Chirp and linewidth enhancement factor of tunable, optically-pumped long wavelength VCSEL H. Halbritter, F. Riemenschneider, J. Jacquet, J.-G. Provost, C. Symonds, I. Sagnes and P. Meissner Small-signal frequency modulation response and chirp characteristics of a tunable, optically-pumped 1.6 mm vertical cavity surface emitting laser (VCSEL) based on microelectromechanical at wavelength tuning are presented for the first time. From the measurements the linewidth enhancement factor aH has been derived and is discussed.
Introduction: The two-chip microelectromechanically (MEMS) tunable singlemode VCSEL features a 20 mm mode diameter. Details of the static performance and the concept can be found in [1]. The wavelength tuning is implemented by a micromachined membrane, which allows wavelength tuning due to current induced heating of the membrane. The characteristics of the VCSEL have been investigated at 1597 nm (Pmax ¼ 0.5 mW) and 1603 nm (0.33 mW).
VCSEL (pump-) powers. For low output powers the response is Vshaped, whereas for higher (pump-) powers the response is flat. At higher frequencies the CPR increases linearly with frequency f, with a decrease of the slope at increased output powers Po (CPR( f ) aHf=2Po), consistent with theory [3] (see inset in Fig. 2). Based on these measurements the linewidth enhancement factor aH has been derived (see Fig. 4), using the method proposed in [5]. Discussion: The VCSEL consists of a periodic gain structure (PGS) with three packets of multiple-quantum wells (MQWs), embedded in absorbing barrier layers. The device is pumped at 980 nm through the top mirror and the air-gap into the solid cavity. The optical pumping from the top leads to a depth-dependent carrier density, e.g. the top barriers and top MQWs capture and generate significantly more carriers than the bottom layers. Subsequently this results in a highly non-uniform carrier density distribution inside the cavity. Based on these carrier density differences a phenomenological approach is presented, which explains both the differences in the CPR shape and the high aH value. 180
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Fig. 2 Chirp-to-power ratio (CPR), magnitude, for l1 ¼ 1597 nm Inset: CPR, linear scale
Experimental results: To measure the frequency modulation (FM) and chirp characteristics, the discriminator method proposed by [2] was employed. The measured FM and chirp-to-power ratio (CPR) characteristics are presented in Figs. 1–3. At low frequencies (up to a few MHz) the response (see Figs. 1 and 2) is dominated by temperature modulation [3]. At higher frequencies the thermal influence becomes less efficient and the behaviour is dominated by carrier density modulation effects [4]. In this frequency range, below the resonance frequency, the FM and CPR response differs for different
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In edge emitters the V-shaped CPR behaviour is typical for clamped carrier density conditions, whereas a flat CPR response can be associated with non-uniform carrier densities inside the mode volume (e.g. comprising barriers and MQWs) [3, 4]. As the mode volume for the VCSEL includes MQWs, barriers and the air-gap, a flat CPR at frequencies below the resonance and above the thermal cutoff frequency is expected. This is confirmed experimentally for high (pump-) powers. Hence at low (pump-) powers the CPR behaviour is significantly different. In this condition the bottom barriers and MQWs are assumed to have a low carrier density, owing to the non-uniform carrier generation within the cavity. As a result the carrier density modulation (in-phase contribution) has a negligible effect on the effective refractive index change of the mode volume compared to the thermal modulation (out-of-phase contribution), even above the
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thermal cutoff frequency, leading to a decreasing CPR value (see Figs. 2 and 3). With rising frequency the term CPR f=Po becomes increasingly dominant and results in a linear increase in the CPR value with rising frequency (see inset in Fig. 2). This leads to the observed V-shaped dip in the CPR characteristics for low pump powers. At high pump powers, the bottom layer carrier densities are higher and the MQWs are clamped, resulting in a more efficient carrier modulation (in-phase contribution), dominating the thermal modulation at frequencies above the thermal cutoff, and leading to the observed flat CPR characteristics. The non-uniform carrier distribution within the cavity leads further to an increased threshold density and, as a result, to a lower differential gain dg=dN [6], compared to a condition where all MQW packets have uniform carrier density distribution at low pump power. In addition, at low pump powers, e.g. low carrier densities, the differential refractive index dn=dN is increased [6]. Using [6] aH ¼ (4p=l)(dn=dN)=(dg=dN) we deduce an increased value of aH at low carrier densities (low pump powers). As a result aH is a function of the pump power and decreases with rising pump power. With a sufficiently high pump power, all three MQW packets are clamped and in similar conditions and aH reaches its minimum value of 4.5. The slightly increased aH at the longer wavelength is caused by wavelength detuning [6]; as for this VCSEL the gain maximum is at shorter wavelengths. The non-uniform carrier density distribution results, as a consequence, in a nonlinear dependence of the VCSEL output power against pump power (see also Fig. 4).
Acknowledgment: This work has been supported by the European Community within the IST project ‘TUNVIC’ (IST-1999-11051). # IEE 2004 Electronics Letters online no: 20040173 doi: 10.1049/el:20040173
H. Halbritter, F. Riemenschneider and P. Meissner (Technische Universita¨ t Darmstadt, Institut fu¨ r Hochfrequenztechnik, Merckstr. 25, Darmstadt 64283, Germany) E-mail:
[email protected] J. Jacquet and J.-G. Provost (Alcatel Research and Innovation, Route de Nozay, Marcoussis 91460, France) C. Symonds and I. Sagnes (Laboratoire de Photonique et de Nanostructures, LPN-CNRS, Route de Nozay, Marcoussis 91460, France)
References 1
2 3
Conclusions: We have presented for the first time the FM and chirp characteristics of a micromechanically tunable, optically-pumped VCSEL. The chirp characteristics have shown significant differences, depending on the pump power. aH was experimentally proven to be dependent on the pump power and reaches its minimum value (aH ¼ 4.5) at higher pump powers. We have further presented a phenomenological explanation, based on a non-uniform carrier density distribution within the periodic gain structure due to optical pumping.
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Riemenschneider, F., Sagnes, I., Bo¨hm, G., Halbritter, H., Maute, M., Symonds, C., Amann, M.-C., and Meissner, P.: ‘A new concept for tunable long wavelength VCSEL’, Opt. Commun., 2003, 222, pp. 341–350 Derickson, D.: ‘Fiber optic test and measurement’ (Prentice Hall, New Jersey, USA, 1998) Tucker, R.S.: ‘High speed modulation of semiconductor lasers’, J. Lightwave Technol., 1985, 3, pp. 1180–1192 Kikuchi, K., Fukushima, T., and Okoshi, T.: ‘Stripe-structure dependence of frequency modulated characteristics of AlGaAs lasers’, Electron. Lett., 1985, 21, pp. 1088–1090 Harder, C., Vahala, K., and Yariv, A.: ‘Measurement of the linewidth enhancement factor a of semiconductor lasers’, Appl. Phys. Lett., 1983, 42, pp. 328–330 Wilmsen, C., Temkin, H., and Coldren, L.A.: ‘Vertical-cavity surfaceemitting lasers’ (Cambridge University Press, Cambridge, UK, 1999)
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