Octave-spanning spectrum of femtosecond
Yb:fiber ring laser at 528 MHz repetition rate in
microstructured tellurite fiber
Guizhong Wang, Tongxiao Jiang, Chen Li, Hongyu Yang, Aimin Wang,* and
Zhigang Zhang
State Key Laboratory of Advanced Optical Communication System and Networks, School of Electronics Engineering
and Computer Science, Peking University, Beijing 100871, China
*
[email protected]
Abstract: The octave-spanning spectrum was generated in a tellurite glass
based microstructured fiber pumped by a 528 MHz repetition rate Yb:fiber
ring laser without amplification. The laser achieved 40% output optical-tooptical efficiency with the output power of 410 mW. By adjusting the
grating pair in the cavity, this oscillator can work at different cavity
dispersion regimes with the shortest dechirped pulse width of 46 fs. The
output pulses were then launched into a high-nonlinearity tellurite fiber,
which has the zero-dispersion wavelength at ~1 μm. The high nonlinearity
coefficient (1348 km−1W−1) and the matched zero-dispersion wavelength
with pump laser enable the octave-spanning supercontinuum generated
from 750 nm to 1700 nm with the coupled pulse energy above 10 pJ.
©2013 Optical Society of America
OCIS codes: (320.7090) Ultrafast lasers; (140.3510) Lasers, fiber.
References and links
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
S. T. Cundiff, “Metrology: new generation of combs,” Nature 450(7173), 1175–1176 (2007).
A. Bartels, R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and T. Dekorsy, “Ultrafast time-domain
spectroscopy based on high-speed asynchronous optical sampling,” Rev. Sci. Instrum. 78(3), 035107 (2007).
T. Wilken, C. Lovis, A. Manescau, T. Steinmetz, L. Pasquini, G. Lo Curto, T. W. Hänsch, R. Holzwarth, and Th.
Udem, “High-precision calibration of spectrographs,” Mon. Not. R. Astron. Soc. 405(1), L16–L20 (2010).
T. Wilken, T. W. Hänsch, Th. Udem, T. Steinmetz, R. Holzwarth, A. Manescau, G. Lo Curto, L. Pasquini, and
C. Lovis, “High precision Calibration of Spectrographs in Astronomy,” Conference on Laser and Electro-Optics
(CLEO), paper CMHH3 (2010).
A. Bartels, D. Heinecke, and S. A. Diddams, “10-GHz self-referenced optical frequency comb,” Science
326(5953), 681 (2009).
M. Hofer, M. E. Fermann, F. Haberl, M. H. Ober, and A. J. Schmidt, “Mode locking with cross-phase and selfphase modulation,” Opt. Lett. 16(7), 502–504 (1991).
D. Ma, Y. Cai, C. Zhou, W. Zong, L. Chen, and Z. Zhang, “37.4 fs pulse generation in an Er:fiber laser at a 225
MHz repetition rate,” Opt. Lett. 35(17), 2858–2860 (2010).
T. Wilken, P. Vilar-Welter, T. W. Hänsch, and Th. Udem, “High repetition rate, tunable femtosecond Yb-fiber
laser,” Conference on Laser and Electro-Optics (CLEO), paper CFK2 (2010).
A. Wang, H. Yang, and Z. Zhang, “503MHz repetition rate femtosecond Yb: fiber ring laser with an integrated
WDM collimator,” Opt. Express 19(25), 25412–25417 (2011).
H. Yang, A. Wang, and Z. Zhang, “Efficient femtosecond pulse generation in an all-normal-dispersion Yb:fiber
ring laser at 605 MHz repetition rate,” Opt. Lett. 37(5), 954–956 (2012).
P. Li, G. Wang, C. Li, A. Wang, Z. Zhang, F. Meng, S. Cao, and Z. Fang, “Characterization of the carrier
envelope offset frequency from a 490 MHz Yb-fiber-ring laser,” Opt. Express 20(14), 16017–16022 (2012).
