A High-Performance, Low-Cost Laser Shutter using a PiezoelectricCantilever Actuator

W. Bowden,1, 2 I. R. Hill,1 P. E. G. Baird,2 and P. Gill11)National Physical Laboratory, Teddington, Middlesex, UK2)University of Oxford, Oxford, Oxfordshire, UK

(Dated: 20 September 2016)

We report the design and characterization of an optical shutter based on a piezoelectric cantilever. Comparedto conventional electro-magnetic shutters, the device is intrinsically low power and acoustically quiet. Thecantilever position is controlled by a high-voltage op-amp circuit for easy tuning of the range of travel, andmechanical slew rate, which enables a factor of 30 reduction in mechanical noise compared to a rapidlyswitched device. We achieve shuttering rise and fall times of 11 µs, corresponding to mechanical slew ratesof 1.3 ms−1, with an timing jitter of less than 1 µs. When used to create optical pulses, we achieve minimumpulse durations of 250 µs. The reliability of the shutter was investigated by operating continuously for oneweek at 10 Hz switching rate. After this period, neither the shutter delay or actuation speed had changedby a notable amount. We also show that the high-voltage electronics can be easily configured as a versatilelow-noise, high-bandwidth piezo driver, well-suited to applications in laser frequency control.

PACS numbers: 65.40.De

I. INTRODUCTION

Optical shutters are ubiquitous in atomic physics andlaser based experiments. The key figures of merit forevaluating the performance of these devices are: shut-ter speed, optical extinction and transmission, repro-ducibility of actuation delays, longevity of operation,and noise—both mechanical (acoustic, vibration) andelectromagnetic. Furthermore, for transportable sys-tems with the potential for space based applications, lowpower consumption and operation in zero gravity is nec-essary. Inevitability, cost considerations must also be ad-dressed, particularly for complex experiments requiringmany shutters.

The majority of commercial and home built shuttersuse solenoid actuators1 which are often relatively slowand noisey. With careful design these issues can bemitigated2,3, however, such actuators require significantpower to maintain shutter arm position and when actu-ated rapidly can broadcast electromagnetic noise to sur-rounding electronics. These drawbacks limit the utilityof solenoid based shutters for many applications such ascompact transportable experimental systems where elec-tronics must be positioned in close proximity and con-sume little power. If sub-microsecond shutter speeds arerequired, acousto-optic4 and electro-optic devices are of-ten used, although typically at the expense of increasedinsertion loss and reduced extinction. For our applica-tion in optical atomic clocks, incomplete extinction canlead to sizeable Stark shifts of the clock transition, ef-fecting clock accuracy. Although piezo based shuttershave been demonstrated previously, they are typicallydiscounted as useful optical shutters due to a limitedactuation range leading to only partial extinction andtransmission5,6. Our cantilever design leverages over adistance of a few centimetres the peizo’s relatively smallactuation range such that the shutter moves on the order

of a millimetre—allowing it to extinguish comfortably ortransmit a focused beam. After writing this paper, theauthors became aware of a similar shutter design usingcantilever piezos7. We build upon this work by present-ing a complete mechanical and electrical design exhibit-ing improved performance.

II. SHUTTER DESIGN

A. Mechanical

The shutter, shown in Figure 1, consists of a thin black-coated foil flag glued to the tip of a cantilever piezoelec-tric actuator8 positioned at the focal point between twoaspheric lenses. The piezoelectric cantilever is either sol-dered, or silver epoxy bonded, to a small PCB which issecured inside an aluminum enclosure measuring 2×3×5cm3. The enclosure has two threaded apertures to holdtwo aspheric lenses for 1) focusing the input beam to theshutter flag and 2) re-collimation on exit. In our ap-plication the shutters are typically placed immediatelybefore optical fibres and provide additional freedom viathe re-collimation lens to optimise fibre input-couplingefficiency. In operation, the piezo receives a high voltagebias (150V) and control voltage (0 to 150V) through twoMCX connectors. Care should be taken when solderingthe cantilever to prevent thermal damage of the piezoceramic.

B. Electronics

The control circuit, shown in Figure 2, uses a high-voltage op-amp9 (U2) powered by a high-voltage DC-DCconverter10 (not shown). Given the low power consump-tion of the device, we can power up to 12 shutter boards

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FIG. 1. The assembled piezo cantilever shutter. The shutterenclosure was designed to have a minimal footprint and offervarious mounting configurations. The shutter flag (top right)can be imaged using the input lens to ensure it is cleanly cutwithout any debris.

