Coherent Power Scaling and Extended Tunability in a Multi chip VECSEL cavity0

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					       Coherent Power Scaling and Extended Tunability in a Multi-chip VECSEL

                                               Li Fan and Mahmoud Fallahi
                            College of Optical Sciences, University of Arizona, Tucson, AZ 85721

                        Jörg Hader, Aramais R. Zakharian and Jerome V. Moloney
Arizona Center for Mathematical Science and College of Optical Sciences, University of Arizona, Tucson, AZ 85721

                                                       James T. Murray
                                    Areté Associates, 3194 N. Swan Road, Tucson, AZ 85712

                                                   Robert Bedford
                  Air Force Research Laboratory, 2241 Avionics Cir. Wright-Patterson AFB, OH 45433

                                    Wolfgang Stolz and Stephan W. Koch
  Department of Physics and Material Sciences Center, Philipps Universität Marburg, Renthof 5, 35032 Marburg,

We report on a multi-chip vertical-external-cavity surface-emitting laser, in which anti-reflection (AR) coated
VECSEL chips serve as folding mirrors in a zigzag fold cavity such as the W-shaped cavity. Compared to the single
chip VECSEL, this multi-chip VECSEL have several potential advantages: (1) The heating is distributed on various
VECSEL chips instead of a single chip. The thermal rollover is delayed since less pump power on each chip will be
needed for achieving high power VECSEL. Thus more pump power can be launched to the laser to achieve higher
power scaling than that of the single chip VECSEL. (2) The multi-chip VECSEL has a much higher round trip small
signal gain than a single chip VECSEL. As a result, we can use an output coupler with low reflectance to increase
the slope efficiency of the laser. (3) The output of a multi-chip VECSEL is a stable coherent beam with good beam
quality which is easily controlled by the fold cavity [1].

A two-chip VECSEL with two slightly different chips is used to prove the concept of the coherent power scaling and
enhancement of the tenability in a multi-chip VECSEL. They are designed for emission around 975 nm and grown
by metal-organic vapor phase epitaxy (MOVPE) on an undoped GaAs substrate. Their active regions of VECSEL
chip 1 and VECSEL chip 2 consist of 14 and 10 InGaAs compressive strained quantum wells , respectively. Each
quantum well is 8 nm thick and surrounded by GaAsP strain compensation layers and AlGaAs pump -absorbing
barriers. The thickness and composition of the layers are optimized such that each quantum well is positioned at an
antinode of the cavity standing wave to provide resonant periodic gain (RPG). A high reflectivity (R > 99.9%) DBR
stack made of 25-pairs of AlGaAs/AlAs is grown on the top of the active region. In addition to the RPG active
region and DBR stack, there is a high aluminum concentration AlGaAs etch-stop layer between the active region
and the substrate to facilitate selective chemical substrate removal. The epitaxial side of the VECSEL wafer is
mounted on chemical vapor deposition (CVD) diamond by indium solder. After the removal of the GaAs substrate
and etch-stop layer, a single layer Si3N4 (n = 1.78 at 980 nm) quarter wave LR coating (for 975-nm signal) is
deposited on the surface of VECSEL chip to achieve a reflectivity of less than 1% at the signal wavelength. A
symmetric W-shaped cavity is designed for this two-chip VECSEL.

Over 19-W output power with 44 % slope efficiency is demonstrated, which almost doubled the output power of a
single-chip VECSEL [1]. Multi-watts high-brightness linearly polarized output with a tuning range of 33 nm is
demonstrated in such a two-chip VECSEL. The experimental results indicate that the two-chip VECSEL has much
larger tuning range and less power variation with the tuned wavelength than the single-chip VECSEL [2]. Multi-chip
VECSEL can be an efficient coherent power scaling scheme and extends the tunability.


[1]   L. Fan, M. Fallahi, J. Hader, A. R. Zakharian, J. V. Moloney, J. T. Murray, R. Bedford, W. Stolz and S. W. Koch, “Multi-chip vertical-
      external-cavity surface-emitting lasers: A coherent power scaling scheme,” Accepted by Opt. Lett.
[2]   Li Fan, Mahmoud Fallahi, Aramais R. Zakharian, Jörg Hader, Jerome V. Moloney, Robert Bedford, James T. Murray, Wolfgang Stolz and
      Stephan W. Koch, “Extended tunability in a two-chip VECSEL,” in preparation for IEEE Photon. Technol. Lett..

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