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Here we report the demonstration of a spectral peaking phenomenon in a fiber laser oscillator. An HCN gas cell was inserted in an ultrashort-pulse Er-doped fiber laser with single-wall carbon nanotubes. Sech2-shaped ultrashort pulses with intense multiple sharp spectral peaks were stably generated. When the generated pulses were coupled into highly nonlinear fiber, enhanced multiple spectral peaks were generated by periodical spectral peaking in the optical fiber. The characteristics and physical mechanism of spectral peaking in the fiber laser were investigated via numerical simulations. As the magnitude of absorption was increased, the magnitude of the generated spectral peaks increased almost exponentially. It was clarified that the spectral peaks were generated through the accumulation of filtering components generated in each round trip.

We demonstrated spectral peaking phenomenon in fiber laser cavity for the first time. Ultrashort soliton pulse with multiple sharp spectral peaks were successfully generated in Er-doped SWNT fiber laser with HCN gas cell.

2.2 µm cavity ring-down spectrometer was developed for tritium and deuterium isotope analysis of water samples. Spectra were obtained during flow-through measurement with H2O and D2O mixed liquid injection and carry-over was negligible.

We quantitatively investigated the image quality in deep tissue imaging with optical coherence microscopy (OCM) in the 1700 nm spectral band, in terms of the signal-to-background ratio (SBR) and lateral resolution. In this work, to demonstrate the benefits of using the 1700 nm spectral band for OCM imaging of brain samples, we compared the imaging quality of OCM en-face images obtained at the same position by using a hybrid 1300 nm/1700 nm spectral domain (SD) OCM system with shared sample and reference arms. By observing a reflective resolution test target through a 1.5 mm-thick tissue phantom, which had a similar scattering coefficient to brain cortex tissue, we confirmed that 1700 nm OCM achieved an SBR about 6-times higher than 1300 nm OCM, although the lateral resolution of the both OCMs was similarly degraded with the increase of the imaging depth. Finally, we also demonstrated high-contrast deep tissue imaging of a mouse brain at a depth up to 1.8 mm by using high-resolution 1700 nm SD-OCM.

Nonlinear fiber effects are useful for controlling optical spectra in a wide variety of ways. Here, we report the demonstration of freely controllable, intense spectral peaking using a high-resolution spectral filter with a liquid-crystal spatial light modulator and nonlinear fibers. A large enhancement of spectral peak components by more than a factor of 10 was achieved by employing phase modulation. Multiple spectral peaks with an extremely high signal-to-background ratio (SBR) of up to 30 dB were generated simultaneously in a wide wavelength range. It was shown that part of the energy from the whole pulse spectrum was concentrated at the filtering part and constructed the intense spectral peaks. This technique is very useful for highly sensitive spectroscopic applications and comb mode selection.

Highly efficient Tm-Ho co-doped ultrashort pulse fiber laser operating at 1.9 μm was developed using single wall carbon nanotube. Wideband supercontinuum at 2.0 μm was generated and high-resolution OCT imaging of human tooth was demonstrated.

Spectral peaking during supercontinuum generation in normal dispersive highly nonlinear fiber was investigated both numerically and experimentally. Sharp multiple spectral peaks were generated stably with 290 GHz frequency interval, and low noise properties were confirmed.

Optical coherence tomography (OCT) is a noninvasive cross-sectional imaging technique with micrometer resolution. The theoretical axial resolution is determined by the center wavelength and bandwidth of the light source, and the wider the bandwidth, the higher the axial resolution. The characteristics of OCT imaging depend on the optical wavelength used. In this paper, we investigated the wavelength dependence of ultrahigh-resolution (UHR) OCT using a supercontinuum for biomedical imaging. Wideband, high-power, low-noise supercontinua (SC) were generated at λ = 0.8, 1.1, 1.3, and 1.7 μ m based on ultrashort pulses and nonlinear fibers. The wavelength dependence of OCT imaging was examined quantitatively using biological phantoms. Ultrahigh-resolution imaging of a rat lung was demonstrated with λ = 0.8–1.0 μ m UHR-OCT. The variation of alveolar volume was estimated using three-dimensional image analysis. Finally, UHR-spectral domain-OCT and optical coherence microscopy at 1.7 μ m were developed, and high-resolution and high-penetration imaging of turbid tissue, especially mouse brain, was demonstrated.

