The variations and trade-off between single-sided-band (SSB) phase noise and supermode noise (SMN) suppression ratio of an actively mode-locked erbium-doped fiber laser (EDFL) with intra-cavity semiconductor optical amplifier (SOA) based high-pass filter are discussed. The insertion of an SOA increases the SMN suppression ratio of the EDFL from 26 dB to 41.4 dB, however, the SSB phase noise at 100 kHz offset frequency from carrier is concurrently degraded from -114 dBc/Hz to -96 dBc/Hz. Such an operation also causes a broadening in the EDFL pulsewidth from 36 ps to 130 ps. The insertion of an optical bandpass filter (OBPF) further reduces the SSB phase noise to -110 dBc/Hz and improves the SMN suppression ratio to 43 dB. Theoretical simulation interprets that the optimized operation of the SOA based high-pass filter at nearly transparent current condition is mandatory to achieve a better SMN suppression ratio and minimize the SSB phase noise of the mode-locked EDFL without sacrificing its output pulsewidth.
The relationship between the backward optical injection waveform and the mode-locked pulse shape of semiconductor optical amplifier fiber laser (SOAFL) is studied. The SOA plays both the roles of a gain medium and an optically controlled modulator in this work. The injected optical comb-like bright and dark pulse-train with 60-ps pulsewidth was generated using a Mach-Zehnder intensity modulator (MZM) which DC-biased voltage of 1.7 V and 7.2 V, respectively. The backward injection of optical dark pulse-train results in a wide gain-depletion width (and a narrow gain window) within one modulation period, which is necessary for perfect mode-locking the SOAFL. In opposite, the backward injection of short optical pulse of bright optical pulse-train only causes a less pronounced gain-depletion effect. Such a broadened gain window can hardly initiate the mode-locking process. The backward comb-like dark pulse-train modulation is much easier to initiate harmonic mode-locking in the SOAFL than the bright pulse-train or sinusoidal-wave injection, which generates pulsewidth as short as 15 ps at 1 GHz. After propagating through 195m-long dispersion-compensating fiber, the pulsewidth of the mode-locked SOAFL can be linearly compressed to 13.5 ps. The linewidth and time-bandwidth product of the compressed SOAFL pulses are 1.78 nm and 0.8, respectively. The pulsewidth can further be nonlinearly compressed by using a 4695m-long single-mode fiber. The shortest mode-locked SOAFL pulsewidth of 3.5 ps at repetition frequency of 1 GHz by using cross-gain modulation technique is reported for the first time.
The wavelength dependent mode-locking performances of a SOAFL under the backward optical injection via a sinusoidal-modulated distributed feedback laser diode (DFBLD) at 1 GHz repetition rate are characterized. The backward optical injection has to be sufficiently high to saturate a SOA and then depletes most of the excited state electrons in the SOA. In order to obtain shorter mode-locking pulsewidth, the DFBLD injecting wavelength should be slightly longer than the peak wavelength of SOA gain to benefit from shorter gain-recovery time and larger modulation depth. As the wavelength of a DFBLD approach the central wavelength of SOA, the shortest pulse was measured via a digital sampling oscilloscope (DSO). The pulses can also be obtained by DFBLD backward injection at 1535 nm in feedback of SOAFL, leading to the optimized pulsewidths of 22.7 ps.
We compare the noise characteristics of optical pulses generated from an actively mode-locked (AML) erbium-doped fiber laser (EDFL) with a semiconductor optical amplifier and an injection-locked EDFL with a gain-switched Fabry-Perot laser diode (FPLD). The mode-locked EDFL pulse exhibits a phase noise of -110.1 dBc/Hz (at 1 MHz offset frequencies from the carrier), the timing jitter of 1.16 ps, and a supermode noise suppression ratio of 47.5 dB. The injection-locked EDFL pulse exhibits a phase noise of -121.1 dBc/Hz (at 1 MHz offset frequencies from the carrier), a timing jitter of 0.31 ps, and a supermode noise suppression ratio of 51 dB. It is demonstrated that the injection-locked EDFL with a gain-switched FPLD has lower noise characteristics than the AML-EDFL.
We compare the frequency detuning properties of the optical pulses generated from erbium-doped fiber lasers (EDFL’s) by using harmonic mode-locking and regenerative amplification techniques. The frequency detuning range of regeneratively amplified pulse (17.78 kHz) is wider than that of harmonic mode-locked pulses (7 kHz). The regeneratively amplidied EDFL pulse has a smaller pulsewidth (22 ps), a higher peak power (40.7 mW), a lower phase noise (-107 dBc/Hz at offset frequency of 100 kHz), and a lower timing jitter (0.33 ps). This is attributed to that the characteristic of the gain-switched optical pulse is remained under regenerative amplification operation. Our harmonic mode-locked erbium-doped fiber laser has a lower phase noise (-100 dBc/Hz @ offset 1 kHz; -105 dBc/Hz @ 10 kHz) than that ever reported in a regeneratively harmonic mode-locked fiber ring laser.
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