Simultaneously suppressing frequency and intensitynoise in a Nd:YAG nonplanar ring
oscillator by means of the current-lock technique
Michèle Heurs, Volker M. Quetschke,* Benno Willke, and Karsten Danzmann
Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut) and Institut für Atom- und Molekülphysik, AbteilungSpektroskopie, Universität Hannover, Callinstrasse 38, D-30167 Hannover, Germany
High-precision metrology experiments, such as in-terferometric gravitational-wave detectors,1 – 4 requireultrahigh-stability laser light sources. Monolithiclaser-diode-pumped Nd:YAG nonplanar ring oscilla-tors (NPROs) offer high intrinsic stability in frequency,intensity, and beam geometry5,6 and are used as mas-ter lasers for injection-locked laser systems in variousinterferometric gravitational-wave detectors.1,7,8 Yetto achieve the required ultrahigh sensitivity for thesedetectors the laser systems have to be further activelystabilized.
The intensity noise of a NPRO can be activelysuppressed by means of a noise eater, a circuitthat feeds back to the current of the pump laserdiodes.9 Conventional frequency stabilization of aNPRO to a rigid cavity in vacuum makes use of oneor more actuators, usually a piezoelectric transducer(PZT) in contact with the NPRO crystal for fastchanges and the temperature of the crystal for slowchanges. Using the pump current as an actuatorfor frequency stabilization is an alternative approachreferred to as current lock.10 This method showsvarious advantages over using the PZT as an actuator;it does not introduce excess beam pointing and showsno resonances up to the resonant relaxation oscilla-tions (RROs).
Using the pump current to tune the frequency of anNPRO is bound to cause a worsening of intensity noisein the NPRO. Instead it was experimentally shownby our group10 that current lock resulted in a smallamount (3 dB) of intensity noise suppression in smallbandwidth. This observation spawned completelynew experiments described in the following, becauseit is desirable to attain a stronger intensity noisesuppression as the by-product of frequency stabiliza-tion by means of the current-lock technique. This isnot possible in a conventional NPRO, however. Themultimode laser diode array, which is the commonNPRO pump source, exhibits strong spatial andtemporal mode f luctuations, even when the pump
0146-9592/04/182148-03$15.00/0
current is held extremely stable. These mode f luctu-ations, even when the pump current is held extremelystable. These mode f luctuations cause intensitynoise as well as frequency noise in the NPRO, andboth noise processes are uncorrelated. We thereforeconstructed an NPRO pumped by a single-mode laserdiode (SM-NPRO). Here the pump mode is spatiallyand temporally unvarying. Stabilization of the pumpcurrent of a single-mode diode can even result insqueezing of the pump light11; in our case it leads toa simultaneous stabilization of the NPRO intensityand frequency noise, because both noise processes arestrongly correlated in the SM-NPRO.
The principle of the current-lock technique is thecoupling of pump power modulations into the outputpower as well as into the laser frequency. On theone hand, pump power f luctuations couple directlyinto f luctuations of the output power of the NPROat Fourier frequencies well below the RRO.12 Onthe other hand, pump power variations are also animportant noise source for frequency f luctuations ofthe NPRO13: Pump power modulation leads to modu-lation of the deposited thermal energy in the activemedium. This thermal modulation changes the indexof refraction of the material as well as the length ofthe crystal; both phenomena result in a change of theoptical path length in the Nd:YAG material. Henceany optical path-length modulation causes a frequencymodulation of the laser light. As the crystal acts as athermal low-pass f ilter, the pump current modulationdecreases with increasing modulation frequency, andthe transfer function of frequency change per currentmodulation shows the typical 1�f behavior.10,14
However, for frequencies approaching the RRO of theNPRO another effect becomes dominant in the transferfunction. Pump power modulation leads to modula-tion in the inversion of the active medium and thereforeto variations in the index of refraction, which entailsfrequency modulation of the laser light. This effect isa small perturbation to the steady state but is clearly
visible in the above-mentioned transfer function at theRRO frequency. Efforts toward an understanding ofthis effect are under way.
In a NPRO with a multimode pump source thefrequency noise suppression from current lock isvery high (more than 100 dB at 100 Hz), and thecorresponding intensity noise suppression is nearlynegligible.10 To increase the degree of simultaneousintensity and frequency noise suppression in currentlock, we constructed a SM-NPRO in cooperation withInnoLight GmbH. It consists of a single-mode laserdiode (SDL�JDS Uniphase, type 5422-H1-810), wave-length nominally 810 nm,15 which is fiber-coupled intoa polarization-maintaining single-mode fiber. Thepump module of the SM-NPRO including beam-shaping optics is constructed on a Microbank system(Schäfter 1 Kirchhoff ). The pump light with a powerof approximately 70 mW after the f iber is matched tothe fundamental mode of a NPRO crystal by an opticaltelescope. The NPRO, designed for low threshold,then emits approximately 10 mW of laser light at1064 nm. Figure 1 shows the schematic setup of theSM-NPRO.
In the experiment described below we stabilized theSM-NPRO frequency to the resonance frequency of arigid reference cavity by use of the current-lock tech-nique to further investigate the effect of frequency andintensity noise coupling in this system.
