Open Access
Add to BibSonomyAbstract
We have fabricated 1240nm GaInNAs high-power semiconductor laser diodes. In pulsed operation 1000 μm × 100 μm laser diodes show record low threshold current densities of 174 Acm-2. Continuous wave output powers exceeding 4.6 Watts at room temperature and 6.2 Watts at a heatsink temperature of -5 °C are obtained from 1300 μm × 200 μm devices. The maximum wallplug efficiency of the device exceeds 40 % and the internal quantum efficiency reaches 0.89. Preliminary lifetime tests were performed for about 1000 h and show stable high-power operation.
©2007 Optical Society of America
1. Introduction
Lasers emitting in the 1.31 μm telecom window are highly interesting for short range fiber optical connections like in metropolitan area networks. Due to absorption losses in the fiber the use of amplifiers considerably extends the range and the performance of transmission systems at 1.31 μm and 1.55 μm. Whereas erbium doped fiber amplifiers are well established for the 1.55 μm wavelength range, amplification at 1.31 μm is usually based on raman amplifiers. The maximum gain of Raman amplifiers occurs near 13.2 THz in common fibers [1], corresponding to a wavelength shift of around 70 nm at a resulting wavelength of 1.31 μm. Direct pumping of Raman amplifiers with semiconductor laser diodes emitting at 1240 nm should reduce costs and noise compared to other systems.
For high-power applications the wavelength of 1.24 μm is out of reach for the conventional InGaAs/GaAs quantum well (QW) system. Since the first proposal of GaInNAs QWs for GaAs-based long wavelength emission by Kondow and coworkers [2] and the first realization of 1.5 μm laser diodes in our group [3], many work has been spent on the improvement of growth conditions and crystal quality of GaInNAs(Sb) QWs [4–6]. Most researchers focused on the telecom wavelength windows around 1.31 μm and 1.55 μm interesting for directly modulated devices, but only few effort has been spent on high power applications [7] and there are no reports in the wavelength regime of 1.24 μm interesting e.g. for direct pumping of raman amplifiers. In this report we show that GaInNAs QWs are an outstanding candidate for the active layer of 1240 nm emitting high-power GaAs-based laser diodes showing some important advantages over the competing InP material system like cheaper and larger substrates, a better temperature stability of the laser diodes and a better high-power performance [4]. By optimizing the high temperature growth of the upper cladding layer, ex-situ annealing steps common for GaInNAs devices were made unnecessary. Fabricated laser diodes show very low threshold current densities, high output powers and a very reliable performance over time.
2. Device fabrication and processing
Laser diodes were grown on Si-doped (001)-oriented GaAs substrates using solid source molecular beam epitaxy. Active nitrogen was supplied by an Applied EPI Uni-Bulb radio frequency plasma source. The active region of the laser diode consist of a 6.5 nm thick Ga0.68In0.32N0.007As0.993 QW. The QW was embedded in 5 nm thick GaAs0.99N0.01 strain-compensating layers (SCLs), which have been shown to improve the optical quality [5]. This active layer was centered in the 400nm wide undoped GaAs waveguide of a separate confinement heterostructure (SCH) with 1.5 μm thick Al0.4Ga0.6As cladding layers doped to ≈ 1x1018 cm -3 with Si and C, respectively. Growth was completed with a heavily C-doped 150 nm thick GaAs contact layer. For basic characterization broad area (BA) lasers were processed with a stripe width of 100 μm and cleaved into pieces with variable cavity lengths. By using standard photolithography and an electron cyclotron resonance (ECR) reactive ion etching (RIE) process high-power BA stripes with a width of 200 μm were also processed. High-power BAs were thinned to 150 μm and mounted epi-side down on a copper heat sink for better heat dissipation and subsequently wire bonded. No facet coatings were applied to any of the examined structures.
Fig. 1. Light output versus drive current characteristics of a 1000 μm × 100 μm BA device in pulsed operation at RT. The inset shows the corresponding spectrum recorded above threshold in pulsed operation.
It is well known, that ex-situ or in-situ annealing can improve the optical properties of GaIn-NAs QWs [8]. AlGaAs cladding layers of GaAs-based laser devices are mostly grown at substrate temperatures in the range of 580 °C to 620 °C or 660 °C to 700 °C. The QW experiences an in-situ annealing effect for about one hour during the high temperature growth of the upper cladding layer. Therefore, the upper cladding growth temperature influences the optical quality of the active region of the laser by annealing. We have investigated the influence of the growth temperature of the upper cladding layer in the range from 580 °C to 720 °C. Lowest threshold current densities are obtained for a growth temperature of 690 °C. Applying additional ex-situ annealing steps to this structure does not result in further reduction of the threshold current density.
