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Advanced Technologies for Optical Frequency Control and Optical Clocks Yoel Fink Thomas Greytak Erich Ippen Franz Kaertner Daniel Kleppner Leslie Kolodziejski Jeffrey Shapiro Franco Wong Massachusetts Institute of Technology Research Laboratory of Electronics Cambridge, MA 02139 Department of Electrical Engineering and Computer Science Department of Materials Science and Engineering Department of Physics <[email protected]> MURI - Technology for Optical Frequency Control and Optical Clocks Outline Few-cycle laser development • Ti:sapphire, Cr:forsterite, Cr:YAG • Phase sensitive nonlinear optics • Attosecond-precision laser synchronization Optical frequency metrology with ultracold hydrogen • Hydrogen spectroscopy and frequency referencing • Locking of femtosecond laser comb to single frequency cw laser Nonlinear optical techniques for comb technology • Efficient SHG and DFG with chirped-grating PPLN • 3-to-1 self phase-locked optical frequency divider • DFG for comb locking to methane-stabilized HeNe laser Ultra-broadband mirrors and saturable absorbers for few-cycle lasers • Novel wide-area GaAlAs oxidized mirrors • Saturable Bragg reflectors for few-cycle lasers Cylindrical photonic bandgap fibers • Novel microstructured fiber for nonlinear optics • Broadband dispersion characteristics of bandgap fibers Ultra-low-jitter modelocked diode laser • Locking to visible wavelength reference MURI - Technology for Optical Frequency Control and Optical Clocks 5-fs Ti:sapphire Laser •double-chirped mirrors •enhanced SPM •octave-spanning spectrum •dispersion-managed modelocking MURI - Technology for Optical Frequency Control and Optical Clocks Chirped and Double-Chirped Mirrors λ TiO2 / SiO2 Bragg-Mirror: B - Layers 4 SiO2 Substrate Chirped Mirror: Air Bragg-Wavelengthλ SiO2 Substrate B Chirped λ1 Negative Dispersion: λ2 λ2 > λ1 --> R. Szipöcs & F. Krausz (Opt. Lett. 19, 201-203, 1994) Computer Optimization Double-Chirped Mirror: Bragg-Wavelength and Coupling Chirped d h = λB /4 SiO2 Substrate ARCoating Air “Impedance” - Matching MURI - Technology for Optical Frequency Control and Optical Clocks DCM-Pairs Covering One Octave Designed Measured 80 400 0 20 0 80 400 200 60 0 40 -400 20 -600 600 800 1000 1200 1400 WAVELENGTH, NM 0 -200 -400 600 -600 800 1000 1200 1400 WAVELENGTH, NM Fabricated by Tschudi Group, Darmstadt MURI - Technology for Optical Frequency Control and Optical Clocks 2 -200 2 40 600 GDD, FS 60 GDD, FS 200 100 REFLECTIVITY (%) 600 REFLECTIVITY (%) 100 5-fs Pulse Characterization IA C • Nonlinear autocorrelation • Spectrum • Recovered amplitude and phase 0 8 1.0 10 6 0.8 10 0.6 10 -1 -2 4 -3 10 0.4 2 0 c) -10 0 10 20 TIM E DELAY , FS FWHM 5 fs -40 -20 0 20 TIME , FS 30 0.2 S P EC TR UM , a.u. 0.0 600 800 1000 1200 b) W AV ELE NGTH, NM P H A SE , R A D IA N -20 PO W ER , a.u. a) 40 -4 10 -5 10 0 - 4 - 8 -12 d) 600 800 1000 1200 W AV ELE NGTH, NM 1400 MURI - Technology for Optical Frequency Control and Optical Clocks Pulse-to-Pulse Phase Slip in a Modelocked Laser VGROUP ≠ VPHASE ∆φ gain Modelocked laser 2nGL/c MURI - Technology for Optical Frequency Control and Optical Clocks RF spectral power density [dB] Phase-Dependent 2nd Harmonic 0 • rf tones reveal dφ/dt -20 -40 -60 -80 -100 0 10 20 30 40 RF Frequency [MHz] 50 60 70 MURI - Technology for Optical Frequency Control and