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International Journal of Pure and Applied Physics. ISSN 0973-1776 Volume 8, Number 2 (2012), pp. 129-134 © Research India Publications http://www.ripublication.com/ijpap.htm The Effect of Magnetic Field on Light/Current and Current/Voltage Characteristics of MQW Laser Firas Sabeeh Mohammed Department of Physics, College of Sciences, AL–Mustansiriyah University, Iraq Abstract The influence of relatively weak magnetic field up to 2 Tesla at room temperature on light/current and current/voltage characteristics of MQW laser was examined. Threshold current, forward bias voltage, light output, and series resistance were studied without and with magnetic field at room temperature. A fundamental change in laser characteristics was observed by applying the magnetic field at different directions with respect to the quantum well plane. The behavior of MQW laser is described in terms of the temperature-induced changes in the junction voltage and internal series resistance as well as the current-density alteration when the magnetic field is applied Introduction The application of quantum well structures to semiconductor laser diodes has received consideration attention because of physical interest as well as its superior characteristics, such as low threshold current density, low temperature dependence of threshold current, wavelength tenability, and excellent dynamic properties [1].The electrical, dynamic and spectral characteristics of laser diode depends on the injection current, laser temperature and composition. However it is also known to depend on magnetic field and pressure [2]. Some preliminary, low temperature investigations, under extremely strong magnetic fields were performed [3]. And to our knowledge, only a little work related to study the characteristics of quantum well laser (QWL) at room temperature in relatively weak magnetic field has been reported [4]. The purpose of this paper is to relate the experimental electrical properties of AlGaInP 130 Firas Sabeeh Mohammed MQWL to existing diode laser models. Light / current, light / voltage, and current / voltage characteristics recorded without and with magnetic field up to (2 Tesla) at room temperature. The Landau level based theory was used for explaining the behavior of laser diode at very low temperature, in strong magnetic field [3]. Our experimental condition differs from theirs. Therefore we measured these changes in magnetic field precisely and introduced a shift mechanism in which the carrier confinement of the laser diode was important. We carried out the experiment by using high accuracy, stability and high speed digital multimeter, it has 0.01 % DC voltage basic accuracy, and in order to reduce the error we used digital video camera to monitor and measure the injection current, output power and bias voltage instantaneously. Experimental setup Figure (1) shows our experimental system and technique used for data acquisition and analyses and illustrated as follow: the semiconductor laser used in our Experiments is (Sanyo DL3149-056. AlGalnP Index Guided Multiple Quantum well active laser) and the lasing wavelength 660 nm at 25°C. The laser diode fixed between poles of a (7.5mm x 1.3cm x 1.3cm) electromagnetic on L-shape copper block, the copper block is mounted through Pelter's element (Thermoelectric module TOM-8-127-4-6,0M) driven by DC current works as a thermal pump. The heat pump is squeezed between the copper plate and the heat sink, the heat sink was supplied with a fan for an efficient heat pumping (CPU cooler). Laser mount, photodiode and lens are fixed on high precise XYZ microposition. For stable operation of laser diode it is necessary that both injection current and laser temperature are controlled. Current control was designed and used in order to stabilizer current of the laser diode within (0.05 mA) and current limit (0-60 mA). The temperature controller was designed both to set the laser operating temperature and to detect the laser temperature fluctuation and correct for them. We project the laser beam from laser diode directly at a photodetector through (colominat lens). We used a Hamamatsou (S2281 Si photodiode). The output power of laser diode measured by using (DVM) through 1.5 kΩ terminal resistance. The fluctuation in (power output, laser diode injection current, laser diode temperature and electromagnet current) are recorded by a digital video camera, and the time resolution of this measurement is determined by frame of the video camera system which is ( 1/60 sec). Based on information gained from [5, 6] we found that the relation between the magnetic field and the laser diode direction is also important. Therefore, the experiment was performed in the following way. First the magnetic flux density vector B and the normal direction n of the layered surface of the laser diode are parallel to each other (B//n) as shown in figure (2a), and second they are perpendicular (B┴n) as shown in figure (2b). The Effect of Magnetic Field on Light/Current 131 Figure 1: Experimental Setuup Figure 2: Orientation of the Magnetic Field Results and Discussion Figure (3) and figure (4) shows the light / current characteristics with and without magnetic field for B//n and B┴n respectively at T=25 ºC. This shows the abrupt onset of laser action at the threshold current and the increase in the threshold when the magnetic field is applied for B//n.In the case of B┴n the effect of magnetic field was small as shown in fig.