Compact, robust, and spectrally pure diode-laser system with a filtered output and a tunable copy for absolute referencing
Emil Kirilov, Manfred J. Mark, Maximilian Segl, Hanns-Christoph Nägerl
mmyjournal manuscript No. (will be inserted by the editor)
Compact, robust, and spectrally pure diode-laser system with a filteredoutput and a tunable copy for absolute referencing
E. Kirilov , M. J. Mark , M. Segl , H.-C. N¨agerl (cid:63) Institut f¨ur Experimentalphysik und Zentrum f¨ur Quantenphysik, Universit¨at Innsbruck, Technikerstrasse 25/4, A-6020 Inns-bruck, AustriaReceived: date / Revised version: date
Abstract
We report on a design of a compact lasersystem composed of an extended cavity diode laser withhigh passive stability and a pre-filter Fabri-Perot cavity.The laser is frequency stabilized relative to the cavity us-ing a serrodyne technique with a correction bandwidthof ≥ ≥
700 MHz. Thefree running laser system has a power spectral density(PSD) ≤
100 Hz /Hz centered mainly in the acousticfrequency range. A highly tunable, 0 . − . Stable and spectrally pure lasers have widespread appli-cations in optical frequency standards [1], measurementof fundamental constants [2,3,4], gravitational-wave de-tection [5], quantum information [6], high-resolution spec-troscopy [7] as well as stimulated adiabatic transfer ofpopulation between atomic/molecular levels [8,9,10,11,12,13]. It has already been demonstrated that the laserfrequency can be locked to better than 0 . (cid:63) H.-C. N¨agerl e-mail: [email protected] a linewidth of <
40 mHz was demonstrated by stabiliz-ing a erbium-doped fiber laser to a silicon single-crystalFabri-Perot cavity held at cryogenic temperatures [14].Diode lasers are generally compact and cost effective,cover wide spectral ranges, can be tuned to the desiredwavelength, and can be optically narrowed. For an ex-tended cavity diode laser (ECDL) with the usual grat-ing feedback the linewidth easily narrows to the 200 kHzrange [15]. To further reduce the linewidth additionaloptical feedback from an external cavity [16] can be em-ployed, combined with a low frequency electronic lock toa reference cavity (RC) [17]. The optical lock is difficultto maintain since one has to stabilize the “half-cavity”between the ECDL and the optical feedback FP. Dif-ferent variations to stabilize and increase the dynamicrange of the optical lock include balanced polarization-sensitive methods or dithering of the half-cavity path[18,19]. For absolute frequency reference and additionalnoise reduction the RC spacer is fabricated out of ULE(Corning), Zerodur, or single-crystal silicon substrates,and the laser is stabilized at the zero crossing of thespacer’s thermal expansion coefficient. The error signalfrom referencing the laser to the RC is generated usingthe Pound-Drever-Hall (PDH) technique [20]. In almostall scenarios for tuning the laser frequency in the range ofthe free spectral range (FSR) of the RC a configurationsof at least 2 double-pass AOMs has to be employed.As an alternative to external cavity optical feedbackone can lengthen the cavity length L of the ECDL [7],since the linewidth ∆ν depends inversely quadraticallyon the length as ∆ν = ∆ν LD / (1 + L/L LD ) , where L LD is the optical length of the laser diode (LD). In addition,the laser is typically locked to a RC with a high band-width (BW) electronic lock. The mode-hop free tunabil-ity is then compromised relative to a short cavity setup.Typically in most designs the slow integrated correctionsignal is applied to the piezo element (PZT) regulatingthe ECDL cavity length and the fast signal is applied tothe injection current of the laser diode. The fast channelBW is typically limited to 1 − a r X i v : . [ c ond - m a t . qu a n t - g a s ] F e b E. Kirilov et al. thermal and electronic charge carrier effects with oppo-site signs of dλ/dI , where λ is the laser wavelength and I is the diode injection current. This is specific to therespective laser diode and requires that one individuallyadapts the design of the proportional-integral-derivative(PID) servo. In some of the above designs the usefuloutput sent to the experiment is taken in transmissionthrough a pre-stabilizing filter cavity (FC) (which couldbe the same one providing the optical feedback [19]) toerase the