Nonlinear Optical Loop Mirror Research Articles

Cavity dynamics of a hybrid mode‐locked figure of eight fiber laser

AbstractMode‐locking is an increased focus area due to the augmented requirement of ultra‐short and high‐intensity optical pulses for varied applications. Tunable mode‐locked lasers capable of operating in the infrared range at high repetition frequency brought innovative opportunities for insensitive and accurate laser surgery. As one important application of optical pulse propagation in an optical fiber, we have discussed a dual‐ring hybrid mode‐locked fiber laser using modulator, and saturable absorber. The active mechanism, saturable absorber, and the nonlinear optical loop mirror result in an output with a peak power of 16 dBm, and 80 fs full‐width at half‐maximum with a high duty cycle. It occupies a small space and is easy to achieve miniaturization and intensification; the laser output can be adjusted by varying multiple parameters such as pump, input, doped fiber length, modulator (including modulation signal), saturable absorber, and other devices.

Experimental demonstration of all-optical aggregation and de-aggregation for a QPSK signal in an elastic optical network.

The aggregation and de-aggregation between one QPSK and two BPSK signals are experimentally demonstrated based on cross-phase modulation (XPM) effects and phase-sensitive amplification (PSA), aiming to improve flexible spectral efficiency in the elastic optical network. Benefiting from the nonlinear-optical loop mirror (NOLM), the two BPSK signals can be extracted without any information redundancy for the de-aggregation scheme. The wavelength of the extracted signals stay the same as the input signal in the NOLM. Moreover, the aggregation from two BPSK signals to one QPSK signal is also successfully achieved. The feasibility of the two schemes can be confirmed by the transfer functions and the input-output constellation. The bit-error-rate (BER) and error vector magnitude(EVM) performance of the two schemes are also investigated and the corresponding OSNRs for error-free signal recovery are obtained respectively. The proposed schemes contribute to the realization of flexible optical networks and can be applied in the future gateway node between optical sub-networks.

Bound-State Soliton and Noise-like Pulse Generation in a Thulium-Doped Fiber Laser Based on a Nonlinear Optical Loop Mirror

We demonstrate a thulium-doped, mode-locked, all-fiber laser capable of operating in two generation regimes: dispersion-managed soliton and noise-like pulse (NLP). Employing a nonlinear optical loop mirror as an artificial saturable absorber, the oscillator generated optical pulses with a fundamental pulse repetition frequency of ~15.795 MHz. The total net dispersion of the laser cavity had a slightly anomalous group delay dispersion value of −0.016 ps2. After appropriate adjustment of a polarization controller, bound states of a dispersion-managed soliton composed of three pulses with fixed soliton separations were also observed. NLP generation, tunable over 35 nm from 1943.5 to 1978 nm, was also presented in the same laser setup. To our knowledge, this is the first report of the generation of tunable NLPs in a mode-locked thulium-doped fiber laser based on a nonlinear loop mirror saturable absorber.

All-Polarization Maintaining Noise-Like Pulse From Mode-Locked Thulium-Doped Fiber Laser Based on Nonlinear Loop Mirror

In this paper, we present a figure-eight all-polarization maintaining thulium-doped mode-locked fiber laser based on a nonlinear optical loop mirror operating at noise-like pulse regimes. The total cavity length is 34.5 m, and the calculated net group delay dispersion is estimated to be ∼−2.616 ps <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . At the maximum pump power, stable noise-like pulse centered at 1943 nm with a spectral full width at half maximum of 32 nm, a pulse energy of 13.51 nJ, and a coherence spike width of 236 fs is generated at a repetition rate of 5.96 MHz with the signal-to-noise ratio ∼64 dB. The polarization extinction ratio is > 15 dB at any mode-locking operation. The slope efficiency and the output power fluctuation of the laser are 5.16% and 0.039%, respectively. The laser does not require polarization state controllers, and it holds very high stability, which is insensitive to temperature changes or mechanical vibrations. Through further amplification, it will be a good laser source for real-time interrogation of fiber Bragg gratings and supercontinuum generation.

