Length Of Single-mode Fiber Research Articles

Analysis of high-quality ultrashort pulse generation based on a dual-drive Mach-Zehnder modulator and chirp compensation

This study reports on high-quality picosecond pulse generation using a single-stage dual-drive Mach-Zehnder modulator (DDMZM) and chirp compensation. Sinusoidal microwave signals with different amplitudes are sent to two RF ports of the DDMZM to form a pulse train and pulse compression is achieved by compensating for the linear frequency chirp with a single-mode fiber (SMF). Three parameters encompassing the power difference between two RF signals, the power of RF signals, and the bias point of the DDMZM that affect the pulse formation have been numerically studied. The optimum length of SMF used for chirp compensation is obtained by simulating the temporal propagation and evolution of the pulse in SMF and 3.56-ps chirp-compensated ultrashort pulses are realized at 1.55 µm with a 25-GHz repetition rate experimentally. Modulator-based flexible ultrashort pulse generation can be achieved easily by tuning the RF generator and light source, and customized high-quality pulses according to practical applications can be expected.

Generation and evolution of stable noise-like pulse with varying dispersion in figure-of-eight erbium-doped fiber laser

A stable noise-like pulse (NLP) in a figure-of-eight nonlinear optical loop mirror (NOLM) erbium-doped fiber (EDF) laser is proposed and demonstrated. The work experimentally investigated the effect of dispersion in the generation and evolution of the NLP by varying the length of single mode fiber (SMF) in the NOLM bidirectional loop (bi-loop). The lengths are 5 m, 10 m, 15.2 m, 20.2 m, 25.9 m, 50.1 m, 76.9 m, and 103.7 m with their net cavity dispersion of −0.275 ps2, −0.386 ps2, −0.500 ps2, −0.611 ps2, −0.737 ps2, −1.269 ps2, −1.859 ps2, and −2.447 ps2, respectively. The stability of the NLP was first evaluated by analyzing a temporal trace of the spike and its pedestal from an autocorrelator, followed by a long-term stability test of 350 min. The work demonstrated a stable NLP was obtained at the SMF length of 25.9 m at a threshold pump of 260.6 mW. At the maximum pump power of 344.7 mW, the pulse energy of 7 nJ was obtained with the highest output power of 29.77 mW. The respective spike and pedestal widths were demonstrated to be at 236.87 fs and 65.74 ps, respectively. The net cavity dispersion was −0.737 ps2 with 49.82 m total cavity length, and the corresponding repetition rate was 4.25 MHz. This work presents the importance of dispersion management of the NOLM to generate a stable NLP at low threshold power.

High resolution and large measurement range voltage sensing based on an optoelectronic oscillator utilizing an unbalanced Mach-Zehnder interferometer.

A voltage sensor with high resolution and large measurement range based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The key component in the cavity to select the oscillating signal is a finite impulse response (FIR)-microwave photonic filter (MPF) which consists of a sinusoidal broadband optical signal, an unbalanced Mach-Zehnder interferometer (MZI), a section of dispersion compensating fiber, and a photodetector. The center frequency of the FIR-MPF is mainly determined by the free spectral range (FSR) of the FIR-MPF. In the lower arm of the MZI, a cylindrical piezoelectric ceramic (PZT) wrapped with a section of optical fiber acts as voltage sensing head. Due to the inverse piezoelectric effect of PZT, the variation of the voltage will cause radial deformation of the cylindrical PZT and then lead to the change of the FSR of the MZI, determining the shift of center frequency of FIR-MPF as well as the frequency of the oscillating signal of the OEO. Thus, by monitoring the shift of the oscillation frequency of the OEO using an electric spectrum analyzer or a digital signal processor, a high-speed interrogation and high-resolution voltage measurement can be realized. Additionally, in the proposed scheme, an infinite impulse response (IIR)-MPF consisting of a fiber ring resonator is cascaded with the FIR-MPF to ensure the single-mode oscillation of the OEO. The experimental results show that a total range of 1700 V voltage sensing from - 200 V to 1500 V is accomplished with the voltage sensitivity of 0.25 GHz/100 V and the resolution of 0.3 V. By adjusting the proportion of the length of single mode fiber between two branches of MZI, the impact of temperature can be greatly reduced. The proposed sensor offers advantages such as a large measurement range, high resolution, high-speed interrogation, and stability to temperature disturbances, making it highly suitable for sensing applications in smart grids.

