Cavity Round-trip Time Research Articles

Cavity dumping by the second harmonic generation in the Q-switched Nd : YAG laser

This paper presents the cavity dumping method by the second harmonic generation in a neodymium laser operating in the Q-switched mode. Theoretical modeling of the laser generation in dynamics is performed. The main characteristics of the output pulses and their dependence on the pump power and coefficient of losses are calculated. The proposed method can be implemented in the laser with a second harmonic crystal inside the cavity and an electro-optical crystal, which is operated by a step voltage pulse. The switching pulse length is defined by the time needed to achieve the maximum giant pulse intensity in the cavity, and is in the order of 0.1–1.0 ms depending on the pump power. Moreover, the voltage pulse jitter should not exceed several nanoseconds. In such case the second harmonic pulses with nanosecond duration and peak intensity of 10 –100 MW/cm2 are generated at the laser output. The output pulses duration is defined only by the cavity length under the conditions of a small response time of the electro-optical crystal compared to the cavity round-trip time. To achieve the maximal peak intensity, one should decrease the coefficient of inactive losses to the possible minimum and increase the pump power.

Generation and pulsating behaviors of loosely bound solitons in a passively mode-locked fiber laser

We demonstrate the experimental observation of loosely bound solitons and pulsations in a passively mode-locked fiber laser. The soliton amount inside one soliton bunch typically increases with the increment of pump power, while the average pulse-to-pulse separation in the regime of hundreds of picoseconds decreases in principle accordingly. Besides, by employing the real-time measurement based on the dispersive Fourier-transform technique, we find that some solitons in a single bunch are experiencing a simultaneous pulsating process with a period corresponding to 100 times longer than the cavity round-trip time. The experimental results can be beneficial for better understanding the formation of multiple solitons and pulsating solitons and enriching the nonlinear phenomena in complex soliton systems.

H-shaped pulse generation with tunable leading edge from a Tm-doped mode-locked fiber laser

A novel h-shaped pulse train with a tunable leading-edge (resembling a h-shaped pulse merging with a weak square pulse) is experimentally generated in a Tm-doped mode-locked fiber laser (MLFL) incorporating a 100 m highly nonlinear fiber. Through carefully adjusting the pump power and cavity polarization states, the leading-edge duration of the obtained h-shaped pulse can be widely tuned from zero (corresponding to the typical conventional h-shaped pulse train) to 567.8 ns (equivalent to the cavity roundtrip time). Our results can further facilitate understanding the intrinsic mechanism and characteristics of novel h-shaped pulse dynamics in MLFLs.

Properties of Phase Solitons in an Optically Driven Semiconductor Ring Laser

A semiconductor ring laser with a long cavity supports propagating localised structures with a chiral charge, named phase solitons. In this paper we study the dependence of the velocity and of the duration of the phase solitons on the characteristic time scales of the laser, namely the photon lifetime and the carrier lifetime. We show numerically that phase solitons are stable over a large range of those parameters and verify that the propagation velocity decreases linearly with the ratio of the carrier lifetime to the photon lifetime, while the duration is proportional to the ratio of the carrier lifetime to the cavity roundtrip time.

Curvature Sensor Based on a Long-Period Grating in a Fiber Ring Resonator Interrogated by an OTDR

The proposed technique demonstrates a fiber ring resonator interrogated by an optical time domain reflectometer (OTDR), for intensity sensing. By using this methodology, a cavity round trip time of 2.85 μs was obtained. For a proof of concept, a long-period grating was inserted in the resonant cavity operating as a curvature sensing device. A novel signal processing approach was outlined, regarding to the logarithmic behavior of the OTDR. Through analyzing the experimental results, an increase in the measured sensitivities was obtained by increasing applied bending. With curvatures performed from 1.8 m−1 to 4.5 m−1, the sensitivity values ranged from 2.94 dB·km−1 to 5.15 dB·km−1. In its turn, the sensitivities obtained presented a linear behavior when studied as a function of the applied curvature, following a slope of 0.86×10−3 dB. The advantages of applying this technique were also discussed, demonstrating two similar fiber rings multiplexed in a series of configurations.

