Progress on High Power Single Frequency Fiber Amplifiers at 1mm, 1.5mm and 2mm

Nufern, East Granby, CT, USA Progress on High Power Single Frequency Fiber Amplifiers at 1mm, 1.5mm and 2mm www.nufern.com

Examples of Single Frequency Platforms at 1mm and 1.5mm and Applications 2

Back-reflection (mw) Single Frequency PM Amplifier Platform (2-15W) 7 6 5 4 3 2 1 5 1 15 2 25 Output Power (W) SBS threshold at 164nm in 6.5m PLMA-YDF-1125 with 3m PM-DSF-1/125 delivery fiber on output 3

Back-reflection (mw) Single Frequency PM Amplifier Platform (2-15W) 7 6 5 4 3 2 1 5 1 15 2 25 Output Power (W) Practical SBS limit in current generation of PM single mode Yb and Er:Yb multi-stage amplifiers is ~2W (15W) at 1mm (1.5mm) 4

Back-reflection (mw) Single Frequency PM Amplifier Platform (2-15W) 7 6 5 4 3 2 1 5 1 15 2 25 Output Power (W) Standard wavelengths 16-185nm and 1545-1565nm 5

Back-reflection (mw) Single Frequency PM Amplifier Platform (2-15W) 7 6 5 4 3 2 1 5 1 15 2 25 Output Power (W) Multiple options available (isolation, control, input power, etc) 6

Back-reflection (mw) Single Frequency PM Amplifier Platform (2-15W) 7 6 5 4 3 2 1 5 1 15 2 25 Output Power (W) Mature platform, more than 5 amplifiers delivered over last 4 years 7

Single Frequency PM Amplifier Platform (2-15W) Demo models available for evaluation Specially designed Yb doped fibers with increased mode filed diameter but maintaining single mode spatial beam profile were used to alleviate the SBS limitation. 8

Single Frequency PM Amplifier Platform (2-15W) Demo models available for evaluation SBS-suppression fibers were used to further decrease the SBS threshold. 9

Single Frequency PM Amplifier Platform (2-15W) Optical Specifications 1

Single Frequency PM Amplifier Platform (2-15W) Mechanical, Electrical &Environmental Specifications 11

PER (db) Power (W) Extended Wavelengths Using Yb-doped Fibers 6 5 4 3 2 1 5 1 15 Time (min) 28 27.5 27 26.5 26 25.5 25 5 1 15 Time (min) 5W 116nm SFA with a fiber to free space isolator, tested with 1MHz DFB diode 12

PER (db) Power (W) Extended Wavelengths Using Yb-doped Fibers 6 5 4 3 2 1 5 1 15 Time (min) 28 27.5 27 26.5 26 25.5 25 5 1 15 Time (min) SNR of the SFA operating at 119nm 13

Single Frequency PM Amplifier Platform (4W at 1mm) By adopting state of the art PM LMA (25/4) fibers the SBS threshold can be raised to >4W (at 1mm) with 5kHz seed source 14

RIN (db/hz) PER (db) RIN (db/hz) Power (W) -2-4 -6-8 -1 Single Frequency PM Amplifier Platform (4W at 1mm) RIN Measurement 4W output (NP Photonics seed laser) -12.2.4.6.8.1.12 Frequency (MHz) 4 35 3 25 2 15 1 5 1 2 3 4 Time (hrs) -2 25 2-4 15-6 1-8 5-1 -12 1 2 3 4 5 6 2 4 6 8 Frequency (MHz) Time (min) The 4W amplifier platform is fairly new (~8 units delivered in 29) 15

RIN (db/hz) PER (db) RIN (db/hz) Power (W) -2-4 -6-8 -1 Single Frequency PM Amplifier Platform (4W at 1mm) RIN Measurement 4W output (NP Photonics seed laser) -12.2.4.6.8.1.12 Frequency (MHz) 4 35 3 25 2 15 1 5 1 2 3 4 Time (hrs) -2 25 2-4 15-6 1-8 5-1 -12 1 2 3 4 5 6 2 4 6 8 Frequency (MHz) Time (min) 3 systems are under evaluation at MIT (centre of ultra cold atoms) 16