Development of a cost-effective optical network based on free space optical (FSO) and optical fiber links for enabling smart city infrastructure: A hybrid approach

Due to the gigantic rise in the use of high data rate-based applications and the ongoing transformation of Indian cities into smart cities, there is a need for a high-speed communication framework to handle the enormous data traffic. The radio frequency (RF) and microwave links seem to be incapable of offering sufficient bandwidth required by the data-hungry applications and consequently, the need for a more effective solution for tackling the issue of scarcity of bandwidth resources has become the necessity of current network infrastructure in India. In this study, a hybrid optical fiber-free space optical (OF-FSO) system for offering high-speed data services has been proposed to assist the implementation of smart city infrastructure in India. This study aims to inspect the system performance of the proposed model under the climatic conditions of mountainous, desert, flatland, and seaside regions of India. The proposed hybrid OF-FSO system comprises a fixed single-mode fiber (SMF) of 20 km and an FSO link (having a varied transmission range between 0–2 km). The proposed hybrid model can support an FSO link of 1.72 km for a fixed SMF of 20 km, corresponding to a bit error rate of the order of 10−4 for atmospheric conditions of the seaside location of Chennai and the range of FSO link varies from 1.32 km to 1.57 km for other locations of India, for the same length of SMF and bit error rate. Furthermore, the proposed hybrid OF-FSO has demonstrated a reduction of 30.8% in Capital Expenditure (CAPEX) cost in contrast to the conventional passive optical network (PON) system.

Spectral dynamics of pseudo-distributed erbium gain in Raman random fiber laser

This work explores the spectral dynamics of pseudo-distributed erbium gain in a Raman-based open cavity hybrid random fiber laser (RFL) and compares it to two other schemes; without erbium-doped fiber (EDF) and with EDF integrated mid-cavity. The pseudo-distribution of erbium gain is simulated by splicing short segments of EDF in between single-mode fiber (SMF) spools. The three schemes have similar total SMF lengths of 78 km, and a total of 30 m EDF if any. It was established that the laser wavelength shifts significantly from 1555.936 nm to 1565.904 nm simply by the insertion of EDF mid-cavity, while the distribution of EDF segments along the cavity allowed the generation of dual-wavelength lasing at 1559.964 and 1565.236 nm. The distribution of EDF also generated a higher maximum output power of 485.60 mW compared to the schemes with lumped EDF and without EDF of 409.17 mW and 352.02 mW output powers, respectively. However, the increment came at the expense of a longer instability event from homogenous gain broadening of SBS induced Stokes and a delayed onset of laser threshold condition. We discuss the formations of these different laser schemes and its dynamics permitted solely due to the uniqueness of random fiber lasers. The scheme presents an interesting opportunity in the development of hybrid random fiber lasers, and the analysis of this structure offers insights for better designed dual-wavelength random fiber lasers and the role that rare-earth Erbium dopant plays in the formation of laser in a Raman-based fiber cavity.

Switchable and tunable multi-wavelength random distributed feedback fiber laser based on parallel Lyot filter

A switchable and tunable multi-wavelength random distributed feedback fiber laser based on a parallel Lyot filter is designed, and the transmission characteristics of the filter are deduced in detail by using the transmission matrix theory. The randomly distributed backward Rayleigh scattering generated in single-mode fiber (SMF) serves as a feedback mechanism. The number of output wavelength increases linearly with the increase of pump power. By adjusting the polarization controller (PC) to control the polarization state of the beam into the polarization maintaining fiber (PMF), laser channels with wavelength interval of 0.88 nm or 0.46 nm can be output. Where the wavelength interval depends on the wavelength phase difference caused by the corresponding branch PMF. The intensity-dependent loss (IDL) caused by the nonlinear optical loop mirror (NOLM) in the experimental structure is not only used to suppress the intense mode competition caused by EDF, but also to control the spectral range of the output wavelength to achieve tunable laser output. When the wavelength interval is 0.88 nm, the wavelength tuning ranges of the three and four laser channels are 4.4 nm and 3.7 nm, respectively. When the wavelength interval is 0.46 nm, the wavelength tuning ranges of the three and four laser channels are 3.7 nm and 4 nm, respectively. Subsequently, the influence of using different lengths of SMF on the output wavelength is analyzed. It is found that longer SMF can increase the linewidth of the laser channel, and the side mode suppression ratio (SMSR) will be improved to 39 dB. In addition, the output power fluctuation of the laser is less than 0.32 dB within 30 min, and the power amplitude and SMSR are stable at −4.47dBm and 34.5 dB, respectively, indicating that the laser can achieve flat and stable multi-wavelength output at room temperature.

