Open Access
Add to BibSonomyAbstract
The emission characteristics of a novel, specially designed broad area diode laser (BAL) with on-chip transversal Bragg resonance (TBR) grating in lateral direction were investigated in an off-axis external cavity setup. The internal TBR grating defines a low loss transversal mode at a specific angle of incidence and a certain wavelength. By providing feedback at this specific angle with an external mirror, it is possible to select this low loss transverse mode and stabilize the BAL. Near diffraction limited emission with an almost single lobed far field pattern could be realized, in contrast to the double lobed far field pattern of similar setups using standard BALs or phase-locked diode laser arrays. Furthermore, we could achieve a narrow bandwidth emission with a simplified setup without external frequency selective elements.
© 2014 Optical Society of America
1. Introduction
Edge emitting diode lasers belong to the most efficient laser sources reaching electro-optical efficiencies of more than 70 % [1]. These devices are used in many applications such as telecommunication and metrology, as pump sources for solid state lasers or lately as direct light sources for material processing. However, the diffraction limited optical output power of diode lasers is strongly limited by catastrophic optical mirror damage, which prevents the widespread usage of these devices in high power applications. Broad area diode lasers (BALs) can achieve output powers of several Watts as single emitters and several hundred Watts when they are combined in laser bars [2]. The main drawbacks of BALs are poor spatial beam quality in the lateral direction due to the broadening of the emitter and longitudinal multimode operation due to the lack of spectral mode selection.
Several techniques to improve the spatial emission characteristics of BALs exist. Mode selective elements for example, can be implemented in the chip itself as diffractive resonators [3], tapered structures [4] or phase locked diode laser arrays (PLDLA) [5]. Recently, PLDLAs with a phase shifter section were realized achieving emission with a single lobed far field [6]. On the other hand external cavities can be used for beam shaping. Especially off-axis feedback was used to improve the emission characteristics of BALs and PLDLAs [7–9], but with the drawback of a double-lobed emission pattern or reduced power extraction efficiency.
Transverse Bragg resonance (TBR) as a mode selection mechanism for waveguiding was first proposed by Yeh et al. [10]. It is based on the resonant Bragg reflection at multiple refraction index steps. This concept was later studied theoretically as longitudinal and transversal mode selective element for diode lasers as part of the chip structure [11]. The work states, that a transverse mode selective operation of a TBR structure is only possible for 1D transversal grating in combination with angled front facets, a special angular selective anti-reflection (AR) coating or an additional longitudinal grating. Recently, TBR structures were used to increase the size of the vertical waveguide structure [12, 13].
Here we investigate the emission characteristics of a TBR BAL in an off-axis external cavity with angular selective feedback. By providing feedback at the TBR angle with an external mirror it is possible to promote the low loss TBR mode and stabilize the BAL. In contrast to standard off-axis external cavity setups most power is emitted into an almost single lobed farfield. Furthermore, a narrow bandwidth was achieved despite the fact that no frequency selective feedback was provided leading to a simplified and more compact setup. To our knowledge, this is the first time that a BAL with lateral TBR structure was operated in an off-axis external cavity. With this proof of concept setup near diffraction limited emission with a single lobed far field pattern could be achieved with an output power of 500 mW and a beam propagation factor of M2 < 1.5 in the slow axis.
2. Chip design and mode selective operation
The TBR BAL, manufactured by the Ferdinand-Braun-Institute, Berlin, had an emitter width of wcore = 91.5 μm and a chip length of 1 mm. In the vertical direction (fast-axis), an InGaAs double quantum well was embedded in an asymmetrical super large optical cavity (ASLOC) [14] with a thickness of 4.8 μm. The confinement and cladding layers of the ASLOC consisted of AlxGa1−xAs, with compositions of x=0.35 and x=0.45, respectively. This vertical structure leads to emission with a narrow fast-axis divergence of 15°. A sketch of the lateral structure of the TBR BAL is shown in Fig. 1(a). In this direction (slow-axis) the TBR BAL had an actively pumped core region surrounded by two unpumped TBR regions. In the TBR regions, a transversal Bragg structure in the form of a refractive index grating was incorporated to achieve transversal mode selection. The Bragg grating layers were etched by i-line wafer stepper lithography parallel to the optical axis (z) and perpendicular to the chip surface. The etch depth was about 2 μm, resulting in an effective-index step of ≈ 0.005. The grating period was wTBR = 3 μm with a duty cycle of 0.5 and each grating extended over 50 periods. To be operated in an external cavity the front facet was anti-reflective (AR) coated, while the back facet had a highly reflective coating. Without the external cavity the TBR amplifier emitted amplified spontaneous emission (ASE) with a broad spectrum up to an injection current of 1.75 A. At higher pump currents the chip started to lase due to feedback from the fast axis collimator (FAC).
