298 / CLEO/EUROPE‘94 / THURSDAY MORNING

0

CThF4 Fig. 2. Interferometric pattem of an Si wafer with a flexible and correspondent surface reconstruction.

CThF4 Fig. 3. Interferometric pattern of a flexible mirror electrostatically deflected by 1.2 pm and correspondent surface reconstruction.

membrane mirror are shown in Figs. 2 and 3. Figure 2 shows the initial shape of the reflecting surface, while Fig. 3 corre- sponds to a mirror, which is deflected electrostatically by 1.2 pm.

The surface profiles have been recon- structed from the fringe patterns using the fast Fourier transform alg~rithm.~ For the initial pattern shown in Fig. 2, the peak to peak and mean square deviations have been established as 6 = 0.8A and U = 0.13X correspondently. Extraction of the initial wafer astigmatism reduces these values to 6 = 0.4A and U = A/16.

To check the yield strength, the flexi- ble mirror has been repeatedly deformed by an external pressure to a deflection amplitude of 50 pm. After removal of the load, membrane elastically went back to its initial shape with no traces of residual deformation.

Optically plane aluminum coated flex- ible mirror have been integrated into Si chip. The fabrication procedure is com- patible with silicon IC processing. Initial optical quality of mirror satisfies the stan- dard demands to reflecting optics. Adap- tive mirror with 10 mm by 10 mm reflect- ing surface, controlled by array of electrostatic actuators is available in a 30 mm by 30 mm IC house.

Author is grateful to Prof. Dr. S. Mid- dlehoek, head of the Laboratory of Elec- tronics Instrumentation, Delft University of Technology, for stimulating discus- sions and support and to Dr. Lina Sarro from DIMES for IC processing. 1. Kurt E. Petersen, “Silicon as a me-

chanical material,” Proc. IEEE 70 420 (1982). T. Kwa, R. Wolfenbuttel, ”Integrated grating/detector array fabricated in silicon using micromachining tech- niques,‘‘ Sensors and Actuators A31, 259 (1992). M. Hisanaga, T. Kourmura et al., ”Fabrication of three-dimensionally shaped Si diaphragm dynamic fo- cusing mirror,” Proc. IEEE MEMS workshop, 30 (1993). L. J. Hombeck, ”128 X 128 deforma-

2.

3.

4.

ble mirror device,” IEEE Trans. ED- 30, 539 (1983). R. P. Grosso, M. Yellin, ”The membrane mirror as an adaptive optical element,” JOSA 67, 399 (1977). S. A. Chetkin, G. V. Vdovin, “De- formable mirror correction of a ther- mal lens induced in the active rod of a solid state laser,” Optics Com- munications 100, 159-165 (1993). M. Takeda, H. Ina, S. Kobayashi, “Fourier-transform method of fringe-pattern analysis for computer-based topography and in- terferometry,” JOSA 72, 156 (1982).

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CThF5 1215

Integrated Mach-Zehnder InCaAsP BRAQWET modulators

B. Dwir, R. Monnard, A. Sadeghi, J.-E Carlin, M.-A. Dupertuis, M. Glick, A. Rudra, E K. Reinhart, Institute of Micro and Optoelectronics, Swiss Federal Inslitute of Technology (EPFL), CH-1015 Lnusanne, Switzerland The BRAQWET (Barrier Reservoir and Quantum Well Electron Transfer) modu- lator is an interesting alternative to quan- tum well modulators based on the Quan- tum Confined Stark Effect and the Wannier Stark Effect. The BRAQWET modulator is based on electrorefraction in a tunable electron-density quantum well. It has excellent high speed potential since it involves only one type of carriers (elec- trons) and the speed of the refractive in- dex change is not limited by carrier life- time. Realization of this structure in InGaAsP materials has potential for even higher speeds due to higher electron mo- bility. However, these materials have a lower bandgap offset, which increases enormously the leakage current of a BRAQWET device (tens of A/cm2 at V = 0.5 V).

We have developed numerical and an- alytical models to accurately describe the electronic band structure of the BRA- QWET. Using these models, we have optimized the layer composition and doping of the BRAQWET for higher ro- bustness against doping level variations in the growth. The resulting structure can tolerate 220% changes in the doping lev- els. The optimized structure shows larger quantum well movement for a given ap- plied voltage, reaching full dipping with less than 0.5 V bias. It also has low leak- age current (theoretically down to A/cm2 at V = 0.5 V).

