Pulsed Tm-doped fiber lasers for mid-IR frequency

Invited Paper
Pulsed Tm-doped fiber lasers for mid-IR frequency conversion
Daniel Creeden*a, Peter A. Budnia, and Peter A. Ketteridgea
a
BAE Systems, Advanced Systems and Technology, PO Box 868, Nashua, NH 03061-0868
ABSTRACT
Fiber lasers are an ideal pump source for nonlinear frequency conversion because they have the capability to generate
short pulses with high peak-powers and excellent beam quality. Thulium-doped silica fibers allow for pulse generation
and amplification in the 2-micron spectral band. This opens the door to a variety of nonlinear crystals, such as ZnGeP2
(ZGP) and orientation patterned GaAs (OPGaAs), which cannot be pumped by Yb- or Er-doped fiber laser directly due
to high losses in the near-IR band. These crystals combine low losses with high nonlinearities and transparency for
efficient nonlinear mid-IR converters. Using such nonlinear crystals and a pulsed Tm-doped master oscillator fiber
amplifier (MOFA), we have demonstrated efficient mid-IR generation with watts of output power in the 3-5μm region.
The Tm-doped MOFA is capable of generating from 10 to 100W of average output power at a variety of repetition rates
(10kHz - >500kHz) and pulse widths (10ns - >100ns). Total mid-IR power is only limited by thermal effects in the
nonlinear materials. The use of Tm-doped fiber-pumped OPOs shows the path toward compact, efficient, and
lightweight mid-IR laser systems.
Keywords: fiber amplifiers, TDFA, pulse amplification, frequency conversion, OPO
1. INTRODUCTION
Recent advancements in thulium-doped fiber laser technologies have allowed for the pursuit of fiber-pumped mid-IR
laser systems. These include the development of fibers, pump diodes, as well as fiber components. Typical mid-infrared
lasers use q-switched diode-pumped bulk crystals doped with thulium or holmium to generate pulses in the 2-micron
spectral region [1]. These optical pulses are then converted to the mid-IR using nonlinear converters in materials such as
ZGP or OPGaAs. However, mid-IR generation using optical parametric oscillators (OPO) pumped directly by fiber
lasers has become attractive due to a fiber laser’s ability generate to high output power and short pulses with high
efficiency and good beam quality [2,3]. In this paper, we discuss our progress with the development of a high power,
pulsed 2-micron fiber laser/amplifier system for driving nonlinear conversion processes and the advantages to using a
pulsed fiber source for mid-IR generation.
2. EXPERIMENTAL SETUP AND RESULTS
We have conducted multiple experiments with our 2-micron fiber system. In our first set of experiments, we used a
single TDFA to amplify the pulses generated in our master oscillator. The amplified pulses were then used to pump a
mid-IR ZGP OPO in a singly resonant format. A schematic of this experimental setup is shown in Fig. 1.
Gain-Switched Oscillator
1550nm
Seed diode
975nm
Diode
980nm
Diode
Isolator
Isolator Er-doped
Fiber
795nm
Diode
Isolator
Er:Yb-doped
Fiber
Tm-doped
Fiber
Fiber
Grating
Isolator
Isolator
25/250 non-PM
Tm-doped Fiber
Lens
Input
Coupler
Half-wave
Plate
ZGP
Fig.1. Schematic of the thulium-doped fiber oscillator/amplifier and ZGP mid-IR OPO
Fiber Lasers VI: Technology, Systems, and Applications, edited by Denis V. Gapontsev,
Dahv A. Kliner, Jay W. Dawson, Kanishka Tankala, Proc. of SPIE Vol. 7195, 71950X
© 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.807208
Proc. of SPIE Vol. 7195 71950X-1
Output
Coupler
Mid-IR
Light
The master oscillator is a gain-switched Tm-doped fiber laser [4]. The 1550nm diode is pulsed at a 40kHz repetition rate
with 200ns pulses by directly modulating the current on the diode. The optical pulses are then amplified in an Er-doped
fiber amplifier followed by an Er:Yb-doped fiber amplifier. The output from the Er:Yb fiber is spliced to an in-line
isolator which is spliced directly to a highly reflective fiber grating, the wavelength of which is in the 2-micron spectral
region. This grating acts as the high reflector for the Tm-doped fiber laser cavity and is directly spliced to a single-mode
Tm-doped fiber. A flat cleave acts as a 4% broadband partial reflector for the gain-switched cavity. The amplified
1550nm signal gain-switches the Tm-doped fiber, generating 30ns pulses at 40kHz repetition rate with 300mW of
average unpolarized output power.