Fig. 1 (a) Lateral design of the transverse Bragg resonance (TBR) broad area diode laser (BAL). (b) Calculated reflectivity of a refraction index step (black dashed line) and TBR grating (red solid line) as a function of the feedback angle.
For a given wavelength the TBR grating defines a specific angle of incidence, where light is reflected constructively in the grating. Figure 1(b) shows the calculated reflectivity as a function of the angle of incidence at one side of the active core with a 50 periods wide TBR grating (red) and a single refraction index step (black) as in a standard BAL. Both curves were derived with the transfer matrix method at a wavelength of λ = 960nm. For small feedback angles the reflectivity at the refraction index step shows total internal reflection. At a certain feedback angle the reflectivity drops rapidly and remains almost zero for higher angles. The reflectivity of the TBR grating shows no total internal reflection at small angles and drops as well at higher feedback angles. However, at a specific resonance angle a peak in the reflectivity is present. The light propagating at this angle experiences reduced internal losses and is called the TBR mode.
3. Experimental setup and results
The experimental setup is shown in Fig. 2. The off-axis external cavity consisted of the TBR BAL, a FAC with a focal length 0.91 mm and a feedback mirror. In fast-axis direction the emitted light was collimated with the FAC and not influenced by the external cavity to preserve the almost diffraction limited beam quality along that axis. No collimation was used along the slow-axis. The divergent emission in the slow-axis in combination with the feedback mirror and the short cavity length allowed angular sensitive feedback and therefore a selection of the low-loss TBR mode. The feedback mirror was placed on a tiltable mount at a distance of 5 mm from the front facet of the TBR BAL. To achieve the highest output power in the TBR mode several feedback mirror reflectivities have been tested and the reflectivity of R = 15% was found to be optimal for our setup. If the mirror is tilted towards the TBR BAL by an angle of θ, feedback will be provided for light exiting the chip at this angle. Thus, this side is called feedback branch. Light exiting the diode under the opposite angle −θ will not be fed back into the diode. This side is called outcoupling branch analogously to a previously reported setup with PLDLAs [9].
Fig. 2 Sketch of the external resonator setup consisting of the TBR BAL, a FAC and a tiltable feedback mirror with 15 % reflectivity.
Figure 3(a) shows the evolution of the far field emission of the TBR BAL at a pump current of 2 A for different feedback angles. The far field emission patterns were measured with a moving slit technique with a slit width of 0.5 mm (angular resolution 0.2°) and a photodiode. At a feedback angle of 0° the far field emission pattern exhibits a broad multimode profile centered around the optical axis. This on-axis emission behavior is typical for a standard BAL. At larger feedback angles, the on-axis emission is suppressed and the emitted far field profile splits up into a double lobe emission. This double lobe emission is well known from off-axis external cavity setups [7, 9] and was recently observed for vertical TBR waveguides as well [13]. For small feedback angles, the fed back light is amplified in the diode laser and emitted mainly to the outcoupling branch (blue). However, at higher feedback angles we observe, that the intensity in the feedback branch rises while the intensity in the outcoupling branch drops. If the feedback angle coincides with the TBR angle of 8.5°, the emission in the outcoupling branch is strongly suppressed and almost all light is emitted in the feedback branch (red). This peculiar behavior is in stark contrast to previous observations with standard BALs or PLDLAs operated in off-axis external cavities. This emission behavior becomes more evident in Fig. 3(b). Here the power was measured behind an aperture (2°) for each branch separately and plotted as a function of the feedback angle. For small feedback angles, the power in the outcoupling branch was higher than for the feedback branch, analogous to results reported with BALs and PLDLAs. At a feedback angle of > 3°, the power in the outcoupling branch started to drop dramatically. When the feedback angle coincides with the TBR angle at 8.5°, which is the position of the maximum emission in the feedback branch, most of the light was emitted in the feedback branch. Thus, by selecting the TBR angle as feedback angle it is possible to enforce a single lobed far field emission for this BAL.
