Harmonic Mode-Locked Ytterbium-Doped Fibre Ring Laser

We demonstrate a harmonic mode-locked ytterbium-doped fibre ring laser, which consists of a polarization-sensitive isolator, two polarization controllers, two 976nm laser diodes as the pump source and a two-segment ytterbium-doped fibre. Utilizing an additive pulse mode-locked technique based on nonlinear polarization evolution, the ytterbium-doped fibre laser can operate in mode-locked state by adjusting the position of polarization controllers. The cavity fundamental repetition rate is 23.78 MHz. We also observe the second- and third-harmonic mode locking in the normal dispersion region, and their repetition rates are 47.66 MHz and 71.56 MHz, respectively. Over-driving of the saturable absorber in the harmonic mode-locking pulse is analysed and discussed in detail.     

Dual-Wavelength Erbium-Doped Fibre Ring Laser by Cascading Tunable Bandpass Filter with Bandstop Filter

A novel fibre ring laser is proposed and successfully demonstrated. By cascading a tunable bandpass filter with a bandstop filter to construct the desired narrow dual-transmission-peak spectrum, and employing a segment of unpumped erbium-doped fibre as a saturable absorber, a stable fibre ring laser is achieved at room temperature. The proposed laser can operate in dual-wavelength with the wavelength spacing of 0.48 nm and the extinction ratio more than 50 dB or switch between the two wavelengths by adjusting a polarization controller. The stability is investigated experimentally and explained theoretically.     

A Proposal and Demonstration for Photonic Generation of a Microwave Signal by Incorporating a Microring Resonator

A proposal for photonic generation of a microwave signal is presented by employing a dual wavelength erbiumdoped fibre ring laser. In the laser, a microring resonator is cascaded with a tunable bandpass filter to serve as a dual-wavelength selector, an unpumped polarization maintaining erbium-doped fibre is used as a saturable absorber. By replacing the microring resonator with a delay interferometer to verify the proposal, a wavelengthtunable dual wavelength single longitudinal mode laser is demonstrated, and a microwave signal at 10.01 GHz with a linewidth of less than 25 kHz is obtained by beating the two wavelengths at a photodetector.     

A Bidirectional, Diode-Pumped, Passively Mode-Locked Nd：YVO4 Ring Laser with a Low-Temperature-Grown Semiconductor Saturable Absorber Mirror

We report the operation of a bidirectional picosecond pulsed ring Nd：YVO4 laser based on a low-temperaturegrown semiconductor saturable absorber mirror. Except for the laser crystal, the six-mirror ring laser cavity has no intra-cavity elements such as focusing lens or mirror. The bidirectional mode locked pluses are obtained at the repetition rate of 117.5 MHz, pulse duration of 81 ps, power of 2 × 200 mW.     

Pulse compression in AO Q-switched diode-pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber

By simultaneously using both an acoustic-optic (AO) modulator and a Cr⁴⁺:YAG saturable absorber in the cavity, for the first time, a diode-pumped doubly Q-switched Nd:GdVO₄ laser has been realized. The pulse duration is obviously compressed in contrast to the actively acoustic-optic Q-switched laser. By considering the Gaussian transversal distribution of the intracavity photon density and the longitudinal distribution of the photon density along the cavity axis as well as the influence of turnoff time of the acoustic-optic (AO) Q-switch, we provide the coupled rate equations for a diode-pumped doubly Q-switched Nd:GdVO₄ laser with both an acoustic-optic (AO) modulator and a Cr⁴⁺:YAG saturable absorber. These coupled rate equations are solved numerically, and the dependence of pulse width, pulse energy and peak power on the incident pump power at different pulse repetition rates is obtained. The numerical solutions of equations agree well with the experimental results.This revised version was published online in August 2005 with a corrected cover date.     

Q-Switched Large-Mode-Area Yb-Doped Fibre Laser Using GaAs as Saturable Absorber

A passive Q-switched large-mode-area Yb-doped fibre laser is demonstrated using a GaAs wafer as the saturable absorber. A high Yb doping concentration double-clad fibre with a core diameter of 30μm and a numerical aperture of 0.07 is used to increase the laser gain volume, permitting greater energy storage and higher output power than conventional fibres. The maximum average output power is 7.2W at 1080nm wavelength, with the shortest pulse duration of 580ns and the highest peak power of 161W when the laser is pumped with a 25W diode laser operating at 976nm. The repetition rate increases with the pump power linearly and the highest repetition rate of 77kHz is obtained in the experiment.     

