Fast loop active filter for titanium:sapphire laser

Abstract

This invention relates to quantum technology and can be used in experiments with ultracold calcium ions. The objective of the proposed invention is to replace a commercial high-speed PID controller in the fast feedback loop of a Ti:sapphire laser with a modular active filter that converts the error signal into a control signal, thereby simplifying and reducing the cost of the circuit. The technical result is the creation of a modular active filter that controls the fast feedback loop of a Ti:sapphire laser, generating a control signal for an intracavity electro-optical modulator from the error signal. This is achieved by the active filter consisting of several easily replaceable signal processing modules, which are active low-pass filters or amplifiers, the outputs of which are summed in a resistive adder.

Description

[0001] The utility model relates to quantum technology and can be used in experiments with ultra-cold calcium ions.

[0002] The prior art includes the implementation of electronic frequency stabilization units for a continuous single-frequency titanium-sapphire laser, described in the work [TL Boyd and HJ Kimble, “Frequency stabilization of a continuous-wave Ti:sapphire laser”, Optics Letters 16(11), 808-810 (1991)].

[0003] This technical solution relates to optics and is used to stabilize the frequency of a continuous single-frequency titanium-sapphire laser relative to an external Fabry-Perot resonator using the Pound-Drever-Hall method.

[0004] Common with the claimed solution is the use of the Pound-Drever-Hall method for stabilizing a continuous single-frequency titanium-sapphire laser, with feedback implemented using PID controllers.

[0005] The disadvantage of this technical solution is the use of a mirror controlled by a piezoelectric element for stabilization, which limits the speed of the feedback response due to the need to move the laser mirror in space to change the frequency.

[0006] The system described in the scientific dissertation [Armin Winkler, "Frequency Stabilization of a 729 nm Ti:Sa Laser for Qubit Manipulation in Trapped Calcium Ions," Institute of Experimental Physics, Innsbruck, published March 12, 2023] was chosen as a prototype. This system stabilizes the frequency of a titanium:sapphire laser relative to an ultrastable Fabry-Perot resonator. This technical solution relates to laser physics and is used to stabilize a titanium:sapphire laser used for qubit manipulation on trapped calcium ions. In this work, the laser frequency is stabilized by three feedback loops (one fast and two slow) using a single error signal obtained by the Pound-Drever-Hall method. The laser frequency control in the fast feedback loop is implemented using an intracavity electro-optical modulator, while control in slow loops is carried out using piezoceramic elements.The feedback loops differ in their operating speed, signal delay, and gain bandwidth, as well as the possible laser tuning range. The fast feedback loop's electronic control unit, which generates the control signal for the intracavity electro-optical modulator from the Pound-Drever-Hall error signal, is built on a commercial FALC PID controller (from Toptica), which implements the proportional-integrating control principle.

[0007] Common with the claimed technical solution is the use of a continuous single-frequency titanium-sapphire laser, an intracavity electro-optical modulator and a Fabry-Perot resonator with a Pound-Drever-Hall error signal unit, as well as the use of a proportional-integrating control principle in a fast feedback loop.

[0008] The disadvantage of this utility model is the need to use a commercial PID controller, which complicates and increases the cost of the system.

[0009] The distinctive features of the claimed utility model are:

[0010] use of a modular filter-amplifier instead of a commercial PID controller, which converts the error signal into a control signal according to a linear proportional-integrating law;

[0011] The use of a modular filter design allows for the selection and optimization of feedback loop control parameters by quickly replacing individual device modules;

[0012] Limit the bandwidth of the low-side filter amplifier to 10-100Hz, to avoid the possibility of controlling the laser with two integrating circuits when the fast and slow feedback loops operate simultaneously, which may lead to instability in the operation of both loops.

[0013] The technical solution utilizes a modular linear active filter to generate the control signal in the fast feedback loop. This filter generates the control signal from the Pound-Drever-Hall error signal by amplifying and integrating the signal in one or more low-pass filters. The active filter is assembled from individual modules, allowing for the selection and optimization of feedback parameters by selecting and replacing them. To ensure stable simultaneous operation of the fast and slow feedback loops of the Ti:sapphire laser, the filter's passband frequency is limited from below to 10-100 Hz.