Fig. 3 (a) Measured far field distribution of the TBR BAL in the slow-axis at a pump current of 2 A for different lateral feedback angles. (b) Optical power of the TBR BAL emission as a function of the feedback angle θ measured in the feedback branch (red solid line) and outcoupling branch (blue dashed line).
The peculiar emission behavior of the TBR BAL can be understood as follows. Due to the chip aspect ratio, half of the light that enters the chip at an angle matching the TBR mode will be reflected first at the TBR grating and then at the back facet and the other half will be reflected first at the back facet and then at the TBR grating. But only light with a specific wavelength matching the resonance condition will be reflected resonantly at the TBR grating. As a result, only light of that wavelength that enters the chip will exit the chip again at the angle of incidence. The spatial mode selectivity is realized by the fact that a higher spatial mode will have a higher divergence than the lowest order mode and the relatively high angular selectivity of the plane mirror architecture. Therefore higher order modes will experience higher losses. Thus, the external cavity setup allows for both spatial and longitudinal mode selection if feedback is provided at the TBR angle.
In Fig. 4 the far field emission of the TBR diode laser with on-axis feedback (black dashed line), off-axis feedback at θ = 4.5° (blue dotted line) and in the TBR mode (red solid line) is shown. For on-axis feedback (θ = 0°), a higher transversal mode with a total far field divergence angle of 4.8° was observed. If feedback is provided off-axis at θ = 4.5°, the emitted far field shows a double lobe structure. Each lobe had a far field divergence angle of < 1°. For a feedback angle of θ = 8.5° which matches the TBR mode, the far field emission pattern showed a strong single lobed emission with a reduced divergence angle of 0.7° in direction of the feedback branch. With off-axis feedback at the TBR mode a maximum optical output power of 500 mW after a 2° aperture (860 mW without aperture) was measured at a pump current of 2 A, giving a electro optical efficiency of 25 %. In this case, the slow axis beam quality was determined to be M2 < 1.5, following the ISO 11146 standard. With on-axis feedback 310 mW of optical power behind a 2° aperture (850 mW without aperture) and a beam quality of M2 ≈ 4.5 were measured at the same pump current.
Fig. 4 Measured far field emission pattern of the TBR BAL emission along the slow-axis for on-axis feedback (black dashed line) and off-axis feedback at 4.5° (blue dotted line) and at the TBR angle (red solid line) of the TBR BAL at a pump current of 2 A.
The spectrum of the TBR BAL emission is shown in Fig. 5 for on-axis feedback (black line), off-axis feedback θ = 4.5° (blue line) and feedback at the TBR angle (red solid line). The spectrum for on-axis feedback shows multiple peaks over a wide spetral range. If feedback is provided under an off-axis angle of θ = 4.5° the emission spectrum is narrowed down, but still shows multiple peaks. Stable single longitudinal mode emission Δλ = 90pm with a good sideband suppression (> 30dB) is only achieved for off-axis feedback in the TBR mode (θ = 8.5°). Even slight deviations from the TBR angle lead to spectral side modes. The behavior described above is consistent for feedback at the TBR mode (θ = 8.5°) from threshold at Ipump = 0.9A up to 2 A. For higher pump currents the TBR mode starts to compete against the lasing from the feedback of the FAC, which can be avoided with a better AR coating.
Fig. 5 Spectra of the TBR BAL emission at 2 A pump current for on-axis feedback 0.0° (black line), off-axis feedback 4.5° (blue line) and feedback at the TBR mode angle of 8.5° (red line).
4. Conclusion
The emission characteristics of a BAL with a lateral TBR grating were investigated in an angular selective external cavity setup. We have observed a strong dependence of the transversal emission behavior of this device on angular feedback. While the lateral emission characteristics of the TBR BAL is similar to that of a standard BAL for small feedback angles, the device shows a new specific behavior for larger feedback angles close to the TBR mode. In contrast to previous results obtained with PLDLAs, which emit a double lobed far field under off-axis feedback, it was possible to enforce emission in direction of the feedback only. When stabilized at the TBR angle of 8.5°, the TBR BAL yielded an output power of 510 mW at a pump current of 2 A. In this case, the far field divergence was determined to be 0.7° and the beam propagation factor was measured to be M2 < 1.5 in the slow-axis direction and M2 < 1.2 in the fast-axis direction. Furthermore, a narrow spectral bandwidth of < 100pm with good sideband suppression was observed when feedback was provided at the TBR angle.
