Acousto-optic modulator based frequency stabilized diode laser system for atomtrappingPeter D. McDowall and Mikkel F. Andersen Citation: Review of Scientific Instruments 80, 053101 (2009); doi: 10.1063/1.3125029 View online: http://dx.doi.org/10.1063/1.3125029 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/80/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High coherent bi-chromatic laser with gigahertz splitting produced by the high diffraction orders of acousto-opticmodulator used for coherent population trapping experiments Rev. Sci. Instrum. 82, 123104 (2011); 10.1063/1.3665986 Creation of arbitrary spectra with an acousto-optic modulator and an injection-locked diode laser Rev. Sci. Instrum. 82, 083108 (2011); 10.1063/1.3626903 A frequency stabilization technique for diode lasers based on frequency-shifted beams from an acousto-opticmodulator Rev. Sci. Instrum. 79, 103110 (2008); 10.1063/1.3006386 Two-frequency acousto-optic modulator driver to improve the beam pointing stability during intensity ramps Rev. Sci. Instrum. 78, 043101 (2007); 10.1063/1.2720725 Optical transfer cavity stabilization using current-modulated injection-locked diode lasers Rev. Sci. Instrum. 77, 093105 (2006); 10.1063/1.2337094

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Acousto-optic modulator based frequency stabilized diode laser systemfor atom trapping

Peter D. McDowall and Mikkel F. AndersenJack Dodd Center for Quantum Technology, Department of Physics, University of Otago,Dunedin 9016, New Zealand

�Received 23 February 2009; accepted 5 April 2009; published online 1 May 2009�

We report on an inexpensive commercial laser diode stabilized to the D2-line in rubidium using asimple scheme. The linewidth was reduced to 1.3 MHz without an external cavity, making it suitablefor laser cooling and trapping. The system is very robust and the laser frequency can be changedrapidly �within 51 �s� while the laser remains in lock. The frequency of the locked laser drifts lessthan 850 kHz peak-to-peak over 25 h. We demonstrate laser cooling and trapping using oursystem. © 2009 American Institute of Physics. �DOI: 10.1063/1.3125029�

I. INTRODUCTION

Many areas of physics research and commercial applica-tions require frequency stabilized lasers. These areas includeatomic and molecular spectroscopy, metrology, optical com-munication, and laser cooling and trapping. Due to their longterm stability, atomic transitions provide excellent stable ref-erence points. The implementation of frequency stabilizinglasers to these references varies widely, as there are advan-tages and disadvantages associated with each technique.

Here we present a robust, simple, and inexpensive fre-quency stabilized laser system based on a commercial laserdiode without an external cavity. Our technique relies on anacousto-optical modulator �AOM� to produce a dispersion-like �DL� error signal with a large capture range comparableto that of Refs. 1–4. The stabilized frequency of the laser isinsensitive to mechanical and acoustic vibrations, vaporpressure inside the reference cell, and beam power.

While being similar in design and characteristics to thesystems reported in Refs. 2–4, our system offers several sim-plifications and new features. The lock frequency can be rap-idly tuned for the purpose of optical molasses. Our simplefrequency tuning scheme does not require a magnetic fieldand avoids the need for high quality polarization optics. Fur-thermore, it requires no complicated electronic circuitry orexpensive lasers yet still maintains long term stability with anarrow linewidth capable of maintaining a magneto-opticaltrap �MOT� for a full working day. The laser frequency isunaffected by acoustic noise, and even hammering the opti-cal table next to the laser did not cause any detectable fre-quency noise.