Optical Clocks Stopping the Optical Phase Slip Trf mTopt = Trf The optical frequency is locked to an exact multiple of the rf rep rate ! MURI - Technology for Optical Frequency Control and Optical Clocks Frequency Locking of Laser Modes •Phase slip in the frequency domain cavity modes c 2ngL f1 Locking an octave by SHG: dφ δ= dt laser modes δ = 0 ⇒ f1 = 2f1 = f2 = f1 + nc 2ngL f2 mc mc ⇒ f1 = 2ngL 2ngL MURI - Technology for Optical Frequency Control and Optical Clocks Second Harmonic Generation with Octave-Spanning Spectrum eiφ Fundamental ωo e2iφ 2nd Harmonic 2ωo => Interference between fundamental and 2nd harmonic MURI - Technology for Optical Frequency Control and Optical Clocks Continuum Generation with Microstructured Fiber Power (arb. units) 10 10 10 0 -2 -4 Input Output 10 -6 400 600 800 1000 1200 1400 Wavelength (nm) Strong guiding shifts the zero-dispersion wavelength to the near visible Spectrum resulting when 80fs pulses from a Ti:sapphire oscillator are focused into a holey fiber J. Ranka et al. MURI - Technology for Optical Frequency Control and Optical Clocks 1600 • All solid state • 1.3 µm wavelengths DCM OC DCM X Spectral Intensity (norm.) 14 fs Cr:forsterite Laser M 1 0 1.2 1.4 Wavelength (µm) 1.6 -40 -20 0 20 Delay Time (fs) 40 DCM L λ /2 P Faraday rotator Nd :YAG Recent result: SBR stabilized Interfer. Autocorrelation (norm.) Autocorrelator DCM 8 4 0 Chuboda et.al. MURI - Technology for Optical Frequency Control and Optical Clocks 20-fs Cr4+:YAG Laser Nd:YVO4 Laser – 1064nm f = 10 cm DCM DCM 2-cm Cr4+:YAG SBR OC DCM SBR Reflectivity 20 fs FWHM Wavelength (µm) MURI - Technology for Optical Frequency Control and Optical Clocks Overlapping Femtosecond Laser Spectra Ti:Sapphire, Cr:Forsterite and Cr:YAG MURI - Technology for Optical Frequency Control and Optical Clocks A Compact Prismless Ti:sapphire Laser Octave-spanning spectrum BaF2 2mm Ti:Al2O3 OC 1 φ = 10ο L = 20 cm PUMP BaF2 - wedges OC 2 Base Length = 30cm for 82 MHz Laser Future: • Even more compact • Scaling to higher repetition rates • >500 MHz, ring laser MURI - Technology for Optical Frequency Control and Optical Clocks Overlapping Femtosecond Laser Spectra 0 Cr:forsterite Spectral Power [log] -10 -20 • Stabilized Cr:forsterite laser 30 dB -30 -40 • Laser synchronization 300 asec timing-jitter Ti:sapphire • Opportunities: extended frequency comb: 600–1600 nm -50 -60 600 • Compact, prismless octave spanning Ti:sapphire laser and/or 800 1000 1200 1400 Wavelength [nm] 1600 single-cycle pulse generation MURI - Technology for Optical Frequency Control and Optical Clocks Balanced Cross-Correlation for Laser Synchronization ∆t Cr:fo -GD/2 Ti:sa SFG + - The residual out-of-loop timing jitter measured from 10mHz to 2.3 MHz is 300 as (a tenth of an optical cycle) Output spectrum: 650-1450nm 3mm GD Fused Silica SFG Cross-Correlation Amplitude Rep.-Rate Control (1/496nm = 1/833nm+1/1225nm). Time [fs] 1.0 -100 0 100 0.8 0.6 Timing jitter 0.30 fs (2.3MHz BW) 0.4 0.2 0.0 0 20 40 Time [s] 60 MURI - Technology for Optical Frequency Control and Optical Clocks 80 100 Optical Frequency Metrology with Ultracold Hydrogen OVERALL GOAL To pursue ultraprecise spectroscopy of ultracold atomic hydrogen for: SCIENCE • Resolve discrepancies in the theory of quantum electrodynamics • Better values for fundamental constants • Test theories of atomic interactions and atomic structure