1/7 For decades, cold atom system and Bose-Einstein condensates (obtained from ultra-cold atoms) have been two of the most studied topics in fundamental physics. Several Nobel prizes have been awarded and hundreds of millions of dollars have been invested in this research. In 1975, cold atom research was enhanced through discoveries of laser cooling techniques by two groups: the first being David J. Wineland and Hans Georg Dehmelt and the second Theodor W. Hänsch and Arthur Leonard Schawlow. These techniques were first demonstrated by Wineland, Drullinger, and Walls in 1978 and shortly afterwards by Neuhauser, Hohenstatt, Toschek and Dehmelt. One conceptually simple form of Doppler cooling is referred to as optical molasses, since the dissipative optical force resembles the viscous drag on a body moving through molasses. Steven Chu, Claude Cohen-Tannoudji and William D. Phillips were awarded the 1997 Nobel Prize in Physics for their work in laser cooling and trapping of neutral atoms. In 2001, Wolfgang Ketterle, Eric Allin Cornell and Carl Wieman also received the Nobel Prize in Physics for realization of the first Bose-Einstein condensation. Also in 2012, Serge Haroche and David J. Wineland were awarded a Nobel prize for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems. Even though this award is more directed toward photons studies, it makes use of cold atoms (Rydberg atoms). Laser cooling Principle Sources: https://en.wikipedia.org/wiki/laser_cooling and https://en.wikipedia.org/wiki/doppler_cooling Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled to near absolute zero through interaction with one or more laser fields. All laser cooling techniques rely on the property that when an object (usually an atom) absorbs and re-emits a photon (a particle of light) its momentum changes. The temperature of a substance is higher when there is a larger distribution of the velocities of the particles that make up the substance. Laser cooling techniques combine atomic spectroscopy with the previously mentioned mechanical effect of light to compress the velocity distribution of a group of particles, thereby cooling the particles. The first example of laser cooling, and also the most common method (so much so that it is often simply referred to as 'laser cooling') is Doppler cooling (see Figure). Doppler cooling uses a laser slightly red-shifted from the molecule s atomic transition with the result that: To a stationary atom the laser is neither rednor blue-shifted and the atom does not absorb the photon. To an atom moving away from the laser the beam is red-shifted and the atom does not absorb the photon. To an atom moving towards the laser the beam is blue-shifted (at its atomic transition wavelength) and the atom absorbs the photon, slowing it. The absorbed photon excites the atom, moving an electron to a higher quantum state. The atom re-emits a photon, but the direction of the photon is random, so no net change in momentum results over many absorption-emission cycles.
Distribution: Internal externalonly 2/7 Cooling atoms requires the photon absorption/emission cycle previously described to be reproduced a large number of times. Photons emitted by a laser source have the same direction. After a large number of absorption/emission cycles, the total force received by an atom resulting from absorption is equal to the sum of the force applied from each absorption multiplied by the number of absorptions. Whereas the total force applied to an atom resulting from spontaneous emissions is quasi null (due to the random direction of photon emissions). For this to happen, it is necessary that the atoms recover their initial fundamental state through spontaneous emission in order to be excited again and so on. This is called cycling transition. Some atoms have a more complex structure, composed by 2 or more energy levels within the atom (hyperfine structure), and may require the use of more than one cooling laser (see D2 transition of the Rb structure). EYLSA fiber laser Wavelength coverage EYLSA fiber lasers are available at different wavelength covering: infrared (15XX and 10XX nm), near-ir (7XX nm), green (5XX nm) ranges. Other wavelengths are currently under development. (See graph 1 page 6) Wavelength accuracy and stability EYLSA wavelength accuracy may be specified at +/-0.01 nm using wavelength meters or atomic cells. Frequency stability obtained from EYLSA 780 nm, wavelength locking loop open. (see graph 2 page 6). 193.7407(46) MHz Cooling 266.6500(90) MHz g F o=o2/3 (0.93 MHz/G) F = 3 5 2 P 3/2 5 2 S 1/2 780.241 209 686(13) nm 384.230 484 468 5(62) THz 12 816.549 389 93(21) cm -1 1.589 049 462(38) ev 72.9112(32) MHz 229.8518(56) MHz 302.0738(88) 88) MHz 2.563 005 979 089 109(34) GHz 4.271 676 631 815 181(56) GHz Figure 1: D2 line of Rb87 and cycling transition 156.9470(70) MHz 72.2180(40) MHz 6.834 682 610 904 290(90) GHz g F o=o2/3 (0.93 MHz/G) g F o=o2/3 (0.93 MHz/G) Repumping g F o=o1/2 (0.70 MHz/G) g F o=o-1/2 (-o0.70 MHz/G) F = 2 F = 1 F = 0 F = 2 F = 1
