Ultra-long Fiber Laser Research Articles

Gain-managed nonlinear amplification of ultra-long mode-locked fiber laser.

In this study, we explored the gain-managed nonlinear (GMN) amplification of ultra-low repetition rate pulses in the range of less than 1 MHz. By seeding the developed 1040 nm ultralong fiber modelocked laser to the GMN amplifier, we achieved high gain and boosted the nonlinear pulse propagation effects. We demonstrated that GMN amplification of low repetition rate pulses provided amplification exceeding 32 dB and spectral broadening up to 91 nm at relatively low pump power levels. Achieving broadband 57 fs pulses with energy exceeding 55 nJ suggests that ultralong fiber lasers paired with GMN amplifiers can be effectively utilized as powerful tools for generating femtosecond broadband pulses at ultra-low repetition rates, with controllable spectral characteristics.

364 km ultra-long Raman fiber laser based on high-order pumping and ultra-low-loss fiber

Raman fiber laser (RFL) based on distributed Raman gain along optical fiber is an ideal platform for realizing ultra-long-cavity laser, which is of great interest in fundamental laser science and multidisciplinary fundamental studies such as complex systems exhibiting turbulence-like characteristics. RFL based on standard single-mode fiber sets the record of the longest laser cavity reaching 270 km with a clearly resolvable mode structure. Here, we explore the effect of pumping scheme and key fiber parameters to further extend the ultimate length of RFL. We theoretically analyze and experimentally demonstrate that by adopting higher-order pumping and ultra-low-loss fiber (ULLF) with low transmission loss and low Rayleigh backscattering coefficient, the laser cavity length of RFL with resolvable mode structure can be significantly increased. As a result, a 364 km-long bi-directional pumped 6th-order ultra-long Raman fiber laser (URFL) based on ULLF is demonstrated with resolvable longitudinal mode, an extension of 35 % over the prior 270 km record. This work paves a way to lengthen cavity length of URFL and provides a new platform for fundamental laser physics study and the ultra-long fiber laser applications in communication and sensing.

Resilience to Passive Attacks of a Secure Key Distribution System Based on an Ultra-Long Fiber Laser Using a Bi-Directional EDFA

In this paper, we study the implementation of a secure key distribution system based on an ultra-long fiber laser with a bi-directional erbium-doped fiber amplifier. The resilience of the system was tested against passive attacks from an eavesdropper. A similarity was observed in the spectra for both secure configurations of the system and no signature that would allow an eavesdropper to obtain the secure state of the system was observed during the state transitions.

Secure key distribution using an ultra-long fiber laser with bi-directional EDFA

In this paper we describe the implementation of a secure key distribution system based on an ultra-long fiber laser with a bi-directional erbium doped fiber amplifier. The resilience of the system was tested against passive attacks from an eavesdropper, having been observed a similarity in spectrum for both secure configurations of the system.

Study of a dissipative soliton in the distributed mathematical model of ultra-long mode-locked fiber lasers

We propose the new, to the best of our knowledge, distributed mathematical model of dissipative soliton evolution in ultra-long fiber lasers. The model is based on a modified Ginzburg–Landau equation, has a stable analytical solution, and takes into account the saturated gain and saturable absorption. The analytical results show a good accordance with the results of numerical simulation of ultra-long ring cavity fiber lasers. The results may be applied to the analysis of a wide range of fiber laser systems.

Hidden Probe Attacks on Ultralong Fiber Laser Key Distribution Systems

In ultralong fiber laser key distribution, two sides use standard optical equipment to create kilometer long fiber lasers in a communication link to establish a secret key. Its security rests on the assumption that any attack to learn the generated key would be unpractical while the legitimate users only need to use standard equipment. We present a challenge to that assumption with a hidden probe attack in which the eavesdropper hides a weak signal in the unavoidable noise floor that appears in the laser during amplification and probes with it the configuration of one or both communication parties. We comment how this attack can compromise different proposals for ultralong laser key distribution and propose possible countermeasures.

Secret key exchange in ultralong lasers by radiofrequency spectrum coding

AbstractWe propose a new approach to the generation of an alphabet for secret key exchange relying on small variations in the cavity length of an ultralong fiber laser. This new concept is supported by experimental results showing how the radiofrequency spectrum of the laser can be exploited as a carrier to exchange information. The test bench for our proof of principle is a 50-km-long fiber laser linking two users, Alice and Bob, where each user can randomly add an extra 1-km-long segment of fiber. The choice of laser length is driven by two independent random binary values, which makes such length become itself a random variable. The security of key exchange is ensured whenever the two independent random choices lead to the same laser length and, hence, to the same free spectral range.

