ELECTRO-THERMALLY TUNABLE DIELECTRIC MIRROR MEMBRANES FOR OPTICAL FILTERS AND VCSELS F. Riemenschneider (1), H. Halbritter (1), G. Hess (2), J. Jacquet (3), A. Plais (3), J. Sigmund (1), P. Meissner (1) 1: Techn. Univ. of Darmstadt, Institut für. Hochfrequenztechnik, Merckstr. 25, D-64283 Darmstadt, Germany,
[email protected], 2: Techn. Univ. of Darmstadt, Institut f. Halbleitertechnik, Germany,
[email protected], 3: OPTO+, Alcatel Research & Innovation, France,
[email protected] Abstract We present a new concept of dielectric mirror membranes for electro-thermally tunable optical filters and VCSELs. The performance of a filter and first results of a VCSEL based on that concept are presented. Introduction Tunable optical filters [1, 2] and tunable lasers [3] play an important role in today’s WDM-systems. Here, we propose a very simple bulk-micromachined two-chip concept of tunable Fabry-Pérot Filters (FP-Filters) and tunable vertical cavity surface emitting lasers (VCSELs). One chip carrying a mirror membrane is placed upside-down on top of a second chip (Fig. 1). The latter can be a distributed Bragg reflector (DBR) on a plane glass substrate for the filter application or in case of the VCSEL application, a “Half-VCSEL”, consisting of a DBR plus active region. Our objective was to combine the favorable optical properties of dielectric DBRs (lower absorption, larger stop band, and higher reflectivity compared to semiconductor DBRs) with the simple way of direct electro-thermal actuation. The idea is to deposit a conventional SiO2 / Si3N4 – DBR on top of a few doped, optically transparent GaAs / AlGaAs – λ/4-layers that will conduct the tuning current. In addition, the GaAs / AlGaAs – layers will increase the reflectivity. Concept and Design of the Membrane Chip The mirror membrane is movable due to flexible suspension beams. Deflection of the membrane as a consequence of electro-thermal heating is achieved by injecting a small electrical current through the thin suspension beams. Usually, a two-chip concept has the disadvantage that assembly costs are very high. In our case, assembly is simply done by placing the membrane chip directly on top of the second chip. The proper initial deflection of the mirror membrane (due to compressive stress) represents the initial cavity length without the necessity of any spacing bars. Via-hole contacts through the substrate are implemented to reach the buried semiconductor layer. In order to obtain a stable cavity the mirror membrane has a rotation-symmetric concave curvature. via-hole Contacts Mirror GaAsSubstrate Membrane (Ni/AuGe/Ni-layer)
Both, the total resonator length and the curvature of the bulk-micromachined mirror membrane have an influence on the modes propagating inside the cavity. For the fabrication of the mirror membrane we use the following mirror material: 13 periods SiO2 / Si3N4 – λ/4-layers, PECVD-grown on top of 1.5 periods doped GaAs / Al0.75Ga0.25As – λ/4-layers (MBE-grown, ND ~ 18 -3 1⋅10 cm ). A 500 nm thick Al0.75Ga0.25As etch-stop layer is between that DBR and the undoped GaAssubstrate. Such a DBR has a theoretic maximum reflectivity of approx. 99.9 % assuming an absorption -1 -1 coefficient of 5 cm for the dielectric and 10 cm for the semiconductor materials. Depending on the length, width, and number of the suspension beams the radius of curvature and the initial deflection, i.e. the initial cavity length, can be adjusted to the application. In case of four suspension beams the membrane mesa is designed so that the two suspension beams fixed at the same edge are shortcircuited (Fig. 2). Earlier experiments have shown, that the current distribution between the four beams has no influence on the deflection behavior of the membrane. Thus, even if the design comprises four beams, only two contacts are necessary. Tunable Fabry-Pérot Filter To fabricate a tunable FP-Filter we simply placed the membrane chip on a second dielectric DBR deposited on a plane quartz glass substrate with anti-reflection coating on the backside. We stick the membrane chip to the second chip by applying a small amount of wax at its edges. With a white light interferometric measurement of the mirror membrane one determines a cavity length of the resonator of approx. L = 23 µm and a radius of curvature of the membrane 200 µm
