Introduction

Pictured above, the system consists of the laser head, an LU660 remote control (on the table), control rack (under) and optional delay generator (on top of the control rack).

Statistics

Hazard Analysis (1064nm operation only - Worst possible case)

Hazard Analysis (1064nm operation only - Practical Limit)

Hazard Analysis (532nm operation)

Controls

The control rack consists of four distinct units as shown. The lower unit (Cooling Group CG603) handles cooling of the laser head. Inside the CG unit a pump circulates deionized (DI) water from an internal reservoir through the laser head and then through a heat exchanger where heat is dumped to flowing tap water. Interlocks are provided in this unit for DI water flow as well as reservoir water level. DI water must be periodically changed - to facilitate this the reservoir features a dump valve and a fill hole. Both are evident when the CG unit is pulled out of the rack.

The power unit (Power Unit PU620) supplies high voltage (800V - 2kV) to charge the capacitor bank of the unit. It also generates the firing signal for the lamp and Q-switch synchronization. Signals from the (Supply Box) SB660 control unit arrive via a DB-9 connector at the rear of the PU unit which then produces a 24V firing signal fed to both the CB630 simmer capacitor bank (to fire the lamp) and to the CU control (to synchronize the Q-switch). Indicators show both power and fault in the unit. This unit has a separate circuit breaker at the rear, accessible only by removing the white cover on the rear of the control stack. Keep the lamp voltage between 1kV and 1.2kV for most experiments - NEVER EXCEED 1.5kV

The CB630 is a capacitor bank with pulse-forming network. Two main capacitors and a pulse-forming network feed the lamp with up to 20J pulses. A series thyristor (SCR) is used to fire the lamp on command.

The SB660 control unit provides Q-switch and firing control via the LU660 logic unit. When the front panel selector on the SB660 is in the EXT position, the Q-switch is fired on command from external signals provided via the BNC jack on the front. When in the INT position, the LU660 provides firing signals for the Q-switch.

Controls on the LU660 are divided into two sections: Firing-rate controls and Q-switch controls. Lamp firing rate may be set to FIXED (the maximum rate), VAR (an intermediate rate), EXT (external control) or MANUAL. MANUAL firing requires the user to press the CHARGE button then the FIRE button in sequence for each pulse. Indicator lamps show when the capacitor bank is ready to fire. Normally, the laser is used in VAR firing mode.

Q-Switch synchronization on the LU660 is accomplished by controls on the right side of the panel. The Q-switch can be made to fire on every lamp pulse (F), every-other pulse (F/2), every fifth pulse (F/5) or manually on the very next pulse each time the FIRE button is pressed. For F, F/2, or F/5 operation the 'SS NORM' switch must be pressed. Q-switch operation in independent of firing rate control and the red 'ON' lamp lights when the internal Q-switch control is active.

The internal Q-switch control has the (usually undesired) feature of a delay between the start of lamp firing and Q-switch operation. With the lamp firing repeatedly, eight seconds will elapse before the ON indicator lights and the Q-switch is active. To overcome this design feature, an external delay may be used to synchronize the Q-switch. The delay box is connected with the 24V trigger signal from the PU unit as an input ("zero time" defined as the point at which the lamp fires) and the output from the delay box fed to the EXT Q-switch input on the front of the SB660 unit (with the switch in the EXT position). A nominal delay of 200 microseconds is required between lamp firing and Q-switch opening - this delay may be adjusted for maximum output. With an external delay, all firing controls will produce immediate Q-switched output including the manual (CHARGE/FIRE) controls.

Where harmonic operation is desired, the appropriate shutter on the front of the laser is opened and the harmonic selected via switch on the laser head. Selecting the desired harmonic has the effect of inserting wavelength-selective dielectric mirrors into the beam output path to separate the wavelength components into separate beams. The harmonic crystal is then phase-matched using the appropriate switch which changes the angle of the crystal relative to the optical axis of the laser.

