Research Topics

Wavelength conversion of lasers via nonlinear-optical effects are essential techniques for expanding operation wavelengths of lasers; by use of wavelength conversion, we can obtain coherent light at the wavelengths at which no laser can directly operate. Then researches on realization of high-performance wavelength-conversion devices are extensively promoted. One of its key factors is choice of appropriate wavelength-conversion materials.

Especially, the refractive indices and the nonlinear-optical coefficients, which are unique material parameters, determine the phase-matching properties and the wavelength-conversion efficiencies, so that it is inherently important to obtain their accurate values for design and evaluation of wavelength-conversion devices. We are performing accurate measurements of refractive indices and the nonlinear-optical coefficients of various newly developed wavelength-conversion materials using our original and unique techniques.

(Left figure: Refractive indices measurement system, Right figure: Second-harmonic power as a function of the sample thickness (Maker fringes))

Much effort has been devoted to develop more compact, higher-power, more highly efficient, and higher-beam-quality solid-state lasers in accordance with emergence of high-power pump diode lasers. Nonlinear wavelength conversion from ultraviolet to infrared has been also intensively investigated to obtain coherent light at the wavelengths at which lasers cannot directly oscillate.

First of all, we try to use newly developed materials to realize high-performance laser devices. However, those materials rarely fulfill all the requirements the optimal lasers satisfy. Then it is important to artificially modify the materials or integrate several materials in order to deduce higher performance. Superior solid-state laser devices and wavelength-conversion devices can be realized by fabricating composite structures from single bulk crystals.

In this case, the fabrication process of such composite structures, i.e., the bonding of materials is an inherently important issue; the bonded interfaces would cause significant scattering or absorption which significantly reduce efficiency.

We have developed a technique to fabricate high-quality composite laser devices using the room-temperature bonding, and realized high-power composite lasers, walk-off compensating wavelength-conversion devices, and quasi-phase-matching devices, as shown in the figures below.

(Upper right figure: Room-temperature-bonding process, Lower right figure: Equipment for room-temperature bonding)

Medical application of lasers

We collaborate with Tokyo Women's medical university to develop optical measurement system of flow rate of cerebrospinal fluid in a shunt tube for treating hydrocephalus.

Hydrocephalus is a disease in which excess cerebrospinal fluid deposited inside a cranium bears hard on a brain, causing various undesirable effects. Shunt operations are carried out as a treat of hydrocephalus, in which a thin tube is inserted in a body through which cerebrosipnal fluid flows from the brain to the stomach, making the brain pressure decrease. However, there does not exist a way to monitor the rate of the cerebrospinal fluid flowing in the tube, so doctors usually control its rate based on their experience and intuitions.

We proposed a method to accurately measure the rate of cerebrospinal fluid flowing in a tube in a non-contact manner using laser light. When we insert air clusters inside the tube, they move at the same velocity with the cerebrospinal fluid. Then we can obtain the rate of fluid by detecting the difference of the reflectivities when the laser light is incident on the cerebrospinal fluid or on the air cluster.

We are now developing a compact portable measurement system for the clinical test.

(Upper right figure: measurement system for the rate of cerebrospinal fluid in a shunt tube, Lower right figure: a typical measurement data)

Laser processing of plastic waste

More than half of recycling of plastic waste in Japan is currently the thermal recycling, in which thermal energy is recovered by burning plastics. It is necessary to effectively sort plastic waste into each type of resin in order increase the ratios of material recycling and chemical recycling. If we can process plastics of different types of resin by several lasers with different wavelengths, we can increase the ratio of recycling by automating the sorting process.

In this work, we measure the transmission spectra of plastic waste to find the specific absorption wavelengths at which to be processed. Here, we show the infrared spectra of more than 500 kinds of plastic waste including packaging of foods, PET bottles, trays, films, containers, and the spectra of visible region for about 50 kinds of transparent plastics.