Ultrafast ChromiumForsterite Laser and its Application to Frequency

Ultrafast Chromium-Forsterite Laser and its Application to Frequency Metrology Ahmer Naweed Group: M. Faheem, K. Knabe, R. Thapa, A. Pung, B. R. Washburn, and K. L. Corwin Thanks: M. Wells, R. Reynolds, and JRM Staff (KSU) S. Diddams and N. Newbury (NIST) J. Nicholson (OFS) Funding: NSF AFOSR

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

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

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

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

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

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

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

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

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

Propagation of Ultrafast Laser Pulses x

Propagation of Ultrafast Laser Pulses x

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.

Frequency Combs Time domain Df E(t) Carrier-envelope phase slip from pulse to pulse because: 2 Df t vg vp tr. t = 1/fr Frequency domain I(f) fo fr It is critical to have an octave spanning spectrum. 0 fn = nfr + fo f 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).

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Existing portable wavelength references for the telecom industry laser C 2 H 2 or LED Pressure-broadened Line centers: ± 130 MHz or ± 13 MHz Used to calibrate Optical Spectrum Analyzers (OSA’s) Line widths ~5 GHz (OSA resolution) pressure → broadening & shift W. C. Swann and S. L. Gilbert, JOSA B 17, 1263 (2000)

Saturation spectroscopy in hollow optical fiber Pump z Probe

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

Self-Referenced Optical Frequency Comb fr I(f) fo f 2 n fn f 0 fn = n fr + fo x 2 2 nfr + 2 fo f 2 n = 2 nfr + fo fo • fo is generated from a heterodyne beat between the second harmonic of the nth mode and the 2 nth 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)

Ti: sapphire vs. Cr: forsterite lasing medium Ti: sapphire Cr: forsterite pump laser 10 W Green (>$ 60, 000) 10 W fiber laser (<$ 15, 000) optical fiber microstructured highly-nonlinear Dispersionshifted frequency range 500 – 1100 nm 1100 – 2200 nm Crystal temp room temp -5 o. C S. Diddams et al. , Science 293 (2001) I. Thomann et al. , OL 28, 1368 (2003)

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

Optimizing Cr: fr Laser: Dispersion Net cavity dispersion = Cr: f dispersion + prism (SF 6 ) dispersion + angular dispersion Pump laser net cavity dispersion* = - 260 fs 2 Cr: f dispersion = 277 fs 2 Prism dispersion = - 588 fs 2 angular dispersion = -1155. 13 fs 2 optimal prism separation = 32. 5 cm third order dispersion = 240. 77 fs 2 Cr: forsterite Laser *I. Thomann et al. , OL 28, 1368 (2003)

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

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

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

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.

Optimizing Cr: fr Laser: Astigmatism Beam diameter (mm) d 2 (cm)

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.

76. 43 nm FWHM Bandwidth 59 nm FWHM Bandwidth I. Thomann et al. , OL 28, 1368 (2003)

103. 452 nm FWHM Bandwidth

Rep. Rate Measurements: 115 MHz

Hyperbolic Secant Pulse: 38 fs. Transform limited pulse for 105 nm bandwidth: 16. 5 fs.

Stability of Mode Locked Laser

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

Supercontinuum Generation Nonlinear Effects cause creation of new optical frequencies

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

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. Aeff =13. 9 mm 2 Dispersion slope = 0. 024 ps/(nm 2 km) Nonlinear coefficient g = 8. 5 ( W km)-1 • Due to break up of these higher order solitons, four-wave mixing generates frequencies at wavelengths shorter than zero dispersion wavelength. J. W. Nicholson et. al, Opt. Lett 28, 643, 2003

Supercontinuum Generation from Cr: fr Laser output 88. 892 nm FWHM Bandwidth Supercontinuum

Current Research Status

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

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

Current Research Status

Saturation Spectroscopy Pump z Probe

Saturation Spectroscopy A= B=

Saturation Spectroscopy Pump Power (m. W) saturation no saturation Distance (m)

Conclusions Robust and efficient Cr: fr femto second laser. FWHM bandwidth of up to 105 nm and output energy of about 220 m. W. 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.

ULTRAFAT LASER BASICS

Chromium-forsterite Lasers: A Brief History

Optimizing Cr: fr Laser: Astigmatism

Frequency Combs for frequency metrology • Transfer stability and accuracy between optical and microwave regimes. Microwave (9. 2 GHz) • • Optical (500 THz) Ti: sapph comb commercially available. Fiber lasers at 1. 5 mm increasingly interesting. – – • Frequency Comb 5 x 104 near IR (telecom) cheaper more portable will require portable references near-IR comb being developed at Kansas State for characterization of new standards.