ABSTRACT: Pulse shaping technology is present to mitigate pulse narrowing and SBS effects in high energy/power ns fiber lasers and to balance SPM and gain narrowing in high energy/power fs fiber lasers. © 2010 Optical Society of America OCIS codes: 140,3510; 140,3500; 140,4050. Introduction High energy pulsed fiber lasers have been considered to be an enabling technology to many applications such as Lidar, free space communications, and material processing  Ultrafast fiber lasers have been evolved into a new era for replacing solid-state ultrafast lasers and exploring new unprecedented applications. Close to kW and mJ level operation for fiber amplifiers have been demonstrate

Farzin Amzajerdian

Jian Liu

Lihmei Yang

Peng Wan

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ns and fs Fiber Lasers
(Invited Paper)
Jian Liu, Peng Wan, Lihmei Yang, Farzin Amzajerdian*
PolarOnyx, Inc., 2526 Qume Drive, Suites 17 & 18, San Jose, CA 95131
*
NASA Langley Research Center, MS 468, Hampton, VA 23681
jianliu@polaronyx.com
ABSTRACT: Pulse shaping technology is present to mitigate pulse narrowing and SBS effects in high
energy/power ns fiber lasers and to balance SPM and gain narrowing in high energy/power fs fiber lasers.
© 2010 Optical Society of America
OCIS codes: 140,3510; 140,3500; 140,4050.
1. Introduction
High energy pulsed fiber lasers have been considered to be an enabling technology to many applications such as Lidar, free space
communications, and material processing [1-13]. In eye safer Lidar applications, 200 ns pulse width is required. However, due to
gain narrowing effects and SBS nonlinear effects in high power and high energy fiber laser, the energy extraction is limited and
the pulse shape is distorted. In this paper, a special pulse shaping fiber laser will be discussed with over 1 W average power
operation and 0.15 mJ pulse energy at a wavelength of 1550 nm and with a pulse width of 200 ns.
Ultrafast fiber lasers have been evolved into a new era for replacing solid-state ultrafast lasers and exploring new
unprecedented applications. Close to kW and mJ level operation for fiber amplifiers have been demonstrated [10,11]. However,
those demonstrations involved many solid state components such as solid state seed oscillator and free space grating stretcher and
practically are hard to be robust systems. This paper also presents a breakthrough approach by introducing a simple MHz seed
fiber oscillator to generate MHz signal pulses and PCF amplifier system to scale the power and energy. It completely eliminates a
pulse picker and reduces significantly the complexity and cost of the high energy fs fiber laser system. 100 W output power and
100 micro-Joule were achieved with a pulse width at 600 fs.
2. Experiment on ns Pulse Shaping Fiber Lasers
We have successfully developed the driver for driving a laser at 1550 nm to achieve any wave format and pulse width for pulse
generation. We can generate muti-micro pulses in one macro-pulse. This achievement provides us with the capability for high
energy pulse amplification at 1550 nm in mitigating both SBS effects (by shaping micro-pulse) and gain narrowing effects (by
shaping macro-pulse).
Assuming a full-width at half maximum (FWHM) Brillouin linewidth, ΔSBS = 13 MHz at 1.5 μm, the dephasing time,
T2 20 ns, can be calculated from the relation:
T2= 1/(πSBS) (1)
T2 describes the required time to phase the created phonons, which establishes the macroscopic acoustic wave in the material. For
pulse widths less than T2, the SBS gain is significantly reduced.
Figure 1 shows experimental results for an amplifier with LMA double cladding Er-doped fibers (EDF). The core
diameter size is 17 micron. It is taken from the circulator port 3 (backward SBS). In Figure 2, the backward SBS generation
clearly shows a threshold for exponential growth. Exponential growth was observed as the amplifier’s pump level increased.
Seed laser was operated at 10 kHz repetition rate and square shape pulses were generated. The pulsed width (FWHM) varies
from 20 ns to 200 ns. Since SBS gain is proportional to laser’s intensity, we presented in Figure 2A the SBS power as a function
of peak power. The SBS power as a function of pulse energy was also plotted as a reference in Figure 2B. The SBS threshold
increases clearly for shorter pulse widths. When the pulse width is 20 ns, which is close to the dephasing time T2, SBS is
significantly suppressed.
Another important phenomena was observed is that the pulse was compressed significantly when it evolves through the
