Mode-locked Fiber Ring Laser Research Articles

Pulse shortening and quality improvement based on spectral filtering in a stretched-pulse mode-locked fiber laser with large third-order dispersion

We demonstrate a passively mode-locked ring fiber laser operating in the stretched-pulse regime. The ring cavity model is constructed with standard single-mode fiber and dispersion delay line for dispersion management. Here, all-solid Yb-doped photonic bandgap fiber is used as the dispersion delay line and provides laser gain simultaneously. As the all-solid Yb-doped photonic bandgap fiber has large third-order dispersion, it is necessary to study its impact on the stretched-pulse operation. The presence of the third-order dispersion reduces pulse quality because of the relatively small group velocity dispersion. To improve the pulse quality, a spectral filter is employed in the model. The simulation results show that spectral filtering could improve the pulse quality and shorten the pulse duration. Meanwhile, the optimum filter bandwidth is studied.

Chirp-Compensated Multichannel Hybrid DWDM/TDM Pulsed Carrier From Optically Injection-Mode-Locked Weak-Resonant-Cavity Laser Diode Fiber Ring

The multichannel chirp compensation of a hybrid pulsed carrier with dense wavelength division multiplexing/ time-division multiplexing (DWDM/TDM), generated from a mode-locked weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) fiber ring under 10-GHz dark-optical-comb injection, is demonstrated with a DWDM channel spacing of 200 GHz. This was implemented by selecting the least-common-multiple repetition frequency in the WRC-FPLD and fiber ring dual cavity. With a chirp parameter of for the WRC-FPLD-based fiber ring, the absolute value of the negative frequency chirp could be linearly suppressed from to with the pulse duration varying from 27 to 20 ps. By using a tunable band-pass filter, nine DWDM channels located between 1533.4 and 1546.2 nm were selected for dispersion compensation within a 55-m long diode fiber ring segment. The original pulsewidth varied from 21.7 to 17 ps for different channels. After chirp compensation, an auto-correlation diagnosis showed that the all-channel pulsewidths of mode-locked WRC-FPLD fiber ring laser changed only from 7.8 to 9.1 ps. The single-channelized WRC-FPLD pulsewidth after linear dispersion compensation and fifth-order soliton compression could be to shortened to 7.8 and 1.4 ps, respectively.

Generation of 10-GHz ultra-short pulses with low time jitter in an actively mode-locked fiber laser

We have experimentally achieved the 8.3-ps ultra-short pulse at 10 GHz repetition rate with the time jitter as low as 590 fs in an actively mode-locked fiber ring laser. The ring-cavity laser is mode-locked by a semiconductor optical amplifier based on cross-gain modulation. The external CW source is modulated with radio frequency signal by an amplitude modulator as the external optical pulses and, then, injected into the fiber ring cavity to achieve active mode locking. Further investigating the laser output characteristics, it indicates that the linewidth of employed CW source affects properties of the generated ultra-short pulse, such as phase noise and time jitter. Ultra-short pulse at high repetition rate with low time jitter can be generated by the optimization of CW laser source.

Switchable dual-wavelength passively mode-locked fiber ring laser using SESAM and cascaded fiber Bragg gratings

We propose and demonstrate an efficient and simple wavelength switchable dual-wavelength passively mode-locked fiber ring laser based on a semiconductor saturable absorber mirror (SESAM) and a cascaded sequence of fiber Bragg gratings (FBGs). The efficient self-starting mode-locked operation was realized with a SESAM. Dual-wavelength pulses with a wavelength spacing of 12.49 nm were obtained which was determined by the reflection peaks of the FBGs used in the experiment. In addition, by rotating the polarization controllers (PCs), the switchable dual-wavelength operation was simply achieved via exploiting wavelength-dependent loss mechanism.