(4). Fig (5) shows the optical output-power shift versus magnetic flux density at (T=25 ºC) and (1.05 Ith) injection current condition for B//n and B┴n. The optical output power decreases when subject to a magnetic field. As the magnetic field was swept from (0 to 2T), the output power drops from (574.21 to 481.4 µ watt) in the case of B//n and from (574.36 to 535.71µ watt) in the case of B┴n. Figure (6) shows the voltage / current characteristics at (T=25 ºC), where the normalized threshold voltage at threshold current and constant light output power plotted without and with magnetic field for B//n and B┴n. The forward bias voltage 132 Firas Sabeeh Mohammed for AlGaInP MQW laser drops and the series resistance of the laser increases when the magnetic field applied at the saturation region, no effect has been recorded at spontaneous and stimulated region. The magnetic field effected currents within and/or around the active layer and the current flow is altered by the Lorentz force [6]. The force on the current depends on the relation in direction between a magnetic field and the current. Thus, the current direction in the active layer is sensitive to the direction of the magnetic field, Hence the change in the current path causes a change in the current density in the active layer leading to a change in temperature and carrier distribution and in turn the oscillation threshold current, forward bias voltage, and output power shift. In particular, the current I and magnetic field B are parallel (B//I) for B//n, suppressing the current diffusion leading to a higher current density than B┴n and a higher temperature, as a result the threshold current takes place toward a higher side and the series resistance, output power, and forward bias voltage drops [5]. Figure 3: Light/current characteristics with and without magnetic field at T=25 ºC for B//n Figure 4: Light/current characteristics with and without magnetic field at T=25 ºC for B┴n The Effect of Magnetic Field on Light/Current 133 Figure 5: The optical output power shift versus magnetic field at T=25ºC and 1.05Ith injection current condition. Figure 6: Light/Voltage characteristics without and with magnetic field (2Tesla) Conclusion We measured the threshold current, optical output power and forward bias voltage shifts of MQW laser diode, in a weak magnetic field at room temperature. We observed the higher threshold current side, the lower power side optical output power shift and lower forward bias voltage especially for B//n. The behavior of MQW laser is described in terms of the temperature-induced changes in the junction voltage and internal series resistance as well as the current-density alteration when the magnetic field is applied. 134 Firas Sabeeh Mohammed References [1] Y. Arakawa, and A. Yariv "Quantum Well Lasers- Gain, Spectra, Dynamics" IEE J. Quantum Electron., vol. QE-22, NO.9, September 1986. [2] S. matsuda, K.Shibata, H. Nakano, T. Sato, M. Ohkawa, T.Maruyama and M. Shimba, " Oscillation wavelength shift characters of a semiconductor laser in a magnetic field, observation by using a beat note ", Electrical Engineering in Japan, vol. 122, No. 3, pp. 46- 54 (1998). [3] Y. Arakawa, H. Sakaki, M. nishioka, M. okamoto, and N. Miura, " Spontaneous emission characteristics of a quantum well laser in a strong magnetic field- an approach to quantum- well- box light source ", Jpn. J. Appl. Phys. Vol. 22, No. 12, pp.L804- L806 December 1983. [4] Y. Sato, T. Miyamoto, T. Sato, M. Ohkawa, T. maruyama, " Some characteristics changes of a semiconductor laser in a magnetic field :Consideration of the shift mechanism by changing the injection current ", TEICE. LQE, Vol. 104, No. 270, pp. 71- 75. (2004). [5] Y. Sato, T. Miyamoto, M. Ohkawa, and T. Maruyama, " Magnetic fields effects on a semiconductor laser characteristics ", SPIE, proceedings Vol. 5722, pp. 90- 97 (2005). [6] T. miyamoto, J. chiba, T. Sato, M. Ohkawa, and T. maruyama, “An investigation into changes in the oscillation characteristics of a semiconductor laser exposed to magnetic fields ", SPIE, Proceedings 61150Z (2006).