remaining noise pedestal due to stochastic laserphase modulation, which typically caries about 10% ofthe laser power and whose width ∼ ∼ The rough scheme is presented on Fig.1. The output ofthe ECDL is passed through a fibered EOM driven bythe superposition of a direct digital synthesizer’s (DDS)sinusoidal output at fixed frequency f pdh and a sawtoothsignal generated by a NLTL powered by a frequency-doubled broadband high-bandwidth voltage-controlledoscillator (VCO) at 2 f tune (cid:29) f pdh (the factor of 2 re-sults from a doubling stage after the VCO; see below).Most of the light entering the EOM at frequency f ecdl isdiffracted into the upper (or lower, but not both; a prop-erty of the serrodyne modulation technique [27], detailsin sec.3) sideband at 2 f tune . The spectral distributionafter the EOM is composed of mostly f ecdl + 2 f tune anda small leftover at f ecdl with both frequencies accom-panied by their small sidebands at ± f pdh (panel 1 ofFig.1). Further the light is directed towards a 2-porttunable FC (tuning is possible by a PZT) that is res-onant with f ecdl + 2 f tune . The laser is locked to the A O M ECDL
EOM WM RCFC
NTL COM X2 SPL MIXAMP DDS tune
LPFVCOPHD SPL DDS PHDPID PIDPZT FC PZT FC PZT GR PZT GR EXP
PBS PBS pDpD
AMP f ecdl1 f ecdl4 f ecdl2 tune f pdh f exp f exp f ecdl3 dds f exp f exp Fig. 1
Overall setup. Panels 1 through 4 indicate the fre-quency components present at different locations in the setupas indicated. Notation that is not defined in the text: PHD:phase detector, SPL: radio-frequency (rf) power splitter,COM: rf combiner, AMP: rf low power ( <
25) dBm ampli-fier, X2: rf doubler, MIX: rf mixer, LPF: low pass filter, PBS:polarizing beamsplitter, PD: photodiode.
FC using a PDH lock at f pdh by slow/fast lock sentto the ECDL-PZT/EOM-NLTL respectively. In trans-mission the filtered light goes to the experiment withfrequency f exp = f ecdl + 2 f tune (panel 4 of Fig.1). Inreflection the light consists now of mostly f ecdl and aleftover at f ecdl + 2 f tune (the last portion results fromthe non-perfect mode matching and mirror losses). Asmall portion of it is necessary for the PDH lock (usingthe lines at f ecdl + 2 f tune ± f pdh ), but the rest is passedthrough a double-pass up-shifting AOM. The AOM isdriven by the frequency difference f tune − f dds , wherethe first portion comes from the same VCO driven by thefast port of the PID used to lock to the FC, but beforethe doubling stage, (Fig.1; denoted X2) and the secondfrom a DDS (subtracted using a rf mixer; denoted MIX).The useful light after the double-pass AOM has the fre-quency f ecdl + 2 f tune − f dds ± f pdh (panel 3 of Fig.1).The result is a filtered, ultra-tightly locked main beamcoming in transmission of the FC with noise dominatedonly by the cavity’s acoustics, and another beam (theone after the AOM) that is shifted 2 f dds away, which,in the acoustic frequency range, perfectly resembles theprimary one. The latter one can be used to lock thewhole system to an ultimate RC by a simple one chan-nel PID with mediocre bandwidth, addressing the PZTof the FC.The system’s advantages apart from the above men-tioned ones over the standard realizations of such sys-tems include: 1) extremely high BW and dynamic rangeof the fibered EOM-NLTL system superior to intra-cavity itle Suppressed Due to Excessive Length 3 EOMs (which have smaller dynamic range) or injectioncurrent locks (which have smaller BW and dynamic rangeand add amplitude noise), 2) comparatively low opticallosses (mainly due to the NLTL), 3) convenient long-range tuning without the need to optimize AOMs whenswitching to another atomic or molecular line as one sim-ply has to change the DC value f tune and accordingly f dds to be in the convenient range of the AOM (the fastlock to the EOM is ac-coupled starting from >
10 Hz), 4)decreased complexity by a) achieving tuning, sidebandmodulation, and locking (to both FC and RC) by justone EOM and one AOM, b) powering all the componentsby <
25 dBm rf amplifiers (still in the power range wherethe costs are minimal), and c) using only one actuator,namely the FC PZT, to lock to the RC in view of its highBW (a small mirror is used and a carefully chosen PZT),and 5) filtered output by the FC, eliminating stochasticlaser noise and unwanted sidebands.
