Investigation of the impact of fiber Bragg grating bandwidth on the efficiency of a fiber Raman laser US-Australia meeting May12, 2015 Leanne J. Henry, Michael Klopfer (1), and Ravi Jain (1) (1) University of New Mexico Integrity Service Excellence 1
General Needs There is a strong need for demonstrating high power (> 100 W) narrow linewidth (NLW) sources in the 1100-1500 nm spectral range for: generation of high power (> 50 W) NLW 589 nm radiation (via SHG) for sodium guidestar applications. generation of high power mid-ir radiation (via DFG) in the 3-5 micron spectral region. nonlinear spectroscopy and long range remote sensing applications 2
1178 nm 100 khz 1069 nm 20 GHz Initial design of narrow linewidth 1178 nm Raman system 1178 nm isolator 1069 nm isolator WDM.01% TAP 3 WDM Yb amp 3 WDM.01% TAP PM 10/125 germanosilicate fiber 3 nm 1121 nm HR 3 nm 1121 nm HR Pumped with broad linewidth 1069 nm and seeded with narrow linewidth 1178 nm 1069 nm is amplified and then Raman converted to 1121 nm in the Raman resonator. The bandwidth of the 1121 nm in the cavity is determined by the length of the cavity and the bandwidth of the fiber Bragg gratings (FBGs) The 1178 nm is amplified as it passes through the Raman resonator Storage of buildup of 1121 nm radiation provides a unique amplifier for 1178 nm 3
Initial results (unusual double -fanged output) Severe 1121 nm linewidth broadening prevented high power levels of 1121 nm intracavity power which necessitated the usage of long Raman cavity in order to obtain significant 1178 nm power levels. Double-fanged output is caused by leakage of this intracavity broadened radiation around the reflectivity band of the FBG. Output power levels of 1178 nm were then limited by Stimulated Brillouin Scattering. 4
Two distinct spectral transformation processes in fiber Raman laser 1121 nm HR 1121 nm HR Process 1: - Broadening of spectrum while propagating in Raman fiber Process 2: - Modification of spectrum upon reflection by fiber Bragg grating 5
Key Design Issues Stimulated Brillouin Scattering arising from the narrow linewidth 1178 nm. limits length of 1121 nm resonator cavity Linewidth broadening of 1121 nm drives usage of a longer resonator cavity due to reduced intracavity 1121 nm power buildup 1121 nm power leakage upstream must be mitigated 6
Modeling: 1 st Generation Nonlinear Model The first generation of the nonlinear model propagated the spectral wave shape in frequency space and included all major nonlinearities in the fiber. This model was found to be applicable for shorter resonator cavities and for the case where the amount of linewidth broadening was somewhat limited. This was the situation since a certain minimum number of spectral points were needed to adequately represent the computational domain and the number of computations scaled as the number of spectral points to the fourth power in the four wave mixing calculation. The model was found to become computationally intractable for cases involving longer resonator cavities and where there is a lot of linewidth broadening which would require a larger number of spectral points, i.e., the model was unable to fully account for the resonator bandwidth. Even with its shortcomings, this model led to some interesting insights in that longer cavities don t necessarily have increased leakage of 1121 nm power. 7
Effect of Four Wave Mixing when the Length of the Resonator Increases Percentage of 1121 nm leaking from cavity may decrease as the fiber length increases % age of leaked 1121 nm power is far less than what is seen experimentally - The resonator cavity that was modeled is defined by 99.8% reflectivity fiber Bragg gratings having a bandwidth of 3 nm centered at 1121 nm. 40W of 1069 nm pump and a.05 W 1178 nm seed in a co-pumped configuration were utilized. Modeling was done in frequency space. - Four wave mixing decreases as the power in the cavity decreased but it also increased as the length of the cavity increased two opposing effects which resulted in a point of maximum 1121 nm power leakage 8