3/7 Doppler cooling requires lasers that emit a very precise wavelength that is precisely shifted from the target cycling transition (typically at 2.5Γ red shifted for optimal atom cooling where Γ is the natural linewidth of the transition; the shift height is determined by the initial atom speeds). The specified wavelength accuracy must be better than the transition linewidth (~MHz). Most of the time, a wavelength locking system is required to reference the laser to the atomic transition during long time periods. Also, laser linewidth must be lower than the atomic transition for better photon efficiency and to avoid excitation of other atomic transitions with out-of-band photons. Atoms excited at another transition may become insensitive to the apparatus and won t be cooled and may even warm other atoms by collisions. For this same reason the laser s optical signal to noise ratio must be excellent. (This requires there are no parasitic laser lines (mode-hop) and very limited amplified spontaneous emission). Note: For atom interferometry experiments (precision measurement techniques using cold atoms which have been developed in many labs), laser linewidth is required to be narrower by few orders, for example khz to Hz linewidth may be necessary. In practice, an atomic cloud of mk temperature is obtained by trapping the atoms in a vacuum chamber and combining the laser cooling principle oriented in 3 directions and a magnetic field. The magnetic field is designed to be 0 at the experiment chamber center and increase proportionally to the distance from the chamber center. The purpose of the magnetic field is to slightly shift the atom s hyperfine structures. Thus, lasers slightly shifted from the transition wavelength associated with this magnetic field will exert a restoring force proportional to the atom s distance from the center of the chamber. This type of trap is called 3D Magneto-Optical Trap (3D MOT). For this trap, 6 beams are required to trap the atoms in each spatial direction and within the 2 propagating paths. In order to exert a force only on atoms that move towards the laser propagating direction, the polarization needs to be set circularly. The laser beam polarization requires an excellent extinction ratio to avoid power fluctuation (due to polarizing optics) and to provide good circular polarization after the quarter-wave plate. Note: 2D MOT also is used to concentrate a cold atom beam. In this case, 4 retro-reflected beams are required. Wavelength locking EYLSA fiber lasers embed: a monitoring fiber output that may be directly connected to a saturated absorption scheme (standard cold atom lab apparatus, not provided with EYLSA). a wavelength locking input (BNC) which directly modifies seeder diode power supply current. Wavelength is tunable on a reproducible and mode-hop-free (MHF) range. This input may be connected to saturated absorption equipment associated with PID (not included with EYLSA) to lock the EYLSA fiber laser wavelength. Successful long-term locking has been demonstrated without relocking: in lab conditions for longer than 2 weeks in high vibration and temperature fluctuation conditions such as the 0G flight ICE experiment, Bordeaux (20 parabolas) Fully available output power Each photon generated from an EYLSA product is fully available for the experiment: No need for mode filtering No need for fiber injection (the photon is already injected into the fiber output) No need to push the current up to keep a constant output power over the lifecycle. Optical signal to noise ratio EYLSA fiber lasers generate visible light from IR light using periodically-poled crystals. Second-harmonic generation is a non-linear effect compared to incident power. The higher the incident power the higher the conversion efficiency is. Periodically-poled crystals have a limited spectral acceptance range of 20 GHz, any photon outside this range is filtered out. Optical signal to noise ratio (OSNR) is measured at better than 55 db for a standard frequency double laser. (see graph 3 page 6). Polarization EYLSA fiber lasers have polarized output as standard. Depending on output type, polarization extinction ratios vary between > 17 db for fibered output and >20dB for free space output.