Experimental measurements of the transfer function of a nonlinear optical loop mirror

Measurement of the average values of the input and output powers of a device can give insight into the transfer function (TF) of that device, but this approach usually hides the real impact of certain propagation phenomena. However, to the best of our knowledge, measurements of the TF of nonlinear optical loop mirrors (NOLMs) have always been carried out using this approach, which may lead to underestimating the impact of dispersive and nonlinear effects. Here, we present the experimental measurement of the TF of a NOLM, made from experimental measurements of the input and output intensity profiles of the device, using a frequency-resolved optical gating technique. Our approach clearly highlights the impact of dispersion effects and Kerr nonlinearity on the NOLM’s TF.

Studying the Operation of an All-PM Yb-Doped Fiber Laser Oscillator at Negative and Positive Net Cavity Dispersion

Chirped fiber Bragg gratings in Yb-doped fiber lasers allow tuning the net cavity dispersion, thus enabling access to pulse dynamics beyond the typical all-normal-dispersion dissipative soliton regime. This Letter demonstrates an ultrafast dispersion-managed all-polarization-maintaining fiber oscillator mode-locked with a nonlinear optical loop mirror. Using a chirped fiber Bragg grating inside the oscillator, it can work at net cavity dispersion in the range from -0.098 ps$^2$ up to +0.067 ps$^2$ and above at the wavelength of 1 $\mathrm{\mu}$m. Depending on the configuration, the system can deliver stable pulses with energies up to 2.3 nJ and pulse durations as short as 98 fs.

Dissipative soliton resonance and noise-like pulse generation of large normal dispersion Yb-doped fiber laser

In this paper, we experimentally demonstrate two types of dissipative soliton resonant (DSR) and noise-like pulse (NLP) in a mode-locked fiber laser using the nonlinear optical loop mirror (NOLM). By appropriately adjusting the polarization states, the switchable generation of DSR and NLP can be achieved from one mode-locked fiber laser. By adjusting the pump power, the pulse width of DSR increases gradually from 2.45 to 13.35 ns with a constant peak intensity, while the NLP just has a little increase, even splitting into two narrower pulses at higher pump power. Two types of DSR and NLP have the same pulse periods of 1.29 μs, corresponding to the cavity length of the fiber laser. The obtained results display the evolution process of DSR pulse and NLP in mode-locked fiber laser and have some application in optical sensing, spectral reflectometry, micromachining, and other relative domains.

Low threshold multi-wavelength erbium-doped fiber ring laser with NOLM and Lyot filter

In this paper, the performance of a switchable multi-wavelength erbium-doped fiber laser (MWEDFL) incorporating Lyot filter and nonlinear optical loop mirror (NOLM) is described. The NOLM consisting of a polarization controller (PC2) and a single-mode fiber is effective to suppress the mode competition caused by the homogeneous broaden line in the erbium-doped fiber amplifier (EDFA). The comb filter is developed by using PC1 and polarization-maintained fiber (PMF). The wavelength interval can be switched among 0.08 nm, 0.34 nm, and 0.89 nm using different lengths of PMF via careful adjusting of the PCs. When the pump power is 45mW, the laser can emit 12 stable lasing lines within the 3 dB bandwidth, and the side mode suppression ratio (SMSR) is greater than 25 dB. The spectral power and wavelength fluctuations of 1.9 dB and 0.16 nm, respectively. Experimental results reveal that the demonstrated laser has good stability and low threshold power.

Performance analysis of an optical TDM transmission link considering fiber dispersion and demultiplexer crosstalk

This paper outlines the performance evaluation of high speed optical time division multiplexed (OTDM) system considering the effect of fiber dispersion and the impairments of nonlinear optical loop mirror (NOLM) used as a demultiplexer. The system performance gets deteriorated by the channel crosstalk and the relative intensity noise (RIN) of the NOLM. Our investigation demonstrates that the system is penalized in terms of power penalty significantly when the intrinsic crosstalk becomes higher than −25 dB. The power coupling ratio must be within the range of 0.47–0.53 in order to maintain the intrinsic crosstalk within −25 dB level. Our analysis also indicates that the system suffers more power penalty with the increasing number of demultiplexing channel and the amount of pulse walk-off between signal and control pulse. Apart from channel crosstalk the system performance is also investigated taking into account the effect of intensity fluctuation of the demultiplexed signal which is called RIN of the NOLM. The system suffers from significant power penalty for the value of such noise higher than 10−5. It is also reported that the power penalty is significant when the chromatic dispersion index becomes higher than 0.30.

