Ti:sapphire Laser

1. Ultrafast Chromium-Forsterite Laserand its Application to Frequency MetrologyAhmer NaweedGroup:M. Faheem, K. Knabe, R. Thapa, A. Pung, B. R. Washburn, and K. L. CorwinThanks: M. Wells, R. Reynolds, and JRM Staff (KSU)S. Diddams and N. Newbury (NIST)J. Nicholson (OFS)Funding:NSFAFOSR

2. Ti:sapphire Laser Verdi 5 - 10 W 530 nm l = 800 nm

3. Cr:forsterite Laser Fiber Laser 10 W 1075 nm l = 1270 nm

4. Cr:forsterite Laser Fiber Laser 10 W 1075 nm l = 1270 nm Frequency standards for the telecom wavelengths

5. Cr:forsterite Laser Fiber Laser 10 W 1075 nm l = 1270 nm Frequency standards for the telecom wavelengths Cr doped forsterite

6. Cr:forsterite Laser Fiber Laser 10 W 1075 nm l = 1270 nm Frequency standards for the telecom wavelengths Cr doped forsterite Poor thermal conductivity

7. Cr:forsterite Laser Fiber Laser 10 W 1075 nm l = 1270 nm Frequency standards for the telecom wavelengths Cr doped forsterite Poor thermal conductivity Sensitive to environmental perturbations

8. Outline Fundamentals of ultrafast lasers Mode locking Dispersion management Frequency combs and their realization Chromium-forsterite lasers: Benefits and Challenges Optimizing Chromium-forsterite laser Operation at KSU Supercontinuum generation Laser performance Future work

9. Tr f t S. Diddams et al., Science 306, 1318 (2004) Ultrafast Lasers: Basics

10. t f Time Bandwidth Product Constant depends upon the pulse shape For a Gaussian pulse,

11. x Propagation of Ultrafast Laser Pulses

12. x Propagation of Ultrafast Laser Pulses

13. Propagation of Ultrafast Laser Pulses Propagation of an ultrafast laser through a transparent material can lead to: • Pulse broadening • Pulse delay • Chirp • Material dispersion is positive. • A prism (or a grating) pair can have both positive or negative dispersion • By using a pair of prisms (or gratings) one can control net cavity dispersion.

14. Time domain 2Df E(t) Df t Frequency domain tr.t = 1/fr I(f) fo fr f fn = nfr + fo 0 Frequency Combs Carrier-envelope phase slip from pulse to pulse because: vg  vp It is critical to have an octave spanning spectrum. Supercontinuum generation in microstructure fiber preserves frequency comb. T. Udem, J. Reichert, R. Holzwarth, and T.W. Hänsch, OL 24, 881, (1999). D. J. Jones, et al. Science 288, 635 (2000).

15. www.nobel.se

16. Existing portable wavelength references for the telecom industry Line centers:±130 MHz or ±13 MHz Used to calibrate Optical Spectrum Analyzers (OSA’s) Line widths ~5 GHz (OSA resolution) pressure → broadening & shift laser or LED C2H2 Pressure-broadened W.C. Swann and S.L. Gilbert, JOSA B 17, 1263 (2000)

17. Pump Probe z Saturation spectroscopy in hollow optical fiber

18. Saturation spectroscopy in hollow optical fiber Significant signal strength at 10 and 20 mW pump powers! 10 mm core R. Thapa, K. Knabe, M. Faheem,K. L. Corwin

19. fn = n fr+ fo x2 2nfr+ 2fo f2n = 2nfr+ fo fo Self-Referenced Optical Frequency Comb fr I(f) fo f2n fn f 0 • fo is generated from a heterodyne beat between the second harmonic of the nth mode and the 2nth mode. • Once fr and fo are referenced to a known oscillator, all the frequency modes of the fs comb are fixed. D. J. Jones, et al. Science 288, 635 (2000)

20. Ti:sapphire vs. Cr:forsterite S. Diddams et al., Science 293 (2001) I. Thomann et al., OL 28, 1368 (2003)

21. Chromium-forsterite Lasers: A Brief History Zhang et al, 90 nm FWHM; 20 fs; 60 mW IEEE J Q. Electronics 1997 V. Yanovsky et al, 90 nm FWHM; 80 nm FWHM; 25 fs, 400 mW OL 1993 Haus et al., 90 nm FWHM; 250 nm FWHM; 14 fs, 80 mW, OL

