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TECHNICAL SPECIFICATIONS FOR KORAD K-2 SERIES LASER SYSTEMS 1.0 INTRODUCTION The K-2 series laser systems deliver intense outputs which are reliable and reproducible. Q-switched mode power levels in the gigawatt range with brightness of the order of 10^13 watts cm ^-2 steradian ^-1 are attainable. Output is limited only by the ability of the materials in the optical cavity to withstand the power intensities. For high reliability, power levels should be held to 500 megawatts. Q-switching is performed by either an electronically controlled pockel cell (Model K-2Q) or a passive bleachable dye cell (Model K-2QP). on conventional mode operation (Model K-2) the ruby can be pumped sufficiently hard to yield outputs of more than 150 joules. Korad laser heads are designed to permit tandem operation (Model K-1500) the reliable output is 1.1 gigawatts. Q-switching is available with a pockel cell (K-1500) or a passive bleachable dye cell (K-1500P). The K-2 series systems are designed to be used in laboratories requiring a wide range of operating conditions to accomplish experimental objectives. The modular concept used in the basic system design permits expansion of the basic K-2 to Q-switched operation or to the oscillator-amplifier combination (K-1500). All systems are designed to provide peak power and radiance commensurate with reliability possible from presently available laser materials and critical optical components. If reliability and long life is sacrificed, then the system can be operated at several times their rated capability. This versatility of the modular system design minimizes early obsolescence since as new requirements arise, the K-2 can be expanded or modified to meet the new needs. K-2 systems available now can include any or all of the following: Conventional ruby output of 150 joules Giant pulse operation with active Q-spoiler (pockel cell) to 500 megawatts (ruby) Giant pulse operation with passive Q-switch to 500 megawatts (ruby) Conventional operation with Nd-glass laser rods Giant pulse operation with active Q-switch (pockel cell) for neodymium Giant pulse operation with Korad proprietary passive Q-switch for neodymium dropped laser rods Expansion into oscillator-amplifier configuration for outputs in the gigawatt range Addition of Korad frequency multipliers for converting the output wavelength of ruby from 6,943 Angstroms to 3,472 Angstroms and Nd output from 10,600 Angstroms to 5,300 or 2,650 Angstroms In addition, a full line of accessory instrumentation such as autocollimators (K-AM), calorimeters (K-J), photodetectors (K-D1) and heat exchangers (K-WC1) are available 2.0 SYSTEM SPECIFICATIONS Operating specifications for the different models of the K-2 series lasers are given below. These measurements were taking using a ruby crystal at room temperature for output at 6,943 Angstroms. These are not the maximum outputs for this laser; but represent conservative values which will provide reliability and reasonable life. Slight variations in performance of production units may occur. PULSE ENERGY PEAK POWER PULSE WIDTH (at 50% power point) RISE TIME OUTPUT WAVELENGTH LINE WIDTH BEAM DIVERGENCE (measured at the half power pulse width - ruby) JITTER PULSE REPETITION RATE CONTROL CABINET SIZE LASER HOUSING SIZE MOUNTING INPUT 3.0 COMPONENTS 3.1 Head The laser head is constructed from heavy castings which are precision machined. The primary components of the laser head; are the laser rod, a flash lamp and a reflector. The laser rod is mounted in the housing surrounded in a closed-coupled optical cavity configuration by the helical flashlamp. A cylindrical reflector around the flashlamp aids in efficiently directing the flash tube pump light into the laser rod. The entire laser housing, including the region occupied by the flashlamp is filled with cooling water. The complete immersion in water improves pumping coupling, pulse repetition rate and prolongs the life of both rod and lamp. The laser head and optical attachments are assembled on a rigid mounting rail. This rail is machined to allow stable precise mounting. Retaining screws for the rail mounted components are accessible from the bottom. Adjustments or removal of individual components can be done without disturbing other components. Provisions are made for mounting the rail rigidly to any laboratory bench or special support fixture as desired. 3.1.1 Laser Rods Various laser rods are available to suit different requirements. The normal k-2 ruby crystal length is approximately 9" long and .625" diameter. It is cut with one Brewster end which of course eliminates reflection and therefore the need of an anti-reflection coating. The other reflective (non-Brewster) end of the ruby is used as the output reflector. Brewster/Brewster as well as Flat/Flat crystals are also supplied when requested. The ruby crystal is available in several qualities as primarily defined by the systems output beam divergence specifications. 