTe in Refs. 4 and 5 , for a laser crystal with 2-mm length and output mir- ror reflectivity 0.9, we have a slope efficiency of 0.275.

The pumping intensity threshold for achieving laser operation can be estimated from

Ith= VI,L,t,/(lU’ U, U, U, U,)

DAY AFTERNOON / CLE0’97 / 433

where L, is the resonator loss, Vis the volume of the laser rod, 1 is the cavity length, t2 is the upper laser level lifetime, and I, is the satura- tion intensity. In our case, we have 1, = 6200 Wkm’, L, = 0.04, 1 = 20 cm, V = 3.14 X 10-3cm3. Then Ith is around 0.3 mJ. Therefore, if we use AlGaAs pumping laser arrays with 80 mJ coupled to the lasing medium, we expect an output intensity up to 22 mJ. 1. L. D. DeLoach et al., IEEE J. Quantum.

Electron. 32, 885 (1996). 2. D. J. Robbins et al., Solid State Commun.

36,61 (1980). 3. L. M. Baranowski et al., Phys. Rev. 160,

627 (1967). 4. A. P. Radlinski, Phys. Stat. Sol. (b) 86,41

(1978). 5. A. P. Radlinski, J. Phys. C: Solid Stat. Phys.

12,4477 (1979). 6. P. Koidl et al., “Near-infrared absorption

of CO’+ in ZnS: Weak Jahn-Teller cou- pling in the 4TZ and 4T1 states” Phys. Rev., B (1973).

7.

8.

G. Cantwell, Eagle-Picher Industries Inc., private communication (1996). W. Koechuer, Solid State Laser Engineering (Springer-Verlag, 1996).

CThU 4:30 pm-6:00 pm Room 318

Frequency-Resolved Optical Gating and Applications

Daniel J. Kane, Southwest Sciences Inc., Presider

CThUl 4:30 pm

Complete characterization of pulse propagation in optical fibers close to the zero-dispersion wavelength with use of frequency resolved optical gating

P. G. Bollond, J. M. Dudley, L. P. Barry, J. D. Harvey, R. Leonhardt, P. D. Drummond,* Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand; E-mail: j.dudley@auckland.ac.nz

Data transmission by use of picosecond pules propagating near the zero-dispersion wave- length (ZDW) will be required for ultrahigh bit-rate optical communication systems. It is thus important to accurately characterize the effects of fiber nonlinearity and higher order dispersion on pulse propagation in this re- gime. Experimental investigations and nu- merical simulations have shown that the com- bined effects of self-phase modulation and third-order dispersion (TOD) may result in the formation of rapid oscillations on the pulse.’ In this earlier work, however, although such severe temporal pulse distortion was clearly evident from numerical simulations, it could not be completely characterized experi- mentally from autocorrelation measure- ments.’

In this paper we use the measurement tech- nique of frequency resolved optical gating (FROG)’ to accurately determine the temporal

434 / CLE0'97 / THURSDAY AFTERNOON

1500 1520 1540 1560 Wavelength (nm)

CThUl Fig. 1 (a) Measuredspectrumand (b) autocorrelation of pulses after propagation through 700 m of DSF; (c) and (d) show respec- tively the corresponding results from numerical simulations of the NLSE.

$770

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Time (ps)

CThUl Fig. 2 (a) Measured FROG trace of pulses after propagation through 700 m of DSF. (b) Retrieved intensity (solid curve, left axis) and phase (dashed curve, right axis) for this FROG data.

profile of an optical pulse after propagation close to the ZDW in a dispersion-shifted fiber (DSF). In our experiments, we used pulses of duration -2 ps, generated from a wavelength- tunable, modelocked erbium-doped fiber laser (EDFL)3 with a pulse peak power of 22 W at an operating wavelength of 1532.5 nm. These pulses were transmitted through 700 m of DSF (measured ZDW of 1545 nm) and the resulting pulses were examined with use of standard optical autocorrelation and spectral measure- ment techniques along with the second har- monic generation (SHG) FROG measurement technique.'

Figures la and Ib show the directly mea- sured spectrum and autocorrelation of the output pulses, while Figs. IC and Id present the results from a numerical simulation of the pulse propagation using a modified nonlinear Schrodinger equation (NLSE) including TOD. The experimental and theoretical results are in good agreement, and from the pulse spectrum we see that the process of four-wave mixing around the ZDW results in the generation of a new spectral component in the anomalous dis- persion regime. The autocorrelation traces clearly show pulse broadening (to 11 ps) and a modulation on the pulse as a result of interfer- ence between components of the pulse spec-

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CThUl Fig. 3 (a) FROG trace and (b) inten- sity and phase of transmitted optical pulse from numerical simulation of the pulse propagation using the NLSE.

trum on either side of the ZDW. The optical autocorrelation measurement, however, does not characterize completely this modulation on the optical pulse, but this can be directly measured with use of the FROG technique.

Figure 2 shows the measured FROG trace and the intensity and phase of the pulse re- trieved using an algorithm based on the method of generalized projections.2 These ex- perimental results are in good agreement with those generated from numerical simulations (Fig. 3) and clearly show the rapid oscillations on the leading edge resulting from the interfer- ence between spectral components on either side of the ZDW. Note that this physical origin of the temporal modulation is particularly e+ dent from the FROG signal, which shows a temporally modulated component around 773 nm while there is no significant amplitude in the broadened pulse spectrum at the funda- mental wavelength of 1546 nm. These results show that the FROG technique can therefore be used to accurately characterize near-ZDW propagation in fibers in a way that is not pos- sible with use of isolated temporal or spectral measurements. *Physics Department, Queensland University, St Lucia, Queensland, Australia 1. J. Schutz et al., Opt. Commun. 95,

357-365 (1993). 2. K. W. DeLong etal., J. Opt. Soc. Am. B 11,

3. W. Margulis et aL, Electron. Lett. 31, 2206-2215 (1994).

645-646 (1995).

CThU2 4:45 pm _____~ ~

Generation and measurement of ultrafast shaped pulses

Andy Rundquist, Erik Zeek, Ivan Christov,* Margaret Murnane, Henry Kapteyn, Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 481 09-2099; E-mail: kapteyn@umich.edu Pulse shaping' and spectral filtering of ultra- short pulses allow us to tailor the excitation

risetime M M M 8.90 f s 49.58fs

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f s

risetime M M M 11.51fs 20 .02 fs

I I I I

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f s

risetime NVHM 13.24fs 21.97fs

4. I E-

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f s

risetime M M M 12.58fs 21.13fs

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1 .OE-1

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f s

CThU2 Fig. 1 Birefringent shaping of a 25 fs Gaussianpulse. (a): n = 5,0 = 45'; (b): n = 7,0 = 40°;(c):n= 15,8=3l0;(d):n= 11,0=35".The y axis is normalized to the peak intensity of the original pulse.