Q. 1. How long is the wavelength of YAG laser? Is it safe to see the laser light?
YAG laser is an invisible near infrared ray and its wavelength is 1064nm. If you see laser light, it is condensed by crystalline lens to reach retina, sometimes causing loss of sight; do not see laser light directly. Most of the laser processor is classified in the most dangerous Class 4 defined by JIS. As both beam and scattered beam are dangerous, don't see or touch laser light (Ref. 1). Laser processor is fitted with red guide beam so that you can see the point on workpiece to which the invisible laser beam is projected. Focusing unit equipped with CCD camera generally shows a cross-hair on monitor display. The intersection is the point to which laser beam is projected.
Q. 2. Advise us of the make-up of YAG laser equipment.
YAG laser equipment is composed of power supply, cooler, oscillator, optical fiber and output unit. Generally, power supply, cooler and oscillator are integrated into one unit. Since optical fiber transmits laser light, only the optical fiber and output unit are installed at manufacturing site. As one unit of laser equipment has several optical fiber deliveries, time-sharing and power-sharing deliveries are available. Time-sharing delivery is to transmit the laser through each optical fiber, depending on start signal given to each fiber. The number of deliveries is 1 to 6 maximum. Power sharing delivery is to transmit laser simultaneously through every fiber. The number of deliveries is 2 to 4, which are common usages.
Figure 1. Make-up of laser equipment
Figure 2. Beam delivery method of laser equipment
Q. 3. Why is the de-ionized water used in YAG laser equipment?
YAG laser equipment employing lamp excitation, installs YAG rod and excitation lamp in a chamber which are surrounded by a gold-plated elliptical reflector cylinder. To oscillate laser, excitation light generated by excitation lamp is projected to YAG rod. The excitation light also generates a great deal of heat, causing the excitation lamp, YAG rod and the inside of the chamber to be heated. Accordingly, cooling water is to be circulated. As ordinary water, however, has lower resistance than lamp tube, lamp current leaks to bypass the lamp. Resultingly, the lamp stops emitting excitation light. Therefore, YAG laser equipment with excitation lamp uses high-resistance de-ionized water to prevent leakage. Even thought de-ionized water circulates, metal ion resolves into the water in circulating path, reducing de-ionized water resistance. Accordingly, metal ion must be removed, using de-ionizer (ion-exchanging resin).
Q. 4. Laser installation site has no water supply. Do you have any countermeasure?
Laser equipment, having laser output of 70W or less, widely used for laser welding of electronic parts requires no water supply because air-cooled laser equipment lineup is available. On this type of laser equipment, cooling water is circulated through heat exchanger and fan cooled. Laser equipment, rated at more than 70W, employs a chiller cooler which keeps water temperature constant and circulates water using pump. It is composed of freezer with refrigerant and water-circulating path. Heat exchange between refrigerant and water is carried out through cooler.
Q. 5. Advise us of consumables for laser equipment.
Consumables are as follows:
(1) Output unit protective glass
(2) Excitation lamp
(3) Deionizer
(4) Water filter
(5) De-ionized water
Photo 1. Consumables
From front: (1) Protective glass (2) Excitation lamp (3) Deionizer (4) Water filter
Q. 6. How do we handle flashlamp, and how does it become when it deteriorates?
Like a halogen lamp of automobile, if you touch a flashlamp with your bare hands, fat will stick to it, causing the lamp to heat and break; accordingly you have to use a pair of clean gloves. Clean the lamp with alcohol when it is stained. Use a pair of protective glasses for safety if the lamp is filled with high pressure gas. When the lamp deteriorates, it will come unlighted or broken.
If you operate laser equipment with a high peak power, material of a flashlamp wall vaporizes and condenses repeatedly, crystallizing to form white powder. If you continue operating, this white coating film absorbs energy, burdening the lamp wall, and causes the lamp to break earlier.
If you operate laser equipment with a high peak power or a high average power, spatter of electrode material will cause aging of lamp. Black spatter sticks to tube wall, decreasing light output. Also aging of electrode causes the lamp to have a difficulty in lighting, or simmer current to be off, which ordinarily flows during lamp not flashing.
Q. 7. What kind of laser oscillation form does the laser have, and how is the magnitude (unit) of laser output expressed?