Tunable 12×10 GHz mode-locked semiconductor fiber laser incorporating a Mach-Zehnder interferometer filter

A stable multiwavelength mode-locked semiconductor fiber ring laser incorporating a fiber Mach-Zehnder interferometer (MZI) filter was proposed and experimentally demonstrated. A semiconductor optical amplifier (SOA) serves as an optically controlled mode-locking element due to gain exhaustion caused by external injected optical pulses. Another SOA severs as a constant-gain medium. A fiber MZI filter with a temperature control is incorporated into the fiber ring cavity to acquire a stable and tunable multiwavelength oscillation. Twelve wavelengths are synchronously mode-locked at 10 GHz, pulse width of mode-locked pulse are about 30 ps. Proposed multiwavelength mode-locked semiconductor fiber ring laser has some distinct advantages, such as simple and compact structure, easy integration, convenient tuning, good stability, potential high repetition rate operating, which has potential application in the future WDM communication system.

Actively mode-locked fiber ring laser by intermodal acousto-optic modulation

We report an actively mode-locked fiber ring laser. A simple and low-insertion-loss acousto-optic modulator driven by standing flexural waves, which couples core-to-cladding modes in a standard single-mode optical fiber, is used as an active mechanism for mode locking. Among the remarkable features of the modulator, we mention its high modulation depth (72%), broad bandwidth (187 GHz), easy tunability in the optical wavelength, and low insertion losses (0.7 dB). The narrowest optical pulses obtained were of 95 ps time width, 21 mW peak power, repetition rate of 4.758 MHz, and 110 mW of pump power.

Long-cavity passively mode-locked fiber ring laser with high-energy rectangular-shape pulses in anomalous dispersion regime

We report on a long-cavity passively mode-locked fiber laser in the anomalous dispersion regime. The nonlinear polarization rotation technique is employed to achieve mode locking. The output pulse from the fiber laser has a rectangular shape and a corresponding gaussian-shape spectral profile. Stable mode-locked pulses at a repetition rate of 278 kHz with single pulse energy as high as 715 nJ are obtained under equal bidirectional pumping power of 500 mW in cavity. The experimental results demonstrate that the passively mode-locked fiber laser operating in the anomalous regime can also realize a high-energy pulse, which is different from the conventional low-energy soliton pulse.

Bidirectional mode-locked fiber ring laser using self-regenerative, passively controlled, threshold gating

We demonstrate a technique to achieve bidirectional mode locking of a fiber ring laser using two amplitude modulators passively driven by a signal regenerated from the oscillation of the laser itself. The new scheme allows generation of two independent-frequency unlocked-countercirculating pulses and active control of their crossing points. The technique minimizes the effects of the dead band associated with crossing at a scattering media. Results show stable bidirectional pulses at a 5MHz repetition rate with no detectable dead band.

Periodic intensity variations on the pulse-train of a passively mode-locked fiber ring laser

We investigate the evolvement of the periodic intensity variations of the pulse-train in a fiber ring laser mode-locked by the nonlinear polarization rotation technique. It is found that the phenomenon of such intensity fluctuations is related to the orientation of the polarization controllers when the pump power is fixed. The numerical results show that the mechanism of the periodic intensity variations is caused by the interaction of the nonlinear polarization rotation and the passive polarizer in the cavity.

Timing-jitter reduction of passively mode-locked fiber laser with a carbon nanotube saturable absorber by optimization of cavity loss

We investigate the relationship between timing jitter and cavity loss of a passively mode-locked fiber ring laser with a carbon nanotube as a saturable absorber. It is the first time that we experimentally demonstrated the reduction of timing jitter by properly increasing laser cavity loss. The lowest timing jitter is achieved when the cavity loss is optimized. Theoretical analysis is in agreement with the experimental observations on the effect of cavity loss for the reduction of timing jitter. Moreover, it is experimentally shown that, at an optimal value of cavity loss, the timing jitter is reduced significantly by 24%, while the relative intensity noise increased by 4% only.

All-optical clock extraction from 40-Gbit/s NRZ data using cascaded long-period fiber grating

All-optical clock extraction from a 40-Gbit/s NRZ input signal is demonstrated using a cascaded long-period fiber grating (CLPG) and a mode-locked fiber ring laser. The CLPG has a Mach–Zehnder configuration with two arms along the core and cladding regions. Using the difference in propagation delay between two arms, the non-return-to-zero (NRZ) signal is converted to the pseudo-return-to-zero (PRZ) signal. To obtain repetitive pulses as a clock signal from the PRZ signal, a ring laser with a semiconductor optical amplifier (SOA) is used. Subsequently, the measured carrier-to-noise ratio (CNR) of the PRZ and clock signals are enhanced up to 30 dB and 31 dB, respectively, compared to that of the original NRZ signal. Also, the clock signal centered at 40 GHz has a low timing jitter of <1.3 ps. It is expected that this method can be applied to high speed fiber-optic systems of >40 Gbit/s due to its small time delay between the core and cladding regions.

