Multiple Side-Band Generation for Two-Frequency Components Injected into a Tapered Amplifier
aa r X i v : . [ phy s i c s . a t o m - ph ] M a r Multiple Side-Band Generation for Two-Frequency ComponentsInjected into a Tapered Amplifier
Hua Luo, Kai Li, Dongfang Zhang, Tianyou Gao, and Kaijun Jiang , , ∗ State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics andMathematics, Chinese Academy of Sciences, Wuhan, 430071, China Center for Cold Atom Physics, Chinese Academy of Sciences, Wuhan, 430071, China ∗ Corresponding Author: [email protected]
Compiled July 16, 2018We have experimentally studied the multiple side-band generation for two-frequency components injected intoa tapered amplifier and demonstrated its effects on atomic laser cooling. A heterodyne frequency-beat measure-ment and a Fabry Perot interferometer have been applied to analyze the side-band generation with differentexperimental parameters, such as frequency difference, injection laser power and tapered amplifier current. Inlaser cooling potassium40 and potassium41 with hyperfine splitting of 1.3GHz and 254MHz, respectively, theside-band generation with a small frequency difference has a significant effect on the number of trapped atoms.c (cid:13)
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Multiple side bands with an equal frequency spacinghave various applications in quantum optics and lasercooling. Zhu et. al. observed the multiple-mode side-band fluorescence around the resonant transition witha strong dichromatic field coupling two energy levels [1].Multiple electromagnetic induced transparencies [2] andfour-wave mixing have been observed with two couplingcomponents and one probing component in a three-levelsystem [3]. And an arbitrary side-band with squeezinghas been demonstrated in the experiment [4]. In lasercooling of alkali atoms, two frequency components arerequired, one for optical pumping and the other for cool-ing. The semiconductor laser has become a standard toolin laser cooling due to its easy operation and low price.But its low output power (generally less than 100mW)puts a limit on the number of trapped atoms. The out-put power of a tapered amplifier can be very high (gen-erally more than 500mW), which enables us to improvethe parameters of trapped atoms. Different spectra havebeen achieved on the base of a tapered amplifier. For ex-ample, two-frequency components can be simultaneouslyinjected into a tapered amplifier [5]. A high power out-put around 670nm has been obtained with an externalcoupling cavity [6, 7]. And a tapered amplifier has beeninjection seeded with a femtosecond comb [8].To get a high-power multi-mode output, multiple fre-quency components can be simultaneously amplified ina tapered amplifier to simplify the experimental setup.But at the same time, the wave-mixing process can resultin a multiple side-band generation, which may have non-negligible effects on atomic behaviors. Here, we will usea heterodyne frequency-beat measurement with a highresolution ( < ν r ) for the heterodyne frequency-beatmeasurement. The transmitting beam with a horizontalpolarization is again divided into two beams in PBS2,and then one beam is frequency shifted from 40MHz to1GHz by using AOM2. The two beams combine togetherin PBS3 and then are reflected by PBS4. Before injectedinto a TA (tapered amplifier), the beam contains two-frequency components ( ν and ν ) both of which have avertical polarization to match the TA polarization. Afterthe TA, The main power is reflected by PBS5 for lasercooling and a small fraction transmits PBS5 for side-band measurement. In PBS6, the reflected beam witha vertical polarization is analyzed by a FPI (free spec-tral range 3GHz) and the transmitting one overlaps withthe reference beam in PBS7 for heterodyne frequency-beat measurement. After PBS8, the two beams with ahorizontal polarization go through a fiber and are de-tected by a high-speed photodetector (Newport, Model1343, 25GHz bandwidth). For our used TA (UniQuantaTech, Model TAL100, spectrum range 750nm-775nm),the output power is 700mW in 21 C when the injectionpower is 14mW and the tapered amplifier current is 1.8A.The apertures of the front and rear facets are 3um and190um, respectively, and both facets are anti-reflectioncoated with 0 . δ = ν − ν = 80 M Hz ,and the reference component is between these two com-ponents, ∆ = ν − ν r = 20 M Hz and ν r − ν = 60 M Hz .For positive side bands, f + n = ∆ + nδ , and for nega-tive side bands, f − n = ( δ − ∆) + nδ , where n is theorder of the side bands. When n = 0, the signals cor-respond to the two injection frequencies ( ν and ν ).All the frequency components (including injection com-ponents and side bands) are indicated in the Fig.2(a).Here, we can observe the 6th order side-band generation,which is the highest order that has been experimentallyobtained in a TA. There are also homodyne frequency-beat signals between different components in the out-put of the TA, f m = mδ , where m = 1 , , , ...p − p is the total number of the frequency compo-nents. Only up to m = 7 signal can be distinguishedin our detection sensitivity and the frequency-beat be-tween higher order side bands are negligible small. Wealso do the heterodyne frequency-beat for a 200MHz dif-ference, where ν − ν = 200 M Hz and ν − ν r = 60 M Hz .The frequency-beat signals are indicated in the insertof Fig.2(a) and only 3 side bands (not include the signof the order) can be detected. When a FPI is applied to perform the measurement, the side-band number isalso 3 as shown in Fig.3(a). For frequency differencesof 280MHz and 360MHz, the measured numbers of theside-band with these two methods are also the same. Asshown in Fig.2(b), the side-band number decreases asthe frequency difference increasing.