Wideband, octave spanning, coherent supercontinuum (SC) with high flatness is very useful for many applications, but it has been difficult to achieve such ideal SC. In this paper, we investigated the octave spanning, coherent SC generation based on Er-doped ultrashort pulse fiber laser system. First, we developed a high power ultrashort pulse system using a single-wall carbon nanotube fiber laser and similariton amplifier. A 1.0–2.2 μ m octave-spanning coherent supercontinuum was successfully generated. Next, by use of a stabilized fiber laser comb system as the seed pulse, an octave spanning SC comb was demonstrated. An octave spanning, highly coherent comb with high flatness was generated with normally dispersive highly nonlinear fiber. The characteristics of the generated supercontinuum comb were examined via balanced heterodyne beat measurements with stable continuous-wave laser diodes. The SC comb was confirmed to have high coherence and low amplitude noise at the observed wavelengths. Finally, the SC generation in two kinds of highly nonlinear fibers with different dispersion properties and the characteristics of the generated comb were examined. The dependence on fiber length, wavelength, and dispersion properties were discussed.

An offset-free, wavelength-tunable, and midinfrared frequency comb working at 184 MHz based on difference frequency generation (DFG) in a PPMgSLT crystal was demonstrated. A coherent Raman soliton pulse and Yb-fiber laser output were used as the seed pulses of DFG, and wavelength tunability in the range 3.1–5.2 μ m was realized. A quantum cascade laser stabilized by the absorption line of N2O was used to measure the beat signal with an MIR comb. The MIR comb had a good spatial beam profile, and a high signal-to-noise ratio (SNR) of 70 dB at a wavelength of 4.52 μ m was confirmed. A stable and sharp beat signal was observed in the MIR region with a linewidth of 160 kHz, and the SNR was larger than 20 dB.

We investigated multiple spectral peak generation during a coherent supercontinuum generation process with normal-dispersion highly nonlinear fibers both numerically and experimentally. Wideband multiple spectral peak generation was achieved in the 1.6–1.7 μm range using an ultrashort-pulse fiber laser, a CH4 gas cell, and highly nonlinear fiber. The maximum signal-to-background ratio was ∼8.4. Thanks to the normal-dispersion fibers, the induced phase shifts in the absorption spectra were clearly observed on the spectra during the spectral peak generation. The intensity and phase noise properties of the generated spectral peaks were examined, and low noise properties were confirmed. The spectral peaking phenomenon was investigated experimentally in a fiber amplifier. Periodical spectral peaking was successfully observed for a soliton pulse with Kelly sidebands, and the optical pulse experienced absorption in HCN gas. It is expected that spectral peaking occurs for pulses that experience absorption in many different kinds of gas species or those with spectral peaks. This light source and phenomenon will be useful for developing novel optical frequency comb techniques and optical wavelength standards.

A direct diode pumped continuous-wave Ti:sapphire laser (DDPTS) is presented. A bow-tie geometry with optical diode is chosen for unidirectional single-mode operation. Frequency selection is performed with a standard combination of birefringent filter and etalon. To accomplish mode-hop free frequency tuning the piezo-driven etalon is stabilized to one of the cavity modes via dither-locking method. To suppress environmental fluctuations the cavity-mode is additionally locked to an external optical cavity with low frequency drift. Feasibility of the setup for high-resolution spectroscopy is demonstrated by saturated absorption spectroscopy of the D2 line of Rubidium.

Optical coherence tomography (OCT) is a non-invasive cross-sectional imaging technique with micrometer resolution. OCT is useful, non-invasive imaging technique of the internal structure, and it has been applied in many fields, especially medicine and industry. The theoretical axial resolution is determined by the center wavelength and bandwidth of the light source, and the wider the bandwidth is, the higher the axial resolution is. Supercontinuum is the high-power, ultrawideband light source. We have been investigating ultrahigh-resolution (UHR)-OCT using supercontinuum. The characteristics of OCT imaging depend on the optical wavelength used. In this talk, we report our recent work of the wavelength dependence of UHR-OCT using a supercontinuum for biomedical imaging. In order to investigate the wavelength dependence of UHR-OCT, the wideband, high-power, low-noise supercontinua were generated at wavelengths of 0.8, 1.1, 1.3, and 1.7 um based on ultrashort pulses and nonlinear fibers. The wavelength dependence of OCT imaging was examined quantitatively using biological phantoms. Ultrahigh-resolution imaging of a rat lung was demonstrated with wavelengths of 0.8 – 1.0 um UHR-OCT. The variation of alveolar volume was estimated using 3D image analysis. We also developed UHR-spectral domain-OCT and optical coherence microscopy (OCM) at 1.7 um. The high-resolution and high-penetration imaging of turbid tissue, especially mouse brain, was demonstrated. The wavelength dependence of OCM was also discussed in terms of mouse brain imaging.

