The Magdalena Ridge Observatory Interferometer (MROI) has completed its design phase and is currently in the
construction phase. The first telescope will be deployed at the MROI site in 2011. Five different vendors are involved
in the design and fabrication of a unit telescope, and a much larger number for the full observatory.
This paper addresses the steps that the MRO Interferometry project will undertake to integrate subsystems developed by
different parties, through commissioning into an operational optical interferometer.
Finally we present the commissioning plan to bring the interferometer to an operational mode. We have developed
"performance verification milestones" that successively increase the "science readiness" of the interferometer and
transitions to an operational phase.
We report on the testing of the modulators within the MROI fringe tracking beam combiner. Modulation in
the beam combiner will be performed via modulators introducing an optical path difference in increments of λ/4
into the beams. Knowledge of the path difference introduced needs to be accurate to within 1!. To achieve this accuracy, the modulators are characterized and the desired step waveform optimized through a Fourier analysis technique. Control is implemented in an FPGA embedded system and performance will be monitored by means of a slow loop Fourier algorithm. Details of the progress on characterization, optimization and future implementation are presented here.
Merging software from 36 independent work packages into a coherent, unified software system with a
lifespan of twenty years is the challenge faced by the Magdalena Ridge Observatory Interferometer (MROI).
We solve this problem by using standardized interface software automatically generated from simple highlevel
descriptions of these systems, relying only on Linux, GNU, and POSIX without complex software such
as CORBA. This approach, based on gigabit Ethernet with a TCP/IP protocol, provides the flexibility to
integrate and manage diverse, independent systems using a centralized supervisory system that provides a
database manager, data collectors, fault handling, and an operator interface.
There is increasing interest in development of high speed, low noise and readily fieldable near infrared (NIR) single
photon detectors. InGaAs/InP Avalanche photodiodes (APD) operated in Geiger mode (GM) are a leading choice for
NIR due to their preeminence in optical networking. After-pulsing is, however, a primary challenge to operating
InGaAs/InP single photon detectors at high frequencies1. After-pulsing is the effect of charge being released from traps
that trigger false ("dark") counts. To overcome this problem, hold-off times between detection windows are used to
allow the traps to discharge to suppress after-pulsing. The hold-off time represents, however, an upper limit on detection
frequency that shows degradation beginning at frequencies of ~100 kHz in InGaAs/InP. Alternatively, germanium (Ge)
single photon avalanche photodiodes (SPAD) have been reported to have more than an order of magnitude smaller
charge trap densities than InGaAs/InP SPADs<sup>2</sup>, which allowed them to be successfully operated with passive quenching<sup>2</sup>
(i.e., no gated hold off times necessary), which is not possible with InGaAs/InP SPADs, indicating a much weaker dark
count dependence on hold-off time consistent with fewer charge traps. Despite these encouraging results suggesting a
possible higher operating frequency limit for Ge SPADs, little has been reported on Ge SPAD performance at high
frequencies presumably because previous work with Ge SPADs has been discouraged by a strong demand to work at
1550 nm. NIR SPADs require cooling, which in the case of Ge SPADs dramatically reduces the quantum efficiency of
the Ge at 1550 nm. Recently, however, advantages to working at 1310 nm have been suggested which combined with a
need to increase quantum bit rates for quantum key distribution (QKD) motivates examination of Ge detectors
performance at very high detection rates where InGaAs/InP does not perform as well. Presented in this paper are
measurements of a commercially available Ge APD operated at relatively short GM hold-off times to examine whether
there are potential advantages to using Ge for 1310 nm single photon detection. A weaker after-pulsing dependence on
frequency is observed offering initial indications of the potential that Ge APDs might provide better high frequency