Digital laser frequency and intensity stabilization based on the STEMlab platform (originally Red Pitaya)
aa r X i v : . [ phy s i c s . i n s - d e t ] S e p Digital laser frequency and intensity stabilization based on the STEMlab platform(originally Red Pitaya)
T. Preuschoff, ∗ M. Schlosser, and G. Birkl † Institut f¨ur Angewandte Physik, Technische Universit¨at Darmstadt,Schlossgartenstr. 7, 64289 Darmstadt, Germany (Dated: September 2, 2020)We report on the development, implementation, and characterization of digital controllers forlaser frequency stabilization as well as intensity stabilization and control. Our design is based on theSTEMlab (originally Red Pitaya) platform. The presented analog hardware interfaces provide allnecessary functionalities for the designated applications and can be integrated in standard 19-inchrack mount units. Printed circuit board layouts are made available as an open-source project.[1, 2]A detailed characterization shows that the bandwidth (1 .
25 MHz) and the noise performance of thecontrollers are limited by the STEMlab system and not affected by the supplementary hardware.Frequency stabilization of a diode laser system resulting in a linewidth of 52(1) kHz (FWHM) isdemonstrated. Intensity control to the 1 × − level with sub-microsecond rise and fall timesbased on an acousto-optic modulator as actuator is achieved. This article appeared in Rev. Sci. Instrum. 91, 083001 (2020) and may be found athttps://doi.org/10.1063/5.0009524. It may be downloaded for personal use only. Anyother use requires prior permission of the author and AIP Publishing.INTRODUCTION
Complex experiments in quantum optics and photon-ics often involve a multitude of laser systems that areactively stabilized in frequency and intensity. For suchapplications, digital controllers are favorable since theyoffer a wide range of control parameters without hard-ware modifications. The quality of the engaged controlelectronics is crucial for the performance of the entirelaser system.We present versatile and cost efficient digital con-trollers for laser frequency and intensity stabilization.[1,2] Both architectures are based on the STEMlab 125-14board.[3] The STEMlab platform has already been suc-cessfully applied to control tasks in optical experiments,such as optical phase locking, [4, 5] laser frequency combstabilization,[6] second harmonic generation,[7] or as alock-in amplifier.[8] In our work, the STEMlab hardwareis embedded in two different analog interfaces that fa-cilitate the application to a considerable variety of lasersystems. We provide a characterization of both systems.As performance benchmarks, frequency stabilization ofan external cavity diode laser system (ECDL) and inten-sity control based on an acousto-optic modulator (AOM)as actuator are presented.
HARDWARE DESIGN
The design comprises ready-to-use open-source printedcircuit board (PCB) layouts[1, 2] that can be integratedin standard 19-inch rack mount units. The STEMlabboard is directly mounted on the PCB which containsa step-down regulator (Linear Technology LT8610) for its power supply. The STEMlab system includes twofast 14-bit analog-to-digital converters (ADC) and twofast 14-bit digital-to-analog converters (DAC) controlledby a field programmable gate array (FPGA). In orderto obtain optimal performance of the DAC outputs, itis essential to deactivate the noisy internal DC offsetcircuits.[7, 9] This shifts the output voltage range from ± .
25 MHz ( π/ ± ). ViaPyRPL, the following locking scheme is implemented: Afirst PI (PI ) directly drives the fast output of the con-troller via one of the DACs (Out ). The signal acts ona fast actuator, e.g. the laser current. This PI is used (a) Frequency controller (b) Intensity controller S T E M l ab + In Out Out PI PI OffsetPiezo driverFastout Piezoout Piezomon.Error mon.Error signal I npu t s t age O u t pu t s t age Buffer In Out PI S T E M l ab Control out PD mon. I npu t s t age O u t pu t s t age HoldPhoto-detectorReferencelevel Error mon. - Buffer C hanne l ( C hanne l i den t i c a l ) P i e z o s t age FIG. 1: Block diagrams of controllers for (a) laserfrequency stabilization and (b) laser intensitystabilization and control.to stabilize the laser frequency to the error-signal zerocrossing. For high-accuracy frequency stabilization it isnecessary to reduce the effect of the finite DAC reso-lution by sufficient attenuation of the output signal atthe actuator input. This restricts the control range ofthe fast actuator. In order to compensate for slow driftswith large amplitude, otherwise leading to saturation ofthe fast output, a second control loop is implemented:The output of the first PI (PI ) also serves as input fora second PI (PI ) that retains the mean output of PI at zero by acting on a slow actuator with large controlrange. For this purpose, its output (Out ) is connectedto a low-noise piezo stage driving a slow piezo actuator.In the piezo stage, the low-pass filtered signal (cut-offfrequency 10 kHz) is amplified and offset in order to op-erate over the full voltage range of the analog interface( ±
10 V). A 250 mA output-buffer (Texas InstrumentsBUF634) allows for driving large-capacitance piezo actu-ators. The piezo output series resistance of 4 . ±
10 V) which effectively ex- tends the controller input voltage range without reducingthe ADC resolution. Additional monitoring outputs forthe amplified photodetector signal and the error signalare available. PyRPL is used to implement a PI drivinga fast control output via an output stage. It consists ofa buffer that can drive a 50 Ω load up to the increasedmaximal output voltage (2 V). An optional sample andhold feature is realized via PyRPL: One of the digitalinput/output pins (DIO) sets the PI gains to zero whenpulled high, resulting in a constant control output aftera delay of 150(10) ns. ) U H T X H Q F \ > + ] @ í í í 1 6 ' [ V r m s / √ H z ] , Q W H Q V L W \ F R Q W U R O O H U L Q S X W V W D J H ) U H T X H Q F \ F R Q W U R O O H U L Q S X W V W D J H 6 7 ( 0 O D E $ ' &