Internal Structure and Optics

This laser consists of a flashlamp-pumped YAG rod (6mm diameter, 115mm length) in an optical cavity consisting of two Brewster polarizers (one on either side of the rod, an electro-optical Q-switch, and two output couplers. Infrared (1064nm) radiation can be emitted from either end of the laser cavity. When configured as an OC, radiation from OC-1 is emitted towards the front of the laser where it may be used inside the laser and passed through the shutter (shutter #1) to become an infrared output beam. When configured as an HR, this beam is absent. Currently this is configured as an HR

Radiation from OC-2 (the primary OC) passes through the non-linear crystal and becomes the SHG component depending on the phase matching condition. With SHG (532nm) selected (via the switch on the laser head), light emitted from the crystal is reflected from two wavelength-selective (dichroic) mirrors which removes the infrared component then through shutter #2 to become the SHG output beam. The system is shown in this position in the photo to the right which details the non-linear crystal and wavelength-selective mirrors. Note that in this condition infrared (1064nm) output is available from shutter #3.

Two motors are also visible in the photo: one adjusts the angle of the crystal to phase-match the harmonic light and the other moves a small deck with the two wavelength-selective mirrors attached. With THG/FHG selected ('355/266' position), the mirrors are moved out of the optical path allowing generated SHG radiation (as well as 1064nm) to pass to the THG/FHG crystal (which is not currently installed in this laser). When the THG/FHG crystal is not installed, the 'raw' output from the laser (a mixture of IR and SHG radiation) passes through shutter #3 and desired components must be separated with external optics.

Operating Procedure

STARTING the laser

1. Don safety glasses suitable for a Q-Switched YAG laser as well as any harmonics in use.
2. Turn the cooling water supply ON. The valve is located behind and to the right of the laser. Water flow may not be evident as it is thermally-controlled: water flows only when the closed-loop DI water system cooling the lamp reaches 85 F.
3. If not already on, turn the three-phase supply (breakers in the black panel) ON.
4. Turn the main breaker behind the laser ON. The breaker is located on the right-rear of the control stack near the main power cord
5. Verify that the FAULT indicator on the PU620 unit is not lit. If it is, a reset is required in which the entire unit is shutdown and powered-on again.
6. Verify that the SECURITY indicator on the LU660 control is lit. If not, an interlock is open. Probable causes include lack of water flow in the DI cooling loop, low water level in the water reservoir in the CG603, the cover is not secure, or an external interlock is open in the room. Correct the problem before proceeding
7. Turn the LOCK switch on the SB660 laser control to ON. The 'LASER EMISSION' indicator will light and, in ten seconds, the lamp will fire according to the selected mode.
8. If an interlock becomes open, the laser will halt. To reset the laser to operating state, the key must be turned off then on again
9. Select a firing and Q-switch mode as per the description above

SHUTDOWN

1. Turn the keyswitch on the SB660 unit OFF. Lasing will cease immediately.
2. LEAVE the WATER and power ON for one minute to remove latent heat from the laser
3. Turn the main power breaker (behind the control stack) OFF
4. Turn the water off

NORMAL OPERATING CONDITIONS

1. With the laser running, select the harmonic operation desired (532) via the toggle switch on the laser head
2. Open the appropriate shutter on the front of the unit: in '532' mode shutter #3 for 1064 for infrared output, or shutter #2 (the middle shutter) for 532nm SHG output. In '355/266' mode, output (IR and SHG) is available from shutter #3 only.
3. Phase-match the harmonic generator using the appropriate switch (separate switches for 532 and 355/266 output). A LIMIT indicator will light to indicate the tuning angle is at the limit for that direction.
4. Laser output may be blocked completely (and independently of the three wavelength-selective shutters) via the main sliding shutter on the front of the unit

MAINTENANCE / REPAIR

1. Periodically, DI water in the cooling unit (CG) must be changed. The resistivity of the water must exceed 100 KOhms/cm as measured in a 10mm diameter hose. To change the water, pull-out the CG unit (be sure to feed hoses from the rear of the rack), drain the internal reservoir using the dump valve, and refill the reservoir via the fill hole.
2. The Pockel's cell is fired via a Marx-bank generator located int he black box adjacent to the cell. The generator is powered via a 300V supply - check this supply first if the cell fails.
3. The "security" lamp indicates interlock status. If diagnostics are required, disassemble the logic unit (LU660) and locate resistor pack U7. Five pins on the pack indicate head temperature, shutter, coolant flow, coolant temperature, and coolant level. Each of these five pins must be at the +15VDC level.

Last updated 2009/11/09 msc