Rectangular dark pulses in all-normal dispersion fiber oscillator

We report the generation of rectangular dark pulses (RDPs) in ytterbium (Yb) doped fiber laser using a nearly 50:50 coupler based nonlinear optical loop mirror (NOLM) in a simple linear standing wave cavity. Width of the dark pulse decreases with increase in the pump power whereas depth of the rectangular pulse profile remains constant. The nonlinear reflectance characteristics of the NOLM used in the cavity has been experimentally investigated to understand its role in formation of RDPs. Numerical simulations were performed to show the evolution of dark pulses in a fiber oscillator in all normal dispersion configuration. Both experimental and numerical simulations indicate that reverse saturable absorption supports formation of dark pulses. With increase in the saturation energy, width of RDP was found to decrease in line with the experimental observations. Further, an ansatz is proposed to model RDPs. The ansatz is used in cubic quntic Ginzburg-Landau equation to establish relationship between pulse parameters and cavity parameters. Results from analytical formalism are in accordance with the experimental measurements and numerical simulations. Detailed theoretical analysis reveals that experimentally observed RDPs do not belong to domain wall type.

Low pedestal sub-17 fs pulse generation through cascaded self-similar compression in photonic crystal fibers

We propose a realistic numerical model based on cascaded self-similar pulse compression to generate low pedestal ultrashort pulses at 2.5 µm. Self-similarity in photonic crystal fibers (PCFs) has been attained by modelling exponentially dispersion decreasing and exponentially nonlinearity increasing chalcogenide PCFs. The tapered PCF (TPCF) model is theoretically designed using the fully vectorial effective index method (FVEIM) to obtain accurate effective refractive index, group velocity dispersion (GVD), and Kerr nonlinearity values with low computation time. Initially, fundamental soliton compression in As2S3 tapered PCF is being investigated for the pedestal-free reduction of the pulse width. Subsequently, the compression factor is further enhanced in the As2Se3 PCF-based nonlinear optical loop mirror. The numerical outcomes reveal a compression of 1 ps pulse down to a pulse width of 16.6 fs with a peak power of 116 W in a total fiber length of L = 61 cm.

Time-bandwidth product of noise-like pulses within the mode-locked regions of a figure-eight fiber laser: theoretical and experimental analysis

We present for the first time, to the best of our knowledge, a theoretical and experimental analysis of the time-bandwidth product (TBP) of noise-like pulses (NLPs) within the mode-locked regions of an erbium-doped figure-eight fiber laser (EDFEFL). The tuning of the mode-locked regions was carried out by varying and recording the values of the angle of the polarization controllers in the ring section and in the nonlinear optical loop mirror (NOLM). Within the mode-locked regions, we obtained a large variability of the TBPs. We found that the variability of the temporal profile is greater than that of the spectral profile in all mode-locked regions; for this reason, the TBPs of all mode-locked regions have a behavior proportional to the temporal profile. We also identify the temporal and spectral profiles corresponding to the maximum and minimum TBP. Finally, among all the NLPs within the mode-locked regions, we identify and study the one that is Fourier-transform-limited. We analyze NLPs with a carrier wavelength of 1562 nm with duration in the order of nanoseconds and a repetition rate of 0.9 MHz.

Parabolic pulses generation based on nonlinear optical fiber loop mirror

A parabolic pulse generator based on non-reciprocal nonlinear optical fiber loop mirror is proposed. When an incident triangular optical pulse is split into two branches of the loop mirror, two coherent light fields with linear intensity profile are counter-propagated in the interferometer and suffer different nonlinear phase shift caused by self-phase modulation. Because the intensity of interference field is determined by the amplitudes and phase relationship between two coherent lights, the interference result gives parabolic pulse. This concept is verified by theoretical analysis and simulation. In the experiment, tunable bright and dark parabolic pulses with repetition frequencies of 8, 9, and 10 GHz are simultaneously generated, which agree with the theoretical expectation well and exhibit large operation bandwidth.