Observation of the extracavity propagation of amplified bound-state soliton pulses emitted from a SWNT mode-locked fiber laser

We have experimentally investigated the extracavity propagation of amplified bound-state soliton pulses from a passively single-wall carbon nanotube mode-locked fiber laser. Under proper configuration of pump power and state of polarization, bound-state pulses exhibit a modulated optical spectrum. Except for the two main center wavelengths of 1557.3 and 1560.5 nm, additional five sidelobes centered at 1548.9, 1552.8, 1565.5, 1569.0 and 1573.1 nm are observed. Besides, the propagation after the extracavity erbium-doped fiber amplification is also investigated. It is demonstrated the significant change of the pulse separation is experimentally obtained when adjusting the extracavity PC before the autocorrelator, which is attributed to the coherent nonlinear soliton-soliton interaction. In addition, when different lengths of single-mode fiber after extracavity amplifier is removed or added, the significant change of the pulse separation is also observed, which matches well with the simulated evolution of pulse separation with respect to the SMF length. The pulse separation change of amplified bound soliton pulses is experimentally and theoretically investigated. These results provide further physical understanding of the generation and extracavity propagation of bound-state soliton pulses.

Effect of Rayleigh backscattering on the multiwavelength random fiber laser bandwidth

We investigated the effect of Rayleigh backscattering on the bandwidth of a multiwavelength random fiber laser (MWRFL). This MWRFL is based on an erbium-doped fiber amplifier (EDFA) as the gain medium and a Lyot filter as the wavelength-selective device. A high number and stable lasing line is generated with the assistance of a nonlinear polarization rotation effect induced by highly nonlinear fiber (HNLF) and optimized by a polarization controller. Longer single-mode fiber (SMF) produces wider multiwavelength bandwidth as compared to shorter SMF lengths, as 10 km, 3 km, and 0.1 km obtained 6.24 nm, 5.76 nm, and 2.75 nm wavelength bandwidth, respectively. When the SMF is removed from the setup, no Rayleigh backscattering is provided in the cavity, which led to a narrower multiwavelength bandwidth of 2.69 nm. Subsequently, the alteration of EDFA power, half-wave plate (HWP) angle, and quarter-wave plate (QWP) angles affected the degradation of multiwavelength performance. The best output spectrum is achieved at an EDFA power of 18 dBm, a HWP angle of 60°, and a QWP angle of 105°. The laser stability over a period of 120 min was excellent with a peak power deviation of 1.33 dB.

Temperature-insensitive refractive index sensor based on a spindle-shaped few-mode fiber modal interferometer.

A designed temperature-insensitive modal interferometer for refractive index measurement based on spindle-shaped few-mode fiber (FMF) is proposed and demonstrated. The interferometer consisting of a specific length of FMF fused between two specific lengths of single-mode fibers is bent into a balloon shape and then burned by a flame into a spindle shape to sensitize. Due to the bending of the fiber, part of the light leaks from the core to the cladding and excites the higher-order modes, and the four modes in the core of FMF interfere with the higher-order modes in the cladding. Therefore, the sensor is more sensitive to the surrounding refractive index. The experimental results show that the highest sensitivity is 237.3nm/RIU from 1.333 to 1.365. The sensor is insensitive to temperature, which solves the problem of temperature cross talk. In addition, with its advantages of a small mechanism, simple fabrication, low loss, and good mechanical strength, the proposed sensor has broad application prospects in chemical production, fuel storage, environmental monitoring, and other fields.