Dual-chirp Fourier domain mode-locked optoelectronic oscillator.

Optoelectronic oscillators (OEOs) have been widely investigated to generate ultra-pure single-frequency microwave signals. Here, we propose and experimentally demonstrate a dual-chirp Fourier domain mode-locked (FDML) OEO to generate dual-chip microwave waveforms. In the proposed FDML OEO, a frequency-scanning dual-passband microwave photonics filter based on phase-modulation-to-intensity-modulation conversion using an optical notch filter and two laser diodes is incorporated into the OEO cavity. Fourier domain mode-locking operation is achieved by synchronizing the scanning period of the filter to the cavity round-trip time. Tunable dual-chirp microwave waveforms with a large time-bandwidth product are generated directly from the FDML OEO cavity in the experiment, which can be used in modern radar systems to improve its range-Doppler resolution.

Harmonically Fourier Domain Mode-Locked Optoelectronic Oscillator

We experimentally demonstrate a harmonically Fourier domain mode-locked optoelectronic oscillator (FDML OEO). Harmonic Fourier domain mode locking operation is achieved when the cavity round-trip time is equal to integer multiples of the scanning period of the filter in the OEO loop. Compared with a fundamentally FDML OEO, frequency scanning microwave signals with increased tuning speed and large bandwidth can be easily generated by the proposed harmonically FDML OEO, which can find applications in spread-spectrum communication and modern radar systems. Up to fourth order, harmonic Fourier domain mode locking operation is achieved in the experiment. The time-bandwidth product of the harmonically FDML OEO is as large as tens of thousands. The maximum chirp rate of the generated $X$ -band linearly chirped microwave waveform is 0.725 GHz/ $\mu \text{s}$ , which is four times higher than that of the fundamentally FDML OEO.

Tunable Fourier Domain Mode-Locked Optoelectronic Oscillator Using Stimulated Brillouin Scattering

We propose and demonstrate a tunable Fourier domain mode-locked optoelectronic oscillator (OEO) using a frequency scanning microwave photonics filter (MPF) based on stimulated Brillouin scattering (SBS). Fourier domain mode locking operation is achieved by synchronizing the tuning period of the SBS-based MPF to the cavity round-trip time of OEO. As a result, thousands of longitudinal modes are stimulated simultaneously in the OEO cavity. Linearly chirped microwave waveforms with a chirp rate up to 0.84 GHz/ $\mu \text{s}$ and a time bandwidth product as high as 22446 are generated in the experiment. The proposed approach provides an important solution for chirped microwave waveforms generation, which is widely used in modern radar and communication systems, and has the potential to be integrated on a chip.

Fourier domain mode locked optoelectronic oscillator based on the deamplification of stimulated Brillouin scattering

A fast frequency scanning optoelectronic oscillator (OEO) based on Fourier domain mode locking (FDML) and the deamplification of stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. The SBS is used to realize a fast frequency scanning microwave photonics filter (MPF). By synchronizing the scanning period of the MPF to the cavity round-trip time, the OEO can be made to operate in the FDML regime. Tunable linearly chirped microwave waveforms are generated in the experiment with a chirp rate up to 0.85 GHz/μs and a time-bandwidth product as large as 23,850. The proposed fast frequency scanning FDML OEO can find applications in modern radar and communication systems.

Short pulse generation in active Q-switched Yb-doped all fiber laser and its amplification

Generation of stable sub-cavity round-trip time pulses have been achieved in all-fiber Ytterbium-doped Q-switched fiber laser by controlling the modulation window ON-time of acousto-optic modulator (AOM). Short pulses (anomalous pulses) of duration in the range of 50–55 ns have been achieved in experimental configurations with the cavity round-trip time of ∼130 ns, when modulation window “ON-time” of AOM was equal to or shorter than the pulse build-up time of the normal Q-switched pulses. In the case of normal AO Q-switching, pulses of duration more than 1.3 µs with an average power of 30 mW and pulse energy of 0.38 µJ at 80 kHz of repetition rate were obtained at 2 W of pump power. At the same value of the pump power, the anomalous pulses of ∼55 ns with an average power of 12 mW were generated, which are ∼24 times shorter than pulses obtained with conventional Q-switching process (normal pulses). The peak power of the anomalous pulses are higher than the normal pulses by the factor of eight at the same value of the pump power. The anomalous pulses from the AO Q-switched oscillator with 55 ns pulse duration have been amplified in single mode Ytterbium doped all fiber amplifier in two stages. Amplified average output power of ∼19 W was achieved with the output pulse energy and peak power of 240 µJ and 4.4 kW respectively in all fiber configuration.