Electron–Hole Plasma-Induced Spectral Blueshift of Optical-Data Injection Mode-Locked Semiconductor Optical Amplifier Fiber Laser

Under an optical nonreturn-to-zero (NRZ) data injection at 10 Gbit/s, the 10-GHz mode-locking and pulsed return-to-zero (RZ) clock extraction from a semiconductor optical amplifier (SOA) based fiber ring is investigated in this paper. The diagnoses on gain and intracavity-power-controlled anomalous blueshifted spectrum and subpicosecond timing jitter are demonstrated. By increasing the injecting power of the optical NRZ data from -3 to 8 dBm into the SOA bias at different currents, the mode locking is completed with a dc level greatly decreasing from 480 to 50 ¿W (only 1.5% of the mode-locked pulse power at 3 mW), corresponding to a pulse/dc amplitude contrast ratio up to 18 dB. Increasing the SOA bias current up to 350 mA significantly suppresses the timing jitter from 1.8 ps to 345 fs, and the extracted RZ clock pulse is shortened from 55 to 27 ps. The pulsewidth of the amplified SOAFL is compressed from 11 ps to 836 fs after dispersion compensation. At constant data injection level, the increasing SOA bias or gain oppositely redshifts the mode-locked SOA fiber ring laser (SOAFL) spectrum by 5 nm. The amplifier spontaneous emission of SOA at short wavelength region ( ~ 1520 nm) is eliminated with increasing NRZ data power, whereas the mode-locking gain peak arises and blueshifts from 1558 to 1552 nm due to the band-filling effect. Such a blueshift in mode-locking spectrum becomes more significant in SOA at lower bias (or gain) condition. A theoretical model interprets the correlation between the nonlinear gain suppression-induced variation of electron-hole plasma in SOA and the blueshifted mode-locking SOAFL spectrum, which is occurred when the gain saturation condition for the SOA becomes more pronounced.

Experimental study on all-normal-dispersion mode-locked fiber laser with optimal spectral filtering position

Employing nonlinear polarization rotation, an all-normal-dispersion mode-locked fiber ring laser was constructed, in which a fiber filter with 10 nm bandwidth was used in the cavity to provide the additional self-amplitude modulation. The important role of the filter position plays in the high-chirped pulse shaping in the laser has been experimentally demonstrated by choosing different locations of the filter in the cavity. A mode-locked pulse series, with the averaged power of 922 mW, output pulse duration of 62 ps, and pulse repetition frequency of 266 MHz, corresponding to the pulse energy of 35 nJ, was obtained in the optimized filter position under a pump power of 320 mW. By the numerical simulation, the propagation dynamics of pulses in the three cases of spectral filter (SF) position in the cavity are obtained and analyzed. The optimal location of the SF is found, which is in agreement with our experimental result.

Multiwavelength actively mode-locked fiber ring laser with a dispersion compensated cavity

A multiwavelength actively mode-locked fiber ring laser that utilises a single semiconductor optical amplifier as both a gain and mode-locking element is presented. It is verified experimentally that the number of mode-locked wavelengths may be maximised by compensating for the anomalous intra-cavity dispersion. This is achieved through the inclusion of an appropriate length of normal dispersion fiber in the laser cavity. Using this technique, the number of wavelengths simultaneously mode-locked at 10 GHz was increased from 2 to 5.