A. Wang, H. Yang, C. Li, and Z. Zhang, “Octave-spanning spectrum generation with a 503MHz repetition rate
femtosecond Yb:fiber ring laser,” Conference on Laser and Electro-Optics (CLEO), paper CTu3G.2 (2012).
A. Chong, W. H. Renninger, and F. W. Wise, “Route to the minimum pulse duration in normal-dispersion fiber
lasers,” Opt. Lett. 33(22), 2638–2640 (2008).
L. Nugent-Glandorf, T. A. Johnson, Y. Kobayashi, and S. A. Diddams, “Impact of dispersion on amplitude and
frequency noise in a Yb-fiber laser comb,” Opt. Lett. 36(9), 1578–1580 (2011).
Y. Song, C. Kim, K. Jung, H. Kim, and J. Kim, “Timing jitter optimization of mode-locked Yb-fiber lasers
toward the attosecond regime,” Opt. Express 19(15), 14518–14525 (2011).
C. Farrell, K. A. Serrels, T. R. Lundquist, P. Vedagarbha, and D. T. Reid, “Octave-spanning super-continuum
from a silica photonic crystal fiber pumped by a 386 MHz Yb:fiber laser,” Opt. Lett. 37(10), 1778–1780 (2012).
#179189 - $15.00 USD
(C) 2013 OSA
Received 2 Nov 2012; revised 29 Jan 2013; accepted 1 Feb 2013; published 19 Feb 2013
25 February 2013 / Vol. 21, No. 4 / OPTICS EXPRESS 4703
17. H. Hundertmark, S. Rammler, T. Wilken, R. Holzwarth, T. W. Hänsch, and P. S. Russell, “Octave-spanning
supercontinuum generated in SF6-glass PCF by a 1060 nm mode-locked fibre laser delivering 20 pJ per pulse,”
Opt. Express 17(3), 1919–1924 (2009).
18. P. Domachuk, N. A. Wolchover, M. Cronin-Golomb, A. Wang, A. K. George, C. M. B. Cordeiro, J. C. Knight,
and F. G. Omenetto, “Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter
segments of highly nonlinear tellurite PCFs,” Opt. Express 16(10), 7161–7168 (2008).
19. A. Ishizawa, T. Nishikawa, S. Aozasa, A. Mori, O. Tadanaga, M. Asobe, and H. Nakano, “Demonstration of
carrier envelope offset locking with low pulse energy,” Opt. Express 16(7), 4706–4712 (2008).
20. D. D. Hudson, S. A. Dekker, E. C. Mägi, A. C. Judge, S. D. Jackson, E. Li, J. S. Sanghera, L. B. Shaw, I. D.
Aggarwal, and B. J. Eggleton, “Octave spanning supercontinuum in an As₂S₃ taper using ultralow pump pulse
energy,” Opt. Lett. 36(7), 1122–1124 (2011).
21. X. Feng, T. M. Monro, V. Finazzi, R. C. Moore, K. Frampton, P. Petropoulos, and D. J. Richardson, “Extruded
singlemode, high-nonlinear, tellurite glass hoely fibre,” Electron. Lett. 41(15), 835–837 (2005).
22. T. Delmonte, M. A. Watson, E. J. O’Driscoll, X. Feng, T. M. Monro, V. Finazzi, P. Petropoulos, J. H. V. Price,
J. C. Baggett, W. Loh, D. J. Richardson, and D. P. Hand, “Generation of mid-IR continuum using tellurite
microstructured fiber,” Conference on Electro-Optics (CLEO), paper CTuA4 (2006).
1. Introduction
Ultrafast fiber lasers and their supercontinuum generation have attracted much attention in
many applications, such as the optical coherence tomography, spectroscopy, and frequency
metrology [1–3]. Particularly, for an optical frequency comb, spectrum over one octave of
bandwidth is necessary for the f–to–2f interferometer technique [4, 5].
The high repetition rate Yb:fiber lasers are desirable to achieve sufficiently large comb
line spacing for high-precision calibration of astronomical spectrographs. The nonlinear
polarization evolution (NPE) mode-locked fiber ring lasers with large modulation depth and
essentially instantaneous response support a broad spectrum for sub-50 fs pulse generations
[6, 7], which becomes a popular technique for the seed pulse source of laser frequency combs.
