Digitally enhanced heterodyne interferometry (DI) is a laser metrology technique employing pseudo-random noise (PRN) codes phase-modulated onto an optical carrier. Combined with heterodyne interferometry, the PRN code is used to select individual signals, returning the inherent interferometric sensitivity determined by the optical wavelength. The signal isolation arises from the autocorrelation properties of the PRN code, enabling both rejection of spurious signals (e.g., from scattered light) and multiplexing capability using a single metrology system. The minimum separation of optical components is determined by the wavelength of the PRN code.

A variation of DI has 100 times reduction in the minimum component separation, allowing measurements of optical components only a few centimeters apart. Instead of the usual electronic decoding, the DI signal is interfered with an appropriately delayed, identically PRN-encoded, local oscillator beam. Optical decoding allows the use of a low-bandwidth signal processing chain with GHz codes, negating the need for high-speed detectors and digital signal processing. This reduced bandwidth also reduces the power consumption of the entire system.

The heterodyne signal is created by off-set phase-locking two lasers with a digital phase-locked loop. The errorpoint is monitored on a dedicated phase-locking photoreceiver. PRN codes are phase-modulated by waveguide modulators onto each laser beam. These are subsequently interfered via a fiber beamsplitter, thus optically demodulating the laser signals, before being detected on a signal photoreceiver. One PRN code is digitally delayed with respect to the other in order to align the codes with respect to the reflected light from an optic under interrogation, thus optically demodulating the signal for that specific mirror. The delay is altered (controlled digitally) to pick out any one of the optics under interrogation.

At the time of this reporting, this is the first known time that DI has been employed to measure optics separated by less than meters, down to a few centimeters. This was achieved by implementing, for the first time, optical demodulation of the encoded laser beams (as opposed to the more traditional/common electronic demodulation). This technique is entirely implemented in software via hardware that would already exist onboard a spacecraft. This reduces complexity, power consumption, volume, and risk of failure.

There are many proposed missions that will employ lasers and require extremely high-resolution metrology. Digital interferometry can be implemented and achieve sub-10-pm resolution. With this new technique, the metrology can be performed on optical components separated by centimeters. This allows measurements of optics on a single optical bench within a single spacecraft, in addition to inter-spacecraft metrology measurements.

This work was done by Glenn De Vine, Daniel A. Shaddock, Brent Ware, Robert E. Spero, Danielle M. Wuchenich, William M. Klipstein, and Kirk McKenzie of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47886

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This article first appeared in the January, 2012 issue of Photonics Tech Briefs Magazine.

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