II. SETUP

A. The laser diode

The laser used in this experiment is a Sharp Microelec-tronics GH0781JA2C laser diode, commercially producedfor high-speed CD writers. The frequency �or wavelength� ofthe free running laser diode can be tuned in two differentways: by changing the injection current or by changing thecase temperature.5 Altering just one of these variables will

tune the laser frequency discontinuously, as the laser willmode-hop,5 making certain frequencies inaccessible. How-ever, by simultaneously adjusting both the injection currentand the case temperature, we were able to access any wave-length within the diode’s tuning range and thereby tune allour diodes to the wavelength resonant with the D2-line inrubidium �Rb� at 780 nm. Once the laser wavelength wastuned to the desired Rb resonance, a feedback circuit held thecase temperature constant. The laser frequency was thenscanned and fine-tuned through modulation of the injectioncurrent. Once on resonance, the laser never mode-hopped,even when performing wide frequency scans of more than 10GHz. The drawback with this method is that the laser outputpower depends on the injection current and is therefore not afree parameter. When tuned to the Rb resonance, we typi-cally got an output power of approximately 50 mW whereasthe diode is rated for 120 mW. The case temperatures were inthe range of 5–10 °C, so we mounted each diode in anair-tight Perspex housing to avoid condensation.

The laser beam was collimated by a lens before directingit to the optical setup described below.

B. Optical setup and feedback circuit

Figure 1 shows the optical setup used to frequency sta-bilize �or lock� the laser frequency to the Rb resonance. Theoutput beam from the laser is directed through an opticalisolator to reduce optical feedback. A small fraction of thelight is reflected from a glass plate and is used to make theerror signal. The reflected beam is passed through a � /2waveplate before a linear polarizer. This enables the user tocontrol the intensity of the beam to ensure the photodiodesare not saturated. The linear polarizer also aligns the polar-ization of the beam with the polarization axis of the AOM.

The AOM creates a relative frequency difference of 300MHz between the first-order diffracted beam and the trans-mitted zeroth order beam. The incident angle of the beamentering the AOM is made such that the intensity of thediffracted beam is maximized, thereby making the diffractionefficiency of the AOM independent of small changes in

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incident angle. The two beams emerging from the AOM aredirected onto a 50/50 beam splitter.

The two beams that are transmitted through the 50/50beam splitter are used to produce a DL error signal. They aredirected through a vapor cell containing natural rubidium inwhich the atoms absorb resonant light. The observed atomiclines are broadened, due to the Doppler effect and excitedstate hyperfine splitting,6 to a linewidth of around 550 MHzfull width at half maximum �FWHM�. The frequency differ-ence between the two beams of 300 MHz is evident from thetwo Doppler broadened absorption profiles that are producedwhen the laser frequency is scanned across resonance and thetransmitted intensity of each beam is measured indepen-dently. Two reverse biased photodiodes in series measure thedifference in transmitted intensity �see Fig. 2�. Since the twoabsorption profiles are shifted in frequency, this produces aDL signal when scanning the laser frequency across anatomic absorption line �see Fig. 3�. The frequency at whichthe DL signal crosses zero �denoted the locking point� isinsensitive to temperature and pressure fluctuations withinthe vapor cell and fluctuations in the laser power up to thefirst order. This latter result was examined by modulating thepower of the beam before the glass plate �see Fig. 1� using anadditional AOM. No discernible change was observed to thelock point frequency even when the laser power was reducedby 50%.

The two beams that are reflected off the 50/50 beam

splitter are used to cancel drift in diffraction efficiency of theAOM. These are directed onto a second set of photodiodes toyield a signal proportional to the difference in intensity of thetwo beams. The polarity of this signal can be switched byinterchanging the beams that hit the two photodiodes, and ischosen such that it has opposite polarity to the correspondingsignal from the two beams that pass through the vapor cell.An opaque metal box covers the optical system and shieldsthe photodiodes from room light.

Figure 2 shows a schematic of the electronics used fordetection and feedback to the laser diode injection current.By adjusting the potentiometer �POT1�, the locking point canbe made independent of imbalance of power in the first andzeroth order beams from the AOM. The locking point nowsupplies a stable frequency reference. When switch SW1 isclosed, the output of the OP177 operational amplifier is 0 V.SW2 connects either the feedback circuit or a signal genera-tor used to scan the laser frequency to the current modulationinput on the laser injection current driver. When SW2 con-nects the feedback circuit to the laser current driver and SW1is opened, the laser frequency is held at the lock point.