However, limited by the ring cavity length, high fundamental repetition rate is still
challenging for Yb:fiber ring lasers. The repetition rate operation up to 570 MHz in Yb:fiber
laser with free-space coupled high pump power (1.4W) was demonstrated by Wilken et al [8].
Recently, a newly-developed technique of wavelength-division-multiplexing (WDM)
collimator [9–11] succeeds to remove the pigtailed fiber and has been proved to be an
efficient way to obtain a higher repetition rate in fiber ring laser. On the other hand, allnormal dispersion (ANDi) fiber laser without the intracavity grating pair can contribute to
higher repetition rate [10, 13]. However, the large net normal dispersion of ANDi fiber laser
increases the noise level and timing jitter, which degraded the signal noise ratio of the
detected carrier envelop offset frequency (fceo) [14, 15]. Therefore, a near zero dispersion
cavity is preferred for a frequency comb, and the grating pair is usually used to adjust the
cavity dispersion.
Higher repetition rate can directly lead to lower delivered pulse energy, which makes the
succeeded spectrum broadening difficult. To date, external amplification has been applied to
obtain enough pulse energy for the octave spanning spectrum in a nonlinear fiber with the
laser repetition rate above 386 MHz [12, 16]. However, the amplification process may
introduce amplified spontaneous emission (ASE) noise, and make the system more complex
and instable. To avoid all of these, high-nonlinearity fibers have been employed such as the
soft glass fibers, tellurite glass fibers and chalcogenide fibers [17–21] to adopt lower pulse
energies. Supercontinuum generation with further lower pulse energy is demanding as the
pulse repetition rate is increased.
In this letter, we report a 528 MHz repetition rate ring cavity femtosecond fiber laser and
its spectrum broadening in a tellurite microstructured fiber to generate octave-spanning
supercontinuum. The spectra obtained are from 750 nm to 1700 nm with the pulse energy
above 10 pJ. To our knowledge, this is the lowest pulse energy to obtain the octave-spanning
spectrum at the repetition rate above 500 MHz.
2. Set up of the 528 MHz Yb:fiber ring laser
The schematic of Yb:fiber laser is shown in Fig. 1. The fiber section comprises 12 cm length
of Yb doped fiber with the absorption of 1600 dB/m at 976 nm. Both of the WDM collimator
#179189 - $15.00 USD
(C) 2013 OSA
Received 2 Nov 2012; revised 29 Jan 2013; accepted 1 Feb 2013; published 19 Feb 2013
25 February 2013 / Vol. 21, No. 4 / OPTICS EXPRESS 4704
and fiber collimator had a length of 4.5 cm, giving a total fiber length 21 cm, the free-space
section was ~22 cm long containing a bulk Faraday rotator, wavelength plates, two
polarization beam splitters (PBS) and a pair of 1000 lines/mm fused silica transmission
gratings providing anomalous dispersion. The output pulses are rejected from the PBS and
dechirped with a pair of gratings outside the cavity.
Fig. 1. Schematic of 528 MHz Yb fiber ring laser (PBS: polarization beam splitter, FR: faraday
rotator, λ/2: half-wave plate, λ /4: quarter-wave plate, YDF: Yb doped fiber).
Compared with our previous experiment [12], the major difference is that the anomalous
dispersion fiber is replaced by a transmission grating pair. This makes the laser more
efficient. Another benefit is the adjustability of the grating separation that allows the fine
tuning of the dispersion for achieving the broadest spectrum.
The cavity dispersion was carefully designed. The calculated group delay dispersion
(GDD) of the total dispersion of for a 12 cm long Yb:fiber is + 2760 fs2. The 9 cm long single
mode fiber is + 2160 fs2. The dispersion of the 1000 lines/mm grating pair is calculated to be
−6300 fs2/mm for double pass and is slightly adjustable.