The Dynamics of a Field in a Cavity with a Slowly Oscillating Metal Mirror

We have examined analytically the behavior of an electromagnetic field in a cavity of one of two metal mirrors that is stationary, while the other mirror oscillates in the vicinity of its equilibrium position. The case in which the period of the mirror oscillations substantially exceeds the characteristic roundtrip time of the electromagnetic radiation in the cavity has been considered. To solve the problem, we have applied the method of two time scales. The method makes it possible to solve the problem by using the expansion with respect to a small parameter, for which the ratio of the cavity roundtrip time to the mirror-oscillation period is used. The solution to the problem in the zeroth-order approximation with respect to the small parameter has been obtained and examined. This solution has been shown to be correct up to the mirror-oscillation amplitude being on the order of the average cavity length. We have showed that this scheme can be used to gain and generate electromagnetic radiation. We have compared our results obtained for the oscillating mirror with the results of an exact solution of the problem when the mirror moves uniformly and rectilinearly. We have showed that, in these two cases, a change in the total field energy in the cavity is inversely proportional to the current value of the cavity length.

A systematic comparison and evaluation of three different Swept-Source interferometers for eye lengths biometry

This study reviews the development of Swept-Source interferometers and compares systematically three different Swept-Source interferometer designs for biometric measurements of the eye. Principles characteristics, conveniences and accessibilities of the three developed systems are presented.The main difference between the three Swept-Source systems is the method of tuning the wavelength at the broadband optical amplifier.The implementation of a “quasi-phase-continuous method” (QPC) for wavelength tuning led to longer measuring depth but was more time-consuming. The wavelength tuning using a rotating polygon mirror scanner was faster.The wavelength tuning via Fourier Domain Mode Locking (FDML), where the tuning frequency ft of the filter must be matched to the inverse cavity roundtrip time τ, achieved the widest tuning range combined with a rather better resolution and signal to noise ratio (SNR).The swept sources were compared using a fiber-optic based Michelson interferometer setup. Measurements of a self-made human model eye demonstrate excellent capturing of the biometric data, with all interfaces of eye optical components and their contours being clearly detected.

Passive and hybrid mode locking in multi-section terahertz quantum cascade lasers

It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking.

Harmonic mode-locking and sub-round-trip time nonlinear dynamics of electro-optically controlled solid state laser

Harmonic mode-locking in a solid state laser due to optoelectronic control is studied numerically on the basis of two methods. The first one is detailed numeric simulation taking into account laser radiation fine time structure. It is shown that optimally chosen feedback delay leads to self-started mode-locking with generation of desired number of pulses in the laser cavity. The second method is based on discrete maps for short laser pulse energy. Both methods show that the application of combination of positive and negative feedback loops allows to reduce the period of regular nonlinear dynamics down to a fraction of a laser cavity round trip time.

Self-organized compound pattern and pulsation of dissipative solitons in a passively mode-locked fiber laser.

We experimentally observe soliton self-organization and pulsation in a passively mode-locked fiber laser. The optomechanical interaction in the optical fiber is key to the formation of equidistant soliton bunches. These solitons simultaneously undergo a pulsation process with a period corresponding to tens of the cavity round trip time. Using the dispersive Fourier transformation technique, we find that the Kelly sidebands in the shot-to-shot spectra appear periodically, synchronizing with the pulsation.

