Algebraic cancellation of inter-channel cross talk in multiplexed heterodyne interferometry.

We demonstrate the algebraic cancellation of residual phase cross talk in digitally enhanced heterodyne interferometry (DEHeI), a code division multiplexing technique for interferometric sensing. By using linear combinations of parallel decoding operations at multiple delays, we synthesize a zero correlation for spurious signals and remove phase cross talk: a method we call offset decoding. We experimentally demonstrate 70 dB of signal isolation and over 40 dB greater isolation than the equivalent standard implementation of DEHeI.

Polarization impedance measurement cavity enhanced laser absorption spectroscopy.

We present a theoretical overview and experimental demonstration of a continuous-wave, cavity-enhanced optical absorption spectrometry method to detect molecular gas. This technique utilizes the two non-degenerate polarization modes of a birefringent cavity to obtain a zero background readout of the intra-cavity absorption. We use a double-pass equilateral triangle optical cavity design with additional feed-forward frequency noise correction to measure the R14e absorption line in the 30012←00001 band of CO2 at 1572.655 nm. We demonstrate a shot noise equivalent absorption of 3 × 10-13 cm-1 Hz-1/2.

Double Rayleigh scattering in a digitally enhanced, all-fiber optical frequency reference.

We demonstrate a passive, all-optical fiber frequency reference using a digitally enhanced homodyne interferometric phase readout. We model the noise contributions from fiber thermal noise and the coupling of double Rayleigh scattering in a digitally enhanced homodyne interferometer. A system frequency stability of 0.1 Hz/Hz is achieved above 100 Hz, which coincides with the double Rayleigh scattering estimate and is approximately a factor of five above the thermo-dynamic noise limit.

Digitally enhanced molecular dispersion spectroscopy.

A frequency and intensity noise immune fiber dispersion spectrometer with a digitally enhanced homodyne phase extraction system is presented. A hydrogen cyanide (H13CN) vapor cell is placed in a digitally enhanced Sagnac interferometer, and the anomalous dispersion at the 1550.515 nm P11 transition is interrogated with a tunable laser. An analytical model of the dispersion induced phase readout shows close agreement with the experimentally obtained phase signal. Immunity to frequency and intensity noise confers sub-microradian phase sensitivity, corresponding to a spectroscopic detection limit of 77ppb×m/Hz.

Infrasonic performance of a passively stabilized, all-fiber, optical frequency reference.

We report the infrasonic performance of a fiber optic laser frequency reference with potential application to space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna. We determine the optimum cross-over frequency between an optical frequency comb stabilized to a Rubidium atomic reference and two passive, all-fiber interferometers interrogated using digitally enhanced homodyne interferometery. By measuring the relative stability between the three independent optical frequency references, we find the optimum cross-over frequency to occur at 1.5 mHz, indicating that our passive fiber frequency reference is superior to the optical frequency comb at all higher frequencies. In addition, we find our fiber interferometers achieve a stability of 20 kHz/Hz at 1.5 mHz, improving to a stability of 4 Hz/Hz above 3 Hz. These results represent an independent characterization of digitally enhanced fiber references over long time scales and provide an estimate of thermal effects on these passively isolated systems, informing future reference architectures.

Multi-target CW interferometric acoustic measurements on a single optical beam.

We present a free-space, continuous-wave laser interferometric system capable of multi-target dynamic phase measurement at acoustic frequencies up to a Nyquist bandwidth of 10.2 kHz. The system uses Digitally-enhanced Heterodyne Interferometry to range gate acoustic signals simultaneously from multiple in-line reflections while isolating coherent cross-talk between them. We demonstrate sub-nanometer displacement sensitivity across the audio band for each individual reflection surface and 1.2 m resolution between successive surfaces. Signals outside the 1.2 m range-gate of the system were suppressed by greater than 30 dB in amplitude, enabling high fidelity independent acoustic measurements.

Compact flexible multi-pass rotary delay line using spinning micro-machined mirrors.