We developed 1.6-1.7 um wavelength tunable, polarization-maintaining dual comb fiber laser. Spectral width was controlled and wideband measurement was realized without bandpass filters. Absorption spectra of CH4 gas was observed in this wide range.

We present three-dimensional (3-D) high-resolution spectral-domain optical coherence microscopy (SD-OCM) by using a supercontinuum (SC) fiber laser source with 300-nm spectral bandwidth (full-width at half-maximum) in the 1700-nm spectral band. By using low-coherence interferometry with SC light and a confocal detection scheme, we realized lateral and axial resolutions of 3.4 and 3.8 μm in tissue (n=1.38), respectively. This is, to the best of our knowledge, the highest 3-D spatial resolution reported among those of Fourier-domain optical coherence imaging techniques in the 1700-nm spectral band. In our SD-OCM, to enhance the imaging depth, a full-range method was implemented, which suppressed the formation of a coherent ghost image and allowed us to set the zero-delay position inside the samples. We demonstrated the 3-D high-resolution imaging capability of 1700-nm SD-OCM through the measurement of an interference signal from a mirror surface and imaging of a single 200-nm polystyrene bead and a pig thyroid gland. Deep tissue imaging at a depth of up to 1.8 mm was also demonstrated. This is the first demonstration of 3-D high-resolution SD-OCM in the 1700-nm spectral band.

We demonstrated a dispersion-managed, high-power, Tm-doped ultrashort pulse fiber laser using a single-wall-carbon-nanotube (SWNT) polyimide film. SWNTs with a diameter of 1.6 nm were synthesized with the enhanced direct injection pyrolytic synthesis (e-DIPs) method, and thin polyimide films in which SWNTs were dispersed were developed as saturable absorbers in the wavelength range λ = 1.8–2.0 µm. An all-fiber type, passively mode-locked, ultrashort-pulse Tm-doped fiber laser was demonstrated using the developed SWNT films. Wavelength tuning operation with gain fiber control and dispersion management of the developed fiber laser were investigated. Stable soliton and dissipative soliton mode locking operations were observed. High-power (102.6 mW) single-pulse mode-locking operation was achieved in a large positive dispersion regime. The repetition rate was 21.6 MHz, and the corresponding pulse energy was 4.75 nJ. To the best of our knowledge, this is the highest power operation of a Tm-doped fiber laser using carbon nanotubes and film-type devices with nano-carbon materials. The developed laser showed self-staring, stable performance and is useful for practical applications.

A linewidth reduction of a distributed feedback quantum cascade laser (DFB-QCL) based on optical feedback for a mid-infrared (MIR) cavity ring down spectroscopy (CRDS) 14C spectrometer is presented. A cat-eye reflector as well as a path-length enhancement by a Herriott cell were employed for a compact optical setup. The laser linewidth was evaluated by monitoring the beat frequency between the DFB-QCL and a MIR optical frequency comb (OFC). The linewidth reduction by optical feedback was clearly observed although slow frequency drifts caused by environmental changes were still visible. A low-bandwidth beat-note lock to the OFC was conducted for long-term stabilization as well as for precise frequency scanning and thereby 14C measurement by MIR CRDS with optical feedback was demonstrated successfully.

Using nonlinear optical effects, we can control the optical spectra in various ways. Here, we discovered a novel phenomenon in ultrafast nonlinear fiber optics, namely, periodical spectral peaking. A narrow spectral dip on a pulse turns into a sharp, intense peak through a nonlinear effect. This phenomenon shows periodical evolution behavior in the soliton regime. If we use absorption spectra with constant frequency separation, like molecular gas absorption spectra, we can generate almost uniform frequency spanning sharp spectra with a linewidth on the order of a few picometers and sub-THz frequency separation. The generated spectral peak almost replicates the characteristics of the absorption spectra. Fundamental characteristics and physical mechanisms were investigated both numerically and experimentally. This phenomenon provides us with a novel approach to control the optical spectra and opens up new aspects and applications of nonlinear fiber optics.

1.6-1.9 um wavelength tunable comb with spectral width of 4 nm was demonstrated using wavelength tunable soliton pulse and comb profile fiber. High SNR of 37 dB was confirmed for generated comb by beat measurement.

