Skip to main content
Engineering LibreTexts

9: Noise and Frequency Control

  1. Last updated
  2. Save as PDF
  • Page ID
    44673

    • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

    \( \newcommand{\dsum}{\displaystyle\sum\limits} \)

    \( \newcommand{\dint}{\displaystyle\int\limits} \)

    \( \newcommand{\dlim}{\displaystyle\lim\limits} \)

    \( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)

    ( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\id}{\mathrm{id}}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\kernel}{\mathrm{null}\,}\)

    \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\)

    \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\)

    \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    \( \newcommand{\vectorA}[1]{\vec{#1}}      % arrow\)

    \( \newcommand{\vectorAt}[1]{\vec{\text{#1}}}      % arrow\)

    \( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vectorC}[1]{\textbf{#1}} \)

    \( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)

    \( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)

    \( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)

    \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

    \(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)

    So far we only considered the deterministic steady state pulse formation in ultrashort pulse laser systems due to the most important pulse shaping mechanisms prevailing in todays femtosecond lasers. Due to the recent interest in using modelocked lasers for frequency metrology and high-resolution laser spectroscopy as well as phase sensitive nonlinear optics the noise and tuning properties of mode combs emitted by modelocked lasers is of much current interest. Soliton-perturbation theory is well suited to successfully predict the noise behavior of many solid-state and fiber laser systems [1] as well as changes in group- and phase velocity in modelocked lasers due to intracavity nonlinear effects [5]. We start off by reconsidering the derivation of the master equation for describing the pulse shaping effects in a mode-locked laser. We assume that in steady-state the laser generates at some position \(z\) (for example at the point of the output coupler) inside the laser a sequence of pulses with the envelope \(a(T = mT_r, t)\). These envelopes are the solutions of the corresponding master equation, where the dynamics per roundtrip is described on a slow time scale \(T = mT_R\). Then the pulse train emitted from the laser including the carrier is

    \[A(T, t) = \sum_{m = -\infty}^{+\infty} a(T = m T_r, t) e^{j[\omega_c (t-mT_R + (\tfrac{1}{v_g} - \tfrac{1}{v_p}) 2mL)]}. \nonumber \]

    with repetition rate \(f_R = 1/T_R\) and center frequency \(\omega_c\). Both are in general subject to slow drifts due to mirror vibrations, changes in intracavity pulse energy that might be further converted into phase and group velocity changes. Note, the center frequency and repetition rate are only defined for times long compared to the roundtrip time in the laser. Usually, they only change on a time scale three orders of magnitude longer than the expectation value of the repetition rate.

    • 9.1: The Mode Comb
      This page examines pulse trains from noise-free mode-locked lasers, focusing on time-domain signals and their steady-state solutions as perturbed solitons. It introduces the impacts of carrier-envelope phase shifts and explores pulse train Fourier transforms. Using soliton-perturbation theory, the author derives self-consistent equations for pulse parameters and repetition rates, addressing fluctuations through the nonlinear Schrödinger equation.
    • 9.2: Noise in Mode-locked Lasers
      This page explores the effects of spontaneous emission noise, phase noise, and timing jitter on mode-locked lasers using master equations and Gaussian distribution. It derives optical and microwave spectra, showing the relationship between linewidth and cavity parameters, while noting that timing fluctuations have minimal impact on central comb linewidths. The significance of soliton perturbation theory is highlighted in calculating timing jitter for stretched pulse modelocked lasers.
    • 9.3: Group- and Phase-Velocity of Solitons
      This page explores the Kerr effect's impact on solitons, leading to self-phase shifts and modifications in the nonlinear Schrödinger equation, especially for ultrashort pulses. It highlights how self-steepening affects timing and velocity changes, alters the carrier-envelope phase, and results in frequency shifts in optical combs.
    • 9.4: Femtosecond Laser Frequency Combs
      This page covers techniques for controlling the optical frequency comb of a femtosecond laser by adjusting cavity length and pulse energy through pump power. It explains the f-to-2f interferometry method for measuring the carrier-envelope offset frequency and details a setup using a 200 MHz Ti:sapphire laser with a phase-locked loop (PLL) system. The page also highlights the importance of measuring carrier-envelope phase fluctuations for enhanced precision in optical frequency comb applications.

    This page titled 9: Noise and Frequency Control is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Franz X. Kaertner (MIT OpenCourseWare) via source content that was edited to the style and standards of the LibreTexts platform.