How to measure laser chirp rate at single-emitter excitation energies

TL;DR

This work addresses measuring laser chirp (group delay dispersion, GDD) at ultra-low pulse energies where nonlinear metrology fails. It introduces a linear, single-photon–sensitive dispersion metrology based on wavelength-to-time mapping, recording arrival times of spectrally filtered components with SNSPDs and time-tagging to extract $D_{1\lambda}$ and $\text{GDD}$ from a linear arrival-time vs wavelength relationship. The method is demonstrated across a range of dispersions using chirped fiber Bragg and CVBG elements and can be implemented with either a 4$f$ pulse shaper or a tunable filter, achieving quantitative agreement with nominal values at attojoule energies. This approach is robust to power, spectral bandwidth, and wavelength sampling, enabling dispersion characterization for single-emitter spectroscopy, ultralow-power communications, and integrated quantum photonics, thereby bridging ultrafast metrology and quantum technologies.

Abstract

We present a simple and direct method for measuring laser chirp rate, i.e., group delay dispersion (GDD) of ultrashort laser pulses at power levels compatible with single-quantum-emitter excitation. Traditional pulse characterization techniques rely on nonlinear optical processes that require high peak powers, making them unsuitable for the attojoule-to-femtojoule regime relevant to quantum photonics. Our approach utilizes a wavelength-to-time mapping method in which the arrival times of spectrally filtered components of a broadband pulse are recorded using a superconducting nanowire single-photon detector and correlated via a high-resolution time-tagging system. The resulting linear relationship between wavelength and arrival time directly yields the dispersion parameter and, subsequently, the GDD. Beyond single-emitter excitation, this technique can be applied in areas such as single-photon spectroscopy, ultralow-power optical communications, and time-domain quantum control, where linear and non-destructive dispersion characterization is essential.

Figures (5)

• Figure 1: Concept: Representation of transform-limited and chirped laser pulses in spectral and temporal domains. The former has flat phase across the spectrum, while the latter carries a parabolic spectral phase. The corresponding wavelength-dependent arrival times are plotted in the bottom panel.
• Figure 2: Experimental scheme:(a) A broadband laser source is directed through a wavelength separator, to tune the peak wavelength $\lambda_0$ and spectral width $\Delta \lambda$ of the spectral component measured using the spectrometer. The laser source triggers the reference start signal and the chirped (i.e., time-delayed) spectral components trigger the stop signals at the arrival time recorder consisting of single-photon detectors (here: superconducting nanowire single-photon detector, SNSPD) and time taggers, (b) chosen methods of wavelength separation, (c) dispersion methods: CF/VBG: chirped fiber/volume Bragg gratings.
• Figure 3: Results of the time-of-arrival measurement as a function of (a) laser power, (b) bandwidth. The measurement sets are vertically shifted for better visibility. (c) Results of all dispersion measurements. In all three cases, the filled circles denote data points and solid lines denote the linear fit, from which the GDD values are computed.
• Figure 4: Results of time-of-arrival dispersion measurement with (a) pulse shaper, and (b) tunable bandpass filter as wavelength selector device.
• Figure 5: (a)-(c) Results of time-of-arrival dispersion measurement as a function of spectral bandwidth and number of wavelength samples (d) Summary of (a)-(c), i.e., computed uncertainty as a function of wavelength samples for various spectral bandwidths.