50 100 150 200 250 300 350 40001234567 F r equen cy B ea t ( d B m ) Frequency (MHz) (b) S i de - band N u m be r Frequency Difference (MHz) (a)
Fig. 2. Generated side bands versus the frequency dif-ference, where the injected power of both components is15mW and the TA current is 1.8A. (a) Frequency beatsignal for a 80MHz frequency difference, and the insert isfor a 200MHz frequency difference. (b) Side-band num-ber versus the frequency difference.Multiple side bands originate from wave mixing whendifferent frequency components overlap in a TA. Thisnonlinear process results in the laser power distribut-ing among different side bands. A FPI is applied tomeasurement the power distribution among different sidebands for a 200MHz frequency difference. As shown inFig.3(a), there exist positive and negative three orderside bands. When injection powers of the two compo-nents are equivalent, side bands distribute almost sym-metrically on both sides. Throughout this manuscript,the side-band power is denoted by its percentage ratioin the total output for simplification. Since the signal forthe third order side band will be indistinguishable in asmall TA current, we only include the experimental re-sults for positive and negative two orders. As shown inFig.3(b), the power of each side band almost remains un-changed with different TA current. This implies that thetwo components undergo wave mixing first to get mul-tiple side bands and then are amplified proportionally.The small power difference of the same order side band(for example + δ and − δ ) mainly comes from the powerimbalance between injection components.Fig.4 shows that the total power in all side bands al-most remains unchanged at 23% with the TA currentvarying as indicated in Fig.3(b). It is also shown in Fig.4that the total power increases from 14% to 27% withinjection laser power increasing. The multiple side-bandgeneration results from a nonlinear process in the solid2aterial and it should be enhanced in the saturationregime. So, the wave-mixing process will become moredominant with a higher injection power. -1000 -800 -600 -400 -200 0 200 400 600 800 10000.40.81.21.62.0 =200MHz I n t e n s it y ( a r b . un it s ) Frequency (MHz) (a) (b) S i de - B and P o w e r ( % ) Tapered Amplifier Current (A)
Fig. 3. (Color online) Power distribution of each sideband with a 15mW injection laser power and a 200MHzfrequency difference. (a) Side-band signal measured witha FPI. In a 1.8A tapered amplifier current, up to +3 δ and − δ side bands are obvious on the spectrum. (b)Power distribution of each side band. The black squareis for + δ , green uptriangle for − δ , red circle for +2 δ andblue downtriangle for − δ . T o t a l S i d e - B a nd P o w e r ( % ) Tapered Amplifier Current (A) T o t a l S i d e - B a nd P o w e r ( % ) Injection Power (mW)
Fig. 4. (Color online) Total power of all side bands. Theblack triangle denotes the total power versus the taperedamplifier current and the red square denotes that versusthe injection power.The multiple side-band generation should have effectson the interaction between atoms and light. For examplein atomic laser cooling, the side-band generation woulddecrease the power of the cooling light and additionalexcitations would destroy trapped atoms. To verify thisprediction, we first do laser cooling of potassium40 witha 1.3GHz hyperfine splitting in the ground state. Twoequivalent frequency components with a 1.3GHz differ-ence are injected into a TA and only 35mw power of theoutput is used to cool atoms. The number of trappedatoms is 4 . . × . As a comparison, we directlyuse two equivalent laser beams with a 1.3GHz frequencydifference and a total 35mw laser power to do the ex- periment. The trapped atom number is 3 . . × inthis condition. The similar numbers of obtained atomsdemonstrate no obvious effect of the side band. But forpotassium41 with only a 254MHz hyperfine splitting,the trapped atom number is 1 . . × with a two-frequency injection and 2 . . × without the two-component injection. The one order of magnitude differ-ence on the trapped atomic numbers shows a significanteffect of the side-band generation. From our measure-ment, the total side-band power is about 20% for a254MHz frequency difference and below our detectionsensitivity for a 1.3GHz difference. This study demon-strates that the two-frequency injection in a TA can af-ford a convenient way to simplify the experimental setupon one side. But on the other side, the multiple side-bandgeneration has a notable effect on the atomic behaviorsif the frequency difference is small. Laser cooling potas-sium40 is our starting point to realize degenerate Fermigases.We emphasize that the heterodyne frequency-beatmeasurement is the first time applied to analyze the side-band generation in a TA. This method is still efficient fora small frequency difference due to its high spectrum res-olution. Also, a FPI is applied to explore the side-bandgeneration with different parameters, such as frequencydifference, tapered amplifier current and injection laserpower. Injection of two frequency components into a TAhas been widely employed in laser cooling (for examplethe reference [9]) and this indicates the significance of aTA with a multiple-frequency injection. But authors ofall these previous works didn’t consider the effect of theside-band generation. In this manuscript, laser coolingpotassium40 and potassium41 demonstrates its obviouseffect on the interaction between atoms and light.This work is supported by NSFC (Grant No 11004224)and NFRF-China (Grant No 2011CB921601). References
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