Identifying the mechanisms of pulse formation and evolution in actively mode-locked Erbium fiber lasers with meters and kilometers-long

Abstract We investigated the dynamics of pulse evolution in Erbium-doped fiber ring lasers with cavity lengths varying from 16.4 m to 100.8 km actively mode-locked at repetition rate of 1 GHz. The novelty of this work is to explore the limits of Kuizenga–Siegman theory in ultralong fiber laser and to demonstrate the dynamics of pulse generation and propagation separately. When we vary the length of the Erbium-doped fiber lasers from meters to kilometers long, three operation regimes were identified: mode-locking regime (for cavity lengths with 16.4 m to 1 km), nonlinearity-dominant regime (1 to 10 km) and dispersion and nonlinearity regime that locked the ratio between soliton period and cavity length: Z S / L cav = 1.35 for cavities with 10 to 100 km in a soliton intracavity condition. The variation of pulse widths and the peak powers are analyzed to define the propagation regimes inside the cavities, depending on the cavity length ( L cav ) , dispersion length ( L D ) and nonlinear length ( L NL ) . When Lcav is shorter than LD and LNL, there is neither dispersive nor nonlinear effect during pulse evolution (pulse has duration of approximately 30 ps). In this regime, its final duration is determined by the standard theory of active mode-locking. For Lcav shorter than LD but ~ L NL , the pulse evolution is in nonlinearity-dominant regime where soliton propagation provides a sech2 profile with a TBP transform limited of 0.315. In addition, for cavities longer than 10 km, L cav ~ L D and longer (or much longer) than LNL, the pulse evolution is in the dispersion and nonlinearity dominant regime with its duration depending on the accumulated dispersion. In this regime the soliton effect takes place and the final pulse duration is defined by the cavity length which is approximately the soliton period.

Dark states ultra-long fiber laser for practically secure key distribution

We present and demonstrate a novel ultra-long fiber laser key distribution system (UFL-KDS). The scheme quenches the lasing process when in its secure states, thus forming "dark states" which provide simple detection on one hand and increased difficulty of eavesdropping on the other. We analyze the practical aspects of previously studied UFL-KDS schemes as well as those of the one presented here and demonstrate successful key distribution across a 200 km link with bit-rates that can exceed 0.5 kbps. Spectral and temporal passive attack strategies are analyzed and discussed in details.

Cavity and random ultralong fibre laser amplification in BOTDAs: a comparison

The predicted performance of BOTDA sensors, assisted by Raman amplification provided through either cavity or bi-pumped mirrorless random ultralong laser schemes, is compared, showing that, although the latter are less affected by RIN transfer for sensing lengths above 100 km, they also require much higher pump powers and suffer more from SPM-induced broadening of the Brillouin pulsed pump.

94 km Brillouin distributed optical fiber sensors based on ultra-long fiber ring laser pumping

A novel optical amplification configuration based on ultra-long fiber laser with a ring cavity was proposed and applied to Brillouin optical time-domain analysis (BOTDA) sensing system, in order to extend the measurement distance significantly. The parameters used in the experiment were optimized, considering the main limitations of the setup, such as depletion, self-phase modulation (SPM) and pump-signal relative intensity noise (RIN) transfer. Through analyzing Brillouin gain spectrum, we demonstrated distributed sensing over 94 km of standard single-mode fiber with 3 meter spatial resolution and strain/temperature accuracy of 28 /1. 4 degree C.

Secure key distribution over a 500 km long link using a Raman ultra‐long fiber laser

AbstractIn traditional communication systems the transmission medium is considered as a given characteristic of the channel, which does not depend on the properties of the transmitter and the receiver. Recent experimental demonstrations of the feasibility of extending the laser cavity over the whole communication link connecting the two parties, forming an ultra‐long fiber laser (UFL), have raised groundbreaking possibilities in communication and particularly in secure communications. Here, a 500 km long secure key distribution link based on Raman gain UFL is demonstrated. An error‐free distribution of a random key with an average rate of 100 bps between the users is demonstrated and the key is shown to be unrecoverable to an eavesdropper employing either time or frequency domain passive attacks.

Distributed Raman amplification using ultra-long fiber laser with a ring cavity: characteristics and sensing application

Distributed Raman amplification (DRA) based on ultra-long fiber laser (UL-FL) pumping with a ring cavity is promising for repeaterless transmission and sensing. In this work, the characteristics (including gain, nonlinear impairment and noise figure) for forward and backward pumping of the ring-cavity based DRA scheme are fully investigated. Furthermore, as a typical application of the proposed configuration, ultra-long-distance distributed sensing with Brillouin optical time-domain analysis (BOTDA) over 142.2 km fiber with 5m spatial resolution and ± 1.5 °C temperature uncertainty is achieved, without any repeater. The key point for the significant performance improvement is the system could offer both of uniform gain distribution and considerably suppressed pump-probe relative intensity noise (RIN) transfer, by optimized design of system structure and parameters.

