Mar 2, 2004 - (CW) laser emission at 1.55 μm of an all InP-based electrically- pumped ... of 2.7 mW has been obtained in quasi-CW operation at a heatsink.
Room-temperature continuous-wave laser operation of electrically-pumped 1.55 lm VECSEL M. El Kurdi, S. Bouchoule, A. Bousseksou, I. Sagnes, A. Plais, M. Strassner, C. Symonds, A. Garnache and J. Jacquet A report is presented on room-temperature (RT) continuous-wave (CW) laser emission at 1.55 mm of an all InP-based electricallypumped vertical external-cavity surface-emitting laser (EP-VECSEL). Threshold currents of 1.4 kA=cm2 and output powers of up to 0.3 mW were measured under CW operation at RT. A maximum output power of 2.7 mW has been obtained in quasi-CW operation at a heatsink temperature of 10.5 C. This first result demonstrates that EPVECSELs are a potential candidate for the realisation of compact vertical-cavity emitting sources.
Introduction: Vertical-cavity semiconductor lasers emitting in the 1.55 mm wavelength range are very attractive sources for fibre-optical communication systems and for some gas sensing applications. However, stable transverse monomode emission is usually obtained for very small active device diameters, which generally limits the optical output power. Vertical external-cavity surface-emitting lasers (VECSELs) are an alternative approach to form surface-emitting devices that was developed in the past years and that overcomes this limitation. The VECSEL consists of a half vertical-cavity surfaceemitting laser (VCSEL) structure comprising the distributed Bragg mirror (DBR) and the active region and of a highly reflective concave mirror, forming a stable plane-concave Fabry-Perot (FP) cavity. This design, using optical pumping, has proven to be very efficient for various applications such as high-power laser emission and modelocked pulse generation [1], intracavity second-harmonic generation [2] or gas sensing application [3]. Among the advantages of such a design are high output power, spatially monomode circular beam, multimode or monomode [4] longitudinal emission depending on the finesse of the FP cavity. However, while optically-pumped devices (OP-VECSEL) have been developed with various III-V compounds and designs and are used in several optical experiments [1–5], compact InP-based EP-VECSELs avoiding the need of a pump laser source have not been reported yet. In this Letter, we present, for the first time to our knowledge, an electrically-pumped VECSEL emitting at 1.55 mm. Half-VCSEL fabrication and experimental setup: The half-VCSEL was grown by molecular beam epitaxy (MBE) on n-doped InP substrate. The n-doped bottom DBR consists of 50 InP=InGaAsP quarter-wavelength pairs. It was designed to provide a maximum reflectivity at a wavelength of l ¼ 1550 nm. A nip junction comprising the nine compressively strained InGaAsP quantum well active layer was grown on top of the DBR, followed by an InGaAsP-based p=pþþ=nþþ=n tunnel junction and an nþ-doped top contact layer. The overall optical thickness of the semiconductor part of the cavity corresponds to 4l. Hþ ion implantation was performed through the structure down to the nþþ=pþþ region and the p-InP spacing layer to provide lateral current confinement, defining 20, 30, and 50 mm diameter devices. Subsequent rapid thermal annealing was performed to improve the electrical characteristics of the implanted devices. Electrodes were formed on the nþ-doped top contact layer and on the backside of the n-InP substrate. The wafer was mounted on a Peltier element to control the substrate temperature from RT down to 10 C. A highly reflective (>99.5% in the 1530–1570 nm wavelength range) spherical (radius of curvature of 10 mm) dielectric mirror was aligned to form a stable plane-concave FP external cavity. The waist of the intracavity beam was adjusted to be lower than the gain region diameter, by precisely adjusting the cavity length slightly below the maximum value of 10 mm. Half-VCSEL characterisation: The average specific resistance has been deduced from I-V measurements on half-VCSEL devices with different current confinement diameters to be 1.2 104 O cm2. The contribution to the overall resistance of the electrical path from the lateral top contact pad to the active region via the nþ-doped contact layer was measured to be around 17 O. The total serial resistance at a
current density of 1 kO=cm2 was 25 and 60 O for the 50 and 20 mm diameter devices, respectively. Fig. 1 shows the reflectivity spectrum of the half-VCSEL measured on a 50 mm diameter device. Furthermore, the Figure displays the calculated reflectivity spectra with and without accounting for the QW absorption. A constant power absorption of 1.4% per quantum well in the 1.5–1.6 mm wavelength range has been used for the calculations. The reflectivity spectrum of the half-VCSEL presents a resonant dip at 1536 nm corresponding to the half-VCSEL cavity mode enhanced by the quantum well absorption. The electroluminescence (EL) spectrum of the half-VCSEL measured at RT and at a current density of 1 kA=cm2 is also shown in the Figure. The EL peak wavelength is 1540 nm. The difference of þ4 nm between the EL peak wavelength and the cavity mode indicates a thermal self-heating effect under CW current injection. A similar effect cannot be observed when the VECSEL is driven in the pulsed regime (pulse of 0.8 ms, duty cycle 10%) where the emission wavelength perfectly matches the resonant dip of the reflectivity spectrum. The RT-CW laser line emission of the same device in a VECSEL configuration is also reported in the Figure.