TECHNOLOGY • Explore the possibilities for an atomic hydrogen optical clock • Develop and apply techniques for measuring the frequencies of optical transitions • Stimulate the development and application of optical frequency combs IMMEDIATE GOAL Measure absolute frequencies of transitions such as 2S →10S (2-photon, 730 nm) using atomic hydrogen as an optical frequency standard MURI - Technology for Optical Frequency Control and Optical Clocks Spectroscopy with a Hydrogen Optical Clock Lock to 1S-2S (243 nm) 486 nm x2 Cold Hydrogen Excite 2S-10S Lock x2 972 nm 764 nm • Excite 1S→2S transition (known to about 1 part in 1014.) • Lock 486 nm laser to 1S→2S • Excite two-photon transition such as 2S→10S • Measure excitation frequencies with an optical frequency comb, referenced to the 486 nm laser, i.e. referenced to hydrogen MURI - Technology for Optical Frequency Control and Optical Clocks Nonlinear Optical Techniques for Comb Technology Ultra-broadband SHG with zero group velocity mismatch • 70 nm bandwidth for 1580 nm → 790 nm in PPKTP Difference-frequency generation of 3.39 µm • • locking Ti:sapphire laser comb to CH-stabilized HeNe laser collaboration between MIT and JILA 3-to-1 frequency divider • • self-phase-locked optical parametric oscillator with cascaded nonlinear interactions in PPLN provides phase-locked markers at 532, 798, and 1596 nm MURI - Technology for Optical Frequency Control and Optical Clocks Difference Frequency Generation of 3.39 µm Servo to lock comb spacing to He-Ne Ti:S fs pulses Ti:S spectrum 667 nm PPLN difference-frequency generator 830 nm cw 3.39 µm output Phase-locked } CH-stabilized He-Ne 4 @ 3.39 µm N pairs of modes yield N2 enhancement N~15,000 MURI - Technology for Optical Frequency Control and Optical Clocks Self-Phase-Locked OPO Yields 3:2:1 Frequency Markers PZT Signal 798 nm Pump PPLN 532 nm Signal 2ω Idler 1596 nm OPO grating SHG grating 2ω 2ω Pump 3ω Self-phase locking (∆ω=0) ω MURI - Technology for Optical Frequency Control and Optical Clocks Ultra-Broadband Saturable Absorber Mirrors GaAs/AlxOy Mirror GOALS • High index-contrast oxidized mirrors for broadband low dispersion • In-based absorber optimized for wavelength – integrated on mirror • Large area for low power density delaminated interface delaminated interface 500 µm AlxOy • Stable, durable structures GaAs substrate PROGRESS • Integrated broadband SBRs for Cr:YAG and Cr:forsterite lasers • Dramatic improvement in layer stability with AlGaAs/AlxOy structures • Wide area oxidization (>500µm diameter) GaAs AlGaAs/AlxOy Mirror GaAs/InGaAs Al0.3Ga0.7As AlxOy 500µm GaAs substrate MURI - Technology for Optical Frequency Control and Optical Clocks Issues Affecting the Distributed Bragg Reflector Oxidation of AlGaAs Layers R946, 5 hr, 415 C Oxidized 11/13/2002 GaAs/InGaAs R946, 5 hr, 415 C Oxidized 11/13/2002 AlxOy Al0.3Ga0.7As GaAs substrate 99µm from mesa edge 13µm from mesa edge • Edges of mesa less oxidized than at center • Mirror bandwidth a function of position MURI - Technology for Optical Frequency Control and Optical Clocks Issues Affecting Saturable Absorber 5hrs Sensitivity to Oxidation Time 8hrs ~230 µm 420C 435C Sensitivity to Oxidation Temperature Center = ~3mm from wafer edge Center = ~11.4mm from wafer edge Center = ~18.4mm from wafer edge Sensitivity to Growth Temperature MURI - Technology for Optical Frequency Control