Mode-locking Characteristics Research Articles

Passive mode locking of inhomogeneously broadened lasers

Passive mode locking of inhomogeneously broadened lasers is studied in three typical regimes: pure self-amplitude modulation (SAM) mode locking, soliton mode locking, and mode locking in the presence of self-phase modulation (SPM) and positive group-delay dispersion (GDD). When the mode-locking strength becomes weak and/or the GDD is large, the lockable spectral width reduces and mode locking becomes unstable, which is a common and unique feature for broadband inhomogeneously broadened lasers. Mode locking by the pure SAM is essentially a phase-locking process. The soliton mode locking best resists the impact of insufficient gain filtering, while mode locking in the presence of strong SPM and positive GDD is the weakest, and a certain amount of gain filtering (narrowing) is necessary for stable mode locking. The scaling relations of mode-locking characteristics with the gain linewidth, GDD, and nonlinearities, and the influence of the degree of inhomogeneity of the gain medium are examined. A linearized saturable absorber may not be able to mode lock a strongly inhomogeneously broadened laser, and a minimum unsaturated absorber loss is required for stable mode locking in most situations.

Generation of 60-nJ sub-40-fs pulses at 70 MHz repetition rate from a Ti:sapphire chirped pulse-oscillator

We have studied the influence of the intracavity dispersion on the mode-locking characteristics of pure-Kerr-lens mode-locked Ti:sapphire chirped-pulse oscillators. Using the results of this study, we have built chirped-pulse oscillators that can be mode-locked when pumped with the highest pump powers commercially available at 532 nm. We generated 62 nJ pulses at 70 MHz repetition rate, with a Fourier limited duration of 33 fs.

Mode-locked performance of hybrid soliton pulse source utilizing fiber grating external cavity lasers

Mode-locking characteristic of hybrid soliton pulse source (HSPS) utilizing linearly chirped raised-cosine flat top apodized fiber Bragg grating (FBG) is investigated by using coupled-mode equations. It is found that the fundamental repetition frequency range of HSPS is significantly extended by using linearly chirped raised-cosine flat top apodized FBG instead of linearly chirped Gaussian apodized FBG. The range of repetition frequencies over which proper mode-locking is obtained is 2–3.3GHz with linearly chirped raised-cosine flat top apodized grating whereas this range is 2.1–2.95GHz with linearly chirped Gaussian apodized grating.

Research on highly Yb3+‐doped passive mode‐locked fiber ring laser

AbstractTheoretical and experimental results on the self‐starting passive mode‐locked Yb fiber‐ring laser generating short pulse are reported. The relations between the laser‐cavity parameters and mode‐locked pulse characteristics are discussed. 980‐nm LD‐pumped laser is used as the pump source and high‐concentration Yb3+‐doped fiber is adopted as the gain medium. Using the nonlinear polarization rotation (NPR) effect of the fiber, a self‐starting stable mode‐locked pulse is obtained, with a center wavelength of 1046 nm, a 3‐dB bandwidth of 6.01 nm, and a 20‐dB bandwidth of 16 nm. The mode‐locked threshold power is 150 mW and output power is 26 mW with a 20‐MHz repetition rate. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 45: 269–270, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20792

External-cavity, actively mode-locked grating-coupled surface-emitting laser and amplification characteristics of a grating-coupled semiconductor optical amplifier.

An external-cavity, actively mode-locked grating-coupled surface-emitting semiconductor laser (GCSEL) is demonstrated for the first time to the authors' knowledge. The mode-locked oscillator generates a train of optical pulses at a 297-MHz pulse-repetition frequency. The optical pulse from the oscillator has a width of 22.6 ps and a spectral bandwidth of 0.07 nm at 975.9 nm, giving a time-bandwidth product of 0.50. In addition, amplification characteristics of a grating-coupled semiconductor optical amplifier are studied with a continuous-wave external-cavity GCSEL.

Mode-locking characteristics and relative intensity noise reduction in hybrid soliton pulse source

The noise and mode-locking phenomena of a hybrid soliton pulse source (HSPS) utilizing different fiber Bragg gratings (FBG) with or without linear chirp are described. The HSPS is modeled by a time-domain solution of the coupled-mode equations including spontaneous emission noise, and relative intensity noise ( RIN ) is calculated using numerical solutions of these equations. It is found that transform-limited pulses over a wide tuning range around the fundamental mode-locking frequency are generated from mode-locked HSPS with linearly chirped uniform and Gaussian apodized FBGs. Mode-locked pulses that are not transform-limited are generated over a wide tuning range from HSPS with Gaussian apodized and uniform FBGs. These gratings can give transform-limited pulses only over a limited tuning range around the fundamental mode-locking frequency. It is also found that RIN reduction is possible for the mode-locked HSPS by selecting a suitable apodization function such as Gaussian and linear chirp rate.