The output from the oscillator is collimated using an aspheric lens and is passed through a free-space optical isolator.
The isolator is polarization dependent, and as a result, half of the power generated by the oscillator is lost. Using another
aspheric lens, 65mW of average 2-micron power is launched into the 25/250 Tm-doped fiber amplifier. The amplifier is
pumped by 20W from a 795nm diode. The performance of the TDFA is shown in Fig. 2.
5
TDFA Output Power (W)
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
Launched 795nm Pump Power (W)
Fig. 2. TDFA output power vs. launched diode pump power
The amplifier has a gain of 18dB and amplifies the seed to 4.2W of average output power with a 33% total slope
efficiency. The low efficiency is due to a large amount of diode light propagating in the cladding of the 100-micron
delivery fiber. This power is lost through the fiber-coupled pump combiner. The slope efficiency with respect to
absorbed pump power of this amplifier is 42%. The amplified 2-micron light is collimated and passed through a freespace optical isolator. After isolation, 2.9W of average power (2.4kW peak) is available to pump the OPO. The output
is not completely randomly polarized due to a tight 2 inch coil of the 25/250 Tm-doped fiber, which adds a slight
birefringence.
An anti-reflection coated spherical lens is used to focus the beam to an effective intensity of 31.2MW/cm2 inside the
ZGP crystal. The input coupler is highly reflective at the signal wavelengths with a 10cm radius of curvature (ROC).
The single resonant (SRO) output coupler is coated for 95% reflection at the signal wavelength with a 10cm ROC. The
slope efficiency of the OPO is shown in Fig. 3. The signal is tunable in the range of 3.4-3.9μm and the idler is tunable
from 4.1-4.7μm. The SRO exhibits an overall slope efficiency of 37.2% with a signal slope of 15% and an idler slope of
22.4%.
Proc. of SPIE Vol. 7195 71950X-2
OPO Output Power (mW)
700
Signal + Idler
Signal
Idler
600
500
400
300
200
100
0
1
1.5
2
2.5
3
3.5
Average Pump Power (W)
Fig. 3. Mid-IR OPO output power vs. 2-micron pump power
Mid-IR conversion can be improved by optimizing the reflectivity of the optics, scaling the pump power, and increasing
the pump spot diameter. We are using a small pump spot to provide a high intensity due to the relatively low average
pump power and high repetition rate. This small spot size also results in a strong thermal lens and a large amount of
signal and idler walk-off in the ZGP, resulting in poor overlap with the pump and reducing the effective crystal length.
Increasing pump power while increasing the pump spot size (given the same pulse parameters) will maintain high
intensity in the crystal while increasing the interaction length, thus improving conversion efficiency and helping to
mitigate the severity of the thermal lens.
In our second set of experiments, we took the amplified output from the MOFA chain shown in Fig. 1 and further
amplified the output power using an additional TDFA prior to pumping a ZGP OPO. These amplified pulses were then
used to pump the same single resonant mid-IR ZGP OPO as in the previous experiment. A full schematic of the system
is shown in Fig. 4.
Gain-Switched Oscillator
980nm Diode
1550nm
Seed diode
Isolator
Isolator
Er-doped
Fiber
795nm
Diode
975nm Diode
Isolator
Er:Yb-doped
Fiber
Tm-doped
Fiber
Fiber
Grating
Isolator
795nm
Diode
Isolator
Isolator
25/250 non-PM
Tm-doped Fiber
25/400 non-PM
Tm-doped Fiber
Input
Coupler
Output
Coupler
Mid-IR
Light
Lens
Half-wave
Plate
ZGP
Fig. 4. Schematic of the power scaled Tm-fiber amplifier chain and ZGP mid-IR OPO
In this set of experiments, we ran the oscillator at a repetition rate of 100kHz, rather than 40kHz to avoid the possibility
of self-switching of the high power amplifier. At the 100kHz repetition rate, the oscillator still generates 30ns optical
pulses at 2-microns. These pulses are amplified in the same 25/250 preamplifier as in the first set of experiments. After
this amplifier, the light is focused into a power amplifier which consists of a 25/400 non-PM Tm-doped fiber and 55W
of 795nm pump power. At the output of the TDFAs, the seed is amplified more than 25dB to 21W of average output
power (30ns pulses, 100kHz PRF) in a near-diffraction-limited beam with an M2 of 1.1±0.05. Fig. 5 shows the
performance of the 25/400 Tm-doped fiber power amplifier.