[0014] The purpose of the proposed utility model is to replace the commercial high-speed PID controller in the fast binding loop of a titanium-sapphire laser with a modular active filter that implements the conversion of an error signal into a control signal, which makes it possible to simplify and reduce the cost of the circuit.

[0015] The technical result is the creation of a modular active filter that controls the fast feedback loop of a titanium-sapphire laser, generating a control signal for an intracavity electro-optical modulator from an error signal.

[0016] The technical result is achieved due to the fact that the active filter consists of several easily replaceable signal processing modules, which are active low-pass filters or amplifiers, the outputs of which are summed on a resistive adder.

[0017] This circuit allows, on the one hand, to achieve a minimum signal delay, which is necessary for the high-quality operation of a fast feedback loop, and on the other hand, it allows for the selection of parameters by quickly replacing modules.

[0018] The essence of the utility model is explained by figures.

[0019] Fig. 1 - Fast feedback loop active filter of titanium:sapphire laser.

[0020] Fig. 2 - The amplitude noise spectrum of the laser error signal in a band of up to 150 kHz. The abscissa axis shows the frequency in kHz, the ordinate axis shows the spectral power density in µV / √Hz.

[0021] The following positions are indicated in Fig. 1:

[0022] 1 - Pound-Drever-Hall error signal block;

[0023] 2 - fast feedback loop active filter;

[0024] 3 - signal processing modules;

[0025] 4 - resistive adder;

[0026] 5 - intracavity electro-optical modulator of titanium-sapphire laser.

[0027] The proposed device works as follows.

[0028] The Pound-Drever-Hall error signal generated by the Pound-Drever-Hall error signal block 1 is fed to the input of the fast-loop active filter 2. The fast-loop active filter 2 consists of individual signal processing modules 3, which process the error signal and act as active low-pass filters or amplifiers, as well as a resistive summer 4, which sums the signals of the individual modules. The resulting signal is fed to the intracavity electro-optical modulator 5 of the titanium:sapphire laser, controlling the laser frequency. The operating principle of this circuit is based on the fact that a first-order low-pass filter at frequencies well above the cutoff frequency is, in fact, an integrator. An n-order filter, in this case, operates as n integrators in a row.Thus, by combining various filters and amplifiers, it is possible to select a bandwidth function that implements a combination of proportional-integrating modules to achieve laser stabilization. The main difference between a low-pass filter and an integrator is its behavior at low frequencies, as the integrator has infinite gain at zero frequency, and thus its operation causes the error signal to approach zero. However, in this case, the use of multiple feedback loops necessitates their operation being separated by frequency, as otherwise, the presence of integrators in all loops could lead to instability. Implementing feedback through low-pass filters, as opposed to integrators, allows for a natural solution to this problem by setting the filter cutoff frequency at 10-100 Hz.

[0029] This utility model is implemented in the stabilization circuit of a continuous-wave single-frequency Ti:sapphire laser installed in the Ultracold Ion Quantum Computing Laboratory of the Russian Quantum Center. Selecting the optimal parameters of the Ti:sapphire laser's fast feedback loop active filter for effective laser noise suppression enabled stable coupling of the laser to a Fabry-Perot resonator using the Pound-Drever-Hall method. Optimization of the signal processing module parameters resulted in the use of two signal processing modules within the Ti:sapphire laser's fast feedback loop active filter: an active low-pass filter with a gain of 9.4× and a cutoff frequency of 3.4 MHz and an active low-pass filter with a gain of 860× and a cutoff frequency of 540 Hz. This configuration reduced the resulting noise of the laser error signal, which was required to be minimized.The resulting spectrum of the amplitude noise of the laser error signal in the band up to 150 kHz is shown in Fig. 2.

Claims

An active filter for a fast feedback loop of a titanium-sapphire laser, designed with the possibility of being installed between the Pound-Drever-Hall error signal unit and the intracavity electro-optical modulator of a continuous single-frequency titanium-sapphire laser, characterized in that the active filter consists of signal processing modules that are active low-pass filters, the outputs of which are summed on a resistive adder.

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