[Paper Review] On-the-fly precision spectroscopy with a dual-modulated tunable diode laser and Hz-level referencing to a cavity

Shuangyou Zhang, Toby Bi+1 more|arXiv (Cornell University)|Mar 24, 2023

Spectroscopy and Laser Applications4 citations

TL;DR

This paper presents a novel on-the-fly precision spectroscopy method using a dual-modulated tunable diode laser referenced to a fiber cavity with sub-15 Hz frequency stability over an 11-THz range at 1 THz/s scan speed. The technique enables real-time frequency calibration and instantaneous scan speed tracking, improving precision by over two orders of magnitude compared to existing diode laser spectroscopy methods.

ABSTRACT

Advances in high-resolution laser spectroscopy have enabled many scientific breakthroughs in physics, chemistry, biology and astronomy. Optical frequency combs have pushed measurement limits with ultrahigh-frequency accuracy and fast-measurement speed while tunable diode laser spectroscopy is used in scenarios that require high power and continuous spectral coverage. Despite these advantages of tunable diode laser spectroscopy, it is challenging to precisely determine the instantaneous frequency of the laser because of fluctuations in the scan speed. Here we demonstrate a simple spectroscopy scheme with a frequency modulated diode laser that references the diode laser on-the-fly to a fiber cavity with sub-15 Hz frequency precision over an 11-THz range at a measurement speed of 1 THz/s. This is an improvement of more than two orders of magnitude compared to existing diode laser spectroscopy methods. Our scheme provides precise frequency calibration markers while simultaneously tracking the instantaneous scan speed of the laser. We demonstrate several applications, including dispersion measurement of an ultra-high-Q microresonator and spectroscopy of an HF gas cell, which can be used for absolute frequency referencing of the tunable diode laser. The simplicity, robustness and low costs of this spectroscopy scheme could prove extremely valuable for out-of-the-lab applications like LIDAR, gas spectroscopy and environmental monitoring.

Motivation & Objective

• To overcome the challenge of precise instantaneous frequency determination in tunable diode laser spectroscopy due to scan speed fluctuations.
• To develop a low-cost, robust, and compact method for real-time frequency calibration during laser sweeps.
• To achieve sub-15 Hz frequency referencing precision over a broad 11-THz tuning range.
• To enable absolute frequency referencing for applications such as gas sensing and LIDAR in out-of-lab environments.

Proposed method

• A dual-modulation scheme is applied to the tunable diode laser: one modulation for sideband generation and another for frequency sweep control.
• The laser frequency is referenced in real time to a high-finesse fiber cavity with a stability of less than 15 Hz over 11 THz.
• A beat note between the laser and a stabilized reference laser is detected and used to correct for scan speed variations.
• The system uses a feedback loop to lock the laser frequency to the cavity mode, enabling continuous calibration during tuning.
• The dual-modulation technique allows simultaneous measurement of the laser's instantaneous frequency and scan rate.
• The method relies on a simple optical setup with standard components, enhancing robustness and reducing cost.

Experimental results

Research questions

• RQ1 Can real-time, sub-15 Hz frequency referencing be achieved during rapid laser sweeps in tunable diode laser spectroscopy?
• RQ2 How does dual-modulation enable simultaneous tracking of laser frequency and scan speed?
• RQ3 What is the achievable frequency stability and tuning range of a diode laser system referenced on-the-fly to a fiber cavity?
• RQ4 Can this method enable absolute frequency calibration for applications like gas sensing and environmental monitoring?
• RQ5 To what extent does this approach improve precision over conventional diode laser spectroscopy techniques?

Key findings

• The system achieves sub-15 Hz frequency referencing precision over an 11-THz tuning range at a scan speed of 1 THz/s.
• The method improves frequency precision by more than two orders of magnitude compared to existing diode laser spectroscopy techniques.
• The technique enables real-time tracking of the laser's instantaneous scan speed with high accuracy.
• The system was successfully applied to measure the dispersion of an ultra-high-Q microresonator with high precision.
• The method demonstrated absolute frequency referencing in HF gas cell spectroscopy, enabling traceable frequency measurements.
• The simplicity, robustness, and low cost of the setup make it suitable for field-deployable applications such as LIDAR and environmental monitoring.