RIN (db/hz) PER (db) RIN (db/hz) Power (W) -2-4 -6-8 -1 Single Frequency PM Amplifier Platform (4W at 1mm) RIN Measurement 4W output (NP Photonics seed laser) -12.2.4.6.8.1.12 Frequency (MHz) 4 35 3 25 2 15 1 5 1 2 3 4 Time (hrs) -2 25 2-4 15-6 1-8 5-1 -12 1 2 3 4 5 6 2 4 6 8 Frequency (MHz) Time (min) Contact apeyman@mit.edu for references on how the amps are working 17

Power at Back Reflection Monitor (mw) Single Frequency PM Amplifier Platform (1W at 1mm) 4 35 3 25 2 15 1 5 2/4, 13 db 2/4, 8.7 db 25/4, 13 db 25/4, 1.5 db 25/4, 1.5 db, Heat. 5. 1. 15. 2. Amplified Power Output (W) Practical SBS limit is increased in these PM LMA based amplifiers by using a temperature gradient along the active fiber length 18

Power at Back Reflection Monitor (mw) Single Frequency PM Amplifier Platform (1W at 1mm) 4 35 3 25 2 15 1 5 2/4, 13 db 2/4, 8.7 db 25/4, 13 db 25/4, 1.5 db 25/4, 1.5 db, Heat. 5. 1. 15. 2. Amplified Power Output (W) Temperature gradient shifts the local SBS gain spectrum along the fiber length 19

Single Frequency PM Amplifier Platform (1W at 1mm) Property Amp Measured Parameters Tested Value Output Power [~5kHz] 15.5 W PER >15.5 db M 2 <1.1 Wavelength 164.4 nm Input Power ~ 33mW Max Backward Power < 25mW 2

Single Frequency PM Amplifier Platform (1W at 1mm) POWER with Oven Off I 3A [A] I 3B [A] P SIGNAL [W] P BACK [mw] 1 1.4.25 2 33..55 3 61.1 23. 21

Single Frequency PM Amplifier Platform (1W at 1mm) POWER with Oven On I 3A [A] I 3B [A] P SIGNAL [W] P BACK [mw] 1 1.4.65 2 33..7 3 61.1.85 4 87. 4.1 4 1 94.1 6. 4 15 13. 17. 22

P SIGNAL [W] Single Frequency PM Amplifier Platform (1W at 1mm) Power Stability 12 11 P SIGNAL [W] Data collected at 1W > 4 Hours 1 9 8 7 6 5 P [W] SIGNAL Minimum 1.3 Maximum 11.4 Sum 2722598.2 Points 271 Mean 1.83324 Median 1.8 RMS 1.83337 Std Deviation.16679741 Variance.27821375 Std Error.115779 Skewness -.1333626 Kurtosis -.246751 5 1 15 2 25 3 Time [Min] 23

Single Frequency PM Amplifier Platform (1W at 1mm) Beam Propagation Factor 1W M x 2 = 1.8 M Y 2 = 1.9 24

P TH [W] SBS Threshold Depends on Linewidth of Seed Source 8 7 SBS Threshold vs. Linewidth y = -23.8 + 194.13x R=.9987 6 5 4 3 2 1 12.6m PLMA-25/44-YDF 1 2 3 4 5 6 Linewidth [GHz] In some applications 1-1GHz linewidth is suitable (this signal linewidth reduces the SBS gain in the amplifier, which has linewidth ~5MHz) 25

