Dual Piezo Reference Cavity for Laser Frequency Locking

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Solution Overview

Problem

Existing laser systems face challenges in maintaining a narrow linewidth due to mechanical noise and thermal drift, especially when tuning and scanning the output frequency, which is critical for applications like optical sensors and communications.

Innovation Solution

An external reference cavity with dual piezoelectric crystals, where one crystal is used for scanning and the other for locking, and a reference cavity drive system that independently controls the mirrors to compensate for thermal expansion and reduce noise, allowing precise frequency control and narrow linewidth operation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If optical elements (birefringent filters and etalons) are inserted into the laser cavity for frequency selection, then single frequency operation is achieved, but mechanical noise from these elements broadens the linewidth of the output field

Engineering Contradiction:

Improvefrequency selection precisionVSAvoidlinewidth broadening

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The patent extracts the frequency selection function from intracavity optical elements and relocates it to an external reference cavity. This removes the mechanical noise source from the laser cavity while preserving the frequency selection capability through external locking and scanning mechanisms.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces an external reference cavity as an intermediary system between the laser source and the frequency selection process. This mediator performs the frequency selection function externally, allowing the laser cavity to operate without the noise-inducing optical elements while still achieving precise frequency control.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Adaptability or versatility

If the laser cavity length is scanned to track the mode and change the output frequency, then frequency tuning is achieved, but thermal expansion and mechanical vibrations cause frequency drift and linewidth broadening

Engineering Contradiction:

Improvefrequency tuning rangeVSAvoidfrequency stability

Core Design Contradiction:

Adaptability or versatilityVSStability of the object's composition

Solution Approach 1:

The patent segments the frequency control function into two independent parts: a scanning function implemented by a piezoelectric actuator for frequency tuning, and a locking function implemented by an external reference cavity for frequency stabilization. This separation allows independent optimization of tuning range and stability.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements a feedback mechanism where the external reference cavity provides a stable frequency reference that locks the laser output. The reference cavity's resonant modes serve as a feedback signal to maintain frequency stability despite thermal expansion or mechanical vibrations of the laser cavity.

Inventive Principle:

Principle #23Feedback

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution effectively reduces the linewidth of the laser output to around 50 kHz and minimizes frequency drift, outperforming traditional methods by halving the induced noise and enabling precise scanning and locking of the laser cavity.

Implementation Method 1

an external reference cavity for locking and scanning a laser cavity comprising a first cavity mirror mounted on a first piezoelectric crystal and a second cavity mirror mounted on a second piezoelectric crystal

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

the first and second piezoelectric crystals are mechanically mounted within the reference cavity such that thermal expansion of the first and second piezoelectric crystals acts to move the first and second cavity mirrors in the same direction along a longitudinal axis of the reference cavity

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentEP2862248B1Method and apparatus for locking and scanning the output frequency from a laser cavity

Publication Date: 2017.03.15 M SQUARED LASERS LIMITED

AI summary

A tunable reference cavity (3) with two independently controllable mirrors (15, 16) is disclosed. The mirrors are mounted on respective piezoelectric crystals (18, 19) so that thermal expansion of the piezoelectric crystals moves the cavity mirrors in the same direction along a longitudinal axis of the reference cavity thereby reducing a change of the cavity length. Also disclosed is a system for locking and scanning the output of a laser cavity (2). An error signal is generated between an output of the laser cavity (28) and the transmission (28) of the laser through the external reference cavity (3). A dual piezo-actuated mirror (6b) permits processing of the error signal (26) with separate signal processing circuits (29a, 29b) used to provide an electrical feedback signal to the dual piezoelectric crystal (22, 23b).