Wavelength-interval-switchable multi-wavelength thulium-doped fiber laser with a nonlinear dual-pass Mach-Zehnder interferometer filter in 2-μm-band

A wavelength-interval-switchable multi-wavelength thulium-doped fiber laser (MWTDFL) is proposed and demonstrated, based on a nonlinear dual-pass Mach-Zehnder interferometer (NDP-MZI) filter for the first time. We investigate the NDP-MZI filter theoretically and experimentally, which has both the transmission characteristics of a dual-pass Mach-Zehnder interferometer (DP-MZI) for wavelength selection and a nonlinear optical loop mirror (NOLM) for suppressing wavelength competition. By tuning two polarization controllers (PCs), two 20-wavelength operations with an opposite phase and a same wavelength-interval of 0.46 nm are obtained and switched between each other flexibly. The maximum power fluctuation and wavelength drift measured are 0.612 dB and 0.04 nm, respectively. In addition, due to the power-equalizing effect resulting from the NOLM, 60 lasing wavelengths within a 3-dB bandwidth with an adjacent wavelength-interval of 0.23 nm are also obtained by adjusting the PCs carefully, and the stability of the MWTDFL is measured experimentally. Furthermore, the performance of the NDP-MZI using an 80 m highly nonlinear fiber has been studied in detail as well. The proposed MWTDFL may find great applications in optical communication and optical sensing.

Nonlinear Absorbing-Loop Mirror Mode-Locked all-Polarization-Maintaining Yb-Doped Fiber Laser

In this paper, a nonlinear absorbing-loop mirror (NAbLM) is constructed in a polarization-maintaining (PM) version and applied in the all-normal dispersion (ANDi) regime, for the first time to our best knowledge. To investigate the properties of NAbLM in contrast with typical nonlinear optical loop mirror (NOLM), an asymmetric optical coupler is used in the NAbLM. In experiment, both the NAbLM and NOLM can be used to mode-lock a Yb-doped fiber (YDF) laser and enable a type of noise-like pulse (NLP) generation. Noticeably, however, it is further found that, in comparison to NOLM, NAbLM evidently enables both temporal and radio frequency (RF) noise suppressions, as well as elimination of RF variations. Further calculations show that the saturable property of the incorporated absorber plays a vital role in differing the NAbLM from typical NOLM. Our results demonstrate some new features of NAbLM, especially in respect to temporal pulse shaping, indicating that NAbLM can be a useful type of fiber loop mirror in nonlinear fiber optics.

High-energy all-PM Yb-doped fiber laser with a nonlinear optical loop mirror

We report on a self-starting Yb-doped fiber laser based on all-normal dispersion polarization maintaining (PM) fiber components. The cavity is mode-locked using a nonlinear optical fiber loop mirror (NOLM) that provides a stable generation of linearly polarized, linearly chirped picosecond pulses. The achieved pulse energy is 5.9 nJ. External grating-pair compression down to 265 fs has been demonstrated.

2 µm noise-like mode-locked fiber laser based on non-linear optical loop mirror

2 µm noise-like mode-locked fiber laser based on non-linear optical loop mirror

Study of the output characteristics of nonlinear optical loop mirror on an erbium-doped mode-locked fiber laser

Study of the output characteristics of nonlinear optical loop mirror on an erbium-doped mode-locked fiber laser

Observation of square- and h-shaped pulse from a mode-locked erbium-doped fiber laser.

We demonstrate the generation of a type of square-shaped pulse in a passively mode-locked erbium-doped fiber laser based on the nonlinear optical loop mirror technique. Through adjusting the pump power and polarization state, square-shaped pulses are generated. Furthermore, we investigate the pulse profile in relation to the optical spectrum. By filtering out short-wavelength spectrum components gradually, pulse shaping is achieved, and the top of the square-shaped pulse becomes flat. Subsequently, by filtering out long-wavelength spectrum components, a type of h-shaped pulse is obtained and the formation reason is also investigated.

The nonlinear optical loop mirror: soliton and noise-like pulse emission in a figure-eight fiber laser

In this article, a symmetrical nonlinear optical loop mirror (NOLM) exhibiting a polarization-dependent transmission is evaluated to generate optical pulse emission in a figure-eight fiber laser in the soliton and noise-like pulse (NLP) regimes. The NOLM structure relies on a 50:50 fiber coupler, a loop with highly twisted single-mode optical fiber and a quarter-wave retarder (QWR) to break the polarization asymmetry. The pulse operation regime is determined by properly adjusting the NOLM low-power transmission, which is easily realized by the rotation of the QWR angle. Soliton pulses of 1.48 ps pulse duration and peak power of 18 W were observed with a peak to peak separation of 1.25 µs, corresponding to a fundamental cavity repetition rate of 0.8 MHz. Moreover, by incrementing the NOLM low-power transmission, NLP emission is generated exhibiting a wide and smooth spectrum of 8.5 nm bandwidth.