Intracavity shaping of mode-locked laser pulses

A laser synthesizer of optical pulses is proposed. It is based on injection locking of numerous weakly coupled laser oscillators with a common resonator. A proper shaping of the susceptibility through shaping the pump beam reduces the dispersion of all orders due to the resonance transition and leads to generation of pulses close to the inverse bandwidth of the gain medium. A simple intracavity shaping mechanism allows synthesis of higher-order time supermodes directly from the laser. This technique could be applied to both active and passive mode-locked lasers with homogeneously and inhomogeneously broadened transitions. The possibility of using a compound gain medium is also discussed     

Repetition-frequency multiplication of mode-locked pulses usingfiber dispersion

In this paper, we describe a novel method for generating a highly repetitive optical pulse train using mode-locked pulses and the group-velocity dispersion (GVD) of optical fiber. An optical pulse train at a multiplied repetition frequency oscillation of an initial mode-locked pulse train is obtained by adjusting waveform fiber length in accordance with the mode-locking frequency and the fiber's GVD. A subterahertz optical pulse train (98-196 GHz) was successfully generated with low pulse intensity fluctuation     

Spectrum and width of mode-locked laser pulses

An analytical theory is given of the spectrum of the pulses generated by a ring laser that is mode locked by a synchronously modulated absorber. A steady-state solution is defined by requiring that, in a round trip, the joint effect of the active medium and the modulated absorber leave the pulses unchanged. The resulting integral equation is solved approximately, and the frequency spectrum of the pulses is obtained in closed form. Although the spectrum cannot be transformed analytically to obtain the pulse shape, the second moment of the shape, which gives a measure of the width, is found. The spectrum and width are found to depend on the curvature of the modulator waveform, near its maximum transmission, and on the gain and bandwidth of the active medium, which is assumed to have a Lorentzian line shape.     

Stabilisation of mode-locked pulses using travelling-wave semiconductor laser amplifier

Numerical and experimental results show, for the first time, that a saturated travelling-wave laser amplifier (TWLA) can be used to amplify and limit the amplitudes of pulses from an actively mode-locked semiconductor laser while removing unwanted secondary pulses.<>     

Temporal compression of light

The 5890 Å output from a CW dye laser was converted into a train of 0.5 ns pulses by frequency modulation and passage through a near-resonant atomic vapor delay line of Na. The theory of the process is discussed in both the time and frequency domains. Using a modulation index of 120 at a frequency of 17.8 MHz, we obtained values for the temporal compression ratio and intensity enhancement of 112 and 14, easily the largest that have been reported.     

Demonstration of down-chirped and chirp-free pulses fromhigh-repetition-rate passively mode-locked lasers

Knowledge and control of the chirp parameters of semiconductor lasers is a prerequisite to obtaining transform-limited pulses and/or to compensate for group velocity dispersion in fiber. Here, we report measurements of the sign and magnitude of chirp in high-repetition-rate mode-locked semiconductor lasers. The chirp of these monolithic lasers is measured in the frequency domain, using filtering and cross-correlation techniques. For different injection currents, a range of different chirp values is measured, including strongly down-chirped pulses at higher injection currents and transform-limited pulses to slightly up-chirped pulses at lower injection currents. The pulse chirp and the resulting broadening are due to the algebraic addition of opposite-signed chirps due to saturation of the absorption section and the gain section. These may cancel each other under some conditions, leading to a soliton-like transform-limited pulse     

Stability of passively mode-locked pulses in the fast absorber limit

We have reexamined the stability criterion derived by Haus in the analysis of a laser system which is passively mode locked by a fast saturable absorber. In his stability analysis, Haus required perturbationsdeltaupsilon(t)of the formdeltaupsilon(t) = (sech (t/tau_{p}))^{nu}where τpis the steady-state pulsewidth and ν is a parameter which is varied, to be stable and arrived at an approximate stability criterion. Upon numerical evaluation, Haus observed that the result was very close to the locus of apices separating the two branches of solutions, and proposed it as a stability criterion. Here we test the stability of arbitrary small perturbations and show that the result proposed by Haus is exact.     

Efficient compression of high-energy laser pulses

Limitations caused by stimulated Raman scattering (SRS) for laser pulse compression in traditional fiber-grating compressors are discussed. It is shown that the scheme utilizing fibers of length L exceeding the length of the dispersion walk off L_{walk off} of pump pulses and SRS permits one to obtain high-contrast compressed pulses, their energy being no more than a few nJ, and the ultimate width being proportional to the square root of the initial pulse width. For the compression utilizing fibers of length L<L_{walk off}, the pulse energies are not limited, but the compression factor is limited to the value of 32, and the compressed pulses have a low-intensity wide pedestal. A theoretical model of high-energy pulse compression with simultaneous pedestal suppression by the polarization technique using nonlinear birefringence of the fiber is discussed. This technique is compared to the spectral windowing technique     

Picosecond synchronisation on mode-locked laser pulses by double constant-fraction triggering

A novel technique is introduced for the synchronisation with better-than-30 ps jitter of electrical pulses to ultrafast light pulses at high repetition rate (74 MHz and more). The technique makes it possible to perform time-correlated single-photon counting measurements at picosecond resolution using mode-locked lasers without cavity dumping.<>     

Compression of Q-switched and mode-locked CW Nd:YAG laser pulses

Using the optical compression technique with a single-mode optical fiber and a grating pair, the 85 ps pulses from aQ-switched, mode-locked CW Nd:YAG laser have been compressed by a factor of 30. On average, an overall enhancement of 7 × in the peak power was achieved.     