DBR-mesa Suspension beam Clearance hole GaAssubstrate
Mirror & substrate of 2nd Chip
Fig. 1: Two-chip concept (cross-sectional view)
Fig. 2: Membrane chip with 4 suspension beams
FSR
-5
rel. output power in dB
Filtertransmission in dB
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Fig. 3: Filter characteristic for different tuning currents
Fig. 5: Tuning characteristic of the VCSEL
of R = 18 mm. In order to excite only the fundamental mode of the resonator the waist w0 of the incoming gaussian light beam must be well adapted to these parameters [4]. The appropriate beam waist radius for single mode excitation (w0 = 17.8 µm) can be determined with the relation:
Tunable VCSELs The proposed mirror membrane concept can be used for the top mirror of a tunable optically or electrically pumped VCSEL. We obtained first results with an electrically pumped “Half-VCSEL”-device, where a membrane with two suspension beams represents the tunable top mirror (Fig. 4). The H+ implantation diameter, which leads to current and optical confinement is 35 µm. The maximum of the photoluminescence peak is at 1510 nm. The tuning range of the laser peak is between 1490 nm and 1515 nm. The output light is partly single mode, partly multimode with a peak power of 3.5 µW in pulsed operation mode. The relatively high threshold current of 30 mA indicates the existence of high losses. However, we expect to increase the output power and to decrease considerably the threshold current by a better adjustment of the cavity length and the membrane curvature to the H+ implantation diameter of the Half-VCSEL.
w02 =
λ L ( R − L) π
The amplified spontaneous emission (ASE) of an erbium-doped fiber amplifier (EDFA) is used as light source. This light is coupled into a fiber lens that produces the desired beam waist. A second fiber lens is used to couple the filter output into a single mode fiber which is connected to an optical spectrum analyzer showing the filter characteristic (Fig. 3). The free spectral range (FSR) of the filter is 52 nm. The full width at half maximum (FWHM) of the filter peak is 0.11 nm. Consequently, the finesse F which is defined as the ratio of FSR and FWHM is higher than 470. The micromachined FP-Filter showed fiber-tofiber insertion loss of around 3 dB. Wavelength tuning of the filter peaks from shorter to longer wavelengths can be observed when increasing the current through the membrane. A tuning range of more than the FSR has been easily achieved with a tuning current of approx. 4 mA. The shape and the amplitude of the transmission peak is almost not affected by tuning. There is a linear relation between transmission wavelength and the square of the tuning current (~ power).
GaAsSubstrate
MM-fiber
Membrane
HalfVCSEL 200 µm Bond wire
Fig. 4: Membrane with two suspension beams on top of a bonded "Half-VCSEL", with multi-mode fiber
Conclusions We have illustrated a new concept of electrothermally tunable dielectric mirror membranes. The advantages of this concept, such as high reflectivity, large stop band, simple actuation, and low-cost assembly have been emphasized. Finally, we have shown its application in tunable Fabry-Pérot filters and tunable VCSELs for WDM-systems. Acknowledgements This work has been supported by the European Union within IST project "TUNVIC" (IST-1999-11051) References 1 P. Tayebati et al., Electronics Letters, Vol. 34 (1998), pp. 1967 – 1968 2 A. Spisser et al., Photonics Technology Letters, Vol. 10 (1998), pp. 1259 – 1261 3 P. Tayebati et al., Photonics Technology Letters, Vol. 10 (1998), pp. 1679 – 1681 4 M. Aziz et al., Phys. Stat. Sol. (a), Vol. 188, No. 3, (2001), pp. 979 – 982