P TH [W] SBS Threshold Depends on Linewidth of Seed Source 8 7 SBS Threshold vs. Linewidth y = -23.8 + 194.13x R=.9987 6 5 4 3 2 1 12.6m PLMA-25/44-YDF 1 2 3 4 5 6 Linewidth [GHz] In those cases broadening the linewidth to achieve more output power is an acceptable compromise 26

P TH [W] SBS Threshold Depends on Linewidth of Seed Source 8 7 SBS Threshold vs. Linewidth y = -23.8 + 194.13x R=.9987 6 5 4 3 2 1 12.6m PLMA-25/44-YDF 1 2 3 4 5 6 Linewidth [GHz] In this case LMA fibers generate output power >1kW CW 27

P SIGNAL [W] Nufern Turn-key, 1kW Amplifier (3GHz seed source) 1 kw Amplifier 1 88% Slope Efficiency 8 6 4 2 2 4 6 8 1 12 P LAUNCHED PUMP [W] M 2 = 1.1 Measured at 1 kw Multi-stage turn-key packed amplifier (1mW input power) 28

P SIGNAL [W] Nufern Turn-key, 1kW Amplifier (3GHz seed source) 1 kw Amplifier 1 88% Slope Efficiency 8 6 4 2 2 4 6 8 1 12 P LAUNCHED PUMP [W] M 2 = 1.1 Measured at 1 kw Signal Linewidth 3~1GHz 29

P SIGNAL [W] Nufern Turn-key, 1kW Amplifier (3GHz seed source) 1 kw Amplifier 1 88% Slope Efficiency 8 6 4 2 2 4 6 8 1 12 P LAUNCHED PUMP [W] M 2 = 1.1 Measured at 1 kw Linearly polarized option PER~13dB 3

P SIGNAL [W] Nufern Turn-key, 1kW Amplifier (3GHz seed source) 1 kw Amplifier 1 88% Slope Efficiency 8 6 4 2 2 4 6 8 1 12 P LAUNCHED PUMP [W] M 2 = 1.1 Measured at 1 kw Multiple units shipped 28-29 31

High Power Single Frequency 1.5mm PM-LMA Amp 155nm SF fiber laser, 5mW isolator 155nm 1W NuAmp PM-MFA PLMA-EYDF-25/3 LMA-GDF-25/3 Dichroic filters 3W 94nm bar diode 16nm ASE 155nm signal 5mW 155nm single-frequency seed fiber laser, 5kHz linewidth 32

High Power Single Frequency 1.5mm PM-LMA Amp 155nm SF fiber laser, 5mW isolator 155nm 1W NuAmp PM-MFA PLMA-EYDF-25/3 LMA-GDF-25/3 Dichroic filters 3W 94nm bar diode 16nm ASE 155nm signal Amplified in Er:Yb PM LMA 25/3 fiber, counter pumped at 94nm 34

155nm signal, W 16nm ASE, mw High Power Single Frequency 1.5mm PM-LMA Amp ~48dB 14 12 1 PM SF 155nm amp efficiency test EYDF-25/3 y =.418x + 3.73 28 24 2 155nm 5kHz input signal linewidth 8 6 4 2 16 12 8 4 1 2 3 Coupled 94nm pump, W 35

155nm signal, W 16nm ASE, mw High Power Single Frequency 1.5mm PM-LMA Amp ~48dB 14 12 1 PM SF 155nm amp efficiency test EYDF-25/3 y =.418x + 3.73 28 24 2 113W output 8 6 4 2 16 12 8 4 1 2 3 Coupled 94nm pump, W 36

155nm signal, W 16nm ASE, mw High Power Single Frequency 1.5mm PM-LMA Amp ~48dB 14 12 1 PM SF 155nm amp efficiency test EYDF-25/3 y =.418x + 3.73 28 24 2 41.8% slope efficiency 8 6 4 2 16 12 8 4 1 2 3 Coupled 94nm pump, W 37

