Standard Operating Procedure
Apollo/Polaris 22HD Double-Pulse Ruby Laser
For a summary of Operating Procedures, Click Here. Because of the unique, and complex, nature of this laser, this SOP contains a general description of the controls as well as the timing system.
Oscillator Rod Specifications from the test report from Saint-Gobain Crystals dated 6-19-01:
Serial # A20K, Boule # 22-3302-06
Size: 3.020" * 0.3756"
Doping: 0.05% +/- 0.005% Cr
Coatings: A/R coated (694nm) on both faces (0.18% measured R at each face)
For EO Modulator specifications, Click Here
The control panel is layed out in such a way that there are several functional groups or controls. The controls may be divided into the following categories. These are: MODE, STATUS, TIMING, and VOLTAGE. In addition, there are BNC receptacles for SYNC and TRIGGERS, a toggle switch for INTERNAL/EXTERNAL Pockels cell control, and a receptacle for plugging in a remote control box. Let us consider each of these in turn.
The STATUS section is made up of three internally illuminated pushbutton switches which indicate and control firing status. Power is turned on by a key switch. This inhibits operation of the laser system by unauthorized personnel. The CHARGE button is pressed to start charging the capacitor bank and remains illuminated as long as the system is charging. The FIRE button is lit (in MANUAL mode) when the system has reached full charge. The FIRE button will remain lit and the system will maintain its charge for approximately one minute. After this time, the system automatically dumps as a safety feature. At any time when the FIRE button is lit (and the safety key is set to the ARM position), depressing the FIRE button will trigger the laser. Pressing the DUMP button will cause the charge on the capacitor to be dumped through a dump resistor - In this case the flashlamp will not fire and no laser action will occur. The DUMP button may be pressed at any time to return the system to a safe condition. The DUMP button is lit whenever the system is in a DUMP status. One of the three buttons, CHARGE, FIRE, or DUMP, should be lit at any time (normally DUMP).
The mode section has three buttons which provide the following modes of operation:
Voltage controls are divided into two types: Voltage for the main power supplies or the capacitor banks, and voltage for the Q-switch.
All timing signals in the Apollo laser system are derived from a quartz crystal oscillator operating at 1 Megahertz. This oscillator drives digital countdown circuitry which, in conjunction with digidial timing controls on the front panel, enable the user to specify the timing of flashlamps and Pockels cell firing to within 1 microsecond. The timing sequence can be understood with reference to the following figure.
In this figure, the zero is defined as that time when the FIRE button is depressed or when the system is triggered with an external pulse. At this instant, a pulse is generated internal to the system. This pulse is called the Master Sync Pulse and all subsequent timing events are measured with respect to this pulse. In single pulse Q-switched systems, the Pockels cell is always fired 1000 microseconds after the Master Sync Pulse. The flashlamps, of course, for the oscillator and amplifier must be fired prior to the firing of the Pockels cell in order to pump the laser rod. The user can select the flashlamp firing time by means of the lamp trigger delay digidial switches on the front panel. The optimum firing time for flashlamps depends on the particular capacitance value of the main capacitor bank. This optimum time is easily found by varying the lamp trigger delay setting and plotting the laser output as a function of this parameter. For most systems, however, a typical value would be a dial setting of 300 to 600 microseconds after the Master Sync Pulse. Since the Pockels cell is fired 1000 microseconds after the Master Sync Pulse, 400 to 700 micro- seconds are available for the flashlamp to pump the laser rod. In double-pulsed Q-switched systems, one of the Pockels cell firing times is fixed at 1000 microseconds and the other one is variable by means of a front panel control. This control also reads directly in microseconds from the Master Sync Pulse.
Several BNC connectors provide sync pulses at the bottom of the control panel. The MASTER SYNC is a positive 10 volt pulse that is generated when the FIRE button is pressed - this pulse is most often used as a trigger for an oscilloscope to capture the output pulse(s). Q-SWITCH SYNC pulses are also available. These pulses are associated with the firing of the thyratron. The time between the sync pulse and the appearance of the laser pulse itself is 600 - 900 nanoseconds with a jitter of approximately +10 nsec.
