1. Fiber Frequency Combs Jennifer Black EE230 Final Presentation

2. 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

3. Mode-locking Pictures from Rick Trebino lecture notes

4. Passively Mode-Locked Lasers MirrorOutput Coupler Gain SA 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. [1] Fourier Transform!

5. Frequency Combs 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. [2] f n = nf r + f offset

6. Applications Breath analyzer, NIST Astronomical measurements, Max Planck Institute Atomic spectroscopy, KSU Optical clock, NIST

7. 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)!

8. CNT Fiber Laser ~ 1550nm EDF CNT!

9. CNT Deposition Nanotubes about fiber taper LD EDFA CNT/ethanol solution: 0.5mg CNT 20mg Ethanol 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

10. CNT Fiber Frequency Combs PROS: High rep rate compared to highly nonlinear optical fiber (HNLF) CONS: Low power threshold VS. HNLF Damaged Nanotubes!

11. Photonic bandgap fibers Hollow capillary fiber: 4% loss per bounce. Cladding Core n 1 = 1.5 n 2 = 1.52 Standard optical fiber: total internal reflection n 1 = 1.5 n 2 = 1.0 Cladding Core PBG fiber ~1.1 1.0 Bragg scattering forbids radial propagation --or-- Photonic crystal forbids propagation everywhere except at defect.

12. Photonic Bandgap Optical Fibers (PBG) d = 10  m Using 10 µm inner diameter PBG fiber Want CNT/PMMA solution in center hole Have PBG guide like solid core fiber

13. Method Taper PBG fiber: Cleave fiber where photonic crystal has collapsed and center hole is all that is left open:

14. Method Apply vacuum to cleaved end of PBG while cleaved end is in CNT/PMMA solution Vacuum chamber CNT/PMMA solution Vacuum Chamber CNT/PMMA solution Microscope FIBERFIBER

15. 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

16. Questions?

17. References [1]: http://www.optik.uni- erlangen.de/mpf/php/abteilung2/index.php?show=res earch&in=precisionmeasurements&and=rim [2]: http://www.rp- photonics.com/frequency_combs.html [3]: http://www.justmeans.com/Carbon-Nanotube- based-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

19. Single Walled Carbon Nanotubes Single wall carbon nanotubes have semiconductor, semimetal or metallic properties depending on the chiral vector of the nanotube 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

20. Carbon Nanotubes (CNT) [3] Mean diameter = 1.35 nm Mean diameter = 1.2 nm Diameter of CNT change transmission wavelength dependence How to incorporate CNT into fiber laser? Transmission vs. Wavelength Curves for CNT of Different Mean Diameter [4]

21. CNT/Polymer Solution Polymer (we use PMMA) used to disperse CNT homogenously – n PMMA = 1.49 Put inside of PBG fiber and guide like solid core fiber Carbon Nanotube precipitant A few days later 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

22. Laser Diode Gain Output Coupler Testing the Fibers Test small piece of sample in a pre-existing fiber laser LD Butt-couple SA (..?) into laser cavity

23. Testing the Fibers Sample is butt-coupled on both sides to a pre-existing fiber laser Fiber Laser Cleaved fiber from the laser PBG sample

24. Optical Spectra Mode-locked – Broad spectra Continuous Wave (CW) – Sharp peak

25. 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

26. Results for Anisole Anisole Sample: – Also lasing CW – Not mode-locking P out = 0.8 mW Length = 2.8 cm P out = 120 µW Length = 3.0 cm

27. Conclusions “Clean PBG” PBG with solution