Modeling: 2 nd Generation Nonlinear Model To enable investigation of a system having a large amount of linewidth broadening or a system having a long Raman cavity, it is necessary to utilize the nonlinear Schrödinger equation to describe the propagation of the spectral envelope down a fiber. - Incorporated within the nonlinear Schrödinger equation are the following nonlinear effects: dispersion, four wave mixing, self-phase modulation, cross-phase modulation, etc. For this model, the number of spectral points needed are only those necessary to adequately describe a continuous envelope in discrete space. In addition, none of the calculations scale as the number of spectral points to the fourth power. 9
Trends associated with linewidth broadening one trip down a 150 m fiber SiO 2 GeO 2 (a) (b) (c) Note that the following parameters were associated with the calculation for each case: (a) 100 W 1121 nm cavity power, 3 nm bandwidth gratings, n 2 = 3.73 10-20 m 2 /W (the value for a 10% germanium doped fiber core), h =.25 m calculation step (note that no wavelength dependence of β 2 was considered in this calculation except for the case that was specific to the fiber involved red square), (b) 100 W 1121 nm cavity power, 3 nm bandwidth gratings, β 2 = 0, h =.25 m, (c) 3 nm bandwidth gratings, β 2 = 0, n 2 = 3.73 10-20 m 2 /W, h =.25 m, and (d) 100 W 1121 nm cavity power, β 2 = 0, n 2 = 3.73 10-20 m 2 /W, h =.25 m. The cavity length was 150 m for all cases. Far less linewidth broadening is predicted by modeling than is observed experimentally (d) 10
1069 nm 20 GHz 1069 nm isolator Redesigned system to explore 1121 nm linewidth broadening.01% TAP WDM Yb amp 3 WDM 1178 nm 1178 nm isolator 3 WDM 3 WDM.01% TAP 3 nm 1121 nm HR PM 10/125 germanosilicate fiber.01% TAP.01% TAP 3 nm 1121 nm HR 100 khz Investigated linewidth broadening as a function of the following cavity lengths: 15, 27, 40, 65, 90, 105, and 140 m Investigated linewidth broadening as a function of 1 and 3 nm FBG bandwidths Measurements made for each case: Output power levels of: 1069 nm (unused), 1121 nm (leakage in the forward direction), 1178 nm (output), BT-HP, FT-HP, BT-cav-input, FT-cav-input, BT-cav-output, FT-cavoutput For the forward direction, spectra out of FT-cav-input, FT-cav-output, and a spectrum of the output PER of 1121 nm (leakage in forward direction), and 1178 nm BT-HP FT-HP BT-cav-input FT-cav-input BT-cav-output FT-cav-output 11
Analysis of data Estimating intracavity 1121 nm power level: for a given 1069 nm pump level, from the FT-cav-input and FT-cav-output spectra, the measured power out of FT-cav-input and FT-cav-output, and the measured splitting ratios of the input and output tap couplers, the intracavity power levels of 1121 nm at the input and output ends of the Raman cavity can be derived. (It was assumed that equivalent power levels at 1069, 1121, and 1178 nm contributed equally to the output spectrum.) Note that equivalent calculations can be performed for the backward propagating waveform. Estimating effective reflectivity of FBGs: The percentage of 1121 nm power leakage in either the forward propagating direction was found by matching up the FT-cavoutput with the output spectrum followed by calculation of the percentage of FTcav-output spectrum missing from the output spectrum. Note that equivalent calculations can be performed for the backward propagating waveform. The effective reflectivity was defined as: R eff = 100 - % leakage 1121 past FBG 12
Performance of cavities with 3 nm FBGs For longer cavities, higher output power levels of 1178 nm and greater Raman conversion levels of 1069 nm to 1121 nm were obtained. But, the amount of 1121 nm leakage in the forward direction initially increased with cavity length, reaching maximum when the cavity was 40 m in length and then, decreased for the same level of 1069 nm pump power above threshold. 13
Comparison of linewidth broadening as light traverses a Raman cavity Broadening in cavity in forward direction 13.8 W of 1069 nm pump 65 m cavity 10/125 germanosilicate fiber 14