We first discuss the laser housing and mount in detail.It is based on a Littrow-type ECDL with a variable cav-ity length in the range of 3 −
10 cm. The passive sta-bility of the ECDL (operating with a laser diode EYP-RWE-1060, Eagleyard) and acoustic immunity are im-proved in comparison to standard designs by a few sim-ple measures (Fig.2). First, a commercial diode collima-tion tube (LT230P-B, Thorlabs) is partially flattened ontwo parallel planes and sandwiched between two Peltierelements (TE-17-1.0-2.0, TE tech.) to control the diode’stemperature using a 10 k Ω thermistor (Vishay BCCom-ponents). The overall square-shaped mount (CM) servesas a heat sink and as a vertical goniometer. Rotationin the vertical plane (around axis Ax0) is necessary tooptimize the optical feedback. It is initiated by verticalprecision screws (VS) pressing against the floor of themain body. Once aligned, the vertical rotation degree offreedom is frozen by tightening the CM to the sidewall ofthe laser body [28] as it is not used in a daily operation.Its contribution therefore to the acoustic noise of thelaser is minimized. The grating (holographic, number oflines/mm chosen such that θ littrow ∼ ◦ ; 33025FL01-239H Richardson gratings) is positioned in a cylinder-flexure structure (CFS) that is allowed to rotate only inthe horizontal plane for tuning the wavelength (aroundAx1) [26]. Rough tuning is achieved by a rotation in-duced by two fine threaded screws (RS) (F3SS10 andN250L3P, Thorlabs) inducing rotation in opposite direc-tions by pushing along the tangent of the CFS (see Fig.2a) and c) for horizontal and vertical views respectively).The screws have ball tips and press against the CFSthrough two sapphire crystals (SC, 43-627, Edmund).Smooth rotation is accomplished by three ball-tip pre-cise screws (CS) that press the upper edge of the cylin-der, which is wedged at 30 ◦ (Fig.2 d)). This feature also CSCSCS RSCS a)b) c)d) .. TSBF
CFSQC Ax0Ax1 Ax2 RLSC SC
Fig. 2 a) ECDL design. Top view cross-section at about midplane. Notation: CFS: cylinder-flexure structure, CM: col-limator mount goniometer, AW: anti-reflection coated win-dows (serving also as vacuum seal with viton O-rings), PZT:grating piezo, PZTt: two tubular concentric PZTs, FC: fil-ter cavity, OC: output coupler, BW: brewster window, PT:Peltier elements, CT: collimator tube for laser diode, VS:precision screws allowing for vertical alignment of the diode(by pressing against the floor of the laser bed), RS: precisionscrews for rough wavelength tuning, CS: centering screws,SC: sapphire crystal, QC: vacuum quick connectors, OR: vi-ton O-rings for vacuum seal, TSB: transmission-sealing bar,GR: holographic grating, MR: mirror for steering the beam,S: clearance holes for holding the CM to the laser body, FT:electrical feedthrough. Ax0: axis of rotation of CM, Ax1: axisof rotation of CFS driven by RS for rough wavelength selec-tion, Ax2: axis with zero velocity during flexing driven byPZT, F1: force exerted by PZT, F2: force exerted on theCFS by each CS. b) 3D view of the CFS with the three CSproviding smooth rotation. c) Zoom in of the vertical crosssection of the rough CFS rotation driving mechanism. RL:ring shaped lip of the CFS. d) Zoom in of the upper roundcleaved edge of the CFS experiencing a force F2 by the balltip of the CS. The scale bar refers only to a). E. Kirilov et al. assures that the screws apply a normal downward forcecomponent to the cylinder (F2), therefore guaranteeingrotation only in the horizontal plane. The CFS contactsthe main body only through the ball tips of the threeCS and a thin 1 mm wide ring-shaped lip left on theouter edge of its bottom surface (denoted RL and visi-ble in Fig.2 c)). After choosing the rough wavelength thehorizontal rotation is also frozen (by applying a force onthe CFS by both RS). Subsequent fine tuning is left toa PZT stack (PCh 150/7x7/2, Piezomechanik) drivingonly the flexure (force F1 on Fig.2 b) ), designed to havea mechanical frequency at ∼