Experimental Investigation of Chaos Synchronization in DFB Diode Lasers with Unsymmetrical Scheme

We experimentally generate high dimension chaotic waveforms with smooth spectrum using a distributed feedback （DFB） semiconductor laser with unidirectional fibre ring long-cavity feedback, and implement the stable chaos synchronization when the chaotic light is injected into a solitary DFB laser diode. The synchronization quality is investigated by time-domain and frequency-domain analysis separately, The frequency-domain analysis indicates that the synchronization has higher quality in the high frequency band. The influences of the injection strength and the frequency detuning on the synchronization are measured. Our experimental results show that the robust synchronization can be maintained with the optical frequency detuning from -11GHz to 40 GHz.     

Control of the pulse duration in one- and two-axis passively Q-switched solid-state lasers

We study theoretically and experimentally different methods to control the pulses emitted by solid-state lasers passively Q-switched by a saturable absorber. We explore one- and two-axis laser schemes allowing to control the pulse duration, which is ruled by the saturation powers of the transitions in the absorber and in the gain medium. In one-axis lasers, it is shown that the adjustment of the pump and laser beam sizes in the active medium and in the absorber provides an efficient means to control the pulse temporal shape and duration. Furthermore, a two-axis laser cavity supporting so-called forked-eigenstate operation permits to freely adjust the parts of the mode power which circulate in the gain medium and in the absorber. In this case, a lengthening of the pulse duration up to 500 ns is obtained with an increase of the average output power. The theoretical results obtained by using rate equations adapted to each cavity geometry are in close agreement with experiments performed on a diode-pumped Nd³⁺:YAG laser Q-switched by a Cr⁴⁺:YAG saturable absorber. The relevance of the different techniques to control the pulse durations in the framework of potential applications is discussed. Received 3 December 2001     

Thermal effects investigation and cavity design in passively mode-locked Nd:YVO4 laser with a SESAM

A diode-pumped passively mode-locked Nd:YVO₄ laser with a five-mirror folded cavity is presented by using a semiconductor saturable absorber mirror (SESAM). The temperature distribution and thermal lensing in laser medium are numerically analyzed to design a special cavity which can keep the power density on SESAM under its damage threshold. Both the Q-switched and continuous-wave mode-locked operation are experimentally realized. The maximum average output power of 8.94 W with a 9.3 ps pulse width at a repetition rate of 111 MHz is obtained under a pump power of 24 W, correspondingly the optical slope efficiency is 39.2%.     

Characteristics of a hybridly mode-locked cw dye laser

We present experimental and theoretical results on the characteristics and random variations of subpicosecond pulses generated by a synchronously pumped cw dye laser with saturable absorber. The analysis of the power spectra indicates rapid fluctuations of the pulse duration of 40–50%, energy fluctuations of 3%, and a jitter of the repetition time of 0.1% corresponding to an absolute jitter of 12 ps. The latter is caused mainly by the temporal jitter of the pump laser. A mismatch of the lengths of the dye and the pump-laser cavity can result in a nonstationary mode-locking regime with a periodic change of the pulse parameters. The interpretation of the experimental results are supported by computer simulations of the pulse evolution process.     

Multiwavelength Dispersion-Tuned Actively Mode-Locked Erbium-Doped Fibre Ring Laser with Gain Competition Suppression

Multiwavelength dispersion-tuned actively mode-locked erbium-doped fibre ring laser is demonstrated by incorporating a section of highly nonlinear fibre （HNLF） in the laser cavity. The HNLF and the time gate element （modulator） in the fibre laser successfully suppress the gain competition in the erbium-doped fibre, and thus enable multiwavelength operation. Simultaneous generation of 10 GHz pulses up to eight different wavelengths is achieved. Wavelength, spacing and modes number tuning are investigated by changing fibre cavity length, dispersion, and erbium-doped fibre amplifier power, respectively.     

Single-walled carbon nanotubes on side polished fiber as a universal saturable absorber for various laser output states

We demonstrated four different laser operational states in the same passive fiber ring laser cavity by controlling solely the polarization state in a saturable absorber. The demonstrated laser operational states were continuous wave, mode-locking, Q-switching, and Q-switched mode-locking. The saturable absorber was fabricated by spin-coating a single-walled carbon nanotubes (SWCNT)/polymer composite on a side polished fiber providing enhanced nonlinear interaction with the SWCNT. The wide tuning range of the saturable absorber allows the simple means to generate different forms of laser output from a single laser cavity.     

Theory of passive mode-locking of CW dye lasers with contacted and non-contacted absorbers

A theoretical treatment of the influence of a saturable absorber in close contact with a highly reflecting mirror on a light pulse is given. The shaping of a Gaussian pulse during a single passage through such an absorber is calculated for various absorber lengths. The influence of absorber position and length on the pulse parameters and the stability of a continuously pumped dye laser are investigated. The shortest pulses and the most favourable operation conditions of the laser are to be expected in the case of a short contacted saturable absorber.     