amplification stage. A 200 ns pulse width was compressed down to 10 ns at high energy level. This is partially due to the gain
dynamics and partially due to the soliton formation in the anomalous dispersion fiber at 1550 nm. The 200 ns pulse evolution at
different pump currents was tested. The seed macro-pulse width is 210 ns, the output pulse width reduces to 150 ns, 50 ns and 15
ns at 1A, 4A and 7A pump current respectively. By manipulating the macro pulse shape launched into the amplifier, the pulse
narrowing issue can be resolved.
We generated 200 ns pulses at the output of the high energy amplifier at various pump currents and repetition rates.
The pump current was tuned from 0 to 8.6 A and the repetition rate varied from 10 kHz to 100 kHz. The relationship between
pump power and pump current is characterized. Because of the phenomena of pulse narrowing during the power amplification,
we manipulate seed macro-pulse shape for each amplifying condition to maintain the square shape and 200 ns pulse width of
output pulses (as shown in Figure 2). The micro-pulse width is controlled at 20 ns. Output power and spectrum were measured in
our experiment. The output power as a function of pump current and the output pulse energy as a function of current is plotted.
The output powers were almost the same at different repetition rates ranging from 10 kHz to 100 kHz. The power was linearly
increased with pump current. The optical efficiency for the power amplifier is about 10%. Higher pulse energy is obtained at
lower repetition rate. The maximum pulse energy of 0.15 mJ was obtained at 10 kHz repetition rate and 8.6A pump current. No
SBS is observed. A prototype is developed and delivered.
3. Experiment on fs Pulse Shaping Fiber Lasers
Ultrafast fiber lasers, by definition, should be all fiber based laser system or with minimum non-fiber based components. At
lease the seed laser, stretcher, amplifiers should be fiber based [12,13]. Those lasers using only one or two pieces of fibers in the
amplifier stages should not be counted as fiber lasers. The progress in this direction is critical to practical industrial applications
by providing compact, robust and cost effective solutions. Up to 100 micro-J has been developed into product by using a pulse
picker method. However, due to the nonlinearity, gain narrowing, and third order dispersion (TOD) mismatch, the pulse width is
limited to sub-ps level. Moreover, the high price is limited for this pulse picker approach to be deployed widely.
In our experiment, 1 MHz all fiber seed oscillator in direct generation of 1 MHz mode locked pulses. We also used a
novel fiber-based dispersion management stretcher (patents 7,430224, 7430, 226, and 7,440,173) in dechirping the pulse and
reducing TOD to the minimum. One of the key pulse-shaping techniques is to manipulate the third-order dispersion (TOD) of the
fiber. Basically, the dispersion in the fiber is controlled by both material dispersion and waveguide dispersion. In the 1020–1090
nm spectral region, material dispersion shows a positive dispersion slope. With traditional fiber designs such as that for Corning’s
(Corning, NY) SMF-28, the TOD is always a positive number around 0.3 ps/nm2-km, which does not match the TOD of the
grating compressor used. However, by manipulating the fiber waveguide structure (with a depressed cladding), waveguide
dispersion can be introduced to modify the material dispersion such that the TOD and dispersion slope of the whole fiber system
are matched and compensated, especially with the grating compressor.
Bulk grating pair was used as a compressor. If photonic bandgap fiber (PBF) can be used in the compression stage, a
truly all fiber solution is provided for the high energy fiber laser without any discrete free space components. This is significant
for many industrial applications.
In the high power PCF amplifier, firstly, a photonic crystal fiber (PCF) is used to provide large mode field diameter (60
m) for signal amplification in the core and large numerical aperture (NA) as high as 0.8 in the cladding for coupling more pump
power into the fiber. This enables the use of a short length of fiber to high average output power and to reduce the effect of
nonlinearity for high energy pulse. Secondly, the PCF is highly doped so more power extraction can be accomplished. Figure 1
shows detailed design of the PCF high power amplifier with integrated pump solution. It has a total of 200 W pump power
