Linewidth Narrowing of Mutually Injection Locked Semiconductor Lasers with Short and Long Delay

Abstract

A simple and effective approach to semiconductor laser linewidth narrowing via mutual injection locking is proposed and demonstrated in both short and long delay regimes. A theoretical analysis is presented to investigate the linewidth behavior of semiconductor lasers under mutual injection locking. Experimental demonstrations in short and long delay regimes are implemented by integrated devices and a fiber link system, respectively. Locking condition and dependence of laser linewidth on coupling parameters in both regimes are studied, confirming mutual injection locking as a practical method for linewidth narrowing. For the short-delayed integrated lasers, a linewidth narrowing factor of 13 is demonstrated and sub-MHz linewidth is achieved, while for the long-delayed lasers coupled by fiber link, the intrinsic linewidth is reduced to sub-100 Hz.Recent research centers on the realization techniques of self-injection, such as low loss Si [11,12] or Si 3 N 4 [13,14] ring resonator, fiber components based feedback circuit [15,16], assembled confocal Fabry-Perot (FP) cavity [17] and CaF 2 cavity [18]. These structures rely on high Q cavity or multiple mode selection components.In this work, we propose and demonstrate a simple but effective approach to linewidth narrowing based on mutual injection locking (MIL) of semiconductor lasers. Previous research on mutual coupling mainly focuses on the nonlinear dynamics [19,20], which can be exploited to realize random sequences generation [20], chaos synchronization [21,22], modulation characteristics enhancement [23] and photonic microwave generation [24]. In addition, it is also used for coherent combining [25,26] and lasing threshold reduction [27]. However, to the best of our knowledge, there have been relatively few reports on laser linewidth or frequency noise characteristics upon mutual injection locking.A systemic study on the linewidth narrowing mechanism, locking condition and frequency noise reduction is carried out in the investigation. Modified mutual coupling rate equations incorporating Langevin noise sources are presented to analyze the mechanisms for frequency noise reduction of mutually injection locked lasers. Previous experiments indicate that laser behaves differently for short delay (τ d < 1/f RO ) and long delay (τ d > 1/f RO ) [19,20,28], where τ d is the coupling delay time and f RO represents the relaxation oscillation frequency. Therefore, experiments are carried out in both regimes: monolithically integrated mutually coupled Distributed Feedback(DFB) lasers are employed to investigate frequency noise performance in short delay regimes, while a fiber link based system is adopted for the study of long delay regimes. Locking condition and the dependence of laser frequency noise on system parameters are investigated in both regimes, confirming mutual injection locking as a practical method for linewidth narrowing.This paper is organized as follows. In Section 2, a theoretical model is presented to clarify the frequency noise suppression in mutually in...

Modulation Bandwidth Enhancement of Monolithically Integrated Mutually Coupled Distributed Feedback Laser

“…The research's detailed enhanced modulation performances are listed in Table 1, where f R is the resonance frequency and f 3dB is the 3-dB modulation bandwidth of the lasers. The enhancement of resonance frequency has also been reported with simultaneous improvements of other laser characteristics, including nonlinear distortion suppression [19], relative intensity noise reduction [23], chirp reduction [24], and linewidth narrowing [27]. Moreover, a modulation bandwidth enhancement of 37 GHz and an open eye of 36 Gb/s have been reported in mutually coupled vertical-cavity surface-emitting laser (VCSEL) arrays [25].…”

“…Several integrated mutually coupled (IMC) lasers are employed to achieve MIL [19][20][21][22][23][24][25][26]. The PPR effect on the IMC lasers is attributed to the red-shift cavity mode due to optical injection from one of the laser sections [27]. The research's detailed enhanced modulation performances are listed in Table 1, where f R is the resonance frequency and f 3dB is the 3-dB modulation bandwidth of the lasers.…”

“…Therefore, it is important to ensure that IMC lasers work under a stable MIL state, as this enables the modulated laser section to work at a higher bias and helps increase the CPR frequency and mitigate its roll-off effect. Moreover, the cavity parameters of IMC lasers need to be well-designed to achieve an appropriate PPR frequency for modulation response enhancement [27]. Additionally, the PPR peak should not be too far from the CPR peak, or there will be a significant gap between the two resonance peaks, which is detrimental to broadband communication [8].…”

Técnica de Aquisição Off-Line da Coerência em Onda Contínua de Lasers Semicondutores com Cálculo do Tempo de Atraso

“…Este trabalho propõe uma forma para se resolver esse problema. Entretanto, devem ser citadas novas tecnologias alternativas para determinação da largura de linha, incluindo-se trabalhos em redução da largura de linha [31][32][33][34].…”