158 / CLEO 2002 / TUESDAY MORNING

2.3 pm double QW p-substrate InGaAsSblAlGaAsSb broad area lasers” IEE Electron. Lett. 35 (1999), p. 298.

4. D. Mehuys, R.J. Lang, M. Mittelstein, J. Salz- man, A. Yariv, “Self-stabilized nonlinear lat- eral modes of broad area lasers”, IEEE I. Quantum. Electron. 23 (1987), p. 1909.

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4 ~ 0.5 CTuE5 9:00 am

Quantum Cascade Semiconductor Amplifiers for High Power Single Mode Emission at h = 7.5 pm

Mariano Troccoli, Federico Capasso, Claire Gmachl, Axel Straub, Deborah L. Sivco, and Alfied Y. Cho, Bell Laboratories, Lucent Technologies, 600 Mountain Ave. Murray Hill NI

We present a mid-IR semiconductor optical am-

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0.0 150 200 250 300 07928, Email: rroccoli@lucent.com

Lateral Dimension (pm)

cTuE4 Fig. 3. Lateral near-field emission pat- plifier based on a quantum cascade (Qc) active region, which has been used to attain high output powers from distributed-feedback (DFB) QC lasers without affecting their sinale mode behav-

tern for a 2.3 pm GaSb-based laser operating at a current I = l.7Ich.

gain spectrum is related to transitions between states distant from quasi-Fermi levels and, conse- quently, is characterized by a lower value of the differential gain. Current increase leads to lower values of a in long-wavelength part of the spec- trum (Fig. 2), suggesting a faster saturation with current of the differential index compared with the differential gain. Several effects, for instance, conduction band nonparabolicity, can explain the differential index saturation.

An independent test for the value of the linewidth enhancement factor or antiguiding pa- rameter was performed based on near-field pat- tern measurements. Total loss and a-factor deter- mine the laser average filament spacing W,, W, - (a-factor*l~ss)-”~.~ Optical losses determined from longwavelength part of the modal gain spectrum (Fig. 1) are 21-22 cm-I and CL is about 3.8 (Fig. 2). The estimated filament spacing is about 15-16 pm. This is in agreement with the number of filaments seen in the near-field pat- tern of a 2.3-pm laser operated at 1.7 times threshold (Fig. 3). 7-8 filaments are observed in the near field pattern of the device with 100-pm stripe.

In conclusion, the values of the a-factor for 2-2.5 um InGa(Al)hSb OW lasers at threshold

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ior. The QC layer structure is the same as outlined in ref [ 11. The device was realized as schematically shown in the inset of Fig. 1, i.e. in a master oscil- lator-power amplifier (MOPA) configuration. The oscillator is a 16 pm wide, 2 mm long ridge waveguide where the single mode laser action is achieved by wet-etching a first-order Bragg grat- ing in the topmost layer of the waveguide.2 The power amplifier is a linearly tapered structure with no grating on top, 0.5 mm long and with a final width of 100 pm. Selective current injection in the different sections is achieved by separate contact areas. In order to keep a high (>30 dB) side mode suppression ratio and to avoid the use of low-reflectivity (R < 0.1%) coatings on the amplifier facet, the MOPA structure is tilted at a 7” angle to the crystal lattice, so that the light re- flected by the cleaved facets does not couple back into the waveg~ide.~

Figure 1 shows the optical power-current characteristics of sample D2744. The maximum peak power emitted from the front facet is 0.39 W. The curves were corrected for current leakage by measuring the resulting voltage on the laser sec- tion when current flows into the amplifier. From the knowledge of the current-voltage characteris- tics of the laser it was possible to estimate the amount of current that leaks from the amplifier

Current (A)

CTuE.5 Fig. 2. Light-current characteristics of D2743 at 300 K for different values of Ia. Dashed line: back facet emission at Ia = 0. Inset: Emission spectra measured at (from left to right): 10, 50, 80,120,160,200,250, and 300 K.

to the laser during the light-current characteriza- tion.

Figure 2 shows the light-current characteris- tics obtained from sample D2743 at T = 300 K. The amplifier section in this case is smaller, to avoid overheating of the device at high currents. Its length is 0.28 mm and the maximum width is 52 pm. The measurement of the back facet emis- sion in this case allowed us to estimate the maxi- mum amplification and the waveguide losses which turned out to be 4.9 dB and 22.1 cm-’, re- spectively. In the inset of Fig. 2 are plotted the spectra measured from sample D2744 at temper- atures varying from 10 to 300 K at currents corre- sponding to the peak optical power. The attain- ment of single mode operation in a large temperature range, facilitated by the tilted design, yields a wide tunability of the emission. Far-field measurements performed on D2743 showed that 90% of the optical power is concentrated within an angle of 20”, in the plane of the epitaxial layers. This value is significantly lower than the beam di- vergence (-60”) measured with smaller ridges4 owing to the weaker diffraction from the wider amplifier facet. Moreover, the transverse beam shape is not affected by the width of the cavity, as it happens in other high-power single-mode de- vices?

