Sucbei Moon

High temperature fiber sensor with high sensitivity based on core diameter mismatch

We report a simple fiber sensor for measurement of high temperature with high sensitivity. The sensing head is a multimode-single mode-multimode (MM-SM-MM) fiber configuration formed by splicing a section of uncoated single mode fiber (SMF) with two short sections of multimode fibers (MMF) whose core is composed of pure silica. Because of the mode-field mismatch at the splicing points of the SMF with 2 sections of MMFs, as well as index matching between the core of the MMF and the cladding of the SMF, optical power from the lead-in fiber can be partly coupled to the cladding modes of the SMF through the MMF. The cladding modes of the SMF then re-coupled to the lead-out fiber, in the same fashion. Due to the effective index difference between the core and cladding modes, an interference pattern in the transmission spectrum of the proposed device was obtained. The interference pattern was found to shift to the longer wavelength region with respect to temperature variation. The temperature sensor can measure temperature stably up to more than 900 degrees C with sensitivity of 0.088 nm/ degrees C.

Ultra-high-speed optical coherence tomography with a stretched pulse supercontinuum source

We introduce a new high-speed Fourier-domain optical coherence tomography (FD-OCT) scheme based on a stretched pulse supercontinuum source. A wide-band short pulse of a supercontinuum source of which output spectrum spanned a wavelength range from 1,200 nm to 1,550 nm was stretched to a long pulse of 70-ns duration by using a dispersive fiber due to the group-velocity dispersion, and it was used directly as frequency-swept light for FD-OCT. The OCT spectral interferogram was acquired in the time domain and converted into the spectral domain by the pre-calibrated time-to-wavelength relation. Using this stretched-pulse OCT (SP-OCT) scheme, we have demonstrated an ultrahigh-speed axial-line scanning rate of 5 MHz. The axial resolution of 8 microm was achieved without re-calibration of the sweep characteristic owing to the passive nature of the frequency-sweeping mechanism.

Generation of octave-spanning supercontinuum with 1550-nm amplified diode-laser pulses and a dispersion-shifted fiber.

We present a compact supercontinuum source using a dispersion-shifted fiber and an amplified diode-laser pulse source. Gain-switched DFB laser operating at 1550-nm wavelength, which provides 30-ps pulses, was used for generating the seeding pulses. And serially cascaded low-cost EDFAs were employed to boost the peak power of the pulses to more than 1 kW. Single-mode supercontinuum spanning nearly the full near-IR band was obtained by passing the amplified high-power pulses through a dispersion-shifted fiber. By investigating various characteristics of the supercontinuum generation, the walk-off between the spectral components was found to limit the effective interaction length of the spectrum-broadening effects. An optimal length of the fiber to obtain a flat spectrum was determined, which minimizes undesirable excessive Raman effect.

Effective single-mode transmission at wavelengths shorter than the cutoff wavelength of an optical fiber

We propose a novel transmission scheme to extend the single-mode operation range of a conventional single-mode fiber (C-SMF) to wavelengths shorter than the cutoff wavelength. It involves the use of a tapered-fiber mode filter (MF) in order to remove the high-order-mode power from the C-SMF and hence, makes the fiber work as a virtually single-mode transmission link. High-order mode suppression of above 10 dB has been achieved at wavelengths of 850 and 980 nm with a fabricated tapered-fiber MF. We have also demonstrated the feasibility of the technique by evaluating the transmission quality with the MF at 850 nm through a 5-km C-SMF having a cutoff wavelength of 1.2 /spl mu/m.

High-speed confocal fluorescence lifetime imaging microscopy (FLIM) with the analog mean delay (AMD) method

We demonstrate a high-speed confocal fluorescence lifetime imaging microscopy (FLIM) whose accuracy and photon economy are as good as that of a time-correlated single photon counting (TCSPC). It is based on a new lifetime determination scheme, the analog mean delay (AMD) method. Due to the technical advantages of multiple fluorescence photon detection capability, accurate lifetime determination scheme and high photon detection efficiency, the AMD method can be the most effective method for high-speed confocal FLIM. The feasibility of real-time confocal FLIM with the AMD method has been demonstrated by observing the dynamic reaction of calcium channels in a RBL-2H3 cell with respect to 4αPDD stimulus. We have achieved the photon detection rate of 125 times faster than a conventional TCSPC based system in this experiment.

Analog mean-delay method for high-speed fluorescence lifetime measurement.