The laser oscillation form and the unit of laser are as follows:
| Pulse oscillation | Continuous oscillation | ||
| Continuous | Q-switched pulse | ||
| Excitation light source | Flashlamp (Xenon), | Arc lamp (Krypton), Laser diode (LD) | |
| Q switch | None | - | AO * |
| Pulse width | 0.1 - 100ms | - | 100 - 200ns |
| Pulse repetition rate | 1 - 500pps | - | 0.5 - 50kHz |
| Peak output | 10kW | - | 10 - 20kW |
| Average output | 600W | 50W | |
*Acoustic optics
Table 1. Classofication by oscillation form.
Figure 3. Oscillation waveform
The magnitude (unit) of laser is as follows:
- 1. In case of continuous wave (CW),
- Average output: W (watt)
- 2. In case of pulse oscillation,
- Energy per 1 pulse: J/P (Joule per pulse)
- Average output: W (watt)
- Average output (W) = Energy per 1 pulse (J/P) x repetition rate (PPS) where repetition rate is the number of repetitions per second expressed in PPS (pulse per second).
- The relation between peak output (W) and pulse width (sec) is as follows:
- Peak output (W) = Energy per pulse (J/P) / Pulse width (sec).
Q. 8. What is laser power feedback control? Advise us of its function and advantage.
As shown in Figure 4, the laser power feedback control is to pick up a portion of laser light by means of beam reflecting mirror and send it to power monitor for sampling. The sampled data is compared with reference waveform signal defined beforehand. Then the lamp current is controlled in real-time so that the difference is reduced to zero, resulting in constant laser output. Thus, laser output is stabilized in both short period of time and long one. The laser power feedback control eliminates lamp voltage control for laser output adjustment, in conventional control, responding to deterioration of excitation lamp. Also, the laser power feedback control adjusts laser output change due to heat-lens effect that occurs in seam welding and high-repetition-rated welding. Owing to this control, an idling run is not needed, that starts oscillation before welding to stabilize the oscillation and then open mechanical shutter to start welding.
Figure 4. Principle of laser power feedback control
Photo 2. Laser-power-feedback-controlled laser ML-2650B
Q. 9. Advise us of the laser output setting on pulse oscillation laser equipment.
Set pulse energy (J) from laser projecting time, i.e., pulse width (ms), and peak power (kW). Here, you can make arbitrary waveform for which you have two choices: a single shot and continuous shots. In the latter, specify repetition rate (PPS) and the number of shots additionally, i.e., the number of laser projections.
Figure 5. Example operating panel (ML-2650A)
Q. 10. What is waveform control?
Absorption factor of laser differs in many respects such as material of metal (Ref. 2), state of surface, melting state, component of metal plating. Iron-group material such as stainless steel absorbs laser light relatively well; therefore, a small laser power can melt such metal. Metal such as copper and aluminum less absorbs laser light at the beginning of projection; then, absorbs well when metal begin to melt. Pulse oscillation laser equipment projects laser light for 0.3 to 30ms. Metal melts during this laser-projection time, and begins to solidify abruptly when the laser projection ends; therefore, if heating and cooling processes are not conducted properly, various unfavorable phenomena occur, such as spatter, porosity, solidification crack, heat distortion (Ref. 3): spatter is what is generated when excessive heat is applied to metal being welded; porosity is that gas generated internally in metal and gas (air, shield gas) taken into metal being welded are sealed in melted metal, making the welded metal porous; solidification crack is what is generated when melted metal is cooled sharply. Waveform control (Ref. 4) is to change projecting laser output with respect to time and supply optimum heat, solving the above-mentioned problems.
Important issues in this control are adequate waveform shaping and accurate laser output; laser-power-feedback control makes them possible.
Figure 6. Reflectance characteristics of metal (Ref. 2)
Figure 7. Waveform-controlled laser waveform
Photo 3. Example welding using waveform control (Ref. 4)
Left: 5ms without waveform control (rectangular wave)
Right: 20ms with waveform control
Q. 11. Advise us of optical fiber make-up.
Optical fiber is composed of core and clad; and these are made of approximately 1mm-diameter glass (mainly quartz). For safety, industry-use fiber is sheathed in approx. 10mm-diameter stainless-steel flexible protective tube.
Two kinds of optical fiber are available: SI (step index) fiber in which core and clad are different by refractive index; and GI (graded index) fiber, whose core is germanium-doped, in which refractive index continuously varies (decreases) from center toward circumference.