Proc. of SPIE Vol. 7195 71950X-3
Output Power (W)
25
20
15
10
5
0
0
10
20
30
40
50
60
Pump Power (W)
Fig. 5. TDFA output power vs. 795nm pump power
An optical isolator is placed in-line after the amplifiers to prevent feedback and to polarize the beam. As a result of the
isolator, only 60% of the power is transmitted, leaving 12.7W of linearly polarized 2-micron light to pump the OPO.
The isolated output from the amplifier is focused through a lens into the ZGP to a 170μm 1/e2 diameter, resulting in a
maximum effective peak intensity of 37.2MW/cm2. In these experiments, the OPO was run in the same SRO
configuration as in the first set of experiments. A plot of OPO output power versus incident pump intensity is shown in
Fig. 6.
2.5
Signal
Idler
OPO Output (W)
2
Total (Signal + Idler)
1.5
1
0.5
0
0
2
4
6
8
10
12
14
Pum p Pow er (W)
Fig. 6. Mid-IR OPO output power vs. 2-micron pump power in the ZGP
The threshold for this OPO is ~21MW/cm2, and at the highest output power, the OPO is running at only 1.8 times above
threshold, generating 2W of total mid-IR output power. More than 1.3W of this is from the idler in the 4.0-4.7μm
spectral region. The remaining power is in the 3.4-3.9μm region. The total mid-IR conversion efficiency in this OPO is
15.7%.
Proc. of SPIE Vol. 7195 71950X-4
The roll-over in the mid-IR output power is caused by thermal effects as a result of the uncooled crystal and the pump
being focused into such a small spot, resulting in a very strong thermal lens in the ZGP. The pump spot size could be
increased slightly to alleviate the thermal lens; however, because of the relatively low peak-power in the pump (4.2kW),
a small spot is required to achieve the high peak intensity needed to reach threshold. This strong thermal lens and the
relatively low level of pump depletion does result in good beam quality for both the signal and idler waves. Signal beam
quality was measured to be M2 = 1.15±0.1, and idler beam quality was measured to have an M2 = 1.2±0.1.
Prior to the thermal roll-over in these experiments, the mid-IR slope is high with respect to the incident pump power,
showing the potential for a high efficiency, high power, fiber-pumped mid-IR OPO. Mitigating the thermal effects
should improve OPO efficiency and increase mid-IR output power. Using OPGaAs rather than ZGP should also help
alleviate some of these effects and improve overall conversion efficiency.
3. CONCLUSION
In these experiments, we have demonstrated pulse amplification in Tm-doped fiber amplifiers and the feasibility to drive
nonlinear converters with Tm-doped fibers. In the first set of experiments, we generated nearly 700mW of average midIR power efficiently by pumping with only 2.9W of 2-micron power. In the second set of experiments, we amplified the
2-micron power to more than 12W and drove a ZGP mid-IR OPO to 2W of average output power. Future experiments
will attempt to increase the peak-power and pulse energy of the pump while increasing the spot size in the crystal to
maintain high pump intensity in a larger spot. This combination should mitigate the effects of the thermal lens which
will result in a higher conversion efficiency with no thermal roll-over. We also plan to use OPGaAs as a nonlinear
converter in future experiments. Due to its quasi-phase matching, walk-off will be eliminated and the interaction length
will be longer, resulting in higher mid-IR conversion efficiency.
This work was supported by BAE Systems and AFRL/RYJW under contract number FA8650-07-2-1209.
REFERENCES
[1]
[2]
[3]
[4]
P. A. Budni, et al., “Efficient mid-infrared laser using 1.9-µm-pumped Ho:YAG and ZnGeP2 optical parametric
oscillators,” JOSA B 17, 723-728 (2000).
S. Desmoulins and F. Di Teodoro, “Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength
conversion of an eye-safe fiber source,” Opt. Lett. 32, 56-58 (2007).
D. Creeden, P. A. Ketteridge, P. A. Budni, S. D. Setzler, Y. E. Young, J. C. McCarthy, K. Zawilski, P. G.
Schunemann, T. M. Pollak, E. P. Chicklis, and M. Jiang, "Mid-infrared ZnGeP2 parametric oscillator directly
pumped by a pulsed 2 μm Tm-doped fiber laser," Opt. Lett. 33, 315-317 (2008).
M. Jiang and P. Tayebati, “Stable 10 ns, kilowatt peak-power pulse generation from a gain-switched Tm-doped fiber
laser,” Opt. Lett. 32, 1797-1799 (2007).
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