Kazuyoshi Itoh

Analysis and experimental assessment of the sensitivity of stimulated Raman scattering microscopy

We theoretically show that the shot-noise-limited sensitivity of stimulated Raman scattering (SRS) microscopy, which enables high-contrast vibrational imaging, is similar to that of coherent anti-Stokes Raman scattering microscopy. We experimentally confirm that the sensitivity of our SRS microscope is lower than the shot-noise limit only by <15 dB, which indicates that the high-sensitivity of SRS microscopy is readily available.

Stimulated Raman scattering microscope with shot noise limited sensitivity using subharmonically synchronized laser pulses

We propose and demonstrate the use of subharmonically synchronized laser pulses for low-noise lock-in detection in stimulated Raman scattering (SRS) microscopy. In the experiment, Yb-fiber laser pulses at a repetition rate of 38 MHz are successfully synchronized to Ti:sapphire laser pulses at a repetition rate of 76 MHz with a jitter of <8 fs by a two-photon detector and an intra-cavity electro-optic modulator. By using these pulses, high-frequency lock-in detection of SRS signal is accomplished without high-speed optical modulation. The noise level of the lock-in signal is found to be higher than the shot noise limit only by 1.6 dB. We also demonstrate high-contrast, 3D imaging of unlabeled living cells.

Sensitivity enhancement of fiber-laser-based stimulated Raman scattering microscopy by collinear balanced detection technique

We propose the collinear balanced detection (CBD) technique for noise suppression in fiber laser (FL)-based stimulated Raman scattering (SRS) microscopy. This technique reduces the effect of laser intensity noise at a specific frequency by means of pulse splitting and recombination with a time delay difference. We experimentally confirm that CBD can suppress the intensity noise of second harmonic (SH) of Er-FL pulses by 13 dB.The measured noise level including the thermal noise is higher by only ~1.4 dB than the shot noise limit. To demonstrate SRS imaging, we use 4-ps SH pulses and 3-ps Yb-FL pulses, which are synchronized subharmonically with a jitter of 227 fs. The effectiveness of the CBD technique is confirmed through SRS imaging of a cultured HeLa cell.

Stimulated parametric emission microscopy.

We propose a novel microscopy technique based on the four-wave mixing (FWM) process that is enhanced by two-photon electronic resonance induced by a pump pulse along with stimulated emission induced by a dump pulse. A Ti:sapphire laser and an optical parametric oscillator are used as light sources for the pump and dump pulses, respectively. We demonstrate that our proposed FWM technique can be used to obtain a one-dimensional image of ethanol-thinned Coumarin 120 solution sandwiched between a hole-slide glass and a cover slip, and a two-dimensional image of a leaf of Camellia sinensis.

10-GS/s 5-bit Real-Time Optical Quantization for Photonic Analog-to-Digital Conversion

We report the experimental demonstration of a 10-GS/s 5-bit real-time optical quantization using the soliton self-frequency shift (SSFS) and multistage spectral compression. We prove the feature of sampling rate transparency at 10 GS/s by transmitting 10-Gb/s multi-intensity level quasi analog signals through our 5-bit optical quantizer. Furthermore, to confirm the performance of our quantizer, we measure the bit-error-rate and calculate the effective number of bits, the integral nonlinearity error, and the differential nonlinearity error.

In situ Micro-Raman Investigation of Spatio-Temporal Evolution of Heat in Ultrafast Laser Microprocessing of Glass

We describe a micro-Raman spectroscopic study of temperature dynamics in glass during the irradiation of focused ultrafast laser pulses. In the experiment, femtosecond pulses followed by nanosecond Raman pump pulses were focused inside a sample. Back-scattered Stokes and anti-Stokes Raman signals were detected by a time-gated polychromator to directly probe local temperature in the sample. We found that the time evolution and spatial diffusion are consistent with thermal diffusion model, whereas their dependences on pulse energy are different in fused silica and borofloat glass.

Stimulated Raman scattering microscopy for live-cell imaging with high contrast and high sensitivity

We describe the principle and experiment of stimulated Raman scattering (SRS) microscopy, which has various advantages such as high-contrast and high sensitivity. To discuss how these advantages are realized in SRS microscopy, we introduce an intuitive picture of SRS, where the SRS process is viewed as homodyne detection of a nonlinear-optical signal by the excitation pulse.

Highly Accurate Compensation Technique for 10-GHz Pulse Intensity Fluctuation Using SPM-Based All-Optical Intensity Limiter

We report a trial of 10-GHz and highly accurate pulse-by-pulse intensity limiting with a single erbium-doped fiber amplifier for low-power-consumption driving optical limiting. A self-phase modulation-based optical limiter with an appropriate pre-chirping made it possible, which provides high limiting accuracy of less than 0.1 dB at a 10-GHz bit rate. The limiting function was tested in a simulation and an experiment for 10-GHz bit sequence with intensity fluctuation. When fed a 1.6-dB sinusoidal intensity modulation at 2 GHz, the experimental setup successfully achieved optical limiting with 0.1-dB accuracy.

Highly Sensitive Signal Detection in Stimulated Parametric Emission Microscopy Based on Two-Beam Interferometry

We present a simple and highly sensitive optical detection method based on two-beam interferometry for application to coherent nonlinear optical microscopy (CNOM). The theoretical sensitivity of this method is higher than that of conventional spectral interferometry (SI). We experimentally applied this technique to stimulated parametric emission (SPE) microscopy and achieved a high sensitivity that is only 4 or 5 dB lower than that of a theoretical shot noise limit. In order to validate the practical applicability of this technique, we demonstrated a noise reduction experiment in the observation of a plant cell with an SPE microscope.

Nonlinear Ultrafast Focal-Point Optics for Microscopic Imaging, Manipulation, and Machining

When ultrafast laser pulses are tightly focused inside a transparent medium, various nonlinear optical phenomena readily occur at the focal point. These phenomena at the focal point can be utilized for nonlinear-optical microscopy, nanosurgery inside biological samples, device fabrication inside transparent materials, and welding of transparent materials. We review these new techniques from a unified viewpoint.