3.1.2 Flash Lamp A helical flashlamp is used for several reasons. Helical lamps provide the most uniform crystal pumping of all the conventional methods of pumping and better beam uniformity and minimization of thermally induced strain are realized. Only a single lamp is necessary Even then, it is considerably underrated. The lamp is nominally rated at 30,000 joules. In the Q-switched mode it is operated at less than 10,000 joules; while no more than 25,000 joules is used in the conventional mode. The use of a single helical lamp allows significant simplification in the power supply system. That is, only a single capacitor bank and associated firing apparatus is required. The high impedance of the lamp, resulting from its long arc length, allows the use of long runs of cable since the energy loss within the cable is still slight. Efficient energy transfer from the capacitor bank to the flashlamp results. 3.1.3 Cooling The laser head is cooled by a flow of distilled, deionized water. The entire head is filled with coolant. This complete immersion provides improved optical coupling of the pump light to the crystal; no subsidiary surfaces are placed between the pumping lamp and the rod. The closer match of index of refraction between the water and crystal increases pumping efficiency compared to air cooling. Immersion of the flashlamp also serves to damp the motion imparted by the firing process. The lamp and its mount are relieved of the full stress that occurs in air firing. The laser system uses a water to air heat exchanger. At a repetition rate of one pulse per minute, only a gentle stream of cooling water is required; approximately 1/2 gallon per minute. The closed cycle cooling system is an option available with the system. Also available is a temperature controlling system which permits holding the temperature of the unit to within 1°F of the preset value. 3.2 Q-spoiling Q-spoiling the laser can be accomplished with active or passive devices. The specific laser application envisioned dictates which Q-spoiler is used. 3.2.1. Pockel Cell The Pockel cell is available in an active Q-spoiler. Korad's Pockel cell is in reality an integral assembly of four essential elements; a) Pockel cell crystal, b) Brewster polarizer stack, c) roof-top prism, d) potted electronics. The Pockel cell crystal is of either K-DP (Potassium Dihydrogen Phosphate) or K-D*P (Potassium Dideuterium Phosphate): K-DP is used for ruby only while K-D*P is used for both ruby and neodymium. The cell itself is submersed in a fluid which prevents any hydroscopic action which would render the cell useless. The fluid also matches the index of the refraction to keep losses to a minimum. The Brewster polarizer stack prevents pre-or-post lasing. The 90° roof prism is accurately cut to within a few seconds of arc to avoid degradation of the laser beam angle. Between the sychronization pulse to the firing circuit of the Pockel cell and the appearance of the giant pulse there is a delay of approximately 700 nanoseconds. The uncertainty (jitter) in the occurrence of the giant pulse is less than 10 nanoseconds. This low jitter is useful in laser applications requiring precise timing. 3.2.2 Passive Q-Switches A passive Q-switch is available for applications requiring extremely narrow linewidth (0.02Å or less). Although this switch does not provide the low jitter available with the pockel cell system, it requires no electronics and therefore reduces cost. A passive Q-switch is also available for neodymium Q-switching. 3.3 Optics The optical components of the laser are secured to holders which are firmly mounted on the optical rail to maintain optical alignment. The holders are equipped with orthogonal adjustments to facilitate optical alignment of the laser. The rail provides a conventional system to change the optical cavity. For example, when changing from Q-spoiled to conventional mode the operator; A) substitutes the Q-switching device (which has it's own 100% reflector) with a 100% reflector, B) inserts an output reflector to the output side of the cavity. The output reflector is needed since conventional mode requires a different output reflectivity. High peak power laser light destroys reflective coatings. Korad has eliminated them in the Q-switched laser. The entrance windows of both the passive and active Q-switch can be at the Brewster angle. The rear of the ruby can be terminated at a Brewster angle to eliminate any reflection at this ruby air interface. The output end of the crystal is normally flat. The air ruby interface at the output end can then be used as the output reflector of the oscillator; A resonant reflector is added if the spiking sometimes associated with the passively Q-switched pulse is undesirable and when increased reflectivity is desired. 3.4 Laser Electronics The electronics are contained in a ruggedly constructed, safety interlocked cabinet. The safety interlock prevents access to the electronics prior to discharging the energy bank. The cabinet is mounted on anti-shock casters. It contains the capacitor bank, the pulse-shaping coil, the charging supply, ignitron trigger circuits and shutter electronics (pockel cell Q-switch controls). The cabinet allows additional space for future expansion. 