Introduction

1. Setup of a Tunable Diode Laser Kevin J. McElwee, ErkanGulturk Bridgewater State University, Bridgewater MA 02325 Mentor: Edward F. Deveney Ph.D • Introduction • Our goal this semester was to setup a recently purchased tunable diode laser from Thorlabs (TLK-L780M). Tunable lasers are important in physics because they allow the user to manipulate the laser to his or her desired frequency (or wavelength). Tunable external-cavity diode lasers (tunable lasers) in general, use a diffraction grating, which allow the user to choose the wavelength at which he or she would like to work at. Our tunable laser uses this type of setup. We desire our wavelength to be around 780 nm. External-cavity diode lasers often have two types of experimental setups: the Littman-Metcalf and Littrow configurations. While our tunable laser kit allows us to use either setup, we chose to use the Littman-Metcalf configuration. • Apparatus • Half-butterfly gain chip • DC Tuning Actuator • DC Servo Motor Controller • Breakout box • This box provides a connection between the laser diode control (LDC) and the thermoelectric cooler (TEC). • Experimental Setup • In our setup, the output beam leaves the gain chip and simultaneously reflects and refracts off the beam splitter. The reflected light, which has lost power (intensity), then enters our photodiode so we can detect the wavelength of our beam. In order to detect the power output of our laser, we slid the power meter in front of the beam’s path before it hit the beam splitter. • Figure (1): The above setup uses a Littman-Metcalf setup. (A) represents our tunable diode laser. (B) represents power meter which measures the output power of our laser. (C) represents a beam splitter. (D) represents our DC Servo Motor Controller which controls the piezo adjustor. • The one problem with this setup is that the refracted light becomes stray light, which can be dangerous if it hits the retina without proper safety precautions. • Littman vs. Littrow Configurations • Figure (2): The above figure portrays the Littrow Configuration. The placement of the grating and the fact it is on a pivot defines this setup. When the beam hits the grating, several orders of beams scatter. One, however (first order beam), reflects directly off the grating back into the gain chip. This allows for stimulated emission. • In a Littrow configuration, the gain chip produces an output beam that reflects the zeroth order beam back into the gain chip. Through stimulated emission, the gain chip will produce the desired wavelength. The wavelength can be altered by changing the angle of the grating itself. The other setup used for external cavity diode lasers is the Littman-Metcalf Configuration. This is the setup we used, which can be seen in Figure (1). The magnified view of the configuration can be seen below in Figure (3). Figure (3): Above, the Littman-Metcalf setup shows the incident beam reflect off the grating. Then, there are several order of beams that reflect off the grating form the incident beam. One, however, reflects off the grating, to the mirror, back to the grating, and back into the gain chip. This allows for stimulated emission. Both positive and negative consequences result from each type of setup. In the Littrow configuration, the angle at which the output beam reflects off the grating changes as the angle of the grating changes. In the Littman-Metcalf configuration, the grating angle never changes, therefore the output beam that reflects off the grating never changes. A few other consequences result as shown below in figure (4). Figure (4): The above chart shows the characteristics of each external cavity diode configuration. What is a Gain Chip? A gain medium can be thought of as anything amplifies light. A gain chip is our gain medium which drives our tunable diode laser. Gain chips have antireflection coating (AR Coating) on one or both ends of the chip. This increases threshold for lasing or completely eliminates lasing altogether. Figure (5): The above image shows a magnified image of our half-butterfly gain chip behind the collimating lens. The diffraction grating is in front of the lens. Lasing Once the laser was turned on and we adjusted the laser to resonate at the desired wavelength, we then tried to produce lasing. When lasing, the power output of a laser will dramatically begin to increase. Lasing works via Bohr’s principle of electron energy orbits: Figure (6): The above equation relates the frequency of a laser to the transition of an electron between two discrete energy levels. The constant, h, is Planck’s constant (6.62510-34 Js). When the electrons in the laser-active atom jump between orbital levels, they must omit or absorb energy in the form of light. The change in energy between orbital levels matches the energy of the light that it interacts with. • There are three different types of interactions between radiation and the electrons of an atom: resonant absorption, spontaneous emission, and stimulated emission. Lasing occurs when stimulated emission is greater than spontaneous emission (population inversion). • Figure (7): In spontaneous emission, electrons drop to discrete, random, lower energy levels. By dropping energy levels, they emit energy in the form of radiation (light). Since they all drop to random lower energy levels, the output beam is incoherent. However, when the desired wavelength re-enters the gain chip (via reflection from the diffraction grating), stimulated emission occurs. Here, an electron that is about to drop to a lower energy level interacts with this incoming radiation (with a specific wavelength) and drops down to a specific energy level that correlates to the energy of the incoming radiation. In doing so, it releases energy in the form of light which matches the wavelength of the incoming light. • Results • According to the manufacturer, lasing should be seen around 37-40 mA of input current. Our results showed lasing right around 40-45 mA of current. • Figure (8): The graph on the left reflects our experimental data, while the graph on the right reflects the manufacturer characteristics of our tunable diode laser. Both graphs compare the power output (mW) of the tunable diode laser with the drive current (mA) that is delivered to the gain chip. Lasing occurs where the graph begins to increase significantly in power. • We also wanted to find where the tunable diode laser began to settle at a specific wavelength regardless of the drive current that we delivered. It appears that the wavelength begins to settle down between 80 mA and 140 mA (~765 nm). • Figure (9): Both graphs show the experimental data. The graph on the left compares the wavelength (nm) with the drive current (mA) we delivered to the gain chip. The graph on the right compares the wavelength (nm) with the thermistor value (kΩ). • Both graphs in figure (9) above show the wavelength settle around 765 nm. This means we want the drive current to be around 80-140 mA and our thermistor setting to be around 12-19 kΩ. • Current Status • Now that we know the thermistor and drive current settings which settle the wavelength of our tunable diode laser, we will aim to start adjusting the wavelength with the piezo adjustor next. By finding the settings that cause the wavelength to settle for our tunable diode laser, we will only have to worry about one variable that changes the wavelength of the laser: the piezo adjustor • References • Ye, C. (2004). Tunable External Cavity Diode Lasers. Hackensack, NJ: World Scientific. • Thorlabs.com