Two 650 mW 976 nm laser diodes were combined into a single mode fiber as the pump
laser which delivers a maximum effective pump power of 1.07 W. The pump power is
coupled into the Yb doped fiber with a WDM collimator, as described in [11]. The laser
threshold is 850 mW. At the maximum pump power, the output power of the mode-locked
laser was 410 mW which exhibits an optical-to-optical efficiency of ~40%. The output power
after the grating pair compressor is 307 mW. The laser was self-starting and last for a week
without covering with a box. This super stability is possibly due to the very short fiber length
and the compact of the laser structure.
The output pulse spectra and the corresponding autocorrelation traces after the grating pair
compressor are presented in Fig. 2. The measured pulse spectra widths were 19 nm, 33 nm
and 50 nm for the intracavity grating separation of 1.4 mm,1.1 mm and 0.8 mm respectively.
The corresponding minimum pulse durations were from 96 fs, 62 fs and 46 fs for the
Gaussian profile assumed. The broadband spectrum and the sub-50-fs pulse agree well with
our prediction that a shorter intracavity fiber will result in short pulse output. Sub-50fs pulses
are favorable for the subsequent spectrum expansion in the high nonlinear fiber.
The measured repetition rate is 527.7 MHz with signal to noise ratio of 60dB (Fig. 3(a)).
The phase noise spectrum was measured till 1 MHz and is shown in Fig. 3(b). The root mean
square of timing jitter was calculated to be 1.7 fs by the integration of the phase noise
spectrum from 1 kHz through 1 MHz. The relative intensity noise was measured to be < 130
dBc/Hz@1MHz.
#179189 - $15.00 USD
(C) 2013 OSA
Received 2 Nov 2012; revised 29 Jan 2013; accepted 1 Feb 2013; published 19 Feb 2013
25 February 2013 / Vol. 21, No. 4 / OPTICS EXPRESS 4705
Fig. 2. (a) (b) (c): spectra with the separation of grating pair at 1.4 mm (a), 1.1 mm (b) and 0.8
mm (c); (d) (e) (f): the corresponding measured autocorrelation traces of the compressed
pulses. The dechirped pulse width was calculated to be 96 fs (d), 62 fs (e) and 46 fs (f) with
Gauss profile assumed.
Fig. 3. (a): Radio frequency spectrum from 0 GHz to 1.1 GHz at the resolution bandwidth of
1MHz. Inset: Radio frequency spectrum at the resolution bandwidth of 10 kHz. (b): phase
noise spectrum. PSD: power spectral density.
3. Description of the tellurite glass based microstructured fiber
The electron microscope image of the cross section of the tellurite fiber used in this
experiment is shown in Fig. 4. The fiber structure is similar as the fiber in [21]: three very
#179189 - $15.00 USD
(C) 2013 OSA
Received 2 Nov 2012; revised 29 Jan 2013; accepted 1 Feb 2013; published 19 Feb 2013
25 February 2013 / Vol. 21, No. 4 / OPTICS EXPRESS 4706
fine filaments 5 μm long and ~100 nm wide supports a 1.2 μm diameter core. The nonlinear
coefficient of this fiber is estimated to be 1348 km−1W−1(with n2 = 2.5 × 10−19m2W−1).
The zero-dispersion-wavelength (ZDW) of the bulk tellurite glass is beyond 2 μm, and the
suspending-core structure can shift the ZDW to shorter wavelength. The modeled dispersion
of this fiber is shown in Fig. 4. It is seen that its ZDW was about 1 μm, a little below the
supercontinuum pump wavelength at 1.03 μm. Thus, the pump pulses propagate in the region
of anomalous dispersion, suitable for supercontinuum generation. The experimental data
about the dispersion at 780 nm and 920 nm were −500 ps/nm/km and −70 ps/nm/km, in
accordance with the modeled results.
Fig. 4. Dispersion curve of the tellurite fiber with the zero dispersion wavelength around 1 μm.
Inset: scanning electron micrograph of the core region in the tellurite fiber.