24. Pump laser Cr:forsterite Laser Optimizing Cr:fr Laser: Dispersion Net cavity dispersion = Cr:f dispersion + prism (SF6 ) dispersion + angular dispersion net cavity dispersion* = - 260 fs2Cr:f dispersion = 277 fs2 Prism dispersion = - 588 fs2angular dispersion = -1155.13 fs2optimal prism separation = 32.5 cm third order dispersion = 240.77 fs2 *I. Thomann et al., OL 28, 1368 (2003)

25. refractive indexn Lens of focal length f h f d Optimizing Cr:fr Laser: Stability Ray transfer matrix (ABCD) analysis is performed to yield optimal cavity parameters that is essential for stable laser operation.

26. Optimizing Cr:fr Laser: Stability Ray transfer matrix (ABCD) analysis is performed to yield optimal cavity parameters that is essential for stable laser operation.

27. Pump laser Cr:forsterite Laser Optimizing Cr:fr Laser: Stability Ray matrix (ABCD) analysis performed to yield optimal cavity parameters that is essential for stable laser operation. Self consistentsolution:

28. Optimizing Cr:fr Laser: Astigmatism Because of a lack of axial symmetry, the beam waist along the sagittal and tangential planes may not necessarily be equal and spatially overlap (astigmatism). Therefore, the effects of astigmatism must be taken into account in cavity stability analysis.

29. Beam diameter (mm) d2 (cm) Optimizing Cr:fr Laser: Astigmatism

30. Mode Locking Cr:fr Laser Unlike Ti-sapphire laser, no well established method for mode-locking the Cr:fr laser is known.Observation of strong and periodic fluctuation in output laser power. This is an indication that the laser is close to ML regime.

31. 59 nm FWHM Bandwidth 76.43 nm FWHM Bandwidth I. Thomann et al., OL 28, 1368 (2003)

32. 103.452 nm FWHM Bandwidth

33. Rep. Rate Measurements: 115 MHz

34. Hyperbolic Secant Pulse: 38 fs. Transform limited pulse for 105 nm bandwidth: 16.5 fs.

35. Stability of Mode Locked Laser

36. Laser Parameters Spectral width: 90-105 nm Pulse Duration: 38 fs Rep. Rate: 115 MHz Output Power: 220 mW Center Wavelength: 1275 nm

37. Supercontinuum Generation Nonlinear Effects cause creation of new optical frequencies

38. Honeycomb Microstructure Optical Fiber J. Ranka, R. Windeler, A. Stentz, Opt. Lett. 25, 25 (2000). courtesy of Jinendra Ranka

39. Highly Nonlinear Fiber • Broadest continuum is generated by the fiber when the ultrafast laser pulse is in the anomalous dispersion region. • The pulse intensity begins to self Raman shift to longer wavelengths. • Due to break up of these higher order solitons, four-wave mixing generates frequencies at wavelengths shorter than zero dispersion wavelength. Aeff =13.9 mm2 Dispersion slope = 0.024 ps/(nm2 km) Nonlinear coefficient g = 8.5 ( W km)-1 J. W. Nicholson et. al, Opt. Lett 28, 643, 2003

40. Laser output 88.892 nm FWHM Bandwidth Supercontinuum Supercontinuum Generation from Cr:fr Laser

41. Current Research Status

42. Current Research Status Fiber in Cr:forsterite Laser Fiber Laser 10 W 1075 nm

43. Fiber out SC BS HNLF stabilized optical frequency comb Synthesizer DM f Loop nonlinear rep Filter crystal Synthesizer f Loop Phase 0 Filter Detector Current Research Status Fiber in Cr:forsterite Laser Fiber Laser 10 W 1075 nm

45. Current Research Status

46. Pump Probe z Saturation Spectroscopy

47. A = B = Saturation Spectroscopy

48. Pump Power (mW) saturation no saturation Distance (m) Saturation Spectroscopy

49. Conclusions Robust and efficient Cr:fr femto second laser.FWHM bandwidth of up to 105 nm and output energy of about 220 mW.Realized supercontinuum generation by coupling Cr:fr pulses to a HNLF. Future Work Octave spanning spectrum.Laser Stabilization.Installation of piezo mounted mirror in laser cavity.