KEYWORDS: Signal to noise ratio, Data acquisition, Acoustics, Sensors, Sensing systems, Polarization, Signal detection, Distributed computing, Fiber lasers, Laser stabilization
In this study, we present a direct detection distributed acoustic sensor based on phase-sensitive optical time domain reflectometer (φ-OTDR) with long sensing range and high signal-to-noise ratio (SNR), which is field-tested over a 50 kmlong fiber. Due to the random nature of Rayleigh backscattered light and fading phenomena, it is hard to characterize the performance of the system. For this reason, the performance of our sensor is specified in a statistical manner in which the mean SNR is determined using the histograms of the SNR. The SNR values are measured for identical acoustic signals in five different days, total of 48 hours and the SNR histograms are obtained for fiber distances of 100 m, 12 km, 21 km, 30 km, 40 km and 50 km. The system is field-tested using external disturbances that are generated from a 50-Hz vibrator. The SNR values are extracted from the power spectral density (psd) of the collected data over the monitored fiber span. Our results show that the φ-OTDR system exhibits a mean SNR of 22.5 dB at 50 km distance.
A regeneratively mode-locked laser with simultaneous low noise radio frequency (RF) tone and optical comb generation is presented. The laser does not need any external RF signal and emits a pulse train at ∼10 GHz repetition rate with a 1.5-ps optical pulse width after compression. The generated RF tone has a signal-to-noise ratio of 121 dB/Hz and an RF fluctuation of 10−9 over 0.1 s. The optical frequency comb spacing is also at ∼10 GHz and the optical comb tooth has a linewidth of <1 kHz.
In this paper, we describe a detailed performance comparison of alternative single-pixel, single-mode LIDAR
architectures including (i) linear-mode APD-based direct-detection, (ii) optically-preamplified PIN receiver, (iii) PINbased
coherent-detection, and (iv) Geiger-mode single-photon-APD counting. Such a comparison is useful when
considering next-generation LIDAR on a chip, which would allow one to leverage extensive waveguide-based structures
and processing elements developed for telecom and apply them to small form-factor sensing applications. Models of
four LIDAR transmit and receive systems are described in detail, which include not only the dominant sources of
receiver noise commonly assumed in each of the four detection limits, but also additional noise terms present in realistic
implementations. These receiver models are validated through the analysis of detection statistics collected from an
experimental LIDAR testbed. The receiver is reconfigurable into four modes of operation, while transmit waveforms
and channel characteristics are held constant. The use of a diffuse hard target highlights the importance of including
speckle noise terms in the overall system analysis. All measurements are done at 1550 nm, which offers multiple system
advantages including less stringent eye safety requirements and compatibility with available telecom components,
optical amplification, and photonic integration. Ultimately, the experimentally-validated detection statistics can be used
as part of an end-to-end system model for projecting rate, range, and resolution performance limits and tradeoffs of
alternative integrated LIDAR architectures.
An optical comb source based on a slab-coupled optical waveguide amplifier (SCOWA) is presented. The laser is
harmonically mode-locked at 10.287 GHz repetition rate and stabilized to an intra-cavity Fabry-Pérot etalon via Pound-
Drever-Hall locking. The Fabry-Pérot etalon serves as a reference for the optical frequency of the comb-lines and
suppresses the fiber cavity modes to allow only a single longitudinal mode-set to oscillate, generating a frequency comb
spaced by the repetition rate. The pulse-to-pulse timing jitter and energy fluctuations are < 2 fs and < 0.03%,
respectively (integrated from 1Hz to 100 MHz). Fundamental to this result is the incorporation of the SCOW amplifier
as the gain medium and the use of an ultra-low noise sapphire-loaded cavity oscillator to mode-lock the laser. The
SCOWA has higher saturation power than commercially available gain media, permitting higher intra-cavity power as
well as available power at the output, increasing the power of the photodetected RF tones which increases their signal-to-noise
ratio. A high visibility optical frequency comb is observed spanning ~3 nm (at -10 dB), with optical SNR > 60 dB
for a cavity with no dispersion compensation. Initial results of a dispersion compensated cavity are presented. A spectral
width of ~7.6 nm (-10 dB) was obtained for this case and the pulses can be compressed to near the transform limit at
~930 fs.