1.6-1.95 μm wavelength tunable, narrow linewidth comb was generated in two different schemes. Comb properties with high SNR were experimentally confirmed using heterodyne beat measurement with cw-LD, and dual comb beat measurement with supercontinuum comb

Bi-directional, Er-doped dual comb fiber laser was demonstrated using polyimide film dispersed with single wall carbon nanotube. Difference of repetition frequency was temporally stable and it could be tuned continuously by pump power control.

We investigated the axial resolution and signal-to-noise ratio (SNR) characteristics in deep-tissue imaging by 1.7-μm optical coherence tomography (OCT) with the axial resolution of 4.3 μm in tissue. Because 1.7-μm OCT requires a light source with a spectral width of more than 300 nm full-width at half maximum to achieve such high resolution, the axial resolution in the tissue might be degraded by spectral distortion and chromatic dispersion mismatching between the sample and reference arms. In addition, degradation of the axial resolution would also lead to reduced SNR. Here, we quantitatively evaluated the degradation of the axial resolution and the resulting decrease in SNR by measuring interference signals through a lipid mixture serving as a turbid tissue phantom with large scattering and absorption coefficients. Although the axial resolution was reduced by a factor of ∼6 after passing through a 2-mm-thick tissue phantom, our result clearly showed that compensation of the dispersion mismatching allowed us to achieve an axial resolution of 4.3 μm in tissue and improve the SNR by ∼5 dB compared with the case where dispersion mismatching was not compensated. This improvement was also confirmed in the observation of a hamster’s cheek pouch in a buffer solution.

Optical coherence microscopy (OCM) is a high-resolution imaging technique based on optical coherence tomography and confocal microscopy. The recent studies on OCM operating at 800-1300 nm spectral region have shown that OCM enables to visualize micrometer- or sub-micrometer-scale structures of animal tissues. Although OCMs offers such high-resolution label-free imaging capability of animal tissues, the imaging depth was restricted by multiple light scattering and light absorption of water in samples. Here, for high-resolution deep-tissue imaging, we developed an OCM in the 1700-nm spectral band by using a supercontinuum (SC) source with a Gaussian-like spectral shape in the wavelength region. Recently, it has been reported that the 1700-nm spectral band is a promising choice for enhancing the imaging depth in the observation of turbid scattering tissues because of the low attenuation coefficient of light. In this study, to clarify that the 1700-nm OCM has a potential to realize the enhanced imaging depth, we compared the attenuation of the signal-to-noise ratio between the 1700-nm and 1300-nm OCM imaging of a mouse brain under the same signal detection sensitivity condition. The result shows that the 1700-nm OCM enables us to achieve the enhanced imaging depth. In this 1700-nm OCM, we also confirmed that the lateral resolution of 1.3 µm and axial resolution of 2.8 µm in tissue were achieved.

We developed full-range, ultrahigh-resolution (UHR) spectral-domain optical coherence tomography (SD-OCT) in 1.7 um wavelength region for high-resolution and deep-penetration OCT imaging of turbid tissues. To realize an ultrahigh axial resolution, the ultra-broadband supercontinuum source at 1.7 um wavelength with a spectral width of 0.4 um at FWHM and home-built spectrometer with a detection range from 1.4 to 2.0 um were employed. Consequently, we achieved the axial resolution of 3.6 um in tissue (a refractive index n = 1.38). To observe deep regions of turbid tissues while keeping the ultrahigh axial resolution, a full-range OCT method to eliminate a coherent ghost image was utilized for our UHR-SD-OCT. Because the full-range method allows us to avoid the formation of a coherent ghost image when the zero delay position is in the inside of specimens, we set the zero delay position to the laser focus position in this study, and then, a region of interest in specimens was moved to the laser focus position where the highest signal intensity is achieved, resulting in the improvement of the observation depth. Thanks to the deep-penetration property of the 1.7 um light and elimination of a ghost image, we successfully demonstrated the visualization of the mouse brain structures at a depth over 1.5 mm from the surface with the 1.7 um UHR-SD-OCT. In this experiment, we confirmed that the brain specific structures, such as corpus callosum, pyramidal cell layer, and hippocampus, were clearly observed.

