Applications of Slow Light. Robert W. Boyd. Institute of Optics and Department of Physics and Astronomy University of Rochester

Transcription

1 Applications of Slow Light Robert W. Boyd Institute of Optics and Department of Physics and Astronomy University of Rochester with special thanks to George M. Gehring, Andreas Liapis, Aaron Schweinsberg, Zhimin Shi, and Joseph E. Vornehm, Jr. Presented at Photonics West, January 27, 2009.

2 Overview of Presentation: Applications of Slow Light Light can be made to go: slow: v g << c fast: v g > c backwards: v g negative Here v g is the group velocity: v g = c/n g n g = n (dn/d) Controllable light velocity leads to many applications buffers / regenerators for optical telecommunications interferometers with novel properties phased and synchronized arrays for beam steering Boyd and Gauthier, Slow and Fast Light, in Progress in Optics, 43, 2002.

3 Slow Light Fundamentals: How to Create Slow and Fast Light I Use Isolated Gain or Absorption Resonance α absorption resonance g gain resonance 0 0 n n ng slow light ng slow light fast light fast light

4 How to Create Slow and Fast Light (Better) Use a Dip in a Gain or Absorption Feature g n dip in gain feature 0 0 α n dip in absorption feature ng slow light ng slow light fast light fast light Narrow dips in gain and absorption lines can be created by various nonlinear optical effects, such as electromagnetically induced transparency (EIT), coherent population oscillations (CPO), and conventional saturation.

5 Light speed reduction to 17 metres per second in an ultracold atomic gas 60 MHz 4 = F =3, M = 2 F 3 = F =2, M = 2 F Lene Vestergaard Hau*², S. E. Harris³, Zachary Dutton*² & Cyrus H. Behroozi* c p D 2 line λ = 589 nm * Rowland Institute for Science, 100 Edwin H. Land Boulevard, Cambridge, Massachusetts 02142, USA ² Department of Physics, Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA ³ Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA 2 = F =2, M = 2 F 1.8 GHz 1 = F =1, M = 1 F NATURE VOL FEBRUARY Transmission Refractive index a b Probe detuning (MHz) Note also related work by Chu, Wong, Welch, Scully, Budker, Ketterle, and many others PMT signal (mv) T = 450 nk τ Delay = 7.05 ± 0.05 μs L = 229 ± 3 μm v g = 32.5 ± 0.5 m s Time ( μs)

6 Goal: Slow Light in a Room-Temperature Solid-State Material Crucial for many real-world applications We have identified three preferred methods for producing slow light (1) Slow light via stimulated Brillouin scattering (SBS) (2) Slow light via coherent population oscillations (CPO) (3) Dispersive slow light (conversion/dispersion) (C/D)

7 Slow Light by Stimulated Brillouin Scattering (SBS) L S S = - Ω z L Ω di S dz = gi LI S g = γ 2 e 2 nv c 3 ρ 0 Γ B. We often think of SBS as a pure gain process, but it also leads to a change in refractive index SBS gain refractive index Stokes gain slow light monochromatic pump laser anti-stokes loss fast light G The induced time delay is where G= g Ip L and is the Brillouin linewidth T d B B

8 Slow-Light via Stimulated Brillouin Scattering Rapid spectral variation of the refractive response associated with SBS gain leads to slow light propagation Supports bandwidth of MHz, large group delays Diode Laser Fiber Amplifier Oscilloscope typical data SBS Generator Polarization Control Circulator Variable Attenuator 50/50 Circulator Detector Modulator Isolator Pulse Generator RF Amplifier SBS Amplifier in out Okawachi, Bigelow, Sharping, Zhu, Schweinsberg, Gauthier, Boyd, and Gaeta Phys. Rev. Lett. 94, (2005). Related results reported by Song, González Herráez and Thévenaz, Optics Express 13, 83 (2005).

