Lab I Safety & Basic Measurement ECE 476 I. Purpose The focus of this experiment is laser light safety and an introduction to the helium-neon laser and the optical power meter. II. Background Laser Safety Lasers and high radiance LEDs may cause eye damage by absorption of light energy inside the retina. This potential hazard has been investigated by a number of scientists and national safety organizations. These organizations contributed to the recommendation of the International Electro-technical Commission (IEC), in which all major industrial nations have representatives. This organization has established maximum permissible exposure limits for light energy. The limit establishes the maximum safe radiation levels at the human eye's entrance surface. For continuous operation these levels depend on wavelength, as shown in Fig. 1 below. Note that the light energy levels are given in units of irradiance (power per unit area). Figure 1: Maximum irradiance chart. The IEC stipulates that all laser products be classified in order to establish suitable means to protect anyone from dangerous laser light levels. Lasers are classified into 4 classes. Class 1 laser products are to be inherently safe. For example, the typical emission limits for class 1 lasers are less than 0.24 mW power and less than 1.28 W cm-2sr-1 radiance at 850 nm. These numbers vary with wavelength. Class 2 laser products are visible lasers ranging from 400 to 700 nm. Within this range the human eye protects itself via the blink reflex, assumed to respond within 0.25 seconds. The CW power from a class 2 source may be 1mW maximum. Class 3A products are not inherently safe. For the visible range from 400 to 700 nm the emission limits may be 5 times those of class 2. 1Class 3B products may emit dangerous radiation levels. The limitation in CW operation of class 3B lasers is 0.5 watt maximum power within the wavelength range from 315 nm up to l mm. Class 4 products produced light levels exceeding those of class 3B limits. These lasers are dangerous, even the reflected light can cause eye damage. These lasers can also burn the skin. The laser products have the following safety requirements: Class 2 lasers must carry the laser warning label. Class 3A, 3B and 4 laser products must carry the laser warning label, a key control, an audible or visible "laser on" warning, a beam stop to automatically disable the laser if no access is required, and safe locations of instrument controls. Users should wear protective glasses. Laser safety goggles are provided for each student in this lab. These goggles are designed to block the light from a specific laser wavelength and let light through at other wavelengths. The blocking ability of laser goggles is measured in a unit called optical density. Optical density (OD) is defined as Optical Power Meter An optical power meter is used to measure power from light sources such as lasers and LEDs. Figure 2: Optical Power Meter (left), photodetector (top middle), attenuator (top right), and manual (bottom right). The photodetector can be used with and without the attenuator (filter) on the detector. The attenuator is the part in the top right of figure 2. The attenuator screws into the photodetector and decreases the intensity of the light by a factor of approximately 1000 (OD=3). In order to take correct measurements with the 23attenuator on the photodetector, the attenuator symbol must be “on” on the photodetector. Take the attenuator off to measure low light levels and put the attenuator on to measure high light levels. Be very careful when handling the optical attenuator. Scratches, cracks, or finger prints on its surface can severely affect both the accuracy and precision of your readings. In general, do not handle, attach or remove the attenuator unless instructed. When you do have to handle the attenuator, wear white gloves to protect it from the oils on your fingers. III. Lab Basics Flip through the green notebook at your lab station. Note that an inventory of equipment (with the part numbers) and specification sheets for the equipment is found there. Look through your equipment cupboard. Note the locations of your safety goggles and lasers. Note that gloves must be worn when handling anything in the lens kit. Answer the following questions by referring to the labels on your 0.5mW He-Ne laser, the laser manual, and the goggles specification sheet. These questions should be filled out before you start conducting the experiment and must be included in your lab report. 1) What is the minimum power output for your He-Ne laser? See the laser datasheet in the green notebook, specifications on the right hand side. The label on the laser gives the RMS maximum power, not the minimum power. 2) What is the class of your He-Ne laser? See the label on the laser. 3) What is the wavelength of your He-Ne laser? 4) What is the maximum permissible irradiance exposure for continuous operation of the He-Ne laser? (Refer to Figure 1) 5) If the laser is focused to a diameter of 3 μm, what is the irradiance? What does this answer imply about the danger of focused laser beams? 6) Calculate the radiance of your laser from the laser datasheet in units of W cm-2sr-1. If you use an approximation, identify it clearly in the calculation. 7) What is the OD of your laser goggles at the wavelength of the He-Ne lasers? Note: You will also be using higher power 4 mW He-Ne lasers which are class 3B in this course. When using the class 3B laser, safety goggles are to be used. IV. Procedure Set-up 0.5 mW Laser 1. Locate all of the parts that you will need: 1-magnetic base, 1-support post (any size), 1-support post holder (any size), 2-coarse threaded inset screws, 1-coarse threaded nut, 1-.5 mW laser, and 1-.5 mW laser power supply. These parts can be seen in figure 3.Figure 3: Parts to mount the 0.5 mW laser. 2. First, take one of the inset screws and screw it as far as it will go into the magnetic base. Next, take the support post holder and screw it all of the way onto the exposed screw on the magnetic base. 3. Look at your support post, on either end there are different sized holes. One hole is for coarse threaded screws and bolts, while the other end is for the fine threaded screws and bolts. For mounting the laser, we will be using the coarsely threaded end. Take the second screw and screw it part way into the support post. Then take the nut and screw it