Panatron, Inc. - Model K2 Series Laser System

TECHNICAL SPECIFICATIONS MODEL K-2 SERIES LASER SYSTEM

1.0 INTRODUCTION

The K-2 series laser systems deliver intense outputs which are reliable and reproducible. Q-switched mode power levels in 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). In conventional mode operation (Model K-2) the ruby can be pumped sufficiently hard to yield outputs of more than 150 joules.

Kordad laser heads are designed to permit tandem operation of two or more heads. In the oscillator-amplifier configuration (Model K-1500) the reliable output is 1.1 gigawatts. Q-switching is available with a pokel 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 Kordad proprietary passive Q-switch for neodymium doped laser rods.

Expansion into oscillator-amplifier configuration for outputs in the gigawatt range.

Addition of Kordad Frequency Multipliers for converting the output wavelength of ruby from 6943 Angstroms to 3472 Angstroms and Nd output from 10,600 Angstroms to 5300 or 2650 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 taken using a ruby crystal at room temperature for output at 6943 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

Conventional mode.................................35 joules nominal with basic capacitor bank.

150 joules with expanded capacitor bank.

Q-spoiled mode......................................5-8 joules nominal with either pockel cell or passive Q-switch.

PEAK POWER

Conventional mode................................400 kilowatts.

Q-spoiled mode......................................500 megawatts for pockel cell or passive cell.

PULSE WIDTH (at 50% power point)

Conventional mode................................500 microseconds typical.

Q-spoiled mode......................................10-20 nanoseconds typical.

(Longer pulse widths to 100 nsec are possible at reduced output. Available on special order.)

RISE TIME

Conventional mode................................Not applicable.

Q-spoiled mode......................................5-7 nanoseconds with pockel cell or passive switch typical.

OUTPUT WAVELENGTH

Conventional mode................................6943 Angstroms (ruby), 10,600 Angstroms (glass)

Q-spoiled mode......................................6943 Angstroms (ruby), 10,600 Angstroms (glass)

LINE WIDTH

Conventional mode................................0.3 Angstroms nominal (ruby), 50 Angstroms nominal (glass).

Q-spoiled mode (ruby)

with passive cell.................................0.02 Angstroms.

with pockel cell...................................0.02 Angstroms per spectral line

BEAM DIVERGENCE (measured at the half power pulse width - ruby)

With standard ruby (48" cavity) 4 milliradians half angle, 1/2 power points.

With selected ruby (48" cavity) 2.5 milliradians half angle, 1/2 power points.

Selection among "selected" rubies is also available when even better beam divergence is required (down to 1.7 mrd 1/2 angle).

JITTER

Q-spoiled mode

with passive cell..................................±50 microseconds

with pockel cell....................................±10 nanoseconds

PULSE REPETITION RATE

Water-cooled system (required) 1 to 4 ppm depending on mode of operation.

Higher repetition rates are available by use of higher rated power supply and associated components.

CONTROL CABINET SIZE

Height.........................................................62 inches

Width..........................................................34 inches

Length.........................................................32 inches

Weight.........................................................1400 pounds

LASER HOUSING SIZE

Height..........................................................5 1/4 inches

Width...........................................................4 3/8 inches

Length..........................................................17 inches

Weight.........................................................15 pounds

MOUNTING

The control cabinet is mounted on 4 anti-shock casters for ease of movement of the laser in the laboratory.

INPUT

....................................................................115V, 60 Cycles, 25 Amperes

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 flashlamp 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. 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 %/8" 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 defind by the system output beam divergence specifications.

3.1.2 FLASHLAMP

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 incluced strain are realized. Only a single lamp is nessary. 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 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 relived 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 as an active Q-spoiler. Kordad'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 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 refraction to keep losses to a minimum. The brewster polarizer stack prevents pre or post lasing. The 90° roof prism is accurately cut within a few seconds of arc to avoid degrading of the laser beam angle.

Between the synchronization pulse to the firing circuit if 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 secured to holders which are firmly mounted on the optical rail to maintain optical 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 its 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. Kordad has eliminated them in the Q-switched laser. The entrance windows of both the passive and active Q-switch can be cut 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 ciol, the charging supply, ignitron trigger curcuits 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 made up of steel encased 10KV oil-filled capacitors. High voltage quick disconnect interconnecting fittings are used between the capacitor bank and an interconnection pannel. A shorting rod is provided as a final means of discharging the capacitor bank. The gravity-operated discharge solinoid 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 electrical 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 charging so that the bank cannot be charged beond 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 CIRCUIT

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 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 rasing 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 consists 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 appling 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 thryratron 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 the 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-cell" 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 time 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 Kordad'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.

5.0 ACCESSORIES

5.1 PHOTODIODE, K-D1

In many applications the efficient use of high peak power laser systems depends to a great degree upon the ability of the experimenter to measure output characteristics. Kordad has devoted a considerable amount of effort to the development of both 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 nanosecond, thereby allowing an undistorted measurement of the time history of the output 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. Kordad has developed the K-J Calorimeter, especially designed to perform this kind of measurement on laser devices. it has been subjected 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 very 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 x 10° watts per square centimeter and energy levels from 0.1 to over 200 joules.

5.3 FREQUENCY MULTIPLIER, K-M

Kordad'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 3470 angstroms and 5300 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 kordad K-AM autocollimator to achieve alignment required for peak performance.

5.5 HEAT EXCHANGERS, K-WC

Kordad has a complete line of heat exchangers providing from 500-5000 watts of cooling depending upon your requirements. Proper cooling is essential to: (A) reliability, (B) repeatibility and (C) wavelength consistency.

6.0 RELIABILITY

System components are designed and are constructed with materials selected to produce the highest possible reliability. Where feasibile service conditions are considerably under manufacturer's ratings.

6.1 CAPACITORS

Kordad 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 lamp. 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 kordad's lasers consists 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 accesible. 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 seperately without readjusting the x-plane.

Kordad 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 surfaces. Finger prints can be "burned" into the surface by the flashlamp.