High-quality photonic sampling streams from a semiconductor diode ring laser

We report on the development of an ultralow-noise, external-cavity, actively mode-locked semiconductor diode laser for application in next-generation photonic sampling systems. A summary of harmonically mode-locked noise characteristics in a 65-MHz ring cavity is presented through the range of pulse repetition frequencies between 130 MHz and 8.3 GHz (2nd-128th harmonic). Important implications regarding the use of gain-versus-loss modulation as the active modelocking mechanisms are discussed. We also report what are, to our knowledge, the lowest noise characteristics achieved to date for a semiconductor diode laser operating at 10 GHz. Individually optimized results of 0.12% rms amplitude noise (10 Hz-10 MHz), and 43 fs rms residual phase jitter (10 Hz-10 MHz) provide a theoretical resolution of 8.6 bits in a 10-GSPS optical analog-to-digital converter. We have also achieved dispersion-compensated pulsewidths; as short as 1.2 ps, and shown successful operation of a novel phase-locked-loop capable of reducing the rms; residual phase noise by as much as 91% within its response bandwidth. Finally, the first measurements of residual phase noise out to the Nyquist frequency (5 GHz) are presented, providing an upper bound on the rms residual phase jitter of 121 fs (10 Hz-5 GHz).

Time-domain analysis of fiber grating semiconductor laser operation in active mode-locking regime

We analyze in detail the mode-locking characteristics of fiber grating semiconductor lasers using a split-step time-domain approach. The accuracy of the simulations is demonstrated by the capability of the numerical code to describe the fine structure of the resonance peak spectral splitting (RPSS) of the intensity modulation (IM) response near the FSR modulation frequency with a very good agreement with analytical results reported in the literature. The mode-locking (ML) frequency range was found to be closely related to the IM characteristic shapes around the resonance peak. The ML pulse characteristics and frequency range of Gaussian gratings with different apodization and dispersion characteristics have been reported as an example.

Efficient continuous-wave TEM00 and femtosecond Kerr-lens mode-locked Cr:LiSrGaF laser

Efficient high-power (900-mW) TEM(00) cw operation of a Cr:LiSrGaF laser pumped by a Kr(+) laser is demonstrated. The results of laser and spectroscopic analysis, including upconversion and excited-state absorption, are presented and used for the optimization of this laser. Kr(+)-laser-pumped mode-locked femtosecond operation (64 fs) was performed. Use of a modified resonator, which can decrease the threshold pump and intracavity power for Kerr-lens mode locking and increase the mode-locked output power (300 mW), is suggested. The inf luence of crystal axis misalignment on the tuning and mode-locking characteristics of the laser is studied.

Amplitude Modulated Harmonic Mode-Locking Characteristics of Er3+ Doped Fiber Ring Laser.

In this paper, characteristics of amplitude modulated harmonically mode locked (AM-HML) erbium doped fiber (EDF) ring laser are presented. It is investigated that the AM-HML method can provide a precise control of the lasing spectral bandwidth. Which in turn, can control the nonlinear effects in single mode fibers, such as stimulated Brillouin scattering in optical fiber communications. The optical spectral bandwidth is found to be broadened from -5GHz to-16.6GHz when the AM modulating low harmonic signal frequency is varied from -3.1609MHz to -44.2526MHz (1 to 14 times of the fundamental cavity frequency fa) at a fixed pulse rate of fh 2.1241 GHz. The effect of the percentage of AM modulation depth is also studied and it is observed that the bandwidth of the AM-HML laser is slightly affected when the percentage of AM modulation is varied from -5% to -50%.

Theoretical study of self-starting mode-locked all-fiber lasers

In this paper, the self-starting mode-locking characteristics of all-fiber lasers are investigated theoretically. The mechanism of self-starting mode-locking is analyzed and the experimental phenomena are explained successfully by introducing effective energy gain. The modulation characteristics of all-fiber lasers and the effect of gain saturation on modulation characteristics are discussed by analyzing the transmissivity of Er-doped fiber loop. Finally, the dynamic processes of all-fiber self-starting mode-locked lasers are simulated numerically.