15 kHz. The center of thegrating moves ∼ µ m for 100 N force applied by thePZT. During flexing the grating experiences a rotationaround Ax2. The CFS is made out of 316 SS. Modeling ofthe above properties is achieved using a standard CADprogram with an elastic properties simulation capability.In practice the resonant frequency is determined by ex-citing the PZT and sweeping a variable sound generatorwhile observing the noise spectral density of the laserwhen locked to the FC. The whole body of the laser ismade out of aluminum and is vacuum sealed, allowingit to be evacuated if desired. It is thermally stabilizedby 4 Peltier elements (TE-127-1.0-1.3, Tetech.) wired inseries. The resistive bridges for both the diode and laserbody are attached on the back of the laser for thermalstability and share the layout with the LD filtering cir-cuit. Rough tuning is possible by engaging the two finelythreaded screws RS with adapted quick-seal connectsQC (B-025-K, Lesker), which allows for rotation withoutbreaking vacuum (applying Apiezon vacuum grease onthe sealing o-rings experiencing the rotation driven bytransmission-seal bar (TSB) from outside). The cham-ber is evacuated through a small inlet (same as V1021-1, DLH Industries) machined on a KF25 flange, whichis economic and does not take much space compared tostandard vacuum valves.The performance of the bare, free-running ECDLwithout engaging the tight locks to the FC and the RCis characterized by analyzing the frequency noise linearspectral density (LSD) of the error signal as shown inFig.3 when the laser is locked to the FC using only theslow ( ∼
10 Hz) servo branch applied to the PZT behindthe grating of the ECDL (the feedback via the EOM isnot applied). There are no visible mechanical resonances.The “fast” linewidth of the laser is ≤
20 kHz mostlydefined by the length of the ECDL cavity. At Fourierfrequencies ≤
100 kHz the spectrum increases from thequantum white noise level due to additional technicalnoise sources. The overall rms linewidth is ∆ν ecdlrms = (cid:82) ∞ S ∆ν ( f ) df = 59 kHz. The corresponding Allan de-viation calculated based on the PSD σ (2 , τ )(Hz ) =2 (cid:82) ∞ S ∆ν ( f ) sin ( f τ ) / ( πf τ ) df is in the ≤
25 kHz rangefor times in the 10 µ s - 10 ms interval. We now use the fibered EOM (EOspace, PM-0K5-10-PFA-PFA-980, 10 GHz BW) as a fast actuator. Almost50% of the light is coupled into the EOM, additionallyspatially filtering the beam and thus delivering a cleanGaussian mode. We note that one typically filters thelaser mode with a polarization maintaining (PM) fiber;the presence of the fibered EOM therefore eliminates thisstep. The light is then directed to the plano-convex FC(finesse ∼ .
481 GHz, spacer made of Zero-dur square block 30x30x100 mm, Helma optics) with onecoupler (both couplers from Layertec, low loss, 12.5 mmdiam.; flat and 500 mm radius) mounted on two concen-tric tubular PZT’s (Meggit, F3270154, F3270055) com-pensating their thermal drifts (Fig.2; PZTt). The FC isplaced in a V-groove and held in place by 8 viton O-rings (3 mm OD) positioned closely under and abovethe diagonal-mid plane (top clamps visible on Fig.2).Almost 80% of the light is coupled into the FC on res-onance. The EOM is driven by a NLTL 7013 (TactronInc., produced by Picosecond Pulse Inc., operating range400 − f is composed of a comb of frequencies withpower scaling roughly as ∼ /n , where n is the Fourierindex. Once the amplitude of the sawtooth signal be-comes equal to the V π voltage of the EOM (i.e. 3-4 V),the modulated phase has an effectively linear time de-pendence and therefore most of the light power ( ∼ f tune (500-1300 MHz) away from thecarrier. This sideband can be tuned with 3 dB BW of ∼
25 MHz limited by the VCO BW and with a dynamicrange of ∼
800 MHz. This allows for an extremely fastand easy to operate lock with a closed loop BW thatcan be pushed to 8 MHz, limited primarily by phase de-lays in the PID (see Fig.4 a) and c) for a schematic) andcable length.The FC is positioned in the same body as the ECDLto minimize space and costs. The whole chamber is cov-ered with silicon (Smooth-on Mold Max 15T) for damp-ing acoustic air-born noise [28]. The sidebands neededfor the PDH lock are imprinted by a 30-MHz wave pro-vided by a fixed DDS (AD9858) right before the NLTL(power splitters resp. combiners are ADP-2-20-75, Mini-Circuits). These sidebands pass the NLTL unaltered.One could perform the mixing after the NLTL with aresistive combiner, but then a 6 dB loss is added andthe V π of the EOM can barely be reached. In reflection10% of the light is used for the PDH to the FC. Thefast correction is applied to the voltage-controlled portof the VCO. The slow correction ( ∼