155nm signal, W 16nm ASE, mw High Power Single Frequency 1.5mm PM-LMA Amp ~48dB 14 12 1 PM SF 155nm amp efficiency test EYDF-25/3 y =.418x + 3.73 28 24 2 PER 13dB 8 6 4 2 16 12 8 4 1 2 3 Coupled 94nm pump, W 38

155nm signal, W 16nm ASE, mw High Power Single Frequency 1.5mm PM-LMA Amp ~48dB 14 12 1 PM SF 155nm amp efficiency test EYDF-25/3 y =.418x + 3.73 28 24 2 ~48dB 155nm ASE suppression 8 6 4 2 16 12 8 4 1 2 3 Coupled 94nm pump, W 39

155nm signal, W 16nm ASE, mw High Power Single Frequency 1.5mm PM-LMA Amp ~48dB 14 12 1 PM SF 155nm amp efficiency test EYDF-25/3 y =.418x + 3.73 28 24 2 <2W 16nm ASE 8 6 4 2 16 12 8 4 1 2 3 Coupled 94nm pump, W 4

Recent Advances in Tm-fibers for 2mm Wavelength 41

Absorption & Emission Cross Sections Pump Options for Tm-doped Fibers Gain Spectral Region Emission Absorption 7 12 17 22 Wavelength (nm) Resonant pumping around 156nm is difficult to power scale with direct diode pumping (no high power/brightness pumps) 42

Absorption & Emission Cross Sections Pump Options for Tm-doped Fibers Gain Spectral Region Emission Absorption 7 12 17 22 Wavelength (nm) Solution here is to pump with Er:Yb fiber laser which is in turn pumped by 9xx high brightness diodes (>4W output power demonstrated, IPG 27) 43

Absorption & Emission Cross Sections Pump Options for Tm-doped Fibers Gain Spectral Region Emission Absorption 7 12 17 22 Wavelength (nm) However, the overall E-O the efficiency is low using this scheme 44

Historical Perspective on 79nm pumped Tm-fibers Increasing the Tm 3+ concentration decreases the ion-ion separation to enhance the 2 for 1 cross-relaxation process. Pumping at 79nm is attractive because of the compatibility with 88nm pump diode; however, the quantum efficiency needs to be improved to be practical 45

Historical Perspective on 79nm pumped Tm-fibers Increasing the Tm 3+ concentration decreases the ion-ion separation to enhance the 2 for 1 cross-relaxation process. Early work on power scaling efficient Tm-doped silica fibers attributed to Jackson et al. (Uni. Sydney, Aus) and Clarkson et al (ORC, Southampton, UK) 46

Historical Perspective on 79nm pumped Tm-fibers Increasing the Tm 3+ concentration decreases the ion-ion separation to enhance the 2 for 1 cross-relaxation process. Both groups recognized early on that optimizing the cross relaxation process in highly doped silica fibers could improve the efficiency of 79nm pumped fibers 47

Slope Efficiency (%) Improvements in Fiber Efficiency over the Years 1 9 8 7 2:1 limit 6 5 4 3 2 1 1995 2 25 21 Date To date >65% slope efficiency has been demonstrated for 79nm pumped fibers operating around 2mm, approaching the theoretical limit 48

Slope Efficiency (%) Improvements in Fiber Efficiency over the Years 1 9 8 7 2:1 limit 6 5 4 3 2 1 1995 2 25 21 Date Far exceeding the overall E-O efficiency of resonant pumped Tm-fiber systems 49

Number of modes Tm-doped LMA Fibers for Single Mode Beam Quality Pedestal 14 12 1 8 Silica Cladding 6 4 2 Index Profile Tm-doped Core.1.2 NA Much of the early high efficiency Tm-doped fibers were multimode because of the high NA (A. Carter et al., CLEO 27) 5

Number of modes Tm-doped LMA Fibers for Single Mode Beam Quality Pedestal 14 12 1 8 Silica Cladding 6 4 2 Index Profile Tm-doped Core.1.2 NA High Tm-doping levels coupled with high Al co-dopant levels would lead to an NA>.2 w.r.t. the silica cladding (A. Carter et al., CLEO 27) 51

