Noise sources and stabilization strategies in frequency combs ICTP � Winter � College � on � Optics Trieste, � Italy February � 17, � 2015 Nathan Newbury National Institute of Standards and Technology, Boulder, CO nnewbury@boulder.nist.gov
Outline � Motivation for frequency combs � Frequency comb � Noise in fiber-based frequency combs – Fixed point � Making a quiet frequency comb � Fiber frequency combs at NIST – Overview of different designs since 2003 – Current “robust” NIST frequency comb � Conclusion
People People Esther Baumann Esther Baumann Hugo Bergeron Hugo Bergeron Mick Cermak Mick Cermak Ian Coddington Ian Coddington Kevin Cossel Kevin Cossel Stefan Droste Stefan Droste Fabrizio Giorgetta Fabrizio Giorgetta Dan Herman Dan Herman Nathan Newbury Nathan Newbury Laura Sinclair Laura Sinclair Bill Swann Bill Swann Gar-Wing Truong Gar-Wing Truong Eleanor Waxman Eleanor Waxman Gabe Ycas Gabe Ycas Other � non � NIST � collaborators: NIST � collaborators: Brian � Washburn � (Kansas � State) Scott � Diddams, � Dave � Leibrandt, � Craig � Jean � Daniel � Deschenes � (U � of � Laval) Nelson, � Scott � Papp, � Frank � Quinlan, � Kevin � Silverman, � Jeff � Shainline, � Rich � Mirin, � ‧ Greg � Rieker � (CU)
Recent review articles RSI � Review � article � on � current � NIST � comb � design: � L.C. � Sinclair, � J. � D. � Deschênes, � L. � Sonderhouse, � W. � C. � Swann, � I.H. � Khader, � E. � Baumann, � N. � R. � Newbury, � and � I. � Coddington, �� Invited � Article: � A � Compact � Optically � Coherent � Fiber � Frequency � Comb, � Review � of � Scientific � Instruments �� 86, � 081301 � (2015); See � also : � http://www.nist.gov/pml/div686/grp07/fpga � based � digital � control � box � phase � stablization � frequency � comb.cfm � Nanophotonics upcoming � review � on � fiber � combs: � S. � Droste, � G. � Ycas, � B. � R. � Washburn, � I. � Coddington, � NRN, � Optical � Frequency � Comb � Generation � based � on � Erbium � Fiber � Lasers , � Nanophotonics, � to � be � published � Fiber � frequency � Comb � noise N. � Newbury, � W. � Swann, � J. � Opt. � Soc. � Am. � B, � Low � noise � fiber � laser � frequency � combs, � 24, � (2007)
Frequency Combs: Why are they special? Intensity Intensity Intensity Laser Frequency frequency frequency frequency Comb ”spectral ”spectral ”spectral comb comb comb ruler„ ruler„ ruler„ ”teeth„ ”teeth„ ”teeth„ � n � n+1 � n � 1 Clock calibrated broad � coherent � frequency � scale � spectrum & � bright laser rf synthesizer
Newbury, � Nat. � Phot., � 5, 186 (2011) Diddams, � JOSA � B, � 27, � B51 � (2010) Applications � of � Frequency � Combs Frequency Comb � Applied to laser-based metrology/sensing systems – As a spectral ruler - As a frequency divider – As a “time” ruler - As a calibrated broadband source
Example � applications Precision � microwave � generation � Precision � molecular � spectroscopy � (for � RADAR) (for � greenhouse � gases) Precision � spectroscopy � (for � exoplanet � searches) Precision � timing � across �� synchronized � network NIST NIST Precision � Ranging Others: Advanced � communications Fundamental � scientific � tests ‧
Outline � Motivation for frequency combs � Frequency comb – Basic picture – Types of Frequency combs � Noise in fiber-based frequency combs – Fixed point – Noise sources – Actuators � Making a quiet frequency comb � Fiber frequency combs at NIST – Overview of different designs since 2003 – Current “robust” NIST frequency comb � Conclusion
A � Mode � Locked � Laser Time � domain Passively Modelocked Laser Outputs � sat. � abs. light � at � gain equally � spaced � modes � of � the � laser � � ��� ��� t Frequency � Domain � ��� Intensity optical frequency
A � Free � Running � Mode � Locked � Laser Time � domain Passively � � ��� Modelocked Laser ��� sat. � abs. gain t Frequency � domain f rep I ( � f ) f o 0 f n = nf rep + f 0
A � Free � Running � Mode � Locked � Laser Time � domain Passively � � ��� Modelocked Laser ��� sat. � abs. gain t Frequency � domain f rep I ( � f ) f o 0 With � noise, � output � moves � around... f n = nf rep + f 0 but � basic � comb � structure � is � preserved. � Comb � can � only � ”translate„ � and � ”breathe„
