RF Linewidth narrowing in Quantum-Dash-Based Passive Mode-Locked Lasers Using Optical Feedback

On-chip integration of semiconductor lasers have shown a growing interest in recent years, especially for the development of photonic integrated circuits (PICs) which are of paramount importance for high-speed communication within and between data centers, and fast on-board data exchanges. For all these applications, a key challenge remains the stability of the laser sources integrated on a PIC in presence of external optical feedback with the view to avoid integrated bulky and costly optical isolation. In this study, the effects of external optical feedback are investigated in hybrid InAs/InP quantum dot comb lasers on silicon. The design of the cavity includes a semiconductor optical amplifier section, a saturable absorber and an on-chip external cavity incorporating a vertical coupler. We measured the resulting feedback properties with respect to the operation conditions (bias current and voltage) and to the length of the saturable absorber. We show that under most operating conditions, the laser remains stable against optical feedback, only few regimes of operation occur, which either improve or degrade the frequency comb and/or the radio-frequency beatnote power of the laser.

The effect of external optical feedback on a packaged monolithic quantum dot passively mode-locked laser is investigated. The radio-frequency (RF) linewidth narrows from 8 KHz in the free-running situation to a value as low as 170 Hz under relatively low feedback and temperature control. The RF linewidth characterization under resonant feedback at a multiple of the laser cavity length agrees well with the published theory. The narrow RF linewidth with external optical feedback makes the chip-scale quantum dot mode-locked laser an attractive source for ultra low noise applications such as optical clocking and optoelectronic oscillators.

A comprehensive study on the small-signal intensity modulation (IM) characteristics of a fiber grating Fabry-Perot (FGFP) laser is numerically investigated. The effect of external optical feedback (OFB), temperature, injection current, cavity volume, nonlinear gain compression factor, and fiber grating (FG) parameters on IM characteristics are presented. The temperature dependence (TD) of IM is calculated according to the TD of laser cavity parameters instead of using the well-known Parkove relationship. It has been shown that the optimum external fiber length (L ext) is 3.1 cm. The optimum range of working temperature for FGFP laser is between 23 to 27 °C. We also show that by increasing the laser injection current from 10 to 60 mA, the IM peak amplitude decreased from 6.3 to 0.2 dB and the relaxation-oscillation frequency (ROF) is shifted from 1.2 GHz towards higher frequency of 5.48 GHz. In addition, the AR coating reflectivity and gain compression factor have no significant effect on the IM. The study indicates that a stable operation and excellent modulation characteristic can be obtained after optimization process.

The effects of external optical feedback mainly introduced by Rayleigh backscattering (RB) in long lead fiber, reflection of fiber end facet, and the fusion splices on the stability of asymmetric distributed feedback fiber laser (DFB-FL) have been investigated. Theoretical analysis based on the compound cavity model is presented, and the critical length Lp,c of lead fiber is obtained to evaluate the tolerance of DFB-FL to RB. Analysis and experimental results show that RB is the main factor causing the instability. The critical length for the shorter end of our asymmetric DFB-FL is measured to be about 170 m. To isolate the laser from the effect of RB, we propose a method using isolator and WDMs, which is experimentally proved to be an efficient way.

The effect of external optical feedback on the linewidth of a single-mode injection laser is considered theoretically. A set of three rate equations with Langevin noise sources is used to obtain the power spectrum. If the high-frequency structure in the power spectrum is ignored, the line shape is Lorentzian and exhibits broadening or narrowing depending on the external-cavity phase shift. Particular attention is paid to the line narrowing after including the effect of carrrier-induced index changes.

The effect of external optical feedback on quantum dot two-section passively mode-locked lasers is investigated. The RF linewidth narrows down from 8 KHz in the free-running situation to a very low value of 350 Hz.

By external optical feedback effect on DMLs butt-coupled with a silica-based AWG, we present that 3-dB bandwidths of a DML submodule can be extended to ~37.5 GHz (@ 90 mA) using commercial 28-Gbaud DML chips.

A new dual-frequency Doppler lidar (DFDL) system is proposed by using a periodic window with Period-6 of laser dynamics subjected to optical feedback. This method has a good velocity resolution while providing ease of implementation.

The high frequency modulation effects on the relative intensity noise (RIN) characteristics of 405 nm InAlGaN laser diodes have been investigated taking into account the strain and thermal effects on the laser diode itself as well as external optical feedback effects. The simulation results indicate that the critical external optical feedback for –125 dB/Hz RIN is around 0.1%, a typical order found in practical DVD operating environments.By optimizing the injection current levels and its frequency, one can reduce the RIN with 0.1∼1% external feedback effect by more than 15 dB/Hz. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We measured the timing jitter, with unprecedented sensitivity and bandwidth, between pairs of similar 80MHz stretched-pulse and 200MHz soliton, passively mode-locked lasers with 400pJ pulse energy, to be 7.3fs and 4.6fs [10kHz, 10MHz], respectively.