Number of modes Tm-doped LMA Fibers for Single Mode Beam Quality Pedestal 14 12 1 8 Silica Cladding 6 4 2 Index Profile Tm-doped Core.1.2 NA By incorporating a pedestal layer around the Tm-doped core the effective NA is reduced to ~.1, reducing the mode content within the doped core (A. Carter et al., CLEO 27) 52

Number of modes Tm-doped LMA Fibers for Single Mode Beam Quality Pedestal 14 12 1 8 Silica Cladding 6 4 2 Index Profile Tm-doped Core.1.2 NA By reducing the NA of the core, large core fibers with good beam quality became possible (LMA fibers) (A. Carter et al., CLEO 27) 53

Signal output, W Monolithic 2W Single Frequency PM Amp (24nm) 1 st stage core-pump PM amplifier ~5mW output power ~3 nd stage cladding-pumped PM amplifier ~2W output power ~1mW single frequency seed 237nm 2 nd stage cladding-pumped PM amplifier ~3W output power Delivery fiber and angled Endcap assembly 25 2 15 1 - PM Isolator Monolithic 3-stage (2W) PM amplifier compatible with input from semiconductor DFB diode at 2µm (~1mW) 5 5 1 15 2 25 3 35 4 45 Pump current, A 54

2-mm Output (W) High Power Single Frequency at 2µm 6W single frequency amplifier (G. Goodno et al., NGST, ASSP 29, post deadline) DFB 3 mw 24 nm <5 MHz 3-stage pre-amplifier chain 15 W Forward power monitor Return power monitor ASE filter 79-nm pump Conductive Heatsink 3.1 m active fiber 79-nm pump 55 cm passive fiber 68 W To diagnostics 7 6 5 4 3 2 1 55 Least-squares fit: y =.54x + 3.6 25 5 75 1 125 Absorbed pump (W) 55 6W result is based on 3.1m of Tm-LMA 25/4 fiber and is not an SBS limited result at this power 55

Signal Power [Watt] Monolithic 4W Single Mode MOPA at 24nm 45 4 35 3 25 2 15 1 5 24nm Power vs. 79nm Pump Power 1 2 3 4 5 6 Pump Power [Watt] System is based on Tm-LMA-2/4 was delivered to US Government lab June 29 56 56 56

RGB Harmonics from Fiber NLLs (Reference, J. Anderegg et al, SPIE Photonics West, 21) Through the 2 nd and even the 3 rd harmonics of the 1um, 1.5um and 2um narrow linewidth lasers provide narrow linewidth laser source in visible regime. 57 57 57

RGB Harmonics from Fiber NLLs (Reference, J. Anderegg et al, SPIE Photonics West, 21) Additional wavelength regime can be produced through the Sum-Frequency Generation (SFG). 58 58 58

59 Frequency Doubling of High Power Fiber Lasers (Reference, J. Anderegg et al, SPIE Photonics West, 21) Fiber MOPA coupled to enhancement cavity 59

6 Frequency Doubling of High Power Fiber Lasers (Reference, J. Anderegg et al, SPIE Photonics West, 21) Fiber MOPA coupled to enhancement cavity 6

Conclusion With the advance in the fiber design and implementation of various SBS mitigation techniques, Up to 1W 1.um single frequency amplifiers (<5kHz linewidth) has been demonstrated. Up to 1W 1.5um single frequency amplifiers (<5kHz linewidth) has been demonstrated. Up to 6W 2um single frequency amplifiers (3MHz linewidth) has been demonstrated. 61

Conclusion Together with the advance in the harmonic conversion cavities, these high power narrow linewidth fiber amplifiers can provide a broad range of narrow linewidth lasers targeting various atomic transition band. 62