Switchable generation of rectangular noise-like pulse and dissipative soliton resonance in a fiber laser

We report on the switchable generation of a rectangular noise-like pulse (NLP) and a dissipative soliton resonance (DSR) in a fiber laser with highly nonlinear effect at very low pump power. The NLP centered at 1530.5nm demonstrates a new characteristic that its profile evolves gradually from rectangular shape to Gaussian-like shape with the increasing pump power. By appropriately manipulating the polarization controller (PC), the laser switches emit a DSR pulse centered at 1551.3nm. The duration of the DSR could broaden from 17.4ns to the cavity round trip time with increasing the pump power, while keeping the pulse profile and the intensity unaltered. This type of fiber laser may not only facilitate further investigations of the characteristics of NLP and DSR but also serve as a multi-functional optical source for potential applications.

Pseudorandom dynamics of frequency combs in free-running quantum cascade lasers

Recent research has shown that free-running quantum cascade lasers are capable of producing frequency combs in midinfrared and THz regions of the spectrum. Unlike familiar frequency combs originating from mode-locked lasers, these do not require any additional optical elements inside the cavity and have temporal characteristics that are dramatically different from the periodic pulse train of conventional combs. Frequency combs from quantum cascade lasers are characterized by the absence of sharp pulses and strong frequency modulation, periodic with the cavity round trip time but lacking any periodicity within that period. To explicate for this seemingly perplexing behavior, we develop a model of the gain medium using optical Bloch equations that account for hole burning in spectral, spatial, and temporal domains. With this model, we confirm that the most efficient mode of operation of a free-running quantum cascade laser is indeed a pseudorandom frequency-modulated field with nearly constant intensity. We show that the optimum modulation period is commensurate with the gain recovery time of the laser medium and the optimum modulation amplitude is comparable to the gain bandwidth, behavior that has been observed in the experiments.

Passive mode-locking at S-band by single-mode thulium-doped fluoride fiber using a thin film PtAg/N-G saturable absorber

We report, for the first time to our knowledge, the usage of platinum silver bimetallic alloy forms on the synthesized N-Gns (PtAg/N-G) as a saturable absorber (SA) for the mode-locking of a thulium-doped fluoride fiber laser. The experimental results have demonstrated that PtAg/N-G exhibits saturable absorption in the 1.5-μm wavelength range and supports ultrashort pulse generation. Mode-locking occurred when the pump power rose to 340 mW, and the output spectrum had a peak at 1506 nm with the 3-dB bandwidth of ∼0.76 nm. The mode-locked pulse train against a cavity roundtrip time of 146 ns that corresponds to a pulse repetition rate around 6.85 MHz. The autocorrelation trace 7.43 ps with a full width at half maximum pulse width of 4.8 ps, and experimental data were closely fit with a sech2 pulse shape and it indicates the generation of soliton pulse. The output power of this fiber laser is 3.24 mW; hence, the pulse energy is 0.31 nJ and peak power is about 56.9 mW as well. We demonstrate graphene-polymer nanocomposites could be an appropriate SA for high-power fiber laser mode-locking.

Spatio-temporal generation regimes in fiber laser systems (Review)

This paper presents a new concept for the study of spatio-temporal generation regimes in which several time scales coexist in the radiation (one of which is related to the cavity round-trip time). The essence of the concept is that the time dependence of the intensity is investigated in two dimensions, one of which corresponds to the evolution over successive cavity round trips. It is shown that fiber lasers of various types, e.g., quasicontinuous lasers and passively and actively mode-locked pulsed lasers, have a variety of spatio-temporal generation regimes. The possibility of experimental detection of localized structures, including solitons, in fiber laser radiation is demonstrated. The prospects of the proposed approach are discussed.

Observation of Mode-Locked Spatial Laser Solitons.

A stable nonlinear wave packet, self-localized in all three dimensions, is an intriguing and much sought after object in nonlinear science in general and in nonlinear photonics in particular. We report on the experimental observation of mode-locked spatial laser solitons in a vertical-cavity surface-emitting laser with frequency-selective feedback from an external cavity. These spontaneously emerging and long-term stable spatiotemporal structures have a pulse length shorter than the cavity round-trip time and may pave the way to completely independent cavity light bullets.