1. Latest developments in 790nm-pumped Tm-doped fiber
laser systems for DIRCM applications.
G. Frith, B. Samson, A. Carter, J. Farroni, K. Farley and K. Tankala,
Nufern, CT, USA
T. McComb
CREOL, UCF, FL, USA
W. Torruellas
JHU-APL, MD, USA
www.nufern.com

2. Application of Tm-doped fibers at 2µm
• Tm-fiber lasers operating CW at 1908nm are an excellent pump source for Ho-YAG
• Many groups are using this scheme to generate high pulse energies at 2.09µm for
IRCM applications
• Q-peak (ASSP 2005), BAe systems (Optics Letters, 2003) and DLR/IB LASER for
example
2

3. Application of Tm-doped fibers at 2µm
• In more recent publications (BAe systems, CLEO 2008) Tm-doped fibers
have been used to amplify ~10nsec pulses at 2µm
• Although fiber lasers are not likely to generate the pulse energies currently
obtained from Ho:YAG MOPA systems generating 10~100mJ pulses, they
could replace a Ho:YAG Q-switch oscillator by generating 0.1~1mJ pulses.
25W, 795nm diode 86W, 795nm
Gain-switched Isolator Isolator
Tm-doped fiber
laser
25/250 Tm-doped 25/400 Tm-doped
fiber fiber
50ns/div
23ns
FWHM
3

4. Application of Tm-doped fibers at 2µm
(BAe Systems, D. Creedon et al, Photonics West 2008)
35
Average Output Power (W)
30 • High average power
25
100kHz
– 30.6W at 100kHz
20
90kHz – 18.6W at 50kHz
15 80kHz
10 70kHz
• 36% slope at 100kHz
60kHz
5
50kHz
0
0 20 40 60 80 100
Pump Power (W)
35.00
30.00
• High peak-power
Peak Power (kW)
25.00
• 15.3kW at 100kHz (20ns pulses) 20.00
• ~300µJ pulse energy 15.00
10.00
• 28.6kW at 50kHz (13ns pulses)
5.00
• ~400µJ pulse energy
0.00
• No nonlinear effects observed 0 20 40 60 80 100
Pump Power (W)
4

5. Some advantages of Tm-fibers at 2µm
• Pulsed fiber systems could offer advantages over solid state lasers at higher rep
rate/average power levels
– Tm-doped fibers offer higher efficiency (important at high average powers)
– Less cooling and often simpler cooling (air rather than water)
– Intrinsic single mode beam quality
• Monolithic all-fiber systems offer advantages for robustness & packaging over solid
state lasers
– Fiber lasers can enable flexible system designs with remote pumps and long
delivery fibers
Tx/Rx Electronics Laser Diodes for
+ Fiber Tray Power Amp
Fiber Lidar with delocalized
hardware
Transport Fiber
Transport Fiber Laser Diode
for Tx/Rx @975nm
Scanner
@1550nm + Power Cables
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6. Some advantages of Tm-fibers at 2µm
• In addition to IRCM applications, high power lasers at “eye-safer” wavelengths are
of interest in directed energy programs
– see for example the recent RFI from JTO (RELI program)
• Tm-fiber lasers are the highest efficiency, high power laser source operating at
these “eye-safer” wavelengths
• For example Er:Yb fibers at 1.5µm have very poor efficiency over 200W CW power
290W at 1576nm from an Er:Yb fiber laser
350 (ORC, Southampton University)
300 19% 4F
9/
Slope efficiency 4I
250 2 9/2
Laser power [W]
200 2F 4I
5 11/
/2 2I
4
150 40% 13/
2
100 Measured
Saturation curve fit
50 2F 4I
7 15/
/2 2
0 Yb3 Er3+
0 200 400 600 800 1000 +
Launched pump power [W]
6

7. Advantages of cladding pumping 790nm
• At Nufern we have been focusing on direct pumping of Tm-silica fibers for high
efficiency and power scaling of 2µm fiber lasers
• Optimized Tm-doped fibers (using cross relaxation) operating with 790nm routinely
deliver
• >60% pump conversion efficiency
• >100W power with single mode beam quality at 2µm
• Offer a higher overall efficiency compared with the alterative scheme based on
resonant pumping at 1560nm with a Er:Yb fiber laser
7

8. Advantages of cladding pumping 790nm
• By optimizing the fiber composition, efficiencies of 790nm pumped Tm-fibers have
steadily improved over the last 5 year
• Including fibers from Southampton, ORC, OFTC (Australia, etc)
8

9. Power scaling at wavelengths <2000nm
• Beyond ~2050nm Tm-doped silica is largely a 4-level system
• Efficient operation at wavelengths less than 2000nm (using 790nm pumping)
requires a fiber design with large core/clad area
• For single mode operation, core diameters are limited to 10-25µm implying clad
diameters must be 125 and 250µm at maximum (rather than 400µm clad fibers)
• Therefore, power scaling at 1900-1950nm requires high power/brightness pumps
9

10. Results on power scaling: 1908nm MOPA.
• 5W seed at 1908nm
• 1.7m of Tm-doped LMA fiber counter-pumped with ~130W (2x65W pumps).
• Fiber mounted on 90mm mandrel with helically cut U-shape channel for highly
effective heat removal
1.7m length of LMA
MO: 5W @ 1908nm Tm-doped fiber
Mode stripper
795nm
pump
FBGs 2+1:1 combiner
Cladding light
stripper Fiber coupled 792nm
pump modules (2™65W)
10