Active-passive modelockingin a single-polarisation erbium fibre laser

Subpicosecond pulses have been generated in an actively modelocked, polarisation-maintaining Er-doped fibre ring laser. A single-polarisation fibre amplifier and an uncompensated polarisation bias section are integrated into the laser. This laser combines the nonlinear pulse-shortening mechanism of polarisation-rotation lasers and the single-polarisation operation, environmental insensitivity, and capacity for active modelocking characteristic of unidirectional single-polarisation ring lasers.

Generation of Steady-state Ultrashort Laser Pulses Based on a Novel Nonlinear Michelson Interferometer

Abstract A pulsed neodymium-doped yttrium aluminium garnet laser mode locked by a novel nonlinear Michelson interferometer, where one branch contains a frequency-doubling potassium titanyl orthophosphate crystal, is reported for the first time. Under the optimal experimental condition, shorter than 10 ps average pulse width at 1·06 μm and up to 95% reproducibility of mode locking can be obtained, and the stability of output energy is about 7·5%. Finally, the reason why its mode-locking output characteristics have been significantly improved is also analysed in this paper.

Cavity-detuning characteristics of CW additive-pulse mode-locked Nd:YLF laser

Temporal and spectral characteristics of the pulses from the self-starting, additive-pulse mode-locked Nd:YLF laser using a nonlinear coupled-cavity are presented. Pulses of tunable widths are obtained over the coarse cavity-detuning range of 1 mm. Temporal shapes measured with a cross-correlation technique show that the pulses are asymmetric when the two cavity lengths are coarsely mismatched. The additive-pulse mode-locking characteristics under the fine cavity-detuning within the range of a wavelength are also investigated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of Experiment and Theory in a Synchronously Mode-Locked NaCl:OH- (F2+)H Color Center Laser

We report a detailed comparison of experiment and theory of a synchronously mode-locked NaCl:OH- (F2+)H color center laser. In particular, we show the dependence of the steady-state pulse-shaping dynamics on the pump pulse power, loss coefficient and pump pulse width. Additionally, we also present a comparison of the mode-locking characteristics of an NaCl:(F2+)H laser and a KCl:Tl0(1) laser.

Femtosecond pulse generation in the 900–950 nm region from a passively modelocked LiF:F +2 colour centre laser

The passive mode-locking characteristics of a LiF:F + 2 colour centre laser operating around 930 nm using the saturable absorber DaQTeC are described. For a standing-wave cavity configuration pulses of 170 fs duration were generated and implementation of colliding-pulse-modelocking in a travelling-wave ring cavity led to pulse duration reduction to 130fs. Experimental data relating to laser configurations involving either the intracavity compensation of group velocity dispersion or the technique of coupled-cavity modelocking are also presented and discussed.

Dynamics of mode-locked laser diodes employing a repetitive short pulse drive current

A novel noniterative modeling technique is described and used to simulate the characteristics of a laser diode source which is mode-locked by a train of short current pulses produced by a comb generator. The basic experimental setup is described, as are the modeling process and a comparison of model predictions and experimental observations. It is shown that the predictions of the numerical model which uses only a single fitting parameter, the coupling efficiency of the laser diode to an external grating, are in good agreement with the measured mode-locking characteristics.

Repetitively pulsed mode-locked Nd:phosphate glass laser oscillator-amplifier system

Performance of a repetitively pulsed mode-locked Nd:glass laser system employing athermal phosphate glass in the oscillator and amplifier stages is described. Improved passive mode-locking characteristics of the oscillator are achieved through use of a l00-microm thick intracavity etalon, enabling reliable generation of transform-limited pulses typically of 5-psec duration. The system produces 1054-nm pulses of high beam quality and ~25-mJ energy at a pulse repetition rate of ~0.2 Hz. Subsequent frequency-doubling steps give conversion efficiencies of ~50% and 25%, respectively.

Investigation of the relationship between the energy of passively mode-locked laser radiation and spectral-luminescence characteristics of saturable absorbers

Investigation of the relationship between the energy of passively mode-locked laser radiation and spectral-luminescence characteristics of saturable absorbers

Optically biased laser gyro

We describe a four-mode ring laser that exhibits none of the mode-locking characteristics that plague laser gyros. This laser is characterized by a bias that changes sign with a change in the direction of rotation and prevents the counterpropagating modes from locking. A theoretical analysis explaining the experimental results is outlined.