A rigid high-finesse three-mirror ring resonator(reference cavity) was placed in an ultrahigh vacuum(pressure p � 1027 Pa) as a frequency referencefor the laser system. Another ring resonator witha piezoelectric actuator for frequency tuning wasplaced in a different vacuum chamber and servedas an independent reference (analyzer cavity) forfrequency noise measurements. In both cases thePound–Drever–Hall technique was used to stabilizethe laser frequency to the reference cavity (unity gainfrequency �4 kHz) and the analyzer cavity length tothe laser frequency (unity gain frequency �2 kHz).The laser light was split by a power beam splitterand individually mode matched into the two cavities.The frequency of the SM-NPRO was stabilized tothe reference cavity by means of the current-locktechnique. Below unity gain the feedback signal ofthis control loop is proportional to the free-runningfrequency noise of the laser relative to the referencecavity. The error signal of this control loop yields theinformation on the free-running frequency noise of thelaser above the unity gain frequency and the in-loopfrequency noise reduction below unity gain.
The analyzer cavity was then stabilized to theSM-NPRO by feeding back to its PZT, and therebythe analyzer cavity resonance frequency was locked tothe laser frequency by means of the Pound–Drever–Hall method. The feedback and error signals ofthis control loop contain the information on the in-dependently measured (out-of-loop) frequency noiseof the SM-NPRO, when it is stabilized to the ref-erence cavity. Below the unity gain frequency ofthe analyzer cavity control loop the feedback signal tothe PZT of the analyzer cavity is a direct measure of theremaining out-of-loop laser frequency noise relative to
the reference cavity. Above the unity gain of this loopthe error signal can be used as a redundant measurefor the free-running frequency noise of the laser. Thefinesse of the analzyer cavity is F � 4440.16 Withthe round-trip length of the analzyer cavity beinglrt � 42 cm, the free spectral range (FSR) according toFSR � c�lrt is 714 MHz. The corresponding linewidthDn (half-width at half-maximum) is therefore 80 kHz[determined by F � FRS��2Dn�], so that it does notact as an optical low pass at the detection frequencies.A photodetector placed behind a pickoff plate in thelaser beam simultaneously measures the intensitynoise of the free-running and the frequency-stabilizedSM-NPRO. The control scheme is displayed in Fig. 2.
Current lock of the SM-NPRO to the reference cavityleads to a strong in-loop frequency noise reduction ofthe laser with a maximum suppression of more than60 dB at 10 Hz. This in-loop frequency noise reduc-tion is limited by the gain of the control loop lockingthe laser by means of current lock to the referencecavity, not by detection noise. A measurement ofthe frequency noise with the independent analyzercavity shows that the out-of-loop frequency noise issuppressed by approximately 30 dB. At low Fourierfrequencies the out-of-loop frequency noise is limitedby detection noise; for Fourier frequencies of approxi-mately 100 Hz the suppression is gain limited. Bothin- and out-of-loop measurements of the frequencynoise of the SM-NPRO are shown in Fig. 3.
Fig. 1. Schematic of the single-mode laser-diode-pumpedNPRO.
Fig. 2. Stabilization scheme for the single-mode laser-diode-pumped NPRO. PD1–PD3, photodetectors; RC,reference cavity; AC, analyzer cavity; CL, current lockstabilization SM-NPRO to reference cavity; ACL, stabiliza-tion analyzer cavity to SM-NPRO; PBS, polarizing beamsplitter; EOM, electro-optic modulator; PP, pickoff plate;LD, laser diode; IN, intensity noise measurement.
Fig. 3. Free-running, in-loop, and out-of-loop frequencynoise of the SM-NPRO with current lock.
Fig. 4. Intensity noise of the SM-NPRO with and withoutcurrent lock
The intensity noise of the SM-NPRO is measuredwith a photodetector behind a pickoff plate. Figure 4shows the free-runnning intensity noise of the laser aswell as the intensity noise of the laser when it is fre-quency stabilized to the reference cavity by means ofcurrent lock. It is obvious that the free-running in-tensity noise is suppressed in current lock by the sameamount as the out-of-loop frequency noise, relative tothe free-running frequency noise (for low Fourier fre-quencies 1�f f licker noise dominates the spectrum andlimits the suppression). This intensity noise suppres-sion is achieved without any further actuation, evenwithout the detection of intensity noise in the f irstplace. It is indeed a positive effect of the couplingof intensity and frequency noise in the NPRO, as de-scribed in the previous section.
We have demonstrated simultaneous frequency andintensity noise suppression of 30 dB in a single-modelaser-diode-pumped monolithic Nd:YAG NPRO. Thiswas achieved by frequency stabilization of the NPROto a rigid cavity through feedback to the pump current(current lock). Because of the strong noise correlation,simultaneous intensity stabilization resulted withoutfurther actuation.
Furthermore, it is interesting to know whether in-tensity stabilization of the pump source has an effecton the intensity noise or even on the frequency noise ofthe NPRO, as in a conventional NPRO pump intensitystabilization that does not improve the intensity noisecharacteristics of the NPRO. The experiments weresuccessfully conducted by our group.17
Using a master oscillator power amplif ier system(single-mode laser diode and amplifier stage with anoutput power of approximately 500 mW) as a pumpsource promises higher NPRO output power of morethan 200 mW, while being able to use the describedtechnique to simultaneously suppress intensity andfrequency noise. This NPRO can then be used as amaster laser for an injection-locked laser system forgravitational-wave detection.
