ON THE EM1 POTENTIAL OF VARIOUS LASER TYPES

M.S. Bhatia* and Girish Kumar **

*L&PT Division, BARC, Mumbai - 400 085, INDIA. **Electrical Engineering Department, IIT Bombay, Powai, Mumbai - 400 076, INDIA.

ABSTRACT - The electrical activity in any laser leads to EM1 emissions, which needs to be controlled if the overall system is to exhibit compatibility. This paper lists the EM1 potential of various laser types and gives some reasons for origin of electrical noise.

1. Introduction

Since its invention in 1960, laser has emerged as a powerful tool that solves many diverse problems in industry, laboratory, medical, warfare and consumer areas. The application areas in which laser has broken new grounds are increasing day by day and this calls for newer laser media and of course new laser designs and technologies. A significant point here is that no single laser type can satisfy all the needs and thus each application requires a different laser. The different laser types are classified in regard to their electrical activity so that their EM1 potential can be assessed.

2. Classification of lasers

Lasers have been classified in various ways depending on the end use. The most general classification that reveals maximal information pertains to the state of the lasing medium. The laser media is medium in which lasing action takes place, it come in all the three phases - solid, liquid and gas. The physical construction, electrical power feed and output power as well as ability to operate in continuous or pulsed mode are distinct for all the three types of lasers. For example, the high average power lasers are invariably gas lasers and this fact can be explained in the following fashion. Gaseous media can be cooled in the best manner and thus they can handle large powers. COn lasers are the best example in this category. The ability to operate in continuous or pulsed mode is again widespread in gas lasers. This can be understood as follows; for laser action to terminate, it is necessary that the lower level involved in the population inversion should depopulate fast enough to enable repetitive pulsing. This is easily possible in gaseous media as the atoms/molecules/ions can de-excite by collisions with themselves or walls. However, most solid state lasers (except diode laser) generally operate in pulsed mode and this is due

to poor heat removal properties of the solid laser medium. Table 1 tabulates some important laser types in each phase. The operation mode and power capability of the laser is indicated with the help of symbols attached along the laser type.

Each of the lasers listed in Table 1 operates at a particular wavelength and this calls for a second classification based on the wavelength region in which the laser light falls; infra-red, visible and ultraviolet. This classification is shown in Table 2. This classification is very useful to the people who are looking for specific application areas. For example, the laboratory physicists look for lasers that can operate in certain wavelength regions - the wavelength region in which they want to do spectroscopic investigations. Similarly, doctors who use lasers for specific end goals find that their jobs are done in the best possible manner with lasers that operate at specific wavelengths. Numerous texts and books have been written to enlighten us to this fact and therefore we can again stress that diversity in laser -types is likely to stay and no single laser can meet all the requirements.

3. Factors that contribute to EM1 potential

Experience on working with many laser types over the last two decades has led us to speculate on the factors that play the major role in EM1 activity of any laser. The factors (not in order of their priority) are as follows:

a) Physical construction, b). Operating power, c) Wall plug efficiency, d) Operating mode, e) Pumping method, f) Accessories used.

The physical construction of a laser plays an important role in deciding its EM1 activity. The more voluminous or long dimensioned lasers tend to be better suited for radiated EM1 emissions (particularly so with pulsed operation). Most gas lasers fall into this category and their construction generally is compartmentalized into two sections coupled by a long high power cable (Figure 1). Typical dimensions are in the range of a few meters and average output powers can be

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as high as a few kilowatts. In contrast, most solid state lasers have a single housing for them and are more compact. Such a construction has been found to be less troublesome as an EM1 emitter.

b) The operating power of any laser decides the total electric power input (efficiency being fixed). High power lasers not only radiate more EM1 but also contribute to more condu’cted noise.

c) A point that is not generally appreciated is the role of conversion efficiency of a laser. Barring CQ laser, most lasers have conversion efficiency of less than one percent. Some lasers have a figure of less than 0.1 percent. Thus, for a one watt laser output, one kilowatt of electrical input is needed in the some cases. This higher electrical activity leads to larger EMI. By contrast, tungsten lamps and halogen lamps are more efficient and thus less EM1 prone.

d) Both continuous or CW mode and pulsed mode operation are possible in many lasing media. Of the two, continuous mode lasers are less noisy as they operate in a steady state and thus low frequency spectrum in their electrical activity. They exhibit more noise during start-up and shut down phase. In contrast, pulsed lasers generate more EM1 (both radiated and conducted) and this increases with the pulse energy and also with shortening of pulse duration (short rise times lead to higher frequency spectrums). If repetition rate is high, the noise activity is further enhanced.

e) The optical pumping method (means for exciting the lasing medium to enable population inversion) contributes to EM1 activity. Gaseous discharge, a popular method of pumping employed in gas lasers leads to more EM1 as the electrical activity in a discharge is quite complex and involves many electrical phenomena occurring at high frequencies. COn laser and Cu vapor laser are two examples in this category. In contrast, the flash lamp pumped Nd-Yag laser is less noisy.

9 The accessories used with the laser such as vacuum pumps/blowers with gas lasers contribute to the overall EM1 activity. Further control electronics and metering equipment also add to this.

4. Conclusions

Considering the diversity in laser types and the applications in which they are used, an attempt has been made to evaluate the factors that contribute to make a particular laser more or less noisy in regard to EMI. For example, it is now easy to point out that high repletion rate pulsed Cu vapor laser is expected to be more noisy than say a ruby laser or a diode laser. Further work on detailed EM1 measurements can further lend quantitative support to the assertions made in this paper.

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Table 1

Classification of lasers on the basis of Dhase of the lasina medium

Solid Liquid Gas

Ruby laser (P) Nd-Yag laser (P) Diode laser (CW & P)

Dye laser (CW & P) He-Ne laser (CW) CO2 laser (CW & P)# Ar' laser (CW) Eximer laser (P)* Cu vapor laser (P)+

# High average power * High peak power + High repetition rate.

Table 2

I nf ra-red

CO2 laser Nd-Yag laser Diode laser

Classification of lasers on the basis of the wavelenath of laser liaht

Visible Ultra-violet

He-Ne !aser Eximer laser Ar ion laser Cu vapor laser Dye laser

Fig. 1 : Construction of a gas laser. (1) Laser head, (2) Cable and (3) Laser power supply.

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