Tunable 19 × 10 GHz L-band FP-SOA based multi-wavelength mode-locked fiber laser

We present a multi-wavelength mode-locked fiber ring laser incorporating a semiconductor optical amplifier (SOA) and a Fabry–Perot semiconductor optical amplifier (FP-SOA). Because the gain of the SOA is depleted by an external injection optical signal, the SOA acts as a loss modulator. The FP-SOA serves as a tunable comb filter. The presented laser source can generate 19 synchronized wavelength channels with the extinction ratio of about 21 dB, each mode-locked at 10 GHz, and mode-locked pulse width is about 40 ps. Oscillation wavelengths band can be tuned by adjusting the bias current of the SOA, and wavelength spacing also can be changed by using a tunable optical delay line (ODL) or a temperature controller. The polarization-insensitive devices ensure that the output power is rather stable. This fiber laser has potential applications in longer waveband (L-band) within the low-attenuation window.

Amplitude-equalized dual-wavelength rational harmonic mode-locked fiber ring laser

We report a dual-wavelength amplitude-equalized rational harmonic mode-locked fiber laser operating at up to 40 GHz based on the technique of nonlinear polarization rotation (NPR) in a highly nonlinear fiber. A theoretical simulation shows that NPR structure can introduce a self-feedback mechanism to stabilize the two simultaneously lasing wavelengths. By carefully adjusting the polarization controllers (PCs) in the cavity, amplitude-equalized pulses at two different wavelengths have been obtained.

All-optical clock extraction from 10-Gbit/s NRZ-DPSK data using modal interference in a two-mode fiber

All-optical clock extraction from a 10-Gbit/s NRZ-DPSK input signal is demonstrated using modal interference in a two-mode fiber (TMF) and a mode-locked fiber ring laser. The TMF has a Mach–Zehnder configuration with two arms along the core and cladding regions. Using the difference in propagation delay between two arms, the non-return-to-zero differential phase shift keying (NRZ-DPSK) signal is converted to the return-to-zero on–off keying (RZ-OOK) signal. To obtain repetitive pulses as a clock signal from the RZ-OOK signal, a ring laser with a semiconductor optical amplifier (SOA) is used. Subsequently, the carrier-to-noise ratio (CNR) of the RZ-OOK and clock signals are enhanced up to 30 dB and 40 dB, respectively, compared to that of the original NRZ-DPSK signal. Also, the clock signal centered at 10 GHz has a low timing jitter of <1.6 ps. It is expected that this method can be applied to high speed fiber-optic systems of >10 Gbit/s due to its small time delay between the core and cladding regions.

Switchable and tunable dual-wavelength ultrashort pulse generation in a passively mode-locked erbium-doped fiber ring laser

A simple switchable and tunable dual-wavelength passively mode-locked erbium-doped fiber ring laser based on nonlinear polarization rotation (NPR) effect is proposed and experimentally demonstrated. The NPR effect effectively induces wavelength- and intensity-dependent loss to readily implement stable dual-wavelength passively mode-locked operation. The wavelength switching and tuning of the dual-wavelength ultrashort pulse laser are achieved only by appropriately rotating the polarization controllers. The side-mode suppression ratio of the output pulse is larger than 41 dB over a wavelength-tuning range of 43.4 nm. Moreover, triple-wavelength ultrashort pulse can also be observed.

Passive harmonically mode-locked fiber ring laser with long single-mode fiber and its application for high-resolution measurement of fiber length

Passive harmonically mode-locked fiber ring laser with long single-mode fiber SMF has been demonstrated. With the increase of nonlinearity due to the incorporation of a long SMF, it is found that the laser can operate in harmonic mode-locking states with various repeti- tion rates. Furthermore, based on the passively mode-locked fiber ring laser, a novel and simple technique for fiber length measurement with resolution on the order of centimeters has been proposed. The fiber length can be easily obtained from the fundamental cavity frequency of the mode-locked fiber laser because the transit time in a laser cavity is exactly proportional to the cavity length. © 2009 Society of Photo-Optical Instru-

Birefringence-Induced Trains of High-Rate Pulses in a Mode-Locked Fiber Laser

The output of a mode-locked erbium-doped ring fiber laser incorporating a section of a polarization-maintaining (PM) fiber is investigated in both numerical simulations and experiments. With proper inline polarization control, the laser can be set to emit a train of pulses, separated by the differential group delay of the PM section. Repetition rates as high as 500 GHz are experimentally observed. The results provide an added insight into the role of birefringence in mode-locked lasers based on nonlinear polarization rotation.