156 / CLE0'97 / TUESDAY AFTERNOON

-50 -25 0 25 50 Angle (Deg.)

(a)

-50 -25 0 25 50 Angle (Deg.)

(b)

CTuQ2 Fig. 1 Far-field patterns in the verti- cal direction of (a) a tapered device and (b) a nontapered reference laser. The FWHMs are 16" and 40" for (a) and (b), respectively.

mode fiber coupled long wavelength laser sources. The monolithic integration of an adiabatic mode transformer has been pro- posed to convert an elliptical laser mode pro- file to a circular, low divergence one. Here, we report a 1.55-pm multiple quantum well laser diode with a built-in vertical mode expander, which exhibits high single mode output power in a circular far-field beam pattern.

The laser material with strained InGaAsP quantum wells was grown with use of MOCVD epitaxy technique. A vertically ta- pered section was formed between the active and the passive waveguide sections for gradual mode expansion. InP cladding and InGaAs contact layers were then regrown by MOCVD. The wafer was processed into ridge waveguide laser diodes. Cleaved laser bars were LWHR coated and individual chips were mounted p-side up on heatsinks.

Figure 1 compares the vertical far-field pat- tern of a typical tapered with that of a reference laser fabricated from a wafer with similar layer structure but no taper. The divergence angle FWHM of the tapered device was 16", which is a significant reduction from the 40' of the ref- erence laser. Far-field patterns in the lateral direction were similar for both types of devices (with and without taper), the FWHM being about 16-19'. Figure 2 shows the L-I charac- teristic of a 750-pm-long tapered device. Single mode laser output power from the facet exceeded 90 mW in cw condition. A prelimi- nary fiber coupling experiment, also shown in

100

8 0

- 6 0 L m

a 2

6 ...

4 0

2 0

0

LRIHR

0 100 200 3 0 0

Current (mA)

CTuQ2 Fig. 2 Light output versus current characteristic for a 750-pm-long tapered device, directly from a chip and after coupling into a lensed fiber.

I I I k - I d 0

4 - 2 0 2 4

Displacement (pm)

CTuQZ Fig. 3 Vertical displacement sensitiv- ity in the lensed fiber coupling for both a tapered and a nontapered reference LDs. The horizontal lines correspond to 1dB loss from the maximum coupling for each curve.

the figure, demonstrated 63% coupling effi- ciency into a lensed single mode fiber. The corresponding 2dB coupling loss is a substan- tial improvement from the 4dB coupling loss (or 40% coupling efficiency) obtained with a standard laser. The misalignment tolerance at 1dB extra loss for the tapered LD was relaxed to ? 1 p m (See Fig. 3 ) .

In conclusion, we have fabricated a 1.55-pm semiconductor laser diode with a cir- cular laser beam profde by integrating a mode expanding feature inside cavity. Over 90 mW single mode output power was obtained with the far-field divergence angles of 16" X 19' at the FWHM. The coupling efficiency of a ta- pered LD was enhanced by 2dB over a nonta- pered reference device, and fiber output ex- ceeding 50 mW was also demonstrated. This work was supported by an ARPA SBIR con- tract DAAHO 1-96-C-R047.

CTuQ3 3:OO pm

Wide laser wavelength range for 1.3 lJtm 10 Gb/s directly modulated transmission resulting from pulse compression effect

I. Kim, T. J. Miller, Y. K. Park, Lucent Technologies, 9999 Hamilton Blvd., Breznzgsville, Pennsylvania 18031

While fiber chromatic dispersion can be con- sidered noncritical at 1.3 k m wavelength, with the data rate increase its impact may signifi- cantly limit systems performance. Thus for ex- ample to keep a dispersion penalty below a certain value (say 1 dB), the operating laser wavelength range needs to be limited. In this paper, we report the pulse compression effect observed in 10 Gb/s transmission using a 1.3 p m low-chirp high-power directly modulated, packaged DFB laser module. Because of the pulse compression, negative dispersion penal- ties have been achieved for the fiber dispersion range of -600 to 0 ps/nm. For use ofthis effect, the laser wavelength range is found to be as wide as 80 nm for transmission over 50 km of embedded standard single mode fiber cables.

