Dewar design for temperature tuned SHG Kenneth G. Spears, Lynn Hoff land, and Robert Loyd We report a design for a wide temperature range dewar for SHG applications. This Dewar has been used for ADP, ADA, and RDP crystals. We include a schematic for a simple proportional temperature controller. In our work with cw dye lasers, mode-locked dye lasers, and cavity dumped dye lasers we have needed tunable uv at higher efficiency and in wavelength re- gions not easily achieved by angle tuning or available temperature tuned systems. We report a design for a variable temperature dewar for cooling and heating SHG crystals from 100 K to 393 K with a temperature stability of better than 0.02 0 C. The design uses a heat leak from a coolant container to a copper block having heaters controlled by a dc proportional temperature controller. The SHG crystal is mounted in a copper holder below the heater block, and a simple proportional controller with a wide control range and good precision allows convenient temperature control. Other workers have briefly reported other SHG Dewar designsl 2 ; we include substantial design details in this report. The dewar design is shown schematically in Fig. 1. Fabrication of the stainless outer can is from a stainless tube of 10-cm diam with 0-ring flanges at each end (Alloy Products Corp.). The outer tube has two welded flanges for 0-ring sealing 2.5-cm diam windows. The inner container is welded from 7.5-cm diam stainless and has a 6.3-mm thick stainless bottom for mounting the heat leak and crystal assembly. The top flange of the coolant container has a 10-P-I-N electrical feed- through and a bellows vacuum value for sealing either a vacuum or an inert atmosphere. The windowsare of X/4 quartz, wedged, and MgF coated to reduce reflec- tions. Figure 2 shows a detailed schematic of the heat leak, heater block, and crystal mounting block. The heat leak is of aluminum, and the center hole reduces ma- terial to achieve a maximum cooling rate of 5C/min with liquid nitrogen cooling from 20 0 C. The 5-cm length of this piece could easily be shortened by de- creasing the cross section of the heat leak. The low rate of heat transfer reduces liquid N 2 loss and prevents cracking of the crystal from thermal stress. Cooling The authors are with Northwestern University, Chemistry De- partment, Evanston, Illinois 60201. Received 5 April 1976. rates depend on the coolant and the starting tempera- ture. For liquid nitrogen our design yields 5 0 C/min starting at 10 0 C, 4C/min starting at -30'C, and 2.60C/min starting at -70'C. A solid aluminum heat leak increases the temperature change by 0.80C/min. The heater block contains two cartridge heaters of 6.3-mm diam (120 V, 100 W from Chromalox) wired in parallel. The heater is controlled by three different thermisters; another thermister and a 200-a platinum resistance sensor provide both a very sensitive and an absolute temperature readout. The thermisters are Yellow Spring Instruments 44001, 44003, and 44006, and the platinum resistance element is from the RDF Corp. The SHG doubling crystals are 1-cm X 1-cm cross section and are in contact with copper mounting surfaces by substantial layers of thermal compound made from Hi-vacuum silicone grease (Dow Corning) and ZnO mixture. This thermal compound is stable and viscous to 1200Cand is used at all mechanical sur- faces and to embed all sensors and heaters. Under normal operating conditions a vacuum is maintained in the dewar. However, for extensive op- eration above 80°C the SHG crystal faces are more stable if a petri dish with a thin layer of dessicant is placed in the dewar bottom, and dry nitrogen is used at 600-760 Torr. The ADA type SHG crystal is most prone to surface degradation above 800C, although nonvacuum operation greatly reduces degradation up to 100°C. We have used this dewar with RDP, ADA, and ADP crystals of 2-cm length. Our applications need long term stability of the doubled output from a cw dye laser, a cw cavity dumped dye laser, and a cw mode-locked and cavity dumped dye laser. The temperature controller is of conventional design except for the nearly linear scaling of a single ten-turn potentiometer; three thermistors allow ranging from 100-393K. The schematic is shown in Fig. 3. Circuit assembly was not of printed circuit design; however, standard precautions of single point ground and shielded connections to sensors are important. The integrated regulator circuit is mounted on a heat sink external to the other circuitry. The use of an integrated circuit to provide both operational amplifier voltageand 1172 APPLIED OPTICS/ Vol. 16, No. 5 / May 1977