from a single high-voltage supply. A TTL controlled ana-logue switch (IC1A) selects for amplification by U2 one oftwo tuneable control voltages (RV1 and RV3) to set theopen and close positions of shutter. This allows conve-nient tuning of the cantilever position and range of travel.Prior to be being amplified, these control voltages arelow-pass filtered to effect a reduction in the mechanicalslew rate of the shutter arm (determined by RV2 and C1).By limiting the range of travel and mechanical slew rateof the cantilever, we reduce vibrations, decrease shut-tering delays, and through reducing stress on the piezopostulate an improvement in lifetime of the device. Toincrease utility of the control electronics the circuit canbe easily reconfigured using on-board jumpers to serve asa tunable piezo driver for use in, for example, laser fre-quency stabilization applications—where both easy tun-ing and fast modulation of the output voltage, togetherwith low voltage noise, are required. In this configura-tion, the output voltage is set using a trim-pot whichdivides down a stable voltage reference (U1)—the samereference is used to derive the two shutter set points. Thisvoltage is filtered by an actively compensated low-passfilter (using U3) with pole determined predominantly byR9 and C3 (f−3dB = 1/2π R9 ·C3), with R10 and C2 re-quired for stability (satisfying R10·C2 > 2 R9·C3) beforebeing combined in U4 with an external control voltageat P1. This control voltage allows for fast modulation of

the output voltage, limited by the power available to thehigh-voltage circuit.11

Shutter SetpointControl

Piezo Driver Setpointand Modulation

10V Reference High VoltageOp-amp

FIG. 2. Schematic of the high voltage driver used to powerthe shutters. By switching jumper pairs (labelled ‘MOD’ and‘SHUTTER’), the circuit can be configured to act as either ashutter driver or as a general purpose low-noise piezo driver.For shutter operation, the shutter is toggled by applying aTTL signal to ‘P1’, with the open and closed positions set by‘RV1’ and ‘RV3’. The slew rate is controlled by ‘RV2’. Foroperation as a general purpose piezo driver, the high volt-age output level is set by ‘RV4’ and can be modulated usingthe input ‘P1’ with a sensitivity controlled by ‘RV3’. In thisapplication, the output impedance ‘R1’ is typically loweredfrom 1 kΩ to 100 Ω. ‘U1’ is a 10V precision voltage reference(LT1021-10V), ‘U2’ is a high voltage op-amp (PA441), ‘U2’and ‘U3’ are single supply low noise op-amps (LT1006), and‘IC1’ is a TTL controlled analogue switch (ADG419). Notshown are voltage regulators for the ±12V rails and any de-coupling or compensation capacitors for the op-amps or volt-age references. These values can be found in their respectivedata sheets. Also not shown is the high voltage supply neededto power ‘U2’ and bias the piezo.

III. PERFORMANCE

A. Shutter Speed and Extinction

To test the shutter, a collimated Gaussian beam from a655 nm laser was focused to an 11 µm 1/e2 beam waist bythe input lens12 and aligned on to the shutter flag. Thepiezo cantilever has a manufacturer’s quoted mechanicalresonance at approximately 200 Hz which is excited inrapid shutter actuation leading to damped oscillations,

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or a settling period, of the cantilever. As a result, the‘closed’ and ‘open’ positions must be set slightly beyondthe minimum required distance to prevent these excur-sions from modulating the optical transmission as thecantilever settles. Reducing the mechanical slew ratehelps to suppress these oscillations, but this comes atthe expense of switching speed. Figure 3 shows the ac-tuation of the shutter for the slowest and fastest shut-tering speeds, corresponding to rise and fall times (mea-sured period between 10%–90% transmission points) of11–33 µs. Given the beam size, this corresponds to me-chanical slew rates between 0.5–1.3 ms−1 making it com-parable to solenoid based shutters1,2. The transmission islimited to approximately 95% by the anti-reflection coat-ing of the two lenses while the extinction when closed isgreater than 100 dB. An exact measurement of the ex-tinction was limited by the noise floor of the photodiodeused in the test setup. Additionally, the shutter can beopened momentarily to produce short optical pulses withdurations as short as 250 µs. The shutter can be easilymodified to produce much shorter pulses by cutting a V-shaped notch in the shutter flag13 such that the beam isboth transmitted and extinguished during a single pass.In such a scheme the pulse duration is easily controlledby tuning the mechanical slew rate.

1 0 1 2 3 4Time (ms)

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FIG. 3. (a) The response of the shutter to a TTL controlsignal at time t=0 ms and t=2 ms. The shutter ‘open’ and‘close’ delays are 0.7 ms and 0.9 ms, respectively. (b) A photo-diode signal showing the rise and fall times, as defined by the10%–90% transmission crossing points, for the shutter withthe fastest (solid line) and slowest (dotted line) slew rates,set by ‘RV2’ (Figure 2). In the slowest case, with the lowestvibration, the switching time is approximately 30 µs while forthe faster setting it is 11 µs. This corresponds to mechanicalslew rates of between 0.5-1.3 ms−1. (c) The profile of a sub-millisecond pulse produced by the shutter.

