Simultaneous Mode-locking Research Articles

Noise characteristics of a polarization-duplex dual-comb fiber laser based on a single gain fiber

We demonstrate gain-sharing polarization-duplex dual-comb laser. Here, a nonlinear amplifying loop mirror (NALM) with a single gain fiber is used to achieve simultaneous mode-locking for the generation of the two frequency combs with slightly different repetition rates. Our system provides stable mode-locking and a good overlap of frequency comb spectra. The repetition rate difference is about 100 kHz and has high stability: we determined a standard deviation of 0.75 Hz, and the Allan deviation is 0.073 Hz at an integration time of 1 s for this free-running system. In the case of simultaneous mode-locking, our system shows a relative intensity noise (RIN) characteristic that has, to the best our knowledge, not yet been reported for dual-comb lasers. On the other hand, the phase noise characteristic of our mode-locked dual-comb laser is similar to the characteristic obtained when one beam path is blocked and the laser is used for single frequency-comb generation.

Multi-Wavelength Spatiotemporal Mode-Locked Fiber Laser at 1.55 μm

We propose and demonstrate an effective and simple multi-wavelength spatiotemporal mode-locked (STML) laser. The laser introduces a multimode interference (MMI) effect using the core diameter mismatch of the fibers, thus enabling simultaneous mode-locking at multiple wavelengths. For the multi-wavelength mode-locking, each wavelength exists a matching pulse sequence, and different wavelengths can possess the same pulse state or different states. Meanwhile, at the dual-wavelength operation, by adjusting the polarization controllers, not only the central wavelength can be continuously adjusted from 1552.93 nm to 1572.64 nm, but also the wavelength interval can be adjusted from 2 nm to 19 nm, which benefits from the tunability of the MMI effect. Moreover, the spectral channels can be consecutively increased from 2 to 4. The obtained results provide a platform for understanding and studying the multi-wavelength STML properties. This flexible all-fiber laser has great potential for applications in the fields of optical signal processing, fiber sensing, and information coding.

Nonlinear multimode interference-based dual-color mode-locked fiber laser

We present a dual-color-soliton fiber laser at two different wavebands by nonlinear multimode interference. A saturable absorber (SA) with single-mode-multimode (MMF)-single-mode fiber structure is placed in the common branch shared by two sub-cavities. Saturable absorption effects can be simultaneously satisfied at 1.5 and 2 µm at a proper length of the MMF. Dual-color solitons can still remain, even by slightly tuning the length of the MMF. The periodical characteristic of this SA provides a flexible choice of MMF length, making it simple for simultaneous mode locking (SML) at two separate wavebands in practice. Our approach not only paves the way for SML at two or more wavebands by the MMF but also could lead to significant applications in pump-probe spectroscopy.

Design of a dual-channel modelocked fiber laser that avoids multi-pulsing.

Multi-channel modelocked lasers and their design have attracted much attention. Here, we use the Swift-Hohenberg equation to study dual-channel simultaneous modelocking (DSML) in a fiber laser. When a quartic filter is added to the laser cavity, the stable dual-channel simultaneous modelocking can be obtained for a given filter bandwidth when frequency separation, ωs, is less than a calculated threshold, ωth. When ωs>ωth, a multipulsing instability occurs. We use a linear stability analysis to determine the limit that the multi-pulsing instability imposes on DSML, and we propose a cavity design that avoids the multi-pulsing instability.

Wavelength-stepped, actively mode-locked fiber laser based on wavelength-division-multiplexed optical delay lines

We propose a new scheme for an actively mode-locked wavelength-swept fiber laser that produces a train of discretely wavelength-stepped pulses from a short fiber cavity. Pulses with different wavelengths are split and combined by standard wavelength division multiplexers with fiber delay lines. As a proof of concept, we demonstrate a laser using an erbium doped fiber amplifier and commercially available wavelength-division multiplexers with wavelength spacing of 0.8 nm. The results show simultaneous mode-locking at three different wavelengths. Laser output parameters in time domain, optical and radio frequency spectral domain, and the noise characteristics are presented. Suggestions for the improved design are discussed.

CVD graphene‐based low pump threshold bidirectional mode‐locked fibre laser

A passively mode-locked femtosecond erbium-doped bidirectional fibre laser with low pump threshold using chemical vapor deposition grown graphene saturable absorbers is demonstrated. Two synchronised and counter propagating soliton output pulse trains with a repetition rate of ~10.38 MHz and pulse widths of 660 fs and 860 fs are obtained simultaneously from the fibre laser. Simultaneous mode-locking of the fibre laser has been realised with a threshold pump power of 56 mW from a single pump laser diode, indicating a ~62% reduction in pump threshold compared to previous results.

