Ultrafast measurement of vector spatial modes by using two-dimensional linear optical sampling.

To the best of the authors' knowledge, we present the first-ever demonstration of the two-dimensional linear optical sampling of fiber spatial modes. The images of the fiber cross sections excited by the LP01 or LP11 modes are directly projected onto a two-dimensional photodetector array and coherently sampled by local pulses with a uniform spatial distribution. Consequently, the spatiotemporal complex amplitude of the fiber mode is observed with a time resolution of a few picoseconds by using electronics with a bandwidth of only a few MHz. Such ultrafast and direct observation of vector spatial modes enables the characterization of the space-division multiplexing fiber itself with high time accuracy and wide bandwidth.

Amplitude-modulated continuous-wave laser rangefinder employing Bessel-Gauss beamforming.

We propose an amplitude-modulated continuous-wave laser rangefinder employing Bessel-Gauss beamforming. Our Bessel-Gauss beam was generated by a newly proposed doublet configuration, which is simple and low cost. Such a beam was propagated >2m with a mainlobe having the diameter <1mm. We have conducted proof-of-concept ranging experiments employing the amplitude-modulated continuous-wave scheme with the Bessel-Gauss beam and obtained ranging results of a measurement distance up to 2 m. To the best of our knowledge, this is the first attempt to apply zeroth-order Bessel-Gauss beamforming to laser rangefinders.

Loss-Modulation-Based Wavelength-Range Shifting of Tunable EDF Ring Laser with Cascaded-Chirped Long-Period Fiber Grating for Temperature Measurement.

A novel tunable Erbium-doped fiber ring laser (EDFRL) with a cascaded-chirped long-period fiber grating (C-CLPG) as a wavelength selection filter is proposed from the viewpoint of the sensor use, in which a variable optical attenuator (VOA) is employed as an intracavity loss modulator to change the oscillation wavelength region so that the resultant tuning wavelength range is widened. In the demonstrative experiment for temperature measurements, oscillation over the wavelength range of 12.85 nm (1557.62~1570.47 nm), which is more than three times range of the previously presented laser and is equivalent to 64 °C in terms of temperature change, was achieved, while a single-wavelength oscillation was maintained. In addition, a practical technique for realizing a temperature measurement by combining with the VOA control is also discussed.

EDF ring laser using cascaded-chirped long period fiber grating for temperature measurement.

A novel tunable erbium-doped fiber ring laser (EDFRL) using a cascaded-chirped long period fiber grating (C-CLPG) as a wavelength selection element is proposed. The oscillation wavelength is determined by the one of the spectral peaks of the C-CLPG used, and the oscillation output provides a high signal to noise (S/N) ratio detection and a highly sensitive measurement of the temperature due to its high power and narrow spectral output. In the experiment, it is confirmed that the wavelength of the output shifts in accordance with the temperature-induced spectral shift of the C-CLPG transmittance spectrum and the temperature sensitivity is obtained to be ∼-0.2 nm/°C within the wavelength range of 1567.30 ∼ 1575.78 nm. The oscillation wavelength range is to be limited depending on the fringe spacing of the channeled spectrum of C-CLPG, which limits the temperature measurement range, but a data processing approach to solve this problem is additionally proposed and its availability is also presented.

Simultaneous mode-by-mode impulse response measurement of multi-mode optical systems based on linear optical sampling.

Linear optical sampling is presented as a useful method to characterize mode-by-mode impulse responses or the spectral transfer matrix of multimode optical systems. In a two non-degenerate linear-polarization (LP) mode transmission line, which actually supported six independent spatial modes, 6 × 6 spectral transfer functions were simultaneously measured using multiplexed mode-by-mode impulse responses in series. According to the singular value analysis of the matrix, the mode-dependent loss spectrum was analyzed over a frequency bandwidth of several hundred GHz, which was determined by the spectra of the probe and reference pulses. The technique would be useful for characterizing various multi-spatial-mode fiber systems that are intensively being developed to overcome the limit in capacity of single-mode optical fibers.

Individual loss distribution measurement in 32-branched PON using pulsed pump-probe Brillouin analysis.

We describe loss distribution measurement in a passive optical network (PON) using pulsed pump-probe Brillouin analysis. A preliminary experiment is demonstrated using a 32-branched PON constructed in the laboratory. We analyze the signal to noise ratio of this measurement and show that the method can realize a 25 dB dynamic range in 90 seconds (10000 times averaging), with an event location resolution of 10 m, and a fiber length identification resolution of 2 m.

Very long range quasi-Fourier spectroscopy for narrowband lasers.

The measurement of the spectral broadening, or temporal coherence property of very narrow linewidth lasers is not an easy task, while such a measurement is essential in any interferometric applications of the lasers. The beat note between two assumingly identical lasers only provides the convolutional spectral profile of the two lasers, but not characterizes the single laser. The delayed self-heterodyne interferometer (DSHI) would not be effective for kHz linewidth range because the finite delay cannot realize complete de-correlation. Here, we demonstrate, for the first time to our knowledge, the complete characterization of the modulus of the degree of coherence (DOC) of kHz linewidth lasers, with a self-referenced fashion where any other reference beam is not used, accordingly, characterize the spectral profile. The method is based on speckle statistical analysis of the Rayleigh scattering in the coherent fiber reflectometry, and would be a novel strong tool to characterize very narrow linewidth lasers.

