III. Feedback control The laser dynamics can be modelled by a system of linked rate equations introduced by Roy et al. (1993) Based on the model it was shown that the stability of the fix point solution can be achieved using a proportional feedback loop of the infrared output intensities on the pump power (the current through laser diode) of the laser The control signal is given by: k x / k y : weighting factors S x / S y : sum intensity in the direction of polarization (x  y) : fix point (setpoint) of the sum intensity III. Feedback control The laser dynamics can be modelled by a system of linked rate equations introduced by Roy et al. (1993) Based on the model it was shown that the stability of the fix point solution can be achieved using a proportional feedback loop of the infrared output intensities on the pump power (the current through laser diode) of the laser The control signal is given by: k x / k y : weighting factors S x / S y : sum intensity in the direction of polarization (x  y) : fix point (setpoint) of the sum intensity Pure electronic intensity noise suppression of a multimode intracavity-doubled solid state laser L. Ehlkes 1, 2, T. Letz 1, 2, F. Lange 1, K. Pyragas 3, 1 and A. Kittel 1 1 Energy and Semiconductor Research Group, Department of Physics, University of Oldenburg 2 Max Planck Institute for the Physics of Complex Systems, Dresden 3 Semiconductor Physics Institute, Lithuania I. Introduction The display technology market needs powerful light sources in the visible spectral range. Diode pumped intracavity frequency doubled solid state lasers (DPSS) are the first choice for the generation of green and blue light, because this type of lasers are compact, robust, efficient and have a higher output power compared to laser diodes.The intracavity frequency conversion by the optical nonlinear crystal is highly efficient but on the other hand destabilizes the dynamic of the laser under multimode operation resulting in periodic or chaotic fluctuations of the output intensity (green problem). However, it is possible to suppress the intensity fluctuations of the laser, but all proposals are based on optical modifications to the laser cavity. These modifications on one hand decrease the efficiency and on the other hand dramatically increase the cost of such a laser, because all the additional parts have to be adjusted carefully which is very time consuming. Our goal is to stabilize the output intensity of an intracavity doubled Nd:YAG laser by using a pure electronic feedback loop which can be produced at low cost in order to deliver a stable high power laser working in multimode operation suitable for e.g. a laser beamer (consumer market). I. Introduction The display technology market needs powerful light sources in the visible spectral range. Diode pumped intracavity frequency doubled solid state lasers (DPSS) are the first choice for the generation of green and blue light, because this type of lasers are compact, robust, efficient and have a higher output power compared to laser diodes.The intracavity frequency conversion by the optical nonlinear crystal is highly efficient but on the other hand destabilizes the dynamic of the laser under multimode operation resulting in periodic or chaotic fluctuations of the output intensity (green problem). However, it is possible to suppress the intensity fluctuations of the laser, but all proposals are based on optical modifications to the laser cavity. These modifications on one hand decrease the efficiency and on the other hand dramatically increase the cost of such a laser, because all the additional parts have to be adjusted carefully which is very time consuming. Our goal is to stabilize the output intensity of an intracavity doubled Nd:YAG laser by using a pure electronic feedback loop which can be produced at low cost in order to deliver a stable high power laser working in multimode operation suitable for e.g. a laser beamer (consumer market). II. Optical setup & Characterization Fig. 1: Optical setup of the laser consisting of a pump diode, collimating and focussing lenses and the cavity of the solid state laser. The left side of the Nd:YAG crystal is coated with a reflective layer and a KTP crystal is used as a nonlinear element Fig. 2: time series of the linear polarized infrared (a,b) and green (c) output intensities Fig. 3: mode spectrum of the infrared laserlight measured by a Scanning-Fabry-Perot-Interferometer Fig. 4: Stabilization of the laser output power via an electronic closed feedback loop of the infrared sum intensities S x / S y on the laser diode current Fig. 5: Theoretical domains of stability for constant mode configuration [1,3], varied pump rate (  = 1,4; 2,1; 10) and limitation of the bandwidth to 1080 kHz (a) as well as for varied mode configuration and constant pump rate (  = 2.1) without limitation of the bandwidth (b). Fig. 6: Experimentally measured domains of stability for  = 1.4 and mode configuration [1,2] (a) as well as for  = 2.1 and mode conf. [1,4] (b). The normalized standard deviation of one infrared output intensity is displayed. Fig. 6: Experimentally measured domains of stability for  = 1.4 and mode configuration [1,2] (a) as well as for  = 2.1 and mode conf. [1,4] (b). The normalized standard deviation of one infrared output intensity is displayed. IV. Extended feedback control laser states with high symmetrical mode configurations (, [n,n]) can not be stabilized using the overall pump power as control force these states show the best efficiency for nonlinear frequency conversion and therefore are preferred in order to stabilize these dynamical states, it is necessary to be able to change the sum intensities S X & S Y independently the direction of polarization of the pumping laser diode has a strong influence on the energy distribution in the cavity eigenmodes of the two polarizations we use a second pump laser diode, which is orthogonal linear polarized to the first one, in order to address and stabilize these states of high symmetry (fig. 7 & 8) IV. Extended feedback control laser states with high symmetrical mode configurations (, [n,n]) can not be stabilized using the overall pump power as control force these states show the best efficiency for nonlinear frequency conversion and therefore are preferred in order to stabilize these dynamical states, it is necessary to be able to change the sum intensities S X & S Y independently the direction of polarization of the pumping laser diode has a strong influence on the energy distribution in the cavity eigenmodes of the two polarizations we use a second pump laser diode, which is orthogonal linear polarized to the first one, in order to address and stabilize these states of high symmetry (fig. 7 & 8) (a) (b) k X / a.u. k Y / a.u. k X / a.u. k Y / a.u. k X / a.u. k Y / a.u. Fig. 7: Schematics of the setup containing the optical components and the electronic feedback loop for intensity noise suppression (LD: laserdiode; BS: beamsplitter; LC: laser-cavity; CF: optical filter; PBS: polarizing beamsplitter; PD: photodiode; HPF: high pass filter; CU: control unit; BP: bypass) Fig. 7: Schematics of the setup containing the optical components and the electronic feedback loop for intensity noise suppression (LD: laserdiode; BS: beamsplitter; LC: laser-cavity; CF: optical filter; PBS: polarizing beamsplitter; PD: photodiode; HPF: high pass filter; CU: control unit; BP: bypass) Fig. 8: Scan through the 4-dim. feedback parameter space while recording the noise intensity (standard deviation of the output intensity normalized by the noise without feedback): Shown are different cross sections leaving the two remaining feedback parameters constant (  X+Y = 12.3). (a) k XY = 1.8 k YX = 1.2 (b) k YY = -2.0k YX = 1.2 (c) k XX = -1.8 k XY = 1.8(d) k XX = -1.8k YY = -2.0 Fig. 8: Scan through the 4-dim. feedback parameter space while recording the noise intensity (standard deviation of the output intensity normalized by the noise without feedback): Shown are different cross sections leaving the two remaining feedback parameters constant (  X+Y = 12.3). (a) k XY = 1.8 k YX = 1.2 (b) k YY = -2.0k YX = 1.2 (c) k XX = -1.8 k XY = 1.8(d) k XX = -1.8k YY = -2.0 (a)(b) (c)(d) LD-X PD-X -Y BSLCCF LD-Y PBS () k xx k yy k yx k xy HPF BP CU f f  the output intensity noise can be reduced significantly  the range of successful operation has been improved LD-X