D. E. McClelland

Demonstration of a passive subpicostrain fiber strain sensor

We demonstrate a fiber Fabry-Perot (FFP) sensor that is capable of detecting subpicostrain signals, from 100 Hz and extending beyond 100 kHz, using the Pound-Drever-Hall (PDH) frequency locking technique. A low-power diode laser at 1550 nm is locked to a free-space reference cavity to suppress its free-running frequency noise, thereby stabilizing the laser. The stabilized laser is then used to interrogate a FFP sensor whose PDH error signal yields the instantaneous fiber strain.

Frequency locking a laser to an optical cavity by use of spatial mode interference

We present a novel technique to frequency lock a laser to an optical cavity. This technique, tilt locking, utilizes a misalignment of the laser with respect to the cavity to produce a nonresonant spatial mode. By observing the interference between the carrier and the spatial mode one can obtain a quantum-noise-limited frequency discriminator. Tilt locking offers a number of potential benefits over existing locking schemes, including low cost, high sensitivity, and simple implementation.

Phase-sensitive interrogation of fiber Bragg grating resonators for sensing applications

This paper discusses a phase-sensitive technique for remote interrogation of passive Bragg grating Fabry-Pe/spl acute/rot resonators. It is based on Pound-Drever-Hall (PDH) laser frequency locking, using radio-frequency phase modulation sidebands to derive an error signal from the complex optical response, near resonance, of a Fabry-Pe/spl acute/rot interferometer. We examine how modulation frequency and resonance bandwidth affect this error signal. Experimental results are presented that demonstrate, when the laser is locked, this method detects differential phase shifts in the optical carrier relative to its sidebands, due to minute fiber optical path displacements.

Balanced homodyne detection of optical quantum states at audio-band frequencies and below

The advent of stable, highly squeezed states of light has generated great interest in the gravitational wave community as a means for improving the quantum-noise-limited performance of advanced interferometric detectors. To confidently measure these squeezed states, it is first necessary to measure the shot-noise across the frequency band of interest. Technical noise, such as non-stationary events, beam pointing, and parasitic interference, can corrupt shot-noise measurements at low Fourier frequencies, below tens of kilo-hertz. In this paper we present a qualitative investigation into all of the relevant noise sources and the methods by which they can be identified and mitigated in order to achieve quantum noise limited balanced homodyne detection. Using these techniques, flat shot-noise down to Fourier frequencies below 0.5 Hz is produced. This enables the direct observation of large magnitudes of squeezing across the entire audio-band, of particular interest for ground-based interferometric gravitational wave detectors. 11.6 dB of shot-noise suppression is directly observed, with more than 10 dB down to 10 Hz.

High-resolution absolute frequency referenced fiber optic sensor for quasi-static strain sensing.

We present a quasi-static fiber optic strain sensing system capable of resolving signals below nanostrain from 20 mHz. A telecom-grade distributed feedback CW diode laser is locked to a fiber Fabry-Perot sensor, transferring the detected signals onto the laser. An H(13)C(14)N absorption line is then used as a frequency reference to extract accurate low-frequency strain signals from the locked system.

Searching for gravitational waves from Cassiopeia A with LIGO

We describe a search underway for periodic gravitational waves from the central compact object in the supernova remnant Cassiopeia A. The object is the youngest likely neutron star in the Galaxy. Its position is well known, but the object does not pulse in any electromagnetic radiation band and thus presents a challenge in searching the parameter space of frequency and frequency derivatives. We estimate that a fully coherent search can, with a reasonable amount of time on a computing cluster, achieve a sensitivity at which it is theoretically possible (though not likely) to observe a signal even with the initial LIGO noise spectrum. Cassiopeia A is only the second object after the Crab pulsar for which this is true. The search method described here can also obtain interesting results for similar objects with current LIGO sensitivity.

Phase-sensitive reflection technique for characterization of a Fabry–Perot interferometer

Using a radio frequency coherent modulation and demodulation technique, we explicitly measure both the amplitude and the phase response of Fabry-Perot interferometers in reflection. This allows us to differentiate clearly between overcoupled and undercoupled cavities and allows a detailed measurement of the full width at half-maximum, the free spectral range, and the finesse of the cavities.

Intensity-noise dependence of Nd:YAG lasers on their diode-laser pump source

Typically, the intensity noise of solid-state lasers is dominated by a resonant relaxation oscillation, RRO, at intermediate frequencies (kilohertz to megahertz) and by pump-source noise at frequencies below the RRO. The RRO is driven by vacuum fluctuations as well as by pump-source fluctuations and is therefore present irrespective of the pump-source noise level. However, the intensity noise at frequencies below the RRO can be substantially lowered by use of a low-noise pump source. This behavior is experimentally studied for diode-pumped Nd:YAG ring lasers. An experimental comparison is made between pumping with a single-element diode laser (SEDL) or with a diode-laser array (DLA). We find good agreement with theory for the SEDL but not for the DLA because the DLAs output intensity noise is spatially variant. We also show that pump-source frequency noise has only a minor effect on the intensity noise of the Nd:YAG laser. The requirements for low-noise operation of solid-state lasers are discussed.

Quantum-noise-limited interferometric phase measurements

Two schemes for interferometric optical phase measurement, with sensitivity limited only by quantum noise in the light, are analyzed. Direct detection is applicable to signals at modulation frequencies away from the technical noise of the light, so that quantum noise dominates the measurement. Alternatively signals otherwise obscured by classical optical noise may be recovered with a phase-modulation technique that shifts the signals to a quantum-noise-limited region of the photocurrent spectrum. The analysis is tested experimentally by using a polarimetric electric-field sensor. In the direct-detection scheme quantum-noise-limited performance produced a phase sensitivity of 0.25 µrad. The indirect scheme allowed subkilohertz signals to be extracted from classical noise 67 dB greater with sensitivity approaching the quantum noise limit.

Picometer level displacement metrology with digitally enhanced heterodyne interferometry

Digitally enhanced heterodyne interferometry is a laser metrology technique employing pseudo-random codes phase modulated onto an optical carrier. We present the first characterization of the techniques displacement sensitivity. The displacement of an optical cavity was measured using digitally enhanced heterodyne interferometry and compared to a simultaneous readout based on conventional Pound-Drever-Hall locking. The techniques agreed to within 5 pm/ radicalHz at 1 Hz, providing an upper bound to the displacement noise of digitally enhanced heterodyne interferometry. These measurements employed a real-time signal extraction system implemented on a field programmable gate array, suitable for closed-loop control applications. We discuss the applicability of digitally enhanced heterodyne interferometry for lock acquisition of advanced gravitational wave detectors.