We propose a new method to extend the path length tunability of rotary delay-lines. This method was shown to achieve a duty cycle of >80% and repetition rates of over 40 kHz. The new method relies on a new multi-segmented micro-machined mirror and serial injection of a single reflection onto separate segments of this mirror. The tunability is provided by the relative positioning of each reflective point on the mirror segments. There are two distinct modes of operation: synchronous and asynchronous. By simply manipulating the spatial position of the returning paths over the respective mirror segments, we can switch between increasing the repetition rate (asynchronous mode) or the total delay path (synchronous mode). We experimentally demonstrated up to 8 m/s scans with repetition rates of up to 42.7 kHz. Furthermore, we present numerical simulations of 18 reflection points to illustrate possibility of achieving a scan speed of up to 80 m/s. Through intermediate combinations of synchronous and asynchronous operation modes with 4 or more passes, we also show that the system can simultaneously increase both repetition rate and scan depth.

Algebraic cancellation of polarisation noise in fibre interferometers.

This experiment uses digital interferometry to reduce polarisation noise from a fiber interferometer to the level of double Rayleigh backscatter making precision fiber metrology systems robust for remote field applications. This is achieved with a measurement of the Jones matrix with interferometric sensitivity in real time, limited only by fibre length and processing bandwidth. This new approach leads to potentially new metrology applications and the ability to do ellipsometry without polarisation elements in the output field.

All-optical low noise fiber Bragg grating microphone.

We present an all-fiber design for a microphone using a fiber Bragg grating Fabry-Perot resonator attached to a diaphragm transducer. We analytically model and verify the fiber-diaphragm mechanical interaction, using the Hänsch-Couillaud readout technique to provide necessary sensitivity. We achieved a noise-equivalent strain sensitivity of 7.1×10-12 ϵ/Hz, which corresponds to a sound pressure of 74 µPa/Hz at 1 kHz limited by laser frequency noise and yielding a signal-to-noise ratio of 47±2 dB with a 1 Pa drive at 1 kHz, in close agreement with modeled results.

Suppressing Rayleigh backscatter and code noise from all-fiber digital interferometers.

We configure an all-fiber digital interferometer to eliminate both code noise and Rayleigh backscatter noise from bidirectional measurements. We utilize a sawtooth phase ramp to upconvert code noise beyond our signal bandwidth, demonstrating an in-band noise reduction of approximately two orders of magnitude. In addition, we demonstrate, for the first time to our knowledge, the use of relative code delays within a digital-interferometer system to eliminate Rayleigh-backscatter noise, resulting in a noise reduction of a factor of 50. Finally, we identify double Rayleigh-backscatter noise as our limiting noise source and suggest two methods to minimize this noise source.

Ultrasensitive real-time measurement of dissipation and dispersion in a whispering-gallery mode microresonator.

We present the characterization of the recently developed cavity enhanced amplitude modulation laser absorption spectroscopy (CEAMLAS) technique to measure dissipation within the evanescent field of a whispering-gallery mode resonator, and demonstrate the parallel use of CEAMLAS and the Pound-Drever-Hall measurement techniques to provide both dissipation and dispersive real-time microresonator measurements. Using an atomic force microscope tip, we introduce a controlled perturbation to the evanescent field of the resonator. In this case, dissipative sensing allows up to 16.8 dB sensitivity improvement over dispersive measurements, providing the possibility for enhanced sensitivity in application such as biomolecule detection.

Bandwidth-division in digitally enhanced optical frequency domain reflectometry.

We demonstrate for the first time the use of digital range-gating in OFDR to allow for orders of magnitude reduction in the required sampling rates. This allows for sensing over long lengths of fiber with fast sweeps of the optical source frequency, without requiring impractical sampling rates. The range-gating is achieved using digitally enhanced interferometry (DI), which isolates individual sections of OFDR signal bandwidth. The reductions in sampling rates permitted by the bandwidth-division are demonstrated both numerically and experimentally.

Critical coupling control of a microresonator by laser amplitude modulation.

We present a laser amplitude modulation technique to actively stabilize the critical coupling of a microresonator by controlling the evanescent coupling gap from an optical fiber taper. It is a form of nulled lock-in detection, which decouples laser intensity fluctuations from the critical coupling measurement. We achieved a stabilization bandwidth of ∼ 20 Hz, with up to 5 orders of magnitude displacement noise suppression at 10 mHz, and an inferred gap stability of better than a picometer/√Hz.