The DFB laser structure is described in Ref. 1. It consists of InGaAsPiInP compressively strained multiple quantum wells. The front and rear facets are AR and HR coated The cw light-current characteristic of the packaged DFB laser module exhibits a threshold current of 12 mA with a 0.22 mW/mA slope efficiency at 25°C. The lasing wavelength is 1309 nm and the measured cw Iinewdth enhancement fac- tor (a) is 1.5.

Figure 1 shows the measured eye diagrams for different fiber dispersion values with vari- ous lengths of dispersion-shifted fiber (DSF). As the amount of negative fiber dispersion in- creases, pulses become compressed and then start to get broadened with further increase. This pulse shaping phenomenon is clearly manifested in the BER performance. Figure 2 shows the dispersion penalties measured at lo-" BER including the result with 51 km of standard single mode fiber The module was biased at 35 m A with an average optical power of5.1 mW(7dBm),andwasdrivenbylOGb/s data with 1.4 VpPp (peak-to-peak voltage of 1.4 V), 223 - 1 NRZ (non return-to-zero) PRBS (pseudo-random bit stream). The back- to-back extinction ratio was 9 dB. For the fiber dispersion range of -600 to 0 pshm, negative

(a) Back-to-back L (b) -240 psinm

(d) .710 psinm (c) -500 psinm I

(40 psidw)

CTuQ3 Fig. 1 Eye diagrams for different fiber dispersions.

TUESDAY AFTERNOON / CLEO'97 / 157

1.5 m 2 1.0 ..

0.5 - b 1 0 - :

E - 1 . 0

f -0.5

2.0 I - -

-

-

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(a)Back to back (b) 240 pslnm (c) -500 psinm (d) 710 pdnm (e) +9 5 psinm (51 km of Standard fiber)

( 4

- 1 5 1 I I ' I 1 I ' I I -800 -700 -600 -500 -400 -300 -200 -100 0 100

Fiber Chromatic Dispersion (psinm)

CTuQ3 Fig. 2 Measured dispersion power penalty at lo-" BER for different fiber disper- sions.

D (ps/nm/km)

CTuQ3 Fig. 3 Dispersion characteristics of embedded standard single mode fiber. AA repre- sents the distribution of the zero-dispersion wavelengths (Ao, L1, AI).

dispersion penalties are obtained as a result of the pulse compression caused by the combined effect of the low-chirp (a = 1.5) and negative chromatic dispersion.' It is noted that with a high output power of the laser module, this wide range of negative dispersion penalty can be used to increase the overall system power budget. By selecting an appropriate laser wave- length shorter than the standard fiber zero- dispersion wavelength, this can be easily achieved.

Figure 3 illustrates the embedded standard fiber dispersion characteristics with the zero- dispersion wavelength distribution of AA cen- tered at A,. For the fiber dispersion of -600 to 0 ps/nm, the DFB laser wavelength (A,) is limitedtoarangeof-80nm< (Adfi- L1) < 0 nm. The fiber length of 50 km, AA = 20 nm, and the dispersion slope of 0.1 ps/nm'/km are assumed. This wide wavelength range of 80 nm is easily achievable in the present capability of DFB laser wavelength control.

In summary, we have shown that by use of the pulse compression effect, the operating la- ser wavelength range is limited to as wide as 80 nm, to achieve a negative dispersion penalty in 1.3 km 10 Gb/s directly modulated transmis- sion over 50 km of embedded standard fiber cables. 1. K. Kojima, 0. Mizuhara, L. J. P. Ketelsen,

I. Kim, R. B. Bylsma, Optical Fiber Com- munication Conference (OFC '96), Post- deadline Paper PD11, San Jose, Calif., Feb. 1996.

