Devices and methods for coherent detection using chirped laser pulses

Inventors

Vakhshoori, Daryoosh • Blanchard, Romain • Mansuripur, Tobias

Assignees

Pendar Technologies LLC

Abstract

We present here systems and methods for generating a heterodyne signal using the naturally occurring chirp of a pulsed single-mode laser. The electrical square-wave pulse used to drive the laser heats the laser cavity, causing the laser frequency to change or chirp during the emission of the optical pulse. This chirped optical pulse can be split into a chirped signal pulse that interacts with a sample and a chirped reference pulse that interferes with the chirped signal pulse on a detector to produce a heterodyne modulation whose instantaneous phase and amplitude depend on the sample's dispersion and absorption, respectively. The chirp is reproducible, so the heterodyne modulation, instantaneous phase, and/or instantaneous amplitude can be average over many measurements, either with multiple pulses from the same laser or multiple pulses from different lasers, each emitting at a different wavelength.

Core Innovation

Chirp-Delay Heterodyne Spectroscopy (CDHS) uses the naturally occurring chirp of a pulsed single-mode laser to generate a heterodyne modulation without requiring an external modulator. A chirped signal pulse interrogates a sample while a chirped reference pulse interferes on a detector, producing an instantaneous-phase/amplitude heterodyne signal. The heterodyne modulation depends on the sample’s dispersion and absorption, so the instantaneous phase is used for dispersion-related information and the instantaneous amplitude is used for absorption-related information.

The method generates a chirped signal pulse from a single-mode laser in response to an electrical pulse, and uses a chirped reference pulse generated with the same single-mode laser. Because the chirp is reproducible, instantaneous phase/amplitude can be extracted per pulse and averaged over many pulses and/or many lasers. This reduces the requirement for coherent beat stability across pulses, enabling averaging without maintaining coherence between separate measurements.

CDHS is described as robust to practical impairments including speckle, phase jitter, and depolarization. The document describes coherent gain by reducing averaging, demonstrated gas absorption mapping, and retrieval of absorption via the modulation envelope and dispersion via instantaneous phase. It also outlines system architectures using beam splitter reference/signal arms, double-pulse reference timing, time-gated extraction, optional phase dithering, and compatibility with broadband and laser-array implementations.

Claims Coverage

The independent claims cover two aspects: a method and a system. Across the independent claims, the inventive features are organized around chirped signal/reference generation from a single-mode laser driven by an electrical pulse, detection of heterodyne modulation from their interference, and determining dispersion and/or absorption from that modulation.

Together, the independent method and system claims define CDHS-like spectroscopic measurement in which a chirped signal pulse and a chirped reference pulse are generated with the same single-mode laser, the interference creates a heterodyne modulation dependent on dispersion and/or absorption, and dispersion and/or absorption are obtained from the detected heterodyne modulation.

Stated Advantages

Documented Applications