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K-1 SERIES LASER SYSTEM Supplementary Information 1.0 INTRODUCTION The K-1 Series are Korad's medium-power laser systems. They are availaable with a number of optional accessories to enable flexible operation for a variety of applications. These systems can be in the conventional (long pulse) or Q-switched (short pulse). For Q-spoiled operation, Pockels cell and passive dye cells are used. Several types of power supplies are available dor different repetition rates. The laser head is designed for water cooling. All systems described in this specification are varieties of the basic K-1 series. For simplification the various K-1 types are identified by model numbers. The model numbers are: 1. K-1, Conventional Mode 2.0 SYSTEM DESCRIPTION 2.1 Mechanical and Electrical Requirements Input Laser Rod Electronics Enclosure Laser Head 2.2 Laser Head Assembly The laser head assembly consists of a housing, laser rod, a helical flashlamp, lamp sockets and reflector. The housing is precision machined from a rugged casting. It is designed to withstand the effects of repeated high energy firings and to hold the laser rod in precise alignment during lasing indefinitely. A special reflector is used to maintain high reflectivity with water. The lamp sockets receive the helical flashlamp essentially on a "plug in" basis and are constructed to carry the high currents and voltages required without over-heating or arcing. For convenient bench mounting of the laser head the housing is provided with tapped holes. 2.2.1 Laser Rods The K-1 laser head operates either with Nd. doped glass rods or rubies. Special ultraviolet shields are provided to prevent color center formation in the glass rods. The glass rods emit laser rediation at 1.06µ while ruby has the characteristic output of 6943 Å at room temperature. The optical quality of glass tends to be better than ruby and consequently results in better beam angle. One grade of glass and two grades of ruby selected on the basis of beam angle are offered. Rubies normally have a 60°C axis orientation and are 4 inches in length and 3/8 or 9/16 inches in diameter. Glass rods are 6 inches long and 1/2 inches in diameter. Other sizes can be provided depending on special requirements of the system. Rubies are finished so that the ends are parallel to within two seconds of arc and are flat to within a fraction of a wavelength. Glass rods are similarly finished. The final test of each laser rod is its performance in a laser system. All Korad laser rods are tested for threshold, output, beam angle, near and far field pattern. 2.2.2 Flashlamp A helical flashlamp is used in the K-1 Series for several important reasons. The arc length of the helix is over 28 inches--over three times the length of two straight lamps (the number usually suplied in straight lamp system of this size). This results in a higher impedance and lower current density for a given output energy. The higher impedance results in less line losses in delivering the stored energy to the helical lamp; also, the higher voltages require less capacitance for a given value of stored energy, resulting in a significant cost savings. The reduced current density results in a high degree of pumping uniformity and consequently a homogenous energy intensity in the laser output. With a helical lamp only a single lamp has to be triggered, whereas with two straight lamps there is always the possibility of poor synchronization if the lamps are in parallel, or uncertainty in firing if they are in series. 2.2.3 Cooling The K-1 laser head is cooled with clean water. This method gives good shot to shot repeatibility, as well as providing optical coupling between the lamp and the laser rod. Passively Q-switched systems require constant gain from shot to shot to match the dye concentration to avoid double pulsing. Since gain is dependent upon temperature, a dependable efficient cooling system is required for reliable operation. Several types of Korad cooling systems are available for K-1 systems. If optimum shot to shot repeatibility as well as high efficiency is required then a refrigerated temperature control system is required. All cooling systems are equipped with a pump, deionizer, heat exchanger and all necessary fittings and tubing. 2.3 The Optical Cavity The basic optical cavity is the head plus two external reflectors. For Q-switching the rear reflector is replaced with a Q-switch assembly and the front reflector becomes a resonant reflector rather than the dielectric mirror used for conventional mode operation. 2.4 Q-Switches Q-switches are classified into three categories according to mode of operation. passive cells are self switching and consist of bleachable dyes in appropriate solutions. Active switches are activated by the application of a voltage and are usually solid state Pockels cell. A rotating mirror can be considered a hybrid since actuation depends on an external driving mechanism but does not offer the degree of firing control attainable with an active Q-switch. All Korad Q-switches are incorporated in optical and electronic systems and are designed to operate with no pre or post lasing. 2.4.1 Passive Q-Switches Bleachable dyes for use with either neodymium doped glass or ruby are offered in cells of appropriate configurations. Passive cells are used when precise firing control is not required. They offer narrow linewidth and low cost. 2.4.2 Active Q-Switches The Pockels Cell is normally used with the K-1 Laser Systems. The Kerr cell has been used with great success with photographic equipment, but is not offered with Korad systems; with the high intensity of laser beams it has not been as reliable as the Pockels cell. The Pockels cell has allowed repetition rates of up to 1 pps. 2.5 Pulse Characteristics 2.5.1 Linewidth Control Linewidth can be decreased to practically single longitudinal mode operation by useing a passive cell and a transmission mode selector in a stretched cavity. Korad engineers can supply detailed information for the optimization of this parameter for special applications. 