This work discusses the development of a frequency chirped, low repetition rate, semiconductor based mode-locked
laser having reduced noise over previous demonstrations. Specifically, we present a major upgrade on the 100 MHz
harmonically mode-locked Theta (Θ) laser cavity design in the form of the introduction of an intra-cavity fiberized
Fabry-Perot etalon. The initial demonstration of the Theta cavity design offered improved energy per pulse and a linearly
chirped pulse output compared to conventional cavity designs. Nonetheless, it suffered from pulse-to-pulse timing and
energy noise. The noisy operation arises from the harmonic nature of the laser. To mitigate this effect we have inserted a
fiberized etalon within the laser cavity.
The intra-cavity etalon stores and inter-mixes the pulses of the harmonically mode-locked laser, as well as enforces
lasing on a single optical mode-set from the multiple interleaved sets supported by the mode-locked laser due to its
harmonic nature. This leads to the generation of a stable optical frequency comb with 100 MHz spacing and the
suppression of the RF super-mode noise spurs, which results in a reduction of the laser noise. Due to fiber length drift in
both the fiberized laser cavity and the fiberized etalon, a long-term stabilization scheme is necessary. An intra-cavity
Hansch - Couillaud scheme is employed. The laser outputs chirped pulses with 10 nm of bandwidth.
This work provides an in depth analysis of both the development of the Theta cavity with the intra-cavity etalon and
the performance of the developed laser system.
Mode-locked lasers have applications in signal processing and communications such as analog to digital conversion,
arbitrary waveform generation and wavelength division multiplexing. For such applications low noise and phase
coherent frequency stabilized optical combs are needed. In this work we report a low noise, Pound-Drever Hall
frequency stabilized, semiconductor mode-locked laser at 10.287GHz centered at 1550nm with 1000-Finesse sealed,
ultralow insertion loss intracavity etalon. The output optical power of the mode locked laser is ~5mW.
KEYWORDS: Modulators, Modulation, Laser sources, Pulsed laser operation, Analog electronics, Transmittance, Electro optics, Electrooptic modulators, Signal generators, Digital signal processing
In this work we present a method for improving the uniformity of the optical spectrum or the temporal intensity
profile of a quasi-CW, linearly chirped laser source covering the time interval between subsequent pulses. A novel laser
cavity design, referred to as the Theta (Θ) cavity, provides linearly chirped pulses directly from the laser oscillator that
having non-uniform optical spectrum, that is mapped into the temporal intensity profile of the pulse, due to the
frequency-to-time mapping nature of this cavity design. The system developed in this work has been designed to
improve the spectral and temporal intensity profile of lasers for photonic signal processing.
A fiberized feed-forward system is implemented to reduce variations in the temporal intensity profile, or the optical
spectrum due to the time-to-frequency mapping, input to the system. In the feedforward scheme presented, the quasi-CW
pulse train generated from the laser is split and part of it is photodetected, while the electrical signal generated alters the
transmittance of the second part of the input as it goes through an amplitude modulator, resulting in increase in the
uniformity of the signal. The contrast of the optical spectrum of a chirped pulse at input to the system is improved from
51% to 16%, or 3.1 times.
It is critical to know the free spectral range (FSR) of an etalon for telecommunication applications. In this
paper, we have improved the Pound-Drever-Hall (PDH) based technique for measuring the FSR of an
etalon by 2 orders of magnitude. This improved technique results 1 part in 106 precision. To our knowledge
this is the most precise measurement of FSR.
We are investigating optical frequency comb generation by direct modulation of CW light. Our scheme is based on three cascaded modulators; one amplitude modulator and two phase modulators. The modulation scheme is optimized for flatness and power efficiency. A stable optical spectrum has been generated with ~100 comb lines with 0.625 GHz spacing and 3 dB flatness. We also investigate comb generation via phase only modulation.
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