Theory of passively mode-locked cw dye lasers

The circulation of an ultrashort light pulse in a continuously pumped modelocked dye laser with a linear cavity configuration containing the active dye, the saturable absorber and a bandwidth-limiting elements is treated. The steady-state condition that the pulse shape reproduces after each cavity round-trip leads to a nonlinear integro-differential equation for this pulse shape. An approximate method for the solution of this equation not limited to the case of low laser gain and small pulse energies is given. The stable single pulse region and characteristic pulse parameters, as energy, duration, intensity and asymmetry, are considered in dependence on the laser parameters.     

Compact sub-100-fs Nd:silicate laser

We present a compact Nd:silicate laser pumped by a single 1-W high-brightness commercial laser diode that generates pulses as short as 88 fs (nearly Fourier limited) when passively mode-locked with a saturable absorber mirror. We also tested a prismless cavity setup employing a single Gires-Tournois mirror, yielding 100-fs pulses. These setups are significantly simpler and more compact than those reported previously for short pulse Nd:silicate lasers.     

Mode-locked fiber laser using an SU8/SWCNT saturable absorber

We report the fabrication of a saturable absorber based on SU8 single wall carbon nanotube (SWCNT) composite material. Thin films with a controllable thickness can be fabricated using a simple and reliable process. These films can be inserted between two FC/APC connectors in order to have an inline saturable absorber. A passive mode-locked laser was built by interleaving the fiberized saturable absorber in an erbium-doped fiber (L-band) ring cavity laser. The laser produces 871 fs pulses with a repetition rate of 21.27 MHz and a maximum average power of 1 mW.     

20 ps pulse generation by an excimer laser pumped double self-Q-switched distributed feedback dye laser

The behaviour of an excimer-laser-pumped distributed feedback dye laser (ELP DFDL) using a mixture of saturable absorber and laser dye as an active medium was studied. In a DFDL self-Q-switching occurs because of the population inversion dependence of the feedback. If the DFDL contains absorber saturation of the absorption by the leading edge of the created laser pulse also causes Q-switching. As a result of this double self-Q-switching, mixing a saturable absorber into the laser dye, the laser pulses become shorter and more stable. This was demonstrated experimentally. Mixing Rhodamine B into the dye solution of a Coumarin 153 ELP DFDL, a six-fold reduction in the pulse duration and a more than two-fold reduction in the pulse energy instability were observed. The experimental results were in good qualitative agreement with the predictions of a simple rate equation model.     

High power single-longitudinal-mode operation in a twisted-mode-cavity laser with a c-cut Nd:GdVO4 crystal

A twisted-mode-cavity laser was established by using a piece of c-cut Nd:GdVO₄ crystal as the lasers active material. Output spectra were scanned by a scanning Fabry–Perot interferometer, which demonstrated that the single-longitudinal-mode laser operation was realized in the twisted-mode-cavity laser configuration. A maximum single-longitudinal-mode laser output power of 2.1 W was obtained when the pump power was 11.5 W. The pump slope efficiency was about 20.0%. A passively Q-switched single-longitudinal-mode laser was also achieved in a twisted-mode cavity by inserting a piece of Cr⁴⁺:YAG as an intracavity saturable absorber. The Q-switched single-longitudinal-mode laser pulse duration was measured to be 100 ns and the single-pulse energy was about 40.0 J.     

Self-Q-switching and mode-locking in an all-fiber Er/Yb co-doped fiber ring laser

Using a section of un-pumped Er/Yb co-doped fiber (EYDF) as a saturable absorber, Self-Q-switching and self-mode-locking pulses have been obtained in an all-fiber EYDF ring laser. Such laser is with the self-Q-switched pulse threshold of 135.22 mW, the repetition rate of approximately 22.2 kHz, and the pulse duration of ∼2.8 μs, respectively. The self-mode-locked threshold is 591.8 mW. By incorporating the saturable absorption in an un-pumped EYDF and a Mach-Zehnder interferometer, when the pump power is increased to 1242.9 mW, the continuous-wave (CW) mode-locking with the pulse width of 26 ns has also been demonstrated experimentally for the first time.     

Single-longitudinal-mode fiber ring laser using fiber grating-based Fabry-Perot filters and variable saturable absorbers

A single-longitudinal-mode (SLM) fiber ring laser is demonstrated by incorporating a fiber Bragg grating (FBG)-based Fabry-Perot (F-P) filter and a variable saturable absorber in a fiber ring cavity. An ultra-narrow bandpass filter is established by the combined mode filtering of the FBG-based F-P filter and the variable saturable absorber. No accurate control for cavity length is required, and stable single longitudinal-mode operation without mode hopping is conveniently achieved.