lauched into the PCF amplifier. An experiment at 100 micro-J level energy (100 W average power) with a pulse width < 600 fs
has been demonstrated at a pulse repetition rate of 1 MHz (Figures 3 and 4) and with M2<1.3. The contrast ratio (peak
voltage/valley voltage) is higher than 36 dB, which indicates very clean pulse amplification.
4. Discussion and Conclusion
In summary, we have demonstrated a pulse shaping technology to generate pulse shapes in any formats for ns pulsed fiber lasers.
This technology is demonstrated to be an efficient way to mitigate SBS effects and pulse narrowing effects in high energy/power
fiber lasers. We successfully generated a 200 ns seed macro-pulse with micro-pulses less than 20 ns. The micro-pulses generation
allows us to suppress SBS significantly, thus dramatically increase the output energy limit. The macro pulse is shaped to be
against the pulse narrowing effects. After power amplification, an average power of over 1 W and the pulse energy of over 0.15
mJ were obtained at 1550 nm. Excellent pulse shape, high extinction ratio and power stability were achieved. A reliable
prototype was developed. To further increase the energy level, we have increased the energy to 0.5 mJ and average power to 5 W.
The laser becomes instable due to SBS and 1 micron lasing. SBS reduction can be achieved by reducing the micro pulse width to
less than 10 ns. More development effort is needed to suppress the 1 micron lasing.
We have also demonstrated a 100 W 600 fs MHz fs fiber laser with all integrated fiber optics components other than
compressor and eliminated pulse picker for reduction of system complexity and cost. This demonstration can be readily
converted to a product by minimum engineering. By adding another stage of amplifier, kW level of output power can be
expected.
Acknowledgement: This paper is supported in part by NASA, AFOSR and NSF contracts.
4. References
[1] G. C. Valley and M. Wright, “Modeling transient gain dynamics in a cladding pumped Yb doped fiber amplifier pulsed at low repetition rate,” CLEO, (2001).
[2] H. Hemmati, M. Wright, and C. Esproles, “High efficiency pulsed laser transmitters for deep space communications,” SPIE 3932, 188-195 (2000).
[3] R. W. Ziolkowski and Justin B. Judkins, “Propagation characteristics of ultrawide bandwidth pulsed Gaussian beams,” J. Opt. Soc. Am. A 9(11), 2021-2030
(1992).
[4] J. B. Hartlay, “NASA’s future active remote sensing missions for earth science,” SPIE 4153, 5-12 (2001).
[5] S. W. Henderson, et al., “Eye safe coherent laser radar for range and micro Doppler measurement,” Proc. IRIS Active Systems 1997, Vol. 1, Tucson, AZ
(1997).
[6] J.B. Abshire, et al., “Laser sounder technique for remotely measuring atmospheric CO2 concentrations”, Eos. Trans. AGU, 82(47), Fall Meet (2010).
[7] G. P. Agrawal, [Nonlinear fiber optics], 3rd equation, Academic Press, Boston (2001)
[8] M. Hildebrandt, et al., “Brillouin scattering spectra in high-power single-frequency ytterbium doped fiber amplifiers”, Opt. Expr. 16, 15970-15979 (2008).
[9] C. E. Dilley, et al., “High SBS-threshold, narrowband, erbium codoped with ytterbium fiber amplifier pulses frequency-doubled to 770 nm”, Opt. Expr. 15,
14389-14395 (2007).
[10] Steffen Hadrich, et al., “High Repetition Rate Fiber Laser System for High Field Physics,” ASSP AMA4, San Diego, CA. February 1, 2010.
[11] Tino Eidam, et al., “830 W average power femtosecond fiber CPA system,” ASSP AWA2, San Diego, CA. February 3, 2010.
[12] Bulent Oktem, et al., “MicroJoule pulse energies at 1 MHz Repetition rate from an all fiber nonlinear chirped pulse amplifier,” ASSP AWA4, San Diego, CA.
February 3, 2010.
[13] William H. Renninger, Andy Chong, and Frank W. Wise, "Giant-chirp oscillators for short-pulse fiber amplifiers," Opt. Lett. 33, 3025-3027 (2008)
Figure 1 SBS Power (from Circulator Port 3) as a functions of: A) Outpu
Figure 2 Manipulating seed pulse shape to generate
(8.6 A), 200 ns macro-pulse width with flat top was obtained.
Figure 3 Spectra and
Figure 4 Pulse train and beam profile for 100 W and 100 micro
PCF fiber design (PM: polarization maintai
A
A Seed
200 ns amplified pu
pulse train of MHz high power/en
-J operation. The s
B
t peak power; B) Output pulse energy.
B
ning).
lses. A) Seed shape; B) At
ergy fs fiber laser
atellite pulse comes with the
0.15 mJ
0.15 mJ
nonperfect PM


ns and fs Fiber Lasers (Invited Paper)

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ns and fs Fiber Lasers (Invited Paper)

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