TY - JOUR

T1 - A Generalized Mode-Locking Theory for a Nyquist Laser with an Arbitrary Roll-Off Factor PART II

T2 - Oscillation Waveforms and Spectral Characteristics

AU - Nakazawa, Masataka

AU - Hirooka, Toshihiko

N1 - Funding Information:
Manuscript received July 30, 2020; revised February 3, 2021; accepted March 9, 2021. Date of publication March 15, 2021; date of current version April 2, 2021. This work was supported by the JSPS Grant-in-Aid for Specially Promoted Research under Grant 26000009. (Corresponding author: Masataka Nakazawa.) The authors are with the Research Organization of Electrical Communication, Tohoku University, Sendai 980-8577, Japan (e-mail: nakazawa@riec.tohoku.ac.jp).
Publisher Copyright:
© 1965-2012 IEEE.

PY - 2021/6

Y1 - 2021/6

N2 - In this paper (PART II), we present output waveforms and the corresponding spectrum of a periodic Nyquist pulse train with a roll-off factor α emitted from a mode-locked Nyquist laser. In the first part, the relationship between the optical filter amplitudes H a and H b installed in a Nyquist laser cavity is derived by using the inverse Fourier transformation of filter F 1 (ω) at a low frequency edge and F 2 (ω) at a high frequency edge. We found that the relationship H b = (4/3) H a for α = 0 is changed into the relationship H b = (1(β Ω m/2)) H a for α ne 0, where β =π/(2α ω N), ω N is the zero-crossing frequency and Ω m is the modulation frequency. This relationship is important for describing the entire spectral profile of the optical filter installed in the laser cavity. In the latter part, we report how we succeeded in generating a Nyquist pulse train with an arbitrary α value by employing computer simulations with analytically derived optical filters consisting of F 1 ($ω) and F 2 (ω), H a, and H b. We found that a Nyquist laser cannot always generate an isolated ideal Nyquist pulse train because there is interference between the wings of adjacent Nyquist pulses. We clarify the differences and similarities as regards filter shape and the corresponding waveform in the time domain of a single Nyquist pulse and a periodic Nyquist pulse train in terms of differences in power P, time-domain distributionτ, spectrum S, and filter shape F. We show that a pure Nyquist pulse train can be obtained with the condition α N >10, where differences in P, S, and F are less than 1 %, and we present a useful chart showing how to generate a Nyquist pulse train in the GHz region. N is the number of modes in the low or high frequency region. We investigated the time domain orthogonality g m,n of the Nyquist pulse train from the laser and found that the orthogonality can be maintained although there is a small interference effect on the wing of the Nyquist pulse.

AB - In this paper (PART II), we present output waveforms and the corresponding spectrum of a periodic Nyquist pulse train with a roll-off factor α emitted from a mode-locked Nyquist laser. In the first part, the relationship between the optical filter amplitudes H a and H b installed in a Nyquist laser cavity is derived by using the inverse Fourier transformation of filter F 1 (ω) at a low frequency edge and F 2 (ω) at a high frequency edge. We found that the relationship H b = (4/3) H a for α = 0 is changed into the relationship H b = (1(β Ω m/2)) H a for α ne 0, where β =π/(2α ω N), ω N is the zero-crossing frequency and Ω m is the modulation frequency. This relationship is important for describing the entire spectral profile of the optical filter installed in the laser cavity. In the latter part, we report how we succeeded in generating a Nyquist pulse train with an arbitrary α value by employing computer simulations with analytically derived optical filters consisting of F 1 ($ω) and F 2 (ω), H a, and H b. We found that a Nyquist laser cannot always generate an isolated ideal Nyquist pulse train because there is interference between the wings of adjacent Nyquist pulses. We clarify the differences and similarities as regards filter shape and the corresponding waveform in the time domain of a single Nyquist pulse and a periodic Nyquist pulse train in terms of differences in power P, time-domain distributionτ, spectrum S, and filter shape F. We show that a pure Nyquist pulse train can be obtained with the condition α N >10, where differences in P, S, and F are less than 1 %, and we present a useful chart showing how to generate a Nyquist pulse train in the GHz region. N is the number of modes in the low or high frequency region. We investigated the time domain orthogonality g m,n of the Nyquist pulse train from the laser and found that the orthogonality can be maintained although there is a small interference effect on the wing of the Nyquist pulse.

KW - Fourier analysis

KW - Mode-locked laser

KW - Nyquist pulse train

KW - roll-off factor

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U2 - 10.1109/JQE.2021.3065935

DO - 10.1109/JQE.2021.3065935

M3 - Article

AN - SCOPUS:85102994604

SN - 0018-9197

VL - 57

JO - IEEE Journal of Quantum Electronics

JF - IEEE Journal of Quantum Electronics

IS - 3

M1 - 9378554

ER -