9 Broadening the SBS Line Width Problem: SBS linewidth Γ B /2π is only 30 MHz (typical value at 1550 nm) This line width is too small to support the Gb/s data rates of telecom Solution: Frequency-broaden the punp laser, which broadens the SBS gain line. This solution works remarkably well, giving linewidths of many GHz SBS gain refractive index Stokes gain anti-stokes loss slow light fast light monochromatic pump laser refractive index BSB gain broadened gain line broadened slowlight bandwidth frequency-broadened pump laser

10 Slow Light by Coherent Population Oscillations in Ruby Recall that n g = n (dn/d). Need a large dn/d. (How?) Kramers-Kronig relations: Want a very narrow feature in absorption line. Well-known trick for doing so: Make use of spectral holes due to population oscillations. Hole-burning in a homogeneously broadened line; requires T << T 2 1. E 3, δ E 1, saturable medium δ measure absorption 1/T 1 absorption profile b 2γ ba = 2 T 2 Γ ba = 1 T 1 PRL 90, (2003). a

11 Observation of Slow Light Propagation in Ruby Function Generator Argon Ion Laser or EO modulator Measurement results: For a 1.2 ms delay, v g = 60 m/s n g = 5x10 6 Digital Oscilloscope Delay (ms) 40 cm Reference Detector Signal Detector Ruby P = 0.25 W P= 0.1 W Experimental setup solid lines are theoretical predictions Modulation F requency (Hz)

12 Time-Domain Explanation of CPO Assume that T pulse << T1 = time scale for saturation changes Then absorption decreases with time during pulse due to saturation reference pulse time delay leading edge sees far more absorption than trailing edge propagation distance Proposed by Basov many years ago Entirely equivalent to frequency-domain explanation, but probably less useful for making simple calculations of the group delay T(response)

13 Observation of Superluminal and Backwards Pulse Propagation A strongly counterintuitive phenomenon - conceptual prediction - laboratory results But entirely consistent with established physics Δt = 0 Δt = 3 G. M. Gehring, A. Schweinsberg, C. Barsi, N. Kostinski, and R. W. Boyd, Science 312, Δt = 6 Δt = 9 Δt = 12 Δt = x10 Normalized length n g Z (m) 5

14 Applications of Slow and Fast Light Buffers and regenerators for telecom Slow/fast light for interferometery Phased- and synchronized-array laser radar

15 Slow Light and Optical Buffers All-Optical Switch input ports NxN switch output ports Use Optical Buffering to Resolve Data-Packet Contention controllable slow-light medium But what happens if two data packets arrive simultaneously? Buffering can lead to dramatic increase in system performance. But requires many pulse lengths of controllable delay. Daniel Blumenthal, UC Santa Barbara; Alexander Gaeta, Cornell University; Daniel Gauthier, Duke University; Alan Willner, University of Southern California; Robert Boyd, John Howell, University of Rochester

16 Regeneration of Pulse Timing Need to recenter each pulse in its time window Crucial for many situations in telecom Removes timing jitter caused by NL and environment effects in fiber Most conveniently done by access to both slow and fast light advance or delay time window Need only approximately ±1 pulse width of delay!

17 Interferometry and Slow Light Under certain (but not all) circumstances, the sensitivity of an interferomter is increased by the group index of the material within the interferometer! Sensitivity of a spectroscopic interferometer is increased Typical interferometer: Beam Splitter #1 Slow Light Medium We use CdS x Se 1-x as our slow-light medium Beam expander L 0 Tunable Laser L Detector Beam Splitter #2 Tunable laser z θ y Slow-light medium Imaging lens P C Here is why it works: d φ d = d d nl c = L c (n dn d ) = Ln g c Z. Shi, R.W. Boyd, D. J. Gauthier, and C.C. Dudley, Opt. Lett. 32, 915 (2007 Z. Shi, R. W. Boyd, R. M. Camacho, P. K. Vudyasetu, and J. C. Howell, Phys, Rev. Lett. 99, (2007) Z. Shi and R.W. Boyd, J. Opt. Soc. Am. B 25, C136 (2008). And also the group of Shahriar on fast-light and interferometry Our experimental results Fringe movement rate (period/nm) experimental data theory for a slow-light medium theory for a non-dispersive medium wavelength (nm) n g

18 SLIDAR: Slow LIght Detection and Ranging A phased- and synchronized-array radar! (Need to control both phase and time delay for each aperture.) time delay modules individual apertures mean wavefront wavefront segment modulated light source phase controllers We are constructing a sparse array We will implement eight or nine channels with 10-cm-diameter subapertures Total baseline approximately 1.5 m.