We present a new high-speed lifetime measurement scheme of analog mean-delay (AMD) method which is suitable for studying dynamical time-resolved spectroscopy and high-speed fluorescence lifetime imaging microscopy (FLIM). In our lifetime measurement method, the time-domain intensity signal of a fluorescence decay is acquired as an analog waveform. And the lifetime information is extracted from the mean temporal delay of the acquired signal. Since this method does not rely on the single-photon counting technique, the signals of multiple fluorescence photons can be acquired simultaneously. The measurement speed can be increased easily by raising the fluorescence intensity without a photon-rate limit. We have investigated various characteristics of our method in lifetime accuracy and precision as well as measurement speed. It has been found that our method can provide excellent measurement performances in various aspects. We hav demonstrated a high-speed measurement with a high photon detection rate of approximately 108 photons per second with a nearly shot noise-limited photon economy. A fluorescence lifetime of 3.2 ns was accurately determined with a standard deviation of 3% from the data acquired within 17.8 micros at a rate of 56,300 lifetime determinations per second.

New optical frequency domain differential mode delay measurement method for a multimode optical fiber.

A novel mode analysis method and differential mode delay (DMD) measurement technique for a multimode optical fiber based on optical frequency domain reflectometry has been proposed for the first time. We have used a conventional OFDR with a tunable external cavity laser and a Michelson interferometer. A few-mode optical multimode fiber was prepared to test our proposed measurement technique. We have also compared the OFDR measurement results with those obtained using a traditional time-domain measurement method.

Normalization detection scheme for high-speed optical frequency-domain imaging and reflectometry.

We introduce a new signal detection method that can effectively suppress the effect of relative intensity noise (RIN) in optical frequency-domain reflectometry or imaging (OFDR/OFDI) schemes to enhance the sensitivity and dynamic range. In this method, spectral interferogram signal is normalized digitally by a spectral reference signal that contains the real-time spectrum and the RIN information of the frequency-swept source. Unlike the conventional balanced detection method that suppresses only additive intensity noises, we found that our proposed scheme removes both the additive and convolutional contributions of the RINs in the final interferogram signals. Experimental demonstrations were performed using a stretched-pulse optical coherence tomography (SP-OCT) system where the high RIN of a supercontinuum source had been a serious drawback. We have experimentally verified the superiority of our proposed scheme in terms of its improved dynamic range in comparison to the balanced detection method. In addition, we have shown that the noise suppression performance is immune to the spectral imbalance characteristics of the optical components used in the system, whereas the common-mode noise rejection of the conventional balanced detection method is influenced by them.

Fourier-domain low-coherence interferometry for differential mode delay analysis of an optical fiber

We propose a novel mode analysis and differential mode delay measurement method for an optical fiber using Fourier-domain low-coherence interferometry. A spectral interferometer based on a Mach-Zehnder interferometer setup was used with a broadband source and an optical spectrum analyzer to detect relative temporal delays between the guided modes of a few-mode optical fiber by analyzing spectral interference signals. We have shown that experimental results of the proposed method agree well with those results obtained by using a conventional time-domain measurement method. We have demonstrated that this new mode analysis technique has high sensitivity (<60 dB) and very good resolution (<1 ps/m).

Referencing techniques for the analog mean-delay method in fluorescence lifetime imaging

The analog mean-delay (AMD) method is a new powerful alternative method in determining the lifetime of a fluorescence molecule for high-speed confocal fluorescence lifetime imaging microscopy. Even though the photon economy and the lifetime precision of the AMD method are proven to be as good as those of the state-of-the-art time-correlated single photon counting method, there have been some speculations and concerns about the accuracy of this method with respect to the absolute lifetime value of a fluorescence probe. In the AMD method, the temporal waveform of an emitted fluorescence signal is directly recorded with a slow digitizer whose bandwidth is much lower than the temporal resolution of the lifetime to be measured. We have found that the drifts and the fluctuations of the absolute zero position in a measured temporal waveform are the major problems in the AMD method. We have proposed electrical and optical referencing techniques that may suppress these errors. It is shown that there may exist more than 2 ns drift in a measured temporal waveform during the period of the first 12 min after electronic components are turned on. The standard deviation of a measured lifetime after this warm-up period can be as large as 51 ps without any referencing technique. We have shown that this error can be reduced to 9 ps with our electronic referencing technique. It is demonstrated that this can be further reduced to 4 ps by the optical referencing technique we have introduced.