2.5.2 Pulse Shape There is some latitude in shaping the giant pulse usually at the expense of other systems parameters.Temperature of laser rod, dye concentration, output reflectivity and cavity length all play a role in the width and shape of the pulse. Korad engineers can advise you as how you can meet the requirements of your work. 2.5.3 Jitter The time uncertainty between the voltage pulse actuating the Q-switch and the appearance of the giant pulse is defined as jitter. The active Q-switch and driving electronics are designed as a system to result in jitter of less than 10 nanoseconds. Jitter for the passive dye switch is 50 microseconds. 2.5.4 Pulse Reproducibility In conventional mode at twice threshold or greater the K-1 pulse energy reproducibility is better than 5% at recommended repetition rates. Q-switched pulses are sensitive to temperature variations in the laser rod; therefore a refrigerated cooler should be used to obtain reproducible Q-switched output. Pockels cell Q-switching typically gives 10% reproducibility at up to 60 ppm with ruby and up to 2 ppm with Nd:glass. With a bleachable dye cell and the repetition rate for ruby limited to 10 pulses per minute, and for neodymium glass to 1 ppm, pulse reproducibility is ~ 20%. 2.6 Laser Electronics The electronic components of the laser system are contained in a sturdy, safety interlocked cabinet. The cabinet is mounted on four anti-shock casters and contains: A capacitor bank, charging supply, flashlamp trigger and shutter curcuits. The cabinet has additional space for future expansion. 2.6.1 Capacitor Bank The energy storage bank consists of steel encased oil filled capacitor and is located in the lower portion of the electronics cabinet. It is conservatively rated, resulting in long lofe and safe operation. The capacitors are designed to withstand current reversals through this condition does not occur in normal operation. 2.6.2 Pulse Shaping Coils The pulse shaping coil contributes to flashlamp life by limiting the initial rate of change of the discharge current. Coils of higher inductance are available if longer pulses are desired. 2.6.3 Charging Supply 2.6.3.1 K-1 Supply The K-1 power supply is designed for simplicity of operation and accuracy of control. It charges the capacitor bank in 15 seconds and is capable of repetition rates at full energy of 4 pulses per minute. The capacitor charging voltage is preset by the operator. This allows convenient and reproducible variation of the laser output. A solid state circuit detects the desired voltage on the capacitor bank and automatucally turns off the charging supply. The response time of this circuit is fast and allows for highly reproducible energy storage. Flashlamp reproducibility is better than 5%. Charging can be stopped at any time and the stored energy can be dumped through a resistor by pressing the "Dump" button. 2.6.3.2 K-15 Rep-Rated Supply The rep-rated charging supply consists of a constant current controlled source that delivers 2 amps up to 5 KV. A repetition rate control is provided which allows selection of rates from 4 to 60 pulses per minute or single shot operation. Charging voltage is pre-selected with a control on the front panel. 2.6.4 Flashlamp Triggering Circuits Lamp triggering is accomplished by the application of a high voltage pulse which ionizes the xenon gas, permitting the energy stored in the capacitor bank to flow into the lamp. The voltage of the triggering pulse is sufficiently high to overcome variations in characteristics during the life of the lamp. 2.6.5 Shutter Electronics (Pockels Cell) The Shutter electronics sub-assemblies providethe voltage required to perform the switching of the Pockels Cell. A pulse-on type of electronics is used with Korad Q-switched laser systems. A high voltage pulse is applied only at the moment of switching. This eliminates space charge effects which occur when a D.C. voltage is applied. The system provides an active optical shutter mechanism with no post or pre lasing and high reliability with excellent repeatibility. The shutter delay circuit accepts a trigger pulse derived from a current rate coil placed in the lamp circuit. After a given delay, adjustable from the front panel, the circuit generates a pulse which fires the active Q-switch. A synchronization pulse is available for external use. Alternatively, the firing of the avtive Q-switch can be synchronized with an external signal. The introduction of this external signal with respect to the firing of the flashlamp can vary ±100 microseconds without appreciable variation in the laser system's output. It can, therefore, be said that the giant laser pulse is "on-call" during an interval of approximately 200 microseconds. When the active Q-spoiler is switched, an external pulse of aproximately 100 volts is generated and made available at a BNC connector on the electronics panel. 3.0 CONTROL AND SAFETY FEATURES 3.1 Laser Head Safety The laser head is designed to provide complete protection from lamp failure and short circuits. No material can be ejected from the head, thus precluding the possibility of injury to personnel or dammage to equipment in the immediate area. 3.2 Cabinet Safety Energy stored at high voltage on the capacitor bank represents a potential danger to personnel. The guiding principle in safe operation is to severly limit access to the bank when the power to the main power supply is turned off, when the back door is opened or when a discharge button, located either on the front panel or on the remote control unit is depressed. High voltage cables and electrodes are covered so that personnel cannot come in contact with them in the event of a failure of the interlocking system proving additional safety. A shorting bar in the cabinet is provided to discharge manually the capacitor bank. This should always be used as a final safety precaution. Stored charge can be dissipated in a resistor by pressing the dump button. The charging and discharging of the capacitor bank and triggering of the flashlamp may all be performed through the use of a remote control unit which allows the user to operate the laser while observing experimental results from a remote location.
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