Diode laser polarity and the Clarendon current drivers: All diode lasers fall into one of two classes: 1. Positive Polarity – where the negative electrode (cathode) is grounded. E.g. Sanyo DL7140-201 780 nm diodes. 2. Negative Polarity – where the positive electrode (anode) is grounded. E.g. SDL-5421-G1 852 nm diodes. In both cases the terminal to be grounded is usually connected to the case of the diode. The two classes are illustrated in Figure 1. It is necessary to match the diode to the correct polarity current source. To this end, two versions of the Clarendon current driver exist: Anode Cathode LD PD LD PD Cathode Earthed to Case Anode Earthed to Case (a) (b) Figure 1: Laser diodes (LD) may be classified as either a) positive or b) negative polarity. Both cases are shown with an optional photodiode (PD) in the casing. EW1291 Positive Polarity Driver Used to drive laser diodes where the cathode is grounded. The pin connections of the input and output are shown in Figure 2. 2 1 3 2 3 1 4 Output Pin 1: 0V Pin 2: +ve Input Pin 1: +16V Pin 3: 0V Figure 2: EW1291 positive polarity current driver. The connectors are shown as viewed from the front of the driver unit. 1 EW1206 Negative Polarity Driver Used to drive laser diodes where the anode is grounded. The pin connections of the input and output are shown in Figure 3. 2 1 3 2 3 1 4 Output Pin 1: 0V Pin 2: -ve Input Pin 1: 0V Pin 3: -28V Figure 3: EW1206 negative polarity current driver. The connectors are shown as viewed from the front of the driver unit. Additional Information Both units require the same cables: 1. Input: 3 Pin DIN Socket (Pins 1 and 3) – Bananas. 2. Output: 4 Pin DIN Socket (Pins 1 and 2) – 4 Pin DIN Socket (Pins 1 and 2) Both units operate in the same fashion. There is a three position ON/OFF switch. When the unit is in the fully OFF position the output pins are shorted together and grounded. Before connecting to a laser ensure the unit is in the fully OFF position and that the current is turned right down. To turn ON, rotate the ON/OFF switch to the fully ON position in gradual steps. Then slowly increase the output current. Before using a unit perform the following checks: 1. Verify the integrity of the cables and that the correct connections are made (E.g. Is Pin 1 at one end connected to Pin 1 at the other end?) 2. Verify the operation of the supply. Insert a few Ohm load resistor into the output socket (There is a 4 Pin DIN connector with a resistor soldered between pins 1 and 2 for this purpose). Turn the supply on and run a small amount of current through the resistor. Use a DVM to measure the potential difference across the resistor. You should find that one end of the resistor is grounded to the case of the driver box and that the other end is either at a positive or negative potential. Diode Protection Circuit The diode protection circuit shown in Figure 4 should be connected across the laser diode in the appropriate manner. This circuit protects the laser diode from damage in two ways. Firstly, the four diodes in series prevent voltage spikes greater than 2.4 V from crossing the laser diode. Secondly, the single diode prevents the laser diode from being reverse biased by more than 0.6 V. 2 Red LD Black Figure 4: Diode protection circuit. The colours refer to the colours of the leads attached to the circuit. The circuit is shown connected to a positive polarity diode where the cathode is grounded. For the positive polarity lasers (used for rubidium) the red lead should be connected to Pin 2 of the laser head 4 Pin DIN connector and the black lead to Pin 1. For the negative polarity lasers (used for caesium) the situation is reversed, so that the red lead should be connected to Pin 1 and the black lead to Pin 2. 3