C. System characteristics

1. Linewidth

Commercial laser diodes without an external cavity of-ten have a linewidth which is too large to make them suitablefor laser cooling of atoms and other atomic physicsexperiments.5,7 To determine the linewidth of the frequency-locked laser diodes used in this experiment, we built twoidentical systems. An additional AOM in the output beam ofthe first laser introduced a frequency shift of 110 MHz. Wecombined the frequency shifted beam with the output fromthe second laser using a 50/50 beam splitter and coupled thebeams into a single mode fiber. A high-speed photodiode at

FIG. 1. Experimental setup used for locking the laser. A schematic of theelectronics used is included in Fig. 2.

FIG. 2. Electronics used to produce the locking signal and provide feedbackto the laser. Four Thorlabs FDS100 photodiodes are used in this electronicsetup. The three operational amplifiers are also connected to the powersupply. The signal generator is used to scan the laser frequency when it isnot in lock.

FIG. 3. �Color online� Locking signal produced when the laser is scannedover the four transitions of the D2-line of Rb, shown by the thick blackcurve. The lock point or stable reference point is where the signal crosseszero on the y-axis. The dashed line shows the model fit of the data made byfitting the difference of two frequency separated Gaussian line shapes toeach DL error signal. Four DL signals appear due to the two naturallyoccurring isotopes in rubidium �85Rb and 87Rb�; each with ground statehyperfine splitting larger than the Doppler broadening.

053101-2 P. D. McDowall and M. F. Andersen Rev. Sci. Instrum. 80, 053101 �2009�

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the output of the fiber measured the beat note correspondingto the difference in laser frequencies and displayed it on aradio frequency �rf� spectrum analyzer.

To ensure that drifts in the relative power of the twobeams emerging from the AOMs in each of our laser systems�see Fig. 1� do not affect the lock point frequency, we modu-lated the rf power to one of the AOMs at a frequency ofaround 1 Hz while the lasers were in lock. The POT1 �seeFig. 2� was then adjusted until the beat note frequency be-came unaffected by the changing rf power. This was per-formed on both laser systems.

We determined the linewidth of the individual laserswhen in lock to be 1.3 MHz FWHM, found by fitting aLorentzian curve to the beat note and assuming the indi-vidual lines to be Lorentzian.8 This measurement was con-firmed by replacing one of the lasers in this experiment witha commercially produced Toptica DL Pro grating stabilizeddiode laser locked to a 87Rb transition using frequencymodulated saturation absorption spectroscopy6 �see Fig. 4�.

A linewidth of 1.3 MHz is significantly smaller than thenatural linewidth of 6 MHz of the D2-line of Rb, indicatingthat this is sufficient for laser cooling and trapping experi-ments. We confirmed this by constructing a MOT using ourlasers as both cooling and repump lasers,9 and observed nodifference in performance of the MOT when using the fre-quency stabilized laser diodes, compared to using commer-cial external cavity diode lasers with a similar amount ofpower. Precise tuning of the laser frequency for MOT obser-vation was achieved by adjusting the AOM frequency �seebelow�.

The linewidth of the unlocked Sharp laser diode wasmeasured by beating one of the unlocked laser diodes withthe stable Toptica laser. A Gaussian model fit of this beatnote produced the most accurate measurement of the lineshape, and was measured to be 4.2 MHz FWHM. Misalign-ment of the optical isolators deteriorated the linewidths. Weattribute this to optical feedback from reflections off the sur-faces of the optical fiber.

2. Tunability

Rapid tuning of the lock frequency was achieved bychanging the AOM frequency. Using an IntraAction Corpo-ration DE3001.5M voltage controlled oscillator �VCO� de-flector AOM driver we were able to tune the lock frequencyboth manually with an analog dial or via a frequency modu-lation �FM� analog input on the driver while the laser re-mained in lock.