100 Hz) goes tothe grating piezo (PZT) of the ECDL. At this point thelaser is locked to sub-Hz linewidth relative to the FC itle Suppressed Due to Excessive Length 5 −6 −4 −2 0 2 4 6−90−80−70−60−50−40−30−20
Frequency (MHz) P S D ( d B ) current lock PZT lockEOM lockFrequency (Hz) L S D ( H z / √ H z ) −1 close loop EOMdet noise free runningECDL (a)(b) Fig. 3
Characterization of the laser noise performance. a)Frequency noise power spectral densities (PSD) of the PDHerror signal as recorded by a spectrum analyzer for the laserstabilized to the FC for 1) just a PZT slow lock not to in-fluence the PSD at frequencies >
10 Hz, 2) applying a fastlock to the injection current, and 3) adding the fast lock tothe fibered EOM (signal taken before the phase detector andthen translated to zero frequency). b) LSD of the PDH signalas measured by an audio analyzer for the cases as indicated. and the PSD is dominated solely by the FC’s acousticnoise. Fig.3 a) and b) compare the PSD/LSD of the er-ror signal to the FC for the free running ECDL and in alocked configuration. For comparison we show the PSDwhen using the injection current of the LD as a fast ac-tuator. We reach a unity gain frequency of ∼ ≥ / √ Hz LSD)and a subtracting high BW amplifier (LMH6624, 1 . .
92 nV / √ Hz LSD) with a fixed voltagegain of 20 dB. This configuration ensures a high in-put impedance optimal for signals coming from phase-detectors and a high usable signal BW while keeping theoverall noise level low. Next a variable attenuator stageallows for a reduction of gain if necessary. Additionallyone can choose between AC and DC coupling of the sig-nal (corner frequency ∼
10 Hz). The first (slow) integra- tor stage has a fixed corner frequency of 10 kHz. Themaximum voltage gain for low frequencies reaches 46dB, the gain in the proportional region is fixed to 6 dB.The amplifier that we use (ADA4817-1, 1 GHz GBWP,4 nV / √ Hz LSD) has FET-input stages, ensuring negli-gible changes in the offset voltage in the whole integralgain region. The full PID stage, using the same ampli-fier as the slow integrator stage, uses DIP-switches toset the corner frequencies of the D- and I-part by chang-ing the corresponding capacitances. The P-part has afixed attenuation of −
20 dB, ensuring that the D-partwith a maximum of 6 dB gain is within a frequency re-gion where the bandwidth of the amplifier is no limita-tion. The available corner frequencies of the I-part witha maximum gain of 20 dB are limited to low frequen-cies such that no overlap with the first (slow) integra-tor could lead to instabilities due to high phase shifts.A full-scale output offset can be added within the laststage (THS3091, 190 MHz Bandwidth, 2 nV / √ Hz LSD),which also serves as an output buffer able to provide upto 250 mA current.Overall, all electronic parts, especially the amplifiers,are chosen carefully to allow a maximum electronic band-width of several tens of MHz such that phase shifts dueto the amplifiers of the PID circuit are negligible in theoverall PID, thereby not limiting the feedback BW. Bodeplots showing the performance of our circuitry are givenin Fig. 4. To minimize electronic noise introduced by thePID, ultra-low-noise amplifiers and small resistor valuesto reduce thermal noise are used. The current and PZTdriver circuits are also home made with noise figures of < µ A rms ( < . / √ Hz @ 2 kHz) for the current and < µ V rms ( < / √ Hz @ 2 kHz) for the piezo in the1 Hz - 20 kHz range (see Fig. 4 c)).Next, for better absolute stability, we reference thelaser to a highly stable RC without any additional com-ponents other than a single AOM (Fig.1). A DC voltagein the range of about 0 −
10 V is added to the correc-tion signal of the PID to tune the laser to a desiredaverage position relative to the carrier 2 f tune . The lightreflected from the FC contains non-diffracted light at thefrequency of the ECDL carrying sidebands at 30 MHz(see panel 2 of Fig.1). Note that this light still has thenoise of the ECDL. It is passed through a double-passup-diffracting AOM (Crystal Technology, model 3200,200 MHz center frequency). The AOM is driven by aportion of the radio-frequency signal that comes fromthe VCO and that is mixed with the signal at f dds froma DDS, resulting in an approximately 200-MHz drivewithin the range of the AOM ( f dds is chosen accord-ingly). The AOM is not a part of the closed loop butsince it is driven by the radio-frequency that contains thefrequency excursions of the ECDL relative to the FC, itcorrects the frequency excursions within its bandwidth.The light after the AOM is shifted by 2 f dds with respectto the light transmitted through the FC (see panel 3 ofFig.1). In comparison to the FC-transmitted beam it has E. Kirilov et al. (b)(a)(c) −9 −8 −7 Frequency (Hz) C u rr e n t N o i s e d e n s i t y I r m s ( A / √ H z ) −8 −6 −4 Frequency (Hz) V o l t a g e V r m s ( V / √ H z ) Frequency (Hz) −150−100−50050 p h a s e ( D e g ) G a i n ( d B ) Frequency (Hz)