Rare-earth-doped nanoparticles are one of the emerging probes for bioimaging due to their visible-to-near-infrared (NIR) upconversion emission via sequential single-photon absorption at NIR wavelengths. The NIR-excited upconversion property and high photostability make this probe appealing for deep tissue imaging. So far, upconversion nanoparticles include ytterbium ions (Yb3 + ) codoped with other rare earth ions, such as erbium (Er3 + ) and thulium (Tm3 + ). In these types of upconversion nanoparticles, through energy transfer from Yb3 + excited with continuous wave light at a wavelength of 980 nm, upconversion emission of the other rare earth dopants is induced. We have found that the use of the excitation of Er3 + in the 1550-nm wavelength region allows us to perform deep tissue imaging with reduced degradation of spatial resolution. In this excitation–emission process, three and four photons of 1550-nm light are sequentially absorbed, and Er3 + emits photons in the 550- and 660-nm wavelength regions. We demonstrate that, compared with the case using 980-nm wavelength excitation, the use of 1550-nm light enables us to moderate degradation of spatial resolution in deep tissue imaging due to the lower light scattering coefficient compared with 980-nm light. We also demonstrate that live cell imaging is feasible with this 1550 nm excitation.

Figure-nine fiber lasers can realize all-polarization-maintaining, self-started, highly stable mode-locked laser sources, and are very attractive for applications such as optical frequency combs, metrology, etc. In this work, we investigated a dispersion-managed, polarization-maintaining, Er-doped, ultrashort-pulse figure-nine fiber laser both experimentally and numerically. Stable, self-started, passive mode-locking operation was achieved in a wide net cavity dispersion region, covering the soliton, stretched pulse, and dissipative soliton mode-locking regimes. A 132 fs ultrashort pulse with spectral width of 46 nm was obtained in the stretched pulse mode-locking regime. The initial mode-locking process and dynamics inside the cavity, in addition to the fundamental characteristics of the output pulses, were examined via numerical analysis. Owing to the asymmetric configuration, the propagation behaviors were different between the two counter-propagation directions. It was found that a large breathing had already started before the passive mode-locking point in stretched pulse mode-locking operation. Intense overshoots were also observed at the beginning of passive mode-locking. Numerical results were almost in agreement with the experimental ones.

We demonstrated a bi-directional, Er-doped dual comb fiber laser consisting of all-polarization-maintaining fiber devices. Polyimide films in which single-wall carbon nanotubes (SWNTs) were dispersed were used as the in-line saturable absorber. In order to avoid synchronization of the two combs and associated damage to the SWNT film, a two-branch configuration with two SWNT films was employed. Soliton pulses with almost the same optical spectra were generated stably in each direction, and dual comb beats were observed simply by overlapping the two outputs. The repetition frequency was 28 MHz, and the frequency difference was 105-140 Hz. Thanks to the small frequency difference, dual comb beats corresponding to the whole optical spectrum were observed without any overlapping. Fourier transform spectroscopy using the developed dual comb source was examined, and the characteristics of an optical filter were successfully obtained.

A Gaussian-like supercontinuum with a bandwidth of 180-nm in 2.1-µm wavelength was generated with an Er-doped fiber laser, Tm-doped fiber amplifier, and highly nonlinear fiber. The seed pulse was generated by Raman soliton effect.

Stable operation of all polarization maintaining optical frequency comb was demonstrated based on Er-doped ultrashort pulse fiber laser with carbon nanotube. The fceo and frep were stabilized and standard deviation of frep was 3.4 mHz.

Offset free mid-infrared optical frequency comb at 4.52 μm was generated through DFG pumped by Yb-doped fiber laser system. Narrow RF beat between MIR comb and quantum cascade laser was observed with high SNR.

All polarization maintaining optical frequency comb was demonstrated based on Er-doped ultrashort pulse fiber laser with carbon nanotube. The f ceo and f rep were stabilized and their linewidths were compressed below 1 Hz by phase locking.

Offset free, 4.4–5.2 μm wavelength tunable, mid-infrared optical frequency comb was generated through DFG pumped by Yb-doped fiber laser system. Coherence verification was performed by beat-note measurement using quantum cascade laser.

Octave spanning supercontinuum comb generation was demonstrated using stabilized fiber laser comb, and highly nonlinear normal dispersive and zero dispersive fibers. Characteristics of spectral shape and coherence were examined and fiber length dependence was discussed.

Supercontinuum is useful light source for ultrahigh resolution optical coherence tomography (OCT) imaging. In this talk, the recent investigations about ultrahigh resolution OCT imaging using supercontinuum and their wavelength dependence are reviewed.

A simple external optical feedback system has been applied to a distributed feedback quantum cascade laser (DFB QCL) for cavity ring-down spectroscopy (CRDS) and a clear effect of feedback was observed. A long external feedback path length of up to 4m can decrease the QCL linewidth to around 50kHz, which is of the order of the transmission linewidth of our high finesse ring-down cavity. The power spectral density of the transmission signal from high finesse cavity reveals that the noise at frequencies above 20kHz is reduced dramatically.