Simultaneous mode-locking at 1565 nm and 1944 nm in fiber laser based on common graphene saturable absorber

We present for the first time to the best of our knowledge an all-fiber thulium (Tm) and erbium (Er) doped fiber laser simultaneously mode-locked by a common graphene saturable absorber. The laser consists of two ring resonators combined with a common saturable absorber (SA). The generated optical solitons have a full width at half maximum (FWHM) of 3.9 nm and 4.2 nm for Tm- and Er-doped laser, respectively. The used graphene layers were grown on copper foils by chemical vapor deposition (CVD) and transferred onto the fiber connector end. Broadband and flat absorption spectrum of used SA supports mode-locked operation at 1565 nm and 1944 nm. The repetition frequency of the resonator with Er-doped fiber was 20.19 MHz while the Tm-doped resonator was around 1 m longer and resulted with repetition rate of 18.43 MHz. The reported experiment unambiguously confirms one of the biggest advantage of the carbon nanomaterial (in this case graphene) SAs over semiconductor saturable absorption mirrors (SESAM), which is broadband operation range, allowing to mode-lock two lasers spectrally separated by almost 400 nm.

Quad-Wavelength Fiber Optical Parametric Oscillator With Equally Distributed Dispersion

We demonstrate a 10-GHz quad-wavelength pulse source based on fiber optical parametric oscillator with equally distributed dispersion. By adopting two spools of single-mode fiber inside the cavity with nearly the same dispersion, simultaneous mode-locking at two different wavelengths with the wavelength spacing of around ~ 1.9 nm in the L-band is achieved. Owing to the parametric process between the pump and the two mode-locked signals, two idlers are generated in the S-band with the same wavelength spacing. Hence, simultaneous generation of 10-GHz pulse train at four different wavelengths located in both S-and L-band is obtained. The wavelength of the generated pulse trains can be tuned over 67 nm, from 1504 to 1536 nm and from 1575 to 1610 nm, with the wavelength span of around ~ 106 nm .

Multiwavelength Pulse Generation Using Fiber Optical Parametric Oscillator

We demonstrate a 10-GHz multiwavelength pulsed generator based on a fiber optical parametric oscillator. By introducing two separated intracavity branches, simultaneous mode-locking at two different wavelengths in the L-band is achieved. Due to the parametric process between the pump and the two mode-locked signals, two idlers are generated in the S-band. Hence, simultaneous generation of a 10-GHz pulse train at four different wavelengths located in both the S- and L-band is accomplished. The wavelength of the generated pulse trains can be tuned over 54 nm, with the wavelength span from 1500 to 1617 nm. The stability of the proposed scheme is also experimentally investigated.

Numerical Investigation and Prediction of Simultaneous Pulse-shaping and Mode-locking of Semiconductor Lasers

Numerical investigations based on a modified multimode laser rate equation model are reported that incorporate both active mode-locking and shaping of ultra-short optical pulses by linear Fourier-optical spectral filtering. At first glance, mode-locking and pulse-shaping seem to exclude each other, since mode-locking requires many longitudinal (axial) modes while pulse-shaping by linear-optical spectral filtering means suppression of many modes or even mode groups. Our results show that both features are possible simultaneously (even in the case with both an amplitude and a phase filter) due to the fact that energy transfer between suppressed modes, on the basis of spontaneous emission, can be strong enough for stable mode-locking. These findings suggest that the usually high signal loss for external pulse-shaping of semiconductor laser pulses can be circumvented by use of a pulse-shaping set-up internal to the mode-locking resonator; in this case only modes useful for pulse-shaping survive competition for inversion.

A diode-pumped high power Q-switched and self-mode-locked Nd:YVO4 laser with a LiF:F2- saturable absorber

Simultaneous mode-locking and Q-switching has been accomplished in a diode-pumped Nd:YVO4/LiF:F2- laser. At an absorbed pump power of 23.6 W, the average output power was 6.0 W and the repetition rate of the Q-switched pulse was 260 kHz. A depth of mode-locking of 100% was obtained and there was no satellite pulse between mode-locked pulse trains. The mode-locked pulse inside the Q-switched pulse had a repetition rate of approximately 148 MHz and its average duration was estimated to be around 75 ps.