Fig. 1 Measured reflectivity spectrum of half-VCSEL compared to simulated spectra otained with and without taking absorption of gain medium into account CW electroluminescence spectrum of half-VCSEL and CW laser line of EP-VECSEL measured at RT are shown for comparison
Fig. 2 VECSEL output power against injected current measured at RT and at 10.5 C in CW Inset: Laser emission spectrum (log-scale) measured at a current density of 1.8 kA=cm2(14 mA)
Lasing and thermal performances: Fig. 2 shows the CW L-I curves of a 20 mm diameter VECSEL at RT and at a heatsink temperature of 10.5 C. A threshold current of 1.4 kA=cm2 (nine QWs) at RT and 1.1 kA=cm2 at a heatsink temperature of 10.5 C can be deduced from measurements. This value is lower than that recently obtained from monolithic VCSELs with highly reflective InP=air-gap top mirror based on the same half-VCSEL structure [6], and is comparable to the state-of-the-art performance of InP-based electrically-pumped
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VCSEL structures [7]. The values for the threshold current lead to a characteristic temperature T0 of 45 K in the 10–25 C temperature range. The maximum emitted power is measured to be 0.5 mW at 10.5 C, and 0.3 mW at RT, limited by a thermal roll-over effect. Significantly higher output power was obtained in quasi-CW operation (0.6 ms current pulse width, duty cycle 30%) with a maximum of 2.7 mW from a 50 mm diameter device at a heatsink temperature of 10.5 C. The T0 value indicates that heating effects still limit the performance of our devices and hence future effort will be invested to improve thermal dissipation and to decrease the self-heating. The inset of Fig. 2 shows a longitudinal multimode emission close to 1540 nm with a longitudinal mode spacing of 15 GHz (0.12 nm) which is in good agreement with the 10 mm cavity length.
M. El Kurdi, S. Bouchoule, A. Bousseksou, I. Sagnes, M. Strassner and C. Symonds (Laboratoire de Photonique et de Nanostructures (LPN), CNRS UPR 020, route de Nozay, Marcoussis 91460, France) A. Plais and J. Jacquet (Alcatel Research and Innovation, route de Nozay, Marcoussis 91460, France) A. Garnache (Centre d’Electronique et de Micro-optoe´ lectronique de Montpellier (CEM2), CNRS UMR 5507, Universite´ Montpellier II, Montpellier cedex 05 34095, France) C. Symonds: Presently with the Institut d’Electronique Fondamentale (IEF), CNRS UMR 8622, Universite´ Paris-Sud, 91405 Orsay, France
References Conclusion: We have demonstrated CW-RT operation at 1.55 mm of an EP-VECSEL with a stable plane-concave FP cavity with near-RT threshold current as low as 1.1 kA=cm2. A maximum output power of 2.7 mW has been obtained in quasi-CW operation. The EP-VECSEL thus appears to be a promising candidate for the realisation of compact 1.55 mm vertical-cavity emitting sources avoiding an additional pump source. Potential applications are, e.g., modelocked pulse sources at multi-GHz rates (10–20 GHz), high power sources at 1.55 mm, or sources for gas spectroscopy and gas detection.
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Acknowledgments: This work is supported by the ‘Action Concerte´e Nanosciences’ 2003 from the CNRS and the French ‘Fonds National pour la Science’. The authors thank K. Merghem for his helpful technical support. # IEE 2004 Electronics Letters online no: 20040445 doi: 10.1049/el:20040445
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Hoogland, S., et al.: ‘Passively mode-locked diode-pumped surfaceemitting semiconductor laser’, IEEE Photonics Technol. Lett., 2000, 12, p. 1135 Raymond, T.D., et al.: ‘Intracavity frequency doubling of a diode-pumped external-cavity surface emitting semiconductor laser’, Opt. Lett., 1999, 24, pp. 1127–1129 Garnache, A., et al.: ‘Application of a diode-pumped broadband verticalexternal-cavity surface-emitting semiconductor laser to high sensitivity intra-cavity absorption spectroscopy’, J. Opt. Soc. Am. B, 2000, 17, pp. 1589–1598 Garnache, A., et al.: ‘1.5 mm high-power circular TEM00 surfaceemitting laser operating in CW at 300 K’. Proc. IPRM, Stockholm, Sweden, 2002 Cerutti, L., et al.: ‘Low threshold pumped Sb-based VECSEL emitting around 2.1 mm’, Electron. Lett., 2002, 39, pp. 290–292 Bouchoule, S., et al.: ‘In-P based wavelength tunable monolithic VCSEL structure with InP=air-gap top mirror compatible with a single epitaxy step’. SPIE Photonics Europe Conf., Strasbourg, France, April 2004 Mereuta, A., et al.: ‘InAlGaAs/InP MQW structures grown by MOVPE in nitrogen atmosphere for 1.55 mm VCSELs’. 10th European Workshop on MOVPE, Lecce, Italy, June 2003
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