National Institute of Standards and Technology

Towards Chip-Integrated Low-Noise

William Loh The is Everywhere National Institute of Standards and Technology

Laser Invention ‒ 1958

Wavelength: 150 nm >250 μm Excimer Quantum Cascade

Power: 1 mW > 1000 W Industrial Cutting

Surgery Imaging Laser Tag

Welding Communications Countermeasures Holography

CDs Ranging Guidance Lasers

Cooling Sensing Photolithography Printers

2 The Lasers of Tomorrow National Institute of Standards and Technology

Greater Higher Tuning Power

Laser

Smaller Lower Size Noise

3 Laser Configuration National Institute of Standards and Technology

Two Conditions T for Oscillation

1) > Loss 2)  Tm 2

Modes

(a) Phase Noise

(b) Linewidth

Power Spectral Density Spectral Power Full-Width Half-Max

Frequency 4 Applications of Low-Noise Lasers National Institute of Standards and Technology Coherent Fiber-Optic Sensing Spectroscopy Communications Q

I

Optical Development of LIDAR Frequency advanced optical- Metrology atomic clocks

Phase Detection Sharp Resonance Need low Need narrow phase noise linewidth

5 Applications of Low-Noise Lasers National Institute of Standards and Technology

Optical Frequency Division T. Fortier et al. Ultrastable cavity (Q ~ 1011) Nature Photon. 2011

CW laser n ~ 50,000 1 Hz linewidth νn = n fr+fo

νn - fo fr f = r n

δνn - δfo δf = r n 1/fr Optical phase noise Noise sidebands reduction ~ 90 dB Spectrum analysis f fr 6 Low-Noise Lasers by Feedback National Institute of Standards and Technology

Stabilize

Erbium-doped fiber laser Sharp Cavity Resonance

Q ~1011

7 Current Technology for Low-Noise Lasers National Institute of Standards and Technology

External- Cavity- Diode DFB Fiber Cavity Stabilized Laser Laser Laser Laser

6 4 5 3 4 Linewidth 10 Hz 10 ‒10 Hz 10 ‒10 Hz 1 Hz Narrower Linewidth Size 0.5 mm 10 mm 100 mm 1000 mm Larger Size

Less Tunability Tunable Want: Narrow Linewidth Compact Size Wavelength Tunable 8 The SBS Microresonator Laser National Institute of Standards and Technology Stimulated Brillouin Scattering (SBS)

Pump Phonon R. W. Boyd, Nonlinear Optics

SBS

SBS Nonlinearity SBS Lasing

D = 6 mm

Output Grudinin et al., PRL, 2009 PumpLee et al., Nature Photon., 2012 9 SBS Versus Conventional Lasers National Institute of Standards and Technology Spontaneous Stimulated Emission Emission Conventional Laser

Pump Output Thermal Stimulated Excitation Brillouin Scattering SBS Laser

Output

Pump 10 The SBS Process ‒ Gain National Institute of Standards and Technology

Pump

Phonon

SBS

1. Pump scatters off of acoustic phonon to create SBS 2. Pump interferes with SBS to reinforce phonon

E~ eiPPSS t k z  e i t k z

11 The SBS Process ‒ Noise National Institute of Standards and Technology

Pump

Phonon

SBS

h Average : Thermal Energy h kT energy e 1 per mode h 1 kT Phonon kT … … SBS process limited by thermal noise EE  11 GHz fcutoff ~ 6 THz reflected pump phonon 12 Microcavity SBS Laser National Institute of Standards and Technology

1. Pump Length ~ 10 m Pump ~ 100 mW

Output

Pump

Loss High-Q Microresonator 2. 6 mm Easy to achieve Output Gain > Loss Pump Fiber Taper Pump ~ 1-10 mW

13 The Importance of SNR National Institute of Standards and Technology

SBS lasers achieve large SNR Low Noise! Im{E} To achieve low phase noise, Noise want large signal to noise: P Signal SBS  PNoise Re{E}

Schawlow-Townes linewidth ~ 1/P

14 Low Noise SBS Lasing National Institute of Standards and Technology

PSBS Diode Lasers SBS Lasers PNoise -1  Want large signal-to-noise αm = 45.6 cm -1 αi = 5 cm

Low Loss Nonlinear Process Q ~ 5×103 Q > 108

Large SNR P = 1 mW P = 1 mW

Δν = 1 MHz Δν < 100 Hz Low Noise Moderate Power (Noise Clamps (mW range) at Threshold) Hertz-class intrinsic linewidth

15 SBS Laser Configuration National Institute of Standards and Technology

• Separate SBS and Pump • Lock Pump to Cavity Pound-Drever-Hall

High-Q Microresonator

Pump  Fiber Taper Phase Photodiode Modulator 1 mm Output H. Lee et al., Nature Photon., 2012 Fiber Taper Fabricated by Vahala group at Caltech 16 SBS Laser Performance National Institute of Standards and Technology