Amplification of mode-locked semiconductor diode laser pulses in erbium-doped fibre amplifier

Amplification of picosecond pulses from an actively mode-locked semiconductor diode laser in an erbium-doped fibre amplifier pumped at 532 nm is demonstrated. Peak emission was centred on 1.536 mu m and a saturated amplifier gain of 13 dB yielded less than 10 ps pulses with a peak power of 0.58 W at 600 MHz. Optimisation of the mode-locked oscillator allowed amplified peak pulse powers greater than 3 W to be generated.<>     

Cross-correlation studies of pulses from the double mode-locked CW dye laser

A detailed investigation of the dual-wavelength pulse trains from the double mode-locked CW dye laser is made using cross-correlation measurements. Direct displays of pulse shapes and of the system jitter distribution are experimentally determined on a picosecond time scale. Techniques for determining the relative time of generation between the two double mode-locked pulse trains are also demonstrated. A variety of operating modes for this system is discussed in detail, providing further insight into the fundamental mechanisms involved in the production of the two pulse trains.     

Coherence analysis of noise-like pulses generated by an erbium-doped fiber mode-locked laser

The noise-like pulses(NLPs)with tunable fraction of the pedestal height in the whole intensity autocorrelation(AC)trace are numerically demonstrated in the designed erbium-doped fiber(EDF)mode-locked laser,which contains the saturable absorber(SA)with nonlinear polarization rotation(NPR),sinusoidal-shaped or Gaussian-shaped filter,two segments of EDFs,and two pieces of single-mode fibers(SMFs)with normal dispersion and anomalous dispersion,re-spectively.The pedestal ratio of the intensity AC trace can be tuned by changing the gain saturation energies of EDFs.The results show that when the net cavity dispersion is 1.06 ps2,the tuning range of the pedestal ratio for the NLPs can reach its maximum values,which are 0.51-0.89 and 0.58-0.88 for the sinusoidal-shaped and Gaussian-shaped fil-ters,respectively.In addition,an appropriate choice of filter bandwidth is also conducive to obtain a wide range of the tuning pedestal ratio for the intensity AC trace.     

Real-time oscilloscope observation of an ultrafast photodiode response to mode-locked laser pulses

Pulses of 100 ps full width at half maximum (FWHM) have been displayed on a direct current to 5-GHz real-time oscilloscope. The 100-ps duration includes contributions from the oscilloscope, the photodetector, and the laser pulse,Maximum current in the linear regime of the photodiode is ≈3 amperes so that electric pulses of ≈70 ps FWHM and ≈150 volts can be obtained with the laser-detector combination. Results of a simple optical method for determining the exposure time of high-speed electronic cameras are also briefly given.     

用级联脉冲压缩器产生皮秒光脉冲

我们将受激后向布里渊散射和受激后向喇曼散射组合,构成一个级联脉冲宽度压缩器,成功地将1.064μm、6ns的光脉冲变为630nm、20ps的光脉冲。     

Picosecond pulses, TEM00mode, mode-locked ruby laser

Picosecond pulses with a single transverse-mode operation have been obtained in a mode-locked ruby laser by using a special rooftop prism cavity. Addition of rhodamine 6G to the dye (DDI in methanol) enabled us to obtain consistently pulses as short as 5 ps.     

Generation of wavelength-tunable transform-limited pulses from amonolithic passively mode-locked distributed Bragg reflectorsemiconductor laser

Generation of wavelength tunable transform-limited picosecond pulses from a monolithic passively mode-locked distributed Bragg reflector (DBR) laser is demonstrated for the first time. A wide tuning range of 12 nm is realized by changing the refractive index of the passive DBR region through a thin-film heater embedded on the DBR section without causing significant changes in pulse characteristics     

An experimental technique for the investigation of the formation of picosecond pulses in a mode-locked Nd-glass laser

A general mode-locking andQ-switching technique which is specially suitable for the investigation of the influence of the non-linear transmission characteristic on the formation of mode-locked pulses is described. In addition, some preliminary results are reported.     

Miniature packaged actively mode-locked semiconductor laser with tunable 20 ps transform limited pulses

Miniature, fibre tailed and packaged, wavelength tunable actively mode locked semiconductor lasers have been developed for use as sources of approximately 20 ps transform limited pulses. Continuous stable operation with >40 nm tuning ranges centred either at 1.3 mu m or 1.55 mu m have been demonstrated. The package has an adjustable cavity length enabling the repetition frequency to be selected between 2 GHz and 8 GHz. Mean output powers in the monomode fibre tail are in the range -10 dBm to -5 dBm.<>