Feature Of The Week: 1/15/12: Review of Work by Nagoya University on Development & Application of Ultrashort Pulse Fiber Lasers
Passively mode-locked fiber lasers are stable and practical ultrashort pulse sources. We have been investigating the highly functional wideband ultrashort pulse fiber laser sources using ultrafast fiber nonlinear effects. These novel light sources are useful for multi-photon memory, multi-photon miscropscopy, spectroscopy, ultrahigh resolution optical coherence tomography, etc. In this talk, the development and applications of the highly functional ultrashort pulse fiber lasers are explained.
Recently, single wall carbon nanotube (SWNT) absorbs a lot of attention as the new nonlinear optical devices. It shows the saturable absorption properties and the recovery time is ~1 ps. The environmental stability is one of the key issues for fiber lasers. We have demonstrated all polarization maintaining (PM) type ultrashort pulse fiber laser using SWNT polyimide film. The polyimide film dispersed with SWNT is easily inserted between the fiber connectors and the self-start passive mode-locking is obtained stably. Since the SWNT has the wide operation bandwidth, we can demonstrate wavelength tunable operation using wavelength filter. The high power operation was also achieved using wavelength filter and variable output coupler. The 12.6 mW high output power was obtained when the output coupling ratio was as high as 98.3 %. A 585 pJ and 45.9 fs ultrashort pulse with peak power of 4.7 kW was generated after nonlinear compression.
Using the anomalous dispersive fibers and ultrashort pulses, we can generate widely wavelength tunable ultrashort pulse generation through soliton effect and intrapulse Raman scattering. In those systems, since the wavelength is continuously shifted by varying the fiber input power, the ultrafast wavelength tuning can be demonstrated using the intensity modulator. Recently, we have demonstrated ultrafast wavelength tunable ultrashort pulse sources using AO modulator. The quasi-super continuum generation is also demonstrated by the programmable rapid wavelength sweep motion. Wavelength tuning of ultrashort pulse in visible region was also demonstrated using pulse trapping phenomena in photonic crystal fibers.
Wavelength-tunable, narrow-linewidth light sources are important for spectroscopy, nonlinear microscopy, metrology, optical communication, and swept-source optical coherence tomography (SS-OCT). If we can compress the optical spectra of the generated soliton pulses, it will be possible to demonstrate wideband and ultrafast wavelength-tunable narrow-linewidth light sources. Recently, we demonstrated wideband spectral compression of wavelength-tunable soliton pulses based on an adiabatic soliton spectral compression technique in dispersion-increasing fiber. A technique of comb-like dispersion profiled fiber (CPF), which has been developed for pulse compression and high-repetition-rate pulse generation, was employed to realize the dispersion-increasing fiber. The CPF was carefully designed by numerical analysis and fabricated by fusion splicing a conventional single-mode fiber and a dispersion-shifted fiber. High-quality spectral compression was successfully demonstrated in a wide wavelength region using this technique. The well-compressed pulse spectra were obtained in the 1620–1840 nm wavelength region. The original pulse spectrum with a width of 12.3–15.1 nm was compressed to 0.54–0.71 nm. This light source is useful for spectroscopy and ultra-high resolution SS-OCT.
Using the ultrashort pulse and highly nonlinear fibers, we can generate wideband super continuum (SC). Generally, the wideband SC has inherent noise and spectral modulation and they cause problems in the practical applications. We have been investigating the generation mechanism of wideband SC and we have succeeded in the generation of high quality SC. The sech2 shaped Raman shifted ultrashort soliton pulse was generated and used as pump pulse. The ultraflat and low noise SC was generated using normal dispersion highly nonlinear fibers. This SC is low noise, almost flat, and stable and it is useful for practical applications. We have also demonstrated the SC generation broadened from 0.45 to 1.4 um wavelength in the ultrashor pulse fiber laser based system. It covers the almost whole visible region.
High quality SC is useful for ultrahigh resolution optical coherence tomography (UHR-OCT). So far, the practical wideband and high power light source has been one of the obstacles in the realization of UHR-OCT. We have been investigating the SC sources for UHR-OCT. Using the normal dispersion fibers, we can generate low noise, Gaussian shaped SC. The UHR-OCT imaging of human retina observed with Gaussian-shaped SC at center wavelength of 830 nm. The side-lobe free clear and precise images of human retina were observed.
Recently, we have demonstrated the ultrafast near infrared spectroscopic measurement using the electrically controlled wavelength tunable narrow line width source by spectral compression of soliton pulse by comb-profile fibers. The spectral fine structures were clearly observed as the temporal shapes.
For more information see recent Article. Courtesy Norihiko Nishizawa from Nagoya University.