IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 3, MAY/JUNE 2018 5100409 Octave Spanning Coherent Supercontinuum Comb Generation Based on Er-Doped Fiber Lasers and Their Characterization Norihiko Nishizawa , Member, IEEE, Toshiki Niinomi , Yoshitaka Nomura, Lei Jin , Member, IEEE, and Yasuyuki Ozeki (Invited Paper) Abstract—Wideband, octave spanning, coherent supercontin- uum (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 devel- oped a high power ultrashort pulse system using a single-wall car- bon nanotube fiber laser and similariton amplifier. A 1.0–2.2 μm octave-spanning coherent supercontinuum was successfully gener- ated. 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 characteris- tics of the generated supercontinuum comb were examined via bal- anced 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 differ- ent dispersion properties and the characteristics of the generated comb were examined. The dependence on fiber length, wavelength, and dispersion properties were discussed. Index Terms—Optical fiber lasers, optical fiber amplifiers, ul- trafast optics, nonlinear optics. I. INTRODUCTION S UPER continuum is the wideband optical spectrum broad- ened by nonlinear optical effects [1], [2]. It has high power, and ultra-wide bandwith, and it outputs from fibers. So it is useful for ultrahigh resolution optical coherence tomography (OCT) [3], [4], multiphoton microscopy [5], spectroscopy [6], etc. The ideal supercontinuum keeps the coherence, and it works as the ultrawideband laser source. Manuscript received August 15, 2017; revised September 29, 2017; accepted November 16, 2017. Date of publication November 22, 2017; date of current version December 12, 2017. (Corresponding author: N. Nishizawa.) N. Nishizawa and L. Jin are with the Department of Electronics, Nagoya University, Nagoya 464-0814, Japan (e-mail: nishizawa@nuee.nagoya-u.ac.jp; kin@nuee.nagoya-u.ac.jp). T. Niinomi and Yoshitaka Nomura are with the Department of Quantum Engineering, Nagoya University, Nagoya 464-0814, Japan (e-mail: niinomi. toshiki@k.mbox.nagoya-u.ac.jp; nomura.yoshitaka@a.mbox.nagoya-u.ac.jp). Y. Ozeki is with Electrical Engineering and Information Systems, University of Tokyo, Tokyo Japan (e-mail: ozeki@ee.t.u-tokyo.ac.jp). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2017.2776521 Optical frequency combs are highly accurate, equal- frequency spanning spectral chains that have allowed break- throughs in the field of metrology [7], [8]. Optical frequency combs have also been applied to various other fields, such as as- tronomy, high-resolution spectroscopy, accurate distance mea- surement, optical frequency standards, and biomedicine [9]– [12]. Fiber laser based comb systems are the most practical and reliable ones [13]–[17], and some groups have demon- strated fiber laser based broadband comb systems [18]–[20]. Broadband combs are useful for highly accurate wideband spec- troscopy, wideband frequency standards, breath analysis, and so on [12], [21]. Using the stabilized comb system and adequate highly nonlin- ear fibers, we can generate ultrawideband SC comb source. Gen- erally speaking, a broadband SC has been generated with highly nonlinear fiber whose zero-dispersion wavelength is close to the wavelength of the pump pulse [20], [21]. In those cases, although they have wide bandwidth, they have many fine structures on the spectral shape [22]. For highly accurate spectroscopy and other demanding applications where stable and highly sensitive measurements are required, a coherent wideband SC comb with good flatness is ideal. In addition, if we generate octave span- ning coherent SC comb, we can demonstrate optical arbitrary waveform generation [23]. In this paper, we investigated octave spanning coherent su- percontinuum comb generation based on ultrashort pulse fiber laser. First, we demonstrated the octave spanning coherent SC gen- eration based on ultrashort pulse Er doped fiber laser system. An ultrashort pulse fiber laser with single wall carbon nanotube (SWNT) was used as the seed pulse source [24], [25]. A high power, clean ultrashort pulse was generated with similariton amplifier [26], and octave spanning SC was generated in highly nonlinear fiber. Next, we demonstrated octave spanning coherent SC opti- cal frequency comb. A stabilized fiber laser comb system was developed, and octave spanning coherent SC comb was real- ized using the output of stabilized fiber laser comb. The co- herence properties were examined by balanced heterodyne beat measurement. 1077-260X © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.