51R 240R160R 240R240R160R 240R160R -5V+5V
GND 4.7pF10pF22pF47pF100pF220pF470pF1nF . +5V-5V +5V-5V Gain I slow
GND
TTL TTL
GND
TTL160R100R 51R+5V-5V-5V +5V 1pF +5V-5V-5V+5V +5V-5V
GND150R ADG417
12 GND3 VDD 4VL56 VEE 78
ADG417
12 GND3 VDD 4VL56 VEE 78
ADG417 +5V-5V +15V-5V+5V-5V +5V +5V+5V +15V-5V
10k +15V5k11kADA4817-1ADA4817-1 THS3091100nF100uF1kGNDDi erential Input Ampli er stageO set Correction Variable Attenuator stage Slow Integrator stage Fast PID stage Output O set stage16kHz3.4MHz 14.2kHz1.42MHz723Hz24R924R9 OP27ADA4899-1ADA4899-1LMH6624
Fig. 4 a) PID circuit for the fast correction fed to the EOM. Frequencies in red are examples for the components values usedin this specific circuit and may be modified depending on the actuators BW for the specific laser system. b) Simulated Bodeplots of the PID circuit with typical parameters of operation; slow integrator (black squares), fast integrator (red diamonds),op. amps. (blue circles), full PID (green triangles), c) LSD of the LD current source and the PZT driver. enhanced noise beyond the AOM’s bandwidth (which istypically 250 kHz). This, however, is irrelevant to thelock to the RC as the only noise that needs to be com-pensated for comes from the acoustic noise of the FC.In Fig.5 we analyze the light that comes out of theAOM. We show the beat signal between the light lockedto and transmitted through the FC and the light diffractedby the AOM when the AOM is either driven by an inde-pendent source or driven by the signal that is correlatedwith the signal that is fed to the EOM. When the AOM isdriven from a fixed 200-MHz source the beat as shown inFig.5 b) reveals the linewidth of the free running ECDL(over the time of the sweep of 2 s). When driven by thehalf frequency of the EOM at f tune − f dds ∼
200 MHzthe beat signal dramatically narrows as shown in Fig.5a). We attribute the spurs at multiples of 50 Hz visiblein Fig.5 a) to the power supply that drives the LD cur- rent source and the PZT drivers (see Fig. 4 c)). Withappropriate filtering, they could be removed.With the feedback to the AOM the diffracted lightcan now be sent to the RC. A single-channel feedback tothe fast PZT of the FC (PZTt in Fig.1) eliminates theacoustic noise of the FC and provides long-term stability.The light after the AOM already carries PDH-sidebands,eliminating the need for an additional EOM. In Fig.6we compare the LSD of the PDH-signal of the free-running ECDL/FC-system to that of the ECDL/FC-system locked to the RC (cylindrical ULE cavity in vac-uum at 10 − Torr with finesse ∼ ∼ itle Suppressed Due to Excessive Length 7 −500 −400 −300 −200 −100 0 100 200 300 400 500−120−110−100−90−80−70−60−50−40 Beat signal (Hz) −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1−110−105−100−95−90−85−80−75
Beat signal (MHz) r e l a t i v e b e a t a m p l i t u d e ( d B ) r e l a t i v e b e a t a m p l i t u d e ( d B ) (a)(b) Fig. 5
Beat spectrum between a) the laser field in transmis-sion of the FC and the laser field through the double-passAOM, when the last is driven from the PID fast port at fre-quency f tune − f dds (see text); in this case the 2 beams (900MHz apart) are locked relative to each other. b) same as a)but the AOM is driven by an independent fixed DDS. Thewidth is representative of the ECDL linewidth over the timeof the 2 s scan. Note the vastly different scales on the hori-zontal axis. −1 Frequency (Hz) L S D ( H z / √ H z ) ECDL/FC locked to RCECDL/FC free running
Fig. 6
Frequency LSD of the PDH error signal as obtained inreflection from the RC when the ECDL/FC system is eithertuned to the peak transmission of the RC without the lock viaPZTt (free running) or locked to the RC by a single channelPID to the PZTt (locked to RC).