B. Durability and Reproducibility

To test the durability of the shutter we operated a de-vice at a 10 Hz switching rate for one week (6 millioncycles). After this period the shuttering speed and delayhad not differed from the values measured prior to test-ing. Recently we operated 10 shutters consistently forthree weeks at a constant switching rate of 2 Hz. Again,we observed no change in shutter behaviour throughoutthis extended period of operation.

Variation in the shot-to-shot transit of the shutter ref-erenced to the TTL control pulse was measured for 1000switching cycles. Figure 4 shows the timing jitter in the50% transmission point to be less than 1 µs. The typicalactuation delay is between 0.75 ms and 1.25 ms and istuned by setting the limits of the piezo actuation rangeand slew rate.

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0

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FIG. 4. An eye-plot showing transmission of the test beamduring transit of the opening shutter for one thousand cycles.A histogram of the time to reach 50% transmission, with meanremoved, is shown in the inset. The timing jitter is less than1 µs.

C. Acoustic Noise and Vibration

An accelerometer was used to measure vibrationscaused by the shutter actuation, see Figure 5. In compar-ison to operating the shutter without any compensation,our piezo driver board can reduce the vibrations by a fac-tor of 30 by limiting the range of travel and mechanicalslew rate. When properly configured, the piezo cantileverbased design generates significantly less vibration in com-parison to our previously used solenoid based shutter, seeinset Figure 5. The shutter is acoustically quiet.

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FIG. 5. Mechanical vibrations measured in the immediatesurroundings of the shutter rigidly mounted to an optical ta-ble. A factor of 30 reduction is achieved by limiting the rangeof travel and mechanical slew rate of the actuator. Slowingthe shutter and limiting its range resulted in approximatelyequal parts reduction in vibration. For comparison, the mea-sured vibration of a solenoid based shutter is shown in theinset.

D. Piezo Driver Electrical Noise

As described in section II B, the control circuit is easilyconfigured for use as a general purpose piezo driver withhigh modulation bandwidth, making it ideal for control-ling laser cavities or tunable Fabry-Perot resonators. Insuch applications it is desirable for the driver to exhibitlow voltage noise. For this configuration, the voltagenoise power spectral density of the high-voltage outputis shown in Figure 6. The low noise properties are intrin-sic to the high voltage op-amp and are not a result of anyexcessive filtering which can limit the modulation band-width of the driver. The small signal modulation band-width of the driver was measured to be 160 kHz (−3 dBpoint for a 1 Vpp output modulation with 1 MΩ load).When driving a piezo, a pole is added with a time con-stant set by the capacitance of the piezo and the outputimpedance of the circuit. For example, with an outputindependence of 100 Ω and piezo capacitance of 10 nF,the −3 dB point dropped to approximately 100 kHz. Ifsuch a piezo driver were to be used in open loop to ac-tuate the length of an external cavity diode laser, thecontribution of this voltage noise to the laser’s spectralnewline can be estimated using the relation between thefrequency noise spectrum and the laser line shape14. As-suming a frequency tuning sensitivity of 50 MHzV−1 forthe laser, the contribution of this voltage noise to theFWHM linewidth with an observation time of one sec-ond would be approximately 7 kHz. In reality this isan upper bound on the estimated broadening as the fi-nite mechanical bandwidth of the piezo-actuated retro

reflector filters the transfer of voltage noise to frequencyexcursions.

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FIG. 6. Power spectral density of the noise on the out-put of the high voltage driver exhibiting a noise floor below1 µVHz−1/2 (1 MΩ load).

IV. DISCUSSION

We have demonstrated a compact, high-performanceoptical shutter based on a cantilever piezoelectric actu-ator. Our characterisation supports these claims show-ing fast shuttering with minimal timing jitter and acous-tic noise for long periods of operation. Previously,such a piezo based shutter solution may have appearedunattractive due to the requirement of switching highvoltages. However, with the current availability of high-performance, high-voltage, low-noise op-amps, the taskof circuit design is a less onerous one. Furthermore, weshowed how the control circuit can be easily modified foruse as a general purpose piezo driver with noise proper-ties rivaling those of commercial offerings and with ex-tended modulation bandwidth.

V. ACKNOWLEDGMENTS

The authors give acknowledgment to Peter Nisbet-Jones and Steven King for the initial work investigat-ing piezo cantilever shutters and helping with the vi-bration noise measurements. Acknowledgment also goesto Richard Hobson for help with shutter testing andCharles Baynham for helpful suggestions regarding themanuscript. This work was done under the auspices ofboth the Future Atomic Clock Technology Initial Train-ing Network funded by the European Union and the UKNMO program.

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