To measure the response time of the lock, we changedthe lock point frequency using a square pulse on the FManalog input of the AOM driver. This caused the laser fre-quency to oscillate about the new lock point as an exponen-tially decaying sinusoid observed on the oscilloscope. Bymodeling the signal using a decaying exponential �see Fig.5�, we measured the recovery time of the lock to the newlock point. The typical time scale for recovery �the e−1 decaytime of the exponential in Fig. 5� was determined to be51 �s and independent of frequency change. This is fastenough for this system to be used for molasses cooling.9 Forpulses which changed the AOM frequency by more than 21MHz, the lock signal became compromised. Above thisthreshold, the intensity of the first order beam began to de-cline until only the zeroth order beam was present. The smallchange in beam angle created when the AOM frequency waschanged by 21 MHz did not affect detection of the beams,due to the large surface area of the Thorlabs FDS100 photo-diodes and the short distance between the AOM and thephotodiodes.

3. Long term stability

The frequency stability of the beat note produced by thetwo identical systems was monitored over a 25 h period. Thecentral frequency of the beat note was observed to be stableto 1.2 MHz peak-to-peak without temperature stabilizing thevapor cell. Assuming the frequency of the two lasers driftedindependently of each other, this corresponds to a frequencydrift of 850 kHz peak-to-peak for the individual lasers. This

FIG. 4. Beat note from the interference of the locked Sharp Microelectron-ics GH0781JA2C laser diode with a Toptica DL Pro grating stabilized diodelaser shown by the solid line. A Lorentzian fit is shown by the dashed line.

FIG. 5. �Color online� The square pulse �dashed line� is fed to the FM inputof the AOM driver and changes the lock point by 10.5 MHz. The exponen-tially decaying sinusoid �solid line� is the signal observed on the oscillo-scope when the laser is in lock. The oscillating signal is modeled by theexponentially decaying dash-dot line.

053101-3 P. D. McDowall and M. F. Andersen Rev. Sci. Instrum. 80, 053101 �2009�

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drift is still small compared to the natural linewidth of theD2-line of Rb. We monitored the frequency stability of oneof the VCOs used as a frequency source for the AOM driversover a similar period of time. We found that it drifted 300kHz, indicating that the stability of the three VCOs in thisexperiment contribute significantly to the measured drift. Wetherefore expect that the drift can be reduced even further byusing better frequency sources.

Given that the laser lock was constructed without anexternal cavity, susceptibility of the output frequency toacoustic vibrations was never observed. This was first exam-ined by observing the beat note and the locking signal of thelocked lasers while manually shaking the optical table andeven banging the table with a large hammer. No effect wasobserved either on the oscilloscope displaying the lockingsignal or on the beat note. This was repeated with the lasersunlocked where again no effect was observed on either sig-nal. We therefore attributed the robustness of the laser sys-tem against acoustic noise to the absence of an external cav-ity, the primary cause of acoustic noise pickup in most diodelaser systems. The large capture range also created by thissystem was not observed to contribute to the robustness ofthe lock in this case because we were unable to change thelaser frequency far enough from the lock point for it to havea discernible effect. However, when using the lock schemewith an external cavity diode laser, the large capture rangecould improve the stability.

III. CONCLUSIONS

We have presented here a simple frequency stabilizedlaser system based on a laser diode without an external cav-ity. The frequency of the laser is insensitive to acoustic vi-brations, fluctuations in laser power, and relative beampower of the two beams emerging from the AOM. The laserfrequency can be changed rapidly while the laser remains inlock. No high-speed electronics are required and those usedare simple to construct. The lock also shows stability within850 kHz over 25 h. We demonstrated laser cooling andtrapping with our system.

ACKNOWLEDGMENTS

This work was supported by University of OtagoResearch Grant and NZ-FRST Contract No. NERF-UOOX0703.

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