In2S3 nanoflakes-based saturable absorber for multi-state soliton generation

Attributing to their excellent photoelectric properties, metal sulfides are potential alternatives for femtosecond photonics. Here, indium sulfide nanoflakes are employed as saturable absorbers. The morphology and chemical composition of In2S3 nanoflakes are characterized. An In2S3 nanoflakes-based SA is fabricated by the method of evanescent light deposition with modulation depth of 2.7 % and saturable intensity of 5 MW/cm2. Stable mode locking with central wavelength of 1530 nm, 3 dB bandwidth of 4.9 nm, pulse duration of 733 fs and fundamental repetition rate of 8.18 MHz is obtained. By carefully adjusting of polarization controller and pumping power, harmonic mode locking with repetition rate of 538 MHz could be achieved. By changing the length of single mode fiber and altering the angle of PC, harmonic soliton molecules centered at 1560 nm are generated with spectral modulation period of 1.3 nm and pulse interval of 4.96 ps. The multi-state soliton generation of the laser confirms the excellent nonlinear absorption characteristics of In2S3 in the communication band, which could be expected to enable more practical instrumentations for spectroscopy, optical sensing and optoelectronics, etc.

Temperature-sensing scheme based on a passively mode-locked fiber laser via beat frequency demodulation

In this paper, we propose a temperature-sensing scheme utilizing a passively mode-locked fiber laser combined with the beat frequency demodulation system. The erbium-doped fiber is used in the laser ring cavity to provide the gain and different lengths of single-mode fibers inserted into the fiber ring cavity operate as the sensing element. Different temperature sensitivities have been acquired in the experiment by monitoring the beat frequency signals at different frequencies. The experimental results indicate that the beat frequency shift has a good linear response to the temperature change. The sensitivity of the proposed sensor is about -44 kHz/°C when the monitored beat frequency signal is about 10 GHz and the ratio of the sensing fiber to the overall length of the laser cavity is 10 m/17.5 m, while the signal-to-noise ratio (SNR) of the monitored signal is approximately 30 dB. The proposed temperature-sensing scheme enjoys attractive features such as tailorable high sensitivity, good reliability, high SNR, and low cost, and is considered to have great potential in practical sensing applications.

Analyzing spectral properties and sensing performance of multi-single-multi mode fiber combination

Here, we have executed a comprehensive investigation on the impact of the single-mode fiber (SMF) lengths on the sensing performance of multi-single-multi mode (MSM) fiber segment combination under different external parameters including temperature, strain and refractive index. An experimental setup is proposed to demonstrate the efficacy in measuring the aforementioned variables with larger sensitivity along with the resolution over the large dynamic range. In addition, different phenomenal characteristics namely fast Fourier transform, fringe visibility, and the free spectral range of the observed transmission spectrum have also been analyzed meticulously. The achieved results show that the sensitivities increase with an increase of the SMF length and are restricted to 12 cm length due to the superimposing and less resolution of the probing dips beyond this. The highest sensitivities of 133.25 pm °C−1, −3.41 pm με −1 and −52.10 nm/RIU for temperature, strain and refractive index, respectively are achieved for the 12 cm length of the SMF which assures potential employment of the proposed sensor in various sensing applications.

Analytical design of fiber-optic FM/PM demodulator

Abstract In this paper, the concept of a novel optical FM/PM demodulator which can be designed from low-cost passive optical components and can have peak-to-peak bandwidth in the GHz region has been described. The bandwidth can be increased by reducing the single mode fiber length incorporated in the design, which is followed by a corresponding fall in frequency sensitivity of the device. The bandwidth and frequency sensitivity can be traded-off keeping their product a constant. The crossover wavelength can be tuned by changing the length of the optical delay line and the length of the single mode fiber (SMF). The Figure of Merit (FOM) of the demodulator, defined as the bandwidth sensitivity product, has a value of π which is greater than that of a passive hybrid Tee microwave discriminator and those of Mach-Zehnder interferometer-based designs. Calculated third harmonic distortion (3HD) of the demodulated optical FM signal increases with the increase in FM index and with the increase in SMF length for a given modulation frequency. Typical 3HD for the device with SMF length of 1 cm is calculated to be −39.9 dB for FM index = 2 and a modulation frequency of 400 MHz.