Simultaneous Q-switching and mode-locking in a diode-pumped Nd:YVO/sub 4/-Cr/sup 4+/:YAG laser

Parametric studies of passive Q-switching and mode-locking in a Nd/sup 3+/:YVO/sub 4/-Cr/sup 4+/:YAG laser were theoretically carried out. Simultaneous mode-locking and Q-switching was also experimentally studied. It was found that over 90% of the output power could be mode-locked in a diode-pumped passively Q-switched Nd/sup 3+/:YVO/sub 4/-Cr/sup 4+/:YAG laser. The average pulse duration of the mode-locked pulse train was estimated to he around 110/spl sim/150 ps. The highest peak power of a single pulse near the maximum of the Q-switched envelope was greater than 100 kW.

10 x 30 GHz pulse train generation from semiconductor amplifier fiber ring laser

A multiwavelength, fiber ring laser source, is demonstrated. It generates 10 wavelength channels, simultaneously mode-locked and synchronized at 30 GHz, each producing 7-ps pulses. The mode-locking technique relies on the gain saturation of the semiconductor amplifier from an external optical pulse train to impose the simultaneous mode-locking of the 10 wavelengths.

Simultaneous modelocking and Q-switching of a Co:MgF 2 laser by loss-modulation frequency detuning

We report the first observation of simultaneous Q-switching and modelocking of a broadly tunable Co:MgF 2 laser operating at 1.5–2.0 μm. This behaviour is obtained by detuning the cavity with respect to the loss modulation. In this regime the modelocked pulses are slightly broadened, but they are increased in amplitude by more than an order of magnitude. The frequency of Q- switching is found to be half the frequency of relaxation oscillation observed for the same pump conditions.

Iodine photodissociation laser with plasma mirror

First experimental results with a pulsed iodine photodissociation laser controlled by target plasma reflectivity are reported. One of the laser mirrors is substituted for a focusing lens and a solid tilted target is located in its focal region. The laser dynamics depends on the geometry of the set-up. Laser pulse durations from 200 to 5 ns were observed if the resonator length was changed from 2 to 5 meters. Simultaneous mode-locking and Q-switching took place if the resonators were longer than 3 meters. Peak power densities of up to 1011 W/cm2 on the target and about a 1 per cent plasma reflectivity, increasing with increasing power density, were measured. A theoretical explanation of the laser behaviour and a comparison of our observations with previous results obtained with TEA-CO2 and Nd-glass amplifiers are presented.

Picosecond duration laser pulse generation by simultaneous active-passive mode-locking in Ar+ ion and dye lasers

We use a rate equation model to analyse a technique for the generation of picosecond duration laser pulses in argon pumped dye lasers. The system is based on passive mode-locking of the Ar+ laser by saturable absorption of the dye which is inserted in the Ar+ laser cavity. Simultaneously, the dye is forced to oscillate in a regime of synchronously pumped mode-locking. The system is relatively simple, does not require the use of an acousto-optical light modulator and can be easily implemented in commercially available lasers. Pulses shorter than 100 ps and having an average power of 20 mW were obtained at a repetition rate of 110 MHz. Good agreement between the model and experimental results was also obtained.

Two-Wave Sum-Frequency Light Generation in Optical Fibers

The generation of phase-matched two-wave sum-frequency light has been observed in optical fibers. A Nd: YAG laser by simultaneous mode-locking and Q-switching or only Q-switching operation is used as a pump laser. The input power dependence and fiber length dependence of the sum-frequency light are investigated in the two pumping methods. The observed coherence length for the sum-frequency light generation is about 200 m.

Two-wave sum-frequency light generation in optical fibers

The generation of phase-matched two-wave sum-frequency light has been observed in optical fibers. A Nd:YAG laser by simultaneous mode-locking and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switching or only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switching operation is used as a pump laser. The input power dependence and fiber length dependence of the sum-frequency light are investigated in the two pumping methods. The observed coherence length for the sum-frequency light generation is about 200 m.

Transverse mode-locking by short pulse injection in large aperture TEA CO_2 oscillators

Single short pulses were injected in TEA-CO(2) laser cavities made of two copper mirrors. Simultaneous longitudinal and transverse mode-locking was evidenced by periodic focusing of the internal short pulse. The focusing action could be eliminated by short train injection.

Internal pulse-code modulation of a 0.6328 ?m He-Ne laser at 150 Mbit s?1

Pulse-code modulation of a 0.6328μm He-Ne laser by simultaneous mode-locking and coupling modulation is achieved at a bit-rate of 150 Mbits−1 using a single modulation element. Both modelocking techniques, loss modulation and phase modulation are used and compared. The phase of the output signal is stabilized by means of a phase-locked loop. The influence of the coupling modulation on the resonator energy causes a time-dependent decrease of the output signal amplitude. This effect can be compensated by controlling the coupling voltage. A calculation of the maximum available bitrate for a 2 mm bore He-Ne laser tube is given.