Implementing Pound-Drever-Hall Locking with Red Pitaya for OPO Squeezed Light Generation

Introduction to Squeezed Light in Quantum Optics

Squeezed light is a fundamental resource in modern quantum optics, characterized by reduced quantum noise in one quadrature component at the expense of increased noise in the orthogonal quadrature. This unique property allows measurements that surpass the standard quantum limit and has critical applications in gravitational-wave detection, quantum computing, and quantum communication.

Drawing on insights shared by Aswin Dev, Experimental Physicist at Quanfluence we explore how precise control over the phase and stability of the optical system—typically achieved through cavity-stabilization techniques such as Pound-Drever-Hall (PDH) locking—enables high-quality squeezed light generation.

Role of Optical Parametric Oscillators in Squeezed Light Production

The optical parametric oscillator (OPO) is the core device used to generate squeezed states of light. An OPO consists of a nonlinear crystal — for example, periodically poled potassium titanyl phosphate (PPKTP) — placed inside an optical resonator. When pumped by a laser at frequency ωₚ, the crystal produces pairs of lower-frequency photons (signal and idler) via parametric down-conversion, satisfying energy conservation: ωₚ = ωₛ + ωᵢ.

Key factors determining the squeezing performance include:

• Pump power near threshold
• Cavity escape efficiency
• Detection efficiency
• Phase stability of the cavity

Phase stability is especially crucial: length fluctuations of the cavity must be controlled to fractions of the optical wavelength to maintain optimal squeezing.

How Pound-Drever-Hall Locking Works

The Pound-Drever-Hall (PDH) technique provides a robust method for stabilizing the length of optical cavities with high precision. A laser beam is phase-modulated at a radio frequency to generate sidebands. When the beam reflects from the cavity, the interference between the carrier and sidebands creates a beat signal.

Demodulating this reflected signal yields an error signal with a steep, linear zero-crossing exactly at the cavity resonance condition. This error signal can then be fed back to actuators that adjust the cavity length, keeping it locked to resonance with sub-wavelength precision.

Red Pitaya 125-14 Implementation for PDH Locking

A practical and accessible implementation of PDH locking can be achieved using the Red Pitaya STEMlab 125-14, an open-source FPGA-based measurement and control platform. In this setup, the Lock-In + PID application developed by Marcelo Luda is used to perform the necessary modulation, demodulation, and feedback control digitally.

Figure 1:Interface for the Lock-In+PID application

Figure 2:Basic setup for PDH locking of a bowtie cavity. LD-1550nm Laser, RP- Red pitaya 125-14,PM-Phase Modulator, PD-Photodiode, PAM-Piezo Actuated Mirror, Xtal – Non-linear crystal

Modulation and Demodulation with Lock-In Detection

In this experiment, Out1 of the Red Pitaya generates an RF signal (~18 MHz) to drive the phase modulator, creating the required sidebands on the laser beam entering the OPO cavity.

The reflected light from the cavity is detected by a fast photodiode. This signal is sent to In1 on the Red Pitaya, where the Lock-In application demodulates the signal to produce the PDH error signal.

Feedback Control with PID Loop

The error signal output is processed through a digital PID controller on the Red Pitaya. This output drives a piezo-actuated mirror inside the OPO cavity via Out2. The piezo amplifier extends the control signal’s dynamic range to adjust the cavity length with nanometer-level precision. This active feedback loop keeps the OPO locked to resonance, ensuring stable squeezing conditions.

Lock Acquisition and Resonance Tracking

Since the PDH error signal only provides useful feedback near resonance, the Lock-In + PID software also includes a scanning routine. During lock acquisition, this algorithm sweeps the cavity length until resonance is detected — at which point the PID loop takes over to maintain stable lock automatically.

Experimental Setup Overview

The typical lab setup includes:

• Laser Diode (LD) at 1550 nm
• Phase Modulator (PM) for sideband generation
• Bowtie or ring cavity containing a nonlinear crystal
• Photodiode (PD) to detect reflected light
• Piezo-actuated mirror (PAM) for cavity length adjustment
• Red Pitaya STEMlab 125-14 for signal generation, detection, and control

This digital approach replaces bulkier analog locking systems, offering flexibility, lower cost, and easy reconfiguration for different cavity or laser parameters.

Alt text for images:

• Figure 1: alt="Red Pitaya Lock-In PID control interface for Pound-Drever-Hall locking"
• Figure 2: alt="Block diagram of PDH locking setup for OPO using Red Pitaya 125-14"

Performance and Squeezing Results

A well-implemented PDH lock allows the OPO to operate continuously for extended periods with minimal drift. Modern systems using this method can achieve squeezing levels of 10–15 dB below the shot noise limit, depending on pump power, cavity finesse, and detection efficiency. The Red Pitaya’s reconfigurability and open-source nature make it ideal for prototyping and refining squeezed light experiments in academic or small-scale quantum optics labs.

Conclusion

The Pound-Drever-Hall locking method remains a cornerstone for stabilizing optical cavities in squeezed light generation. The Red Pitaya 125-14 offers a compact, affordable, and flexible solution for implementing this technique digitally. By lowering barriers to high-performance control, platforms like Red Pitaya support research at the frontiers of quantum optics and precision measurement.