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Fiber Frequency Combs Jennifer Black EE230 Final Presentation Mode-Locked Lasers • Laser that produces a series of ultrashort pulses (infinite pulse train) • Two techniques: – Active: uses optical modulator – Passive: may use a saturable absorber Mode-locking Pictures from Rick Trebino lecture notes Passively Mode-Locked Lasers Use of a saturable absorber (SA) in the cavity creates the pulse train. SA are materials with non-linear optical properties that attenuate low optical intensities. SA [1] Gain Fourier Transform! Mirror Output Coupler Frequency Combs [2] • If the carrier envelop offset can be set then the comb is stabilized • Frequencies used as optical ruler • Beat notes are measured The frequency comb (red) can be beat against an unknown frequency (blue). If the comb frequencies are known, then the unknown frequency can be determined. fn = nfr + foffset Applications Breath analyzer, NIST Optical clock, NIST Astronomical measurements, Max Planck Institute Atomic spectroscopy, KSU Table Top vs. Fiber lasers • Optical fibers can be used as waveguides for lasers that are: – Cheap – Portable – Robust – “Easy” Table top frequency comb at Center for Quantum Technologies in Singapore Fiber frequency comb shipped via FedEx (worked first try)! CNT Fiber Laser CNT! ~ 1550nm EDF CNT Deposition • Process: •10mW at 1560 nm through SMF and put into solution for 30s •Out of solution for 1 min • Throughput checked •Continue until loss = -3dB •Put into fiber laser cavity and mode-locks Nanotubes about fiber taper LD EDFA CNT/ethanol solution: •0.5mg CNT •20mg Ethanol CNT Fiber Frequency Combs • PROS: High rep rate compared to highly nonlinear optical fiber (HNLF) • CONS: Low power threshold VS. HNLF Damaged Nanotubes! Photonic bandgap fibers n1 = 1.5 n2 = 1.52 Core Cladding Standard optical fiber: total internal reflection n1 = 1.5 n2 = 1.0 Core Cladding Hollow capillary fiber: 4% loss per bounce. ~1.1 1.0 PBG fiber Bragg scattering forbids radial propagation --or-Photonic crystal forbids propagation everywhere except at defect. Photonic Bandgap Optical Fibers (PBG) •Using 10 µm inner diameter PBG fiber •Want CNT/PMMA solution in center hole d = 10 mm • Have PBG guide like solid core fiber Method • Taper PBG fiber: • Cleave fiber where photonic crystal has collapsed and center hole is all that is left open: Method • Apply vacuum to cleaved end of PBG while cleaved end is in CNT/PMMA solution CNT/PMMA solution Microscope Vacuum chamber Vacuum Chamber CNT/PMMA solution F I B E R Conclusion • Frequency combs used for precision measurements. • Fiber offers robust, “easy”, cheap and portable frequency combs. • Design challenges remain for desired threshold powers. • Work continues on CNT/PMMA SA at KSU Questions? References • [1]: http://www.optik.unierlangen.de/mpf/php/abteilung2/index.php?show=res earch&in=precisionmeasurements&and=rim • [2]: http://www.rpphotonics.com/frequency_combs.html • [3]: http://www.justmeans.com/Carbon-Nanotubebased-Batteries-for-HEVs/11428.html • [4]: Sze Y. Set, H. Yaguchi, Y. Tanaka, M. Jablonski. Ultrafast Fiber Pulsed Lasers Incorporating Carbon Nanotubes. IEE Journal of Selected Topics in Quantum Mech., Vol. 10, No. 1 Single Walled Carbon Nanotubes Single wall carbon nanotubes have semiconductor, semimetal or metallic properties depending on the chiral vector of the nanotube C na1 ma 2 Metallic n m integer multiple of 3 Semiconductor n m integer multiple of 3 Semimetal nm 0 Excitonic absorption in the semiconductor nanotube is responsible for the saturable absorption property Ultrafast recovery of the saturable absorber is due to metallic nanotubes serving a recombination centers Carbon Nanotubes (CNT) Transmission vs. Wavelength Curves for CNT of Different Mean Diameter [4] Mean diameter = 1.35 nm Mean diameter = 1.2 nm [3] • Diameter of CNT change transmission wavelength dependence • How to incorporate CNT into fiber laser? CNT/Polymer Solution • Polymer (we use PMMA) used to disperse CNT homogenously – nPMMA = 1.49 • Put inside of PBG fiber and guide like solid core fiber Step 1: 3mg CNT and 10mL of a solvent are sonicated for 3 hours Step 2: 37mg of PMMA are added and sonicated for an additional 2.5 hours Solvents used: - Acetone - Anisole A few days later Carbon Nanotube precipitant Testing the Fibers • Test small piece of sample in a pre-existing fiber laser Laser LD Diode Butt-couple SA (..?) into laser cavity Output Coupler Gain Testing the Fibers Cleaved fiber from the laser Fiber Laser PBG sample Sample is butt-coupled on both sides to a pre-existing fiber laser Optical Spectra 0.1 • Mode-locked – Broad spectra Power (mW/nm) 0.01 1E-3 1E-4 1E-5 1E-6 1540 1550 1560 1570 Wavelength (nm) • Continuous Wave (CW) – Sharp peak 1580 1590 Results for Acetone • Acetone Sample: – Lasing CW but not mode-locking… – Not stable – Poor solvent for this process – Laser possibly boiling away solvent – optical limiting – … Try a different solvent! Pout = 0.5 mW; length = 4 cm Results for Anisole • Anisole Sample: – Also lasing CW – Not mode-locking Pout = 120 µW Length = 3.0 cm Pout = 0.8 mW Length = 2.8 cm Conclusions “Clean PBG” PBG with solution