High-accuracy laser metrology enhances the VLTI
The Very Large Telescope Interferometer (VLTI) at the Paranal Observatory (Paranal, Chile) allows the coherent superposition of the stellar light collected by two 8-m-diameter telescopes separated by more than 100 m
by Samuel Lévéque, Yves Salvadé, René Dandliker, and Olivier Scherler
The Very Large Telescope Interferometer (VLTI) at the Paranal Observatory (Paranal, Chile) allows the coherent superposition of the stellar light collected by two 8-m-diameter telescopes separated by more than 100 m (see Fig. 1). With this interferometric setup, astronomers can observe astronomical objects with a spatial resolution in the milli-arc-second range. This is equivalent to the angle subtended by a €1 coin with a diameter of 23.25 mm from a distance of 4800 km.
FIGURE 1. The VLT array on Paranal Mountain in Chile performs high-resolution stellar optical interferometry.
The light captured by the two telescopes follows a train of 25 mirrors distributed along a subterranean path of approximately 200 m long, before reaching the observatory's interferometric laboratory (see Fig. 2). Here, the stellar beams interact coherently to produce interference fringes. In order to achieve this, the optical path must be kept constant to within a fraction of a micron using movable delay lines.
In particular, forthcoming ambitious stellar-interferometry programs with the VLTI include the detection of extra-solar planets. The presence of a planet orbiting around a target star can be revealed by recording simultaneously the interference fringes produced by this target star and a nearby reference star.1 However, optical path differences and fluctuations occurring inside the stellar interferometer affect the fringe signals.
The European Southern Observatory (Munich, Germany), in collaboration with the Institute of Microtechnology of Neuchâtel (Neuchâtel, Switzerland), is developing a laser metrology system for the Paranal Observatory to monitor these instrumental disturbances to an accuracy of 5 nm. The system is equivalent to two heterodyne Michelson interferometers operating simultaneously and having a common optical path with both of the stars observed through the VLTI optical train (see Fig. 3). The disturbance to be monitored corresponds to the difference between the path variations recorded by the two Michelson interferometers.
FIGURE 3. The Nd:YAG interferometer consists of two reference beams and two measurement beams propagating through the VLTI, the phase difference of which is measured by a digital phase meter.
Each interferometer arm reaches up to 276 m with an optical path difference of 120 m, varying at a maximum speed of about 25 mm/s. The interferometers use different heterodyne frequencies (650 and 450 kHz) to suppress crosstalk signals. These frequencies are generated using four fiber-coupled acousto-optic modulators (IntraAction series FCM-40) connected to a single Nd:YAG laser emitting at 1319 nm (Lightwave Electronics Model 125).
After photodetection and filtering, the system mixes individual heterodyne signals such that the disturbance to be monitored is directly coded in the phase of a 200-kHz carrier signal. This superheterodyne detection is tailored to our operating conditions. The disturbance has a low amplitude and bandwidth compared to the optical path difference occurring in each interferometer, which is a consequence of having two interferometers with an equivalent large path in common. "Super-heterodyne" detection avoids the need for two independent high-bandwidth and highly synchronized phase measurements. Instead, the system uses the bandwidth of the phase meter for phase averaging to possibly increase the overall system accuracy.
The fiber-pigtailed photodetectors offer a 10-MHz bandwidth and a noise equivalent power of 0.2 pW/√Hz, which is very close to the Johnson noise limit. All bandpass filters are designed to minimize any differential phase shift in the detection band. The phase measurement of the 200-kHz carrier is based on the principle of time interval measurement using a 200-MHz clock signal with programmable logic components of the Altera Max7000B family. To minimize instrumental phase shifts, the clock signal is derived from the 200-kHz interferometric reference signal using a phase-lock loop. The measured phase can be averaged over a user-defined integration time ranging from 20 μs to 0.65 s. All components, including the fiber-pigtailed photodetectors, are mounted on VME boards (see Fig. 4).
FIGURE 4. The laser head and acousto-optic modulators (right) generate the necessary heterodyne frequencies. Interference fringes are detected by pigtailed photodetectors mounted on VME boards, before phase measurement.
We have evaluated the performance of the overall photodetection and phase-measurement chain on a test bench at the Institute of Microtechnology. The resolution of the digital board is 2π/1024 (or 0.64 nm in double pass) with a maximum sampling frequency of 200 kHz. The standard deviation of the phase noise is less than 2π/1024, for an optical power higher than 100 nW per interferometric arm and a fringe contrast of 70%. For the same fringe contrast, the measured accuracy is 2π/800 (or 0.8 nm in double pass) for a 50-kHz bandwidth and a photodetected power of 20 nW per interferometric arm.
Testing phase accuracy performance at the 2π/1000 level is a very difficult task in laboratory conditions because it requires an equivalent mechanical and environmental stability. Our approach is to take advantage of two-wavelength interferometry, in which the phase accuracy is measured on a synthetic wavelength much larger than an optical wavelength.2 We use two Nd:YAG lasers operating with a frequency difference of 1.5 GHz to generate a synthetic wavelength of 200 mm. In this case, we need only to introduce a known path variation with an accuracy of 0.1 mm to demonstrate a system accuracy of 2π/1000. The frequency difference of the two lasers is stabilized at the 5 x 10-5 level by monitoring the 1.5-GHz beat frequency and by controlling accordingly the length and temperature in the cavity of one of the lasers.
In the next step, absolute frequency stabilization of the laser will be implemented. The goal is to limit the error introduced by the laser-head instability to 1 nm over a maximal disturbance range of 100 mm. Finally, a full-scale testing of the laser interferometer will be performed at the Paranal Observatory in the spring of 2002.
1. F. Delplancke et al., Astron. Telescope and Instrum. 2000, SPIE 4006.
2. R. Dändliker, R. Thalmann and D. Prongué, Opt. Lett. 13, 339 (1988).
3. Internet homepage for the VLTI project: www.eso.org.
4. Internet homepage for the optics groups of IMT: www-optics.unine.ch.
Samuel Lévéque is a photonic physicist at the European Southern Observatory, Munich, Germany; Yves Salvadé is senior scientist, René Dandliker is director of the applied optics group, and OLIVIER SCHERLER is research assistant at the Institute of Microtechnology, University of Neuchâtel, Neuchâtel, Switzerland. E-mail: firstname.lastname@example.org.