19 2-D Hybrid SLIDAR System ν AOM νδν DCF delay for y-direction DCF polarizer SMF φ phase controller φ SMF #1 #2 #3 #4 sub-aperture outputs y C/D SBS x Possible array layouts broadened pump field EOM L 1 L 2 L 3 L 2 L 3 L 3 delay compensators AFG 2 signal pulse train EOM AFG 1 coupler L 1 L 2 L 3 delay elements for x-direction

20 1-D Conversion/Dispersion System or - Dispersive Slow Light Risley prism collimator star coupler νδν AOM ν signal pulse train EOM AFG 1 polarizer SMF Agilent external cavity SDL; 3 mw power; tunable 1530 to 1580 nm; 100 khz linewidth φ φ DCF Note: each group channel feeds two phase channels

21 Characterization of the Dispersive Delay Unit Finally, we have constucted a fully compensated delay unit Channel 1: Length 3L of SMF Channel 2: Length 2L of SMF and L of DCF Channel 3: Length L of SMF and 2L of DCF Channel 4: Length 3L of DCF (That is, each channel has roughly the same group delay. The differential delay between channels is controlled by wavelength.) SMF = single mode fiber DCF = dispersion compensated fiber L = 537 m SMF SMF SMF DCF DCF DCF The system performance is described in the accompanying movie.

22 Demonstration of Four-Channel Dispersive Slow Light click within frame to play movie

23 1-D SBS SLIDAR System Risley prism collimator star coupler AOM ν νδν broadened pump field ( 500 MHz) EOM polarizer φ φ L 1 L 2 L 3 L 2 L 3 L 3 delay compensators additional laser or SBS oscillator AFG 2 signal pulse train EOM AFG 1 coupler L 1 L 2 L 3 delay elements for x-direction

24 Channelization to Increase Slow-Light Delays A key parameter in slow light systems is the delay measured in pulse width, also known as the delay-bandwidth product, Δν τ d. There is no fundamental limit to Δν τd, and values as large as 80 have been achieved. Miller (PRL, 2007) showed that to achieve a given Δν τd, the system must obey g n avg L n/ 3 0. But this limit assumes the use of a single channel system. A multichannel system can overcome this limit. Multichannel schemes have been proposed and demonstrated previously, but not in a manner that can exceed the Miller limit We recently (PRA, 2008) proposed a channelization architecture that can overcome this limit

25 Our New Channelization Architecture This is the idea of our approach n n (ν 0 ) 2δn δn 0 δn 2δn n eff of the channelized device δn various channels 2 c c 0 c 2 c ideal slow-light medium ν ν 0 choose parameters so that phase jump is 2πN 3 This system is discretely tunable This is our proposed implementation flat-top broadband pump pump spectral slicer n'g / n' g (1) 2 1 signal spectral slicer SBS delay module of one channel signal spectral combiner t / T 0 signal input single mode fiber signal output Z. Shi and RWB, PRA

26 Thank you for your attention!

27 SLIDAR: Slow LIght Detection and Ranging A phased- and synchronized-array radar! time delay modules individual apertures mean wavefront wavefront segment modulated light source phase controllers Need to control both phase and time delay for each aperture Our Approach: We will first construct an 8-channel, 1-D system using a tunable source and group velocity dispersion to control the group delay We will then construct a 9-channel, 2-D system using C/D for one transverse dimension and SBS for the other