11. Results on power scaling: 1908nm MOPA
• 70W output at 1908nm, pump power limited. Ideal source for pumping Ho:YAG
• 53% slope efficiency - artificially low due to diodes shifting off wavelength (9dB
absorption length at threshold to 6dB at full power).
1.0
Atmospheric transmittance
0.8
Laser spectrum (AU)
Ho:YAG absorption
80 0.6
1908nm output power (W)
Laser spectrum
0.4 Atmospheric transmittance
60 Ho:YAG absorption
0.2
40 0.0
1905 1906 1907 1908 1909 1910
Slope = 52.7%
Wavelength (nm)
20
0
0 20 40 60 80 100 120 140
Launched pump power (W)
11

12. Results on power scaling: 1920nm oscillator
• Example of power scaling air cooled platform at short wavelength,
>65W output power 1920nm, single mode laser
• Full arbitrary modulation capability with <10µs rise / fall
• ~19% WPE (neglecting cooling) – We are currently developing high-
temp diodes to relax cooling requirements
**Observed roll-off due to diode wavelength, not fiber
12

13. Results on scaling Tm-lasers towards kW
• Output power from a single Tm-doped fiber is now approaching 1kW level
• Change in slope is due to the diodes, latest results show efficiency >50% at >850W
output power from a single Tm-fiber
13

14. Diode developments for efficient platforms
• High brightness pump diodes at 793nm are critical for efficient operation at shorter
wavelengths 1900-1950nm
• High power/brightness pumps at 793nm delivering 20W into 105/125 0.22NA fiber
are now becoming available
• E-O efficiency ~43% (ex-fiber)
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15. Pulsed 2µm system for non-linear conversion
• Previous work (Jiang, Optics Lett. 2007) on gain switched Tm-doped fiber lasers
used an amplified 1560nm diode as the source
• Our oscillator uses a Q-switched Er:Yb fiber laser as the pulse sources, which has
the advantage of producing higher power (no need for the amplifier stage)
Er/Yb:Fiber Coil
Fiber coupled Acousto-Optic Q-Switch
15

16. Latest generation PM-LMA Tm-doped fibers
• After producing the gain switched pulses, we amplify them in a PM-LMA fiber to
produce higher pulse energy, with linear polarized single mode beam quality
• Linear polarized output from the amplifier increases the efficiency for non-linear
conversion
• These PM-LMA fibers require a raised refractive index pedestal to lower the effective
core NA for robust single mode operation
14
Number of modes
Stress member
Pedestal 12
10
8
6
er
4
Outer
2 LMA
Cladding
Core 0
0.0 0.1 0.2 0.3
Fibre NA
16

17. Pulsed 2µm systems for non-linear conversion
• Variable PRF (25-100kHz), up to ~5kW peak power and up to
6W ave. (PRF dependent), single polarisation.
AOQ-switched 3.5m length of PM-TDF
Er:Yb fiber laser
20cm PM-TDF
Isolator
1908nm FBGs 2+1:1 combiner
IRFS aplanatic
Fiber coupled 790nm
lenses
pump modules (2 18W)
1.2
70 6 6
1.0
Maximum stable power (W)
5 5
60
0.8
Amplitude (AU)
4 4
Peak power (kW)
Pulse width (ns)
50 0.6
3 3 29ns FWHM
0.4
40
2 2 0.2
30 0.0
1 1
-0.2
20 0 0 0 40 80 120 160 200
20 40 60 80 100 120 20 40 60 80 100
Time (ns)
PRF (kHz) PRF (kHz)
17

18. Pulsed 2µm systems for non-linear conversion
• System generates kW peak powers, polarized output, single mode beam quality
• Ideal for frequency doubling
• Using PPLN we frequency doubled 1908nm to 954nm
• 60% conversion efficiency demonstrated
• ~1W of average power generated at 954nm (full results submitted to Photonics
West, 2009)
1000
PM amplifier 20W average power
GHG Power @ 954.5nm (mW)
900
800
700
600
500
400
300
200
100
0
0 200 400 600 800 1000 1200 1400
Polarized Input Power (mW)
18

19. Example of long wavelength tuning in Tm-
doped fibers to 2.125µm
25
20
Output power (W)
Amplitude (AU)
15
1960 2000 2040 2080 2120
Wavelength (nm) 10
Slope = 41.4%
5
0
2123 2124 2125 2126 2127 0 20 40 60
Wavelength (nm) Launched pump power (W)
• Lower efficiency attributed to cavity finesse
• Onset of ASE seen at ~22W
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20. Preliminary Lifetest of High Efficiency Tm-lasers
• New Tm-fiber compositions have been designed to maximize cross-
relaxation whilst minimizing energy transfer up-conversion.
• Preliminary testing of 20W laser (500hrs) operating at 1950nm pumped at
792nm and 100W laser (25hrs) show stable operation
• Life testing of high power 790nm diodes is also underway
25 110
20 100
Output power (W)
90
Power (W)
15
80
10
70
5
60
0 50
0 100 200 300 400 500 0 5 10 15 20 25
Time (hours) Time (hours)
20
20

21. Conclusions and Acknowledgements
• Efficient pumping of optimized Tm-doped fibers at 790nm is becoming a
mature technology, replacing resonant pumping of Tm-fibers at 1560nm
– Slope efficiencies between 55-65% are typical in packaged monolithic
systems
• Concerns over degradation even at >100W output power using this scheme
have not been observed
– Including the demonstration of ~1kWatt level device that shows no
saturation or roll-over
• High brightness, high power diodes at 790nm are becoming available that
enable air cooled fiber lasers designs at >100W
– Diode E-O at ~45% enable high power 2µm sources with wall-plug
efficiencies approaching 20% for the first time
• These developments are enabling new lasers/amplifiers at wavelengths
between 1850-2150nm for various DoD applications as well as commercial,
sensing and medical
• Thanks to Q-peak, BAe systems (Nashua), JH-APL, and the JTO for MRI
funding.
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