2. Y. Miyamoto, K. Hagimoto, F. Ichikawa, M. Yamamoto, T. Kagawa, Electron. Lett. 27,853-855 (1991).

CTuQ4 3:15 pm

Effect of p-doping on carrier leakage and characteristic temperature To of 1.3 pm strained InGaAsP/lnP multiple quantum well lasers

D. V. Donetsky, G. L. Belenky, C. L. Reynolds Jr.,** R. F. Kazarinov, S. Luryi, State University of New York at Stony Brook, Stony Brook, New York 11 794 In Ref. 1 leakage current of electrons from the active region of InGaAsP/InP multiple quan- tum well (MQW) was measured with use of a purely electrical technique within the current density range 0 -2 kA/cm2 in cw regime. In this paper we will discuss the results of simulta- neous measurements of the leakage current and threshold current in InGaAsP/InP lasers within the range of current densities 0-10 kA/cm'. Measurements were carried out within 10-80°C temperature range under the pulse operation with 50 ns pulse width and 100 kHz repetition rate.

We studied broad area devices with electron collector with undoped and 1.5 X lo'* cmp3 Zn doped separate confinement heterostruc- tures (SCH). The laser active region consists of 9 quantum wells 70-A-thick and 100 A barriers surrounded by 500 A SCHs. The results of leakage current measurements are presented in Fig. 1.

One can see that the dependence of the leakage current versus injection current has a tendency to saturate for undoped structures and remains linear within of measured range for devices with a doped SCH. The observed nonlinearity of the dependency of leakage cur- rent versus injection contributes to the nonlin- earity of light-injection current curves and thus increases the composed second order (CSO) distortion value for analog devices. At the same time, even for the devices with un- doped heterointerfaces at SOT, the magnitude of carrier leakage measured at 10 kA/cm2 in- jection current density does not exceed 4% and will not significantly affect external efficiency.

Figure 2 shows the temperature depen- dence of threshold current for devices with undoped and doped interfaces. Measured typi- cal characteristic temperatures To were close to 60 K for doped structures. In lasers with un- doped SCH, the threshold values rapidly in-

inlectla" current OenSltr (W") InjMlon current oenr,ty (LA,")

(a) (b)

CTuQ4 Fig. 1 Dependencies of the leakage current density on the injection current density for the lasers with undoped (a) and doped (b) p-cladding interfaces.

0 20 40 60 80 100 Temperature (OC)

CTuQ4 Fig. 2 Dependencies of the threshold current density versus temperature for the lasers with undoped (open symbols, dashed lines) and doped (solid symbols, solid lines) p-cladding in- terfaces.

crease with temperature above 40°C. Com- parison of the data presented in Figs. 1 and 2 shows that the difference in the threshold cur- rent temperature behavior cannot be ex- plained by the differences in the leakage cur- rent. Possible mechanisms of this behavior are discussed. *Lucent Technologies Bell Laboratories, Murray Hill, New Jersey 07974 **Lucent Technologies Bell Laboratories, Brein- igsville, Pennsylvania 18031 1. G. L. Belenky, C. L. Reynolds, R. F. Ka-

zarinov, V. S. Swaminathan, S. Luryi, J. Lopata, IEEE J. Quantum. Electron. 32, 1450 (1996).

CTuQ5 3:30 pm

A new model for the polarization bistability in ridge-waveguide InGaAsP/lnP lasers

A. Richter, Ch. Lienau, T. Elsaesser, A. Klehr,* Max-Born-Institut fur Nichtlineare Optik und Kurzzeitspektroskopie, Rudower Chaussee 6, 0-12489 Berlin, Germany Switching processes in semiconductor lasers are important both from theviewpoint ofnon- linear dynamics and for numerous applica- tions in optoelectronics and optical communi- cations. Recently, anomalous polarization- dependent emissions have been observed in strained InGaAsP/InP semiconductor lasers. In particular, an S-shaped TE-TM polarization bistability (Fig. 1) at room temperature was found in strained ridge-waveguide 1.3 p m InGaAsP/InP lasers by Klehr et aL' Such struc- tures are of particular interest for device appli- cations like optical switches as the switching times between TE and TM emission are on the order of 50 ps for impulsive current modula- tion.

Here, we employ for the first time near-field scanning optical microscopy (NSOM) to clarify the physical mechanisms underlying the polarization bistability. NSOM allows us to overcome the diffraction-limited resolution of