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