3.4.1 Capacitor Bank The energy storage capacitor bank is located in the lower portion of the cabinet. The bank is maid up of steel encased 10 KV oil-filled capacitors. High voltage quick disconnect interconnection panel. A shorting rod is provided as a final means of discharging the capacitor bank. The gravity-operated discharge solenoid discharges the bank whenever power is removed. Typical storage bank capacities are: 10,000 joules in the Q-switched and 25,000 joules with conventional mode. 3.4.2 Pulse-Shaping Network A pulse-shaping network limits the rate of increase as well as the magnitude of the current pulse to the flashlamp. It consists of an inductor wired in series with the flashlamp. This feature contributes significantly to prolonged flashlamp life and improves the efficiency of converting electoral energy to useable light energy. 3.4.3 Charging Supply An 800 watt 10-KV power supply charges the capacitor bank. A front panel knob provides preselection of the final capacitor bank charging voltage. An electronic circuit detects the desired voltage on the capacitor bank and automatically turns off the supply at the preselected voltage. The response time of this electronic circuit is fast, and allows for highly reproducible energy storage. If the electronic circuit should fail, a backup system set just above 10 KV provides the stop charge so that the bank cannot be charged beyond its rated value. This consists of a solid state comparator circuit which deactivates the charging circuit relay. Approximately 15 seconds are required to charge a 10,000 joules bank. 3.4.4 Flashlamp Triggering Circuits Since the lamps used in the K-2 laser system will usually break down at voltages lower than the storage bank voltage, a holdoff device is required. It has been determined that the most reliable type to use is an ignitron. An ignitron together with its firing circuit is standard equipment with all K-2 laser systems. Triggering of the lamp is affected by raising the potential of the lamp reflector to a level to cause the gas in the lamp to ionize. 3.4.5 Shutter Electronics The shutter electronics consist of the circuits required to actuate the Pockel cell. The design of the electronics is such that the voltage appears across the cell only at the time of firing. This "Pulse on Technique" greatly extends the life of the Pockel cell crystal. Actual switching of the cell is accomplished by applying a voltage pulse to "normally off" Pockel cell. The design provides fast stable triggering. A delay circuit adjustable from the front panel accepts a trigger pulse from the Rogowski coil of the lamp firing circuit. After a preset delay, a pulse triggers a small thyratron network which in turn fires a larger thyratron. The thyratron activates the potted electronics which cause the voltage pulse to appear across the Pockel cell crystal. This design provides extremely stable triggering. The jitter of the laser pulse is normally less than ±10 nanoseconds with the use of a thyratron. A synchronizing pulse is available from the Pockel cell control panel. Alternatively, the firing of the Pockel cell can be synchronized with some external signal derived from experimental apparatus. The variation in timing of this external signal with respect to the firing of flashlamp may be at least ±100 microseconds without any appreciable variation in the laser system's output. It can therefore be said that the laser pulse is "on-call" during an interval of approximately 200 microseconds relative to the shutter delay circuit output pulse. 4.0 SAFETY FEATURES A gravity-operated discharge solenoid provides a safety feature to insure that the capacitor bank is fully discharged when the power supply is turned off. This solenoid can also be controlled by the operator who at any tome may push the "discharge" button. This completely dumps the stored energy. The charging, discharging and triggering of the flashlamp may all be controlled through the use of a remote control unit attached to the end of a 10 foot cable. This optional unit allows the operator to have complete control of laser firing while observing the experimental results from a remote position. In keeping with Korad's continued emphasis on safety, the cabinet is completely interlocked. None of the high voltages are exposed when the rear access door is opened. It is actually necessary to remove protective covers in order to expose dangerous terminals. A shorting bar at the rear of the cabinet provides a means of manually discharging the capacitor bank prior to exposing any components. 5.1 Photodiode, K-D1 In many applications the efficient use of high peak power laser systems depends to a great degree upon ability of the experimenter to measure output characteristics. Korad has devoted a considerable amount of effort to the development of both power and energy measurement techniques in the high peak power range. Our high speed laser detector (Model K-D1) is especially suited for measuring very short high peak power pulses. Its response time is approximately 0.3 nanoseconds, thereby allowing an undistorted measurement of the time history of the output power pulse pattern (assuming that an oscilloscope with a sufficiently fast response is available). Circuitry is provided to monitor simultaneously the power and energy of the laser pulse. 