4. Octave-spanning spectrum generation
A 13 cm long tellurite fiber was used for supercontinuum generation. The dechirped pulses
were directly coupled into the tellurite fiber with a 1.5 mm focal length aspheric lens. The
launched pulse energy was increased from 1.3 pJ to 17.5 pJ. The resulting spectra, shown in
Fig. 5, are plotted on a logarithmic scale for different launched pulse energies. An octavespanning spectrum from 750 nm to 1700 nm was found at the pulse energy below 20 pJ.
It can be seen that the spectral broadening appeared at the pulse energy as low as 1.3 pJ.
After the pulse energy was increased to 3.7 pJ, the Raman peak shifted to 1150 nm in the
long-wavelength regime, and a peak appeared at 850 nm in the normal dispersion regime of
the fiber. Further broadening to 800nm on the short-wavelength side of the pump and the
Raman peak around 1650 nm were observed, when the launched energy reached 9.8 pJ. As
the pulse energy is further increased from 9.8 to 17.5 pJ, more Raman soliton peaks filled in
gaps between 1250 nm to 1600 nm, together with the spectral intensity increased. There are
two peaks located at 770 nm and 1540 nm in the spectrum. To the best of our knowledge, this
is the lowest pulse energy reported for the generation of the octave-spanning spectrum
directly from Yb-based fiber laser oscillator.
Such a low-pulse-energy broadened spectrum not only offer the possibility for the direct
generation of frequency comb from a Yb:fiber laser, but is of particular interest in the
development of astro-combs, which requires high repetition rate up to tens of GHz.
The octave-spanning spectra with longer tellurite fiber lengths were also tested. It is easy
to obtain octave-spanning spectrum across two peaks at 700-800 nm and 1400-1600 nm,
#179189 - $15.00 USD
(C) 2013 OSA
Received 2 Nov 2012; revised 29 Jan 2013; accepted 1 Feb 2013; published 19 Feb 2013
25 February 2013 / Vol. 21, No. 4 / OPTICS EXPRESS 4707
respectively. These peaks offer the source for the f -to-2f interference signal for Yb fiber laser
combs.
Mid-infrared signals beyond 1700 nm were also recorded by an infrared spectroscopy
(ocean optics NIR quest) in this experiment. With 17.5 pJ launched pulse energy, Raman
peaks at 2100 nm and 2300 nm were observed, with an intensity of 27 dB lower than the
maximum signal at 1030 nm. Because the tellurite glass has a high transmission through the
mid-infrared to 5 μm [22], this technique is potentially used for mid-infrared spectroscopy
and mid-infrared frequency comb. Further study in this wavelength region will be reported in
the near future.
Fig. 5. Spectrum evolution for the coupled pulse energy from 1.3 pJ to 17.5 pJ in the tellurite
fiber.
5. Conclusions
We have developed a compact 528 MHz repetition-rate and 46 fs Yb:fiber ring laser, and
obtained octave-spanning spectrum pumped by this laser in a microstructured tellurite fiber at
the coupled pulse energy of tens of pJ. The transmission grating pair and compact cavity
make the laser more efficient and deliver shorter pulses. Owing to the high output power and
short pulses, the spectrum expands a wavelength range from 750 nm to 1700 nm in a 13 cm
tellurite fiber. This is the first time to show the octave spanning spectrum at >500 MHz
repetition rate and at the lowest pulse energy. Such a broad spectrum can be used to develop a
simple and compact frequency comb with the f-to-2f interference technique. Furthermore, the
experiment also shows the capability of the fiber to expand the spectrum to mid-infrared with
low pulse energies.
Acknowledgments
The authors thank Tanya Monro and Yinlan Ruan of the University of Adelaide for providing
tellurite fibers. This work was supported in part by the National Natural Science Foundation
of China (60927010, 10974006, 110274046, 61177047, and 60907040), and the Templeton
Foundation.
#179189 - $15.00 USD
(C) 2013 OSA
Received 2 Nov 2012; revised 29 Jan 2013; accepted 1 Feb 2013; published 19 Feb 2013
25 February 2013 / Vol. 21, No. 4 / OPTICS EXPRESS 4708