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Chinese Physics Letters

General Laser Intensity Langevin Equation in a Single-Mode Laser Model

Ke Sheng-zhi, Cao Li, Wu Da-Jin, K.L. Yao

Published Feb 8, 2001• 19 citations
Publisher

Abstract

A two-dimensional single-mode laser model is investigated, with cross-correlations between the real and imaginary parts of the quantum noise as well as the pump noise. The general closed form of the laser intensity Langevin equation (GILE) is obtained under a stable locked phase resulting from the cross-correlation λ q between the real and imaginary parts of the quantum noise. Because of the presence of a new term containing λ q , we can unify the two opposite intensity Langevin equations which correspond to the two special cases for |λ q |→0 and |λ q |→1 in the GILE. It is expected that the transient and stationary properties of the laser model can be changed qualitatively when λ q varies.

DOI
10.1088/0256-307x/18/3/320

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A single-mode laser noise model driven by quadratic pump noise and quantum noise with cross-correlation between the real and imaginary parts of the noises is proposed. The approximate Fokker-Planck equation (AFPE) of the model for the laser phase and the laser amplitude is derived. It is found that the laser phase is controlled intensively by the correlation between the real and imaginary parts of the pump noise and that of the quantum noise. The correlation between the real and imaginary parts of quantum noise ${\ensuremath{\lambda}}_{q}$ tends to lead the laser phase to be locked at some values and the correlation between the real and imaginary parts of the pump noise ${\ensuremath{\lambda}}_{p}$ tends to destroy or confine the laser phase lock. Quantitative results are presented and discussed in detail. As an important application of the above-mentioned results, we take a phase lock approximation to get a Langevin equation for the laser field amplitude and an AFPE of the laser intensity.
We study the effects of correlations between the real and imaginary parts of quantum noise on the intensity fluctuation for a single-mode laser. The analytic expressions of the intensity correlation function C (τ) and the associated relaxation time T c in the case of a stable locked phase resulting from the cross-correlation λ q between the real and imaginary parts of quantum noise are derived by means of projection operator method. Based on numerical computations it is found that the presence of cross-correlations between the real and imaginary parts of quantum noise causes the intensity fluctuation to increase. A slowing-down phenomenon exists in the sense that T c increases as a function of |λ q |. Thus the decay of intensity fluctuation becomes slower.
We study the effects of correlations between the real and imaginary parts of quantum noise on the intensity fluctuation for a single-mode laser. The analytic expressions of the intensity correlation function C(r) and the associated relaxation time Tc in the case of a stable locked phase resulting from the cross-correlation λq between the real and imaginary parts of quantum noise are derived by means of projection operator method. Based on numerical computations it is found that the presence of cross-correlations between the real and imaginary parts of quantum noise causes the intensity fluctuation to increase. A slowing-down phenomenon exists in the sense that Tc increases as a function of |λq|. Thus the decay o[ intensity fluctuation becomes slower.
This paper studies the effects of cross-correlations between the real and imaginary parts of quantum noise on the laser intensity in a saturation laser model. It derives the analytic expressions of the intensity correlation function C(τ) and the associated relaxation time T (C) in the case of a stable locked phase resulting from the cross-correlation λ q between the real and imaginary parts of quantum noise. Based on numerical computations it finds that the presence of cross correlations between the real and imaginary parts of quantum noise slow down the decay of intensity fluctuation, i.e., it causes the increase of intensity fluctuation.
By means of the unified coloured noise approximation and phase lock, we study in this paper the stationary intensity distribution of the single-mode laser cubic model driven by coloured pump noise with cross-correlation between the real and imaginary parts of the quantum noise. We present a thorough discussion of how the cross-correlation λ q between the real and imaginary parts of the quantum noise and the self-correlation time τ of the pump noise determine the behaviours of the mean intensity ⟨ I ⟩ and variance λ 2 (0) for both below and above the threshold operation and many new phenomena are discovered. When the laser is operated above the threshold, increasing the cross-correlation intensity λ q makes the ⟨ I ⟩–τ curves exhibit a ``re-entrant phase transition''. Whether the laser is above or below the threshold, the whole ⟨ I ⟩–τ curve moves down as λ q increases; however, when λ q = 1 (perfect cross-correlation), the curve abruptly runs up. A nonzero value of cross-correlation time τ (τ≠0) leads to the entire λ 2 (0)– p ′ curve being suppressed (here p ′ is the pump noise intensity). This indicates the increasing precision of the laser beam due to the existence of the self-correlation time τ. The behaviour of the λ 2 (0)– p ′ curve as a function of λ q is similar to that of the ⟨ I ⟩–τ curve against λ q : that is, whether the laser is above or below the threshold, the λ 2 (0)– p ′ curve moves up as λ q increases; however, when λ q = 1, the curve suddenly moves down. Through the study in this paper, we can find a way to optimize for output laser intensity.
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