LSD of the PDH error signal and does not include apossible residual amplitude modulation (RAM) at thePDH frequency. Choosing a smaller (i.e. lighter) cavitymirror and a FC PZT with a smaller inner diameter weexpect to be able to increase the BW of the second lockto at least ∼
50 kHz and not be limited by the mechan-ical resonance of the FC PZT-mirror [29] that can stillbe seen in the error-signal spectrum.
In conclusion, we have demonstrated a simple, compact,and versatile laser system that is easy to operate andthat achieves simultaneously high spectral purity, hightunability, long-term stability, and robustness. It shouldfacilitate experiments in high precision spectroscopy, me-teorology, interferometry, and quantum state control. Wenote that this locking scheme does not involve electronicfeedback to the laser diode itself. Laser diodes and fiberedEOMs cover a large portion of the visible and near-infrared spectrum of interest for many experiments inatomic and molecular physics, making the setup widelyapplicable. The fibered EOM’s highly defined interactionregion within the crystal makes the system less suscep-tible to problems arising from crystal inhomogeneity oranisotropy of the applied field [14]. One can also use theEOM for canceling RAM by simply feeding a DC offseton top of the rf correction signal [30]. An ultimate testfor our system would be a beat measurement involvingtwo identical systems locked to independent ultra-stablereference cavities. Our system will first be applied tomolecular ground-state transfer on KCs molecules simi-lar to work presented in Ref.’s [8,9,10,11,12,13].
We thank J. Berquist for providing the ULE cavity, J.Danzl for helping with the design of the PID circuit, andK. Aikawa for useful suggestions on the manuscript. Weacknowledge generous support by R. Grimm. The workis supported by the European Research Council (ERC)under Project No. 278417.
References
1. S. A. Diddams, T. Udem, J. C. Bergquist, E. A. Curtis,R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee,C. W. Oates, K. R. Vogel, and D. J. Wineland, An opticalclock based on a single trapped 199Hg+ ion. Science ,825–828 (2001)2. C. W. Chou, D. B. Hume, T. Rosenband, and D. J.Wineland, Optical clocks and relativity. Science ,1630–1633 (2010)3. H. Marion, F. Pereira Dos Santos, M. Abgrall, S. Zhang,Y. Sortais, S. Bize, I. Maksimovic, D. Calonico,J. Gr¨unert, C. Mandache, P. Lemonde, G. Santarelli,P. Laurent, A. Clairon, and C. Salomon, Search for vari-ations of fundamental constants using atomic fountainclocks. Phys. Rev. Lett. , 150801 (2003)4. C. Eisele, A. Y. Nevsky, and S. Schiller, Laboratory testof the isotropy of light propagation at the 10 − level.Phys. Rev. Lett. , 090401 (2009)5. B. P. Abbott, LIGO: The laser interferometergravitational-wave observatory. Rep. Prog. Phys. , 076901 (2009) E. Kirilov et al.6. P. Schindler, D. Nigg, T. Monz, J. T. Barreiro, E. Mar-tinez, S. X. Wang, S. Quint, M. F. Brandl, V. Nebendahl,C. F. Roos, M. Chwalla, M. Hennrich, and R. Blatt, Aquantum information processor with trapped ions. NewJournal of Physics , 123012 (2013)7. N. Kolachevsky, J. Alnis, C. G. Parthey, A. Matveev,R. Landig, and T. W. H¨ansch, Low phase noise diodelaser oscillator for 1S-2S spectroscopy in atomic hydro-gen. Opt. Lett. , 4299–4301 (2011)8. J. G. Danzl, E. Haller, M. Gustavsson, M. J. Mark,R. Hart, N. Bouloufa, O. Dulieu, H. Ritsch, and H.-C. N¨agerl, Quantum gas of deeply bound ground-statemolecules. Science , 1062 (2008)9. K.