1. C. Gmachl, A. Tredicucci, F. Capasso, A.L. Hutchinson, D.L. Sivco, J.N. Baillargeon, and A.Y. Cho, “High power h = 8 pm quantum cascade lasers with near optimum perfor- mance’: Appl. Phys. Lett. 72, 3130-3132

~~ . , were in the range 3-4 and are in agreement with values obtained from above-threshold near-field measurements.

This work was supported by AFOSR, grant F496200110108.

(1998). 2. C. Gmachl, F. Capasso, J. Faist, A.L. Hutchin-

son, A. Tredicucci, D.L. Sivco, J.N. Bail- largeon, S.N.G. Chu, and A.Y. Cho, “Contin- uous-wave and high-power pulsed operation of index-coupled distributed feedback quan- tum cascade laser at h = 8.5 pm”, Appl. Phys.

0.4

s 0.3 v References

D.Z. Garbuzov, H. Lee, V. Khalfin, R. Mar- tinelli, J.C. Connolly, G.L. Belenky “2.3-2.7 mm room temperature CW operation of In- GaAsSblAlGaAsSb broad waveguide SCH- QW diode lasers” IEEE Photon. Tech. Lett. 11 (1999), p. 794. K. Shim, H. Rabitz, P. Dutta, “Band gap and lattice constant of GaxInl-xAs,Sbl-y’:J. Appl. Phys. 88 (ZOOO), p. 7157. D.V. Donetsky, G.L. Belenky, D.Z. Garbuzov, H. Lee, R.U. Martinelli, G. Taylor, S. Luryi, J.C. Connolly “Direct measurements of het- erobarrier leakage current and modal gain in

Lett. 72,1430-1432 (1998). 5 0 2 1, , , , ,p ifAI 3. C.E. Zah, J.S. Oshinski, C. Caneau, S.G.

a Menocal, L.A. Reith, J. Zalsman, F.K. * 01 Shokoohi, and T.P. Lee,“Fabrication and per- E formance of 1.5 pm GaInAsP travelling-wave

laser amplifier with angled facets”, Electron. Lett. 23,990-991 (1987).

4. C. Gmachl, F. Capasso, A. Tredicucci, D.L. Sivco, R. Kohler, A.L. Hutchinson, and A.Y.

CTuE5 Fig. 1. Light-current characteristics of Cho, “Dependence of the device perfor- sample D2744 at T = 80 K. Duty cycle is 0.05%. mance on the number of stages in quantum- Different values of the amplifier current (Ia) are cascade lasers’: IEEE J. Sel. Topics Quantum shown. Inset: schematics of the MOPA device. Electron. 5, 808-816 (1999).

0 0 1 2 3

Current (A)

TUESDAY MORNING / CLEO 2002 / 159

5. D. Hofstetter, T. Aellen, M. Beck, and J. Faist, “High average power first-order distributed feedback quantum ‘cascade lasers”, IEEE Phot. Technol. Lett. 12, 1610-1612 (2000).

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CTuE6 9:15 am

Low Current Operation of GaAs/AIGaAs Based Quantum Cascade Lasers

Hideaki Page, Alfred0 de Rossi, Phillipe Collot, Valentin Ortiz, and Carlo Sirtori, Hideaki. Page@Thalesgroup.com, Thales Research and Technology, Orsay, France

Although the threshold current density of quan- tum cascade lasers is continuously decreasing, the true current at threshold remains large. This is mainly imposed by the long emission wave- lengths of these devices which require larger ridge sizes. Therefore a considerable reduction of the current could come from a decrease of the device dimensions. The major obstacles to this strategy are the increased scattering losses of the optical mode with the edges of the laser ridge and the higher mirror losses for shorter cavity lengths.

To overcome the problem of the scattering losses we have developed a processing technology based on ion-implantation, which separates the electrical from the optical confinement. A sketch of the facet of the processed laser is shown on Fig. 1. Figure (2a) shows the light- and voltage-cur- rent characteristics of an implanted sample com- pared to a reference device. The width of current injection has been reduced from 30 to 8 pm and the threshold current from 1.8 A to 500 mA, re- spectively. In the device with selective current in- jection, the optical field extends over the whole ridge. Therefore, the gain region has the strongest overlap with the fundamental lateral mode which is preferentially excited, as can be seen in the far field measurements, Fig. (2b).