Centimeter-level spatial resolution over 40 km realized by bandwidth-division phase-noise-compensated OFDR.

We present a bandwidth-division phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) technique, which permits a fast sweep of the optical source frequency. This method makes it possible to reduce the influence of environmental perturbation, which is the dominant factor degrading the spatial resolution of frequency-domain reflectometry at a long measurement range after compensation of the optical source phase noise. By using this approach, we realize a sub-cm spatial resolution over 40 km in a normal laboratory environment, and a 5 cm spatial resolution at 39.2 km in a field trial.

Coherence characterization of narrow-linewidth beam by C-OFDR based Rayleigh speckle analysis.

A novel method for characterizing the amplitude of a coherence function with respect to a delay between two optical waves is proposed and demonstrated by using a distributional Rayleigh speckle analysis based on C-OFDR. This technique allows us to estimate both the coherence time of the laser and that of the spectral profiles from the measured amplitude of the coherence function, if the symmetry of the spectrum can be assumed. The spectral width obtained in the experiment agrees roughly with that obtained using a delayed self-heterodyne method.

High-reflectivity-resolution coherent optical frequency domain reflectometry using optical frequency comb source and tunable delay line.

We propose a high-reflectivity-resolution coherent optical frequency domain reflectometry (OFDR) with a novel scheme of delay shift averaging (DSAV) by using an optical frequency comb source and a tunable delay line to suppress the fading noise. We show theoretically and experimentally that the novel DSAV scheme is equivalent, in realizing a high reflectivity resolution, to our previously reported frequency shift averaging (FSAV) of the same number of teeth of the optical frequency comb [1], but does not need the expensive narrow-pass-band tunable optical filter required in the previous scheme. Furthermore, by using this new method in combination with FSAV, better reflectivity-resolution is obtained compared to using only conventional FSAV with a single-wavelength laser source.

Full polarimetric phase-noise-compensated optical-frequency-domain reflectometry for distributed measurement of high-PMD fibers.

Full polarimetric reflectometry is realized with a spatial resolution of 10 cm over 20 km based on phase-noise-compensated optical-frequency-domain reflectometry, with a polarization-diversity-detection scheme. The distributed differential-group-delay measurement of high-polarization-mode-dispersion fibers is realized using this technology.

Channel-allocation-adaptive WDM signal observation based on sequential ultrafast field sampling.

A novel technology for simultaneous WDM signal monitoring is presented based on ultrafast field sampling. Its flexibility regarding channel allocation and/or channel bandwidth is unique and attractive, as is its ultrafast nature. Two- and four-channel WDM signals, with the total bandwidth of 800 GHz, are successfully discriminated and observed with the proposed scheme.

Simultaneous WDM signal detection realized by ultrafast field sampling.

This paper proposes a novel detection technique for wavelength-division multiplexing (WDM) signals that uses the ultrafast field sampling approach. The proposed technique simultaneously samples the total field of the WDM signal with no wavelength-demultiplexing; its electrical post-processing provides a filtering function in the digital domain. As a result, the individual fields of the WDM channels, including mutual phase relationship, can be jointly reconstructed. As a preliminary demonstration, the simultaneous monitoring of a WDM signal composed of two channels with independent polarization states, is successfully performed using a dual-channel field sampling system with polarization diversity. This demonstration can be expanded to the detection of a WDM signal with N-channels by using an N-channel field sampling system. It is also experimentally-verified that the reconstructed fields preserve the mutual phase relationship of the original fields. This 'field sensitive' WDM detection may open new possibilities for advanced WDM monitoring systems and even the electrical compensation of linear and/or non-linear inter-channel cross-talk in digital coherent detection systems.

Cost-effective bandwidth-reduced Brillouin optical time domain reflectometry using a reference Brillouin scattering beam.

We propose a simple and cost-effective technique that can reduce the electrical bandwidth for Brillouin frequency-shift sensors with heterodyne detection without the need for expensive instruments or a complicated system. With this technique we use only a reference fiber. The reference Brillouin scattering light in the reference fiber is used as a local light for heterodyne detection. We confirm that this method can be used for measuring the Brillouin scattering spectrum distribution with a much lower frequency bandwidth (0.2 GHz) than that employed for conventional heterodyne detection (11 GHz). The method operates in a way similar to conventional Brillouin optical time domain reflectometry with comparable accuracy. Moreover, we successfully demonstrate temperature distribution sensing and show that we can compensate for temperature variation in the reference fiber.

Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method.

A novel type of optical frequency domain reflectometry with a measurement range much longer than the laser coherence length is proposed and experimentally demonstrated. To reduce the influence of laser phase noise, the measurement signal is compensated by using reference signals generated from a single auxiliary interferometer supported by a newly proposed compensation process. The compensation is accomplished numerically with a computer for each section of the delay fiber length in an auxiliary interferometer after only one data acquisition. By using the proposed technique, it is confirmed experimentally that the laser phase noise is well compensated even beyond the coherence length.