Synchronization of the laser to external experimental events is made possible by using the BNC inputs labeled TRIGGERS at the bottom right- hand corner of the control panel. When the toggle switch labeled INT/EXT is in the INT (internal) position, application of a positive 10 volt pulse to the BNC labeled MASTER TRIGGER will cause firing of the entire laser sequence, just as if the FIRE button had been manually depressed. When the toggle switch is in the EXT (external) position, application of a positive 10 volt pulse to the MASTER TRIGGER will cause the flashlamps to be fired in the usual fashion with the timing shown on the digidials. However, the Pockels cell will not be fired. It is now necessary to introduce a positive 10 volt pulse at the Q-SWITCH TRIGGER BNCs in order to fire the Pockels cell. This feature permits synchronization of the laser firing relevant to external experimental events with low jitter.
A receptacle is provided at the bottom righthand corner of the control panel for a remote control unit which allows charging, firing and dumping of the laser from a remote location.
Four indicators on the lower-left corner of the panel show open interlocks as follows:
When the power supply is turned on the initial status ot the system should always be in the "DUMP" mode, i.e. the dump button should be the only one lighted on the control panel.
On depressing the charge button the following sequence of events will occur:
The system is now ready to be fired. If no "FIRE" signal is received, the system monitors the capacitors voltage and recharges if it falls below 1.0% of the desired voltage. The system will continue to do this for a period of 1-minute at which time the system dumps itself for safety reasons.
During the "CHARGE" mode the Pockels cell electronics are also being charged to a value dependent on the Q-switch dial setting on the control panel.
Depressing the "FIRE" button on the control panel initiates the 1-msec firing sequence. In the firing sequence the lamps are first fired followed by the triggering of the Q-switches. The time of firing of the lamps is determined by the dial settings on the control panel. Similarly the triggering of the #1 Q-switch is dependent on the corresponding dial setting. The second Q-switch is triggered at the end of 1-msec.
The lamps are fired after a time delay (in ms) set by the digi-dials on the control panel. The clock card sends a pulse at the dialed-in time to the lamp trigger card, which in turn generates a negative 400 volt pulse for each lamp. The pulse causes the first of two ignitions to become, thus providing a path for the ionization capacitor to dump into the lamp causing preionization.
As the ionization capacitor discharges its voltage becomes equal to the voltage of the storage capacitor. At that instant the second ignitron turns on. The stored energy is then dumped via this ignitron through the PFN (Pulse forming network) into the flashlamp.
The first of two pulses is applied to the Pockels cell at the time pre-set on the digi-dial. This pulse turns on the Q-switch and the first laser pulse is emitted from the cavity. At the end of lmsec the Pockels cell is again turned on, and the second laser pulse is emitted. The voltage pulses for the Pockels cell are initiated at the Pockels cell trigger card. Each trigger pulse from the card causes the isolation capacitor (in box on rail) to be grounded through the thyratron, creating a corresponding pulse of 400 nsec duration across the KD*P Pockels cell. This completes the laser firing sequence. Shown in the following figure is a schematic and waveform of the Pockels cell circuitry.
The following oscilloscope output, captured using a high-speed silicon photodetector on the laser output, shows typical timing of a single Q-switched laser pulse. At t=0 the laser is fired and the oscilloscope triggered ... the trigger point is 200μs to the left of the screen and so is not seen here. In this case the OSC LAMP delay setting is 350μs and a spike appears when the lamp fires (electrical noise generated by the ignition process). Similarly, the AMPLIFIER lamp fires at 600μs. The baseline is seen to rise gradually at this point from (amplifier) pump light which leaks from the amplifier housing (scattering from the rod) striking the detector. The Q-switch delay is set for 900μs which is where the first laser pulse appears - if the Q-switch voltage is set properly. The laser pulse does not occur here - only the electrical 'spike' caused by the pulse generator. The fact that the 'spike' is not actually laser output can be verified by the fact that the height is proportional to the applied Q-switch voltage throughout the entire range! The second laser pulse (QS2) is fixed in time and occurs at 1000μs with the spike seen here.
In the lower photo, the trace is re-adjusted so that the trigger point is 100μs to the left. Pulse #1 is not firing here (the thyratron misfired in this example) and pulse #2 is seen to produce lasing - evident here by the fact that the optical output not only 'spiked' (the width of the laser pulse is about 50ns and the horizontal scale is 100μs per division so the laser output appears as a 'spike' in the figure), but the detector also saturated with the intense energy of the pulse.