Structures were fabricated by Chemi- cal Beam Epitaxy based on the new de- sign. The structures include a single BRAQWET in the waveguide to mini- mize the operating voltage and leakage current. The quantum well parameters have been chosen for operation close to 1.55 Km. The current-voltage curves of relatively large samples (area 0.1 cm’) show a leakage current down to 10 mA/ cm2 at an applied voltage of 0.5 V. The results of differential absorption spectros- copy clearly show the BRAQWET char- acteristics of band filling effects for posi- tive voltages and Quantum Confined Stark Effect for negative voltages.

We fabricated single rib waveguides

BRAQWET phase modulation

CThF5 Fig. 1. Modulation of a 6-mm- long BRAQWET sample as measured by external Mach-Zehnder interferometer at A = 1.56 pm. Light intensity is shown as function of applied voltage. Inset: change in index of refraction, as calculated from the interference curve.

and integrated Mach-Zehnder structures on the BRAQWET wafer. We used photo- lithography and reactive ion etching to define the 3 pm X 0.4 pm (w X h) rib. The guides and Mach-Zehnder structures were isolated by reactive-ion-etched trenches and planarized with poliymide. Electrical contacts were then deposited on top of each guide. The Mach-Zehnder structure used Y-couplers of 2” angle to minimize losses.

Measured leakage current of the rib waveguides is about 7 mA/cm2 at V = 0.5 V. The absorption is 20 cm-’ at A = 1.56 pm. The phase modulation was measured by an external Mach-Zehnder interferometer and the results are shown in the figure below. The change in the re- fractive index, calculated from the inter- ference curve, is shown in the inset. Pre- liminary measurements of the integrated Mach-Zehnder structure show modula- tion of up to 35% using single arm drive of 0 to 2 V and up to 45% using sym- metrical drive of 2’2 V.

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CThC 1100 Rooms C & D

Single Mode lasers and Spectral Properties J. Buus, Gayton Photonic, Ltd., Presider

CThGl 1100

Single-mode monolithic ring laser with vertical grating coupler

R. van Roijen, J. J. L. Horikx, J. M. M. van der Heijden, T. van Dongen, B. H. Verbeek, M. J. N. van Stralen,* Philips Optoelectronics Centre, Prof. Holstlaan 4, 5656 AA Eindkoven, The Netherlands Monolithic ring lasers in InP are well suited for application in integrated com-

THURSDAY MORNING / CLEO/EUROPE'94 I 299

[ Ring Waveguide //output

ilaveguide I

Reflection 1 U

! - \--,-,,

Vertical grating

CThCl Fig. 1. Schematic drawing of the ring laser. Light coming out of one of the ring branches is diffracted by the grating. The first-order reflection is reflected to the output waveguide, the second-order coupled in to the ring.

ponents in the telecommunication optical window. A number of methods for cou- pling light out of a ring have been re- ported,'"? but often these methods are very demanding on technology. Wave- length selection and stability in monolithic ring lasers are also important issues.

We have used a vertical grating, partly similar to the grating applied in a number of wavelength (de)multiplexers: but now to act as an outcoupler and a wavelength selecting element simultane- ously. By coupling the second-order re- flection back into the ring, and coupling the first order to an output waveguide, a stable single-mode laser with a side mode suppression ratio better than -20 dB has been obtained.

The fabrication technology is as fol- lows: an X = 1.5 ym InGaAsP active layer, an InP layer, and a contacting layer are grown on InP by MOVPE. Deep ridge waveguides, etched through the active layer, are defined by NE. A Si,N, dielec- tric layer is deposited for electrical and optical isolation and etched open on top of the ridge. Finally, metal is applied to both sides for electrical contacting. The ridge waveguides are 2.5 ym wide and support only the fundamental mode. The diameter of the ring is 300 pm. The end facet of the outcoupling waveguide is AR coated with a HfO, layer.

A schematic diagram is shown in Fig. 1. Light enters from one of the ring wave- guides and is diffracted by the vertical grating. The second-order reflection is di- rected into the other ring waveguide, the first-order reflection goes towards the outcoupling waveguide.