 2 

• > 102 SBS noise reduction at high frequencies

• SBS noise comparable or better than commercial solid-state and fiber lasers

Gorodetsky et al., JOSA B, 2004 Thermorefactive Noise: Matsko et al., JOSA B, 2007 17 Dual-Microcavity SBS Laser National Institute of Standards and Technology Pound-Drever-Hall

Microdisk

SOA Pump  Phase Photodiode Modulator

Heat Output

SBS Pump 18 SBS Tuning National Institute of Standards and Technology

Tuning by microdisk Tuning by other temperature resonant modes

Laser tuning up to Terahertz

19 Dual-Microcavity SBS Laser National Institute of Standards and Technology Pound-Drever-Hall

Microdisk

SOA Pump  Microrod Phase Photodiode Modulator  Photodiode

Pound-Drever-Hall

Papp, PRX 2013 Del’Haye, APL, 2013

20 Microrod Resonators National Institute of Standards and Technology

Microrod Fab: Fused Silica

Papp, PRX 2013 Del’Haye, APL, 2013

Q = 0.9 Billion

21 Microrod Resonator Fabrication National Institute of Standards and Technology

Video Courtesy of Dr. Pascal Del’Haye 22 Dual-Microcavity SBS Laser Noise National Institute of Standards and Technology

SBS noise improved through locking to a second microresonator

23 Dual-Microcavity SBS Laser Spectrum National Institute of Standards and Technology

Pump Laser: 10 kHz linewidth

SBS Microdisk: 4 kHz linewidth

Dual-microcavity SBS Laser: 90 Hz linewidth

24 SBS Laser Spectroscopy National Institute of Standards and Technology

4-kHz cavity resonance only resolved by the locked SBS laser

W. Loh et al., Noise and dynamics of stimulated Brillouin scattering microresonator lasers, PRA 2015 W. Loh et al., Dual-microcavity narrow-linewidth Brillouin laser, Optica 2015 25 Current Technology for Low-Noise Lasers National Institute of Standards and Technology

External- Cavity- Dual- Diode DFB Cavity Fiber Laser Stabilized Microcavity Laser Laser Laser SBS Laser

Linewidth 106 Hz 104‒105 Hz 103‒104 Hz 1 Hz 100 Hz

Size 0.5 mm 10 mm 100 mm 1000 mm 10 mm

• Size comparable to an external cavity laser • Linewidth many orders of magnitude better

26 Takeaways National Institute of Standards and Technology

1. There is a real need for low noise lasers that are both compact and tunable

2. SBS lasing in a microresonator allows for large ratios of signal to noise at low pump

powers

Low noise lasers

3. Obtain four orders of magnitude noise reduction by locking to a microrod 27 Combining Two Resonators into One National Institute of Standards and Technology Pound-Drever-Hall

MicrorodMicrodisk

SOA Pump  Microrod Phase Photodiode Modulator  Photodiode

ChallengesPound-Drever -Hall 1. Dimensions 2. Power/Area

28 SBS Microrod Laser Noise National Institute of Standards and Technology

SBS microrod laser is 102 better than microdisk laser at low freq. 29 SBS Microrod Laser Spectrum National Institute of Standards and Technology

Pump Laser: 10 kHz linewidth

SBS Microdisk: 4 kHz linewidth

Dual-microcavity SBS Laser: 90 Hz linewidth

SBS Microrod: 400 Hz linewidth

30 Future Directions ‒ SBS Laser National Institute of Standards and Technology

Stabilize

6 mm Δf3dB = 90 Hz 1 Hz

Cesium Clock: Δf/f ~ 2×10-15 Optical-Atomic Clock: Δf/f ~ 2×10-18 N. Hinkley et al. Science 2013

10 Hz Stabilize 1 m

31 Future Directions ‒ SBS Laser National Institute of Standards and Technology

Optical Frequency Division T. Fortier et al. Ultrastable cavity (Q ~ 1011) Nature Photon. 2011 CW laser 6 mm n ~ 50,000 1 Hz linewidth νn = n fr+fo

νn - fo fr f = r n

δνn - δfo T. Herr et al. Nature Photon. 2014 δf = r n 1/fr Optical phase noise Noise sidebands reduction ~ 90 dB Spectrum analysis f fr 32 Acknowledgments National Institute of Standards and Technology

• Postdoctoral Advisors – Dr. Scott Diddams and Dr. Scott Papp

• NRC Postdoctoral Fellowship, DARPA Pulse

NIST OFM Lab Caltech Vahala Group Adam Green Hansuek Lee Joseph Becker Kerry Vahala Fred Baynes Daniel Cole Frank Quinlan Pascal Del’Haye Aurélien Coillet Katja Beha Scott Papp Scott Diddams 33 Questions? National Institute of Standards and Technology

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