Two distinct regimes of pulse separation times can be identified. For short pulse separation from 200 nsec to about 50 μsec, the flashlamp does not appreciably repump the laser rod between pulses. Thus, the total stored laser energy available for use is that which is present in the rod at the time the first pulse is emitted In general this energy may be distributed between the two pulses in an arbitrary manner. In order to produce a sequence of two pulses, it is necessary to apply the first Pockels cell voltages to only open the Q-switch part way and leave some energy for the second pulse. The long pulse separation regime is characterized by pulse separations of 100 μsec or greater. For these times, the rod will be sjgnficantly repumped between pulses. The Pockels cell voltage may then be adjusted to give optimum Q for both pulses.
Once the desired pulse separation has been set, there are only three control settings which must be varied to obtain two Q-switched pulses of equal energy. There are: vary the timing of the flashlamp with respect to the first Pockels cell pulse; vary the input energy to the flashlamp; and vary the Pockels cell voltage on Pockels cell #1 (Pockels cell #2 on extended pulse separation systems). It is assumed that Pockels cell #2 (Pockels cell #1 on extended pulse separation systems) is always set to optimum. In the case of short pulse separations we may use the following procedure to obtain the desired pulse train. The attached curves are helpful in arriving at initial settings.
For long pulse separations of 100 to about 500 μsec, both Pockels cell voltages may be set for optimum. Energy is distributed between the two pulses in this case by varying the lamp timing control. It may also be necessary to vary the lamp voltage control.
Pulse separations of greater than 500 μsec can be achieved with extended pulse separation systems. The technique involves firing the oscillator lamp twice in succession with two separate capacitor banks. The timing for the second capacitor bank (Bank #2) is variable from 500 μsec to 1 sec; thus, pulse separations up to 1 second are possible.
Type: QX1020, Serial # 9214
Electrical Specification (Halfwave voltage vs. polarity @ 633nm):
Halfwave Voltage at User Wavelength (λuser):
V1/2 - User = V1/2 - 633 nm * (λuser / 633nm)
Preparing the water cooling system:When the system has been sitting for a period of time, stagnant water in the system becomes contaminated and conductivity of the cooling water becomes low. Each time before running the system the water cooling system must be purged and refilled with fresh deionized water.
STARTING the laser
OPERATING the laser (summary)
SHUTTING DOWN the laser
Water Cooling System
The water cooling system must not incorporate any brass fittings - only stainless steel or plastic must be employed in the system. Main coolant lines are 'food safe' poly lines. The chiller has been modified with a lowered intake allowing almost complete draining of the reservoir tank. Tees bring DI water from the V13C (cleanroom) system into the system. A low-flow GEMS flowswitch (0.1 gpm min) is installed in the loop immediately beside the chiller. The flowswitch is powered from 24 VDC from two adapters and a simple contact closure indicates adequate water flow. Electrical connection is made to the laser power supply via a specially-keyed 5-pin DIN connector.
Plumbing inside the laser rail features two valves allowing the isolation of the amplifier from the rest of the coolant system. Should the experimenter desire, the amplifier rod could be substituted for the oscillator rod by isolating the coolant system and re-installing the amplifier housing where the oscillator normally resides.
Periodically the chiller reservoir must be cleaned thoroughly to remove particulate matter which accumulates.
Power Supply System
Interlocks were added to comply with CDRH and ANSI standards. As shipped, the unit featured only a single interlock loop for the power supply door and the rack. A single relay under the main chassis (lower left side) is used to indicate interlock status - the coil has a white/org and white/red wire. The white/red wire connects to the interlock switches which are in series (Green ground -> switches in series -> white/red relay coil). Unregulated Power is derived from the logic board (Drawing 100388) in slot J1. Additional interlocks are added on the (-) side of the relay coil as a series circuit incorporating the water flow sensor, two external inputs (one on the back of the power supply - intended for the room door interlock, one at the laser head - intended for a class-I interlocked experiment), and a cover interlock switch.
If the dump solenoid fails to pull-in, or pulls-in intermittently (this is often seen as a failure for capacitors to reach full charge), replace the PUMP relay (a Potter&Brumfield K10P-11D15-12 unit) under the HV chassis (pull the supply out of the cabinet half-way). This relay is socketed.