In order to reduce the mirror losses we have developed metallic facet coatings for high reflec- tivity. These mirrors consist of a thin Ti0 isolat- ing layer followed by 200 nm of a gold coating that can be easily deposited on the facet. These mirrors are applied to the back facet of the laser and have shown a near 100% reflectivity. Thus, very short cavity lasers (500 pm), previously lim- ited to cryogenic temperatures by gain saturation effects, operated up to near room temperature. Moreover, high average optical power, 11 mW, at

Reference device Selective pumping Activer ion

a) w m b, implated rgions implated rgions

CTuE6 Fig. 1. Schematic representations of the lasers processed using ion implantation for the electrical insulation: a) reference sample. The ion-implantation is outside the two deep trenches etched to define the lateral waveguide (30 pm wide). b) Selective injection ridge laser. The implantation has been extended into the zone defined by the two trenches. The current channel is 8 pm wide whereas the optical mode is defined by the wider zone between the trenches.

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b) CTuE6 Fig. 2. a) L-I and V-I characteristics for the reference and the selective current injec- tion devices. Note the reduction in threshold cur- rent when the current channelled by the implan- tation in the center of the ridge. The near identical slope efficiencies indicate that no extra optical losses have been incurred. b) shows the lateral far field emission for the 8 and 30 p n sam- ples. The geometry of the implantation allows se- lective pumping of the optical mode.

-30°C has been measured for Peltier cooled lasers (A -9 pm) using these mirror coatings.

CTuE7 9:30 am ~ ~~ ~~

Relative Frequency Stabilization of Two 8.5-micron Quantum Cascade Lasers to 5.6 Hz

Matthew S. Taubman, Tanya L. Myers, Richard M. Williams and Bret D. Cannon, Pacific Northwest National Lab, Richland WA 99352, Email: Matthew. Tau bman@pnl.gov Federico Capasso, Claire Gmachl, Deborah Sivco, Albert]. Hutchinson and Aljred Y; Cho, Bell Laboratories, Lucent Technology, 600 Mountain Avenue, Murray Hill, New Iersey 07974

The recent advent of quantum cascade lasers (QCLs)”’ emitting in the mid- to long-wave in- frared has greatly increased the availability of tunable lasers in this spectral region. Applications for such laser sources include high-resolution spectro~copy,3’~ optical communications and chemical en sing.^ While these have intrinsically low frequency noise compared to diode lasers due to their near-zero alpha parameter,’’6’7’8 a vast in- crease in their utility is obtained by frequency sta- bilizing them to a reference such as a molecular transition or a cavity resonance.’

An application such as precision sub-Doppler spectroscopy requires reduction of the high fre-

quency spectral noise components of the laser, re- sulting in a narrower effective linewidth. This in turn requires servo control loops with minimal delay and sufficient bandwidth to allow useful ac- cess to this high frequency information, but also very high gains at lower frequencies to remove l/f noise and drifts. We demonstrate the successful frequency stabilization of two QCLs to separate optical cavities using highly optimized servo loops based on the Pound-Drever-Hall tech- nique.”-” We present 5.6 Hz FWHM beat notes between these two systems that are stable for pe- riods up to a second. This represents an effective reduction from the recently recorded free-run- ning linewidth of these same lasers of 150 kHz13 of over 25,000.

Such measurements between two lasers are typically taken using adjacent modes of a single optical ~avi ty .””~ This relaxes some of the strin- gent engineering requirements for two fully inde- pendent optical cavities stable enough to facilitate this meas~rement.’~ We present a technique that not only allows this measurement to be per- formed using two optical cavities, but relieves the engineering constraints for these cavities even further than those in a traditional single-cavity experiment. In fact, the two cavities we employed have measured finesse values of only 206 and 171 respectively, are made of stainless steel vacuum fittings with rubber O-rings, are bolted directly to a standard optical table with only rudimentary vibration isolation and exhibit significant acoustic resonances! Never intended for such pre- cision measurements, these cavities were simple prototypes of sub-optimal construction at best. However, a third servo loop, of gain and band- width inferior to the two main laser servo loops, forces one laser-cavity system to track the other at frequencies in the low audio band. While insuffi- cient to further modify the fast laser linewidth of either system, it sufficiently removes the relative drifts and low frequency noise to allow a highly stable 5.6 Hz relative linewidth beat measure- ment to be taken.

The central feature in Figure 1 shows a typical heterodyne beat note obtained during our exper- iments. It has a Lorentzian width of 5.6 Hz, and a Gaussian width of 2 Hz, the latter corresponding to the analyzer bandwidth. The other peaks at ap- proximately 2 kHz spacings are caused by

CTuE7 Fig. 1. Heterodyne spectrum of two cavity-locked quantum cascade laser systems, showing principal beat and 2 kHz-spaced acoustic noise structures. Central feature fitted with a Voigt profile (not shown) with 5.6 Hz Lorentzian width and 2 Hz Gaussian width, the latter corresponding to the analyzer bandwidth.