-K. Ni, S. Ospelkaus, M. H. G. de Miranda, A. Pe’er,B. Neyenhuis, J. J. Zirbel, S. Kotochigova, P. S. Julienne,D. S. Jin, and J. Ye, A high phase-space-density gas ofpolar molecules. Science , 231 (2008)10. M.J. Mark, J.G. Danzl, E. Haller, M. Gustavsson, N.Bouloufa, O. Dulieu, H. Salami, T. Bergeman, H. Ritsch,R. Hart, and H.-C. N¨agerl, Dark resonances for ground-state transfer of molecular quantum gases. Appl. Phys.B , 219–225 (2009)11. J. G. Danzl, M. J. Mark, E. Haller, M. Gustavsson,R. Hart, J. Aldegunde, J. M. Hutson, and H.-C. N¨agerl,An ultracold high-density sample of rovibronic ground-state molecules in an optical lattice. Nature Phys. , 265–270 (2010)12. K. Aikawa, J. Kobayashi, K. Oasa, T. Kishimoto,M. Ueda, and S. Inouye, Narrow-linewidth light sourcefor a coherent Raman transfer of ultracold molecules.Opt. Express, , 205301 (2014)14. T. Kessler, C. Hagemann, C. Grebing, T. Legero,U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye,A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity. Nature Photonics , 687–692(2012)15. F. Riehle, Frequency Standards. Wiley VCH Verlag(2004)16. C. H. Breant, Frequency noise analysis of optically self-locked diode lasers. IEEE J. Quantum Elect. , 1131–1142 (1989)17. E. Kirilov and S. Putterman, 2-Photon ionization for ef-ficient seeding and trapping of strontium ions. EuropeanPhys. J. D , 683–691 (2009)18. K. D¨oringshoff, I. Ernsting, R.-H. Rinkleff, S. Schiller,and A. Wicht, Low-noise, tunable diode laser for ultra-high-resolution spectroscopy. Opt. Lett. , 2876–2878(2007)19. J. Labaziewicz, P. Richerme, K. R. Brown, I. L. Chuang,and K. Hayasaka, Compact, filtered diode laser systemfor precision spectroscopy. Opt. Lett. , 572–574 (2007)20. E. D. Black, An introduction to Pound-Drever-Hall laserfrequency stabilization, Am. J. Phys. , 79–87 (2001)21. A. N. Matveev, N. N. Kolachevsky, J. Alnis, andT. W. H¨ansch, Spectral parameters of reference-cavity-stabilised lasers. Quantum Electron. , 391–400 (2008)22. L. P. Yatsenko, B. W. Shore, and K. Bergmann, Detri-mental consequences of small rapid laser fluctuations on stimulated Raman adiabatic passage. Phys. Rev. A ,013831 (2014)23. C. Petridis, I. D. Lindsay, D. J. M. Stothard, andM. Ebrahimzadeh, Mode-hop-free tuning over 80 GHzof an extended cavity diode laser without antireflectioncoating. Rev. Sci. Instrum. , 3811–3815 (2001)24. R. Kohlhaas, T. Vanderbruggen, S. Bernon, A. Bertoldi,A. Landragin, and P. Bouyer, Robust laser frequencystabilization by serrodyne modulation. Opt. Lett., ,1005–1007 (2012)25. E. Afshari, S. Member, and A. Hajimiri, Nonlinear trans-mission lines., IEEE J. Solid-St. Circ. , 744–752 (2005)26. A commercial ECDL design based on a flexture de-sign is available by the company Toptica (patentsDE102007028499 and US7970024).27. D. M. S. Johnson, J. M. Hogan, S.-W. Chiow, and M. A.Kasevich, Broadband optical serrodyne frequency shift-ing. Opt. Lett. , 745–747 (2010)28. E. C. Cook, P. J. Martin, T. L. Brown-Heft, J. C. Gar-man, and D. A. Steck, High passive-stability diode-laserdesign for use in atomic-physics experiments. Rev. Sci.Instrum. , 043101 (2012)29. T. C. Briles, D. C. Yost, A. Cing¨oz, J. Ye, and T. R. Schi-bli, Simple piezoelectric-actuated mirror with 180 kHzservo bandwidth. Opt. Express , 9739–9746 (2010)30. N. C. Wong and J. L. Hall, Servo control of ampli-tude modulation in frequency-modulation spectroscopy:demonstration of shot-noise-limited detection. J. Opt.Soc. Am. B2