Generation of dissipative solitons in an actively mode-locked ultralong fibre laser

A single-pulse actively mode-locked fibre laser with a cavity length exceeding 1 km has been developed and investigated for the first time. This all-fibre erbium-doped laser has a normal intracavity dispersion and generates dissipative 8-ns solitons with a fundamental repetition rate of 163.8 kHz; the energy per pulse reaches 34 nJ. The implemented mode locking, based on the use of intracavity intensity modulator, provides self-triggering and high stability of pulsed lasing. A possibility of continuous tuning of the centre lasing wavelength in the range of 1558 – 1560 nm without any tunable spectral selective elements in the cavity is demonstrated. The tuning occurs when controlling the modulation signal frequency due to the forced change in the pulse repetition time (group delay) under the conditions of intracavity chromatic dispersion.

Hybrid lasing in an ultra-long ring fiber laser

In this paper, we reported the realization of an ultra-long ring fiber laser (RFL) with hybrid emission related to both random lasing and cavity resonance. Compared with a linear random fiber laser (LRFL), the Rayleigh scattering (RS) inducting distributed feedback effect and the cavity inducting resonance effect exist simultaneously in the laser, which reduces the lasing threshold considerably and provides a hybrid way to form random lasing (RL). The laser output can be purely modeless RL when pump power is high enough. It is also discovered that the laser is insensitive to temperature variation and mechanical disturbance, this is unique and quite different from conventional RFLs which are environmentally unstable due to existence of the cavity modes.

Optical turbulence and spectral condensate in long fibre lasers

We study numerically optical turbulence using the particular example of a recently created, ultra-long fibre laser. For normal fibre dispersion, we observed an intermediate state with an extremely narrow spectrum (condensate), which experiences instability and a sharp transition to a fluctuating regime with a wider spectrum. We demonstrate that the number of modes has an impact on the condensate's lifetime. The smaller the number of modes, the more resistant is the condensate to perturbations. Experimental results show a good agreement with numerical simulations.

Detailed theoretical investigation on improved quasi-lossless transmission using third-order Raman amplification based on ultralong fiber lasers

An alternative structure using third-order Raman amplification based on ultralong fiber lasers (UL-FLs) is investigated numerically to further improve the quasi-lossless transmission performance. The performance comparisons on flatness of signal distribution, optical signal–noise ratio (OSNR), and noise figure (NF) among first-order, second-order, and third-order Raman pumping based on UL-FLs are discussed numerically. The results display that the gain distribution is pushed more uniformly along UL-FLs using third-order Raman pumping, which is very useful to further extend the quasi-lossless midspan distance. For transparency transmission, enhanced quasi-lossless transmission could be achieved without sacrifice to OSNR and NF. We also analyze the transmission dynamics of loss-managed fundamental solitons for different pumping configurations. The amplifier span and transmission quality are improved considerably, even for lower dispersion lengths, by employing the proposed scheme.

RIN transfer in 2nd-order distributed amplification with ultralong fiber lasers

We investigate numerically the effect of ultralong Raman laser fiber amplifier design parameters, such as span length, pumping distribution and grating reflectivity, on the RIN transfer from the pump to the transmitted signal. Comparison is provided to the performance of traditional second-order Raman amplified schemes, showing a relative performance penalty for ultralong laser systems that gets smaller as span length increases. We show that careful choice of system parameters can be used to partially offset such penalty.

Long-distance soliton transmission through ultralong fiber lasers

We present the first experimental demonstration (to our knowledge) of long-distance unperturbed fundamental optical soliton transmission in conventional single-mode optical fiber. The virtual transparency in the fiber required for soliton transmission, over 15 complete periods, was achieved by using an ultralong Raman fiber laser amplification scheme. Optical soliton pulse duration, pulse bandwidth, and peak intensity are shown to remain constant along the transmission length. Frequency-resolved optical gating spectrograms and numerical simulations confirm the observed optical soliton dynamics.

Enhanced key-establishing rates and efficiencies in fiber laser key distribution systems

The secure key-establishing efficiency of an ultra-long fiber laser key distribution system (UFL-KDS) is limited by half a key bit per cycle. We purpose and analyze novel schemes and protocols for breaking this limit while retaining system security. These schemes increase the probability of a secure bit transition by expanding the variable and coding parameters of the UFL-KDS. Furthermore, a new wavelength division multiplexing UFL implementation, which enables an additional key-establishing rate enhancement, is proposed and analyzed. These schemes further emphasize the potential in the UFL concept, in particular its security and its practical implementation properties.