Figure 2 is a light vs current charac- teristic of the device under continuous operation. The threshold current is rela- tively high because all of the waveguide material, including the diffraction grat- ing, is active and has to be electrically pumped. The threshold current density of 4.6 kA/cm2, however, is comparable to Fabry-Perot lasers made from the same material, indicating there is no excessive

bends. Figure 3 shows a typical spec- trum. The mode separation of 0.68 nm corresponds to the ring round-trip. The side mode suppression is in excess of 20 dB due to two factors: first, the grating is wavelength selective; however, a few

loss through light radiating from the

CThCl Fig. 2. Power vs current characteristic of the monolithic ring laser under continuous operation at room temperature. Due to the modal stability the curve is quite straight. The insets show the stability of the wavelength at different current.

I

0 1 CW,T=20°C

I I

CThCl Fig. 3. Intensity as a function of wavelength at 383 mA injection current under continuous operation at room temperature. The peak wavelength is 1573 nm, the side mode suppression 21 dB.

other longitudinal ring modes will have a round-trip gain, which is slightly different (of the order 0.05 dB). Second, a small re- flection from the outcoupling waveguide, leading to coupled-cavity behavior, also acts as a selection mechanism. In this way the ring has a single stable longitudinal laser mode and no kinks in the L-I char- acteristic, in contrast to multimode inter- ference based ring lasers?

The results are in agreement with de- sign calculations for the grating coupler.

The ring laser with grating coupler is a device with a straightforward technol- ogy, offering the possibility to integrate a stable single-mode laser without facets on an InP optoelectronic chip. *Delft University of Technology, PO. Box 5031, 2600 GA Delft, The Netherlands 1. D. R. Scrifes, R. D. Burnham, W.

Streifer, Appl. Phys. Lett. 28, 681 (1 976).

2. J. P. Hohimer, G. R. Hadley, G. A. Vawter, Appl. Phys. Lett. 63, 278 (1993). M. J. N. van Stralen, R. van Roijen, E. C. M. Pennings, J. M. M. van der Heijden, T. van Dongen, B. H. Verbeek, Proc. European Conf. on Integrated Optics, 1993, Neuchatel, Switzerland, 2-24, 25.

Proc. U) 139, 383 (1992). J. B. D. Soole, K. R. Poguntke, A. Scherer, H. P. LeBlanc, C. Chang- Hasnain, J. R. Hayes, C. Caneau, R. Bhat, M. A. Koza, Appl. Phys. Lett. 61, 2750 (1992).

3.

4. T. Krauss, l? J. R. Layboum, IEEE

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CThG2 1115

Surface mode-coupling in CaAs/AIGaAs laser diodes-a new technique for a single mode laser

A. Kock, C. Gmachl, E. Gornik, L. Korte,* M. Rosenberger,"" P. Lustoza de Souza,*+* lnstitut fur Festkorperelektronik, Gusshausstrasse 25-29, A-1040 Wien, Austria Single longitudinal mode laser diodes are of high interest for optical fiber transmis- sion systems. Among the many types of single mode laser diode structures the distributed feedback (DFB) laser diode presently seems to be most promising.

Recently we have reported on novel surface emitting laser diodes.'.' The cru- cial feature of this type of laser diode is the utilization of a surface mode emission (SME) technique, which is based on the coupling of the laser light propagating in the active region to a surface mode on top of the laser diode. By this we have achieved surface emission from conven- tional, horizontal cavity GaAs/AlGaAs laser diodes with a beam divergence of 0.2". Now we have found that the SME technique has a high potential for the re- alization of a single mode laser diode.

The strong influence of the SME proc- ess to the emission spectrum of a laser diode is shown in Fig. 1 and Fig. 2. Figure 1 shows the emission spectrum of a laser diode without coupling the laser mode to a surface mode. Figure 2 shows the emis- sion spectrum of the same laser diode, when modified as shown in the inset for utilizing the SME technique. The laser stripe shows a 30 ym X 150 ym large window, where a surface grating (period 1300 nm) is etched into the p-AlGaAs cladding layer. The grating is necessary for a coupling between the laser mode and the surface mode.

The significant "quasi" single mode emission is due to a strong feedback of the surface mode to the laser mode, as is schematically shown in the inset of Fig. 2. The TE,-surface mode is excited by the laser mode (process (1)) and propagates in the polyimid film waveguide in a zig- zag manner along the grating. The sur- face mode decays radiatively and emits light into the air space (process (3)). The crucial point is the re-radiation of the sur- face mode into the laser (process (4)). The

1

0.8 - m Y 0.6 * .- 2 Y

'* 0.4 Ti c .g c

0.2

874 876 878 880 882 wavelength [nm]

CThGZ Fig. 1. Emission spectrum of a laser diode without surface mode coupling.