and Optical Clocks OmniGuide Mirror Fibers primary gap centered around 1.5 µm MURI - Technology for Optical Frequency Control and Optical Clocks Bandgap Fiber with and without Defect Regular OmniGuide structure 100µm Modified (defect) structure 1µm MURI - Technology for Optical Frequency Control and Optical Clocks Mirror-guide Dispersion Characteristics Fiber with cavity mode Bandgap fiber 20 LH 15 Dispersion D (ps/nm-km) Dispersion D (ps/nm-km) 20 10 Metallic 5 0 -5 -10 -15 -20 1.2 15 1.55 µm LH 10 5 Metallic ZD 0 -5 -10 -15 1.4 1.6 1.8 2 Vacuum wavelength (µm) 2.2 -20 1.2 1.4 1.6 1.8 Vacuum wavelength (µm) MURI - Technology for Optical Frequency Control and Optical Clocks 2 2.2 Modelocked External External Cavity Diode Laser • 500 µm gain section • 50 µm saturable absorber section • λ = 1547nm , f = 9.35 GHz • BPF = 0.7 nm : τ = 6.7 ps • BPF = 5 nm : τ = 3 ps VSA Poseidon SBO OSC I gain 5-nm BPF MIRROR R = 99.9% SELFOC LENS OUTPUT NEC external cavity MLLD MURI - Technology for Optical Frequency Control and Optical Clocks Residual Phase Noise Measurement • Vector signal analyzer for low frequency offsets (1-10 MHz) • RF spectrum analyzer for high frequency offsets (10 MHz – 4.5 GHz) VECTOR SIGNAL ANALYZER OSC AMP 3-dB AMP CH1 MLLD DELAY CH2 RF ANALYZER (RF analyzer after mixer has higher sensitivity than for direct detection.) MURI - Technology for Optical Frequency Control and Optical Clocks Single-Sideband Phase Noise - Results • Hybrid MLLD, 9GHz, 6.7 ps, Poseiden SBO • Vector signal analyzer (0 – 10 MHz) • RF spectum analyzer (10 MHz – 4.5 GHz) -80 Vector Signal Analyzer Data L(f) dBc/Hz -90 Spectrum Analyzer Data -100 -110 -120 -130 -140 -150 Vector Signal Analyzer Noise Floor -160 10 100 1k Spectrum Analyzer Noise Floor 10k 100k 1M 10M 100M 1G Offset Frequency (Hz) Integrated timing jitter from 10 Hz to 4.5 GHz = 86 fs 1kHz – 10MHz : 40 fs 154 fs including all noise spurs SBO jitter = 5.6 fs (10 Hz to 10 MHz) MURI - Technology for Optical Frequency Control and Optical Clocks Synchronization of Modelocked Diode Laser to Visible Standard MIT-JILA collaboration D. J. Jones et al. Opt. Lett. 2003 MURI - Technology for Optical Frequency Control and Optical Clocks Timing Jitter Spectral Density of Stabilized MLLD 20 fs jitter (1Hz – 10 MHz) MIT-JILA collaboration D. J. Jones et al. Opt. Lett. 2003 MURI - Technology for Optical Frequency Control and Optical Clocks Outline Few-cycle laser development • Ti:sapphire, Cr:forsterite, Cr:YAG • Phase sensitive nonlinear optics • Attosecond-precision laser synchronization Optical frequency metrology with ultracold hydrogen • Hydrogen spectroscopy and frequency referencing • Locking of femtosecond laser comb to single frequency cw laser Nonlinear optical techniques for comb technology • Efficient SHG and DFG with chirped-grating PPLN • 3-to-1 self phase-locked optical frequency divider • DFG for comb locking to methane-stabilized HeNe laser Ultra-broadband mirrors and saturable absorbers for few-cycle lasers • Novel wide-area GaAlAs oxidized mirrors • Saturable Bragg reflectors for few-cycle lasers Cylindrical photonic bandgap fibers • Novel microstructured fiber for nonlinear optics • Broadband dispersion characteristics of bandgap fibers Ultra-low-jitter modelocked diode laser • Locking to visible wavelength reference MURI - Technology for Optical Frequency Control and Optical Clocks