Kazuhiko Sumimura

All-polarization-maintaining Er-doped ultrashort-pulse fiber laser using carbon nanotube saturable absorber

We present an all-polarization-maintaining Er-doped ultrashort-pulse fiber laser using a single-wall carbon nanotube polyimide nanocomposite saturable absorber. The maximum average power for single-pulse operation is 4.8 mW, and the repetition frequency is 41.3 MHz. Self-start and stable mode-locking operation is achieved. The RF amplitude noise is also examined and it is confirmed that the noise figure is as low as that of a solid-state laser. Using a polarization-maintaining anomalous dispersive fiber, a 314 fs output pulse is compressed to 107 fs via higher-order soliton compression. The peak power of the compressed pulse is up to 1.1 kW.

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.

Polarization-maintaining, high-energy, wavelength-tunable, Er-doped ultrashort pulse fiber laser using carbon-nanotube polyimide film

A high-energy, wavelength-tunable, all-polarization-maintaining Er-doped ultrashort fiber laser was demonstrated using a polyimide film dispersed with single-wall carbon nanotubes. A variable output coupler and wavelength filter were used in the cavity configuration, and high-power operation was demonstrated. The maximum average power was 12.6 mW and pulse energy was 585 pJ for stable single-pulse operation with an output coupling ratio as high as 98.3%. Wide wavelength-tunable operation at 1532-1562 nm was also demonstrated by controlling the wavelength filter. The RF amplitude noise characteristics were examined in terms of their dependence on output coupling ratio and oscillation wavelength.

Stimulated Raman hyperspectral imaging based on spectral filtering of broadband fiber laser pulses

We demonstrate a technique of hyperspectral imaging in stimulated Raman scattering (SRS) microscopy using a tunable optical filter, whose transmission wavelength can be varied quickly by a galvanometer mirror. Experimentally, broadband Yb fiber laser pulses are synchronized with picosecond Ti:sapphire pulses, and then spectrally filtered out by the filter. After amplification by fiber amplifiers, we obtain narrowband pulses with a spectral width of <3.3 cm(-1) and a wavelength tunability of >225 cm(-1). By using these pulses, we accomplish SRS imaging of polymer beads with spectral information.

Ultralow-repetition-rate, high-energy, polarization-maintaining, Er-doped, ultrashort-pulse fiber laser using single-wall-carbon-nanotube saturable absorber

An ultralow-repetition-rate, all-polarization-maintaining (PM), Er-doped, ultrashort-pulse fiber laser was demonstrated using a single-wall-carbon-nanotube polyimide film. Using a ring cavity configuration, output pulses with pulse energy of 0.7-2.6 nJ were obtained at an ultralow repetition rate of 943-154 kHz for a fiber length of 0.1-1.3 km. A novel θ (theta) cavity configuration was proposed, which enabled us to reduce the required fiber length by half. A repetition rate of 132 kHz was achieved using this configuration with 909 m of PM fiber.

Quasi-super-continuum generation using ultrahigh-speed wavelength-tunable soliton pulses

Quasi super continuum generation at 1.1-1.4 µm is demonstrated using ultrahigh speed wavelength tuning of femtosecond soliton pulses for the first time. The center wavelength, bandwidth, and spectrum shape can be changed arbitrarily.

Quasi-supercontinuum generation using 1.06 μm ultrashort-pulse laser system for ultrahigh-resolution optical-coherence tomography.

We demonstrate quasi-supercontinuum (quasi-SC) generation around the 1.3 μm wavelength region using ultrahigh-speed, wavelength-tunable, femtosecond soliton pulses based on an ultrashort-pulse laser system operating at a wavelength of 1.0 μm. The wavelength tuning range was from 1.0 to 1.9 μm, and the scanning speed was up to 1.3 MHz. A Gaussian-like quasi-SC with a bandwidth of 220 nm was generated at 1220 nm. The generated quasi-SC was used in an optical-coherence tomography system. High axial resolutions of 5.1 μm in air and 3.7 μm in tissue were obtained. A maximum sensitivity of 100 dB was achieved, and ultrahigh-resolution images of a hamster cheek pouch were observed.

Generation of 0.45-1.38 μm visible to near-infrared widely broadened supercontinuum using Er-doped ultrashort-pulse fiber laser system

Visible to near-infrared, widely broadened supercontinuum generation is demonstrated using an ultrashort-pulse fiber laser system. A 229 mW, 83 fs high-power ultrashort pulse was generated at 1550 nm in an Er-doped fiber-chirped pulse amplification system. An almost pedestal-free 107 fs second-harmonic pulse was generated at 780 nm using periodically poled LiNbO3. A 0.45-1.38 μm widely broadened supercontinuum was generated in a highly nonlinear photonic crystal fiber. In terms of spectral flatness, the total modulation bandwidth was within 14 dB. All of the fiber devices were fusion spliced so that the system showed good stability. The effect of a pulse-trapping phenomenon in the supercontinuum generation process was discussed from experimental considerations.

Laser Oscillation of Nd-Doped Silica Glass with High Thermal Shock Parameter

Laser oscillation with the input/output characteristics of bulk-type Nd-doped silica glass with a high thermal shock parameter was demonstrated for the first time. The thermal shock parameter was estimated to be 12.0 W/cm, which is 1.5 times larger than that of Nd:YAG crystal (7.9 W/cm). The maximum output energy of 154 mJ was obtained with an output mirror of 20% transmittance. Although the output energy was 154 mJ, if without loss, output energy increases up to 1.69 J, eleven times larger than the present state. These results mean that Nd-doped silica glass can work as a high-average or a high-peak-power-laser.