Physics Package Integration

The NIST microfabricated atomic clock. The black lines in the photographs B-F indicate 1 mm. The layers in diagram A are a: baseplate/laser, b: spacer, c: light attenuator, d: lens/spacer, e: light attenuator, f: quarter-wave plate (not shown), g: heater, h: cell (glass), i: cell (silicon), j: cell(glass), k: heater, l: photodiode/spacer, m: baseplate.

The individual physics package subassemblies are assembled together to form the CSAC physics package. First a VCSEL die is mounted onto an AlN baseplate on which has been deposited a series of gold traces to electrically connect the device to the local oscillator and control system. The VCSEL top contact is wire-bonded to the RF lead. The optics assembly is mounted above the VCSEL, and ND filters are added or subtracted to the top of the optics assembly to optimize the intensity of the laser beam. The alkali atom cell, with heaters attached is mounted to the top of the optics assembly, and finally a photodetector subassembly caps the structure. Six gold wire bonds connect the CSAC with the baseplate: four provide current to the ITO heaters and two enable readout of the photodiode. In addition the RF lead enables current to reach the laser. The entire structure, shown in the figure below, has a volume of 9.5 mm³, not including the base plate. An external thermistor (just behind the CSAC assembly in the photograph above at right) monitors the temperature of the cell, which is stabilized to near 80 °C using an active servo. The baseplate is heated to 46 °C in order to tune the laser wavelength to the optical transition while simultaneously optimizing the output power, which is controlled by the laser DC injection current. Roughly 70 mW of power is required to maintain the 34° temperature difference between the cell and the baseplate.

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References:

S. Knappe, L. Liew, V. Shah, P. Schwindt, J. Moreland, L. Hollberg and J. Kitching, "A microfabricated atomic clock," Appl. Phys. Lett., 85, 1460, 2004.