In SI fiber, light propagates, repeating total reflection within@critical angle of reflection because refractive indexes of core and clad are different. In GI fiber, light propagates windingly and radially in core. Make-up of optical fiber, refractive index, and propagation characteristics are shown in the sketches below:
If an optical fiber is bent sharply, propagation loss (loss in propagating in clad, and loss in transmitting into stainless-steel tube) and poor light-condensing characteristics due to increased diffusion angle at fiber outlet end will result. Unlike common electric wiring, close attention must be paid to bending radius.
Figure 8. Make-up and light propagation in optical fiber
Characteristics of penetration in welding differ by optical fiber type: SI fiber presents a bowl-shaped penetration because laser light intensity (power density) distribution at its outlet is trapezoidal; GI fiber, a deep cone-shaped penetration because laser light intensity distribution is near Gaussian distribution. At present, SI fiber is more broadly used because it is more common and easier in maintenance.
Photo 4. Shapes of penetration in SI and GI optical fibers
Q. 12. Advise us of the caution in handling optical fiber.
Allowable bending radius of optical fiber is defined, corresponding to core diameter; so if a fiber is bent more tightly, it may be broken. Generally, allowable radius is 100mm for 0.1mm-diameter core; 200mm for 0.8mm-diameter core. When you remove fiber from such as laser equipment, output unit, use great caution in stain, dust, not sticking to fiber. As power density is highest at optical fiber end, the laser equipment may be damaged if dust sticks to it.
Figure 9. Bending radius of optical fiber
Q. 13. What kind of output unit do you have?
Output unit is to condense the laser light emitted from optical fiber. A standard straight output unit that condenses laser light transmitted from laser equipment through optical fiber to output unit; a multi-lens output unit that divides laser light into 2 beams by itself, processing at 2 sites simultaneously; and output unit with CCD camera that welds while checking the point being welded, are available. Select the unit by such as workpiece, focal distance. Recently, galvano-scanning output unit has come into the market, whose laser beam scans using 2 axes (X- and Y-axis) mirrors. This type of laser performs a high-speed welding.
Figure 10. Output unit with CCD camera
Figure 11. Galvano-scanning output unit
Q. 14. What are the " f " and " W.D." of output unit? Also, what is the protective glass used for?
" f " is focal distance of lens; generally focal distance of condensing lens.
"W.D." (work distance) is the distance between the external frame of output unit and workpiece. The protective glass is used to protect an expensive lens against such as stain, spatter; so, replace it when stained. Generally, such as an AR-coated white glass plate, BK7 glass is used.
Figure 12. Standard output unit
Q. 15. Advise us of the shape of beam at a workpiece being processed.
Laser light projected from output unit is condensed at the focus of output unit as the image of optical fiber diameter.
Spot diameter is expressed by the equation:
Spot diameter = Fiber diameter x f2/f1
where f1 is focal distance of the lens in output unit, that parallels laser light projected from optical fiber, and f2 is focal distance of the lens that condenses the parallel light.
Figure 13. Output unit optics
The equation shows that the spot diameter is variable with respect to optical fiber diameter and focal distance of condensing lens f2. Actually, laser power inputted to optical fiber which depends on diffusion angle of the laser light, such as workpiece to which laser light is projected, and construction of manufacturing equipment limit f2.
In addition, when you design a fixture, you have to take depth of focus into consideration in calculating allowance of work distance. When output unit condenses the laser light, distribution of condensed light is not symmetrical in front of and behind focus; therefore, depth of focus is defined as the range in which good welding is done. Generally, the depth of focus mentioned above remarkably differs from that of laser light having ideal Gaussian distribution of laser intensity.
Furthermore, you have to design such as fixture, considering the optical characteristics of output unit. For example, if you reduce spot diameter using f2, focal distance and depth of focus are to be shortened, and laser light projection angle comes to be an abtuse angle, causing spatter. If you elongate focal distance using f2, depth of focus also becomes longer and spot diameter increases; therefore laser power must be increased in proportion to spot diameter. Recently, laser equipment is furnished with some functions to accept optical fiber easily.
Table 2. Optical characteristics of output unit
| f2 | Focal distance | W.D. | Depth of focus | Spot diameter | Projection angle | Laser power |
| Long | Long | Long | Long | Large | Acute angle | Large |
| Short | Short | Short | Short | Small | Abtuse angle | Small |
* References
1.
2. After R. W. Ohsein "The Industrial Laser Annual Handbook 1990 Edition" Penn Well Books, D Belforte and M. LeittEds. , Tulsa, Oklahoma USA,(1990)
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