Advances in ultrashort laser pulse technology have recently generated several new applications in fields as diverse as laser processing, medical and bio-optics, opto-electronics, etc. However, ultrashort-pulse lasers require quiet laboratory environments and water cooling, which prevents their use for practical applications. Spectroscopic applications also require a wavelength-tunable ultrashort pulses. The tuning range, however, is generally limited by the gain bandwidth of the laser. Since these pulses are generated with wavelength conversion using nonlinear crystals and mechanical tuning, it is difficult to generate wideband tunable pulses.
Of significant interest is the recent development of compact ultrashort-pulse fiber lasers. Since these lasers consist of fiber-optic devices, they produce stable pulses and do not require water cooling. Fiber lasers are also maintenance-free and can work wherever there is electricity. The wavelength of the output pulses, however, can only be changed within the gain bandwidth of the fiber amplifier and the bandwidth is less than 100nm. In our work, we investigate all-fiber wideband utrashort-pulse sources based on ultrashort-pulse fiber lasers and study nonlinear effects in optical fibers.
Specialty fibers, such as highly nonlinear fibers and photonic crystal fibers, were recently developed. Their characteristics are determined by their structure and they have demonstrated high energy density and dispersion tunability. Using ultrashort-pulse lasers and suitable specialty fibers, we can generate ultrawideband optical spectra, referred to as a super continuum.1 All-fiber super-continuum sources have also been reported.2 However, their noise level is generally high and the super continuum is characterized by excessive fine structure. To date, no practical ultrawideband super continuum has been generated. Our first efforts were accordingly focused on producing widely-tunable ultrashort pulses, subsequently used to generate—for the first time ever—a high-quality super continuum.
Our wavelength-tunable ultrashort-pulse source consists of only an ultrashort- pulse fiber laser and nonlinear fibers.3 When the ultrashort pulse couples with the optical fibers, pulse breakup occurs through stimulated Raman scattering and a wavelength-tunable soliton pulse is generated from the nonlinear optical phenomena occurring in the fibers. As the fiber input power is increased, the magnitude of the wavelength shift is also increased monotonically. The soliton pulses and the highly nonlinear fibers then generate a low-noise, ultraflat super continuum.
At first, we generated ultrashort pulses tunable from 1.55 to 2.0μm using a 1.55μm Er-doped ultrashort-pulse fiber laser (see Figure 1).3 We then generated 1.0 to 1.7μm tunable pulses with a 1.0μm Yb-doped ultrashort-pulse fiber laser using photonic-crystal fibers as the nonlinear fibers (see Figure 2).4 Super-continuum generation was again achieved.5
Figure 1. Optical spectra of wavelength-tunable soliton pulses that were generated through nonlinear effects. As the fiber input power is increased, the center wavelength is continuously red-shifted. We can tune the wavelength from 1.55 to 2.0μm.
Figure 2. Wavelength-tunable soliton pulse generation using a 1.0μm ultrashort-pulse fiber laser and photonic-crystal fibers is shown with a 1.06 to 1.7μm wavelength shift.
The characteristics of the pump pulse and of the nonlinear fibers represent key parameters in the production of an ultraflat super continuum. In our work, we use a soliton pulse and highly nonlinear fibers with small dispersion. As a result, the self phase modulation becomes the dominant nonlinear effect for spectrum broadening, generating an ultraflat super continuum with low noise levels and high coherence that makes it an excellent optical frequency comb for metrology applications (see Figure 3).
Figure 3. Spectra of our high-quality super continuum are shown on both linear (top) and log scales (bottom). The spectrum is broadened from 1.3 to 2.0μm and the flatness is within ±1dB over the 500nm bandwidth.
The high-power soliton pulse is generated by increasing the power of the pump laser diode and the large mode area of the photonic-crystal fiber. Since the nonlinear coefficient of large-mode-area photonic-crystal fibers is small, we can produce high-energy fundamental soliton pulses. Using normal-dispersion highly-nonlinear fibers, a more-than-an-octave-spanning high-quality super continuum can be generated, which is useful for optical frequency standards applications.
In conclusion, we have developed practical, all-fiber wideband ultrashort-pulse sources in which we can tune the center wavelength continuously by merely varying the optical power. These soliton pulses can be used to generate a high-quality super continuum. These systems are very compact and stable, and very promising for practical applications.