4/7 For 3D MOT, the laser power stability and beam quality are paramount factors in avoiding fluctuation of the cold atomic cloud (warming). Output power stability EYLSA fiber laser output is controlled by photodiode. A feedback loop automatically adjusts the pumping diode power supply current in order to deliver stable laser output power. EYLSA fiber lasers have less than 2% output power instability over 12 hours of operation. (see graph 4 page 6). Figure 2 : 3D MOT schematic Temperatures colder than tens of μk may be obtained using optical dipolar trapping techniques (other techniques are available using electro-magnetic traps or hybrid traps). Dipolar trapping requires additional lasers with emitting wavelengths away from the cycling transition. A dipole potential is created using an electro-magnetic field from the laser. Cold atoms are trapped in this dipole potential to be further cooled to hundreds of nk temperatures. These lasers require very low intensity noise, high output power, and excellent beam pointing stability and beam quality. Typical wavelengths are 1064 nm and 1560 nm. Other laser parameters must be considered when using such complex experimental setups. The first consideration is the laser reliability. In an experiment using several lasers one failure might compromise the entire experiment. Wavelength stability over a vibration and temperature range is necessary for a long period to obtain accurate or reproducible measurements. Excellent beam pointing stability also reduces time consuming beam path resetting of the experimental setup. Beam quality EYLSA fiber lasers have TEM00, M²<1.2 output. Also they offer single mode fiber delivery (standard for 780 nm, optional for other wavelength). Relative intensity noise EYLSA fiber lasers deliver excellent relative intensity noise results due to the single frequency operation. Typical values are below 0.1% on a large frequency band. (see graph 5 page 6). Beam pointing stability EYLSA fiber lasers beam pointing stability is unsurpassed due to its single mode fiber delivery.
5/7 Reliability EYLSA fiber lasers use industrial grade components which have been submitted to long-term testing at Quantel. Mean time before failure has been evaluated to be better than 100K hours. EYLSA 780 fiber lasers are warrantied for 2 years. Compactness The EYLSA fiber laser all-in-one package contains everything from the power supply to second harmonic generator. Only output connectors are needed to be ready for use with your optical table. With EYLSA there is no need to integrate the full laser on a vibration-damped optical table. Figure 2 : 3D MOT schematic Figure 3: Typical optical table for Rb atom cooling experiments Atom cooling applications are the basis of the most accurate atomic clocks, gravimeters, inertial sensors, etc. Users are more and more likely to require a system with a better reliability and stability as well as more compactness and functionality in harsher environmental conditions than are present within a lab. Environmental conditions EYLSA fiber lasers undergo burn-in tests and thermal cycling between 5-35 C before final characterization to validate their ability to operate in lab conditions. Their insusceptibility to vibrations has also been tested by operation in 0 G flight (0 to 2 G acceleration, high vibrations during take-off, landing and parabolas).
6/7 20 WAVELENGTH (nm) >< 10 3 1 1: wavelength coverage. 4: output power stability. RIN (db/hz) OUTPUT POWER (W) >< 0 450 550 650 750 850 950 1 050 1 150 1 250 1 350 1 450 1 550 1 650 2: wavelength stability (unlocked). 3: optical spectrum (780 nm). Note: Optical signal to total integrated optical noise has been measured using hot atoms by INLN, Nice, France. The ratio was better than 40 db (limited by measurement). Frequency (khz) 5: relative intensity noise obtained from a 1064 nm, 10 W fiber laser.
7/7 Laser cooling technologies (based on 780 nm) LASER TECHNOLOGIES COMPARISON FOR Rb ATOM COOLING Parameters FIBER LASER ECDL ECDL+TA Ti:Sa Available output power (single mode, single frequency) 1 W 1 0.1 W 3 W 2 Isolation required (15% losses) Fiber injection for mode filtering (50% loss) 3.5 W 3 Tunability + +++ +++ +++ Linewidth +++ ++ ++ ++ Beam quality +++ +++ - ++ Beam pointing stability +++ + + + Wavelength stability +++ + + ++ Output power stability +++ ++ - ++ Signal to noise ratio ++ ++ - +++ Reliability +++ ++ + + Ease of use +++ +++ ++ + Price ++ +++ ++ - Usage cost +++ ++ + - Need alignment / cleaning No Yes Yes Yes Warranty Extra 1 Warrantied power with excellent beam quality and out of single mode fiber 2 Beginning of life, without mode filtering 3 High power 532 nm pumping laser 2 years Fiber output, Repumping and cooling wavelength at same time Only electronic is warrantied 12 months Only electronic is warrantied 12 months Modularity 3 months