3. Device performance and high-power operation
In the following the performance of the processed BA devices based on the grown GaInNAs material is discussed. Figure 1 shows the light output characteristics of a BA laser with dimensions 1000 μm × 100 μm in pulsed operation (pulse width: 300 ns; period: 1 ms) at room temperature (RT), emitting at 1236 nm as shown in the inset of Fig. 1. A very low threshold current density of only 174 Acm-2 and a slope efficiency of 0.29 W/A per facet is obtained. The threshold current density is significantly lower than observed for GaInNAs QWs in the 1.3 μm wavelength regime by other groups (for comparison: values around 300 Acm -2 are reported in [9–11]). Furthermore the threshold current density of the 1.24 μm lasers investigated here is comparable to the values obtained for highly strained InGaAs QWs emitting at an even shorter wavelength of 1.20 μm (e.g. 184 Acm-2 in [12]), which are not suitable for high-power continuous wave (cw) application as the high strain in the active region potentially leads to device degradation. The present threshold is even slightly lower than values reported for InAs/InGaAs QD high-power lasers, where a value of 190 Acm-2 is reported for a device with dimension 1600 μm × 100 μm in [13].
Fig. 2. Light output and voltage versus drive current characteristics curve of an epi-side down mounted 1300μm × 200μm BA device under cw operation at RT. The inset shows the spectrum at I = 1.6∙Ith.
Fig. 3. Light output versus drive current characteristics curve of the 1300 μm × 200 μm BA device under cw operation at -5 °C
Figure 2 shows light current characteristics of a high-power BA device mounted epi-side down with dimensions 1300 μm × 200 μm. Here, a further reduced threshold current density of 142 Acm-2 and a slightly higher slope efficiency of 0.36 W/A per facet is obtained under cw operation. A maximum output power of more than 4.6 Watts at RT is obtained. A series resistance of 3 ∙ 10-4 Ωcm2 and a turn-on voltage of 1.1 V can be derived from the voltage versus drive current characteristics in Fig. 2. The emission wavelength of the device is in the range of 1240 nm as shown by the spectrum obtained at I = 1.6 ∙ Ith and shown in the inset of Fig. 2. Maximum output power is limited by thermal rollover. A reduction of the heat sink temperature increases the achievable output power. Figure 3 shows the light versus drive current characteristics of the identical device with a heat sink temperature of -5 °C. A maximum output power of more than 6.2 Watts is achieved, still limited by thermal rollover. Figure 4 shows the wallplug efficiency of the device at RT with a maximum value exceeding 40%.
Figure 5 shows the cavity length dependence of the reciprocal external differential efficiency. According to with the internal quantum efficiency ηi, the external differential efficiency ηe, the internal absorption αi and the cavity length L, a value of ηi=0.89 can be derived from the data. All observed values are comparable or better than those obtained for high-power laser diodes in this wavelength range using QDs [13] or GaInNAs QWs [7] as active layers.
Fig. 5. Cavity length dependence of the reciprocal external differential efficiency for determination of internal device parameters derived from BA lasers with a stripe width of 100 μm under pulsed operation
Although there are reports about long lifetime of GaInNAs laser diodes in cw operation [7], lifetime properties are still challenging due to incorporated nitrogen and low growth temperature related defects, especially for high-power operation. Using our optimized structures, reliable device performance is observed even for high-power operation. Figure 6 shows the output power over time for a fabricated BA laser. The measurement was started at a total output power of about 2.2 Watts. With no degradation occurring within the first 110 h, the injection current was increased corresponding to an output power of about 3.0 Watts. After almost 1000 h of high-power operation no device degradation is observable for output powers of several Watts.
Fig. 6. Total output power versus operating time of a GaInNAs high-power BA device under cw operation at RT. The discontinuity at about 110h is due to an intentional increase of the drive current.
The presented data shows the high potential of laser diodes with active layers consisting of GaInNAs QWs for high-power applications in the 1240 nm wavelength regime. For applications like direct pumping of raman amplifiers lasing characteristics like beam quality will become important for an efficient fiber coupling and could e.g. be realized using tapered laser diodes [14].
