Subpicometer length measurement using heterodyne laser interferometry and all-digital rf phase meters.

We present an all-digital phase meter for precision length measurements using heterodyne laser interferometry. Our phase meter has a phase sensitivity of 3 µrad/√Hz at signal frequencies of 1 Hz and above. We test the performance of our phase meter in an optical heterodyne interferometric configuration, using an active Sagnac interferometer test bed that is flexible and low noise. We demonstrate more than 70 dB of laser frequency noise suppression to achieve an optical phase sensitivity of 5 µrad/√Hz and a corresponding displacement sensitivity of 0.5 pm/√Hz at signal frequencies above 10 Hz. In addition, we demonstrate the ability of our phase meter to follow full fringe signals accurately at 100 Hz and to track large signal excursions in excess of 10(5) fringes without cycle slipping. Finally, we demonstrate a cyclic error of ≤1 pm/√Hz, above 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.

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.

Photothermal effects in fiber Bragg gratings.

Writing a fiber Bragg grating in optical fiber generates an intrinsic broadband absorption term that can result in photothermal heating during subsequent use with fiber core guided light. This, in turn, can cause a significant shift of a grating resonance via the thermo-optic coefficient, even at low in-fiber light powers. The magnitude of the absorption term and its dependence on the grating strength are detailed. We further show how the degree of heating can be influenced by the particular environment in which the grating is placed and that, while the shift can be quite deleterious for some applications, its effect can be mitigated. A simple conductive model is developed.

Laser frequency-noise-limited ultrahigh resolution remote fiber sensing.

When a fiber Fabry-Perot is used in an ultra-sensitive strain detection system via a radio-frequency interrogation scheme, its frequency discrimination properties can be enhanced by reducing the linewidth of its resonance. This increases the signal-to-noise ratio, and thus suppresses the strain equivalent noise floor. We demonstrate this improvement in a long-distance high performance remote sensing system and show that in reflection, it can mitigate the effects of random phase noise introduced by Rayleigh back-scattering. In transmission, it improves the remote system sensitivity to sub-picostrain resolution, which surpasses any other long-distance remote sensing system to date. With the reduced fiber Fabry-Perot linewidth, all noise sources in the delivery fiber become irrelevant, as the transmission system is limited only by the pre-stabilized laser frequency noise.

Sampled Bragg gratings in chalcogenide (As(2)S(3)) rib-waveguides.

We have written a sampled Bragg grating into a highly photosensitive chalcogenide (As(2)S(3)) rib-waveguide using a scanning Sagnac interferometer. The grating exhibits evenly spaced rejection peaks over a 40 nm bandwidth. We estimate the induced refractive index change of the waveguide to be over 0.03 by matching the measured spectrum to numerical solutions of the coupled mode equations while accounting for an induced chirp. The sampled Bragg grating presented is comparable in strength and bandwidth to the best sampled Bragg gratings obtained to date in silica optical fibre.

Error free all optical wavelength conversion in highly nonlinear As-Se chalcogenide glass fiber.

We present the first demonstration of all optical wavelength conversion in chalcogenide glass fiber including system penalty measurements at 10 Gb/s. Our device is based on As2Se3 chalcogenide glass fiber which has the highest Kerr nonlinearity (n(2)) of any fiber to date for which either advanced all optical signal processing functions or system penalty measurements have been demonstrated. We achieve wavelength conversion via cross phase modulation over a 10 nm wavelength range near 1550 nm with 7 ps pulses at 2.1 W peak pump power in 1 meter of fiber, achieving only 1.4 dB excess system penalty. Analysis and comparison of the fundamental fiber parameters, including nonlinear coefficient, two-photon absorption coefficient and dispersion parameter with other nonlinear glasses shows that As(2)Se(3) based devices show considerable promise for radically integrated nonlinear signal processing devices.

Integrated all-optical pulse regenerator in chalcogenide waveguides.

We report a fully integrated, passive, all-optical regenerator capable of terabit per second operation, based on a highly nonlinear chalcogenide (As2S3) glass rib waveguide followed by an integrated Bragg grating bandpass filter. We demonstrate a clear nonlinear power transfer curve with 1.4 ps optical pulses, capable of improving the signal-to-noise ratio and reducing the bit error rate for digital signals.

Soliton compression and pulse-train generation by use of microchip Q-switched pulses in Bragg gratings.

Pulse compression and pulse-train generation are demonstrated by use of kilowatt 580 ps pulses generated by a compact (15 cm x 3 cm x 3 cm) microchip Q-switched laser followed by a fiber Bragg grating. A 12-fold pulse compression to 45 ps with five times peak power enhancement is achieved at 1.4 kW through soliton effect compression in the fiber grating. At 2.5 kW, modulational instability leads to a train of high-contrast sub-100 ps pulses. These demonstrations take advantage of the ultrastrong dispersion at frequencies close to the edge of the photonic bandgap. Experimental results are discussed in the context of the nonlinear Schrödinger equation and are compared with simulations of the nonlinear coupled-mode equations.

Effect of group delay ripple on picosecond pulse compression schemes.

We show experimentally, through autocorrelation and frequency-resolved optical gating measurements, that a simple dispersive fiber Bragg grating with group delay ripple approximately 10 ps peak-to-peak may be used effectively to stretch ultrashort optical pulses for linear amplification before recompression to a higher-power pulse. We further investigate, through simulations, the effect of group delay ripple on the pulses and show that there are regimes, defined by both ripple magnitude and ripple period as a function of wavelength, in which the pulses are nearly perfectly compressed. A map with contours of equal figures of merit indicates favorable regions of operation.

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.

Photothermal effects in passive fiber Bragg grating resonators.

Photothermal effects in passive Fabry-Perot resonators are caused by the conversion of circulating optical energy into heat as a result of absorption. This results in thermal change in the resonator's optical path length, the round-trip phase, and hence the resonance condition. We describe a simplified dynamic numerical model for photothermal effects in passive fiber Bragg grating resonators and present results of their experimental observation.