Ken Kashiwagi

In-situ monitoring of optical deposition of carbon nanotubes onto fiber end

Carbon nanotubes (CNTs) emerged as an attractive material for nonlinear optical devices. Their quasi-one-dimensional structure provided their unique nonlinear characteristics. However, one of their drawbacks is the handling method. We have proposed and demonstrated optical manipulation of CNTs to deposit them onto cores of optical fiber ends with a simple technique. Although the method is very simple, it requires precise control of the optical power. The method does not posses controllability of the CNT-layer properties. In this paper, we employed optical reflectometry to solve these problems. A 15 microm diameter circular region was area-selectively coated by CNTs using highly uniform solution. The preferentially-deposited CNTs were directly, for the first time, observed by a field emission scanning electron microscope (FE-SEM).

Planar waveguide-type saturable absorber based on carbon nanotubes

The authors propose and demonstrate a planar waveguide-type optical device based on carbon nanotubes (CNTs). They realize saturable absorbers utilizing the interaction between the evanescent field of guiding mode and the CNTs. Saturable absorption characteristics are observed and insertion loss decreases ∼3% by high power pulsed laser. The device shows that polarization dependence originated from the asymmetric structure of their device and over 15dB polarization dependent loss. The saturable absorption is observed only in the polarization state that shows a maximum loss caused by the CNTs.

Novel cost effective carbon nanotubes deposition technique using optical tweezer effect

Carbon nanotubes (CNTs) have been intensively studied for optical applications because of their useful characteristics. However, handling of the CNTs is one of the largest problems for device applications. Several methods have been reported to fabricate optical devices, such as spray method, direct synthesis method, and polymer embedding method. These methods require complicated process and dissipate excessive amount of CNTs. Therefore, an easy and cost effective handling technique of CNTs is required. In this paper, we propose and demonstrate a novel technique to deposit CNTs onto only the core region of end facets of optical fibers. We successfully realized area selective deposition using optical tweezers. This technique requires a very simple setup and consumes only a small amount of CNTs. We confirmed presence of CNTs at the selected region by microscopic Raman spectroscopy. As an optical device application, we inserted the CNT deposited fiber into the fiber ring laser cavity as a saturable absorber, and realized passive modelocking. This technique will allow us to realize low-cost CNT-based photonic devices.

All optical switching using carbon nanotubes loaded planar waveguide

We realized, for the first time, NOLM-type all optical switching with carbon nanotubes loaded planar waveguide. ~20% splitting ratio contrast has been achieved using just 1 cm-long carbon nanotubes loaded planar waveguide.

Optically formed carbon nanotube sphere

Carbon nanotubes (CNTs) offer vast possibilities for future ultra-fast photonic devices. One of the biggest challenges to realize the devices is handling of carbon nanotubes. To achieve efficient handling, we have proposed and demonstrated the optical deposition of CNTs onto optical fiber ends. We found that a sphere-shaped super-structure made of carbon nanotubes can be fabricated by changing the light intensity. Chemical functionalization was not necessary in contrast to the conventional techniques of fabricating super-structures. We demonstrated physical manipulation of the sphere, and realized a passively mode-locked fiber laser employing the sphere.

Frequency-shifted multiwavelength FBG laser sensor

We propose and demonstrate a novel frequency-shifted FBG (fiber Bragg grating) laser sensor system. It is composed of an EDFA (erbium-doped fiber amplifier) broadband light source, 3-FBG array, single mode fiber, circulator, acousto-optic modulator (AOM) and an output 3dB fiber coupler. The frequency-shifted feedback by the AOM allows the laser to operate at multiwavelength at room temperature. Compared to a conventional passive system, the optical signal power and SN ratio were enhanced by 20.1 dB, and 17.0 dB, respectively, and the linewidth was reduced to 1/5.3.

Frequency-Shifted Multiwavelength Fiber Bragg Grating Laser Sensor

We propose and demonstrate a novel active multiwavelength fiber Bragg grating (FBG) laser sensor system. Simultaneous multiwavelength oscillation by the frequency-shifted feedback technique markedly enhances output power and signal-to-noise ratio (SNR) compared with a conventional passive FBG sensor system. The principle of the simultaneous measurement of the dynamic strain signal from wavelength multiplexed FBGs is experimentally confirmed by employing an arrayed waveguide grating (AWG) as a wavelength demodulator.

Fabrication of Silica-Based Glass Optical Waveguide by UV Beam Scanning

UV induced refractive index increase, the photosensitivity, in Ge-doped silica glass has been utilized for the fabrication of fiber Bragg gratings and long period fiber gratings. Recently, this phenomenon has also been applied to the fabrication of optical channel waveguides in order to achieve cost efficiency and fabrication flexibility. In this work, the fabrication of various optical waveguides by UV beam scanning is demonstrated. 10-mm-long straight optical waveguides and a Y-Branch waveguide with a branching ratio of nearly 50% are realized. We also propose and demonstrate a technique for the simultaneous fabrication of an optical waveguide and a Bragg grating by scanning UV beam through a phase mask.

Distance measurement over 30 km using highly sensitive two-photon detection

Elongation of distance measurement based on two-photon detection was achieved by using a lock-in amplifier. The multiple reflection points in an optical fiber distributed at 1 km to 30 km away was successfully identified.

Optical reflectometry for in-situ monitoring of carbon nanotubes deposition by optical tweezers

Reflectometry is adopted for in-situ monitoring of carbon nanotubes deposition to fiber end by optical tweezers. Reflectivity increases drastically once CNTs are deposited, and enhancement of layer uniformity is observed through damping of reflectivity fluctuation.