To our knowledge, this is the first time that such a lateral TBR BAL structure was investigated in an external cavity setup. The use of an external cavity made it possible to achieve transversal mode selection even with a one dimensional on-chip TBR grating. The high angular selectivity of the external cavity allowed a thorough investigation of the mode selection behavior of the TBR structure. Our results represent a first proof of concept that stabilization of a TBR BAL is feasible in a simplified setup compared previous results with PLDLAs and BALs.
Acknowledgments
We thank Dr. Götz Erbert and Dr. Bernd Eppich from the Ferdinand-Braun-Institute, Berlin, for providing the TBR diode laser chip and for fruitful discussions.
References and links
1. M. Kanskar, T. Earles, T.J. Goodnough, E. Stiers, D. Botez, and L.J. Mawst, “73% CW power conversion efficiency at 50 W from 970 nm diode laser bars,” Electron. Lett. 41, 245–247 (2005). [CrossRef]
2. H. Li, I. Chyr, X. Jin, F. Reinhardt, T. Towe, D. Brown, T. Nguyen, M. Berube, T. Truchan, D. Hu, R. Miller, R. Srinivasan, T. Crum, E. Wolak, R. Bullock, J. Mott, and J. Harrison, “>700W continuous-wave output power from single laser diode bar,” Electron. Lett. 43, 27–28 (2007). [CrossRef]
3. A. Büttner, U. D. Zeitner, and R. Kowarschik, “Design considerations for high-brightness diffractive broad-area lasers,” J. Opt. Soc. Am. B 22, 796–806 (2005). [CrossRef]
4. J. N. Walpole, E. S. Kintzer, S. R. Chinn, C. A. Wang, and L. J. Missaggia, “High-power strained-layer In-GaAs/AlGaAs tapered traveling wave amplifier,” Appl. Phys. Lett. 61, 740–742 (1992). [CrossRef]
5. D. R. Scifres, R. D. Burnham, and W. Streifer, “Phase-locked semiconductor laser array,” Appl. Phys. Lett. 33, 1015–1017 (1978). [CrossRef]
6. L. Liu, J. Zhang, S. Ma, A. Qi, H. Qu, Y. Zhang, and W. Zheng, “High-brightness diode laser arrays integrated with a phase shifter designed for single-lobe far-field pattern,” Opt. Lett. 38, 2770–2772 (2013). [CrossRef] [PubMed]
7. M. Chi, B. Thestrup, and P. M. Petersen, “Self-injection locking of an extraordinarily wide broad-area diode laser with a 1000-μm-wide emitter,” Opt. Lett. 30, 1147–1149 (2005). [CrossRef]
8. A. Jechow, V. Raab, and R. Menzel, “High CW power using an external cavity for spectral beam combining of diode laser-bar emission,” Appl. Opt. 45, 3545–3547 (2006). [CrossRef]
9. A. Jechow, V. Raab, R. Menzel, M. Cenkier, S. Stry, and J. Sacher, “1 W tunable near diffraction limited light from a broad area diode in an external cavity with a line width of 1.7 Mhz,” Opt. Commun. 277, 161–165 (2007). [CrossRef]
10. P. Yeh and A. Yariv, “Bragg reflection waveguides,” Opt. Commun. 19, 809–812 (1976). [CrossRef]
11. L. Zhu, A. Scherer, and A. Yariv, “Modal gain analysis of transverse bragg resonance waveguide lasers with and without transverse defects,” IEEE J. Quantum Electron. 43, 934–940 (2007). [CrossRef]
12. B. J. Bijlani and A. S. Helmy, “Bragg reflection waveguide diode lasers,” Opt. Lett. 34, 3734–3736 (2009). [CrossRef]
13. L. Wang, Y. Yang, Y. Zeng, L. Wang, C. Tong, X. Shan, H. Zhao, R. Wang, and S. Yoon, “High power single-sided Bragg reflection waveguide lasers with dual-lobed far field,” Appl. Phys. B 107, 809–812 (2012). [CrossRef]
14. C. Fiebig, G. Blume, M. Uebernickel, D. Feise, C. Kaspari, K. Paschke, J. Fricke, H. Wenzel, and G. Erbert, “High-power DBR-tapered laser at 980 nm for single-path second harmonic generation,” IEEE J. Sel. Top. Quantum Electron. 15, 978–983 (2009). [CrossRef]