Offset � Frequency � Stabilization Jones, � et � al. � Science 288 , 635 � (2000) Passively T = f r -1 Modelocked Laser J. Hall sat. � abs. T. Hänsch gain t Spectrally � broaden � to � an � octave I ( � f ) f o f r 0 0 f o x2 ”Self � referenced � Lock„ phase � locked � loop f 0 = 2( nf rep + f 0 ) - 2 nf rep + f 0
Stabilization � of � the � Second � Degree � of � Freedom J. Hall Passively T. Hänsch T = f r -1 Modelocked Laser sat. � abs. gain t A choice: Stabilize to an Optical or RF oscillator f r I ( � f ) f o 0 0 Phase � lock � (stabilize) offset � frequency, �� f o
Frequency Comb needs a Reference Oscillator Optical � Oscillator RF � oscillator (cavity � stabilized � Laser) (Quartz � / � DRO � / � H � maser) Signal � @ � 200 � THz � Signal � @ � 10 � MHz � — 10 � GHz � Amplitude (lin. units) 1 � Hz -40 -20 0 20 40 Frequency offset (Hz) Pound � Drever � Hall � Cavity � Lock • Quartz/DRO: � small, � compact, � cheap • Not � small, � not � compact, � not � cheap • RF � comb � stabilization � easy • Optical � comb � stabilization � hard • No � optical � coherence � in � comb • Optically � coherent � comb • Broad � optical � teeth • ”Delta � function � teeth„
RF � Stabilization J. Hall Passively T. Hänsch T = f r -1 Modelocked Laser sat. � abs. gain t RF � oscillator RF � phase � I ( � f ) locked � loop f o f r 0 0 f n = nf rep + f 0 Phase � locked offset � frequency, �� f o
Optical � Stabilization J. Hall Passively T. Hänsch T = f r -1 Modelocked Laser sat. � abs. gain t Optical � ”Clock„ � = � Narrow � linewidth laser Laser Optical � phase � locked � loop I ( � f ) f o 0 0 f Opt Phase � locked offset � frequency, �� f o
Optical � Phase � Locked � Loop Optical � heterodyne � -6 ResBW 10 93 Hz spectrum Optical � Intensity -8 10 Reference � -10 10 Laser -12 10 f rf =f Opt � f Laser 71.85 72.00 72.15 Frequency (MHz) Frequency � f Laser Phase/Frequency � Noise � PSD comb filter Rad 2 /Hz � Hz 2 /Hz rf f Opt f rep Loop Fourier � Freq. filter Counted � Frequency � � clk Frequency Deviation Rf Phase f rf synth. comparison Allan � deviation Time ”in � loop„ � measures � of � comb � phase � coherence � and � frequency � stability
RF � vs � Optical � Stabilization: � Lever � Arm � Difference RF � For � an � RF � lock: Phase � locked �� f o � RF � phase � noise � is � multiplied � by � n 2 up � to � optical � � Broad � optical � linewidths Cavity � Stabilized � Optical � teeth � central � position � defined � absolutely Laser Optical � phase � � For � an � Optical � Lock � : locked � loop I ( � f ) � Optical � phase � noise � divided � by � n 2 down � to � rf � Narrow � optical � linewidths � across � comb � (if � reference � laser � narrow) 0 0 f Opt Phase � locked �� f o
Other �� Stabilization � Options: � double � pinning J. Hall Passively T. Hänsch T = f r -1 Modelocked Laser sat. � abs. gain t Laser Laser I ( � f ) f o 0 0 f Opt, 1 f Opt, 2 NO � Offset � frequency � stabilization �� > � no � need � for � octave � supercontinuum But � no � absolute � frequency � knowledge � (unless � cavity � separately � measured)
Other �� Stabilization � Options: � free � running � laser � J. Hall Passively T. Hänsch T = f r -1 Modelocked Laser sat. � abs. gain t RF � frequency � Free � counter � running � (nf rep ) Laser I ( � f ) f o 0 0 0 f n = nf rep + f 0 Phase � locked Avoids � need � for � cavity � stabilized � laser offset � frequency, �� f o Retains � qbsolute frequency � knowledge
Femtosecond Laser Frequency Combs A unique source for sensing and spectroscopy • an array of millions of phase-coherent CW oscillators • large spectral coverage: 300 nm - 10 microns • precisely known frequencies (~1 Hz resolution) • high peak power for efficient nonlinear optics Difference Frequency Generation Harmonic Generation and Continuum and Continuum Er:fiber Yb:fiber Tm:fiber Ti:sapphire 300 500 1000 1500 2000 10000 wavelength (nm) Ti:Sapphire laser Er:fiber laser courtesy of S. Diddams et al.
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