Dependence of 1121 nm power leakage on 1121 nm intracavity power for cavities having 3 nm FBGs P1069nm = 4802mW Percentage 1121 nm power leakage depends linearly on the 1121 nm intracavity power level for all cavity lengths explored. For the longer cavity lengths explored (>= 40 m), similar levels of linewidth broadening are seen for a given 1121 nm intracavity power. This seems to indicate that maybe cavity length isn t a big effect which is consistent with modeling. But, for the shorter cavity lengths explored, noticeably less leakage of 1121 nm output power was seen for a given 1121 nm intracavity power which seemed to indicate the existence of another regime and that maybe cavity length does have a significant role. 15
Dependence of resonator performance on FBG bandwidth 65 m cavities having 1 and 3 nm FBGs were investigated. The cavity with 1 nm FBGs had performance inferior to that of the cavity having 3 nm FBGs with less 1121 nm intracavity power buildup and less amplification of the 1178 nm Also, the percentage leakage of 1121 nm for the cavity having 1 nm FBGs was significantly greater than that from a cavity having 3 nm FBGs for identical 1121 nm intracavity power levels. 16
Raman modeling using effective reflectivities of 3 nm FBGs L = 65 m P1121 nm (forward) [mw] Reff L = 90 m P1121 nm (forward) [mw] Reff 0.01 99.91 22.47 99.91 494.69 99.91 1248.62 99.91 2990.35 98.156 3437.92 97.8 5054.63 96.77 5163.80 96.6 6957.16 95.02 6863.00 95.6 8341.71 93.97 8606.95 94.447 10106.30 92.92 9941.99 93.4 11468.23 91.63 11428.75 92.45 13014.18 90.72 12740.21 91.5 14165.37 90.21 13981.49 90.67 15525.58 89.36 15018.46 90 16850.84 88.5 15933.70 89.1 18037.84 87.7 16626.75 88.47 Model was anchored by adjusting attenuations at the maximum level of P1069 nm (pump). The experimentally derived Reff for this pump level was utilized. Model was then run for remainder of pump levels using experimentally derived values for Reff. Agreement within 10-30% was obtained. Differences between the model and experiment may be attributable to the experimental error associated with Reff. 17
Impact of using experimentally derived values of Reff in a model (3 nm FBGs) Usage of the manufacturers value for the reflectivity of the grating, R = 99.91%, results in an overshoot of resonator performance whereas, usage of too low of a value of Reff of 87.7% results in an undershoot of the performance. This occurs since Reff decreases as the degree above threshold and the intracavity 1121 nm power increases. Best agreement between the experiment and modeling is obtained when experimentally DISTRIBUTION STSTEMENT derived A. Approved values for public of release; Reff distribution are used is unlimited in the model. 18
1069 nm 20 GHz 1069 nm isolator Finalization of the low power.01% TAP WDM Yb amp 3 WDM 1178 nm 100 khz 1178 nm isolator stage 3 WDM 3 WDM.01% TAP BT-HP 3 nm 1121 nm HR FT-HP PM 10/125 germanosilicate fiber 3 nm 1121 nm HR Cavity Length [ m] Reff P1069 (out) [W] P1121 nm (forward) [W] P1178 nm (out) [W] SBS 15 88.5 4.139 25.29 1.244 5.022 27 84.209 2.809 19.2 1.666 10.73 40 88.07 1.197 19.91 3.36 24.51 65 89.86 0.7198 14.49 4.723 50.15 90 92.695 0.534 11.15 5.599 78.1 115 92.996 0.474 8.84 5.701 101.3 Will seed with a 1 W 1178 nm source Modeling assumes.5w 1178 nm and 12.67 W 1069 nm pass through input FBG into cavity Modeling shows that longer cavities result in higher output levels of 1178 nm, lower intracavity 1121 nm levels, and increased SBS. Maximum cavity length 65 m 19
Conclusions The percentage of 1121 nm spectral power leakage past a FBG: is nearly linearly dependent on the intracavity 1121 nm power especially for shorter cavity lengths, shows a dependence on the length of the resonator cavity Broader bandwidth FBGs seem to enable greater 1121 nm intracavity power buildup and better system performance 3-5 W of 1178 nm output power from this system should be achievable Further model development is needed 20