This paper describes the effects of optical feedback on the sensitivity of VCSEL tunable-diode laser spectroscopy (TDLS). Three VCSELs, emitting at different wavelengths in the near-infrared, were used. A TDLS system, subjected to optical feedback, exhibited a common signal-to-noise ratio profile for all three lasers. A catastrophic degradation of TDLS sensitivity occurred when feedback exceeded a level which we associate with coherence collapse. The TDLS system had a CH4 minimum detection limit of 7.5 ppmm without optical feedback. Optical feedback of less than ten percent reduced this sensitivity by two orders of magnitude. This reduction of system sensitivity was accompanied by a second-harmonic absorption signal baseline shift which degraded the system accuracy.

The effect of optical feedback on hysteresis loop (HL) is investigated. Hysteresis loop occurs between two eigenstates of the He-Ne laser (x , y) and it changes by optical feedback at angles from 0° to 180°. Hysteresis loop size decreases and increases with the rotation angle (θ) of the polarization. Hysteresis loop effects on vertical (⊥) and horizontal (//) polarization and the intensity for two eigenstates. When HL size increases, then the intensity (I⊥ or I//) decreases vice versa clear. Best results at 70-110 angles from the external cavity (optical feedback cavity), whereas the intensity very high and HL disappear.

Effects of optical feedback on period-one nonlinear dynamics of an optically injected semiconductor laser are numerically investigated. The optical feedback can suppress the period-one dynamics and excite other more complex dynamics if the feedback level is high except for extremely short feedback delay times. Within the range of the period-one dynamics, however, the optical feedback can stabilize the period-one dynamics in such a manner that significant reduction of microwave linewidth and phase noise is achieved, up to more than two orders of magnitude. A high feedback level and/or a long feedback delay time are generally preferred for such microwave stabilization. However, considerably enhanced microwave linewidth and phase noise happen periodically at certain feedback delay times, which is strongly related to the behavior of locking between the period-one microwave oscillation and the feedback loop modes. The extent of these enhancements reduces if the feedback level is high. While the microwave frequency only slightly changes with the feedback level, it red-shifts with the feedback delay time before an abrupt blue-shift occurs periodically. With the presence of the laser intrinsic noise, frequency jitters occur around the feedback delay times leading to the abrupt blue-shifts, ranging from the order of 0.1 GHz to the order of 1 GHz.

We present a numerical study of the effects of optical feedback on the static and dynamic characteristics of vertical-cavity surface-emitting lasers (VCSELs) under both single-mode and two-mode operations. Our model includes transverse effects such as carrier diffusion and spatial hole burning (SHB) and is therefore capable of including gain-saturation-induced coupling among transverse modes. For short external cavities (/spl sim/30 /spl mu/m), optical feedback can completely suppress higher-order transverse modes. For relatively long external cavities (/spl sim/1 cm), VCSELs follow a period-doubling route to chaos. Numerical results show that multimode VCSELs exhibit self-pulsations at higher feedback levels compared with the single-mode case.

The purpose of this study was to determine the effect of visual feedback and verbal encouragement on concentric peak torque of the quadriceps and hamstrings muscles of males and females at an isokinetic velocity of 60 deg/s. All testing was performed on the Biodex B-2000 isokinetic dynamometer in three sessions, separated by 7 to 14 days. Fifteen males and 15 females (age = 25.4 ± 2.4 yrs, wt = 76.6 ± 16.5 kg, ht = 173.61 ± 9.5 cm) were tested under the following conditions: (a) visual feedback, (b) verbal encouragement, (c) combined visual feedback and verbal encouragement, and (d) no feedback (control). Testing order was counterbalanced to control for possible learning effects. Two separate 2 x 4 repeated measure analyses of variance (ANOVA) were used to examine the difference among test conditions. Examination of quadriceps force production revealed that subjects generated greater peak torque when visual feedback was provided than when verbal encouragement or no feedback were provided. Similarly, quadriceps force production was greater when combined visual feedback and verbal encouragement was provided than when verbal encouragement or no feedback were provided (p < 0.05). Examination of hamstrings force production revealed that subjects generated greater peak torque when combined visual feedback and verbal encouragement was provided than when verbal encouragement, and no feedback were provided. Additionally, hamstrings force production was greater when visual feedback was provided than when no feedback was provided (p < 0.05). These results indicate that the use of visual and combined visual and verbal feedback increased quadriceps and hamstrings force production when compared to the control condition where no feedback was provided.