Wide-range and highly sensitive pH sensor based on a figure-eight fiber structure coated with copper/polyvinyl alcohol hydrogel

Construction of pH sensors based on optical fiber encapsulated copper (Cu-NPs) has been accomplished. Briefly, a length of single-mode fiber (SMF) of about 25 cm has been wrapped in two rings by twisting one side of the fiber on the other from both ends to form a figure-eight shape. To upsurge the sensitivity, the sensor configuration has been immobilized with copper nanoparticles/polyvinyl alcohol hydrogel (Cu/PVA) composite. The Cu/PVA composite was employed to shape a membrane structure on the sensing active length by laminating it using the dip-coating method. The wavelength interrogation method was employed to evaluate the sensitivity of the fabricated pH sensor. The fabricated fiber pH sensor exhibits a sigmoidal response above a wide range of pH from 1 to 14. The result displays a superior sensitivity of ∼4.8 nm/pH for a range of 1-7 pH and 3.86 nm/pH for the range of 8-14 pH with an excellent linear response. Besides the great sensitivity, the dual-parameters measurement of pH and refractive index was effectively accomplished with perfect stability. The proposed fiber sensor possesses a superior performance compared with the other sensors.

One KHz Order Narrow Linewidth Fiber Laser Using Rayleigh Backscattering Mechanism in an Additional Piece Optical Fiber

A simple, low-cost, single-longitudinal mode, C-band narrow-linewidth optical fiber laser is presented based on the methodology of the Rayleigh backscattering (RBS). In this paper, a 1551 nm fiber ring laser is developed, and single-mode fiber (SMF) is added to compress the line width. When the SMF length of the RBS cavity is 120 m, the laser has better performance than that in other SMF lengths with a laser line width of 1.46 KHz with housing shield. The optical signal-to-noise ratio (OSNR) is 59.86 dB, and its maximum output power is 9.4 mW. It can quickly achieve the single longitude-mode operation by controlling the variable optical attenuator (VOA). The bit error rate at 10 Gb/s PRBS NRZ modulation is measured to be 10−9 when the optical receiving power is −16.2 dBm.

Experimental demonstration of single-span 100-km O-band 4×50-Gb/s CWDM direct-detection transmission.

We report on what is to the best of our knowledge the longest 50-Gb/s/λ O-band wavelength-division multiplexed (WDM) transmission. A pair of in-house built bismuth-doped fiber amplifiers (BDFAs) and the use of Kramers-Kronig detection-assisted single-sideband transmission are adopted to overcome the fiber loss and chromatic dispersion, respectively, in a reach-extended O-band coarse WDM (CWDM) system with a channel spacing of ∼10 nm. Through experiments on an amplified 4×50-Gb/s/λ direct-detection system based on booster and pre-amp BDFAs, we show the superior performance of single-sideband transmission in terms of both optical signal-to-noise ratio sensitivity and uniformity in performance amongst CWDM channels relative to double-sideband transmission after both 75-km and 100-km lengths of single-mode fiber. As a result, up to 100-km reach with comparable performance at all 50-Gb/s channels was achieved without the need for in-line optical amplification.

Demonstration of passively Q-switched and mode-locked operations through dispersion control in Er-doped fiber lasers with a cylindrite-based saturable absorber

Two-dimensional (2D) materials have been extensively investigated in the fields of electronics, biology, and chemistry. Especially in the field of optoelectronics, profiting from 2D layered materials-based photonic devices, such as optical switchers, optical modulators, and optical polarizers, new research areas of ultrashort pulse lasers have been opened up. Cylindrite is a kind of sulfosalt mineral with highly anisotropic optical responses. Two isotropic layers, a SnS2-like layer and a PbS-like layer, are stacked together to form a natural van der Waals heterostructure. Thin cylindrite flakes can be obtained through both mechanical exfoliation (ME) and liquid-phase exfoliation (LPE) methods. In this work, cylindrite nanosheets are prepared successfully by the LPE method. Nonlinear optical absorption properties are studied firstly in advance of exploring the potential of cylindrite as optical modulators. Dispersion management is realized by increasing the length of single-mode fiber. And with the intracavity net dispersion (IND) changing from −0.24 ps2 to −4.47 ps2, both Q-switched and mode-locked operations could be observed in a cylindrite-based Er-doped fiber laser (EDFL) in turn. Q-switched pulses outputs are obtained when the IND are about −0.24 ps2 and -0.68 ps2. The maximum average output power is 1.71 mW. The obtainable maximum single pulse energy is 53.89 nJ. Mode-locked operations are demonstrated when the IND are about −0.91 ps2, -2.01 ps2, -3.44 ps2, and -4.47 ps2. Besides, a double-ended pumped mode-locked EDFL with a signal-to-noise ratio of about 51 dB can also be obtained. This work reveals the nice nonlinear optical properties of the cylindrite-based saturable absorber and indicates that the cylindrite-based optical modulator holds tremendous potential and feasibility in the field of ultrashort pulse fiber lasers.