5.2 Calorimeter, K-J The real basis for laser output measurements, is a calorimetric measurement. Korad has developed the K-J Calorimeter, especially designed to perform this kind of measurement on laser devices. It has been subject to through testing under a wide variety of conditions. These K-J Calorimeters have features which are unique in the industry -- they have a high absolute accuracy (absolute error less than ±3%), the ability of maintaining calibration over extended operation, and an ability to measure the energy of vary high peak power pulses. The fluid absorption medium used in the K-J Calorimeters does not saturate or otherwise change its absorption characteristics at power intensities up to 5 to 10^8 watts per square centimeter and energy levels from 0.1 to over 200 joules. 5.3 Frequency Multiplier, K-M Korad's K-M Frequency Multiplier is available as a K-2 series accessory. It can be furnished for use with a ruby or neodymium doped laser rods. These Frequency Multipliers give the capability to generate 3,470 Angstroms and 5,300 Angstroms wavelength respectively. This device is supplied with mounts and a precision cell alignment mechanism all located in a single housing. Reliable conversion efficiencies of 3-5% for neodymium and ruby respectively are easily attainable - while 8% to 10% is not unrealistic. 5.4 Alignment Apparatus, Autocollimator, K-AM In any laser system, installation changes or relocation of any components in the optical system necessitate a precise alignment. It is relatively simple to use the Korad K-AM autocollimator to achieve alignment required for peak performance. 5.5 Heat Exchangers, K-WC Korad has a complete line of heat exchangers providing from 500-5,000 watts of cooling depending upon your requirements. Proper cooling is essential to: a) reliability, b) repeatability and c) wavelength consistency. 6.0 RELIABILITY System components are designed and are constructed with materials selected to provide the highest possible reliability. Where feasible service conditions are considerably under manufacturer's ratings. 6.1 Capacitors Korad uses steel-encased oil-filled capacitors in the energy storage bank because of these proven reliability and protection personnel from hazards due to rupture or explosion. Their selection is based on the results of rigorous life testing and comparison with performance of other types of capacitors. 6.2 Flashlamps The helical flashlamp used in the laser results in decreased energy density due to the increased length compared to straight lamps. The pulse shaping coil limits the initial surge of current in the flashlamp, which increases lamp life. The lamp is supported by mounting brackets and immersed in water which reduces shock and vibration associated with firing the lamp. The helical lamp provides a more uniform pumping of the laser rod and results in more uniform beam intensities. 6.3 Electronics The electronics of Korad's laser consist of components carefully selected to give reliable service. In most cases standard, readily available parts are used. 7.0 MAINTAINABILITY The K-1 and K-2 series lasers have been designed to permit "ease of maintainability". Every component within the system provides sufficient room for accessibility. The control cabinet is constructed large enough so that the components within are neatly arranged to optimize space and are also accessible. The individual drawers, such as the shutter electronics and power supply can be removed for trouble shooting and conducting preventative maintenance. Interconnecting cables are provided to permit complete removal of drawers. Capacitors are connected with quick change interconnecting cables. The electronics are provided with external connection jack for monitoring and check out. The laser head construction permits simple removal of all the components. The crystal slides out to permit flashlamp removal. Since the flashlamp is helical, no alignment is necessary. Elliptical cavities require very delicate lamp alignment for proper pumping. The optics of the system are equipped with orthogonal adjustments. Mirror adjustments can be extremely time consuming. Orthogonal mirror adjustments allow an adjustment in the x-plane without a corresponding adjustment in the y-plane. The y-plane can then be adjusted separately without readjusting the x-plane. Korad ruby rods are supplied with holders, which allow easy initial alignment when spare crystals are installed. They permit handling the rod without touching the optical surface. Finger prints can be "burned" into the surface by the flashlamp. ----------------------------------------------------------------------------------------------- 1) R. L. Townsend, C. M. Stickly and A. D. Maio, Applied Phys. Letters, Vol. 7, No. 4, 15 August 1965. 2) B. H. Soffer and R. H. Hoskins, Nature, Vol. 204, No. 4955 17 October 1964. ----------------------------------------------------------------------------------------------- NOTE: All detailed specifications contained herein represent typical methods of design and manufacture to guarantee the critical laser output parameters of peak power, energy, beam divergence, linewidth, pulsewidth and wavelength. Korad must retain the right to change components, individual component specifications or specific details of construction from time to time without notice provided the key laser output specifications are not affected. |