4. Conclusion
In conclusion, we have realized GaAs-based 1240 nm high-power laser diodes using GaInNAs active layers. Record low threshold current densities were obtained. A high internal quantum efficiency of almost 90% permitted to reach ouput powers of more than 4.6 Watts at RT and 6.2 Watts at -5°C. The maximum wallplug efficiency reaches more than 40%. The devices reliably operate in high-power cw operation, showing no output power degradation over the total time investigated (1000h). By an appropiate processing and mounting, these laser diodes could have high potential to serve as light sources for the direct pumping of Raman amplifiers operating in the XS-band at a wavelength of 1.31 μm.
Acknowledgments
Financial support of this work by the Bundesministerium für Bildung und Forschung (BMBF) under the frame of the HOPTRA project is gratefully acknowledged.
References and links
1. R.H. Stolen and E.P. Ippen, “Raman gain in glass optical waveguides,” Appl. Phys. Lett. 22, 276–278 (1972). [CrossRef]
2. M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, and Y. Yazawa, “GaInNAs: A novel material for long-wavelength-range laser diodes with excellent high-temperature performance,” Jpn. J. Appl. Phys. 35, 1273–1275 (1996). [CrossRef]
3. M. Fischer, M. Reinhardt, and A. Forchel, “GaInAsN/GaAs laser diodes operating at 1.52 μm,” Electron. Lett. 36, 1208–1209 (2000). [CrossRef]
4. J.S. Harris, “GaInNAs long-wavelength lasers: progress and challenges,” Semicond. Sci. Tech. 17, 880–891 (2002). [CrossRef]
5. M. Fischer, D. Gollub, M. Reinhardt, M. Kamp, and A. Forchel, “GaInNAs for GaAs based lasers for the 1.3 to 1.5 μm range,” J. Cryst. Growth 251, 353–359 (2003). [CrossRef]
6. J.S. Harris, R. Kudrawiec, H.B. Yuen, S. R. Bank, H. P. Bae, M.A. Wistey, D. Jackrel, E.R. Pickett, T. Sarmiento, L.L. Goddard, V. Lordi, and T. Gugov, “Development of GaInNAsSb alloys: Growth, band structure, optical properties and applications,” Phys. Status Solidi B 244, 2707–2729 (2007). [CrossRef]
7. D.A. Livshits, A.Y. Egorov, and H. Riechert, “8W continous wave operation of InGaAsN lasers at 1.3 μm,” Electron. Lett. 36, 1381–1382 (2000) [CrossRef]
8. E.M. Pavelescu, J. Slotte, V.D.S. Dhaka, K. Saarinen, S. Antohe, G. Cimpoca, and M. Pessa, “On the optical properties of quantum well GaIn(N)As/GaAs semiconductors grown by molecular-beam epitaxy,” J. Cryst. Growth 297, 33–37 (2006). [CrossRef]
9. K. Adachi, K. Nakahara, J. Kasai, I. Kitatani, I. Tsuchiya, M. Aoki, and M. Kondow, “Low-threshold GaInNAs single-quantum-well lasers with emission wavelength over 1.3 μm,” Electron. Lett. 42, 1354–1355 (2006) [CrossRef]
10. M. Hopkinson, C.Y. Jin, H.Y. Liu, P. Navaretti, and R. Airey, “1.34 μm GaInNAs quantum well lasers with low room-temperature threshold current density,” Electron. Lett. 42, 923–924 (2006) [CrossRef]
11. S.M Wang, Y.Q. Wei, X.D. Wang, Q.X. Zhao, M. Sadeghi, and A. Larrson, “Very low threshold current density 1.3μm GaInNAs single-quantum well lasers grown by molecular beam epitaxy,” J. Cryst. Growth 278, 734–738 (2005) [CrossRef]
12. T.K. Sharma, M. Zorn, F. Bugge, R. Hulsewede, G. Erbert, and M. Weyers, “High-power highly strained InGaAs quantum-well lasers operating at 1.2 μm,” IEEE Photon. Technol. Lett. 14, 887–889 (2002) [CrossRef]
13. A. Wilk, A.R. Kovsh, S.S. Mikhrin, C. Chaix, I.I. Novikov, M.V. Maximov, Yu.M Shernyakov, V.M. Ustinov, and N.N. Ledentsov, “High-Power 1.3 μm InAs/GaInAs/GaAs QD lasers grown in a multiwafer MBE production system,” J. Cryst. Growth 278, 335–341 (2005) [CrossRef]
14. W. Kaiser, J.P. Reithmaier, A. Forchel, H. Odriozola, and I. Esquivias, “Theoretical and experimental investigations on temperature induced wavelength shift of tapered laser diodes based on InGaAs/GaAs quantum dots,” Appl. Phys. Lett. 91, 051126 (2007) [CrossRef]