Dispersion Management and Pulse Characterization of Graphene-Based Soliton Mode-Locked Fiber Lasers

This paper presents the generation and characterization of femtosecond pulses utilizing graphene-polymethyl-methacrylate (PMMA) thin-film saturable absorber (SA), which is subjected to different lengths of single-mode fiber (SMF) in an erbium-doped fiber laser cavity. The graphene/PMMA-SA is prepared by using a simple transfer procedure of the thin-film on a fiber ferrule. By increasing the SMF length from 0 to 4 m, the corresponding group velocity dispersion of the entire cavity is estimated to change from −0.033 to −0.121 ps2. Analysis of the pulse performance shows that the pulse width behavior varies from 820 fs to 710 fs against different cavity lengths. Similarly, the pulse repetition rate and the spectral bandwidth can be adjusted from 12.5 to 10.0 MHz, and from 8.2 to 5.6 nm, respectively. A comprehensive discussion on the pulse performance is presented, which can contribute to widening the knowledge on the operation of graphene-based soliton mode-locked erbium-doped fiber lasers based on dispersion management by controlling the cavity length.

Wavelength switchable and tunable noise-like pulses from a 2 μm figure-eight all-fiber laser

We propose the generation of wavelength switchable and tunable noise-like pulses (NLPs) in 2 μm all-fiber laser mode-locking with nonlinear amplifier loop mirror technique. An properly segment of ultra-high numerical aperture fiber is inserted in the cavity to reduce the pump power required for achieving mode-locking at different wavelengths, and a suitable length of single mode fiber is employed to induce more intracavity birefringence. By increasing the pump power and adjusting the intracavity polarization state, the fiber laser can deliver NLPs with switchable wavelengths between 1979.4 nm and 2003.8 nm. Based on the spectral filter effect induced by fiber birefringence, continuous wavelength tunable NLP operation at these two wavebands can be achieved by adjusting polarization controllers. The center wavelength of NLP can be continuously tuned from 1952.1 nm to 1999.0 nm and from 2002.6 nm to 2014.4 nm, with corresponding spectral tuning ranges of 46.9 nm and 11.8 nm, respectively. This wavelength switchable and tunable NLP fiber laser has great potential in fiber sensing and industrial processing.

Efficient method to improve the distribution probability of dissipative soliton and noise-like pulse in all-normal-dispersion fiber lasers.

Inspired by the chirped pulse amplification technique, herein, we show an efficient method to improve the distribution probability of dissipative soliton and noise-like pulse in all-normal-dispersion fiber lasers by using an intracavity pulse power editing (PPE) technique for the first time. The dissipative-soliton fiber laser is thus simplified into three parts: a PPE link, a saturable absorber (SA), and a spectral filter. Pulse with different peak powers can be edited in the PPE link, then undergo the positive- or reverse-saturable absorption of the SA, and finally pass through the filter. Further, just by assigning the length of single-mode fiber (SMF) at different positions in the PPE link with a fixed cavity length, four pulse patterns, including dissipative soliton (DS), DS molecules, a bound pattern of DS and noise-like pulse (NLP), and pure NLP, can be controllably produced in fiber lasers. The observed bound pattern of DS and NLP is a new addition to the pulse dynamic pattern family. It is found that the longer the SMF after the gain fiber is, the pulse will be severely broadened. This pulse can easily enter the positive-saturable absorption region of most saturated absorption curves, which will increase the probability of DS radiation; if the SMF behind the gain fiber is shorter, the pulse is not severely broadened. The pulse has a high probability of entering the reverse-saturable absorption range of most saturated absorption curves, resulting in a higher likelihood of generating NLP. In experiments, it is only necessary to increase the SMF length between the gain fiber and the isolator to build a DS fiber laser; however, to construct an NLP fiber laser, only the SMF length between the gain fiber and the isolator needs to be shortened. The experimental results agree well with the numerical predictions. The results significantly broaden the design possibilities for pulse lasers, making them much more accessible to produce specific pulse patterns.