He-Ne laser glow discharge properties

Introduction

The task of designing or tailoring the power supply to a gas laser tube (He-Ne, cw CO₂, etc.) is not entirely trivial in relation to the nonlinear characteristics of the V-A glow discharge curve. The glow discharge is used in the above-mentioned types of lasers to excite atoms or molecules of the gain medium.

If the electric field is placed between electrodes located in pure gas or gas mixture, one of the following effects can be observed:

• gas acts as an insulator, there is no flow of charged particles between the electrodes
• discharge column formation and record of the electric current between the electrodes

The main parameter determining which of these two effects occurs is the electric field intensity. The first effect occurs when the electric field intensity is insufficient to initiate the process of ionization of neutral gas molecules by using electrons (obtained e.g. by emission from the cathode). If the field intensity is large enough, the electrical charged particles in the gas (e.g. the electrons from the cathode) can have so high energy that causes ionization of neutral gas molecules, i.e. the separation of one or more electrons. The further movement of the resulting electrons and ions can ionize other molecules, and in this way the amount of charge can grow like an avalanche. At the same time, the current is increasing and the glow discharge is formed in the gas.

If this discharge is bound to the outer charge carrier, then the process is called non-self-sustaining. If an avalanche-like increase in the number of current carriers results in the discharge that occurs at any small initial external discharge, then the process is called self-sustaining. The second type is used in cw gas lasers pumped by glow discharge at low pressure, e.g. He-Ne laser or CO₂ laser.

In quantitative derivation of the glow discharge V-A characteristics, the current calculation is the best first step. The work done per unit of time by applying the external voltage source to the discharge electrodes is consumed to change the energy of the moving charge, i.e. the following equation is valid:

(5.1)

, where U is the voltage, I denotes the discharge current, e means the charge of the electron, and ν stands for the electron velocity. In this simple case, the current is inversely proportional to the voltage.

If more charged particles are moved in the vacuum, the field cannot be considered as given and the influence of the bulk field must be taken into account. Therefore, the dependence of the current on the applied voltage is not linear, i.e. the Ohm law is not valid. In the case of the self-sustaining glow discharge, the situation is even more complicated. The complex structure of this type of discharge is primarily caused by various electron and ion motions and the ionization process. The intensity of gas ionization in the electric field can be characterized by the number of ion-electron pairs. The pairs are generated by charged particles per path unit in the direction of the field. These numbers are called ionization coefficients. In the case of electrons the coefficient is denoted by the α symbol, in the case of ions the β symbol is used. The electron ionization coefficient follows the relation:

(5.2)

, where E is the electric field intensity, p describes gas pressure, and A and B are coefficients related to a particular gas. For the anode current the following relation can be derived:

(5.3)

where I₀ is an initial current, d is a distance between electrods, and η is an ionisation efficiency, the number of secondary electrons from one ion impact. The Eq. 5.3 shows that the dependence between current and voltage is complicated. From the overall dynamics of the ionization processes for the V-A characteristics follows that at low currents (at the beginning of the discharge process) the voltage decreases to a certain value, the current passes through a minimum, and then the voltage rises non-linearly with the increase in current.

The described glow discharge V-A characteristics have regions with negative differential resistance. In the case of improperly designed external parameters, these regions may cause current oscillations. At low currents, even small parasitic capacitances of discharge tube structure and inlets are sufficient to the relaxation oscillations. The parasitic capacity is powered through large resistance from the power supply until the capacitor voltage reaches the ignition voltage of the discharge. The gas is ionized within the tube. The capacitor discharges and a glow discharge occurs. A significant increase in discharge current can be observed and this value is much larger than the value corresponding to the power supply resistor and the source voltage. After the capacitor is discharged, the current decreases again. The glow discharge is stopped and the whole process is repeated. In V-A characteristics, a minimum discharge current can be found for a particular experimental design. Below this value, the instability of the discharge and its extinction always occur.

If V-A discharge characteristics are measured by DC devices and relaxation oscillations are generated, then only mean current and voltage values are observed. These mean values do not provide any information about the real discharge process. Therefore, oscilloscope must be used to observe the oscillations of glow discharge and to measure its V-A characteristics.

Goal

Fig. 5.1: He-Ne source wiring for pulse mode operation.

Measure glow discharge V-A characteristics and determination of main macroscopic parameters of current and voltage. Determination of dependencies between active laser media (glow discharge) parameters and laser optical output.

Instructions

1. Connect He-Ne laser tube and voltage source according schematic on fig. 5.1. Observe temporal dependence of current and voltage on oscilloscope. When voltage on capacitor C get over latch voltage level, discharge is starting and capacitor is discharging trough resistor R to discharge. If current decrease below I_MIN then discharge is break and capacitor is recharging trough resistor R_n from high voltage source VN. The current is monitored on resistor R_I and voltage is measured on divider R_D1 and R_D2.
2. Measure glow discharge latch voltage level and characteristic current rise time. Measure the repetition rate of discharge.
3. 
   Fig. 5.2: Photodetector wiring.
   Measure glow discharge minimum current.
4. Record values of current and voltage for variable time of discharge and deduce V-A characteristics of glow discharge.
5. Connect power to PIN diode and place it into laser beam. The PIN diode is wired according to the diagram on fig. 5.2.
6. Measure temporal dependence of laser pulse power by using the PIN diode, simultaneously record current through the tube. Deduce laser power as a function of current through the tube, assuming the PIN diode sensitivity 0.5 A/W.
7. Switch laser to commercial power source, laser will be operated in cw mode.
8. Measure the laser power in cw mode

Requested results

1. The list of parameters: latch voltage, characteristic current rise time, laser repetition rate, glow discharge minimum current, output power in cw mode.
2. Table and paramatric graph of V-A characteristics of glow discharge.
3. The graph of laser pulse power temporal profile.
4. The graph of laser output power as a function of current through tube.

References

Chen, F.F., Introduction to plasma physics

theory of glow discharge

Yariv, A., Quantum Electronics

gas laser theory