Subfrequency noise signal extraction in fiber-optic strain sensors using postprocessing.

Laser frequency fluctuations typically limit the performance of high-resolution interferometric fiber strain sensors. Using time delay interferometry, we demonstrate a frequency noise immune fiber sensing system, where strain signals were extracted well below the noise floor normally imposed by the frequency fluctuations of the laser. Initial measurements show a reduction in the noise floor by a factor of 30, with strain sensitivities of a nanostrain/Hz at 100 mHz and reaching 100 ps/Hz at 1 Hz. Further characterization of the system indicates the potential for at least 4.5 orders of magnitude frequency fluctuation rejection.

Laser frequency noise immunity in multiplexed displacement sensing.

Digitally enhanced interferometry (DI) can be used to distinguish between interferometric signals and simultaneously monitor in-line object displacements with subnanometer sensitivity. In contrast to conventional interferometry-where these signals interfere with each other and degrade performance-we experimentally show that by using DI, each of these signals can be isolated and measured at the same time. We present what we believe to be the first demonstration of DI's signal multiplexing capabilities, showing simultaneous length sensing of three sections of an optical fiber. The cross talk between length measurements was less than 2.6×10(-3) with a displacement noise floor of 200 pm/√Hz, which corresponds to a strain sensitivity of less than 80 picostrain(pϵ) in each sensor. We also enhance our system's displacement sensitivity at low frequencies by combining information from multiple lengths to suppress errors due to laser frequency noise.

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.

Experimental demonstration of impedance match locking and control for coupled resonators.

We describe and verify the dynamic behavior of a novel technique to optimize and actively control the optical impedance matching condition of a coupled resonator system. The technique employs radio frequency modulation and demodulation to interrogate the reflection amplitude response of the coupled cavity system. The sign and magnitude of the demodulated signal is used in a closed loop feedback system which controls the coupling condition of a three-mirror resonator. This was done by actuating on the spacing between two of mirrors, effectively using the pair as a variable reflectivity compound mirror. We propose that this technique can be used for controlling the signal bandwidth of next-generation gravitational wave detectors, as well as optimizing circulating optical carrier power in the instrument.

Optical fiber sensing based on reflection laser spectroscopy.

An overview on high-resolution and fast interrogation of optical-fiber sensors relying on laser reflection spectroscopy is given. Fiber Bragg-gratings (FBGs) and FBG resonators built in fibers of different types are used for strain, temperature and acceleration measurements using heterodyne-detection and optical frequency-locking techniques. Silica fiber-ring cavities are used for chemical sensing based on evanescent-wave spectroscopy. Various arrangements for signal recovery and noise reduction, as an extension of most typical spectroscopic techniques, are illustrated and results on detection performances are presented.

Pico-strain multiplexed fiber optic sensor array operating down to infra-sonic frequencies.

An integrated sensor system is presented which displays passive long range operation to 100 km at pico-strain (pepsilon) sensitivity to low frequencies (4 Hz) in wavelength division multiplexed operation with negligible cross-talk (better than -75 dB). This has been achieved by pre-stabilizing and multiplexing all interrogation lasers for the sensor array to a single optical frequency reference. This single frequency reference allows each laser to be locked to an arbitrary wavelength and independently tuned, while maintaining suppression of laser frequency noise. With appropriate packaging, such a multiplexed strain sensing system can form the core of a low frequency accelerometer or hydrophone array.

Using active resonator impedance matching for shot-noise limited, cavity enhanced amplitude modulated laser absorption spectroscopy.

We introduce a closed-loop feedback technique to actively control the coupling condition of an optical cavity, by employing amplitude modulation of the interrogating laser. We show that active impedance matching of the cavity facilitates optimal shot-noise sensing performance in a cavity enhanced system, while its control error signal can be used for intra-cavity absorption or loss signal extraction. We present the first demonstration of this technique with a fiber ring cavity, and achieved shot-noise limited loss sensitivity. We also briefly discuss further use of impedance matching control as a tool for other applications.